外文翻译循环流化床锅炉中飞灰含量的研究.docx
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外文翻译循环流化床锅炉中飞灰含量的研究.docx
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外文翻译循环流化床锅炉中飞灰含量的研究
外文原文
ResearchonCarbonContentinFlyAshfromCirculatingFluidizedBedboilers
Abstract
ThecarboncontentintheflyashfrommostChinesecirculatingfluidizedbed(CFB)boilersismuchhigherthanexpected,whichdirectlyinfluencesthecombustionefficiency.Inthepresentpaper,carbonburnoutwasinvestigatedinbothfieldtestsandlaboratoryexperiments.Theeffectofcoalproperty,operationcondition,gas-solidmixing,chardeactivation,residencetime,andcycloneperformanceareanalyzedseriatimbasedonalargeamountofexperimentalresults.Acoalindexisproposedtodescribethecoalrank,havingastrongeffectonthecharburnout.Badgas-solidmixinginthefurnaceisanotherimportantreasonofthehighercarboncontentintheflyash.Somecharsintheflyasharedeactivatedduringcombustionoflargecoalparticlesandhaveverylowcarbonreactivity.Severalsuggestionsaremadeaboutdesign,operation,andmodificationtoreducethecarboncontentintheflyash.
Introduction
Withtheadvantagesoffuelflexibilityandlowpollutantemission,circulatingfluidizedbed(CFB)combustiontechnologyhasbeendevelopedrapidlyinpowergeneration.ThecapacityofCFBpowerplantshasbeengrowingsteadilyeversincethecommercializationofthetechnologyinthelate1970s.Currently,themaximumcapacityofasingleCFButilityboilerisontheorderof300MWeandmorelargecapacityunitsof600and800MWesupercriticalpressureCFBboilersaredeveloping.ThedevelopmentofCFBboilersinChinastartedinthe1980s,andthemaximumcapacityforasingleunithasbeenincreasingyearafteryear,asshowninFigure1.Thereareover1000CFBboilersinoperationupto2004,includingover20unitsabove135MWe.Inaddition,80unitsof135MWeand10unitsof300MWeCFBboilersareinconstructionoronorder.
Figure1Maximumcapacityforasingleunit.
However,itisafactthatthecombustionefficiencyofCFBboilersislowerthanthatofpulverizedcoalfired(PC)boilers,thoughhighcombustionefficiencywasreportedintheCFBmarketabroad.1Browncoals,withhighactivity,arecommonlyburnedabroad,whichisthemainreasonforlowcarboncontentintheflyash.InChina,onthecontrary,mostCFBboilersburnhardcoalssuchasanthracite,bituminous,andcoalwastes;thecarboncontentintheflyashismuchhigherthanexpected,2especiallyforthelarge-capacityCFBboilersmanufacturedwithimportedtechnology.Atthesametime,thehighcarboncontentintheflyashlimitsthepotentialutilizationascementmaterials.3IthasbecomethebottleneckreducingthecompetitivepoweroftheCFBboiler.
CoalcombustionprocessesinCFBboilerareverycomplex,undergoingthefollowinginterrelatedsequences:
4heatinganddrying,devolatilizationandvolatilecombustion,swellingandfragmentation,andburningofchar.Theunburnedcarboncontentintheflyashisbelievedtobethefinalresultsforcharacterizingthecombustionefficiency.Tooptimizetheprocessandmakepredictionsofthecombustorperformanceinabroadrangeofoperatingconditions,itisnecessarytounderstandthecombustionefficiency,especiallytheunburnedcarbonintheflyash.
ThecarbonburnoutinChineseCFBboilerswasinvestigatedinbothfieldtestsandlaboratoryexperimentstoidentifythemajorfactorsaffectingthecarboncontentintheflyash.
ExperimentalSection
Theexperimentalinvestigationsincludeagreatdealoffieldtestsandlaboratoryexperiments,suchassamplingofcoalandflyash,controlandmeasurementsoftheoperationparameters,measurementsofoxygenconcentrationandsolidsuspensiondensity,sizedistributionmeasurementofflyash,measurementofcarboncontentintheflyash,coalanalysis,andcharreactivitymeasurement,etc.
Thefieldtestshavebeencarriedoutinmorethan20unitsofcommercialCFBboilerssinceseveralyearsago.Coalandflyashweresampledrespectivelyfrom17-unit220t/hboilersandthenanalyzedinthelaboratory,tocomparetheeffectofcoalrank.Thecarboncontentintheflyashwasmeasuredbythelossofignition(LOI)method.
Onbedtemperature,bedpressure,andexcessairratio,experimentswerecarriedoutforsixkindsofcoalinthree75t/hboilersindetail,whicharemanufacturedbythesameboilermanufacturerandhavenearlythesamestructure.
Ongas-solidmixing,theoxygenconcentrationandsolidsuspensiondensityinthefurnacewereinvestigatedinone75t/hboiler.Thecross-sectionofthefurnaceis3m(depth)and6m(width).Fourmeasuringpointswerelocatedatthecenterlineofthesidewallalongtheheight.Thegassamplingprobeandsolidbulkdensityprobewereinsertedthroughthesidewall,whichareillustratedinFigure2andFigure3.
Figure2Gassamplingprobe.
Figure3Solidbulkdensityprobe.
Onchardeactivation,charsampleswerepreparedinatubefurnaceatvarioustemperaturesforvariousdurations,andthereactivitymeasurementwasperformedbythermalgravimetricanalysis(TGA).
Moredetailsabouttheexperimentalmethods,instruments,andprocessesaredescribedinrefs6,9,10,and12.
ResultsandDiscussion
CarbonContentintheFlyAshfromCFBBoilers.
Thedistributionofcarboncontentintheflyashisnonhomogeneous,andthehighercarboncontentisconcentratedwithinthesizerangeof30-50
m.AtypicalexamplefromoneboilerisshowninFigure4.Theformationofcarboncontentintheflyashrelatesthemasstransfer,heattransfer,andtheformationofashlayerduringcombustion.5Fordifferentsize,theashlayerresistance,surfacetemperature,residencetime,andburnouttimearedifferent,resultinginthenonhomogeneousdistribution.
Figure4Distributionofcarboncontentinflyash.
BedTemperature
Generallyspeaking,theincreaseofthebedtemperaturecanpromotethechemicalreactionrateandthenincreasethecombustionefficiency.Experimentswerecarriedoutforsixkindsofcoalinthree75t/hCFBboilers,whicharemanufacturedbythesameboilermanufactureandhavenearlythesamestructure.ItcanbeseeninFigure5that,for
allkindsofcoals,thecarboncontentintheflyashhasastrongrelationshipwiththebedtemperaturewherethebedinventory,theexcessairratio,andtheairsupplyarekeptasconstantaspossible.6Forthesixdifferentcoals,theincreaseofbedtemperaturewillcausethedecreaseofthecarboncontent.
Figure5Effectofbedtemperatureoncarboncontentinflyash.
Itclearlyprovesthepromotionofthebedtemperaturecanreducethecarboncontentintheflyashandincreasethecombustionefficiency.However,otherdisadvantages,suchaslowerdesulfurizationefficiencyandmoreNOxemission,wouldlimittheincreasingtemperature.7,8Thus,itisnotanadvisablewayforperformanceoptimizationofCFBboilers.
CoalRank
ACFBboilercanbedesignedtoburnalmostanykindofsolidfuels,but,asnotedabove,thepracticalexperienceofCFBboilerinChinaprovesthatthecarboncontentintheflyashisnotaslowasexpected.Figure5alsoshowsthatthecoalpropertywillinfluencethecarboncontentintheflyash.Itisbelievedthatthekeyfactoraffectingthecombustionefficiencyisthecoalcharacteristics,includingvolatilecontent,heatvalue,charreactivity,andcharstructure,etc.Thehighvolatilecoals,suchasbrowncoalandbituminouscoal,usuallyhavehigherreactivityandareeasytoburnout,whilethelowvolatileandhighashcoals,suchasanthraciteandleancoal,areusuallyonthecontrary.Operatingconditions,carboncontentintheflyashes,andcoalpropertiesfrom17-unit220t/hCFBboilersareshowninTable1.
Table1.OperatingConditionsandCarbonContentinFlyAshfromCFBBoilers
boiler
furnacetemp(
C)
excessairratio
carboncontentinflyash(%)
Vdaf
Qar,net,p(MJ/kg)
A
880-905
1.21
17.17
21.49
22.76
B
880-905
1.23
13.74
20.55
20.52
C
860-890
1.23
8.91
22.51
19.83
D
880-910
1.22
7.05
28.88
20.80
E
892-915
1.21
8.72
30.52
19.87
F
900-915
1.26
22.20
11.25
21.00
G
885-900
1.31
8.38
27.79
20.10
H
900-910
1.24
6.79
35.54
18.00
I
890-900
1.25
6.91
46.65
17.10
J
890-920
1.28
8.72
30.52
18.84
K
875-885
1.32
6.30
29.93
18.82
L
880-900
1.26
5.61
40.58
18.00
M
890-915
1.26
5.30
45.58
13.13
N
880-900
1.26
18.01
13.00
21.56
O
875-895
1.26
5.01
49.59
15.90
P
900-930
1.25
27.12
5.98
21.39
Q
860-890
1.26
16.3
19.28
22.70
Itisclearthatthecarboncontentintheflyashdependsonthecoaltypestrongly.AcoalindexIwasdefinedasthevolatilecontent,Vdaf(dryash-freebasis,magnitudeon1-basis)dividedbythelowerheatingvalue,Qar,net,p(MJ/g):
And,therelationshipbetweenthecarboncontentintheflyashandthecoalindexcanbeeasilyseeninFigure6.
Figure6Relationshipbetweencarboncontentinflyashandcoalindex.
AlthoughthefurnacetemperatureinboilerP,burninganthracite,ishigherthanotherboilers,thecarboncontentintheflyashisstillthehighest.Actually,itisalsothegeneralexperienceinChineseCFBboilersburninganthracitethatthecarboncontentintheflyashisalwaysexcessivelyhigh.Evenforsomekindsofbituminous,thecarboncontentintheflyashisstillrelativelyhigh.Onthecontrary,theCFBboilersburningbrowncoal,whichhashighcoalindex,normallyhavelowcarboncontent.Thecoalindex,presentedhere,issuggestedausefulparametertorepresentthecoalreactivityandtoanalyzethecharburnout.
Gas-SolidMixingandAirSupply
Alargeamountofsolidparticlesareelutriatedfromthede
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