燃煤锅炉-外文翻译-外文文献-英文文献-中英翻译.doc
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燃煤锅炉-外文翻译-外文文献-英文文献-中英翻译.doc
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ControllingtheFurnaceProcessinCoal-FiredBoilers
Theunstabletrendsthatexistinthemarketoffuelsuppliedtothermalpowerplantsandthesituationsinwhichtheparametersoftheiroperationneedtobechanged(orpreserved),aswellasthetendencytowardtheeconomicalandenvironmentalrequirementsplacedonthembecomingmorestringent,arefactorsthatmaketheproblemofcontrollingthecombustionandheattransferprocessesinfurnacedevicesveryurgent.Thesolutiontothisproblemhastwoaspects.Thefirstinvolvesdevelopmentofacombustiontechnologyand,accordingly,thedesignofafurnacedevicewhennewinstallationsaredesigned.Thesecondinvolvesmodernizationofalreadyexistingequipment.Inbothcases,thetechnicalsolutionsbeingadoptedmustbeproperlysubstantiatedwiththeuseofbothexperimentalandcalculationstudies.
TheexperienceCentralBoiler-TurbineInstituteResearchandProductionAssociation(TsKTI)andZiOspecialistsgainedfromoperationofboilersandexperimentalinvestigationstheycarriedoutonmodelsallowedthemtoproposeseveralnewdesignsofmulticellandmaneuverable—inotherwords,controllable—furnacedevicesthathadbeenputinoperationatpowerstationsforseveralyears.Alongwiththis,anapproximatezero-one-dimensional,zonewisecalculationmodelofthefurnaceprocessinboilershadbeendevelopedattheTskTi,whichallowedTskTispecialiststocarryoutengineeringcalculationsofthemainparametersofthisprocessandcalculatestudiesoffurnacesemployingdifferenttechnologiesoffiringandcombustionmodes.
Naturally,furnaceprocessadjustmentmethodslikechangingtheairexcessfactor,stackgasrecirculationfraction,anddistributionoffuelandairamongthetiersofburners,aswellasotheroperationswrittenintheboileroperationalchart,areusedduringboileroperation.However,theeffecttheyhaveontheprocessislimitedinnature.Ontheotherhand,controlofthefurnaceprocessinaboilerimpliesthepossibilityofmakingsubstantialchangesintheconditionsunderwhichthecombustionandheattransferproceedinordertoconsiderablyexpandtherangeofloads,minimizeheatlosses,reducetheextenttowhichthefurnaceiscontaminatedwithslag,decreasetheemissionsofharmfulsubstances,andshifttoanotherfuel.Suchacontrolcanbeobtainedbymakinguseofthefollowingthreemainfactors:
(i)theflowsofoxidizerandgasesbeingsettomoveintheflameinadesiredaerodynamicmanner;
(ii)themethodusedtosupplyfuelintothefurnaceandtheplaceatwhichitisadmittedthereto;
(iii)thefinenesstowhichthefuelismilled.
Thelattercaseimpliesthataflame-bedmethodisusedalongwiththeflamemethodforcombustingfuel.Thebedcombustionmethodcanbeimplementedinthreedesignversions:
mechanicalgrateswithadensebed,fluidized-bedfurnaces,andspouted-bedfurnaces.
Aswillbeshownbelow,thefirstfactorcanbemadetoworkbysettingupbulkyvorticismstransferringlargevolumesofairandcombustionproductsacrossandalongthefurnacedevice.Iffuelisfiredinaflame,theoptimalmethodoffeedingittothefurnaceistoadmitittothezonesnearthecentersofcirculatingvorticisms,asituationespeciallytypicalofhighlyintensefurnacedevices.Thecombustionprocessinthesezonesfeaturesalowairexcessfactor(α<1)andalonglocaltimeforwhichthecomponentsdwellinthem,factorsthathelpmakethecombustionprocessmorestableandreducetheemissionofnitrogenoxides.
Alsoimportantforthecontrolofafurnaceprocesswhensolidfuelisfiredisthefinenesstowhichitismilled;ifwewishtominimizeincompletecombustion,thedegreetowhichfuelismilledshouldbeharmonizedwiththelocationatwhichthefuelisadmittedintothefurnaceandthemethodforsupplyingitthere,fortheoccurrenceofunburnedcarbonmaybeduenotonlytoincompletecombustionoflarge-sizefuelfractions,butalsoduetofineonesfailingtoignite(especiallywhenthecontentofvolatilesDaff<20%).
Owingtothepossibilityofpictoriallydemonstratingthemotionofflows,furnaceaerodynamicsisattractingagreatdealofattentionofresearchersanddesignerswhodevelopandimprovefurnacedevices.Atthesametime,furnaceaerodynamicsliesattheheartofmixing(masstransfer),aprocessthequantitativeparametersofwhichcanbeestimatedonlyindirectlyorbyspecialmeasurements.Thequalitywithwhichcomponentsaremixedinthefurnacechamberproperdependsonthenumber,layout,andmomentaofthejetsflowingoutfromindividualburnersornozzles,aswellasontheirinteractionwiththeflowoffluegases,withoneanother,orwiththewall.
Itwassuggestedthatthegas-jetthrowdistancebeusedasaparameterdeterminingthedegreetowhichfuelismixedwithairinthegasburnerchannel.Suchanapproachtoestimatinghowefficientthemixingismaytoacertaindegreebeusedinanalyzingthefurnaceasamixingapparatus.Obviously,thegreaterthejetlength(anditsmomentum),thelongerthetimeduringwhichthevelocitygradientitcreatesinthefurnacewillpersistthere,aparameterthatdetermineshowcompletelytheflowsaremixedinit.Notethatthehigherthedegreetowhichajetisturbulenceattheoutletfromanozzleorburner,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)currentwithflowrateG1G2,andthethirdmodelisforthecasewhenjetsflowintoadriftstreamwithflowrateG1 S0=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[7]. Themethodtraditionallyusedtocontrolthefurnaceprocessinlargeboilersconsistsofequippingthemwithalargenumberofburnersarrangedinseveraltiers.Obviously,ifthedistancebetweenthetiersisrelativelysmall,operationsondisconnectingorconnectingthemaffecttheentireprocessonlyslightly.Afurnacedesignemployinglargeflat-flameburnersequippedwithmeansforcontrollingtheflamecorepositionusingtheaerodynamicprincipleisastepforward.AdditionalpossibilitiesforcontrollingtheprocessinTPE-214andTPE-215boilerswithasteamoutputof670t/hwereobtainedthroughtheuseofflat-flameburnersarrangedintwotierswithalargedistancebetweenthetiers;thismadeitpossiblenotonlytoraiseorlowertheflame,butalsotoconcentrateordispersethereleaseofheatinit[1].Averytangibleeffectwasobtainedfrominstallingmulticell(operatingoncoalandopen-hearth,coke,andnaturalgases)flat-flameburnersintheboilersofcogenerationstationsatmetallurgicalplantsinUkraineandRussia. Unfortunately,wehavetostatethat,evenatpresent,thoseinchargeofselectingthetype,quantity,andlayoutofburnersinafurnacesometimesadopttechnicalsolutionsthatarefarfrombeingoptimal.Thisproblemshouldthereforebeconsideredinmoredetail. Ifweincreasethenumberofburnersnbinafurnacewhileretainingtheirtotalcross-sectionalarea(ΣFb=i
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