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Application conditions for
highly reactive lime hydrates
in dry sorption
IChemE Waste Symposium 2003
Sheffield/UK, 30. June/2. July 2003
E. Bechoux, Dr. B. Naffin, X. Pettiau,
Dipl.-Ing. M. Sindram, M. Tomlinson
4/03

Eugene Bechoux; Lhoist Western Business Unit, Wavre; Belgium
Dr. Burkhard Naffin; Rheinkalk, Wülfrath, Germany (Speaker)
Xavier Pettiau; Lhoist Coordination Centre, Limelette, Belgium
Martin Sindram; Rheinkalk GmbH, Wülfrath; Germany
Mark Tomlinson, Lhoist UK, Hindlow, United Kingdom
"Application conditions for highly reactive lime hydrates in dry sorption"
Flue-gas treatment (FGT) processes can be roughly classified into three categories:
1. wet flue-gas cleaning processes,
2. semi-dry flue-gas cleaning processes,
3. dry flue-gas cleaning processes.
Wet FGT processes are primarily used in power stations for flue-gas desulphurisation
(FGD). Because of the process technology the flue gas temperatures of approx. 60 -
70°C in the scrubbing stage is achieved. In the case of semi-dry processes, a lime
milk suspension is introduced into the stream of flue gas so that the flue gas is
cooled by evaporation of the water to a temperature of 140 – 170 °C. The reaction
products are separated as a dry powder by a downstream filter. In the case of dry
processes, the reaction material is introduced as a powder into the flue gas. The high
(850 – 1000 °C), medium (300 – 450 °C) and low (80 – 220 °C) temperature ranges
are used for this (in particular for flue gas treatment of waste incineration plants). Dry
processes have become standard in modern flue-gas cleaning since they offer the
user decisive advantages.
The common factor in all processes is the general use of lime-based products (milled
limestone - CaCO3, lime - CaO, hydrated lime – Ca(OH)2) to neutralise the acid
forming pollutants (HCl, SO2, HF). Criteria for the application of these products to
scrub gases are, in addition to chemical purity (CaO content and secondary
components), for
· milled limestone: reactivity and grain-size distribution,
· lime: hydrating behaviour (t60 value, dispersibility),
· hydrated lime: grain-size distribution, specific surface area, pore volume.
From the chemical point of view, hydrated lime is generally used for dry and semi-dry
processes.
The Lhoist Group has been a driving force since the mid-1980s in the development of
products for use in semi-dry and dry processes (WÜLFRAsorp ® , WÜLFRAgran ® ), in
particular. The result of this intensive research was the first hydrated lime with a high
surface area, namely WÜLFRAsorp ® A. While standard hydrated lime usually has a
specific surface area of approx. 18 m 2 /g (according to BET), the specific surface area
of WÜLFRAsorp ® A is approx. 38 m 2 /g. In this way, the surface area available for the
gas-solid reactions in the dry sorption process is, in principle, more than doubled.
Furthermore, the number of particles and dispersibility is markedly increased owing
to the fineness of the product (d50 approx. 3 µm compared to 6 µm for standard
commercial hydrated lime). The intensive development to improve the standard
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hydrated lime by the Lhoist research department led to the product Spongiacal ®
(brand name in Germany: WÜLFRAsorp ® D SP and in France Captacal ® SP). Here,
in addition to a further increase in the specific surface area to approx. 45 m 2 /g, the
pore volume, which is particularly important for the difficult removal of SO2, was
decisively increased. While standard hydrated lime has a pore volume of approx.
0.08 cm 3 /g, the corresponding value for Spongiacal ® is greater than 0.2 cm 3 /g (N2
desorption).
Particle Size d 50
[10 -6 m]
8
6
4
2
0
Normal Hydrated Lime WÜLFRAsorp® A Spongiacal®
Graph 1: Comparison of different Hydrated Limes
Specific Surface Area [m²/g] BET
50
45
40
35
30
25
20
15
10
5
0
Normal hydrated lime WÜLFRAsorp ® A Spongiacal ®
Graph 2: History of Development
Porevolume [cm³/g]
0,25
0,2
0,15
0,1
0,05
0
2

Dry processes have become standard in modern flue-gas cleaning since they offer
the user decisive advantages. In addition to the safe compliance with mandatory
limiting values and high flexibility, they also lead to lower investment and operating
costs, low personnel expenditures as well as a simple concept and space-saving
design. The reaction products are produced in the dry state. Examples for its use can
be found in almost all fields of application, e.g.:
• wood and biomass-fired power stations,
• municipal waste incineration plants,
• hazardous waste incineration plants,
• conventional power stations,
• non-ferrous metal processing,
• glassworks,
• ceramics industry,
• process firing.
In addition to the reduction of acid-forming pollutants, the removal of ecotoxic
substances, such as heavy metals (e.g. Hg, Pb, Zn) and dioxins/furans
(PCDD/PCDF), is of particular interest. These adsorption processes are carried out
advantageously in the low-temperature range using suitable mixed products. For this
application, the Lhoist Group can supply products tailored to the customer's
requirements and which are based on Spongiacal ® , hydrated lime and limestone
powder. The adsorption materials used are, for example, lignite coke and activated
carbon. The products offered by Lhoist for this purpose offer a higher inherent safety.
They can be used, if necessary, without additional apparatus and personnel
expenditures. This fulfils the demand for more simple and more reliable plant
engineering.
specific surface (BET) [m²/g]
60
50
40
30
20
10
0
Normal hydrated lime Spongiacal® Hydrated lime
with 3% lignite coke
Graph 3: Lime Carbon Mixtures
adsorptive carbon surface
Spongiacal®
with 3% lignite coke
3

The use of products that are optimally customised to the process provide particular
advantages for the operator of semi-dry and dry processes. Therefore, highly
reactive hydrated lime (Spongiacal ® ) is generally used since this
• minimises the consumption of operating materials,
• allows a safe compliance with mandatory limiting values,
• reduces the amount of residual waste (requirement to minimise residual waste!).
In the low-temperature range, the highly reactive hydrated lime products
(WÜLFRAsorp ® A and Spongiacal ® ) have been used successfully for some time. An
important further development was the combination of semi-dry and dry process
steps with high porous products (MCD process).The medium temperature range
offers very good application conditions for the use of special hydrates, particularly
those to remove SO2. For example, in the glass industry, flue-gas cleaning has been
very successful using WÜLFRAsorp ® A at 350 – 500 °C 4 . In the high-temperature
range at approx. 850 –1000 °C, products with high surface areas that were specially
developed for flue-gas cleaning (e.g. WÜLFRAsorp ® A) have also been used
successfully.
WÜLFRAsorp ® A Spongiacal ®
WÜLFRAsorp ® A
NHL / Milled Limestone
~ 1200 - 1000 °C
~ 1000 - 850 °C
SO2 NHL
~ 850 - 450 °C ~ 300 - 220 °C ~ 220 - 80 °C
~ 450 - 300 °C
SO 2 + HCl
WÜLFRAsorp ® D SP
NHL
SO 2 + HCl
Max. 180°C
PCDD/DF + Hg
Graph 4: Temperature application range of lime based products
In addition to the temperature a number of other factors influence the removal of
pollutant gases. They can be summarised under the headings gas properties (given),
process engineering (can be influenced) and adsorbent (can be influenced). The
influence of temperature for the use of hydrated lime in semi-dry and dry process is
discussed here, in particular.
Results and operating experience are discussed on the basis of practical examples
from the field of thermal waste treatment.
4

Thermal waste treatment plants
At the beginning of the 1990s retrofitting or construction of flue-gas cleaning plants
was initiated for municipal waste and hazardous waste incineration plants (MWI /
HWI). The driving force for this was the 17th BImSchV in Germany which was nearly
completely transferred to European legislation, which moreover also affected other
industrial fields.
Important objectives in this respect include high efficiencies in the removal of HCl
and SO2 in addition to the already much-discussed ecotoxic pollutants. In order to
meet the mandatory objectives, very complicated wet processes were installed in the
first phase while some operators used retrofitting or upgrading existing semi-dry or
dry processes. The installation of the MCD process proved to be a successful
measure for the cost-effective compliance with the 17th BImSchV. The various
methods include combinations of spray sorption and dry sorption as well as a
combination of two dry sorption steps. Of considerable importance for this is the use
of a highly reactive Spongiacal ® product in the dry sorption stage. The application of
mixed adsorbents is widely used to reduce ecotoxic pollutants.
SO2-separation [%]
100
80
60
40
20
0
Schematic description of the MKT-process
one stage spray
dry sorption
Stoichiometric ratio
savings ~ 30 - 50 %
0,0 1,0 2,0 3,0 4,0
Stoichiometric ratio [-]
Graph 5: Description of the MCD process (Two-Stage-Process)
The operating temperatures of the dry sorption stages have recently been used to
optimise operational processes. The temperatures in the fabric filter stage usually lie
between 150 °C and 190 °C. Current investigations have also been carried out at
220 °C.
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Incinerator
Spray dry adsorption Bag house filter
Milk of lime & water
Spongiacal ® + Active carbon
Graph 6: Application example of the MCD process
ca. 170°C
Clean gas
Reaction products
In one application, the operating trials are used to check the performance of the MCD
process with regard to the removal of the acid-forming pollutants HCl and SO2 as well
as Hg. Combinations of spray and dry sorption with a lime milk suspension and
Spongiacal ® with active carbon were used. This plant is operated in the dry sorption
stage at approx. 170 °C. During the measuring period, the concentrations of HCl in
the raw gas lay between 500 and 1200 mg/m³N.tr., while the concentrations of SO2 in
the raw gas were 200 – 500 mg/m³N.tr.. The average removal efficiencies attained
were > 99 % for HCl and > 94 % for SO2. These values were obtained with a
stoichiometric ratio of 2.0 - 2.5. An important factor was also the adjustability of the
dry sorption stage with regard to the minimum dosage. The specific consumption of
lime-based products in these trials was approx. 15.7 kg/t waste † .
In addition to the overall mass balances some tests with high HCl and SO2
concentrations were carried out. In one case the HCl concentrations on the raw gas
side increased from 1.500 mg/m³N.tr. to app. 6.000 mg/m³N.tr.. The injection of
Spongiacal was activated and it was possible even under these unfavourable
conditions to fulfil the requirements regarding HCl reduction (< 10 mg/m³N.dry )
† Burnt lime + Spongiacal ®
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Graph 7: Test results: HCl peak
During another test SO2 concentrations were high and increased from 350 mg/m³N.dry
to nearly 1.000 mg/m³N.dry. Due to the injection of the high reactive Spongiacal it was
possible to enhance the desulfurisation.
Graph 8: Test results: SO2 peak
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In the next picture operating results from different municipal waste incineration plants
are compared. By the use of the two-stage-process (MCD) it is possible to reduce not
only the lime consumption between 30 – 50 % but the residue as well. The
application of such a process allows very flexible operation of a flue gas cleaning
system. Even peaks on the raw gas side are controllable.
spec. consumption [kg/t]
40
35
30
25
20
15
10
5
0
MWI I (MKT)
MWI II (MKT)
MWI III (MKT)
MWI IV
MWI V
→ MCD Process: Specific consumption ca. 18 - 22 kg/t waste
spec. consump.
Spongiacal
spec. consump. HL
(lime based)
→ One Stage Spray Dry Sorption: Specific consumption ca. 28 - 35 kg/t waste
Graph 9: Comparison of operational results
Conclusions
• Even in the temperature ranges previously regarded as being unfavourable for
the use of hydrated lime, the removal of acid-forming pollutants, such as HCl
and SO2, by means of a highly reactive additive tailored to the application case,
was highly effective.
• The limiting values of the 17th BImSchV were observed.
• In works trials, it was proved that the use of Spongiacal ® gave reductions of
more than a half in the consumption of operating materials.
• This gave corresponding reductions in the amount of residual waste.
• According to currently available operating experience, the use of Spongiacal ® in
a MWI with a single or two-stage dry sorption plant can safely comply with the
limiting values for SO2 and HCl specified in the 17th BImSchV. It is
advantageous to use these products in a mixture with a carbon component for
the removal of ecotoxic pollutants. This means that complicated flue-gas
cleaning processes are not necessary. It also ensures an economically viable
operation on a long-term basis with low expenditures of personnel, equipment
and materials.
• If high contents in the raw gas occur frequently, the use of the MCD process
with its high effectiveness and low consumption of additives allows compliance
with mandatory limiting values, even under conditions that were previously
considered to be unfavourable. Moreover, this process allows operation with low
expenditures of personnel, equipment and materials.
8

In the next picture operating results from different municipal waste incineration plants
are compared. By the use of the two-stage-process (MCD) it is possible to reduce not
only the lime consumption between 30 – 50 % but the residue as well. The
application of such a process allows very flexible operation of a flue gas cleaning
system. Even peaks on the raw gas side are controllable.
spec. consumption [kg/t]
40
35
30
25
20
15
10
5
0
MWI I (MKT)
MWI II (MKT)
MWI III (MKT)
MWI IV
MWI V
→ MCD Process: Specific consumption ca. 18 - 22 kg/t waste
spec. consump.
Spongiacal
spec. consump. HL
(lime based)
→ One Stage Spray Dry Sorption: Specific consumption ca. 28 - 35 kg/t waste
Graph 9: Comparison of operational results
Conclusions
• Even in the temperature ranges previously regarded as being unfavourable for
the use of hydrated lime, the removal of acid-forming pollutants, such as HCl
and SO2, by means of a highly reactive additive tailored to the application case,
was highly effective.
• The limiting values of the 17th BImSchV were observed.
• In works trials, it was proved that the use of Spongiacal ® gave reductions of
more than a half in the consumption of operating materials.
• This gave corresponding reductions in the amount of residual waste.
• According to currently available operating experience, the use of Spongiacal ® in
a MWI with a single or two-stage dry sorption plant can safely comply with the
limiting values for SO2 and HCl specified in the 17th BImSchV. It is
advantageous to use these products in a mixture with a carbon component for
the removal of ecotoxic pollutants. This means that complicated flue-gas
cleaning processes are not necessary. It also ensures an economically viable
operation on a long-term basis with low expenditures of personnel, equipment
and materials.
• If high contents in the raw gas occur frequently, the use of the MCD process
with its high effectiveness and low consumption of additives allows compliance
with mandatory limiting values, even under conditions that were previously
considered to be unfavourable. Moreover, this process allows operation with low
expenditures of personnel, equipment and materials.
8