关于生物质燃烧技术及气化技术概要英文.docx

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