Mercury Removal from Cement Plants by Sorbent Injection ... - CHEC
Mercury Removal from Cement Plants by Sorbent Injection ... - CHEC
Mercury Removal from Cement Plants by Sorbent Injection ... - CHEC
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TECHNICAL UNIVERSITY OF DENMARK<br />
DEPARTMENT OF CHEMICAL AND BIOCHEMICAL ENGINEERING<br />
Ph.D. Thesis, June 2011<br />
<strong>Mercury</strong> <strong>Removal</strong> <strong>from</strong> <strong>Cement</strong> <strong>Plants</strong> <strong>by</strong> <strong>Sorbent</strong> <strong>Injection</strong><br />
upstream of a Pulse Jet Fabric Filter<br />
Yuanjing Zheng
I<br />
Preface<br />
This thesis is written for partial fulfillment of the requirements to obtain the<br />
Ph.D. degree at the Technical University of Denmark. The work has been carried out<br />
at the <strong>CHEC</strong> (Combustion and Harmful Emission Control) Research Centre at the<br />
Department of Chemical and Biochemical Engineering under the supervision of Prof.<br />
Anker Degn Jensen <strong>from</strong> <strong>CHEC</strong>, Department Manager Christian Windelin and<br />
Flemming Jensen <strong>from</strong> FLSmidth A/S. The project is financially supported <strong>by</strong> the<br />
Industrial PhD programme of the Danish Ministry of Science, Technology and<br />
Innovation, Danish Advanced Technology Foundation as part of the Research<br />
Platform on New <strong>Cement</strong> Production Technology.<br />
I would like to thank my supervisors, particularly Anker Degn Jensen, for<br />
their support, fruitful discussion and comments. Technician Thomas Wolfe and<br />
department workshop are gratefully acknowledged for help building the fixed-bed<br />
reactor system. I am very grateful to Mr. Peter Paone <strong>from</strong> FLSmidth A/S for reading<br />
part of the manuscript. Student Jacob Clement Nielsen is acknowledged for<br />
performing some of the screening tests. Thanks to all the other people at <strong>CHEC</strong> and<br />
FLSmidth, not mentioned here, for their great help received during my study.<br />
Finally, I would like to thank my family and friends for their support and<br />
encouragement.<br />
Yuanjing Zheng<br />
Kgs. Lyng<strong>by</strong>, June 2011
II<br />
Abstract<br />
There are growing concerns over mercury emissions due to their toxicity,<br />
volatility, persistence, and bioaccumulation in the environment. <strong>Mercury</strong> emissions<br />
<strong>from</strong> cement plants are being regulated <strong>by</strong> environmental agencies in most countries.<br />
Among the available technologies for mercury removal <strong>from</strong> flue gas, sorbent<br />
injection upstream of a polishing fabric filter is considered as the most promising and<br />
suitable technology for cement plant application. <strong>Cement</strong> plants are quite different<br />
<strong>from</strong> power plants and waste incinerators regarding the flue gas composition,<br />
temperature, gas and solid residence time, and inherent material circulation. Thus<br />
knowledge obtained <strong>from</strong> mercury removal in power plants and incinerators might<br />
not be applied to cement plants directly and fundamental investigation under well<br />
controlled cement kiln condition is imperative.<br />
Tests in simulated cement kiln flue gas show that the red brass converter<br />
developed for waste incinerator application does not work properly for either<br />
elemental or total mercury measurement. Sodium sulfite converter is developed and<br />
optimized for oxidized mercury reduction and total mercury measurement. The<br />
response time of the sulfite converter is short, which makes it appropriate for<br />
dynamic measurement of mercury adsorption and oxidation <strong>by</strong> sorbents.<br />
Screening tests of sorbents for mercury removal <strong>from</strong> cement plants have<br />
been conducted in the fixed-bed reactor system using simulated cement kiln flue gas<br />
with elemental mercury and mercury chloride sources. The tested sorbents include<br />
commercial activated carbons, commercial non-carbon sorbents, and cement<br />
materials. With elemental mercury present in the flue gas, no mercury adsorption or<br />
oxidation <strong>by</strong> non-carbon based sorbents and cement materials is observed. Generally<br />
larger amount of adsorbed mercury is obtained with sorbents that have larger mercury<br />
oxidation capacity. While all the non-carbon based sorbents and cement materials<br />
show some adsorption of mercury chloride. Among the tested sorbents the Darco Hg
activated shows the best performance of adsorption of both elemental and oxidized<br />
mercury and is recommended as the reference sorbent for fundamental investigation.<br />
Parametric studies of mercury adsorption <strong>by</strong> activated carbon have been<br />
conducted in the fixed-bed reactor regarding the effects of adsorption temperature,<br />
flue gas rate, mercury level, carbon particle size, carbon load, and flue gas<br />
composition. The mercury adsorption isotherm follows Henry’s law for the applied<br />
mercury inlet levels in this project. Henry’s constant and heat of adsorption are<br />
derived for model input. The mercury adsorption capacity does not change with O2,<br />
CO, and NO levels in the flue gas, but decreases when CO2, H2O, SO2, and NO2<br />
concentrations increase. Slight promoting effects of HCl on mercury adsorption are<br />
observed with HCl in the flue gas up to 20 ppmv. Larger mercury adsorption capacity<br />
is obtained when HCl is removed <strong>from</strong> the gas. Similar adsorption behaviors of<br />
mercury chloride and elemental mercury <strong>by</strong> Darco Hg activated carbon are observed<br />
using simulated cement kiln flue gas, due to the effective catalytic oxidation of<br />
elemental mercury <strong>by</strong> the activated carbon.<br />
Mathematical models are developed to simulate mercury adsorption <strong>by</strong> a<br />
single carbon particle, fixed carbon bed, in the duct and fabric filter. The developed<br />
fixed bed model can reasonably simulate the mercury breakthrough curve of the fixed<br />
carbon bed. Comparison with fabric filter model simulations and experimental data<br />
<strong>from</strong> slipstream tests at a cement plant shows that the developed two-stage model is a<br />
valuable tool and can reasonably predict the mercury removal <strong>from</strong> cement plants <strong>by</strong><br />
carbon injection upstream of a fabric filter.<br />
III
Resumé (summary in Danish)<br />
Der er voksende bekymringer over kviksølvemissioner grundet disses<br />
giftighed, flygtighed, bestandighed og biologisk akkumulation i miljøet.<br />
Kviksølvemissioner fra cementfabrikker reguleres i de fleste lande af miljøorganer.<br />
Blandt de tilgængelige teknologier til fjernelse af kviksølv fra røggas anses<br />
sorbentinjektion opstrøms for et posefilter for den mest lovende og velegnede<br />
teknologi til anvendelse på cementfabrikker. <strong>Cement</strong>fabrikker er temmelig forskellige<br />
fra kraftværker og affaldsforbrændingsanlæg med hensyn til røggassammensætningen,<br />
temperatur, opholdstid af gas og faststof samt iboende materialecirkulation. Derfor<br />
kan viden opnået fra kviksølvfjernelse i kraftværker og affaldsforbrændingsanlæg<br />
ikke anvendes direkte på cementfabrikker og fundamental undersøgelse under<br />
velkontrollerede forhold svarende til cementfremstilling brændingsovn er essentielt.<br />
Test i simuleret røggas fra cementbrændingsovn viser, at en kommerciel<br />
konverter udviklet til anvendelse på affaldsforbrændingsanlægs ikke virker godt for<br />
hverken elementær kviksølvmåling eller total kviksølvmåling. Som en del af<br />
projektet er der udviklet en natriumsulfitkonverter til reduktion af oxyderet kviksølv<br />
samt total kviksølvmåling. Sulfit konverterens responstid er kort hvilket gør den<br />
velegnet til dynamisk måling af kviksølv adsorption og oxidation med sorbenter.<br />
Screeningsforsøg af sorbenter til fjernelse af kviksølv fra cementfabrikker er<br />
udført i et fixed bed reaktorsystem ved brug af simuleret røggas fra cementsovne med<br />
både elementært kviksølv samt kviksølvklorid. De testede sorbenter inkluderer<br />
kommercielle aktivt kul- og kommercielle ikke-kulstofsorbenter samt<br />
cementmaterialer. Med elementært kviksølv tilstede i røggassen blev hverken<br />
kviksølvadsorption eller -oxidation observeret med de ikke kulstofbaserede sorbenter<br />
og cementmaterialer. Generelt opnås større adsorberet mængde kviksølv med<br />
sorbenter der har større kviksølvoxidationskapacitet. Alle de ikke-kulstofbaserede<br />
sorbenter og cementmaterialer viser nogen adsorption af kviksølvklorid. Blandt de<br />
testede sorbenter udviser Darco Hg aktivt kul den bedste evne til adsorption af både<br />
IV
elementært og oxideret kviksølv og anbefales som referencesorbent i den<br />
fundamentale undersøgelse.<br />
Parameterstudier af kviksølvadsorption med aktivt kul er blevet udført i en<br />
fixed bed reaktor med hensyn til effekter af adsorptionstemperatur, røggasmængde,<br />
kviksølvniveau, kulstofpartikelstørrelse, kulstofbelastning og røggassammensætning.<br />
I dette projekt følger kviksølvadsorptionsisotermen Henrys lov for den anvendte<br />
koncentration af kviksølv. Henrys konstant og adsorptionsvarmen er fundet til<br />
indsættelse i model. Kviksølvadsorptionskapaciteten ændres ikke som følge af O2,<br />
CO og NO niveauer i røggassen, men falder når CO2, H2O, SO2, og NO2<br />
koncentrationerne stiger. En mindre positiv effekt af HCl på kviksølvadsorption er<br />
observeret med HCl i røggassen op til 20 ppmv. Større kviksølv adsorptionskapacitet<br />
opnås når HCl fjernes fra gassen. Lignende adsorptionsmønster for kviksølvklorid og<br />
elementært kviksølv med Darco Hg aktivt kul er observeret ved brug af simuleret<br />
røggas fra cementsovne, på grund af den effektive katalytiske oxidation af elementært<br />
kviksølv med det aktive kul.<br />
Matematiske modeller er udviklet til at simulere kviksølvadsorption på en<br />
enkel kulpartikel, i en fixed bed af aktivt kul, i kanalen og i posefilteret. Den<br />
udviklede fixed bed model med god nøjagtighed simulere kviksølv<br />
gennembrydningskurven for fixed bed forsøgen. Sammenligning af posefiltermodel<br />
simuleringer med eksperimentelle data fra slipstrømstests på en cementfabrik viser at<br />
den udviklede to-trins model er et værdifuldt værktøj der på fornuftigvis kan<br />
forudsige kviksølvfjernelsen fra cementfabrikker med kulstofinjektion opstrøms for et<br />
posefilter.<br />
V
VI<br />
Table of contents<br />
Preface ...........................................................................................................................I<br />
Abstract.........................................................................................................................II<br />
Resumé (summary in Danish).....................................................................................IV<br />
Table of contents.........................................................................................................VI<br />
1. Introduction............................................................................................................... 1<br />
1.1 Project background ............................................................................................. 1<br />
1.2 Project objectives................................................................................................ 3<br />
1.3 Outline of the thesis ............................................................................................ 3<br />
1.4 References........................................................................................................... 4<br />
2. <strong>Mercury</strong> emissions and transformations in cement plants........................................ 6<br />
2.1 <strong>Cement</strong> production processes ............................................................................. 6<br />
2.2 <strong>Mercury</strong> contents in fuels and cement raw materials ....................................... 12<br />
2.3 <strong>Mercury</strong> emissions............................................................................................ 14<br />
2.3 <strong>Mercury</strong> transformation during combustion..................................................... 15<br />
2.3.1 <strong>Mercury</strong> transformation in coal combustion flue gas ................................ 17<br />
2.3.2 <strong>Mercury</strong> transformation within cement kiln system.................................. 23<br />
2.4 Conclusions....................................................................................................... 27<br />
2.5 Further work ..................................................................................................... 28<br />
2.6 References......................................................................................................... 28<br />
3. Review of technologies for mercury removal <strong>from</strong> flue gas .................................. 32<br />
3.1 Introduction....................................................................................................... 32<br />
3.2 <strong>Mercury</strong> avoidance technology......................................................................... 33<br />
3.2.1 Coal cleaning ............................................................................................. 33<br />
3.2.2 <strong>Cement</strong> raw material cleaning ................................................................... 33<br />
3.2.3 Fuel switching............................................................................................ 34<br />
3.3 <strong>Mercury</strong> removal <strong>by</strong> powdered activated carbon injection .............................. 35<br />
3.3.1 Parameters affecting mercury removal <strong>by</strong> activated carbon injection....... 35<br />
3.3.2 Tests of mercury sorbents in lab-scale fixed-bed reactors......................... 38<br />
3.3.3 <strong>Sorbent</strong> injection in power plants .............................................................. 49<br />
3.3.5 Carbon surface chemistry and mechanisms of mercury capture on carbons<br />
............................................................................................................................ 58<br />
3.3.6 Processing and reuse of mercury laden activated carbon .......................... 63<br />
3.3.7 Applicability of sorbent injection in cement plants................................... 65
3.4 <strong>Mercury</strong> removal <strong>by</strong> activated carbon bed ....................................................... 65<br />
3.5 <strong>Mercury</strong> control <strong>by</strong> flue gas desulphurization systems .................................... 67<br />
3.6 <strong>Mercury</strong> removal <strong>by</strong> sodium tetrasulfide injection........................................... 68<br />
3.7 Enhanced mercury removal <strong>by</strong> oxidation ......................................................... 69<br />
3.8 <strong>Mercury</strong> removal <strong>by</strong> roaster process................................................................. 72<br />
3.9 Conclusions....................................................................................................... 73<br />
3.10 Further research requirement.......................................................................... 75<br />
3.11 Abbreviations.................................................................................................. 75<br />
3.12 References....................................................................................................... 76<br />
4. Experimental methods and materials...................................................................... 86<br />
4.1 Description of the fixed-bed reactor system..................................................... 86<br />
4.1.1 Gas mixing system..................................................................................... 88<br />
4.1.2 <strong>Mercury</strong> vapor addition system ................................................................. 88<br />
4.1.3 Humidifier for water vapor addition.......................................................... 90<br />
4.1.4 Low temperature furnace and fixed-bed reactor........................................ 92<br />
4.1.5 <strong>Mercury</strong> analysis system............................................................................ 93<br />
4.2 Converter and sorbent materials ..................................................................... 100<br />
4.3 Flue gas composition ...................................................................................... 103<br />
4.4 <strong>Sorbent</strong> load in fixed-bed test ......................................................................... 103<br />
4.5 Experimental procedure.................................................................................. 105<br />
4.6 <strong>Sorbent</strong> characterization ................................................................................. 106<br />
4.6.1 Scanning electron microscopy ................................................................. 106<br />
4.6.2 Particle size distribution........................................................................... 107<br />
4.6.3 Analysis of mercury in sorbent................................................................ 108<br />
4.7 References........................................................................................................... 108<br />
Appendix............................................................................................................... 110<br />
4A Check of mercury analyzer ............................................................................. 110<br />
4B Water addition verification ............................................................................. 112<br />
5. Dynamic measurement of mercury adsorption and oxidation on activated carbon in<br />
simulated cement kiln flue gas.................................................................................. 117<br />
5.1 Review of gaseous mercury measurement technology................................... 117<br />
5.2 Performance test of the mercury analyzer ..................................................... 119<br />
5.3 Performance test of the red brass converter.................................................... 121<br />
5.4 Performance of the sulfite converter............................................................... 125<br />
5.5 Examples of dynamic measurement of mercury adsorption and oxidation on<br />
activated carbon .................................................................................................... 131<br />
5.6 Suggestions for practical application of the converter.................................... 132<br />
5.7 Conclusions..................................................................................................... 133<br />
VII
5.8 References....................................................................................................... 134<br />
6. Effects of bed dilution and carbon load on mercury adsorption capacity of activated<br />
carbon........................................................................................................................ 137<br />
6.1 Introduction..................................................................................................... 137<br />
6.2 Effects of carbon load..................................................................................... 137<br />
6.3 Effects of bed dilution..................................................................................... 141<br />
6.4 Effects of sand load......................................................................................... 143<br />
6.5 Effects of carbon loading location.................................................................. 144<br />
6.6 Effects of bed materials .................................................................................. 145<br />
6.7 Effects of carbon type and particle size .......................................................... 146<br />
6.8 Tests with only Portland cement..................................................................... 147<br />
6.9 Conclusions..................................................................................................... 148<br />
6.10 References..................................................................................................... 149<br />
7. Screening tests of mercury sorbents ..................................................................... 151<br />
7.1 Introduction..................................................................................................... 151<br />
7.2 <strong>Sorbent</strong> properties and compositions.............................................................. 153<br />
7.3 SEM-EDX analysis of fresh sorbents ............................................................. 157<br />
7.4 Baseline test .................................................................................................... 160<br />
7.5 Screening tests in nitrogen.............................................................................. 160<br />
7.6 Screening tests in simulated cement kiln flue gas with elemental mercury<br />
source.................................................................................................................... 162<br />
7.7 Screening tests in simulated cement kiln flue gas with HgCl2 source............ 166<br />
7.8 Conclusions..................................................................................................... 170<br />
7.9 References....................................................................................................... 172<br />
8. Fundamental investigation of elemental mercury adsorption <strong>by</strong> activated carbon in<br />
simulated cement kiln flue gas.................................................................................. 176<br />
8.1 Introduction..................................................................................................... 176<br />
8.2 Effect of adsorption temperature .................................................................... 177<br />
8.3 Isotherm tests .................................................................................................. 179<br />
8.4 Effect of carbon particle size .......................................................................... 183<br />
8.5 Effect of flue gas flow rate ............................................................................. 185<br />
8.6 Effects of flue gas compositions..................................................................... 186<br />
8.6.1 Effect of CO2 ........................................................................................... 186<br />
8.6.2 Effect of O2 .............................................................................................. 188<br />
8.6.3 Effect of H2O ........................................................................................... 189<br />
8.6.4 Effect of CO............................................................................................. 192<br />
8.6.5 Effect of SO2............................................................................................ 193<br />
8.6.6 Effect of HCl............................................................................................ 195<br />
VIII
8.6.7 Effect of NO............................................................................................. 197<br />
8.6.8 Effect of NO2 ........................................................................................... 198<br />
8.7 Conclusions..................................................................................................... 201<br />
8.8 References....................................................................................................... 202<br />
9. Fundamental investigation of mercury chloride adsorption <strong>by</strong> activated carbon in<br />
simulated cement kiln flue gas.................................................................................. 206<br />
9.1 Introduction..................................................................................................... 206<br />
9.2 Effect of temperature ...................................................................................... 207<br />
9.3 Effect of flue gas composition ........................................................................ 210<br />
9.4 Conclusions..................................................................................................... 212<br />
9.5 References....................................................................................................... 213<br />
10. Simulation of mercury adsorption <strong>by</strong> fixed carbon bed ..................................... 215<br />
10.1 Adsorption equilibrium................................................................................. 215<br />
10.2 Transport consideration in adsorption process ............................................. 216<br />
10.2.1 External transport................................................................................... 216<br />
10.2.2 Internal transport.................................................................................... 218<br />
10.3 Modeling of adsorption in a single particle .................................................. 220<br />
10.4 Fixed bed adsorption model.......................................................................... 226<br />
10.5 Conclusions................................................................................................... 239<br />
10.6 List of symbols.............................................................................................. 239<br />
10.7 References..................................................................................................... 241<br />
11. Simulation of mercury removal <strong>by</strong> activated carbon injection upstream of a fabric<br />
filter........................................................................................................................... 243<br />
11.1 Common assumptions for mercury removal in the duct and fabric filter..... 243<br />
11.2 Duct model.................................................................................................... 246<br />
11.3 Model for the filter cake ............................................................................... 253<br />
11.4 Fabric filter model ........................................................................................ 257<br />
11.5 Two-stage model........................................................................................... 263<br />
11.6 Conclusions................................................................................................... 269<br />
11.7 List of symbols.............................................................................................. 270<br />
11.8 References..................................................................................................... 271<br />
12. Concluding remarks............................................................................................ 273<br />
13. Suggestions for further work .............................................................................. 277<br />
IX
1.1 Project background<br />
1<br />
1<br />
Introduction<br />
There are growing concerns over mercury emissions due to its toxicity, volatility,<br />
persistence, and bioaccumulation in the environment. According to an inventory of<br />
global mercury emissions to the atmosphere <strong>from</strong> anthropogenic sources <strong>by</strong> Pacyna et al.<br />
[1], the largest emissions of mercury are <strong>from</strong> combustion of fossil fuels. <strong>Mercury</strong><br />
emissions <strong>from</strong> cement and mineral production are the second largest anthropogenic<br />
sources.<br />
While mercury emissions <strong>from</strong> waste incinerators and power plants have been<br />
and continue to be regulated <strong>by</strong> the authorities in many countries, strict mercury emission<br />
limits for cement plants are also established <strong>by</strong> different countries [2-6]. U.S.<br />
Environmental Protection Agency (EPA) recently set the nation’s first limits on mercury<br />
emissions <strong>from</strong> existing cement kilns and strengthened the limits for new kilns [7-9]. The<br />
mercury emission limit for existing and new cement plants is 55 and 21 pound/million<br />
tons of clinker, respectively. These emission limits correspond to 10 and 4 µg/Nm 3 .<br />
When fully implemented in 2013, EPA estimates the annual mercury emissions will be<br />
reduced about 92% [8]. It is estimated that few cement kilns in U.S. can achieve this new<br />
mercury emission limit without some changes to the system, either through operational<br />
adjustment or use of add-on technology.<br />
<strong>Mercury</strong> is present in both cement raw materials used for kiln feed and fuels used<br />
in the cement production process. Due to rising energy costs and ever stricter energy and<br />
environmental regulations, alternative fuel technology is becoming an important factor in<br />
controlling costs. To gain a competitive edge, many cement and mineral producers<br />
worldwide have set ambitious targets for increasing their future usage of alternative fuels
- both waste-derived fuel and biomass. High mercury containing alternative fuels such as<br />
chemical waste, domestic waste and sewage sludge are also incinerated in cement plants<br />
and high mercury emission problems have been encountered. To ensure that the mercury<br />
emission limit is met, FLSmidth has initiated research on mercury removal <strong>from</strong> cement<br />
plants.<br />
Due to the extremely low concentration range of mercury in the flue gas, mercury<br />
emission control techniques are technically challenging and expensive. Currently,<br />
activated carbon injection upstream of a particulate control device such as fabric filter<br />
has been shown to have the best potential to remove both elemental and oxidized<br />
mercury <strong>from</strong> the flue gas for combustion facilities not equipped with a wet flue gas<br />
desulphurization plant [10]. This also applies to cement plants where typically no wet<br />
flue gas desulphurization unit is installed. In cement plant application sorbent will be<br />
injected upstream of a polishing filter instead of an existing filter in order to separate<br />
carbon <strong>from</strong> the cement materials and save the disposal cost of sorbent and cement<br />
materials mixture.<br />
Although activated carbon is the most studied sorbent for capturing mercury <strong>from</strong><br />
power plant flue gas, mercury adsorption <strong>by</strong> activated carbon is not clearly understood<br />
yet, and research and development efforts are still needed before carbon injection may be<br />
considered as a commercial technology for wide use [2]. New sorbents need to be<br />
developed, the sorbent costs need to be reduced and the amount of carbon injected needs<br />
to be kept to a certain level to minimize the cost. Furthermore, mercury adsorption<br />
stability <strong>by</strong> sorbents needs to be proved.<br />
Extensive research has been carried out to reduce mercury emissions <strong>from</strong> coal<br />
combustion and waste incineration, but very little efforts have been concentrated on<br />
mercury removal in cement plants. The mercury removal not only depends on the sorbent<br />
but also on the speciation of mercury, flue gas composition and temperature, and the<br />
system configuration. The mercury emissions and gas stream characteristics <strong>from</strong> coal<br />
combustion and waste incineration are quite different <strong>from</strong> those <strong>from</strong> cement kilns [4].<br />
Thus knowledge obtained <strong>from</strong> mercury removal in power plants and incinerators might<br />
2
not be applied to cement plant directly. Non-carbon based cement-friendly sorbent is<br />
desired so that the mercury containing sorbent can be used in cement production instead<br />
of costly disposal.<br />
Despite the considerable experimental research that has been carried out to date,<br />
few models for mercury adsorption <strong>by</strong> activated carbon injection in power plant or<br />
incinerator flue gas have been proposed. A comprehensive model is desired to estimate<br />
appropriate design and operating strategies that would lead to efficient and economic<br />
control of mercury.<br />
1.2 Project objectives<br />
The overall goal of this project is to develop and advance improved mercury control<br />
technologies using sorbent injection upstream of a pulse jet fabric filter for cement plant.<br />
Specific objectives are as follows:<br />
1. To obtain updated knowledge of mercury control technologies relevant to cement<br />
plant <strong>by</strong> comprehensive literature review.<br />
2. To develop an experimental lab setup and screen sorbents for capturing mercury<br />
<strong>from</strong> cement kiln flue gas.<br />
3. To test and develop thermal catalytic converters for oxidized mercury reduction and<br />
total mercury measurement.<br />
4. To develop an understanding of sorbent chemistry and provide mechanistic<br />
understanding and kinetic rates for sorbents of interest.<br />
5. To develop mathematic models that can describe mercury removal in fixed-bed and<br />
predict mercury removal efficiency in cement plant <strong>by</strong> injecting sorbent upstream of a<br />
fabric filter.<br />
1.3 Outline of the thesis<br />
The thesis starts with a chapter (Chapter 2) on introduction of cement production<br />
process and mercury emission and transformation in cement kiln systems. Then in<br />
Chapter 3 available knowledge on mercury removal technologies <strong>from</strong> flue gas is<br />
3
eviewed and the applicability of the reviewed technologies in cement kilns is analyzed.<br />
Properties and performance of typical sorbents are also presented.<br />
Experimental methods and materials are presented in Chapter 4. Chapter 5<br />
particularly deals with the test of a red-brass based converter and development of a<br />
sulfite-based oxidized mercury reduction unit for total gaseous mercury measurement.<br />
Effects of bed dilution and carbon load on equilibrium mercury adsorption capacity of<br />
the activated carbon are investigated in chapter 6. Screening tests of different sorbent<br />
materials in the fixed-bed reactor under simulated cement kiln flue gas are reported in<br />
Chapter 7. Chapter 8 deals with fundamental investigation of mercury adsorption <strong>by</strong><br />
activated carbon in simulated cement kiln flue gas using elemental mercury source.<br />
<strong>Mercury</strong> adsorption mechanism and kinetics <strong>by</strong> the activated carbon will be reported.<br />
The fundamental investigation of mercury chloride adsorption <strong>by</strong> the activated carbon in<br />
simulated cement kiln flue gas will be reported in Chapter 9.<br />
Chapters 10 and 11 will deal with simulations of mercury adsorption <strong>by</strong> the<br />
activated carbon. Chapter 10 focuses on simulation of mercury adsorption <strong>by</strong> a single<br />
carbon particle and a fixed carbon bed. Simulation of mercury adsorption <strong>by</strong> activated<br />
carbon injection upstream of a fabric filter is the topic of Chapter 11. Validation of the<br />
developed duct-fabric filter two-stage model <strong>by</strong> available pilot-scale data is reported.<br />
Finally, conclusions <strong>from</strong> the project are presented in Chapter 12. Suggestions for<br />
further work are given in Chapter 13.<br />
1.4 References<br />
[1] E.G. Pacyna, J.M. Pacyna, F. Steenhuisen, S. Wilson, Global anthropogenic mercury<br />
emission inventory for 2000, Atmospheric Environment. 40 (2006) 4048-4063.<br />
[2] The European Parliament and the Council of the European Union, Union directive<br />
2000/76/EC on the incineration of waste, 2000.<br />
[3] J. Werther, Gaseous emissions <strong>from</strong> waste combustion, Journal of Hazardous Materials. 144<br />
(2007) 604-613.<br />
[4] G. Ebertsch and S. Plickert, German contribution to the review of the reference document on<br />
best available techniques in the cement and lime manufacturing industries, Part I: Lime<br />
manufacturing industries, 2006.<br />
4
[5] German <strong>Cement</strong> Works Association, Environmental protection in cement manufacture, VDZ<br />
activity report 2003-2005.<br />
[6] Canadian Council of Ministers of the Environment, Canada-wide standards for mercury<br />
emissions, 2000.<br />
[7] U.S. EPA, EPA sets first national limits to reduce mercury and other toxic emissions <strong>from</strong><br />
cement plants, http://yosemite.epa.gov/opa/admpress.nsf, accessed September 6, 2010.<br />
[8] U.S. EPA, Fact sheet, Final amendments to national air toxics emission standards and new<br />
source performance standards for Portland cement manufacturing, 2010.<br />
[9] U.S. EPA, National emission standards for hazardous air pollutants <strong>from</strong> the Portland cement<br />
manufacturing industry and standards of performance for Portland cement plant, 40 CFR Parts 60<br />
and 63, EPA-HQ-OAR-2007-0877, FRLRIN 2060-AO42; EPA-HQ-OAR-2002-0051, FRLRIN<br />
2060-AO15, http://www.epa.gov /ttn/oarpg/t1/fr_notices/portland _cement_fr_080910.pdf,<br />
accessed January/17, 2011.<br />
[10] J.H. Pavlish, E.A. Sondreal, M.D. Mann, E.S. Olson, K.C. Galbreath, D.L. Laudal, S.A.<br />
Benson, Status review of mercury control options for coal-fired power plants, Fuel Processing<br />
Technology. 82 (2003) 89-165.<br />
5
<strong>Mercury</strong> emissions and transformations in cement<br />
6<br />
2<br />
plants<br />
Knowledge of mercury emissions, speciation, and transformation in cement<br />
plants is important for understanding the transport and fate of mercury released to air<br />
pollution control systems. In this chapter cement production processes are first<br />
introduced and compared with power plants and waste incinerators regarding the flue gas<br />
composition, temperature, residence time, and inherent material circulation. Then<br />
mercury contents in fuels and raw materials applied in cement production and mercury<br />
emission <strong>from</strong> Portland cement plants are presented. Finally mercury transformations in<br />
combustion flue gas and cement kiln system are reviewed.<br />
2.1 <strong>Cement</strong> production processes<br />
Although cement production also involves combustion, the flue gas temperature<br />
and residence time in cement kilns are quite different <strong>from</strong> power plants and waste<br />
incinerators. To help understand the mercury chemistry in the cement kiln systems, a<br />
brief description of the cement production process is necessary. Differences regarding the<br />
gas temperature, residence time, flue gas composition, and material cycles among cement<br />
kilns, power plants and waste incinerators are discussed below.<br />
Depending on how the raw material is handled before being fed to the rotary kiln,<br />
the processes can be categorized as dry, semi-dry, semi-wet and wet processes [1].<br />
Presently, about 78% of Europe's cement production is <strong>from</strong> dry process kilns [1], about<br />
16% of production is <strong>by</strong> semi-dry/semi-wet process kilns, and approximate 6% of cement<br />
production is <strong>from</strong> wet process kilns. Today, all new plants are based on the dry process<br />
and many old wet plants are either replaced or converted to the dry or semi-dry process.
In the dry process the feed material enters the kiln in a dry, powdered form.<br />
Production of cement can be subdivided into the areas of supply of raw materials,<br />
burning of cement clinker in the rotary kiln, and final cement production <strong>by</strong> adding<br />
interground additives [2].<br />
Raw materials for the manufacture of Portland cement clinker consist basically of<br />
limestone and aluminosilicates. At times, certain corrective materials such as bauxite,<br />
iron ore, and sand are used to compensate the specific chemical shortfalls in the raw mix<br />
composition. Apart <strong>from</strong> natural raw materials, waste materials containing lime,<br />
aluminate, silicate, and iron are also used as raw materials substitutes.<br />
Figure 2.1 illustrates a typical dry cement production process [2].The mixture of<br />
raw materials is milled in a raw mill and dried <strong>by</strong> the hot kiln flue gas. In a downstream<br />
electrostatic precipitator (ESP) or fabric filter (FF), the raw meal is separated and<br />
subsequently transported to raw meal silos. The raw meal is fed into the kiln system,<br />
which is comprised of a tower of cyclone preheaters. The calcination process can almost<br />
be completed before the raw material enters the kiln if part of the fuel is added in a<br />
precalciner, which is located between the kiln and the preheater.<br />
7
Figure 2.1. Sketch of a dry cement production process [2].<br />
In the burning of cement clinker it is necessary to maintain material temperatures<br />
of up to 1450°C to ensure the sintering reactions required [1]. This is achieved <strong>by</strong><br />
applying peak combustion temperatures of about 2000°C with the main burner flame.<br />
Figure 2.2 shows the gas temperature in the kiln system as a function of residence time<br />
and comparison with the gas temperature profiles in a pulverized coal-fired boiler and<br />
waste incinerator [1,3,4]. The combustion gases <strong>from</strong> the main kiln burner remain at<br />
temperatures above 1200°C for at least 5-10 seconds. An excess of oxygen, typically 2-4<br />
vol.%, is also required in the combustion gases of the rotary kiln as the clinker needs to<br />
be burned under oxidizing conditions. The residence time of the solid materials in the<br />
rotary kiln is 20-30 min and up to 60 min depending on the length of the kiln. The hot<br />
flue gas flows through the rotary kiln and preheater in opposite direction to the solids.<br />
The burning conditions in kilns with precalciner firing depend on the precalciner design.<br />
8
Gas temperatures <strong>from</strong> a precalciner burner are typically around 1100°C, and the gas<br />
residence time in the precalciner is approximately 3 seconds. In the cyclone preheater<br />
zone, the gas temperatures typically range <strong>from</strong> approximately 880-890°C at the inlet of<br />
the bottom preheater cyclone to 350°C at the exit of the top preheater and can have a<br />
residence time of 10 to 25 s. The post-preheater zone consists of the cooler, the mill dryer<br />
and the air pollution control device, with gas temperature typically in the range <strong>from</strong><br />
approximately 350-90°C <strong>from</strong> the top of the preheater to the exit stack outlet.<br />
Fig. 2.2. Gas temperature and retention time profiles in a cyclone preheater/precalciner<br />
kiln system, pulverized coal-fired boiler, and waste incinerator. Data are <strong>from</strong> [1,3,4].<br />
Generally the gas temperature and residence time in a kiln system is much higher<br />
and longer than those in a pulverized coal-fired boiler and waste incinerator. The<br />
temperature profile in the waste incinerator shown in Figure 2.2 is in the region <strong>from</strong> the<br />
furnace exit to the boiler exit [4] and the gas temperature is much lower than those <strong>from</strong><br />
cement kiln and pulverized coal-fired boiler.<br />
The clinker leaving the rotary kiln is cooled down <strong>by</strong> grate or planetary coolers.<br />
After cooling, the clinker is ground with a small amount of gypsum to produce Portland<br />
cement, which is the most common type of cement. In addition, blended cements are<br />
9
produced <strong>by</strong> intergrinding cement clinker with materials like fly ash, granulated blast<br />
furnace slag, limestone, natural or artificial pozzolanas [5].<br />
Table 2.1 compares the flue gas compositions among coal-fired power plant,<br />
waste incinerator and cement kiln. The major difference between cement kiln flue gas<br />
and other flue gases is the larger water and CO2 content in the kiln flue gas. The oxygen<br />
content in the kiln gas is lower than in coal combustion and waste incineration flue gas.<br />
The emission of HCl <strong>from</strong> cement kilns is normally much lower than those <strong>from</strong> waste<br />
incinerators. This could be due to the fact that the environment in cement plants is<br />
effective for absorbing acid gasses [6], such as a range of gas temperatures <strong>from</strong> 100 to<br />
1650°C, gas residence time of about 30s, high levels of turbulence, high concentrations<br />
of alkaline solids including sodium and potassium oxides, and freshly created CaO in<br />
high concentrations. Therefore, gaseous species such as HCl or HF are nearly completely<br />
captured <strong>by</strong> the inherent and efficient alkaline sorption effect of the cement kiln system<br />
[1].<br />
Table 2.1 Typical flue gas compositions in coal-fired boiler, waste incinerator, and<br />
cement kiln before air pollution control device (APCD).<br />
Pulverized coalfired<br />
boiler [1,7-<br />
10,10,11]<br />
10<br />
Waste<br />
incinerator [7,8]<br />
<strong>Cement</strong> kiln<br />
[7-9,12]<br />
O2 (vol.%) 4-6 6-15 2-4<br />
CO2 (vol.%) 10-16 5-14 14-33<br />
H2O (vol.%) 5-12 10-18 5-35<br />
CO (ppmv) 10-100 10-100 600-2600<br />
NO (ppmv) 100-1000 100-1000 475-1900<br />
NO2 (ppmv) 5-50 5-50 25-100<br />
N2O (ppmv)
<strong>Cement</strong> kilns also differ <strong>from</strong> conventional boilers and incinerators in having the<br />
dust recycles in the kiln systems. There are two material cycles in the cement kiln system,<br />
i.e., the internal and external cycle. Because of the countercurrent flow of combustion<br />
products and solids in cement kilns, volatile elements such as mercury, alkalis, sulphur<br />
and chlorine evaporated <strong>from</strong> the solids at the hot end of the kiln near the combustion<br />
zone are carried to the cold end <strong>by</strong> the combustion gases. Some of the volatile<br />
compounds pass through the entire system and exit in vapor phase through the stack.<br />
However, as the flue gas cools, some volatile compounds may adsorb/condense onto dust<br />
particles and surrounding walls in the cooler regions of the kiln system. With the raw<br />
meal, they are reintroduced to the hot zone thus establishing the internal cycle of volatile<br />
elements.<br />
The external cycle comprises the mass flows that include the raw mill and dust<br />
collectors downstream of the preheater. A small part of the circulating elements leaves<br />
the kiln with the exhaust gas dust and is precipitated in the dedusting device of the<br />
system. The collected cement kiln dust (CKD) often is blended into the raw meal for<br />
reintroduction, or part of it is fed directly to the cement mill to lower the alkali content of<br />
the clinker and meet product specifications. The CKD typically accounts for about 7% of<br />
the solid flow in cement plant with a precalciner [13].<br />
With excessive input of volatile elements, the installation of a kiln gas <strong>by</strong>pass<br />
system may become necessary in order to extract part of the circulating elements <strong>from</strong><br />
the kiln system. This <strong>by</strong>pass dust, which is usually highly enriched in alkalis, sulphur or<br />
chloride, is cooled down and then passed through a dust collector before being<br />
discharged.<br />
The operation modes of the cement plants are important for understanding<br />
mercury transformations in the kiln systems as presented in section 2.3. There are two<br />
operation modes [2], i.e., compound operation (raw-mill-on) and direct operation (rawmill-off),<br />
as shown in figure 2.3. Usually these modes are run alternately. The raw mill<br />
operates typically 80-90% of the time the kiln operates [14]. During compound operation<br />
11
the dust-containing off-gas <strong>from</strong> the cyclone preheater is used for drying and transporting<br />
the raw meal <strong>from</strong> the raw mill. Water injection in the cooler is not applied to cool down<br />
the gas. The raw meal and fly dust <strong>from</strong> the kiln system are collected <strong>by</strong> the ESP or FF<br />
and passed on to the raw meal silo. During direct operation, the raw mill is not used. The<br />
dust-containing off-gas <strong>from</strong> the kiln is cooled down in the off-gas cooler <strong>by</strong> the injection<br />
of water and subjected to subsequent dedusting in the ESP or FF.<br />
Figure 2.3. Operation models in cement production [2].<br />
These different modes of operation considerably influence the temperatures and<br />
material flows between the mill, kiln system, and dust filter. These changes also affect<br />
the trace element mass flows in the plant. Increased off-gas temperature during direct<br />
operation causes higher mercury emission level than in the compound mode [2].<br />
Moreover, regular alternation of the operation modes results in weekly cycles of mercury<br />
flows in the cement plant, as discussed in section 2.2.<br />
2.2 <strong>Mercury</strong> contents in fuels and cement raw materials<br />
A comprehensive analysis of mercury content in 291 raw material samples <strong>from</strong><br />
57 cement plants in Canada and U.S. was conducted <strong>by</strong> Hills and Stevenson [15]. Table<br />
2.2 shows the mercury contents in the fuels and raw materials applied in cement<br />
production. There is a wide range of mercury level in both fuels and cement raw<br />
materials. The reported average mercury content in the raw materials except for fly ash<br />
and recycled cement kiln dust is less than 80 ppb. In terms of fuel sources, the majority<br />
of studies reported that the average and maximum levels of mercury in coal, tire-derived<br />
12
fuel, and petroleum coke are under 0.2 and 1 ppm, respectively. Fly ash has a high<br />
mercury content and application of fly ash in cement production results in increased<br />
mercury input to the cement kiln and potentially higher mercury emissions. Process<br />
changes in cement plants such as substitution with alternative fuels may result in more<br />
plants needing solutions for mercury emission control.<br />
Table 2.2. <strong>Mercury</strong> contents in raw materials and fuels for cement production. All on dry<br />
weight basis.<br />
Material/fuel Category Sample Average Minimum Maximum<br />
number (ppm) (ppm) (ppm)<br />
Limestone [15] Primary 90 0.017
associated primarily with the sulphide phase [21]. In cement production, most of the<br />
mercury is <strong>from</strong> the kiln feed rather than the fuels when considering the amount of fuels<br />
and raw materials used [22].<br />
2.3 <strong>Mercury</strong> emissions<br />
The U.S. Portland cement association summarized 50 mercury emission tests in<br />
the U.S. during 1989-1996 [23]. All the mercury emission data for long dry, preheater,<br />
and precalciner kilns were essentially obtained with the raw-mill-on operating mode. The<br />
emission data are only for plants not burning hazardous waste. The information on<br />
mercury speciation is not available. The mercury emission concentrations varied <strong>from</strong><br />
0.02 μg/Nm 3 to 385.6 μg/Nm 3 with a mean value of 28.0 μg/Nm 3 @dry, 7% O2 and a<br />
standard deviation of 62.7 μg/Nm 3 . The maximum mercury concentration was three<br />
times higher than the second highest value.<br />
The U.S. Portland cement association has later gathered and analyzed mercury<br />
emissions and process data <strong>from</strong> 645 stack tests in 42 cement plants up to 2007 [24]. The<br />
mercury emissions include particle-bound mercury (Hgp), elemental mercury (Hg 0 ), and<br />
oxidized mercury (Hg 2+ ). The mercury emissions and speciation <strong>from</strong> cement kilns can<br />
vary over time and depend on raw materials and fuels used, and process operation. The<br />
average mercury speciation percentages for cement plants with preheater or precalciner<br />
not firing waste are 5% Hgp, 56% Hg 2+ , 39% Hg 0 for raw-mill-on mode [24], and 4%<br />
Hgp, 62% Hg 2+ , 34% Hg 0 during raw-mill-off mode.<br />
Large variations of mercury speciation during raw-mill-on and -off modes have<br />
been observed in some plants with higher mercury emission during the raw-mill-off<br />
period [25]. Measurements at Ash Grove’s Durkee plant showed that the average<br />
mercury concentration during raw-mill-on and raw-mill-off period was 410 and 2250<br />
μg/Nm 3 , respectively [25]. The larger mercury emission during raw-mill-off period is<br />
probably due to high flue gas temperature and lack of mercury adsorption <strong>by</strong> cement raw<br />
materials. Due to the high mercury emission, the Ash Grove’s Durkee plant has<br />
14
volunteered to install a sorbent injection process for removing at least 75% of the<br />
mercury [26].<br />
The complex mercury mitigation cycles within the cement kiln system make it<br />
difficult to obtain an equilibrium state due to the periodical shut down of raw mills for<br />
maintenance. It typically takes weeks to reach long term equilibrium of the mercury<br />
emission [27].<br />
The German cement manufacturing association has reported mercury emission<br />
results <strong>from</strong> 216 measurements on 44 kilns [28]. Twenty of the results were below the<br />
detection limit. Most of the measurements were below 40 μg/Nm 3 . Only six of the results<br />
were 60 μg/Nm 3 or higher.<br />
The emitted elemental mercury <strong>from</strong> Powder River Basin (PRB) coal-fired power<br />
plants ranges <strong>from</strong> approximately 10 to100 μg/Nm 3 [29]. <strong>Mercury</strong> concentrations in the<br />
flue gas <strong>from</strong> municipal solid waste combustion (200 to 1000 μg/Nm 3 ) are one to two<br />
orders of magnitude higher than for coal combustion sources (5 to 20 μg/Nm 3 ) [30,31].<br />
<strong>Mercury</strong> levels in cement kiln flue gas are generally closer to those found in coal-fired<br />
boilers and lower than those found in waste incinerators.<br />
Pacyna et al. [32] presented an inventory of global mercury emissions to the<br />
atmosphere <strong>from</strong> anthropogenic sources for the year 2000. The largest emissions of<br />
mercury to the global atmosphere are <strong>from</strong> combustion of fossil fuels, mainly coal in<br />
utility, industrial, and residential boilers. Emissions of mercury <strong>from</strong> coal combustion are<br />
between one and two orders of magnitude higher than emissions <strong>from</strong> oil combustion.<br />
Various industrial processes account for additional 30% of mercury emissions <strong>from</strong><br />
anthropogenic sources worldwide in 2000. <strong>Mercury</strong> emissions <strong>from</strong> cement and mineral<br />
production are the second largest anthropogenic sources.<br />
2.3 <strong>Mercury</strong> transformation during combustion<br />
Knowledge of mercury transformations in combustion flue gas is important for<br />
selection of the mercury control technology and understanding the fate and behavior of<br />
mercury <strong>from</strong> combustion processes. Major chemical forms of mercury <strong>from</strong> combustion<br />
15
sources are oxidized mercury and elemental mercury [33,34]. Another form is particulate<br />
mercury, which is the portion of mercury deposited on fine particles. Oxidized mercury<br />
species, such as HgCl2 and HgO, are easily removed <strong>by</strong> existing wet type air pollution<br />
control devices like flue gas desulphurization (FGD), due to its water-soluble property.<br />
Also particulate mercury is readily removed <strong>by</strong> the main dust removal control devices<br />
such as ESPs and FFs. On the other hand, elemental mercury is difficult to control<br />
because of its high vapor pressure and insolubility in water.<br />
Table 2.3 presents properties of selected mercury compounds. Metallic mercury is<br />
a heavy, silvery-white liquid metal at typical ambient temperatures and pressures, and it<br />
vaporizes under those conditions. Mercurous (Hg +1 ) and mercuric (Hg +2 ) mercury form<br />
numerous inorganic and organic chemical compounds, but the mercurous mercury is<br />
rarely stable under ordinary environmental conditions [23]. The solubility of the mercury<br />
compounds varies greatly <strong>from</strong> negligible (Hg2Cl2, HgS) to very soluble (HgCl2).<br />
Mercuric sulfate reacts with water to produce yellow insoluble basic mercuric subsulfate<br />
and sulfuric acid.<br />
Table 2.3. Properties of selected mercury compounds [23,35,36]. n.a.: not available<br />
Hg 0 Elemental<br />
mercury<br />
Hg2Cl2<br />
HgCl2<br />
Hg2SO4<br />
HgSO4<br />
Name Molar<br />
weight<br />
(g/mol)<br />
Mercurous<br />
chloride<br />
Mercuric<br />
chloride<br />
Mercurous<br />
sulphate<br />
Mercuric<br />
sulphate<br />
Melting<br />
point<br />
(�C)<br />
Boiling<br />
point<br />
(�C)<br />
16<br />
Decomposition<br />
/sublimate<br />
temperature<br />
(�C)<br />
Density<br />
(g/cm 3 )<br />
Aqueous<br />
solubility<br />
(g/l at 25�C)<br />
200.59 -38.8 356.7 n.a. 13.53 5.6�10 -7<br />
472.09 525 n.a. 383 7.15 0.002<br />
271.50 277 302 n.a. 5.43 28.6<br />
497.24 n.a. n.a. n.a. 7.56 0.51<br />
296.66 n.a. n.a. 450 6.47 decomposes
HgS <strong>Mercury</strong><br />
sulfide<br />
HgO Mercuric<br />
oxide<br />
Hg2Br2<br />
HgBr2<br />
Hg2I2<br />
HgI2<br />
Hg2F2<br />
Name Molar<br />
weight<br />
(g/mol)<br />
Mercurous<br />
bromide<br />
Mercuric<br />
bromide<br />
Mercurous<br />
iodide<br />
Mercuric<br />
iodide<br />
Mercurous<br />
fluoride<br />
HgF2 Mercuric<br />
fluoride<br />
Hg2(NO3)2 Mercurous<br />
nitrate<br />
Hg(NO3)2 Mercuric<br />
nitrate<br />
Melting<br />
point<br />
(�C)<br />
Boiling<br />
point<br />
(�C)<br />
232.66 n.a. 446-<br />
583<br />
17<br />
Decomposition<br />
/sublimate<br />
temperature<br />
(�C)<br />
Density<br />
(g/cm 3 )<br />
Aqueous<br />
solubility<br />
(g/l at 25�C)<br />
580 8.10 insoluble<br />
216.59 n.a. 356 500 11.14 insoluble<br />
560.99 405 n.a. 340-350 7.31 3.9�10 -4<br />
360.44 237 322 n.a. 6.03 slightly<br />
soluble<br />
654.98 n.a. n.a. 140 7.70 Slightly<br />
soluble<br />
454.40 259 350 n.a. 6.36 0.06<br />
439.18 n.a. n.a. 570 8.73 decomposes<br />
238.59 645 650 645 8.95 soluble,<br />
reacts<br />
525.19 n.a. n.a. 70 (dihydrate) 4.80<br />
(dihydrate)<br />
slightly<br />
soluble, reacts<br />
324.7 79 n.a. n.a. 4.3 0 soluble<br />
2.3.1 <strong>Mercury</strong> transformation in coal combustion flue gas<br />
Figure 2.4 illustrates the potential mercury transformation paths during coal<br />
combustion [33]. All forms of mercury in the coal decompose in the combustion flame to<br />
form Hg 0 (g) [30,33]. In the post combustion section where the gas temperature decreases,<br />
Hg 0 (g) may remain as a monatomic species or react to form inorganic mercurous and<br />
mercuric compounds. The principal oxidized forms of mercury in coal combustion flue<br />
gas are assumed to be Hg 2+ compounds. Oxidation of mercury via halogenation does not<br />
reach equilibrium under conditions of rapid quenching [4,7]. The degree of oxidation of<br />
mercury via gas-phase reactions therefore depends on the cooling rate of the flue gas.
After mercury chlorination, the resulting HgCl2(g) may remain in the flue gas or adsorb<br />
onto inorganic and carbonaceous ash particles entrained in the flue gas. In addition to<br />
HCl(g) and Cl2(g), O2(g) and NO2(g) are potential mercury oxidants in the flue gas<br />
[30,33].<br />
Figure 2.4. Potential mercury transformation during coal combustion and subsequently in<br />
the resulting flue gas, modified after [33].<br />
Many parameters can potentially affect the formation of various mercury species<br />
throughout a combustion system [30], including fuel type and composition, combustion<br />
environment, heat transfer/cooling rate, residence time at lower temperatures during<br />
convective cooling, configuration of APCD, and operating practices.<br />
As a starting point, the distribution of mercury species in coal combustion flue<br />
gas can be calculated using thermodynamic equilibrium calculations. Senior et al. [3]<br />
calculated the equilibrium mercury speciation in the flue gas <strong>from</strong> Pittsburgh bituminous<br />
coal combustion. Typical results <strong>from</strong> 227 to 827�C are shown in figure 2.5. At<br />
temperatures below 150�C condensed HgSO4 is the only preferred specie (not shown in<br />
figure 2.5). Similar observations were also observed <strong>by</strong> Frandsen et al. [37]. As<br />
illustrated in figure 2.5, below 450�C all of the mercury is predicted to exist as HgCl2.<br />
Above about 700�C 99% of mercury is predicted to exist as gaseous elemental mercury.<br />
18
The remaining 1% is predicted to be gaseous HgO. Between 450 and 700�C the split<br />
between HgCl2 and elemental mercury is determined <strong>by</strong> the chlorine content of the coal.<br />
%Hg<br />
100<br />
80<br />
60<br />
40<br />
20<br />
HgCl 2(g)<br />
HgO(g)<br />
0<br />
200 300 400 500 600 700 800 900<br />
Temperature (oC)<br />
19<br />
Hg(g)<br />
Figure 2.5. Equilibrium distribution of mercury species in flue gas <strong>from</strong> combustion of<br />
Pittsburgh bituminous coal. Modified after [3]. Coal composition: 4.98 wt% H, 1.48 wt%<br />
N, 1.64 wt% S, 8.19 wt% O, 7.01 wt% ash, 980 ppmm Cl, 0.11 ppmm Hg. Gas<br />
composition at a stoichiometric ratio of 1.2: 14.44 vol.% CO2, 5.69 vol.% H2O, 3.86<br />
vol.% O2, 76.59 vol.% N2, 1166 ppmv SO2, 62 ppmv HCl, 1.24 ppbv Hg, 15.5 ppmv SO3.<br />
The effect of HCl concentration on equilibrium partitioning between elemental<br />
mercury and HgCl2 is illustrated in figure 2.6 [38]. The crossover temperature between<br />
the elemental and oxidized forms increases <strong>from</strong> 530 to 740�C as the HCl concentration<br />
increases <strong>from</strong> 50 to 3000 ppm. The studied HCl level is much higher than real level in<br />
the flue gas and study using low HCl concentration will be more relevant. The crossover<br />
point is not influenced <strong>by</strong> the mercury concentration as long as hydrochloric acid is<br />
present in excess. At low temperatures, approximately 10% of the mercury is predicted to<br />
be present as HgO (not shown in figure 2.6). This is probably due to the fact that the<br />
calculations do not use simulated flue gas or include gases such as SO2.
Figure 2.6. Equilibrium distribution of elemental mercury and mercury chloride for<br />
different HCl concentrations [38]. Other gas concentrations include 7.4% O2, 6.2% CO2,<br />
12.3% H2O and N2 as balance.<br />
The high levels of mercury oxidation are most strongly correlated with high<br />
chlorine concentrations in the coal [33]. Iron is thought to catalyze the oxidation and<br />
subsequent capture of mercury [30]. Calcium likely reacts with chlorine and sulphur<br />
during the combustion process and there<strong>by</strong> reduces its ability to promote the oxidation of<br />
mercury [33]. The high percentages of elemental mercury typically found in emissions<br />
<strong>from</strong> lignite and subbituminous coal combustion can likely be attributed to their high<br />
calcium and low chlorine contents.<br />
Full-scale measurements showed that elemental mercury was dominant in the<br />
stack of coal-fired power plants, while oxidized mercury was dominant in the stack of<br />
incinerators [34,39]. This could be due to the formation of mercury compounds in<br />
furnaces and APCDs configuration differences between them. For the study of mercury<br />
removal <strong>by</strong> sorbent injection upstream of dust collectors, it is important to know the<br />
mercury speciation at the APCD inlet rather than at the stack. The data of mercury<br />
speciation in the flue gas at the inlets of APCDs are very scattered [25,40-43]. This is<br />
again due to different parameters that potentially affect the mercury speciation. Therefore,<br />
20
to develop a mercury control system for a specific plant, measurement of the mercury<br />
speciation at the APCDs’ inlet is necessary.<br />
There is disagreement in the publications on the relative importance of mercury<br />
halogenation in the flue gas <strong>by</strong> chlorine and bromine. Most literatures suggest that<br />
chlorine plays the most important role in oxidation of mercury [30,33]. However,<br />
research <strong>by</strong> Vosteen et al. [44] shows that the critical species for the halogenation of<br />
mercury in the flue gases is not chlorine, but rather bromine. The stable form of the<br />
halogens at high combustion temperatures are HCl and HBr. On cooling of the gases, the<br />
diatomic and molecular form of the halogens become stable according to the Deacon<br />
type of reactions [33,44]:<br />
4HCl �O�2HO� 2Cl(Chlorine-Deacon-reaction)<br />
(R2.1)<br />
2 2 2<br />
4HBr �O�2HO� 2Br<br />
(Bromine-Deacon-reaction) (R2.2)<br />
2 2 2<br />
The kinetics of the bromine-Deacon-reaction is more favorable [33,44]. Moreover,<br />
molecular chlorine is consumed during boiler passage <strong>by</strong> SO2 through the chlorine<br />
Griffin reaction:<br />
SO �Cl �HO�SO� HCl (Chlorine-Griffin-reaction) (R2.3)<br />
2 2 2 3 2<br />
In contrast to chlorine, the bromine-Griffin-reaction is not thermodynamically<br />
favored at temperatures above 100�C, because the Gibbs free reaction enthalpy of the<br />
bromine-Griffin-reaction is strongly positive within the whole boiler temperature range.<br />
Therefore, SO2 is not consuming Br2 during boiler passage. To summarize, the primary<br />
reason that bromine is a much more effective mercury oxidizer than chlorine is that HBr<br />
dissociates much more extensively into reactive atomic species than HCl at typical postflame<br />
conditions [45].<br />
The world average Cl contents in coals for bituminous and lignite coals are,<br />
respectively, 340±40 and 120±20 ppm [46]. The typical bromine content in the coal is<br />
about 1-10 ppm [33,44,47]. Although the chlorine content in the coal is far higher than<br />
the bromine content in the coal, the amount of molecular bromine Br2 in the flue gas may<br />
be many times higher than the amount of Cl2 in the flue gas downstream the combustion<br />
zone [44]. Recently, Niksa [45,48] also stated that homogeneous chemistry with bromine<br />
21
species is much faster than with chlorine species because the bromine atom<br />
concentrations at the furnace exit are three to four orders of magnitude greater. There<br />
might be ample supply of Br2 to oxidize the typical amounts of mercury in the coal flue<br />
gases through direct mercury bromination:<br />
Hg � Br � HgBr (Direct Hg bromination) (R2.4)<br />
2<br />
2<br />
Based on this knowledge, direct bromine injection into the flue gas has been proposed<br />
and patented to enhance mercury capture <strong>by</strong> fly ash or sorbents, or mercury oxidation<br />
followed <strong>by</strong> removal in wet flue gas desulphurization (FGD) unit [44,49]. However, the<br />
higher concentration of Br2 in the post-combustion zone is not verified <strong>by</strong> full-scale<br />
investigation due to the lack of Br2 and Cl2 measurements.<br />
The arguments on the relative importance of mercury adsorption <strong>by</strong> bromine are<br />
supported <strong>by</strong> simulation and full-scale demonstration in power plants [45,50].<br />
Simulations with only homogeneous reaction mechanism <strong>by</strong> Niksa et al. [45] show that<br />
50% mercury oxidation is obtained for a typical thermal history along a power plant gas<br />
cleaning system with 10 ppmv Br in the flue gas. In contrast, no mercury oxidation is<br />
achieved <strong>by</strong> 20 ppmv HCl in the flue gas. Homogeneous mercury oxidation <strong>by</strong> bromine<br />
begins as the flue gas cools below 600�C and accelerates sharply when the temperature<br />
drops to below 300�C. At the furnace exit, bromine atoms are present in concentrations<br />
that are comparable to HBr levels, in contrast to the much lower concentrations of<br />
chlorine atoms at these conditions.<br />
Liu et al. [50] estimated that a 50% mercury oxidation could be obtained <strong>by</strong><br />
injecting 52 ppm Br2 in the flue gases without fly ash for a reaction time of 15 s at 137°C.<br />
Laboratory study of Br2 in the simulated flue gas showed that fly ash in the flue gas<br />
significantly promoted the oxidation of Hg 0 <strong>by</strong> Br2 and the unburned carbon in the fly<br />
ash played a major role in the promotion primarily through the rapid adsorption of Br2<br />
[50]. Hg 0 oxidation in the gas phase was found to be less important than fly ash-induced<br />
oxidation <strong>by</strong> Br2. However, there is an increasing concern on the stability of bromine<br />
impregnated in the AC, added to fuels, or injected directly to the flue gas, which could<br />
22
lead to downstream pollution and pipeline corrosion due to the strong acidic nature of<br />
bromine.<br />
2.3.2 <strong>Mercury</strong> transformation within cement kiln system<br />
Larsen et al. [51] made a thermodynamic calculation of potential mercury species<br />
distribution in a cement kiln preheater. In order to get closer to a preheater environment,<br />
chloride as well as sulphide and sulphate compounds were included in the oxygencontaining<br />
system. Detailed compositions of the solid and flue gas can be found in the<br />
figure caption. The alkaline dust was represented <strong>by</strong> CaO in the calculations, which is in<br />
excess compared to the acidic components such as HCl and SO2. Figure 2.7 illustrates the<br />
equilibrium distribution of mercury species as a function of temperature when the<br />
mercury input in the solid is in ppmm level. The dominant species below 180°C is<br />
oxidized mercury in forms of HgO and HgCl2, while all mercury compounds<br />
thermodynamically preferred above 200°C are gas-phase species and the main species is<br />
Hg 0 (g).<br />
23
Figure 2.7. Equilibrium distribution of mercury species as a function of temperature in<br />
the preheater environment with mercury input in the range of ppmm [51].<br />
Thermodynamic calculation input: solid: 5.00 kmol CaO, 0.000025 kmol HgCl2,<br />
0.000025 kmol HgSO4, 0.000025 kmol HgS, 0.000025 kmol Hg, gas: 0.03 kmol HCl, 1<br />
kmol H2O,1 kmol O2, 30.00 kmol CO2, 0.05 kmol SO2, 67.95 kmol N2.<br />
Figure 2.8 illustrates the equilibrium distribution of mercury species as a function<br />
of temperature when the mercury input is in ‰ level. Presence of CaO and HCl are not<br />
included in the calculation assuming that HCl can be captured <strong>by</strong> large amount CaO in<br />
the cement raw materials. The results are completely different <strong>from</strong> the calculation with<br />
ppmm level of mercury input in the solid. The dominant species below 200°C is HgSO4,<br />
while a certain amount of HgCl2(g) is formed above 200°C. The HgSO4(g) decomposes<br />
at around 450°C, thus the dominant species above 450°C are Hg 0 (g) and HgCl2(g).<br />
24
Fig. 2.8. Equilibrium distribution of mercury species as a function of temperature in the<br />
preheater environment with mercury input in the range of ‰ [51]. Thermodynamic<br />
calculation input: solid: 0.025 kmol HgCl2, 0.025 kmol HgSO4, 0.025 kmol HgS, 0.025<br />
kmol Hg, gas: 1 kmol H2O,1 kmol O2,30.00 kmol CO2, 0.05 kmol SO2, 97.95 kmol N2.<br />
General conclusions <strong>from</strong> thermodynamic calculations for a preheater<br />
representative environment are [51]: HgS will most probably be converted to other<br />
mercury species when entering the preheater, provided the reaction rates are sufficiently<br />
high compared to residence time. <strong>Mercury</strong> species are preferentially gas-phase<br />
compounds at temperatures above about 400°C. In a CaO rich environment, the<br />
thermodynamically preferred mercury species above 300°C is Hg 0 (g). This may be<br />
primarily because CaO acts as an HCl drain. Calculation indicates that the<br />
thermodynamically favored mercury species present at the extraction point for a typical<br />
kiln <strong>by</strong>-pass is Hg 0 (g).<br />
Detailed experimental information of mercury transformation in cement kiln<br />
system has not been reported. Although cement production also involves combustion, the<br />
25
flue gas composition, temperature and residence time in cement kiln are quite different<br />
<strong>from</strong> power plants and waste incinerators as explained earlier. When looking at mercury<br />
chemistry in cement kilns, these factors should be taken into consideration.<br />
Schreiber et al. [22] investigated the fate and inherent control of mercury in<br />
cement kiln systems using material balance studies and comprehensive stack tests that<br />
were conducted over the past two decades. They concluded that mercury does not simply<br />
volatilize out <strong>from</strong> combusted fuels and heated kiln feed materials and leave directly out<br />
of the stack. The cement kiln systems have some inherent ability to control mercury stack<br />
emissions.<br />
Besides adsorption of mercury on the raw material, as shown earlier, new<br />
mercury compounds such as mercury silicates might be formed through reaction of<br />
mercury with silicate in the raw material and exit the system with the clinker product.<br />
The formation of complex silicates in a kiln system is possible due to the high silica<br />
content in the raw feed (typically 13-15 wt.%) and sufficient residence time for reactions<br />
to take place as vaporized mercury cycles through a kiln system. Edgarbaileyite is the<br />
first reported structure to contain both Hg and Si [52,53]. It has the stoichiometry<br />
Hg6Si2O7 with all of the Hg occurring within the structure as (Hg2) 2+ dimers. Although<br />
the mineral data of Edgarbaileyite is available, it has not been possible to identify the<br />
thermodynamic properties of the mineral. A chemical equilibrium study was conducted<br />
to estimate probable conditions for the formation of mercury silicates in high temperature<br />
systems [54]. Results <strong>from</strong> the study suggest that HgSiO3 may form over a temperature<br />
range of 225 to 325°C. However, the equilibrium calculations also indicate that mercury<br />
silicate formation may be inhibited <strong>by</strong> the presence of chlorine and sulfur. It is reported<br />
<strong>by</strong> the European cement association that volatile metals are retained in the clinker to a<br />
very small extent only [1]. Unfortunately, there are no laboratory studies to date that<br />
confirm that mercury silicates are stable above temperatures of 325°C. Fundamental<br />
research is required to identify formation of mercury silicates in the cement kiln systems.<br />
26
2.4 Conclusions<br />
<strong>Cement</strong> plants are quite different <strong>from</strong> power plants and waste incinerators<br />
regarding the flue gas composition, temperature, residence time, and inherent material<br />
circulation. The flue gas temperature and residence time in a kiln system are much higher<br />
and longer than those in a pulverized coal-fired boiler and waste incinerator. There are<br />
larger water and CO2 contents in the cement kiln flue gas.<br />
In cement production the raw materials contain mercury – often at much higher<br />
levels than in the fuels. The flue gas mercury level is highly dependent on the type of fuel<br />
and raw materials. The mercury concentrations in the flue gas <strong>from</strong> cement kilns are<br />
typically in the range of 1-50 μg/m 3 . Instead of fuel, cement raw materials are the<br />
dominant sources of mercury in the cement kiln flue gas. Higher mercury emissions,<br />
however, are observed for cement plants firing waste.<br />
The mercury emissions and speciation <strong>from</strong> cement kilns can vary over time and<br />
depend on raw materials and fuels used, and process operation. The average mercury<br />
speciation percentages for cement plants with preheater or precalciner not firing waste<br />
are 5% Hgp, 56% Hg 2+ , 39% Hg 0 for raw-mill-on mode, and 4% Hgp, 62% Hg 2+ , 34%<br />
Hg 0 during raw-mill-off mode.<br />
<strong>Mercury</strong> transformations in combustion flue gas have been investigated<br />
intensively to get an understanding of the transport and fate of mercury into to air<br />
pollution control systems. All forms of mercury in the fuel decompose in the combustion<br />
flame to form Hg 0 (g), which is oxidized to Hg 2+ in the post combustion section. <strong>Mercury</strong><br />
halogenation <strong>by</strong> chlorine and bromine is the dominant mercury transformation<br />
mechanism in coal combustion flue gas. The resulting HgCl2(g) may remain in the flue<br />
gas or adsorb onto inorganic and carbonaceous ash particles entrained in the flue gas<br />
stream. Equilibrium calculations and experiments show that bromine is a much more<br />
effective mercury oxidizer than chlorine.<br />
The cement kiln systems have some inherent ability to retain mercury in the solid<br />
materials. The mercury evaporated <strong>from</strong> the solids at the hot end of the kiln is carried to<br />
the cold end <strong>by</strong> the combustion gases. As the flue gas cools, some mercury may<br />
27
adsorb/condense onto dust particles in the cooler regions of the kiln system. When the<br />
plant is running in raw-mill-on mode, the kiln gas containing volatilized mercury is used<br />
to sweep the mill of the finely ground raw feed particles and some mercury is adsorbed<br />
<strong>by</strong> the fine particulates. However, the adsorbed mercury is either carried back to the kiln<br />
hot zone or added to the kiln system together with the raw meal, thus forming mercury<br />
cycles in the kiln system.<br />
2.5 Further work<br />
There is limited literature regarding mercury characteristics, emissions, and<br />
removal <strong>from</strong> cement kilns. Essentially all of the published data and information apply to<br />
waste incinerators and coal-fired boilers, all of which have mercury emissions and gas<br />
stream characteristics that are quite different <strong>from</strong> those <strong>from</strong> cement kilns. Therefore,<br />
comprehensive studies on mercury chemistry in the cement kiln and mercury removal<br />
<strong>from</strong> cement plants are imperative.<br />
The inherent recycle of mercury in the kiln system should be further investigated.<br />
The interactions between mercury and cement raw materials play an important role in<br />
understanding of mercury chemistry in the cement kiln system. Research is required to<br />
break the mercury cycle in the kiln system, regenerate and implement beneficial<br />
utilization of removed mercury-contained CKD. These treatment systems minimize net<br />
CKD generation <strong>by</strong> removing mercury, alkalies and other contaminants and returning<br />
treated dust to the system without compromising product quality.<br />
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May 4-9 2003.<br />
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29
[22] R.J. Schreiber, C.D. Kellett and N. Joshi, Inherent mercury controls within the Portland<br />
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31
Review of technologies for mercury removal <strong>from</strong><br />
32<br />
3<br />
flue gas<br />
This chapter reviews the available technologies for mercury removal <strong>from</strong> flue<br />
gas, and the applicability of the technologies in cement plant is discussed. Focus is put on<br />
mercury removal <strong>by</strong> sorbent injection. Tests of sorbents in lab-scale fixed-bed reactors,<br />
slipstream pilot-scale reactors and full-scale plants are reported.<br />
3.1 Introduction<br />
The options for mercury control include mercury avoidance <strong>by</strong> coal and raw<br />
material cleaning, mercury removal <strong>by</strong> sorbent injection upstream of existing air<br />
pollution control devices (APCDs), and enhanced mercury removal <strong>by</strong> oxidation.<br />
Differences in fuel type and composition and pollution control devices make it necessary<br />
to develop customized solutions for each plant. The suitable mercury control method for<br />
a specific plant depends on the plant’s configuration, fuel types, and existing flue gas<br />
controls used for other pollutants. In addition, the complicated chemistry and multiple<br />
mechanisms governing mercury speciation in combustion facilities makes it necessary to<br />
investigate mercury emission control technologies at conditions relevant to each specific<br />
plant [1]. <strong>Mercury</strong> control technologies applied in power plants and waste incinerators<br />
are reviewed in this section and the applicability of these technologies in cement plants is<br />
discussed.
3.2 <strong>Mercury</strong> avoidance technology<br />
3.2.1 Coal cleaning<br />
Physical coal cleaning is used primarily to reduce the ash and pyritic sulfur<br />
content of coal [2-4]. Approximately 75% of the U.S. Eastern and Midwestern<br />
bituminous coals undergo physical coal cleaning prior to shipment to power plants. Less<br />
than 20% of the western coals, such as Powder River basin (PRB) coal and Colorado<br />
bituminous coals, are cleaned [5].<br />
Ash and pyritic sulfur are removed due to the difference in the densities of these<br />
materials compared to the organic constituents in the coal. <strong>Mercury</strong> present in a sulfide<br />
form also has a high density and can be removed during physical coal cleaning.<br />
Reduction in mercury levels in coals ranging <strong>from</strong> 10% up to 78% have been reported<br />
[3,6,7]. The average mercury reduction resulting <strong>from</strong> physical coal cleaning is estimated<br />
to be in the range of 20% to 37% [6,8]. The cost of waste water treatment is very high.<br />
As most of the mercury is <strong>from</strong> the raw material in cement production, the extent of<br />
mercury removal through coal cleaning is expected to be very limited.<br />
3.2.2 <strong>Cement</strong> raw material cleaning<br />
Two methods are proposed in the patents for removing mercury <strong>from</strong> cement raw<br />
materials, i.e., washing and gasification prior to feeding to the kiln [9,10]. By water<br />
washing the water-soluble mercury in the raw materials is removed. In the gasification<br />
process, raw materials are introduced into a heating furnace and mercury and its<br />
compounds contained in the raw materials are gasified. The resulting gas is introduced<br />
into an activated carbon adsorption tower, mercury and its compounds are adsorbed and<br />
separated. However, no results on the mercury removal efficiency <strong>by</strong> raw material<br />
cleaning are reported in the publications. Due to large amount of raw materials applied in<br />
the cement production, the cost of raw material cleaning is expected to be extremely high<br />
and this technology appears not suitable for mercury removal <strong>from</strong> cement plants.<br />
33
3.2.3 Fuel switching<br />
The use of tire-derived fuels (TDF) and substitution of coal and petroleum coke<br />
with natural gas could potentially result in a modest reduction in the mercury emissions<br />
due to the replacement of mercury-containing fossil fuels with low mercury fuels. Cofiring<br />
of TDF with a subbituminous coal in a 55 kw pilot-scale pulverized coal<br />
combustor had a significant effect on mercury speciation in the flue gas [11]. With 100%<br />
coal firing, there was only 16.8% oxidized mercury in the flue gas compared to 47.7%<br />
when 5 wt.% TDF was co-fired and 84.8% when 10% TDF was co-fired. The<br />
significantly enhanced mercury oxidation may be the result of additional homogeneous<br />
gas phase reactions between elemental mercury and the additional chlorine <strong>from</strong> TDF<br />
combustion. The chlorine content in TDF is about 600 ppmm. However, co-firing of<br />
TDF in the pilot-scale combustor with a hybrid filter for mercury removal demonstrated<br />
only limited improvement on mercury-emission control <strong>by</strong> the hybrid filter without<br />
sorbent injection. The enhanced mercury oxidation <strong>from</strong> co-firing TDF has potential in<br />
mercury emission control for power plants equipped with a wet flue gas desulphurization<br />
unit, since oxidized mercury is easily captured in the scrubber. Typically, kilns using<br />
TDF have a replacement rate no greater than 30% of the total fuel requirement. Richards<br />
et al. [12] summarized the available air emissions data for cement plants firing TDF and<br />
literature applicable to cement kilns and concluded that the variability in mercury<br />
concentrations and speciation overshadowed any beneficial impact on emissions due to<br />
the firing of TDF.<br />
Natural gas firing in boilers that are presently being fired with coal results in<br />
direct and significant reductions in mercury. However, as mentioned previously the fuel<br />
is usually not the dominant source of mercury in cement kiln flue gas. The limestone and<br />
possibly other raw materials in the kiln feed provide most of the mercury that is<br />
evaporated and emitted. Accordingly, the substitution of solid fuels would have only<br />
limited impact.<br />
34
3.3 <strong>Mercury</strong> removal <strong>by</strong> powdered activated carbon injection<br />
Powdered activated carbon (PAC) injection systems are well established as<br />
commercial air pollution control processes for a variety of volatile organic compounds,<br />
dioxin-furan, and heavy metals control applications [5]. The following three versions of<br />
PAC processes are being considered for widespread use in coal-fired power plants [6]: (1)<br />
PAC injection upstream of the existing dust collector system; (2) Gas cooling followed<br />
<strong>by</strong> PAC injection upstream of the existing dust collector system; (3) Gas cooling of the<br />
effluent gas stream of the existing dust collector system followed <strong>by</strong> PAC injection<br />
upstream of a second dust collector for removal of the adsorbent.<br />
The activated carbon particles remain suspended in the moving gas stream for<br />
periods of one to three seconds. They then deposit onto the dust cake formed on the filter<br />
bags. Additional mercury capture takes place when the mercury-containing gas stream<br />
passes through the sorbent-containing dust cake. Electrostatic precipitators (ESPs) are<br />
rarely used as the downstream polishing dust collector because the precipitated activated<br />
carbon is partially isolated <strong>from</strong> the gas stream once it reaches the collection plate<br />
surface.<br />
3.3.1 Parameters affecting mercury removal <strong>by</strong> activated carbon<br />
injection<br />
There are a large number of variables that affect the adsorption of mercury on<br />
powdered activated carbon. These include [5]: mercury speciation and concentration,<br />
sorbent physical and chemical properties such as particle size distribution, pore structure<br />
and distribution, and surface characteristics, gas temperature, flue gas composition,<br />
sorbent concentration, mercury-sorbent contact time, and adequacy of sorbent dispersion<br />
into the mercury containing gas stream.<br />
Due to the differences of these variables among plants, there are large variations<br />
in the reported PAC injection rates and mercury removal efficiencies in various studies<br />
and commercial systems [13]. Therefore, it does not make sense to compare the mercury<br />
35
emoval efficiencies and sorbent injection rates without considering the actual conditions<br />
in the specific plants.<br />
<strong>Mercury</strong> speciation determines the mercury capture capacity of sorbents at a<br />
given temperature. Pavlish and others [13] concluded that virgin activated carbon has a<br />
higher rate of capture for mercuric chloride than for elemental mercury. Ho and others<br />
[14] reported that sulphur-impregnated activated carbons have enhanced rates of<br />
elemental mercury capture.<br />
Pavlish et al. [13] conducted a detailed review on possible rate-controlling<br />
mechanisms for mercury removal <strong>by</strong> sorbent injection. The overall reaction rates may be<br />
limited <strong>by</strong> mass transfer <strong>from</strong> the bulk gas to the sorbent surface, the equilibrium<br />
adsorption capacity, and the rates of reactions occurring on the sorbent surface.<br />
All adsorption processes, especially those dependent on physisorption operate<br />
more effectively at low temperatures due to the large adsorption capacity at low<br />
temperatures. Adsorption processes for flue gas cleaning usually are operated in the<br />
temperature range of 150°C to 200°C. The pilot plant studies of PAC injection indicate<br />
that the mercury removal efficiency is strongly dependent on the gas temperatures [5].<br />
Efficiencies of 10% to 70% have been measured at 170°C, and removals of 90% to 99%<br />
have been measured at 100°C. <strong>Mercury</strong> capture takes place <strong>by</strong> both physisorption and<br />
chemisorption [13]. With increasing temperature, physical adsorption decreases due to<br />
the nature of exothermal adsorption process whilst chemisorption might be enhanced on<br />
kinetics [15].<br />
<strong>Mercury</strong> competes with a variety of gases for the adsorption sites on the activated<br />
carbon. Water vapor is important because it is present at concentrations many orders of<br />
magnitude above mercury. At moisture levels above 5% to 10%, moisture competition<br />
can be significant. There are indications that high moisture levels in the flue gas will<br />
suppress the capture of mercury <strong>by</strong> activated carbon [5,16]. It was postulated that water<br />
molecules are able to fill micropores, there<strong>by</strong> blocking adsorption sites for mercury.<br />
Although it is agreed that water plays an important role in the mechanism of<br />
mercury capture, there is disagreement in the literature about the effect of water on<br />
36
mercury removal. Pavlish et al. [13] reported that reintroducing water into flue gas in a<br />
lab-scale reactor at 135�C after a period of sorption testing on dry flue gas resulted in an<br />
immediate release of mercury <strong>from</strong> the activated carbon. However, in another lab-scale<br />
study [17] the presence of moisture on the carbon surface was reported to promote<br />
mercury bonding. About 75–85% reduction in Hg 0 adsorption capacity was observed<br />
when the carbon samples’ moisture at a level of 2 wt.% was removed <strong>by</strong> heating at<br />
110�C prior to the Hg 0 adsorption experiments at room temperature. These observations<br />
suggest that the moisture adsorbed on activated carbons plays a critical role in retaining<br />
Hg 0 . It was postulated that adsorbed H2O is closely associated with surface oxygen<br />
complexes and the removal of the H2O <strong>from</strong> the carbon surface <strong>by</strong> low-temperature heat<br />
treatment reduces the number of active sites that can chemically bond Hg 0 or eliminates<br />
the reactive surface conditions that favor Hg 0 adsorption [17]. Liu et al. [18] found that<br />
the mercury adsorption capacity of sulfur impregnated activated carbon did not change<br />
significantly when 5 vol.% water was added to the dry gas at 140�C, however, the<br />
adsorption capacity decreased 25% when the water content in the gas increased to 10<br />
vol.%. These observations indicate that it is important to investigate the sorbent using the<br />
same water vapor content as in the full-scale plant flue gas or in a wide range of moisture<br />
content. Further investigations of the effect of water on mercury adsorption are desired to<br />
reveal the dominating effects.<br />
Miller et al. [19] and Ochiai et al. [20] conducted full factorial design<br />
experiments in fixed-bed reactors to determine the relative effects of SO2, HCl, NO, and<br />
NO2 on the elemental mercury capture ability of commercial activated carbons. Without<br />
acid gases present, upon exposure to a baseline gas mixture of 6% O2, 12% CO2, 8%<br />
H2O, and N2, the lignite activated carbon sorbent provided only about 10–20% initial<br />
mercury capture of Hg 0 for about 30 min and then fast breakthrough at 107°C [19,20].<br />
Adding 50 ppmv HCl alone with the baseline gas improves the mercury adsorption<br />
significantly [19,20]. It was also found out that adding NO or NO2 alone with the<br />
baseline gas also improves the mercury adsorption capacity significantly. The mercury<br />
capture increases <strong>from</strong> 10-20% for about 30 min using the baseline gas to 90-100% for<br />
37
more than 2-6 h when 300 ppmv NO or 20 ppmv NO2 was added one at a time to the<br />
baseline gases at 107°C.<br />
When 1600 ppmv SO2 is added to the baseline gas the mercury adsorption<br />
capacity will not be changed or only improved slightly [19,21]. Addition of 20 ppm SO3<br />
to the gas reduced mercury capture <strong>by</strong> nearly 80%, and higher SO3 concentrations led to<br />
further reductions in the mercury capture [21-23]. The competition between SO3 and<br />
mercury for binding sites on the surface of activated carbon decreases the mercury<br />
adsorption capacity.<br />
The combination of 1600 ppmv SO2 and 20 ppmv NO2 additions resulted in<br />
significant different mercury breakthrough profile compared to adding NO2 alone [19,20].<br />
A highly significant interaction between SO2 and NO2 caused a rapid breakthrough of<br />
mercury and is the controlling mechanism responsible for poor sorbent performance. The<br />
detailed mechanism of SO2 and NO2 interaction is presented in section 3.3.5.2.<br />
3.3.2 Tests of mercury sorbents in labscale fixedbed reactors<br />
A good sorbent is expected to have high mercury adsorption capacity and fast<br />
kinetics. A sorbent with good capacity but slow kinetics is not a good choice as it takes<br />
mercury compound molecules too long time to reach the particle interior [24]. On the<br />
other hand, a sorbent with fast kinetics but low capacity is not good either as a large<br />
amount of sorbent is required for a given mercury removal. To satisfy these two<br />
requirements, the sorbent must have a reasonably high surface area or micropore volume<br />
and a pore network of relatively large pores for the transport of molecules to the interior.<br />
3.3.2.1 Carbonbased sorbents<br />
As mentioned previously mercury capture is very sensitive to the flue gas<br />
composition and temperature, and for this reason only the mercury capture capacities of<br />
sorbents tested under simulated flue gas conditions are reported and compared here.<br />
Extensive research has been conducted to study the sorbent mercury capture capacity<br />
mainly using lab-scale fixed-bed reactors [25-36]. The mercury sorbents can be divided<br />
38
into three groups, i.e., virgin carbon sorbents, chemically treated carbons, and noncarbon<br />
based sorbents. The majority of the publications focused on elemental mercury<br />
capture and only few studies investigated capture of HgCl2.<br />
Typical properties of selected sorbents are reported in table 3.1. Coal source and<br />
ash content relate to the composition of the coal and characterize the state of the carbon<br />
such as fixed carbon/volatile ratio [37,38]. High ash content reduces the overall activity<br />
of the activated carbon. Particle surface area is a measure of adsorption capacity and<br />
describes the available surface for mercury adsorption. Pore size and distribution is an<br />
indicator of sorbent quality, with smaller pores preferred. Particle size is used to describe<br />
the degree of sorbent physical preparation. The mean particle size and distribution of<br />
particle size are important parameters for evaluating mercury removal rate and pressure<br />
drop. Smaller size provides faster rate of adsorption and results in larger pressure drop.<br />
Content of bromine/chlorine/sulfur is considered as an indicator of the chemical<br />
characteristics of sorbent responsible for mercury adsorption. Bulk density reflects a<br />
gross approximation of the processing or surface area of a given carbon. The most<br />
investigated sorbents in the literature is Darco FGD, which is a commercial lignite based<br />
powdered activated carbon and is developed for heavy metal removal <strong>from</strong> incinerators<br />
and power plants.<br />
Table 3.1. Properties of selected sorbents.<br />
<strong>Sorbent</strong>s Sources Ash<br />
(wt%)<br />
Norit<br />
Darco<br />
FGD<br />
Norit<br />
Darco<br />
Insul<br />
Norit<br />
Darco<br />
Hg<br />
Norit<br />
Darco<br />
Hg-LH<br />
Lignite coal 28.2-<br />
32.1<br />
Sulfur<br />
(wt%)<br />
0.86-<br />
1.1<br />
Surface<br />
area<br />
(m 2 /g)<br />
481-<br />
600<br />
Pore<br />
volume<br />
(cm 3 /g)<br />
0.535-<br />
0.610<br />
39<br />
Average<br />
pore<br />
size<br />
(nm)<br />
Mass<br />
mean<br />
particle<br />
size(µm)<br />
Bulk<br />
density<br />
(g/cm 3 )<br />
3.2 6.8-15 0.51 0.49-<br />
0.58<br />
Porosity Reference<br />
[25-27,29-<br />
32,35,39]<br />
Lignite coal,<br />
based on<br />
Darco FGD,<br />
fine,<br />
chemically<br />
treated<br />
700 6 0.32 [33,34]<br />
Lignite coal 1.2 600 16-19 0.51 [40,41]<br />
Lignite coal,<br />
bromine<br />
treated<br />
1.2 550 16-19 0.60 [41,42]
<strong>Sorbent</strong>s Sources Ash<br />
(wt%)<br />
Calgon<br />
FluePac<br />
AC<br />
Calgon<br />
HGR<br />
HOK<br />
standard<br />
HOK<br />
super<br />
Masda<br />
GAC<br />
Bituminous<br />
coal<br />
Sulfur<br />
(wt%)<br />
Surface<br />
area<br />
(m 2 /g)<br />
Pore<br />
volume<br />
(cm 3 /g)<br />
40<br />
Average<br />
pore<br />
size<br />
(nm)<br />
Mass<br />
mean<br />
particle<br />
size(µm)<br />
Bulk<br />
density<br />
(g/cm 3 )<br />
Porosity Reference<br />
5.8 0.7 606 0.285 32 0.585 [33,34,43]<br />
Bituminous,<br />
10.9- 413- 0.130 2.0 9.8 0.590 [44-46]<br />
sulfur treated<br />
15 486<br />
Lignite coal 10.0 0.60 300 0.620 63 0.55 [47]<br />
MnO2-AC MnO2<br />
solution<br />
impregnated<br />
activated<br />
carbon<br />
FeCl3-AC FeCl3<br />
solution<br />
impregnated<br />
activated<br />
Shanghai<br />
activated<br />
carbon<br />
Damao<br />
activated<br />
carbon<br />
1%<br />
ZnCl2<br />
Damao<br />
5%<br />
ZnCl2<br />
Damao<br />
Lignite coal 10.0 0.60 300 24 0.44 [47]<br />
carbon<br />
produced<br />
<strong>from</strong> wood<br />
<strong>by</strong> zinc<br />
chloride<br />
method<br />
Bituminous<br />
coal<br />
Zncl2<br />
treated<br />
Damao<br />
carbon<br />
Zncl 2<br />
treated<br />
Damao<br />
carbon<br />
6.0 735 0.300 2.0 [48]<br />
0.43 865 0.290 90 0.45 [28]<br />
0.44 1470 0.920 90 0.58 [28]<br />
0.55 1850 1.050 90 0.67 [28]<br />
770 0.330 1.7 280 [49]<br />
608 0.27 1.8 280 [49]<br />
277 0.19 2.7 280 [49]<br />
There is disagreement in the publications on the effect of mercury species on<br />
activated carbon and char adsorption capacity. Yang et al. [13,50] reported that the<br />
capture capacities of HgCl2 <strong>by</strong> bituminous char are larger <strong>by</strong> a factor of two than those of<br />
elemental mercury using only CO2, O2, H2O and N2. However, other studies, as shown<br />
in figure 3.1, show the opposite trend [25-27]. The elemental mercury adsorption<br />
capacity for the studied carbons is about 0.5-4 times larger than the HgCl2 adsorption<br />
capacity in the temperature of 110-160 �C using simulated flue gas with 6% O2, 12%<br />
CO2, 7% H2O, 50 ppmv HCl and 1600 ppmv SO2.
Adsorption capacity, �g Hg/HgCl2/g-carbon<br />
3000<br />
2500<br />
2000<br />
1500<br />
1000<br />
500<br />
0<br />
45 �g/m3 Hg0,Darco FGD [26, 27]<br />
54 �g/m3 Hg0,lab AC <strong>from</strong> high S coal [25]<br />
Figure 3.1. Effect of mercury speciation on mercury adsorption capacity on activated<br />
carbon. Adsorption temperature is 135�C and data are <strong>from</strong> [25-27]. Simulated flue gas<br />
with 6% O2, 12% CO2, 7% H2O, 50 ppmv HCl and 1600 ppmv SO2.<br />
Generally, the mercury adsorption capacities of carbon sorbents decrease when<br />
the gas temperature is increased [13,50]. However, tests of Darco FGD at 100-135�C<br />
(see figure 3.2) <strong>from</strong> different studies are not in agreement with the trend. This is<br />
probably due to the short exposure time for the test at 100�C and the presence of SO3 in<br />
the simulated gas for test at 120�C. The adsorption capacity at 100�C was measured after<br />
2 h and before the complete breakthrough, while other tests were run until the complete<br />
breakthrough of the sorbent bed was obtained. As discussed previously, the presence of<br />
SO3 in the flue gas will decreases the sorbents’ mercury adsorption capacity. This<br />
observation again illustrates the difficulty of analyzing the results <strong>from</strong> different studies<br />
that are not conducted under the same conditions.<br />
41<br />
59 �g/m3 Hg0,Darco FGD [25]<br />
60 �g/m3 HgCl 2 ,Darco FGD [25]<br />
61 �g/m3 HgCl 2 ,lab AC <strong>from</strong> high S coal [25]
Hg adsorption capacity, �g Hg/g_carbon<br />
3000<br />
2500<br />
2000<br />
1500<br />
1000<br />
500<br />
0<br />
100oC, 400 �g Hg 0 /m3 [29-31]<br />
120oC, 290 �g Hg 0 /m3, SO 3 [13,32]<br />
Figure 3.2. Effect of adsorption temperature on mercury adsorption capacity on Darco<br />
FGD activated carbon. Data are <strong>from</strong> [13,25-27,29-32]. Gas composition: 100�C: 1000<br />
ppmv SO2, 50 ppmv HCl in N2; 120�C: 8.3% O2, 14.8% CO2, 7.2% H2O, 278 ppmv NO,<br />
650 ppmv SO2, 107 ppmv CO, 46 ppmv HCl, 27 ppmv SO3 in N2; 135�C: 6% O2, 12%<br />
CO2, 7% H2O, 1600 ppmv SO2, 50 ppmv HCl in N2;<br />
Previous studies showed that the low chlorine concentration in the flue gas <strong>from</strong><br />
combustion of low-rank coals is a major limiting factor in the mercury control<br />
performance using the virgin activated carbons [13,50]. Various chemically treated<br />
carbons were developed to compensate for the lack of halogens in the combustion flue<br />
gas. These include chloride-impregnated carbons [49,51-55], sulfur-impregnated carbons<br />
[25,51,53,56-61], brominated carbons [50,55,62,63], iodine-impregnated carbons [52,53],<br />
ozone-treated carbon [64], and carbon impregnated with metal compounds such as MnO2,<br />
FeCl3 and CuCl2 [28,65-67]. The price of the chemically treated carbon is typically<br />
higher than the non-treated one. The price of the non-treated Norit Darco Hg was about<br />
1.1 US$/kg in 2007, while the bromine-treated carbons cost about 1.9-2.6 US$/kg [41].<br />
42<br />
135oC, 45 �g Hg 0 /m3 [26,27]<br />
135oC, 59 �g Hg 0 /m3 [25]
Chlorine impregnation of a virgin activated carbon using dilute solutions of<br />
hydrogen chloride leads to increases in fixed-bed capture of both elemental mercury and<br />
mercuric chloride <strong>by</strong> a factor of 2-3 for a simulated flue gas without HCl, but with 7.1%<br />
O2, 6.9% H2O, 3.4% CO2, 4.5 ppmv CO, 200 ppmv NOx and 500 ppmv SO2 [51]. It is<br />
not reported how the chlorine impregnated carbon behaviors if HCl is present in the gas.<br />
Coal-derived activated carbon <strong>from</strong> high-organic-sulfur coals was reported to<br />
have a greater equilibrium Hg 0 adsorption capacity than that prepared <strong>from</strong> low-organicsulfur<br />
coal when tested using a simulated flue gas with 6% O2, 7% H2O, 12% CO2, 50<br />
ppmv HCl, and 1600 ppmv SO2 [25]. At 135�C the equilibrium Hg 0 adsorption capacity<br />
of carbon derived <strong>from</strong> high-organic-sulfur coal, which contained 3.7 wt % total sulfur<br />
and 2.9 wt% organic sulfur is 2718 µg Hg 0 /g_carbon, on the other hand the equilibrium<br />
Hg 0 adsorption capacity of carbon prepared <strong>from</strong> low-organic-sulfur coal with 1.2 wt%<br />
total sulfur and 0.7 wt% organic sulfur was only 1304 µg Hg 0 /g_carbon. When the loworganic-sulfur<br />
coal-derived activated carbon is impregnated with elemental sulphur at<br />
600°C, its equilibrium Hg 0 adsorption capacity is comparable to the adsorption capacity<br />
of the activated carbon <strong>from</strong> the high-organic-surfur coal. Elemental sulphurimpregnated<br />
carbons enhance elemental mercury removal due to the formation of<br />
mercury sulphide on the carbon surface [68]. A portion of the inherent organic sulphur<br />
in the starting coal, which remained in the activated carbon, plays an important role in<br />
adsorption of elemental mercury. Besides organic sulphur, the surface area and<br />
micropore area of the activated carbon also influence Hg 0 adsorption capacity [25]. The<br />
HgCl2 adsorption capacity is not as dependent on the surface area and concentration of<br />
sulphur in the activated carbon as for adsorption of Hg 0 .<br />
Another method for modifying carbon surfaces is oxidation, using reagents that<br />
include oxygen, ozone, hydrogen peroxide, nitric acid, and permanganate [64]. Ozone<br />
treatment of carbon surfaces leads to large increases in the elemental mercury capture<br />
capacity <strong>by</strong> more than a factor of 100 when tested in Argon gas, but the activity is easily<br />
destroyed <strong>by</strong> exposure to air, to water vapor, or <strong>by</strong> mild heating at 120�C [64]. Freshly<br />
ozone-treated carbon surfaces are shown to form labile C–O containing oxidizing groups,<br />
43
which are likely to be epoxides or secondary ozonides. However, this ability fades with<br />
aging. The finding opens the possibility of in-situ carbon ozonolysis to create fresh,<br />
super-active sorbents with the additional benefit of sorbent hydrophilicity useful in<br />
certain applications. Ozone treatment of fly ash carbon has been reported to inhibit the<br />
adsorption of commercial surfactants in concrete paste, thus mitigating the known<br />
negative effects of carbon on ash utilization [69-73]. Therefore, the enhanced mercury<br />
removal could be a co-benefit of the ozone treatment.<br />
3.3.2.2 Noncarbon sorbents<br />
For cement plant application a non-carbon sorbent is more attractive if the used<br />
sorbent can be added to the final cement product or can be separated and regenerated.<br />
Carbon can deteriorate the cement quality if the used carbon is not separated <strong>from</strong> the<br />
cement materials <strong>by</strong> installing an expensive polishing filter. As inspiration to cement<br />
plant application, research on development of non-carbon sorbents that do not adversely<br />
impact sales of fly ash as a coal combustion <strong>by</strong>product for Portland cement and concrete<br />
production are reported in this section.<br />
Chemically synthesized manganese oxides powder has been demonstrated in<br />
power plants to remove mercury, NOx and SO2 <strong>from</strong> flue gas [74-77]. The reacted<br />
sorbent can be regenerated <strong>by</strong> a wet chemical process if the sorbent is injected just before<br />
the added polishing fabric filter [74-77]. The simplified capture reactions for these<br />
pollutants are suggested as following:<br />
0<br />
Hg + MnO2 Mn*Hg complex<br />
� (R3.1)<br />
NO x + MnO 2 � Mn(NO 3) 2<br />
(R3.2)<br />
SO x + MnO2 � MnSO4<br />
(R3.3)<br />
Non-carbonaceous materials or mineral oxides including silica gel, alumina,<br />
molecular sieves, zeolites, and montmorillonite have been modified with various<br />
functional groups such as amine, amide, thiol, urea, and additives such as elemental<br />
sulfur, sodium sulfide, and sodium polysulfide to examine their potential as sorbents for<br />
the removal of mercury vapor at coal-fired utility power plants [78]. A number of sorbent<br />
44
candidates such as amine-silica gel, urea-silica gel, thiol-silica gel, amide-silica gel,<br />
sulfur-alumina, sulfur-molecular sieve, sulfur-montmorillonite, sodium sulfidemontmorillonite,<br />
and sodium polysulfide-montmorillonite, were synthesized and tested in<br />
a lab-scale fixed-bed system under an argon flow for screening purposes at 70°C and<br />
140°C. Several functionalized silica materials used for effective control of heavy metals<br />
in the aqueous phase showed insignificant adsorption capacities for mercury control in<br />
the gas phase, suggesting that mercury removal mechanisms are different in these two<br />
phases. Among the synthesized samples, sodium polysulfide-impregnated<br />
montmorillonite showed a moderate adsorption capacity at 70°C.<br />
The commercial Amended Silicates sorbent uses silicate minerals as substrate<br />
particles on which a chemical reagent with a strong affinity for mercury and mercury<br />
compounds is impregnated [79,80]. A phyllosilicate substrate, for example, vermiculite<br />
or montmorillonite, is used as an inexpensive support to a thin layer for a polyvalent<br />
metal sulfide, ensuring that more of the metal sulfide is engaged in the sorption process.<br />
The sorbent is prepared <strong>by</strong> ion exchange between the silicate substrate material and a<br />
solution containing one or more of a group of polyvalent metals including tin, iron,<br />
titanium, manganese, zirconium, and molybdenum. Controlled addition of sulfide ions to<br />
the exchanged silicate substrate produces the sorbent. The silicates provide a low-cost<br />
substrate material with average particle size of a few microns and extended surface area<br />
for the amendment process. Due to their high silicate content, they have been proven<br />
compatible with the continued sale of fly ash as a pozzolan material for concrete and<br />
cement production. The price of the Amended Silicates sorbent is about 2.2-4.4 US$/kg<br />
[81], which is comparable to the price of the chemically treated carbons [41]. However,<br />
the performance data of the Amended Silicates sorbent are rarely reported due to the<br />
concern of intelligent property.<br />
A comparison between Darco FGD activated carbon and Ca(OH)2 indicated that<br />
non-carbon-based sorbents with relatively high Ca contents can be fairly effective HgCl2<br />
sorbents [29,30]. The Ca-based sorbents exhibited HgCl2 removal as high as half of the<br />
removal shown <strong>by</strong> the Darco FGD activated carbon when 100 mg sorbent was tested in a<br />
45
ench-scale fixed-bed reactor using simulated flue gas containing 10% CO2, 7% O2, 5%<br />
H2O, and 173 ppmv SO2 [29,30]. However, the carbon-based sorbent showed superior<br />
efficiency of elemental removal compared to Ca-based sorbent.<br />
Full-scale investigations in coal-fired power plants have observed mercury<br />
capture <strong>by</strong> unburned carbon in the fly ash [82]. <strong>Mercury</strong> removal <strong>by</strong> fly ash has also been<br />
extensively studied to find a solution to the expensive mercury sorbents [36,83-87]. As<br />
shown in figure 3.3, the amount of carbon in the fly ash has a strong effect on mercury<br />
adsorption capacity of the fly ash. The mercury adsorption capacity increases with<br />
carbon content in the fly ash, however, it is not directly proportional to the carbon<br />
content. The mercury adsorption capacity of Nixon fly ash with 2% residual carbon is<br />
about 30% of the commercial activated carbon Darco G60 [86].<br />
Adsorbed Hg (g Hg/106 g ash)<br />
900<br />
800<br />
700<br />
600<br />
500<br />
400<br />
300<br />
200<br />
100<br />
0<br />
Nixon, 2% C<br />
Cherokee, 8.7% C<br />
Clark, 32.7% C<br />
Huntington, 35.9% C<br />
0 10 20 30 40<br />
Carbon content in the fly ash (%)<br />
Figure 3.3. <strong>Mercury</strong> adsorption capacity on fly ashes with different carbon content. Data<br />
are <strong>from</strong> [86]. The applied adsorption temperature is 121�C and elemental mercury<br />
concentration is 4 mg/m 3 with nitrogen as balance gas.<br />
Dunham et al. [85] investigated 16 fly ash samples <strong>from</strong> a variety of sources and<br />
coal types in a fixed-bed reactor at 121-177�C using elemental mercury or HgCl2 in<br />
simulated flue gas mixtures of O2, SO2, NO, NO2, H2O and HCl. While many of the ash<br />
46
samples oxidized elemental mercury to HgCl2 in a range of 15-85%, not all of the<br />
samples that oxidized mercury also captured elemental mercury. However, no capture of<br />
elemental mercury was observed without accompanying oxidation. In general, oxidation<br />
of elemental mercury increased with increasing amount of magnetite (Fe3O4) in the ash.<br />
However, one high-carbon subbituminous ash with no magnetite showed considerable<br />
mercury oxidation that may have been due to the carbon. Dunham et al. [85] suggested<br />
that an iron oxide with a spinel-type structure is active in fly ash with respect to mercury<br />
oxidation. Surface area as well as the nature of the surface, such as the oxygen<br />
functionality and presence of halogen species appeared to be important for oxidation and<br />
adsorption of elemental mercury. For the applied gas composition in Dunham’s study<br />
[85], the capacity of the ash samples for HgCl2 was similar to that for elemental mercury.<br />
There was a good correlation between the capacity for HgCl2 and the surface area. The<br />
correlation between HgCl2 and loss on ignition was not as strong, suggesting that it is<br />
not the carbon content alone but also properties of the ash, such as surface area, that<br />
influence capture of HgCl2.<br />
Based on the research of interactions between mercury and fly ash, carbon that<br />
remains in pulverized coal fly ash could be used as an inexpensive adsorbent for mercury<br />
removal. The fly ash would be injected into the flue gas prior to the particulate control<br />
device [86,88] similarly to the way in which activated carbon is used, thus eliminating<br />
large capital and sorbent costs. Due to the low carbon content and small mercury<br />
adsorption capacity of fly ash, however, a large amount of fly ash may be required.<br />
Another alternative to activated carbon might be the use of noble metal-based<br />
sorbent. Noble metals such as gold and silver form reversible amalgams with mercury<br />
[89]. A class of magnetic zeolite composites with supported silver nanoparticles has been<br />
tested for elemental mercury removal <strong>from</strong> power plant flue gas [89]. Gaseous mercury is<br />
captured <strong>by</strong> the sorbent and the mercury-laden sorbent particles are collected <strong>by</strong> an<br />
existing dust collector and separated <strong>from</strong> the fly ash <strong>by</strong> magnetic separation. After mild<br />
heat treatment to release captured mercury the sorbent is regenerated for the next cycle of<br />
mercury capture. The technology is still in early stage and research is required regarding<br />
47
the stability of the sorbent and possible regeneration cycles. Since noble metal is used in<br />
synthesis of the sorbent it is expected that the sorbent is expensive. It is not clear how<br />
this process could be cost effective compared to the activated carbon injection system<br />
and the released mercury also must be captured <strong>by</strong> some sort of process.<br />
3.2.2.3 Insitu produced sorbents<br />
To reduce the cost of sorbents, methods for in-situ production of activated carbon<br />
<strong>from</strong> coal-fired power plants have been invented [33,34,90]. In the so-called Thief<br />
process, partially combusted coal <strong>from</strong> the furnace of a pulverized coal power generation<br />
plant is extracted <strong>by</strong> a lance and then re-injected into the ductwork downstream of the air<br />
preheater [33,34,90]. Tests show that the Thief sorbents exhibit capacities for mercury<br />
<strong>from</strong> flue gas streams that are comparable to those exhibited <strong>by</strong> commercially available<br />
activated carbons. The process extracts 0.1-0.5% of the furnace gas in the boiler<br />
depending on the desired sorbent injection rate and mercury removal level. The mass of<br />
solids extracted <strong>from</strong> the furnace is very small in comparison to the mass of coal being<br />
burned. The estimated heat loss is less than 0.3% for a 500 MWe power plant burning<br />
PRB subbituminous coal.<br />
Another process uses an oxy-fuel burner to devolatilize and activate the coal to<br />
produce activated carbon [33,34,90]. In the burner natural gas is combusted together with<br />
an oxygen stream, producing a high temperature oxygen-rich stream which passes<br />
through a nozzle. Downstream of the hot oxygen nozzle the parent coal mixes with the<br />
hot oxygen and begins to burn. Devolatilization and activation take place in a reactor<br />
which leads to a particle separation step where the product is separated <strong>from</strong> the syngas<br />
stream. The syngas can then be ducted to the boiler to provide added fuel value. At<br />
several points in the process additives can be introduced to dope the product, or to<br />
control the product morphology.<br />
48
3.3.3 <strong>Sorbent</strong> injection in power plants<br />
Many activated carbons have been tested in U.S. power plants. Table 3.2 presents<br />
the tested sorbents and applied APCDs and coals. The mercury removal efficiencies are<br />
not included in the table due to various mercury removal efficiencies obtained at<br />
complicated test conditions. Instead the mercury removal efficiencies as a function of<br />
sorbent injection rate are shown in figures.<br />
The most studied sorbents are Darco FGD, Darco Hg, and Darco Hg-LH. The<br />
Darco Hg is formerly known as Darco FGD manufactured specifically for the removal of<br />
mercury in coal fired utility flue gas emission streams [80], while Darco Hg-LH is<br />
bromine impregnated. Although the mercury levels at the inlet of ACPDs are generally<br />
similar, the extents of mercury removal <strong>by</strong> the existing APCDs without sorbent injection<br />
are quite different. This is due to fact that different ranks of coal and APCD<br />
configurations are applied <strong>by</strong> different power plants.<br />
Without looking at the detailed data of the specific plants, it is difficult to<br />
evaluate the sorbent performance <strong>by</strong> comparing the mercury removal efficiency.<br />
However, some trends can be observed <strong>by</strong> comparing the results obtained under similar<br />
conditions. Figure 3.4 compares the mercury removal at Holcomb and Stanton power<br />
station <strong>by</strong> injection of Darco Hg-LH upstream of SDA and baghouse. At Holcomb and<br />
Stanton power station the <strong>by</strong>product <strong>from</strong> SDA is disposed and therefore activated<br />
carbon is injected before the SDA and baghouse. Figure 3.5 illustrates the mercury<br />
removal <strong>by</strong> Darco FGD injection upstream of a new added so-called COHPAC compact<br />
hybrid particle collector. COHPAC is an EPRI-patented design that places a high air-tocloth<br />
ratio fabric filter downstream of an existing ESP to improve overall particulate<br />
collection efficiency. The results of mercury removal <strong>by</strong> Darco Hg injection upstream of<br />
cold-side ESP are presented in figure 3.6. Up to 80 mg/m 3 activated carbon is applied for<br />
systems with FF, while up to 320 mg/m 3 carbon is injected upstream of cold-side ESP.<br />
49
Table 3.2. Summary of full-scale tests conducted in U.S. power plants. LNB: low NOx<br />
burner, COHPAC: compact hybrid particulate collector<br />
Location Test<br />
load<br />
MW<br />
Holcomb, unit 1, 360<br />
MW [91-93]<br />
180,<br />
360<br />
Coal APCD Inlet mercury<br />
�g/Nm 3 ,dry<br />
PRB SDA@143�C<br />
+Baghouse,<br />
LNB<br />
50<br />
<strong>Mercury</strong><br />
removal<br />
without<br />
sorbent, %<br />
<strong>Sorbent</strong>s<br />
10-12 0-13 Darco Hg, Darco<br />
Hg-LH, Calgon<br />
208CP<br />
Stanton, unit 10, 60<br />
MW [93]<br />
60 Lignite SDA, baghouse - - Darco Hg-LH<br />
Stanton unit 1, 150 - PRB Cold-side ESP - 15 Brominated PAC<br />
MW [94]<br />
(B-PAC)<br />
Gaston, unit 3, 270 135 Low sulfur COHPAC@143� 7-10 6 Darco FGD, fine<br />
MW [95,96]<br />
bituminous C, hot side ESP,<br />
FGD, Insul, ESP<br />
coal<br />
LNB<br />
ash<br />
Big Brown, unit 2, 150 30%PRB/70 ESP, COHPAC - - Darco FGD,<br />
600 MW[97]<br />
% lignite,PRB @ 177�C, LNB<br />
FGD/NaCl/CaCl2 Presque Isle, unit 7-9, 90 PRB Polishing - - Darco FGD<br />
90 MW[98,99]<br />
baghouse<br />
Meramec, unit 2, 140 70 PRB Cold-side ESP 10-12 15-30 Darco Hg-LH,<br />
MW [91,93,100]<br />
@160�C, LNB<br />
Darco Hg<br />
Pleasant Prairie, unit 150 PRB Cold-side 16-17 5 Darco FGD, Darco<br />
2, 600 MW [96,101]<br />
ESP@138�C,<br />
Hg, Insul, lime,<br />
SO3 conditioning<br />
Sorbalit<br />
Brayton Point, unit 1, 125 Low sulfur Cold-side ESP 17 - Darco FGD, Darco<br />
250 MW [102]<br />
bituminous @138�C, SO3<br />
Hg, HOK, LAC<br />
coal<br />
conditioning<br />
Leland Olds, unit 1, 220 Lignite Cold-side ESP, 6-7 - Darco FGD Hg<br />
220 MW [93]<br />
LNB<br />
/CaCl2<br />
St. Clair, unit 1, 145 145 85%PRB/15 Cold-side ESP - - Brominated PAC<br />
MW [93]<br />
% bituminous<br />
coal<br />
(B-PAC)<br />
Laramie River unit 3, 140 PRB SDA+cold-side 10-12 4 Darco Hg-LH<br />
550 MW[91,103]<br />
ESP<br />
Darco Hg<br />
Monroe, unit 4, 775 196 PRB/bitumino Cold-side 5-10 10-30 Darco Hg-LH,<br />
MW [91]<br />
us coal ESP@125�C,<br />
Darco Hg, Darco<br />
SCR<br />
XTR<br />
Conesville, unit 6, 400 Bituminous Cold-side ESP, 15-30 50 Darco Hg-LH,<br />
400 MW [91]<br />
coal<br />
wet FGD<br />
Darco Hg<br />
Plant Yates, unit 1, 100 Bituminous Cold-side ESP, - - Super HOK<br />
100 MW[93]<br />
coal<br />
wet FGD<br />
Salem Harbor unit 1, 85 Low sulfur Cold-side - - Darco FGD<br />
85 MW [104]<br />
bituminous ESP@125�C,<br />
coal<br />
LNB<br />
Ameren Labadie unit 630 PRB Cold-side 5-12
As shown in figure 3.4 and 3.5, mercury can be efficiently removed <strong>by</strong> activated<br />
carbon injection upstream of SDA/baghouse or a polishing baghouse. When 32 mg/m 3<br />
Darco Hg-LH in SDA/baghouse system and Darco Hg in polishing baghouse system are<br />
applied, about 80% of mercury can be removed. Further increase of the carbon injection<br />
rate above 32 mg/m 3 results in a slow increase of the mercury removal efficiency. The<br />
mercury removal efficiency in SDA/baghouse system <strong>by</strong> Darco Hg-LH is larger than that<br />
<strong>by</strong> Darco Hg. This is due to the applied Darco Hg-LH sorbent, which is bromine<br />
impregnated and has larger mercury adsorption capacity than Darco FGD. The waste<br />
disposal cost of sorbent injection upstream of SDA/baghouse is expected to be higher<br />
since used activated carbon cannot be separated <strong>from</strong> the desulphurization product and<br />
regenerated. Tests at Gaston power station showed that carbon injection significantly<br />
increased the cleaning frequency of the COHPAC baghouse [95,96]. At an injection<br />
concentration of 32 mg/m 3 the cleaning frequency increased <strong>from</strong> 0.5 to 2<br />
pulses/bag/hour, most likely due to the small particle size of the PAC causes a high<br />
pressure drop.<br />
<strong>Mercury</strong> removal efficiency, %<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
Stanton, lignite, Darco Hg-LH<br />
Holcomb, PRB, Darco Hg-LH<br />
Holcomb, PRB, Darco Hg<br />
0 20 40 60 80 100<br />
PAC injection rate, mg/m 3<br />
Figure 3.4. <strong>Mercury</strong> removal as a function of injection rate of Darco Hg-LH sorbent in<br />
power plants using SDA and baghouse as APCDs. Data are <strong>from</strong> [91-93].<br />
51
<strong>Mercury</strong> removal efficiency, %<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
Gaston, bituminous<br />
Big Brown, 30%PRB/70%lignite<br />
Presque Isle, PRB<br />
0 20 40 60 80<br />
PAC injection rate, mg/m 3<br />
Figure 3.5. <strong>Mercury</strong> removal as a function of injection rate of Darco FGD sorbent in<br />
power plants <strong>by</strong> sorbent injection upstream of a polishing baghouse. Data are <strong>from</strong> [95-<br />
99].<br />
As shown in figure 3.6, much more than 32 mg/m 3 Darco Hg activated carbon are<br />
required to obtain 80% mercury removal <strong>by</strong> carbon injection upstream of cold-side ESP,<br />
where the flue gas temperature is about 125-160�C. This is due to the short contact time<br />
between mercury vapor and injected carbon in the ESP and mercury is mainly captured<br />
during the carbon particle in-flight period. When bromine treated carbons B-PAC and<br />
Darco Hg-LH is used, the mercury removal efficiency across the cold side ESP increases<br />
significantly.<br />
52
<strong>Mercury</strong> removal efficiency, %<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 50 100 150 200 250 300 350<br />
PAC injection rate, mg/m 3<br />
53<br />
Pleasant, PRB,Darco Hg<br />
Brayton, bituminous,Darco Hg<br />
Meramec, PRB,Darco Hg<br />
Leland Olds, lignite,Darco Hg<br />
Monroe,PRB,SCR <strong>by</strong>pass, Darco Hg<br />
Stanton unit 1, PRB, B-PAC<br />
Meramec, PRB, Darco Hg-LH<br />
Figure 3.6. <strong>Mercury</strong> removal as a function of injection rate of Darco Hg sorbent in power<br />
plant <strong>by</strong> sorbent injection upstream of a cold side ESP. Data are <strong>from</strong> [91,93,96,100,101].<br />
Tests at Pleasant Prairie showed that there was no significant effect on mercury<br />
removal with PAC injection when SO3 was used as flue gas conditioning agent to obtain<br />
optimal dust resistivity and improve ESP performance [96,101]. The level of applied SO3<br />
at Pleasant Prairie was not reported. However, tests at Labadie unit 2 showed that the<br />
presence of SO3 in the flue gas can decrease mercury capture <strong>by</strong> activated carbon [105].<br />
The applied SO3 concentration in the flue gas at Labadie unit 2 was about 5-10 ppmv.<br />
This is probably due to the competitive adsorption between Hg and SO3 since both<br />
mercury and SO3 bind to the Lewis acid base sites on the activated carbon surface<br />
[21,22].<br />
In some plants burning PRB coals, it was observed that when the carbon injection<br />
rate was increased above 160 mg/m 3 the mercury removal efficiency <strong>by</strong> the cold side<br />
ESP leveled off at about 60% [91,93,96,100,101]. At Brayton Point plant bituminous<br />
coal was fired and the mercury removal increased with carbon injection rates in all the<br />
tested ranges up to 320 mg/m 3 reaching 90% mercury removal [102]. This is probably<br />
due to the fact that at the Brayton Point the predominant species of mercury is in the
oxidized form since there is a significant amount of HCl present in the flue gas <strong>from</strong><br />
Brayton [102], in contrast to Pleasant Prairie where the majority of vapor phase mercury<br />
was in the elemental form.<br />
3.3.4 <strong>Sorbent</strong> injection tests at cement plant<br />
There are very limited studies on mercury removal <strong>by</strong> sorbent injection in cement<br />
plants. In 2007 a six-week test was conducted at Ash Grove <strong>Cement</strong> Company’s Durkee<br />
plant using a slipstream fabric filter after the main bag filter [106,107].<br />
The overall goal of the tests at Durkee was to perform a parametric test on a<br />
slipstream of actual flue gas to obtain an understanding of how various operating and<br />
design parameters are likely to impact mercury control in the Durkee plant. The<br />
evaluated parameters included activated carbon type, filter bag type, powdered activated<br />
carbon injection rate, and filter air-to-cloth ratio.<br />
The slipstream filter had twelve �152 mm�3658 mm bags, corresponding to a<br />
filtration area of 21 m 2 . The filter chamber and inlet duct were insulated and heated to<br />
maintain a temperature of about 138°C. <strong>Mercury</strong> concentrations at the filter inlet and<br />
outlet were measured <strong>by</strong> a Horiba/Nippon Instruments Corporation DM-6B, as well as<br />
the Ontario hydro method. The tested carbons include Darco Hg, Darco Hg e-11, Darco<br />
Hg LH, and Envergex e-sorb e11. The last two carbons are chemically treated. The<br />
Darco Hg is prepared <strong>from</strong> lignite coal and the Darco Hg e-11 is a coarser version of the<br />
Darco Hg. The particle size of the Darco Hg e-11 carbon is not reported. The test<br />
duration for each parametric study was only about one hour. The flue gas compositions<br />
are not publically reported and baghouse cleaning cycle is unknown.<br />
Figure 3.7 shows the mercury removal efficiency as a function of PAC injection<br />
rate for different carbons. For the Darco Hg and Darco Hg e-11, the mercury removal<br />
efficiency increases only slightly <strong>from</strong> 80-90% to 90-95% when the PAC injection rate is<br />
further increased above 48 mg/m 3 . The mercury removal efficiencies <strong>by</strong> the untreated<br />
carbons are generally larger than those <strong>by</strong> the treated carbons when low injection rates<br />
are applied. Treated carbons (Darco Hg LH and Envergex) have been shown to perform<br />
54
etter than untreated carbons in coal-fired boilers, especially in systems with ESP where<br />
reaction times are short [108]. The halogens in the carbon act to oxidize the Hg in the<br />
system and allow faster adsorption onto the carbon. This is critical in systems with higher<br />
SOx concentrations, because SOx species have been shown to compete for active sites on<br />
the carbon surface [21,22], as discussed earlier. The halogens on the treated carbon allow<br />
the oxidized Hg to bind to the carbon surface before the SOx species consume the active<br />
sites [108]. At the Durkee plant, the SOx concentrations in the slipstream baghouse are<br />
very low compared to a coal-fired utility system. Thus the promoting effects of halogen<br />
treated carbon are less pronounced.<br />
<strong>Mercury</strong> removal efficiency, %<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 20 40 60 80 100<br />
PAC injection rate, mg/m 3<br />
55<br />
DARCO Hg<br />
DARCO Hg e-11<br />
Envergex<br />
DARCO Hg LH<br />
Figure 3.7. <strong>Mercury</strong> removal efficiency as a function of PAC injection rate for different<br />
sorbents at 138°C. The applied bag material is polyphenylene sulphide (PPS) and the airto<br />
cloth ratio is 1.22 m/min. Data are <strong>from</strong> [106].<br />
At rates higher than 80 mg/m 3 , the untreated carbons appear to perform similarly<br />
as the treated carbons with a mercury removal efficiency of about 90-95%. However,<br />
injection of the treated carbons at 80 mg/m 3 does not result in a significant increase in the<br />
mercury control efficiency as compared to untreated carbon injected at 48 mg/m 3 . This<br />
shows that there is no reason to choose halogenated carbon over untreated carbon,
particularly in light of the higher price and potential concerns associated with the use and<br />
disposal of halogen-treated materials.<br />
The trend of mercury removal efficiency of Darco Hg e-11 is similar to that of<br />
finer Darco Hg, but the Hg removal results were lower <strong>by</strong> 10%–15%. This was most<br />
likely caused <strong>by</strong> the larger particle sizes leading to more severe diffusion limitation.<br />
Three bag types were tested, namely, polyphenylene sulphide (PPS), membrane<br />
and fiberglass with membrane, while other conditions of the baghouse are the same. The<br />
primary aim of testing different bag types is to investigate whether retention of carbon<br />
particles on the bag surface can enhance the mercury removal efficiency. The<br />
comparison of the performance of the tree bag types is presented in figure 3.8. At PAC<br />
injection rate above 48 mg/m 3 of Darco Hg all three bag types perform quite similarly at<br />
138°C. Considering the uncertainty caused <strong>by</strong> the variability between the inlet and the<br />
outlet mercury measurements, it is likely that the bags are performing essentially the<br />
same at these conditions [108]. Then the only controlling factor for choosing the bag<br />
type is the working temperature. The flue gas temperature in the bag filter area of the<br />
cement process varies a lot and can exceed 200°C. Among the tested bag types, only the<br />
membrane/fiberglass bag can withstand continuous operating temperatures at 260°C and<br />
is therefore recommended.<br />
56
<strong>Mercury</strong> removal efficiency, %<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 20 40 60 80 100<br />
PAC injection rate, mg/m 3<br />
57<br />
PPS<br />
membrane/fiberglass<br />
Membrane<br />
Figure 3.8. Effects of bag material on mercury removal efficiency at 138°C. The applied<br />
air-to-cloth ratio is 1.22 m/min and the sorbent is Darco Hg. Data are <strong>from</strong> [106].<br />
The effect of air-to-cloth ratio on mercury removal was tested using the<br />
membrane/fiberglass bag. It is reported that at a PAC injection rate of 16 mg/m 3 the<br />
mercury removal efficiency increased with increasing the air-to-cloth ratio in the range of<br />
1.2-3.0 m/min and when the injection rates were higher than 48 mg/m 3 the mercury<br />
removal efficiency increases only slightly with further increasing the injection rate since<br />
the mercury removal efficiency is higher than 90% [106]. The increase of mercury<br />
removal efficiency with filtration velocity might be due to the fast accumulation of<br />
carbon on the bag surface. However, care must be taken when discussing the observation.<br />
The cleaning control of the bags was not specified. It is unknown whether the bags were<br />
cleaned at a fixed time interval or defined pressure drop over the filter. Most of the tests<br />
were conducted for a period of only about 20 min, while only several tests were run for<br />
up to 1-2 h.<br />
<strong>Injection</strong> of Darco Hg before the fabric filter with membrane/fiberglass was also<br />
tested in the raw-mill-off operating period. The mercury concentration at the filter inlet<br />
during the raw-mill-on period was about 485 µg/Nm 3 , but increased to about 2600
µg/Nm 3 during raw-mill-off operating period. Using an air-to-cloth ratio of 2.4 m/min,<br />
moderate mercury removal efficiencies of 52% and 58% were obtained at PAC injection<br />
rates of 48-80 mg/m 3 , respectively. A mercury removal efficiency of 88% was achieved<br />
when the PAC injection rate was increased to 160 mg/m 3 .<br />
Based on the parametric study at 138�C, design parameters for the full-scale<br />
sorbent injection upstream of a polishing filter at Durkee cement plant were<br />
recommended. The untreated carbon, fiberglass with membrane bag type, and air-tocloth<br />
ratio of 1.8-2.4 m/min were suggested. The proposed sorbent injection rate is 48<br />
mg/m 3 and 80 mg/m 3 for the raw-mill-on and raw-mill-off operation period, respectively.<br />
The estimated mercury removal efficiency is 90% during raw-mill-on conditions and<br />
60% during raw-mill-off conditions. The weighted mercury removal efficiency expected<br />
is about 77% on annual average.<br />
3.3.5 Carbon surface chemistry and mechanisms of mercury capture on<br />
carbons<br />
3.3.5.1 Carbon surface chemistry<br />
The surface chemistry of carbons determines their moisture content, catalytic<br />
properties, acid-base character, and adsorption of polar species. It is related to the<br />
presence of heteroatoms other than carbon within the carbon matrix. The most common<br />
heteroatoms are oxygen, nitrogen, phosphor, hydrogen, chlorine, and sulphur [109].<br />
During preparation of carbon and particularly during cooling and storage, carbon<br />
materials are in contact with the ambient air so that elements such as H and O are fixed<br />
on the surface, leading to oxygenated chemical functional groups [110]. Several<br />
structures of oxygen functional groups have been proposed as shown in table 3.3.<br />
Functional groups can be acidic, basic, or neutral in character. Surface oxygen groups on<br />
carbon materials decompose upon heating <strong>by</strong> releasing CO and CO2 at different<br />
temperatures. A CO2 peak results <strong>from</strong> carboxylic acids at low temperatures, or lactones<br />
at higher temperatures; carboxylic anhydrides originate both a CO and a CO2 peak;<br />
phenols, ethers, carbonyls, and quinones originate a CO peak.<br />
58
Table 8. Surface oxygen groups on carbon and their decomposition <strong>by</strong> TPD, after<br />
[110,111].<br />
Group name Decomposition Decomposition<br />
product temperature (�C)<br />
Carboxyl CO2 100-400<br />
Lactone CO2 190-650<br />
Carboxylic anhydrides CO+CO2 350-627<br />
Phenolic CO 600-700<br />
Ether CO 700<br />
Carbonyl CO 700-980<br />
Quinone CO 700-980<br />
Besides oxygenated functions, nitrogenated functions can be introduced on<br />
carbon surface <strong>by</strong> reaction of a carbon with a nitrogen-containing reactant or preparation<br />
of a carbon <strong>from</strong> a nitrogen-containing precursor [110].<br />
3.3.5.2 Mechanisms of mercury capture on carbons<br />
In order to understand the mercury capture mechanisms, it is important to<br />
understand the chemical and physical nature of the mercury-sorbent interaction. X-ray<br />
absorption fine structure (XAFS) spectroscopy and X-ray photoelectron spectroscopy<br />
(XPS) are techniques that have been previously used to determine information about the<br />
speciation and binding of mercury on a variety of materials [112,113]. XAFS spectra can<br />
be defined <strong>by</strong> two regions which include X-ray absorption near-edge spectroscopy<br />
(XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy. XANES<br />
spectra provide information on the oxidation state and characteristics of the first neighbor<br />
coordination environment. EXAFS spectroscopy provides more robust information on<br />
the identity of nearest-neighboring elements, coordination values, and interatomic bond<br />
distances.<br />
XAFS spectroscopy was used to distinguish between elemental and oxidized<br />
mercury in the sorbents <strong>by</strong> comparing the XAFS spectrum. Elemental mercury exhibits a<br />
59
single peak only in the first-derivative of the mercury XANES spectrum, whereas most<br />
mercuric compounds exhibit a two-peak spectrum [112]. The sorbents were tested for<br />
mercury capture at temperatures lower than 200°C. The studied sorbents included<br />
carbonaceous materials and inorganic-based material, such as lime-derived sorbents and<br />
zeolites.<br />
The XANES data imply that the capture of elemental mercury must involve an<br />
oxidation process, either in the gas phase before interacting with the sorbent, or<br />
simultaneously as the Hg 0 atom interacts with the sorbent [112]. This is consistent with<br />
the fact that all Hg-sorbed materials examined exhibit the characteristic dual inflection<br />
point structure in their XANES spectra that is indicative of the formation of Hg–anion<br />
chemical bonds. The anion could be virtually any available electronegative species, as<br />
evidence has been seen for the formation of Hg–I, Hg–Cl, Hg–S, Hg–O, and Hg-Br<br />
[112,113]. Modeling of mercury capture <strong>by</strong> activated carbon using density functional<br />
theory shows that the mercury binding energies increase with the addition of the<br />
following halogen atoms, F>Cl>Br>I [114]. Data <strong>from</strong> S and Cl XANES spectra, as well<br />
as <strong>from</strong> the Hg XAFS data, strongly support the hypothesis that interaction of acidic<br />
species (HCl, HNO3, H2SO4, HBr, etc.) in the flue gas with the sorbent surface is an<br />
important mechanistic process that is responsible for creation of active sites for mercury<br />
capture <strong>by</strong> chemisorption. The mechanisms of elemental mercury capture on the carbon<br />
sorbents likely consist of surface-enhanced oxidation of the elemental mercury via<br />
interaction with surface-bound halide species with subsequent binding <strong>by</strong> surface halide<br />
or sulphate species [113].<br />
The catalytic effects of carbon sorbents for mercury capture were investigated <strong>by</strong><br />
Olson et al. [115]. The studied carbons were lignite- and bituminous-derived carbon and<br />
catalytic carbon, which were available commercially with enhanced catalytic<br />
functionality for aqueous reactions such as decomposition of peroxides. Catalytic<br />
carbons are produced <strong>by</strong> recarbonization of urea or ammonia-treated oxidized activated<br />
carbons or <strong>by</strong> impregnation of nitrogen-containing polymers and pitches [115]. Without<br />
acid gases in the gas stream at 150�C, 50% mercury breakthrough was observed after 8<br />
60
min for the catalytic carbon, while less than 1 min for the lignite- and bituminous-derived<br />
carbons. Thus, a catalytic chemisorption mechanism predominates for the sorption of<br />
mercury at these conditions.<br />
The mercury adsorption capacity of the sorbent is inversely proportional to the<br />
temperatures in a studied range of 50-150°C, indicating that a preliminary physisorption<br />
step with mercury associating with a surface site takes place [115]. The chemisorption of<br />
Hg 0 is likely a multistep reaction. When the temperature is increased, the rate of each<br />
chemical reaction step increases and the exothermic physisorption of Hg 0 at nonoxidizing<br />
binding sites will decrease. If the sorption process includes a preliminary<br />
physisorption equilibration where Hg 0 binds and desorbs at the active site, the<br />
equilibration will show a negative temperature effect on the overall reaction rate, since<br />
desorption is favored at higher temperatures. Although chemisorption may account for<br />
the main sorption of mercury, the extent to which increasing the temperature may affect<br />
the sorption rate cannot be predicted.<br />
A detailed mechanism has been proposed to explain the effects of SO2 and NO2<br />
as shown in figure 3.9 [116]. In the presence of NO2, Hg 0 is catalytically oxidized on the<br />
carbon surface to form the nonvolatile nitrate Hg(NO3)2, which is bound to basic sites on<br />
the carbon. The Lewis base site refers to the zigzag carbon atom positioned between<br />
aromatic rings [117]. Capture continues until the binding sites are used up and<br />
breakthrough occurs. In the presence of SO2, some of the catalytic sites are converted to<br />
a sulfate form where Hg(NO3)2 is no longer formed. <strong>Mercury</strong> is still oxidized on the<br />
surface with NO2 acting as the oxidizing agent, but the product formed is a labile sulfur<br />
compound, mercury bisulfate [Hg(SO4H)2]. The bisulfate in turn reacts with NO3 - to<br />
form a stable but volatile acidic form of the mercuric nitrate. The emission of Hg(NO3)2<br />
or the hydrate Hg(NO3)2�H2O has been confirmed <strong>by</strong> solvent trapping and gas<br />
chromatography analysis. Sulfurous acid that accumulates <strong>from</strong> the hydration of SO2<br />
converts the previously formed nonvolatile basic mercuric nitrate into the volatile form,<br />
which explains the slow release of previously captured mercury over time in the presence<br />
of NO2 and SO2.<br />
61
Figure 3.9. Proposed heterogeneous model for mercury capture on carbon showing<br />
potential impact of acid gases [116].<br />
Sulphur trioxide can be present in power plant flue gas through one of the<br />
following paths [21]: (1) During combustion, coal-S is converted to SO2 and a small<br />
fraction of the sulfur is further oxidized to SO3. During combustion of high-sulfur coals,<br />
a minor part of the sulfur is converted to SO3, leading to flue gas concentrations in the<br />
range of 1-40 ppm. (2) SO3 is sometimes added to a level above 10 ppm to flue gas<br />
upstream of an ESP as a conditioning agent and to improve ESP performance. SO3 and<br />
H2SO4 have a low vapor pressure and can condense on fly ash and this reduces the<br />
resistivity of the ash and allows it to be removed more efficiently <strong>by</strong> the ESP. (3) SO2<br />
can be oxidized to SO3 <strong>by</strong> SCR catalysts installed for NOx reduction [118-120]. SCR<br />
catalysts typically contain vanadium oxides, which are known catalysts for the oxidation<br />
of SO2 to SO3 and Hg to HgCl2.<br />
The inhibiting effect of SO3 on mercury capture <strong>by</strong> activated carbon injection has<br />
been observed in full-scale power plant tests [21,121]. Possible mechanisms for the SO3<br />
effect on mercury capture <strong>by</strong> activated carbon are postulated <strong>by</strong> Presto and Granite<br />
[21,22]. In addition to removing mercury, activated carbon is also used as catalyst for<br />
oxidation of SO2 to sulphuric acid [122,123]and as SO2 sorbent. There is competitive<br />
adsorption between Hg and SO3 since both mercury and SO3 bind to the Lewis acid base<br />
62
sites on the activated carbon surface. The adsorption of SO3 could be favored both<br />
kinetically and thermodynamically. The concentration of SO3 in flue gas is typically in<br />
the range of 1-40 ppm and this is orders of magnitude larger than typical mercury<br />
concentrations. The bond formed between the S 6+ species, such as sulfuric acid and<br />
sulfates, and the carbon surface is stronger than the bond between mercury and the<br />
surface. SO2 can oxidize to sulphate and form a chemical bond with the carbon surface<br />
with a heat of adsorption of >80 kJ/mol. Some activated carbon catalysts for converting<br />
SO2 to H2SO4 are self-poisoned <strong>by</strong> SO3 or sulfate buildup on the surface. A similar<br />
phenomenon might explain the inhibiting effect of SO3 on mercury capture.<br />
3.3.6 Processing and reuse of mercury laden activated carbon<br />
The existing production capacity for powdered activated carbon is only 10% of<br />
the capacity required for full implementation of the activated carbon injection technology<br />
to control mercury emissions [124]. The mercury sorption capacity of the activated<br />
carbon is very low, about 1-4 mg of mercury per gram of sorbent, depending on the<br />
mercury concentration in the flue gas [125]. This implies that 250 to 1000 g of activated<br />
carbon are needed to remove 1 g of mercury in the flue gas. Therefore, a large quantity of<br />
spent sorbents contaminated with various forms of mercury is produced.<br />
Presently the PAC with adsorbed mercury must be disposed after use. In addition<br />
to the purchase expense, the disposal of this material is also quite costly. There are strict<br />
regulations for disposal of mercury-containing wastes [126]. Hazardous wastes<br />
containing less than 260 mg/kg of total mercury are required to be treated to 0.20 mg/L,<br />
measured using the toxicity characteristic leaching procedure (TCLP) for mercury<br />
residues <strong>from</strong> retorting, and 0.025 mg/L TCLP for all other low mercury wastes. Wastes<br />
that contain greater than 260 mg/kg total mercury are required to undergo roasting or<br />
retorting in a thermal processing unit capable of volatilizing mercury and subsequently<br />
condensing the volatilized mercury for recovery.<br />
To reduce the PAC purchase expense and disposal cost of mercury-containing<br />
PAC, a process has been developed to regenerate the used PAC and recover mercury<br />
[124]. To separate PAC <strong>from</strong> the fly ash, PAC is injected between the main filter and the<br />
63
polishing FF. The collected PAC is periodically removed <strong>from</strong> the filter and regenerated<br />
in nitrogen process gas and is directed to a multiple activated carbon column gas<br />
treatment system to remove the gaseous mercury <strong>from</strong> the cooled process gas stream.<br />
After passing through the sulphur impregnated carbon columns, the carrier gas is injected<br />
into the flue gas stream ahead of the carbon injection site. In this way only a small<br />
amount of carbon with high mercury content requires disposal.<br />
An inert atmosphere is required for the tray desorption furnace to avoid<br />
significant losses of the PAC material during mercury desorption. Using a desorption<br />
temperature of 550°C and a duration of 30 minutes, the PAC can be recycled at least 10<br />
times without significant degradation of the adsorption characteristics in nitrogen [124].<br />
It is unknown whether the cycled sorbent works satisfactorily in the real flue gas.<br />
There are only few studies on the mercury desorption <strong>from</strong> exposed sorbents. It is<br />
worth noting that mercury desorption is relevant both to recover the mercury and to<br />
detoxify the adsorbing material in order to avoid its stabilization before land-filling or to<br />
allow its reuse.<br />
A study of mercury desorption in nitrogen <strong>from</strong> sulphur impregnated activated<br />
carbon showed that the adsorption rate was faster than the desorption rate [59]. <strong>Mercury</strong><br />
desorption <strong>from</strong> sorbents is strongly affected <strong>by</strong> desorption temperature, with faster<br />
desorption at high temperature and the mercury-sorbent pair. The desorption rate is<br />
relatively fast initially and then levels off close to zero at a certain concentration of<br />
mercury in sorbents.<br />
Desorption of mercury <strong>from</strong> activated carbon and fly ash mixture was also carried<br />
out in a fluidized bed reactor at temperatures up to 500°C [127]. All the mixtures had<br />
constant mercury content, i.e., no mercury desorption was observed, until a critical<br />
temperature was reached and then with rapidly decreasing mercury content as the<br />
temperature was increased to higher levels. The critical temperature was found to be a<br />
linear function of carbon contents in the mixtures, increasing <strong>from</strong> 330°C at 17% carbon<br />
to 370°C at 33% carbon. The temperature at which all of the mercury was removed was<br />
in the 450 to 500°C range.<br />
64
3.3.7 Applicability of sorbent injection in cement plants<br />
As PAC systems are adapted for control of boilers, it will be possible to evaluate<br />
the feasibility of these control techniques for cement kiln applications having<br />
approximately the same mercury concentrations. Considering the differences between<br />
boiler and kiln applications the possible application of PAC systems to cement kilns<br />
appears to be considerably more challenging than to coal-fired boilers.<br />
Powdered activated carbon injection systems do not appear to be appropriate<br />
upstream of a cement kiln fabric filter system. <strong>Cement</strong> kilns must recycle a major portion<br />
of the collected dust. Some kilns use the fabric filter system as an integral part of the raw<br />
material processing system. Recycling the mercury laden activated carbon would result<br />
in the revolatilization of the large majority of the mercury. Disposal of the activated<br />
carbon containing cement kiln dust (CKD) also would be complicated because it might<br />
be classified as a hazardous waste due to the presence of mercury.<br />
Due to these issues, a powdered activated carbon injection system would have to<br />
be installed downstream of the main kiln fabric filter to avoid the CKD recycling and<br />
disposal issues. A second fabric filter would have to be installed after the main fabric<br />
filter. The activated carbon injection system would have to be positioned to provide one<br />
to two seconds residence time prior to entering the second fabric filter. The temperature<br />
of this system would have to be controlled to less than 200�C to ensure proper mercury<br />
adsorption and reduce the risk of activated carbon fires in the fabric filter or solids<br />
handling system.<br />
3.4 <strong>Mercury</strong> removal <strong>by</strong> activated carbon bed<br />
Fixed and moving bed systems for mercury and dioxin-furan control are also used<br />
in Europe [5]. In both types of systems, contaminant-laden gas is forced through a bed of<br />
granular activated carbon.<br />
One of the fixed bed systems used Sorbalit sorbent instead of activated carbon.<br />
The Sorbalit sorbent consists of Portland cement, lime, carbon, and sulfur compounds<br />
such as sublimed sulfur, Na2S, NaHS, and Na2S4 [16].<br />
65
The quantity of mercury that can be retained on the sorbent at equilibrium is<br />
important. The sorbent can be used at levels that approach the saturation capacity of the<br />
sorbent at the operating gas temperature and gas stream conditions. However, the control<br />
system must have the capability to remove the sorbent on at least a semi-continuous basis.<br />
In fixed bed systems, the activated carbon must be replaced with fresh carbon at a<br />
rate that is dependent primarily on the rate of approach to the mercury saturation level,<br />
and the rate of static pressure increase. Spent carbon can be disposed of <strong>by</strong> combustion if<br />
the unit is equipped with a wet scrubbing system. The combustion process destroys the<br />
organic compounds captured in the carbon, and the wet scrubber collects the heavy<br />
metals and acid gases. In this case, however, the elemental mercury might not be<br />
removed due the insolubility of elemental mercury in the water. Another disposal option<br />
is to dispose the carbon in a landfill. Because of the adsorbed pollutants, this waste may<br />
require disposal as a hazardous waste. Another option is to heat the carbon and desorb<br />
the pollutants <strong>from</strong> the carbon.<br />
Slipstream tests of the activated carbon bed have been recently conducted in<br />
several U.S. power plants [128]. Direct adaptation of existing carbon bed technology to<br />
mercury removal <strong>from</strong> utility power plant flue gas is very costly because of the large flue<br />
gas volumes and low mercury concentrations involved [129]. A thorough engineering<br />
and economic analysis would be necessary to determine the feasibility of modifications<br />
that reduce bed size and the amount of carbon in the bed. The effectiveness of the<br />
modified beds for mercury removal under various flue gas conditions needs to be<br />
determined. Furthermore, the tradeoff between gas velocity to the bed, bed sorbent size<br />
and bed thickness, pressure drop, mercury and ash collection effectiveness, and bed<br />
lifetime should be examined.<br />
For cement plant application, the fixed bed activated carbon systems could not be<br />
installed upstream of the main kiln bag filter or ESP. The high dust loadings in these<br />
locations would quickly blind both types of beds and result in very high activated carbon<br />
usage rates and disposal requirements. Accordingly, it would be necessary to install these<br />
systems downstream of the main particulate matter control system. Similar to the power<br />
66
plant application, installing a fixed bed carbon system in a cement plant will also be very<br />
costly.<br />
3.5 <strong>Mercury</strong> control <strong>by</strong> flue gas desulphurization systems<br />
Dry and wet scrubbers, commonly used in large scale combustion systems for<br />
SO2 and HCl control can be simultaneously used for mercury retention, taking advantage<br />
of the same sorbents used for sulphur or adding a new material for mercury [130-133].<br />
Wet scrubbing systems predominately collect oxidized mercury [5,131]. In the purge<br />
stream, mercuric chloride is collected as a precipitated solid along with the calcium<br />
sulfate. When used as stand alone systems, they have the capability to achieve moderateto-high<br />
removal efficiencies for oxidized mercury. They are entirely ineffective in the<br />
removal of the highly insoluble elemental mercury.<br />
There are limited number of lime-based scrubbing systems used primarily for<br />
particulate and SO2 control at lime kilns. There are presently only few cement kilns in the<br />
United States equipped with wet SO2 scrubbing systems [5].<br />
The stability of oxidized mercury captured in the flue gas desulphurization (FGD)<br />
systems has been investigated and it was found that the captured oxidized mercury can be<br />
reduced <strong>by</strong> aqueous phase reactions to form elemental mercury [6]. The insoluble<br />
elemental mercury is rapidly released to the gas stream. Occurrence of mercury in FGDgypsum<br />
may threaten its re-use for wallboards since mercury can be released during the<br />
heating steps in wallboard manufacturing [131].<br />
Spray dryer absorbers (SDA) with a Ca(OH)2 slurry have been used for sulfur<br />
dioxide and hydrogen chloride control at waste incinerators and coal-fired boilers. SDA<br />
has been applied recently to cement kilns to control HCl that contributes to secondary<br />
plume formation [5].<br />
SDA systems would have to be installed after the main particulate matter control<br />
system to ensure that captured mercury remains with a solid waste product and is not<br />
recycled to the feed end of the kiln. With respect to fossil fuel fired boilers, the reported<br />
mercury removal efficiencies <strong>by</strong> SDA systems are in the range of 50% to 60% for eastern<br />
67
ituminous coals and 0% to 20% for western lignite and subbituminous coals [5]. The<br />
difference is caused <strong>by</strong> the lower fraction of oxidized mercury for the lignite coals. With<br />
respect to cement kilns, it appears unlikely that SDA systems will be more effective than<br />
inherent adsorption in cement kiln systems.<br />
3.6 <strong>Mercury</strong> removal <strong>by</strong> sodium tetrasulfide injection<br />
Sodium tetrasulfide (Na2S4) has been used as a sorbent to remove mercury <strong>from</strong><br />
flue gas in a number of waste-to-energy plants [1]. This technology should not be<br />
confused with sodium sulfide Na2S that was tried in both Europe and U.S. without<br />
success [134]. The shortcomings of Na2S are that it can leave a strong odor of hydrogen<br />
sulfide (H2S) in the ash and it does not control all species of Hg. The major advantages of<br />
the Na2S4 technology are that it controls elemental as well as ionic forms of Hg.<br />
An aqueous Na2S4 solution is injected into the flue gas duct and such a system<br />
can easily be retrofitted to an existing flue gas cleaning plant [134]. The sodium<br />
tetrasulfide reacts with vapor phase mercury to form solid mercuric sulfide (HgS), which<br />
is a solid at temperatures below about 580°C, and is insoluble [134]. By converting<br />
vapor-phase mercury to an insoluble solid, it may be removed in a FF or ESP. Sodium<br />
tetrasulfide can react with both oxidized and elemental mercury in accordance with the<br />
following simplified reactions [134]:<br />
Na2S4 �HgCl2 � HgS �2NaCl � 3S<br />
(R3.4)<br />
Hg�S � HgS<br />
(R3.5)<br />
Decomposition of Na2S4 <strong>by</strong> an acid such as HCl can provide excess elemental<br />
sulfur. It can also generate an alternate form of ionic sulfur, H2S, for reaction with<br />
oxidized mercury as shown in the following reactions:<br />
Na S �2HCl �HS�3S� 2NaCl<br />
(R3.6)<br />
2 4 2<br />
HgCl2 �H2S�HgS � 2HCl<br />
(R3.7)<br />
In the absence of HCl, carbon dioxide may act as an acid for decomposition:<br />
Na S �2CO�2HO�HS�3S� 2NaHCO<br />
(R3.8)<br />
2 4 2 2 2 3<br />
68
Therefore, it is possible to eliminate both the elemental and ionic forms of mercury in the<br />
flue gas.<br />
However, H2S will still be produced in the process as shown in R3.6 and R3.8.<br />
The problem of H2S odor in the ash cannot be avoided. This process is not suitable to<br />
mercury removal <strong>from</strong> cement plant due to the presence of sodium, which could<br />
deteriorate the cement quality.<br />
3.7 Enhanced mercury removal <strong>by</strong> oxidation<br />
Oxidation pretreatment systems may convert elemental mercury to oxidized<br />
mercury upstream of wet scrubber systems and even upstream of conventional particulate<br />
matter control systems. Once in the oxidized form, mercury is captured in these air<br />
pollution control systems at efficiencies approaching 85% [5]. The oxidation<br />
pretreatment systems must be able to withstand the gas stream conditions upstream of the<br />
air pollution control system used for capture of the oxidized mercury. Oxidation<br />
pretreatment systems are only effective for the vapor phase mercury that is not adsorbed<br />
on particle surfaces.<br />
Selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR)<br />
systems are used in coal-fired boilers and waste incinerators for the control of nitrogen<br />
oxides. The impact of SNCR systems on the chemical form of mercury in a gas stream<br />
appears to be minimal [5].<br />
SCR systems use a catalyst to react ammonia and nitrogen oxides to provide<br />
nitrogen and water. SCR systems are used extensively for coal-fired boilers and waste<br />
incinerators. Full-scale tests have been performed in four U.S. power plants and the<br />
results are presented in table 3.4 [135,136]. Significant oxidation of elemental mercury<br />
across the SCR was observed in plant 2 and 4. While slight mercury oxidation over SCR<br />
was experienced in pant 1 and 3. General conclusions <strong>from</strong> these tests are the oxidation<br />
effect was quite variable and appears to be coal-specific and possibly catalyst-specific. In<br />
particular, the catalyst type, space velocity, and catalyst age may all be important<br />
variables. Plant 1 burns PRB coal with lower chlorine content and the catalyst is older.<br />
69
More than 90% of the mercury in the flue gas at plant 1 is elemental mercury at the SCR<br />
inlet. One possible explanation for the relatively low oxidation rate of the SCR at plant 3<br />
is the relatively high space velocity, which is nearly double the space velocity compared<br />
to other plants. In addition, the total inlet mercury concentration was more than twice the<br />
levels seen at the other test sites.<br />
Table 3.4. Test conditions and results of mercury oxidation over SCR catalyst in four U.S.<br />
power plants. Data are <strong>from</strong> [135,136].<br />
Plant 1 2 3 4<br />
Coal PRB<br />
subbituminous<br />
Ohio<br />
bituminous<br />
high-sulfur<br />
70<br />
Pennsylvania<br />
bituminous<br />
low-to-medium<br />
sulfur<br />
Kentucky<br />
bituminous<br />
medium-sulfur<br />
Hg in coal (ppmm) 87 168 400 131<br />
Cl in coal (ppmm)
ange (300-350°C) in a real flue-gas atmosphere [139], while NH3 shows a small<br />
detrimental effect. Adding 2000 ppmv SO2 to baseline gases that contain 6% O2, 12%<br />
CO2, 8% H2O, 550 ppmv NH3, 600 ppmv NO, 18.5 ppmv NO2 without HCl only<br />
increases the mercury oxidation over SCR <strong>from</strong> 3% using baseline gases to 7% with<br />
2000 ppmv SO2 [139,142].<br />
Adding 50 ppmv SO3 to the baseline gases improves the mercury oxidation to<br />
20% [139,142]. Adding 50 ppmv HCl to the baseline gases without SO2/SO3 results in<br />
71% oxidized mercury in flue gas across the SCR compared to 45% mercury oxidation<br />
when 50 ppmv SO3 was further added to the flue gas. With 50 ppmv HCl and 2000 ppmv<br />
SO2 were added to the baseline gases, mercury oxidation recovered to 64%. The<br />
combination of 2000 ppmv SO2/50 ppmv SO3 and 50 ppmv HCl showed a 63% mercury<br />
oxidation. These observations indicate that both SO2 and SO3 had a negative effect on<br />
mercury–chlorine oxidation over the SCR as a result of slower mercury oxidation <strong>by</strong> the<br />
sulfated site compared to that of the chlorinated site. The extent of the mitigating effect<br />
<strong>by</strong> the 2000 ppmv SO2 was not as severe as the 50 ppmv SO3 since the concentration of<br />
SO3 derived through SO2 oxidation over SCR was much lower than the 50 ppmv SO3.<br />
There is possible competition between HCl, SO2, and SO3 over the SCR catalyst.<br />
Conventional SCR catalyst with higher mercury oxidation capacity has been<br />
closely related to higher oxidation of SO2 to SO3 [144]. Higher SO2 oxidation in coalfired<br />
applications can cause negative downstream impacts such as air heater fouling, flue<br />
duct corrosion and visible stack plumes. The SO2 to SO3 conversion is designed to be less<br />
than 1.5% at SCR operating conditions. Research has been focused on developing new<br />
SCR catalyst that has higher oxidation rate of elemental mercury and very low SO2 to<br />
SO3 conversion [144].<br />
Two studies have demonstrated that ultraviolet radiation in the presence of solid<br />
titanium dioxide (TiO2) results in the photocatalytic conversion of elemental mercury to<br />
HgO when HCl is not present in the gas [6,145,146]. The TiO2 is readily available as a<br />
major component of conventional SCR catalysts. Accordingly, the combination of<br />
ultraviolet light and SCR catalysts could in principle be used to oxidize elemental<br />
71
mercury. Compared to lab-scale study where ultraviolet light can be readily applied,<br />
application of ultraviolet light in monolith catalyst could be a technical challenge.<br />
The application of SCR systems to cement kilns continues to be precluded <strong>by</strong><br />
problems associated with alkali metal and arsenic related catalyst poisoning, SO2<br />
oxidation to SO3, particulate matter loadings that can be 5 to 20 times higher than coalfired<br />
boiler high dust systems, and non-ideal temperature ranges for SCR catalysts<br />
[147,148]. For these reasons, it is unlikely that an SCR system will be used for the<br />
control of nitrogen oxides and oxidation of elemental mercury in cement plants in the<br />
near future.<br />
3.8 <strong>Mercury</strong> removal <strong>by</strong> roaster process<br />
Recently a new process for mercury removal <strong>from</strong> the cement kiln flue gas <strong>by</strong> a<br />
roaster was invented and patented [149-151]. As mentioned earlier recycled cement kiln<br />
dust <strong>from</strong> the main bag filter has high mercury content. The mercury flow in the<br />
collected cement kiln dust is about 60% of the mercury inlet to the cement kiln [152].<br />
This indicates that dust captured in the main baghouse acts as a natural sorbent for<br />
mercury. This mercury enriched dust is taken to the new mercury roaster process for<br />
cleaning before the dust is returned to the system. Figure 3.10 illustrates an example of a<br />
mercury roaster installation. The baghouse dust is fed to a roasting system which uses a<br />
heat source (for example kiln <strong>by</strong>pass gas, cooler vent gas, or hot gas generator) to heat<br />
the dust above the boiling point of mercury compounds. While the mercury is still in the<br />
gas phase, the gas stream enters a hot ESP which removes most of the cleaned dust. This<br />
dust is taken back to the blending silo to be part of the kiln feed. After the ESP, the gas<br />
stream is cooled below the mercury boiling point so that the mercury can condense on<br />
the dust particles that were not captured in the ESP and additional sorbent is added to the<br />
gas stream here to capture the mercury. The cleaned gas after the baghouse is vented to<br />
the atmosphere. Depending on the type of applied sorbents the mercury enriched<br />
dust/sorbent collected in the baghouse can be transported to the finish mill area to be<br />
72
added to the cement or disposed as waste. The air and sorbent flow rates are expected to<br />
be smaller than what would be seen with a full carbon injection system.<br />
Figure 3.10. Sketch of the roaster process [149].<br />
It should be pointed out that the process is still under development. Information is<br />
lacking on the achievable mercury removal efficiency and operating cost. Since most<br />
dust is removed <strong>by</strong> the ESP it appears that sorbent is still required for capture the<br />
mercury evaporated <strong>from</strong> the roaster. Calcium chloride may be required to oxidize the<br />
elemental mercury and enhance mercury capture <strong>by</strong> the sorbent. It is unclear how<br />
effectively the elemental mercury can be oxidized and how much calcium chloride is<br />
required.<br />
3.9 Conclusions<br />
<strong>Mercury</strong> can be removed <strong>from</strong> the flue gas <strong>by</strong> fuel cleaning and switching, raw<br />
material cleaning, sorbent injection, sorbent bed, oxidation <strong>by</strong> catalyst and subsequent<br />
removal <strong>by</strong> wet scrubber, spray drier absorber, and roaster process with smaller sorbent<br />
injection system. Presently sorbent injection is considered as the most promising and<br />
developed mercury removal technology. <strong>Mercury</strong> removal <strong>by</strong> sorbent injection can be<br />
affected <strong>by</strong> many factors such as mercury speciation and concentration, flue gas<br />
73
composition and temperature, mercury vapor-sorbent contacting time, sorbents and<br />
sorbent dispersion, etc. Due to the high moisture level and lack of carbonaceous particles<br />
in the cement kiln flue gas, and release of the captured mercury during recirculation to<br />
the kiln, the application of sorbent injection to cement kilns will be more challenging and<br />
the obtained knowledge <strong>from</strong> coal-fired power plants and waste incinerators cannot be<br />
applied to cement kiln directly. The PAC injection system should be installed<br />
downstream of the main kiln fabric filter and upstream of a new added polishing fabric<br />
filter to avoid the cement kiln dust recycling and increased disposal issues.<br />
Powdered activated carbon is the most widely used sorbent for mercury removal<br />
<strong>from</strong> flue gas. However, there is a lack of fundamental investigation of mercury<br />
adsorption <strong>by</strong> activated carbon in simulated cement kiln flue gas. Even for power plant<br />
application, mercury capture kinetics is not available in most of the publications and<br />
many of the studies were carried out in air or nitrogen without acid gases presence. The<br />
majority of the publications focused on elemental mercury capture and only few studies<br />
investigated capture of HgCl2 which is a major mercury species. To reduce the cost of<br />
sorbent and possible disposal expense, non-carbon based and concrete/cement friendly<br />
sorbents such as Amended Silicate TM have been developed. Other developments include<br />
regeneration and recirculation of sorbents and in-situ generation of activated carbon. The<br />
performance of these sorbents needs to be proved in the full-scale application.<br />
The carbon-oxygen surface complexes and flue gas composition play an<br />
important role in mercury removal <strong>by</strong> activated carbon injection. Both physisorption and<br />
chemisorption are involved in mercury capture <strong>by</strong> carbons. The mechanisms of elemental<br />
mercury capture on the carbons consist of surface-catalyzed oxidation of the elemental<br />
mercury via interaction with surface-bound halide species with subsequent binding <strong>by</strong><br />
surface halide or sulphate species. Co-presence of SO2 and NO2 in the flue gas results in<br />
a poor performance of carbons. There is competitive adsorption between Hg and SO3<br />
since both mercury and SO3 bind to the Lewis acid base sites on the activated carbon<br />
surface.<br />
74
3.10 Further research requirement<br />
Activated carbon injection is a promising technology, but further research is<br />
needed to provide the best sorbent with effective mercury capture at a low cost.<br />
Investigation of mercury capture <strong>by</strong> the activated carbon using simulated cement kiln<br />
flue gas is imperative to evaluate whether the activated carbon is also a promising<br />
sorbent for cement plant application. Lab-scale tests are desired to obtain kinetics and<br />
study the effects of different operating parameters.<br />
More focus is needed on developing alternative sorbents. Fly ash and cement raw<br />
materials such as clay and silica might be used as cement-friendly sorbents and<br />
alternatives for activated carbon. A better understanding of mercury removal <strong>by</strong> fly ash<br />
and other cement-friendly sorbents is therefore needed. Fundamental investigation on the<br />
regeneration of the sorbents and enhancement of the sorbents <strong>by</strong> adding chemical agents<br />
during regeneration is required. Focus should be put on desorption temperature,<br />
separation and purification of collected mercury compounds and possible regeneration<br />
cycle of the sorbents. Possibility of regenerating the sorbent <strong>by</strong> hot flue gas <strong>from</strong> the kiln<br />
system and in-situ enhancement of the sorbent should be investigated.<br />
3.11 Abbreviations<br />
APCD: Air pollution control device<br />
CKD: <strong>Cement</strong> kiln dust<br />
COHPAC: Compact hybrid particulate collector<br />
ESP: Electrostatic precipitator<br />
EXAFS: Extended X-ray absorption fine structure<br />
FF: Fabric filter<br />
FGD: Flue gas desulphurization<br />
LNB: Low NOx burner<br />
PAC: Powdered activated carbon<br />
PPS: Polyphenylene sulphide<br />
PRB: Powder River Basin<br />
75
SCR: Selective catalytic reduction<br />
SDA: Spray dryer absorber<br />
SNCR: Selective non-catalytic reduction<br />
TCLP: Toxicity characteristic leaching procedure<br />
TDF: Tire-derived fuel<br />
XAFS: X-ray absorption fine structure<br />
XANES: X-ray absorption near-edge spectroscopy<br />
XPS: X-ray photoelectron spectroscopy<br />
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85
Experimental methods and materials<br />
To provide a simple means for screening the performance of candidate sorbents<br />
and derive mercury capture kinetics for promising sorbents in a mercury-laden simulated<br />
cement kiln flue gas, a fixed-bed reactor system was designed and built in this project. In<br />
this chapter the entire reactor setup will be described. Furthermore, the choice of some of<br />
the core parts, i.e., the mercury vapor generator, humidifier for water vapor addition and<br />
mercury analysis system are described in more details. Materials and methods applied in<br />
this project are also presented.<br />
4.1 Description of the fixedbed reactor system<br />
Tests of the oxidized mercury converter and sorbents were conducted in a fixedbed<br />
reactor system as illustrated in figure 4.1. A photo of the system is shown in figure<br />
4.2. Main equipments in the reactor system include a gas mixing system with water vapor<br />
addition <strong>by</strong> a humidifier and mercury source in a calibration gas generator to simulate the<br />
cement kiln flue gas, a low temperature oven with a glass reactor, mercury analysis<br />
system, and mercury traps for exhaust gas treatment. To avoid mercury condensation and<br />
accumulation in the system, all the gas lines before the analyzer are heated to 150�C. All<br />
temperatures including temperatures of heated lines, ovens, reactors, converter, and<br />
analytical cell in mercury analyzer are sampled. The mercury source, reactor and hot<br />
panel are located in a dedicated ventilation hood.<br />
86<br />
4
N 2<br />
N 2<br />
CO 2<br />
O 2<br />
HCl<br />
SO 2<br />
NO x<br />
N 2<br />
Flow meter<br />
MFC<br />
MFC<br />
MFC<br />
MFC<br />
MFC<br />
MFC<br />
MFC<br />
MFC<br />
Vent Vent<br />
Hg source<br />
Evaporator<br />
Rotameter<br />
Filter<br />
87<br />
Heat trace<br />
Reactor<br />
Distribution<br />
box<br />
Analyzer<br />
Converter<br />
Figure 4.1. Sketch of the fixed-bed reactor system for converter and sorbent tests.<br />
Figure 4.2. Photo of the fixed-bed reactor system.<br />
Air<br />
Filter<br />
Vent
4.1.1 Gas mixing system<br />
The gas mixing system consists of valves and mass flow controllers for adding<br />
different gases to simulate cement kiln flue gas. Gas addition includes carrier nitrogen to<br />
the mercury source, carrier nitrogen to humidifier, CO2, O2, HCl, SO2, NO, NO2 and<br />
balance nitrogen. The addition of the gases is controlled <strong>by</strong> the mass flow controllers and<br />
the actual flow rate of each gas is measured <strong>by</strong> a bubble flow meter.<br />
4.1.2 <strong>Mercury</strong> vapor addition system<br />
<strong>Mercury</strong> sources are added using a commercial calibration gas generator <strong>from</strong><br />
VICI Metronics, Dynacalibrator Model 150-06e-C with a gas flow capacity up to 750<br />
ml/min. The Model 150 calibration gas generator is a constant temperature system<br />
designed to generate precise ppm or ppb concentrations of chemical compounds in a gas<br />
stream, using a permeation tube as the trace gas source. Figure 4.3 shows a picture and<br />
sketch of the calibration gas generator. A passivated glass-coated permeation chamber<br />
houses the permeation device, with measured inert carrier gas nitrogen sweeping the<br />
calibration gas/vapor <strong>from</strong> the chamber. A digital temperature controller maintains the<br />
chamber temperature at a set point with an accuracy of ±0.01°C and a wide range of<br />
temperature settings (5°C above ambient to 110°C).<br />
Figure 4.3. Left: picture of the calibration gas generator. Right: sketch of the calibration<br />
gas generator [1].<br />
88
The permeation tubes are small, inert capsules containing pure liquid elemental<br />
mercury or solid mercury chloride in a two- phase equilibrium between its gas phase and<br />
its liquid or solid phase, respectively. At a constant temperature, the device emits the<br />
compound through its permeable portion at a constant rate. Figure 4.4 illustrates the<br />
working principle of elemental mercury permeation tube.<br />
Figure 4.4. Sketch of the permeation tube with elemental mercury [2].<br />
The amount of mercury released <strong>from</strong> the tube is governed <strong>by</strong> the permeability of<br />
the material used for the tube, the length of the tube, and the temperature at which the<br />
tube is maintained. When the permeation rate at that temperature and the carrier flow rate<br />
are known, the concentration of the calibration stream can be estimated. Table 4.1 shows<br />
the specifications of the permeation tubes used in this project.<br />
Table 4.1. Specifications of the elemental mercury and mercury chloride permeation<br />
tubes used in this project.<br />
Elemental <strong>Mercury</strong> <strong>Mercury</strong><br />
mercury<br />
chloride chloride<br />
Working temperature (�C) 70 70 50<br />
Tube diameter (mm) 9.8 9.8 9.8<br />
Tube length (mm) 100 13 70<br />
Release rate (ng/min) 378 2445 823<br />
89
It is difficult to control the release rate of the mercury chloride tube to a similar<br />
value as the elemental mercury tube. The originally supplied mercury chloride tube has a<br />
release rate that is five times larger than the quoted tube at 70�C. After several trials<br />
VICI can provide a tube with a release rate of 823 ng/min at 50�C.<br />
4.1.3 Humidifier for water vapor addition<br />
The water vapor is not removed before mercury analyzer and mercury<br />
concentration is therefore measured on a wet basis. Thus it is important to get a precise<br />
control of the water addition.<br />
Survey and quotation of water vapor addition methods and equipments were<br />
carried out to get a reliable water vapor addition device. Current water addition methods<br />
include direct liquid injection, bubblers, porous membrane contactor and non-porous<br />
membrane contactor. The first three methods are already applied in <strong>CHEC</strong>. However,<br />
recent studies show that fluctuation is a main problem [3,4]. Due to the small amount of<br />
water injected in the direct injection method, it is difficult to control such small flow rate.<br />
Water condensation test of the bubbler evaporator shows that the water saturation<br />
fluctuates and cannot reach the calculated level [3,4]. This method is low cost, but has<br />
inaccuracies due to the temperature of the gas and liquid, operating pressure, and liquid<br />
level.<br />
Porous membrane contactor uses Nafion selective permeable membrane tube and<br />
water to continuously humidify gas streams. The producer suggests recirculation of the<br />
water at 4% of the gas flow [5]. It is difficult to find such a small pump that works at<br />
temperatures above 50�C. Flow with greater pressure needs to be flowing inside tubes to<br />
prevent tubing collapse. <strong>CHEC</strong> has made an evaporator using the membrane tube.<br />
However, a lot of humidity fluctuation has been observed and the reasons have not been<br />
identified [3,4]. Swedish company Cellkraft produces commercial evaporator using the<br />
membrane tube [6]. The system uses similar design of <strong>CHEC</strong>’s membrane evaporator,<br />
but has a water trap to remove water droplet. The water tank is heated <strong>from</strong> outside and<br />
there is an integrated heating tape for the gas lines. The water level in the tank is<br />
90
controlled and automatically filled. The producer can provide calibration curve and<br />
guarantee for water droplet free and working properly at small carrier gas flow rate of<br />
300 ml/min. The evaporator system without a dew point sensor costs about 64,000 DKK.<br />
American company Rasirc produces a water vapor addition unit using the nonporous<br />
membrane tube and integrated water temperature, level and dew point control<br />
[7,8]. The unit purifies and controls water vapor addition for a wide range of flow rates<br />
and process pressures. The membrane excludes particles, micro-droplet, volatile gases<br />
and other opposite charged species and ensures only water vapor is added. Figure 4.5<br />
illustrates the configuration of the Rasirc humidifier. Carrier gas to be humidified flows<br />
into the humidification unit. The water is heated to match the desired dew point<br />
temperature or humidification level. Water diffuses across the membrane to saturate the<br />
gas to be humidified. Temperature of the humidified gas is measured and fed back to a<br />
temperature controller to adjust the humidification level. Internal pressure control<br />
maintains independence <strong>from</strong> variations in downstream process pressures which allows<br />
operation into atmospheric and vacuum pressure environments. The unit with integrated<br />
humidity sensor costs about 50,000 DKK. The Rasirc humidifier has been widely used in<br />
the fabrication of semiconductors, nanotechnology, photovoltaics, fuel cells and other<br />
applications [8]. After comparison, a Rasirc RHS-IP-3-HT humidifier with an internal<br />
dew point sensor to regulate the dew point of the saturated gas was purchased.<br />
91
Figure 4.5. Sketch of the Rasirc humidifier [8]. Internal dew point sensor is not shown in<br />
the sketch.<br />
4.1.4 Low temperature furnace and fixedbed reactor<br />
The low temperature oven is a three-zone electrically heated furnace and can heat<br />
up to 300�C. The oven has an internal diameter of 50 mm and a length of 450 mm. The<br />
heating tape for the top, middle and bottom zone is 170W/1m, 700W/4m, and 170W/1m,<br />
respectively. Three thermocouples are installed to measure the temperature at each zone<br />
and the heating is controlled <strong>by</strong> a center control box of the whole setup. The bottom of<br />
the oven is closed and the glass reactor has a u-shape. To avoid losses of sorbent powder<br />
in the gas stream, a downward flow is applied in the reactor. Quartz wool plugs are used<br />
at both ends of the sorbent bed. The top of the reactor and oven is heated <strong>by</strong> a heating<br />
tape.<br />
The temperature profiles at different setpoints are measured, as shown in figure<br />
4.6. The location of the sorbent bed is also illustrated in the figure. The measurements<br />
confirm that an isothermal reactor zone of about 300 mm is obtained with an estimated<br />
temperature uncertainty of ±2�C to the setpoints.<br />
92
Oven temperature ( o C)<br />
300<br />
250<br />
200<br />
150<br />
100<br />
50<br />
0<br />
Setpoint: 250 o C<br />
Setpoint: 200 o C<br />
Setpoint: 150 o C<br />
Setpoint: 120 o C<br />
0 50 100 150 200 250 300 350 400<br />
Distance to oven bottom (mm)<br />
93<br />
<strong>Sorbent</strong> bed<br />
position<br />
Figure 4.6. Temperature profile of the low temperature reactor oven.<br />
The glass reactor applied in this project is shown in figure 4.7. The reactor has an<br />
outer diameter of 20 mm (internal diameter of 18 mm) and a glass fiber porous plate to<br />
hold sorbent sample. With the dimension of the reactor shown in figure 4.7, the sorbent<br />
bed is located in the middle height of the low temperature furnace.<br />
Figure 4.7. Pictures with dimensions for the glass reactor.<br />
4.1.5 <strong>Mercury</strong> analysis system<br />
The mercury analysis system consists of a Lumex RA-915 AMFG elemental<br />
mercury analyzer, a gas distribution box and a oxidized mercury converter. Figure 4.8<br />
illustrates the sketch of the analysis system. A photo of the analysis system is presented<br />
in Figure 4.9.
Figure 4.8. Sketch of the mercury analysis system.<br />
Figure 4.9. Picture of the mercury analysis system with box open. The oxidized mercury<br />
converter is behind the mercury analyzer and gas distribution box.<br />
94
4.1.5.1 The Lumex analyzer<br />
The Lumex mercury analyzer has a measuring range of 0-500 �g/Nm 3 and<br />
automatic zero and span calibration functions. The lower detection limit is about 2<br />
�g/Nm 3 . All the gas lines and analytical cell inside the analyzer are heated. The analyzer<br />
can analyze gas of up to 30% water, and therefore no drying of the gas is needed. The<br />
analyzer determines the mercury concentration <strong>by</strong> Zeeman atomic absorption<br />
spectrometry using high frequency modulated polarized light. It is possible to measure<br />
only elemental mercury <strong>by</strong>passing the converter and only total mercury passing the gases<br />
through the converter, and to change the frequency of elemental and total mercury<br />
measurement switching through the sampling software.<br />
A block diagram of the analyzer is shown in figure 4.10 and a photo the analyzer<br />
internal parts are shown in figure 4.11. A membrane pump P draws flue gas <strong>from</strong> a<br />
sampling point via heated lines through a gas distribution box. There are four valves in<br />
the gas distribution box. The flue gas stream is either directed through the converter<br />
which reduces oxidized mercury to elemental mercury (valve V3 opened, valve V4<br />
closed) or is passed directly to the analytical cell AC, which is kept at a temperature of<br />
about 150°C. In the cell AC, which has an optical path length of about 0.4 m, a<br />
spectrometer determines the mercury concentration <strong>by</strong> Zeeman atomic absorption<br />
spectrometry using high frequency modulated polarized light (ZAAS-HFM). After<br />
leaving the cell, the gas is passing through a heated gas line and is then vented to a<br />
carbon trap before the ventilation. Temperature of the cell is constantly monitored <strong>by</strong><br />
temperature sensors T. The whole unit is controlled <strong>by</strong> an industrial panel PC, and<br />
powered <strong>by</strong> a power module PM.<br />
95
Figure 4.10. Block diagram of the mercury analyzer. The gas distribution box and<br />
converter are not integrated in the analyzer.<br />
96
Figure 4.11. Photo of the analyzer internal parts.<br />
The measurement principle of the analyzer is illustrated in figure 4.12 [9]. A<br />
mercury electronic discharge lamp is placed in a strong magnetic field H, <strong>by</strong> which the<br />
mercury resonance line at 254 nm is split into the three polarized Zeeman components �-,<br />
�, and �+. Only the �-components of the electromagnetic radiation will be registered <strong>by</strong><br />
the photo detector D. �- and �+ are separated <strong>by</strong> a polarization modulator. As long as<br />
mercury vapor is absent in the multipath cell, the intensities of both �-components are<br />
equal. When mercury is admitted to the cell, the difference in intensities between the two<br />
�-components increases as a function of the mercury concentration. As the spectral shift<br />
between the �-components is significantly smaller than the widths of molecular<br />
absorption bands and scattering spectra, background absorption <strong>by</strong> interfering<br />
compounds can be neglected.<br />
97
Figure 4.12. Illustration principle of the Zeeman atomic absorption spectrometry using<br />
high frequency modulated polarized light (ZAAS-HFM) [9].<br />
Sample gas connection to the analyzer is maintained at ambient pressure, with<br />
any excess flow vented to the atmosphere. Heated inlet and outlet lines are connected to<br />
the analytical cell inside the analyzer <strong>by</strong> means of 6 mm Swagelok-type fittings. The<br />
analyzer requires between 1 and 12 l/min of sample gas at all times and the flow can be<br />
controlled <strong>by</strong> the needle valve before the pump.<br />
4.1.5.2 Gas distribution box<br />
The gas distribution box contains valves for switching the gas and air to the<br />
analyzer and converter. The valves are controlled <strong>by</strong> the sampling software. The box is<br />
heat traced and isolated. There is a switch valve before the gas distribution box. Addition<br />
of air or sample gas to the analysis system can be selected.<br />
98
4.1.5.3 The oxidized mercury converters<br />
Two converters are used in this project. Originally Lumex supplied a low<br />
temperature converter for the red brass catalyst. Figure 4.13 shows a picture of the<br />
converter with a glass container loaded with red brass chips. The converter is designed to<br />
work at 180�C and the highest temperature is about 250�C. The glass container has an<br />
outer diameter of 20 mm and can hold about 20 g red brass chips.<br />
Figure 4.13. Picture of the low temperature converter with a glass container loaded with<br />
20 g red brass chips.<br />
Later a high temperature converter was used for the sulfite based converter<br />
material. The high temperature oven is a three-zone electrically heated furnace with a<br />
quartz reactor, which has an inner diameter of 17 mm and can hold up to 30 g sulfitebased<br />
converter pellets. The converter is a fixed-bed reactor made of quartz as shown in<br />
figure 4.14. The inner and bottom tubes of the reactor were removable. The sulfite-based<br />
pellets are placed on the porous quartz plate. The converter temperature is measured<br />
below the porous quartz plate <strong>by</strong> a thermocouple shielded in a quartz tube.<br />
99
Figure 4.14. Sketch of the high temperature converter with quartz reactor in the furnace.<br />
4.2 Converter and sorbent materials<br />
The red brass chips are obtained through Lumex. The idea of using red brass at<br />
low temperature is to bind free halogens in the flue gas and thus prevent back reaction<br />
into mercury halides and corrosion problem caused <strong>by</strong> SO2 oxidation at high<br />
temperatures [10]. Figure 4.15 shows picture of the red brass chips which have a<br />
thickness of about 0.5 mm and are rolled to a diameter of about 2 mm and a length of<br />
about 10 mm.<br />
100
Figure 4.15. Picture of the red brass chips supplied <strong>by</strong> Lumex.<br />
The sulfite converter material is prepared according to the work of Akiyama et al.<br />
[11]. Alumina pellets or zeolite pellets are first dried at 600�C for 24 h. Then the pellets<br />
are impregnated with water glass <strong>by</strong> forming a thin layer of water glass on the surfaces of<br />
the pellets. Sodium sulfate powders are added and mixed with the impregnated pellets.<br />
To inhibit crystallization of the salts, CaSO4 is added to the sulfite salts at a ratio of 50<br />
wt.%. About 15 to 45 wt.% of the sulfite salts and CaSO4 mixture are adhered almost<br />
uniformly to the thin layer of water glass. Immediately after mixing the product is placed<br />
in an oven and vacuum-dried at room temperature for 1 h, then it is vacuum-dried at<br />
150�C for 12 h. Figure 4.16 shows the picture of the prepared sulfite-based converter<br />
material. White powders of sodium sulfite and calcium sulfate are doped on the zeolite<br />
pellets with a diameter of 3 mm.<br />
101
Figure 4.16. Picture of the prepared sulfite-based converter material.<br />
To be able to quantify the oxidized mercury reduction efficiency, the oxidized<br />
mercury is produced <strong>by</strong> passing the flue gas with known concentration of elemental<br />
mercury to the reactor with 4 g catalyst for selective catalytic reduction of NOx. The<br />
catalyst piece was cut <strong>from</strong> a corrugated-type monolith obtained <strong>from</strong> Haldor Topsøe<br />
A/S. The catalyst is based on a fiber reinforced titania (TiO2) carrier, which is<br />
impregnated <strong>by</strong> vanadium (V2O5) and tungsten (WO3). The vanadium loading (3 wt.%<br />
V2O5) was uniformly distributed across the wall thickness of the monolith [12,13]. The<br />
efficiency of the converter is evaluated <strong>by</strong> the recovery extent of measured total mercury<br />
through the SCR catalyst and converter compared to the elemental mercury level at the<br />
inlet of the SCR catalyst.<br />
The most investigated sorbents in this project is Darco Hg activated carbon,<br />
which is a commercial lignite based powdered activated carbon and is developed for<br />
heavy metal removal <strong>from</strong> incinerators and power plants. The Darco Hg carbon has a<br />
bulk density of 0.51 g/cm 3 and a surface area of about 600 m 2 /g. The average particle<br />
102
size is 16 �m and the porosity is about 58% [14-22]. Properties of other sorbents are<br />
presented in the chapter of sorbent screening.<br />
4.3 Flue gas composition<br />
The total flow rate through the reactor is 2.75 Nl/min of which about 2 Nl/min is<br />
passed through the analyzer. The typical composition of the simulated cement kiln flue<br />
gas applied in this work includes 21% CO2, 6% O2, 1% H2O, 10 ppmv HCl, 1000 ppmv<br />
NO, 23 ppmv NO2, and 1000 ppmv SO2. The applied mercury concentration is about<br />
160-180 µg/Nm 3 <strong>by</strong> keeping the elemental mercury and mercury chloride source at 70ºC<br />
and 50ºC, respectively, and using 0.275 Nl/min nitrogen as carrier gas. The water level in<br />
the simulated flue gas is lower than real level in the cement kiln flue gas. This is due to<br />
the limitation of the humidifier. Although the humidifier can add water vapor relatively<br />
precisely, it is not robust. The membrane can be easily broken and the unit cannot stand<br />
high over pressure. After short period of operation, the unit was repaired twice <strong>by</strong><br />
changing the membrane and installing of a pressure release valve. It seems that the unit<br />
can run properly only for short period. Another reason is the fluctuation measurement of<br />
the mercury analyzer with more than 5% water in the simulated flue gas. Therefore it is<br />
decided to use 1% water in most of the tests <strong>by</strong> adding water through a bubbling bottle.<br />
In few cases high water contents are used to cover a wide range of water level in the<br />
simulated flue gas.<br />
4.4 <strong>Sorbent</strong> load in fixedbed test<br />
For the applied reactor in this project, at least 500 mg activated carbon is need to<br />
form a fixed-bed covering the cross area of the reactor. The amount of the sorbent sample<br />
is determined <strong>by</strong> the sample saturation time. Literature reported that approximately 600<br />
mg of sorbent initially was placed into the reactor; however, the samples were reduced<br />
<strong>from</strong> 600 mg to between 100 and 150 mg after it was observed that extremely long<br />
durations (up to weeks) would be required to saturate the larger quantity of sorbent [23].<br />
To avoid channeling the sorbent sample is usually mixed with some inert materials such<br />
103
as sand and glass beads. Application of dilution <strong>by</strong> sand powder can also accelerate the<br />
tests. The reaction gas flows downward through the bed to minimize the chance of<br />
selective flow or channeling through the bed. Reactor sizes and sample dilutions applied<br />
in the literature are reviewed and summarized in table 4.2.<br />
Table 4.2. Reported reactor sizes, flow rates and sorbent sample loads in the literature.<br />
<strong>Sorbent</strong> loading Reactor<br />
size<br />
ID (mm)<br />
50 mg fly ash mixed with 3 g<br />
glass beads, 2.5 mm bed<br />
thickness, additional 57.5 mm<br />
glass beads upstream to the bed<br />
for better flow distribution<br />
0.2-0.6 g sample (copper<br />
compound based sorbents and<br />
commercial proprietary sorbents)<br />
held <strong>by</strong> a glass wool plug, 16 mm<br />
bed thickness<br />
20-30 mg sorbent (Norit FGD<br />
carbon and functionalized silica)<br />
in 6 g silica, glass fiber filter at<br />
two ends<br />
0.61 g Darco G60 carbon with 3 g<br />
glass beads, 4 mm bed length<br />
20 mg activated carbon mixed<br />
with 1 g sand<br />
5 mg carbon on 3 g glass beads, 4<br />
mm bed thickness<br />
20 mg carbon in 10 g sand,<br />
supported <strong>by</strong> quartz wool<br />
10-100 mg sorbent mixed with 2<br />
g sand, bed thickness of about 5<br />
mm, quartz wool at two ends<br />
104<br />
Flow rate<br />
(Nl/min)<br />
Superficial<br />
velocity (cm/s)<br />
@150�C<br />
35 3.2 8.7 [24]<br />
4 0.15 30.8 [25]<br />
12.7 0.91 18.4 [26]<br />
35 2.8 7.5 [27]<br />
6.35 0.17 13.8 [28]<br />
35 4.2 7.52 [29]<br />
12.7 1 20.3 [30]<br />
References<br />
18 2.75 22.1 This work
4.5 Experimental procedure<br />
An experimental procedure is developed for sorbent tests and measures are taken to<br />
avoid mercury accumulation in the system. The detailed procedure is as following:<br />
� The mercury source is maintained at the operating temperature with carrier gas<br />
through all the time.<br />
� Increase the sulfite converter temperature setpoints <strong>from</strong> 100�C to 500�C.<br />
� Check the temperatures of mercury source, heated lines, low temperature oven,<br />
gas distribution box and analyzer, set the low temperature reactor oven to desired<br />
temperature.<br />
� Weight desired amount of sorbent and mix with 2g sand powder, load the sample<br />
to the glass reactor.<br />
� Check and measure the flow rates of different gases.<br />
� When the converter temperature reaches 500�C for about 30 min, change the<br />
mercury analyzer measurement mode <strong>from</strong> elemental to total mercury<br />
measurement.<br />
� Switch the valve before the glass reactor to <strong>by</strong>passing the reactor position; add<br />
gases except mercury to the system.<br />
� Add the gases to the mercury analyzer to check whether there is some mercury<br />
accumulated in the hot system; if there is some mercury detected, wait the<br />
mercury reading decreases to zero value and then switch mercury source to the<br />
system.<br />
� Start the test and data sampling, make note in the sampling program, measure the<br />
mercury inlet concentration for at least 30 min to ensure that the glass reactor is<br />
heated for about 1 h and stable reactor temperature is obtained.<br />
� Switch the gases to fixed sorbent bed, make note in the sampling program.<br />
� After full mercury breakthrough is observed for 30 min, switch air to the analyzer<br />
system while keeping simulated flue gas to the reactor, and switch the mercury<br />
analyzer to measure elemental mercury.<br />
105
� When the elemental mercury measurement mode is ready, switch the simulated<br />
flue gas to the analysis system.<br />
� After 20 min, switch the simulated flue gas to <strong>by</strong>pass the reactor and measure the<br />
inlet mercury concentration for another 20 min.<br />
� Stop the test, switch air to the analysis system, switch mercury source to the<br />
carbon trap and ventilation, and stop other gases.<br />
� Remove the reactor <strong>from</strong> the oven and remove the sample after the reactor is<br />
cooled, store the sample in a closed plastic bottle with label.<br />
� At the end of the day, turn all gases off except nitrogen and H2O when elemental<br />
mercury source is applied and add also HCl when mercury chloride is applied to<br />
flush the system overnight. Decrease the converter temperature to 100�C, to<br />
ensure the mercury analyzer is in elemental mercury measurement mode and air<br />
is added to the analysis system.<br />
� Always keep the whole reactor system hot.<br />
4.6 <strong>Sorbent</strong> characterization<br />
4.6.1 Scanning electron microscopy<br />
Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-<br />
EDX) analysis is used to understand mercury capture mechanisms <strong>by</strong> different powder<br />
sorbent. The main goals of the SEM-EDX analysis is to study the sorbents’ topography<br />
(surface features), morphology (shape and size), and composition. Morphology study<br />
will be used to identify particle agglomeration and compare with particle size<br />
measurement.<br />
The SEM-EDX analysis is conducted at Center for Electron Nanoscopy, DTU.<br />
Micrographs and EDX analysis of carbon samples are carried out using Quanta FEGSEM<br />
200F. The carbon samples are not coated, while the non-carbon samples are coated with<br />
14 nm carbon and analyzed on Inspect ‘S’ SEM. The FEGSEM is a high resolution<br />
flexible microscope with field emission gun (FEG). The Inspect ‘S’ is a scanning<br />
electron microscope with a tungsten filament electron source. To support the surface<br />
106
information obtained <strong>by</strong> the imaging detectors both SEMs are equipped with Oxford<br />
Instruments INCA EDX analyzer which gives possibilities to analyze chemical elements<br />
position on the sample surface in single spots or over a selected area. The microscope can<br />
operate in as well high- and low vacuum as in environmental mode. A typical working<br />
distance of 10 mm is applied.<br />
A thin layer of sample powders is spread on a double sided conductive carbon<br />
table. If particles are piled on each other charge-up easily takes place, causing them to<br />
move during observation. A low accelerating voltage of 5 kV is applied during imaging<br />
on the FEGSEM to obtain detailed information on the particle surface and minimize<br />
specimen charging problem.<br />
4.6.2 Particle size distribution<br />
The particle size distributions of the sorbent powders are analyzed <strong>by</strong> a Malvern<br />
Mastersizer S analyzer using laser diffraction. The technique of laser diffraction is based<br />
around the principle that particles passing through a laser beam will scatter light at an<br />
angle that is directly related to their size. Large particles scatter light at narrow angles<br />
with high intensity [31], whereas small particles scatter at wider angles but with low<br />
intensity. The analyzer consists of a laser to provide a source of coherent, intense light of<br />
fixed wavelength, a sample presentation system to ensure that the material under test<br />
passes through the laser beam as a homogeneous stream of particles in a known,<br />
reproducible state of dispersion, and a series of detectors which are used to measure the<br />
light pattern produced over a wide range of angles.<br />
Based on previous analysis experience at <strong>CHEC</strong>, the samples are dispersed either<br />
in ethanol or distilled water for one minute before measurement to avoid agglomeration<br />
and the result is the average of 5 measurements.<br />
107
4.6.3 Analysis of mercury in sorbent<br />
<strong>Mercury</strong> content in the exposed sorbent is analyzed <strong>by</strong> a DMA-80 analyzer <strong>from</strong><br />
Milestone at FLSmidth Dania lab. 100 mg of powder sample is first weighted and loaded<br />
in a sample boat. The boats are transported automatically into the furnace. The sample is<br />
initially dried and then thermally decomposed in a continuous flow of oxygen.<br />
Combustion products are carried off and further decomposed in a hot catalyst bed [32].<br />
<strong>Mercury</strong> vapors are trapped on a gold amalgamator and subsequently desorbed for<br />
quantization. The mercury content is determined using an atomic absorption<br />
spectrophotometer at 254 nm. The instrument determines the absolute amount of Hg and<br />
then the software calculates its concentration in the sample.<br />
4.7 References<br />
[1] VICI Metronics Inc., Dynacalibrator ® Model 150 calibration gas generator brochure, 2008.<br />
[2] VICI Metronics Inc., Dynacal permeation tubes, 2011.<br />
[3] B. Maribo-Mogensen and J. Christensen, Internal steam reforming in solid oxide fuel cells,<br />
Bachelor, Department of Chemical and Biochemical Engineering, Technical University of<br />
Denmark, 2008.<br />
[4] A.F. Castells. Steam reforming kinetics over Ni-YSZ used as anode material for solid fuel<br />
cells, Master, Department of Chemical and Biochemical Engineering, Technical University of<br />
Denmark, 2009.<br />
[5] Perma Pure, MH TM -series humidifier user manual, http://www.permapure.com /PDF%20Files<br />
/MH%20Manual.pdf, accessed March/20, 2009.<br />
[6] Cellkraft AB, P-series humidifier manual, http://www.cellkraft.se/humidity_and_steam/P-<br />
Series.html, accessed March 20, 2009.<br />
[7] RASIRC, RASIRC Rainmaker TM humidification system manual, http://www.rasirc.com<br />
/resources/datasheets/datasheet_RASIRC_RainMaker_HS.pdf, accessed March 20, 2009.<br />
[8] Jeffrey Spiegelman, RainMaker humidification system for precise delivery of water vapor<br />
into atmospheric and vacuum applications, http://www.rasirc.com/resources/whitepapers<br />
/whitepaper_RHS.pdf, accessed March 20, 2009.<br />
[9] Lumex Ltd, RA-915 AMFG automatic mercury monitor for flue gas operational manual,<br />
2009.<br />
[10] R. Kanefke, H. Köser, B. Vosteen, F. Kristina, B. Frank, S. Raik, Method for the production<br />
of elemental mercury <strong>from</strong> mercury compounds, patent WO 2008/064667 A2, 2008.<br />
108
[11] S. Akiyama, J. Kato, F. Koga, K. Ishikawa, Catalysts for reducing mercury, a mercury<br />
conversion unit, and an apparatus for measuring total mercury in combustion exhaust gas <strong>by</strong><br />
using the same, patent US2007/0232488 A1, 2007.<br />
[12] Y. Zheng, A.D. Jensen, J.E. Johnsson, Deactivation of V2O5-WO3-TiO2 SCR catalyst at a<br />
biomass-fired combined heat and power plant, Applied Catalysis B: Environmental. 60 (2005)<br />
253-264.<br />
[13] Y. Zheng, A.D. Jensen, J.E. Johnsson, J.R. Thøgersen, Deactivation of V2O5-WO3-TiO2<br />
SCR catalyst at biomass fired power plants: Elucidation of mechanisms <strong>by</strong> lab- and pilot-scale<br />
experiments, Applied Catalysis B: Environmental. 83 (2008) 186-194.<br />
[14] H.C. Hsi, S. Chen, M. Rostam-Abadi, M.J. Rood, C.F. Richardson, T.R. Carey, R. Chang,<br />
Preparation and evaluation of coal-derived activated carbons for removal of mercury vapor <strong>from</strong><br />
simulated coal combustion flue gases, Energy & Fuels. 12 (1998) 1061-1070.<br />
[15] M. Rostam-Abadi, S.G. Chen, H.C. Hsi, M. Rood, R. Chang, T.R. Carey, B. Hargrove, C.<br />
Richardson, W. Rosenhoover, F. Meserole, Novel vapor phase mercury sorbents, Proceedings of<br />
the EPRI-DOE-EPA Combined Utility Air Pollutant Control, Washington, DC, Aug 25–29,1997.<br />
[16] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang and F.B. Meserole, Factors<br />
affecting mercury control in utility flue gas using sorbent injection, Proceedings of the Air &<br />
Waste Management Association's 90th Annual Meeting, Toronto, Ontario, Canada, June 8-13,<br />
1997.<br />
[17] B. Ghorishi and B.K. Gullett, Fixed-bed control of mercury: Role of acid gases and a<br />
comparison between carbon-based, calcium-based, and coal fly ash sorbents, Proceedings of the<br />
EPRI/DOE/EPA Combined Utility Air Pollutant Control Symposium, Washington, DC, August<br />
25–29,1997.<br />
[18] S.B. Ghorishi and C.B. Sedman, Combined mercury and sulfur oxides control using<br />
calcium-based sorbents, Proceedings of the EPRI-DOE-EPA Combined Utility Air Pollutant<br />
Control Symposium, Washington, DC, August 25–29, 1997.<br />
[19] S.B. Ghorishi and B.K. Gullett, An experimental study on mercury sorption <strong>by</strong> activated<br />
carbons and calcium hydroxide, The Fifth Annual North American Waste-to-Energy Conference,<br />
Research Triangle Park, NC, 22-25 April, 1997.<br />
[20] N. Hutson, C. Singer, C. Richardson, J. Karwowski and C. Sedman, Practical applications<br />
<strong>from</strong> observations of mercury oxidation and binding mechanisms, EPRI-DOE-EPA-AWMA<br />
Combined Power Plant Air Pollutant Control Mega Symposium, Washington DC, August 30 -<br />
September 2, 2004.<br />
[21] Norit Americas Inc., Datasheet of Darco FGD powdered activated carbon, 2008.<br />
[22] R. Bhardwaj. Impact of temperature and flue gas components on mercury speciation and<br />
uptake <strong>by</strong> activated carbon sorbents. Master thesis. University of Pittsburgh, 2007.<br />
[23] D.J. Hassett, K.E. Eylands, <strong>Mercury</strong> capture on coal combustion fly ash, Fuel. 78 (1999)<br />
243-248.<br />
109
[24] D. Karatza, A. Lancia, D. Musmarra, Fly ash capture of mercuric chloride vapors <strong>from</strong><br />
exhaust combustion gas, Environ. Sci. Technol. 32 (1998) 3999-4004.<br />
[25] J.W. Portzer, J.R. Albritton, C.C. Allen, R.P. Gupta, Development of novel sorbents for<br />
mercury control at elevated temperatures in coal-derived syngas: results of initial screening of<br />
candidate materials, Fuel Processing Technology. 85 (2004) 621-630.<br />
[26] J.Y. Lee, Y. Ju, T.C. Keener, R.S. Varma, Development of cost-effective noncarbon<br />
sorbents for Hg 0 removal <strong>from</strong> coal-fired power plants, Environ. Sci. Technol. 40 (2006) 2714-<br />
2720.<br />
[27] D. Karata, A. Lancia, D. Musmarra, F. Pepe, Adsorption of metallic mercury on activated<br />
carbon, Symposium (International) on Combustion. 26 (1996) 2439-2445.<br />
[28] G. Skodras, I. Diamantopoulou, G. Pantoleontos, G.P. Sakellaropoulos, Kinetic studies of<br />
elemental mercury adsorption in activated carbon fixed bed reactor, Journal of Hazardous<br />
Materials. 158 (2008) 1-13.<br />
[29] D. Karatza, A. Lancia, D. Musmarra, C. Zucchini, Study of mercury absorption and<br />
desorption on sulfur impregnated carbon, Experimental Thermal and Fluid Science. 21 (2000)<br />
150-155.<br />
[30] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang, F.B. Meserole, Factors affecting<br />
mercury control in utility flue gas using activated carbon, Journal of the Air & Waste<br />
Management Association. 48 (1998) 1166.<br />
[31] Malvern Instruments Ltd, Understanding how laser diffraction works,<br />
http://www.malvern.com/LabEng/technology/laser_diffraction/laser_diffraction.htm, accessed<br />
January/3, 2011.<br />
[32] Milestone Srl, DMA-80: Principle of operation, http://www.milestonesrl.com<br />
/analytical/products-mercury-determination-dma-80-and-dma-803-principle-of-operation.html,<br />
accessed January 6, 2011.<br />
[33] A. Ocklind. Calculation <strong>from</strong> gas dewpoint to water content, 2009.<br />
[34] R.H. Perry, D.W. Green, J.O. Maloney, (Eds.), Perry’s chemical engineers’ handbook, 7th<br />
ed. The McGraw-Hill Companies, Inc., 1997.<br />
Appendix<br />
4A Check of mercury analyzer<br />
Since many valves and fittings are used in the mercury analysis system, it is<br />
important to make a leakage test of the system. Lumex service technicians performed the<br />
leakage test. The Lumex analyzer cannot stand high pressure and the system therefore<br />
cannot be tested <strong>by</strong> plugging the system and checking the pressure change. Instead, flow<br />
rates at the gas distribution box, converter, and analyzer are measured simultaneously<br />
110
using rotating flow meters and a separate pump <strong>from</strong> Lumex. Flow rates measured <strong>by</strong> the<br />
portable rotating flow meters at the analysis system inlet and outlet and flow rate<br />
measured <strong>by</strong> the integrated flow meter in the analyzer are the same at both cold and hot<br />
condition, indicating that there is no leakage in the Lumex analysis system. The function<br />
of heating the gas panel and lines to avoid mercury accumulation in the system was also<br />
checked. The test was conducted <strong>by</strong> first passing mercury contained gas through the hot<br />
panel and then stopping mercury addition and the hot panel was flushed <strong>by</strong> nitrogen. No<br />
mercury was detected in the flushing nitrogen, confirming that no mercury was<br />
accumulated in the lines.<br />
The required flow rate for the mercury analyzer was determined. The needle<br />
valve for controlling the flow rate was changed <strong>from</strong> <strong>by</strong>pass line between the analytical<br />
cell and the pump to on the line between the analytical cell and the pump. The flow rate<br />
is better controlled; however, it cannot be reduced to as low as l l/min. The flow rate<br />
through the analyzer is set to 2 l/min. The analyzer requires overflow. Therefore a 3<br />
l/min gas through the reactor is used and the excess flow will <strong>by</strong>pass the analyzer and<br />
exhaust through the ventilation.<br />
Lumex service technician brought a portable mercury analyzer RA-915+, which<br />
is the same as the Haldor Topsøe analyzer except that it has only a single analytical cell.<br />
These two analyzers were compared <strong>by</strong> making tests at Haldor Topsøe’s mercury<br />
research facility. Using a single analytical cell at analyzers, the measured mercury<br />
concentration <strong>by</strong> Lumex and Haldor Topsøe analyzer was 9936 and 10080 ng/m 3 ,<br />
respectively. <strong>Mercury</strong> concentration measured <strong>by</strong> Haldor Topsøe analyzer using multiple<br />
cells was 10200 ng/m 3 . This indicates that the Lumex portable analyzer works properly<br />
and seems reliable.<br />
Comparison of Lumex AMFG monitor, which is used in present project, with<br />
Lumex portable analyzer was conducted <strong>by</strong> running these analyzers simultaneously. The<br />
measured mercury concentrations <strong>by</strong> the portable analyzer are about 10% higher than the<br />
AMFG, as shown in table 4.3.<br />
111
Table 4.3. Comparison of mercury measurement <strong>by</strong> the portable Lumex RA-915+ and<br />
the Lumex AMFG analyzer applied in this project.<br />
Concentration measured <strong>by</strong><br />
portable analyzer, �g/Nm 3<br />
Concentration measured<br />
<strong>by</strong> AMFG analyzer,<br />
�g/Nm 3<br />
249 220 1.13<br />
217 201 1.08<br />
153 137 1.11<br />
112<br />
Ratio of portable/AMFG<br />
Lumex said that the difference is caused <strong>by</strong> different conditions used during<br />
calibration at the factory and test at <strong>CHEC</strong> lab. The gas lines before the analyzer at the<br />
factory was not heated and the analytical cell was tested at a temperature which was<br />
50�C lower than at <strong>CHEC</strong> lab. Since comparison with Haldor Topsøe analyzer shows<br />
that the portable analyzer works properly, the AMFG is calibrated using linearity test. A<br />
factor of 1.10 was used as the calibration coefficient.<br />
4B Water addition verification<br />
To verify the water addition stability and accuracy condensation tests are<br />
conducted. The configuration of the test system is shown in figure 4.17. A water column<br />
of a height of about 500 mm is hanged 1 m above the humidifier as water source. The top<br />
of the water column is open. A stop valve is installed below the water column to allow<br />
disconnection of the water column for weight measurement. Nitrogen is used as the<br />
carrier gas. The water level in the humidifier is always kept at full level <strong>by</strong> a liquid level<br />
switch and a micro pump. The gas line between the humidifier and the condensation<br />
water bottle is heated at 110�C to avoid water condensation in the gas line. The water<br />
bath temperature is kept at 4�C. The condensation bottle is filled with some water to<br />
enhance the heat exchange and capture of water. It takes about 70 min for the humidifier<br />
to reach the desired dew point. During this period the gas is <strong>by</strong>passed to the water bath<br />
and passes through a water bottle before ventilated. When the desired dew point is<br />
reached and stabilized, the water tank is disconnected for weight measurement. The gas<br />
is still running through the humidifier and water is continuously added. During the
measuring of the water tank weight about 80 mm water inside the ¼’’ Teflon tube is<br />
added and the corresponding weight is about 1 g. When the water tank is connected back<br />
there will be an 80 mm air plug inside the Teflon tube. This means that 1 g water is not<br />
added and should be deducted <strong>from</strong> the theoretical calculation for comparison with the<br />
measured water addition. The amount of water added can be evaluated either <strong>by</strong><br />
measuring weight change of the water tank or weight of water collected in the<br />
condensation bottles.<br />
Figure 4.17. Sketch of the condensation test system.<br />
When the dew point is reached and stabilized, the dew point reading of the<br />
saturated gas is changing about � 0.1�C <strong>from</strong> the set point. This indicates that the relative<br />
humidity of the gas is quite stable and close to 100%.<br />
To calculate the amount of water added the saturated water vapor pressure at<br />
given dew point should be calculated first. Table and figure of saturated water vapor<br />
pressure can be readily found in text book. The saturated water vapor pressure can be<br />
calculated <strong>by</strong> using the empirical expression <strong>from</strong> Cellkraft, which is a Swedish<br />
membrane humidifier produce [6,33]:<br />
113
lnP �10.4592 �4.05�10 T �4.18�10T�3.69�10 T �1.02�10 T �8.65�10 T<br />
� � � � � � � � � �<br />
(4.1)<br />
where the water saturated water vapor pressure Ps is in MPa, T is in K.<br />
Partial pressures of the gases are proportional to the gas flow rates:<br />
P HO F<br />
2 HO 2 �<br />
P F<br />
(4.2)<br />
�3 �5 2 �7 3 �9 4 �13<br />
5<br />
s<br />
�16 6<br />
9.04 10 T<br />
�18 7<br />
2.00 10 T<br />
�22 8<br />
7.79 10 T<br />
�25<br />
9<br />
1.91 10 T 3968.06 /( T 39.57)<br />
N2 N2<br />
Ptot � PH2O� PN<br />
(4.3)<br />
2<br />
where T is the desired dew point of the gas, 82 �C, 355.15 K<br />
�3 �5 2 �7<br />
3<br />
lnPs<br />
�10.4592 �4.05�10 �355.15 �4.18�10�355.15 �3.69�10 �355.15<br />
� � � �<br />
�1.02�10 �355.15 �8.65�10 �355.15 �9.04�10 �355.15 �2.00�10�355.15 �22 8 �25<br />
9<br />
�7.79�10 �355.15 �1.91�10 �355.15 �3968.06 /( �355.15 �39.57) ��2.9686<br />
(4.4)<br />
and Ps � 513.76 mbar<br />
With room temperature of 25�C, carrier nitrogen flow, 0.3l/min, equals 0.275 Nl/min,<br />
9 4 13 5 16 6 18 7<br />
one can calculate:<br />
Water addition rate (g/min):<br />
(18 / 22.4) �0.275 �(513.76<br />
/1013.25)<br />
� 0.227<br />
1� 513.76 /1013.25<br />
Water flow rate (Nl/min):<br />
0.275 �(513.76<br />
/1013.25)<br />
� 0.283<br />
1� 513.76 /1013.25<br />
The water vapor pressure can also be calculated using Antoine equation [34]:<br />
Ps<br />
log10 �8.07131�1730.63/(233.426 � T )<br />
(4.5)<br />
where Ps is in Torr (1 mmHg), T is in �C.<br />
Ps<br />
log10 �8.07131�1730.63/(233.426 �82) � 2.5847<br />
then P � 512.35 mbar<br />
s<br />
Calculated water addition rate (g/min):<br />
(18 / 22.4) �0.275 �(512.35<br />
/1013.25)<br />
� 0.226<br />
1� 513.76 /1013.25<br />
Water flow rate (Nl/min):<br />
114
0.275 �(512.35<br />
/1013.25)<br />
� 0.281<br />
1� 513.76 /1013.25<br />
Table 4.4 presents the calculated water addition rates and flow rates at different<br />
dew points. Calculations using Cellkraft equation and Antoine equation give almost the<br />
same results.<br />
Table 4.4. Calculated water addition rate and flow rate at different dew points.<br />
Dew point (�C) Cellkraft equation Antoine equation<br />
H2O addition H2O flow rate H2O addition H2O flow rate<br />
rate (g/min) (Nl/min) rate (g/min) (Nl/min)<br />
82 0.227 0.283 0.226 0.281<br />
77 0.156 0.194 0.155 0.193<br />
72 0.112 0.139 0.111 0.138<br />
63 0.064 0.080 0.064 0.080<br />
4 0.002 0.002 0.002 0.002<br />
The comparison between the measured water addition and calculated water<br />
addition is presented in table 4.5. For the first two tests only one condensation bottle is<br />
used and about 1/3 of the bottle is filled with water. The water collected in the<br />
condensation bottle is only 50-60% of the calculated value. This is probably due to the<br />
short gas residence time in the condensation bottle and small amount of water filled in<br />
the condensation bottle. The amount of water added <strong>by</strong> water tank weight measurement<br />
is reasonably in agreement with the calculation. Later more water is filled in the bottle<br />
and two or four bottles are connected in series. Then the amounts of water added <strong>by</strong><br />
measuring the water tank weight change and water collected in the bottle are similar and<br />
about 90-95% of the calculated values. For the four bottle in-series tests the weight<br />
change of each bottle is measured. Almost all the water is collected in the first bottle and<br />
the water collected in other bottles is negligible. The test results show that the humidifier<br />
works well.<br />
115
Table 4.5. Comparison between the measured water addition and calculated water<br />
addition.<br />
Dew point (�C) 82 82 82 82 82 82<br />
N2 flow (ml/min) 300 300 300 300 300 400<br />
Room temp. (�C) 24.9 24.7 26.3 25.7 24.9 24.9<br />
Duration (min) 450 318 368.5 240 143 120<br />
Calculated water addition (g) 100.25 70.55 82.17 52.52 28.89 35.12<br />
Measured water addition<br />
through tank weight (g)<br />
102.4 63.5 74.10 47.7 27.6 31.8<br />
Measured/calculated (%) 102 90 90.5 90.8 95.5 90.6<br />
Measured water addition<br />
through collection in water<br />
bottle (g)<br />
61.48 36.27 73.99 47.08 - 32.32<br />
Water bottle number 1 1 2 2 4 2<br />
Measured/calculated (%) 61.4 51.4 90.3 89.6 - 92<br />
116
Dynamic measurement of mercury adsorption and<br />
oxidation on activated carbon in simulated cement<br />
117<br />
5<br />
kiln flue gas<br />
This chapter starts with a review of available gaseous mercury measurement<br />
technologies. Pros and cons of the technologies will be discussed. Then tests of the<br />
commercial red brass converter in simulated cement kiln flue gas are presented. Finally<br />
development of sulfite-based converter for oxidized mercury reduction in simulated<br />
cement kiln flue gas is reported. Suggestions for practical applications of the sulfite<br />
converter in both lab and cement plants are presented.<br />
5.1 Review of gaseous mercury measurement technology<br />
Presently, the accepted methods for mercury measurement are wet-chemistry<br />
procedures such as EPA methods 29 and 101 A for total mercury measurement and the<br />
Ontario Hydro method for total mercury and speciation measurement [1,2]. These<br />
methods often have 2-week or more turn-around time for results. The sorbent trap<br />
method was developed to shorten the analysis time of the collected samples. These<br />
methods can only provide an average mercury concentration over a 1-2 hour period, and<br />
cannot characterize the variability in mercury emissions due to process and operating<br />
changes with time.<br />
To obtain an understanding of the process of mercury removal <strong>by</strong> sorbent<br />
injection upstream of a fabric filter, it is necessary to study them under more controlled<br />
conditions such as in a laboratory scale setup, for example using a fixed bed reactor. In<br />
such experiments it is also necessary to use a continuous emission monitor (CEM) to
obtain knowledge of uptake of total and speciated mercury in simulated flue gas to fully<br />
evaluate the control technologies under development. Fixed-bed experiments have been<br />
used <strong>by</strong> many laboratories to test the relative effectiveness of different mercury sorbents.<br />
The critical assumption of this experimental method is that the performance of a sorbent<br />
over a long exposure time (hours) reflects the filtration/reaction on bags, where the<br />
sorbent contacts the flue gas for about 25 minutes [1]. Therefore, it is preferable to verify<br />
this assumption <strong>by</strong> supplementing the final mercury content data with breakthrough data<br />
obtained using a CEM.<br />
Real- or near-real-time mercury emission measurement can in principle be<br />
obtained depending on the applied detection method. Generally, real-time measurements<br />
can be achieved <strong>by</strong> analyzers using cold vapor atomic absorption spectroscopy. The cold<br />
vapor atomic fluorescence spectrophotometer collects mercury in flue gas on alternating<br />
gold traps and thermally desorbs the mercury in about five minute intervals allowing for<br />
semi-continuous measurements [3].<br />
Available commercial mercury analyzers can only measure elemental mercury.<br />
The measurement of total mercury as well as mercury speciation can only be achieved<br />
indirectly. For this purpose, all oxidized mercury is reduced to its elemental form <strong>by</strong> a<br />
converter system. It should be noted that the technology for the analytical part of the<br />
detection system is somehow matured and provides accurate and sensitive detection of<br />
elemental mercury [3]. The conversion unit, on the other hand, is a subject of continuing<br />
research and improvement efforts [2].<br />
The converter can be based either on wet chemistry or dry conversion. In a wetchemistry<br />
conversion unit the Hg 2+ is converted to Hg 0 via a liquid phase reducing agent,<br />
often stannous chloride (SnCl2), prior to entering the analysis unit. There is interference<br />
with SO2, which can affect the reduction of Hg 2+ when using SnCl2 [2,4,5]. Furthermore,<br />
the wet chemicals themselves are very corrosive and need frequent replenishment.<br />
For on-line measurements, a dry converter is usually preferred over a wet<br />
chemical converter for the reasons mentioned above [6]. Several dry converter types<br />
exist. In a pure thermal conversion unit, the flue gas is heated to reduce all Hg 2+ to Hg 0 .<br />
118
However, reoxidation of the reduced mercury before reaching the analysis unit is a<br />
concern. Furthermore, the required temperature depends on the HCl concentration in the<br />
gas. In case of a thermocatalytic conversion, the potential short lifetime of the catalyst is<br />
an issue due to the possible poisoning <strong>by</strong> acidic gases in the sample gas [2,4,5].<br />
Compared to mercury measurements in power plants and waste incinerators, there<br />
is a lack of experience related to continuous measurement of mercury emissions <strong>from</strong><br />
cement kilns. Furthermore, the experience gained <strong>from</strong> power plant and waste incinerator<br />
may not be applied directly to cement plant due to the different process conditions and<br />
flue gas compositions [7]. In this work a commercial red brass converter, which is<br />
developed for application in waste incinerators, is tested in simulated cement kiln flue<br />
gas and an improved sodium sulfite-based converter is developed and tested.<br />
5.2 Performance test of the mercury analyzer<br />
The analyzer has an internal mercury source for span calibration. However, the<br />
span calibration is conducted at an elemental mercury concentration of about 16 �g/Nm 3 ,<br />
which is much lower than typical mercury concentration of about 180 �g/Nm 3 applied in<br />
this project. If the linearity of the analyzer is poor then the measured mercury<br />
concentration at typical mercury levels in this project could be wrong. To check the<br />
analyzer linearity some tests were conducted. The carrier nitrogen flow rate through the<br />
mercury source was kept at 275 Nml/min. Firstly, the mercury concentration in the outlet<br />
gas <strong>from</strong> the mercury source was calculated using the measured mercury concentration in<br />
the mixed gas and applied flow rates. Then part of the gas <strong>from</strong> the outlet of mercury<br />
source was <strong>by</strong>passed to ventilation and more nitrogen was added to the empty reactor to<br />
dilute the mercury-contained gas and keep the total flow through the reactor at 2.75<br />
Nl/min. Figure 5.1 shows the comparison between the measured and calculated mercury<br />
concentration in the mixed gas. Very good agreement is obtained between the measured<br />
and calculated mercury concentration, confirming that the linearity of the analyzer is<br />
good.<br />
119
Measureed Hg concentration (�g/Nm3)<br />
250<br />
200<br />
150<br />
100<br />
50<br />
0<br />
0 50 100 150 200 250<br />
Calculated Hg concentration (�g/Nm3)<br />
Figure 5.1. Linearity of the Lumex mercury analyzer. Measured elemental mercury<br />
concentration is compared with the calculated values in the range of 0-250 �g/Nm 3 .<br />
The effects of different gases on elemental mercury measurement were<br />
investigated <strong>by</strong> adding gases separately. Figure 5.2 shows the measured mercury<br />
concentration under different conditions. Nitrogen, water, and CO2 were used as baseline<br />
gas. Further addition of O2, SO2, NOx and HCl step <strong>by</strong> step to the baseline gases gives<br />
the same mercury level. This indicates that these gases at the applied level do not have<br />
influence on elemental mercury measurement. The consistent mercury concentration also<br />
implies no mercury oxidation in the lines.<br />
120
Hg concentration (�g/Nm3)<br />
240<br />
200<br />
160<br />
120<br />
80<br />
40<br />
0<br />
-40<br />
Baseline<br />
gas<br />
Baseline<br />
gas,O 2<br />
air to analyzer<br />
Baseline<br />
gas,SO 2<br />
0 10 20 30 40 50 60 70<br />
Time (min)<br />
Figure 5.2. Effects of different gases on elemental mercury measurement <strong>by</strong>passing the<br />
converter.<br />
121<br />
Baseline<br />
gas,NO X<br />
Baseline<br />
gas,HCl<br />
5.3 Performance test of the red brass converter<br />
Baseline gas<br />
O 2, SO 2, NO X,HCl<br />
The red brass chips are obtained through the analyzer supplier Lumex. The<br />
typical composition of red brass includes 85% Cu, 5% Sn, 5% Zn and 5% Pb [8]. The<br />
idea of using red brass at low temperature is to bind free halogens in the flue gas and thus<br />
prevent back reaction into mercury halides as illustrated in following reaction [9]:<br />
Cl2 �Cu � CuCl2<br />
(5R1)<br />
The red brass converter is designed to convert Hg 2+ to Hg 0 at low temperatures of<br />
120-250�C to minimize the corrosion problem caused <strong>by</strong> SO2 oxidation at high<br />
temperatures [9]. The principle of the converter is to convert oxidized mercury according<br />
to following reaction:<br />
HgCl2 �Me�Hg� MeCl2<br />
(5R2)<br />
where Me could be Cu, Sn, Zn, and Pb that are contained in the red brass.
The performance of the red brass converter on elemental mercury measurement<br />
was first investigated. Test of the converter at 180�C in nitrogen atmosphere with only<br />
elemental mercury shows that the converter works well, since measurements through and<br />
<strong>by</strong>pass of the converter give the same mercury concentrations and the response time is<br />
short. However, tests of the converter using simulated flue gas and elemental mercury<br />
show that the performance of the converter degrades as a function of time. After short<br />
term exposure to the simulated flue gas the measured mercury level through the<br />
converter starts to decrease and is lower than that measured <strong>by</strong>passing the converter.<br />
Detailed investigations were then conducted to study the possible effects of gases<br />
on Hg 0 measurement through the converter. The applied gas concentrations are: 15 ppmv<br />
HCl, 1000 ppmv NO, 30 ppmv NO2, 1000 ppmv SO2, 1% H2O. Figure 5.3 shows the<br />
measured Hg 0 through the converter after adding different gases. When HCl, SO2 and<br />
NOx is added alone with water, the measured Hg 0 after the converter are the same as the<br />
inlet. However, when HCl is added either with SO2 or NOx the measured Hg 0 through the<br />
converter decreases with time, indicating that the catalyst surface is modified and starts<br />
to adsorb mercury or oxidize it to HgCl2.<br />
122
Hg concentration (�g/Nm 3 )<br />
300<br />
250<br />
200<br />
150<br />
100<br />
50<br />
0<br />
N 2+Hg+H 2O<br />
N 2+Hg+<br />
H 2O+NO x<br />
N 2+Hg+<br />
H 2 O+HCl N 2+Hg+H 2O+SO 2 N 2+Hg+H 2O<br />
+HCl+SO 2<br />
0 60 120 180 240 300 360 420 480<br />
Time (min)<br />
Figure 5.3. Measured elemental mercury concentration through the converter with 20 g<br />
red brass chips at 180�C after adding different gases.<br />
123<br />
N 2+Hg+H 2O<br />
+HCl+SO 2<br />
+NO x<br />
N 2+Hg+H 2O<br />
+SO 2 +NO x<br />
N 2+Hg+H 2O<br />
+HCl+NO x<br />
Besides reaction with mercury chloride, copper in the red brass can also react<br />
with other gases and form oxidized copper compounds. Possible reactions include:<br />
2Cu �O� 2CuO<br />
(5R3)<br />
2<br />
CuO �2HCl �CuCl � H O<br />
(5R4)<br />
2 2<br />
2CuO �2SO�O� 2CuSO<br />
(5R5)<br />
2 2 4<br />
Similar reactions could also take place for metals such as Sn, Zn and Pb contained in the<br />
red brass. It has been reported that NO2 is a very good oxidizing agent for preparing ZnO<br />
<strong>from</strong> metallic zinc through the reaction [10]<br />
NO2 �Zn � NO � ZnO<br />
(5R6)<br />
A similar reaction might take place between NO2 and copper. Copper chloride and<br />
copper sulfate have been used as promoters to improve mercury oxidation and adsorption<br />
<strong>by</strong> different sorbents [11-15]. These possible reactions might explain why elemental
mercury adsorption and oxidation takes place on the red brass chips in the simulated<br />
cement kiln flue gas.<br />
The oxidized mercury is added and produced <strong>by</strong> passing gases to the reactor with<br />
4 g SCR catalyst plate at 150�C. Oxidation of Hg 0 <strong>by</strong> the SCR system has been reported<br />
in both power plants[16,17] and in bench-scale tests [18-21]. The oxidation of mercury<br />
<strong>by</strong> the SCR catalyst is fast and about 70% mercury oxidation is obtained when 4g SCR<br />
catalyst is exposed to the simulated flue gas with15 ppmv HCl, 1% H2O, 1000 ppmv NO,<br />
30 ppmv NO2 and 1000 ppmv SO2 at 150�C. Figure 5.4 shows the result for using only<br />
15 ppmv HCl, 1% H2O and with N2 as balance. It takes about 5 h for the converter to<br />
obtain full oxidized mercury reduction, indicating that red brass converter cannot be used<br />
for dynamic measurement.<br />
Hg concentration (�g/Nm3)<br />
240<br />
200<br />
160<br />
120<br />
80<br />
40<br />
0<br />
Hg0, <strong>by</strong>pass reactor<br />
Hg0 through reactor with SCR<br />
0 2 4 6 8 10 12<br />
Time (hour)<br />
124<br />
Hgtotal through converter<br />
Figure 5.4. Measured total mercury concentration using 4g SCR catalyst at 150�C and<br />
2.75 Nl/min flue gas containing only 15 ppmv HCl and 1% H2O, 20 g red brass in the<br />
converter at 180�C.<br />
Based on these tests it was suggested <strong>by</strong> the supplier that the converter material<br />
reaches stability only after a period of several hours of operation under the gas mixture<br />
investigated in this project [22]. Tests were then conducted <strong>by</strong> conditioning the converter
with gases excluding Hg 0 addition. Detailed results are shown in figure 5.5. After<br />
conditioning the red brass catalyst with 15 ppmv HCl, 1000 ppmv SO2, 500 ppmv NO,<br />
15 ppmv NO2 and 1% H2O for 40 h, the measured mercury level through the converter is<br />
about 90 �g/Nm 3 compared to Hg 0 inlet level of 210 �g/Nm 3 . Furthermore, the measured<br />
mercury concentration through the converter keeps decreasing with time. The results<br />
show that the red brass converter does not work for the present conditions.<br />
Hg concentration (�g/Nm3)<br />
240<br />
200<br />
160<br />
120<br />
80<br />
40<br />
0<br />
Hgtotal through converter<br />
0 1 2 3 4 5<br />
Time (hour)<br />
125<br />
Hg0 <strong>by</strong>pass<br />
converter<br />
Figure 5.5. Measured total mercury concentration after passing SCR reactor with 4g SCR<br />
catalyst at 150�C. 20 g red brass in the converter at 180�C was preconditioned <strong>by</strong> 2.75<br />
Nl/min flue gas containing 15 ppmv HCl, 1000 ppmv SO2, 1% H2O, 500 ppmv NO, 15<br />
ppmv NO2 for 40 h.<br />
5.4 Performance of the sulfite converter<br />
The principle of the sulfite converter is that oxidized mercury such as HgCl2 can<br />
be reduced to Hg 0 through following reaction [23]:<br />
HgCl �NaSO �Hg�2NaCl �SO � O<br />
(5R7)<br />
1<br />
2 2 3 2 2 2
It is reported that 95% or more HgCl2 reduction efficiency can be obtained at<br />
300-500�C [23]. It is not stated in the patent for which gas composition this conversion<br />
was obtained and for how long. Thus a fundamental investigation of the sulfite converter<br />
under simulated cement kiln flue gas is necessary. The effect of converter temperature on<br />
Hg 0 recovery was tested and the results are illustrated in table 5.1. With 5 g sulfite<br />
compounds at 350�C, full mercury recovery can only be obtained for 1 h. Then the Hg 0<br />
recovery decreases with time and drops to 43% after another 3.5 h. For 10 g sulfite<br />
compounds at 450�C, full mercury recovery was obtained for 2 h. Then the mercury<br />
recovery decreased with time and dropped to 87% after another 4 h.<br />
Fast deactivation of sodium sulfite can take place at high temperatures [23].<br />
Sodium sulfite is water soluble and can recrystallize when water is present in the gas.<br />
When recrystallization occurs, the resistance of a layer of the sodium sulfite to gas<br />
transport is increased and the oxidized mercury reduction efficiency may be reduced. The<br />
higher the temperature the more recrystallization of sodium sulfite may take place. To<br />
minimize the deactivation, the converter was first tested at 250�C. However, only about<br />
50% mercury recovery was obtained immediately after switching gas to the converter<br />
and the Hg 0 recovery kept decreasing to 36% after another 1.5 h. Then the converter<br />
temperature was increased to 500�C. The mercury concentration after the converter<br />
increased sharply to 180% of the inlet elemental mercury level right after increasing the<br />
converter temperature. This is probably due to the fact that mercury is first adsorbed on<br />
the converter material at 250�C and then desorbs at high temperatures. Full mercury<br />
recovery was obtained for 1 h and then the mercury recovery decreased slowly to 88%<br />
after another 13 h.<br />
126
Table 5.1. Test results of elemental mercury recovery <strong>by</strong> the sulfite converter in 2.75<br />
Nl/min simulated cement kiln flue gas containing 21% CO2, 6% O2, 1% H2O, 1000 ppmv<br />
NO, 30 ppmv NO2, and 1000 ppmv SO2.<br />
Sulfite<br />
compound load<br />
(g)<br />
Converter<br />
temperature<br />
(�C)<br />
HCl level<br />
(ppmv)<br />
127<br />
Short time<br />
performance<br />
5 350 15 Full Hg 0 recovery<br />
for 1 h<br />
10 250 15 50% Hg 0 recovery<br />
for 0.5 h<br />
10 450 15 Full Hg 0 recovery<br />
for 2 h<br />
20 500 (after 15 Full Hg<br />
250�C test)<br />
0 recovery<br />
for 1 h<br />
20 500 2 Full Hg 0 recovery<br />
for 72 h<br />
20 500 6 Full Hg 0 recovery<br />
for 24 h<br />
20 500 10 Full Hg 0 recovery<br />
for 15 h<br />
Long time performance<br />
Hg 0 recovery decreases to<br />
43% after 3.5 h<br />
Hg 0 recovery decreases to<br />
36% after 1.5 h<br />
Hg 0 recovery decreases to<br />
87% after 4 h<br />
Hg 0 recovery decreases to<br />
88% after 13 h<br />
Not tested<br />
Not tested<br />
Hg 0 recovery decreases to<br />
95% after 35 h<br />
The level of HCl in the flue gas is a key factor that determines the efficiency and<br />
lifetime of the converter. Since only short time of full oxidized mercury reduction was<br />
observed with 15 ppmv HCl in the simulated flue gas, the HCl level was decreased to<br />
study the effects. With 2, 6, and 10 ppmv HCl in the simulated gas, full oxidized<br />
reduction can be obtained for at least 72, 24, and 15 h, respectively, for short-term test.<br />
With 10 ppmv HCl in the simulated cement kiln flue gas continuous operation of the<br />
converter with 20g sulfite material up to 2-3 months has been achieved. The presence of<br />
HCl in the gas can result in mercury oxidation both in the flue gas and on the sorbent.<br />
The recombination of elemental mercury and HCl after the converter might also be<br />
enhanced with high levels of HCl in the gas.<br />
It should be noted that the sulfite can be oxidized to sulfate <strong>by</strong> oxygen in the flue<br />
gas:<br />
Na SO � O � Na SO<br />
(5R8)<br />
1<br />
2 3 2 2 2 4
To study the effects of sodium sulfite oxidation to sulfate on oxidized mercury reduction<br />
efficiency, the sulfite pellets were first exposed to 12% O2 at 350�C for 18 h. Figure 5.6<br />
shows that the maximum Hg 0 recovery is about 90% after preconditioning <strong>by</strong> oxygen and<br />
slowly decreases with time. This indicates that the sodium sulfite was partly oxidized to<br />
sodium sulfate during preconditioning <strong>by</strong> oxygen. The formed sulfate is not active for<br />
reduction of oxidized mercury to elemental mercury. Normally the analyzer is running all<br />
the time to avoid damage of the lamp <strong>by</strong> restarting of the analyzer. When the experiment<br />
is not run, air is added to the analyzer. The test of oxygen precondition indicates that the<br />
converter should be closed to avoid oxidation of the sulfite compound when the<br />
experiment is not running. Instead the analyzer is running in Hg 0 measurement mode<br />
with air to the analyzer, <strong>by</strong>passing the converter.<br />
Hg concentration (�g/Nm 3 )<br />
200<br />
160<br />
120<br />
80<br />
40<br />
0<br />
Hgtotal<br />
<strong>by</strong>pass<br />
reactor<br />
0 30 60 90 120<br />
Time (min)<br />
128<br />
Hg0 inlet<br />
Hgtotal through reactor<br />
Figure 5.6. Test of 20 g sodium sulfite converter materials at 350�C using 2.75 Nl/min<br />
simulated flue gas with 15 ppmv HCl. The sulfite pellets were pre-conditioned <strong>by</strong> 12%<br />
O2 for 18 h. Oxidized mercury is produced <strong>by</strong> passing gases through 4 g SCR catalyst at<br />
150�C.
The dynamics of the converter were investigated <strong>by</strong> studying the response time of<br />
mercury measurement to the change of mercury addition and switching between the<br />
reactor and <strong>by</strong>pass. This was carried out <strong>by</strong> step up and step down tests [7]. The<br />
dynamics of Hg 0 measurement <strong>by</strong>passing the converter were first investigated. The steps<br />
of the dynamics test are illustrated in figure 5.7 Air was used as zero gas and added to the<br />
analyzer directly until a stable reading was achieved for about 5 min. Then air addition<br />
was stopped and Hg 0 in simulated flue gas was added <strong>by</strong>passing the reactor with SCR<br />
catalyst and the sulfite converter to measure the Hg 0 inlet level. After a stable reading of<br />
the inlet Hg 0 level was obtained for about 10 min, zero air was added to the analyzer and<br />
step down test of Hg 0 measurement was finished when stable reading was obtained. The<br />
95% response time of both step up and down is less than 0.5 min for elemental mercury<br />
measurement <strong>by</strong>passing the sulfite converter. The step change is very similar to that seen<br />
in the elemental measurement shown in figure 5.2.<br />
Hg concentration (�g/Nm3)<br />
140<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
Bypass<br />
SCR<br />
Air to<br />
converter,<br />
gas to SCR<br />
Through<br />
SCR<br />
0 30 60 90 120<br />
Time (min)<br />
129<br />
Air to<br />
converter<br />
Bypass<br />
SCR<br />
Bypass<br />
SCR<br />
stop Hg<br />
addition<br />
Figure 5.7. Response of total mercury measurement with elemental Hg inlet level of 112<br />
�g/Nm 3 . 20 g sodium sulfite converter materials is used in the converter at 500�C using<br />
2.75 Nl/min simulated flue gas with 10 ppmv HCl. Oxidized mercury is produced <strong>by</strong><br />
passing gases through 4 g SCR catalyst at 150�C.
Then the measurement was switched to total mercury measurement through the<br />
converter. The Hg 0 in the simulated flue gas was added to the reactor with SCR catalyst<br />
and the converter. The step up test of total mercury measurement was finished when<br />
stable mercury measurement through the converter was achieved for 50 min. Then air<br />
was added to the converter for a step down test. The dynamic tests were conducted at<br />
different Hg 0 inlet levels of 41, 112, and 150 µg/Nm 3 . Figure 5.7 shows the response of<br />
both Hg 0 measurement <strong>by</strong>passing the converter and SCR catalyst and total mercury<br />
measurement through the SCR catalyst and converter with a Hg 0 inlet level of 112<br />
µg/Nm 3 . Close look at the response for step change in figure 5.8 shows that the response<br />
of mercury measurement is very fast for both step up and down tests. The 95% response<br />
time for step up and down change is 1.5 and 0.6 min, respectively.<br />
Hg concentration (�g/Nm 3 )<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
1.50 min<br />
95% step<br />
up change<br />
34 36 38 40 42 44 46 48 50<br />
Time (min)<br />
130<br />
0.60 min<br />
95% step<br />
down change<br />
78 80 82 84 86<br />
Time (min)<br />
Figure 5.8. Close look of the response time test shown in figure 5.7. Left: step up test <strong>by</strong><br />
switching gas addition to the sulfite converter <strong>from</strong> air to simulated flue gas with<br />
oxidized mercury produced <strong>by</strong> SCR catalyst. Right: step down test <strong>by</strong> switching gas<br />
addition to the sulfite converter <strong>from</strong> simulated flue gas with oxidized mercury produced<br />
<strong>by</strong> SCR catalyst to air.
5.5 Examples of dynamic measurement of mercury adsorption and<br />
oxidation on activated carbon<br />
Commercial activated carbons, Darco Hg and HOK standard were investigated in<br />
simulated cement kiln flue gas at 150�C. Figure 5.9 shows the mercury profiles for the<br />
Darco Hg activated carbon. The experiments are conducted twice using separate total and<br />
elemental mercury measurement. Comparison of the elemental and total mercury<br />
measurement shows that both adsorption and oxidation of mercury <strong>by</strong> the carbon occur.<br />
After mercury breakthrough is achieved, the mercury oxidation is stable at about 92%.<br />
Gaseous Hg (�g/Nm3)<br />
200<br />
160<br />
120<br />
80<br />
40<br />
0<br />
<strong>by</strong>pass<br />
reactor<br />
0 0.5 1 1.5 2 2.5 3 3.5<br />
Time (hour)<br />
131<br />
through<br />
reactor<br />
Gaseous Hg total<br />
Gaseous Hg 0<br />
Figure 5.9. Total and elemental mercury profile of 30 mg Darco Hg activated carbon<br />
mixed with 2 g sand at 150�C using 2.75 Nl/min simulated cement kiln flue gas with 10<br />
ppmv HCl, two separate tests and measurements.<br />
Rather than running the test of the same carbon twice as shown in figure 5.9, it is<br />
possible to run the test once and evaluate both the mercury adsorption and oxidation <strong>by</strong><br />
the carbon. The mercury breakthrough was first obtained <strong>by</strong> measuring total mercury<br />
through the converter, then zero air is added to the converter and the analyzer was<br />
changed to Hg 0 measurement mode. During this period mercury in simulated gas was<br />
still added to the sorbent to avoid possible desorption of mercury <strong>from</strong> the carbon. When
the analyzer was running for Hg 0 measurement, gases after the reactor were switched to<br />
the analyzer to measure Hg 0 . Figure 5.10 illustrates the mercury adsorption and oxidation<br />
<strong>by</strong> HOK standard carbon at 150�C. In this case 57% mercury oxidation was observed.<br />
Both the tests of Darco Hg and HOK in simulated cement kiln flue gas show that the<br />
sulfite converter and analysis system are capable of following the transient mercury<br />
outlet concentration in a satisfactory way.<br />
Gaseous Hg (�g/Nm 3 )<br />
200<br />
160<br />
120<br />
80<br />
40<br />
0<br />
<strong>by</strong>pass<br />
reactor<br />
Hg total<br />
through<br />
reactor Hg total<br />
Gas to reactor,<br />
air to analyzer<br />
change <strong>from</strong> Hg total<br />
to Hg 0 measurement<br />
0 1 2 3 4 5<br />
Time (min)<br />
132<br />
through<br />
reactor Hg 0<br />
Figure 5.10. Total and elemental mercury profile of 30 mg HOK standard activated<br />
carbon mixed with 2 g sand at 150�C using 2.75 Nl/min simulated cement kiln flue gas<br />
with 10 ppmv HCl.<br />
5.6 Suggestions for practical application of the converter<br />
The conditions in full-scale application are much more demanding than in the labscale<br />
investigation. In this work no particles in the gas stream were applied. On the other<br />
hand, the dust load in the flue gas between the raw mill and filter could be up to 800-<br />
1000 g/Nm 3 . The sampling probe needs to be able to separate the particles <strong>from</strong> the flue<br />
gas efficiently to avoid plugging of probe. Adsorption of mercury <strong>by</strong> the dust and probe<br />
should be minimized <strong>by</strong> high sampling flow rate and high filter temperature.
The HCl content in the cement kiln flue gas can be up to 20-25 ppmv [24]. As<br />
found in this work, the full Hg 0 recovery can only be maintained for short period when<br />
more than 10 ppmv HCl is present in the flue gas. It is therefore necessary to remove HCl<br />
before or in the converter. Lime pellets can be used together with the sulfite compounds<br />
and the converter temperature should be high enough to avoid mercury adsorption on the<br />
lime pellets. Alternatively, large amount of converter material might be used.<br />
Compared to power plants and incinerators, the emission levels of CO and<br />
volatile organic compounds such as hydrocarbons are higher in cement plants. The<br />
emission level of volatile organic compound in the stack gas of cement kilns is usually<br />
between 10 and 100 mg/Nm 3 , with a few excessive cases up to 500 mg/Nm 3 [25]. The CO<br />
concentration in the stack gas can be as high as 1000 mg/Nm 3 , even exceeding 2000<br />
mg/Nm 3 in some cases. High levels of CO and hydrocarbons in the flue gas will cause<br />
fast contamination of the windows in the analytical cells and interruption of the mercury<br />
measurement [22]. Measures such as dilution should be applied to minimize the problem.<br />
The sulfite material should be kept in a closed box to avoid oxidation <strong>by</strong> air and<br />
moisture. It is important that the sulfite powders are adhered uniformly to the surface of<br />
the thin layer of water glass on the zeolite pellets. Thoroughly mixing the water glass<br />
with zeolite pellets in a plastic container can improve the sulfite converter performance.<br />
In this way, the sulfite converter can work well up to months, as observed in this work.<br />
For both lab-scale and full-scale application of the analysis system, it is important<br />
to avoid cold parts in the system. All the connections, Teflon lines and gas contacting<br />
parts in the analyzer before the spectrometry should be heated above 150�C. When the<br />
converter is not used, the converter temperature should be decreased to 100�C. The<br />
converter should be closed to avoid deactivation of the converter material due to<br />
oxidation of sulfite to sulphate <strong>by</strong> air.<br />
5.7 Conclusions<br />
To be able to perform dynamic measurement of mercury adsorption <strong>by</strong> sorbents,<br />
red brass chips and sulfite converter were investigated in simulated cement kiln flue gas<br />
133
in a fixed-bed reactor system. The converter with red brass chips works only when<br />
measuring elemental mercury in nitrogen (i.e., without carrying out actual conversion)<br />
and does not work properly even when only elemental mercury was added to the<br />
simulated flue gas. The red brass is poisoned or oxidized within a short time and adsorbs<br />
elemental mercury. When oxidized mercury was produced <strong>by</strong> passing gases through a<br />
separate reactor with an SCR catalyst, the red brass converter cannot fully reduce HgCl2<br />
to elemental mercury under any relevant condition.<br />
Sodium sulfite converter material was prepared <strong>by</strong> dry impregnation of sodium<br />
sulfite and calcium sulfate powders on zeolite pellets using water glass as binder. The<br />
optimal operating temperature of the sulfite converter is 500�C. The level of HCl in the<br />
flue gas is a key factor that determines the efficiency and lifetime of the converter. Full<br />
elemental mercury recovery can only be obtained for short period with 15 ppmv HCl in<br />
the simulated gas, but the sulfite converter works well at 500�C with up to 10 ppmv HCl<br />
in the simulated cement kiln flue gas. When the converter is not used, the converter<br />
temperature was decreased to 100�C without air passing through to avoid deactivation of<br />
the converter material <strong>by</strong> oxidation of the sodium sulfite to sodium sulfate. The response<br />
time of the sulfite converter is short and typically within at most two minutes, which<br />
makes it appropriate for not too fast dynamic measurements, as verified <strong>by</strong> dynamic<br />
mercury adsorption tests on commercial activated carbons Darco Hg and HOK standard<br />
in a fixed-bed reactor. Suggestions for practical application of the sulfite converter in<br />
cement plant with high dust load are provided.<br />
5.8 References<br />
[1] R.J. Schreiber and C.D. Kellett, Compilation of mercury emissions data, PCA R&D Serial No.<br />
SN3091, 2009.<br />
[2] D.L. Laudal, J.S. Thompson, J.H. Pavlish, L.A. Brickett, P. Chu, Use of continuous mercury<br />
monitors at coal-fired utilities, Fuel Processing Technology. 85 (2004) 501-511.<br />
[3] J. Wu, Y. Du and W. Pan, J. Ren, P. He, W. Wang, M. Shen, X. Leng, Y. Jin, Z. Dai, L. Zhao,<br />
X. Ming, Y. Cao, W. Pan, Study on different measurement methods of mercury emission in the<br />
134
coal-fired power station, 3 rd International Conference on Bioinformatics and Biomedical<br />
Engineering, Beijing, China, June 11-13, 2009.<br />
[4] V. Schmid, Continuous monitoring of mercury emissions <strong>from</strong> stationary sources, 2002.<br />
[5] M. Holmes and J. Pavlish, <strong>Mercury</strong> information of clearinghouse, Quarterly 2–mercury<br />
measurement, 2004.<br />
[6] J. Wang, Z. Xiao, O. Lindqvist, On-line measurement of mercury in simulated flue gas, Water,<br />
Air, & Soil Pollution. 80 (1995) 1217-1226.<br />
[7] M.L. Jones, D.L. Laudal and J.H. Pavlish, <strong>Mercury</strong> emission monitoring for the cement<br />
industry, <strong>Cement</strong> Industry Technical Conference Record, 2008 IEEE, Miami, Florida, May 18-22,<br />
2008.<br />
[8] Wikipedia, Brass, http://en.wikipedia.org/wiki/Brass, accessed December 7, 2010.<br />
[9] R. Kanefke, H. Köser, B. Vosteen, F. Kristina, B. Frank, S. Raik, Method for the production<br />
of elemental mercury <strong>from</strong> mercury compounds, patent WO 2008/064667 A2, 2008.<br />
[10] J.A. Rodriguez, T. Jirsak, J. Dvorak, S. Sambasivan, D. Fischer, Reaction of NO2 with Zn<br />
and ZnO: Photoemission, XANES, and density functional studies on the formation of NO3, The<br />
Journal of Physical Chemistry B. 104 (2000) 319-328.<br />
[11] S. Lee, J. Lee, T.C. Keener, Bench-scale studies of in-duct mercury capture using cupric<br />
chloride-impregnated carbons, Environ. Sci. Technol. 43 (2009) 2957-2962.<br />
[12] S. Lee, J. Lee, T.C. Keener, The effect of methods of preparation on the performance of<br />
cupric chloride-impregnated sorbents for the removal of mercury <strong>from</strong> flue gases, Fuel. 88 (2009)<br />
2053-2056.<br />
[13] A. Makkuni, R.S. Varma, S.K. Sikdar, D. Bhattacharyya, Vapor phase mercury sorption <strong>by</strong><br />
organic sulfide modified bimetallic iron-copper nanoparticle aggregates, Ind Eng Chem Res. 46<br />
(2007) 1305-1315.<br />
[14] D.E. Meyer, S.K. Sikdar, N.D. Hutson, D. Bhattacharyya, Examination of sulfurfunctionalized,<br />
copper-doped iron nanoparticles for vapor-phase mercury capture in entrainedflow<br />
and fixed-bed systems, Energy & Fuels. 21 (2007) 2688-2697.<br />
[15] D.E. Meyer, N. Meeks, S. Sikdar, N.D. Hutson, D. Hua, D. Bhattacharyya, Copper-doped<br />
silica materials silanized with bis-(triethoxy silyl propyl)-tetra sulfide for mercury vapor capture,<br />
Energy Fuels. 22 (2008) 2290-2298.<br />
[16] D.L. Laudal, J.S. Thompson, J.H. Pavlish, L. Brickett, P. Chu, R.K. Srivastava, J. Kilgroe,<br />
C.W. Lee, <strong>Mercury</strong> speciation at power plants using SCR and SNCR control technologies, EM:<br />
Air and Waste Management Association's Magazine for Environmental Managers. (2003) 16-22.<br />
[17] H.G. Pedersen, L.S. Pedersen, H. Rostgaard and K. Pedersen, Oxidation of mercury on DNX<br />
catalysts. Proceedings of the Air Quality V: <strong>Mercury</strong>, Trace Elements, SO3, and Particulate<br />
Matter Conference, Arlington, VA, Sept 19–21, 2005.<br />
135
[18] S. Straube, T. Hahn, H. Koeser, Adsorption and oxidation of mercury in tail-end SCR-<br />
DeNOx plants—Bench scale investigations and speciation experiments, Applied Catalysis B:<br />
Environmental. 79 (2008) 286-295.<br />
[19] Y. Zhuang, J. Laumb, R. Liggett, M. Holmes, J. Pavlish, Impacts of acid gases on mercury<br />
oxidation across SCR catalyst, Fuel Processing Technology. 88 (2007) 929-934.<br />
[20] Y. Cao, Z. Gao, J. Zhu, Q. Wang, Y. Huang, C. Chiu, B. Parker, P. Chu, W. Pan, Impacts of<br />
halogen additions on mercury oxidation in a slipstream selective catalyst reduction (SCR) reactor<br />
when burning sub-bituminous coal, Environ. Sci. Technol. 42 (2008) 256-261.<br />
[21] H. Kamata, S. Ueno, T. Naito, A. Yukimura, <strong>Mercury</strong> oxidation over the V2O5(WO3)/TiO2<br />
commercial SCR catalyst, Ind Eng Chem Res. 47 (2008) 8136-8141.<br />
[22] R. Moeseler. Issues about red brass converter, personal communication, Lumex Analytical<br />
GmbH, 2010.<br />
[23] S. Akiyama, J. Kato, F. Koga, K. Ishikawa, Catalysts for reducing mercury, a mercury<br />
conversion unit, and an apparatus for measuring total mercury in combustion exhaust gas <strong>by</strong><br />
using the same, patent US2007/0232488 A1, 2007.<br />
[24] C. Senior, A. Sarofim and E. Eddings, Behavor and measurement of mercury in cement<br />
kilns, presented at the IEE-IAS/PCA 45 th <strong>Cement</strong> Industry Technical Conference, Dallas, Texas,<br />
May 4-9 2003.<br />
[25] CEMBUREAU, the European <strong>Cement</strong> association, Best available technologies for the<br />
cement industry, 1999.<br />
136
Effects of bed dilution and carbon load on<br />
mercury adsorption capacity of activated<br />
137<br />
6<br />
carbon<br />
This chapter reports the effects of bed dilution and carbon load on the equilibrium<br />
mercury adsorption capacity of the activated carbon. The mercury adsorption capacity<br />
per unit mass of the activated carbon decreases when the carbon load is increased.<br />
Detailed investigations are conducted to reveal the cause.<br />
6.1 Introduction<br />
Most of the studies on mercury adsorption use bed dilution [1-9], while only few<br />
investigations apply pure sorbent bed [10-14] when the mercury sorbents are evaluated in<br />
fixed-bed reactors. The sorbent beds are often diluted with inert particles to suppress<br />
other potential disturbing effects such as axial dispersion and <strong>by</strong>passing [15,16]. Lowsurface-area<br />
materials such as glass beads and sand/quartz powder are preferred as<br />
diluting solids because of their relative inertness and good heat transfer properties. The<br />
effects of sorbent load on the mercury adsorption capacity of the sorbent are rarely<br />
reported in the literature.<br />
6.2 Effects of carbon load<br />
The direct result of the fixed-bed test is the mercury adsorption breakthrough<br />
curve. The percentage breakthrough is determined as a function of time <strong>by</strong> normalizing<br />
the measured total mercury concentration at the outlet of the sorbent bed to the inlet<br />
mercury concentration.
From the mercury breakthrough curve, the amount of mercury adsorbed on unit<br />
mass of the sorbent as a function of time can be calculated <strong>from</strong> the expression:<br />
q<br />
t<br />
�<br />
F<br />
W<br />
t<br />
� ( Cin<br />
� Cout,<br />
t ) dt<br />
(6.1)<br />
0<br />
where F is the flow rate through the sorbent bed, W is the mass of the sorbent, Cin is the<br />
inlet mercury concentration, Cout,t is the mercury concentration at the reactor outlet at<br />
time t. The mercury adsorption capacity of a known weight of a sorbent is calculated in<br />
terms of µg Hg adsorbed/mg_sorbent <strong>from</strong> the breakthrough curve for the sorbent. The<br />
equilibrium adsorption capacity is defined <strong>by</strong> the time when the outlet Hg concentration<br />
is first equal to the inlet concentration.<br />
Figure 6.1 presents the mercury breakthrough curves for different loads of Darco<br />
Hg activated carbon mixed with 2 g sand powder at 150�C using simulated cement kiln<br />
flue gas with elemental mercury. Faster mercury breakthrough is observed for smaller<br />
carbon load as expected. The calculated amount of adsorbed mercury and equilibrium<br />
mercury adsorption capacity per unit mass of the activated carbon are illustrated in figure<br />
6.2. The calculated amount of mercury adsorbed in the carbon does not increase<br />
proportionally to the mass of carbon, i.e., the mercury adsorption capacities of the carbon<br />
apparently decreases when the carbon load is increased. It seems that there is promotion<br />
of mercury adsorption <strong>by</strong> the sand when it is mixed with activated carbon. The trend line<br />
indicates that about 8.39 µg mercury is adsorbed <strong>by</strong> 2 g sand powder. Stuart [17] also<br />
reported that activated carbon mixed with sand had larger mercury uptake capacity than<br />
the carbon tightly packed in the reactor. He postulated that the incoming gas might be<br />
short circuiting and allowing the gas flow through the reactor without encountering all<br />
the tightly packed carbon. This argument is doubtful since even if the contact is poor<br />
uptake of mercury would just be lower and eventually the same uptake will be reached.<br />
138
C out/C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
A B C D<br />
0 1 2 3 4 5 6<br />
Time (hour)<br />
139<br />
A: 10 mg<br />
B: 30 mg<br />
C: 60 mg<br />
D: 100 mg<br />
Figure 6.1. <strong>Mercury</strong> breakthrough curves of different Darco Hg activated carbon loads<br />
mixed with 2 g sand powder and tested at 150�C using 2.75 Nl/min simulated flue gas<br />
with 170 µg/Nm 3 elemental mercury, 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 10<br />
ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
Adsorbed Hg (�g)<br />
50<br />
40<br />
30<br />
20<br />
10<br />
Adsorbed Hg<br />
Adsorbed Hg<br />
Y=0.3797X+8.3921,R<br />
0.2<br />
0<br />
0<br />
0 20 40 60 80 100 120<br />
2 =0.99<br />
Adsorption capacity<br />
Carbon load in 2 g sand (mg)<br />
Figure 6.2. Calculated amount of adsorbed mercury and equilibrium mercury adsorption<br />
capacity of Darco Hg activated carbon mixed with 2 g sand powder and tested at 150�C<br />
with different carbon loads using 2.75 Nl/min simulated flue gas with 170 µg/Nm 3<br />
elemental mercury, 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 10 ppmv HCl, 1<br />
vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
Hg Adsorption capacity (�g Hg/mg_carbon)
To investigate whether the oxidation of elemental mercury could influence the<br />
mercury adsorption capacity, mercury adsorption <strong>by</strong> different carbon loads using HgCl2<br />
source are conducted. As shown in figure 6.3, similar trends as tests with elemental<br />
mercury source are observed. This implies that the decrease of mercury adsorption<br />
capacity with increased carbon load in the sand is not caused <strong>by</strong> the oxidation of<br />
elemental mercury.<br />
Adsorbed mercury (�g)<br />
35<br />
30<br />
25<br />
20<br />
15<br />
10<br />
5<br />
0<br />
Adsorbed Hg<br />
0.4<br />
Adsorbed Hg<br />
Y=0.426X+6.739,R 0.2<br />
0<br />
0 10 20 30 40 50 60 70 80<br />
2 =0.96<br />
Adsorption capacity<br />
Carbon load (mg)<br />
Figure 6.3. Calculated amount of adsorbed mercury and equilibrium mercury adsorption<br />
capacity of Darco Hg activated carbon mixed with 2 g sand powder and tested at 150�C<br />
with different carbon loads using 2.75 Nl/min simulated flue gas with 170 µg/Nm 3<br />
mercury <strong>from</strong> HgCl2 source, 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 10 ppmv<br />
HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
One possible cause of the decrease of mercury adsorption capacity with increased<br />
carbon load could be the wall effect. Carbon particles separate <strong>from</strong> sand powders during<br />
loading the sample to the reactor. A layer of carbon particles deposits on the sample<br />
holder. Later these carbon particles are loaded to the reactor <strong>by</strong> knocking the sample<br />
holder. During loading of the sample to the reactor, carbon particles stick on the reactor<br />
wall. A quartz wool plug is used to move the carbon particles that adhere on the reactor<br />
wall to the top of the carbon bed. As a result some carbon particles are loaded to the area<br />
140<br />
2<br />
1.8<br />
1.6<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
Hg adsorption capacity (�g Hg/mg_carbon)
close to the reactor wall. More carbon particles adhere on the reactor wall when larger<br />
carbon load is applied.<br />
Another reason might be the leakage of the system and mercury adsorption <strong>by</strong> the<br />
quartz wool and sand powder. However, leakage tests of the system at different stages of<br />
the project show that the system is tight. The pressure drop over the carbon bed is about 5<br />
mbar. The deviation of the flow rates at the reactor inlet and outlet to the flow rate after<br />
the gas mixing panel is within 2.2%. Tests with empty reactor, reactor with quartz wool,<br />
and sand powder do not show any adsorption of either elemental mercury or mercury<br />
chloride.<br />
6.3 Effects of bed dilution<br />
Negative deviation of conversion caused <strong>by</strong> dilution of the catalyst bed with inert<br />
particles in gas-solid systems has been reported [15,16]. Dilution of activated carbon <strong>by</strong><br />
inert sand powder is applied in this work; it is therefore relevant to evaluate the possible<br />
effects caused <strong>by</strong> the dilution. The extent of negative effect depends on the amount of<br />
dilution, the reaction/adsorption kinetics, the particles and reactor geometry, and the<br />
degree of segregation of carbon and sand. Since the mercury removal fraction <strong>by</strong> the<br />
carbon bed changes with time, the dilution effect � as a relative measure of the deviation<br />
in the conversion can be calculated for different time:<br />
xundiluted () t � xdiluted () t<br />
�() t � (6.2)<br />
xundiluted<br />
where xdiulted(t) and xundiluted(t) is the mercury removal fraction at time t for diluted and<br />
undiluted bed, respectively.<br />
For practical application the relative deviation in conversion can be estimated<br />
<strong>from</strong> observable parameters [15,16] :<br />
b d p xdiluted () t<br />
�() t �(<br />
)<br />
(6.3)<br />
1�b hbed<br />
2<br />
where b is the volume of inert sand as fraction of total volume of solids, dp is carbon<br />
particle diameter, and hbed is the bed height.<br />
For 10 mg Darco Hg carbon mixed with 2 g sand, the b is calculated as:<br />
141
msand<br />
/ �sand<br />
�3<br />
2�10 /1602<br />
carbon �carbon � sand �sand<br />
�<br />
�6 � �<br />
�3<br />
b � � �0.98<br />
m / m / 10 10 / 510 2 10 /1602<br />
Figure 6.4 presents the calculated relative deviation in mercury adsorption as a<br />
function of time for different loads of Darco Hg carbon tested at 180�C in simulated<br />
cement kiln flue gas. Larger relative deviation in short period is observed for smaller<br />
carbon loads, i.e., larger dilution ratio. However, the area under the relative deviation<br />
curve and above the zero deviation appears to be similar for different carbon loads. This<br />
indicates that the influence of bed dilution on the equilibrium mercury adsorption<br />
capacity of the carbon is similar. Therefore the decrease of mercury adsorption capacity<br />
with increase of carbon loads is probably not caused <strong>by</strong> the bed dilution.<br />
Relative deviation, �<br />
0.08<br />
0.07<br />
0.06<br />
0.05<br />
0.04<br />
0.03<br />
0.02<br />
0.01<br />
0<br />
-0.01<br />
A<br />
B<br />
C<br />
0 0.5 1 1.5 2 2.5<br />
Time (hour)<br />
142<br />
A: 10 mg carbon, b=0.98<br />
B: 30 mg carbon, b=0.96<br />
C: 60 mg carbon, b=0.91<br />
Figure 6.4. The calculated relative deviation � as a function of time for tests at 180�C<br />
with different carbon loads using 2.75 Nl/min simulated flue gas with 170 µg/Nm 3<br />
mercury <strong>from</strong> HgCl2, 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 10 ppmv HCl, 1<br />
vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
Tests using same dilution ratio, i.e., 10 mg carbon is mixed with 2 g sand, 30 mg<br />
carbon with 6 g sand, 60 mg carbon with 12 g sand, are also performed at 180�C using<br />
simulated cement kiln flue gas with HgCl2. Figure 6.5 shows the calculated amount of<br />
adsorbed mercury and equilibrium mercury adsorption capacity of the activated carbon.
Similar mercury adsorption capacity is still not obtained for different carbon loads, which<br />
behaves as with 2 g sand.<br />
Adsorbed mercury (�g)<br />
30<br />
25<br />
20<br />
15<br />
10<br />
5<br />
0<br />
Adsorbed Hg<br />
Adsorbed Hg<br />
Y=0.3777X+7.0386,R<br />
0.2<br />
0<br />
0 10 20 30 40 50 60 70 80<br />
2 =0.97<br />
Adsorption capacity<br />
Carbon load (mg)<br />
Figure 6.5. Calculated amount of adsorbed mercury and equilibrium mercury adsorption<br />
capacity of Darco Hg activated carbon mixed with sand powder using same dilution rate<br />
and tested at 180�C with different carbon loads using 2.75 Nl/min simulated flue gas<br />
with 170 µg/Nm 3 mercury <strong>from</strong> HgCl2 source, 1000 ppmv NO, 23 ppmv NO2, 1000<br />
ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
6.4 Effects of sand load<br />
If the mercury adsorption <strong>by</strong> activated carbon is promoted <strong>by</strong> the sand mixing, it<br />
would be interesting to investigate the effects of sand load on the promotion of mercury<br />
adsorption capacity of the carbon <strong>by</strong> running tests with different masses of sand. Table<br />
6.2 presents the calculated amount of adsorbed mercury and equilibrium mercury<br />
adsorption capacity of 10 mg Darco Hg activated carbon mixed with different amounts of<br />
sand powder at 150�C in simulated cement kiln flue gas with HgCl2. When the sand load<br />
is above 20 mg the mercury adsorption capacities of the Darco Hg do not increase further<br />
and level off at a value of about 1.135 µg Hg/mg_carbon. This also indicates that the<br />
repeatability of the experiment is reasonable.<br />
143<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
Hg adsorption capacity (�g Hg/mg_carbon)
Table 6.2. Calculated amount of adsorbed mercury and equilibrium mercury adsorption<br />
capacity of 10 mg Darco Hg activated carbon mixed with different amounts of sand<br />
powder and tested at 150�C using 2.75 Nl/min simulated flue gas with HgCl2 source,<br />
1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2,<br />
and 21 vol.% CO2.<br />
Sand load (g) Hg inlet (µg/Nm 3 ) Adsorbed Hg (µg) Adsorption capacity<br />
(µg Hg/mg_carbon)<br />
0 164 5.06 0.506<br />
0.01 161 9.81 0.981<br />
0.02 160 11.36 1.136<br />
0.05 166 11.48 1.148<br />
0.10 164 11.07 1.107<br />
0.25 166 12.17 1.217<br />
0.5 174 10.50 1.050<br />
1 217 10.24 1.024<br />
2 183 12.24 1.224<br />
2 163 11.80 1.180<br />
4 206 11.29 1.129<br />
6.5 Effects of carbon loading location<br />
The carbon sample was separated <strong>from</strong> the sand powder with quartz wool plug to<br />
test possible effect of carbon loading location. Different locations of the carbon sample<br />
are applied to investigate whether the promotion of mercury adsorption is caused <strong>by</strong> the<br />
preconditioning of the gas <strong>by</strong> the sand. When the carbon is on top of the sand, the flue<br />
gas first contacts the carbon powder. However, as shown in figure 6.6, the equilibrium<br />
mercury adsorption capacity is almost the same when the carbon sample is loaded on top<br />
of and under the sand powder. This implies that the promotion of mercury adsorption<br />
only occurs when the carbon is mixed with sand powder. The slightly larger mercury<br />
adsorption capacity with sand powder in the reactor compared to only carbon in the<br />
reactor might be due to the improved contact of carbon with gas flow.<br />
144
Hg adsorption capacity (�g Hg/mg-carbon)<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
No sand<br />
0<br />
Figure 6.6. Equilibrium mercury adsorption capacity of 10 mg Darco Hg activated<br />
carbon on top of and under 1 g sand powder at 150�C using 2.75 Nl/min simulated flue<br />
gas with 170 µg/Nm 3 mercury <strong>from</strong> HgCl2 source, 1000 ppmv NO, 23 ppmv NO2, 1000<br />
ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
6.6 Effects of bed materials<br />
Different bed materials are applied to investigate the possible effects of bed<br />
materials on the mercury adsorption capacity of activated carbon. The investigated bed<br />
materials include sand powder, fine quartz powder, and glass beads and the mean<br />
diameters of these materials are 215, 2, and 180 µm, respectively. Baseline tests with<br />
only fine quartz powder and glass beads show that fine quartz powder is inert for<br />
mercury adsorption and the mercury adsorption <strong>by</strong> the glass beads is negligible. Figure<br />
6.7 compares the mercury adsorption capacity of Darco Hg activated carbon tested with<br />
different bed materials. The mercury adsorption capacity of Darco Hg carbon tested with<br />
fine quartz powder is much smaller than those with sand powder and glass beads. The<br />
fine quartz powder behaves like paste and might hinder the contact of gas with the<br />
carbon particles. Inconsistent mercury adsorption capacity is still obtained when different<br />
amounts of carbon are mixed with 2 g sand powder, fine quartz powder, and glass beads.<br />
145<br />
Carbon on top<br />
Sand on top
Hg adsorption capaity (�g Hg/mg_carbon)<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
Sand powder<br />
Fine quartz powder<br />
Glass beads<br />
10 mg carbon in<br />
2 g bed material<br />
146<br />
Sand powder<br />
Fine quartz powder<br />
Glass beads<br />
30 mg carbon in<br />
2 g bed material<br />
Figure 6.7. Equilibrium mercury adsorption capacity of Darco Hg activated carbon<br />
mixed with different bed materials and tested at 150�C using 2.75 Nl/min simulated flue<br />
gas with 170 µg/Nm 3 mercury <strong>from</strong> HgCl2 source, 1000 ppmv NO, 23 ppmv NO2, 1000<br />
ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
6.7 Effects of carbon type and particle size<br />
To study the effects of carbon type and particle size on the mercury adsorption<br />
capacity, commercial activated carbon pellets of Norit RB4 are crushed and sieved to<br />
size of 165 and less than 32 µm in diameter. Figure 6.8 shows the mercury adsorption<br />
capacity obtained with different carbon loads, carbon types and particle size. Inconsistent<br />
mercury adsorption capacity at different carbon loads is observed for both Darco Hg and<br />
Norit RB4 carbons with different sizes. The effects of carbon load are much smaller for<br />
Norit RB4 carbon.
Hg adsorption capacity (�g Hg/mg_carbon)<br />
2<br />
1.8<br />
1.6<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 10 20 30 40 50 60 70 80<br />
Carbon load (mg)<br />
147<br />
Darco Hg, 16 �m<br />
Norit RB 4, 165 �m<br />
Norit RB 4,
Adsorbed Hg (�g)<br />
10<br />
9<br />
8<br />
7<br />
6<br />
5<br />
4<br />
3<br />
2<br />
Adsorbed Hg<br />
Adsorption capacity<br />
1 2 3 4 5<br />
Portland cement load (g)<br />
148<br />
0.0025<br />
0.002<br />
0.0015<br />
0.001<br />
0.0005<br />
Figure 6.9. Calculated amount of adsorbed mercury and equilibrium mercury adsorption<br />
capacity as a function of Portland cement load at 150�C using 2.75 Nl/min simulated flue<br />
gas with 170 µg/Nm 3 mercury <strong>from</strong> HgCl2 source, 1000 ppmv NO, 23 ppmv NO2, 1000<br />
ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
6.9 Conclusions<br />
Inconsistent mercury adsorption capacity of activated carbon is observed at<br />
different carbon loads when mixed with sand. Smaller mercury adsorption capacity is<br />
obtained with larger carbon load. Tests with elemental mercury and mercury chloride,<br />
different carbon type and particle sizes show the same trend. Effects of bed dilution at<br />
fixed carbon load on the equilibrium mercury adsorption capacity appear to be limited.<br />
The mercury adsorption capacity of activated carbon obtained using sand and<br />
carbon mixture is larger than that obtained with only activated carbon. The mercury<br />
adsorption capacity with 10 mg carbon increases with sand load up to 20 mg and then<br />
levels off when the sand load is further increased.<br />
Similar mercury adsorption capacities are obtained with different Portland cement<br />
loads in the reactor. This implies that the inconsistent mercury adsorption capacity of<br />
carbon obtained using different carbon loads might be due to possible adsorption of<br />
0<br />
Hg adsorption capacity (�g Hg/mg_cement)
mercury <strong>by</strong> sand when it is mixed with carbon, rather than the failure of the experimental<br />
setup. The sand powder alone is inert for mercury adsorption, while after modification<br />
with chemical reagent it can be used for mercury adsorption [18,19]. In-situ analysis<br />
technology is required to reveal whether mercury is adsorbed <strong>by</strong> the sand when it is<br />
mixed with activated carbon.<br />
The problem of inconsistent mercury adsorption capacity was encountered in the<br />
late stage of the project when performing a fundamental parametric study. Although<br />
detailed tests are conducted to reveal the cause, the problem is not solved due to the lack<br />
of analysis techniques and time. It is impossible to repeat and run all the tests with only<br />
large carbon load within the time schedule of the project. For a full-scale application in<br />
the cement plant it is impossible to exclude all the cement materials in the flue gas even<br />
with a polishing filter. Instead of providing actual kinetics data relevant to full-scale<br />
application conditions, this work aims at evaluation the effects of different operating<br />
parameters and mathematical model development. Therefore, mercury adsorption<br />
kinetics obtained using 10 mg activated carbon mixed with 2 g sand powder is used in<br />
the following chapters dealing with parametric study and model development. It may be<br />
argued that it is difficult to state what the real capacity is if it depends on the mixing<br />
condition. In reality there will always be least 20 mg diluter with the sorbent and this is<br />
where it has stabilized.<br />
6.10 References<br />
[1] S. Sjostrom, T. Ebner, T. Ley, R. Slye, C. Richardson, T. Machalek, M. Richardson, R.<br />
Chang, Assessing sorbents for mercury control in coal-combustion flue gas, J. Air & Waste<br />
Manage. Assoc. 52 (2002) 902.<br />
[2] S.J. Lee, Y. Seo, J. Jurng, T.G. Lee, <strong>Removal</strong> of gas-phase elemental mercury <strong>by</strong> iodine- and<br />
chlorine-impregnated activated carbons, Atmospheric Environment. 38 (2004) 4887-4893.<br />
[3] D. Karatza, A. Lancia, D. Musmarra, Fly ash capture of mercuric chloride vapors <strong>from</strong><br />
exhaust combustion gas, Environ. Sci. Technol. 32 (1998) 3999-4004.<br />
[4] J.W. Portzer, J.R. Albritton, C.C. Allen, R.P. Gupta, Development of novel sorbents for<br />
mercury control at elevated temperatures in coal-derived syngas: results of initial screening of<br />
candidate materials, Fuel Processing Technology. 85 (2004) 621-630.<br />
149
[5] J.Y. Lee, Y. Ju, T.C. Keener, R.S. Varma, Development of cost-effective noncarbon sorbents<br />
for Hg 0 removal <strong>from</strong> coal-fired power plants, Environ. Sci. Technol. 40 (2006) 2714-2720.<br />
[6] D. Karata, A. Lancia, D. Musmarra, F. Pepe, Adsorption of metallic mercury on activated<br />
carbon, Symposium (International) on Combustion,. 26 (1996) 2439-2445.<br />
[7] G. Skodras, I. Diamantopoulou, G. Pantoleontos, G.P. Sakellaropoulos, Kinetic studies of<br />
elemental mercury adsorption in activated carbon fixed bed reactor, Journal of Hazardous<br />
Materials. 158 (2008) 1-13.<br />
[8] D. Karatza, A. Lancia, D. Musmarra, C. Zucchini, Study of mercury absorption and<br />
desorption on sulfur impregnated carbon, Experimental Thermal and Fluid Science. 21 (2000)<br />
150-155.<br />
[9] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang, F.B. Meserole, Factors affecting<br />
mercury control in utility flue gas using activated carbon, Journal of the Air & Waste<br />
Management Association. 48 (1998) 1166.<br />
[10] R. Yan, Y.L. Ng, D.T. Liang, C.S. Lim, J.H. Tay, Bench-scale experimental study on the<br />
effect of flue gas composition on mercury removal <strong>by</strong> activated carbon adsorption, Energy &<br />
Fuels. 17 (2003) 1528-1535.<br />
[11] R. Yan, D.T. Liang, L. Tsen, Y.P. Wong, Y.K. Lee, Bench-scale experimental evaluation of<br />
carbon performance on mercury vapour adsorption, Fuel. 83 (2004) 2401-2409.<br />
[12] G.E. Dunham, R.A. DeWall, C.L. Senior, Fixed-bed studies of the interactions between<br />
mercury and coal combustion fly ash, Fuel Processing Technology. 82 (2003) 197-213.<br />
[13] B.A.F. Mibeck, E.S. Olson, S.J. Miller, HgCl2 sorption on lignite activated carbon: Analysis<br />
of fixed-bed results, Fuel Process Technol. 90 (2009) 1364-1371.<br />
[14] G.E. Dunham, S.J. Miller, <strong>Mercury</strong> capture <strong>by</strong> an activated carbon in a fixed-bed benchscale<br />
system, Environmental Progress. 17 (1998) 203.<br />
[15] R.J. Berger, J. Pérez-Ramírez, F. Kapteijn, J.A. Moulijn, Catalyst performance testing: the<br />
influence of catalyst bed dilution on the conversion observed, Chem. Eng. J. 90 (2002) 173-183.<br />
[16] R.J. Berger, J. Pérez-Ramírez, F. Kapteijn, J.A. Moulijn, Catalyst performance testing: bed<br />
dilution revisited, Chemical Engineering Science. 57 (2002) 4921-4932.<br />
[17] J.L. Stuart. Development of an experimental system to study mercury uptake <strong>by</strong> activated<br />
carbon under simulated flue gas conditions, Master thesis, University of Pittsburgh, 2002.<br />
[18] M. Holmes and J. Pavlish, <strong>Mercury</strong> information clearinghouse, Quarter 3- Advanced and<br />
developmental mercury control technologies, July 2004.<br />
[19] J.R. Butz, T.E. Broderick and C.S. Turchi, Amended Silicates for <strong>Mercury</strong> Control,<br />
project final report, DOE Award Number: DE-FC26-04NT41988, 2006.<br />
150
Screening tests of mercury sorbents<br />
This chapter first deals with screening of mercury sorbents in simulated cement<br />
kiln flue gas using elemental mercury source. Then screening tests using mercury<br />
chloride source are reported. The results are used to suggest promising sorbents for<br />
application in cement plant and provide explanation of mercury adsorption in cement<br />
production <strong>by</strong> cement materials.<br />
7.1 Introduction<br />
<strong>Sorbent</strong>s are screened in the laboratory using simulated flue gas before fieldtesting<br />
in actual flue gas. The purpose of these laboratory tests is to evaluate a number of<br />
sorbents at conditions similar to those expected at typical cement plants. These test<br />
results then are used to determine the most appropriate samples for large scale tests.<br />
Basing on the screen tests, promising sorbents will be further investigated in the lab in<br />
detail to obtain adsorption kinetics and study the influence of different operational<br />
parameters. The fixed bed tests are not intended to simulate the conditions where a<br />
sorbent is injected continuously upstream of a fabric filter but they provide a good<br />
indication of sorbent effectiveness, providing the exposure conditions are similar.<br />
Screening measurements are used to evaluate mercury capture effectiveness, oxidation<br />
potential, and capacity for the selected sorbents.<br />
16 sorbent materials are collected and compared. The selected sorbents are tested<br />
in the fixed-bed reactor with continuous mercury measurement, following closely the<br />
experimental procedure as described in chapter 4.<br />
The empty reactor, quartz wool plug, and sand for sorbent dilution are tested first<br />
to investigate whether there is some mercury adsorption <strong>by</strong> the empty reactor, quartz<br />
151<br />
7
wool and sand powder. Then the sorbents are tested in the simulated cement kiln flue gas<br />
using either elemental mercury or mercury chloride source to study whether the sorbents<br />
behavior differently using different mercury sources.<br />
During mercury adsorption tests, the elemental Hg can be fully or partially<br />
oxidized because of reactions between the elemental Hg, sorbent, and flue gas<br />
components. Then the extent of mercury oxidation is calculated <strong>by</strong> comparing the<br />
measured elemental mercury after breakthrough and the inlet level of added elemental<br />
mercury.<br />
The direct result of the fixed-bed test is the mercury adsorption breakthrough<br />
curve. The percentage breakthrough is determined as a function of time <strong>by</strong> normalizing<br />
the measured total mercury concentration at the outlet of the sorbent bed to the inlet<br />
mercury concentration.<br />
From the mercury breakthrough curve, the amount of mercury adsorbed on unit<br />
mass of the sorbent as a function of time can be calculated <strong>from</strong> the expression:<br />
F t<br />
qt� ( C<br />
0<br />
in �Cout,<br />
t ) dt<br />
W � (7.1)<br />
where F is the flow rate through the sorbent bed, W is the mass of the sorbent, Cin is the<br />
inlet mercury concentration, Cout,t is the mercury concentration at the reactor outlet at<br />
time t.<br />
The mercury adsorption capacity of a known weight of a sorbent is calculated in<br />
terms of µg Hg adsorbed/g_sorbent material <strong>from</strong> the breakthrough curve for the sorbent.<br />
The area under the inlet mercury concentration line and a breakthrough curve is used to<br />
determine how much mercury is adsorbed <strong>by</strong> the sorbent. By material balance, the area<br />
between the curve and the line provides the information on the total mercury adsorbed<br />
onto a sorbent if the entire bed reaches equilibrium with mercury vapor. The equilibrium<br />
adsorption capacity is defined <strong>by</strong> the time when the outlet Hg concentration is first equal<br />
to the inlet concentration.<br />
Previous bench-scale studies have reported performance of the sorbents in terms<br />
of the adsorption capacity and/or the time taken for complete breakthrough of Hg <strong>from</strong> a<br />
sorbent [1-13]. However, the final application of the sorbent injection is in a full-scale<br />
152
plant where the sorbent is injected in the duct and captured either in a fabric filter, where<br />
the contact time of mercury with carbon particles is very short. Therefore, the adsorption<br />
rate within these time scales is the most important parameter when evaluating sorbent<br />
performance.<br />
The amount of mercury adsorbed at time t (mt) can be calculated using<br />
m �m �( C �C ). F. � t<br />
(7.2)<br />
t��t t in out, t�1 2�t<br />
where � C � C ) 2<br />
Cout, t�<br />
1 (<br />
2�t<br />
out,<br />
t out,<br />
t�<br />
�t<br />
The adsorption rate at each time step is calculated using<br />
mt��t mt<br />
ratet<br />
dt<br />
�<br />
� (7.3)<br />
The initial rate is evaluated as the slope of the cumulative adsorption curve in the<br />
first 25 min.<br />
7.2 <strong>Sorbent</strong> properties and compositions<br />
The collected sorbent candidates include both commercial sorbents and cement<br />
materials. Virgin activated carbon Dacro Hg, formerly known as Darco FGD [14] is<br />
prepared <strong>from</strong> lignite coal and has been widely studied in the literature [1,2,5-7,15-24]<br />
and is therefore tested here as a reference sorbent. Darco Hg-LH is Darco Hg treated with<br />
bromine and developed for application with low chlorine concentration in the flue gas<br />
<strong>from</strong> combustion of low-rank coals. Activated Lignite HOK is produced according to the<br />
so-called rotary-hearth furnace process [25-27]. Unlike activated carbon, activated<br />
Lignite HOK is produced as mass product with an annual output of 200,000 tons at a<br />
much lower price than that of activated carbon. HOK is the most widely used sorbent for<br />
waste incinerator flue gas cleaning in Europe. Sorbalit is a mixture of reagents, surfaceactive<br />
substances and chemical additives [28]. Reagents are calcium based compounds<br />
such as CaCO3, CaO and Ca(OH)2. Examples of surface-active substances are activated<br />
carbon, aluminum oxide and zeolite. Chemical additives are sulfur and sulfur compounds<br />
such as Na2S, NaHS, Na2S4. Sorbalit can be produced with carbon contents ranging <strong>from</strong><br />
4% to 65%. Minsorb DM and ME are non-carbon based sorbent and for removal of<br />
153
dioxin/furan and mercury, respectively [29,30]. The hydrated lime is standard Sorbacal<br />
product used for SO2 and SO3 removal [31].<br />
<strong>Cement</strong> materials are obtained <strong>from</strong> FLSmidth Dania lab. Clay contains<br />
essentially hydrous aluminum silicates, with minor amount of magnesium, iron, alkalies<br />
or alkaline earths [32]. <strong>Cement</strong> kiln dust is a fine-grained solid material with high<br />
alkaline content removed <strong>from</strong> the cement kiln exhaust gas <strong>by</strong> filters. The cement kiln<br />
dust contains mainly incompletely reacted raw material, including a raw mix at various<br />
stages of burning, and particles of clinker. The primary constituents are silicates, calcium<br />
oxide, carbonates, potassium oxide, sulfates, chlorides, various metal oxides, and sodium<br />
oxide [33].<br />
The kaolin sample is <strong>from</strong> Prolabo Merck. Hydroxyapatite has a formula of<br />
Ca5(PO4)3(OH) and has been used as sorbent to removal of heavy metals <strong>from</strong> waste<br />
incinerators [34]. Initial tests show that hydroxyapatite is a new promising sorbent for<br />
heavy metal removal <strong>from</strong> waste incineration flue gas [34]. Hydroxyapatite is chemically<br />
similar to the mineral component of bones and suitable for biomedical application.<br />
Properties of the sorbents are presented in table 7.1. Carbon-based sorbents have<br />
much larger surface area than the non-carbon based sorbents and cement raw materials.<br />
The volume median diameter D(v,0.5) is the diameter where 50% of the distribution is<br />
above and 50% is below. D(v,0.9) diameter means that 90% of the volume distribution is<br />
below this value. Similarly D(v,0.1) diameter means that 10% of the volume distribution<br />
is below this value. Generally the cement materials have a smaller particle size than the<br />
commercial sorbents. The cement materials are the cheapest due to the availability of<br />
large quantity in the cement plant and saving of transport cost. The bromine treated<br />
Darco Hg-LH carbon is much more expensive than the virgin activated carbons and noncarbon<br />
sorbents. The high price of hydroxyapatite is because it is pharmaceutical grade.<br />
Table 7.1. Properties of sorbents studied in this work.<br />
<strong>Sorbent</strong> D(v,0.1)<br />
�m<br />
D(v,0.5)<br />
�m<br />
154<br />
D(v,0.9)<br />
�m<br />
BET<br />
area<br />
Bulk<br />
density<br />
Price<br />
USD/kg
m 2 /g g/cm 3<br />
Darco Hg 1.27 15.99 43.07 600 0.51 1-2<br />
Darco Hg-LH 1.12 15.36 44.70 550 0.60 2-4<br />
HOK standard 63 300 0.55 1-2<br />
HOK super 24 300 0.44 1-2<br />
Sorbalit 0.85 12.60 52.24 58.5 0.42 1-2<br />
Minsorb DM 6.76 52.02 168.95 120 0.60 1-2<br />
Minsorb ME 3.20 39.17 177.07 70 1.10 1-2<br />
Hydrated lime 0.30 3.35 13.15 21.5 0.35 0.2<br />
Saklei fly ash 3.77 36.15 115.07 0.7 - 0.1<br />
Gypsum 1.63 18.78 62.20 18.5 - 0.1-0.15<br />
Raw meal 0.30 9.47 77.56 1.8 - 0.1<br />
Portland cement 0.32 16.16 46.10 1.8 - 0.1-0.2<br />
<strong>Cement</strong> kiln dust 0.33 3.36 63.93 6.5 - 0.05<br />
Clay 0.36 9.73 58.40 15.2 - 0.1<br />
Kaolin 1.24 5.60 20.65 13.0 - 0.1-0.2<br />
Hydroxyapatite 0.20 3.80 47.99 70.2 - 160<br />
Table 7.2 presents chemical composition of some selected sorbents. The<br />
compositions of HOK carbons and Minsorb sorbents are <strong>from</strong> the literature published <strong>by</strong><br />
the manufacture and the product datasheet [26,27,29,30]. The compositions of Darco<br />
carbons and Sorbalit are obtained <strong>by</strong> averaging 10-20 spot analyses of the samples <strong>by</strong><br />
SEM-EDX. Compositions of other materials are obtained <strong>by</strong> inductively coupled plasma<br />
(ICP) spectrometry. The Darco carbons have larger ash content than the HOK carbons.<br />
SEM-EDX analyses show that the Darco Hg-LH has a bromine content of about 7.8 wt%.<br />
The main elements of Sorbalit are C and Ca, in agreement with the statement <strong>by</strong> the<br />
producer [28]. Minsorb ME has larger Al and Fe contents than Minsorb DM. Ca is the<br />
main element in the raw meal. Kaolin and Saklei fly ash <strong>from</strong> bituminous coal<br />
combustion have similar composition with large Al and Si contents.<br />
155
Table 7.2 Chemical composition of selected sorbents. All in wt%<br />
<strong>Sorbent</strong> Moisture Ash C Cl S K Na Mg Ca Fe Al Si Reference<br />
Darco Hg
7.3 SEMEDX analysis of fresh sorbents<br />
The main goals of the SEM-EDX analysis is to study the sorbents’ topography<br />
(surface features), morphology (shape and size), and composition. Morphology study<br />
will be used to identify particle agglomeration and compare with particle size<br />
measurement.<br />
Figure 7.1 shows a typical micrograph of the fresh Darco Hg carbon which<br />
has various single carbon particles of irregular surface with different shape and<br />
brightness. Close observation of the big particles at higher magnification shows that<br />
there are many small floc-like particles agglomerated on the big particle. Images at<br />
lower magnification (not shown in figure 7.1) show that most of the particles are<br />
within the range of 5-30 �m and this is in reasonable agreement with the particle size<br />
measurements <strong>by</strong> laser diffraction. However, it should be noted that these SEM<br />
pictures provide only semi-quantitative results of particle sizing since the technique<br />
uses two-dimension information to infer a three-dimensional quantity<br />
Figure 7.1. SEM micrographs of the fresh Darco Hg activated carbon at different<br />
magnifications. Scale bar <strong>from</strong> left to right is 10 and 5 �m, respectively.<br />
Figure 7.2 illustrates the difference in information provided <strong>by</strong> secondary<br />
electron (SE) image and backscattered electron (BSE) image. The SE image is<br />
superior for displaying surface detail and particle morphology but does not generally<br />
show chemical heterogeneity. EDX analysis shows that in the BSE image the small<br />
bright spots in the left (area 7) have high iron content, the bright spot in the center on<br />
157
the big particle (area 4) and big bright particle on the up-right corner (area 1) have<br />
high silica content. Area 5 has high content of calcium and the particle is crystal-like.<br />
The carbon particles have similar brightness level as the carbon substrate on the<br />
carbon table and are not clearly seen in the BSE image.<br />
Figure 7.2. SE (left) and BSE (right) images of the fresh Darco Hg sorbent at the<br />
same location. Positions for SEM-EDX analysis are marked on the BSE image. Scale<br />
bar is 30 �m.<br />
As shown in figure 7.3 the morphology of the fresh Darco Hg-LH is very<br />
similar to that of the fresh Darco Hg and this is not surprised since the Darco Hg-LH<br />
is prepared <strong>from</strong> Darco Hg <strong>by</strong> a brominating process.<br />
Figure 7.3. SE images of fresh Darco Hg-LH activated carbon. Scale bar is 5 �m.<br />
158
Compared to the fresh Darco Hg activated carbon there are high contents of<br />
Na, S, and Br in the Darco Hg-LH sample. The average molar ratio of Na/Br is about<br />
1.74, while the molar ratio of Na/(0.5S+Br) is about 1.18, suggesting the sample is<br />
brominated <strong>by</strong> exposing to NaBr and Na2SO4/Na2SO3/Na2S compounds instead of to<br />
HBr or Br2.<br />
The SE images of fresh Sorbalit sorbent are presented in figure 7.4. The<br />
particles are much less porous than the carbon particles. A thin layer of small crystallike<br />
flakes agglomerate on the big particles. The small dots on the background are<br />
<strong>from</strong> the carbon table for sample holding.<br />
Figure 7.4. SE images of fresh Sorbalit sorbent. Scale bar is 5 �m.<br />
The SE images of the fresh Minsorb ME sorbent are shown in figure 7.5. The<br />
particle size is generally lager than carbon particle size and in agreement with the<br />
particle size measurement <strong>by</strong> the laser diffraction.<br />
159
Figure 7.5. SE images of fresh Minsorb ME sorbent. Scale bar is 50 �m.<br />
7.4 Baseline test<br />
As a starting point, baseline tests of the empty glass reactor, quartz wool plug<br />
and sand powder are conducted first to investigate whether mercury can be adsorbed<br />
<strong>by</strong> these parts and materials. Tests are conducted in both nitrogen and simulated<br />
cement kiln flue gas with either elemental mercury or mercury chloride sources. In all<br />
cases, simultaneous mercury breakthroughs are observed indicating no mercury<br />
adsorption is adsorbed <strong>by</strong> these materials. The mercury exposed sand powder is<br />
analyzed for mercury content in the sample. No mercury is detected in the exposed<br />
sand, which again verifies that no mercury adsorption <strong>by</strong> the sand takes place.<br />
7.5 Screening tests in nitrogen<br />
Preliminary tests of some sorbents were conducted <strong>by</strong> mixing 5-10 mg<br />
sorbent with 2 g sand in nitrogen using elemental mercury source. Only elemental<br />
mercury was measured due to the fact that the problem of converter for total mercury<br />
measurement was not solved at that time. Tests at 150�C show that instantaneous<br />
mercury breakthrough was observed for all the sorbents except the bromine treated<br />
Darco Hg-LH carbon. As shown in figure 7.6, even in nitrogen the bromine treated<br />
160
Darco Hg-LH carbon can both oxidize and adsorb some mercury. Compared to<br />
instantaneous mercury breakthrough observed <strong>by</strong> the non-treated Darco Hg carbon,<br />
mercury adsorption <strong>by</strong> the Darco Hg-LH carbon is due to the promoting effects of<br />
bromine in the Darco Hg-LH carbon. Part of the mercury is probably oxidized on the<br />
Darco Hg-LH carbon <strong>by</strong> the bromine compounds. However, most of the mercury is<br />
still in the form of elemental mercury. Figure 7.6 also shows the breakthrough curve<br />
of 10 mg Darco Hg tested in nitrogen at 150�C with HgCl2 source and total mercury<br />
measurement. In contrast to test using elemental mercury source, it takes about 15 h<br />
to reach the breakthrough. These tests indicate that mercury oxidation is an important<br />
step during mercury adsorption <strong>by</strong> the sorbent. Elemental mercury needs to be<br />
oxidized first either in the gas phase or on the sorbent before being adsorbed <strong>by</strong> the<br />
sorbent.<br />
Gaseous Hg, C out /C in<br />
1.2<br />
0.8<br />
0.4<br />
0<br />
1<br />
1, Hg 0 source, 230 �g/Nm 3 , 5 mg<br />
Darco Hg-LH, Hg 0 measurement<br />
2, HgCl 2 source, 209 �g/Nm 3 , 10 mg<br />
Darco Hg, Hg total measurement<br />
0 2 4 6 8 10 12 14 16 18<br />
Time (hour)<br />
Figure 7.6. <strong>Mercury</strong> breakthrough curves at 150 �C for 5 mg Darco Hg-LH carbon<br />
tested in N2 with elemental source and elemental mercury measurement and 10 mg<br />
Darco Hg carbon tested in N2 with HgCl2 source and total mercury measurement. 2 g<br />
sand as bed mixing material.<br />
161<br />
2
7.6 Screening tests in simulated cement kiln flue gas with<br />
elemental mercury source<br />
Total mercury measurement was conducted using the sulfite-based converter<br />
to obtain mercury breakthrough curves using elemental mercury source in the<br />
simulated cement kiln flue gas. Figure 7.7 illustrates the screening results of 30 mg<br />
different sorbents in 2 g sand at 150�C. From the mercury breakthrough curves, the<br />
amount of adsorbed mercury <strong>by</strong> the sorbent and the average initial adsorption rate for<br />
the first 25 min are calculated and presented in table 7.3. The extents of mercury<br />
oxidation <strong>by</strong> different sorbents are illustrated in figure 7.8.<br />
C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
A B C D<br />
0 1 2 3 4 5 6<br />
Time (hour)<br />
162<br />
E<br />
A:Minsorb ME, 169�g/Nm 3 Hg 0<br />
B: Darco Hg, 180�g/Nm 3 Hg 0<br />
C: HOK super, 167�g/Nm 3 Hg 0<br />
D: Sorbalit, 164�g/Nm 3 Hg 0<br />
E: HOK standard, 171�g/Nm 3 Hg 0<br />
F: Darco Hg-LH, 167�g/Nm 3 Hg 0<br />
Figure 7.7. <strong>Mercury</strong> breakthrough profiles for 30 mg sorbets in 2 g sand tested at<br />
150�C in simulated cement kiln flue gas with 164-180 µg Hg 0 /Nm 3 , 1000 ppmv NO,<br />
23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6<br />
vol.% O2.<br />
F
<strong>Mercury</strong> oxidation (%)<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
Darco Hg 16 �m<br />
Darco Hg-LH 15 �m<br />
Figure 7.8. Percentages of mercury oxidation <strong>by</strong> 30 mg sorbets in 2 g sand tested at<br />
150�C in simulated cement kiln flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO,<br />
23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6<br />
vol.% O2.<br />
Table 7.3. <strong>Mercury</strong> 99% breakthrough time, adsorbed mercury and initial adsorption<br />
rates for 30 mg sorbets in 2 g sand tested at 150�C in simulated cement kiln flue gas<br />
with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv<br />
SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
<strong>Sorbent</strong> 99%<br />
breakthrough<br />
time (min)<br />
HOK Super, 24 �m<br />
163<br />
HOK Standard 63 �m<br />
Sobalit super 13 �m<br />
Adsorbed Hg (µg<br />
Hg/g_sorbent)<br />
Minsorb ME 40 �m<br />
Initial adsorption rate<br />
(µg Hg/g_sorbent/h)<br />
Hydroxyapatite 0 0 0<br />
Minsorb DM 0 0 0<br />
Minsorb ME 28 31 70<br />
Sorbalit 110 270 310<br />
HOK super 111 632 690<br />
HOK standard 164 699 560<br />
Darco Hg 90 726 890<br />
Darco Hg-LH 320 1305 690
Neither mercury adsorption nor oxidation is observed <strong>by</strong> 30 mg non-carbon<br />
based sorbents Minsorb DM, hydroxyapatite, and cement materials at 150�C. The<br />
bromine treated carbon Darco Hg-LH has larger adsorption capacity but smaller<br />
adsorption rate compared to the non-treaded Darco Hg carbon. As shown in figure<br />
7.8 and table 7.3, there is a clear trend between the extent of mercury oxidation and<br />
amount of adsorbed mercury. Generally larger amount of adsorbed mercury is<br />
obtained with sorbents that have larger mercury oxidation capacity. The initial<br />
adsorption rate of coarse HOK standard carbon is slightly smaller than the fine HOK<br />
super due to the larger diffusion resistance within the larger carbon particles. Sorbalit,<br />
which is a mixture of lime and carbon, shows poorer performance than the carbons.<br />
Minsorb ME, which is aluminumsilicates based sorbent shows the poorest<br />
performance among the tested commercial sorbents despite that it has much larger<br />
surface area than the Sorbalit sorbent. This is probably due to its capacity for mercury<br />
oxidation is much smaller than the Sorbalit sorbent.<br />
The adsorption of mercury in the Darco Hg carbon is attempted <strong>by</strong> analyzing<br />
the mercury content in the exposed carbon sample. Table 7.4 compares the measured<br />
and calculated mercury contents in the carbons <strong>from</strong> the breakthrough curve. The<br />
calculated mercury contents are much larger than the measured values for the carbon<br />
and sand mixtures. The analysis of mercury content in the sample uses only 100 mg<br />
of the sample for analysis and one reason for the disagreement could be that the<br />
sample analyzed might not be representative. Only 30 mg carbon is mixed with 2 g<br />
sand and the carbon may separate <strong>from</strong> the sand. This is often observed during<br />
loading the sample to the reactor. To ensure most of the carbon is loaded to the<br />
reactor, the sample holder is shaken to remove the carbon deposited on the sample<br />
holder and a big quartz wool plug is used to clean carbon deposited on the reactor<br />
wall and move the carbon to the fixed-bed bed. The carbon particle might deposit on<br />
the container wall and therefore the analyzed sample could contain relatively more<br />
sand powder. The mercury content in the carbon is calculated <strong>from</strong> the measured<br />
mercury level in the carbon-sand mixture and the carbon-sand mixing ratio. Since no<br />
mercury adsorbed <strong>by</strong> the sand powder, analysis using non-representative carbon-sand<br />
mixture could result in small mercury content in the carbon.<br />
164
Table 7.4. Comparison of measured and calculated mercury contents in the carbons<br />
<strong>from</strong> the breakthrough curve. Flue gas composition: 141-183 µg Hg 0 /Nm 3 , 1000<br />
ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.%<br />
CO2, and 6 vol.% O2.<br />
<strong>Sorbent</strong> description Measured Hg in Calculated results <strong>from</strong><br />
carbon (ppmm) breakthrough (ppmm)<br />
30 mg Darco Hg in 2g sand, 150°C,<br />
141 µg/Nm 3 Hg in flue gas<br />
2.22 5.96<br />
30 mg Darco Hg in 2g sand, 150°C,<br />
183 µg/Nm 3 Hg in flue gas<br />
2.27 13.14<br />
30 mg Darco Hg in 2g sand, 150°C.<br />
143 µg/Nm 3 Hg in flue gas<br />
1.49 7.06<br />
500 mg Darco Hg, 200°C, 160 µg/Nm 3<br />
Hg in flue gas<br />
46.79 50.87<br />
To check whether the method for calculating the mercury content in the<br />
carbon is reasonable, a new test was performed <strong>by</strong> using only carbon sample to avoid<br />
the problem of non-representative sample caused <strong>by</strong> carbon-sand mixing. As shown<br />
in table 7.4, the measured mercury content in the carbon is about 92% of the<br />
calculated value <strong>from</strong> the breakthrough curve. This reasonable agreement between the<br />
measured and calculated value confirms that the method of calculating mercury<br />
content in the carbon <strong>from</strong> the breakthrough curve works to a satisfactory extent.<br />
To be able to observe some mercury adsorption <strong>by</strong> the cement materials, the<br />
adsorption temperature was decreased to 75�C. However, still no mercury adsorption<br />
was observed <strong>by</strong> 30 mg cement materials at 75�C. Then the sorbent load is increased<br />
to 2 g. Among the tested cement materials only raw meal shows some mercury<br />
adsorption as shown in figure 7.9. This can to some extent explain the low mercury<br />
emission <strong>from</strong> cement plants during raw mill-on period. The dust load in the flue gas<br />
after the raw mill could be up to 800-1000 g/m 3 and therefore noticeable amount of<br />
mercury could be adsorbed <strong>by</strong> the raw meal both in and after the raw mill.<br />
165
Gaseous Hg (�g/Nm 3 )<br />
180<br />
160<br />
140<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
<strong>by</strong>pass<br />
reactor<br />
through reactor<br />
Gaseous Hg total<br />
Calculated Hg in sorbent<br />
0 30 60 90 120 150<br />
Time (min)<br />
166<br />
0.0016<br />
0.0012<br />
0.0008<br />
0.0004<br />
Figure 7.9. <strong>Mercury</strong> breakthrough profile and calculated mercury adsorption in 2 g<br />
cement raw meal tested at 75�C in simulated cement kiln flue gas with 160-170 µg<br />
Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.%<br />
H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
7.7 Screening tests in simulated cement kiln flue gas with HgCl2<br />
source<br />
The collected sorbents are also tested in simulated cement kiln flue gas with<br />
HgCl2 source. Figure 7.10 shows the mercury breakthrough curves for 10 mg<br />
sorbents in 2 g sand at 150�C using simulated cement kiln flue gas with170±10<br />
µg/Nm 3 mercury <strong>from</strong> HgCl2 source. The 99% breakthrough time, calculated amount<br />
of adsorbed mercury <strong>by</strong> the sorbent <strong>from</strong> the breakthrough curve, and the average<br />
initial adsorption rate for the first 25 min are presented in table 7.5.<br />
0<br />
Calculated Hg in sorbent (mg Hg/g_sorbent)
Figure 7.10. <strong>Mercury</strong> breakthrough profiles of 10 mg sorbents tested in 2g sand at<br />
150�C in simulated cement kiln flue gas with HgCl2 source. The inlet mercury level<br />
is 170±10 µg/Nm 3 , other gases include 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl,<br />
1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
Table 7.5. <strong>Mercury</strong> 99% breakthrough time, adsorbed mercury and initial adsorption<br />
rates for 10 mg sorbets in 2 g sand tested at 150�C in simulated cement kiln flue gas<br />
using HgCl2 source. The inlet mercury level is 170±10 µg/Nm 3 , other gases include<br />
1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21<br />
vol.% CO2, and 6 vol.% O2.<br />
<strong>Sorbent</strong> 99%<br />
breakthrough<br />
time (min)<br />
Adsorbed Hg (µg<br />
Hg/g_sorbent)<br />
167<br />
Initial adsorption rate<br />
(µg Hg/g_sorbent/h)<br />
Hydroxyapatite 0 0 -<br />
Minsorb DM 5 12 -<br />
Minsorb ME 49 363 780<br />
Sorbalit 86 429 730<br />
HOK standard 74 1021 1390<br />
HOK super 55 1153 2170<br />
Darco Hg 43 1224 2510<br />
Darco Hg-LH 52 1290 2510
The hydroxyapatite sorbent still does not adsorb any mercury even using the<br />
mercury chloride source. This means that the mercury adsorption <strong>by</strong> hydroxyapatite<br />
is not only limited <strong>by</strong> its ability of oxidizing mercury, but also other properties. The<br />
Minsorb DM sorbent shows low adsorption of HgCl2 compared to no adsorption of<br />
elemental mercury. Minsorb ME and Sorbalit show similar mercury adsorption in<br />
terms of mercury adsorption capacity and initial adsorption rate. All the carbons show<br />
similar mercury adsorption capacity; while the HOK standard has the smallest initial<br />
adsorption rate. Compared to similar initial adsorption rate of elemental mercury, the<br />
initial adsorption rate of HgCl2 for HOK super is about 50% larger than the HOK<br />
standard. The elemental mercury adsorption capacity of Darco Hg-LH is about 79%<br />
larger and initial Hg 0 adsorption rate is about 23% smaller in comparison with the<br />
virgin Darco Hg carbon. Similar HgCl2 adsorption capacity and initial adsorption rate<br />
of Darco Hg and Darco Hg-LH indicate that Darco Hg is a better choice at least for<br />
removing HgCl2 <strong>from</strong> cement kiln flue gas.<br />
<strong>Cement</strong> materials were also tested for HgCl2 capture <strong>from</strong> simulated cement<br />
kiln flue gas using a sorbent load of 2 g without mixing with sand powder. Figure<br />
7.11 presents the mercury breakthrough curves of 2 g cement materials tested at<br />
150�C using simulated cement kiln flue gas with170±10 µg/Nm 3 mercury <strong>from</strong><br />
HgCl2 source. The 99% breakthrough time and calculated amount of adsorbed<br />
mercury <strong>by</strong> the sorbent <strong>from</strong> the breakthrough curve are given in table 7.7. The<br />
fluctuation of some breakthrough curves is due to the aging of the sulfite-based<br />
converter material. After changing the converter material used for about 3 months<br />
smooth mercury breakthrough is obtained again.<br />
168
Gaseous Hg (C out /C in )<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
A B<br />
F<br />
E<br />
C<br />
0 0.2 0.4 0.6 0.8 1<br />
Time (hour)<br />
D<br />
A: 2 g hydrated lime<br />
B: 2 g clay<br />
C: 2 g kaolin<br />
D: 10 mg Darco Hg in 2 g sand<br />
E: 2 g cement kiln dust<br />
F: 2 g gypsum<br />
Figure 7.11. <strong>Mercury</strong> breakthrough profiles of 2 g cement materials tested at 150�C<br />
in simulated cement kiln flue gas with HgCl2 source. The inlet mercury level is<br />
170±10 µg/Nm 3 , other gases include 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl,<br />
1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. Data of 10 mg Darco<br />
Hg carbon in 2 g sand are shown for comparison.<br />
Table 7.7. <strong>Mercury</strong> breakthrough time and adsorbed mercury for 2 g cement materials<br />
tested at 150�C in simulated cement kiln flue gas using HgCl2 source. The inlet<br />
mercury level is 170±10 µg/Nm 3 , other gases include 1000 ppmv NO, 23 ppmv NO2,<br />
10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
<strong>Sorbent</strong>s Breakthrough time (min) Adsorbed Hg (µg<br />
Hg/g_sorbent)<br />
Hydrated lime 11 0.60<br />
Clay 14 0.90<br />
Kaolin 23 1.77<br />
<strong>Cement</strong> kiln dust 60 1.81<br />
Gypsum 49 1.73<br />
Raw meal 54 0.90<br />
Saklei fly ash 38 1.01<br />
Portland cement 60 2.28<br />
169<br />
Gaseous Hg (C out /C in )<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
G<br />
I<br />
H<br />
D: 10 mg Darco Hg in 2 g sand<br />
G: 2 g raw meal<br />
H: 2 g Portland cement<br />
I: 2 g Saklei fly ash<br />
0 0.2 0.4 0.6 0.8 1<br />
Time (hour)<br />
D
Compared to null adsorption of elemental mercury at temperature as low as<br />
75�C, all the tested cement materials show some adsorption of mercury chloride<br />
at150�C. The lack of ability for mercury oxidation is probably the main limitation for<br />
these materials to be used for mercury removal <strong>from</strong> flue gas, which contains both<br />
elemental and oxidized mercury. 2 g of kaolin, cement kiln dust, gypsum, and<br />
Portland cement adsorb similar amount of mercury chloride as 10 mg Minsorb ME<br />
and Sorbalit commercial sorbents. Raw meal, hydrated lime, clay, Saklei fly ash have<br />
similar mercury adsorption capacities which are about half of the capacities of kaolin,<br />
cement kiln dust, gypsum, and Portland cement. These results can further explain low<br />
mercury emission <strong>from</strong> the cement plant during raw mill-on period, as about 55-65 %<br />
of the mercury in the cement kiln flue gas is oxidized mercury [35]. Considering the<br />
low cost and abundance of the cement materials, injection of cement materials for<br />
mercury control in cement plant is feasible provided that the elemental mercury in the<br />
flue gas can be oxidized <strong>by</strong> adding an oxidant. However, raw meal, hydrated lime,<br />
clay, cement kiln dust, and kaolin have to be recycled to the kiln in the cement<br />
production and the adsorbed mercury will be released again in the hot zone. If these<br />
materials are not recycled the disposal cost will be high since larger amount of these<br />
materials have to be used compared to activated carbon for same amount of mercury<br />
removal. Gypsum and Saklei fly ash can be added to the final cement product and the<br />
release of captured mercury and high disposal cost are avoided. This also applied for<br />
the Portland cement. However, the stability of mercury in the final cement product<br />
requires further investigation.<br />
7.8 Conclusions<br />
Screening tests of sorbents for mercury removal <strong>from</strong> cement plants have<br />
been conducted in the fixed-bed reactor system. The tested sorbents include<br />
commercial activated carbons, commercial non-carbon sorbents, and cement<br />
materials. Screening measurements are used to evaluate initial mercury capture rate,<br />
oxidation potential, and capacity for the selected sorbents. The amount of mercury<br />
adsorbed is calculated <strong>from</strong> the mercury breakthrough curve and the initial mercury<br />
170
adsorption rate is further evaluated for application regarding sorbent injection<br />
upstream of a fabric filter.<br />
Baseline tests of empty reactor, quartz wool plug, and sand powder show that<br />
no mercury adsorption is observed either in nitrogen or simulated cement kiln flue<br />
gas with elemental mercury or mercury chloride source.<br />
Initial tests of sorbent in nitrogen with elemental mercury at 150�C find that<br />
only the bromine treated Darco Hg-LH activated carbon shows some mercury<br />
adsorption among the collected sorbents. However, the virgin Darco Hg carbon<br />
adsorbs mercury chloride in nitrogen. This indicates that mercury oxidation is an<br />
important factor for mercury adsorption <strong>by</strong> the sorbents. Elemental mercury needs to<br />
be oxidized either in the flue gas or on the sorbent.<br />
Tests a collection of sorbents (30 mg in 2 g sand) at 150�C in simulated<br />
cement kiln flue gas with elemental mercury show that no mercury adsorption or<br />
oxidation takes place on the non-carbon based sorbents Minsorb DM, hydroxyapatite,<br />
and cement materials. The mercury adsorption capacity of bromine treated carbon<br />
Darco Hg-LH is 79% larger than the non-treated Darco Hg carbon, but the initial<br />
adsorption rate is 23% smaller. Generally a larger amount of adsorbed mercury is<br />
obtained with sorbents that have larger mercury oxidation capacity. A lower amount<br />
of mercury is adsorbed <strong>by</strong> the HOK carbon compared to Darco Hg carbon, probably<br />
be due to both the smaller surface area and mercury oxidation capacity of the HOK<br />
carbon. The initial adsorption rate of coarse HOK standard carbon is slightly lower<br />
than the fine HOK super due to the larger diffusion resistance within the larger HOK<br />
standard carbon particles. Sorbalit shows poorer performance than the carbons, while<br />
Minsorb ME shows the poorest performance among the tested commercial sorbents<br />
despite that it has much larger surface area than the Sorbalit sorbent. Among the<br />
tested sorbents Darco Hg has the largest initial adsorption rate of elemental mercury.<br />
The collected sorbents are also tested in simulated cement kiln flue gas with<br />
mercury chloride using 10 mg sorbents in 2 g sand at 150�C. The hydroxyapatite<br />
sorbent still does not adsorb any mercury. The Minsorb DM sorbent shows negligible<br />
adsorption of HgCl2 compared to no adsorption of elemental mercury. Minsorb ME<br />
and Sorbalit show similar mercury adsorption of about 400 µg Hg/g_sorbent. All the<br />
171
carbons show similar mercury adsorption capacity; while the HOK standard has the<br />
smallest initial adsorption rate. Similar HgCl2 adsorption capacity and initial<br />
adsorption rate of Darco Hg and Darco Hg-LH indicate that Darco Hg is a better<br />
choice at least for removing HgCl2 <strong>from</strong> cement kiln flue gas.<br />
Compared to non-observable adsorption of elemental mercury on 30 mg<br />
sample at temperature as low as 75�C, all the tested cement materials show some<br />
adsorption of mercury chloride at 150�C using a sorbent load of 2 g. Similar amount<br />
of mercury chloride adsorption is observed <strong>by</strong> 2 g of kaolin, cement kiln dust,<br />
gypsum, Portland cement, and 10 mg Minsorb ME, Sorbalit commercial sorbents.<br />
Among the tested sorbents the Darco Hg activated shows the best<br />
performance of adsorption of both elemental and oxidized mercury, with the largest<br />
initial adsorption rate and second largest mercury adsorption capacity and a lower<br />
price than the treated carbon. Therefore, the Darco Hg carbon is recommended as the<br />
reference sorbent for a fundamental investigation of mercury adsorption in simulated<br />
cement kiln flue gas and large-scale tests. Adsorption <strong>by</strong> cement materials at larger<br />
load can explain the phenomena of low mercury emission <strong>from</strong> the cement plant<br />
during raw mill-on period, when larger amount of cement materials are present in the<br />
flue gas at relative low temperature. Considering the low cost and abundance of the<br />
cement materials, injection of cement materials for mercury control in cement plant is<br />
feasible provided that the elemental mercury in the flue gas can be oxidized <strong>by</strong><br />
adding of oxidant. Compared to raw meal, clay, kaolin, and cement kiln dust, gypsum,<br />
Saklei fly ash, and Portland cement are more preferred due to avoidance of captured<br />
mercury release in the kiln and high disposal cost since these materials will be added<br />
to the finished cement product. However, the stability of mercury in the exposed<br />
cement materials requires further investigation to study whether it will be released<br />
<strong>from</strong> the final cement product.<br />
7.9 References<br />
[1] H.C. Hsi, S. Chen, M. Rostam-Abadi, M.J. Rood, C.F. Richardson, T.R. Carey, R. Chang,<br />
Preparation and evaluation of coal-derived activated carbons for removal of mercury vapor<br />
<strong>from</strong> simulated coal combustion flue gases, Energy & Fuels. 12 (1998) 1061-1070.<br />
172
[2] M. Rostam-Abadi, S.G. Chen, H.C. Hsi, M. Rood, R. Chang, T.R. Carey, B. Hargrove, C.<br />
Richardson, W. Rosenhoover, F. Meserole, Novel vapor phase mercury sorbents,<br />
Proceedings of the EPRI-DOE-EPA Combined Utility Air Pollutant Control, Washington,<br />
DC, Aug 25–29, 1997.<br />
[3] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang and F.B. Meserole, Factors<br />
affecting mercury control in utility flue gas using sorbent injection, Proceedings of the Air &<br />
Waste Management Association's 90th Annual Meeting, Toronto, Ontario, Canada, June 8-13,<br />
1997.<br />
[4] C.X. Hu, J.S. Zhou, Z.Y. Luo, S. He, G.K. Wang, K.F. Cen, Effect of oxidation treatment<br />
on the adsorption and the stability of mercury on activated carbon, Journal of Environmental<br />
Sciences-China. 18 (2006) 1161-1166.<br />
[5] B. Ghorishi and B.K. Gullett, Fixed-bed control of mercury: Role of acid gases and a<br />
comparison between carbon-based, calcium-based, and coal fly ash sorbents, Proceedings of<br />
the EPRI/DOE/EPA Combined Utility Air Pollutant Control Symposium, Washington, DC,<br />
August 25–29, 1997.<br />
[6] S.B. Ghorishi and C.B. Sedman, Combined mercury and sulfur oxides control using<br />
calcium-based sorbents, Proceedings of the EPRI-DOE-EPA Combined Utility Air Pollutant<br />
Control Symposium, Washington, DC, August 25–29, 1997.<br />
[7] S.B. Ghorishi and B.K. Gullett, An experimental study on mercury sorption <strong>by</strong> activated<br />
carbons and calcium hydroxide, The Fifth Annual North American Waste-to-Energy<br />
Conference, Research Triangle Park, NC, 22-25 April, 1997.<br />
[8] N. Hutson, C. Singer, C. Richardson, J. Karwowski and C. Sedman, Practical applications<br />
<strong>from</strong> observations of mercury oxidation and binding mechanisms, EPRI-DOE-EPA-AWMA<br />
Combined Power Plant Air Pollutant Control Mega Symposium, Washington DC, August 30<br />
- September 2, 2004.<br />
[9] E.J. Granite, M.C. Freeman, R.A. Hargis, W.J. O'Dowd, H.W. Pennline, The thief process<br />
for mercury removal <strong>from</strong> flue gas, J. Environ. Manage. 84 (2007) 628-634.<br />
[10] W.J. O’Dowd, H.W. Pennline, M.C. Freeman, E.J. Granite, R.A. Hargis, C.J. Lacher, K.<br />
Andrew, A technique to control mercury <strong>from</strong> flue gas: The thief process, Fuel Processing<br />
Technology. 87 (2006) 1071-1084.<br />
[11] R. Bhardwaj. Impact of temperature and flue gas components on mercury speciation and<br />
uptake <strong>by</strong> activated carbon sorbents . Master thesis. University of Pittsburgh, 2007.<br />
[12] M.M. Maroto-Valer, Y. Zhang, E.J. Granite, Z. Tang, H.W. Pennline, Effect of porous<br />
structure and surface functionality on the mercury capacity of a fly ash carbon and its<br />
activated sample, Fuel. 84 (2005) 105-108.<br />
[13] S. Eswaran, H.G. Stenger, Z. Fan, Gas-phase mercury adsorption rate studies, Energy &<br />
Fuels. 21 (2007) 852-857.<br />
[14] J.R. Butz, T.E. Broderick and C.S. Turchi, Amended Silicates for <strong>Mercury</strong> Control,<br />
project final report, DOE Award Number: DE-FC26-04NT41988, 2006.<br />
173
[15] J.H. Pavlish, E.A. Sondreal, M.D. Mann, E.S. Olson, K.C. Galbreath, D.L. Laudal, S.A.<br />
Benson, Status review of mercury control options for coal-fired power plants, Fuel<br />
Processing Technology. 82 (2003) 89-165.<br />
[16] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang, F.B. Meserole, Factors<br />
affecting mercury control in utility flue gas using activated carbon, Journal of the Air &<br />
Waste Management Association. 48 (1998) 1166.<br />
[17] N. Hutson, C. Singer, C. Richardson and J. Karwowski, Practical applications <strong>from</strong><br />
observations of mercury oxidation and binding mechanisms, The fifth mega symposium on<br />
air pollutant controls for power plants, Washington, DC, August, 2004.<br />
[18] S. Sjostrom, Evaluation of sorbent injection for mercury control: Topical report for DTE<br />
Energy’s Monroe Station, DOE Award Number DE-FC26-03NT41986, Report Number<br />
41986R16, 2006.<br />
[19] S. Sjostrom, Evaluation of sorbent injection for mercury control: Topical report for<br />
Sunflower Electric’s Holcomb Station, DE-FC26-03NT41986, Topical Report No. 41986R07,<br />
2005.<br />
[20] S. Sjostrom, Evaluation of sorbent injection for mercury control: Topical report for<br />
AmerenUE’s Meramec Station Unit 2, DE-FC26-03NT41986, Topical Report No. 41986R09,<br />
2005.<br />
[21] S. Sjostrom, Evaluation of sorbent injection for mercury control: Topical report for<br />
Basin Electric Power Cooperative’s Laramie River Station, DE-FC26-03NT41986, Topical<br />
Report No. 41986R11, 2006.<br />
[22] ADA-ES, Field test program to develop comprehensive design, operating, and cost data<br />
for mercury control systems: Final site report for E.C. Gaston Unit 3 sorbent injection into<br />
COHPAC for mercury control, DE-FC26-00NT41005, Topical Report No. 41005R11, 2003.<br />
[23] ADA-ES, Field test program to develop comprehensive design, operating, and cost data<br />
for mercury control systems: Final site report for Brayton Point Generating Station Unit 1<br />
sorbent injection into a cold-side ESP for mercury control, DE-FC26-00NT41005, Topical<br />
Report No. 41005R21, 2005.<br />
[24] ADA-ES, Field test program to develop comprehensive design, operating, and cost data<br />
for mercury control systems: Final site report for PG&E NEG Salem Harbor Station Unit 1<br />
sorbent injection into a cold-side ESP for mercury control, U.S. DOE Cooperative Agreement<br />
No. DE-FC26-00NT41005, 2004.<br />
[25] J. Wirling, Implementation of process-integrated waste gas cleaning using activated<br />
lignite, A&WMA specialty conference on hazardous waste combustors, Kansas City, Kansas,<br />
March 28-30, 2001, .<br />
[26] J. Wirling, Adsorptive waste gas cleaning in an industrial-scale coal-fired power plant,<br />
A&WMA specialty conference on mercury emissions, Chicago, Illinos, August 21-23, 2001, .<br />
[27] J. Wirling, J. Jablonski, Safety aspects in the use of carbonaceous sorbents during waste<br />
gas treatment, Metallurgical Plant and Technology. 3 (2007) 144.<br />
174
[28] A. Licata, M. Babu and L. Nethe, Acid gases, mercury, and dioxin <strong>from</strong> MWCs,<br />
National Waste Processing Conference Proceedings ASME, Boston, Massachusetts, June 5-8,<br />
1994.<br />
[29] Lhoist Group, Data sheet of Minsorb ® DM sorbent, 2010.<br />
[30] Lhoist Group, Data sheet of Minsorb ® ME sorbent, 2010.<br />
[31] Lhoist Group, Sorbacal flue gas treatment, http://www.sorbacal.com/, accessed January<br />
25, 2011.<br />
[32] J.I. Bhatty, F.M. Miller, S.H. Kosmatka, (Eds.), Innovations in Portland cement<br />
manufacturing, Portland <strong>Cement</strong> Association, Skokie, Illinois, U.S.A, 2004.<br />
[33] U.S. EPA, Materials characterization paper in support of the proposed rulemaking:<br />
Identification of nonhazardous secondary materials that are solid waste-cement kiln dust<br />
(CKD), 2010.<br />
[34] C. Verwilghen, S. Rio, J. Ramaroson, A. Nzihou and P. Sharrock, The use of<br />
hydroxyapatite for the removal of heavy metals <strong>from</strong> industrial flue gas PART B:<br />
Investigation in pilot scale, DustConf 2007, Maastricht, The Netherlands, April 23-24, 2007.<br />
[35] R.J. Schreiber and C.D. Kellett, Compilation of mercury emission data, PCA R&D<br />
Serial No. SN3091, 2009.<br />
175
Fundamental investigation of elemental<br />
mercury adsorption <strong>by</strong> activated carbon in<br />
simulated cement kiln flue gas<br />
This chapter reports a fundamental investigation of elemental mercury<br />
adsorption <strong>by</strong> Darco Hg activated carbon in simulated cement kiln flue gas. The<br />
investigation includes the effects of temperature and gas composition on mercury<br />
adsorption kinetics and equilibrium uptake.<br />
8.1 Introduction<br />
The investigation is mainly conducted using 10 mg Darco Hg mixed with 2 g<br />
sand in simulated cement flue gas of a baseline composition of 1000 ppmv NO, 23<br />
ppmv NO2, 1000 ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 21 vol.% CO2, 6 vol.% O2.<br />
Table 8.1 shows the investigated parameters and the tested ranges. The idea of using<br />
wide range of parameters is to simulate possible cement kiln flue gas composition<br />
and derive kinetics correlations that can be used to predict mercury adsorption <strong>by</strong><br />
activated carbon under different conditions. The isotherms and kinetics are obtained<br />
using different adsorption temperatures, mercury inlet levels, and gas composition.<br />
The percentages of mercury oxidation are also investigated <strong>by</strong> measuring the<br />
elemental mercury level after the complete mercury breakthrough is obtained and<br />
comparison with inlet elemental mercury concentration.<br />
Another commercial carbon, Norit RB4, is also investigated to the study the<br />
effects of carbon particle size. The granular Norit RB4 has a diameter of 4 mm and<br />
length of 10 mm, respectively. The Norit RB4 pellet has a surface area of 1060-1320<br />
m 2 /g, a microporous volume of 0.41-0.54 cm 3 /g, 0.8% of water, and 5.6% of ash [1,2].<br />
The pellets are crushed and sieved for studying the effects of particle size.<br />
176<br />
8
Table. 8.1. Parameters for lab-scale fundamental investigation of elemental mercury<br />
adsorption <strong>by</strong> the activated carbon.<br />
Parameters Baseline values Range tested<br />
Flue gas rate (Nl/min) 2.75 1.1-2.75<br />
Adsorption temperature (�C) 150 75-250<br />
Gas composition<br />
Hg 0 (µg/Nm 3 ) 160-170 0-170<br />
NO (ppmv) 1000 100-1000<br />
NO2 (ppmv) 23 0-100<br />
SO2 (ppmv) 1000 100-1000<br />
HCl (ppmv) 10 0-20<br />
CO (ppmv) 0 0-1000<br />
H2O (vol.%) 1 0-15<br />
CO2 (vol.%) 21 1-31<br />
O2 (vol.%) 6 1-16<br />
8.2 Effect of adsorption temperature<br />
Figure 8.1 shows the effect of adsorption temperature on mercury<br />
breakthrough profiles after the carbon bed. As expected, faster mercury breakthrough<br />
is obtained at higher adsorption temperature. This pronounced effect of temperature<br />
on the mercury adsorption capacity of the activated carbon evidences a physical<br />
adsorption mechanism between the mercury and Darco Hg carbon. Physical<br />
adsorption <strong>from</strong> the gas phase is accompanied <strong>by</strong> a decrease in free energy of the<br />
system [3,4]. The gaseous molecules in the adsorbed state have fewer degrees of<br />
freedom than in the gaseous state. This results in a decrease in entropy during<br />
adsorption. Using the thermodynamic relationship:<br />
�G ��H �T� S<br />
(8.1)<br />
It follows that the term ΔH, which is the heat of adsorption, must be negative<br />
indicating that adsorption is always an exothermic process, respective of the nature of<br />
the forces involved in the adsorption process.<br />
177
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
1 2 3<br />
0 0.5 1 1.5 2 2.5 3 3.5 4<br />
Time (hour)<br />
178<br />
1: 150 o C<br />
2:120 o C<br />
3:75 o C<br />
Figure 8.1. Effect of adsorption temperature on mercury breakthrough of 10 mg<br />
Darco Hg mixed with 2 g sand using 2.75 Nl/min simulated flue gas with 160-170 µg<br />
Hg 0 /Nm 3 ,1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.%<br />
H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
The effect of temperature on the extent of mercury oxidation <strong>by</strong> the Darco Hg<br />
carbon is present in figure 8.2. The adsorption temperature does not affect the<br />
oxidation of mercury <strong>by</strong> the Darco Hg carbon. The mercury oxidation is always<br />
larger than 92% and the average mercury oxidation percentage is about 97% in the<br />
studied temperature range of 75-250�C.
<strong>Mercury</strong> oxidation (%)<br />
110<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
50 100 150 200 250 300<br />
Temperature ( 0 C)<br />
Figure 8.2. Effect of adsorption temperature on mercury oxidation <strong>by</strong> 10 mg Darco<br />
Hg mixed with 2 g sand using 2.75 Nl/min simulated flue gas with 160-170 µg<br />
Hg 0 /Nm 3 ,1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.%<br />
H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
8.3 Isotherm tests<br />
In order to simulate the performance of a given sorbent, the adsorption<br />
equilibrium information such as the isotherm and characteristics of the sorbent must<br />
be known. The adsorption isotherm is the most extensively employed method for<br />
representing the equilibrium states of an adsorption system [3,4]. It can give useful<br />
information regarding the adsorbate, the adsorbent, and the adsorption process. It<br />
helps in the determination of the heat of adsorption, and the relative absorbability of a<br />
gas on a given adsorbent.<br />
<strong>Sorbent</strong> equilibrium data can be generated <strong>by</strong> conducting adsorption<br />
breakthrough tests in the fixed-bed reactor. Figure 8.3 illustrates the mercury<br />
breakthrough curves of 10 mg Norit Hg activated carbon tested at 120�C with<br />
different elemental mercury inlet levels. The time necessary for saturation of 10 mg<br />
carbon is in the order of 0.6-1.2 h for the elemental mercury inlet level of 27-95<br />
µg/Nm 3 . It takes longer time to reach the complete breakthrough when the mercury<br />
inlet level is lower. This is in agreement with the observation <strong>by</strong> Karatza et al. [5] that<br />
179
the saturation time decreased when the inlet mercury level was increased <strong>from</strong> 1 to<br />
5.5 mg/m 3 . The driving force of mercury adsorption is the difference between the<br />
amount of adsorbed mercury <strong>by</strong> unit carbon at a particular mercury inlet<br />
concentration and the theoretical amount of mercury that could be adsorbed <strong>by</strong> unit<br />
carbon at that concentration and this driving force disappears when the adsorption<br />
gradually approaches its equilibrium state. Initially the rate of adsorption is large as<br />
the whole carbon surface is bare but as more and more of the surface becomes<br />
covered <strong>by</strong> the mercury molecules, the available bare surface decreases and so does<br />
the rate of adsorption. The driving force theory can therefore explain the sigmoidal<br />
shape of the breakthrough curve. The driving force for higher mercury inlet level is<br />
larger at the initial stage of the adsorption (the first 12 min as shown in figure 8.3)<br />
and becomes smaller and similar for all the applied mercury inlet levels due to the<br />
accumulation of mercury in the carbon. As a result, faster mercury breakthrough is<br />
obtained for larger mercury inlet concentration.<br />
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 0.2 0.4 0.6 0.8 1 1.2<br />
Time (hour)<br />
180<br />
1 2 3<br />
1: 95 �g/Nm 3 Hg 0<br />
2: 57 �g/Nm 3 Hg 0<br />
3: 27 �g/Nm 3 Hg 0<br />
Figure 8.3. Effect of elemental mercury inlet level on mercury breakthrough of 10 mg<br />
Darco Hg mixed with 2 g sand tested at 120�C using 2.75 Nl/min simulated flue gas<br />
with 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21<br />
vol.% CO2, and 6 vol.% O2.<br />
The isotherm studies are conducted at 75, 100, 120, and 150�C. As shown in<br />
figure 8.4 there is a linear correlation between the amounts of mercury adsorbed on<br />
the unit mass of carbon and the inlet mercury concentrations. For the mercury levels
applied in this work (
plots lnk as a function of the reciprocal of temperature. Since adsorption is<br />
exothermic, the Henry constant decreases with temperature. There is a linear relation<br />
between lnk and 1/T and <strong>from</strong> the slope and intercept the calculated value for k0 and<br />
�H ads is 0.869 m 3 /g and -8543 J/mol, respectively. In the work of Karatza et al. [5] a<br />
heat of adsorption of -22000 J/mol was found for Darco G60 activated carbon tested<br />
in nitrogen. The Darco G60 carbon has surface area of 600 m 2 /g and is typically used<br />
for treating fine chemicals and pharmaceutical intermediates [9]. Calculated binding<br />
energy of elemental mercury on activated carbon at room temperature using density<br />
functional theory and fused-benzene ring cluster approach is -18100 J/mol [10].<br />
Effects of other flue gas constituents have not been considered in the simulations. The<br />
derived heat of adsorption for Darco Hg carbon in simulated cement kiln flue gas is<br />
about half of both the experimental data for Darco G60 in nitrogen and theoretical<br />
calculation of binding energy of elemental mercury on activated carbon in nitrogen at<br />
room temperature.<br />
ln(k)<br />
-4<br />
-4.2<br />
-4.4<br />
-4.6<br />
Data<br />
Y=1027.4881X-7.0484<br />
R 2 =0.93<br />
-4.8<br />
0.0022 0.0024 0.0026 0.0028 0.003<br />
1/T (1/K)<br />
Figure 8.5. Plot of lnk as a function of 1/T. The Henry’s constants k are derived <strong>from</strong><br />
isotherms at 75, 100, 120, and 150�C. 10 mg Darco Hg mixed with 2 g sand is tested<br />
using 2.75 Nl/min simulated flue gas with 1000 ppmv NO, 23 ppmv NO2, 10 ppmv<br />
HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
The oxidation of mercury <strong>by</strong> the Darco Hg carbon seems not to be affected <strong>by</strong><br />
the elemental mercury inlet level, as shown in figure 8.6. The mercury oxidation is<br />
182
always larger than 90% and the average mercury oxidation percentage is about 96%<br />
in the studied elemental mercury inlet level of 18-180 µg/Nm 3 . While almost no<br />
mercury oxidation takes place on the activated carbon tested in nitrogen. It is<br />
expected that the heat of adsorption is different for elemental mercury and oxidized<br />
mercury adsorption <strong>by</strong> the carbon. This might explain the difference between the<br />
derived heat of adsorption <strong>from</strong> this work and both the experimental data for Darco<br />
G60 in nitrogen and theoretical calculation of binding energy of elemental mercury<br />
on activated carbon in nitrogen.<br />
<strong>Mercury</strong> oxidation (%)<br />
110<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 40 80 120 160 200<br />
Hg inlet concentration (�g/Nm 3 )<br />
Figure 8.6. Effect of elemental mercury inlet level on mercury oxidation <strong>by</strong> 10 mg<br />
Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated flue gas<br />
with 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21<br />
vol.% CO2, and 6 vol.% O2.<br />
8.4 Effect of carbon particle size<br />
The Darco Hg carbon has a mean diameter of only 16 µm. To be able to<br />
observe the influence of carbon particle size on mercury adsorption, the Norit RB4<br />
pellets are crushed and sieved to fractions having mean diameter of 38, 98, 165, and<br />
325 µm. Figure 8.7 presents the mercury breakthrough curves for crushed Norit RB4<br />
carbon with different particle sizes tested at 150�C using simulated cement kiln flue<br />
gas. Figure 8.8 further illustrates the effect of carbon particle size on the percentage<br />
of mercury oxidation and initial adsorption rate. Faster mercury breakthrough is<br />
183
observed for smaller carbon particle. The final mercury adsorption capacity is the<br />
same at a value of 0.725 µg Hg/mg_carbon, which is about 65% of the Darco Hg<br />
adsorption capacity at 150�C. Higher mercury oxidation and initial adsorption rate are<br />
also observed for smaller carbon particles.<br />
Gaseous Hg (C out /C in )<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
1 2 3<br />
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6<br />
Time (hour)<br />
184<br />
1: 38 �m<br />
2: 98 �m<br />
3: 325 �m<br />
Figure 8.7. Effect of particle size on mercury breakthrough for 10 mg crushed Norit<br />
pellets in 2 g sand tested at 150�C using 2.75 Nl/min simulated flue gas with 160-170<br />
µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.%<br />
H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
<strong>Mercury</strong> oxidation (%)<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
Thiele modulus �<br />
0 2 4 6 8<br />
10<br />
Hg oxidation<br />
Adsorption rate for first 25 min<br />
0<br />
0<br />
0 50 100 150 200 250 300 350<br />
Carbon particle size (�m)<br />
2<br />
1.5<br />
1<br />
0.5<br />
<strong>Mercury</strong> adsorption rate (�g Hg/mg_carbon/h)
Figure 8.8. Effects of particle size on mercury oxidation and initial adsorption rate for<br />
10 mg crushed Norit RB4 pellets in 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
The observation on effect of carbon particle size can be quantified using the<br />
well-known Thiele modulus, expressed for the first order kinetics as [11]:<br />
k<br />
S<br />
�<br />
� � R p<br />
ox a<br />
De<br />
p<br />
(8.3)<br />
where Rp is the carbon particle radius, kox is the oxidation rate constant for the first<br />
order reaction, Sa is the surface area per unit mass of carbon, �p is the carbon particle<br />
density, and De is the effective diffusivity. The Thiele modulus is defined as the ratio<br />
of an intrinsic reaction rate in the absence of mass transfer limitations to the rate of<br />
diffusion into the particle under specified conditions. When the carbon particle size<br />
increases, Thiele modulus becomes larger. For the smallest particle size of 38 µm the<br />
calculated Thiele modulus is about 1. For the larger particles the Thiele modulus are<br />
much larger than 1, indicating that the mercury oxidation <strong>by</strong> the large particles might<br />
be limited <strong>by</strong> the internal diffusion resistance. Similar trends are observed for the<br />
mercury oxidation percentage and initial adsorption rate, which again indicates that<br />
mercury oxidation is an important step in elemental adsorption <strong>by</strong> the activated<br />
carbon.<br />
8.5 Effect of flue gas flow rate<br />
The effect of flue gas flow rate on mercury breakthrough profile is presented<br />
in figure 8.9. With larger flue gas flow rate the active carbon is saturated and reach<br />
the equilibrium capacity in shorter time. The initial mercury breakthrough time,<br />
which is defined as time when the mercury concentration after the carbon bed starts<br />
to increase, decreases when the flow rate is increased. The final adsorption capacities<br />
for all tested flow rates are almost the same.<br />
A higher superficial velocity is associated with a higher total mercury input,<br />
resulting in a faster consumption of the sorption capacity of the activated carbon and<br />
185
corresponding higher mercury outlet concentration. While a higher superficial<br />
velocity enhances the mass transfer rate and the corresponding mercury sorption rate.<br />
Gaseous Hg C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 0.4 0.8 1.2 1.6 2<br />
Time (hour)<br />
186<br />
2750 Nml/min<br />
1830 Nml/min<br />
1100 Nml/min<br />
Figure 8.9. Effect of flue gas flow rate on mercury breakthrough of 10 mg Darco Hg<br />
mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated flue gas with 160-<br />
170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1<br />
vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
8.6 Effects of flue gas compositions<br />
Most of the previous work conducted the tests either in nitrogen or added<br />
single gas to the baseline gas of either only N2 or a mixture of CO2, O2, H2O and N2<br />
[5,11-19]. Some studies use simulated flue gas that does not contain all the relevant<br />
gases, especially the acid gases [20-26]. It is more reasonable to change ranges of the<br />
relevant gases instead of completely removing the gases <strong>from</strong> the baseline to simulate<br />
the real flue gas. In this work, the concentration of relevant gas component is varied<br />
while the concentrations of the other flue gas components remain at the baseline<br />
values.<br />
8.6.1 Effect of CO2<br />
The effects of CO2 concentration in the flue gas on mercury adsorption<br />
capacity of Darco Hg carbon at 150�C are illustrated in figure 8.10. The mercury
adsorption capacity slightly decreases when the CO2 level in the gas is increased <strong>from</strong><br />
1 to 31 vol.%. The negative effects of CO2 in the flue gas on mercury adsorption <strong>by</strong><br />
the virgin activated carbon were also observed <strong>by</strong> Yan et al. [13]. The decrease of the<br />
adsorption capacity in the presence of CO2 is probably due to the reduction in the<br />
active sites for mercury adsorption due to the competitive adsorption of CO2 and<br />
mercury on the carbon. The weak effect of CO2 on mercury adsorption might due to<br />
the fact that significant CO2 adsorption on the activated carbon only occur at low<br />
temperature (
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, and 6 vol.% O2.<br />
8.6.2 Effect of O2<br />
Figure 8.11 shows the effects of oxygen concentration in the flue gas on the<br />
mercury adsorption capacity of Darco Hg at 150�C. The mercury adsorption capacity<br />
hardly changes with increasing the oxygen level <strong>from</strong> 1 vol.% to 16 vol.% when<br />
taking the experimental uncertainty into account. In all the cases the mercury<br />
oxidation <strong>by</strong> the carbon is about 97%.<br />
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 4 8 12 16 20<br />
O 2 level in gas (%)<br />
Figure 8.11. Effect of O2 concentration in the flue gas on mercury adsorption capacity<br />
of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated<br />
flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl,<br />
1000 ppmv SO2, 1 vol.% H2O, and 21 vol.% CO2.<br />
The effects of oxygen level in the gas on mercury adsorption <strong>by</strong> activated<br />
carbon were investigated using a gas mixture of Hg, N2, and O2 at 140�C in the<br />
literature [24]. When the oxygen concentration was increased <strong>from</strong> 0 to 3 vol.%, the<br />
mercury adsorption capacity on the studied activated carbon remained almost<br />
unchanged. The mercury adsorption capacity increased <strong>by</strong> 16 and 33% when the<br />
oxygen level was further increased to 6 and 9 vol.%, respectively.<br />
188
The possibility of carbon-oxygen complexes formation during the fixed-bed<br />
test and their impact on mercury adsorption was investigated [24]. Pretreatment of the<br />
unoxidized carbon <strong>by</strong> air for 7 days had no impact on the mercury adsorption<br />
performance of the carbon. The air can oxidize carbon surface and increases its acidic<br />
surface functional group content. However, these changes have no impact on the<br />
performance of active carbon for mercury adsorption.<br />
Thermodynamic calculations of mercury-oxygen reactions suggest that about<br />
30% of the mercury could be present as HgO(g) at 200�C, while at lower<br />
temperatures HgO(s) is the dominant form, when acid gases such HCl and NOx are<br />
not present in the gas [33,34]. This could lead to very high mercury adsorption<br />
capacity as it is not adsorption of mercury but precipation of HgO(s) on the carbon<br />
that takes place. However, the exact temperature range at which HgO(s) is the<br />
dominant form was not reported. Homogenous gas phase reaction of mercury with<br />
oxygen in an atmosphere of N2, O2, and Hg was investigated <strong>by</strong> Hall et al [34].<br />
Results suggest that a homogeneous gas phase reaction between oxygen and<br />
elemental mercury is not an important factor in flue gas reaction processes. The<br />
enhanced mercury adsorption in the presence of oxygen can be explained <strong>by</strong> the<br />
conversion of mercury to mercuric oxides as there is no reaction between oxygen and<br />
mercury in the absence of activated carbon surface.<br />
When HCl is present in the gas even at ppmv level, compared to vol.% level<br />
of oxygen, elemental mercury will be mainly oxidized to HgCl2 instead of HgO as<br />
shown <strong>by</strong> the thermodynamic calculations presented in chapter 2 [35,36]. Oxygen<br />
was found to be a weak oxidant of mercury [37]. This could explain the weak effect<br />
of oxygen on the mercury adsorption <strong>by</strong> Darco Hg carbon tested in simulated flue gas<br />
in this work.<br />
8.6.3 Effect of H2O<br />
The effect of water in the flue gas on the mercury breakthrough profile and<br />
mercury adsorption capacity of Darco Hg at 150�C is shown in figure 8.12 and 8.13,<br />
respectively. The presence of water in the flue gas generally accelerates the mercury<br />
breakthrough and therefore decreases the amount of mercury adsorbed on the carbon.<br />
189
The mercury adsorption capacity is increased significantly when water is removed<br />
<strong>from</strong> the simulated flue gas. The mercury adsorption capacity of Darco Hg carbon<br />
tested without water in the flue gas is about 5.5 times of that with 1 vol.% water in<br />
the gas. However, this result is not practically important since full-scale flue gas<br />
always contains water in percentage level [38]. Compared to CO2, the effects of H2O<br />
in the flue gas on mercury adsorption are more pronounced. The mercury oxidation<br />
percentage is about 97% for water level in the range of 0-15 vol.%, however, the<br />
mercury oxidation decreases to 68% when 25 vol.% of water is added to the flue gas.<br />
As a result the mercury adsorption capacity of Darco Hg with 25 vol.% water is only<br />
about 44% of that with 1 vol.% water in the flue gas.<br />
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
1 2 3<br />
0 1 2 3 4 5 6<br />
Time (hour)<br />
190<br />
1: 8% H 2O<br />
2:1% H 2 O<br />
3:0% H 2O<br />
Figure 8.12. Effects of water concentration in the flue gas on mercury breakthrough<br />
profile of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1000 ppmv SO2, 6 vol.% O2, and 21 vol.% CO2.
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 5 10 15 20 25 30<br />
H 2O level in gas (%)<br />
191<br />
Data<br />
Y=1.1277X -0.261<br />
R 2 =0.98<br />
Figure 8.13. Effects of water concentration in the flue gas on mercury adsorption<br />
capacity of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1000 ppmv SO2, 6 vol.% O2, and 21 vol.% CO2. The regressed equation<br />
of mercury adsorption capacity is not valid for H2O level of zero.<br />
Tests in N2, Hg, and H2O at 140�C show that the mercury adsorption capacity<br />
of sulfur impregnated activated carbon does not change significantly when the water<br />
content in the gas is increased <strong>from</strong> 0 to 5 vol.% [24]. While the adsorption capacity<br />
decreases about 25% when the water content is further increased to 10 vol.%. This is<br />
in contrast to tests with simulated flue gas in this work, as the mercury adsorption<br />
capacity always decreases with water addition in the flue gas.<br />
The negative effects of water presence in the gas on mercury adsorption <strong>by</strong><br />
the carbon could be due to the competitive adsorption of water on the carbon. Water<br />
adsorption on carbon has been studied <strong>by</strong> several researchers [39-42]. Due to the<br />
strong chemisorption of water molecules with the acidic oxygen functional group on<br />
the carbon, the initial water adsorption occurs at the functional groups, and further<br />
water adsorption will occur on top of the chemisorbed water molecules via hydrogen<br />
bonding [42].
8.6.4 Effect of CO<br />
Unlike most other combustion processes, organic constituents in the raw<br />
material for clinker production result in CO emissions, even under optimized<br />
combustion conditions. In the preheater, these organic components in the raw<br />
material are liberated, part of them being emitted with the exhaust gas. The carbon<br />
monoxide concentration in the exhaust gas <strong>from</strong> cement rotary kiln systems ranges<br />
between 0.1 and 5 g/Nm³ (80-4000 ppmv) [43]. There is no reported investigation on<br />
possible effect of CO on mercury adsorption <strong>by</strong> the activated carbon.<br />
The effect of CO concentration in the flue gas on the mercury adsorption<br />
capacity of Darco Hg carbon at 150�C is illustrated in figure 8.14. Similar to the<br />
effects of oxygen, the mercury adsorption capacity is not affected <strong>by</strong> the presence of<br />
CO in the range of 0-1000 ppmv.<br />
<strong>Mercury</strong> adsorption capacity (�g Hg/mg_carbon)<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 200 400 600 800 1000 1200<br />
CO level in the gas (ppmv)<br />
Figure 8.14. Effects of CO concentration in the flue gas on mercury adsorption<br />
capacity of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
As shown in figure 8.15, there is a slight decrease of mercury oxidation<br />
percentage when the CO level in the flue gas is increased. The mercury oxidation<br />
decreases <strong>from</strong> 98% when less than 100 ppmv CO is present in the flue gas to 85%<br />
192
with 1000 ppmv CO in the flue gas. This is probably because oxidized mercury is<br />
reduced at higher CO levels.<br />
<strong>Mercury</strong> oxidation (%)<br />
110<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 200 400 600 800 1000<br />
CO level in flue gas (ppmv)<br />
Figure 8.15. Effects of CO concentration in the flue gas on mercury oxidation <strong>by</strong> 10<br />
mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated flue<br />
gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000<br />
ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
8.6.5 Effect of SO2<br />
The effect of SO2 in the flue gas on mercury breakthrough profile and<br />
adsorption capacity of Darco Hg at 150�C is presented in figure 8.16 and 8.17,<br />
respectively. There are strong effects of SO2 in the flue gas on the mercury adsorption<br />
<strong>by</strong> the activated carbon. The mercury adsorption capacity decreases when the SO2<br />
level in the flue gas is increased. The mercury adsorption capacity of Darco Hg<br />
carbon tested with 100 ppmv SO2 in the flue gas is about 4 times of that tested with<br />
1000 ppmv SO2 in the flue gas. Dunham et al. [44] also found that the mercury<br />
adsorption capacity of activated carbon is inversely affected <strong>by</strong> SO2 in the flue gas<br />
with reductions in adsorption capacity noted at concentrations as low as 100 ppmv<br />
SO2. The mercury oxidation is not affected <strong>by</strong> changing the SO2 level in the flue gas<br />
and is about 97% for SO2 added in the range of 100-1000 ppmv.<br />
193
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
1 2<br />
0 0.5 1 1.5 2 2.5 3 3.5<br />
Time (hour)<br />
194<br />
3 4<br />
1: 1000 ppmv SO 2<br />
2: 500 ppmv SO 2<br />
3: 300 ppmv SO 2<br />
4: 100 ppmv SO 2<br />
Figure 8.16. Effects of SO2 concentration in the flue gas on mercury breakthrough<br />
profile of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
Data,Hg<br />
Y=69.176X -0.592<br />
R 2 =0.99<br />
0 200 400 600 800 1000 1200<br />
SO 2 level in gas (ppmv)<br />
Figure 8.17. Effects of SO2 concentration in the flue gas on mercury adsorption<br />
capacity of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2. The regressed equation of<br />
mercury adsorption capacity is valid for SO2 level higher than 100 ppmv.
Without the presence of NOx in the gas, the mercury adsorption capacity of<br />
activated carbon is also reported to decrease when the SO2 concentration increases<br />
[38]. This might be explained <strong>by</strong> the oxidation of SO2 to SO3 on the activated carbon<br />
and the inhibiting effect of SO3 on mercury capture <strong>by</strong> activated carbon injection has<br />
been observed in full-scale power plant tests [45,46]. In addition to removing<br />
mercury, activated carbon is also used as catalyst for oxidation SO2 to sulphuric acid<br />
and as SO2 sorbent [47,48]. There is competitive adsorption between Hg and SO3<br />
since both mercury and SO3 bind to the Lewis base sites on the activated carbon<br />
surface [45,49]. Some activated carbon catalysts for converting SO2 to H2SO4 are<br />
self-poisoned <strong>by</strong> SO3 or sulfate buildup on the surface. Therefore, a similar<br />
phenomenon might explain the inhibiting effect of SO3 on mercury capture.<br />
Previous results demonstrated that the oxidation of SO2 on carbon particles<br />
was greatly enhanced <strong>by</strong> the presence of trace quantities of gaseous NO2 [50-53].<br />
NO2 is an efficient oxidant for SO2 sorbed on carbon. According to the mechanisms<br />
of flue gas and mercury interactions on activated carbon proposed <strong>by</strong> Dunham et al.<br />
[44] and Olson et al. [54], sulfurous acid that accumulates <strong>from</strong> the hydration of SO2<br />
converts the previously formed nonvolatile basic mercuric nitrate into the volatile<br />
form. This results in the slow release of previously captured mercury over time in the<br />
presence of NO2 and SO2.<br />
8.6.6 Effect of HCl<br />
Figure 8.18 illustrates the effects of HCl in the flue gas on the mercury<br />
adsorption capacity of Darco Hg tested at 150�C. There are weak effects of HCl<br />
concentration in the flue gas on mercury adsorption capacity when 0.5-20 ppmv HCl<br />
is added to the flue gas. The mercury adsorption capacity increases gradually when<br />
the HCl level is increased <strong>from</strong> 0.5 to 5 ppmv and then it levels off when the HCl<br />
level is further increased. The mercury oxidation percentage is about 97% for all the<br />
tested HCl levels except that only 87% mercury oxidation is obtained when 0.5 ppmv<br />
HCl is added to the gas.<br />
195
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 4 8 12 16 20 24<br />
HCl level in gas (ppmv)<br />
196<br />
Data<br />
Y=0.9728X 0.071<br />
R 2 =0.82<br />
Figure 8.18. Effects of HCl concentration in the flue gas on mercury adsorption<br />
capacity of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 1000<br />
ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2. The regressed equation of<br />
mercury adsorption capacity is valid for HCl level higher than 0.5 ppmv.<br />
Strong promoting effects of HCl on mercury adsorption <strong>by</strong> activated carbon<br />
in the simulated flue gas without NOx at 135�C have been reported <strong>by</strong> Carey et al.<br />
[38]. Addition of HCl to the simulated gases of 1600 ppmv SO2, 6% O2, 12% CO2<br />
and 7% H2O results in an increase of equilibrium adsorption capacity for elemental<br />
mercury <strong>from</strong> 0 at 0 ppmv HCl to a value approaching 3 µg Hg/mg_carbon in the<br />
range of 50–100 ppmv HCl. The adsorption capacity does not change significantly<br />
above 50 ppm HCl.<br />
When NOx is included the simulated flue gas, the promoting effects of HCl on<br />
adsorption capacity of activated carbon becomes less pronounced. <strong>Mercury</strong> can be<br />
adsorbed <strong>by</strong> the carbon without HCl presence in the gas, provided that NOx is present.<br />
To study whether mercury can be adsorbed <strong>by</strong> the activated carbon in the absence of<br />
HCl, one test was conducted <strong>by</strong> removing HCl <strong>from</strong> the baseline flue gas applied in<br />
this work. Figure 8.19 shows the mercury breakthrough curve for 10 mg Darco Hg in<br />
2 g sand at150�C.
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 0.4 0.8 1.2 1.6<br />
197<br />
Time (hour)<br />
10 ppmv HCl<br />
0 ppmv HCl<br />
Figure 8.19. Comparison of mercury breakthrough curves with 0 and 10 ppmv HCl in<br />
the simulated flue gas, 10 mg Darco Hg mixed with 2 g sand and tested at 150�C<br />
using 2.75 Nl/min simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23<br />
ppmv NO2, 1000 ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
Figure 8.19 compares the mercury breakthrough curves of 0 and 10 ppmv<br />
HCl in the simulated cement kiln flue gas. Test without HCl in the gas shows that<br />
large amount of mercury is adsorbed on the carbon and mercury breakthrough is not<br />
obtained after 10 h (not shown in figure 8.19). This might be due to the fact that<br />
HgCl2 will form when HCl is present in the gas and on the other hand HgO or HgSO4<br />
will form when HCl is not present through following reactions:<br />
2Hg O2<br />
2HgO<br />
� � (8R1)<br />
Hg NO � HgO � NO<br />
� 2 (8R2)<br />
Hg � O � SO � HgSO<br />
(8R3)<br />
2<br />
2<br />
4<br />
HgO(s) is easily captured <strong>by</strong> the carbon since it might condense on the carbon at the<br />
applied adsorption temperature of 150�C.<br />
8.6.7 Effect of NO<br />
As shown in figure 8.20, changing of NO concentration in the simulated flue<br />
gas does not affect the adsorption capacity of Darco Hg tested at 150�C. The mercury
oxidation extent <strong>by</strong> the Darco Hg carbon is about 98% for tested NO in the range of<br />
100-1000 ppmv.<br />
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 200 400 600 800 1000 1200<br />
NO level in gas ( ppmv)<br />
Figure 8.20. Effects of NO concentration in the flue gas on mercury adsorption<br />
capacity of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 10 ppmv HCl, 23 ppmv NO2, 1000<br />
ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
Strong promoting effects of NO on mercury adsorption <strong>by</strong> the activated<br />
carbon have been reported when 300 ppmv NO was added to baseline gas with 8<br />
vol.% H2O, 6 vol.% O2, and 12 vol.% CO2 at 107�C [5,11-19]. Liu et al. [24] reported<br />
that adding 500 ppmv NO to nitrogen did not change the mercury adsorption <strong>by</strong> the<br />
activated carbon at 140�C. Fan et al. [55] proposed that the promoting effects of NO<br />
on mercury adsorption <strong>by</strong> the activated carbon is due to the reaction of NO with O2 to<br />
form NO2 and active O atoms that could further react with elemental mercury. Thus<br />
the effects of NO on mercury adsorption depend on the presence of other acid gases<br />
in the flue gas. With HCl and NO2 presence in the gas the effects of NO are less<br />
significant.<br />
8.6.8 Effect of NO2<br />
The effect of NO2 in the flue gas on mercury breakthrough profile and<br />
adsorption capacity of Darco Hg at 150�C is presented in figure 8.21 and 8.22,<br />
198
espectively. The mercury oxidation <strong>by</strong> the Darco Hg carbon is about 98% for tested<br />
NO2 in the range of 0-100 ppmv. Dunham et al. [44] also found that the mercury<br />
adsorption capacity of activated carbon is inversely proportional to the concentrations<br />
of NO2 in the simulated flue gas. A decrease in the mercury adsorption capacity of<br />
activated carbon was observed at concentrations as low as 2.5 ppmv NO2. The<br />
negative effects of NO2 are again due to the interaction between NO2 and SO2, as<br />
discussed in section 8.6.5.<br />
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
1 2 3<br />
0 0.2 0.4 0.6 0.8 1 1.2<br />
Time (hour)<br />
199<br />
1: 100 ppmv NO 2<br />
2: 23 ppmv NO 2<br />
3: 5 ppmv NO 2<br />
Figure 8.21. Effects of NO2 concentration in the flue gas on mercury breakthrough<br />
curves of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 10 ppmv HCl, 1000 ppmv NO, 1000<br />
ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.6<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 20 40 60 80 100 120<br />
NO 2 level in gas (ppmv)<br />
200<br />
Data<br />
Y=2.0685X -0.199<br />
R 2 =0.89<br />
Figure 8.22. Effects of NO2 concentration in the flue gas on mercury adsorption<br />
capacity of 10 mg Darco Hg mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 10 ppmv HCl, 1000 ppmv NO, 1000<br />
ppmv SO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2. The regressed equation of<br />
mercury adsorption capacity is not valid when NO2 is not presented in the gas.<br />
Table 8.2 summarizes the regressed equations of Darco Hg carbon mercury<br />
adsorption capacity as a function of flue gas composition at 150�C. From these<br />
equations it is possible estimate mercury adsorption capacity in a wide range of glue<br />
gas composition.<br />
Table 8.2 Summary of regressed equations of Darco Hg carbon mercury adsorption<br />
capacity as a function of flue gas composition at 150�C.<br />
Gases Concentration unit Concentration range Equations<br />
CO2 % >0 Y=1.3779X -0.066<br />
H2O % >0 Y=1.1277X -0.261<br />
SO2 ppmv ≥100 Y=69.176X -0.592<br />
HCl ppmv ≥0.5 Y=0.9728X 0.071<br />
NO2 ppmv >0 Y=2.0685X -0.199
8.7 Conclusions<br />
A parametric study of elemental mercury adsorption <strong>by</strong> activated carbon has<br />
been conducted in the fixed-bed reactor <strong>by</strong> mixing 10 mg Darco Hg carbon with 2 g<br />
sand and using simulated cement kiln flue gas. Equilibrium mercury adsorption<br />
capacity, initial adsorption rate and mercury oxidation percentage are evaluated.<br />
Increasing adsorption temperature results in decreased equilibrium mercury<br />
adsorption capacity of the activated carbon. The mercury adsorption isotherm follows<br />
Henry’s law for the applied mercury inlet levels in this project at all tested<br />
temperatures. All these are consistent with a physical adsorption mechanism. The<br />
derived heat of adsorption is -8543 J/mol for elemental mercury adsorption <strong>by</strong> Darco<br />
Hg activated carbon in simulated cement kiln flue gas.<br />
The effects of carbon particle size were investigated using the crushed Norit<br />
RB4 pellets. Higher mercury oxidation and initial adsorption rate are observed for<br />
smaller carbon particles, while the equilibrium mercury adsorption capacity is the<br />
same.<br />
The effects of flue gas composition are investigated <strong>by</strong> varying the<br />
concentrations of relevant gases instead of complete removal of the single gas <strong>from</strong><br />
the baseline to simulate the real flue gas. The mercury adsorption capacity does not<br />
change with changes in the O2, CO, and NO levels in the flue gas. The mercury<br />
adsorption capacity decreases when CO2, H2O, SO2, and NO2 concentrations in the<br />
flue gas increase. The following correlation between mercury adsorption capacity and<br />
these gas concentrations are obtained at 150�C: mercury adsorption capacity is<br />
proportional to CCO2 -0.066 , CH2O -0.261 , CSO2 -0.592 , CNO2 -0.199 . The decrease of mercury<br />
adsorption capacity is due to the competition for active site with mercury <strong>by</strong> CO2 and<br />
H2O, and conversion of the previously formed nonvolatile basic mercuric nitrate into<br />
the volatile form <strong>by</strong> interactions between SO2 and NO2.<br />
Slight promoting effects of HCl on mercury adsorption are observed when<br />
HCl concentration is varied in the range of 0.5-20 ppmv. A larger mercury adsorption<br />
capacity is obtained when HCl is removed <strong>from</strong> baseline gas. This might due to the<br />
fact that HgCl2 will form when HCl is present in the gas while HgO(s) will form<br />
201
when HCl is not present. HgO is more easily captured <strong>by</strong> the carbon since it<br />
condenses on the carbon at the applied adsorption temperature of 150�C.<br />
Significant mercury oxidation is observed <strong>by</strong> the activated carbon. Even for<br />
10 mg carbon typically an oxidation level of 94-97% is found. Increasing CO and<br />
water level in the flue gas causes a slight decrease of mercury oxidation. Larger<br />
mercury oxidation percentage is obtained with smaller carbon particle size. All these<br />
observations indicate that mercury oxidation <strong>by</strong> HCl, when this is present in the gas,<br />
is an important step in elemental adsorption <strong>by</strong> the activated carbon.<br />
8.8 References<br />
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evaluation of carbon performance on mercury vapour adsorption, Fuel. 83 (2004) 2401-2409.<br />
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uptake of gas-phase mercury, Environ. Sci. Technol. 31 (1997) 2319-2325.<br />
[17] S. Lee, Y. Park, Gas-phase mercury removal <strong>by</strong> carbon-based sorbents, Fuel Process<br />
Technol. 84 (2003) 197-206.<br />
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chlorine-impregnated activated carbons, Atmospheric Environment. 38 (2004) 4887-<br />
4893.<br />
[19] D. Karatza, A. Lancia, D. Musmarra, C. Zucchini, Study of mercury absorption and<br />
desorption on sulfur impregnated carbon, Experimental Thermal and Fluid Science. 21 (2000)<br />
150-155.<br />
[20] H.C. Hsi, S. Chen, M. Rostam-Abadi, M.J. Rood, C.F. Richardson, T.R. Carey, R.<br />
Chang, Preparation and evaluation of coal-derived activated carbons for removal of mercury<br />
vapor <strong>from</strong> simulated coal combustion flue gases, Energy Fuels. 12 (1998) 1061-1070.<br />
[21] S.B. Ghorishi and B.K. Gullett, An experimental study on mercury sorption <strong>by</strong> activated<br />
carbons and calcium hydroxide, The Fifth Annual North American Waste-to-Energy<br />
Conference, Research Triangle Park, NC, 22-25 April, 1997.<br />
[22] S. Eswaran, H.G. Stenger, Z. Fan, Gas-phase mercury adsorption rate studies, Energy<br />
Fuels. 21 (2007) 852-857.<br />
[23] R. Ochiai, M.A. Uddin, E. Sasaoka, S. Wu, Effects of HCl and SO2 concentration on<br />
mercury removal <strong>by</strong> activated carbon sorbents in coal-derived flue gas, Energy & Fuels. 23<br />
(2009) 4734.<br />
[24] W. Liu, R.D. Vidic, T.D. Brown, Impact of flue gas conditions on mercury uptake <strong>by</strong><br />
sulfur-impregnated activated carbon, Environ. Sci. Technol. 34 (2000) 154-159.<br />
[25] G.E. Dunham, S.J. Miller, <strong>Mercury</strong> capture <strong>by</strong> an activated carbon in a fixed-bed benchscale<br />
system, Environmental Progress. 17 (1998) 203.<br />
[26] S. Lee, J. Lee, T.C. Keener, Novel sorbents for mercury emissions control <strong>from</strong> coalfired<br />
power plants, Journal of the Chinese Institute of Chemical Engineers. 39 (2008) 137-<br />
142.<br />
[27] M. Molina-Sabio, A.M.A. Muñecas, F. Rodríguez-Reinoso, B. McEnaney, Adsorption<br />
of CO2 and SO2 on activated carbons with a wide range of micropore size distribution,<br />
Carbon. 33 (1995) 1777-1782.<br />
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[28] T.C. Drage, O. Kozynchenko, C. Pevida, M.G. Plaza, F. Rubiera, J.J. Pis, C.E. Snape, S.<br />
Tennison, Developing activated carbon adsorbents for pre-combustion CO2 capture, Energy<br />
Procedia. 1 (2009) 599-605.<br />
[29] M.G. Plaza, S. García, F. Rubiera, J.J. Pis, C. Pevida, Post-combustion CO2 capture with<br />
a commercial activated carbon: Comparison of different regeneration strategies, Chem. Eng.<br />
J. 163 (2010) 41-47.<br />
[30] C. Shen, C.A. Grande, P. Li, J. Yu, A.E. Rodrigues, Adsorption equilibria and kinetics<br />
of CO2 and N2 on activated carbon beads, Chem. Eng. J. 160 (2010) 398-407.<br />
[31] Y. Sun, Y. Wang, Y. Zhang, Y. Zhou, L. Zhou, CO2 sorption in activated carbon in the<br />
presence of water, Chemical Physics Letters. 437 (2007) 14-16.<br />
[32] B. Guo, L. Chang, K. Xie, Adsorption of carbon dioxide on activated carbon, Journal of<br />
Natural Gas Chemistry. 15 (2006) 223-229.<br />
[33] B. Hall, O. Lindqvist, E. Ljungstroem, <strong>Mercury</strong> chemistry in simulated flue gases related<br />
to waste incineration conditions, Environ. Sci. Technol. 24 (1990) 108-111.<br />
[34] B. Hall, P. Schager, J. Weesmaa, The homogeneous gas phase reaction of mercury with<br />
oxygen, and the corresponding heterogeneous reactions in the presence of activated carbon<br />
and fly ash, Chemosphere. 30 (1995) 611-627.<br />
[35] C.L. Senior, A.F. Sarofim, T. Zeng, J.J. Helble, R. Mamani-Paco, Gas-phase<br />
transformations of mercury in coal-fired power plants, Fuel Process Technol. 63 (2000) 197-<br />
213.<br />
[36] R.N. Sliger, J.C. Kramlich, N.M. Marinov, Towards the development of a chemical<br />
kinetic model for the homogeneous oxidation of mercury <strong>by</strong> chlorine species, Fuel Process<br />
Technol. 65-66 (2000) 423-438.<br />
[37] Niksa, S. and J. J. Helble, Interpreting laboratory test data on homogeneous mercury<br />
oxidation in coal-derived exhausts. EPA-DOE-EPRI Combined Power Plant Air Pollution<br />
Control Symposium: The Mega Symposium and the A&WMA Specialty Conference on<br />
<strong>Mercury</strong> Emissions: Fate, Effects, and Control, Chicago, Illinois, August 2001.<br />
[38] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang, F.B. Meserole, Factors<br />
affecting mercury control in utility flue gas using activated carbon, Journal of the Air &<br />
Waste Management Association. 48 (1998) 1166.<br />
[39] M.M. Dubinin, V.V. Serpinsky, Isotherm equation for water vapor adsorption <strong>by</strong><br />
microporous carbonaceous adsorbents, Carbon. 19 (1981) 402-403.<br />
[40] M.M. Dubinin, Water vapor adsorption and the microporous structures of carbonaceous<br />
adsorbents, Carbon. 18 (1980) 355-364.<br />
[41] F. Stoeckli, T. Jakubov, A. Lavanchy, Water adsorption in active carbons described <strong>by</strong><br />
the Dubinin-Astakhov equation, Journal of the Chemical Society, Faraday Transactions. 90<br />
(1994) 783-786.<br />
[42] D.D. Do, H.D. Do, A model for water adsorption in activated carbon, Carbon. 38 (2000)<br />
767-773.<br />
204
[43] ECRA (European cement research academy), Carbon monoxide formation and burn-out<br />
during the clinker burning process, News letter 2, 2008.<br />
[44] G.E. Dunham, E.S. Olson and S.J. Miller, Impact of flue gas constituents on carbon<br />
sorbents, McLean, VA, September 19-21, 2000.<br />
[45] A.A. Presto, E.J. Granite, Impact of sulfur oxides on mercury capture <strong>by</strong> activated<br />
carbon, Environ. Sci. Technol. 41 (2007) 6579-6584.<br />
[46] J. Jarvis, F. Meserole, SO3 Effect on <strong>Mercury</strong> Control, Power Eng. 112 (2008) 54-60.<br />
[47] E. Raymundo-Piñero, D. Cazorla-Amorós, C. Salinas-Martinez de Lecea, A. Linares-<br />
Solano, Factors controlling the SO2 removal <strong>by</strong> porous carbons: relevance of the SO2<br />
oxidation step, Carbon. 38 (2000) 335-344.<br />
[48] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, Temperature programmed<br />
desorption study on the mechanism of SO2 oxidation <strong>by</strong> activated carbon and activated<br />
carbon fibres, Carbon. 39 (2001) 231-242.<br />
[49] A.A. Presto, E.J. Granite, A. Karash, Further investigation of the impact of sulfur oxides<br />
on mercury capture <strong>by</strong> activated carbon, Ind Eng Chem Res. 46 (2007) 8273-8276.<br />
[50] W.R. Cofer III, D.R. Schryer, R.S. Rogowski, The enhanced oxidation of SO2 <strong>by</strong> NO2<br />
on carbon particulates, Atmospheric Environment (1967). 14 (1980) 571-575.<br />
[51] W.R. Cofer III, D.R. Schryer, R.S. Rogowski, The oxidation of SO2 on carbon particles<br />
in the presence of O3, NO2 and N2O, Atmospheric Environment (1967). 15 (1981) 1281-1286.<br />
[52] W.R. Cofer III, D.R. Schryer, R.S. Rogowski, Oxidation of SO2 <strong>by</strong> NO2 and O3 on<br />
carbon: Implications to tropospheric chemistry, Atmospheric Environment (1967). 18 (1984)<br />
243-245.<br />
[53] J.A. Rodriguez, T. Jirsak, J. Dvorak, S. Sambasivan, D. Fischer, Reaction of NO2 with<br />
Zn and ZnO: Photoemission, XANES, and density functional studies on the formation of NO3,<br />
The Journal of Physical Chemistry B. 104 (2000) 319-328.<br />
[54] E.S. Olson, B.A. Mibeck, S.A. Benson, J.D. Laumb, C.R. Crocker, G.E. Dunham, et al.,<br />
The mechanistic model for flue gas-mercury interactions on activated carbons: The oxidation<br />
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[55] X. Fan, C. Li, Zeng Guangming, Z. Gao, L. Chen, W. Zhang, et al., <strong>Removal</strong> of gasphase<br />
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(2010) 4250-4254.<br />
205
Fundamental investigation of mercury<br />
chloride adsorption <strong>by</strong> activated carbon in<br />
simulated cement kiln flue gas<br />
This chapter deals with a fundamental investigation of mercury chloride<br />
adsorption <strong>by</strong> the Darco Hg activated carbon in simulated cement kiln flue gas. The<br />
results are compared with tests using elemental mercury.<br />
9.1 Introduction<br />
Compared to research on elemental mercury adsorption [1-13], there are few<br />
studies on mercury chloride adsorption <strong>by</strong> activated carbon [1,2,14-16]. Some of<br />
these studies conducted tests using simulated flue gases containing 1600 ppmv SO2,<br />
50 ppmv HCl, 12 vol.% CO2, 7 vol.% H2O, 6 vol.% O2, but without NOx [1,2,14].<br />
Carey et al. [15] performed tests using simulated flue gas with 1600 ppmv SO2, 1-50<br />
ppmv HCl, 10-12 vol.% CO2, 8 vol.% H2O, 6 vol.% O2, and 200-400 ppmv NOx. The<br />
research [15] focused on a comparison of mercury adsorption capacity obtained in a<br />
fixed-bed reactor using simulated flue gas and real power plant flue gas. Mibeck et al.<br />
[16] investigated the effects of acid gases <strong>by</strong> adding 1600 ppmv SO2, 50 ppmv HCl,<br />
400 ppmv NO, and 20 ppmv NO2 alone or in combination to baseline gas of 12 vol.%<br />
CO2, 8 vol.% H2O, 6 vol.% O2. Only breakthrough curves are presented, neither<br />
adsorption capacity nor kinetics is reported in their work.<br />
In this work, mercury chloride adsorption capacity and kinetics are<br />
investigated <strong>by</strong> varying the relevant gas concentrations and operating parameters.<br />
206<br />
9
9.2 Effect of temperature<br />
The effect of temperature on mercury chloride adsorption is investigated <strong>by</strong><br />
applying the same conditions as for the study with elemental mercury source reported<br />
in chapter 8. The breakthrough curves using HgCl2 source are compared with those<br />
obtained with elemental mercury at 100, 120, and 150�C, as illustrated in figure 9.1-3.<br />
In contrast to the breakthrough curve obtained with elemental mercury, the<br />
breakthrough curve of mercury chloride often has an introduction period. It takes<br />
some time to reach the lowest outlet mercury concentration after switching the flue<br />
gas with mercury chloride to the carbon bed. While the outlet mercury decreases to<br />
the lowest value almost simultaneously after switching the flue gas with elemental<br />
mercury to the carbon bed. This phenomenon is also observed <strong>by</strong> Mibeck et al. [16].<br />
They also reported that about 90-95% of mercury after the carbon bed is oxidized<br />
mercury. Measurement in this work shows that all the mercury after the carbon bed is<br />
oxidized mercury, i.e., no reduction takes place.<br />
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 0.2 0.4 0.6 0.8 1<br />
Time (hour)<br />
207<br />
Hg 0 , 170�g/Nm 3 Hg,<br />
1.183 �g Hg/mg_carbon<br />
HgCl 2, 183�g/Nm 3 Hg,<br />
1.224 �g Hg/mg_carbon<br />
Figure 9.1. Comparison of breakthrough curves obtained using mercury chloride and<br />
elemental mercury source for 10 mg Darco Hg carbon mixed with 2 g sand tested at<br />
150�C using 2.75 Nl/min simulated flue gas with 1000 ppmv NO, 23 ppmv NO2,<br />
1000 ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 0.2 0.4 0.6 0.8 1 1.2<br />
Time (hour)<br />
208<br />
Hg 0 , 166�g/Nm 3 Hg,<br />
1.335 �g Hg/mg_carbon<br />
HgCl 2, 156�g/Nm 3 Hg,<br />
1.375 �g Hg/mg_carbon<br />
Figure 9.2. Comparison of breakthrough curves obtained using mercury chloride and<br />
elemental mercury source for 10 mg Darco Hg carbon mixed with 2 g sand tested at<br />
120�C using 2.75 Nl/min simulated flue gas with 1000 ppmv NO, 23 ppmv NO2,<br />
1000 ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
Gaseous Hg, C out /C in<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6<br />
Time (hour)<br />
Hg 0 , 167�g/Nm 3 Hg,<br />
1.506 �g Hg/mg_carbon<br />
HgCl 2, 159�g/Nm 3 Hg,<br />
1.429 �g Hg/mg_carbon<br />
Figure 9.3. Comparison of breakthrough curves obtained using mercury chloride and<br />
elemental mercury source for 10 mg Darco Hg carbon mixed with 2 g sand tested at<br />
100�C using 2.75 Nl/min simulated flue gas with 1000 ppmv NO, 23 ppmv NO2,<br />
1000 ppmv SO2, 10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2.<br />
The equilibrium mercury adsorption capacities are very similar for tests using<br />
elemental mercury and mercury chloride under same gas conditions at different<br />
temperatures. This is probably due to the catalytic oxidation of elemental mercury <strong>by</strong>
the activated carbon. As shown in chapter 8, almost all elemental mercury is oxidized<br />
<strong>by</strong> the activated carbon. Thus it is not surprising to observe similar adsorption<br />
behavior of elemental mercury and mercury chloride adsorption <strong>by</strong> the activated<br />
carbon for the present flue gas containing HCl. For sorbents with poor mercury<br />
oxidation ability, the adsorption behavior of elemental mercury and oxidized mercury<br />
is expected to be different. Increasing adsorption temperature also decreases the<br />
adsorption capacity of mercury chloride.<br />
Similarly to tests with elemental mercury, the adsorption constants of HgCl2<br />
are also derived. Figure 9.4 shows that there is a linear relation between lnk and 1/T<br />
and <strong>from</strong> the slope and intercept the calculated value for k0 and �H ads is 1.595 m 3 /g<br />
and -6587 J/mol, respectively. The corresponding k0 and �H ads for tests with<br />
elemental mercury is 0.869 m 3 /g and -8543 J/mol, respectively.<br />
ln(k)<br />
-4<br />
-4.2<br />
-4.4<br />
-4.6<br />
Hg 0 data<br />
Y=1027.4881X-7.0484<br />
R 2 =0.93<br />
HgCl2 data<br />
Y=792.2246X-6.4409<br />
R 2 =0.93<br />
-4.8<br />
0.0022 0.0024 0.0026 0.0028 0.003<br />
209<br />
1/T (1/K)<br />
Figure 9.4. Plot of lnk as a function of 1/T. The Henry’s constants k are derived <strong>from</strong><br />
isotherms at 100, 120, 150, and 180�C. 10 mg Darco Hg mixed with 2 g sand is tested<br />
using 2.75 Nl/min simulated flue gas with 1000 ppmv NO, 23 ppmv NO2, 10 ppmv<br />
HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. Data for tests<br />
using elemental mercury are shown for comparison.
9.3 Effect of flue gas composition<br />
Since negligible effects of CO2, O2, CO, and NO on elemental mercury<br />
adsorption <strong>by</strong> the activated carbon are observed and preliminary tests of mercury<br />
chloride adsorption <strong>by</strong> the activated carbon shows similar behavior as elemental<br />
mercury, only effects of H2O, SO2, HCl, and NO2 on mercury chloride adsorption <strong>by</strong><br />
the Darco Hg activated carbon are investigated. The effects of H2O, SO2, HCl, and<br />
NO2 on equilibrium adsorption capacity of mercury chloride are presented in figure<br />
9.5-8, respectively. Data for tests using elemental mercury are shown for comparison.<br />
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 5 10 15 20 25 30<br />
H 2O level in gas (%)<br />
210<br />
Data,Hg 0<br />
Y=1.1277X -0.261<br />
R 2 =0.98<br />
Data, HgCl2<br />
Y=1.2389X -0.240<br />
R 2 =0.98<br />
Figure 9.5. Effects of water in the flue gas on mercury chloride adsorption capacity of<br />
10 mg Darco Hg carbon mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 10 ppmv<br />
HCl, 6 vol.% O2, and 21 vol.% CO2.
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
0 200 400 600 800 1000 1200<br />
SO 2 level in gas (ppmv)<br />
211<br />
Data,Hg 0<br />
Y=69.176X -0.592<br />
R 2 =0.99<br />
Data,HgCl2<br />
Y=58.23X -0.565<br />
R 2 =0.99<br />
Figure 9.6. Effects of SO2 in the flue gas on mercury chloride adsorption capacity of<br />
10 mg Darco Hg carbon mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 6 vol.% O2, 1<br />
vol.% H2O, and 21 vol.% CO2.<br />
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.6<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
0 4 8 12 16 20 24<br />
HCl level in gas (ppmv)<br />
Data,Hg 0<br />
Y=0.9728X 0.071<br />
R 2 =0.82<br />
Data,HgCl2<br />
Y=0.9162X 0.1158<br />
R 2 =0.76<br />
Figure 9.7. Effects of HCl in the flue gas on mercury chloride adsorption capacity of<br />
10 mg Darco Hg carbon mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2, 6 vol.% O2,<br />
1 vol.% H2O, and 21 vol.% CO2.
<strong>Mercury</strong> adsorption capacity (�g_Hg/mg_carbon)<br />
1.6<br />
1.4<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0<br />
Data, Hg 0<br />
Y=2.0685X -0.199<br />
R 2 =0.89<br />
Data, HgCl2<br />
Y=2.0732X -0.197<br />
R 2 =0.88<br />
0 20 40 60 80 100 120<br />
NO 2 level in gas (ppmv)<br />
Figure 9.8. Effects of NO2 in the flue gas on mercury chloride adsorption capacity of<br />
10 mg Darco Hg carbon mixed with 2 g sand tested at 150�C using 2.75 Nl/min<br />
simulated flue gas with 1000 ppmv NO, 1000 ppmv SO2, 10 ppmv HCl, 6 vol.% O2,<br />
1 vol.% H2O, and 21 vol.% CO2.<br />
The adsorption capacity of mercury chloride is slightly larger than the<br />
elemental mercury when the water content in the flue gas is above 1 vol.%. Almost<br />
the same tendency of adsorption capacity as a function of SO2, HCl, and NO2<br />
concentration in the flue gas is observed for mercury chloride and elemental mercury.<br />
This is again due to the high oxidation rate of elemental mercury <strong>by</strong> the Darco Hg<br />
carbon.<br />
9.4 Conclusions<br />
Similar adsorption behaviors of mercury chloride and elemental mercury <strong>by</strong><br />
Darco Hg activated carbon are observed using simulated cement kiln flue gas at<br />
different temperatures. Increasing adsorption temperature also decreases the<br />
adsorption capacity of mercury chloride.<br />
The effects of H2O, SO2, HCl, and NO2 on mercury chloride adsorption <strong>by</strong><br />
Darco Hg are investigated <strong>by</strong> varying their concentrations in the baseline gas.<br />
Compared to elemental mercury adsorption, a slightly larger adsorption capacity of<br />
mercury chloride is obtained when the water content in the flue gas is above 1 vol.%.<br />
212
The dependence of mercury chloride adsorption capacity on SO2, HCl, and NO2<br />
concentrations in the flue gas is the same as elemental mercury adsorption capacity.<br />
The similar behavior of mercury chloride and elemental mercury is due to the<br />
effective catalytic oxidation of elemental mercury <strong>by</strong> the activated carbon in the<br />
presence of HCl.<br />
9.5 References<br />
[1] H.C. Hsi, S. Chen, M. Rostam-Abadi, M.J. Rood, C.F. Richardson, T.R. Carey, R. Chang,<br />
Preparation and evaluation of coal-derived activated carbons for removal of mercury vapor<br />
<strong>from</strong> simulated coal combustion flue gases, Energy & Fuels. 12 (1998) 1061-1070.<br />
[2] M. Rostam-Abadi, S.G. Chen, H.C. Hsi, M. Rood, R. Chang, T.R. Carey, B., Hargrove, C.<br />
Richardson, W. Rosenhoover, F. Meserole, Novel vapor phase mercury sorbents,<br />
Proceedings of the EPRI-DOE-EPA Combined Utility Air Pollutant Control, Washington,<br />
DC, Aug 25–29, 1997.<br />
[3] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang and F.B. Meserole, Factors<br />
affecting mercury control in utility flue gas using sorbent injection, Proceedings of the Air &<br />
Waste Management Association's 90 th Annual Meeting, Toronto, Ontario, Canada, June 8-13,<br />
1997.<br />
[4] C.X. Hu, J.S. Zhou, Z.Y. Luo, S. He, G.K. Wang, K.F. Cen, Effect of oxidation treatment<br />
on the adsorption and the stability of mercury on activated carbon, Journal of Environmental<br />
Sciences-China. 18 (2006) 1161-1166.<br />
[5] B. Ghorishi and B.K. Gullett, Fixed-bed control of mercury: Role of acid gases and a<br />
comparison between carbon-based, calcium-based, and coal fly ash sorbents, Proceedings of<br />
the EPRI/DOE/EPA Combined Utility Air Pollutant Control Symposium, Washington, DC,<br />
August 25–29, 1997.<br />
[6] S.B. Ghorishi and C.B. Sedman, Combined mercury and sulfur oxides control using<br />
calcium-based sorbents, Proceedings of the EPRI-DOE-EPA Combined Utility Air Pollutant<br />
Control Symposium, Washington, DC, August 25–29, 1997.<br />
[7] S.B. Ghorishi and B.K. Gullett, An experimental study on mercury sorption <strong>by</strong> activated<br />
carbons and calcium hydroxide, The Fifth Annual North American Waste-to-Energy<br />
Conference, Research Triangle Park, NC, 22-25 April, 1997.<br />
[8] N. Hutson, C. Singer, C. Richardson, J. Karwowski and C. Sedman, Practical applications<br />
<strong>from</strong> observations of mercury oxidation and binding mechanisms, EPRI-DOE-EPA-AWMA<br />
Combined Power Plant Air Pollutant Control Mega Symposium, Washington DC, August 30<br />
- September 2, 2004.<br />
[9] E.J. Granite, M.C. Freeman, R.A. Hargis, W.J. O'Dowd, H.W. Pennline, The thief process<br />
for mercury removal <strong>from</strong> flue gas, J. Environ. Manage. 84 (2007) 628-634.<br />
213
[10] W.J. O’Dowd, H.W. Pennline, M.C. Freeman, E.J. Granite, R.A. Hargis, C.J. Lacher, A.<br />
Karash, A technique to control mercury <strong>from</strong> flue gas: The thief process, Fuel Processing<br />
Technology. 87 (2006) 1071-1084.<br />
[11] R. Bhardwaj. Impact of temperature and flue gas components on mercury speciation and<br />
uptake <strong>by</strong> activated carbon sorbents . Master thesis. Master, University of Pittsburgh, 2007.<br />
[12] M.M. Maroto-Valer, Y. Zhang, E.J. Granite, Z. Tang, H.W. Pennline, Effect of porous<br />
structure and surface functionality on the mercury capacity of a fly ash carbon and its<br />
activated sample, Fuel. 84 (2005) 105-108.<br />
[13] S. Eswaran, H.G. Stenger, Z. Fan, Gas-phase mercury adsorption rate studies, Energy &<br />
Fuels. 21 (2007) 852-857.<br />
[14] T.R. Carey, O.W. Hargrove Jr, C.F. Richardson, R. Chang, F.B. Meserole, Factors<br />
affecting mercury control in utility flue gas using activated carbon, Journal of the Air &<br />
Waste Management Association. 48 (1998) 1166.<br />
[15] T.R. Carey, C.F. Richardson, R. Chang, F.B. Meserole, M. Rostam-Abadi, S. Chen,<br />
Assessing sorbent injection mercury control effectiveness in flue gas streams, Environ. Prog.<br />
19 (2000) 167-174.<br />
[16] B.A.F. Mibeck, E.S. Olson, S.J. Miller, HgCl2 sorption on lignite activated carbon:<br />
Analysis of fixed-bed results, Fuel Process Technol. 90 (2009) 1364-1371.<br />
214
215<br />
10<br />
Simulation of mercury adsorption <strong>by</strong> fixed<br />
carbon bed<br />
To properly understand an adsorption process, two basic ingredients, i.e.,<br />
equilibrium and transport processes must be investigated. Understanding of the<br />
adsorptive capacity is within the domain of equilibrium, and understanding of the<br />
diffusion resistance is within the domain of transport process. These two aspects are<br />
first introduced in this chapter. The remaining part of this chapter deals with<br />
mathematical models describing the behavior of isothermal adsorption of mercury in<br />
a carbon particle and a fixed carbon bed including adsorption isotherm, mass balance<br />
for the gas phase and mass balance inside the adsorbent.<br />
10.1 Adsorption equilibrium<br />
A summary of commonly used isotherm equations for pure gas adsorption is<br />
given in the following table 10.1. Considering the typical mercury level in the flue<br />
gas of ppb level, the simple Henry’s law is able to describe the isotherm well, as<br />
shown in chapter 8.
Table 10.1. Summary of commonly used isotherm equations for pure gas adsorption<br />
[1].<br />
Isotherm Equation Remarks<br />
Henry law C�� KC<br />
Low pressure range<br />
Langmuir<br />
bC<br />
C� � C�s<br />
1�<br />
bC<br />
Has Henry law limit and<br />
finite saturation limit<br />
Frendlich<br />
1/n<br />
C�KFC � Does not have Henry law<br />
limit and no saturation limit<br />
Langmuir-Frendlich<br />
1/ n<br />
( bC)<br />
C� � C�s<br />
1/ n<br />
1 � ( bC)<br />
Does not have Henry law<br />
limit, but has finite<br />
saturation limit<br />
Toth<br />
bC<br />
C� � C�s<br />
t 1/ t<br />
[1 � ( bC)<br />
]<br />
Has Henry law limit and<br />
finite saturation limit<br />
Unilan s<br />
C�s 1�<br />
be C<br />
C��ln( ) �s<br />
2s 1�be<br />
C<br />
Has Henry law limit and<br />
finite saturation limit<br />
K: Henry constant; C: gaseous adsorbate concentration; C�: adsorbed concentration in<br />
the sorbent; C�s: saturation adsorbed concentration in the sorbent; b: Langmuir<br />
constant; KF: Frendlich constant; s: heterogeneity parameter<br />
10.2 Transport consideration in adsorption process<br />
Adsorption of an adsorbate molecule on to the porous surface of an adsorbent<br />
include following steps [2]:<br />
1. External (or interphase) mass transfer of the adsorbate <strong>from</strong> the bulk fluid <strong>by</strong><br />
convection through a thin film or boundary layer.<br />
2. Internal (intraphase) mass transfer of the adsorbate <strong>by</strong> pore diffusion <strong>from</strong> the<br />
outer surface of the adsorbent to the inner surface of the internal porous structure.<br />
3. Surface diffusion along the porous surface.<br />
4. Adsorption of the adsorbate onto the porous surface.<br />
10.2.1 External transport<br />
Rates of convection mass and heat transfer between the outer surface of a<br />
particle and the surrounding bulk fluid during an adsorption process are given,<br />
respectively, <strong>by</strong> [2]:<br />
216
dN<br />
�kmA�cb � cs�<br />
(10.1)<br />
dt<br />
dQ<br />
�hA�Ts� Tb<br />
�<br />
(10.2)<br />
dt<br />
where km is the external mass transfer coefficient, A is the particle external surface<br />
area, Cb and Cs is the gas concentration in the bulk and at the particle surface,<br />
respectively. h is the heat transfer coefficient, Tb and Ts is the gas temperature in the<br />
bulk and at the particle surface, respectively.<br />
When fluid flows past a single particle, experimental transport data<br />
correlations are usually developed for coefficients averaged over the particle surface.<br />
Some typical correlations published <strong>by</strong> Ranz and Marshall for Nusselt numbers as<br />
high as 30, and Sherwood numbers to 160 are the following [2]:<br />
N �2� 0.60N<br />
N<br />
(10.3)<br />
Nu<br />
1 1<br />
2 3<br />
Re Pr<br />
N �2� 0.60N<br />
N<br />
(10.4)<br />
1 1<br />
2 3<br />
Sh Re Sc<br />
C p�<br />
where Prandtl number NPr= ; Schmidt number NSc=<br />
k<br />
217<br />
�<br />
; Reynolds number<br />
�Di<br />
d pG<br />
NRe= , and G is the fluid mass velocity.<br />
�<br />
When particles are packed in a bed, the fluid flow patterns are restricted, and<br />
the single particle correlations cannot be used to estimate the average external<br />
transport coefficients for particles in the bed. A correlation of 37 sets of mass-transfer<br />
data including Sherwood number corrections for axial dispersion result in an<br />
expression of the form [2]:<br />
k d<br />
N �<br />
m p<br />
0.<br />
5 1<br />
3<br />
Sh � � 2 1.<br />
1N<br />
Re N Sc<br />
(10.5)<br />
Di<br />
This equation covers a Schmidt number range <strong>from</strong> 0.6 to 70600, a Reynolds number<br />
range <strong>from</strong> 3 to 10000. Particle shapes applicable include spheres, short cylinders,<br />
flakes and granules. By analogy, the corresponding equation for fluid-particle<br />
convection heat transfer in packed beds is:<br />
N<br />
Nu<br />
hd p<br />
0.<br />
5 1<br />
3<br />
� � 2 �1.<br />
1N<br />
Re N Pr<br />
(10.6)<br />
k
When these equations are used with beds packed with non-spherical particles, dp, is<br />
the equivalent diameter of a spherical particle.<br />
10.2.2 Internal transport<br />
Porous particles in most cases have a sufficiently high effective thermal<br />
conductivity so that temperature gradients within the particle are negligible. In<br />
contrast, internal mass transfer within the particle must be considered. In sorption<br />
processes, transport is <strong>from</strong> the exterior to the interior for adsorption and <strong>from</strong> the<br />
interior to the exterior for desorption processes. The flux of mercury transported to<br />
the carbon particle can be expressed as:<br />
dC<br />
N A � �De<br />
(10.7)<br />
dr<br />
where De is the effective diffusion coefficient. There are basically three modes of<br />
transport of molecules inside a porous medium: Knudsen diffusion, molecular<br />
diffusion, and surface diffusion [1].<br />
10.2.2.1 Molecular Diffusion<br />
When the adsorbate is in a macropore or in the fluid phase, the frequency of<br />
collision with a surface is minimal and transport of the molecule occurs via<br />
intermolecular collisions only. This mode of transport is due to a partial pressure<br />
gradient of a continuum fluid mixture.<br />
For binary gas mixtures at low pressure (
parameters of the individual species in the system. MWA and MWB are molecular<br />
weights of species A and B. The molecular collision diameter, �AB, is calculated as<br />
the arithmetic average of the two species:<br />
� � ( � � � )<br />
(10.9)<br />
1<br />
AB 2 A B<br />
�D,AB is a dimensionless function of temperature and the intermolecular potential<br />
field for a molecular of A and B. The interaction is described <strong>by</strong> the individual<br />
Lennard-Jones 12-6-potentials, �A and �B, in accordance with following equation:<br />
�AB k �<br />
�A�B k. k<br />
(10.10)<br />
�D,AB can be calculated according to:<br />
1.06036 0.19300 1.03587 1.76474<br />
�DAB , � � � �<br />
0.15610<br />
( kT ) exp(0.47635 kT ) exp(1.52996 kT ) exp(3.89411 kT )<br />
� AB �AB �AB �AB<br />
(10.11)<br />
where k is the Boltzmann’s constant.<br />
The Lennard-Jones potential parameter for N2 can be easily found while for<br />
elemental mercury only few data are available. Table 10.2 lists the Lennard-Jones<br />
potential parameter for N2 and elemental mercury.<br />
Table 10.2. Lennard-Jones potential parameter for N2 and elemental mercury [3].<br />
� (Å) �/k (K)<br />
N2 3.681 91.5<br />
Hg 3.23 627<br />
10.2.2.2 Knudsen Diffusion<br />
This diffusion process occurs when the mean free path of the adsorbate is<br />
much larger than the diameter of the channel in which the diffusing molecules reside.<br />
This normally occurs at very low pressure and channels of small size, usually of order<br />
of 10 nm to 100 nm [1]. The flow is induced <strong>by</strong> collision of gaseous molecules with<br />
the pore wall.<br />
2r<br />
8RT<br />
g<br />
T<br />
DK� � 9700r<br />
(10.12)<br />
3 � MW MW<br />
where r is the pore radius in cm, T in K, MW in g/mol, Dk, in cm 2 /s.<br />
219
10.2.2.3 Surface Diffusion<br />
In most cases, the surface diffusion coefficient is unknown as the heat of<br />
adsorption is not available. Furthermore, the surface diffusivity is a strong function of<br />
the amount of mercury adsorbed and the sorbent surface coverage is low due to the<br />
low mercury level in the flue gas, it is therefore reasonable to assume that the surface<br />
diffusion resistance can be neglected.<br />
10.3 Modeling of adsorption in a single particle<br />
The final aim of this project is to develop a mathematical model that can<br />
simulate mercury adsorption <strong>by</strong> a carbon cake on the fabric filter bags. A single<br />
particle model is the core and starting points of the filter model. The single particle<br />
model can be used to study how an adsorption process would vary with parameters<br />
such as particle size, bulk concentration, pressure, temperature, pore size, and<br />
adsorption affinity. Analytical solution of the single particle model is available when<br />
linear adsorption isotherm is used. However, the single particle model works only at<br />
constant gas atmosphere. The gas concentration changes in time for both the fixedbed<br />
and fabric filter adsorption processes. Therefore a numerical solution of the<br />
single particle model is required in order to incorporate it to fixed-bed and fabric<br />
filter models.<br />
Do [1] has made a detailed description of the single particle adsorption model<br />
using linear isotherm. Both analytical solution and numerical solution using<br />
orthogonal collocation method with subroutines in MATLAB are provided in his<br />
book. Fixed-bed and fabric filter models in this work are further developed on the<br />
basis of the single particle model.<br />
Since the mercury level in the flue gas is very low, the adsorption system can<br />
be treated as isothermal. Mass balance around a thin shell element in the particle<br />
gives [1]:<br />
�C �C�<br />
1 � s �C<br />
� p �(1 �� p) � De ( r )<br />
s<br />
�t �t r �r �r<br />
220<br />
(10.13)
where �p is the porosity of the particle, C is gaseous mercury concentration, C� is the<br />
mercury concentration in the adsorbed phase, De is the pore diffusivity, and s is the<br />
particle shape factor (s=0, 1, and 2 for slab, cylinder, and sphere, respectively).<br />
The free molecules of mercury in the pore space and the adsorbed mercury<br />
molecules at any point within a particle are assumed in equilibrium with each other.<br />
The local linear isotherm takes the form:<br />
C�� KC<br />
(10.14)<br />
where K is the Henry constant.<br />
Substituting the local equilibrium into the mass balance equation, we can<br />
obtain [1]:<br />
�C D 1 � s �C<br />
� D � C �<br />
( r )<br />
�t � � K r �r �r<br />
2<br />
e<br />
app<br />
� p (1 � p)<br />
s<br />
221<br />
(10.15)<br />
with initial condition: t=0, C=Ci, (10.16)<br />
and typical boundary conditions:<br />
�C<br />
r �0, �0<br />
�r<br />
�C<br />
r � Rp, �De �km( C �Cb)<br />
Rp<br />
�<br />
r r�R (10.17)<br />
(10.18)<br />
For slab object R is the half thickness, while for cylindrical and spherical objects, R is<br />
their respective radius. Cb is the concentration of the adsorbate in the bulk<br />
surrounding the particle, km is the external mass transfer coefficient.<br />
An analytical solution of the concentration distribution within the particle is<br />
given in the form of an infinite series [1]. The solution is only valid for a particle<br />
surrounded <strong>by</strong> a gas atmosphere not changing in time. To get a numerical solution,<br />
equation 10.15 is written in a dimensionless form <strong>by</strong> defining following nondimensional<br />
variables and parameters:<br />
C r D t C C<br />
y � ; x� ; � � ; y � ; y �<br />
C R R C C<br />
app b i<br />
2 b i<br />
0 p p<br />
0 0<br />
�y 1 � s �y<br />
� ( x )<br />
(10.19)<br />
s<br />
��x�x �x<br />
Initial condition: �=0, y=yi (10.20)
Boundary conditions:<br />
�y<br />
x �0, �0<br />
�x<br />
222<br />
(10.21)<br />
x 1; y<br />
Bi(<br />
yb<br />
y)<br />
x<br />
� � �<br />
�<br />
(10.22)<br />
�<br />
where Bi is the Biot number � km<br />
R p De<br />
.<br />
The problem has symmetry at x=0, and it is useful to utilize this <strong>by</strong> making<br />
the transformation of u=x 2 , and the differential equation becomes [1]:<br />
2<br />
�y � y �y<br />
�4u �2( s�1)<br />
2<br />
���u � u<br />
(10.23)<br />
The equation is solved <strong>by</strong> the orthogonal collocation method [4]. The domain u�(0,1)<br />
is represented <strong>by</strong> n interior collocation points. Taking the boundary point (u=1) as the<br />
(n+1) -th point, we have a total of n+1 interpolation points. The first and second<br />
derivatives at these interpolation points are related to the functional values at all<br />
points as given below:<br />
�y<br />
n�1<br />
� Aij y j<br />
�u i j<br />
� (10.24)<br />
�<br />
y<br />
2 n�1<br />
� 2 � Bij y j<br />
(10.25)<br />
�u i j<br />
The matrices A and B are constant matrices once n+1 interpolation points have been<br />
chosen. The mass balance equation is valid at any point within the u domain.<br />
Evaluating the equation at the i th interior collocation point we get:<br />
n�1<br />
�yi � Cy ij j<br />
�� j�1<br />
� (10.26)<br />
For i=1, 2,…n, where<br />
C �4uB �2(1 � s) A<br />
(10.27)<br />
ij i ij ij<br />
Numerical calculation of the average gaseous mercury concentration inside the<br />
particle is obtained <strong>by</strong>:<br />
1<br />
() (, ) ( 1) (, ) s<br />
Ct � CtxdV� s� Ctxxds<br />
V<br />
V<br />
1<br />
� � (10.28)<br />
0
1 s�1<br />
2<br />
( s �1)<br />
Ct () � Ctxu (, ) du<br />
2 � (10.29)<br />
0<br />
The integration is evaluated <strong>by</strong> Radau quadrature [1,4]:<br />
1 s�1 1<br />
n�1<br />
2<br />
� �<br />
s �1<br />
Ctxu (, ) du� (1 �u) uCtxdu (, ) ��wC k k;<br />
� �0; � �<br />
2<br />
� � (10.30)<br />
0 0<br />
where the weight factors wk are the Radau quadrature weights.<br />
The calculated Radau quadratue weights <strong>from</strong> the program are normalized [4]:<br />
k Wk ( � , � )<br />
I<br />
k �1<br />
w<br />
� (10.31)<br />
( �� , ) �( � �1) �( � �1) 2 s �1<br />
I � � for � �0and � �<br />
�( � �� �2) s �1<br />
2<br />
2<br />
w � W<br />
s �1<br />
k k<br />
1 s�1 2<br />
n�1<br />
0<br />
k �1<br />
223<br />
(10.32)<br />
(10.33)<br />
( s �1)<br />
Ct () � Ctxu (, ) du��WC k k<br />
2 � (10.34)<br />
The boundary condition at the particle surface becomes:<br />
�y<br />
Bi<br />
u �1; � ( yb� y)<br />
�u<br />
2<br />
Bi<br />
(10.35)<br />
N �1<br />
� An�1, jyj � ( yb � yn�1)<br />
(10.36)<br />
j�1<br />
2<br />
From which we can solve for the concentration at the boundary in terms of other<br />
dependent variables [1]:<br />
y<br />
n�1<br />
y<br />
n<br />
2<br />
� � A y<br />
�<br />
2<br />
1�<br />
An�1,<br />
n�1<br />
Bi<br />
b n�1, j j<br />
Bi j�1<br />
(10.37)<br />
The mass balance equation together with initial and boundary conditions are solved<br />
numerically <strong>by</strong> combination of collocation and Runge-Kutta methods using<br />
MATLAB [1]. Both the analytical solution and numerical solution give the same<br />
results for the single particle adsorption model using local linear isotherm [1].<br />
The developed model is used to simulate mercury adsorption <strong>by</strong> a single<br />
Darco Hg activated carbon particle exposed to elemental mercury at 150�C in
simulated cement kiln flue gas with 1000 ppmv NO, 23 ppmv NO2, 1000 ppmv SO2,<br />
10 ppmv HCl, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2. The inputs to the model<br />
are presented in table 10.3. The Henry’s constant is derived <strong>from</strong> fixed-bed<br />
experiments using 10 mg Darco Hg carbon in 2 g sand. Table 10.4 presents the<br />
calculated diffusion coefficients, external mass transfer coefficient and Biot number<br />
<strong>by</strong> the program.<br />
Table 10.3. Inputs to the single particle adsorption model.<br />
Parameters Unit Value<br />
Temperature �C 150<br />
Carbon particle diameter µm 16<br />
Carbon true density kg/m 3 2200 [1]<br />
Carbon particle porosity - 0.73 [1]<br />
Carbon pore radius nm 10 [1]<br />
Bed porosity - 0.5<br />
Reactor diameter mm 18<br />
Flow rate Nl/min 2.75<br />
Hg inlet concentration µg/Nm 3 170<br />
Henry’s constant m 3 /g 10.24<br />
Collocation point number - 10<br />
Table 10.4. Calculated diffusion coefficients, external mass transfer coefficient and<br />
Biot number <strong>by</strong> the single particle adsorption model.<br />
Parameters Unit Value<br />
Diffusion coefficient of Hg 0 in N2, DHg,N2 m 2 /s 2.44e-5<br />
Knudsen diffusion coefficient of Hg 0 , DK m 2 /s 1.41e-6<br />
Pore diffusion coefficient, Dp m 2 /s 1.33e-6<br />
Effective pore diffusion coefficient, De m 2 /s 7.10e-7<br />
Apparent diffusion coefficient, Dapp m 2 /s 1.17e-13<br />
External mass transfer coefficient, km m/s 3.37<br />
Biot number - 37.95<br />
The concentration profiles of elemental mercury inside the particle at different<br />
times are illustrated in figure 10.1. At the beginning there is a sharp concentration<br />
profile inside the particle, indicating larger diffusion resistance inside the particle<br />
compared to the boundary layer. At later stage, the concentration profile becomes flat<br />
224
until the mercury concentration reaches the bulk level at all positions inside the<br />
particle when the adsorption equilibrium is obtained.<br />
Figure 10.1. Simulated mercury concentration profile inside the particle at different<br />
time. The corresponding time for each curve <strong>from</strong> bottom to top is 0.06, 0.29, 0.77,<br />
2.20, 5.40, 14.04, 33.84, 94.32, 323.64, 1425.24, 3600 s, respectively. Inputs to the<br />
model are given in table 10.3.<br />
The amount of elemental mercury adsorbed as a function of time for different<br />
particle sizes is illustrated in figure 10.2. The calculated external mass transfer<br />
coefficient, Biot number, and time for equilibrium adsorption are given in the table<br />
10.5. The larger the particle is, the larger the Biot numbers are. This indicates that the<br />
larger particle has larger internal mass transfer resistance. As a result, it takes the<br />
larger particle longer time to reach the equilibrium. For a 16 µm particle it takes 7.2<br />
min to reach the equilibrium, while for a 100 µm particle it takes more than four<br />
hours.<br />
225
Figure 10.2. Simulated amount of mercury adsorbed in the Darco Hg carbon particle<br />
as a function of time for particles with different diameters.<br />
Table 10.5. Calculated external mass transfer coefficient, Biot number, and simulated<br />
equilibrium approach time for Darco Hg carbon with different particle sizes.<br />
d=5 �m d=16 �m d=50 �m d=100 �m d=200 �m d=300 �m<br />
External<br />
mass transfer<br />
coefficient,<br />
km (m/s)<br />
10.26 3.37 1.18 0.64 0.36 0.26<br />
Biot number 36.14 37.95 41.48 45.15 50.73 55.24<br />
99%<br />
equilibrium<br />
approach<br />
time (h)<br />
0.01 0.12 0.89 4.28 17.04 36.41<br />
10.4 Fixed bed adsorption model<br />
In this project a plug flow model with linear equilibrium isotherm, external<br />
and intraparticle mass transfer resistances is developed. Due to the low level of<br />
226
mercury applied in this project the system can be treated as isothermal. The plug-flow<br />
model means that the fluid velocity profile is uniform at all radial positions, a fact<br />
which generally involves turbulent flow conditions. In addition, it is assumed that the<br />
fixed-bed adsorption reactor is packed randomly with adsorbent particles. The<br />
adsorption process is supposed to be very fast relative to the convection and diffusion<br />
effects; subsequently, local equilibrium will exist inside the adsorbent particles [5].<br />
If the solid particles are small, the axial diffusion effects can be ignored and<br />
the main mode of transport in the mobile fluid phase is <strong>by</strong> convection [6]. Consider a<br />
section of the fixed bed column with a length of �z, cross section area of A, and bed<br />
porosity of �b, as shown in figure 10.3, a mass balance of the mercury contained in<br />
both phase, we get [6]:<br />
�C<br />
�q<br />
v0 AC(<br />
z,<br />
t)<br />
� v0<br />
AC(<br />
z � �z,<br />
t)<br />
� � b A�z<br />
� ( 1�<br />
� b ) A�z<br />
(10.38)<br />
�t<br />
�t<br />
where v0 is the superficial fluid velocity. Dividing through <strong>by</strong> A�z and taking limit,<br />
we get the overall balance of the mercury [6]:<br />
�C �C �q<br />
v0<br />
��b �(1 ��b) �0<br />
�z �t �t<br />
q is the volume-average mercury loading per unit volume of porous pellet,<br />
Figure 10.3. Sketch of a fixed-bed absorber.<br />
Using the void velocity u, we get:<br />
�C �C 1�<br />
�b<br />
�q<br />
u � � �0<br />
�z �t � �t<br />
b<br />
227<br />
(10.39)<br />
(10.40)
q can be expressed as [2]:<br />
3 P R<br />
� � (10.41)<br />
2<br />
q r qdr<br />
3<br />
RP<br />
0<br />
where Rp is the radius of the carbon particle.<br />
Equation 10.40 gives the concentration of the mercury in the bulk gas as a<br />
function of time and location in the bed. The concentration of mercury in the gas<br />
within the pores of a carbon particle is obtained <strong>by</strong> solving equation 10.13.<br />
The simultaneous solution of equation 10.13 and10.41 is a hard task, which<br />
can be avoided <strong>by</strong> using the tank-in-series method. The fixed-bed is divided into N<br />
equal size well-mixed tanks and the mercury mass balance in the bulk gas phase for<br />
each tank can be written as:<br />
dCbi<br />
,<br />
� (10.42)<br />
bVi� FCbini , �FCbi , �kmNpAs( Cbi , �Csi<br />
, )<br />
dt<br />
where Vi is the volume of each tank, F is the flow rate through the bed, Cbin,i and Cb,i<br />
is the inlet and outlet mercury concentration in tank i, respectively, Np is the particle<br />
number in tank i, As is the outer surface area of one particle, Cs,i is the gaseous<br />
mercury concentration at the particle surface in tank i. Giving the bed cross area A,<br />
bed height h, total mass of sorbent in the bed M, void velocity u, particle radius Rp,<br />
and density �p, the above equation can be expressed as:<br />
Ah dCbi<br />
,<br />
3M<br />
�b �u�bA( Cbin, i �Cb, i) �km ( Cb, i �Cs,<br />
i)<br />
N dt NRp�p(1<br />
��<br />
p)<br />
Further arranging equation 10.43 into:<br />
dC uN 3Mk<br />
� ( C �C ) � ( C �C<br />
)<br />
dt h R Ah<br />
bi ,<br />
m<br />
bini , bi ,<br />
�b p�p(1 ��<br />
p)<br />
bi , si ,<br />
228<br />
(10.43)<br />
(10.44)<br />
Initial condition:<br />
t=0, C=0, Cbin,i=Cb0, all tanks (10.45)<br />
t>0 Cbin= Cb0, tank 1 (10.46)<br />
Boundary conditions:<br />
�C<br />
r �0, �0<br />
�r<br />
�C<br />
r � R, �D �k ( C �C<br />
)<br />
e m R b, i<br />
�r<br />
r�R (10.47)<br />
(10.48)
Dimensionless equation can be written as:<br />
2<br />
dybi , R � p uN 3Mk<br />
�<br />
m<br />
� � ( ybini , � ybi , ) � ( ybi , � y ) 1, i �<br />
d� Dapp �� h �bRp�p(1 ��<br />
p)<br />
Ah ��<br />
Initial condition:<br />
229<br />
(10.49)<br />
�=0, y=0, ybin,i=1, all tanks (10.50)<br />
�>0 ybin= 1, first tank (10.51)<br />
Boundary conditions become:<br />
�y<br />
u �0, �0<br />
�u<br />
(10.52)<br />
�y<br />
Bi<br />
u �1; � ( yb� y)<br />
(10.53)<br />
�u<br />
2<br />
The boundary-value partial differential equation along the particle radius<br />
(equation 10.19) is solved <strong>by</strong> the orthogonal collocation method [4]. The particle<br />
radius is represented <strong>by</strong> n interior collocation points. The boundary point is the<br />
(n+1) th point. For each tank the initial-value ordinary equations contain n+1 equation<br />
for the particle collocation points and another equation for the bulk phase mercury in<br />
the bed:<br />
2<br />
dyn 2 R �uN 3Mk<br />
�<br />
�<br />
m<br />
� � ( ybin, i � yn�2) � ( yn�2 � yn�1)<br />
�<br />
d� Dapp �� h �bR�p(1 ��)<br />
Ah ��<br />
(10.54)<br />
For the whole fixed-bed the resulting system of N(n+2) ordinary differential<br />
equations are solved <strong>by</strong> the MATLAB routine ode15s.<br />
Besides the inputs to the single particle adsorption model, the inputs to the<br />
developed fixed bed adsorption model also include tank number of 20 after parameter<br />
study the effect of tank number, a bed thickness of 5 mm, which corresponds to a<br />
mixture of 10 mg Darco Hg carbon with 2 g sand powder in the reactor, actual and<br />
baseline concentrations of SO2, NO2, and H2O in the simulated cement kiln flue gas,<br />
preexponential factor of Henry’s constant of 0.869 m 3 /g and heat of adsorption of -<br />
8543 J/mol as presented in chapter 8.
A parameter study of the model was first conducted to evaluate the effect of<br />
collocation point number inside the carbon particle and tank number on the mercury<br />
breakthrough curve of the carbon bed. Figure 10.4 illustrates the effects of collocation<br />
point number inside the carbon particle on the mercury breakthrough curve of 10 mg<br />
Darco Hg carbon tested at 75�C with 90 µg/Nm 3 mercury in the simulated cement<br />
kiln flue gas. Reasonable agreement between the simulation and experimental data is<br />
already obtained using one collocation point inside the carbon particle. Simulations<br />
using 2, 5, and 10 collocation points generate the same mercury breakthrough curve.<br />
Generally more accurate solutions can be obtained using more collocation points.<br />
Since the simulation can be done within 30 s, a collocation point of 10 is used as<br />
default input to the program.<br />
Figure 10.4. Effects of collocation point number inside the carbon particle on the<br />
simulated mercury breakthrough curves of 10 mg Darco Hg mixed with 2 g sand<br />
tested at 75�C using 2.75 Nl/min simulated flue gas with 90 µg Hg 0 /Nm 3 , 10 ppmv<br />
HCl, 1000 ppmv NO, 1000 ppmv SO2, 23 ppmv NO2, 1 vol.% H2O, 6 vol.% O2, and<br />
21 vol.% CO2. A tank number of 20 is applied in the simulation.<br />
230
Figure 10.5 presents the effects of applied tank number in the simulation on<br />
the predicted mercury breakthrough curve of 10 mg Darco Hg carbon tested at 75�C<br />
with 90 µg/Nm 3 mercury in the simulated cement kiln flue gas. Better agreement<br />
between the simulation and experimental data is obtained when larger tank number is<br />
applied. When the tank number is above 20, the produced breakthrough profile is<br />
almost the same.<br />
Figure 10.5. Effects of applied tank number on the simulated mercury breakthrough<br />
curves of 10 mg Darco Hg mixed with 2 g sand tested at 75�C using 2.75 Nl/min<br />
simulated flue gas with 90 µg Hg 0 /Nm 3 , 10 ppmv HCl, 1000 ppmv NO, 1000 ppmv<br />
SO2, 23 ppmv NO2, 1 vol.% H2O, 6 vol.% O2, and 21 vol.% CO2. A collocation point<br />
number of 10 inside the carbon particle is applied in the simulation.<br />
Validation and parametric study of mercury adsorption <strong>by</strong> the activated<br />
carbon is conducted <strong>by</strong> simulation and comparison with the experimental data as<br />
shown in figure 10.6-10.12. The developed fixed bed model can reasonably simulate<br />
the effects of temperature, mercury inlet concentration, flow gas rate, carbon particle<br />
size, and SO2, H2O, NO2 level in the flue gas on the mercury breakthrough curve of<br />
fixed bed with 10 mg carbon in 2g sand powder. Isotherm study of crushed Norit<br />
231
RB4 pellets is not performed and the Henry’s constant for Norit RB4 carbon is<br />
calculated <strong>by</strong> comparing the equilibrium adsorption capacity with Darco Hg carbon at<br />
the same conditions.<br />
Figure 10.6. Comparison of simulation and experimental data for effect of adsorption<br />
temperature on mercury breakthrough of 10 mg Darco Hg mixed with 2 g sand using<br />
2.75 Nl/min simulated flue gas with 90 µg Hg 0 /Nm 3 ,1000 ppmv NO, 23 ppmv NO2,<br />
10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
The model can clearly simulate the effect of adsorption temperature on<br />
mercury breakthrough curve of the carbon bed, i.e., faster mercury breakthrough is<br />
obtained at higher adsorption temperature. The Henry constants at each temperature<br />
are calculated <strong>from</strong> the derived preexponential factor and heat of adsorption. The best<br />
agreement between the simulation and experimental data is obtained for adsorption<br />
test at 75�C as shown in figure 10.6.<br />
232
Figure 10.7. Comparison of simulation and experimental data for effect of elemental<br />
mercury inlet level on mercury breakthrough of 10 mg Darco Hg mixed with 2 g sand<br />
tested at 120�C using 2.75 Nl/min simulated flue gas with 1000 ppmv NO, 23 ppmv<br />
NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
Figure 10.7 shows that the best agreement between the simulation and<br />
experimental data regarding the effects of mercury inlet level is the mercury<br />
breakthrough curve of 57 µg/Nm 3 mercury level. The simulation slightly overpredicts<br />
the mercury adsorption with 95 µg/Nm 3 and underpredicts the mercury adsorption for<br />
tests with 27 µg/Nm 3 in the flue gas. When taking the experimental uncertainty into<br />
account, the simulation is acceptable as the average uncertainty of the equilibrium<br />
mercury adsorption capacity is about ±10%.<br />
233
Figure 10.8. Comparison of simulation and experimental data for effect of flue gas<br />
flow rate on mercury breakthrough of 10 mg Darco Hg mixed with 2 g sand tested at<br />
150�C using 2.75 Nl/min simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv<br />
NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and<br />
6 vol.% O2.<br />
Figure 10.8 illustrates that there are good agreements between the simulations<br />
and experimental data regarding the initial mercury breakthrough time when the<br />
mercury concentration after the carbon bed starts to increase and the model correctly<br />
predicts that. The model overpredicts the effect of changing gas flow especially for<br />
high flow rates.<br />
234
Figure 10.9. Comparison of simulation and experimental data for effect of particle<br />
size on mercury breakthrough for 10 mg crushed Norit RB4 pellets in 2 g sand tested<br />
at 150�C using 2.75 Nl/min simulated flue gas with 160-170 µg Hg 0 /Nm 3 , 1000 ppmv<br />
NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and<br />
6 vol.% O2.<br />
There are good agreements between the simulations and experimental data for<br />
the effect of particle size for Norit RB4 over the size range of 38-325 µm, as<br />
illustrated in figure 10.9.<br />
235
Figure 10.10. Comparison of simulation and experimental data for effect of SO2<br />
concentration in the flue gas on mercury breakthrough profile of 10 mg Darco Hg<br />
mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated flue gas with 160-<br />
170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1 vol.% H2O, 6<br />
vol.% O2, and 21 vol.% CO2.<br />
236
Figure 10.11. Comparison of simulation and experimental data for effect of water<br />
concentration in the flue gas on mercury breakthrough profile of 10 mg Darco Hg<br />
mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated flue gas with 160-<br />
170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 6<br />
vol.% O2, and 21 vol.% CO2.<br />
237
Figure 10.12. Comparison of simulation and experimental data for effect of NO2<br />
concentration in the flue gas on mercury breakthrough curves of 10 mg Darco Hg<br />
mixed with 2 g sand tested at 150�C using 2.75 Nl/min simulated flue gas with 160-<br />
170 µg Hg 0 /Nm 3 , 10 ppmv HCl, 1000 ppmv NO, 1000 ppmv SO2, 1 vol.% H2O, 6<br />
vol.% O2, and 21 vol.% CO2.<br />
Figure 10.10 and 10.11 show that there are good agreements between the<br />
simulations and experimental data on the effects of SO2 and H2O levels in the<br />
simulated cement kiln flue gas. The model slightly overpredicts the mercury<br />
adsorption rate with 100 and 5 ppmv NO2 in the flue gas and slightly underpredicts<br />
the mercury adsorption with 23 ppmv NO2 as shown in figure 10.12. The effects of<br />
these gases on the mercury adsorption capacity are evaluated <strong>by</strong> the derived<br />
correlations between mercury adsorption capacity and gas concentrations, as<br />
presented in chapter 8.<br />
238
10.5 Conclusions<br />
Mathematical models for mercury adsorption <strong>by</strong> a single carbon particle and a<br />
fixed carbon bed are developed. Local equilibrium within the carbon particle is<br />
assumed and the adsorption system is assumed to be isothermal due to the low<br />
mercury concentration presented in the flue gas. The models account for both the<br />
external and internal mass transfer resistances. The orthogonal collocation method is<br />
used to solve mercury diffusion and adsorption inside a sorbent particle. The fixedbed<br />
model is solved <strong>by</strong> a tank-in-series method.<br />
Henry’s constant obtained <strong>from</strong> fixed-bed investigation of mercury adsorption<br />
<strong>by</strong> activated carbon in the simulated cement kiln flue gas is used as input to the<br />
models. The single particle model can simulate the mercury concentration profile and<br />
amount of adsorbed mercury inside the carbon particle as a function of adsorption<br />
time.<br />
The developed fixed bed model can reasonably simulate the effects of<br />
adsorption temperature, mercury inlet concentration, flow gas rate, carbon particle<br />
size, and SO2, H2O, NO2 level in the flue gas on the mercury breakthrough curve of<br />
fixed bed with 10 mg carbon in 2 g sand powder. The developed models are useful<br />
tools for understanding the mercury adsorption <strong>by</strong> the activated carbon and<br />
interpretation of the experimental results.<br />
10.6 List of symbols<br />
A: carbon particle external surface area (m 2 )<br />
A: cross area of the fixed-bed (m 2 )<br />
A: matrix in equation 10.24<br />
As: outer surface area of one carbon particle (m 2 )<br />
b: Langmuir equilibrium constant (m 3 /g)<br />
B: matrix in equation 10.25<br />
Bi: dimensionless Biot number<br />
C: gaseous mercury concentration (µg/m 3 )<br />
C: matrix in equation 10.26<br />
Cb: gas bulk mercury concentration (µg/m 3 )<br />
Cbo: initial gas bulk mercury concentration (µg/m 3 )<br />
Cb,i: outlet mercury concentration in tank i (µg/m 3 )<br />
239
Cbin,i: inlet mercury concentration in tank i (µg/m 3 )<br />
Ci: initial mercury concentration (µg/m 3 )<br />
Cs,i : gaseous mercury concentration at the particle surface in tank i (µg/m 3 )<br />
cp: specific heat (J/(kg.K)<br />
Cs: gaseous mercury concentration at the particle surface (µg/m 3 )<br />
Cµ: adsorbed mercury concentration in the sorbent (µg/m 3 )<br />
Cµs: saturated concentration of adsorbed mercury in the sorbent (µg/m 3 )<br />
DAB: binary molecular diffusion coefficient (m 2 /s)<br />
Dapp: apparent diffusion coefficient (m 2 /s)<br />
De: effective diffusion coefficient (m 2 /s)<br />
Di: molecular diffusion coefficient (m 2 /s)<br />
Dk: Knudsen pore diffusion coefficient (cm 2 /s)<br />
dp: particle diameter (m)<br />
F: gas flow rate (m 3 /s)<br />
G: fluid mass velocity (kg/(m 2 .s))<br />
h: heat transfer coefficient (W/(m 2 .K))<br />
h: bed height (m)<br />
k: thermal conductivity (W/(m.K))<br />
k : the Boltzmann’s constant (J/K)<br />
K: Henry’s constant<br />
KF: Frendlich constant<br />
km: gas film mass transfer coefficient (m/s)<br />
M: carbon load in the fixed-bed (mg)<br />
MW: mole weight (g/mol)<br />
n: exponent in isotherm equations of Frendlich, and Langmuir-Frendlich<br />
n: number of interior collocation points<br />
N: amount of transported mercury in equation 10.1 (µg)<br />
N: tank number<br />
Np: carbon particle number in the tank<br />
NPr: Prandtl number<br />
NSc: Schmidt number<br />
NRe: Reynolds number<br />
P: pressure (atm)<br />
q: mercury concentration in the sorbent (µg/m 3 )<br />
Q: heat (W)<br />
r: radial coordinate (m)<br />
r: pore radius (cm)<br />
Rg: universal gas constant, 8.314 (J/(mol.K))<br />
Rp: sorbent particle radius (m)<br />
s: heterogeneity parameter in Unilan isotherm equation<br />
240
s: the particle shape factor (s=0, 1, and 2 for slab, cylinder, and sphere, respectively)<br />
t: exponent in isotherm equations of Toth<br />
t: time (s)<br />
T: temperature (K)<br />
Tb : gas temperature in the bulk (K)<br />
Ts: gas temperature at the particle surface (K)<br />
u: dimensionless parameter, u=x 2<br />
u: void velocity (m/s)<br />
v0: superficial fluid velocity (m/s)<br />
Vi: volume of the tank (m 3 )<br />
Vp: volume of the carbon particle (m 3 )<br />
wk: Radau quadratue weight<br />
Wk: normalized Radau quadratue weight<br />
x: dimensionless radius<br />
y: dimensionless mercury concentration<br />
yb: dimensionless gas bulk mercury concentration (µg/m 3 )<br />
yi: dimensionless initial mercury concentration (µg/m 3 )<br />
z: axial coordinate (m)<br />
Greek symbols<br />
�: parameter defined <strong>by</strong> equation 10.31<br />
�: parameter defined <strong>by</strong> equation 10.31<br />
�AB: Lennard-Jones 12-6-potentials for specie A and B<br />
�b: bed void fraction<br />
�p: sorbent particle porosity<br />
�p: sorbent particle density (kg/m 3 )<br />
µ: dynamic viscosity (kg/(m.s))<br />
�AB : molecular collision diameter (Å)<br />
�D,AB: dimensionless parameter in equation 10.8 and 10.11<br />
�Hads: heat of adsorption (J/mol)<br />
�: dimensionless time<br />
10.7 References<br />
[1] D.D. Do. Adsorption analysis: equilibria and kinetics, Imperial College Press, 1998.<br />
[2] J.D. Seader, E.J. Henley, Separation process principles, John Wiley & Sons, Inc. 1998.<br />
[3] P.J. Gardner, P. Pang, S.R. Preston, Binary gaseous-diffusion coefficients of mercury and<br />
of zinc in hydrogen, helium, argon, nitrogen, and carbon-dioxide, J. Chem. Eng. Data. 36<br />
(1991) 265-268.<br />
241
[4] J. Villadsen, M.L. Michelsen, Solutions of differential equation models <strong>by</strong> polynomial<br />
approximation, Prentice-Hall, Inc., 1978.<br />
[5] D.M. Ruthven, Principles of adsorption and adsorption processes, John Wiley & Sons,<br />
Inc., 1984.<br />
[6] R.G. Rice, D.D. Do. Applied mathematics and modelling for chemical engineers, John<br />
Wiley & Sons, Inc, 1995.<br />
242
243<br />
11<br />
Simulation of mercury removal <strong>by</strong> activated<br />
carbon injection upstream of a fabric filter<br />
This chapter deals with the development of a two-stage model for simulation<br />
of mercury removal <strong>by</strong> carbon injection upstream of a fabric filter. First the<br />
development of duct-fabric filter models is presented, and then the models are<br />
compared with available experimental data <strong>from</strong> pilot-scale investigation.<br />
11.1 Common assumptions for mercury removal in the duct and<br />
fabric filter<br />
<strong>Mercury</strong> removal <strong>by</strong> the sorbent injection upstream of a fabric filter consists<br />
of two stages, i.e., the duct and filter sections as illustrated in figure 11.1. Powdered<br />
sorbent such as activated carbon is metered to the injection point at a rate<br />
proportional to the gas stream flow. Once dispersed, mercury species diffuse to the<br />
particle surface and migrate into pores of the activated carbon particle. The carbon<br />
particles remain suspended in the moving gas stream in the duct for periods of one to<br />
three seconds. It then deposits onto the carbon cake formed on the filter bags.<br />
Additional mercury capture takes place when the mercury-containing gas stream<br />
passes through the carbon cake. The carbon cake grows with filtration time and after<br />
a certain time the pressure drop across the filter reaches its threshold value and the<br />
cleaning process is initiated <strong>by</strong> pulse injection of compressed air. A fraction of the<br />
filter bags is periodically cleaned to relieve the pressure drop across the fabric filter.
Figure 11.1. Sketch of the mercury removal process <strong>by</strong> carbon injection upstream of a<br />
fabric filter.<br />
A mathematical model is a useful tool to simulate the mercury capture and<br />
evaluate the mercury removal efficiency for various operational conditions. An<br />
advanced model can provide a rational basis for describing and characterizing the<br />
effectiveness of mercury removal <strong>by</strong> sorbent injection and provide guidelines for<br />
developing new types of sorbents and improve of the process.<br />
To make the mathematics tractable, following assumptions are made:<br />
1. The relevant mercury species in the gas phase is assumed to be either<br />
elemental mercury or mercuric chloride. Elemental mercury is much more difficult to<br />
remove if the sorbent cannot oxidize it. As shown in chapter 8 and 9, similar<br />
adsorption behavior of elemental mercury and mercury chloride <strong>by</strong> activated carbon<br />
is observed since significant oxidation of mercury <strong>by</strong> the activated carbon occurs if<br />
HCl is present in the gas above few ppmv. This is the case in most practical systems<br />
and will be assumed here.<br />
2. Activated carbon particles are spherical, uniform in size and uniformly<br />
dispersed in the duct and filter cake.<br />
244
3. The temperature is constant and uniform through the system. <strong>Mercury</strong><br />
adsorption heat effects are neglected due to the trace level mercury concentrations.<br />
The adsorption equilibrium is described <strong>by</strong> Henry’s law as shown in chapter 8.<br />
4. Both the gas and the solid flow rates are constant. In reality, there are changes<br />
of both differential pressures over the filter bag and cake porosity with time. The<br />
cleaned section of the filter <strong>by</strong> pulse jet would have less hydraulic resistance,<br />
resulting in a larger fraction of the flow diverted to this section. There would be a<br />
dynamic redistribution of the flow as the cake grows on the filter bag surface. Flora et<br />
al. [1] evaluated the effect of the dynamic redistribution of flow on removal of<br />
mercury using activated carbon injection in a fabric filter system. The magnitude of<br />
this impact is small compared with the potential impact caused <strong>by</strong> uncertainties in the<br />
isotherm and mass transfer parameters. When the differential pressure over the bag<br />
increases the system fan speed will be increased correspondingly to maintain the<br />
same filtration velocity. It is therefore reasonable to assume constant gas flow<br />
through the filter cake.<br />
5. <strong>Mercury</strong> adsorption on the duct walls and filter fabric is negligible.<br />
Equilibrium conditions are reached between the gas phase and walls/fabric so that no<br />
net exchange of mercury is present.<br />
6. <strong>Removal</strong> of mercury <strong>from</strong> the bulk gas phase is caused solely <strong>by</strong> adsorption<br />
on the activated carbon.<br />
7. A mass transfer boundary layer causes resistance to mass transfer <strong>from</strong> the<br />
bulk gas phase to the activated carbon particle external surface, and mass transfer<br />
within the carbon particle is controlled <strong>by</strong> pore diffusion.<br />
8. Since the mercury level in the flue gas is very low and the surface diffusivity<br />
is a strong function of the amount of mercury adsorbed, it is reasonable to assume<br />
that the surface diffusion resistance can be neglected.<br />
9. The free gaseous mercury molecules in the pore and the adsorbed mercury<br />
molecules at any point within a particle are in equilibrium with each other. The local<br />
adsorption kinetics is much faster than the diffusion process into the particle.<br />
245
11.2 Duct model<br />
Part of the mercury is removed in the duct section. Simulation of mercury<br />
removal in the duct is presented in this section taking into account relevant<br />
mechanisms.<br />
The flue gas is assumed to travel in plug flow along the duct. To verify<br />
whether the slip velocity between the activated carbon particles and the gas is<br />
relevant, Scala evaluated the terminal velocity of the particles for the particle sizes of<br />
interest, e.g., less than 100 �m [2-4]. Results indicated that terminal velocities are<br />
always more than one order of magnitude lower than typical flue gas velocity so that<br />
it is reasonable to assume that particles travel at the same velocity as the flue gas. The<br />
particle Reynolds number was always smaller than one, justifying the assumption of<br />
Stokes regime.<br />
Mass balance around a thin shell element in the spherical particle gives:<br />
�C �C�<br />
1 � 2 �C<br />
� p ��p(1 �� p) � De ( r )<br />
(11.1)<br />
2<br />
�t �t r �r �r<br />
where �p is the porosity of the particle, C is the gaseous mercury concentration, C� is<br />
the adsorbed mercury per unit mass of the particle, �p is the particle density and De is<br />
the effective diffusivity.<br />
The local linear isotherm takes the form:<br />
C�� KC<br />
(11.2)<br />
where K is the Henry’s constant.<br />
Substituting the local equilibrium into the mass balance equation, we can get:<br />
�C D 1 � �C<br />
� Dapp� C �<br />
( r )<br />
�t � � K r �r �r<br />
2 e<br />
2<br />
� p (1 � p) �p<br />
2<br />
246<br />
(11.3)<br />
Assuming plug flow and no slip velocity, mercury adsorption in the duct can be<br />
treated as a batch adsorber. Assuming perfect mixing, the mass balance of mercury in<br />
the bulk phase is:<br />
dCb<br />
V �� AJ<br />
(11.4)<br />
Rp<br />
dt<br />
where V is the volume of the adsorber, Cb is the concentration of mercury in the<br />
adsorber, A is the total exterior surface area of all carbon particles in the adsorber, and
J is the mass transfer into the carbon particle per unit interfacial area. If the<br />
Rp<br />
particles are spheres, the total exterior surface area is<br />
mp<br />
3<br />
A �<br />
� (1 � � ) R<br />
p p p<br />
where mp is the mass of the particles and Rp is the particle radius. Equation 11.4 can<br />
be rearranged into:<br />
dC m<br />
b p 1 3 3�<br />
3�km<br />
�� J �� J �� ( Cb�C )<br />
Rp Rp Rp<br />
dt V � (1 �� ) R � (1 �� ) R � (1 ��<br />
) R<br />
p p p p p p p p p<br />
247<br />
(11.5)<br />
(11.6)<br />
where � is the carbon load in the flue gas (kg/m 3 ), km is the external mass transfer<br />
coefficient,<br />
C is the gaseous mercury concentration at the carbon particle surface.<br />
Rp<br />
Initial condition: t=0, C=0, Cb=Cb0, (11.7)<br />
Boundary conditions:<br />
�C<br />
r �0, �0<br />
�r<br />
�C<br />
r � R , �D �k ( C �C<br />
)<br />
p e m R b<br />
p<br />
�r<br />
Rp, t<br />
(11.8)<br />
(11.9)<br />
Equations 11.3 and 11.6 are written in a dimensionless form <strong>by</strong> defining the<br />
following non-dimensional variables and parameters:<br />
C r D t C<br />
y � ; x� ; � � ; y � ;<br />
C R R C<br />
app b<br />
2 b<br />
b0 p p b0<br />
where y is the interparticle gas concentration.<br />
�y 1<br />
� 2<br />
��x � 2 �y<br />
( x )<br />
�x �x<br />
dy<br />
d� �<br />
k<br />
� R<br />
R<br />
D<br />
( )<br />
3�k<br />
R<br />
� � D<br />
( )<br />
2<br />
b 3�<br />
m<br />
p m p<br />
�� yb �y �� y 1 b �y<br />
1<br />
p(1 � p) p app p(1 � p) app<br />
(11.10)<br />
(11.11)<br />
Initial condition: �=0, y=0, yb=1 (11.12)<br />
Boundary conditions become:<br />
�y<br />
x �0, �0<br />
�x<br />
(11.13)
�y<br />
kmRp x �1; � ( yb � y) � Bi( yb � y)<br />
�x<br />
D<br />
e<br />
248<br />
(11.14)<br />
The problem of diffusion and adsorption in the carbon particle has symmetry at x=0,<br />
and it is useful to utilize this <strong>by</strong> making the transformation of u=x 2 , and the<br />
differential equation becomes:<br />
2<br />
�y � y �y<br />
�4u�6 2<br />
���u � u<br />
(11.15)<br />
Boundary conditions become:<br />
�y<br />
u �0, �0<br />
(11.16)<br />
�u<br />
�y<br />
Bi<br />
u �1; � ( yb� y)<br />
(11.17)<br />
�u<br />
2<br />
The equation is solved <strong>by</strong> the orthogonal collocation method [5]. The domain<br />
u�(0,1) is represented <strong>by</strong> n interior collocation points. Taking the boundary point<br />
(u=1) as the (n+1) -th point, we have a total of n+1 interpolation points. The first and<br />
second derivatives at these interpolation points are related to the functional values at<br />
all points as given below:<br />
�y<br />
n�1<br />
� Aij y j<br />
�u i j<br />
� (11.18)<br />
�<br />
y<br />
2 n�1<br />
� 2 � By ij j<br />
(11.19)<br />
�u i j<br />
The matrices A and B are constant matrices once n+1 interpolation points have been<br />
chosen. The mass balance equation is valid at any point within the u domain.<br />
Evaluating the equation at the i th interior collocation point we get:<br />
n�1<br />
�yi � Cy ij j<br />
�� j�1<br />
� (11.20)<br />
For i=1, 2,…n+1, where<br />
C �4uB � 6A<br />
(11.21)<br />
ij i ij ij<br />
�y � �<br />
n<br />
i Cy ij j Ci, n�1yn�1 �� j�1<br />
� (11.22)<br />
The boundary condition at the carbon particle surface is:
Bi<br />
n�1<br />
� An�1, jyj � ( yb � yn�1)<br />
(11.23)<br />
j�1<br />
2<br />
From which we can solve for the concentration at the boundary in terms of other<br />
dependent variables [6]:<br />
y<br />
n�1<br />
y<br />
n<br />
2<br />
� � A y<br />
�<br />
2<br />
1�<br />
An�1,<br />
n�1<br />
Bi<br />
b n�1, j j<br />
Bi j�1<br />
Including the equation for the bulk phase mercury,<br />
dy 3�k<br />
2<br />
mR n�<br />
p<br />
�� ( yn�2 �yn�1)<br />
d� � (1 � � ) D<br />
p p app<br />
249<br />
(11.24)<br />
(11.25)<br />
n+1 initial-value ordinary differential equations are solved simultaneously <strong>by</strong><br />
MATLAB routine ode15s.<br />
The developed duct model is very similar to that developed <strong>by</strong> Scala [2-4].<br />
The main difference between the models is that Scale used Langmuir isotherm and<br />
dynamic adsorption, i.e., local equilibrium is not assumed.<br />
The model input parameters are listed in table 11.1 for simulation of mercury<br />
adsorption <strong>by</strong> injection of Darco Hg activated carbon into the duct. The Henry’s<br />
constant is derived <strong>from</strong> fixed-bed investigation as presented in chapter 8. When the<br />
gas composition is different <strong>from</strong> the baseline test, the effect of individual gas on the<br />
mercury adsorption is evaluated using correlations derived <strong>from</strong> chapter 8. Since fullscale<br />
data are not available for comparison it is the intention here to test the model<br />
ability instead of simulating the full-scale application. The simulation results are<br />
analyzed <strong>by</strong> selecting a set of operating variables as a base case for computations and<br />
to assess the influence of the relevant input variables on the process <strong>by</strong> varying them<br />
one at a time.
Table 11.1. Inputs to the duct adsorption model.<br />
Parameters Unit Value<br />
Temperature �C 75-150<br />
Actual SO2 concentration ppmv 1000<br />
Baseline SO2 concentration ppmv 1000<br />
Actual NO2 concentration ppmv 23<br />
Baseline NO2 concentration ppmv 23<br />
Actual H2O concentration % 1<br />
Baseline H2O concentration % 1<br />
Hg inlet concentration µg/Nm 3 170<br />
Carbon particle diameter µm 5-200<br />
Carbon true density kg/m 3 2200<br />
Carbon particle porosity - 0.73<br />
Carbon pore radius nm 10<br />
Carbon injection rate g/m 3 0.05-10<br />
Residence time in the duct s 0-10<br />
Henry’s constant<br />
preexponential factor<br />
m 3 /g 0.869<br />
Heat of adsorption J/mol -8543<br />
Collocation point number - 10<br />
Figure 11.2 illustrates the simulated bulk mercury concentration in the duct as<br />
a function of flight time for different injection rates of 16 µm Darco Hg carbon at<br />
150�C to the baseline gas of 170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2, 10<br />
ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. The gas<br />
bulk mercury concentration is normalized with the inlet mercury concentration. The<br />
mercury concentration decreases with increasing of residence time in the duct and the<br />
carbon load. To obtain 80% mercury removal <strong>by</strong> injection of 16 µm Darco Hg carbon<br />
at 150�C, it needs either a long residence time in the duct, i.e., long duct (> 1 g/m 3<br />
load and 10 s) or large carbon injection rate (10 g/m 3 load and 0.16 s). After 10 s in<br />
the duct the mercury removal efficiency is 77.9% and 97.6% for a carbon injection<br />
rate of 1 and 10 g/m 3 , respectively.<br />
250
Figure 11.2. Simulated gaseous mercury concentration as a function of residence time<br />
in the duct and carbon injection rate. Darco Hg carbon with a diameter of 16 µm is<br />
injected at 150�C to simulated cement kiln flue gas with 170 µg Hg 0 /Nm 3 , 1000<br />
ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.%<br />
CO2, and 6 vol.% O2.<br />
The effects of carbon particle size on mercury concentration in the duct are<br />
illustrated in figure 11.3. The gas bulk mercury concentration decreases faster for<br />
smaller carbon particles during the first 2 s in the duct and a larger mercury removal<br />
is obtained <strong>by</strong> smaller carbon particles at all residence time, indicating that diffusion<br />
resistance is relevant for mercury adsorption on carbon particles. Decreasing the<br />
particle size <strong>from</strong> 16 to 5 µm can increase the mercury removal efficiency <strong>from</strong> 77.9<br />
to 87.6% using an injection rate of 1 g/m 3 at 150�C. The improvement of mercury<br />
removal efficiency <strong>by</strong> further lowering the particle size is less pronounced (not shown<br />
in figure 11.3).<br />
251
Figure 11.3. Simulated gaseous mercury concentration as a function of residence time<br />
in the duct and carbon particle size. 1 g/m 3 Darco Hg carbon is injected at 150�C to<br />
simulated cement kiln flue gas with 170 µg Hg 0 /Nm 3 , 1000 ppmv NO, 23 ppmv NO2,<br />
10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
The effects of particle size on mercury removal can be explained <strong>by</strong> Biot<br />
numbers for different particle sizes as shown in table 10.5. Calculations show that the<br />
larger the particle is, the smaller external mass transfer coefficient, the larger the Biot<br />
numbers are. This indicates that the larger particle has relatively larger internal<br />
transfer resistance. As a result, it takes the larger particle longer time to reach the<br />
equilibrium. In all the cases calculated here, the Biot numbers are much larger than<br />
36, indicating that the internal diffusional resistance is much larger than the external<br />
mass transfer resistance.<br />
The effects of temperature on mercury removal in the duct are presented in<br />
figure 11.4. Similar mercury outlet concentrations are observed for injection 0.5 g/m 3<br />
Darco Hg carbon with a size of 16 µm for the first 2 s in the duct, and then lower<br />
mercury outlet concentrations are obtained with lower flue gas temperature and<br />
longer residence time in the duct.<br />
252
Figure 11.4. Simulated gaseous mercury concentration as a function of residence time<br />
in the duct and flue gas temperature. 0.5 g/m 3 Darco Hg carbon with a diameter of 16<br />
µm is injected at 150�C to simulated cement kiln flue gas with 170 µg Hg 0 /Nm 3 ,<br />
1000 ppmv NO, 23 ppmv NO2, 10 ppmv HCl, 1000 ppmv SO2, 1 vol.% H2O, 21<br />
vol.% CO2, and 6 vol.% O2.<br />
11.3 Model for the filter cake<br />
The filter cake formed on the bags is similar to a fixed-bed with the difference<br />
that new adsorbent is continuously fed. The fixed-bed model with constant bed<br />
thickness developed in chapter 10 is extended here to deal with this situation. During<br />
filtration without cleaning of the bag the carbon cake thickness grows with time. At<br />
the beginning no carbon particles are collected on the bag surface. After a short time<br />
�t a layer of carbon is collected and the corresponding carbon cake thickness is:<br />
�u� b<br />
L � �t<br />
(11.26)<br />
� (1 �� )(1 ��<br />
)<br />
p p b<br />
where � is the carbon load in the flue gas, u is the face velocity on the filter bags.<br />
This thin layer of carbon particles can be treated as a continuous stirred-tank reactor<br />
253
(CSTR). The bed volume is AL, the mass of carbon in the tank<br />
is M � � (1 �� ) V � � (1 �� )(1 � � ) AL.<br />
The mass balance for mercury in the bulk<br />
b p b p p b<br />
phase in the tank becomes:<br />
dCb u 3(1 ��<br />
b) km<br />
� ( Cbin �Cb) � ( Cb� C )<br />
(11.27)<br />
Rp<br />
dt L � R<br />
b p<br />
The dimensionless equation can be written as:<br />
2<br />
dy R �<br />
b p u 3(1 ��<br />
b) k �<br />
m<br />
� � ( ybin � yb) � ( yb� y ) 1 �<br />
(11.28)<br />
d� Dapp ��L �bRp��<br />
n+1 collocation points are used for the carbon particle. The bulk phase mercury<br />
balance equation becomes:<br />
2<br />
dy R �<br />
n�2pu3(1 ��<br />
b) k<br />
�<br />
m<br />
� � ( ybin � yn�2) � ( yn�2 � yn�1)<br />
�<br />
d� Dapp ��L �bRp��<br />
Initial and boundary conditions:<br />
254<br />
(11.29)<br />
�=0, y=0, ybin=1 (11.30)<br />
�y<br />
u �0, �0<br />
�u<br />
(11.31)<br />
1; ( �2 �1)<br />
2<br />
� �<br />
�y<br />
Bi<br />
u � yn<br />
yn<br />
(11.32)<br />
�u<br />
Theses n+1 equations are solved in a time interval of [0 �t] <strong>by</strong> MATLAB<br />
routine ode15s. After another �t, another layer of carbon with thickness of L is<br />
formed on the bag surface and on top of the first layer and is termed as tank 2. Now<br />
the system contains 2(n+1) initial-value ordinary differential equations which are<br />
solved simultaneously in a time interval of [0 �t] <strong>by</strong> MATLAB routine ode15s. The<br />
initial conditions for equations in tank 1 are the calculated concentration <strong>from</strong> last �t<br />
interval. The initial conditions for tank 2 are y=0, ybin,2=1. At �>0, ybin,1= yb,2. The<br />
cycle is conducted to the desired filtration time. Here it is assumed that the carbon<br />
particles are injected just at the filter inlet. The combination of duct injection and<br />
filter cake model will be presented in section 11.5. In the later case the carbon<br />
particles arriving in the filter will have already adsorbed mercury with some radial<br />
profile, i.e., y�0.
Simulation of mercury removal <strong>by</strong> the fixed-bed with moving boundary is<br />
performed using the conditions <strong>from</strong> the Durkee pilot plant study [7,8]. The inputs to<br />
the model are given in table 11.2. The flue gas temperature is taken <strong>from</strong> the field test<br />
report [7,8] and the flue gas compositions are supplied <strong>by</strong> Paone [9]. Other gas<br />
concentrations are the same as the baseline gas. The effects of CO2 and HCl are not<br />
accounted for since the effects of these gases are less pronounced compared to SO2,<br />
NO2 and H2O. Referring to results <strong>from</strong> chapter 8, when the CO2 level in the flue gas<br />
is above 21 vol.%, which is used in baseline test and deriving of the adsorption<br />
kinetics, the mercury adsorption capacity of the carbon is only slightly decreased.<br />
With HCl in the gas up to 15 ppmv, the mercury adsorption capacity is almost not<br />
affected <strong>by</strong> changing the HCl level in the gas. Large adsorption capacity is obtained<br />
without HCl in the gas.<br />
Table 11.2. Inputs to the filter cake model.<br />
Parameters Unit Value<br />
Temperature �C 138<br />
Actual SO2 concentration ppmv 5<br />
Baseline SO2 concentration ppmv 1000<br />
Actual NO2 concentration ppmv 5<br />
Baseline NO2 concentration ppmv 23<br />
Actual H2O concentration % 15<br />
Baseline H2O concentration % 1<br />
Hg inlet concentration µg/Nm3 200<br />
Carbon particle diameter µm 16<br />
Carbon true density kg/m 3 2200<br />
Carbon particle porosity - 0.73<br />
Carbon pore radius nm 10<br />
Carbon injection rate mg/m 3 8-80<br />
Filtration time s 1500<br />
Air to cloth ratio m/min 1.2<br />
Time for new cake layer min 0.5-5<br />
Henry’s constant<br />
preexponential factor<br />
m 3 /g 0.869<br />
Heat of adsorption J/mol -8543<br />
Collocation point number - 10<br />
255
The effect of the time for new carbon layer addition �t on the mercury<br />
removal efficiency of the fabric filter is illustrated in figure 11.5. Generally the more<br />
frequently the new layer is added the larger mercury removal efficiency is obtained<br />
for filtration time less than 1200 s. Smoother mercury removal efficiency curve will<br />
be obtained using a smaller time interval for adding a carbon layer. For short<br />
filtration time the new carbon layer should be added very fast in the simulation,<br />
otherwise the simulated mercury removal efficiency will be smaller due to the delay<br />
of new carbon layer addition. For filtration time larger than 1200 s same mercury<br />
removal efficiency is predicted with 5 min interval for new carbon layer addition as<br />
with smaller time interval. However, the computation time of the program is<br />
considerably reduced using an interval of 5 min compared to 1 min.<br />
<strong>Mercury</strong> removal, %<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
30 s<br />
60 s<br />
120 s<br />
300s<br />
0<br />
0 300 600 900 1200 1500 1800<br />
Time (s)<br />
Figure 11.5. Simulated effects of new cake layer addition frequency on the mercury<br />
removal efficiency of a fabric filter without cleaning of the bags. 16 mg/m 3 Darco Hg<br />
carbon with a diameter of 16 µm is injected at 138�C to simulated cement kiln flue<br />
gas with 200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2,<br />
15 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.<br />
256
Figure 11.6 presents the simulated mercury removal efficiency <strong>by</strong> a fabric<br />
filter without cleaning of the bags as a function of filtration time and injection rate of<br />
carbon. Every minute a layer of carbon is added to the bag surface. As expected,<br />
larger mercury removal efficiency is obtained with higher carbon injection rate. The<br />
mercury removal efficiency increases fast with time after initiating carbon injection<br />
up to 600 s and then it slowly increases with filtration time.<br />
Figure 11.6. Simulated mercury removal efficiency <strong>by</strong> a fabric filter without cleaning<br />
of the bags as a function of filtration time and injection rate of carbon. Darco Hg<br />
carbon with a diameter of 16 µm is injected at 138�C to simulated cement kiln flue<br />
gas with 200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2,<br />
15 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. �t= 1 min.<br />
11.4 Fabric filter model<br />
In reality the filter bags are periodically cleaned. Therefore the filter cake<br />
model needs to be extended with periodical cleaning of the bags in order to simulate<br />
mercury adsorption in the bag filter. Assuming the cleaning cycle interval is tclean,<br />
fraction of filter cleaned per cycle is fclean. After first tclean, the carbon cakes on the<br />
257
ag surface have different exposure time to mercury and the mercury removal<br />
efficiency is the average of the mercury removal efficiency in different sections of the<br />
filter. Table 11.3 illustrates the carbon cake life time in different sections of the fabric<br />
filter and the calculation of average mercury removal efficiency across the filter as a<br />
function of filtration time. Pulse duration is selected as 0.1 second. Symbol * means a<br />
pulse with 0.1/60 min and indicates that a fraction of the filter bags is cleaned.<br />
Table 11.3 Illustration of carbon cake lifetime for different filter sections due to<br />
periodic cleaning of bags. Here tclean=25 min; fclean=0.1.<br />
Filtration time Exposure time of Average mercury removal efficiency<br />
(min) different filter sections<br />
0 0 0<br />
0-25 10@[0 25] �<br />
25 10@[25]<br />
25* 1@[0], 9@[25]<br />
25*-50 1@[0 25], 9@[0 50]<br />
50 1@[25], 9@[50]<br />
50* 1@[0], 1@[25], 8@[50]<br />
50*-75 1@[0-25], 1@[0 50],<br />
8@[0 75]<br />
75 1@[25], 1@[50], 8@[75]<br />
75* 1@[0], 1@[25], 1@[50],<br />
7@[75]<br />
75*-100 1@[0 25], 1@[0 50],<br />
258<br />
�<br />
[0 25]<br />
[25]<br />
9<br />
[25]<br />
10 �<br />
1<br />
��[0 25] 9�[0<br />
50] �<br />
10 �<br />
1<br />
��[25] 9�[50]<br />
�<br />
10 �<br />
1<br />
�� [25] 8�<br />
[50] �<br />
10 �<br />
1<br />
��[0 25] ��[0 50] � 8�[0<br />
75] �<br />
10<br />
1<br />
��[25] ��[50] � 8�[75]<br />
�<br />
10<br />
1<br />
��[25] ��[50] � 7�[75]<br />
�<br />
10<br />
1<br />
�[025] ��[050] ��[075] � 7�[0100]<br />
10<br />
1<br />
�[25] ��[50] ��[75] � 7�[100]<br />
10<br />
1<br />
�[25] ��[50] ��[75] � 6�[100]<br />
10<br />
1 ��[025] � �[050] ��[075] ��[0100] ��<br />
�<br />
10 �<br />
�<br />
6�<br />
�<br />
� [0 125]<br />
�<br />
1 ��[25] ��[50] ��[75] ��[100] ��<br />
�<br />
10 � �<br />
6�<br />
�<br />
� [125]<br />
�<br />
1 ��[25] ��[50] ��[75] ��[100] ��<br />
�<br />
10 � �<br />
5�<br />
�<br />
� [125]<br />
�<br />
1@[ 0 75], 7@[0 100] � �<br />
100 1@[25], 1@[50],<br />
1@[75], 7@[100] � �<br />
100* 1@[0], 1@[25], 1@[50],<br />
1@[75], 6@[100] � �<br />
100*-125 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
6@[0 125]<br />
125 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
6@[125]<br />
125* 1@[0], 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
5@[125]
Filtration time Exposure time of<br />
(min) different filter sections<br />
125*-150 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
1@[0 125], 5@[0 150]<br />
150 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 5@[150]<br />
150* 1@[0], 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 4@[150]<br />
150*-175 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
1@[0 125], 1@[0 150],<br />
4@[0 175]<br />
175 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
4@[175]<br />
175* 1@[0],1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
3@[175]<br />
175*-200 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
1@[0 125], 1@[0 150],<br />
1@[0 175],3@[0 200]<br />
200 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 3@[200]<br />
200* 1@[0], 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 2@[200]<br />
200*-225 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
1@[0 125], 1@[0 150],<br />
1@[0 175], 1@[0 200],<br />
2@[0 225]<br />
225 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 1@[200],<br />
2@[225]<br />
225* 1@[0], 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 1@[200],<br />
1@[225]<br />
259<br />
Average mercury removal efficiency<br />
1 ��[0 �<br />
10 �<br />
��[0 25] ��[050] ��[0 125] � 5�[0<br />
150]<br />
75] ��[0100] ��<br />
�<br />
�<br />
�<br />
1 ��[25] ��[50] ��[75] ��[100] ��<br />
�<br />
10 � �<br />
�[125] 5�<br />
�<br />
� � [150]<br />
�<br />
1 ��[25] ��[50] ��[75] ��[100] ��<br />
�<br />
10 � �<br />
�[125] 4�<br />
�<br />
� � [150]<br />
�<br />
1 ��[0 �<br />
10 �<br />
��[0 25] ��[050] ��[075] ��[0100] ��<br />
�<br />
125] ��[0 150] �4�<br />
�<br />
[0 175] �<br />
1 �� 10<br />
�� �� �� ��<br />
[25] [50] [75] [100]<br />
�� �<br />
�[125] �[150] 4�<br />
�<br />
� � � [175] �<br />
1 �� 10<br />
�� �� �� ��<br />
[25] [50] [75] [100]<br />
�� �<br />
�[125] �[150] 3�<br />
�<br />
� � � [175] �<br />
1 �� 10<br />
�� �� �� � �<br />
[0 25] [0 50] [0 75] [0 100]<br />
�<br />
�<br />
�<br />
�[0 125] �[0 150] �[0 175] 3�<br />
�<br />
� � � � [0 200] �<br />
1 �� 10<br />
�� �� �� � �<br />
[25] [50] [75] [100]<br />
�� �<br />
�[125] �[150] �[175] 3�<br />
�<br />
� � � � [200] �<br />
1 �� 10<br />
�� �� �� � �<br />
[25] [50] [75] [100]<br />
�� �<br />
�[125] �[150] �[175] 2�<br />
�<br />
� � � � [200] �<br />
��[025] ��[050] ��[075] ��[0100] � �<br />
1 � �<br />
�� �� �� �� ��<br />
10<br />
2<br />
[0 125] [0 150] [0 175] [0 200]<br />
� �<br />
� �[0<br />
225]<br />
�<br />
�� �� �� �� � �<br />
1<br />
10<br />
2<br />
[25] [50] [75] [100]<br />
� �<br />
��[125] ��[150] ��[175] ��[200] ��<br />
�<br />
�<br />
�<br />
� [225]<br />
�<br />
�� �� �� �� � �<br />
1<br />
10<br />
[25] [50] [75] [100]<br />
� �<br />
��[125] ��[150] ��[175] ��[200] ��<br />
�<br />
�<br />
�<br />
� [225]<br />
�
225*-250 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
1@[0 125], 1@[0 150],<br />
1@[0 175], 1@[0 200],<br />
1@[0 225], 1@[0 250]<br />
��[025] ��[050] ��[075] ��[0100] � �<br />
1 � �<br />
��[0 125] ��[0 150] ��[0 175] ��[0 200] ��<br />
10 � �<br />
��[0225] ��[0250]<br />
�<br />
250 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 1@[200],<br />
1@[225], 1@[250]<br />
��[25] ��[50] ��[75] ��[100] � �<br />
1 � �<br />
��[125] ��[150] ��[175] ��[200] ��<br />
10 �<br />
�[225] �<br />
�<br />
� � [250]<br />
�<br />
250* 1@[0], 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 1@[200],<br />
1@[225]<br />
��[25] ��[50] ��[75] ��[100] � �<br />
1 � �<br />
��[125] ��[150] ��[175] ��[200] ��<br />
10 �<br />
�<br />
�<br />
� [225]<br />
�<br />
250*-275 1@[0 25], 1@[0 50],<br />
1@[0 75], 1@[0 100],<br />
1@[0 125], 1@[0 150],<br />
1@[0 175], 1@[0 200],<br />
1@[0 225], 1@[0 250]<br />
��[025] ��[050] ��[075] ��[0100] � �<br />
1 � �<br />
��[0 125] ��[0 150] ��[0 175] ��[0 200] ��<br />
10 � �<br />
��[0225] ��[0250]<br />
�<br />
275 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 1@[200],<br />
1@[225], 1@[250]<br />
��[25] ��[50] ��[75] ��[100] � �<br />
1 � �<br />
��[125] ��[150] ��[175] ��[200] ��<br />
10 �<br />
�[225] �<br />
�<br />
� � [250]<br />
�<br />
275* 1@[0], 1@[25], 1@[50],<br />
1@[75], 1@[100],<br />
1@[125], 1@[150],<br />
1@[175], 1@[200],<br />
1@[225]<br />
��[25] ��[50] ��[75] ��[100] � �<br />
1 � �<br />
��[125] ��[150] ��[175] ��[200] ��<br />
10 �<br />
�<br />
�<br />
� [225]<br />
�<br />
… … …<br />
The filter cake model is run for time interval of [0 250] min. The calculated<br />
mercury removal efficiencies at different time are used to calculate the corresponding<br />
average mercury removal efficiency across the whole fabric filter.<br />
The input parameters <strong>from</strong> Durkee slipstream tests listed in table 11.2 are<br />
again used as model inputs to the fabric filter model. Other inputs include a bag<br />
cleaning interval of 25 min and a cleaning fraction of 0.1. It is assumed that a new<br />
sorbent layer is accumulated on the filter bag every 5 min.<br />
Figure 11.7 shows the simulated mercury removal efficiency if the filter was<br />
running without periodical cleaning up to 4 h. Compared to the short filtration time of<br />
25 min as shown in figure 11.5, the mercury removal efficiency reaches a stable value<br />
after about 1 h for the applied injection rates of powdered activated carbon. This<br />
260
ehavior is due to the growing thickness of the carbon cake. Fresh carbon is<br />
continuously injected to the filter, providing increased mercury adsorption. At long<br />
times, the inner layers of the carbon cake, consisting of almost fully spent carbon,<br />
gives negligible contribution to the process so that asymptotic conditions are reached.<br />
Figure 11.7. Simulated mercury removal efficiency <strong>by</strong> 1/10 of the fabric filter without<br />
cleaning of the bags as a function of filtration time and injection rate of carbon. Darco<br />
Hg carbon with a diameter of 16 µm is injected at 138�C to simulated cement kiln<br />
flue gas with 200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv<br />
SO2, 15 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. �t=5 min, Air to cloth ratio<br />
ufilter= 1.2 m/min.<br />
Figure 11.8 presents the simulated mercury removal efficiency across the<br />
fabric filter with periodical cleaning 10% of the bags every 25 min. When 10% of the<br />
bags is cleaned the mercury removal efficiency decreases. At the beginning the<br />
mercury removal efficiency decreases slightly and it decreases more at later stage.<br />
This is due to the fact that more carbon is collected on the filter bag and is removed<br />
<strong>by</strong> pulse cleaning. The model assumes that all the carbon collected on the bag is<br />
completely removed <strong>from</strong> the bag surface and the corresponding mercury removal<br />
efficiency for this fraction of bags drops to zero when the pulse cleaning is initiated.<br />
The mercury removal efficiency across the whole filter reaches a stable level after all<br />
261
the bags have been cleaned once. The less smooth curve is due to the applied time<br />
interval of 5 min for a new carbon layer addition. Only five data points are used in a<br />
cleaning interval of 25 min. This can be easily improved <strong>by</strong> decreasing the time<br />
interval of new carbon cake layer addition at the expense of longer computation time.<br />
Figure 11.8. Simulated mercury removal efficiency of the fabric filter with cleaning<br />
of the bags as a function of filtration time and carbon injection rate. Darco Hg carbon<br />
with a diameter of 16 µm is injected at 138�C to simulated cement kiln flue gas with<br />
200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2, 15<br />
vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. tclean=25 min, �t=5 min, ufilter=1.2 m/min.<br />
Figure 11.9 shows the simulated effects of flue gas temperature on the<br />
mercury removal efficiency of the fabric filter. When the flue gas temperature is<br />
reduced <strong>from</strong> 138�C to 75�C an improvement of about 8% mercury removal<br />
efficiency is obtained. However, whether this improvement is economical needs to be<br />
compared with additional costs <strong>by</strong> cooling down the flue gas.<br />
262
Overall bag filter mercury removal efficiency, %<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 1 2 3 4 5 6 7<br />
Time (hour)<br />
263<br />
1: 75 degree C<br />
2: 100 degree C<br />
3: 115 degree C<br />
4: 138 degree C<br />
Figure 11.9. Simulated effects of flue gas temperature on mercury removal efficiency<br />
of the fabric filter. 16 mg/m 3 Darco Hg carbon with a diameter of 16 µm is injected to<br />
simulated cement kiln flue gas with 200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2,<br />
10 ppmv HCl, 5 ppmv SO2, 15 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. tclean=25<br />
min, �t=5 min, ufilter= 1.2 m/min.<br />
11.5 Twostage model<br />
Models developed in previous sections simulate mercury removal <strong>by</strong> separate<br />
parts of the full-scale process. In this section mercury adsorption in the duct section is<br />
coupled with the fabric filter section. First the duct model is run. The mercury<br />
concentrations inside the particle and the bulk mercury concentration at the end of the<br />
duct are used as initial conditions for the fabric filter model. Then the fabric filter<br />
model is run to desired filtration time. The model inputs are the same as the fabric<br />
filter model. A flight time of 1 s in the duct is applied.<br />
Figure 11.10 shows the simulated mercury removal efficiency in the duct<br />
section at Durkee cement plant. When a smaller carbon injection rate is applied the<br />
mercury removal efficiency after 1 s in the duct is negligible. As shown in figure<br />
1<br />
2<br />
3<br />
4
11.10, about 2% mercury removal is obtained when 16 mg/m 3 Darco Hg carbon is<br />
injected. However, at larger carbon injection rates the mercury removal efficiency<br />
after 1 s in the duct is noticeable. Therefore, high mercury removal efficiency can be<br />
obtained <strong>by</strong> increasing the residence time of carbon particles in the duct, i.e., <strong>by</strong><br />
applying long duct, provided that there is enough space in the plant and large carbon<br />
injection rate is applied.<br />
Figure 11.10. Simulated mercury removal efficiency in the duct as a function of<br />
residence time and injection rate of carbon. Darco Hg carbon with a diameter of 16<br />
µm is injected at 138�C to simulated cement kiln flue gas with 200 µg Hg 0 /Nm 3 ,<br />
1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2, 15 vol.% H2O, 21 vol.%<br />
CO2, and 6 vol.% O2.<br />
Figure 11.11 compares the simulated and measured mercury removal<br />
efficiency of the fabric filter at Durkee slipstream plant. Generally there is good<br />
agreement between the simulation and pilot-scale data. This indicates that the<br />
adsorption kinetics derived <strong>from</strong> 10 mg Darco Hg carbon in 2 g sand is reasonable<br />
and the developed model is a useful tool to simulate and optimize the carbon injection<br />
process.<br />
264
<strong>Mercury</strong> removal over the filter (%)<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 10 20 30 40 50 60 70 80 90 100<br />
Activated carbon injection rate (mg/m 3 )<br />
265<br />
Model<br />
Data<br />
Figure 11.11. Comparison of simulated and measured mercury removal efficiency of<br />
the fabric filter at Durkee slipstream plant. Darco Hg carbon with a diameter of 16<br />
µm is injected at 138�C to simulated cement kiln flue gas with 200 µg Hg 0 /Nm 3 ,<br />
1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2, 15 vol.% H2O, 21 vol.%<br />
CO2, and 6 vol.% O2. tclean=25 min, �t=5 min, ufilter= 1.2 m/min.<br />
The overall mercury removal efficiency of the sorbent injection system refers<br />
to mercury removal <strong>from</strong> the carbon injection point to the fabric filter outlet and can<br />
be evaluated as following:<br />
� �100 �(1 �(1 �� %)(1 � � %))<br />
(11.33)<br />
total duct filter<br />
Table 11.4 summarizes the calculated mercury removal efficiencies in the duct, fabric<br />
filter and the whole carbon injection system for different carbon injection rates. The<br />
contribution of mercury removal in the duct is much smaller to the mercury removal<br />
in the whole carbon injection system. However, data regarding overall mercury<br />
removal for the Durkee slipstream plant are not available for comparison.
Table 11.4 Simulated mercury removal efficiencies in the duct, fabric filter and the<br />
whole carbon injection system for different carbon injection rates. Darco Hg carbon<br />
with a diameter of 16 µm is injected at 138�C to simulated cement kiln flue gas with<br />
200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2, 15<br />
vol.% H2O, 21 vol.% CO2, and 6 vol.% O2. Carbon particle residence time in the<br />
duct is 1 s. Every five minute a layer of carbon is added to the bag surface. Air to<br />
cloth ration is 1.2 m/min.<br />
Carbon load<br />
(mg/m 3 <strong>Mercury</strong> removal <strong>Mercury</strong> removal in Total <strong>Mercury</strong><br />
) in duct (%) fabric filter (%) removal (%)<br />
8 1.0 54.0 54.5<br />
16 2.0 69.6 70.2<br />
48 5.9 86.0 86.9<br />
80 9.7 90.3 91.2<br />
The applied carbon injection rate for mercury control is much smaller than the<br />
typical dust load in the flue gas for particulate emission control process. Therefore,<br />
the pressure drop over the fabric filter is expected to increase slowly with filtration<br />
time for mercury control process. It is then feasible to extend the bag cleaning<br />
interval, i.e., use less frequent cleaning of the bags. Figure 11.12 illustrates the<br />
simulated effects of bag cleaning frequency on the mercury removal efficiency of the<br />
fabric filter. The mercury removal efficiency slightly increases when the bag cleaning<br />
interval is increased. Extending the bag cleaning interval <strong>from</strong> 25 min to 100 min<br />
results in a 1.3% improvement of mercury removal efficiency. A longer bag cleaning<br />
cycle results in longer retention time of the carbon particles on the bags, which allows<br />
the carbon particles to adsorb more mercury <strong>from</strong> the flue gas.<br />
266
Hg removal efficiency, %<br />
88<br />
87.6<br />
87.2<br />
86.8<br />
86.4<br />
86<br />
25 50 75 100 125<br />
Bag cleaning interval (min)<br />
Figure 11.12. Simulated effects of bag cleaning frequency on mercury removal<br />
efficiency of the fabric filter. 48 mg/m 3 Darco Hg carbon with a diameter of 16 µm is<br />
injected at 138�C to simulated cement kiln flue gas with 200 µg Hg 0 /Nm 3 , 1000<br />
ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2, 15 vol.% H2O, 21 vol.% CO2,<br />
and 6 vol.% O2. tclean=25 min, �t=5 min, ufilter= 1.2 m/min.<br />
The only carbon injection system for mercury control <strong>from</strong> cement production<br />
is operated at Ash Grove’s Durkee cement plant [10]. Instead of continuous injection<br />
of powdered activated carbon, the fabric filter works in fixed-bed adsorber mode.<br />
During the first run period, the activated carbon was added to the system through the<br />
first day and removed <strong>from</strong> the system through the eighth day. The average removal<br />
efficiency during those intervening 6 days was 92.8% [10]. However, the carbon<br />
injection rate and duration in the first day is not reported.<br />
Simulations are performed to simulate the fabric filter fixed-bed operation<br />
mode. Firstly activated carbon is injected at high load for 30 min to form a carbon<br />
cake on the bags. Then the activated carbon injection is stopped and the fabric filter<br />
works as a fixed-bed adsorber. When the initial mercury breakthrough occurs or the<br />
mercury emission limit is reached, the bags are cleaned <strong>by</strong> pulse-jet compressed air<br />
and later activated carbon is injected again. The injection-adsorption-cleaning cycle is<br />
repeated. The carbon injection without bag cleaning period is simulated <strong>by</strong> the filter<br />
cake model and the fixed-bed adsorption period is simulated <strong>by</strong> the fixed-bed model.<br />
267
Figure 11.13 shows the simulated mercury breakthrough curves of the fabric<br />
filter injected with 0.5-2.0 g/m 3 Darco Hg carbon for 30 min. The actual carbon<br />
injection rate at full-scale test is not available and high carbon injection rates are<br />
tested here to show the effect. The initial mercury breakthrough time for carbon load<br />
of 0.5, 1.0, 1.5, and 2.0 g/m 3 is 17.9, 35.7, 53.6, and 71.5 h, respectively. This means<br />
that about 100% mercury removal efficiency is obtained within the initial mercury<br />
breakthrough periods. With an activated carbon injection of 1-2 g/m 3 for 30 min, the<br />
fabric filter can work for 53.6 to 71.5 h before the initial breakthrough occurs. This<br />
means that the activated carbon injection is only required 2-3 times per week. This is<br />
in agreement with the information obtained <strong>from</strong> Paone [9] on practice of the Durkee<br />
sorbent injection plant. However, the actual carbon injection rate and duration at<br />
Durkee plant are unknown and are required for validation of the simulation.<br />
Gaseous mercury outlet (Cout/Cin)<br />
1.2<br />
1<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
A<br />
B C D<br />
0<br />
0 50 100 150 200 250 300 350 400<br />
Time (hour)<br />
268<br />
A: 0.5 g/m3<br />
B: 1.0 g/m3<br />
C: 1.5 g/m3<br />
D: 2.0 g/m3<br />
Figure 11.13. Simulated mercury breakthrough curves of fabric filter injected with<br />
large carbon loads for 30 min and then in fixed-bed operation mode. Darco Hg<br />
carbon with a diameter of 16 µm is injected at 138�C to simulated cement kiln flue<br />
gas with 200 µg Hg 0 /Nm 3 , 1000 ppmv NO, 5 ppmv NO2, 10 ppmv HCl, 5 ppmv SO2,<br />
15 vol.% H2O, 21 vol.% CO2, and 6 vol.% O2.
To compare with mercury removal <strong>by</strong> continuous injection of carbon,<br />
simulations are conducted <strong>by</strong> injection of the same total amount of carbon within the<br />
initial mercury breakthrough period. The corresponding carbon injection rate is 14<br />
mg/m 3 and the simulated mercury removal efficiency is only 66.8%. The fabric filter<br />
works more efficiently for mercury removal in fixed-bed operation model than<br />
continuous injection of carbon for the same total amount of carbon injected. This is<br />
probably due to the larger amount of carbon accumulated on the bags in the fixed-bed<br />
operation mode. The fixed-bed operation model is limited <strong>by</strong> the pressure drop over<br />
the fabric filter. It is expected that a larger pressure drop over the filter and power<br />
consumption of the system is required when a large carbon injection rate is applied in<br />
a reasonable period.<br />
11.6 Conclusions<br />
The developed single particle and fixed-bed adsorption models are further<br />
extended to duct and fabric filter models to simulate mercury removal <strong>by</strong> carbon<br />
injection upstream of a fabric filter. The fabric filter model is accounted for <strong>by</strong> adding<br />
a new carbon layer on the bag surface after a short time and treating each layer as a<br />
well mixed tank. Finally the duct model and fabric filter model are coupled to a twostage<br />
model. The mercury concentrations inside the particle and the bulk mercury<br />
concentration at the end of the duct are used as initial conditions for the fabric filter<br />
model. The models are based on materials balances in both gaseous and adsorbed<br />
phase along the duct length/growing filter cake and inside the carbon particles. The<br />
models account for adsorption kinetics, both the external and internal mass transfer<br />
resistances, accumulation of carbon layer on the bags, and periodical cleaning of the<br />
bags.<br />
Henry’s constant obtained <strong>from</strong> fixed-bed investigation of mercury adsorption<br />
<strong>by</strong> activated carbon in the simulated cement kiln flue gas is used as input to the<br />
models. The effects of SO2, H2O, NO2 levels in the flue gas on mercury removal are<br />
accounted <strong>by</strong> using correlations derived <strong>from</strong> the fixed-bed investigation.<br />
Duct model simulations indicate that large carbon loading in the flue gas are<br />
required to obtain high mercury removal efficiency due to the short residence time.<br />
269
To minimize the carbon feed rate it is advisable to lower the operating temperature.<br />
Improvements in the mercury removal efficiency can be obtained also <strong>by</strong> increasing<br />
the in-duct particle residence time and decreasing the carbon particle size.<br />
In contrast to the in-duct removal process, simulations of mercury adsorption<br />
in the fabric filter show that higher mercury removal efficiency can be achieved with<br />
moderate carbon consumption due to the effective gas/carbon contact on the filter<br />
bags. The effects of carbon load, temperature, frequency of new carbon layer addition<br />
and bag cleaning on mercury removal efficiency are simulated. The fabric filter<br />
model can predict the mercury removal profile with jagged nature because of the<br />
intermittent partial cleaning of the bags. Comparison with simulation and<br />
experimental data <strong>from</strong> Durkee cement plant slipstream tests shows that the<br />
developed two-stage model can reasonably predict the mercury removal <strong>from</strong> cement<br />
plants <strong>by</strong> carbon injection upstream of a fabric filter.<br />
Minor benefits can be obtained <strong>by</strong> increasing the cleaning cycle time of the<br />
fabric filter compartments. The fabric filter works more efficiently on mercury<br />
removal when it is operated as fixed-bed adsorbed <strong>by</strong> injection of high carbon load in<br />
short time and then stopping carbon injection and cleaning of the bags.<br />
11.7 List of symbols<br />
A: total exterior surface area of all particles in the adsorber (m 2 )<br />
A: matrix in equation 11.18<br />
B: matrix in equation 11.19<br />
Bi: dimensionless Biot number<br />
C: gaseous mercury concentration (µg/m 3 )<br />
C: matrix in equation 11.20 and 11.21<br />
Cb: gas bulk mercury concentration (µg/m 3 )<br />
Cbo: initial gas bulk mercury concentration (µg/m 3 )<br />
Cbin: inlet mercury concentration in tank (µg/m 3 )<br />
Cµ: adsorbed mercury concentration in the sorbent (µg/m 3 )<br />
Dapp: apparent diffusion coefficient (m 2 /s)<br />
De: effective diffusion coefficient (m 2 /s)<br />
fclean: fraction of bags cleaned per pulse cleaning<br />
J: mercury flux (µg/m 2 )<br />
K: Henry’s constant<br />
270
km: gas film mass transfer coefficient (m/s)<br />
L: thickness of carbon cake (m)<br />
mp: mass of carbon particle in the adsorber (g)<br />
M: carbon load in the tank (mg)<br />
n: number of interior collocation points<br />
r: radial coordinate (m)<br />
Rp: carbon particle radius (m)<br />
t: time (s)<br />
tclean: time interval for bag cleaning (25)<br />
u: dimensionless parameter, u=x 2<br />
u: face velocity on the filter bags (m/s)<br />
ufilter: air to cloth ratio (m/min)<br />
V: volume of the adsorber (m 3 )<br />
Vb: volume of the adsorber (m 3 )<br />
x: dimensionless radius<br />
y: dimensionless mercury concentration<br />
yb: dimensionless gas bulk mercury concentration (µg/m 3 )<br />
ybin: dimensionless inlet mercury concentration in tank (µg/m 3 )<br />
Greek symbols<br />
�b: bed void fraction<br />
�p: carbon particle porosity<br />
�p: carbon particle density (kg/m 3 )<br />
�: dimensionless time<br />
�: carbon load in the flue gas (kg/m 3 )<br />
�: mercury removal efficiency (%)<br />
11.8 References<br />
[1] J.R.V. Flora, R.A. Hargis, W.J. O'Dowd, A. Karash, H.W. Pennline, R.D. Vidic, The role<br />
of pressure drop and flow redistribution on modeling mercury control using sorbent injection<br />
in baghouse filters, J. Air Waste Manage. Assoc. 56 (2006) 343-349.<br />
[2] F. Scala, Simulation of mercury capture <strong>by</strong> activated carbon injection in incinerator flue<br />
gas. 1. In-duct removal, Environ. Sci. Technol. 35 (2001) 4367-4372.<br />
[3] F. Scala, Simulation of mercury capture <strong>by</strong> activated carbon injection in incinerator flue<br />
gas. 2. Fabric filter removal, Environ. Sci. Technol. 35 (2001) 4373-4378.<br />
[4] F. Scala, Modeling mercury capture in coal-fired power plant flue gas, Ind Eng Chem Res.<br />
43 (2004) 2575-2589.<br />
[5] J. Villadsen, M.L. Michelsen, Solutions of differential equation models <strong>by</strong> polynomial<br />
approximation, Prentice-Hall, Inc., 1978.<br />
271
[6] D.D. Do, Adsorption analysis: equilibria and kinetics, Imperial College Press, 1998.<br />
[7] L. Hayes-Gorman, Regulating mercury emissions: Ash Grove <strong>Cement</strong> in Durkee, Air<br />
toxics summit 2008, Boise, Idaho, 4-7 August, 2008.<br />
[8] Schreiber & Yonley Associates, <strong>Mercury</strong> emissions test report, Ash Grove <strong>Cement</strong><br />
Company Durkee, Oregon, Project No. 060204, 2007.<br />
[9] P. Paone, Personal communication about flue gas compositions and temperature for pilotscale<br />
sorbent injection tests at Ash Grove Durkee plant and FLSmidth Mineral Lab, 2010.<br />
[10] Curtis D. Lesslie, Mail to U.S.EPA about initial results of Ash Grove's Durkee sorbent<br />
injection system, http://www.whitehouse.gov/sites/default/files/omb/assets/ oira_2060/2060_<br />
07292010-3.pdf, visited March 21, 2011.<br />
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273<br />
12<br />
Concluding remarks<br />
To develop and get a better understanding of mercury removal <strong>from</strong> cement<br />
plant <strong>by</strong> sorbent injection upstream of a pulse jet fabric filter, this project has focused<br />
on four areas: comprehensive review of mercury emission <strong>from</strong> cement plants and<br />
analysis applicability of available technologies for mercury removal <strong>from</strong> cement<br />
plants, test and development of thermal catalytic converters for oxidized mercury<br />
reduction and dynamic total mercury measurement, screening tests and fundamental<br />
investigation of mercury adsorption <strong>by</strong> sorbents in simulated cement kiln flue gas,<br />
and development of mathematic models that can describe mercury removal <strong>by</strong> fixedbed<br />
and carbon injection upstream of a fabric filter.<br />
<strong>Cement</strong> plants are quite different <strong>from</strong> power plants and waste incinerators<br />
regarding the flue gas composition, temperature, residence time, and inherent<br />
material circulation. Instead of fuel, cement raw materials are the dominant sources of<br />
mercury in the cement kiln flue gas. The mercury emissions and speciation <strong>from</strong><br />
cement kilns can vary over time and depend on raw materials and fuels used, and<br />
process operation. Among the available technologies for mercury removal <strong>from</strong> flue<br />
gas, sorbent injection upstream of a polishing fabric filter is considered as the most<br />
promising and suitable technology for cement plant application.<br />
To be able to perform dynamic measurement of mercury adsorption <strong>by</strong><br />
sorbents, a red brass chips and sulfite converter were investigated in simulated<br />
cement kiln flue gas. The red brass converter works only when measuring elemental<br />
mercury in nitrogen and does not work properly even when only elemental mercury is<br />
added to the simulated flue gas. The red brass converter cannot fully reduce HgCl2 to<br />
elemental mercury under any relevant condition.<br />
The sodium sulfite converter material was prepared <strong>by</strong> dry impregnation of<br />
sodium sulfite and calcium sulfate powders on zeolite pellets using water glass as
inder. The sulfite converter works well at 500�C when less than 10 ppmv HCl is<br />
present in the simulated cement kiln flue gas. The response time of the sulfite<br />
converter is short and typically within at most two minutes, which makes it<br />
appropriate for not too fast dynamic measurements.<br />
Inconsistent mercury adsorption capacity of activated carbon is observed at<br />
different carbon loads in 2 g sand. A smaller mercury adsorption capacity is obtained<br />
with larger carbon load. Tests with elemental mercury and mercury chloride, different<br />
carbon type and particle sizes show the same trend. Effects of bed dilution on the<br />
equilibrium mercury adsorption capacity appear to be limited.<br />
Screening tests of sorbents for mercury removal <strong>from</strong> cement plants have<br />
been conducted in the fixed-bed reactor system using simulated cement kiln flue gas<br />
with elemental mercury and mercury chloride source. The tested sorbents include<br />
commercial activated carbons, commercial non-carbon sorbents, and cement<br />
materials. Screening measurements are used to evaluate initial mercury capture rate,<br />
oxidation potential, and capacity for the selected sorbents.<br />
The sorbents don’t adsorb any mercury when tested with elemental mercury<br />
in nitrogen. Tests of a range of 30 mg collected non-carbon based sorbents and<br />
cement materials as sorbents in 2 g sand at 150�C in simulated cement kiln flue gas<br />
with elemental mercury do not show any mercury adsorption or oxidation. Generally<br />
a larger amount of adsorbed mercury is obtained with sorbents that have larger<br />
mercury oxidation capacity. While all the non-carbon based sorbents and cement<br />
materials show some adsorption of mercury chloride. This indicates that mercury<br />
oxidation is an important factor for mercury adsorption <strong>by</strong> the sorbents. Elemental<br />
mercury needs to be oxidized either in the flue gas with HCl or on the sorbent.<br />
Among the tested sorbents the Darco Hg activated shows the best performance of<br />
adsorption of both elemental and oxidized mercury and is recommended as the<br />
reference sorbent for fundamental investigation.<br />
A parametric study of elemental mercury adsorption <strong>by</strong> activated carbon has<br />
been conducted in the fixed-bed reactor <strong>by</strong> mixing 10 mg Darco Hg carbon with 2 g<br />
sand. Increasing adsorption temperature results in decreased equilibrium mercury<br />
adsorption capacity of the activated carbon. The mercury adsorption isotherm follows<br />
274
Henry’s law for the applied mercury inlet levels in this project. The derived heat of<br />
adsorption is -8540 J/mol for elemental mercury adsorption <strong>by</strong> Darco Hg activated<br />
carbon in simulated cement kiln flue gas. Higher mercury oxidation and initial<br />
adsorption rate are also observed for smaller carbon particles, while the equilibrium<br />
mercury adsorption capacity is the same.<br />
The mercury adsorption capacity does not change with O2, CO, and NO levels<br />
in the flue gas, but decreases when CO2, H2O, SO2, and NO2 concentrations increase.<br />
The decrease of mercury adsorption capacity is due to the competition for active site<br />
with mercury <strong>by</strong> CO2 and H2O, and conversion of the previously formed nonvolatile<br />
basic mercuric nitrate into the volatile form <strong>by</strong> interactions between SO2 and NO2.<br />
Slight promoting effects of HCl on mercury adsorption are observed when HCl<br />
concentration is varied in the range of 0.5-20 ppmv. Larger mercury adsorption<br />
capacity is obtained when HCl is removed <strong>from</strong> baseline gas because HgO(s) is<br />
formed on the carbon.<br />
Similar adsorption behaviors of mercury chloride and elemental mercury <strong>by</strong><br />
Darco Hg activated carbon are observed using simulated cement kiln flue gas. This is<br />
due to the effective catalytic oxidation of elemental mercury <strong>by</strong> the activated carbon.<br />
Mathematical models are developed to simulate mercury adsorption <strong>by</strong> a<br />
single carbon particle, fixed carbon bed, in the duct and fabric filter. Orthogonal<br />
collocation method is used to solve mercury diffusion and adsorption inside a carbon<br />
particle. The fixed-bed model is solved <strong>by</strong> tank-in-series method. The fabric filter<br />
model is accounted for <strong>by</strong> adding a new carbon layer on the bag surface after a short<br />
time as a well mixed tank. The two-stage duct-fabric filter model accounts for<br />
adsorption kinetics, both the external and internal mass transfer resistances,<br />
accumulation of carbon layer on the bags, and periodical cleaning of the bags.<br />
Henry’s constant obtained <strong>from</strong> fixed-bed investigation are used as input to<br />
the models. The developed fixed bed model can reasonably simulate the effects of<br />
adsorption temperature, mercury inlet concentration, flow gas rate, carbon particle<br />
size, and SO2, H2O, NO2 level in the flue gas on the mercury breakthrough curve of<br />
the fixed carbon bed.<br />
275
Duct model simulations indicate that a large carbon load is required to obtain<br />
a high mercury removal efficiency due to the short residence time. Simulations of<br />
mercury adsorption in the fabric filter show that higher mercury removal efficiency<br />
can be achieved with moderate carbon consumption due to the effective gas/carbon<br />
contact on the filter bags. The effects of carbon load, temperature, frequency of new<br />
carbon layer addition and bag cleaning on mercury removal efficiency are simulated.<br />
The fabric filter model can predict the mercury removal profile with jagged nature<br />
because of the intermittent partial cleaning of the bags. Comparison with simulation<br />
and experimental data <strong>from</strong> Durkee cement plant slipstream tests shows that the<br />
developed two-stage model can reasonably predict the mercury removal <strong>from</strong> cement<br />
plants <strong>by</strong> carbon injection upstream of a fabric filter.<br />
276
277<br />
13<br />
Suggestions for further work<br />
This work has investigated the mercury removal <strong>by</strong> carbon injection upstream<br />
of a fabric filter under more controlled conditions using a fixed bed reactor. Pilot or<br />
full-scale tests are desired to demonstrate the ability of the studied sorbents and<br />
technology to control emissions of mercury <strong>from</strong> cement plant over a typical range of<br />
operating conditions for an extended period of time and to further validate the<br />
developed models. The condition in full-scale application is much more demanding<br />
than in the lab-scale investigation. Further development and test of the sulfite<br />
converter is required for dynamic measurement of mercury in large scale<br />
investigation. A sampling probe is needed to separate the particles <strong>from</strong> the flue gas<br />
efficiently without plugging. Adsorption of mercury <strong>by</strong> the dust and probe should be<br />
minimized <strong>by</strong> high sampling flow rate and high heating temperature.<br />
The problem of inconsistent mercury adsorption capacity for different carbon<br />
loads could not be solved within the project. More thorough investigation is<br />
necessary to reveal the cause. New analysis technology is required to reveal whether<br />
mercury is adsorbed <strong>by</strong> the sand when it is mixed with activated carbon.<br />
This project investigates only mercury removal <strong>by</strong> the activated carbon. In the<br />
future multipollutants control <strong>by</strong> the activated carbon should be studied <strong>by</strong> measuring<br />
also other harmful species such as SO2 and NOx. When more than one component is<br />
involved in the adsorption system, adsorption equilibrium involving competition<br />
between molecules of different types is needed for the understanding of the system as<br />
well as for the design purposes.<br />
To reduce the sorbent cost, regeneration of used sorbents should be<br />
investigated. Recycling sorbent collected <strong>by</strong> the fabric filter to the injection process<br />
also requires more investigation. Modification of cement materials <strong>by</strong> additives that
can oxidize mercury is attractive. However, the influence of the additives on the<br />
cement quality needs to be investigated.<br />
Models developed in this work assume that all the particles have a uniform<br />
size. It is interesting to take the particle size distribution into account in the more<br />
advanced model. The developed fabric filter model does not include pressure drop<br />
over the filter. It is useful to incorporate the pressure development of the fabric filter<br />
and pulse jet cleaning instead of assuming a constant bag cleaning interval. Current<br />
models assume local equilibrium inside the carbon particle, simulation with a full<br />
kinetics description of the adsorption process is necessary to investigate whether this<br />
assumption is reasonable.<br />
278