热动文献Effect of Ash in Coal on the Performance of Coal .doc

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Energy Sources, Part A: Recovery, Utilization, and

Environmental Effects

Publication details, including instructions for authors and subscription information:Effect of Ash in Coal on the Performance of Coal Fired

Thermal Power Plants. Part II: Capacity and Secondary

Energy Effects

M. Siddhartha Bhatt

aa Central Power Research Institute, Trivandrum, India

Published online: 18 Aug 2006.

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EnergySources,PartA,28:43–58,2006

Copyright©Taylor&FrancisGroup,LLC

ISSN:1556-7036print/1556-7230online

DOI:10.1080/009083190523244

EffectofAshinCoalonthePerformanceof

CoalFiredThermalPowerPlants.

PartII:CapacityandSecondaryEnergyEffects

M.SIDDHARTHABHATT

CentralPowerResearchInstitute

Trivandrum,India

Thisarticlereportsthesecondaryenergyeffects(wear/erosion/abrasion,slagging,

andfouling)ofashincoalontheenergyperformanceofcoal?redthermalpower

plantsofcapacityrange30–500MW.Italsogivestheextentofcapacityreductionin

equipmentdueto?ringofcoalswithhigherashcontents.

Atanashcontentof75%incoal,theeffectsonthesystem(withoutfueloil

support)follow:(a)decreaseinHardgroveindexfrom80to44;(b)20%ofthe

speci?cenergyconsumption(SEC)ofinduceddraft(ID)fans,10%–12%ofthatof

forceddraft(FD)andprimaryair(PA)fans,17%ofthatofdrummills,and12%–13%

ofthatofball-racemillsandbowlmills,areaccountedforbywear/erosion/abrasion

effects;(c)decreaseoffanef?cienciesby5%–6%pointsduetowear/erosion/abrasion

effects;(d)capacitylossoriginatingfromwear/erosion/abrasioneffectsaloneis8%

duetoIDfans,1%duetoPAfans,and6%duetomills;(e)foulingeffectsarehigh

foulingfactor,decreaseinboileref?ciencyby3%,andcapacityreductionof2%;and

(f)CFsbasedonoverallunitperformanceare31%forunitsbelow210MW,26%

for210MWunits,and40%for500MW.Consideringthecapacityrestrictionsdueto

individualequipment,CFatanashcontentof57%is85%duetotheboilerfans,84%

duetoRaymondbowlmillsanddrumtypeballmills,71%duetoslowspeedlarge

ballandracemills,and88%duetoashslurrypumps.Whenthecoalexceeds70%

andtendstoward76%,theheatingvalueofcoaltendstowardzero.Theeffectsof

slagging(independentofashcontentincoal)area20%decreaseinboilerwaterwall

loading,a3.5%pointsdecreaseinboileref?ciency,andcapacityreductionof14%.

Keywordsashcontent,equipmentef?ciency,speci?cenergyconsumption,capacity

reduction,ef?ciencyunderrating

Introduction

IndianCoals

Indiannoncokingcoals(sub-bituminousandofdriftorigin),whicharethemainsourceoffuelforpowergenerationinIndia,canbebroadlyclassi?edintothreecategories:(a)steamcoalandrubble,(b)slackcoalandwasherymeddlings,and(c)runofminecoal.Thesearesubdividedintolong?ameandnon-long?amecoals(GazetteofIndia[GOI],1986).Mostofthecoalreservesofaround200billiontonnesareoflow-gradevariety

AddresscorrespondencetoM.SiddharthaBhatt,CentralPowerResearchInstitute,P.O.Box8066,SirC.V.RamanRoad,Bangalore-560080,India.E-mail:msbhatt@powersearch.cpri.res.in

43Downloaded by [Inner Mongolia University of Technology] at 19:34 23 May 2013

44M.S.Bhatt

withlowheatingvalueandhighash.Beingofdriftorigin,themineralsandashareinti-matelymixedinthecoalmatrix.Indiancoalsarerichinkaolinite(35%–60%),α-quartz(4%–30%),illite(shale;5%–10%),siderite(1%–5%),andcaclite(1%–5%)(Mandal,1999).Theotherlesscommonmineralsaremarcasite,pyrites,dolomite,hematite,andgypsum.

Besidestheheatingvalue,therateofheatreleaseanditsreactivitytowardignitionandcombustionin?uencestheutilityofcoalforpowergeneration.Ashreferstotheinorganicresidueofthemineralmatterincoal,whichhasbeenincineratedat800?Cinair.Whileincreaseinashcontentseriouslyaffectsthecombustionin?xedbedorstokerboilers,ithardlyaffectsthedegreeofcombustioninpulverizedburnersandcirculating?uidizedbedcombustors(Chatenet,1965).

ThelargestofIndianrun-of-minecoalis230mm.IngeneralIndiancoalsareharderthanEuropean,Australian,andAmericancoals.

Coalscanberankedbasedontheirusefulheatingvalue(UHV—ahigherheatingvaluethatconsiderstheeffectsofoxygen).Coalslowerinrankcontainahigherpercent-ageofoxygen.Theoxygenpercentageincreaseswithanincreaseinmoisturecontent.Coalshigherinoxygenandhigherinmoisturethoughlowerinrank,demonstratehighercombustionreactivity(CentralFuelResearchInstitute[CFRI],1967).Thus,therearetwoopposingfactors—theheatvalueandtheheatreleaserate.Asthemoisturecontentincreases,theUHVdecreases.Sinceoxygencontentincreaseswithincreaseinmoisturecontent,combustionreactivityandheatreleaseratealsoincrease.Thus,higherO2incoalsisapremiuminlowrankingcoals.

Volatilematter(combustiontemperature600?C)isanindicatorofcoalreactivitytowardignitionandheatreleaserate.TypicalvolatilematterinIndiancoalsis25%–45%.

Petrographicanalysis(surfacemorphology)ofIndiancoalsindicatespredominantlyvitrinitecontent(20%–60%)andinertinite(20%–65%;Mandal,1999).Thevirtriniteandinertinitetogetherconstitutemorethan90%ofthecoal.Thebalanceisexiniteandasmallamountofsemivitrinite.Whilevitriniterepresentshighreactivityinertinitedenoteslowreactivity.Exiniteismorereactivethanvitriniteinthepresenceofcalciumandlessreactivethaninertiniteinitsabsence.Semivitriniteislessreactivethanvitrinite.AshfromIndianCoals

Contaminationofcoalwithashhasbeenaproblemsincecoalcametobeusedasafuelforsteamgenerationandmotivepower.Coalsusedinthermalpowerplantsgenerallycontainash(asreceivedbasis)from8%toashighas55%.Ashincoalisclassi?edasinherentash(cannotbereducedbycleaning),associatedash(presentincoalseams,lenticels,andpartings),andadventitiousash(introducedextraneouslyduringcutting;CentralElectricityGeneratingBoard[CEGB],1971).Highashincoalisoccurringmainlybecauseofopencastminingwherecoaliscontaminatedwithsurfacemineralimpurities(increaseinratioofopencasttoundergroundminingfrom26%/74%in1974–1975to75%/25%in1995–1996)andpoorerseams.Kaoliniteparticles(generallylessthan3µm)andα-quartz(lessthan100µm)arethemosttroublesomecontaminantsinash.Chemically,ashcontainsprimarilysilicaandalumina.AshfromIndiancoalscontainpredominantlysilica(SiO2;55%–65%),alumina(Al2O3;25%–35%),andironoxide(Fe2O3;Meenakshietal.,1999).Thebalanceiscalciumoxide(CaO),magnesiumoxide(MgO),sodiumoxide(Na2O),potassiumoxide(K2O),titania(TiO2),andsulphurtrioxide(SO3).Themajorconstituents,viz.,SiO2andAl2O3,areacidicwhilemostoftheothersexcept(TiO2)arebasic.Thebasicoxidesaremorepronetoslagging,clinkering,anddepositformation.Downloaded by [Inner Mongolia University of Technology] at 19:34 23 May 2013

EffectofAshinCoal(CapacityandSecondaryEnergyEffects)45

PresentWork

Themineralspecies,whichaffectstheenergyperformanceofpowerplants,isquartz,whichisfreesilica.Ashoriginatingfromcoalaffectspowerplantperformanceinavarietyofways.Theprimaryeffectsofashcombustion,heattransfer,and?owhydrodynamicsalreadyhavebeendiscussedbyBhatt(2003).Therearesecondaryenergyeffects—capacityreduction,wearanderosion,slagging,andfouling,whichleadtoperformancedeteriorationeffects.Inthisarticle,thesecondaryenergyeffectsonthesystemandunitperformanceindicesarepresented.

ExperimentalWork

Detailedstudieshavebeenundertakeninmorethan30coal?redthermalpowerunits.Theproximateandultimateanalysisofcoalandashhavebeenconductedonsamplescollectedfromtheseunitsspanningawiderangeofashcontents.Allmajoroutputvariablesoftheplanthavebeenrecordedoveraperiodof24hours.Thestudiespertaintoconditionswithoutfueloilsupportinthecompleteashrange.

Basedon?eldstudiesconductedintheunitsandstudiesatthelaboratorytheresultsareobtainedintheformofthefollowingcurve?ts:

i.CurvesofType1(Linear)

Y=A0+A1X(1)

ii.CurvesofType2(Powerseries)

Y=A0+A1X+A2X2(2)

iii.CurvesofType3(Exponential)

Y=B0exp(B1X)(3)

iv.CurvesofType4(Nonlinear)

Y=D0XD1(4)

Intheabovecurve?ts,XandZaretheindependentvariables,Yisthedependentvariable,andA,B,Dareconstants.

Theindicesforquanti?cationoftheenergyeffectsofashincoalareasfollows:(i)Ef?ciencyistheratiooftherateofenergyoutputtotherateofenergyinputtothesystem.

(ii)Capacityfactor(CF)duetoelectricauxiliaryequipmentistheinstantaneous

plantloadfactor(PLF)correspondingtounityauxiliaryloadfactor(ALF)as

CF=PLF/ALF(5)

ItistheinstantaneousPLFcorrespondingto100%electricalloadingontheauxiliary.

(iii)Speci?cenergyconsumption(SEC)istheelectricalenergyperunitoutputof

theauxiliaryequipment.Downloaded by [Inner Mongolia University of Technology] at 19:34 23 May 2013

46M.S.Bhatt

(iv)Thespeci?cenergyconsumptionratio(SECR)istheratiooftheSECata

givenashtothatatstandardash.Astheashcontentincoalincreases,theSECRdecreases.

(v)TheSECRprovidestheSECratioduetoincreasedashincoalalone.Butwhen

ashincoalincreases,inordertomaintainthegivensystemoutputlevel,thecoal?owthroughthesystemalsoincreasesbecauseofreducedUHVofcoal,loweref?ciencyofequipment,andsoon.Theincreasedcoal?owthroughthesystemcanbequanti?edbytheSFCRwhichistheratiooftheSFCoftheunitatagivenashcontenttothatatstandardash.

(vi)Toaccountforthedualeffectofincreaseincoal?owthroughthesystemas

wellasincreaseinSECduetoqualityeffects,aSECindex(SECI)isde?nedas

SECI=SECR·SFCR(6)

Thisindexcoversbothquantityeffects(excessiveenergyconsumptionduetoexcessive?ownecessitatedbyreducedef?ciencyorcapacity)andqualityeffects(excessiveenergy/poorperformanceduetowear,erosion,etc.).Hence,itrepresentstheoverallbehaviorofthesystem,whichincreaseswiththeashcontentofcoal.

ResultsandDiscussion

EffectofAshinCoalonSizeReduction

ThehardnessofcoalisrepresentedbytheHardgroveindex(HGI),whichisgivenbythepercentageofparticlesbelow75µm(Hardgrove,1932).Itvariesbetween100foreasilygroundcoalsto40fordif?culttogrindcoals.

Theupperlimitofmoistureinrawcoalisgenerally10%.Duringtherainyseasonsomecoalsthataresoftinnature(HGI=90to130)duetohighclaycontent(20%–40%)containhighmoisture(15%–20%;Mandal,1999).Althoughtheyaresoft,theycause?owproblemsandconsumeexcessivepowerforpulverization.

TheHGIoffreshlyminedcoal,whichwas60afterexposuretotheatmosphereforayear,ledtovaluesof85–95(Banerjeeetal.,2000).Althoughthemillingpowerisreduced,thecoaldisintegratesintosmallparticlesandposeshandling,crushing,andmillingproblems(Banerjeeetal.,2000).

Thecoef?cientsinthecurve?tsforvariablesrelatedtosizereductionaregiveninTable1.Thevariablethataffectsthemillingpower,thatis,theHGIvariesbetween44and80foranashrangeof6%–75%.

PerformanceDeteriorationinPowerPlantEquipmentCausedby

Wear/Erosion/AbrasionArisingfromAshinCoal

Abrasionisawearprocessthatoccurswhensolidsmoveoverasurface.Whenthekineticenergyoftheparticleishighitiscallederosion.Thetotalwearisthesumtotaloftheabrasionanderosion.Themainingredientincoal/ashthatisresponsibleforthehighrateofwear/abrasion/erosionisalphaquartz.

Wearincoal?redthermalpowerplantsiscausedbythefollowing:(a)?owofcoalbecauseofashinherentinitand(b)?owof?yash/bottomashleadingtoperformancedeteriorationgeneratedaftercombustionofcoal.Downloaded by [Inner Mongolia University of Technology] at 19:34 23 May 2013