外文翻译--麦秸燃烧和二氧化碳气化的加压热重分析反应研究 英文版.pdf

JfPRESSURIZEDTHERMOGRAVIMETRICREACTIVITYSTUDYOFWHEATSTRAWCOMBUSTIONANDC02-GASIFICATION.OleRathmannandJytteB.IllerupDepartmentofCombustionResearchRiseNationalLaboratoryDK-4000Roskilde,DENMARK:Keywords:Wheat-strawbiomass,Pressurized,Gasificationreactivity.Abstract.Biomassfuelreactivityisinterestingsincebiomass,ase.g.wheatstraw,isarelevantfuelforadvancedpressurizedpowerplantsduetotheCO,neutrality.Inthisstudycombustion-andC0,-gasificationreactivitiesofpulverizedwheatstrawcharupto40barwereinvestigatedbyisothermalthermogravimetricanalysis,andtheresultswerecomparedtoatypicalsubbituminouscoal.Arecentlybuiltpressurizedthermogravimetricanalyzerofthehorizontaltype,withope-ratingconditionsatleastupto1200C,45bar,wasused.InthestudytheeffectsoftemperatureandpartialpressureofthereactantsO2andC02,respectively,wereseen,forgasificationalsotheinhibitioneffectofCO,whereasadistincttotalpressureeffectcouldnotbeobserved.Alsotheeffectofusingnon-pulverizedstrawpieceswasinvestigated.INTRODUCTIONTheuseofbiomassfuelsforpowergenerationandheatproductionhasanumberofadvantageswiththeC02neutralityandsavingofdepletablefossilfuelsamongthemostobvious.Inaddition,nationallegislationinDenmarkurgetouselocallyavailablebiomassase.g.woodwasteandstraw.Consideringthefuture,characterizationofbiomassfuelwillthusberelevantinrelationtocombustionandgasificationforhighefficiencypressurizedpowerplanttypes(asIGCC).Asaresult,researchgroupsinScandinaviaandelsewherehavebeenactiveincharacter-izingthebiomassfuelsrelevantintheirrespectivecountries,andmainissueshavebeeninfluenceofpressure,temperatureandgascompositiononthereactionrate.Especiallywood,strawandblackliquorchargasificationreactivitiesatmoderatereactionrateshavebeenstudiedusinglarge-sampleverticalPressurizedThermoGravimetricAnalyzer(PTGA)instruments.Blackadder&Rensfelt(1985)studiedpyrolysisofwood,celluloseandlignineupto800Cat1to40bar.Moilanenetal.(1993)investigatedatmosphericsteamgasificationofcharsofwood,peatandblack-liquorincomparisonwithcoalandbrowncoal,whileWhittyetal.(1993b)investigatedpyrolysisandcharC02-gasificationofcoarselypulverizedwoodandpeatat850Cat1-20bar.StoltzeetaL(1993)studiedatmosphericsteamgasificationoflargesamplesofbarleystrawcharat75OoC-950C.BlackliquorcharwasinvestigatedbyBackmanetal.(1993)andbyWhitty(1993)regardingpressurizedC02-andsteamgasificationinpresenceofCOat650-800Cand1-30bar.Thefollowingoverallpictureisseen:Inpyrolysis,woodcharyieldincreasesclearlywithincreasingpyrolysispressure,whilethereactivitydisplaysaweakdecrease.Woodcharreactivityissimilartothatofpeatandbrowncoalandabout1orderofmagnitudefasterthanforcoal,blackliquordisplaysareactivity2ordersofmagnitudefasterthanwood,andgasificationwithsteamhasareactivityabout4timesfasterthanwithCO,.Thereactvityincreasesstronglywithincreasingtemperatureandasseenforcoal.Increaseintotalpressureatconstantgaspercen-tagecompositionhasaweak,enhancingeffectforwoodwhileforblackliquortheeffectisdirectlyreducing,attributedtoinhibitionbyCOorH,insidethecharpores.InthisworkwehaveinvestigatedcombustionandCO,-gasificationreactionratesofstrawasthisfuelisexpectedtobeoneoftheimportantbiomassfuelsinfutureadvancedDanishpowerheatingstations.TheworkwasperformedundercontractwithandtheDanishutilityassociationsElsaandElkraftwithintheEuropeanUnionAPASprogramme,andwassup-portedbytheDanishMinistryofEnergy.EXPERIMENTALThefuelsampleswerepreparedfromwheatstrawnormallyusedinDanishelectrical-poweddistrict-heatingstations.Entirestrawsweregroundtopulverized,averagestmwsampleswithaparticlediametersmallerthan200fim.Charofpulverizedstraw,strawinter-nodepieces,strawnodesandgrainswereproducedinafurnacebya7minutes900Cpyrolysisheattreatmentatatmosphericpressurewithoutoxygenaccess.Therawfuelcompositionsgivenintable1indicaterathersimilarproximatecompositionswhileashcompositionsdiffer,especiallywithrespecttoK20andP,O,.704ThePTGA-instrumentusedforthestudy(fig.1)isamodifiedDuPontthermogravimetricanalyzerofthehorizontaltype,recentlybuiltatRISQ.ThefuelsampleisplacedonasmallIcmdiameterPlatinumtrayhangingonahorizontalbalancearm,situatedinareactiontubetogetherwiththermocouples.Thereactiontubeisheatedfromoutsidebyaheaterelement,andtheentirethermobalancearrangementisplacedinsidea35Ipressurevessel.Pressureandflowcontrolareperformedbyanumberofvalvesandgasmassflowcontrollers.Thetestgas,selectablefromeitheraninertgas(N,)sourceorapremixedreactivegassource,issuppliedtotherearendofthereactiontubewithafillerelementtoheatthetestgastooperatingtemperature.Atestgasratecorrespondingtoalinearvelocityinthereactiontubeofabout1cm/sat1000Cisusedgivingaswitchingtimeforthegascompositionatthesamplepositionofabout100s.ThebalancesytemistheoriginalDuPontcomponent,whilereactiontube,ovenandallexternalsystemsarenew.ThePTGAcanoperateupto45barand1200Csimultaneously.Samplesuptoabout150mgcanbeinvestigated,butforlowdensityfuelsthepracticallimitissetbythefuelvolumeandmaybe5toIOmg.Dataloggingisperformedonceper2seconds.Pyrolysisexperimcntswereperformedinaninerttestgaswitharampedtemperature.Thecombustionandgasificationreactivityexperimentswereperformedisothermallywiththeoperatingtemperatureestablishedininerttestgasafterwhichthereactivetestgas(N2/02orN,/C02/C0mixture)wasselected.Themaximumconversionratethatcouldbemeasuredwassetbythetestgasswitchingtimeandthemaximumdiffusionrateofreactanttothesampleandwithinthesample.Theexperimentalweightsignalswerecompensatedforbouyancyandbalancearmlengthexpansion,basedoncalibrationtests.ForthereactivityexperimentsatransformationthenfollowedofthecompensatedweighttoaconversiondegreeXandareactivityR(=reactionrateperremainingchar)x=(mco-mc)hR=-dmJdt/m,=dX/dt/(1-X)(1)(2)wherem,andmCoaretheinstantaneousandinitialcharmassondryandashfreebasis,respec-tively.Eachreactivityexperimentwascharacterizedbythereactivityat50%conversion,b,andthevariationofRwithconversionXwas,asanapproximation,describedbyasinglenormalizedreactivityprofilef(X)(normalizedto1.0atX=0.5)foreachofthereactiontypes,i.e.R(X)*R,sf).Ancstimateofthereactiontimetoacertainconversiondegreecouldthenbecalculatedasrxtx=If(X)/(I-X)dX(3)JoRESULTSAminornumberofpyrolysisexperimentswereperformedonraw,pulverizedstrawsamplestoseetheinfluenceofheatingrateandpressureinthetemperaturerange150-1000C.At20bar,heatingratesof10,30or5OWminhadnoeffectoncharyield.Withaheatingrateof30C/minthecharyieldseemedtoincreaseweaklywithpressure:15%,20%and22%charyield(2.5%uncertainty)for1.5,20barand40bar,andthemaximumpyrolysisrateoccurredinanarrowrangefrom342Cto355C.Moreaccurateexperimentalresultsarenecessarytoquantifythepressureeffectonthecharyield.Astheseresultsshowedlittleeffectofpyrolysispressureitwasassumedthattheatmosphericpyrolysispressurewasofnoimportancefortheresultingchar.Combustionreactivitywasmeasuredattemperaturesfrom300Cto410C,pressuresfrom1.5.barto40barand0,pressuresfrom0.08barto0.8bar.Theresultsfor&,5areshowninfig.2.Thereactivityincreasedwithincreasingconversiondegree,withvaluesatX=0.2andX=0.8about0.6and1.7timesthevalueatX=0.5,respectively.Whileresultsatsuchlowtemperaturesarenotdirectlyapplicabletopracticalconditionstheypermitcomparisontocorrespondinreferencecoaldata.At400Cand0.26bar02,thereactivitywasfoundtobeabout0.002s-,about30timeshigherthanthereactivityforthereferencecoal.ThedependenceontemperatureandO2partialpressurecorrespondstoanactivationenergyof110kJ/moleandareactionorderof0.61.Theactivationenergyissimilartothatofthereferencecoalbutthereactionorderisalittlelower.Nosignificanteffectofthetotalpressurewasobserved.P705LIco2gasificationreactivitywasmeasuredinthefluidized-bedrelevanttemperaturerange850C-lO5O0Cat20barandwith3differentgascompositions,includingonewiththeinhibitor-componentCO,asshowninfig.3.Afewoftheseexperimentswererepeatedat4and40bartotalpressurewithunchangedC02/C0partialpressures.At1000Cthereactivityb,swasfoundtobeabout0.006sin0.7barCo2andnoCO.Thereactivityincreasedwithconversiondegree,withValuesatX=O.2andX4.8about0.6and2timesthevalueatX4.5,respectively.ThepresentStrawreactivityisabout16timesfasterthanforthereferencecoaland1.5-2timesfasterthanforthewoodreactivityasmeasuredbyWhittyetal.(1993a).Theresultsforb.5couldbeinterpretedintermsofLangmuir-Hinshelwoodkinetics(seee.g.Laurendeau(1978):%.s=kPco,/(l+aPco+bPcoz),wherek,a,andbhaveArrheniustemperaturedependence,foraandb,however,withnegativeactivationenergies.Inspiteofsomeunsystematicvariationintheexperimentaldata,saturation(describedbythebparameter)wasclearlyobservedasthereactivityincreasedlessthanproportionallytotheincreaseinC02pressurefrom0.7to2.2barinabsenceofCO.Also,thecleardecreaseinreactivityfrom1.5to5times,whenchangingfroma2/0bartoa2/2barCO,/COtestgas,indicatesclearlyinhibition(describedbytheaparameter)atthetesttemperatures.Boththesaturationandinhibitioneffectsshowedanexpectedreductionwithincreasingtemperature.Thepresentresultsaretooscarcetopermitanunambiguousdetermina-tionofkineticparameters,butasshowninfig.3,aLangmuir-Hinshelwoodrepresentationcouldbeobtainedusinga-andb-parametersfromthereferencecoalincombinationwithafittedkl-parameter.However,thevalueofthisk,-parameterindependenceoftemperatureisstronglydependentontheappliedvaluesofthea-andb-parameters.Themeasurementssuggestamoderatereactivityreductionwithincreasingtotalpressure,maybeupto2timeswhenthepressureisincreasedfrom4to40baratconstantC02andCOpartialpressures.Incontrasttoblackliquorreactivity,thepresentdatasuggestanincreasingreactivitywhenincreasingthepressureatconstantgaspercentagecomposition.Thereactivityofcharofnon-pulverizedstrawparticleswasmeasuredat900-1000Cwith20barpressureand2/2barC02/C0partialpressure.Relativetothecharofaverage,pulverizedstrawtheinter-nodeparticlesshowedareactivitymorethan2.5timesfaster,node-particlesshowedareactivity2timesslowerat900Cwhileat950Cand1000Citwasmorethan3timesfasterthanforpulverizedstraw.Grainsshowedareactivityabout2.5timesslowerthanpulverizedstraw.Thesurprisingfasterreactionsofinter-nodeandnodeparticlesmaybeduetothenon-packedstructureofthesample,butforthenodesthisbehaviormayalsobeduetothehigherconcentrationofthecatalystK20intheash(seetable1).DISCUSSIONANDCONCLUSIONThepresentPTGAinstrumenthasprovedtobeavaluableinstrumentforbiomassfuelreactivityanalysisatmoderatereactionrates,inthepresentstudyofwheatstraw.Pyrolysisofpulverizedstrawsuggestedasmallincreaseincharyieldwithincreasingpressure.Pressurizedcombustionandgasificationreactivityofstrawcharsamples,pyrolyzedatlowheatingrate,wasinvestigatedupto40barbutwiththemainpartoftheinvestigationsat20bar.Thecombustionreactivityatlowtemperature(around350C)with0.08to0.8bar02wassomewhatmorethanoneorderofmagnitudehigherthanforareferencecoalandwiththesamerelativetemperaturedependence.Thedependenceon0,partialpressurewasweaker,andnosignificantdependenceontotalpressure(atconstantpartialpressure)couldbeobserved.COiGasificationreactivitywasinvestigatedaround950Catco,andcopressuresaround1bar,Theresults.displayingbothCO,-saturationandCo-inhibition,couldbeunderstoodi,ofh&muh-Hhshelwoodkinetics,butmoreaccuratemeasurementsarenecessarytoactuallydeterminethekineticParameters.InabsolutemagnitudethemawcharreactivitywasfoundtobeanorderofmagnitudehigherthanfortheWferenCecoalandalsoalittlehigherthanforwoodbiomass.Charofnon-pulverizedsuaw-particles,includinggrains,displayedreactionratesmoderatelyscatteringabove(3times)andkhw(2times)thatOfthepulverizedsamples,presumablyasaofsizeeffect,compactnessandcompositionofthenon-pulverizedparticles.nefuelreactivityresultsdisplayedamxkratedecreasewithtotalpressure,butmoreaccurateareneededtotellwhetherthisisasignificanteffectAlso,experimentswithcharspyrolyzedathighheatingratesatvariouspressureswouldberelevanttodistinguishbetweentheeffectsoftotalpressurethroughPyrolysisandthroughchargasification,706IREFERENCESBac!unan,R.,Frederick,W.J.andHupa,M.,BasicStudiesonBlack-LiquorPyrolysisandCharGasification.BioresourceTechnology46(1993)pp153-158.Blackadder,W.,Rensfelt,E.(I985).Apressurizedthermobalanceforpyroly