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Reduction of SO2 emissions from a fluidized bed under staged combustion by fine limestone
Enviroomcnth&national, Vol. 23, No. 2, pp. 227-236.1997 C&.&t 01997 Elseviu ScienceLtd Printedin the USA. All rights resuwd 0160-4120/97S17.00+.00
Pergamon
PIISOMO-4120(97)00009-3
REDUCTION OF SO, EMISSIONS FROIUIA FLUIDIZED BED UNDER STAGED COMBUSTION BY FINE LIMESTONE W.Z. Khan Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
B.M. Gibbs Department of Fuel and Energy, University of Leads, Leeds, LS2 9JT, U.K.
EI 951 l-364 M(Received 18 November 1995; accepted 8 January 1997)
This paperconsidersthe effectsof operatingvariableson SO, emissionin stagedcombustionwith a stainlesssteelcombustor2 m high with a 0.3 x 0.3 m crosssection.Fluidizing air was supplied through a multihole distributor.The experimentswere carried out for fluidizing velocities of l-2 m/s, bed temperatureof 830-88O”C,2040% excessair, andbed particlesixesof 0.313and 0.655mm. Thebedtemperahue, fluidiig velocity,andexcessair level hada significanteffect on SO, emissionsreductionduring stagedcombustion.A maximumreductionof 64% was obtained at 70:30stage,830°C bedtemperature and 1.0m/s fluidizing velocity. The zeroor negativeemissionsreductionresultedfrom the balancebetweentire SO,pick-upby limestonein the regionwhere oxidizingconditionsprevail(dueto a higherstagelevel, i.e. 60:40)andthe releaseof SQ, already cqtured elsewherein the combustor,from CaSO,in reducingregions(dueto higherstageand low eXCeSS air).
Copyright 91997 Elsevier Science Lrd
INTRODUCTION
The fluidized-bed combustion of coal has the potential for simultaneous reduction in NO, and SO, emissions. Previous studies(Gibbs et al. 1976, 1988; Gibbs and Salam 1989; Khan and Gibbs 1990,1995; Kunii et al. 1980; Valk and Bramer 1989) have shown that substantial reduction in SO, emissions can be achieved using limestone and a reduction of 50% or more in NO, emissions by the operation of a combustor in an air-staged mode. The limestone can be introduced either mixed with the coal or separately.Limestone caltines, on entering the fluidized-bed combustor, form CaO, which then undergoessulphationwith SO,. In airstaged operation, the combustion air is separatedinto a primary air streamwhich is suppliedto the bed and a secondaryair streamwhich is injected higher up in the bed or freeboard space. For example, in 60:40 stage, 227
40% of the total air is injected as a secondaryair. All the coal is injected into the primary stage so that the bed is maintainedat substoichiometric conditions promoting NO destruction. The sulphation experiments in a thermo-gravimetric analyzer(TGA) apparatusconductedby Mulligan et al. ( 1989) showedthat limestone has a maximum sulphation efficiency at about 850°C. They confirmed that, in the temperaturerangeof 800-969°C sulphation occurs by reaction and subsequentlyby the diffusion of calcium and oxide ions through a coherent calcium sulphate layer to the gas/sulphateinterface. Allen and Hayhurst (1989) reported the formation of calcium sulphide and sulphate below 900°C in the absenceof oxygen. In the presenceof oxygen,calcium sulphateis formed with calcium sulphide and calcium sulphite as intermediates.
W.Z. Khan andB.M. Gibbs
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Hot vertical probe
r
L.S. hopper
Coal hopper
:TC4 TC3 TC2 ,TCl
L
Ash and spent sorbent collector
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Fig. 1. Main featuresof fluid&d-bed combustorand ancillaries.
Simonset al. (1988) usedan entrainment-flowreactorto test the adsorptionof sulphurdioxide and oxygenat a temperatureof around900’C. They reportedthat temperaturecontrol is much more importantthan eitherthe oxygenor water concentration. Tatebayashiet al. (1980) carried out unstagedcombustion experiments on a pilot-scale, fluidized-bed combustorto investigatesulphur reduction. Their data indicatedthat the desulphurizationefficiency decreases with a decreasein excessair. The optimum temperature for maximum sulphur reduction was in the range of 800-830°C. Modrak et al. (1982) used a 1.8 mz fluidized-bed combustor for unstagedexperiments. They observedthat sulphur removal is a strong function of the amount of fresh limestone feed. However, other factors like particle size, bed temperature,sulphur-release level, and entrainment lossesmay influence the desulphurizationefficiency. Stagedcombustion gives higher SO2and CO in the bed. Secondaryair allows further combustion in the freeboard and increases SO, emission, thereby decreasing overall SO, retention (Bramer et al. 1988; Modrak et al. 1982;Nack et al. 1980;Tatebayashiet al. 1980; Valk et al. 1987; Valk and Bramer 1989; Zaunder et al. 1987). According to Valk et al. (1987), SO, emissionsare doubled at a 0.6 primary air factor, which is equivalentto 60:40 stage.Regardingthe effect
of temperature,Valk et al. (1987) reported a 70% increase in SO, emissions when bed temperaturewas increasedfrom 790°C to 880°C. At 88O”C, 0, in the flue decreasedfrom 10 to 6.7%. A two-fold increasein SO, emissionsabove the zero level at a bed temperatureof 890°C and at an in-bedair ratio of 0.75 reportedby Barnes(1988), indicatesthat decomposition of CaSO, could also occur at a temperature of 880°C. Reactions(1) to (3) show the decomposition mechanismof CaSO,: CaSO,+ CO + CaO + SO, + CO, CaSO,+ 4C0 - CaS+ 4C0, 3CaS0, + CaS -* 4Ca0 + 4S0,
(1) (2)
(3)
Apart from CO, other reducing agents like H, and carbon can also reduce CaSO,. The laboratory scale investigationsof Spitsbergenet al. (1988) showedthat only small amountsof CaO and SO2were producedat 850°C and a considerableamountof CaS was not converted. CaS + 20, -t CaSO, (4) CaS+ 3/2 0, - CaO + SO, (5) Their findings, basedon reactions(4) and (5), give supportto the ideathat during stagedcombustion, SO, is producedby reaction (5).
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Reductionof Sq emissionsunderstagedcombustion
Air-staged combustion is a proven technique to reduceNO, emissions,but at the sametime it causesan increasein SO, emissions.This increaseis due to the presenceof secondaryair in the freeboardthat allows further combustion. The available literature on the reduction of SO, emissionsby limestone at stagedcombustion is still under dispute and more research is needed to confirm the mechanism of SO* reduction during stagedcombustion conditions. APPARATUS AND PROCEDURE
The main featuresof the fluidized-bedcombustorand ancillariesare presentedin Fig. 1. The bed consistedof silica sandof mean size 0.313 and 0.665 mm. The fine sand of 0.3 13 mm was used for the experiments of 1.Om/s fluidizing velocity and 0.665 mm was used for the experimentsof 1.5 and 2.0 m/s fluidizing velocity. Fluidizing air was suppliedby a fan, meteredand introduced through a distributor plate. For staged combustion,the secondaryair was introducedinto the combustor through a stainlesssteel pipe 100 cm abovethe bed surface. The minimum fluidizing velocity II,,,, for small size sandparticlescan be calculatedby the following equation of Kunii and Levenspiel(l969) if Re of sandparticles is < 20:
Y?ll= where, d, = particlediameterin cm; p, = densityof sandin gm/cm3; pB = densityof gas in g/cm’; g = accelerationdueto gravity (980 cm/s’);and, p = viscosity of gas in g/cm-s. The fluidizing velocity correspondsto the airflow rate required to burn a known weight of coal. Therefore, fluidizing velocity can be calculatedfrom the following formula: Fluidizing velocity = Airflow rate x (273.15 + bed temp. in “C) Area of bed x time x 293.15”C The accuracyof fluidizing velocity was checkedby metering the airflow rate and measuringthe oxygen in the flue.
Table 1. Typical analysisof Coventry (U.K.) coal* Proximateanalysis(dry basis)g/kg Ash Volatile matter Fixed carbon
62.2 330.0 607.8
Ultimate analysis(dry basis)gikg 775.1 Carbon Hydrogen 48.0 Oxygen 85.0 Nitrogen 14.3 15.0 Sulfur Moisture 50.0 Grosscalorific value (M&K) 31.185 * Coal ResearchEstablishment- Stock Orchard,U.K.
The bed was preheatedby a propane burner fixed above the bed, and the fluidizing airflow rate was adjusted to the lowest level to minimize heating time. Coal was fed in the combustor when the bed temperature reached 550°C. When the bed temperature reached8OO”C,the desiredcoal feed rate was adjusted to a constantvalue, the propaneburner was switched off, and the fluidizing air was adjustedto the required level. The bed temperaturewas maintained constant using an adjustable cooling coil with regulated circulating water. Concentrations of 0, (by TaylorServomex OA570 Paramagnetic Analyzer), CO (by ADC-nondispersiveintrared analyzer),CO* (by SiegaIrga 120nondispersiveanalyzer),and Sq (by ADC-RF Infrared gas analyzer)were recordedcontinuously. The experimentswere carried out at bed temperatures of 830-88O”C,fluidizing velocities of 1-2 m/s and excess air levels of 20-40%. Static bed height was 30 cm. Bituminous coal of 3- 16 mm (large) diameterin size was fed overbed and the Penrith limestone of 0.85 mm meandiameterin size was fed underbed.The composition of coal is shown in Table 1. The molar ratio of calciumto sulfur was 3 : 1. Threeair-stagelevels, i.e., 85: 17, 70:30, and 60:40, were used to investigate the effect of variousvariableson SO,. The experiments were conductedat 40 and 20% excessair. For 1.Oand 2.0 m/s fluidizing velocity, two air-stage levels were used while for 1.5 m/s velocity, the afore-mentioned three levelswere used.The axial concentrationprofiles of SO, and 0, through the combustor were measured for fluidizing velocities of l-2 m/s. The excess air combustionwas maintainedby changing the coal feed rate.
W.Z. Khan andB.M. Gibbs
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80 70 --
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o!::::::::::::::::::: 800
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Fig. 2. Effect of bed temperatureon sulfur dioxide reductionat 1.Om/s fluidking velocity and 40% excessair. 80 70 -60 -iE
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Fig. 3. Effect of bed temperatureon sulfur dioxide reductionat 1.5 m/s fluidizing velocity. RESULTS AND DISCUSSION Effect of bed fempefefufe
The effect of bed temperatureon SO, reduction is shown in Figs. 2 to 4. Figure 2 gives the results at 1.0 m /s and excess air of 40%. Figure 3 shows the plot of results at 1.5 m /s velocity and excessair of 40 and 20%, and Fig. 4 illustrates the results obtained at 2.0 m /s velocity and 40 and 20% excessair levels.For
1.Om /s fluidizing velocity, fine sandof 0.313 m m was used. The bed temperaturevariation has a dramatic effect on sulphurreduction,which is true for all fluidizing velocities and sizes of bed material. A maximum reduction of 64% was obtained for 70:30 stagingat 830°C bed temperatureand at 1.Om /s reductionat 830°C was 51%, and upon increasingthe bed temperature to 88O”C, reduction decreasedto 25%.
Reductionof SQ emissionsunderstagedcombustion
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70 T
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Fig. 4. Effect of bed temperatureon sulfur dioxide reductionat 2.0 m/s fluidizing velocity. 60 50 40 2% 30 5 20 .33 + 10 p! Ei 0 L z
-10 -20 -30 -40 0.5
0.6
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Fig. 5. Effect of PACR on sulfur dioxide reductionat 1.5 fluidizing velocity and 850°C bed temperature.
At 1.5 m/s fluidizing velocity, the maximum reduction was 61%, 43%, and 14% for 85:15, 70:30, and 60:40 stageat 830°C; this decreasedto 17%, 13%, and 5% at 880°C. The SO, reduction was further reduced when the excessair level was reduced from 40% to 20%, and a negative reduction was observedat a temperatureof 880°C. The negativereductionincreasedas the air-stage level was increased.
At 2.0 m/s overall fluid&kg velocity and 20% excess air, the maximum reduction was 22% and 8% (at 830°C) for 70:30 and 60:40 stage,and this decreased to -12% and -37% at 880°C. The higher negative reduction at 60:40 stageindicatesthe generationof SO, in the reducingregion of the combustor.The rapid fall in SO, reduction might be caused by two possible mechanisms:1) by lower in-bedoxygenpartial pressure
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during stagedcombustion, limiting the sulphationreaction; and 2) by decompositionof calcium sulphate under reducing conditions. However, the maximum reductionat 830°C is in agreementwith the findings of Tatebayashiet al. (1980) and Valk et al. (1987). Effect of excess air
The effect of excessair on SO, reduction during airstaged combustion can also be seen in Figs. 2 to 4. These figures present the results of SO, reduction at threeoverall fluidizing velocitiesof 1.O,1.5,and2.0 m /s. A decreasein SO, reduction can be seenin theseplots when the excessair level was reduced, and this was true for all fluidizing velocities and primary to secondary air ratios. The lower excessair level brings about a changein primary air to coal ratio and the primary stage(the lower bed region) becomesmore substoichiometric. The zero or negativereduction results from the balancebetweenthe SO*pick-up by limestone in the region where oxidizing conditions prevail and the releaseof SO, (from CaSO, in reducing regions) already capturedelsewherein the combustor.At 40% secondaryair, the negative reduction is probably becausea large proportion of the bed is under reducing conditions. Effect of primary air to coal ratio
The effect of variation in primary air to coal ratio (PACR) is shown in Fig. 5. The results obtained at a bed temperatureof 850°C are compared.The results show that the extent of SO, reduction during staged combustion is strongly influenced by the amount of primary air injected;this was true for variousfluidizing velocities. The reduction of SO, decreasessteadily with decreasingPACR. This decreasecould be due to the combination of lower oxygen concentrationin the bed availablefor sulphation and sulphur generafionin the reducing region. At low PACR, an increase in carry-over of unburnt fuel sulphur species into the freeboard,where it subsequentlyoxidizesto SO*,could also increaseSO, emission,thus lowering the overall SO, reduction. Effect of fhidizing velocity
The plot in Fig. 6 shows the effect of fluidizing velocity on SO, reduction at 70:30 stage,40% excess air and a bed temperatureof 850°C. A decreasein SO, reduction was observedwith an increasein fluidizing velocity. The decreasein SO2reductionat a high fluid-
W.Z. Khan and B.M. Gibbs
izing velocity can be attributed to the rapid mixing of SO, reducingspecieswith secondaryair which results in the oxidation of some of these speciesto end-products. Another explanationcould be the decreasedreaction time (shorterresidencetime in oxidizing zone). A third reasoncould be the lowering of the Ca/S ratio due to elutrition of fines of sorbent (limestone) at higher velocity. Axial concentration profiles of sulp~or dioxide and oxygen through the combustor
Axial concentration profiles of sulphur dioxide through the combustor for a wide range of operating conditionsare shown in Figs. 7 to 11. The variation in in-bed and freeboardSO, pick-up by limestoneduring air stagingis clearly seenin theseplots. They show that SO, capture is sensitive to parameterslike fluidizing velocity and excessair. All axial profiles were taken at a bed temperatureof 850°C. The SO2reduction is also influencedby the presenceof partly spentlimestone,if the level of stageor excessair is changedduring the experiment.Figure 7 illustratesthe resultsof axial concentration of SO, and 0, for 1.0 m /s fluidizing velocity, 40% excessair and a sandparticle size of 0.3 13 m m . The axial profile of SO, at 70:30 stage shows a continuousdecreasein SO, in the bed between40 and 100 cm height and then a slight increasein the fi-eeboard. The extent of sulphation in the bed can be understoodby linking this axial profile with the axial profile of oxygen for the corresponding operation conditions.The gradualincreasein the oxygen concentration justifies the observedtrend in SO, reduction. The axial profile of SO, at 60:40 stageshows the SO, reductionat a height of 42 cm, the generationof SO*at 72 cm, and then a reductionat 100 cm followed by the generationof SO, abovea height of 100 cm. This trend clearly indicatesthe creationof oxidizing and reducing regions in the bed and freeboard. Unlike 70:30, the 60:40 stageproducestwo oxidizing and two reducing regions. The oxygen profile at 60:40 shows that at heightsof 42 and 72 cm, the oxygen concentrationwas zero (substoichiometricbed) and at 102 cm, oxygen concentrationrapidly rises, due to secondaryair injection at this height. Axial profiles of SO, and oxygen for 1.5 m /s fluidizing velocity and 40% excessair are shown in Fig. 8. This figure presentsthe axial profile at three stage levels: 85: 15, 70:30, and 60:40. The maximum in-bed SO, captureseemsto occur at the lower stagelevels of 85:15 and 70:30. At 60:40 stage, the bed is under a
Reductionof SQ emissionsunderstagedcombustion
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60 ,
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Fluidizing velocity (m/s) Fig. 6. Effect of fluidizing velocity on sulfur dioxide reductionat 40% excessair and 850°C bed temperature.
--+-
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6CHOstage ---+-
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6CY40stage
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Height above the distributor (cm) Fig. 7. Axial concentrationprofile of sulfur dioxide aud oxygen through the combustorat 1.Om/s fluidizing velocity, 850°C bed temperature,and 40% excessair.
reducing environment and a slight reduction in SO2at 102 cm (near the secondaryair injection point) was observed. However, the trend in the SO, profile indicatesthe creation of oxidizing and reducing zones in the combustor. The unexpectedtrend in the caseof 70:30 staging could be due to the error of taking readingsafter 20 min in this particularexperiment.The axial oxygen profile for the correspondingconditions
showsa similar trend, except during 85: 15 stagewhen the higher oxygen concentrationat a height of 72 cm could be due to less secondaryair and rapid mixing of primary and secondaryair. Figure 9 shows axial profiles of sulphurdioxide and oxygen for 1.5 m/s velocity and 20% excessair. The deviation from the expected trend at a lower excessair level (in the case of 70:30 and 60:40) was due to the presenceof limestone from
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W.Z. Khan andB.M. Gibbs
I-II-
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Fig. 8. Axial concentrationprofile of sulfur dioxide and oxygen through the combustorat 1.5 m/s fluidking velocity, 850°C bed temperature,and 40% excessair. +
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Fig. 9. Axial concentrationprofile of sulfur dioxide and oxygen through the combustorat 1.5 m/s fluidizing velocity, 850°C bed temperature,and 20% excessair.
a previous run and fluctuation in the volumetric flow rate of the air usedfor the pneumaticinjection of limestone. Figure 10 gives the axial profile of SO2and oxygen for 2.0 m/s and 40% excess air. During both stage levels of 70:30 and 60:40, the spent limestone of the
previous run was already present. The zero or lower reductionof SO,, at a height of 42 cm for 30 and 40% secondaryair, indicates the generationof SO* by the decomposition of calcium sulphate under a reducing environmentcoupledwith the short residencetime (due to the high velocity) and lowering of the Ca/S ratio
235
‘Reductionof SQ emissionsunderstagedcombustion
+
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Height above the distributor (cm) Fig. 10. Axial concentrationprofile of sulfur dioxide and oxygen through the combustorat 2.0 m/s fluidizing velocity, 850°C bed temperature,and 40% excessair. +
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Height above the distributor (cm) Fig. 11. Axial concentrationprofile of sulk dioxide and oxygen through the comb&or at 2.0 m/s fluidizing velocity, 850°C bed temperature,and 20% excessair.
(due to elutrition of fines in the sorbent). In-bed variation in the oxygen profile could be the result of higher primary air velocity (in the caseof 2.0 m/s overall fluidizing velocity). Axial profiles of SO, and oxygen for 2.0 m/s and 20% excess air are shown in Fig. 11. The expected
axial profile of SO, was observed under the given operatingconditions.Higher in-bed SO, reduction was found at 70:30 stage,which is justified by the oxygen profile for the correspondingoperatingparameters.The reduction in SO, capture at a height of 72 cm is apparent from the curve and indicates the presenceof a
236
reducing environment. The axial profile of oxygen showsa similar oxygen concentrationat heights of 42, 72, and 102 cm for 70:30 staging, but for 60:40 stage, oxygen concentrafion was further reduced at 42 and 72 cm, with a higher concentrationat 102 cm height. This trend in the profile presentsthe picture of oxidizing and reducingregionsproducedby the stageand above-bedmixing. CONCLUSIONS
Sulphur dioxide reduction is sensitive to bed temperature, excess air, and air-stage level. Among these, the bed temperature variation has a dramatic effect on sulphur dioxide reduction. A maximum reduction of 64% was obtainedby underbedfeedingof fine limestone at 70:30 stage,830°C bed temperature and 1.Om/s fluidizing velocity. High fluidizing velocity during stagedcombustiondeterioratesthe environment for maximum SQ reduction.The negativereductionat 880°C continues to increaseas the level of air stage increases.The zero or negative reduction results from the balancebetween the SO, pick-up by limestone in the region where oxidizing conditions prevail and the release of SO, (from CaSO, in reducing regions) alreadycapturedelsewherein the combustor.The axial concentrationprofiles of SO, and 0, through the combustor confirm the creation of oxidizing and reducing regions in the bed and freeboard which justify the release of more captured sulphur dioxide at a higher stage. REFERENCES Allen, D.; Hayhurst,A.N. A study of the reactionbetweenSQ and solid calcium oxide both with and without oxygen present.In: Proc. joint meeting of the British and French Section of the Combustion Institute. Rotten, France. 1989: 73-76. Available from: The CombustionInstitute, Washington,DC. Barnes, J.P. Abatement of NOx emissions from a coal fired fluidized-bed. Ph.D. Thesis. U.K.: Dept. of Fuel and Energy, University of Leeds; 1988. Bramer, E.A. et al. Sulphur captureunder reducing conditions at atmosphericfluidized-bedcombustion.In: Proc.4th international fluidized-bed combustion conference.Vol. 1. 1988: 1/11/ll/l l/l 1. Available from: The Institute of Energy, London, U.K. Gibbs, B.M.; Pereira,F.J.; Beer, J.M. The influence of air staging on the “NO” emission from a fluidized-bed combustor. In: Sixteenthsymposium(international)on combustion.Pittsburgh, PA, USA. 1976: 46I-473. Available from: The Combustion Institute, Washington,DC. Gibbs, B.M.; Salam, T.F.; Sibtain, S.F. The reduction of NOx emissionsfrom a fluidized-bedcombustorby stagedcombustion
W.Z. Khan and B.M. Gibbs’
combined with ammonia injection. In: 21nd symposium (international) on combustion. 1988. Available from: The CombustionInstitute, Washington,DC. Gibbs, B.M.; Salam,T.F. Reductionof NOx in the freeboardof a fluidized-bed combustor using ammonia injection. In: Joint meeting of the British and French Section of the Combustion Institute. Rouen, France. 1989: 53-56. Available from: The CombustionInstitute, Washington,DC. Khan, W.Z.; Gibbs, B.M. The SO2captureby limestoneat staged combustionin a fluid&d combustor.In: Proc. 13&world energy engineering congress Atlanta, GA, USA. 1990: 273-278. Available from: Associationof Energy Engineers,Atlanta, GA. Khan, W.Z.; Gibbs, B.M. The inthtence of air staging in the reductionof SO*by limestonein a fluidized-bedcombustor.Fuel 74: 800-805; 1995. Kunii, D.; Levenspiel,0. Fluidizationengineering.New York, NY: John Wiley and Sons,Inc.; 1969. Kunii, D.; Furusawa,T.; Kuang Tsai Wu. NOx emissioncontrol by a stagedfluid&d-bed combustorof coal In: Internationalfluidization conference.Henniker,N.H. Aug. 3-8. 1980: 175-183. Modrak, T.M.; Tang, J.T.; Auliso, C.J. Sulphur captureand NO, reductionon the 6’x 6’AFBC test facility. PreprintsACS, Div. Fuel Chem. 1982: 226-242. Available from: The American Chemical Society,Washington,DC. Mulligan, T.; Pomeroy, M.; Bannard, J.E. The mechanism of sulphationof limestoneby SQ in the presenceof oxygen.J. Inst. Energy. 62(450):40-47; 1989. Nack, H. et al. Control of SO2 and NOx emissions by baflIes multisolid fluidized-bed combustion process. In: Proc. 6th internationalconferenceon fluidized-bed combustion.Atlanta, USA. 1980:979-985.Available from: The American Society of MechanicalEngineers,New York, NY. Simons, G.A.; Parker, D.E.; Monency, J.R. The oxygen reaction order of SO2with CaO. Comb. and Flame 74: 107-l 10; 1988. Spitsbergen,U.; Van Werren, W.C.; Akse, H.A. Desulphurization quality of limestone-basedsorbentstested under multistaged fluidized-bedcombustionconditions. In: Proc. 5th annual internationalPittsburghcoal conference.Pittsburgh,PA. 1988: 690704. Available from: Pittsburgh Energy Technology Center, Pittsburgh,PA. Tatebayashi,J.; Okada,Y.; Yano, K.; Ikeda, S. SimultaneousNO, and SO, emissionsreductionwith fluidized-bed combustion.In: Proc. 6” internationalconferenceon fluidized-bed combustion. Atlanta, GA. 1980: 986-995. Available from: The American Society of MechanicalEngineers,New York, NY. Valk, M.; Bramer, E.A.; Toissant, H.H.J. Effect of stagedcombustionof coal on emissionlevelsof NOx and SO2in a fluidizedbed. In: Proc. ninth international conferenceon fluidized-bed combustion.Boston,MA, USA. 1987:784-794.Available from: The AmericanSocietyof MechanicalEngineers,New York, NY. Valk, M.; Bramer, E.A. Optimal stagedcombustionconditions in a fluidized-bed for simultaneouslow NO, and SO, emission levels.In: Proc.tenth internationalconferenceon fluidized-bed combustion. San Francisco, CA 1989: 995-1001. Available from: The American Society of Mechanical Engineers,New York, NY. Zaunder, B. et al. SOx-NO, control in a staged cyclone coal combustorwith limestoneinjection. Preprint of paperpresented at 194thnationalmeetingof ACS. 32:(4): 375-382;1987.Available from: The American Chemical Society, Washington,DC.
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REDUCTION OF SO, EMISSIONS FROIUIA FLUIDIZED BED UNDER STAGED COMBUSTION BY FINE LIMESTONE W.Z. Khan Department of Chemical Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
B.M. Gibbs Department of Fuel and Energy, University of Leads, Leeds, LS2 9JT, U.K.
EI 951 l-364 M(Received 18 November 1995; accepted 8 January 1997)
This paperconsidersthe effectsof operatingvariableson SO, emissionin stagedcombustionwith a stainlesssteelcombustor2 m high with a 0.3 x 0.3 m crosssection.Fluidizing air was supplied through a multihole distributor.The experimentswere carried out for fluidizing velocities of l-2 m/s, bed temperatureof 830-88O”C,2040% excessair, andbed particlesixesof 0.313and 0.655mm. Thebedtemperahue, fluidiig velocity,andexcessair level hada significanteffect on SO, emissionsreductionduring stagedcombustion.A maximumreductionof 64% was obtained at 70:30stage,830°C bedtemperature and 1.0m/s fluidizing velocity. The zeroor negativeemissionsreductionresultedfrom the balancebetweentire SO,pick-upby limestonein the regionwhere oxidizingconditionsprevail(dueto a higherstagelevel, i.e. 60:40)andthe releaseof SQ, already cqtured elsewherein the combustor,from CaSO,in reducingregions(dueto higherstageand low eXCeSS air).
Copyright 91997 Elsevier Science Lrd
INTRODUCTION
The fluidized-bed combustion of coal has the potential for simultaneous reduction in NO, and SO, emissions. Previous studies(Gibbs et al. 1976, 1988; Gibbs and Salam 1989; Khan and Gibbs 1990,1995; Kunii et al. 1980; Valk and Bramer 1989) have shown that substantial reduction in SO, emissions can be achieved using limestone and a reduction of 50% or more in NO, emissions by the operation of a combustor in an air-staged mode. The limestone can be introduced either mixed with the coal or separately.Limestone caltines, on entering the fluidized-bed combustor, form CaO, which then undergoessulphationwith SO,. In airstaged operation, the combustion air is separatedinto a primary air streamwhich is suppliedto the bed and a secondaryair streamwhich is injected higher up in the bed or freeboard space. For example, in 60:40 stage, 227
40% of the total air is injected as a secondaryair. All the coal is injected into the primary stage so that the bed is maintainedat substoichiometric conditions promoting NO destruction. The sulphation experiments in a thermo-gravimetric analyzer(TGA) apparatusconductedby Mulligan et al. ( 1989) showedthat limestone has a maximum sulphation efficiency at about 850°C. They confirmed that, in the temperaturerangeof 800-969°C sulphation occurs by reaction and subsequentlyby the diffusion of calcium and oxide ions through a coherent calcium sulphate layer to the gas/sulphateinterface. Allen and Hayhurst (1989) reported the formation of calcium sulphide and sulphate below 900°C in the absenceof oxygen. In the presenceof oxygen,calcium sulphateis formed with calcium sulphide and calcium sulphite as intermediates.
W.Z. Khan andB.M. Gibbs
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Hot vertical probe
r
L.S. hopper
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:TC4 TC3 TC2 ,TCl
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Fig. 1. Main featuresof fluid&d-bed combustorand ancillaries.
Simonset al. (1988) usedan entrainment-flowreactorto test the adsorptionof sulphurdioxide and oxygenat a temperatureof around900’C. They reportedthat temperaturecontrol is much more importantthan eitherthe oxygenor water concentration. Tatebayashiet al. (1980) carried out unstagedcombustion experiments on a pilot-scale, fluidized-bed combustorto investigatesulphur reduction. Their data indicatedthat the desulphurizationefficiency decreases with a decreasein excessair. The optimum temperature for maximum sulphur reduction was in the range of 800-830°C. Modrak et al. (1982) used a 1.8 mz fluidized-bed combustor for unstagedexperiments. They observedthat sulphur removal is a strong function of the amount of fresh limestone feed. However, other factors like particle size, bed temperature,sulphur-release level, and entrainment lossesmay influence the desulphurizationefficiency. Stagedcombustion gives higher SO2and CO in the bed. Secondaryair allows further combustion in the freeboard and increases SO, emission, thereby decreasing overall SO, retention (Bramer et al. 1988; Modrak et al. 1982;Nack et al. 1980;Tatebayashiet al. 1980; Valk et al. 1987; Valk and Bramer 1989; Zaunder et al. 1987). According to Valk et al. (1987), SO, emissionsare doubled at a 0.6 primary air factor, which is equivalentto 60:40 stage.Regardingthe effect
of temperature,Valk et al. (1987) reported a 70% increase in SO, emissions when bed temperaturewas increasedfrom 790°C to 880°C. At 88O”C, 0, in the flue decreasedfrom 10 to 6.7%. A two-fold increasein SO, emissionsabove the zero level at a bed temperatureof 890°C and at an in-bedair ratio of 0.75 reportedby Barnes(1988), indicatesthat decomposition of CaSO, could also occur at a temperature of 880°C. Reactions(1) to (3) show the decomposition mechanismof CaSO,: CaSO,+ CO + CaO + SO, + CO, CaSO,+ 4C0 - CaS+ 4C0, 3CaS0, + CaS -* 4Ca0 + 4S0,
(1) (2)
(3)
Apart from CO, other reducing agents like H, and carbon can also reduce CaSO,. The laboratory scale investigationsof Spitsbergenet al. (1988) showedthat only small amountsof CaO and SO2were producedat 850°C and a considerableamountof CaS was not converted. CaS + 20, -t CaSO, (4) CaS+ 3/2 0, - CaO + SO, (5) Their findings, basedon reactions(4) and (5), give supportto the ideathat during stagedcombustion, SO, is producedby reaction (5).
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Reductionof Sq emissionsunderstagedcombustion
Air-staged combustion is a proven technique to reduceNO, emissions,but at the sametime it causesan increasein SO, emissions.This increaseis due to the presenceof secondaryair in the freeboardthat allows further combustion. The available literature on the reduction of SO, emissionsby limestone at stagedcombustion is still under dispute and more research is needed to confirm the mechanism of SO* reduction during stagedcombustion conditions. APPARATUS AND PROCEDURE
The main featuresof the fluidized-bedcombustorand ancillariesare presentedin Fig. 1. The bed consistedof silica sandof mean size 0.313 and 0.665 mm. The fine sand of 0.3 13 mm was used for the experiments of 1.Om/s fluidizing velocity and 0.665 mm was used for the experimentsof 1.5 and 2.0 m/s fluidizing velocity. Fluidizing air was suppliedby a fan, meteredand introduced through a distributor plate. For staged combustion,the secondaryair was introducedinto the combustor through a stainlesssteel pipe 100 cm abovethe bed surface. The minimum fluidizing velocity II,,,, for small size sandparticlescan be calculatedby the following equation of Kunii and Levenspiel(l969) if Re of sandparticles is < 20:
Y?ll= where, d, = particlediameterin cm; p, = densityof sandin gm/cm3; pB = densityof gas in g/cm’; g = accelerationdueto gravity (980 cm/s’);and, p = viscosity of gas in g/cm-s. The fluidizing velocity correspondsto the airflow rate required to burn a known weight of coal. Therefore, fluidizing velocity can be calculatedfrom the following formula: Fluidizing velocity = Airflow rate x (273.15 + bed temp. in “C) Area of bed x time x 293.15”C The accuracyof fluidizing velocity was checkedby metering the airflow rate and measuringthe oxygen in the flue.
Table 1. Typical analysisof Coventry (U.K.) coal* Proximateanalysis(dry basis)g/kg Ash Volatile matter Fixed carbon
62.2 330.0 607.8
Ultimate analysis(dry basis)gikg 775.1 Carbon Hydrogen 48.0 Oxygen 85.0 Nitrogen 14.3 15.0 Sulfur Moisture 50.0 Grosscalorific value (M&K) 31.185 * Coal ResearchEstablishment- Stock Orchard,U.K.
The bed was preheatedby a propane burner fixed above the bed, and the fluidizing airflow rate was adjusted to the lowest level to minimize heating time. Coal was fed in the combustor when the bed temperature reached 550°C. When the bed temperature reached8OO”C,the desiredcoal feed rate was adjusted to a constantvalue, the propaneburner was switched off, and the fluidizing air was adjustedto the required level. The bed temperaturewas maintained constant using an adjustable cooling coil with regulated circulating water. Concentrations of 0, (by TaylorServomex OA570 Paramagnetic Analyzer), CO (by ADC-nondispersiveintrared analyzer),CO* (by SiegaIrga 120nondispersiveanalyzer),and Sq (by ADC-RF Infrared gas analyzer)were recordedcontinuously. The experimentswere carried out at bed temperatures of 830-88O”C,fluidizing velocities of 1-2 m/s and excess air levels of 20-40%. Static bed height was 30 cm. Bituminous coal of 3- 16 mm (large) diameterin size was fed overbed and the Penrith limestone of 0.85 mm meandiameterin size was fed underbed.The composition of coal is shown in Table 1. The molar ratio of calciumto sulfur was 3 : 1. Threeair-stagelevels, i.e., 85: 17, 70:30, and 60:40, were used to investigate the effect of variousvariableson SO,. The experiments were conductedat 40 and 20% excessair. For 1.Oand 2.0 m/s fluidizing velocity, two air-stage levels were used while for 1.5 m/s velocity, the afore-mentioned three levelswere used.The axial concentrationprofiles of SO, and 0, through the combustor were measured for fluidizing velocities of l-2 m/s. The excess air combustionwas maintainedby changing the coal feed rate.
W.Z. Khan andB.M. Gibbs
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80 70 --
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o!::::::::::::::::::: 800
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Fig. 2. Effect of bed temperatureon sulfur dioxide reductionat 1.Om/s fluidking velocity and 40% excessair. 80 70 -60 -iE
50--
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Fig. 3. Effect of bed temperatureon sulfur dioxide reductionat 1.5 m/s fluidizing velocity. RESULTS AND DISCUSSION Effect of bed fempefefufe
The effect of bed temperatureon SO, reduction is shown in Figs. 2 to 4. Figure 2 gives the results at 1.0 m /s and excess air of 40%. Figure 3 shows the plot of results at 1.5 m /s velocity and excessair of 40 and 20%, and Fig. 4 illustrates the results obtained at 2.0 m /s velocity and 40 and 20% excessair levels.For
1.Om /s fluidizing velocity, fine sandof 0.313 m m was used. The bed temperaturevariation has a dramatic effect on sulphurreduction,which is true for all fluidizing velocities and sizes of bed material. A maximum reduction of 64% was obtained for 70:30 stagingat 830°C bed temperatureand at 1.Om /s reductionat 830°C was 51%, and upon increasingthe bed temperature to 88O”C, reduction decreasedto 25%.
Reductionof SQ emissionsunderstagedcombustion
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70 T
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Fig. 4. Effect of bed temperatureon sulfur dioxide reductionat 2.0 m/s fluidizing velocity. 60 50 40 2% 30 5 20 .33 + 10 p! Ei 0 L z
-10 -20 -30 -40 0.5
0.6
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0.9
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Fig. 5. Effect of PACR on sulfur dioxide reductionat 1.5 fluidizing velocity and 850°C bed temperature.
At 1.5 m/s fluidizing velocity, the maximum reduction was 61%, 43%, and 14% for 85:15, 70:30, and 60:40 stageat 830°C; this decreasedto 17%, 13%, and 5% at 880°C. The SO, reduction was further reduced when the excessair level was reduced from 40% to 20%, and a negative reduction was observedat a temperatureof 880°C. The negativereductionincreasedas the air-stage level was increased.
At 2.0 m/s overall fluid&kg velocity and 20% excess air, the maximum reduction was 22% and 8% (at 830°C) for 70:30 and 60:40 stage,and this decreased to -12% and -37% at 880°C. The higher negative reduction at 60:40 stageindicatesthe generationof SO, in the reducingregion of the combustor.The rapid fall in SO, reduction might be caused by two possible mechanisms:1) by lower in-bedoxygenpartial pressure
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during stagedcombustion, limiting the sulphationreaction; and 2) by decompositionof calcium sulphate under reducing conditions. However, the maximum reductionat 830°C is in agreementwith the findings of Tatebayashiet al. (1980) and Valk et al. (1987). Effect of excess air
The effect of excessair on SO, reduction during airstaged combustion can also be seen in Figs. 2 to 4. These figures present the results of SO, reduction at threeoverall fluidizing velocitiesof 1.O,1.5,and2.0 m /s. A decreasein SO, reduction can be seenin theseplots when the excessair level was reduced, and this was true for all fluidizing velocities and primary to secondary air ratios. The lower excessair level brings about a changein primary air to coal ratio and the primary stage(the lower bed region) becomesmore substoichiometric. The zero or negativereduction results from the balancebetweenthe SO*pick-up by limestone in the region where oxidizing conditions prevail and the releaseof SO, (from CaSO, in reducing regions) already capturedelsewherein the combustor.At 40% secondaryair, the negative reduction is probably becausea large proportion of the bed is under reducing conditions. Effect of primary air to coal ratio
The effect of variation in primary air to coal ratio (PACR) is shown in Fig. 5. The results obtained at a bed temperatureof 850°C are compared.The results show that the extent of SO, reduction during staged combustion is strongly influenced by the amount of primary air injected;this was true for variousfluidizing velocities. The reduction of SO, decreasessteadily with decreasingPACR. This decreasecould be due to the combination of lower oxygen concentrationin the bed availablefor sulphation and sulphur generafionin the reducing region. At low PACR, an increase in carry-over of unburnt fuel sulphur species into the freeboard,where it subsequentlyoxidizesto SO*,could also increaseSO, emission,thus lowering the overall SO, reduction. Effect of fhidizing velocity
The plot in Fig. 6 shows the effect of fluidizing velocity on SO, reduction at 70:30 stage,40% excess air and a bed temperatureof 850°C. A decreasein SO, reduction was observedwith an increasein fluidizing velocity. The decreasein SO2reductionat a high fluid-
W.Z. Khan and B.M. Gibbs
izing velocity can be attributed to the rapid mixing of SO, reducingspecieswith secondaryair which results in the oxidation of some of these speciesto end-products. Another explanationcould be the decreasedreaction time (shorterresidencetime in oxidizing zone). A third reasoncould be the lowering of the Ca/S ratio due to elutrition of fines of sorbent (limestone) at higher velocity. Axial concentration profiles of sulp~or dioxide and oxygen through the combustor
Axial concentration profiles of sulphur dioxide through the combustor for a wide range of operating conditionsare shown in Figs. 7 to 11. The variation in in-bed and freeboardSO, pick-up by limestoneduring air stagingis clearly seenin theseplots. They show that SO, capture is sensitive to parameterslike fluidizing velocity and excessair. All axial profiles were taken at a bed temperatureof 850°C. The SO2reduction is also influencedby the presenceof partly spentlimestone,if the level of stageor excessair is changedduring the experiment.Figure 7 illustratesthe resultsof axial concentration of SO, and 0, for 1.0 m /s fluidizing velocity, 40% excessair and a sandparticle size of 0.3 13 m m . The axial profile of SO, at 70:30 stage shows a continuousdecreasein SO, in the bed between40 and 100 cm height and then a slight increasein the fi-eeboard. The extent of sulphation in the bed can be understoodby linking this axial profile with the axial profile of oxygen for the corresponding operation conditions.The gradualincreasein the oxygen concentration justifies the observedtrend in SO, reduction. The axial profile of SO, at 60:40 stageshows the SO, reductionat a height of 42 cm, the generationof SO*at 72 cm, and then a reductionat 100 cm followed by the generationof SO, abovea height of 100 cm. This trend clearly indicatesthe creationof oxidizing and reducing regions in the bed and freeboard. Unlike 70:30, the 60:40 stageproducestwo oxidizing and two reducing regions. The oxygen profile at 60:40 shows that at heightsof 42 and 72 cm, the oxygen concentrationwas zero (substoichiometricbed) and at 102 cm, oxygen concentrationrapidly rises, due to secondaryair injection at this height. Axial profiles of SO, and oxygen for 1.5 m /s fluidizing velocity and 40% excessair are shown in Fig. 8. This figure presentsthe axial profile at three stage levels: 85: 15, 70:30, and 60:40. The maximum in-bed SO, captureseemsto occur at the lower stagelevels of 85:15 and 70:30. At 60:40 stage, the bed is under a
Reductionof SQ emissionsunderstagedcombustion
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60 ,
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Fluidizing velocity (m/s) Fig. 6. Effect of fluidizing velocity on sulfur dioxide reductionat 40% excessair and 850°C bed temperature.
--+-
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6CHOstage ---+-
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6CY40stage
60
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Height above the distributor (cm) Fig. 7. Axial concentrationprofile of sulfur dioxide aud oxygen through the combustorat 1.Om/s fluidizing velocity, 850°C bed temperature,and 40% excessair.
reducing environment and a slight reduction in SO2at 102 cm (near the secondaryair injection point) was observed. However, the trend in the SO, profile indicatesthe creation of oxidizing and reducing zones in the combustor. The unexpectedtrend in the caseof 70:30 staging could be due to the error of taking readingsafter 20 min in this particularexperiment.The axial oxygen profile for the correspondingconditions
showsa similar trend, except during 85: 15 stagewhen the higher oxygen concentrationat a height of 72 cm could be due to less secondaryair and rapid mixing of primary and secondaryair. Figure 9 shows axial profiles of sulphurdioxide and oxygen for 1.5 m/s velocity and 20% excessair. The deviation from the expected trend at a lower excessair level (in the case of 70:30 and 60:40) was due to the presenceof limestone from
234
W.Z. Khan andB.M. Gibbs
I-II-
1100
85/?5
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, 8
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Fig. 8. Axial concentrationprofile of sulfur dioxide and oxygen through the combustorat 1.5 m/s fluidking velocity, 850°C bed temperature,and 40% excessair. +
86/?5 stage
+
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+
8506 stage
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8CV40stage
i6
1100 1000 + 900 800 700 600 500 400 300 200 100 40
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Height above the distributor (cm)
Fig. 9. Axial concentrationprofile of sulfur dioxide and oxygen through the combustorat 1.5 m/s fluidizing velocity, 850°C bed temperature,and 20% excessair.
a previous run and fluctuation in the volumetric flow rate of the air usedfor the pneumaticinjection of limestone. Figure 10 gives the axial profile of SO2and oxygen for 2.0 m/s and 40% excess air. During both stage levels of 70:30 and 60:40, the spent limestone of the
previous run was already present. The zero or lower reductionof SO,, at a height of 42 cm for 30 and 40% secondaryair, indicates the generationof SO* by the decomposition of calcium sulphate under a reducing environmentcoupledwith the short residencetime (due to the high velocity) and lowering of the Ca/S ratio
235
‘Reductionof SQ emissionsunderstagedcombustion
+
7w3Ostage -&--
6Olwttage
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B(y40
StI3QIB
1000 900 800 700 800 500 400 40
60
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Height above the distributor (cm) Fig. 10. Axial concentrationprofile of sulfur dioxide and oxygen through the combustorat 2.0 m/s fluidizing velocity, 850°C bed temperature,and 40% excessair. +
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1200 1100 1000 900 800 700 600 500 400 300 200 100 40
60
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Height above the distributor (cm) Fig. 11. Axial concentrationprofile of sulk dioxide and oxygen through the comb&or at 2.0 m/s fluidizing velocity, 850°C bed temperature,and 20% excessair.
(due to elutrition of fines in the sorbent). In-bed variation in the oxygen profile could be the result of higher primary air velocity (in the caseof 2.0 m/s overall fluidizing velocity). Axial profiles of SO, and oxygen for 2.0 m/s and 20% excess air are shown in Fig. 11. The expected
axial profile of SO, was observed under the given operatingconditions.Higher in-bed SO, reduction was found at 70:30 stage,which is justified by the oxygen profile for the correspondingoperatingparameters.The reduction in SO, capture at a height of 72 cm is apparent from the curve and indicates the presenceof a
236
reducing environment. The axial profile of oxygen showsa similar oxygen concentrationat heights of 42, 72, and 102 cm for 70:30 staging, but for 60:40 stage, oxygen concentrafion was further reduced at 42 and 72 cm, with a higher concentrationat 102 cm height. This trend in the profile presentsthe picture of oxidizing and reducingregionsproducedby the stageand above-bedmixing. CONCLUSIONS
Sulphur dioxide reduction is sensitive to bed temperature, excess air, and air-stage level. Among these, the bed temperature variation has a dramatic effect on sulphur dioxide reduction. A maximum reduction of 64% was obtainedby underbedfeedingof fine limestone at 70:30 stage,830°C bed temperature and 1.Om/s fluidizing velocity. High fluidizing velocity during stagedcombustiondeterioratesthe environment for maximum SQ reduction.The negativereductionat 880°C continues to increaseas the level of air stage increases.The zero or negative reduction results from the balancebetween the SO, pick-up by limestone in the region where oxidizing conditions prevail and the release of SO, (from CaSO, in reducing regions) alreadycapturedelsewherein the combustor.The axial concentrationprofiles of SO, and 0, through the combustor confirm the creation of oxidizing and reducing regions in the bed and freeboard which justify the release of more captured sulphur dioxide at a higher stage. REFERENCES Allen, D.; Hayhurst,A.N. A study of the reactionbetweenSQ and solid calcium oxide both with and without oxygen present.In: Proc. joint meeting of the British and French Section of the Combustion Institute. Rotten, France. 1989: 73-76. Available from: The CombustionInstitute, Washington,DC. Barnes, J.P. Abatement of NOx emissions from a coal fired fluidized-bed. Ph.D. Thesis. U.K.: Dept. of Fuel and Energy, University of Leeds; 1988. Bramer, E.A. et al. Sulphur captureunder reducing conditions at atmosphericfluidized-bedcombustion.In: Proc.4th international fluidized-bed combustion conference.Vol. 1. 1988: 1/11/ll/l l/l 1. Available from: The Institute of Energy, London, U.K. Gibbs, B.M.; Pereira,F.J.; Beer, J.M. The influence of air staging on the “NO” emission from a fluidized-bed combustor. In: Sixteenthsymposium(international)on combustion.Pittsburgh, PA, USA. 1976: 46I-473. Available from: The Combustion Institute, Washington,DC. Gibbs, B.M.; Salam, T.F.; Sibtain, S.F. The reduction of NOx emissionsfrom a fluidized-bedcombustorby stagedcombustion
W.Z. Khan and B.M. Gibbs’
combined with ammonia injection. In: 21nd symposium (international) on combustion. 1988. Available from: The CombustionInstitute, Washington,DC. Gibbs, B.M.; Salam,T.F. Reductionof NOx in the freeboardof a fluidized-bed combustor using ammonia injection. In: Joint meeting of the British and French Section of the Combustion Institute. Rouen, France. 1989: 53-56. Available from: The CombustionInstitute, Washington,DC. Khan, W.Z.; Gibbs, B.M. The SO2captureby limestoneat staged combustionin a fluid&d combustor.In: Proc. 13&world energy engineering congress Atlanta, GA, USA. 1990: 273-278. Available from: Associationof Energy Engineers,Atlanta, GA. Khan, W.Z.; Gibbs, B.M. The inthtence of air staging in the reductionof SO*by limestonein a fluidized-bedcombustor.Fuel 74: 800-805; 1995. Kunii, D.; Levenspiel,0. Fluidizationengineering.New York, NY: John Wiley and Sons,Inc.; 1969. Kunii, D.; Furusawa,T.; Kuang Tsai Wu. NOx emissioncontrol by a stagedfluid&d-bed combustorof coal In: Internationalfluidization conference.Henniker,N.H. Aug. 3-8. 1980: 175-183. Modrak, T.M.; Tang, J.T.; Auliso, C.J. Sulphur captureand NO, reductionon the 6’x 6’AFBC test facility. PreprintsACS, Div. Fuel Chem. 1982: 226-242. Available from: The American Chemical Society,Washington,DC. Mulligan, T.; Pomeroy, M.; Bannard, J.E. The mechanism of sulphationof limestoneby SQ in the presenceof oxygen.J. Inst. Energy. 62(450):40-47; 1989. Nack, H. et al. Control of SO2 and NOx emissions by baflIes multisolid fluidized-bed combustion process. In: Proc. 6th internationalconferenceon fluidized-bed combustion.Atlanta, USA. 1980:979-985.Available from: The American Society of MechanicalEngineers,New York, NY. Simons, G.A.; Parker, D.E.; Monency, J.R. The oxygen reaction order of SO2with CaO. Comb. and Flame 74: 107-l 10; 1988. Spitsbergen,U.; Van Werren, W.C.; Akse, H.A. Desulphurization quality of limestone-basedsorbentstested under multistaged fluidized-bedcombustionconditions. In: Proc. 5th annual internationalPittsburghcoal conference.Pittsburgh,PA. 1988: 690704. Available from: Pittsburgh Energy Technology Center, Pittsburgh,PA. Tatebayashi,J.; Okada,Y.; Yano, K.; Ikeda, S. SimultaneousNO, and SO, emissionsreductionwith fluidized-bed combustion.In: Proc. 6” internationalconferenceon fluidized-bed combustion. Atlanta, GA. 1980: 986-995. Available from: The American Society of MechanicalEngineers,New York, NY. Valk, M.; Bramer, E.A.; Toissant, H.H.J. Effect of stagedcombustionof coal on emissionlevelsof NOx and SO2in a fluidizedbed. In: Proc. ninth international conferenceon fluidized-bed combustion.Boston,MA, USA. 1987:784-794.Available from: The AmericanSocietyof MechanicalEngineers,New York, NY. Valk, M.; Bramer, E.A. Optimal stagedcombustionconditions in a fluidized-bed for simultaneouslow NO, and SO, emission levels.In: Proc.tenth internationalconferenceon fluidized-bed combustion. San Francisco, CA 1989: 995-1001. Available from: The American Society of Mechanical Engineers,New York, NY. Zaunder, B. et al. SOx-NO, control in a staged cyclone coal combustorwith limestoneinjection. Preprint of paperpresented at 194thnationalmeetingof ACS. 32:(4): 375-382;1987.Available from: The American Chemical Society, Washington,DC.