煤气化工艺中煤渣煤灰的利用.docx

煤气化工艺中煤渣煤灰的利用.docx

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煤气化工艺中煤渣煤灰的利用

CoalGasificationashandApplications

1.Gasifierashdepositionandagglomeration

Figure1.Gasification-basedenergyproductionsystemconcepts

（SandeepTandon,2008AsiaCleanEnergyForum）

Conceptually,coalgasificationisarelativelysimplyprocess.Acarbonaceousfuel-usuallycoal,petroleumcoke,orheavyoil-isco-fedwithwaterandoxygeninareducingatmosphereathighpressure（upto1,000psig）toproducethedesiredproductsofcarbonmonoxide（CO）andhydrogen（H2）.SulfurintheformofH2SandsomeamountofCO2isalsoproducedandremovedintheprocess.Thisgaseousmixtureiscommonlyreferredtoassynthesisgasorsyngas.Commercialgasifiersdifferwidelyinthewayinwhichtheyproduceash,eitheradryash,anagglomeratedash,orslagmayresult.TheLurgiandotherfixed-bedgasifiersoperatebypassingairoroxygenandsteamunderpressureupthroughabedofcoal,whichisfedtothetopofthebedthroughalockhopper.Coalandcharmovetothebottomastheyaregasified,andthedryashisremovedthroughabottomgrate.Alternatively,afixed-bedgasifiercanbedesignedtooperateathightemperatures,producingabottomslagthatistappedthroughahearth,i.e.,theBritishGasLurgi（BGL）process.Fluidized-bedgasifiers,includingtheUSKelloggRustWestinghouse（KRW）andInstituteofGasTechnology（IGT）processesandtheGermanWinklerprocess,operateinagasificationmodeusingsteamandairoroxygeninafashionresemblingPFBC（pressurizedfluidized-bedcombustion）.Fluidized-bedgasifiersmayproduceeitheradryashorafusedagglomeratedash,dependingonthedesign,theoperatingtemperatures,andthefusiontemperatureoftheash.Entrained-flowgasifiers,includingDow,Texaco,andShelldesigns,alloperateatveryhightemperaturesandproduceavitreousslag.Integratedgasificationcombined-cycle（IGCC）systemsdirectlylinkthesevarioustypesofgasifierswithagasturbine-steamturbinecycletoachievehighconversionefficiency.

（a）

（b）

Figure2.（a）Fluidized-bedgasifier;（b）Entrained-bedgasifier（up-flow）

（1999Energy&EnvironmentalResearchCenterTopicalReport）

Inadry-ashfixed-bedgasifierorgrate-typecombustor,bedtemperaturesaremaintainedbelowthefusiontemperatureoftheash,andthebulkoftheashisconsolidatedonthegratedischarge.Inveryhigh-temperatureslagginggasifiersandcombustors,allofthephysicaltransformationsdescribedareoperative,buttheconsolidationofashandslagdependsonreactorconfiguration.Inafixed-bedslaggingreactor（e.g.,theBGLgasifier）,virtuallyalloftheinorganicreactionproductsarerecapturedintherelativelycooldescendingfuelbedandconsolidatedintotheslagdischarge.（StevenA.Benson,et.al.FuelProcessingTechnology44（1995）1-12）

Inanentrained-flowreactor（e.g.,Texaco,Dow,andShellgasifiersorcyclone-typecombustors）,slagispartiallyseparatedbyimpingementorcyclonicaction,whilea（potentiallysmall）fractioniscarriedforwardwiththehotgas.Hotmineralmatterisdepositedonthewallasslag.Theslaggingbehaviorisacriticalforprotectingtherefractory-linedwallsofthegasifierfromtheharshenvironmentwithinthegasifier.Inadequateslaggingcanleadtoexcessiverefractorywear.

Figure3.Gravityinducedflowofaviscousslaglayerdownasolidsurface.

（MichaelJ.Bockelie,et.al.ReactionEngineeringInternational）

Coalswithlowashcontentarepreferredforbotheconomicalandtechnicalreasons.Ifgasifieroperatingconditionsarekeptconstant,anincreaseincoalashcontentwillleadtoadecreaseingasificationefficiencyandanincreaseinslagproductionanddisposal.However,eachtechnologyhasslightlydifferentcoalashrequirementsdependingontheirdesign.ThereisaminimumashcontentrequiredfortheSCGP（ShellCoalGasificationTechnology）（>8wt%）,theBBP（>1wt%）andtheHitachigasifiersbecauseofaslagself-coatingsystemonthewallofthegasifiers,whichhastobecoveredbyslagtofunctionandminimizeheatlossthroughthewall（Figure4）.

Figure4.ThePrenflogasifier

（AndrewJ.Minchener.Fuel84（2005）2222–2235）

2.Propertiesandapplicationsofgasifierash

Thechemical,mineralogical,andphysicalcharacteristicsofgasifierashhavebeeninvestigated,andthecharacteristicsofashproducedfromtheShellpilot-scaletestingandTexacotestinghavebeenreported.Slagandashsampleshavebeencharacterizedfromeightgasifiers.Thetypesofmaterialsexaminedincludedcoarseashorslagandcyclonedust.Thematerialswerefoundtobenonhazardous,butthephysicalcharacteristicsandchemicalcompositionsvariedsignificantlyasafunctionofprocessconfiguration,operation,coalfeedcomposition,andcoalhandling.Theelementalcompositionsoftheslagsproducedingasificationsystemsweresimilartothebottomashfromconventionalcoalcombustionsystems.Thebulkcompositionsofcyclonedustsampleswerefoundtobesimilartoconventionalcoalcombustionflyash.Themineralogicalexaminationofslagsindicatedthatmanyofthesamehigh-temperaturesilicatemineralsarepresentintheslagsamples,alongwithreducediron-bearingcompounds.Thekeydifferenceincoalgasificationashandslagcomparedtocombustionashisthelackofsulfur.Sulfurispresentinsmallquantitiesintheash,usuallyintheformofasulfide.Inaddition,theotherashspeciesinthesystemmayalsobeinreducedform.Theentrained-flowslagginggasifiersrecycleallflyashbacktothevitreousslag.SlagsamplesproducedintheShellprocesswereshowntobedepletedinseveraltraceelements.Thefineflyslagcontainedcarbonandahigherleveloftracemetalsandothervolatileinorganiccomponents.

Thecharacteristicsandsubsequentutilizationpotentialofresiduesfromnewcoalusetechnologieswillbedeterminedbytheuniquelydifferentthermaltransformationsofthecoalash,newprocessprovisionsforsulfurandNOxcontrol,andnewparticulatecollectionmethods（hotgascleaning）.Inmanynewtechnologies,thesulfuriscapturedalongwiththeashresidues,producinglargevolumesofhigh-calciumandhigh-sulfurresidues.TheseresiduesdonotexhibitpropertiesatallsimilartothoseforconventionalCCBssuchasflyash,bottomash,orboilerslag,andtheyarenotgenerallysuitableforuseinthesameapplications.Theclassificationofthesetypicallyhigh-calciumandhigh-sulfurresiduesbyapplicationisnecessarilyspeculativeatthistime,butsomegeneralobservationscanbemade.

Flyashfrompressurizedcombustorsthatdonotintroducecalciumforsulfurcaptureshouldbesimilartoconventionalpulverizedcoal-firedflyashandwouldhaveapplicationsinthesametypesofcementreplacementuses.High-strengthvitreousslagsfromentrained-flowgasifierswouldbesuitableformostapplicationswherefineaggregate（e.g.,sand）iscustomarilyused,whichcanincludeconcreteproducts,asphaltfiller,controlledlow-strengthmaterial（CLSM）,roofinggranules,andbrickorotherceramicproducts.Agglomeratedash,suchasthatobtainedfromhigh-temperaturefluidized-bedgasifiers,issimilartobottomashfromconventionalboilersandwouldbeusedinsimilarapplicationsinroadbaseandaggregate.Dryashfromfixed-bedgasifierswillbeintermediatebetweenflyashandbottomashandincorporatesomereducedchemicalphases;themostlikelyusesareinhigh-volumefillandspecialtymanufacturedproducts.Aspreviouslynoted,calciumsulfideproducedbyinsitusulfurcaptureinsomegasifiersmustbeoxidizedtosulfatepriortouseordisposal.Additionalresearchisneededtocharacterizemanyofthesematerialsforregulatoryclassificationandoptimumbeneficialuse.

Researchtodatehasnotadequatelyaddressedtheutilizationpotentialofthesematerials,buttheyhavebeenshowntobeenvironmentallyacceptableformanyuseapplicationsanddisposalbyconventionalmethods.Moreworkisneededtoachievehighlevelsofutilizationandtoreducethelandfillingthatwouldotherwiseberequired.

3.CoalashuseinIntegratedCoalGasificationCombinedCycle（IGCC）system

Morethan43milliontonnes（metric）ofcoalflyash（FA）areproducedannuallyintheEuropeanUnion.AlthoughthemainproportionsoftheseFAsareproducedbyPulverizedCoalCombustion（PCC）process,smallamountofFAaregeneratedfromtheIntegratedCoalGasificationCombinedCycle（IGCC）.InthePCCplantsthe80%ofcoalcombustionby-productsareproducedasFAsand20%asslag.Conversely,mostofthegasificationby-productsareobtainedasslag（around90%）,andasmallproportion（around10%）iscollectedasFA.Apartfromthesedifferences,thePCCandIGCCprocessesgenerateFAswithdifferentmineralogicalcompositions.ThePCCFAsaremadeupofaluminosillicateglass（52-90%forEuropeanFAs）,withCa,Fe,Na,K,Ti,Mnimpurities,andvariableminoramountsofquartz,mullite,lime,hematite,magnetite,gypsumsandfeldspars,aswellastracesofsillimanite,cristobalite-trydimite,wollastonite,andFe-Alspinels.IGCCFAsarecharacterizedbyapredominantaluminosilicateglassmatrixandawidevarietyoffinecrystallinereducedspecies（mainlysulphides）.

IntegratedGasificationCombinedCycle（IGCC）plantsproducealargevolumeofsolidbyproducts,justlikepulverizedcoalcombustion（PCC）plants.Itisestimatedthata425netMWIGCCplantburning10%ashcoalwillproduceabout450to500tonsperdayofsolidbyproducts.Thesecanbeseparatedintotwofractions:

acoarsevitreousfraction（FRIT）andacarbonaceousfinesfraction（CFF）.TheCFFfraction,whichhasahighheatingvalue,isgenerallyrecycledthroughthegasifierasafuelsourcetoimprovecoalutilization.TheFRITfractionmaybeused