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//fddfVjj VfRJTJjVVDj //5$6/ #/iJjV`jJVjjVTjJj / / $5 / /  Tjl`VjGDjgVjjVdljdJj//1$:/#/  DlfVji]jYVjjTiVjRdlj"// #/1$:/#/ jJYldjVVjJjJiGjjTJjJ//2$A// jGVjYVjJj Jdjl]fNjJf /#/30$09/ #/ Gj]iljdJjGJj JdigYVV`//;$0A/ #/ `jjGTTDjGJjJTjiRjGi"//#/C<0$0Ah/ ##//YjGgDjJVjNjjGVTDjR/! / <0$298%)VOCdestruction.Completenessofcombustioninaflareisgovernedbyflametemperature,residencetimeinthecombustionzone,turbulentmixingofthegasstreamcomponentstocompletetheoxidationreaction,andavailableoxygenforfreeradicalformation.CombustioniscompleteifallVOCareconvertedtocarbondioxideandwater.IncompletecombustionresultsinsomeoftheVOCbeingunalteredorconvertedtootherorganiccompoundssuchasaldehydesoracids.Flaresaregenerallycategorizedintwoways:(1)bytheheightoftheflaretip(i.e.,groundorelevated),and(2)bythemethodofenhancingmixingattheflaretip(i.e.,steamassisted,airassisted,pressureassisted,ornonassisted).Elevatingtheflarecanpreventpotentiallydangerousconditionsatgroundlevelwheretheopenflame(i.e.,anignitionsource)islocatednearaprocessunit.Elevatingtheflarealsoallowstheproductsofcombustiontobedispersedaboveworkingareastoreducetheeffectsofnoise,heat,smoke,andobjectionableodors.Inmostflares,combustionoccursbymeansofadiffusionflame.Adiffusionflameisoneinwhichairdiffusesacrosstheboundaryofthefuel/combustionproductstreamtowardthecenterofthefuelflow,formingtheenvelopeofacombustiblegasmixturearoundacoreoffuelgas.Thismixture,onignition,establishesastableflamezonearoundthegascoreabovetheburnertip.Thisinnergascoreisheatedbydiffusionofhotcombustionproductsfromtheflamezone.Crackingcanoccurwiththeformationofsmallhotparticlesofcarbonthatgivetheflameitscharacteristicluminosity.Ifthereisanoxygendeficiencyandifthecarbonparticlesarecooledtobelowtheirignitiontemperature,smokingoccurs.Inlargediffusionflames,combustionproductvorticescanformaroundburningportionsofthegasandshutoffthesupplyofoxygen.Thislocalizedinstabilitycausesflame -I)2 flickering,whichcanbeaccompaniedbysootformation.Asinallcombustionprocesses,anadequateairsupplyandgoodmixingarerequiredtocompletecombustionandminimizesmoke.Thevariousflaredesignsdifferprimarilyintheiraccomplishmentofmixing.Steamassistedflaresaresingleburnertips,elevatedabovegroundlevelforsafetyreasons,thatburntheventedgasinadiffusionflame.Theyreportedlyaccountforthemajorityoftheflaresinstalledandarethepredominantflaretypefoundinrefineriesandchemicalplants.Toensureanadequateairsupplyandgoodmixing,thistypeofflaresysteminjectssteamintothecombustionzonetopromoteturbulenceformixingandtoinduceairintotheflame.Someflaresuseforcedairtoprovidethecombustionairandthemixingrequiredforsmokelessoperation.Theseflaresarebuiltwithaspidershapedburner(withmanysmallgasorifices)locatedinsidebutnearthetopofasteelcylinder0.6meters(24inches)ormoreindiameter.Combustionairisprovidedbyafaninthebottomofthecylinder.Theamountofcombustionaircanbevariedbyvaryingthefanspeed.Theprincipaladvantageofairassistedflaresisthattheycanbeusedwheresteamisnotavailable.Althoughairassistanceisnotusuallyusedonlargeflares(becauseitisgenerallynoteconomicalwhenthegasvolumeislarge)thenumberoflargeairassistedflaresbeingbuiltisincreasing.Thenonassistedflareconsistsofaflaretipwithoutanyauxiliaryprovisionforenhancingthemixingofairintoitsflame.Itsuseislimitedtogasstreamsthathavealowheatcontentandalowcarbon/hydrogenratiothatburnreadilywithoutproducingsmoke.Thesestreamsrequirelessairforcompletecombustion,havelowercombustiontemperaturesthatminimizecrackingreactions,andaremoreresistanttocracking.Pressureassistedflaresusetheventstreampressuretopromotemixingattheburnertip.Severalvendorsnowmarketproprietary,highpressuredropburnertipdesigns.Ifsufficientventstreampressureisavailable,theseflarescanbeappliedtostreamspreviouslyrequiringsteamorairassistforsmokelessoperation.Pressureassistedflaresgenerally(butnotnecessarily)havetheburnerarrangementatgroundlevel,andconsequently,mustbelocatedinaremoteareaoftheplantwherethereisplentyofspaceavailable.Theyhavemultipleburnerheadsthatarestagedtooperatebasedonthequantityofgasbeingreleased.Thesize,design,number,andgrouparrangementoftheburnerheadsdependontheventgascharacteristics.Anenclosedflare'sburnerheadsareinsideashellthatisinternallyinsulated.Theshellreducesnoise,luminosity,andheatradiationandprovideswindprotection.Enclosed,orgroundbasedflaresaregenerallyusedinsteadofelevatedflaresforaestheticorsafetyreasons.Ahighnozzlepressuredropisusuallyadequatetoprovidethemixingnecessaryforsmokelessoperationandairorsteamassistanceisnotrequired.Inthiscontext,enclosedflarescanbeconsideredaspecialclassofpressureassistedornonassistedflares.Theheightmustbeadequateforcreatingenoughdrafttosupplysufficientairforsmokelesscombustionandfordispersionofthethermalplume.Theseflaresarealwaysatgroundlevel.  Enclosedflaresgenerallyhavelesscapacitythanopenflaresandareusedtocombustcontinuous,constantflowventstreams,althoughreliableandefficientoperationcanbeattainedoverawiderangeofdesigncapacity.Stablecombustioncanbeobtainedwithlowerheatcontentventgasesthanispossiblewithopenflaredesigns(1.9to2.2_MJ_/sm3(50to60Btu/_scf_)),probablyduetotheirisolationfromwind (D$, effects.Enclosedflaresaretypicallyusedatlandfillstodestroylandfillgas.(EPA,1995) Advantages:  m+&/   AdvantagesofflaresoverothertypesofVOCoxidizersinclude(EPA,1992;EPA,1991): -9)2 L P5h(pUJKLMNORSL  1. J3   0  J3VE25h  a  .3  0(#(#    Canbeaneconomicalwaytodisposeofsuddenreleasesoflargeamountsofgas;    2.   J3   0  J3F25h  a  .3  0(#(#  Inmanycasesdonotrequireauxiliaryfueltosupportcombustion;and    3.   J3   0  J3G25h  a  .3  0(#(#  Canbeusedtocontrolintermittentorfluctuatingwastestreams. V  12.0  Disadvantages: O(#(#   Disadvantagesofflaresinclude(EPA,1995): K;   K;H0  25h  d  .3  0(#(#  Canproduceundesirablenoise,smoke,heatradiation,andlight;K;HH݌N (#(# Ќ   K;   K;I0  25h  e  .3  0(#(#  CanbeasourceofSOx,NOx,andCO;K;II݌! q (#(# Ќ   K;   K;J0  25h  f  .3  0(#(#  Cannotbeusedtotreatwastestreamswithhalogenatedcompounds;andK;JJ݌ D (#(# Ќ   K;   K;K0  25h  g  .3  0(#(#  Releasedheatfromcombustionislost.K;KL݌  (#(# Ќ  & | OtherConsiderations:  m  '|mL  Flaringisconsideredasacontroloptionwhentheheatingvalueoftheemissionstreamcannotberecoveredbecauseofuncertainofintermittentflowasinprocessupsetsofemergencies.Ifthewastegashasaheatingvaluehighenoughtosustaincombustion(i.e.greaterthan11MJ/sm3or300Btu/scf),   thestreammayserveasafuelgasforanincineratorifoneisemployedatthesite(EPA,1991).B.` hp x (#B & | References:  5 AWMA'|5,O,1992.Air&WasteManagementAssociation,AirPollutionEngineeringManual.VanNostrand . Reinhold,NewYork.EPA,1991.U.S.EPA,OfficeofResearchandDevelopment, ControlTechnologiesforHazardousAirPollutants,EPA/625/691/014,Washington,D.C.,June.EPA,1992.U.S.EPA,OfficeofAirQualityPlanningandStandards, ControlTechniquesforVolatileOrganicEmissionsfromStationarySources,EPA453/R92018,ResearchTrianglePark,NC.,December.EPA,1995.U.S.EPA,OfficeofAirQualityPlanningandStandards, SurveyofControlTechnologiesforLowConcentrationOrganicVaporGasStreams,EPA456/R95003,ResearchTrianglePark,NC.,May.EPA,1996.U.S.EPA,OfficeofAirQualityPlanningandStandards, OAQPSControlCostManual,FifthEdition,EPA453/B96001,ResearchTrianglePark,NC.February.Gonzalez,1999.SteveGonzalez,Kaldair,Inc.,Houston,Texas,(800)5253247,personalcommunicationswithEricAlbright,April15and16,1999.