动力.docx
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动力.docx
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动力
thisagegroup,therewereabout226op-posedwall-fired,143tangential-
fired,andabout15cyclone-firedboilersinoperationintheU.S.in2011.
Theseunits--thebackboneofthebase-
loadcoal-firedfleet--willbeartheburden
ofensuringthattheusualhighstandardsof
electricalgridperformance,availability,and
reliabilityaremetinthefuture.Thoughmost
oftheseunitshavehigh-gradeAQCSs,they
willrequireupgradestocomplywithmaxi-
mumachievablecontroltechnology,butthe
costisnotforecasttoadverselyimpactunit
competitivenessintermsofgenerationcost.
However,theadditionalAQCSupgradesre-
quiredforenvironmentalcompliancewill
addadditionalcomplexitytoplantsnow
strainingtomaintainunitavailabilityand
capacityfactor.
Avitalpartofanycoal-firedunitisits
fueldeliverysystem,asshowninFigure
3.Forthepurposesofthesubcommittee's
analysis,theFDSconsistsofthefeeders,
pulverizers(mills),classifiers,coalpiping,
andburners.Thesesystemsarevitalfor
efficientandreliableplantoperationsbut
alsorequiresubstantialmaintenancedueto
theabrasivenatureofcoal.
IdentifyPossibleFuelSystem
Upgrades
Inrecentyears,improvementsinmonitor-
ingequipmenthaveledtosignificantper-
formanceimprovementsinFDSequipment
andthereforetheavailabilityandreliabil-
ityofaplant.Manyofthesehavebeenin
flow-measuringdevicesforenhancedcon-
troloffuel(suchasfeedercoalflowand
pulverizedcoalflowincoalpipes)and
airflow(includingprimaryandsecondary
air,pulverizerpreheatedair,coalpipeair,
windboxair,andindividualburnerair--
bothsecondaryandtertiary).
Inaproperlyinstrumentedsystem,the
amountofcoalandairsenttoindividual
burnerssovitalforlow-NOxandCOop-
erationcanbemeasuredandmonitored,
ensuringthatgoodcombustiontakesplace.
Pleasenotethatcycloneboilersutilize
adifferentFDSthatconsistsoffeeders,
crushers,andcycloneburners.Thesedif-
ferenceswillbeaddressedintheproject
economicssectionofthisarticle.
UpgradestotheFDStoimproveplant
operatingeconomicsarenumerousand
oftensite-andboiler-specific(Table1).
EachFDScomponentupgradecanhave
benefits,mostofwhichcanbequantified.
Anexampleistheretrofitofadynamic
classifier,whichimprovescoalfineness
andvirtuallyeliminatesthecoarsecoal
particles(>50mesh).Coarseparticlesare
amaincauseoffoulinganddepositionin
thefurnaceandtheconvectionsectionsof
theboiler.Theyalsoimpedegoodlow-
NO~burnerperformance.Improvingfine-
nessalsoreducesunburnedcarboninthe
flyash,thusimprovingcombustionand
boilerefficiency.
AdditionalAssumptions
Membersofthesubcommitteedecidedthat
threeboilertypes,representativeofthe35-
to50-year-oldfleet,wouldbeselected:
opposedwall-,tangential-,andcyclone-
firedboilers.Thegroupalsodecidedthat
representativeboilersystemswouldform
thebasisoftheupgradeanalyses,although
theplantitselfwouldremainanonymous.
Acommonfactorsharedamongboilers
ofthisvintageisthattheywerenormally
designedtoburneasternorIllinoisBasin
coalandhavebeenorarebeingconsidered
forconversiontoasubbituminousPRB
coalforemissionsreduction.Mostboilers
havehadsomeburnermodificationsover
theiroperationallife,andnewmodifica-
tionswouldbeconsideredmoreforper-
formanceimprovementthanforemissions
reduction.InfocusingontheFDS,itwas
furtherassumedthatairpollutionretrofits
wouldnotbepartoftheFDSupgrades.
ItwasalsoassumedthatanyFDSup-
gradewouldnotincreasetheheatinput
overitsoriginaldesignratingandthatthere
wouldbenoincreaseinemissions,toavoid
theneedforaNewSourceReview.Inad-
dition,emissionswouldbelessthan100
tons/yearforindividualpollutants,thus
nottriggeringthePreventionofSignificant
Deteriorationprocess.Itwasassumedthat
theopposedwallandcycloneboilershad
selectivecatalyticreduction(SCR)sys-
temsinstalledforNO~control,butthatthe
tangential-firedunitdidnotyetrequirein-
the50-meshmaterial,whichisaprimary
causeofslaggingandfouling.Alesser-
knownfeatureofDCsisthattheycanalso
increasepulverizercapacity(dependingon
DCspeed),finenessathigherspeed,and
throughputatlowerspeed.Typically,this
isaccomplishedwithlittleornoincreasein
pressuredrop.
BecausePRBisavolatilecoal,thereis
littleneedforanincreaseincoalfineness,
butthereisaneedforincreasedpulverizer
throughputovertheoriginaleasterncoal
designconditions.DCcostswereestimated
as6x$300,000x2(costofinstallation)=
$3,600,000,acknowledgingthat$300,000
isonthehighsideandnormalinstalled
costisusuallylessthandoubletheequip-
mentcost.
AswitchfromeasterntoPRBcoaland
itsassociatedlowerheatingvaluemeans
thatgreaterquantitiesofPRBcoalpassing
throughtheFDSandtheverticalshaftmills
wererequiredtoproducethes~imeheatin-
put.Inthisplant,heatinputtotheboiler
wasreducedduetocoalthroughputlimita-
tions.Thus,theDCs'abilitytorecoverlost
pulverizerandboilercapacity,especially
whenoneormorepulverizersareoutof
serviceforrequiredmaintenancework,is
vital.Aconservativevalueof5%lossin
boilercapacitywasused.Thereductionof
50-meshcoalparticlesandimprovedcom-
bustionwasestimatedtorecovertwodays
ofoperationatfullload.Otherbenefitsin-
cludedimprovedloadresponse,improved
coaldrying,oftenlessvibration,butmodest
NOxandunburnedcarbon(UBC)reduction.
Thesavingsfromthe5%capacityrecovery
was$8,760,000,andtwodaysoffull-load
operationat$0.05/kWhwas$960,000,for
atotalsavingsof$9,720,000.
CoalPipes.Thesubcommitteeconsid-
eredcoalpipeflowmetersbecausecoal
flowmeasurementsmaynotbeexact.The
individualflowmetersallowcomparison
ofcoalflowbetweenpipestoensuremore
equalandconsistentcoalflowtotheburn-
ersthatwillinturnensuregoodairtofuel
ratiosateachburner.
Therewassomediscussionaboutre-
placingcoalpipesonthebasisofincreased
pressuredroporfanlimitation.Itwasfelt
thatcoalpipesareseldomreplaced,sothis
optionwasdiscarded.Insum,upgrades
studiedincludedthecoalpipeflowmodi-
fications,$500,000,andprimaryairflow
modifications,$200,000,foratotalof
$700,000.
Itisvitaltomeasurebothcoalandairasa
precursortoaggressiveefficiencyimprove-
ment.ItwasalsofeltthattheDCswould
resolveapotentialpressuredropissue,and
thepressuredropissuewasfurtherclarified,
asshownbelowintheBCSsection.
BurnerModernization.Theexisting
low-NOxburners(LNBs)areathirdgen-
eration;afourth-generationupgradeis
plannedthatwillincludeburnermodern-
izationandairflow-monitoringdevices.
Overfireairisalsoincluded.NOxreduc-
tionwiththesenewLNBsandOFAwould
beabout10%(~0.02lb/106Btu),butthey
serveprimarilytoreduceNH3consumption
bytheSCR.UBCwouldnotdrop,andCO
wouldbeheldto<100ppm.
ThecombinationofretrofitDCsand
LNBswouldreduceslaggingandfouling,
improvefluegasflowtotheSCRandair
heater,providebettercombustion,andim-
proveboilerperformance.Typically,LNBs
areupgradedeverysixtoeightyearsdueto
newdesignimprovements.Costswereesti-
matedfortheLNBsas$75,000x24burners
x2(costofinstallation)=$3,600,000;OFA
$25,000x8+$50,000x8(costofinstal-
lation)=$600,000;electrical$1,000,000
(costofinstallation);and$200,000for
FuelPreparationSystem.Thefeeder-
crusher"island"wasincludedintheFDS
boundary,becausethisequipmentisavital
partofcoalsizingnecessaryforgoodcom-
bustion.Itwasgenerallyagreedthatsome
modificationwouldhavebeenmadeto
thefeeder-crusherislandwhenswitching
toPRBcoal,whichwouldhaveincluded
dustcontrolandothersafety-relatedissues.
Thesamewouldhavebeenincludedinall
thecoalconveyors,butthecoalconveyors
fromthefeeder-crusherislandtothecy-
clonearenotpartoftheFDS.
AspartoftheFDSupgrade,someup-
gradeswillbemadetothefeeder-crusher
islandtoimprovecoalgrindatagivenor
increasedthroughput,andthesecostswere
estimated.Indiscussingthecyclonemodi-
fication,itwasagreedthatthe12cyclones,
feeders,andburnerswouldhavebeenpre-
viouslymodified,butthattheupgradesto
reduceUBC,whichisinsomecases20%
to30%,wouldberequired.Upgradesin-
cludeanewposimetricfeeder,finegrind
cagecrusher,andmotorupgradesat
$1,200,000.
CycloneModernization.Cyclones
wereupgradedwithnewsplitsecondary
airdampersanddamperactuators.Bene-
fitsincludereducedNO,andUBCbyim-
provingcombustion(allowinglowexcess
airoperation);moreevenfluegasdistri-
butioninthefurnaceconvectionsections,
SCR,andairheater;andreducedslagging.
Upgradeswereestimatedat$2,800,000.
Thebenefitswerereducedoperationat
lowerloadswithindividualcyclones
forcedoutofserviceduetocycloneslag
buildupsanddowntimeforboilerdeslag-
ging.Longertimeisusuallyrequiredto
coolcyclonefurnacesfordeslaggingand
maintenancework.
Thesavingsestimatesarebasedonan
additionalsevenfulldaysofoperationin
ayearbyreducingforcedoutagesforcy-
clonecleaning.Modificationcostswere
calculatedat$3,360,000.
Itwasnotedthatifthecyclonesthem-
selveswerenearingtheendoftheiruse-
fullifeandweretobereplaced,thereare
numerousupgradesthatshouldbeincor-
poratedinthereplacementcyclonesto
further
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