王建昕+清华+缸内直喷发动机氧化模型预测

ProceedingsCombustionInstitute

/locate/proci

ofthe

ProceedingsoftheCombustionInstitute33(2011)

3151–3158

Adetailedoxidationmechanismforthepredictionofformaldehydeemissionfrommethanol-gasoline

SIengines

FanZhang,ShijinShuai ,ZhiWang,XiaZhang,JianxinWang

StateKeyLaboratoryofAutomotiveSafetyandEnergy,TsinghuaUniversity,Beijing100084,China

Abstract

Iflow-contentmethanol-gasolineblendedfuelsareutilizedincurrentPFIgasolineengines,formalde-hydeemissionneedstobeintensivelyevaluated.Inthisstudy,adetailedcomprehensivemethanoloxida-tionmechanismwasdeveloped,basedonpresentreactionrateconstantandpathinformation.Thein uenceofCH,CH2(S),andCH2(T)radicalspeciesandnitricoxidewasconsideredinthemechanism.Shock-tubeand ow-reactordatawereusedtovalidatethemechanism.Numericalsimulationsofallsys-temswereconductedbyCHEMKIN-basedprograms.Inordertoconstructamethanol-gasolinemecha-nism,anoxidationmechanismofgasolinesurrogatewascombinedwiththemethanolmechanism.Thegasolinesurrogatemechanismwasformedwithiso-octane(iso-para nrepresentative),toluene(aromaticrepresentative),and1-hexene(ole nrepresentative).Themethanol-gasolinemechanismwasvalidatedbythejet-stirredreactor(JSR)experimentdata.Thesimulationresultsoftheproposedmechanismhaveagenerallygoodagreementwiththeexperimentaldata.Sequentially,theBoostenginecyclemodelwasestablishedandcoupledwiththemethanol-gasolinemechanismtosimulatetheformaldehydeemissionsofthelow-percentmethanol-gasolineblendedfuelsfromaSIengine,andalsoappliedtopredicttheemis-sionsofthehigh-percentblendedfuels.TheexperimentaldatafromtheSIenginewereobtainedbytheFTIR(Fouriertransforminfrared)spectrometer.ThesimulationresultsofSIenginesachieveagoodcon-sistencywiththeexperimentalresults.

Ó2010TheCombustionInstitute.PublishedbyElsevierInc.Allrightsreserved.

Keywords:Methanol;Gasoline;Oxidationmechanism;Emissions;Formaldehyde

1.Introduction

Asaliquidfuel,methanolcanbeproducedfromagreatnumberofdi erentrawandrenew-ablematerialresources.Theutilizationofcoal-generatedmethanolasapracticalalternativefuelisoneofthemostrealisticoptionsforChina,due

Correspondingauthor.Fax:+861062772515.

E-mail(S.

Shuai).

address:sjshuai@

tothe“oil-lean,gas-lacking,andcoal-rich”struc-tureofChineseenergyresources[1].However,formaldehydeemissionfrommethanolenginesisharmfultotheenvironmentaswellastohumanhealth.Thus,iflow-contentmethanol-gasolineblendedfuelsareutilizedincurrentPFI(PortFuelInjection)gasolineengines,formaldehydeemissionneedstobeintensivelyevaluated.

Thenumericalsimulationmethodcouldbedevelopedasausefulengineeringtooltoinvesti-gateunburnedmethanolandformaldehydeemissionsfrommethanolengines.Thus,detailed

1540-7489/$-seefrontmatterÓ2010TheCombustionInstitute.PublishedbyElsevierInc.Allrightsreserved.doi:10.1016/j.proci.2010.07.029

3152F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–3158

chemicalkineticmechanismsformethanoloxida-tionshouldberesearched.Thereareabout10kindsofmethanoloxidationmechanismsshowninthepastliterature.Bowman[2]developedthe rstdetailedmechanismsbasedonshock-tubeexperiments,despitefacingdi cultiesinpredict-ingmethanolignitiondelayperiodinlowtemper-ature(<1800K)causedbyalackofelementaryratedata.Subsequently,WestbrookandDryer[3]developedthe rstcomprehensivekineticmodelofmethanoloxidation,whichhadasuc-cessfulveri cationwith ow-reactorandshock-tubedatainawidetemperaturerange.TheshortageisthatsomeelementaryrateconstantandreactionpathinformationwerelackedandCH3Owasneglectedinthemechanism.NortonandDryer[4]undatedtheWestbrookandDryermechanismusingmorecurrentrateconstantsandthermochemicalparameters,andidenti edtheimportanceoftheHO2radicalinthemethanolcombustionprocess.Anewsetof ow-reactordatawasusedtoverifythemechanism.Themeth-anoloxidationmechanism,madebyEgolfopoulosetal.[5],hadanexcellentagreementwiththeexperimentallaminarspeedand ow-reactordataoverarangeofinitialtemperaturesandpressures.However,therewasabadpredictionofmethanolignitiondelayperiodcomparedwithBowman’sshock-tubemeasurements.Grotheeretal.[6–8]subsequentlydevelopedacomprehensivemecha-nism,whichcouldpredictbothlaminarburningvelocitiesandauto-ignitioninspark-ignition(SI)engines.Accordingtosensitiveanalysis,itcon-cludedthatsomereactionshadagreatimpactonlaminarburningvelocities,suchasthebranch-ingratiobetweenreactionsCHandCH3OH+OH=CH2OH+H2Oradical3OH+OH=CHHspeciessuchas3O+2O.AlthoughCH,CH2(S),andCH2(T)wereomittedintheHeldandDryer[9]mechanism,ithasagoodagreementwithshock-tube, ow-reactor,andpremixedlam-inar amesexperimentaldataoverawideapplica-blerange.Lindstedtetal.[10,11]providedamethanoloxidationmechanism,containingCH,CH2(S),andCH2(T)radicalspecies.Itcanbeusedtocalculatethelaminar ameburningspeed,ignitiondelayperiod,andvariousemissions.BasedontheHeldandDryermechanism,LiandWilliams[12,13]constructedaclassi edmeth-anoldetailedmechanism,usingCO/H2O/H2/O2,CH2O,andCH3OHreactionsandnewreactionrateconstantandthermodynamicdata.Christianetal.[14]hadstudiedtheoxidationofmethanolina ow-reactorexperimentallyunderdiluted,fuel-leanconditionsat650–1350K,overawiderangeofOout2concentrations(1–16%),andwithandwith-thepresenceofnitricoxide.Atpresenttheresearchontheoxidationmechanismofmetha-nol-gasolineblendedfuelisrare.Casimiretal.[15]developedadetailedchemicalkineticreactionmechanismresultingfromthemergingofvali-

datedkineticschemesfortheoxidationofthecomponentsofthepresentM85(methanol-gaso-lineblendedfuelcontaining85%methanol)surro-gate.GoodagreementbetweentheexperimentalresultsandthecomputationswasobservedunderthepresentJSRconditions.

Inthisstudy,adetailedcomprehensivemetha-noloxidationmechanismwasdeveloped,basedonpresentreactionrateconstantandpathinforma-tion.Thein uenceofCH,CHicalspeciesandnitricoxidewas2(S),andCHconsidered2(T)rad-inthemechanism.Shock-tubeand ow-reactordatawereusedtovalidatethemechanism.Inordertoconstructamethanol-gasolinemechanism,anoxi-dationmechanismofgasolinesurrogatewascom-binedwiththemethanolmechanism.Thegasolinesurrogatemechanismwasformedwithiso-octane(iso-para nrepresentative),toluene(aromaticrep-resentative),and1-hexene(ole nrepresentative).Themethanol-gasolinemechanismwasvalidatedbythejet-stirredreactor(JSR)experimentdata.Sequentially,theBoostenginecyclemodelwasestablishedandcoupledwiththemethanol-gasolinemechanismtosimulatetheformaldehydeandnitricoxideemissionsofthelow-percentmethanol-gaso-lineblendedfuelsfromaSI(Sparkignition)engine,andalsoappliedtopredicttheemissionsofthehigh-percentblendedfuels.TheexperimentaldatafromtheSIenginewereobtainedbytheFTIR(Fouriertransforminfrared)spectrometer.Numericalsimu-lationsofallsystemswereconductedbyCHEM-KIN-basedprograms.2.Methanolmechanism

Themethanolmechanismisbasedonthatpro-posedpreviouslyfortheoxidationofmethanol.Theproposedkineticreactionmechanismhas46speciesand247reversiblereactions.ThebasicC/H/Oreactionrateconstantsandpathinforma-tionoftheproposedmechanismarebasedontheHeldandDryer[9]mechanism.Theclassi edCO/H2O/H2/O2,CH2O,andCHmechanism3OHreactionsfromtheLiandWilliams[12,13]areadded.ThereactionsinvolvingCH,CH(T)radicalspeciesarebasedontheLindstedt2(S),andCHetal.2[10,11]mechanism.ThereactionsrelatedwithnitricoxidefromtheChristianetal.[14]mecha-nismarealsoaddedintotheproposedmechanism.Thefullmechanism,includingthermochemicaldata,isavailableasSupplementalmaterial.

Predictionsutilizingthepresentmethanoloxi-dationmechanismhavebeencomparedwiththeexperimentaldata.Forhightemperaturecombus-tion,shock-tubeexperimentsareoperatedatthehighertemperaturerangeof1500–2500K.Formethanoloxidationconditions,theexperimentaldataofBowman[2]weremodeledbythepresentmethanolmechanism.Theshock-tubecon gura-tionwasnumericallysimulatedasanadiabatic,

F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–3158

Table1

InitialconditionsofCH3OH/O2/Armixturesinashock-tube.

CH3OH(%)

MixtureMixtureMixtureMixture

1234

2.0010.751

O2(%)4.0021.501

Ar(%)94.0097.0097.7598.00

Pressure(atm)1.453.254.153.1

3153

Table2

InitialconditionsofCH3OH/O2/N2mixturesin ow-reactors.T(K)10001030949

P(atm)112.5

U1.61.220.83

CH3OH0.007350.009430.00333

O20.0068910.0115940.006018

N20.9857590.9789760.990652

constantvolumesystem.Theinterestparameterofshock-tubedataisignitiondelaytimes.Itisde nedasthetimeintervalwhichtheproductofCO-andO-atomconcentrationsreachesthemax-imum.Table1showstheInitialconditionsofCH3OH/O2/Armixturesina3.8cmstainlesssteelshock-tubeperformedbyBowman[2].Allmen-tionedmixturesweremodeledandrepresentativeresultsformixturesofCH3OH/O2/ArconditionsareshowninFig.1.Itindicatesthereisanexcel-lentagreementbetweencomputedandexperimen-talvaluesofignitiondelaytimes.

Flow-reactorexperimentsessentiallybridgethegapbetweenstatic-reactorandshock-tubeexperi-mentsfortheunderstandinganddevelopmentofkineticschemes.Reactor-typeexperimentsprovideinformationforthelowandintermediatetempera-tureregimestypicallyintherangeof800–1200K.Adiabaticsolutionswereadoptedforthenumericalmodelingofthe ow-reactordata.Inthepresent

investigationthedataofAronowitzetal.[16],NortonandDryer[4],andHeldandDryer[17]foroxidationconditionsweresimulated.Theexperimentalconditionsof parisonsinFigs.2–4demonstratethatthecalculatedspeciesandtem-peraturepro lesareingenerallygoodagreementwiththespeciestimehistorymeasurementsin ow-reactors[16,4,17].Forallcases,thepredictedpeakvaluesandlocationsofCOareclosetotheexperimentaldata.CH2O,thekeyspeciesformeth-anoloxidation,alsohasagoodagreementbetweencomputedandexperimentalconcentrationpro les.However,thecomputedlevelofH2istoohigh.3.Methanol-gasolinemechanism

Themethanol-gasolineoxidationmechanismisanoxidationmechanismofgasolinesurrogatecombinedwiththemethanolmechanism.Thegasolinesurrogatemechanismwasformedwithiso-octane(IC8H18,iso-alkanerepresentative),

3154F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–3158

toluene(C7H8,aromaticrepresentative),and1-hexene(IC6H12,alkenerepresentative).Theiso-octaneand1-hexenemechanismsarebasedonCurranetal.[18]mechanism.Thetoluenemecha-nismisobtainedfromGustavssonandGolovit-chev[19]mechanism.Theproposedmethanol-gasolinemechanismhas113speciesand669reversiblereactions.Thebasechemistryofthemethanol-gasolinemechanismisthesamewiththatofthemethanolmechanism.Thus,thecom-binationwiththesurrogatedfuelmechanismwillnota ectthepredictabilitywiththemethanolmechanism.Thefullmechanism,includingther-mochemicaldata,isavailableasSupplementalmaterial.

Inthedetailedmechanism,CH3OH rstlyreactswithsmallmolecular(H,OH,HO2andO2)bythedehydrogenationorthermalpyrolysisreac-tion.CH2OHandCH3Oaregeneratedbythedehy-drogenation.Themainformationofmethanol

F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–3158

Table3

InitialconditionsoftheM85surrogatemixturesinajet-stirredreactor.U12

CH3OH0.0038050.003805

IC8H180.0000980.000098

C7H80.0000680.000068

IC6H120.0000290.000029

O20.0078080.003904

N20.988260.992164

3155

P(atm)1010

thermalcrackingisCH2(S).Afterthefurtheroxi-dationofCH2OHandCH3O,theimportantmid-dleproductCH2Oisgenerated.HCOisgeneratedbythefurtherdehydrogenation.HCOistrans-formedintoCObythedehydrogenation.Eventu-allyCOisoxidizedintoCO2.TheoxidationreactionapproachofIC8H18andairhasastrongselectivityvariedwiththetemperature.Itcanbedividedintolowtemperature,intermediatetemper-atureandhightemperaturereactionphases.Inthelowtemperaturestage(600K<T<900K),aftertwostageofoxygenationIC8H18isdecomposedintoaldehydeandOH.Inintermediatestage(900K<T<1050K),thehydroxylC8H17istrans-formedintoole nsandH2O2.Inthehightempera-turestage(T>1050K),OHisgeneratedinalargenumber.FuelmoleculesarequicklytransformedbyOH.ThegeneratedCOreactswithOHunderhightemperatureandCO2isgenerated.C7H8 rst

reactswithO2,OH,O,andHtogenerateC7H7bythedehydrogenation.C7H7istransformedintoC6H5afterthefurtherdehydrogenation.ThedecompositionofC6H5formsC2H2andC4H3low-carbonhydrocarbons.Aftertheoxidationreaction,COandCO2aregenerated.C6H11is rstlygeneratedbythereactionbetweenIC6H12andOH,CH3.AfterthefurtherpyrolysisreactionC3H7andC3H5low-carbonhydrocarbonsaregen-erated.CH3CHOandCH2Oareformedbytheoxi-dationreactions.FinallyCO2isgeneratedaftertheoxidationreaction.

Inthestudy,thedetailedmethanol-gasolineoxidationmechanismisusedtosimulatetheoxida-tionofM85surrogatemixturesinajet-stirredreac-tor(JSR).InitialconditionsoftheM85surrogatemixturesaregiveninTable3.TheoxidationofthesemixturesisperformedinaJSRata xedres-idencetimeof0.7sandat10atm[15].Figures5

3156F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–3158

and6showthecomparisonbetweenexperimentalandmodelingresultsatU=1and2,respectively.Itindicatesthatthesimulationresultsofthepro-posedmechanismhaveagenerallygoodagreementwiththeexperimentaldata.However,thedi erencebetweensimulatedandexperimentaldatabecomeslargerinthehightemperatureregion.

4.SIenginesimulation

Inthisstudy,theBoostenginecyclesimulationsoftwareiscoupledwiththeCHEMKINprogramtosimulateSIengines.Boostisutilizedtosimu-latetheheatreleaseprocessofenginecombustion.AteachcalculationtimestepBoostprovidesthecorrespondingtemperatureTandpressureP,whicharede nedastheinputconditionsofCHEMKINtocalculatethevariationofeachspe-cies.Inthisway,thesimulationcanprovideamoreaccuratedescriptionoftheheatreleasepro-cessofSIengines.Thecoupledchemicalreactionkineticsisalsoabletocalculatethereactionvari-ationofeachspecies.

ingtheFouriertransformmethod,theabsorptionspectrum(intensity/wavelength)iscalculatedfromthedetectedinterferogram(inten-sity/time).Theindividualexhaustgascomponents

Table4

EQ491ienginesimulationparameters.Ignitionadvanceangle(°CA)26Enginespeed(r/min)2400EngineTorque(Nm)120Bore(mm)90.82Stroke(mm)

76.95Compressionratio

8.9Connectingrodlength(mm)

127

Table5

BoundaryandinitialconditionsofSIenginesimulation.Intakepressure0.1MPaIntaketemperature30°CExhaustpressure0.1MPaCylinderpressureat0.5MPaexhaustvalveopen

Cylindertemperatureat730°C

exhaustvalveopen

aredeterminedfromthespectrumbyusingrefer-encespectra(fromtheFTIRevaluationmethodpackages)andspeciallydevelopedmathematicalfunctionstominimizecrossinterference.ForHCHOcalibration,avaporizerwasembeddedintheFTIRwithaddingasolutionofformalde-hyde/H2O(0.01mol/L)tocalibratetheformalde-hydeemission.Forformaldehydemeasurementrangeof0–1000ppm,themeasurementaccuracyis3ppm.Thesamplingturnoverfrequencyis2Hz.Thevolumeofthesamplecellis200mL.Theopticalpathlengthis2m.Thewavenumberresolutionis0.5cmÀ1.Thesamplelinesandgascellareheatedto190°C(gascell185°C)topre-ventthecondensationandpolymerization.

Figure7givesthecalculatedcylinderpressurecurveofM30fuelusingthecombustionmodel,comparedwiththetestdata.Seenfromthe gure,thecylinderpressurecurveofthesimulatedresultsandexperimentaldataarebasicallythesame.

Figure8givesthecomputedformaldehydeemissionsofdi erentproportionalmethanol-gasolineblendedfuels.Thedual-zonecombustionmodelisadoptedinthecombustionphaseshown

F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–31583157

inFig.8.Themolefractioninthisphaseisthecombinationofburnedareaandunburnedarearesults.SeenfromFig.8,formaldehydeisgradu-allygeneratedafterthestartofthecombustion.After0°CAthereisacertainamountofformal-dehydeconsumptionduetothetemperaturerise.ThemodelsetsthattheintakeairstartsatÀ368°CA.Atthistimethemolefractionofform-aldehydereducesduetothechargeoffreshair.AstheendoftheintakeprocessatÀ120°CA,thelowtemperatureoxidationofmethanolreactiontakesplaceinthecylinder.Thus,someofformaldehydebeginstogenerate.Meanwhile,theamountofgen-eratedformaldehydealsoincreaseswiththemetha-nolproportioninmethanol-gasolineblendedfuelsincreasing.Afterthestartofthemixturecombus-tionthecylinderformaldehydestartstoquicklygenerateandthendecreasesbytherapidoxidation.Figure9givesthecomparisonsbetweensimulatedandexperimentalformaldehydeemissionsfromdi erent-contentmethanol-gasolineblendedfuels.AsshowninFig.9,formaldehydeemissionshaveanearlylineargrowthwiththeincreaseofthemeth-anolproportion.TheexperimentaldatafromtheSIenginewereobtainedbytheFTIRspectrometer.Themaximumerrorbetweensimulatedandexper-imentalvaluesofpuregasoline,M10,M20,andM30islessthan50%.Thesimulationresultsachieveagoodconsistencywiththeexperimentalresults.

Theexperimentsusinghighpercentageofmethanol-gasolineblendedfuelsarerestrictedbythefactoroffuelinjectionpulsewidth.Thus,usingtheestablishedenginemodelcanprovideapredic-tionontheformaldehydeemissionsofhigh-contentmethanol-gasoline,asshowninFig.10.Itindicatesthatthehigh-contentmethanol-gasolineblendedfuelgeneratesalargeamountofformalde-hydeemission,about6–8timeshigherthangasoline.

5.Conclusions

Inthisstudy,adetailedcomprehensivemetha-noloxidationmechanismwasdeveloped,basedonpresentreactionrateconstantandpathinfor-mation.Thein uenceofCH,CHnitricoxide2(S),andCH2(T)radicalspeciesandwascon-sideredinthemechanism.Shock-tubeand ow-reactordatawereusedtovalidatethemechanism.Numericalsimulationsofallsystemswerecon-ductedbyCHEMKIN-basedprograms.Inordertoconstructamethanol-gasolinemechanism,anoxidationmechanismofgasolinesurrogatewascombinedwiththemethanolmechanism.Thegas-olinesurrogatemechanismwasformedwithiso-octane,toluene,and1-hexene.Themethanol-gaso-linemechanismwasvalidatedbythejet-stirredreactorexperimentdata.Thesimulationresultsoftheproposedmechanismhaveagenerallygoodagreementwiththeexperimentaldata.Sequen-tially,theBoostenginecyclemodelwasestablishedandcoupledwiththemethanol-gasolinemecha-nismtosimulatetheformaldehydeemissionsofthelow-percentmethanol-gasolineblendedfuelsfromaSIengine,andalsoappliedtopredicttheemissionsofthehigh-percentblendedfuels.ThesimulationresultsofSIenginesachieveagoodcon-sistencywiththeexperimentalresults.Acknowledgments

Thisworkwas nanciallysupportedbytheNationalHighTechnologyResearchandDevel-opmentProgramofChina(“863”Program)“AdaptabilityResearchonMethanolVehicle”,underGrant2006AA11A1A4.AppendixA.Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,intheonlineversion,atdoi:10.1016/j.proci.2010.07.029.

3158F.Zhangetal./ProceedingsoftheCombustionInstitute33(2011)3151–3158

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