Handbook of Combustion, Volume 5 (New technologies) PDF
Preview Handbook of Combustion, Volume 5 (New technologies)
j 1 1 HCCI Combustion Chemistry, Reduced Kinetic Mechanisms and Controlling Strategies HatimMachrafi 1.1 Introduction 1.1.1 PresentSituation In our daily life combustion engines are present everywhere. They are used for purposes such as driving trucks, cars, and busses. This results in large-scale con- sumptionoflimitedfossil-fuelreservesandtheproductionofexhaustgasesthatharm boththeenvironmentandourhealth.Incontrast,italsoprovidesadvantages,suchas mobility,thatarelinkedtothewidespreadavailabilityofpassengercars.Regardlessof onesperspective,itseemsreasonabletoexpectthatthenumberofvehicles(witha combustionengine)willriseinthefuture,especiallygivenfactorssuchastherapid economicdevelopmentinseveraldenselypopulatedareas.Furthermore,trendsfor passengercarsindicatethatvehicleweightandsizearelikelytoincrease,togetherwith thepowerrequirementsofthegrowingnumberofelectronicdevicesonboard.Allof these factors will increase fuel consumption. Furthermore, legislation restricts the amount of pollutants that are emitted by gasoline and diesel engines (Figure 1.1) (http://www.itsnottingham.info/Commuter4_Pres3.htm). Consequently, world-wide fuel consumption and exhaust emissions can only realisticallybereducedifanalternativefortheIC(internalcombustion)engineis developedwithcharacteristicssignificantlybetterthanthoseofpresentengines. 1.1.2 HCCIEngines,ANewAlternative Ideally,anyalternativetoICenginesshouldnotbedependentonfossilfuels,emitno harmfulproducts,andhaveabetterefficiency.Thewordsfuelcellarepromising. Unfortunately,large-scaleintroductionofthispropulsiontechnologyisexpectedto takemanyyears,mainlyduetohighcosts,problemswithon-boardhydrogenstorage, andthelackofhydrogeninfrastructure.Furthermore,thewell-to-wheelefficiency offuelcellpowertrainswithallthenecessaryauxiliaryequipmentisestimatedtobe HandbookofCombustionVol.5:NewTechnologies EditedbyMaximilianLackner,FranzWinter,andAvinashK.Agarwal Copyright(cid:1)2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim ISBN:978-3-527-32449-1 j 2 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies Figure1.1 Europeanon-highwayemissionlimits–D0(1990)toD5(2010). similar, or even lower, than that of optimized diesel engines [1, 2]. Production of hydrogeninthevehiclewouldsolvesomeoftheaboveproblems,butaslongasfossil fuelreformationisapplied,thiswouldnotbeasustainablesolution.Trulycleanfuel celltechnologywouldbebasedonarenewablesource,suchaswindorsolarenergy. However,withpresenttechnologicalstandardsintherenewableenergyfield,itisfar fromrealistictobelievethatasubstantialportionofthefossilfuelusedforvehicle propulsioncouldbereplacedbysustainablealternativesinthenearfuture.There- fore,theuseofICengineswillmostlikelycontinueforawhileand,aslongasno outstanding alternative is available, research focusing on improving the internal combustionengineconceptisjustified.GiventhatICengineshaveexistedforover acentury,whatimprovementscanbeachieved?Animpressivedevelopmentofthe ICenginehastakenplace:fuelconsumptionandemissionshavebeenconsiderably reduced,whileincreasingthepoweroutput.Thecombustionprocessesindieseland sparkignitionenginesarediscussedinVol.3Ch.14andVol.3Ch.15[3,4].Other systemshavebeenintroducedrecently:flexiblevalveoperation;cylinderde-activation cycle-to-cycle, model-based engine control systems; and direct-injection of fuel. Withoutquantifyingthepossiblefutureimprovements,itisconcludedthatdevel- opmentofICengineswillprobablycontinueduringtheyearstocome.Logically,one major possibility could now be to improve the conventional Otto and/or diesel engines. Another major possibility that seems to hold much promise is homoge- neouschargecombustionignition(HCCI). 1.2 HCCICombustion 1.2.1 Definition HCCIcanbedefinedasapremixed,leanburncombustionprocess,precededbya homogeneousair/fuelport-injection.Figure1.2showsthiscombustionmode. j 1.2 HCCICombustion 3 Figure1.2 SchemeoftheHCCIcombustionmode[5]. The HCCI engine generally runs on a lean, diluted mixture of fuel, air, and combustion products, which is not ignited by a spark but by compression auto- ignition instead. To speed up the kinetics, the temperature of the charge at the beginningofthecompressionstrokecanbeincreased.Thiscanbedonebyheating theintakeairorbykeepingpartofthewarmcombustionproductsinthecylinder (integralgasrecirculation).Bothstrategiesresultinahighergastemperatureafter compression, which in turn speeds up the chemical reactions that occur in the (nearly) homogeneous mixture in the following cycle. In contrast with the spark ignition(SI)engine,whereaspark-plugisusedtogenerateapropagatingflame,and thecompressionignition(CI)engine,wheretheinjectioncausesadiffusionflame, ignitionintheHCCIenginewilloccurwhenthetemperatureandpressurearehigh enough for fuel oxidation chemistry to become very rapid. Figure 1.3 depicts the differencesbetweentheHCCIengineandtheSI/CIengines. Figure1.3 DifferencebetweentheHCCIengineanddiesel/sparkignitionengines[5]. j 4 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies Figure1.4 DifferenceinpressurerisebetweentheHCCIengineanddieselengine[5]. Auto-ignitioninHCCIenginesissaidtooccursimultaneouslyatmanylocations, calledhotspots,throughoutthecylinder.Thiscausesthepressurerisetotakeplace inashortertimespanthaninadieselengine,whilstthetemperatureremainslower. Figure1.4showsthispressurerise. Furthermore, it is claimed that turbulence is of minor importance for the combustionprocessinanHCCIengine,thoughmoreresearchisneededinthis domain,andthatflamepropagationoccursinalesssignificantway.Itisgenerally agreedthattheHCCIprocessismainlydrivenbychemistry.WhileOttoengines arecharacterizedbyalowefficiencyatpartloadanddieselenginesexhausthigh concentrations of particulate matter and NO , the HCCI engine has neither of x these problems: as the peak temperature in the cylinder is low for lean, diluted mixtures, NO concentrations are reported [6, 7] to be two orders of magnitude x lower than for the Otto and diesel processes and particulate emissions are negligible.Figure1.5showsthezoneofNOandsootformationasafunctionof thetemperatureandequivalenceratioaswellastheoperatingzonesforengines thatwouldrunintheSI,CI,andHCCImodes.Owingtoitslowemissionsandhigh efficiency,theHCCIprocessmightinfuturereplacetheOttocycleduringpart-load operation. 1.2.2 ProblemofImplementingtheHCCIMethod Atpresent,theHCCIconceptcannotbeimplementedinautomotiveapplications duetoseveralproblems:complexandexpensivesolutions(e.g.,variablevalvetrain systems)maybenecessarytomakeHCCIcombustionpossibleandtheoperating windowofsmoothHCCIoperationisstilloflimitedsize.Butthemajorproblemis the difficulty in controlling the moment of auto-ignition and the energy release j 1.3 ChemicalKineticsinHCCICombustion 5 Figure1.5 NO andparticulatereductionatlowtemperatureswiththeHCCImethod[8]. x rate. As the mixture in the cylinder is premixed and no spark plug is used, the chemicalprocessesdeterminetheonsetofauto-ignition,aswellasthefuelburn rate.IncontrasttotheSIandCIprocesses,HCCIcombustioncannotbecontrolled directly;thespark-timingcanbemanipulatedinspark-ignitionengines,whilethe moment and amount of fuel injected can be controlled in diesel engines. If we assumethattheadditionoffuelinHCCIenginesiseitherbyport-injectionorearly direct-injection, to obtain a near-homogeneous mixture, HCCI engines have no directwayofcontrollingthecombustionprocess.Atpresent,HCCIresearchhas cometothestageatwhichworkersseemtobeattemptingtosolvethisproblem. Forthiscontrol,abetterunderstandingisneededofhowthechemicalreactions interferewitheachotherandwhatthemostimportantaspectsarethatsetoffthe ignition.Forthispurpose,ashortreviewfollowsfirst(Section1.3),explainingthe typicalchemicalreactionpathwaysoflinear,branched,andaromaticalkanefuels andtheroletheyplayintheauto-ignitionprocess.Section1.4thendiscussesthe use of reduced kinetic mechanisms to study HCCI auto-ignition. Section 1.5 discusses how a kinetic mechanism can propose a controlling strategy for the HCCIengine. 1.3 ChemicalKineticsinHCCICombustion The fuels that are used in present conventional engines are mostly gasoline and diesel. These fuels contain primarily linear alkanes, branched alkanes, and aro- matics.Previousresearch[9–12]hasinvestigatedtheHCCIauto-ignitionprocessfor j 6 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies manycompounds.AgreatpartoftheHCCIandauto-ignitioninvestigationsuseso- called Primary Reference Fuels such as iso-octane and n-heptane [10, 12, 13]. Therefore, the chemical reaction pathways of n-heptane and iso-octane will be explainedindepth,forabetterunderstandingoftheHCCIcombustionphenomena. Little research has been performed on the auto-ignition of aromatics [14, 15]. Nonetheless,themostinvestigatedaromaticcompoundseemstobetoluene[14–18]. Many studies concerning the features of auto-ignition have been performed. The fundamental origins of these features have been discussed in detail in the litera- ture [19–28]. Auto-ignition can be defined [29] as the ignition of a combustible material,commonlywithair,asaresultofheatgenerationfromexothermicoxidation reactionswithouttheinterventionofexternalenergysourcessuchasasparkora flame.Asstatedabove,n-heptane,iso-octane,andtoluenearetakenasexamplesfor, respectively,thelinearalkanes,branchedalkanes,andaromatics.Here,emphasisis laid on the chemical reactions and the general pathways upon which most of the literatureisagreed. 1.3.1 ChemicalCombustionMechanismofIso-Octane The most important reaction branches through which the oxidation of n-heptane proceedsarewellknown[13,30–33].Theinitiationreactionisdescribedby: C H þO !C H .þHO . ð1:1Þ 7 16 2 7 15 2 Owingtoitshighendothermicity,thisreactionisnotanimportantroutetothe formationofthen-heptylradical(C H .).Oncethereactionsystemhascreatedother 7 15 radicals,suchasOH.,.O.,andH.,hydrogenabstractionbysuchradicalsisfavored morethantheabstractionbymolecularoxygen[34].Thesetypesofreactionsareoften denotedastheconsumptionreactions: C H þX.!C H .þXH ð1:2Þ 7 16 7 15 Here,X.isaradical.Thefastestrateofattackisbythehydroxylradical(OH.).The Hatomcanbeabstractedfromdifferentsitesofthen-heptanemoleculeandtherate constantisdependentonthetypeofC(cid:1)Hbondthatisbroken.n-Heptanehasonly primaryandsecondarysites,whereasiso-octanealsohastertiarysites. 1.3.1.1 Low-TemperatureInterval After the H-abstraction step, shown previously, the heptyl radical can react with molecularoxygentoformanalkylperoxyradical: C H .þO !C H OO. ð1:3Þ 7 15 2 7 15 According to Curran et al. [12], this is the most important reaction for low- temperature oxidation. Furthermore, they state that the equilibrium constant of this reaction is very strongly temperature dependent: at higher temperatures the equilibriumofthereactionbeginstoshifttowardsdissociation,therebyindicating j 1.3 ChemicalKineticsinHCCICombustion 7 the switch from low- to high-temperature chemistry, via the intermediate-temper- aturechemistry. The low-temperature path continues with an isomerization step, in which the alkylperoxyradicalistransformedintoahydroperoxyalkylradical: C H OO.!.C H OOH ð1:4Þ 7 15 7 14 Above 800K, the ketohydroperoxide species will decompose into several frag- ments,atleasttwoofwhichareradicals[33].Chainbranchingbecomesquiterapid duetothelargenumberofradicalsformed[36].ThisaddsmoreenergyandOH. radicalstothesystem.Hereby,olefins,aldehydes(whichsubsequentlydecompose, forming CO), and OH. radicals are formed. The low-temperature reactions are exothermic,havingreleasedheatduringthislow-temperatureinterval,raisingthe systems temperature [34].Additionally OH. radicals are produced that react with thefuel.Thiscorrespondstothebeginningofatemperaturerisethatisreferredtoas thecoolflame.Formaldehydealsoisproducedinthecoolflame.Therefore,cool flamescanbetracedbyformaldehydeanalysis. 1.3.1.2 Intermediate-TemperatureInterval Whenthetemperaturebecomeshighenough,theintermediate-temperaturereac- tions become more important than the low-temperature branch. These reactions become dominant at intermediate temperatures, because the activation energy barrier is more easily overcome at these elevated temperatures. Moreover, the additionofthehydroperoxyalkylstomolecularoxygenstartstoshift.Theinterme- diate-temperaturereactionsdescribetheconversionof.C H OOH.radicalradicals 7 14 into conjugate olefins, cyclic ethers, and beta-scission products, instead of the reactionpathwaysthroughketohydroperoxidesformedatlowtemperatures[10,31]. Thebeta-scissionreactionsformnexttostableoxygenatedproducts,alsoOH.radical radicals, eventually, and are, relatively, the most reactive pathway of the three mentioned. The increasing importance of these propagation channels leads to a lower reactivity of the system, which is observed as the NTC region [10]. NTC (negativetemperaturecoefficient)referstothetemperatureintervalduringwhich system reactivity decreases despite a rise in temperature. The reason that the chemistry slows down is that the low-temperature chain-branching reactions are replacedbypropagationreactionsatintermediatetemperatures,whichdonotlead toincreasingnumbersofreactiveradicals.Atintermediatetemperatures,H O can 2 2 beformedasfollows[35]: H.þO þM!HO .þM ð1:5Þ 2 2 AlkaneþHO .!AlkylradicalþH O ð1:6Þ 2 2 2 Inthefirstreaction,Misathirdbody.AccordingtoLietal.[37],H O isalsoformed 2 2 bythefollowingreaction: HO .þHO .!H O þO ð1:7Þ 2 2 2 2 2 j 8 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies CoxandCole[38]alsodiscussthisreaction: RCHOþHO .!H O þRCO ð1:8Þ 2 2 2 Inthisreaction,RCHOisanaldehyde.Thesereactionsdescribethebuildupof H O attemperatureslowerthanapproximately1000Kuntilthetemperatureishigh 2 2 enoughforH O decompositiontobecomedominant.Thisprocessisverytypicalfor 2 2 hydrocarbonignitionandisthoroughlydiscussedbyWestbrook[35]. 1.3.1.3 High-TemperatureInterval Whenthetemperaturebecomes higherthan theintermediate-temperature range, the high-temperature interval is reached. One might wonder how such a high temperature is achieved when reactions become slower during the NTC period. According to Amneus [39], a pre-requisite for the cool flames to induce full combustion is that sufficient energy must be released by the low-temperature reactions to raise the system temperature to the high-temperature region. More importantly, the temperature can be raised sufficiently by external effects as well: whenthecompressionratioishighenough,thegastemperatureinthecombustion chamberofaninternalcombustionenginewillbeforcedintothehigh-temperature regimebymeansofthecompressionworkduringthecompressionstroke.Athigher temperatures(higherthanapproximately1000K),theconversionreactionsofheptyl radicalsintobetadecompositionproductsandconjugateolefinstakeoverfromthe additiononmolecularoxygen. Anotherpossiblereactioninvolvingthealkylradicalisthefollowingisomerization reaction: C H .!isomer ð1:9Þ 7 15 AccordingtoCurranetal.[12],thereactionslikereaction(1.9)areonlyimportantat temperatures above 850K due to the relatively high activation energy of these endothermic beta-scission reactions. If the temperature rises above 1200K, the relativelyhighactivationenergyofthereactionH. þ O ! .O. þ OH.isovercome 2 anditbecomesthedominantchainbranchingstep[35,40];thereactants,including oneradical,leadtotwoproductradicals.Sincemolecularoxygenparticipatesinthis reaction, lean fuel mixtures are more reactive in this high-temperature regime, whereas rich fuel mixtures are oxidized quickly at low temperatures due to chain branching, which depends on radical species formed directly from the parent fuel[10].Whenthehigh-temperatureintervalisdiscussedbelow,tworeactionsare considered[41]: H O þM!OH.þOH.þM ð1:10Þ 2 2 H.þO !.O.þOH. ð1:11Þ 2 These radical-branching reactions are very important for the high-temperature regime, because the products contain more radicals than the reactants. The first chain-branchingreactiontakesplaceataround1000Kanditmarksthemomentof j 1.3 ChemicalKineticsinHCCICombustion 9 ignition [41]. Above 1200K, the reaction between the H. radicals and molecular oxygennolongerformsHO butinsteadformsoxygenandhydroxylradicals.The 2 resultingOH.radicals,formedfromthereactionsdiscussedabove,rapidlyconsume anyfuelandarapidincreaseintemperaturefollows.Thus,decompositionofH O 2 2 andconsumptionoffuelresultinignition[35]. 1.3.2 ChemicalCombustionMechanismofIso-octane The iso-octane oxidation scheme is very similar to that for n-heptane. The main differenceliesinthedistributionofthemassflowsthroughthedifferentreaction branches. The distribution of the products formed changes accordingly. This differenceintheproductdistributionismainlydependentonthemolecularstructure ofthefuel(Figure1.6).Thenumber1inthefigureindicatestheprimaryH-atoms, 2 indicates secondary H-atoms, and 3 indicates tertiary H-atoms. One of the differences between n-heptane oxidation and iso-octane oxidation is discussed by Dagautetal.[30],whostatethatthesumofsecondaryandtertiaryHatoms(whichare more reactive than primary H atoms, since primary C(cid:1)H bonds are stronger) is muchlowerinthecaseofiso-octane.n-Heptanehastwoprimaryandfivesecondary H-atoms. Iso-octane has five primary, one secondary and one tertiary H-atom. However, while iso-octane has one tertiary H-atom, the much higher number of secondaryH-atomsforn-heptaneoutweighthis,andn-heptanecanundergomuch easierH-abstractions. Consequently, H-abstractions and isomerization (including internal H abstrac- tion)reactionsaresloweddown.Therefore,thepropagationsequencethroughalkyl peroxyradicals,hydroperoxyalkylradicals,andcyclicethersisfavoredmoreforiso- octanethanforn-heptane.Inturn,mostlystablespecies,suchasolefinsandcyclic ethers,areproducedduringlow-temperatureoxidationofiso-octane,whiletherateof oxygenatedspeciesformationislow.Togetherwiththechainbranchingpaththrough ketohydroperoxides and their products, favored for n-heptane, this explains why theheatreleaseduringthecoolflameperiodismuchmoresubstantialforn-heptane than for iso-octane. Li et al. [37], who measured molar fractions of intermediate species under motored engine conditions, also found that the formation of cyclic etherswasmuchlargerforiso-octanethanforn-heptane. Figure1.6 Molecularstructuresofn-heptaneandiso-octane. j 10 1 HCCICombustionChemistry,ReducedKineticMechanismsandControllingStrategies Westbrooketal.[42]haveusedsensitivityanalysistostudytheproductionofOH. radicalsforfuelswithvaryingRONnumbers.Thecitedauthorsconcludedthatthe productionrateofOH.decreasesgloballywithincreasingoctanenumber.Sincethe fastestreactionsconsumingfuelarethosewithOH.,thisisanimportantconclusion. Theseobservationsaretrendsratherthanfacts.Theoctanenumberdoesnotindicate preciselyandconsequentlythebehavioroftheauto-ignitiondelay.Asaresultofall this, n-heptane is less resistant to auto-ignition than iso-octane. Consequently, ignitiondelaytimesaretypicallyshorterforn-heptane. 1.3.3 ChemicalCombustionMechanismofToluene 1.3.3.1 InitiationReactions The major difference between aromatics and alkanes can be explained by the structureofaromatics.Figure1.7showsasexamplesbenzeneandtoluene. Benzeneandtoluenepresentaspecialprobleminthat,toaccountforallthebonds, theremustbealternatingcarbondoublebonds(Figure1.7a).UsingX-raydiffraction, researchers discovered that all of the carbon–carbon bonds in benzene are of the samelengthof140picometres(pm).TheC(cid:1)Cbondlengthsaregreaterthanadouble bond(134pm)butshorterthanasinglebond(147pm).Thisintermediatedistanceis explainedbyelectrondelocalization:theelectronsforC(cid:1)Cbondingaredistributed equallybetweeneachofthesixcarbonatoms.Onerepresentationisthatthestructure existsasasuperpositionofso-calledresonancestructures,ratherthaneitherform individually. This delocalization of electrons is known as aromaticity, and gives benzeneandtoluenegreatstability,oftencalledstabilitybyresonance.Thisenhanced stabilityisthefundamentalpropertyofaromaticmoleculesthatdifferentiatesthem frommoleculesthatarenon-aromatic.Thisalsocausesthegreaterdifficultyinauto- ignitingaromatics. Theinitiationreactionfortolueneis: C H CH þO !C H CH .þHO . ð1:12Þ 6 5 3 2 6 5 2 2 C C C C (a) C C C C C C C C C benzene toluene (b) C Figure1.7 Molecularchemicalstructuresofbenzeneandtoluene(a)andtheirresonance structures(b).
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