Hot Mix Asphalt PlantsWord格式文档下载.docx
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Hot Mix Asphalt PlantsWord格式文档下载.docx
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Hotmixasphaltpavingmaterialscanbemanufacturedby:
(1)batchmixplants,
(2)continuousmix(mixoutsidedryerdrum)plants,(3)parallelflowdrummixplants,and(4)counterflowdrummixplants.ThisorderoflistinggenerallyreflectsthechronologicalorderofdevelopmentandusewithintheHMAindustry.
In1996,approximately500milliontonsofHMAwereproducedatthe3,600(estimated)activeasphaltplantsintheUnitedStates.Ofthese3,600plants,approximately2,300arebatchplants,1,000areparallelflowdrummixplants,and300arecounterflowdrummixplants.Thetotal1996HMA
productionfrombatchanddrummixplantsisestimatedatabout240milliontonsand260milliontons,respectively.About85percentofplantsbeingmanufacturedtodayareofthecounterflowdrummixdesign,whilebatchplantsandparallelflowdrummixplantsaccountfor10percentand5percentrespectively.Continuousmixplantsrepresentaverysmallfractionoftheplantsinuse(#0.5percent)and,therefore,arenotdiscussedfurther.
AnHMAplantcanbeconstructedasapermanentplant,askid-mounted(easilyrelocated)plant,oraportableplant.AllplantscanhaveRAPprocessingcapabilities.VirtuallyallplantsbeingmanufacturedtodayhaveRAPprocessingcapability.Mostplantshavethecapabilitytouseeithergaseousfuels(naturalgas)orfueloil.However,baseduponDepartmentofEnergyandlimitedState
inventoryinformation,between70and90percentoftheHMAisproducedusingnaturalgasasthefueltodryandheattheaggregate.
11.1.1.1BatchMixPlants!
Figure11.1-1showsthebatchmixHMAproductionprocess.Rawaggregatenormallyisstockpiledneartheproductionunit.Thebulkaggregatemoisturecontenttypicallystabilizesbetween3to
5percentbyweight.
Processingbeginsastheaggregateishauledfromthestoragepilesandisplacedintheappropriatehoppersofthecoldfeedunit.Thematerialismeteredfromthehoppersontoaconveyerbeltandistransportedintoarotarydryer(typicallygas-oroil-fired).Dryersareequippedwithflightsdesignedtoshowertheaggregateinsidethedrumtopromotedryingefficiency.
Asthehotaggregateleavesthedryer,itdropsintoabucketelevatorandistransferredtoasetofvibratingscreens,whereitisclassifiedintoasmanyasfourdifferentgrades(sizes)andisdroppedintoindividual“hot”binsaccordingtosize.Atnewerfacilities,RAPalsomaybetransferredtoaseparateheatedstoragebin.Tocontrolaggregatesizedistributioninthefinalbatchmix,theoperatoropensvarioushotbinsoveraweighhopperuntilthedesiredmixandweightareobtained.Concurrentwiththeaggregatebeingweighed,liquidasphaltcementispumpedfromaheatedstoragetanktoanasphaltbucket,whereitisweighedtoachievethedesiredaggregate-to-asphaltcementratiointhefinalmix.
3/04
EMISSIONFACTORS
11.1-2
3
Figure11.1-1.Generalprocessflowdiagramforbatchmixasphaltplants(sourceclassificationcodesinparentheses).
Theaggregatefromtheweighhopperisdroppedintothemixer(pugmill)anddry-mixedfor
6to10seconds.Theliquidasphaltisthendroppedintothepugmillwhereitismixedforanadditionalperiodoftime.Atolderplants,RAPtypicallyisconveyeddirectlytothepugmillfromstoragehoppersandcombinedwiththehotaggregate.Totalmixingtimeusuallyislessthan60seconds.Thenthehotmixisconveyedtoahotstoragesiloorisdroppeddirectlyintoatruckandhauledtothejobsite.
11.1.1.2ParallelFlowDrumMixPlants!
Figure11.1-2showstheparallelflowdrummixprocess.Thisprocessisacontinuousmixingtypeprocess,usingproportioningcoldfeedcontrolsfortheprocessmaterials.Themajordifferencebetweenthisprocessandthebatchprocessisthatthedryerisusednotonlytodrythematerialbutalsotomixtheheatedanddriedaggregateswiththeliquidasphaltcement.Aggregate,whichhasbeenproportionedbysizegradations,isintroducedtothedrumattheburnerend.Asthedrumrotates,theaggregates,aswellasthecombustionproducts,movetowardtheotherendofthedruminparallel.
Liquidasphaltcementflowiscontrolledbyavariableflowpumpelectronicallylinkedtothenew(virgin)aggregateandRAPweighscales.Theasphaltcementisintroducedinthemixingzonemidwaydownthedruminalowertemperaturezone,alongwithanyRAPandparticulatematter(PM)fromcollectors.
ThemixtureisdischargedattheendofthedrumandisconveyedtoeitherasurgebinorHMAstoragesilos,whereitisloadedintotransporttrucks.Theexhaustgasesalsoexittheendofthedrumandpassontothecollectionsystem.
Parallelflowdrummixershaveanadvantage,inthatmixinginthedischargeendofthedrumcapturesasubstantialportionoftheaggregatedust,thereforeloweringtheloadonthedownstreamPMcollectionequipment.Forthisreason,mostparallelflowdrummixersarefollowedonlybyprimarycollectionequipment(usuallyabaghouseorventuriscrubber).However,becausethemixingofaggregateandliquidasphaltcementoccursinthehotcombustionproductflow,organicemissions(gaseousandliquidaerosol)maybegreaterthaninotherasphaltmixingprocesses.Becausedataarenotavailabletodistinguishsignificantemissionsdifferencesbetweenthetwoprocessdesigns,thiseffectonemissionscannotbeverified.
11.1.1.3CounterflowDrumMixPlants!
Figure11.1-3showsacounterflowdrummixplant.Inthistypeofplant,thematerialflowinthedrumisoppositeorcounterflowtothedirectionofexhaustgases.Inaddition,theliquidasphaltcementmixingzoneislocatedbehindtheburnerflamezonesoastoremovethematerialsfromdirectcontactwithhotexhaustgases.
Liquidasphaltcementflowiscontrolledbyavariableflowpumpwhichiselectronicallylinked
tothevirginaggregateandRAPweighscales.ItisinjectedintothemixingzonealongwithanyRAPandparticulatematterfromprimaryandsecondarycollectors.
Becausetheliquidasphaltcement,virginaggregate,andRAParemixedinazoneremovedfromtheexhaustgasstream,counterflowdrummixplantswilllikelyhaveorganicemissions(gaseousandliquidaerosol)thatarelowerthanparallelflowdrummixplants.However,theavailabledataareinsufficienttodiscernanydifferencesinemissionsthatresultfromdifferencesinthetwoprocesses.AcounterflowdrummixplantcannormallyprocessRAPatratiosupto50percentwithlittleornoobservedeffectuponemissions.
11.1-4
Figure11.1-2.Generalprocessflowdiagramforparallel-flowdrummixasphaltplants(sourceclassificationcodesinparentheses).3
11.1-5
MineralProductsIndustry
Figure11.1-3.Generalprocessflowdiagramforcounter-flowdrummixasphaltplants(sourceclassificationcodesinparentheses).3
11.1.1.4RecycleProcesses393!
Inrecentyears,theuseofRAPhasbeeninitiatedintheHMAindustry.ReclaimedasphaltpavementsignificantlyreducestheamountofvirginrockandasphaltcementneededtoproduceHMA.
Inthereclamationprocess,oldasphaltpavementisremovedfromtheroadbase.Thismaterialisthentransportedtotheplant,andiscrushedandscreenedtotheappropriatesizeforfurtherprocessing.Thepavingmaterialisthenheatedandmixedwithnewaggregate(ifapplicable),andtheproperamountofnewasphaltcementisaddedtoproduceHMAthatmeetstherequiredqualityspecifications.
11.1.2EmissionsAndControls2-3,23
EmissionsfromHMAplantsmaybedividedintoductedproductionemissions,pre-productionfugitivedustemissions,andotherproduction-relatedfugitiveemissions.Pre-productionfugitivedustsourcesassociatedwithHMAplantsincludevehiculartrafficgeneratingfugitivedustonpavedandunpavedroads,aggregatematerialhandling,andotheraggregateprocessingoperations.Fugitivedustmayrangefrom0.1µ
mtomorethan300µ
minaerodynamicdiameter.Onaverage,5percentofcoldaggregatefeedislessthan74µ
m(minus200mesh).FugitivedustthatmayescapecollectionbeforeprimarycontrolgenerallyconsistsofPMwith50to70percentofthetotalmasslessthan74µ
m.UncontrolledPMemissionfactorsforvarioustypesoffugitivesourcesinHMAplantsareaddressedinSections11.19.2,“CrushedStoneProcessing”,13.2.1,“PavedRoads”,13.2.2,“UnpavedRoads”,13.2.3,“HeavyConstructionOperations”,and13.2.4,“AggregateHandlingandStoragePiles.”Production-relatedfugitiveemissionsandemissionsfromductedproductionoperationsarediscussedbelow.EmissionpointsdiscussedbelowrefertoFigure11.1-1forbatchmixasphaltplantsandtoFigures11.1-2and11.1-3fordrummixplants.
11.1.2.1BatchMixPlants!
Aswithmostfacilitiesinthemineralproductsindustry,batchmixHMAplantshavetwomajorcategoriesofemissions:
ductedsources(thoseventedtotheatmospherethroughsometypeofstack,vent,orpipe),andfugitivesources(thosenotconfinedtoductsandventsbutemitteddirectlyfromthesource
totheambientair).Ductedemissionsareusuallycolle
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