外文翻译石灰石脱硫对循环流化床中NOX排放的影响.docx
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外文翻译石灰石脱硫对循环流化床中NOX排放的影响.docx
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外文翻译石灰石脱硫对循环流化床中NOX排放的影响
英文原文
LimestoneEffectsonNOxFormationinCFBCombustors
Abstract
CirculatingFluidizedBed(CFB)combustiontechnologyhasbeenwidelyusedinpowergenerationwiththeconsiderationsofitsadvantagesineconomicallycontrollingSO2andNOxemissions.However,itisfoundthatNOXemissionincreasedupto30%whenlimestoneisaddedintothecombustorfordesulphurization,especiallywithratherhighCa/Sratio.ThephenomenonofNOxaugmentwasdiscussedbasedonassociatedmechanismsandchemicalkinetics.Thecatalyticoxidizationeffect,preferentialconversioneffectfromHCNtoNH3,andreductioneffectonNOxwereanalyzed.Itisfoundlimestonecanactasacatalyst,increasingthereactionratesforthereactionassociatedwithNOxformation.Furthermore,CaOdecomposedCaCO3favorstheconversionofHCNtoNH3whentheyarereleaseinde-volatilizationprocess.DuetopreferentialoxidizationofNH3toNOratherthantheunstableN2O,NOXemissionincreaseswithlimestoneinjection.WhenhighCa/Sisused,thereductioneffectbecomesdominatedandreducestheoverallNOXemission.
KeywordsCFB,limestone,desulphurization,NOXformation,chemicalkinetics
1.Introduction
CirculatingFluidizedBed(CFB)combustiontechnologyhasbeenwidelyusedinpowergeneration,becauseofitsadvantagesinsuchasthehighfuelflexibilityforburningvariouskindsofcoalsandthehighfeasibilityineconomicalemissioncontrol.GiventhatthecombustiontemperatureinCFBcombustors,e.g.,boilers,isusuallybetween1020~1120Kandsecondaryaircanbeinjectedatdifferentportsalongtheaxialdirection,theNOXconcentrationinthefluegascanbecontrolledtobemuchlowerthanthoseusingothercombustiontechnologies.ShowninFig.1,NOxemissionfromaCFBcombustorsarelowest,intherangeof100-220ppm,comparedwithotherkindsofcombustors[1].
Fig.1NOXemissionlevelfordifferentcombustionsystems[3]
Whencoalswithhighsurfacecontentareburned,limestoneisaddedintoCFBboilersfordesulphurization.Withinthenormaltemperatures,theefficiencyoflimestonedesulphurizationcanbeupto90%[1].Inadditiontothedesulphurization,limestonemayplayseveralotherpositiverolesinimprovingtheboilerperformanceofsuchascombustion,heattransfer,materialbalanceandashseparationintheseparator.However,observedinpracticaloperationandexperiments,limestoneadditioninCFBcombustorsmightcauseanegativeeffectontheNOXemissions.Figure2depictstheNOxemissionofacommercialCFBboilerwithlimestonedesulphurization,operatingwithbedtemperaturebetween1150Kand1200KwithCa/sratioof2.2.Itcanbeseen,whenlimestoneisinjected,whileN2Oemissionwaslittleaffected,theNOremarkablyincreasedby50ppmorabout30%[2].Figure3furtherdepictssomeexperimentalresultsoftheinfluenceCa/SratioontheNOxemission[3].WhenCa/Sratiochangesissmallerthan2,theNOXconcentrationincreaseswiththeCa/Sratio.WhenCa/Sratioishigherthan2,theNOXconcentrationdecreaseswiththeCa/Sratio.
ThoughthemechanismsforNOxformationinhomogeneousreactionsareratherclear,thestudiesonthemechanismsforNOxformationinheterogeneousatmosphere,especiallywiththepresenceoflimestone,arelimited.
Fig.2NOXemissionwith/withoutFig.3NOXconcentrationwithdifferentlimestonedesulphurizationCa/Sratios(Tb=1165K,[O2]=6%)
2.NOxformationmechanismsandchemicalkineticsinaCFBcombustor
Therearemainlythreewell-knownmechanismscountingfortheNOXformationincoalcombustion[4]:
(1)ExtendedZeldovich(orthermal)mechanisminwhichO,OH,andN2speciesareinequilibriumvaluesandNatomsareinsteadystate.
(2)PromptMechanismswhereNOisformedmorerapidlythanpredictedbythethermalmechanismabove,eitherby(i)FenimoreCNandHCNpathways,or(ii)theN2O-intermediateroute,or(iii)asaresultofnon-equilibriumconcentrationsofOandOHradicalsinconjunctionwiththeExtendedZeldovichscheme.(3)FuelNitrogenMechanism,inwhichfuel-boundnitrogenisconvertedtoNO.
Apparently,foraCFBcombustor,neithertheExtendedZeldovichmechanismwhichdependsstronglyontemperatureandonlybecomessignificantwhentemperatureisabove1750K,northePromptmechanismwhichbecomessignificantonlywithabundantCHiradicalandlowO2concentrationisimportant.ThedominatemechanismforNOXformationinCFBcombustorisfuelnitrogenmechanism.
FuelNitrogenmechanismisrathercomplicated.PartofnitrogenincoalisusuallyreleasedintheformofHCNandNH3asvolatilenitrogen,andtherestremainsasfixednitrogenduringthede-volatilizationprocess.Theratioofvolatilenitrogentofixednitrogen,asaresultofde-volatilization,dependsoncoaltype,temperatureandheatingrateofcoalparticles[5].Normally,theoxidizationofvolatilenitrogenoccursinhomogeneousgas-phasereactionsimmediatelyafterde-volatilization,whiletheoxidizationoffixdnitrogeninheterogeneousgas-solidreactionsalongwithfixedcarboncombustion[6,7].
3.LimestoneeffectsonNOformationaCFBcombustor
3.1Catalyticeffectforfuel-Noxidization
Giventheresidencetimeoffluegasinthecombustorisshort,itisimpossibleforeveryreactiontoreachequilibrium.Oncethefluegasexitsthecombustor,itiscooledandtheassociatedreactionsstop.ThereactionrateswithNOtakingpartincanbeexpressedas:
Where,KcisexpressedbyArrhenius’law:
Therefore,smallactivateenergiesandlargecollisionfrequenciesfavorhighreactionrate.OnlyreactionswithhighreactionratesaresignificanttoinfluenceNOXformation.WithCaOpresenceinaCFBcombustor,theimportantreactionsassociatedtoNOxformationarelistedinTab.1[6].
Table1.ImportantNOrelatedreactionsinaCFBcombustor
Reaction
(molcms)
(kg/mol)
N2+O=NO+N
N+O2=NO+O
N+HO=NO+H
NO+HO2=NO2+HO
NO+M=NO+M
NO+O+M=NO2+M
N2+O+M=N2O+M
1.301014
6.40109
4.001013
1.001013
6.401016
9.401014
1.401013
0.0
1.0
0.0
0.0
0.5
0.0
0.0
315.89
26.15
0.00
0.00
0.00
8.075
86.609
ItcanbeseenthatmostofthereactionsforNOXformationwithisfasterthanthoseforNOXdecomposition.Thereforeinthelimitedresidencetime,thepresenceoflimestonefavorsmoreNOX.
3.2PreferentialconversioneffectfromHCNtoNH3
Thegaseousnitrogenmatterexistsinformsofaromaticoraminohydrocarbonsinthebeginningofthede-volatilizationprocess,dependingonthecoaltype,heatingrateandtemperatureetc.MostnitrogeninaromatichydrocarbonsisthenconvertedtoHCNwhilemostnitrogeninaminohydrocarbonsisdecomposedtoNH3.Later,HCNandNH3areoxidizedfollowingdifferentpathways.
InaCFBcombustor,HCNispreferentiallyoxidizedtoN2Oinfollowingpathways[7,8]:
HCN+O→NCO+H(R1)
NCO+NO→N2O+CO(R2)
And,NH3ispreferentiallyoxidizedtoNOXinfollowingthreesteps:
Step1:
NH3→NH2
NH3+OH→NH2+H2O(R3)
NH3+O→NH2+OH(R4)
NH3+O→NH2+OH(R5)
Step2:
NH2→NH
NH2+OH→NH+H2O(R6)
NH2+O→NH+OH(R7)
NH2+H→NH+H2(R8)
Step3:
NH→NO
NH+O2→NO+OH(R9)
NH+O→NO+H(R10)
NH+OH→NO+H2(R11)
Itcanbeseenfromabovechemicalschemes,twomajorformsofnitrogencompoundsexistinaCFBcombustor:
N2OandNOandtheyarepreferent-iallyoxidizedfromHCNandNH3respectively.Whenlimestoneisaddedfordesulphurization,itproducesCaOduringthepyrolysisprocess.ThenCaOcanreactwithHCN,convertingHCNintoNH3.
CaO+2HCN→CaCN2+CO+H2(R12)
CaCN2+3H2O→CaO+CO2+2NH3(R13)
CaCN2+H2O+2H2+CO2→CaO+2NH3+2CO(R14)
Basedonthepreferentialoxidizationschemesof(R3)to(R11),NH3ispronetoformNO,resultinginenhancementofNOxformatio-n.
However,eventhoughN2OisthemainpollutantinaCFBcombustor,itislittleaffectedbylimestoneinjection.Ononehand,N2Oformationisenhancedinwithhomogeneousreaction(R2)andtheheterogeneousreactionsoncarbonsurfacesuchas(R15)and(R16);onetheotherhand,thedisassociationofN2Oisalsoenhancedinreaction(R17)and(R18).
CNO+NO→N2O+CO(R15)
CN+NO→N2O+C(R16)
2N2O→2N2+O2(R17)
N2O+C→N2+CO(R18)
Thereaction(R17)issensitivetotemperature.Whentemperatureisabove1250K,morethan90%ofN2OisdecomposedintoN2andO2.Experimentalstudy[9]showedthatthetemperaturethresholdforinitialingN2Odecompositionisloweredbylimestonetobefrom1100Kto950Kwithlimestone,andmorethan70%oftheN2Oisdecomposedattemperatureof1100K.
3.3ReductioneffectsofCaOonNOxformation
AsshowninFig.3,althoughCaOdecomposedfromCaCO3facilitateNOXformationwhenCa/Sisrathersmall,itcanalsosuppressNOXformationwhenCa/Sislarge.Thepossiblereasonsare:
1.SO2favorstoconvertHCNtoNH3,resultinginmoreNOproductaccordingtothekineticsdiscussedinprevioussection.Fromtheotherview,thedepletionofSO2atlargeCa/SratiosobstaclesHCNconversion,resultinginlessNOproduct.
2.Inthehotcombustor,CaOabsorbsSO2toformCaSO3,whichactsasareductionagentforNOinreaction[9]:
2NO+CaSO3→N2O+CaSO4(R19)
3.CaOandsomeothermaterialsinaCFBcombustorwillpromotetheconversionreactionofNOtoN2[10,11].ThematerialsincludetheAl2O3andMgOinashandFe2O3onthecombustorwall.Itreactionisasfollowing:
4NH3+6NO→5N2+6H2O(R20)
4.NOadheringonthesurfaceofCaOparticlesiseasiertobereducedbyCOorotherreductionagents.
4.Conclusions
Whilelimestonefavorsdesulphurizationinacoal-firedCFBcombustor,butitmightincreasepollutantNOemission.LimestonecanactasacatalysttoenhanceNOxformation,influencingtheassociatedreactionrates.Thechemicalkineticssho
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