关于生物质燃烧技术及气化技术概要英文.docx
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关于生物质燃烧技术及气化技术概要英文
Chapter5MajorTechnologiesfortheExploitationofRenewableEnergySources
5.1Technologycharacterisation
Accordingtothedegreeofmaturity,biomassconversiontechnologiescanbeclassifiedintothreecategories:
-Traditionaltechnologies
-State-of-the-arttechnologies
-Emergingtechnologies
Traditionaltechnologies
Traditionaltechnologiesareconventionaltechnologieswhichareusedforalongtimewithoutanytechnologicalbarrier.Themajortraditionaltechnologiesare:
Pileburners.Approximatelyfiftyyearsago,allcommonwaysofburningwoodwasteinvolvedsomeformofpileburning.Thepileburningcombustorwastypicallycomposedofnumberofcells.Eachofthecellsconsistedofalower,refractorylinedcombustionchamberwithagratefloortosupportfuelpileandanupper,secondcombustionchamber.Thefurnaceandtheboilerwereseparatedwiththefurnacegenerallylocatedabovethesecondarycombustionchamber.Thepileburningboilerdesignsweresimpletodesignandinexpensive.
Inpileburningsystem,woodwasteisnormallypiledashighastentotwelvefeet.Combustionairflowupwardthroughthegratesfromunderneaththepileandinwardfromthecellwallsprovidingoxygenforcombustion,coolingairforthegratesandpromotingturbulenceandfueldrying.Mostofthefuelwillbeburnedonthegrates.Burningofthevolatileswillbecompletedinthesecondarycombustionzonewhereoverfireairisintroduced.Operatingtemperaturestypicallyrangedfrom1100°Cto1400°C.Typically,pileburnershadaslowresponsetimetodemandfluctuations.
Thecombustionprocessisverydifficulttoaccuratelycontrolandthecellshadtobeshutdownperiodicallyforcleaning.Theloadandtheamountofexcessairaredifficulttocontrol.Duetohighoperatingtemperatures,thewoodashactuallyslaggedandpooledontopofthegrate.Theashisthenmanuallybrokenupandremovedfromthefurnace.
Pileburnersarecapableofhandlingwoodfuelwithahighmoisturecontentwithlargequantitiesofdirtandcontaminantsmixedinwiththefuel.Fuelsizingislesscriticalwithpileburnersthanwithothercombustionsystems.
Relativetoothercombustiontechnologies,theefficiencyislow.Especiallyinolderdesigns,boilerefficiencyislow,generally50%to60%,duetolargesurfaceareaofthefurnaceandtheabsenceofradiantairheating.ThehighoperatingtemperatureswillbasicallybeadisadvantagewithrespecttothermalNOxcreation.
Combustionongrate.Therearethreekinds:
stationaryslopinggratesystem,travellinggratesystem,andvibratinggratesystem.
Stationaryslopinggratesystem.Theconceptofthestationaryslopinggratewasalreadydevelopedinthelate1920s.Fuelisintroducedatthetopofthegrateandslidesdownthegrate.Thefuelburnsasitproceedsthebottom.Thissystemisalsoreferredtoassemi-pileburning.Characteristicproblemswithearlydesignsincludedavalanchingofthefuelonthegrateanddifficultyincontrollingboththesteamloadandtherateofcombustion.
Travellinggratesystem.Intravellinggratespreaderstokerdesigns,theentirebottomsofthefurnaceisaslowmovingplatformorconveyorformingthegrate.Thegrateiscooledbyairfedfromunderthegrate.Inthisway,thegratemechanismanditscoolingsystemdefinesthemaximumacceptableundergrateairtemperaturewhich,correspondingly,definesthemoisturecontentofthefuelthatcanbeburned.Watercooledwallscouldbeusedtopreventslagformationadjacenttothestoker.Fuelisfedfromapneumaticspreaderstokersystemlocatedonthefrontofthefurnace.Smalleranddryerfuelparticlesareburnedinsuspension,whilethelargerparticlesfallinathinlayeronthemovinggrate.Thefuelhastoburnatauniformrateandasufficientspeed.
Vibratinggratesystem.Thevibratinggratesystemoffersthebenefitofspreadingthefuelsothatsmallpilesthatmightformonthegratearelevelledout.Therearelessmovingpartsthanwiththemovinggratesandthereforelessmaintenanceisrequired.Fuelcanbemechanicallydistributedbyscrewfeederslocatedatthetopofthegrate.Recentboilersusewater-cooledvibratinggrates,allowingtheuseofhightemperatureundergrateairandahigherpercentageofoverfireair.ThisatitsturnenableslowercombustiontemperaturesandthereforebettercontrolofNOxformation.Anotheradvantageoflowerquantitiesofunderfireairislowerunburnedparticlecarry-over.Otheradvantagesofthesystemare:
-loadcontrolcapabilitiescomparabletothoseofanoilburner,becauseofthefactthatalargeamountofthefuelisburnedinsuspension;and
-possibilitytoswitchto100%firingofalternativefuelssuchasoilorgaswithoutanyfurtherprotectionofthegrate.
Usually,fuelswitchingcapacityofgratesystemsislimited.Moisturecontentshouldtypicallybekeptwithinabout10%ofthedesignrate.Fluctuationsinmoisturecontentoutsidethisrangeresultinsignificantchangesinfluegasflowsandinheattransferrates.Fuelswithlowmeltingashes,likemanyagriculturalwastes,aretypicallykeptbelowapproximately15%(heatinput)ofthetotalboilerfuel.Thesimplicityandflexibilityofthegratesystemmakesthisdesignoneofthemostadaptableunitstoco-firesolidfuels.Thedifferenceinbulkdensityoffuelsmaycreatedifficultiesinusingthesamespreaderordistributorforbothfuels.
Efficienciesofrecentdesignsrangeuptoabout84%(LHV)fortravellinggratesand96%(LHV)forvibratinggrates.Inthe1980smanyofthesesystemsadoptedastagedcombustionprocessinordertomeetwithNOxemissionsstandards.
ExamplesofcombustionongratesystemsarepresentedinTable21.
Table21.Combustionongrate-examples
Parameter
Unit
Value
Value
Value
Plantsize
MWe
2.5
50
34
Technologytype
-
inclinedmoving
grate
travellinggrate
watercooledvibratinggrate
Plant
-
ChiaMeng
McNeilPlant
MåbjergvæketCHPPlant4
Country
-
Thailand
USA
Denmark
Startupyear
-
1997
1984
1993
Technology
Fuelused(moisture)
%
ricehusk(10)
wood(47)
straw(16)
wood(40)
msw1(23)
naturalgas(0)
Steamdata:
-temperature
-pressure
-flow
°C
bar
kg/s
420
34
17
510
92
61
520
100
35
Efficiencies:
-boiler(LHV)
-turbine(gross)
-net(LHV)
%
%
%
-
-
-
83
39
30
89
36
30
Cost
Investmentcosts(1992US$)
$/kWe
15502
1800
2900
Emissions
Emissions:
-NOx
-CO
-particulates
mg/Mjin
mg/Mjin
mg/Mjin
1503
333
-
74
177
4
108
130
-
1municipalsolidwaste
2In1997US$
3Averageemissioninppmunit
4ForCHPplants,atheoreticalestimationhasbeenmadeofhowmuchelectricitycouldbegeneratedwhentherewasnoheatsupply
Source:
Broeketal.,1995,COGEN
State-of-the-arttechnologies
State-of-the-arttechnologiesarethetechnologies,thatcanbeusedatthepresenttimewithminimaldevelopmentalbarriers.Themajortechnologiesare:
Suspensionburning.Suspensionfiredboilersresemblepulverisedcoalboilersinthatthecombustionoccurswhilethefuelparticlesarepneumaticallysuspendedinanairstream.Animportantattractionofthesuspensionfiredboilersisthereducedfurnacesizeduetodrierfuel.
Twobasictypesofsuspensionburnersareavailableforuseonsteamgenerators,namelycyclonicburnersandsolid-fuelburners.Cyclonicburnersaredesignedtomixfuelandairinthecorrectproportionandtocompletecombustionbeforeswirlingparticlesoffuelreachtheendoftherefractorychamber.Solid-fuelburnersmixtheairandfueltogetherinthecorrectproportionandignitethecombustiblemixture.Burnoutoffuelparticlesiscompletedinaverticalcylindricalfurnace.
Elaboratefuelpreparationandfeedingsystemarerequiredforsuspensionfiringsystem.Forpropercombustion,biomassfuelsarerequiredtohaveamoisturecontentoflessthan15%andafuelparticlesizeoflessthan6mm.Iffuelslikewoodchipsorstrawareused,ithastobedriedandprocessedthroughahammer-milltoreducetheparticlesize.Thepresenceofdryfinefuelparticlescreatesapotentialexplosionhazard.Thus,suspensionburningfuelhandlingsystemsrequiremorecarefuldesignthanconventionalbiomassfuelhandlingsystems.
Theefficiencyofasuspensionfiredboileris,ashighas80%(HHV).Thisispartlycausedbylowexcesscombustionairrequired,whichresultsinabetterheattransferbecauseoflowerfluegasvelocity.NOxemissioncontrolcanbeundertakeninasimilarwayaswithpulverisedcoalfiring.Burnersshouldbeadjustedinsuchawaythattemperaturepeakswithinthecombustionareaareprevented.
Atmosphericfluidisedbedcombustion.Ofthedifferentfluidisedbedboilerswhichareusedatthemoment,mostofthemareeitherbubbling(BFB)orcirculatingfluidisedbed(CFB)boilers.
Influidisedbedcombustion,theprimarycombustionairfromthebottomofthefurnaceisinjectedwithsuchhighvelocitythatthematerialinsidethefurnacebecomesaseethingmassofparticlesandbubbles.Thisseethingmassconsistsofboththefuelandofgranularinertmaterial.Whenstartingupboiler,thisinertmaterialisheatedtoignitionpointofthefuelatwhichpointthefuelisfedfromabovethebubblingbed.Asteadycombustiontakesplace,inwhichthefuel,becauseofthefluidityofthesystem,israpidlymixedthroughoutthebedandinwhichthereisahighheattransferbecauseoftheintimatecontactoffuelandinertmaterial.Thispermitscombustiontotakeplacewithaminimumofexcessairandatalowcombustiontemperature,typically800-900°C,ascomparedwithstokerfiredboilers.Anotheradvantageofthefastheattransferisthattheinstallationshaverela