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11concentrationsofheavymetalareattheirlowestinthefirstcollectorchamberandhighestinthelastchamber.Theconcentrationofcadmiuminflyashusedasfertilisercanbereducedbyasmuchas70byapplyingelectrostaticprecipitationfractionation.Theremovalofotherheavymetalsisnotasefficientasthatofcadmium.Theresultsshowthatelectrostaticprecipitationisanadequatemethodintheasincreasingamountsofbiofuelsareused.Woodandpeatashcanbespreadontoforestlandsorarablelandasfertilablylower,beingabout6in1997.materialsincreasetheashcontainedCdconcentrations,andthenscreenoutthecadmiumcontainingmaterials.entsinash,onthesolubilityofnutrientsintheashandthesoil,andonsoilproperties,e.g.acidityandnutrientconcentrationsOravaetal.,2004Silfverberg,1996.Table2showstheheavymetalconcentrationsoffourashtypes.Manysubstancescontainedinashareinextremelypoorlysolubleforms.Astheheavymetalse.g.cadmium,Correspondingauthor.Emailaddresseshanne.oravamikkeliamk.fiO.Hanne,timo.nordmanoulu.fiN.Timo,hannu.kuopanporttimikkeliamk.fiK.Hannu.MineralsEngineering192006iserorassoilimprovementmaterial,andwiththepurposeofaddingcalciumtothesoil.TheuseofashhasbeenconstrainedbyfactorssuchasitsdustcontentandheavymetalconcentrationsthelatterhavinginmanycasesexceededthemaximumpermittedlevelsimposedinFinlandonsoilimprovementsubstancesTable1.In2001,theutilisationrateofcoalash84wasconsiderablyhigherthanthatofpeatandmixedfuelash43.WoodflyashutilisationrateshavebeenconsiderSmallamountsofflyashareusedasafertiliserbothinagricultureandinforestry.Generally,variousashtypesaremoresuitableasasoilimprovementmaterialthanfertilisersinagriculturebecausetheamountsofsolubleplantnutrientsinasharefairlylow.Peatashisusedmainlyasaphosphatefertiliserandwoodashinlimingofmineralsoilsandasabasicandsupportfertiliserinthegrowingofcerealcrops.Thelimingandfertilisereffectsofashinthesoildependontheconcentrationsofcalciumandnutrifractionatingofflyashtobeusedasafertiliserorsoilamendment.C2112006ElsevierLtd.Allrightsreserved.KeywordsElectrostaticseparationSizingClassificationFluedustsRecycling1.IntroductionHeatingenergyplantsandpowerplantsinFinlandgenerateapprox.400,000tonnesofashofbiofueloriginperyear.TheamountsofsuchashwillincreaseinthefutureTheextractionofheavymetalsfromflyashcouldenableitsmoreefficientutilisation.Currently,itappearsthatmanipulatingthepowerplantfuelqualityistheonlymethodavailableforthispurpose.Thismeansthatwemustknowthecombustiblefuelsexactconsistency,andwhichIncreasetheutilisationofflyashOravaHannea,,NordmanTimoaYTIResearchCentre,MikkeliPolytechnic,bUniversityofOulu,P.O.Box4300,Received28April2006AvailableonlineAbstractThebasicideainthisstudyistolookintothepossibilitiesofreducingtrostaticprecipitation.Theutilisationofflyashasfertiliserishamperedvariable.Flyashfractionationexperimentsweredoneusingelectrostatic08926875/seefrontmatterC2112006ElsevierLtd.Allrightsreserved.doi10.1016/j.mineng.2006.07.002withelectrostaticprecipitationb,KuopanporttiHannuaP.O.Box181,FI50101Mikkeli,FinlandFI90014Oulunyliopisto,Finlandaccepted7July2006September2006theheavymetalconcentrationsofflyashbymeansofelecbyitshighconcentrationsofheavymetals,whicharehighlyprecipitatorsatfourpowerplants.Basedontheresults,theThisarticleisalsoavailableonlineatwww.elsevier.com/locate/mineng1596–1602lead,nickelinashareinverypoorlysolubleforms,itcanTable2HeavymetalconcentrationsofvariousashtypesPalola,1998Heavymetalsmg/kgCoalashPeatashWoodashBarkashArsenicAs2.3–2002–2000.2–607–28CadmiumCd0.01–2500.05–80.4–404–20ChromeCr3.6–740015–25015–25040–81CopperCu30–300020–40015–30057–144MercuryHg0.01–800.001–10.02–10.012–0.4NickelNi1.8–80015–20020–25036–52LeadPb3.1–18005–15015–100053–140ZincZn14–13,00010–60015–10,0001100–5100Table1MaximumpermittedconcentrationsofheavymetalsinsoilimprovementmaterialsandinflyashfrompowerplantAOrava,2003ElementPowerplantAflyashmg/kgMaximumpermittedconcentrationmg/kgYear2002Year2001Year1999MercuryHg2.50.31–2.0CadmiumCd2.695.056.33.0ArsenicAs18.3419.733550NickelNi––100LeadPb56.59106.552.7150CopperCu86.7290.1178600ZincZn189.7376.97061500O.Hanneetal./MineralsEngineerinbeassumedthatashfertilisationwillnotresultinsignificantheavymetalimpacts,e.g.inwatersystems,withinashortperiodoftimefollowingfertilisation.Inthelongrun,harmfulheavymetalsmay,however,bereleasedfromashinsolubleformsandbetherebytranslocatedintothevegetationNieminen,2003.Thelimingeffectofashlowersthesolubilityofheavymetalsinthesoil.Ashmayatfirstraisethecadmiumconcentrationintreestands,butoncetreegrowthhasimprovedtheconcentrationsoftraceelementsandheavymetalsmayfallevenbelowtheinitiallevel.TheriseintheCdconcentrationinsomeplantspeciescanlastforalongtimeMoilanen,2003.Cadmiumisconsideredtobethemostharmfulofallheavymetalsbecauseitremainsinthesoil,itbecomesenrichedinfoodchains,anditistoxictoorganisms.ElectrostaticprecipitationFig.1iscurrentlythemostcommonmethodusedinseparatingthesolidmatterfrompowerplantfluegases.Theadvantagesofelectrostaticprecipitationincludehighcollectionefficiencyashighas99.9anditssuitabilityfordealingwithparticlesofdifferentsizesevenparticlesizesbelow1lmandvariablefluegasvolumes.Itsfurtheradvantagesarelongservicelife,goodoperationalreliability,andlowoperatingandmaintenancecostsWalsh,1997Immonen,1987.Thefunctioningoftheelectrostaticprecipitatorissignificantlydependentonthepropertiesoftheflyashtobecollected.Theamountandsizedistributionoftheparticlestoberemovedhaveasignificantimpactonthefunctioningoftheelectrostaticprecipitator.Althoughthecollectionefficiencyofelectrostaticprecipitatorismoreorlessconstantirrespectiveoftheparticlemass,theeffectivemigrationvelocityislowerinthecaseofsmallparticles.Duetothedifferentchargingpropertiesoftheparticles,thecollectionefficiencyoftheparticlesvariesasafunctionofparticlesize.Themostdifficultparticlesizefromthepointofviewofseparationis0.2–0.5lmNyka¨nen,1993Kouvo,2003.Theconcentrationsofheavymetalinashcanbereducedbyfractionationofthefinestashparticlesfromfluegasesbymeansofmultichamberelectrostaticprecipitators.Thefractionatingpropertiesoftheprecipitatorcanbeinfluencedbyactionssuchasrestrictingandpulsatingthecurrent.OurresearchresultshaveshownthatheavymetalsFig.1.AlstomFinlandOyselectrostaticprecipitatorJalovaaraetal.,2003.g1920061596–16021597areconcentratedinfineashparticlesOravaetal.,2004Orava,2003.AccordingtotheresultsofThunandKorhonen1999,the3fieldelectrostaticprecipitatorwasstopped,dependingontheoperatingconditions,84–95oftheoverallamountofashinthefirstfield,4–15inthesecondfield,andapprox.1inthelastfield.Thecadmiumconcentrationofashcanbereducedatleastby15–25bymeansoffractionatingtheashusingelectrostaticprecipitatorsOravaetal.,2004ThunandKorhonen,1999.Dependingontheboilerinquestion,ashesfrombarkfuelledandwoodchipfuelledpowerplantsgrateboilersaredividedintoweightpercentagecategoriesasfollowsbottomash70–90,cycloneflyash10–30,electrostaticprecipitatorflyash2–8anddustemissions0.1–3.0AgarwalandAgarwal,1999.Industcombustionandfluidizedbedcombustion,theshareofflyashgenerationis80–100.Asmuchas75–90oftheheavymetalsCdandZnarecontainedinthefineparticlefractionoftheflyash,whichisseparatedbyelectrostaticprecipitatorsDahletal..Fig.2setsoutthezinc,leadandcadmiumcontentsmg/kgandminbottomash,cycloneashandelectrostaticprecipitatorflyash.Basedonthefigure,ashcanthusbereducedtobelowthemaximumpermittedconcentrations.2.MaterialsandmethodsThefractionatingtrialswithflyashwereperformedatfourpowerplantsA,B,CandD.TheelectrostaticprecipitatorswereoperatedatthepowerplantsatdifferentvoltagelevelsandsamplesweretakenfromtheESPsvariousfields.Allthesamplestakenfromtheelectrostaticprecipitatorsweretakenfromtheashfeederslocatedundertheelectrostaticprecipitatorsbeforetheashwasfedintothesilo.ThesampleswereanalysedforthepresenceofPb,Cu,Zn,Ni,AsandCdusingthegraphitemethod1598O.Hanneetal./MineralsEngineering1920061596–1602itmaybestated,forexample,thatelectrostaticprecipitatorflyashhasahigherCdcontentthancycloneash,whichispartlyduetothefactthat,comparedtocyclones,electrostaticprecipitatorsseparatesmallerparticlesthatcontainthemajorityofheavymetals.Inthiscase,theportionoftheflyashthatissuitableforuseasafertiliser,intermsofitsconsistency,remainsatthecycloneObernbergerandBiedermann,1997.Theelectrostaticprecipitatorcanmoreeffectivelyfractionateflyashthanthetraditionalmethodswhenamechanicalclassifiercycloneisconnectedbeforetheashreachestheprecipitator.Fig.3showsabasiclayoutdrawingofapowerplantfiredbyusingbiofuelsandwhichisprovidedwithamulticyclonebeforetheelectrostaticprecipitator.Asmuchas75–90oftheheavymetalsCdandZncontainedinflyashareboundtothefineflyashfractionseparatedbytheelectrostaticprecipitatorDahletal.,2002.ProperlydesignedandadjustedelectrostaticprecipitaFig.2.Heavymetalconcentrationsandtheirdivisionasbulkpercentagefiguresinbottomash,cycloneflyashandfilterflyashAgarwalandAgarwal.tionisinprinciple,capableofseparatingthatfractionofthefluegases,whichcontainsthegreatestamountofheavymetalsbutonlyafractionoftheoverallamountofash.TheheavymetalconcentrationsinthemainpartoftheFig.3.TheashfractionsproducedbyabiofuelfiredandparticlesizedeterminationwasdoneusingaMalverndevice.PowerplantAusespeat,forestchipandoilandthebyproductsofthemechanicalwoodprocessingindustryasitsfuels.Theboilercapacityavailabletothepowerplantis150MW.Thefractionatingtrialswereperformedwiththepowerplantscurrent3fieldelectrostaticprecipitator.PowerplantBusestwoboilers,oneaPyroflowcirculatingfluidizedbedboilercapacity55MWandtheotherafluidizedbedboilercapacity42MW.Thetrialswerecarriedoutusingthefluidizedbedboiler.Thepowerplantsprincipalfuelismilledpeatwithwoodfuels,sootandaluminiumoxidemixedinwithit.Theflyashfrombothboilersisconveyedvia2fieldelectrostaticprecipitatorstoacommonashsilo.PowerplantCisequippedwithtwopowerplantboilers.Boiler1isafluidizedbedboilerwithafuelcapacityof267MW.Boiler2isaPyroflowcirculatingfluidizedbedboilerwithafuelcapacityof315MW.ThetestswereperformedusingthePyroflowcirculatingfluidizedbedboiler.Thefuelsusedatthepowerplantweremainlymilledpeatandvariouswoodfuels.Bothboilersareequippedwith3fieldelectrostaticprecipitatorsfromwhichtheboilersflyashisblownpneumaticallytoacommonashsilo.TheelectricalpowergeneratedbypowerplantDsfluidizedbedboilerplantis77MWanditsheatingcapacityis246MW.ThefuelsusedinthefluidizedbedboilerarepowerplantAgarwalandAgarwal.mainlymilledpeatandwoodwaste.Theflyashisseparatedfromthefluegasesbymeansofa3fieldelectrostaticprecipitator.3.ResultsDuringtrialswithpowerplantAselectrostaticprecipitatortrials1–7thefuelusedwascomposed49peatand51woodfuels.Theashfunnelsoffields1–3oftheelectrostaticprecipitatorweresampledandanalysedFig.4.Onthebasisoftheresults,theCdconcentrationwasatitslowestinfield1oftheelectrostaticprecipitatorandatitshighestinfield3.Thisisduetothebiggerflyashparticlesaccumulatinginfield1andfield3containingashwiththeparticlesinthefirstfieldoftheelectrostaticprecipitator.22.22.42.62.833.23.4024681012CBOratioCdmg/kgFig.5.TheeffectoftheCBOratiofromelectrostaticprecipitatorfield1onflyashCdconcentrationsmg/kgduringtrialrunswithpeatfuel.22.22.42.62.833.23.43.63.84253035404550VoltagekVCdmg/kgFig.6.TheeffectofthefiltervoltagelevelkVfromelectrostaticO.Hanneetal./MineralsEngineering1920061596–16021599smallestparticles.TheCdconcentrationinfield1varieswithintherangeof2.2–3.6mg/kgandinthelastfieldwithintherangeof7.2–12.4mg/kg.ConcentrationsareaffectedbypropertiessuchastheESPvoltage,fuelquality,andfluegasflowrate.Inalmosteveryelectrostaticprecipitatorsfield1theCdconcentrationisbelowthepermittedmaximumlimit3.0mg/kgsetdownforashintendedforfertiliseruse.Duringtrialruns,theelectrostaticprecipitatorfieldsCBOratiocycleblockinoperationwascontrolledwithintherange0–12.Thevalue0meansthatallthehalfcyclesofthefieldinquestionarecurrentlyactive,andforexample,thevalue2meansthatonlyathirdofthehalfcyclesareactive.TherebytheCBOvaluedeclareshowmanysequentialhalfcyclesareclosed,thatis,howoftentheseparatorssupplycurrentispulsated.Inthepresentresearch,theCBOvaluewascontrolledbytheMicroKraftcontrollerwhosemaintaskistokeepthevoltageneartothebreakdownvoltage.Themostimportantthingistobeabletoinfluenceandchangethepropertiesoffield1intheelectrostaticprecipitator.Thefirstfieldenablestheproductionofflyashwithheavymetalconcentrationlevelsthatmakeitsuitableasafertiliser,forexample.Fig.5showshowtheelectrostaticprecipitatorsCBOratiocontrolforfield1affectstheflyashCdconcentrationlevels.Amongotherthings,theincreasingordecreasing024681012141234567Cdmg/kgField1Field2Field3Fig.4.Cdconcentrationsmg/kgoftheflyashinfields1–3duringtrialruns1–7withpeatfuelwhenusingtheelectrostaticprecipitator.numberofelectrostaticprecipitatorfieldflashoversisexcludedfromthiscontrol.Themorefieldhalfcyclestherearedeactivated,thelowertheheavymetalcontentoftheflyashbeinggenerated.Cdconcentrationvariationsarealsocausedbyfuelqualityvariations,inadditiontothecontrolitself,amongotherfactors.Fig.6showshowtheelectrostaticprecipitatorsfiltervoltageaffectstheCdconcentrationleveloffield1.Theheavymetalconcentrationlevelincreasesinaccordancewiththerisingvoltage.Thisisduetothefactthathighervoltagescanmoreeffectivelyseparatefineparticlesthatalsocontainheavymetals.ThefiltervoltagelevelindicatesthefieldsactualstatusmoreeffectivelythandoestheCBOratio.Amongotherthings,italsotakesintoaccountanyelectricbreakdownsthatoccurwithinthefield.Fig.7showstheparticlesizeclasseslmD10andD50offields1–3resultingfromtrials5,6and7.D10isaparticlesizewithrespecttowhich90ofthesamplesparticlesarelargerand10aresmaller.D50isahalvingparticlesizeclass,ortheparticlesizewithrespecttowhichsamplesparticlesarelargerandsmallerintheratioof50/50.Onthebasisofthisfigure,thesmallersizedparticlestendtobeconcentratedinthelastfieldandthebiggersizedprecipitatorfield1onflyashCdconcentrationsmg/kgduringtrialrunswithpeatfuel.
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