11concentrationsofheavymetalareattheirlowestinthefirstcollectorchamberandhighestinthelastchamber.Theconcentrationofcadmiuminflyashusedasfertilisercanbereducedbyasmuchas70%byapplyingelectrostaticprecipitationfractionation.Theremovalofotherheavymetalsisnotasecientasthatofcadmium.Theresultsshowthatelectrostaticprecipitationisanadequatemethodintheasincreasingamountsofbiofuelsareused.Woodandpeatashcanbespreadontoforestlandsorarablelandasfertil-ablylower,beingabout6%in1997.materialsincreasetheash-containedCdconcentrations,andthenscreenoutthecadmiumcontainingmaterials.entsinash,onthesolubilityofnutrientsintheashandthesoil,andonsoilproperties,e.g.acidityandnutrientcon-centrations(Oravaetal.,2004；Silfverberg,1996).Table2showstheheavymetalconcentrationsoffourashtypes.Manysubstancescontainedinashareinextremelypoorlysolubleforms.Astheheavymetals(e.g.cadmium,*Correspondingauthor.E-mailaddresses:hanne.oravamikkeliamk.fi(O.Hanne),timo.nord-manoulu.fi(N.Timo),hannu.kuopanporttimikkeliamk.fi(K.Hannu).MineralsEngineering19(2006)iserorassoilimprovementmaterial,andwiththepurposeofaddingcalciumtothesoil.Theuseofashhasbeencon-strainedbyfactorssuchasitsdustcontentandheavymetalconcentrations；thelatterhavinginmanycasesexceededthemaximumpermittedlevelsimposedinFinlandonsoilimprovementsubstances(Table1).In2001,theutilisationrateofcoalash(84%)wascon-siderablyhigherthanthatofpeatandmixedfuelash(43%).Woodflyashutilisationrateshavebeenconsider-Smallamountsofflyashareusedasafertiliserbothinagricultureandinforestry.Generally,variousashtypesaremoresuitableasasoilimprovementmaterialthanfertilis-ersinagriculturebecausetheamountsofsolubleplantnutrientsinasharefairlylow.Peatashisusedmainlyasaphosphatefertiliserandwoodashinlimingofmineralsoilsandasabasicandsupportfertiliserinthegrowingofcerealcrops.Thelimingandfertilisereectsofashinthesoildependontheconcentrationsofcalciumandnutri-fractionatingofflyashtobeusedasafertiliserorsoilamendment.C2112006ElsevierLtd.Allrightsreserved.Keywords:Electrostaticseparation；Sizing；Classification；Fluedusts；Recycling1.IntroductionHeating-energyplantsandpowerplantsinFinlandgen-erateapprox.400,000tonnesofashofbiofueloriginperyear.TheamountsofsuchashwillincreaseinthefutureTheextractionofheavymetalsfromflyashcouldenableitsmoreecientutilisation.Currently,itappearsthatmanipulatingthepowerplantfuelqualityistheonlymethodavailableforthispurpose.Thismeansthatwemustknowthecombustiblefuelsexactconsistency,andwhichIncreasetheutilisationofflyashOravaHannea,*,NordmanTimoaYTIResearchCentre,MikkeliPolytechnic,bUniversityofOulu,P.O.Box4300,Received28April2006；AvailableonlineAbstractThebasicideainthisstudyistolookintothepossibilitiesofreducingtrostaticprecipitation.Theutilisationofflyashasfertiliserishamperedvariable.Flyashfractionationexperimentsweredoneusingelectrostatic0892-6875/$-seefrontmatterC2112006ElsevierLtd.Allrightsreserved.doi:10.1016/j.mineng.2006.07.002withelectrostaticprecipitationb,KuopanporttiHannuaP.O.Box181,FI-50101Mikkeli,FinlandFI-90014Oulunyliopisto,Finlandaccepted7July2006September2006theheavymetalconcentrationsofflyashbymeansofelec-byitshighconcentrationsofheavymetals,whicharehighlyprecipitatorsatfourpowerplants.Basedontheresults,theThisarticleisalsoavailableonlineat:/locate/mineng15961602lead,nickel)inashareinverypoorlysolubleforms,itcanTable2Heavymetalconcentrationsofvariousashtypes(Palola,1998)Heavymetals(mg/kg)CoalashPeatashWoodashBarkashArsenic(As)2.320022000.260728Cadmium(Cd)0.012500.0580.440420Chrome(Cr)3.6740015250152504081Copper(Cu)303000204001530057144Mercury(Hg)0.01800.00110.0210.0120.4Nickel(Ni)1.880015200202503652Lead(Pb)3.11800515015100053140Zinc(Zn)1413,000106001510,00011005100Table1MaximumpermittedconcentrationsofheavymetalsinsoilimprovementmaterialsandinflyashfrompowerplantA(Orava,2003)ElementPowerplantA:flyash(mg/kg)Maximumpermittedconcentration(mg/kg)Year2002Year2001Year1999Mercury(Hg)2.50.312.0Cadmium(Cd)2.695.056.33.0Arsenic(As)18.3419.733550Nickel(Ni)100Lead(Pb)56.59106.552.7150Copper(Cu)86.7290.1178600Zinc(Zn)189.7376.97061500O.Hanneetal./MineralsEngineerinbeassumedthatashfertilisationwillnotresultinsignifi-cantheavy-metalimpacts,e.g.inwatersystems,withinashortperiodoftimefollowingfertilisation.Inthelongrun,harmfulheavymetalsmay,however,bereleasedfromashinsolubleformsandbetherebytranslocatedintothevegetation(Nieminen,2003).Thelimingeectofashlow-ersthesolubilityofheavymetalsinthesoil.Ashmayatfirstraisethecadmiumconcentrationintreestands,butoncetreegrowthhasimprovedtheconcentrationsoftraceelementsandheavymetalsmayfallevenbelowtheinitiallevel.TheriseintheCdconcentrationinsomeplantspeciescanlastforalongtime(Moilanen,2003).Cadmiumiscon-sideredtobethemostharmfulofallheavymetalsbecauseitremainsinthesoil,itbecomesenrichedinfoodchains,anditistoxictoorganisms.Electrostaticprecipitation(Fig.1)iscurrentlythemostcommonmethodusedinseparatingthesolidmatterfrompowerplantfluegases.Theadvantagesofelectrostaticpre-cipitationincludehighcollectioneciency(ashighas99.9%)anditssuitabilityfordealingwithparticlesofdier-entsizes(evenparticlesizesbelow1lm)andvariablefluegasvolumes.Itsfurtheradvantagesarelongservicelife,goodoperationalreliability,andlowoperatingandmainte-nancecosts(Walsh,1997；Immonen,1987).Thefunctioningoftheelectrostaticprecipitatorissignif-icantlydependentonthepropertiesoftheflyashtobecol-lected.Theamountandsizedistributionoftheparticlestoberemovedhaveasignificantimpactonthefunctioningoftheelectrostaticprecipitator.Althoughthecollectione-ciencyofelectrostaticprecipitatorismoreorlessconstantirrespectiveoftheparticlemass,theeectivemigrationvelocityislowerinthecaseofsmallparticles.Duetothedierentchargingpropertiesoftheparticles,thecollectioneciencyoftheparticlesvariesasafunctionofparticlesize.Themostdicultparticlesizefromthepointofviewofseparationis0.20.5lm(Nykanen,1993；Kouvo,2003).Theconcentrationsofheavymetalinashcanbereducedbyfractionationofthefinestashparticlesfromfluegasesbymeansofmulti-chamberelectrostaticprecipitators.Thefractionatingpropertiesoftheprecipitatorcanbeinfluencedbyactionssuchasrestrictingandpulsatingthecurrent.OurresearchresultshaveshownthatheavymetalsFig.1.AlstomFinlandOyselectrostaticprecipitator(Jalovaaraetal.,2003).g19(2006)159616021597areconcentratedinfineashparticles(Oravaetal.,2004；Orava,2003).AccordingtotheresultsofThunandKorho-nen(1999),the3-fieldelectrostaticprecipitatorwasstopped,dependingontheoperatingconditions,8495%oftheoverallamountofashinthefirstfield,415%inthesecondfield,andapprox.1%inthelastfield.Thecad-miumconcentrationofashcanbereducedatleastby1525%bymeansoffractionatingtheashusingelectrostaticprecipitators(Oravaetal.,2004；ThunandKorhonen,1999).Dependingontheboilerinquestion,ashesfrombarkfuelledandwoodchipfuelledpowerplants(grateboilers)aredividedintoweightpercentagecategoriesasfollows:bottomash7090%,cycloneflyash1030%,electrostaticprecipitatorflyash28%anddustemissions0.13.0%(AgarwalandAgarwal,1999).Industcombustionandflu-idized-bedcombustion,theshareofflyashgenerationis80100%.Asmuchas7590%oftheheavymetals(CdandZn)arecontainedinthefineparticlefractionoftheflyash,whichisseparatedbyelectrostaticprecipitators(Dahletal.).Fig.2setsoutthezinc,leadandcadmiumcontents(mg/kgandm-%)inbottomash,cycloneashandelectrostaticprecipitatorflyash.Basedonthefigure,ashcanthusbereducedtobelowthemaximumpermittedconcentrations.2.MaterialsandmethodsThefractionatingtrialswithflyashwereperformedatfourpowerplants(A,B,CandD).Theelectrostaticpre-cipitatorswereoperatedatthepowerplantsatdierentvoltagelevelsandsamplesweretakenfromtheESPsvar-iousfields.Allthesamplestakenfromtheelectrostaticpre-cipitatorsweretakenfromtheashfeederslocatedundertheelectrostaticprecipitatorsbeforetheashwasfedintothesilo.ThesampleswereanalysedforthepresenceofPb,Cu,Zn,Ni,AsandCdusingthegraphitemethod1598O.Hanneetal./MineralsEngineering19(2006)15961602itmaybestated,forexample,thatelectrostaticprecipitatorflyashhasahigherCdcontentthancycloneash,whichispartlyduetothefactthat,comparedtocyclones,electro-staticprecipitatorsseparatesmallerparticlesthatcontainthemajorityofheavymetals.Inthiscase,theportionoftheflyashthatissuitableforuseasafertiliser,intermsofitsconsistency,remainsatthecyclone(ObernbergerandBiedermann,1997).Theelectrostaticprecipitatorcanmoreeectivelyfrac-tionateflyashthanthetraditionalmethodswhenamechanicalclassifier(cyclone)isconnectedbeforetheashreachestheprecipitator.Fig.3showsabasiclayoutdraw-ingofapowerplantfiredbyusingbiofuelsandwhichisprovidedwithamulti-cyclonebeforetheelectrostaticpre-cipitator.Asmuchas7590%oftheheavymetals(CdandZn)containedinflyashareboundtothefineflyashfractionseparatedbytheelectrostaticprecipitator(Dahletal.,2002).Properlydesignedandadjustedelectrostaticprecipita-Fig.2.Heavymetalconcentrationsandtheirdivisionasbulkpercentagefiguresinbottomash,cycloneflyashandfilterflyash(AgarwalandAgarwal).tionisinprinciple,capableofseparatingthatfractionofthefluegases,whichcontainsthegreatestamountofheavymetalsbutonlyafractionoftheoverallamountofash.TheheavymetalconcentrationsinthemainpartoftheFig.3.Theashfractionsproducedbyabiofuel-firedandparticlesizedeterminationwasdoneusingaMalverndevice.PowerplantAusespeat,forestchipandoilandtheby-productsofthemechanicalwoodprocessingindustryasitsfuels.Theboilercapacityavailabletothepowerplantis150MW.Thefractionatingtrialswereperformedwiththepowerplantscurrent3-fieldelectrostaticprecipitator.PowerplantBusestwoboilers,oneaPyroflowcirculat-ingfluidized-bedboiler(capacity55MW)andtheotherafluidized-bedboiler(capacity42MW).Thetrialswerecar-riedoutusingthefluidized-bedboiler.Thepowerplantsprincipalfuelismilledpeatwithwoodfuels,sootandalu-miniumoxidemixedinwithit.Theflyashfrombothboil-ersisconveyedvia2-fieldelectrostaticprecipitatorstoacommonashsilo.PowerplantCisequippedwithtwopowerplantboilers.Boiler1isafluidized-bedboilerwithafuelcapacityof267MW.Boiler2isaPyroflowcirculatingfluidized-bedboilerwithafuelcapacityof315MW.Thetestswereper-formedusingthePyroflowcirculatingfluidized-bedboiler.Thefuelsusedatthepowerplantweremainlymilledpeatandvariouswoodfuels.Bothboilersareequippedwith3-fieldelectrostaticprecipitatorsfromwhichtheboilersflyashisblownpneumaticallytoacommonashsilo.TheelectricalpowergeneratedbypowerplantDsfluid-ized-bedboilerplantis77MWanditsheatingcapacityis246MW.Thefuelsusedinthefluidized-bedboilerarepowerplant(AgarwalandAgarwal).mainlymilledpeatandwoodwaste.Theflyashissepa-ratedfromthefluegasesbymeansofa3-fieldelectrostaticprecipitator.3.ResultsDuringtrialswithpowerplantAselectrostaticprecipi-tator(trials17)thefuelusedwascomposed49%peatand51%woodfuels.Theashfunnelsoffields13oftheelec-trostaticprecipitatorweresampledandanalysed(Fig.4).Onthebasisoftheresults,theCdconcentrationwasatitslowestinfield1oftheelectrostaticprecipitatorandatitshighestinfield3.Thisisduetothebiggerflyashparticlesaccumulatinginfield1andfield3containingashwiththeparticlesinthefirstfieldoftheelectrostaticprecipitator.2.833.23.4024681012CBO-ratioCdmg/kgFig.5.TheeectoftheCBOratiofromelectrostaticprecipitatorfield1onflyashCdconcentrations(mg/kg)duringtrialrunswithpeatfuel.2.83.84253035404550VoltagekVCdmg/kgFig.6.Theeectofthefiltervoltagelevel(kV)fromelectrostaticO.Hanneetal./MineralsEngineering19(2006)159616021599smallestparticles.TheCdconcentrationinfield1varieswithintherangeof2.23.6mg/kgandinthelastfieldwithintherangeof7.212.4mg/kg.ConcentrationsareaectedbypropertiessuchastheESPvoltage,fuelquality,andfluegasflowrate.Inalmosteveryelectrostaticprecip-itatorsfield1theCdconcentrationisbelowthepermittedmaximumlimit(3.0mg/kg)setdownforashintendedforfertiliseruse.Duringtrialruns,theelectrostaticprecipitatorfieldsCBOratio(cycleblockinoperation)wascontrolledwithintherange012.Thevalue0meansthatallthehalf-cyclesofthefieldinquestionarecurrentlyactive,andforexam-ple,thevalue2meansthatonlyathirdofthehalf-cyclesareactive.TherebytheCBOvaluedeclareshowmanysequentialhalf-cyclesareclosed,thatis,howoftenthesep-aratorssupplycurrentispulsated.Inthepresentresearch,theCBOvaluewascontrolledbytheMicro-Kraftcontrol-lerwhosemaintaskistokeepthevoltageneartothebreakdownvoltage.Themostimportantthingistobeabletoinfluenceandchangethepropertiesoffield1intheelectrostaticprecipi-tator.Thefirstfieldenablestheproductionofflyashwithheavymetalconcentrationlevelsthatmakeitsuitableasafertiliser,forexample.Fig.5showshowtheelectrostaticprecipitatorsCBOratiocontrolforfield1aectstheflyashCdconcentrationlevels.Amongothert