毕业设计外文翻译燃煤锅炉的燃烧进程控制.docx
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毕业设计外文翻译燃煤锅炉的燃烧进程控制
ControllingtheFurnaceProcessinCoal-FiredBoilers
Theunstabletrendsthatexistinthemarketoffuelsuppliedtothermalpowerplantsandthesituationsinwhichtheparametersoftheiroperationneedtobechanged(orpreserved),aswellasthetendencytowardtheeconomicalandenvironmentalrequirementsplacedonthembecomingmorestringent,arefactorsthatmaketheproblemofcontrollingthecombustionandheattransferprocessesinfurnacedevicesveryurgent.Thesolutiontothisproblemhastwoaspects.Thefirstinvolvesdevelopmentofacombustiontechnologyandaccordingly,thedesignofafurnacedevicewhennewinstallationsaredesigned.Thesecondinvolvesmodernizationofalreadyexistingequipment.Inbothcases,thetechnicalsolutionsbeingadoptedmustbeproperlysubstantiatedwiththeuseofbothexperimentalandcalculationstudies.
TheexperienceCentralBoiler-TurbineInstituteResearchandProductionAssociation(TsKTI)andspecialistsgainedfromoperationofboilersandexperimentalinvestigationstheycarriedoutonmodelsallowedthemtoproposeseveralnewdesignsofmultifuelandmaneuverable—inotherwords,controllable—furnacedevicesthathadbeenputinoperationatpowerstationsforseveralyears.Alongwiththis,anapproximatezero-one-dimensional,zonewisecalculationmodelofthefurnaceprocessinboilershadbeendevelopedattheTsKTI,whichallowedTsKTIspecialiststocarryoutengineeringcalculationsofthemainparametersofthisprocessandcalculatestudiesoffurnacesemployingdifferenttechnologiesoffiringandcombustionmodes.
Naturally,furnaceprocessadjustmentmethodslikechangingtheairexcessfactor,stackgasrecirculationfraction,anddistributionoffuelandairamongthetiersofburners,aswellasotheroperationswrittenintheboileroperationalchart,areusedduringboileroperation.However,theeffecttheyhaveontheprocessislimitedinnature.Ontheotherhand,controlofthefurnaceprocessinaboilerimpliesthepossibilityofmakingsubstantialchangesintheconditionsunderwhichthecombustionandheattransferproceedinordertoconsiderablyexpandtherangeofloads,minimizeheatlosses,reducetheextenttowhichthefurnaceiscontaminatedwithslag,decreasetheemissionsofharmfulsubstances,andshifttoanotherfuel.Suchacontrolcanbeobtainedbymakinguseofthefollowingthreemainfactors:
(1)Theflowsofoxidizerandgasesbeingsettomoveintheflameinadesiredaerodynamicmanner;
(2)Themethodusedtosupplyfuelintothefurnaceandtheplaceatwhichitisadmittedthereto;
(3)Thefinenesstowhichthefuelismilled.
Thelattercaseimpliesthataflame-bedmethodisusedalongwiththeflamemethodforcombustingfuel.Thebedcombustionmethodcanbeimplementedinthreedesignversions:
mechanicalgrateswithadensebed,fluidized-bedfurnaces,andspouted-bedfurnaces.
Aswillbeshownbelow,thefirstfactorcanbemadetoworkbysettingupbulkyvorticestransferringlargevolumesofairandcombustionproductsacrossandalongthefurnacedevice.Iffuelisfiredinaflame,theoptimalmethodoffeedingittothefurnaceistoadmitittothezonesnearthecentersofcirculatingvortices,asituationespeciallytypicalofhighlyintensefurnacedevices.Thecombustionprocessinthesezonesfeaturesalowairexcessfactor(α<1)andalonglocaltimeforwhichthecomponentsdwellinthem,factorsthathelpmakethecombustionprocessmorestableandreducetheemissionofnitrogenoxides.
Alsoimportantforthecontrolofafurnaceprocesswhensolidfuelisfiredisthefinenesstowhichitismilled;ifwewishtominimizeincompletecombustion,thedegreetowhichfuelismilledshouldbeharmonizedwiththelocationatwhichthefuelisadmittedintothefurnaceandthemethodforsupplyingitthere,fortheoccurrenceofunburnedcarbonmaybeduenotonlytoincompletecombustionoflarge-sizefuelfractions,butalsoduetofineonesfailingtoignite(especiallywhenthecontentofvolatilesVdaf<20%).
Owingtothepossibilityofpictoriallydemonstratingthemotionofflows,furnaceaerodynamicsisattractingagreatdealofattentionofresearchersanddesignerswhodevelopandimprovefurnacedevices.Atthesametime,furnaceaerodynamicsliesattheheartofmixing(masstransfer),aprocessthequantitativeparametersofwhichcanbeestimatedonlyindirectlyorbyspecialmeasurements.Thequalitywithwhichcomponentsaremixedinthefurnacechamberproperdependsonthenumber,layout,andmomentumofthejetsflowingoutfromindividualburnersornozzles,aswellasontheirinteractionwiththeflowoffluegases,withoneanother,orwiththewall.
Itwassuggestedthatthegas-jetthrowdistancebeusedasaparameterdeterminingthedegreetowhichfuelismixedwithairinthegasburnerchannel.Suchanapproachtoestimatinghowefficientthemixingismaytoacertaindegreebeusedinanalyzingthefurnaceasamixingapparatus.Obviously,thegreaterthejetlength(anditsmomentum),thelongerthetimeduringwhichthevelocitygradientitcreatesinthefurnacewillpersistthere,aparameterthatdetermineshowcompletelytheflowsaremixedinit.Notethatthehigherthedegreetowhichajetisturbulizedattheoutletfromanozzleorburner,theshorterthedistancewhichitcovers,and,accordingly,thelesscompletelythecomponentsaremixedinthefurnacevolume.Oncethroughburnershaveadvantagesoverswirlonesinthisrespect.
Itiswasproposedthattheextenttowhichoncethroughjetsaremixedastheypenetratewithvelocityw2anddensityρ2intoatransverse(drift)flowmovingwithvelocityw1andhavingdensityρ1becorrelatedwiththerelativejetthrowdistanceinthefollowingway
Whereksisaproportionalityfactorthatdependsonthe“pitch”betweenthejetaxes(ks=1.5~1.8).
Theresultsofanexperimentalinvestigationinwhichthemixingofgaswithairinaburnerandtheninafurnacewasstudiedusingtheincompletenessofmixingasaparameterarereportedin5.
Aroundoncethroughjetisintensivelymixedwiththesurroundingmediuminafurnacewithinitsinitialsection,wheretheflowvelocityatthejetaxisisstillequaltothevelocityw2atthenozzleorificeofradiusr0.Thevelocityofthejetblownintothefurnacedropsveryrapidlybeyondtheconfinesoftheinitialsection,andtheaxisithasinthecaseofwall-mountedburnersbendstowardtheoutletfromthefurnace.
OnemayconsiderthattherearethreetheoreticalmodelsforanalyzingthemixingofjetswithflowrateG2thatenterintoastreamwithflowrateG1.Thefirstmodelisforthecasewhenjetsflowintoa“free”space(G1=0),thesecondmodelisforthecasewhenjetsflowintoatransverse(drift)currentwithflowrateG1
G2,andthethirdmodelisforthecasewhenjetsflowintoadriftstreamwithflowrateG1S0=0.67r0/a,
(2)whereaisthejetstructurefactorandr0isthenozzleradius.
Ata=0.07,thelengthoftheroundjet’sinitialsectionisequalto10r0andtheradiusthejethasatthetransitionsection(attheendoftheinitialsection)isequalto3.3r0.Themassflowrateinthejetisdoubledinthiscase.Thecorrespondingminimumfurnacecross-sectionalareaFfforaroundoncethroughburnerwiththeoutletcross-sectionalareaFbwillthenbeequaltoandtheratioFf/Fb≈20.Thisvalueisclosetotheactualvaluesfoundinfurnacesequippedwithoncethroughburners.Infurnacesequippedwithswirlburners,a=0.14andFf/Fb≈10.Inbothcases,theintervalbetweentheburnersisequaltothejetdiameterinthetransitionsectiondtr,whichdifferslittlefromthevaluethathasbeenestablishedinpracticeandrecommendedin.
Themethodtraditionallyusedtocontrolthefurnaceprocessinlargeboilersconsistsofequippingthemwithalargenumberofburnersarrangedinseveraltiers.Obviously,ifthedistancebetweenthetiersisrelativelysmall,operationsondisconnectingorconnectingthemaffecttheentireprocessonlyslightly.Afurnacedesignemployinglargeflat-flameburnersequippedwithmeansforcontrollingtheflamecorepositionusingtheaerodynamicprincipleisastepforward.AdditionalpossibilitiesforcontrollingtheprocessinTPE-214andTPE-215boilerswithasteamoutputof670t/hwereobtainedthroughtheuseofflat-flameburnersarrangedintwotierswithalargedistancebetweenthetiers;thismadeitpossiblenotonlytoraiseorlowertheflame,butalsotoconcentrateordispersethereleaseofheatinit.Averytangibleeffectwasobtainedfrominstallingmultifold(operatingoncoalandopen-hearth,coke,andnaturalgases)flat-flameburnersintheboilersofcogenerationstationsatmetallurgicalplantsinUkraineandRussia.
Unfortunately,wehavetostatethat,evenatpresent,thoseinchargeofselectingthetype,quantity,andlayoutofburnersinafurnacesometimesadopttechnicalsolutionsthatarefarfrombeingoptimal.Thisproblemshouldthereforebeconsideredinmoredetail.
Ifweincreasethenumberofburnersnbinafurnacewhileretainingtheirtotalcross-sectionalarea(ΣFb=idem)and