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ACOMPARISONSTUDYOFACFBANDPCFBASHCHARACTERISTICS

FosterWheelerDeνelopmentCorporation,12PeachTreeHillRoad,
Livingston,NJ07039,USA

WesternResearchInstitute,,Laramie,WY,82070,USAAbstract:Theadventoffluidizedbedcombustiontechnologyhasprovidedavenues
forenvironmentalissues-
BedCombustion(PCFB)technologyintheveryne缸future,thereisa
needtounderstandthesimilaritiesanddi旺ereIleesin由echaracteristicsofsolid by-products from the conventional Atmospheric Circulating
FluidizedBedCombustion(ACFB)
ACFBresidues,themaincomponentsinPCFBresidues缸efrom:
-theinorganicconstituentsinthecoalandincorporatedsediments(Al,
Fe,andSi),
-theinorganicconstituentsinthecoalandincorporatedsediments(Al,
Fe,andSi),
ve巧'highsulfurdonotnorma11yfindacceptanceinpulverizedcoa1(.)
,ifthesulfurcontentislarge,thennecess缸ysorbenthastobeaddedtocaptureS02insolidform.
CFBboilersgeneratetwomajorwastestreams,flyashandbottomash,
whichareamixtureoffuelash,unbumedcarbonresidues,'
,inadditiontothefueland sorbentcharacteristics,
ofca1ciumasanoxideandasasu1fate,;theseby-products町ecomposedprimarilyoffuelashconstituents,,the
,dueto由eeffectofsorbentontheoverallashchemistry.
Pressurizedfluidizedbedcombustion(PFBC)representsoneofthe
mostpromisingemergingCleanCoa1Technologies(CCT).CirculatingPFBCtechnologyisbeingdemonstratedatthepilot-sca1eatFosterWheelerEnergiaOyinKarhula,
(WRI)hascompletedathree-ye町projectundersponsorshipoftheElectricPowerResearchInstitute(EPRI),FosterWheelerEnergyInternational,Inc.,(DOE)FederalEnergyTechnologyCenterσ卫TC)thataddressedashusemarketsand
-volumeuseapplications.
TheEnvironmentalProtectionAgency(EPA)madeafmalrule由at
effectiveSeptember2,1993,fourlarge-volumefossilfuelcombustion
(FFC)wastestreamsfromelec位icutilitypowerplants缸eexemptedfromRCRA(ResourceConservationandRecoveryActof1976)SubtitleCforhazardouswasteregulations(FederalRegister1993).τbewastes位eams
includeflyash,bottomash,boilerslags,andfluegasdesulfurization
(FGD),thefluidizedbedcombustionσBC),extensivestudieshavebeen
conductedontheACFBandPCFBashes(Young,1996;Connand
Sellakumar,1997,Conn,Wu,andSellakumar1997;Bland,1998).
Inthispaper,areviewoftheACFBandPPCFBprocesses,key
唱'A
p,‘飞hysicalandchemicalproperties缸。,thecurrent
‘./、
andpotentialusesoftheby-
Influidizedbedcombustors,limestoneisaddedinthebedforsulfur
(Koskinenetal.,1993).Numerousstudieshaveconfrrmedthatatatmosphericconditions,thefirst
stepinthesulfurcaptureprocessisthecalcinationofCaC03.
CaC03 (s) → CaO (s) + C02 (g)
Thecalcinationreactionproceedssignificantlyfasterthanthenextstepthatisthesulfurcapturestep,calledthesulfationreaction.
CaO(s)+S02(g)+1/202(g)→ CaS04(s)
(2)
Underpressurizedfluidizedbedcombustionconditions,thehighpartialpressureofC02thatexistsbyvirtueofthehighcombustorpressure,,thedesulfurizationreactioninpressurizedconditionsisviathe
"d让ect"su1fationofCaC03.
CaC03(s)+S02(g)+1/202(g)→CaS04(s)+C02(g)(3)
toS03(g),andthefollowingreactionisalsopossible(Hajaligoletal.
1988):
CaC03(s) + S03(g) @ CaS04 (s) + C02(g)(3a)
,accordingtoLygnfeltand
Leckner(1989)significant缸nountsofS02maybereleasedfromthe
sorbentinthepressurizedfluidizedbedcombustionifconditionsbecomereducinginthereactor.
Insumm町y,itcanbeconcludedthatunderPCFBconditions:
-thesulfationofsorbentstakesplacebyadirectreactionbetween
CaC03andSOx..
-thesulfationrateincreaseswithtemperatureand
-thetotalpressureinthecombustordoesnotlimit由esulfurcapturebythesorbent.
-τbesurfaces位uctureoftheashparticles仕omthePCFBprocessislikelytobedifferentfromthatoftheashfromtheACFBprocess.
CHARCTERIZATIONOFACFBASH
'hereisageneraldecreaseinCaOandS03with
,alumina,-raydiffraction(XRD)'hedataconfirmtheobservation由attheashes缸ecomposedprincipallyofanhydrite[CaS04],lime[CaO],quartz[Si02],andassociatedoxidesof让on,magnesium,anddehydroxylatedclaysoriginatingfromthefuelash
components.
PhysicalProperties:Thegeneralphysicalpropertiesoftheasheswerealsodete口nined,includingpour,巳dandpackedbulkdensities,spec近icgravity,particlesizedistribution,'heflyashesallwererelativelyfinewithgreater由an80%passinga200-meshscreen(74mm).Asaresult,theseashescanreadilybemadeintocement-(lowSfuel)to916(highSfuel)kglm3;thecompactedbulkdensityoftheflyasheswereslightlyhigherandrangedfrom840(lowS)to1167仙ighS)kg/
(lowS)(highS)fortheflyashes.
CHARACTERIZATIONOFPCFBASH
ThestudyofPFBCashuseoptionshasincludedtwodifferentashes:
combustionofhigh-sulfurlllinoisBasincoalwithalimestonesorbent.
(Bland,1998).
GeneralChemistry:Withtheexceptionofrelativelyhighmineralcarbon,thechemistryofthePCFBashesistypicalofashes企omACFBoflow-sulfurandhigh-sulfurcoalsusinglimestoneanddolomitesorbents.
PhaseanalysesoftheashesbyX-raydi岱actionshowthatthePCFBashesarecomposedprincipallyofanhydrite(CaS04),calcite(CaC03),coalashoxides,(CaO)inthePFBCashesisdistinctlydifferentfromAFBCashes,,inPFBCsystems,
limeandhighcarbonates(calcite)(LOI)'heLOIinthePFBC
%andtheorganiccarboncontents缸elessthan2%.ηlefreelime(CaO)contentofthePFBCasheswasdeterminedbyASτMC-25tobeinthe
%.Them司jorityofthelimeappearstostillbe
carbonatedintheformofCaC03.
PhysicalProperties:Thegeneralphysicalpropertiesoftheasheswerealsodetermined,includingbulkdensities,specificgravity,and
densitiesof795to948kg/
DISCUSSION
AshCharacteristics:(TCLP)(EPACFRPart241).Theashleachatedatasubstantiatethatnone
oftheleachatesgeneratedfromtheACFBandPFBCashesexceededthe
,-firedpowersystemsarea1readycategorizedasno由azardousandhavebeengivenanexc1usionfromtheseRCRAreq山rements(Table2).
AshUtilizationStudies::
construction applications: cement substitute,concrete blockproduction,brick production,soilstabilizer,roadbase/subbasematerials,structuralfillmaterials,andsynthetic
aggregates;
-agriculturalapplications:limingandsoilamendment;
-wastestabilization:acidicwastestabilizerandsludgestabilizer.
ηlekeyfuelandsorbentproperties,whichcaninfluenceash
characteristics,缸'esulfurcontent,ashcontent,andashcomposition
inc1udingtheformofCa(CaC03orCaO).Otherimportantfuelpropertiesinfluencingashcharacteristics缸''hesepropertieswillimpactonhowthesemineralswill
exittheCFBintheflyorbottomashstream,whichcanhaveasignificant

1980s.(,Kilgouretal,1991,,
;;Bland,1998;Bland,1999).DetailsonPCFBashusehasbeenreportedsince1993(;Bland,1998).Asumm缸yofthecurrentknow-howisgivenbelow.
AshUseinConstructionApplications:ACFBandPCFBflyashescannotbec1assifiedasClassForC,becauseoflowFAS(ferricoxide,alumina,andsilica)andhighS03content(Table1).EventhoughCFBflyashmaynotquali句rasaPortlandcementadmixture,,
,thecarboncontentoftheflyashmustbelimitedtolessthan5%,sincesegregationcanoccur