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thefurnaceprocessincoal-firedboilers.ThermalEngineering,55,1,72-77.

ControllingtheFurnaceProcessinCoal-FiredBoilers

Theunstabletrendsthatexistinthemarketoffuelsuppliedtothermalpowerplantsandthesituationsinwhichtheparametersoftheiroperationneedtobechanged（orpreserved）,aswellasthetendencytowardtheeconomicalandenvironmentalrequirementsplacedonthembecomingmorestringent,arefactorsthatmaketheproblemofcontrollingthecombustionandheattransferprocessesinfurnacedevicesveryurgent.Thesolutiontothisproblemhastwoaspects.Thefirstinvolvesdevelopmentofacombustiontechnologyand,accordingly,thedesignofafurnacedevicewhennewinstallationsaredesigned.Thesecondinvolvesmodernizationofalreadyexistingequipment.Inbothcases,thetechnicalsolutionsbeingadoptedmustbeproperlysubstantiatedwiththeuseofbothexperimentalandcalculationstudies.

TheexperienceCentralBoiler-TurbineInstituteResearchandProductionAssociation（TsKTI）andZiOspecialistsgainedfromoperationofboilersandexperimentalinvestigationstheycarriedoutonmodelsallowedthemtoproposeseveralnewdesignsofmultifuelandmaneuverable—inotherwords,controllable—furnacedevicesthathadbeenputinoperation

atpowerstationsforseveralyears.Alongwiththis,anapproximatezero-one-dimensional,zonewisecalculationmodelofthefurnaceprocessinboilershadbeendevelopedattheTsKTI,whichallowedTsKTIspecialiststocarryoutengineeringcalculationsofthemainparametersofthisprocessandcalculatestudiesoffurnacesemployingdifferenttechnologiesoffiringandcombustionmodes.

Naturally,furnaceprocessadjustmentmethodslikechangingtheairexcessfactor,stackgasrecirculationfraction,anddistributionoffuelandairamongthetiersofburners,aswellasotheroperationswrittenintheboileroperationalchart,areusedduringboileroperation.However,theeffecttheyhaveontheprocessislimitedinnature.Ontheotherhand,controlofthefurnaceprocessinaboilerimpliesthepossibilityofmakingsubstantialchangesintheconditionsunderwhichthecombustionandheattransferproceedinordertoconsiderablyexpandtherangeofloads,minimizeheatlosses,reducetheextenttowhichthefurnaceiscontaminatedwithslag,decreasetheemissionsofharmfulsubstances,andshifttoanotherfuel.Suchacontrolcanbeobtainedbymakinguseofthefollowingthreemainfactors:

（i）theflowsofoxidizerandgasesbeingsettomoveintheflameinadesiredaerodynamicmanner;

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（ii）themethodusedtosupplyfuelintothefurnaceandtheplaceatwhichitisadmittedthereto;

（iii）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,andmomentaofthejetsflowingoutfromindividualburnersornozzles,aswellasontheirinteractionwiththeflowoffluegases,withoneanother,orwiththewall.

Itwassuggestedthatthegas-jetthrowdistancebeusedasaparameterdeterminingthedegreetowhichfuelismixedwithairinthegasburnerchannel.Suchanapproachtoestimatinghowefficientthemixingismaytoacertaindegreebeusedinanalyzingthefurnaceasamixingapparatus.Obviously,thegreaterthejetlength（anditsmomentum）,thelongerthetimeduringwhichthevelocitygradientitcreatesinthefurnacewillpersistthere,aparameterthatdetermineshowcompletelytheflowsaremixedinit.Notethatthehigherthedegreetowhichajetisturbulizedattheoutletfromanozzleorburner,theshorterthedistancewhichitcovers,and,accordingly,thelesscompletelythecomponentsaremixedin

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thefurnacevolume.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，andthethirdmodelisforthecase,

whenjetsflowintoadriftstreamwithflowrateG1<

G2.Thesecondmodelrepresentsmixinginthechannelofagasburner,andthethirdmodelrepresentsmixinginafurnacechamber.Weassumethatthemixingpatternwehaveinafurnaceisclosertothefirstmodelthanitistothesecondone,since0<

G1/G2<

1,andwewillassumethatthethrowdistancehofthejetbeingdriftedisequaltothelengthS0ofthe―free‖jet’sinitialsection.Theejectionabilityofthejetbeingdriftedthenremainsthesameasthatofthe―free‖jet,andthelengthoftheinitial

sectioncanbedeterminedusingthewell-knownempiricalformulaofG.N.Abramovich[6]:

S0=0.67r0/a,

（2）whereaisthejetstructurefactorandr0isthenozzleradius.

Ata=0.07,thelengthoftheroundjet’sinitialsectionisequalto10r0andtheradiusthejethasatthetransitionsection（attheendoftheinitialsection）isequalto3.3r0.Themassflowrateinthejetisdoubledinthiscase.Thecorrespondingminimumfurnacecross-sectionalareaFfforaroundoncethroughburnerwiththeoutletcross-sectionalareaFbwillthenbeequaltoandtheratioFf/Fb?

20.Thisvalueisclosetotheactualvaluesfoundinfurnaces

equippedwithoncethroughburners.Infurnacesequippedwithswirlburners,a=0.14andFf/Fb?

10.Inbothcases,theintervalbetweentheburnersisequaltothejetdiameterinthetransitionsectiondtr,whichdifferslittlefromthevaluethathasbeenestablishedinpracticeandrecommendedin[7].

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Themethodtraditionallyusedtocontrolthefurnaceprocessinlargeboilersconsistsofequippingthemwithalargenumberofburnersarrangedinseveraltiers.Obviously,ifthedistancebetweenthetiersisrelativelysmall,operationsondisconnectingorconnectingthemaffecttheentireprocessonlyslightly.Afurnacedesignemployinglargeflat-flameburnersequippedwithmeansforcontrollingtheflamecorepositionusingtheaerodynamicprincipleisastepforward.AdditionalpossibilitiesforcontrollingtheprocessinTPE-214andTPE-215boilerswithasteamoutputof670t/hwereobtainedthroughtheuseofflat-flameburnersarrangedintwotierswithalargedistancebetweenthetiers;

thismadeitpossiblenotonlytoraiseorlowertheflame,butalsotoconcentrateordispersethereleaseofheatinit[1].Averytangibleeffectwasobtainedfrominstallingmultifuel（operatingoncoalandopen-hearth,coke,andnaturalgases）flat-flameburnersintheboilersofcogenerationstationsatmetallurgicalplantsinUkraineandRussia.

Unfortunately,wehavetostatethat,evenatpresent,thoseinchargeofselectingthetype,quantity,andlayoutofburnersinafurnaces