Energy&Sustainability

Lecture14:FossilFuels‐Coal

February26,2009

Fossilfuels

•  Oldestofalltechnologies:Burningfueltoprovideheat

•  Morethan¾ofourpresent‐dayuseofprimaryenergyisfromfossilfuels

•  Processofcombustion:chemicalreactionbetweenoxygen,usuallyfromsurroundingair,andtheconstituentelementsofthefuel,mainlycarbonandhydrogen

•  Energyreleaseasheatwhichiscarriedawayinitiallybythecombustionproducts

2000

Coal•  Incontrasttooil/gasmightbecalledignoblefuel– Lessconvenienttransportation,storageanduse– ProducestwicetheamountofCO2

Coal•  Incontrasttooil/gasmightbecalledignoblefuel–  Itsextractionleadstolandsubsidenceandspoilheaps

– Stripmining– Deathsofhundredsofminersinanaverageyear

Coal•  Consumption:CoalRise<TotalEnergyRise

CoalCombustion

•  Extremelycomplexnature•  ThecompositionofCoal:

– Carbonandhydrogen– Oxygenandnitrogen– Sulphurandothers–  Inertmaterials

– moisture

CompositionofCoal

Amounts of inert materials and moistures obviously important when determining the heat value of coal

TheCombustionProcess•  Earlystageofcombustion:

– moistureevaporates(moremoistureinbrowncoalthanhardcoal,between1%and10%)

– Heatingandevaporationusessomeoftheenergyofthecoal,butlessthan1%

•  Temperaturecontinuestorise:arangeofgasesevolve,calledvolatilematter(VM):– Arisefromthedissociationofcoalstructure– Carrymostofthehydrogenandoxygen,someofitscarbon:CO,CH4,otherformsofhydrocarbons(bitumens)

– Releaseheatastheyburn,about50%ofthecoalsenergy

TheCombustionProcess•  Combustiblepartofthematerialremains:

–  fixedcarbon(charcoal,coke)– Canburnathightemperatureinoxygen:

– Dependingontypeofcoalthisaccountsforvirtuallyalltheheatoutputornomorethanhalf

•  Finally:Ash(inertmaterialthatremains)– Bestcoalhaslessthan10%ofit– 15%notuncommon,andinsomecountries30%istoleratedifpriorityistouselocalcoal

DifferentTypesofRanksofCoal

•  Fixedcarbonisafactordeterminingtherank

CombustionProducts•  Chemicalchangeofmaincomponents:

•  Nitrogen:NOXorNox•  Sulphur:SO2

– Sulphurcanaccountforasmuchas5%ofthemass

–  Inhighsulphurcoalsabouthalfofitmaybeintheinertmaterial(removablebywashing)

– Evenlessthan1%releases20kgofSO2pertonneofburnedcoal

Howmuchcarbondioxideisreleasedinthecombustionofone

tonneofcoal?

HowmuchCO2incombustionofonetonneofcoal?

•  Assumeallthecarboninthecoalreactswithoxygen:

rel.atomicmass:122*1644

•  So12kgofCproduces44kgofCO2or

•  1kgofCproduces3.67kgofCO2

•  NowconsideronetonneoflowVMbituminouscoal:10%ismoistureorash‐>900kgofdryandashfreecoal

•  88%ofthiscoalcarbon‐>792kgofcarbon•  SothemassofCO2releasedinthecombustionof1tonneofcoalis792kg*3.67=2.9tonnes

Fires,furnacesandboilers

•  Thedestinationofapprox.¾offossilandbiofuelsisafire,furnaceorboiler

•  Drivingfactorindesignofboilers:efficiencywithwhichenergyisextracted

•  Betterdesign– Lowerfuelrequirements

– Lowercosts– LoweremissionofCO2andotherundesirableproducts

Howmuchwaterisneededtobringaliterofwatertoboil?

•  Inputdata:– specificheatcapacityofwater=4200Jkg‐1K‐1

– Massof1literofwater=1kg

– Heatvalueofwood=15MJ/kg– Densityofwood=600kgm‐3

– 1cubiccentimeter=10‐6m3

Howmuchwaterisneededtobringaliterofwatertoboil?

•  Calculation– Heatenergyneededtoheat1literofwaterfrom20oCto100oC

=80*4200J=336kJ

– Heatenergyreleasedinburning1cm3ofwood

=15*600*10‐6MJ=9.0kJ– Volumeofrequiredwood=336÷9.0cm3=37cm3

Suggests only one thin stick of about a food of wood is needed

Surprising result?

Why not that simple?

Sideremark:LatentHeat

amount of energy in the form of heat released or absorbed by a chemical substance during a change of state

Fires,furnacesandboilers•  Thestovemighthaveonlyafuel‐to‐usefulheatefficiencyof10%,whichcanbecomparedto70%ofawellrundomesticgaswater‐heateror>90%foramodernpowerstationboiler

•  Themajorconsumersofcoal:largepowerstationboilerstoproducesteam

•  Usefultostudybecause:– Mostefficientfuel‐burningsystemswehave– Wasteproductscreatesomeoftheworld’smajorpollutionproblems

–  Interestingtechnologicalsolutionsexist

PowerStationBoilers•  Toextractthemaximumenergyfromthesolidfuel,bothfixedcarbonandtheVMmustbefullyburnt(notsimplesinceoneissolidandtheothergas)

•  Bothmustbeburntataboutthesamerate•  Purposeoftheplant:toproducesteam‐>thirdrequirementisforthebestpossibleheattransferfromtheburningfuelintothecirculatingwater

•  Finally:minimizeby‐productsandincludeamethodtodealwithunavoidablewaste:ashandfluegas

•  Aim:powerstationthatcandealsafelyandefficientlywithfuelswithverydifferentphysicalpropertiesandrangeofheatvalues

Coal‐firepowerstation

Threetypesofsolidfuelpower‐station

boiler

TheGrateboiler•  Fuelisinpiecesofafewmillimetersacross

•  Fedinfromahopperoronaconveyerbelt

•  Moveacrossthegrateinanupwardflowofair

•  Fixedcarbonburnsonthegrateandthevolatilematterinthespaceabove

•  Radiantheatfrombothreachesarrayoftubesthroughwhichthewatercirculates

•  Thehotgasesfromthecombustionreachanothersetoftubes

TheGrateboiler

•  Boilersofthistypearestillusedforcoal,butmainlyforbiofuelssuchaswoodchips,processeddomesticwastesetc.,notsuitableforpulverizedfuelboilers

•  Increasinglyreplacedwithcleanermoreefficientboilers

PulverizedFuelBoilers

•  Mostcommonboilertypeinpresent‐daycoal‐firedpowerstations

•  inuseformorethanhalfacentury

•  Cantransfer>90%oftheenergycontentofthecoaltothecirculatingwaterorsteam

•  Pulverizedfuel(PF):coalentersthefurnaceintheformofparticleslessthanabout100microns

•  Coaldustsweptinacontrolledflowofairtotheburnerjets•  Tinyparticles‐>thefixedcarbonburnscompletelyinashorttime‐>VMandfixedcarbonburntogetherinroughlythesamepartofthefurnace‐>efficiencyincreaseinheattransfer

PulverizedFuelBoilers

•  Shorttimethatfuelspendsinthefurnace‐>reducestheproductionofNOXandothercombustionproducts

•  Carefulcontroloftheair/fuelmixtureisrequired(notenoughair:unburntcharintheashorCOinthefluegases

toomuchair:promotesproductionofundesirableoxidesandreducestheefficiencybycarryingawaymoreheatinthefluegases)

•  On‐linemonitoring(controloffluegasflow,particlesize)•  DisadvantageofPFboilers:ashisfinedustthatwillbecarriedintotheatmospherewithoutpreventivemeasures

FluidizedBedBoilers•  Fluidizedbedcombustion(FBC)

–  offerssolutionstosomeofthepollutionproblemsofcoalcombustion

–  Possibilityofburningotherfuelscleanly•  1980s:firstplantsonstream•  Bytheendof2000:afew1000•  Essentialfeature:thicklayerofinertmaterial(sandorgravel(particlesizesof0.3‐2.0mm)onthebaseplate

•  Baseplatehassmallaperturesthroughwhichjetsofairareblown

•  Atacertainairspeed:thicknessofmaterialexpandstoadepthof>1mandstartstobehavelikeliquid

FluidizedBedBoilers•  Airflowfurtherincreased:formsbubblesrisingthroughthebed‐>particlesbouncearoundasiftheywereaboilingliquid

•  Fuelparticlesarefedintothebed•  Constantmotionandairflow‐>bothfixedcarbonandVMburnquicklyandheattheentirebed

•  Watertubesareburiedinthebedandorcontainmentwalls‐>excellentthermalcontact‐>goodheattransferdoesnotrequirethehightemperaturesofanopenfurnace

•  Bubblingfluidizedbedcombustion(BFBC):majorityofplants

Sowhyhasbituminouscoalbetterheatvaluethananthracite?

The relative proportion of fixed carbon and volatile matter generally indicative of the different ranks.

FBB:Othersystems•  Circulatingfluidizedbedcombustion(CFBC):

–  Developedalittlelater,butrapidgrowth(>1000since2000)–  Increasedairflowdrivesparticlesinthespaceabovethefluidizedbed‐>behavelikeahotgas

–  Circulatingsystem:constantlyreturnsparticlestothebed‐>hightemperatureismaintained‐>increaseoftimeforcombustion‐>widerrangeofcoalsandotherfluidscanbeused

•  Pressurizedfluidizedbedcombustion(PFBC):– Mostadvanced,canproduce250MW–  BasedontheBFBC,buthigherpressureinthefurnace(~10atm)‐>hotgasesfromthefurnacecanbeusedinagasturbineaswellasraisingsteamforthesteamturbine(combined‐cycle)

•  Pressurizedcirculatingsystemsunderdevelopment

FlueGases•  Quantitieswhichmightbereleasedintotheatmospherebyamodern660MWcoal‐firedpowerstationinonehour(figuresareapproximate):–  2500tonnesofNitrogen,80%oftheair,passunchangedthroughthewholesystembutheatingitaccountsforabouthalfoftheenergylossintheboiler

–  700tonnesofcarbondioxide‐>climate–  >150tonnesofsteam(moistureinthecoal,combustionproduct),notcondensingitaccountsfortheotherhalfoflostenergy,butfluegasesneedtostayhotiftheyaretorisethroughatallchimney

FlueGases•  Quantitieswhichmightbereleasedintotheatmospherebyamodern660MWcoal‐firedpowerstationinonehour(figuresareapproximate):–  AtonneofNOX:ThehigherthefurnacetemperaturethegreatertheproductionofNOX‐>acidrain,otherhealthdamages

–  1–20tonnesofsulphurdioxide,powerstationofthisexampleproducesabout4tonnesofSO2inonehourfromcoalwith1%sulphur‐>acidrain

–  10‐20tonnesofflyash(particulates)resultingfromburningpulverizedcoal‐>visibleasdirt,tinyparticlescandamagelungsandcontainpoisonousimpurities.

FlueGas

•  Therearetwowaysofdealingwiththesepollutants:removethemordon’tproducethem

FlueGasandFBC

•  Fluidizedbedboilers’approach:Don’tproducethem:– Sulphurcomponentsarereducedatsourcebyintroducinglimestone:SO2reactswithlimestone

–  ‐>calciumsulphate(canberemovedfromthebed)– NOXproductionreducedbykeepingbedtemperature<1000degC(2000degCinconventionalplant)

– Smallerquantitiesofparticulatesthaninpulverizedfuelplant

FlueGasRemovel•  ThefluegasesofFBCboilersarecleanedusingthesamemethodsasinPFplants

•  InthecaseofPFBCparticulateremovalisanintegralfeatureoftheplantsincethegasesmustbecleanedtoahighstandardbeforetheycanbeusedinagasturbine

FlueGasRemovel•  Particulateremoval:

•  bagfilters(methodofatraditionalhouseholdvacuumcleaner,inefficientforparticlesizesof<10microns)

•  cyclonefilters(particlesarethrownoutwardsfromthefastspinningair,inefficientforparticlesizesof<10microns)

•  Electrostaticprecipitation(fineparticlesacquireanelectricchargebypassingnearahigh‐voltagewireandarepulledside‐waysoutofthegasstreambyanelectricalfield,effectiveforverysmallparticles,butcapitalcostsconsiderablyhigher)

•  Combinationoftheabove

FlueGasDesulphurization

•  FGDusuallyinvolvesreactingtheSO2withfinelydividedlimestone(CaCO3,calciumcarbonate)

•  Usuallyaslurryorsprayorjetsofwaterareusedtobringthelimestoneintocontactwiththefluegas‐>insolublecalciumsulphateprecipitatesandcanberemoved

•  Involvedcosts:– Electrostaticprecipitation:5%tothecapitalcostofanewpowerstation

– FGDasmuch15%andusesenergy‐>reducesoverallefficiencyofplant‐>increasescostsperenergyunit

Whydoveryfewpowerplantsworld‐widereachthebestachievablereductionsofthepollutants?

•  Putintocontext:– 3000tonnes/hourofgasesleavetheboiler– Hotterthanthehottestdomesticoven(1000‐2000

oC)

– ParticulatelevelgreaterthanintheworsteverSaltLakeCitysmog

– aconcentrationofSO2a1000timesworsethandowntownL.A.onabadday

– Enoughmoisturetocauseittostartraininginthegasstreamifthetemperaturefallsbelowthatofamoderateoven

Whydoveryfewpowerplantsworld‐widereachthebestachievablereductionsofthepollutants?

•  Anycleaningsystem– mustbeabletohandlethishot,dirtycorrosivemassonacontinuousbasis

– Shouldremovemostofthepollutants,currentaims:~90%ofSO2and99%ofparticulates

– Shoulduseaslittleenergyaspossible– Shouldleaveenvironmentallyacceptableresidues– Andshouldbecheap

•  Thereisnosuchsystem

EmissionTrading•  Administrativeapproachusedtocontrolpollution,akacapandtrade

•  Howitworks:– Centralauthority(government,internationalbody)setsacapontheamountofapollutantthatcanbeemitted

– Companiesetc.areissuedemissionpermitsandarequiredtoholdanequivalentamountofallowancesorcredits

– Totalofallowancesmustnotexceedcap– Companiesthatneedtoincreasetheirallowancesmustbycreditsfromthecompanieswhopolluteless

EmissionTrading•  Effect:buyerispayingchargeforpollutant,sellerisbeingrewardedforreducingpollution

•  Intheory:thosewhocanreduceemissionsmostcheaplywilldoso‐>pollutionreductionatthelowestpossiblecoststosociety

•  Examples:GreenhousegasestradingprogramintheEU,SO2andNOXtradingprogramintheUS

•  Contrast:directemissiontaxes•  Pro:mostofthemoneystaysinthesystemandisspentonsustainableprojects,reducescosttocontrolacidrain(incaseofofSO2)

•  Contra:complexity,monitoring,enforcement,dispute,manipulation,toomanyemissioncredits

EmissionTrading

•  FinancialTimesin2007:“Carbonmarketscreateamuddle”and“…leavemuchroomforunverifiableinformation”

ComparisonofEmissionTradingwithruledbasedsystem

•  Europe:ruledbased•  US:sulfurdioxidetradingsysteminstitutedin1990

CleanCoalInitiative

•  "Coalisanabundantresourceintheworld...Itisimperativethatwefigureoutawaytousecoalascleanlyaspossible.”

Dr.StevenChu,SecretaryofEnergy

SenateConfirmationHearing

January13,2009

CleanCoalInitiative

•  FromtheDOEwebsite:””Cleancoaltechnology"describesanewgenerationofenergyprocessesthatsharplyreduceairemissionsandotherpollutantsfromcoal‐burningpowerplants.”

CleanCoalInitiative

•  FromtheDOEwebsite:"Inthelate1980sandearly1990s,theU.S.DepartmentofEnergyconductedajointprogramwithindustryandStateagenciestodemonstratethebestofthesenewtechnologiesatscaleslargeenoughforcompaniestomakecommercialdecisions.Morethan20ofthetechnologiestestedintheoriginalprogramachievedcommercialsuccess.”

CleanCoalInitiative•  FromtheDOEwebsite:"Theearlyprogram,however,wasfocusedontheenvironmentalchallengesofthetime‐primarilyconcernsovertheimpactofacidrainonforestsandwatersheds.Inthe21stcentury,additionalenvironmentalconcernshaveemerged‐thepotentialhealthimpactsoftraceemissionsofmercury,theeffectsofmicroscopicparticlesonpeoplewithrespiratoryproblems,andthepotentialglobalclimate‐alteringimpactofgreenhousegases.”

CleanCoalInitiative•  Therewillbegovernmentco‐financedprograms,e.gUtah:

CleanCoalInitiative•  ThreeprojectsattheUniversityofUtah:①  FundamentalsofMercuryOxidationinFlueGas‐

This$539,000project(DOEshare:$397,000):•  developknowledgeandmodelsneededbyutilityoperatorstomeetexpectedEPAmercuryregulations

•  focusonunderstandingmercuryoxidationreactionchemistryincludingtheeffectsofchlorine,nitrogenoxide,sulfurdioxide,andashparticlereactions.

②  MaterialsforPowerPlantSensors(DOEup$608k):•  developnovelmicroscalegassensingdevicessuitableforapplicationinexhaustgasstreamsofpowerplants.

•  devicesthatcantoleratetheseconditionswhilestillaccuratelysensingverysmalllevelsofgasessuchascarbondioxideandnitrogenoxides.

CleanCoalInitiative•  ThreeprojectsattheUniversityofUtah:③  Corrosion‐ResistantCoatingsforPowerPlants–

(DOEshare:$200,000):•  developacommericiallyviablecoatingtechnology

basedonnanocrystallineintermetallicmaterialsforadvancedcoal‐firedpowergenerationsystemsforwhichcorrosionresistanceandcreepstrengthathightemperaturesarecritical