developmentsin the control of fine particulate.pdf

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Developmentsin the control of fine particulateair emissions*

K.DARCOVICH,K.A.JONASSONandC.E.CAPES**Institutefor ChemicalProcessandEnvironmentalTechnology, NationalResearchCouncilof Canada,Ottawa,Ontario,KIAOR6CanadaReceived21October 1996;accepted26 November 1996

Abstract-Acomprehensivereviewof emergingtechnologiesandrecentdevelopmentsinair pollutioncontrolequipmentand processesspecificallyaddressing particulateremovalis providedhere.Emphasisis

placedonapproacheswithahighdegreeof noveltyand/or involvingnewmodificationstoconventionalsystems.Someemergingareas,suchaswasteincineration,have producednew particulatematter controlneedswhichwill bediscussed.Thesubjectandkey problemareasareintroduced,followed bya brief descriptionof thevarious particlecapturemechanismsandthe principaltechnologygroupswhichexploitthesemechanismstoachieve particlecapture.Moredetailsarethengivenfor specificinnovative projects,organized principally bytechnology,aswellas byindustrysector.Finally,alternativestoend-of-pipetreatmentstrategiesarediscussed.Anextensivelistof referencesisincluded,representativeof athoroughliteraturesurvey.

1.INTRODUCTIONIn the1970s,it becamewidelyrecognizedthatfine particlesfromindustrial pro-cessesandautomobileshadasignificantlymorenegativeenvironmentalimpactthantherelativelylarge particlesof windblowndustregulated bytheUSEnviromentalProtectionAgency(EPA)as 'totalsuspended particulatematter'(TSP).Intermsof air pollutioncontrol, particleswithaerodynamicdiameterslessthan10¡Lm,referredto asPM10,have beentargettedfor removal.StudiesinCanadaandtheUShavetypicallyidentifiedsourcessuchasfugitivedust,construction,agricultureandfuelcombustionasthemaincontributorstoPM10loadings.At presentintheUS,PM10is aguidelineusedfor air qualitystandards.Pressurehas been buildingtotightenthestandardtoPM2.5.With particlesinthisfiner sizefraction,impactsonhumanhealtharemore pronouncedduetoanincreasedrelativetoxicsconcentration,aswellasreadyrespirability.

*NRCC No.37624.**Towhomcorrespondenceshould beaddressed.



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Advancedfine particleremovaltechnologiesareconsideredto beendof pipesys-temscapableof treatingPM10(or evenPM2.5)materialto producegasstreamemis-sionsincompliancewithair qualitystandards,andwithsomemarginto beeffectiveunder

tighter anticipatedfuture

regulations.Emission

preventionstrategiesarealso

included.In1990theUSEPA produceda listof 189HazardousAir Pollutants(HAPs)under theCleanAir ActAmendments.ThisActidentifiesspecificchemicalelementsandcompoundsas harmfulto healthandenvironment,and placesrestrictionsonthe permissibleemissionslevels.Thecontrolof HAPsimpacts particulateremoval

practiceinthatsomeHAPsarereleasedin solidform(mostlymetals),or in thecaseof somevolatileorganiccompounds(VOCs),areadsorbedonto particulatesalso

presentintheemissionstream.Theremovalof particles(liquids,solidsor mixtures)fromagasstreamrequirestheir

depositionandattachmenttoa surface.Thesurfacemay becontinuous,suchasthewallandconeof acycloneor thecollection platesof anelectrostatic precipitator,or thesurfacemay bediscontinuoussuchasspraywater dropletsinascrubbingtower.Oncedepositedona surface,a meansmust be providedto removethecollected

particlescontinuouslyor atintervalswithoutappreciablereentrainmentintothegasstream.

Themagnitudeof theforcetomovea particletowardacollectingsurfaceisinflu-encedmarkedly bythesizeandshapeof the particle.Gravitysettlingwill beefficientonlyonlarge particles;40-50/-tmis thelower limitof effectiveness.Flowline

interceptionandinertialimpactionwill beeffectiveon particlesdownto2-3¡Lm.Diffusionaldepositionandthermal precipitation becomeincreasinglyefficientwithdecreasein particlesizeandarehighlyefficienton particlessmaller than0.2itm.Electrostaticforcesarethestrongestforcesthatcanactonfine particles(looselydefinedas particlessmaller than2-3um).

Controlequipmentfor particulatesfallintofivegeneralclasses,i.e.gravity/inertialsettlers;centrifugalseparators(cyclones);electrostatic precipitators;fabric, packed

bedor rigid barrier filters;andwetscrubbers.Suchequipmentmust bematchedtothe particlecharacteristicsaswellas variablessuchasflowrate,temperatureand

requiredremovalefficiency.It is wellrecognizedthatnouniversal particleremovalmethodexistswhichwillsatisfyall problemsandconditions.Thechoiceis often basedonacompromise betweentechnicalandeconomicfactors.

This presentreviewstemsfrom participationin a techno-economicassessmentof emergingair pollutioncontroltechnologies[1].Other overviewsof thefieldof fine

particleremovaltechnologyareavailableina number of monographshavingvarious perspectives[2-7].

Thisarticlefirst presentsa brief overviewof particleremovalmechanisms,thenasummaryof thegenerictechnologygroupsthathave beendevelopedtoexploitthese

mechanisms.The bulk of thereviewwillthenfocusonspecificnoveltechnologies, processesandfundamentalmechanismsthathave beenidentifiedfromanextensiveliteraturesurveyonthesubject.Theemphasiswill betohighlightinnovative pre-commercialsystemsaswellasrecentresearchfindingswith potentialapplicationtoadvancedfine particleremovaltechnologies.



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1. I.Particleremovalmechanisms

1.1.1.Sieving.Whena gasstreamcontaining particulatematter passesthrougha collectiondevice(typicallya screen)wherethegaps betweenthecollectorsaresmaller thanthe particledimension,thesolidis retainedonthe barrier.Generally,thismechanismis bestsuitedfor larger particlesandisnotcommonfor industrialgascleaning.

1.1.2.Gravitysettling.All particlesaresubjecttogravityandwillattaina terminalsettlingvelocityunder quiescentconditions.Brownianmotion,thermal,turbulentandconvectiveforcesmayactonverysmall particlesto stabilizetheir suspension

inair.Generally,however,a netdownwardmotionwillarisefor particleslarger than5ttm.Thecollector may befibrous,agranular bedor simplythefloor of asettlingchamber.Gravitysettlingis nota commonfine particulatematter removalmechanism.

1.1.3.Inertialimpaction.Larger particlesina gasstreamwill possessenoughmomentumtodeviatefromthefluidstreamlinesandcollidewithacollectingobstacleintheir path.These particlesareseparated byimpaction.Collectionefficiencyisenhanced bylarger particlediameter or conversely,a smaller collector dimension

thatwillenhancecontact.Certaintypesof fibrefilters, packed beds,spraysystems,impingingstreamcollectorsandcyclonesemploythisremovalmechanism.

1.1.4.Interception.A particle beingcarried byafluidmayexperienceagrazingcollisionwithacollector,whichis knownasinterception.Thismechanismdiffersfromimpactioninthesensethatthe particledoesnotactuallyneedtodepartfromthefluidstreamlinetocontactthecollector.Thenatureof thefluidflowislessimportantfor thismechanism.Interceptionandimpactionmayoperatesimultaneously,althoughthelatter becomesdominantat

higher gasvelocities.Internal

tortuosityina filter

mediumwillcausenumerousgasstreamdirectionchangesandwillincreasethe probabilitythatinterceptionor impactionwouldoccur.

Collectionefficiencygenerallyincreaseswithincreasing particletocollector sizeratio.Examplesincludethecollectionof particlesinfibrousor granular filter media.

1.1.5.Diffusion.Concentrationgradientsof particlesin agaseousmediumwillinducemigrationof particulatematter toregionsof lower localconcentration.Thegas phase particleconcentrationnexttocollector surfacesisnormallysmall,sothat

thereisusuallya diffusionalforceactingonthe particlestocausethemtoapproachthecollector.Smallgas phasevelocitiesand particlediametersenhancecollection bythismechanism.Alargecollector surfaceareaisnormallyrequiredfor separationsto beeffectedinthisway.Fibrousandgranular mediaandsmallliquiddropletscanremove particulatematter bythismechanism.



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1.1.6.Electrostatics.Inherently, particulatematter will possesssomedegreeof surfaceelectricalcharge.Variouswaysof conditioningthefeedstreamcanenhancethischarging.Anexternallyimposedelectricalfieldwillapplya forceonchargedmatter andwillsetit in motiontowardsa collector of

oppositeelectrical

charge.Electrostaticcollectionisimproved byincreasingthemagnitudeof theelectricalattraction,decreasing particlesizeanddecreasingtheflowrateof thegasstream.Electrostatic precipitatorsareatechnologyemployingthiscapturemechanism.The

performanceof other typesof collectorssuchasfabricfiltersandsomescrubbersisenhancedwithelectrostaticcharging.

1.1.7.Other mechanisms.Other phenomenaexistwhichcannot beusedaloneascapturemechanisms, butknowledgeof their effectsis constructiveindesigningadvancedfine

particleremoval

equipment.Particlessuspendedinagasareknowntodescendthermalgradients,havingatendencytomovetowardscoldsurfacesor awayfromhotsurfaces.Thisis knownasthermophoresis.Basically,a coldcollectionsystemwillenhance particleremovalfroma hotgasstream.Ina parallelfashion,anyspeciesof gasdiffusingwithina carrier streamwillinduce

a flux.For example, particleswouldgenerally becarriedtowarda surfacewherecondensationisoccurringandawayfroma surfacewhereevaporationisoccurring.

1.1.8.Generalremovalmechanismconsiderations.Whileallof theabove particle

removalmechanismscanactatthesametime,theyhavemoremarkedeffectsfor specific particlesizes.Figure1showsschematicallytheeffectof particlediameter ontheefficiencyof thevariouscapturemechanisms[8].It can beseenherethatif thecontributionsof thevariousmechanismsaresummed,thereexistsa particlediameter

Figure1.Relativeextentof particulateremovalmechanismsasafunctionof diameter.Adaptedfrom[8].



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for whichthe particulatematter removalis leastefficient.Ingeneral,allowingfor variationsinthematerialof whichthe particulatematter iscomposed,thediameter rangeof 0.2-2ttm presentsthemostdifficultyfor air pollutioncontrolequipment.

InFig.1,theremovalefficiencyonthey-axisisdefinedastheratioof the particulatematter concentrationinthegasstreamenteringasystemtothatleavingit.

1.1.9.Additionalconsiderations.Inadditiontothetechnologiessummarizedabove,therearea fewmoretechnicaloptionstoconsider for theremovalof airborne partic-ulatematter.Processcontrolequipmentcan beusedtooptimizethe performanceof almostalltypesof existingequipment.

Changescan bemadeto processeswhichgenerate particulateemissions.For example,cleaner burningfuelsor combustor temperaturecontrolscansignificantlyreduce

theamountof particulatematter createdandcarried byfluegases,therebymakingitsremovallessdifficult.Modellingof unit performancecanalso be beneficial.Anunderstandingof how

alltherelevantvariablesinteractcan pointtosystemmodificationswhichenhance particulatematter removalfromgasstreams.Areviewandanalysisof various perfor-mancemodelsfor filtersandelectronicair cleanerswasdone byLawlessetal.[10].Computationalfluiddynamic(CFD)simulationshaverecently beenmadetoevaluatedustladengasstreamtwo-phaseflow patternsinsideair pollutioncontrolequipment[11, 12].

2.RECENTADVANCESINUNITOPERATIONSFOR AIRBORNEPARTICULATECONTROL

Described belowarethevariousconventionaltechnologiesthathave beendevelopedtoremove particulatematter fromgasstreams.Thecapturemechanismsdescribed

previouslyareexploitedintheseunitsor processes.Eachunitoperationwill betreatedin itsownsection.Theywill beintroduced

withagenericdescription

of the process

andequipment.

Thisisthenfollowed by

adiscussionof specificrecentinnovationsandtechnologydevelopments.Theemphasisinthissectionis onthedesignandoperationof the physicalequipment.Section3discusses particulateemissioncontrolfroma process perspective.

Figure2 is agraphicalguideline,mappinga suggestedtreatmentmethodfor a particle-ladengasstreamdefined byadesired particulateremovallevelandoperat-ingfiltrationvelocity[3].Thereis aroughmonotoniccorrespondence betweenthefiltrationefficiencyandthe pressuredropacrossthefilter for agivenflux.

2.1.CyclonesThegeneral principleof inertialseparationisthatthe particulateladengasisforcedto

changedirection.Asthegaschangesdirection,theinertiaof the particlescausesthemtocontinueintheoriginaldirectionand beseparatedfromthegasstream,exploitingtheinertialseparationmechanism.



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Figure2.Unitoperationmapfor airborne particulatetreatment,with parametersof filtrationefficiencyandfiltrationvelocity.Adaptedfrom[3].

Cyclones,wherethegasisforcedtospinina vortexthroughatube,arethemostcommontypesof inertialseparators.Atypicalcycloneconfigurationis showninFig.3.

Inertialseparatorsarewidelyusedfor thecollectionof medium-sizedandcoarse particles.Their relativelysimpleconstruction,low pressuredropsandabsenceof moving partsmeanthat boththecapitalandthemaintenancecostsarelower than

for alternativemethods.Ingeneral,however,cycloneefficiencydropsif thefinescontentof the particulatematter issignificant.Theyaretypicallyusedas precleanersupstreamof other devicestoreduce particulateloadingsandtoremovelarger,abrasive

particles.Conventionalcycloneseparatorsareseldomcapableof thenecessaryremovaleffi-

cienciesfor anentire particlesizerangeof anindustrialgascleaningapplicationunder presentPM10guidelines,if designedaccordingtostandardelementarycyclonetheo-ry[13].Thedeviation between predictionand performancearisesfromthe presenceof secondarycurrentswithinthecyclone body,whichdisturbthe predicted process

of separation.Primaryflowcharacter can berestored byattentionto thedetaileddesignof thecyclone,includingthegeometryof theseparationchamber,the positionof openings,useof flowguideswithinthecyclone,thedimensionsandthegeometryof thehopper, bleedingand bypassingof thegas,useof multicyclones,andmeansfor dustagglomeration.Alternatively,a 'duststrand'cyclonecan bedesigned,exploiting



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Figure3.Simplifiedschematicof acyclone.

thesecondaryflows(Deanvortices)whichattainhigher localsolidsconcentration,enhancing particleremoval[14].Someinnovativemodificationshave beenmadetoconventionalcycloneunits.The

applicationof anexternalelectricfieldtoacycloneunitfilteringlowresistivitysilicaroughlydoubledthecollectionefficiencyof particleslessthan5Amindiameter atlowflowrates[15].Built-in particulatematter removalcapabilitywasconsideredwiththedesignof acyclonicsuspensioncombustor for the burningof pulverizedcoalor coal-water slurry[16].Cyclonicseparatorsarealso beingconsideredasdieselsootcollectors.Aunitdesigned byKudosCorp.intheUK involvesspinningagasthrough

a tubewhilealsospinningthetubeitself,toseparate particlesdownto0.11 /,Lm.Thissystemexploitssecondaryflows(Rankinevortices)andoperatesatrotationratesof 60000r.p.m.[17].It hasyetto befieldtested.Acompetingsystemmakesuseof a sprayelectrodewhichcharges(presumably,neutralizes) particlessothattheyagglomerateandarecaptured byacyclone.Between75and90%of the particulateemissions,downtodiametersof about0.6>m,werecaptured bythissystemduringtestswithaPeugeotdieselengine[18].

2.2.Electrostatic precipitators(ESPS)

ESPisa particlecontroldevicethatuseselectricalforcestomove particlesoutof theflowinggasstreamandontocollector plates.The particlesaregivenanelectriccharge byforcingthemto passthroughacorona,aregioninwhichgaseousionsflow.Theelectricalfieldthatattractsthecharged particlestothewallscomesfromelectrodesmaintainedathighvoltagein thecenter of theflowchannel.Thisarrangement



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is shownschematicallyinFig.4. Chapter 10of [4]containsa number of goodillustrationsshowingtheconstructionandconfigurationof entireESPunits.

Oncethe particlesarecollected,theymust beremovedfromthe plateswithout

anyappreciablereentrainmentintothe

gasstream.Thisis

usuallyaccomplished byknockingthemloosefromthe plates,allowingthecollectedlayer of particlestoslidedownintoahopper.Some precipitatorsremovethe particles byintermittentor continuouswashingwithwater.

ESPshaveenjoyedverywidespreadusefor fine particulatematter removalappli-cations.Theyareamature,establishedtechnologyandthemovefor innovationintheESPareaappearssomewhatlessurgentthanelsewhere. Nevertheless,thereisstillsignificantandinterestingactivityfor updatingESPtechnologyfor theimprovedcollectionof ultrafine particulates.Quiteextensivereportingonrecentwork appears

regularly[19, 20].Inviewof the presentlargenumber of ESPinstallations,aconsiderablemarketexistsfor retrofitopportunities.Thetighteningof emissionsstandardsfor S02and NOxandsub-100 >m particulatematter mayimpactonthecapabilitiesof theESPinfrastructure.Anoverviewof someof theissuesfacingtheESPuser community,andanintroductionto someof the basictypesof retrofitoptionswasgiven byOffenandAltman[21].Beyondtheneedtoconditionthefluegas,typicalupgradeoptionsincludeenlargingtheESP,sectionalizingtheESP(dividingfluegas passagesintoagreater number of segmentswhichcanindividually beoperatedatmoreoptimumlevels),spacingthe

collector platesfurther apart,implementing processcontroland/or improvedrappingandcollectionequipment[22].Twoindependentstudies,focussingonthetechno-economicsof ESPusefoundthatwiththeincreasinguseof high-resistivitycoals,ESPscannotcompetewith pulse-jetfabricfilter systemswhen particulateemissionsregulationsallowonly0.01lb/MBtu[23,24].

Figure4.Schematicof atypicalelectrodeandcollector arrangementfoundinsideanelectrostatic pre-cipitator.



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Anumber of recent papersdetailspecific physicalmodificationstoESPstohandlehigh-resistivityflyash.CasestudiesarediscussedwhereexistingESPinstallationswererebuilttohouselarge plateheightsandwider platespacings[25].TheESPcan

be placed

downstreamof anon-leaking,high

dustloadgas-gas

heater (withwater astheheatexchangemedium)toreducethegastemperaturethroughtheESPtothe90-100°Crangerather thantheconventional130-140°Crange[26].Pilottesting

hasshownthatthedecreasedtemperaturedecreasestheelectricalresistivityof thedust,improvingcollectionefficiency.Hitachihasintroducedtwonewconfigurationsfor ESPs[27].ThemovingelectrodeESPhasthe platessectionedintoshort panelsthatcirculatelikea verticalconveyor belt.Thedustis collected byrotating brushesinahopper zonewherenogasflowsand particulatematter reentrainmentisminimal.Theyhavealsodevelopeda 'saw-edged'wirethatshowssuperior staticdischargecharacteristics,usefulfor

rapidlychargingsubmicron

particulatematter whichismore

abundantwhenthe boiler fuelis oilinsteadof coal.Operatingexperiencewithintermittentenergizationof theESPtosuppress back coronaandthusincreaseESPefficiencyfor thecaptureof high-resistivity particulatematter wasreported[28,29].Exampleswerecitedongasstreamtreatmentfromcoal-fired boilers,oil-fired boilers,Kraft pulp boilersandiron-oresintering plants.

Anoveltwo-stageESPwithanelectrostaticagglomerator betweenthestageshas beentestedatthelaboratoryscale[30].AquadrupoleACelectricfieldisappliedtotheremainingfinesafter thefirstESPstage,inducingoscillatorymotionandelectrical

polarization,whichhave beenshowntoincreasesubmicronmeandiameters by400%andincreasecollectionefficiencyfrom95to98%.Asystemhas beendesignedwithflowdiversionduringrappingtominimizefinesentrainment[31].Costestimatesfor thistechnologyshowthatthistechnique provides benefitsfor emissionslimits

below50mg/Nm3.Specialhorizontalstripcollectorswithout baffleshave beenshownto increaseESP performanceonlargeinstallations[32].Seoul-Sharp-CEDhasdevelopedwhattheyterma 'SUPER-ESP'[33].Byseparatingthechargingandcollectingfunctions, bothstepscan bedoneindependentlyina moreconsistentandoptimizedway.Ahigh particlechargecan beimpartedfor anylevelof resistivity,andthenthese particlescan besubjectedtotheappropriateelectricfieldfor collection.Thetemperaturecan becontrolledintheseparatechargingareatomodifytheresistivity.Insteadof anESPwithseverallongconventionalsections,a smaller ESPresultsfromthisconfigurationwhena number of chargingandshortcollector pairsareused.For low-resistivity particulatematter thisdesigncan provideaunitaboutone-thirdthe

physicalsizeof aconventionalESPandthesizeisfurther reducedto one-sixthfor high-resistivity particulatematter.

Theelectricalnatureof precipitatorsmakethemespeciallysuitablefor digitalcon-trol.A recentexpertsystemcontroller employingfuzzylogicallowssomeof themoredifficultoperationsto beruninamoreoptimizedfashion,suchasdiagnosingfaults,start-upandshutdownroutinesaswellascontrollingtheoperatingefficien-cy[34].Fieldtestinghasshownsignificantemissionsreductionsaswellasloweredenergyconsumption.Other typesof controllersfor spark rateandspark anticipationcan better enableexistingESPunitstohandlefluegasandflyashvariabilityin anenergyefficientmanner [35].



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WetESPsystemscontinueto playa keyrolefor specialized particulatematter removalapplications.For certainlower temperatureapplications(belowthedew-

point)onlywetelectrostaticsystemscan beused.Dissolvingchemicalagentsinthewash

liquidcan

providea meanstofixunwanted

gaseouscomponentsalongwiththe

suspended particulatematter [36].InJapan,MitsubishiHeavyIndustriesLtdhavedevelopedandtesteda 'highvelocity'wetESP,capableof workingatfaceveloci-tiesof 10m/s,therebyallowingitsinstallationwithinexisting plantconfigurationsinsomecases[37].WetESPsarewellsuitedtoapplicationssuchascoal-firedmag-netohydrodynamicsystemswherethefinescontentinthefluegasissubstantial[38].Asa retrofitoptionfor wasteincineration plants,wetESPshave beenaddedafter awetscrubber totreat particulatematter andacidgas.AunitknownastheSonicKleenwetESPsystemconsistsof a bundleof hexagonal(tosavespace)downflowtubesmadeof a

conductingsteel

alloy,whichcontaindischargeelectrodes[39].Condensed

liquidsfromthegasstreamirrigateandhelpcarry particulatestothe bottomof theunit.Variousinnovationshave beenmadeto addressspecificapplicationneeds.The

final particulatematter cleanupfromwoodfired boilersis oftendonewithESPssincetheflyashhasa lowresistivity.Rappingfor suchsystemsis of paramountimportanceasresinsandunburnedmaterialmay be presentonthecollector surface,andanysparkingcouldcauseit toignite.Sonichornswereinstalledandtested,replacingrappersfor cleaning precipitator internalsfor woodfiredapplications[40].Emissionsof particulatematter withsonichornrappingwereshowntohavedecreased.Anoptionalwayof treatingdieselsootis withanelectrostaticagglomerator [41].Acorona-less pipe-typeESPisusedtocollectandagglomeratedieselsoot particleswhichgrowintolarger particlesandmay becollecteddownstreamwitha simpleinertialdevice.

Anumber of theoreticalmodelshave beendevelopedrecentlytocharacterizeESP behaviour.The performanceof existingESPscan beenhancedwithimprovedfeedstreamdistributionthroughtheir internalvolume.ACFDstudywasconductedtohelpdesigna flowdistributor for ESPsusedinthecementindustry[42].Thedesignrecommendationswereusedto bringadry-processcement plantintoregulatorycom- plianceinIndiana,USA.Ona moremicro-scale,a CFDstudywasmadeonionicwind,whichisthesecondaryflowthatdevelopsunder massconservationrequirementswhenionizedgasdriftstowardsthecollecting plate.It wasfoundthationicwindeffectsaresignificantfor superficialflowvelocitiesunder 0.6m/sanda collectionefficiencyreductionof over 10%mayoccur whenthesuperficialflowvelocityfalls

below0.2m/s[43].Conditioningthecarrier gasoftenincreasesthefeedstreamhumidity.Theinfluenceof humidityonthechargedensityandtheelectricfieldina

precipitator wasmodelledandsolvedwithgoodresults[44].

2.3.FabricfiltersFabricfiltersremovedustfromagasstream by passingthestreamthrougha porousfabric.Dust particlesforma porouscakeonthesurfaceof thefabric.Itisnormallythiscakethatactuallydoesthe bulk of thefiltration.



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Themanner inwhichthedustisremovedfromthefabricisa crucialfactor inthe performanceof a fabricfilter system,sothatfluxlossesfromexcessive pressuredropwitha thick cakeare balancedagainstdustleakageoccurringwithtoothina cake.

Fabricfilter systemsarefrequentlycalled baghouses,sincethefabricisusuallymountedincylindrical bags.Thetwomostcommon baghousedesignsarethereverse-air andthe pulse-jettypes.Thesenamesdescribetheaffiliatedfabriccleaningsystem.

Therehas beenanincreasingrecenttrendtowardsselectingfabricfiltersfor newequipmentinstallationsandupgradesinexisting plants.Improvementsinfabricfilter

performance,simplicityof operationandlowcapitalcosthave beenmotivatingfactors.Themostsignificantinnovationshavecomeintheareaof fabricmaterials.

Sincethelate1980s, baghouseinstallationshave beguntoappear featuring pulse-jetfabricfilters(PJFFs).Thesesystemsarequicklyreplacingconventionalreverse-gas-

cleaned baghouses(RGB),as themostwidelyusedfabricfilter unit.PJFFsarecharacterized bytheir useof periodicshort, powerful burstsof air tocleanthe bags,allowingtheunitto beoperatedattwotothreetimesthefacevelocitiesof thereverse-gas-cleanedsystems.Ingeneraltheyexhibitimproved pressuredropcharacteristicsandcan beretrofittedintoESPcasings.SeeFig.5.

Figure5. Schematicof a pulse-jetfabricfilter baghouse.Thefilter bagsontheleft-handsideareshowninfiltrationmodeandthoseontheright-handsideareshowninthe pulse-jetcleaningmode.



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Athree-partseriesof paperswas publishedto provideanoverviewof the per-formanceandoperatingexperienceswithPJFFsfor coal-fired boilers,aleadingap-

plication[45-47].ExtensivedataonvariousPJFFinstallationswere provided.In

general,theseunits

operate below0.03lb/MBtu

(0.054mg/Kcal)whichistheUS

EPA particulatematter emissionsstandard.WovenfeltssuchasDralonT8,Rytonand Nomexwererecommended bagmaterials.Teflon bagshaveshownsomediffi-cultywith particulatematter collection, possiblyowingtothelargefibrediameters,despitetheir abilitytooperateinhotter andmorecorrosiveatmospheres.Part2 of theseriesreportedon pilottestsfor fossilfuelinstallations,includingoil-fired boilerswithuniformlygoodresults.Higher frequency bagcleaningwasrequiredfor highashcoals.Part3 of theseries providesaneconomiccomparisonwithESPsandRGBs.ESPscouldonlycompetewithPJFFsystemswithlow-resistivityfeeds.ComparedtoRGBs,thePJFFsystemsoffer a 10%lesslevelizedcostover a 30year plantlifeaccordingtothisanalysis.TherearesomecommercialPJFFsystemsavailable.TheOPTIPULSEsystemfromABBisaimedatsteelmakingapplications[48].TheHosokawaMicropulsystemwasdesignedwithinletgasdiffusersto provideuniformface pressuresover thefilter bagsurface,givinganoverallimprovementin performance[49,50].Similarly,filter cagedesignsmadefrom perforatedmetalsheet,rather thanwiremesh,subject pulse-jet

bagstolessstressand prolongtheir lives[51].Baghousesareoftenemployedtocollect particulatematter fromsomeof thefeed

conditioning processesdiscussedinSection3.3.Filter bagsmadefrom3M Nextel

312wovenalumina/boria/silicaceramicfibrehave beeninstalledintheSNRB pro-cessandhave beensuccessfullytestedattemperaturesupto590°CagainsttheEPAstandards[52,53].

Thefabricmaterialsthemselvesareof coursethesubjectof considerableresearch.Theextramechanicaldemands placedon pulse-jetsystemsmakeglass-basedfilter media pronetofailure.Asaconsequenceof this,therehave beennumeroussyntheticfeltsdevelopedfor someof theseapplications,suchasindustrial boilers,combustingfluidized bedsandincinerators[54].Polytetrafluoroethylene(PTFE)or Gore-Texefilter materialshave beendeveloped.Someearlywork showsthatthefavourable

PTFEsurface properties permiteasydustlayer removal,andoperationattemperaturesupto 260°C.Fine particulatematter still posesa problem,with50%escapeat0.86 jum[55].Pulpand paper bark boilers presentspecial problemsas thesegasstreamscancontainincandescent particles.Temperatureresistanceupto600°Cis

providedwithBiothermicastainlesssteelfabricfilters.Afault-free4-year testwasconductedatanindustrialsitewiththismaterial[56,57].

Ona morefundamentallevel,therehave beenstudiesconductedonsomeof thedetailedfabric properties.A parametricstudywasconductedontheeffectsof yam

processingandweavingvariablesonthefiltrationefficiencyof wovenglassfabrics.

It wasfoundthat processingconditionswhichkeeptheglassfilamentsintheyarn bundleseparated producedthe best performance[58].A number of commerciallyavailableconductivefabricshave beendesignedtoreduceelectrostaticchargeindustcollectorswhereflammabledustsare present,suchasinflour mills,woodworkingoperationsand polyvinylchloridemanufacturing[59].



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Analternativetothefilter bagisthefilter 'box'or cartridgefilter.Theseunitsaremadeof stiff corrugatedfilter materials,whichcanoffer muchhigher filter surfaceareacomparedto baghousesof thesamefootprint.Manyof thetechnicalcapabilities

aresimilar to baghouses, buttheir designmakestheminherentlylesscleanable.Theyareratedfor particulatematter loadingsof about25%of thatfor baghouses[60].Owens-ComingFiberglashave beendevelopingavanadium/titaniumcatalystcoated

wovenglassfabricfor simultaneous NOxand particulatematter control.Over 90% N0?removalhas beenobtained, butmanyoperating parametersrequiredtoachievethisleveleconomicallyhaveyetto beresolved[61,62].

Fabricfilter systemscan beelectrostaticallyenhanced.Anearlystudyshowedthatanelectricfieldcanalter thedustdeposition patterninsidea bag,givingalower pressuredrop[63].A number of analyticalandexperimentalinvestigationsof this

phenomenaarereviewed

byRaoandMurthy[64].A portableelectrostaticcartridge-typefilter hasrecently beendeveloped,for applicationssuchascookingodour controlandsmoketreatment.Over 98% particulatematter capturehas beenachievedatdiameters between0.01and1.0 however facevelocitiesmust belessthan0.1m/s[65].

2.4.WetscrubbersIntheseunits,a particleladengasstreamissubjectedto a sprayof liquidwhichcontactsandcapturesthesolidmaterial.Avarietyof geometriesanddesignsareavailablefor implementingthistechnique.Themechanismsof impaction,interceptionanddiffusionall playa rolein particulatematter captureinwetscrubbers.

Condensationscrubberscan beconsideredto bea subclassof wetscrubbers.Incondensationscrubbers,usuallysupersaturatedwater vapour is broughtintocontactwith particulatematter andliquidcondensationoccursonthesurfaceof thesolids.Thiscancreatea nucleationsitefor agglomerationwithother wetted particles.Themassof the particlesgrowsinthiswayandtheycan bemorereadilycollected byinertialimpactionor evengravitysettling.Thethermophoresismechanismis alsoexploitedincondensationscrubbers[9].

Wetscrubber technologiescontinuetoenjoyfavour for particulatematter removal,especiallyincoal-fired power generationapplicationswherethefluegasalsocontainsS02and NOxandcan beremovedsimultaneously bythescrubbingliquid.Con-ventionalunitsincludeventuriscrubbers,mechanicallyaidedscrubbers, pump-aidedscrubbers,wetted-filter typescrubbersandtrayor sievetypescrubbers.Anoverviewof recentwork inthisfieldis providedinan NTIS bibliographicdatabase[66].SeeFig.6.

Somerecentdevelopmentsinwetscrubbingequipmentareintheareaof rotaryor centrifugaldesigns.Onesuchsystem, basedonaConfinedVortexScrubber (CVS),for

fine particulateremovalfromcombustionfluegaseshas beendevelopedandconsistsof acylindricalvortexchamber withmultipletangentialfluegasinlets[67].Water isintroducedintothechamber andis confinedwithinthevortexchamber bytheextremelyhighcentrifugalforcesgenerated bythegasflow.Theconfinedwater formsalayer throughwhichthefluegasis forcedto bubble,leadingtocollection



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Figure6.Componentsof astandardventuriscrubber.Theoutletisoftenconnectedtoamisteliminator wherethewater andgasstreamsareseparated.efficienciesinexcessof 99.5%for fineflyashof diameter 3gmand98%for 0.3¡Lmdiameter particles.A commercialunitknownas theRotorfilterTMmakesuseof scrubbingwater inconjunctionwitha seriesof rotatingairfoils,arrangedsothatthetwostreamscarryingtheliquidandsolid phasescontacteachother atrightangles.Inthisway, particulatecaptureoccursinrecirculationregionscreated bythechevronshapedspokes[68].Theseunitsaredesignedfor smallindustryandcanhandle

between50and500m3gas per minute.Upwardsof 99%of particulatesat0.25diameter can berecovered.Asystemdesignedfor the power industriesemploysa

jet bubblereactor andvesselsmadefromfibreglassreinforced plasticstowithstandthecorrosiveelementsinthestreams[69].DownstreamfromanESP,thisscrubber

wasabletoremoveupwardsof 90%of theescaping particulatematter whilealsoachievinggoodS02control. Newdevelopmentscontinuetooccur withventuriscrubbers.Aspecialintegrated

systemhas beendesignedwitha two-fluidnozzlewhichcan becontrolledtoenhance particleremoval,aided byimpingement platesinthevessel[70].Another suchsystem,aimedatacidgascontrolwhichscrubs particulatesaswell,isintegratedwitha steamstripper toremoveandseparatesaleableS02andVOCs[71].Amulti-stagedsystemusinga saturatedvapour chamber to aidin thenucleationof water-wettablesub-micron particulatematter canachieve99%removalat0.155 >m,withhighturbulence

contributingto particlecaptureatthescrubber walls[72].Somerecentwork has beencarriedoutontheelectrificationof wetscrubbingsys-tems.Anionizingwetscrubber has beendevelopedtoremovefine particulatematter downto0.05itm,whilesimultaneouslyremovingacidgases[73].Itsmainareaof applicationiswasteincineration.Theunitcombinesthe principlesof wetscrubbing



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andelectrostatic precipitation,chargingandagglomerating particulatematter withsubsequentcollection byinertialimpactionandimageforceattractionwithinanirri-gated packed bed.Ascrubber withoppositelychargedrotating brushesdownstream

of anionizingatomizer toremove particulatematter is presentlyinthedevelopmentstage[74].Thisdesigntargets particulatematter below5ttmdiameter,andcanoperateatcomparativelylowliquidadditionrates.Treatmentof metalfumefromahazardouswasteincinerator inanionizingwetscrubber wasinvestigated[75].Col-lectionefficienciesrangedfrom22to71%andweregenerallyhigher for lessvolatilemetals.Afundamentalstudyhasmodelledacomplexinterdependenceof theappliedfield,the particlechargeandthedirectionof thefieldrelativeto particlemotionfor electrifiedwetscrubbers[76].Additionally,afundamentalstudyexaminingtheinflu-enceof surfaceelectric propertiesof spraydropletsand particulates,inconjunctionwithsurfactantadditionfor dust

suppressionhas beencarriedout

[77].Themostim- portanteffectfor enhanced particulatematter capturewasanelevateddropletchargefor cationicdropletsolutions.Somerecentwork hasaddressedsector-specificwetscrubbingneeds.Mistelimina-

torsusedinfluegasdesulfurizationareexpectedtohavesomedifficultycomplyingwithanticipated0.002lb/MMBtu(14mg/Nm3)emissionsregulations[78].It has

beenshownthathorizontalflowconfigurationscanupgradethe performanceof sys-tems,whichmayneedtoremoveupwardsof 99.5%of PM10dust.Atwo-stagescrubber designedfor thealuminumindustryhas beenshowntoremoveover 99%of

particulatematter aswellasHClandmorethan90%of

C12fromaluminumreduc-

tioncast-housefurnaceoffgas[79].Irrigationwithchemicalcleansingagentsallowscrubbedmaterialsto berecycledintothereduction process.Finally,a lessrecent, butrarelyimplementedtechnologyis thatof counterflow

inertialimpactionwetscrubbers[5].Anearlystudyfor thecollectionof alunitedust(analumino-silicate)hadtwocollidingaxisymmetricdustygasstreams,sprayedwithfluidattheimpactionzone.Decelerationof the particlesandturbulentmixingattheimpactionzone bringaboutagglomerationof the particulates,wherebytheydropoutof thestreamandcan beremoved[80].

2.4.1.Condensationscrubbers.Condensationscrubberscan beconsidereda sub-classof wetscrubbers,althoughtherearekeydifferencesinthemechanismsemployedtoremove particulatematter.Inthecaseof condensationscrubbers,the particlestravelinavapour saturatedgasstream,wherevapour condensesontheir surfacesandactsasanagglomeratingagent.Onesuchdesign,for thetreatmentof sub-0.25itm particulatematter frommunicipalwastewater incineratorsis documented[81].Existingwetscrubberscan beretrofittedtooperateinthecondensationmode.Asimilar designfor municipalwasteincineratorsmakesuseof fluegasmoisturemixedwitha coldgasstreamresultinginfluxforcecondensation

scrubbing[9].

2.5.Granular bed filters(GBFS)In ananalogousfashionto wetscrubbers,a particleladenstreamcan be passedthrougha bedof solidmaterialwherethe particulatematter iscaptured.The bedcan



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bestationary,movingor fluidized.Thecollector materialtypically possessessomefavourablesurface propertiestoenhancecaptureof the particulatematter.Granular

bedfiltersareoftenusedwhenwastewater problems posed bywetscrubbingsystemsare

unacceptable,suchasfor fluorinecontrolinsomealuminum

plants.Fluidized

GBFsarecurrentlyunder considerationfor hotgascleanupapplications,especiallyasanalternativetoceramiccandlefilterswhichhavehada troublesomedebut.Are-centstudyhasfoundthattheeconomic performanceof GBFscomparesfavourablytocandlefiltersfor coal-fired power generationfacilities[82,83].Theseunitsareme-chanicallysimpleandrequirelessfine-tuningandmaintenance.Themovingcollector

bedhassome by-productvalueandalsoeliminatestheneedfor costlyfilter mediacleaning.Morethan97%of flyashcan beremoved byGBFsata facevelocityof 1.6m/s(basedonthestandpipecross-section), buttheyareknowntohave problemscollecting particulatematter under 2ttm.Atfacevelocitiesover 4m/s,thecollectionefficiencydropstoabout80%[84].MovingGBFshavealso beeninvestigatedcombinedwithelectrostaticenhancement.The basicunitdeveloped bytheCombustionPower Co. performedwellfor particulatematter over 2 >m.Variousfieldstrengths, bothunder ACandDCmodeweretested.TheACenhancementat75Hz producedaslightoptimum.DCoperationalsoshowedvery promisinginitialresults, butitsimplementationisconsideredto beless practical[85, 86].Atlowfacevelocities(0.11m/s),upwardsof 98%of 0.5>mflyashcould

becollected.Theefficiencydropsoff to50%atfacevelocitiesnear 0.7m/s.Anovelsystemhas beendevelopedwitha fluidizedGBFexitingacrossa metal

filter screen[87].Asketchof theunitis showninFig.7. Thescreensurfacecan becleaned bymomentarilyinterruptingthegasflow.The pressuredropinthissystemislow,althoughmaterialfiner thanabout1.5 pmisdifficulttocapture.Thissystemallowsrelativelyhighfacevelocities,over 2.5m/s.Therearea fewinstanceswhereGBFsystems provideagood particulatematter

removalsolution.Mosttraditionalair pollutioncontroldevicesareineffectivefor collectingsubmicronmetalfume,whichoftenarisesin wasteincineration.Oneapproachhas beentotestvarioussorbentsina fluidized bedfilter [88];another has

beentouseactivatecharcoal packed beds[89].

Anumber of fundamentalstudieshave beenconductedrecentlytofurther theunder-standingof granular bedfilters.Finematerialwasfoundto bemorereadilycollectedatelevatedtemperatures[90].Reynoldsnumber effectswereconsideredinanother study[91].

Sometimesgranular bedfiltersarereferredtoasdryscrubbers, butmoreoftendryscrubber meansa somewhatsimilar, butdifferenttypeof unit.Anumber of systemsexistwherethestreamdustmay beremoved, butthena sorbentisintroducedfor NOxand/or SOxremoval.Thiscreatesa needfor subsequent particulatematter removalfromthegasstream.Inviewof this,thesorbentmaterialcan bechosensothat

it issufficientlylargeanduncohesiveto beeasilyseparated bylow pressuredrop baghouses.Withanappropriatechoiceof sorbent,thesulfur andnitrogencontaining productcan beusedtomakea number of saleable by-products[92,93].Anelectron beamdryscrubber existsinwhichtheelectronsgenerateOH-radicalswhichoxidizeS02and NOxintosulfuricandnitricacid, precursorsfor makingfertilizers[94].



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Figure7.Fluidizedgranular bedfilter fittedwithmetalscreento preventfilter materialentrainment.Adaptedfrom[87].

2.6.CeramicfiltersCeramicfiltersarerigid barrier filtersconstructedwith poresizesdesignedtoremovefineairbornesolids.The physical propertiesof theceramicmaterialallowtheseunits

to beoperatedunder hightemperaturesandincorrosiveenvironmentsaswellas beingsuitablefor cleaningwithcorrosiveagents.Theyaremadeintwostandardformats:dead-endlowdensity'candle'filtersandmulti-channelcross-flowmodules.

Intherapidlyevolvingfieldof hotgasfiltration,ceramicfilter elementshave provento besuitablefor efficient particulateremoval,withtheabilityto performinharshenvironments.Ceramicmembraneswerefirstusedinthe1940sfor uraniumenrich-ment,yetit took untilthemid-1980sfor particulatematter removalapplicationsfromgasstreamstofirstappear.Generaloverviewsof thisfieldare provided byEggerstedtetal.[95]aswellas byWhiteetal.[96].Recently,ceramicmaterialshavealsofound

their wayintohotgasapplicationsasstructuralfilter components,replacingexoticalloysandsimplifyingdesignapproaches.Anumber of differentceramicfiltrationde-vicesarecurrentlyavailable,eachwithadvantagesanddisadvantages.Thesedevicesincludeceramiccrossflowor ceramicmembranefilters,ceramicfabricfilter bags,and porousceramicfilter tubesor 'candles'.Inaddition,thesedesignsareavailablein



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differentceramicmaterials,eachhaving propertieswhichmay or maynot besuitablefor agiven processapplication.

Despitetheroughly40%higher cost per unitfiltrationareacomparedto bagfilters(1991),ceramicunitsaregaining popularity[97].Inadditionto benefits previouslymentioned,theycanalsohandleheavydustloadingsandfacevelocitiesupwardsof 3cm/s,comparedto about1.5cm/sfor fabricsystems.Providedthatthesystemselectediscleanable,theunitlifecan beseveraltimeslonger thanwithother materials.

High-efficiency particulateair (HEPA)filtershave beenin existencefor over 50years.Theyaretypicallymadeof glassmicrofibresandareusedfor veryhighefficiencyremovalof submicron particulatematter.HEPAfiltersaresuitablefor applicationswhereaveryhighlevelof air purityisrequiredor for theremovalof hazardous particulatematerials[98].

2.6.1.Candle filters.Figure8 showsa schematicunitwhereceramiccandlefiltersareusedtoremove particulatematter fromagasstream[3].Here,thefilter mediumis providedin theformof long,hollowtubesor 'candles',closedat oneendandhungverticallyfromasupport platesothatthegas passesfromtheoutsideinwards.Adustcakeaccumulatesontheoutsidesurfaceof thecandleandiscleanedatregular intervalswithastrong pulseor jetof gassentfromtheinsideoutviathe pulsenozzles.

Therearetwocommontypesof filtrationmediausedinceramiccandles.Thefirstisahigh-densitysinteredor bondedgranular siliconcarbide(SiC).Theother type

Figure8.Simplifiedschematicof aceramiccandlefilter unitwith pulseair cleaning.



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is primarilyalow-densityalumina(ash03)intheformof bondedfibres.Thelow-densityunitsshowlessresistancetoflow,areless pronetothermalshock andarelessexpensive[99].

Themostattractivefeatureof ceramiccandlefiltersistheir abilityto performgascleaningathightemperatures, presentlynear 1000°C.Thedemandfor hotgascleaningisstrongestincoal-fired power plantsusing pressurizedfluid bedcombustion(PFBC)or integratedgasificationcombinedcycle(IGCC) processes.Conventional

bagfiltershavethermaltoleranceswhichrequirethatgasstreams becooled,typically bydilutingthestreamwithatmosphericair.Directhotfiltrationeliminatestheneedfor extra processequipment,reducesthegasvolumefor treatmentandallowswasteheatto berecovered,givinga 2-3%increaseinoperatingefficiency[100].

TheSchumacher SiCcandlefiltersweresomeof thefirsttoundergoextensivetestingin pilot

facilities.In1988atGrimethorpe

intheUK,

a pilot

hotgas particulateremovalsystemusingceramiccandlefilterswas builtina PFBCtestfacility[101].EarlytestswereconductedwithSchumalithSC40modelcandles(mean poresize100/tm),as

wellastheDiaSchumalithF40candleswhicharemadewithanadditionalsurfacelayer of aluminafibres.TheDiaSchumalithF40candleswerefoundto besuperior

becausetheywereabletowork inasurfacefiltrationmodeathigher facevelocitiesandrequiredlower stressesfor dustcakedetachment.TheF40candleswereusedinmostsubsequentwork atGrimethorpewheretheygave promisingdustremovalresultsover 800h testsconductedat850°Cand10 bar pressure[102].Further testingatGrimethorperevealedthatextendedusewithcold

pulsecleaningcausedthecandlesto

liftanddescendrepeatedlyresultinginfractures bothinthecandlesandtheir sealinggaskets[103].Asimilar pilottestwasreported bySchumacher themselves,withover 4000hattemperatures between780and850°C.Attemperaturesabove700°Cmorethan99.999%of all particulatematter wasremoved,withthemost penetrationoccurringinthe0.2-0.4fLmsizerange(cf.Fig.1)[104].

Increasingly,demandingtestingof theDiaSchumalithF40has beenreportedmorerecently.Concernsover performancehavefocussedoncandleintegrityover repeatedhot-coldcleaningcycles.Thestrengthof thecandlewasfoundtodecrease by2.2%over a 4 month periodattestsconductedat 850°C.Athermalshock (temperaturechange)of over 600°Cwasfoundtooccur bynumericalsimulationof thecleaning pulseflowinsidethecandle[105].By1993,newtestshadshownimprovedversionsof theDiaSchumalithF40candles, butstilla recommendationfor hot pulsingwasmadeto minimizetheseverematerialsduty[106].Other testshaverevealedthat

breakageof candlefiltersarosefromthemechanicalstressesassociatedwithliftinganddropping.Specifically,thedeteriorationmechanismsweretheoxidationof SiCtoformcristobalite(Si02),microcrackingandcrystallizationintheglass bondnecksandglass/SiCinterfaceand pittingof theglass bond byCaS04[107].Servicelivesof uptoaboutoneyear could beexpectedfromtheSiCcandlefilters.Thethermalshock

resistanceof theDiaSchumalithF40filterswascomparedtoother SiCcandlesmade byRefractonaswellascandlesfromAsahiGlassCo.madeof cordierite[108,109].Schumacher SiCcandlefiltershave been pilottestedfor hotgascleaningata low-

level-wasteincinerator ata Germannuclear facility,andwerefoundto be promisingwhenthefeedstream particulatematter containsverylittlesubmicronmaterial[110].



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Fibrous,layered,low-densityalumina-silica basedcandlefiltersmade byFoseco(FosecoCerafil2000i)have beentestedfor PFBChotgascleaning[111].Thesetestswereconductedattemperaturesupto740°Candtypicallyupwardsof 99.9%of the

particulatematter wasremoved.Thenominal

poresizefor theseunitsis lessthan

10fLm[3].Althoughnoneof thefiltersinthesetestsfailed,thetesting periodof 340h wasconsideredtooshorttocommentontheir durability.Candlefiltersmadeof ?-cordierite bytheAsahiGlassCo.weretestedonPFBC

dusts[112].Of noteisthatthesecandlesfilter fromtheinsideout,usinga positive pressurerather thansuction.Atcomparable permeationrates,theywerefoundtogivea downstreamdustlevelunder 3 p.p.m.w.,comparedtolessthan1 p.p.m.w.for theSchumacher SiCcandles.TheAsahifiltersweretestedfor over 2000h attemperaturesupto 880°C.Likelyduetotheir quitelarge physicalsize(lengthof 5.8

m)and

beingfastenedtothevesselat both

ends,thethermalshocksrelatedto

the pulsecleaning produceda muchhigher rateof filter mechanicalfailurecomparedwithother candles.Anupdateonthestatusof theAsahicandlefilterswas provided byHigashiandMaeno[113].

Testinghas beenconductedonsurfacetreatedSiCcandles(LayCer 50/5)made bytheIndustrialPumpandFilter ManufacturingCo.(USA)[114].Theyhavea nominal

poresizeof 24ttm,weretestedat870°Candgavea filtrationefficiencynear 100%.Thisstudyfocussedonthenatureof thecake builduponthefilter surfaceandhowdifferentoperatingconditionsrelatedtothe pressure-fluxrelationship.

Westinghousehas

developeda new

housingunitfor candlefiltersandhastestedthe

DiaSchumalithF40candles[115].TheywereexpectedtoreplacetheSiCfilterswithalumina/mullitecandles.A projectsponsored bytheUSDOEis beingconductedtodevelop practicalhotgasfilter designsthatmeettheoperationalrequirementsof PFBC[116].

Theeconomicsand processoptionstoconsider for implementingceramiccandlefilter systemsarediscussed byZievers etal.[117].

2.6.2.Cross-flowfilters.Particulatefilterscanalso bemadefromdensesinteredce-

ramicconstructedinto box-likestructures.Suchdesignsincludeseparate passagewaysfor thefeedandcleanedgaswhicharelinked bythe porousceramicfilter material.Figure9showsa schematicsketchof onecomer of ageneric block shapedceramiccross-flowfilter.Ascan beseen,thisarrangementgivesaveryhighfiltrationareatovolumeratiocomparedtoconfigurationsusedinother technologies.Alltheother

benefitsattributedtoceramiccandlefiltersinthe previoussectionapplytoceramiccross-flowfilters[95].Backflow pulsingisusedtocleanthe particulatematter whichcakesonthe passagewalls.

Theemissionof sootfromdieselengineshas poseda problemfromaregulatory

standpoint.Tomeetthesestandards,andsinceit is not practicalto coolexhaustgasesfromavehicle,various particulatetrapsmakinguseof ceramiccross-flowfilter technology(andother typesof ceramicfilters)have been beinginvestigatedsincetheearly1980s[118-120].The proposedEPAair qualitystandardsfor 1994haverenewedinterestinthistopic[121].



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Figure9.Simplifiedschematicof aceramiccross-flowfilter.Shownhereisthetopcomer of a box-shapedunit.

TheAsahiGlassCo.hasdevelopeda new particulatetrapsystem[122,123].Itfeatures botha cross-flowtypeceramicfilter andreversecleaningregeneration.Thefilter ismadeupof thin plateelementsof modifiedcordierite.Each platehasa rowof sometwodozenopen-end,oval-sectionedchannels.A periodicreverse jetcleaning

bycompressedair injectionremovesthe particulatefromthefilter wallanddropsitintothevessel bottom.It is burned byanelectricheater.Thetrapefficiencyof thissystemrangesfrom88to95%.Thedurabilityhas beenconfirmed by bothlaboratorytestsandfieldtestswith public busesinLosAngeles,USA. NGK InsulatorsLtdalso producea cordieritediesel particulatetrap[124].Recent

attentionhasfocussedoncleaningrequirementsunder specifiedcombustionflowrateswithminimalthermalshock.The NGK unithas beentestedextensivelyon busesinItalyand performedwellin principle[125].Itwasrecommendedthatthecross-flowfilter unit beenabledwithcontinuousfilter loadingcontrol,to bring particulatematter collectionefficiencytoacceptablelevelsfor practicaloperatingconditions.

In NorthAmerica,GeneralMotorsincooperationwithCominghavedevelopeda

cordieritewall-flowfilter for lightdutydieselengines[126].Thethermaldurabilitywastestedanddesignedtoachievearequired120000vehiclemiles( 192 000km),withoutexceedingthe0.13g/mile particulateemissionsregulation.Theuseof Com-ing particulatetrapsonheavydieselequipmentinundergroundmineshas beenshowntoimproveair quality[127].

Inrelatedresearchonfilter regeneration,ithas beenshownthatitis possibleto pro-videa moreuniformenergyinputtoa filter byincorporatingmicrowave-susceptiblematerialsinthefilter bodyandthenusingRF(microwave)energytoinitiatecombus-tion[128].Thismethodgivesimprovedcontrolover thesootcombustionandlower

thermalstresseswithinthefilter.For genericair pollutioncontrolapplications, preliminarycommercialunitshave beenmadeavailable byCeraMem(USA)[129].The particulatematter removalabilityisnear 100%withnominal poresizesfrom0.5to0.004 >m,dependingonthesurfacemembranecoatingused.Asetof 36moduleseachwithexternaldimensions



200

of 12x 12x30cmandover 3.4m2filter areawasabletocleansatisfactorilythe0.28m3/sfluegasat120°CfromanEPRI baghousefacilityover a periodof 1100h.Testingonhotgasstreamsfromamagnetohydrodynamic(MHD) power generationcombustor revealedsomeunit

sealing problemsalongwith

unsatisfactory pressuredropsandfacevelocities[130].Ceramiccross-flowfiltersfor MHD plantswere,however,stillconsideredto bea promisingtechnology.Arecentadvancewastheuseof acatalyticvanadiacoatingfor simultaneous NOand particulatematter removalfromhotgasstreams[131].TheUSDOEisalsoinvestigatingacatalyticmixedmetaloxidecoatingonceramiccross-flowfiltersfor integratedS02, N0?and particulatematter removal[132].

Across-flowceramicfilter unithas beenunder development byWestinghouseandCoorsCeramics[133].Itunderwenttestsat900°Cfor periodsof 1000hwithvarious

typesof PFBC

flyash.In

general, particulatematter removalwas

quitehigh, but

posttestinspectionrevealeda number of materialflaws.Whenthemeansizeof theairborne particulatematter fell below5tim,theflowresistancefromthedustcakeandtherequiredcleaning pulseintensityincreasedmarkedly.

Anew product byCeraNovaandSpecificSurfaceCorp.usesa novel powder pro-cessingmethodtotailor ceramiccrossflowfilter elements[134].Theyemploya'three-dimensional printing' process,wherebyfunctionalceramiccomponentsarecre-ateddirectlyfromcomputer generatedmodelstructureswhichcontrolafabricatingunitwhich'prints'themonolith,layer bylayer.Thecompositionwithineach build-

planecan beselectivelycontrolled byvaryingthecompositionof powder andsus- pendingfluid.Anink jet printingheadisusedtometer thematerials.Inthiswaythemicrostructurecan becontrolled bytherelativeinfluenceof cohesiveandcapillaryforcesof thematerialsintroducedtoeachlayer.Functioningfiltershaveyetto bemadeinthisway, butsamplemonolithshave been preparedinthelaboratory.

3.RECENTPROCESSINGINNOVATIONSIn additiontoimprovementsmadeonair pollutioncontrolequipment,therehas

beenmuchrecentattentiongiventoaddressingairborne particulateemissionremovaland/or reductionfromanintegrated perspective.Researchanddevelopmenton pro-cessmodificationsaimedat producingahigher qualitygasemissionarediscussed

below.

3.1.HybridsystemsTheneedfor simultaneouscontrolof particulatematter andtoxicgases,or even

particulatematter of variablesizeandcharacter often,resultsintheuseof hybrid

systems.Thesesystemscompriseeither asingleunitor a processinwhichvarious particulatematter collectionmethodsareemployedin seriestoobtainsignificantlyreducedsolidsemissions.

For the purposeof thistreatiseahybridsystemwill beconsideredto beasingle physicalunitwhichcombinestwoor more particulatematter removaltechnologies.



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Atechnologydeveloped byEPRIintheUScombineselectrostatic precipitationwithfabricfiltration.ThissystemisknownastheCompactHybridParticulateCollector (COHPAC)[135].The baghousefollowstheESP,either inseriesor actuallyinside

theESPhousing.Ina COHPACsystem,theair leavinga power plantflowsfirstthroughanESP,thenthrougha baghouse.TheinitialESPtreatmentsignificantlyreducestheconcentrationandoftenimpartssomeelectricchargeto the particlesenteringthe baghouse.Thecharged particlesaremoreliabletorepeleachother aswellasthefabricfilter material,sothatthelevelof porecloggingis reducedand

bagcleanabilityis increased.For thesereasons,thefacevelocityintheCOHPAC baghousecan befour toeighttimeshigher thaninconventionalinstallations.Inturn,theincreasedthroughputallowsa muchsmaller physicalunitsize[136].Anumber of pilotandcommercialscaletestsof theCOHPACsystemhave beenmade.The

particulatematter emissionrequirementsweremetsatisfactorily, but baglifeconcernswereraised, perhapsowingtothecomparativelyhigher facevelocitiestowhichthe bagsaresubjected[137].

Afewmiscellaneoushybrid-typetechnologieshave beendeveloped.A processandapparatusfor treating particulatematter andacidgasinoneunithas been patented[138,139].Here,theexhaustgas(i.e.fromsintering plantsor fuel-fired plants)isfirstde-dustedinacyclonestage,thenafter carefullycontrolledmoistureaddition, passedtoanESPstagewherethe particulatematter collectedis humidified butessentiallydry,thusallowingit to beseparatedfromtheacidgas.A smallindustrialmodule

has beendevelopedwhichcombinescentrifugationandwetscrubbingfor particulatematter removal,followed by passingthegasstreamthroughactivatedcarbonsponge bagsfor demisting[140].Thedemistingstepcanincludeacidgasfixationand/or odour removal.

3.2.Feedsamplingand processcontrolTheelectricalnatureof ESPsmakesthemwellsuitedfor implementingon-linecontrolsystems, principallyfor power efficiency.Someof thesecontrolstrategieshave beennotedinSection2.2.On-linemonitoringof emissionstotheatmospherefor regulatorycomplianceisa differentmatter andsometechnologiesfor this purposehaverecently beendeveloped.

A prototypeESPdevicewasdevelopedcontaininga total-reflectionX-rayfluores-cencespectrometer (TXRF),intendedtodetectheavyelementsandmetals[141].Atflowratesunder 1.31/min,essentially100%of submicron particulatematter is col-lectedandtheconcentrationof HAPsinthestreammay beaccuratelymonitored.Anon-lineair samplingapparatushas beendevelopedfor measuringVOCconcen-trationsingasstreams[142].Twoinitialfiltrationstagesremove particulatematter above2.5fLmindiameter,atwhich pointimpactionseparatorsmakesizeclasses be-

low2.5,1.5,0.8and0.32itm.Thecaptured particulatematter canthen beanalyzedfor adsorbedVOCs.Somedeviceshave beendevelopedfor on-linemeasurementof particulateconcentrationsingasstreams.Aglassfibrefilter has beenincorporatedintoagravimetricdevicefor continuoussamplingof particulatesdownto0.011 >minsize.Theglassfibreshavesuperior opticalcharacteristicswhichcan beexploitedfor



202

other analyticalmeasurementsandareavailablewiththissampler [143].Similarlyaconceptualdesignhas been patentedfeaturingaseriesof impingingcollectorswhich

provideagasstream particulateconcentrationandsizedistribution[144].Practical

operatingrangeshavenot

yet beenestablished.

TheBHAGrouphavea lineof continuous particulatemonitoringdevicesusingamodulatedLEDlightsignalwhichrespondsonlytomoving particulates,soitremainsunaffected byaccumulationof dustor moisture[145].Theextentof signalmodulationcan becalibratedtodustconcentrationinthegasstream.A primaryintendeduseis in baghouseinstallationsasa monitor for filter bagleaksor failures.Another

potentiallyusefuldevicefor baghouseinstallationsistheHansentekTMspark detector andextinguisher [146].Sparksof diametersdownto0.4itmcan bedetected byinfraredsensorsandanappropriateextinguisher can betriggeredtoeliminatethem.Sucha

systemwouldallowtheintroductionof fabricfiltersto control

particulateemissionsfromwood-fired pulpand paper boilers.Fabricfiltershave beenslowtogainwidespreaduseinthissector duetotherisk of firefromairbornemicro-embers.

Processcontrolsystemshavefounda morelimitedapplicationin particulatecontrolinstallations.TheHarwellLaboratoryinEnglandhas beendevelopinganexpertsystemfor thecontrolof fabricfilters[147].Thestateof theartin thisareaissomewhatrudimentary becausethe primaryoperatingvariable,the pressuredropatsteadystate,is notreadily predictable.Usingon-line pressuredropmeasurements,theflow,compressor pressureand pulse periodscan beadjustedinrealtime,and

earlyresultshaveshown

upto20%

savingsin power consumption.3.3.Feedstreamconditioning

Oftenthe performanceof air filtrationunitscan beenhancedif the particulatematter in thefeedstreamenteringtheseunitsisgivensome preliminarytreatment.Feedstream'conditioning'mayconsistof suchtreatmentsasinjectingadditivesto thestream,irradiatingitor subjectingittoacousticshock.Theobjectivestypicallyaretomodifythesurface propertiestoimprovethe particulatematter collectionefficiencyor toagglomeratethe particulatematter tolarger,morereadilyremovableentities.

Awellknownconditioningapproachistheinjectionof S03intotheexhauststreamsfromlow-sulfur coal burning power plants,whichisknownasflue-gasconditioning(FGC).The presenceof theS03reducestheresistivityof theflyashinthesestreamsfor easier capture byESPs.Areviewof recentadvancesinthisareais provided byKrigmontandCoe[148].Someinnovationsincludehot-sideinjection,co-injectionof ammonia(dual-conditioning),anddual-conditioninginfrontof fabricfilterswheretheresultingfilter cakedragcan besignificantlyreduced.A patented processwasdesignedwhere NH3andS03areinjectedataratioof 2:1,inconcentrationsof 1to100 p.p.m.[149].Fabricfilter pressuredropsfor cleaningflyashfromPittsburgh#8

andMonticellocoalsampleswerefoundto betypicallytwoto threetimeslower,andthe particulatematter capturewasalsoimproved.Thisresultwasfollowedup bytestsinAustraliawhichshowedoppositeresults,whereahighlycohesivefilter cakewasreported[150].Thedisparityintheresultswasattributedtodifferencesinthecoalandfabricmaterials.



203

Testshave beenconductedwithanumber of differentsulfur andammoniacontainingconditioningcompoundsfor ESPfeeds.Regardingcosteffectiveness,ammoniumsulfate[(NH4)2S041inlowdosageswasfoundto perform bestfor lowsulfur coal

fiyash[151].A relatedtechnologyis knownas sorbentinjection.In thiscase,solidsorbent particulatessuchassilicates,calciumsalts,limes,etc.areinjectedintoagasstreamto adsorbSOxfromthegas.Arecentreview presentssixsuchcommercial pro-cesses[152].Afollowuparticleonhigh-temperaturesorbentinjectionreportsthattheelectricalresistivityof the particulatematter is increasedsubstantiallyandthe

performanceof theelectrostatic precipitator isdegradedaccordingly.Theresistivitymay be brought back to anacceptablelevel bytheuseof fluegashumidificationor conditioningwithS03.Certainmixturesof ashandhighlyreactivesorbentdonot

appear to

respondto

S03conditioning.Various

catalyticadditiveswereviewed

asa potentialmeanstomakehigh-temperaturesorbentinjectiona moreviable pro-cess[153].Sorbentinjection putsincreasedloadson particulatematter collectionequipmentandalsomodifiesthe particulate properties,especiallywithhumidifiedsorbentinjection.Recentresearchhasfocussedondesigningsorbentsystemswhich

possibly benefit,or atleastdonothinder the performanceof the particulatematter removalequipment.TheADVACATE processdeveloped bytheUSEPAusesaspe-cially preparedcalciumsilicatesorbentwhichappearsto bequitecohesiveandentersthesysteminanagglomeratedstatewhichaidstheoverall particulatematter control

bynot

introducingadditionalsubmicronmaterial[154].Babcock &Wilcoxhavede-

velopeda sorbent processdesignedtowork infrontof baghouses[155].ItisknownastheSO,-NO,-RoxBoxTMsystem(SNRB)andisanadvancedair pollutioncontrol processthatsignificantlyreducessulfur oxide(SOx),nitrogenoxide(NOx)and par-ticulateemissionsfromcoal-fired boilers[156].The processusesahigh-temperaturecatalytic baghousefor integratingSOxreduction byinjectinganalkalisorbent(suchashydratedlimeor sodium bicarbonate), NOxremovalthroughammoniainjectionandselectivecatalyticreduction,and particulatecollection.

TheWestinghouseILEC(IntegratedLowEmissionsCleanup)systemrepresentsadifferentapproachtofeedconditioning.Theuseof ceramicfiltersisenvisionedfor

particulatecontrolattemperaturesabove900°Cindirectcoal-firedturbineapplica-tions.Someadhesiveflyash particleswill beemittedthatcouldforma difficult-to-removefilter cake.TheILEC processinjectssorbent particlestoeither removetheadhesive particles beforetheyreachtheceramic barrier filters,or modifytheadhesivenatureof thefilter cakesothatit ismoreeasilyremovable[157].

Froma morefundamental perspective,a recentreviewis availableonhowthe performanceof fabricfiltersandelectrostatic precipitatorsusedtocollectashfromthecombustionof coaldependson bulk propertiesof theash,suchascohesivityor electricalresistivity[158].Anumber of recentfindingshave beencited;thesurface

of flyashwasfoundto beextremelycomplexintermsof chemicalspecies;sorbentmaterialscan beaddedtoenhancesurfacecharge propertiesfor streamstreated byESPs; NH3addedaloneor withS03changesthecohesivityof ash,reducingrappingemissionsfromESPs;smallsizedsorbent particulateshave beenfoundtoactasstericinhibitorstoreducecohesivityof coalashcleaned byfabricfilters;ahumidifiedstream



204

infabricfilterswillexhibitalower pressuredropduetothehigher porosityinthedustcake,whichformsmorecompactashagglomerates.AUSDOEexperimental

programisnowunderwaytofollowuponcharacterizingthemechanismsatwork in

fluegasconditioning[159].Acousticagglomerationhas beenrecentlyemployedtoenlargethesizeof submi-cronmaterialsothattheycan beremovedmorereadily byESP.At present,somelaboratorytestinghasshownimprovedresultsonmodelsystems[160,161].Sometheoreticalanalysessuggestthatorthokinetic,hydrodynamicandturbulentmecha-nismscontributetoanincreaseof particle-particlecollisionsfor systemssubjectedtoacousticalvibration[162,163].Anumericalsimulationwasruntomodel bimodal(flyashwithsorbent)acousticagglomeration,withsomecomparisontoexperimentaldata provided.Theneteffectof thecontributingcollisionmechanismswasestimated

bycurve-fitting;no

modellingof theadhesion

process between

particleswas

provid-ed[164].TheUSDOEisaimingtodevelopacousticagglomerationmethodsthatwouldallowtheuseof conventionalcyclonestoachieveveryhigh particulatecollec-tionefficiencyandeliminatetheneedfor barrier filters,whichhave been problematicin viewof durabilityandeconomicsfor proposeddirectcoal-firedturbines[165].A patenthas been procured[166]andinitialtestingisunderway[167].

Yetanother noveltechnologyto beinvestigatedis knownas theelectron beam precharger [168],alsoenvisionedfor coalflyashremoval.Anelectron beamisem- ployedtoionizegasmoleculesina precharger region, producinga bipolar plasmaof ionsandelectronsinacontinuousstateof formationandrecombination.Whensub-

jectedtoastrongelectricfield,the plasmaisseparatedintotwomonopolar fractions.Onlythenegativefractionisusedandisdrawnacrosstheducttochargethedusttoveryhighlevels,whichfacilitatesitsremoval byESP.Theelectron beam precharg-er wasfoundtoimprovethe particulatecollectionefficiencyof aconventionalESP.Testinghasalso beenconductedtocompare pulsecharginginsteadof directcurrentcharging,whichcan bedoneathigher strengthwhereit shouldcreatea morestableenvironmentfor freeelectronsto precharge particles.

3.4.Processmodifications

toreduceemissionsAs partof anoverallstrategytocomplywithenvironmentalstandards,a preferredlonger termsolutionissimplytomodifythe processwhichemitsthe particulatesto lower emissionlevels.Thus,reducedloadis placedonthe particulateremovalequipment.Withoutaugmentationof existingequipment,lower emissionlevelsof

particulatematter can beattained.For example,inthesteelmakingindustry,ithas beendemonstratedthat byreducing

thequantityof limeaddedto thefurnaceearlyintheoxygen blowingcycle,theresultingslag becomesmoreviscousandfoamy,reducing particulateemissions by

upto59%[180].Thechromeelectroplating process producesa toxichexavalentchromiumemission.Changesinfour processconditionswereimplementedwiththeobjectiveof reducingemissionsfromthe process[181].Theseincludedthefreeboardheightof theelectro-

plating bath,eliminationof compressedair for bathagitation,theuseof floating balls



205

onthetank surfaceandananti-mist bathadditive.Infour differenttestfacilitiesusingtheir existingcommon particulatematter controlequipment,reductionsinchromiumemissionsof 85-95%wereobserved,allmeetingthequantitativeindustryobjective

of 0.006mg/Ah.Unfortunatelyatthistime,therearenotmanyexamplesof such processesintheliterature, butuseof emissionreductionstrategiesisexpectedtoincreaseinthefuture.

3.5.Sector-specificairborne particulatematter controlconsiderationsAsubjectreceivingconsiderableattentionrecentlyisthetreatmentof emissionsfromincinerationsites.Medical,municipalandindustrialwastes beingincineratedhave

beensubjecttoincreasinglytightregulationsdueto the potentiallytoxiccontentof theemissions.ThewetESPsystem(alsodescribedin Section2.2)has beenfoundto beaneffectivemeansfor reducing particulateandtoxicemissionsfromsewagesludgeincineratorsunder the proposedPart503USEPA(February1989)regulations[169].Part503wouldlimitarsenic, beryllium,cadmium,chromium,lead,mercuryandnickelcontentsof particulateemissionstomaximumconcentrationsof 6.3,42.3,1.8,233.9,125.1,135.5and72.6mg/kg,respectively.TheUSEPAhas

projectedthatmorethan50%of theexistingsewagesludgeincineratorswill beoutof compliancewiththese proposedregulations.Pilottestingwitha post-scrubber feedconsistingof particulatematter whichwas50% below0.6ftm,wasabletoincreasetheoverall particulatematter removalefficiencyfrom98.3to 99.88%.Equipmentoptionsandcostconsiderationsarediscussedfor cleanair actcompliancefor wasteincinerators[170].Astudyof thethenatureandcompositionof incinerator generated

particulatematter asrelatedtofueltypewasdone byHackfortandBorchardt[171].Ashortreviewisavailablewhichdiscusses particulatematter treatmentintheminer-

al processingindustries[172].Four mainsourcesof pollutionareconsidered:miningandsitetreatment,smelting,recyclingandlargescalecombustion processes.Atten-tionisgivento processmodificationswhichreduce particulatematter emissionsor

producegasstreamsmoreamenableto particulatematter removal.A paper devotedtorecentdevelopmentsinironoresinteringdiscussesenvironmentalaspectsinclud-

ing particulatecontrol[173].PertinentESPandscrubbingsystemsarenotedaswellassome processes byBritishSteelandLurgiwhichhave beenupdatedto producelower particulatematter emissionlevels.TheUSDOEconducteda study provid-inganoverviewof particulatematter controltechnologiesfor hotgassteamsfromcoal-firedinstallations[174].Themajor particulatecontroldeviceissuesaddressedincludedtheintegrationof the particulatecontroldevicesintocoalutilizationsystems,on-linecleaningtechniques,chemicalandthermaldegradationof components,fatigueor structuralfailures, blinding,collectionefficiencyasa functionof particlesize,andscale-upof particulatecontrolsystemstocommercialsize.

Particulatecontrolis alsoanissuein the pulpand paper sector.Powers etal.conductedasurveyof applicabletechnologiesfor magnesiumor ammoniumsulfiterecoveryfurnacesin pulpmills[175].WetsystemsarerecommendedinthiscaseassimultaneousS02removalis alsodesired. Normally, pulpmillshavea wastewater treatmentsystem,sotransfer of airbornecontaminantstoanaqueousstreamdoesnot



206

poseanadditionalenvironmentalconcern.Successfulsystemstestedona pilotscalewerewetESPsfromFluid-IonicsandBeltran,andMonsanto'sDynawaveScrubber.A similar approachis takenin the particleboardindustry.Fine particulatecleaninghas beensuccessfulwithwetESPsfor

treatingthegasstreamfroma sander dustfireddrumdryer [176].Withtighteningenvironmentalregulations,recoveryof heavymetalemissionsis becomingamorecrucialissue.Of the189HAPsregulatedunder thenewCleanAir Act,11areheavymetalsandmostarecapturedasPM10.Theviabilityof variousexistingandemergingtechnologyfor particulateheavymetalcaptureisdiscussed by

Nudo[177].Ageneraloverviewonthescrubbingof inorganictoxicsisgiven byMcInnesetal.[178].Wetscrubbingor absorptionarethemostcommon particulatematter removalmethodsfor nonmetallicinorganics,whileESPsand baghousesarethe

primaryeffectivemeansof controllingmetals.Aspecialventuriscrubber operatedatalarger pressuredropwhencomparedtostandardventuriscrubbers,wasdesignedtoremoveheavymetalsfromwasteincinerator gasstreamsdowntoUSEPAguidelinelevels[179].Theincreasedoperatingcostsof thissystemmayhinder itswidespreadacceptance.

4.SUMMARYIngeneral,manyof theconventional processunitsusedfor airborne particulatecon-trolareconsideredto

employeffectivemature

technology.Thechoiceof

specificunitsandthewaytheyareinsertedintoanengineeringdesigndependonthesourceof theairborne particulatesandthefinalemissionrequirements.Asurveyof recentrelevantscientificandengineeringliteratureonthissubjecthasrevealedthatmostof theresearchanddevelopmentinthisareaisincremental,andis oftenundertak-entoachievesitespecificobjectivestoreachcompliancewithtighteningemissionsregulations,i.e.air pollutioncontrolunitsaretypicallyinstalledasadd-onsandsomeresearchmay bedonetofinetunetheunittofunction properlyfor eachspecificgasstreaminquestion.Mostreportsof newtechnologiesdescribeenhancedversionsof conventionalunits,whicharesometimesimproved bycombiningcapturemechanismsthatarenormallyexploitedinisolation(i.e.electrificationof fabricfilters).TheUSCleanAir ActAmendmentsof 1990didnotspecificallyupdateregulationsfor all particulateemissions, butthereisgrowingexpectationthatstricter legislationisforthcominginthisarea[182].Asstated previously, particulateemissioncomplianceat presentdoesnot posea serioustechnologicalchallenge.However, particulateemissionsareoftenconcentratedin the0.1-2.0Amdiameter range,sinceexisting

processunitsarecollectivelyleastefficientinremovingsolidsof thissizefromgasstreams.Further,thelistof 189HazardousAir Pollutantstargetedfor controlintheUSCleanAir ActAmendmentscontainsmanyheavymetalsandheavyorganics

whichexistassubmicron particulatematter [183].Healthrelatedstudiesrecommendcleaningof sub-2.5Am particulates,sincematerialinthissizeclassisrespirableandcontributestohumanillness.

TheUSEPAhasanAir andEnergyEngineeringResearchLaboratorywhichalsomonitorstrendsanddevelopmentsinair pollutioncontrol.Theycitedevelopment



207

occurringinthe processdesignareawherevariationsof combinationsandsequencesof particulatecontrolunitscan producesomedesired benefit[1].Theconceptof

processdesignandintegrationisincreasinglyimportantfor newinstallationswhere,

for example,novelheat-resistantfilter materialsmay beusedfor particulateremovalatelevatedtemperaturessothattheheatenergymay bemoreefficientlyretained.Coupledwith processdesignaremethodstoreducetheat-sourceemissionof par-

ticulatessothattheir post-processingrequirementsareminimized.For regulationswhichallowemissionsasmass per unitof energygenerated(i.e.fossilfuel based

power stations),upgradingof thefuelsource prior to combustionmay becomein-creasinglynecessary.Scrubbersandagglomeratingsystemswhere particulatesandgaseoustoxicsaresimultaneouslyremovedarealsounder consideration.

Oneareawherenewadvancesare beingmadeinairborne particulatecontroltech-

nologiesis thatof filter materials.Manyof thefabricsusedin baghousesarenowavailablewithhigher thermalandchemicalresistance,longer servicelifeandfiner particlediameter control.Cross-flow, pulsed-jet,ceramictubular andrigidcandle-typefiltersarealsoemployedandshow promisingresults.Manyof thesefilter typesoffer theadvantageof beingreadilyandeasilycleaned.Ceramiccross-flow particulatetrapshave beenshownto performatlevelsmeeting1994EPAstandards.

For the problematicsubmicron particles,muchattentionhas beendevotedtoag-glomerationof these particlestoformlarger conventionallytreatableunits.Modi-ficationsto wetscrubbinghave beenmadewitha pre-treatmentstepwherethegas

streamwassubjectedtoacondensingsteaminjector whichinduced particleagglom-eration[81].Similar pre-treatmentsare beingstudiedfor streamssenttoESPs[184].Other newdirectionsin particulatecontrolrelatetothesourcesgeneratingthe pol-

lution.Municipalwasteincinerationisarelativelyrecent processand posesdifficultyowingto thevariabilityin thefeedstock.Specialcatalyticscrubbersand processdesigncombinationshave beenstudiedfor particulatesfromwasteincineration.

Fromthe presentscientificandtechnical perspective,thefollowingtrendscan beidentifiedasemerginginthedevelopmentof advancedfine particlecontroltechnolo-gies.

(1)Processmodelling,simulationandcontrolfor improved performance.(2)Asaresultof,or closelyrelatedto(1),areopportunitiestointegrateseveraltech-

nologiesthrough processmodelswhichidentifysynergy betweenthetechniques.(3)Sizeenlargementtechniqueswhichmovesub-3fLm particlesintothelarger more

treatablerange.(4)Filtrationmodulesandmaterialsapplicabletohightemperaturesituationsand/or

whicharecleanableandanti-fouling.

AcknowledgementsTheliteraturesurveyusedin preparingthisreviewwascommissioned byDr K.Ra-machadranof EnvironmentCanadafor their TechnologyAdvancementProgramfor AdvancedFineParticleRemoval,under DSScontractno.K2610-4-2050.



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