Some Investigations on Fly Ash Resistivity Generated in ... - isesp

<strong>Some</strong> <strong>Investigations</strong> on Fly Ash Resistivity Generated in Indian Power Plants 399<strong>Some</strong> <strong>Investigations</strong> on Fly Ash Resistivity Generated in Indian Power PlantsAvinash Chandra(Centre for Energy Studies, Indian Institute Of Technology, Delhi, Hauz Khas, New Delhi-110016, India.E-mail: Hchandra@ces.iitd.ernet.inH)Abstract: The design and operation of Electrostatic Precipitator depends on the properties of the coal burned and fly ashgenerated in the boilers of the power plant. The properties of the coal used in different power plants cross the India vary widely,but most of the times the sulfur contents are low in the coals. As a result the resistivity of the fly ash is in general, very high andit leads to develop back corona at the collecting electrodes even at comparatively low current densities flowing through thedifferent fields of ESP. All this results in detorieting the Collection efficiency of ESP and large size of ESP is required to meetthe emission standard for similar level of power generation elsewhere. Knowledge of electrical resistivity is essential for sizingof ESP and to determine a strategy to improve its performance. A special facility has been developed at IIT Delhi to investigatethe variation of resistivity at different temperature and moisture levels under simulated conditions those exiting in side an ESP.A series of experiments have been conducted on the fly ash samples obtained from different power plants in India. Empiricalrelations developed by Bickelhaupt, which are based on chemical composition of fly ash for calculating the electrical resistivity,are used to calculate as theoretical value for given experimental conditions. New empirical relations based on experimentalresults and operating conditions have been developed to provide better agreements.Keywords: resistivity, India power plant, prediction, air conditioning1 INTRODUCTIONThe performance of the electrostatic precipitator dependsupon many parameters viz. gas flow, gas temperature, gascomposition, dust composition, gas moisture; electrical properties such as dust resistivity, voltage waveform. Themechanical design features like geometry of dischargeelectrode, collecting electrode, electrode spacing and rappingmechanism are also important for the efficient working ofESP. However, a single most important parameter, which hasgot significant impact over the sizing and performance of theprecipitator, is the fly ash resistivity. In coal-fired powerplants, the design and operation of electrostatic precipitator(ESP) depends largely on the properties of the coal burnedand fly ash generated. The electrical resistivity affects theprecipitator performance as follows [1,2].(1) The adhesive forces of the dust particles on thecollecting electrodes depend on the resistivity. The holdingforce on the electrodes increase as the value of resistivityincrease and more force are required to dislodge the dust layerfrom the collecting electrodes. The deposits of dust particlesat the discharge electrodes alter the characteristics of coronadischarge and thereby affect the precipitator performance.(2) The high resistivity dust produces a high electric fieldacross the dust layer deposited on collecting electrodes whencurrent passes through them. This electric field is high enoughto cause local ionization at the dust layer. This process oflocal ionization in precipitator parlance is called back-corona.Back corona generates charges of opposite polarity and thepositive charges emanating from the dust layers neutralize thenegatively charged dust particles and thereby precipitationprocess is impaired. The neutralized dust particles travelalong the gas flow with out being captured. Under backcorona conditions, the voltage available for the charging ofdust particles reduces and as a consequence the migrationvelocity reduces and so is the collection efficiency.(3) The low resistivity ash, on the other lead to poorholding force of the dust on the collecting electrode. Therewill be re-entrainment loss, under such condition andprecipitator performance will be impairedThe electrical system is thus disturbed due to (a) changein operating voltage, (b) change in power fed to the systemand (c) change in sparking behavior of the precipitator.The performance of ESP with respect to fly ashresistivity may be summarizing as follows:ABCDTable 1 Performance of ESP with respect toFly Ash Resistivity(10 4 -10 8 )Ω·cm(10 8 -10 10 )Ω·cm(10 10 -10 11 )Ω·cm(10 11 -10 13 )Ω·cmConductiveNormalResistivityModerateResistivityHighResistivityLowmigrationvelocityModeratemigrationvelocityHighmigrationvelocityVery lowmigrationvelocityLow collectionefficiency ofESPModeratecollectionefficiencyof ESPHigh collectionefficiency ofESPLow collectionefficiency ofESPThe fly ash resistivity on Indian coal is in the range of(10 11 -10 13 ) Ω·cm and as a result they are of big sizes to meetthe emission standard.

40011th International Conference on Electrostatic PrecipitationChemical Composition and Electrical ResistivityThe electrical properties of fly ash depend on thechemical composition of coal ash feed and combustionconditions prevailing in the boiler of the power plant. It is,therefore important to understand the relation between thechemical composition of fly ash coming out from the boilerand its electrical resistivity besides its composition itstemperature and moisture contents are also important. Theaverage characteristic of Indian coal is given in Table 2 [3]. Inmany of the power plants in India, the ash content of coal isas high as 45%. Thus compared with the U.S. and Europeancoals, Indian coals generate about 6 t0 7 times more ash forcollection by the ESPs for similar electricity generation.Moreover, the properties of fly ash generated in India are alsosignificantly different. Because of the low sulfur content (lessthan 0.6%), and chlorine content is less than 0.1%, [4] theresistivity of fly ash is 100 to 1000 times higher than thatgenerated, say, in U.S. As a result, the ESPs in India, despitebeing much larger, have lower collection efficiencies, than theESPs in U.S. Thus knowledge of fly ash resistivity is essentialfor the design improvements of ESPs. The electricalresistivity of fly ash strongly depends on the chemicalcomposition of the fly ash. Major constitute of the fly ash aresilica (SiO 2 ), alumina (Al 2 O 3 ) and iron oxides (Fe 2 O 3 ), Table3 indicates the typical fly ash chemical compositions data forsome of the Indian thermal power stations.Coal MinesCharacteristicsProximateAnalysis (%)MoistureVolatile matterFixed CarbonAshSulphurHHV(kJ/kg)Ultimate (Ash)Analysis (%)SiO 2Al 2 O 3Fe 2 O 3TiO 2P2O5CaoMgOSO 3Na 2 O 3K 2 OMn OTable 2 Average characteristics of Indian coalSingareni Kushm–anda Singrauli Jharia Neyveli9.623.332.934.00.3634133.359.3522.048.05––5.572.26––––10.023.025.040.50.285590.061.327.425.281.700.541.420.970.231.07–0.05512.020.127.940.00.313641.660.7325.76.41.760.71.20.930.260.261.73–13.017.5128.2236.080.413300.057.6426.3910.191.430.821.780.600.590.20––42.5224.519.57.50.632850.065.213.273.6––11.25.01.370.320.04–Totalaveragevalue17.4221.6826.7031.610.393902.9860.8422.966.701.630.684.231.950.610.460.880.05S. No. Power stations1.2.3.4.5.6.7.8.9.10.11.BadarpurDadriRihandUnchaharKorbaVindhyanchalRamagundamVijayawadaNeyveliKahalgoanFarakkaTable 3 Chemical composition of Indian fly ashFly Ash Compounds (weight %)SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 Mn O MgO Cao K 2 O Na 2 O 3 LOI57.3652.7459.7559.6062.0962.8960.8361.6338.0360.3560.3031.7837.8034.130.6031.3027.0826.6330.9243.3830.1230.901.650.900.51.501.821.101.131.721.821.811.304.623.416.14.203.336.124.193.334.055.625.020.21b.d.0.40.10b.d.b.d.0.08b.d.0.12b.d.b.d.0.230.240.350.400.010.100.800.050.020.400.600.621.000.20.900.030.803.031.117.670.800.900.590.660.450.700.040.270.900.610.050.560.500.230.140.30.200.090.100.400.130.430.120.152.663.010.45–1.211.501.810.403.400.200.30

<strong>Some</strong> <strong>Investigations</strong> on Fly Ash Resistivity Generated in Indian Power Plants 401Table 4 Effect of fly ash chemistry on resistivityChemical constituent Property Effect on ResistivitySilica (SiO 2 )Alumina (Al 2 O 3 )Calcium Oxide (CaO)Iron Oxide (Fe 2 O 3 )Sodium Oxide (Na 2 O)Phosphorous pentaoxide (P 2 O 5 )Pottasium Oxide (K 2 O)Sulphur Trioxide (SO 3 )Lithium Oxide (Li 2 O 3 )Titania (TiO 2 )Magnesia(MgO)InsulatorInsulatorSO 3 absorberIncreases alkali ion solubility and mobilityIon ContributorIon ContributorIon ContributorIon ContributorIon Contributor––––IncreaseIncreaseIncreaseDecreaseDecreaseDecreaseDecreaseDecreaseDecrease––––In view of the importance of the resistivity of the fly ashcomposition (Table 4) as a primary factor of the performanceof a precipitator, it become necessary the knowledge of the flyash chemistry effect on resistivity (as shown in Table 4). Itmay be noted that silica, alumina, and calcium oxide rangebetween 85 to 90% leading to higher resistivity. [6] Theelectrical resistivity depends on its chemical composition,temperature, and moisture. A number of empirical relationshave been developed to predict the electrical resistivity of flyash as a function of various parameters mentioned earlier [5,6]. A set of correlations for predicting fly ash resistivity basedon the composition and the coal analysis have developed byBickelhaupt [7,8] and are widely used in USA, primarily forthose fly ashes which are based on western coals. SouthernResearch Institute Birmingham USA is also using similarrelations [9] for evaluating the resistivity. In India, however,there are varieties of coal used in different power plantsacross the country. They differ significantly from those usedin USA. The composition of fly ash is too different fromthose generated in power plants in USA. The objectives of thepresent studies are: (a) to measure resistivity of representativefly ash samples from different Indian power plants andcompare the experimental results with those obtained fromBickelhaupt relations. (b) To develop mathematic model topredict the fly ash resistivity for Indian coals, in case there issignificant departure from Bickelhaupt relations. (c) To studythe effect of NH 3 dosing and sodium conditioning of ash onthe resistivity of fly ash. The samples have been obtainedfrom the Indian power plants, wherever such experimentshave been performed to improve the collection efficiency ofESP.2 EXPERIMENTAL ARRANGEMENT AND PROCE-DUREAn experimental test arrangement was set up per IEEEstandard criteria and guidelines [10] for the fly ash resistivitymeasurements as shown in Fig. 1. The current across the flyash layer under test is limited to 2×10 -5 amp/cm 2 to avoid theohmic heating of the fly ash sample. The test apparatusincludes four electric resistivity test cells enclosed in such amanner that the test cells are housed in a thermally controlledchamber so that resistivity can be determined at temperaturerange of 90 ℃-455 ℃. A dc high voltage power supply wasused to impress the required magnitude of electric fieldstrength. The environment was maintained as per thestandard. The environmental water concentration wasintroduced by bubbling a portion of dry gas through distilledwater maintained at a selected temperature in athermostatically controlled water bath. It was 9% by volumeat the specified temperature in the present study. The oven iscapable of operating in the desired temperature range, within0.01 ℃ accuracy. The resistivity test cell has parallel plateconstruc-tion made from SS 304 steel. The resistivity cellcurrent was measured using a sensitive electrometer capableof reading current in the range o 10 -3 to 10 -11 amp. with anaccuracy of ± 2% of the full-scale reading. Fly ash sampleswere prepared in accordance with the IEEE standard andplaced in the test cell in a grounded environmental chamber.The upper electrode is gently placed on the top of ash with adefined pressure. The oven is started and once the desiredtemperatures are reached, the readings are taken for thetemperature, voltage and current using the instrumentationprovided in the test facility. The fly ash resistivity, ρ iscalculated from standard relation:⎛V⎞⎛ A⎞ρ = ⎜ ⎟⎜ ⎟⎝ I ⎠⎝ l ⎠where, V and I are the voltage and current across the fly ashsample, l and A represent the thickness and area of crosssectionof sample of fly ash cell. The resistivity is calculatedfor more than 250 different fly ash samples from Indian coalfired thermal power plants for the temperature range of 90 ℃to 460 ℃. However, 20 representative samples have beenselected whose chemical composition is known, for thedevelopment of model for ash resistivity.3 EMPIRICAL RELATIONS FOR PREDICTION OFFLY ASH RESISTIVITYBased on the chemical compositions of representative flyash samples, shown in Table 5, the electrical resistivity has

40211th International Conference on Electrostatic Precipitationbeen calculated using Bickelhaupt [11] correlations, whichare described as the following section.Fig. 1 Schematic diagram of ash resistivity measurement setup at IIT DelhiTable 5 Chemical composition of the typical fly ash samples (weight percent as the oxide) used for calculatingthe Electrical ResistivitySample No. Na 2 O K 2 O MgO CaO Fe 2 O 3 Al 2 O 3 SiO 2 TiO 2 P 2 O 5 SO 35 0.13 1.10 0.88 1.13 5.21 31.60 55.45 1.91 0.18 0.3813 0.09 0.59 0.54 1.63 4.22 35.95 52.97 2.19 0.10 0.5529 0.06 0.64 0.26 1.43 5.08 28.24 61.33 2.01 0.07 0.2537 0.13 0.48 0.45 2.97 7.56 24.76 60.20 1.59 0.06 0.2056 0.15 1.34 0.74 2.57 6.58 31.03 51.61 2.60 0.13 0.2364 0.15 1.04 0.73 2.30 5.46 23.70 59.64 1.90 0.9 0.033.1 Bickelhaupt RelationsBickelhaupt [11] proposed correlations to calculate thefly ash resistivity from the results of coal and the fly ashanalysis. The correlation fly ash resistivity in terms of volumeresistivity, surface resistivityAnd adsorbed acid resistivity is: The volume resistivityis:ρV= exp[( −1.8916lnX −0.9696lnY+ 1.237ln Z + 3.62876) − ( 0.069078)E(1)+ ( 9980.58 )]TThe surface resistivity is:ρS= exp[27.59774 −2.233348lnX− 0.00176W− 0.069078E(2)− 0.00073895W( exp ) 2303.3 ]TThe adsorbed acid resistivity is:ρ exp[59.0677 0.854721CSO 13049.47a= −3−T− 0.069078 E](3)For Z > 3.5% or K < 1.0 %The resultant resistivity is:1 1 1= +ρvsaρvsρa(4)1 1 1where = +ρvs ρv ρs(5)3.2 Proposed Correlation for Fly Ash Resistivity of IndianCoalThe Bickelhaupt model in equations is used to calculatethe resultant resistivity for three fly ash samples of Indianpower plants. Comparison between calculated and experimentalfly ash resistivity values for these samples arepresented in Figs. 2-7. It can be observed that Bickelhauptmodel results differ appreciably from experimental values inthe lower temperature range 90℃-160 ℃. It may be due to

<strong>Some</strong> <strong>Investigations</strong> on Fly Ash Resistivity Generated in Indian Power Plants 403significant difference in concentration of elements like sulfur,lithium, sodium and moisture contents as well as alumina plussilica components among the Indian and US coals. The sulfurconcentration in coal regulates the adsorbed acid resistivity.Fig. 5Resistivity (Ohm-cm)1.0E+141.0E+131.0E+121.0E+111.0E+101.0E+09Sample 051.0E+080 100 200 300 400 500Temperature (0C)Fig. 2ExperimentalBhpt modelPredictedResistivity (Ohm-cm)1.0E+141.0E+131.0E+121.0E+111.0E+101.0E+091.0E+08Sample 56ExperimentalBhpt modelPredicted0 100 200 300 400 500Temperature (0C)Fig. 6Resistivity (Ohm-cm)1.0E+141.0E+131.0E+121.0E+111.0E+101.0E+09Sample 131.0E+080 100 200 300 400 500Temperature (0C)Fig. 3ExperimentalBhpt modelPredictedResistivity (Ohm-cm)1.0E+141.0E+131.0E+121.0E+111.0E+101.0E+091.0E+08Sample 64ExperimentalBhpt modelPredicted0 100 200 300 400 500Temperature (0C)Fig. 7Resistivity (Ohm-cm)Resistivity (Ohm-cm)1.0E+141.0E+131.0E+121.0E+111.0E+101.0E+091.0E+081.0E+141.0E+131.0E+121.0E+111.0E+101.0E+091.0E+08Sample 290 100 200 300 400 500Temperature (0C)Fig. 4Sample 37ExperimentalBhpt modelPredictedExperimentalBhpt modelPredicted0 100 200 300 400 500Temperature (0C)Keeping these points in view, the fly ash resistivity forIndian coals was re-calculated in terms of Surface and volumeconduction. Since the concentration of sulfur is very less inIndian coals, it is worthwhile to assume that very little or zeroadsorbed acid conductivity is present. The Negligibleadsorption of SO 3 conduction may also be due to formation ofglassy alumina-silicate Surface that hinders the adsorption ofSO 3 on the fly ash surface. The total conduction in fly ash isthus assumed entirely due to surface and volume conduction.The Bickelhaupt expressions for surface and volumeresistivity are therefore, modified for the Indian coals.Regression procedure based on the Marquardt-Levenbergalgorithm is used to find the coefficients of the independentvariables of volume and surface resistivity that give the bestfit between the proposed correlations and the experimentaldata. The modified correlation for the volume and surfaceresistivity is:Correlation for the volume resistivity:( a X b Y c Z d )ρ = exp[ − ln − ln + ln +V v v vg− ( e ) (vvE + )]TCorrelation of the surface Resistivity:⎡gexp ln ( expS⎤ρS= as bs X cSW dsE eSW⎛ ⎞⎢− − − − ) ⎜ T ⎟⎣ ⎝ ⎠⎥⎦ (7)v(6)

40411th International Conference on Electrostatic PrecipitationThe resultant resistivity is:1 1 1= + (8)ρ ρ ρVS4 TO STUDY THE EFFECT OF NH 3 DOSING OF FLUEGASES AND SODIUM CONDITIONING OF ASHFly ash sample were collected from power plant No.2and power plant No.3 [12]. In power plant No.2, the ammoniadosing to flue gas was varied and the inlet and outlet dustloading were measured. The fly ash samples were collectedfor no dosing and varied amount of NH 3 . The experimentalmeasurements of electrical resistivity were made as per IEEEstandards. Similarly fly ash samples were collected frompower plants No.3 for untreated and treated with sodiumconditioned coal before feeding to boiler. Corresponding datafor dust loading was also available. In the following sectionswe describe investigations related with fly ash resistivity.4.1 Investigation on Electrical Resistivity Due to Dosing ofAmmonia to Flue GasesResistivity measurement was made on the fly ashsamples between the temperatures 90 ℃-455 ℃ with 9%moisture. Measurements were made on ascending anddescending mode of temperatures for different amount ofdosing. The following observations are made:(1) There are significant changes in resistivity duringascending mode between no dosing and dosed samples.(2) These changes are more pronounced at lowertemperatures (≤ 200 ℃); where surface conduction dominatesthrough fly ash deposited on the collecting electrodes.(3) There is significant drop in resistivity correspondingto enhanced value of migration velocity, collection efficiencyof ESP and much lower out let emissions.(4) The difference in resistivity between the dosing andno dosing conditions reduce as one goes to highertemperatures.(5) There is very little difference in resistivity betweenno dose and dosed fly ash samples in descending mode.One may conclude from these observations thatammonia dosing enhances surfaceConduction (≤ 200 ℃), thereby reducing the resistivityby about an order of magnitude. As a result, collectionefficiency of ESP improves and emission levels dropdrastically (from 166 mg/Nm 3 to 48 mg/Nm 3 ). As thetemperature raise (≥ 200 ℃), the effect of NH 3 is barelyobserved. This also explains why there is very little differencebetween dosed and undoes samples of fly ashes in descendingmode.4.2 Effect of Sodium ConditioningFly ash samples were collected from power plant No.2.Sodium sulphate salt was added to coal mass in such amanner as to increase the sodium oxide content of fly ash by0.5% [12]. Fly ash samples from treated and untreatedconditions were obtained and resistivity measurements weremade as per IEEE norms. Based on the experimentalinvestigations (Fig 8), the following observations may bemade:(1) there is significant decrease in resistivity at alltemperatures in the range 90 ℃-455 ℃.(2) This decrease of resistance corresponds to enhancecollection efficiency of ESP resulting in drastic reduction inemission levels [12].(3) There is enhanced electric conduction due to increasedconduction of sodium concentrations both at lower (surfaceconduction) and higher (volume conduction) temperatures. Atoperating temperatures of ESP 140 ℃-180 ℃ the resistivity isdecreased by an order of magnitude resulting in highermigration velocities.

40611th International Conference on Electrostatic Precipitation2. D.Visuvasam and S.Sekar, Role of fly ash Reseistivityon performance of electrostatic precipitator, BHEL’sexperience, Work shop on ESP Performance on fly ashResistivity,IIT Delhi September 23-24, 2004.3. Chandra, A., Kumar, S. and Kumar Sanjeev.<strong>Investigations</strong> on fly ash Resistivity of varieties of Coalsused in Indian power plants, Proceeding, 9thInternational Conference on Electrostatic Precipitation,Mpumalanga, South Africa B-05, 2004.4. Chandra, A., Sabberwal, S.P., & Mukherjee, A.K.).Performance evaluation of an ESP unit Using low gradecoal. 6th International Conference on ElectrostaticPrecipitators, Budapest, 209-214, 1996.5. White H.J., Industrial electrostatic precipitation. AddisonWesley, Reading MA. 1963.6. Chandra, A., Sharma, P.K., Sanjeev, K., & Kumar, S.Effect of fogging, flue gas conditioning and sodiumdosing of coal on fly ash Resistivity, an experimentalinvestigation. International Conference on Energy andEnvironment: Strategies for Sustainable Development,New Delhi, 418-423, 2004.7. Bickelhaupt, R.E. Electrical volume conduction in flyash. Journal of Air Pollution Control Association, 24,251-255, 1974.8. Bickelhaupt, R.E. Surface Resistivity and the chemicalcomposition of fly ash. Journal of Air Pollution ControlAssociation, 25, 248-152, 1975.9. Dismukes, Edward. B. Conditioning of fly ash withsulfur trioxide and Ammonia. Report No.EPA-600/2-75-015, Southern research Institute, Birmingham. AlabamaUSA. ,1975.10. IEEE Standard criteria and guidelines for the laboratorymeasurement and reporting of fly ash Resistivity. IEEE-Standard 548, 1991.11. Bickelhaupt, R.E. A technique for predicting fly ashResistivity. US EPA Report No. EPA-600/7-79-204,114, 1979.12. Merchant, G.H. Jr. Evaluation of Sodium Conditioning,Water Fogging and Coal Washing for EnvironmentalPerformance Improvement of ESP’s at BALCO CaptivePower Plant. Research Report under USAID-India:Green house Gas Pollution Prevention Project, Sept.1999.