...
,PB83-2637231111111111111111111111111111111111111EPA-600!2-83-070August1983EVALUATION
OF THE EFFICIENCYOF INDUSTRIAL FLARES:Background - Experimental
Design - FacilitybyDo Joseph, J. Lee, C. McKinnon,R. Payne, and J.
PohlENERGY AND ENVIRONMENTAL RESEARCH CORPORATION18 MasonIrvine,
California 92714EPAContract No. 68-02-3661EPAProject Officer: Bruce
A. TichenorIndustrial Processes BranchIndustrial Environmental
Research LaboratoryResearchTriangle Park, NorthCarolina
27711Preparedfor:U. S. ENVIRONMENTAL PROTECTION AGENCYOfficeof
Research and DevelopmentWashington, DC 20460, - ~I REPRODUCED BY;~
I'" . u.s. Department or CommerceNational Technical Infonnalion
ServicelSpringfield, Virginia 22161 ~_. -----./TECHNICALREPORT
DATA(Plcau read bUJrlJctions onthe rercrsebefore completing)1.
REPORTNO.12. 3.REPBSNiS AC2E6S372' eEPA- 600 / 2- 83-0704. TITLEAND
SUBTITLE 5. REPORTDATEEvaluationoftheEfficiencyof Industrial
Flares:August1983Background- -Experi'11entalDes ign- - Fac ility6.
PERFORMINGORGANIZATIONCODE7. AUTHORIS) B.
PERFORMINGORGANIZATIONREPORTNO.D. Joseph, J. Lee, C. ~ c K i n n o
n , R. Payne, and J. Pohl9. PERFORMINGORGANIZATIONNAMEANDADDRESS
10. PRCIGRAMELEMENTNO.Energyand Environmental
ResearchCorporation18Mason11. CONTRACT/GRANTNO.Irvine, California
9271468-02-366112..SPONSORINGAGENCYNAMEANDADDRESS 13.
TYPEOFREPORTANDPERIODCOVEREDEPA, OfficeofResearch and
DevelopmentPhase1. II: 10/80-1/82IndustrialEnvironmental
ResearchLaboratory14. SPONSORINGAGENCYCODEResearchTrianglePark, NC
27711EPA/600/1315. SUPPLEMENTARYNOTESIERL-RTPproject officer is
Bruce A. Tichenor, 'YIail DropOJ,919/541- 2547.~16. ABSTRACT The
reportsummarizesthetechnical literatureontheuseof industrialflares
andreviewsavailableemiss ion esti'11ates. Technicalcritiques of
past flareeffic iency studiesareprovided. l\Il:athematical modelsof
flamebehavior areexploredandrecommendations for
flareflamemodelsaremade. Theparametersaffectingflareefficiency
areevaluated, andadetailed experi'11ental test planis
developed.Thedesignofaflaretestfacility isprovided,
includingdetails ontheflaretips,fuel and steamsupplies, flowcontrol
and measurement, em iss ions saTYlplingandanalys is', anddata
acquis it ionandpFocess ing.-17. KEYWORDSANDDOCUMENTANALYSISa.
DESCRIPTORS b.IDENTIFIERS/OPENENDEDTERMS C. COSATI
Field/GroupPollution "'1easurement Pollut ionControl 13BExhaust
Gases FlowControl Stat ionarySources 2lB 20DEfficiency Sampiing
Industrial Flares 14G 14B1\1athematical Models Analyzing
12AFlamesFuels 21DHI. DISTRIBUTIONSTATEMENT 19. SECURITYCLASS(This
Repo;/j 21. NO. OFPAGESUnclass Hied 2g"ReleasetoPublic20,
SECURITYCLASS(Thispage) 22. PRICEUnclassified;" "-EPAForm22201
(973)i: 'IThis document has been reviewed inaccordance withu.s.
Environmental ProtectionAgencypolicyandapproved for publication.
Mentionof trade namesor commercial products does not constitute
endorse-ment or recommendationfor use.ii,....ABSTRACTThe U.S.
Environmental ProtectionAgency has contractedwith
EnergyandEnvironmental Research Corporationtoconduct aresearch
programwhichwillresult inthe quantificationof emissions fromand
efficiencies of industrialflares. The studyis beingconducted in
four phases:I - Experimental DesignII - Design of Test
FacilitiesIII - Development of Test FacilitiesIV- Data
CollectionandAnalysisThis report provides the results of Phases I
and II of the study.The report summarizes the technical literature
on the use of flares andreviews available emissionestimates.
Technical critiques of past flareefficiencystudies are provided
..Mathematical models of flame behavior areexplored, and
recommendations for flare flame models are made.The parameters
affectingflare efficiencyareevaluatedandadetailedexperimental test
plan is developed. The designof aflare test facilityisprovided,
includingdetails on theflare tips, fuel and steamsupply,
flowcontrol andmeasurement, emissions samplingandanalysis, anddata
acquisi-tion and processing." AQualityAssurance/QualityControl
Programis alsodescribed.Results of the testing program(Phases III
and IV) will be included ina later report.iiiTABLE OF CONTENTS2.1
Use of Industrial FlaresSUMMARY .BACKGROUND2.4 Experimental
Information on
Flares.2-482-492-602-682-682-70-Page1-11-11-31-71-92-1 2-22-32-82-8
2-132-132-172-19 2-192-202-342-362-402-44
2-452-46PetroleumRefiningPetroleumProductionBlast FurnacesCoke
OvensChemical Process Industry .Summary of Use of Industrial
FlaresDesignof FlaresFuels Flared.. ."Flare Operating
ConditionsFlare Sizeand Capacity ..Summaryof Commercial FlaresFlare
Characteristics .Characteristics of Previous ExperimentalStudies on
Flares .....The Structureof Flare Flames .Flare Efficiency
.Productionof Soot in FlaresThe Effect of Wind on the Performance
of FlaresThe Effect of SteamInjection/Forced Draft onthe
Performance of Fl ares . . . . . . . . . .
.2.1.12.1.22.1.32.1.42.1. 52.1.62.3.12.3.2
.2.3.32.3.42.3.52.4.12.4.22.4.32.4.42.4.52.4.62.4.71.1 Pollutant
Emissions FromFlares1.2 Deficiencies in Previous Flare Emission
Studies1.3 Technical Approach.1.4 Report Organization2.2 Emissions
fromFlares2.3 Commercial Flares ..1.02.0SectionvPrecedingpageblank"
,-Section3.04.0TABLE OF CONTENTS (Continued)2.5 Modelingof Flares
.....2.5.1 Models of Flare Behavior2.5.2 Previous Models of
Jets.2.5.3 Solutions of the Transport Equations2.5.4 Scaling
Considerations .2.5.5 The Broadwell Model of Turbulent Flames.2.5.6
Recommendations for Modelingof FlaresTHE NEED FOR WORK . . . .
.TECHNICAL APPROACH AND EXPERIMENTAL PLAN4.1 Overall Approach .4.2
The Need for Studyof Pilot Scale Flares4.3 Size of Pilot Scale
Flares4.4 Operating Conditions4.5 Selectionof Gases.4.6 The Effect
of Steam4.7 The Effect of Wind.4.8 Experimental Measurements4.9
Modelingthe Emissionof Pollutants FromFlares4.10Experimental Plan
.Page2-772-78 2-87 2-100 2-1032-1092-1133-1 4-14-14-44-5 4-10 4-12
4-12 4-13 4-134-154-164.10."1 RequiredScope of the Pilot ScaleTest
FacilHy .4-164.10.2 Experimental Program . 4-17,-5.06.0FACILITIES
REVIEW .....5.1 FacilityRequirements5.2 Existing Facilities5.3
Proposed Flare FacilityDESIGN OBJECTIVES .6.1 Design Criteria6.2
Approach ..6.3 Parameters ...viPrecedingpageblank5-1 5-15-65-7. .
6- 1 6-1 6-1 6-1TABLE OF CONTENTS (ContinuedSectionAPPLICATION AND
ANALYSIS OF DATA7.8 Control, Data Acquisitionand Processing6.4
FacilityCapabilityFACILITY DESIGN .....8.1 Independent Variables8.2
,Cal cul ati on of Emi ssi ons . . . .8.3 Interpretationof Flame
Structure6-26-26-2. . 6-26-36-36-37-17-17-37-97-9. .
7-97-127-127-157-157-177-197-227-267-46. 7-527-537-588-18-1.
8-28-4, .Flare Size .....Flare Gas PropertiesNozzle Exit
VelocityWind ConditionsAir EntrainmentMeasurementsFuel Metering
.Tracer Meteri ngSteamMetering6.3. 16.3.26.3.36.3.46.3.56.3.67 ~ 5
. 17.5.27.5.37.1 Flare Stack and Flare Tip7.2 Fuel Supplyand
Handling7.3 Tracer Supply .7.4 SteamSupply .7.5 Input FlowControls
and Metering.7.6 Ambient Conditions Measurement and Control.7.7
Measurement Techniques .........7.7.1 Global (Overall) Combustion
Efficiency7.7.2 Applicationof Tracer Gas7.7.3 Extractive Sampling
.7.7.4 VelocityMeasurement .7.7.5 TemperatureMeasurements7.7.6
Characterizationof Flame Structures7.08.0vii".....Section9.0TABLE
OF CONTENTS (Continued)8.4 Scaling and Modeling8.5
ConclusionsREFERENCESAPPENDICESAppendixA- Comparisonof Commercial
FlaresAppendixB - Summaryof CARB (CaliforniaAirResources Board)
Survey .AppendixC - QualityAssurance Plan .....Appendix D- Emission
Factors for Flare CombusitonAppendix E - Calculationof Flame Shape
and LengthviiiPage8-58-6 9-1 A-1 B-1 C-1, D- 1 E-1LIST OF
FIGURESFigure2-1 (a) Variation in gas densityflared froma German
refinery;(b) Actual floWrateof test flare used bySiegel (1980) .
2-62-2 Volume ratios of hydrocarbons in theflare and off-gas
forthree tests on aflare at a German refinery (Siegel, 1980) ..
2-92-3 Components of an elevatedflare (Klett and Galeski, 1976).
2-212-4 Rectangularmulti-jet ground flare (Klett and Galeski,1976)
. . . . . . . . . . . . . . . . . . . . . 2-.222-5 Designof flare
tips (Klett and Galeski, 1976) . 2-262-6 Stack height and allowable
radiation intensity (Oenbringand Sitterman, 1980) . . . . ...
2-302-7 Commercial flare heads. (a) 20 Mscfd pipe flame; (b)
60Mscfd smokeless flare anda 20Mscfd smokyflare,(Peabody/Kaldair,
1979) 2-412-8 Distributionof flare nozzle sizes reported by
Californiarefineries toCalifornia Air Resources Board Survey
(1980). 2-432-9 Estimates of flare emissions due to incomplete
combustionof eddies . . . . . . . . . . . . . . . . . 2-522-10
Temperature profiles inacommercial flare . 2-532-112-12Effect of
propaneemissions on combustionefficiency.Propane as fuel . . . . .
. . . . . . . . . . . .Effect of CO emissions on
combustionefficiency.COas fuel .2-572-58'.-2-13 The effect of
throughput on concentrationprofiles intwo flare flames (Siegel,
1980) ' . . . . . 2-592-14 Radial concentrationprofiles in aflare
flame (Siegel,1980) . .. . . . . . . . . . . . . . . . . ... . . .
. 2-612-15 Species centerline concentrations as afunction of
heightabove burners (Lee andWhipple, 1981) . . . . . . . ... .
2-662-16 Summaryof flare emission, excludingsoot, as afunction
ofheight above burner tip (Lee and Whipple, 1980). . . ... 2-672-17
Effect of soot concentration on combustionefficiency.Propane as
fuel . . . . .. 2-69ixFigure2-18LISTOF FIGURES
(Continued)PageTheeffect of steaminjection on flame length (Siegel,
1980) 2-72.2-19 The effect of steaminjectionon
concentrationprofiles,3and 6 meters above flame tip (Siegel, 1980)
. . . . . 2-742-20 The effect of steaminjectionon local flare
efficiency(Siegel, 1980) . . . . . . . . . . . . . . . . . .
2-752-21 Theeffect of steaminjection on temperature (Siegel, 1980)
2-762-22 Short time photographs.of turbulent flame ( B e ~ k e r ,
et al,1981) . . . . . . . . . . . . . 2-792-23 Conceptionof aflare
shedding eddies . . 2-802-24 Reaction pnd life of eddies shed
fromaflare . 2-812-25 Eddy frequency . 2-83:;-2-26 Concentrationof
CO in eddies . 2-842-27 Decayof an eddyfromapool {ire (Bratz, et
al, 1980) 2-852-28 Geometric dimensions of the largest and smallest
eddies inn-hexane pool flames as,a function of pool diameter
(Bratz,et al, 1980) . . . . . . . . . . . . . . . . . . . . .. .
2-862-29 Progressive change in flame type with increase
innozzlevelocity (Hottel and Hawthorne, 1949) . . . . . . . .
2-882-30 Theoretical predictionof entrainment in buoyant
jets(RicouandSpalding, 1961) . . . . 2-902-31 Entrainment by
buoyant jets and flames (Ricou and Spalding,1961) . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 2- 912-32 Definitions of x,
z, the curvilinear coordinate ~ , horizon-tal and vertical
velocitycomponents uand w, velocity U,and flame radius 0
.................... 2-922-33 Computed profiles of compositionand
densityfor d =0.005m,w= 22.1 m/sec, and uQ) - 2.55m/sec . . . . . .
. . ... 2-962-34 Experimental soot concentrations on the axis of
the C2H2diffus;'on flame (Re =7000) comparedwith
predictions(Magnussen, 1980) 2-99xLIST OF FIGURES
(Continued)Figure2-35, .Mixingfactor (M2) anddegree of oxidation
(N) for"Constant Residence Time" scaled natural gas flames--axial
traverses (Salvi and Payne, 1980) ..... 2-1082-36 Comparison of
model predictions of NOmass fractionagainstmeasurements of Bilger
and Beck (1975) for hydrogen-airflames for x/d > 30. The
prediction indicates afrozenNO flux, whereas the data indicate NO
destruction. . 2-1124-1 Experimental flare tipconcept 4-117-1
Six-inchflare head. . . . . 7-47-2 Gas consumption for different
nozzle sizes and nozzles gasvelocities . . . . 7-67-3 Fuel
supplysystem 7-87-47-57-67-77-8.7-97-10a7-10b7-11Flare - tracer
supplyandmeteringFuel control andmetering systemSteammeteringWind
generator, support structures, and sample hoodsExhaust gas
collection hood for pilot scale flare ..Distributionof
intermittencyfactor, n, and mean forwardvelocity inaround free jet
(Corrsinand Kistler, 1954)Approximate streamlines ina turbulent
roundjet. Thespreadingangle is shown for two values of
intermittencyfactor, n .Airflowstreamline drawn intoa hood, fromAir
PollutionEngineeri ng Manual, 1973 . . . . . . . . . . . . . . .
.Errors in solid concentrationfor samples drawnanisokinetically
(Badzioch, 1959and 1960)7-11 7-14 7-167-187-217-237-247-24 7-297-12
Water-cooledsoot-samplingprobewithexchangeablefiltersandfiltertips
(Chedaille and Braud, 1972) .. 7-317-13 Sample systemschematic '
7-37: ,.....7-14 Tedlar bag sample concentrations changes with
time(Polasek andBullin, 1978) .xi7-38LIST OF FIGURES (Continued)Fi
gure7-15 Design of multi-rake probe .. 7-407-16
Photographicanalysis. 7-57A-l Principal elements of a "steam-ring"
smokeless flare tip 1976) .. A-2A-2 Principal elements of the
Flaregas FS antipol1utant flaretip1976) . . . . . . . . . . . . . .
. .. A-3'.--A-3A-4A-5A-6Principal elements of the Indair smokeless
flare 1976) .Principal elements of the Smoke-Ban model SVL
flaretip1976) .Principal elements of the Zink Services SAfield
flaretip1976) ...........Principal elements of atypical groundflare
(Brzustowski 1976) . . . . . . . . . . . . . . . . . . . . .
.xiiA-4A-5A-6A-7LISTOF TABLESTable1-1 Previous flare emission
studies . . . . . . . . . 1-4i2-1 Surveyof Californiaoil
refineryflares (CARB, 1980) ... 2-42-2 Gas flared in U.S.
refineries (Klett and Galeski, 1976) . 2-72-3 Flare gas composition
froma German refinery (Siegel, 1980) .2-10.
;....2-42-52-62-72-82-92-102-112-122-132-142-152-162-172-183-14-1Characteristics
of gases flared in the UnitedStatesCompositionof gases flared
during petroleumproduction .Surveyof gases flared fromblast
furnaces (Klett andGaleski, 1976) .Surveyof gases flared fromcoke
ovens (Klett andGaleski, 1976) . .Surveyof gases flared in the
chemical industry (Klettand Galeski, 1976) . .Estimate of the
amount of gas flared in the U.S. in 1980Capacityof different flare
types ...Properties used inexample of flare designRelative cost of
suppressing soot inflares (Klett andGaleski,1976) . ... .
...Properties of fuels flaredSootingtendencies of fuel (Mandell,
1978) .Experimental measurements on flaresRange of concentrations
measured inflares studiesExperimental measures of
combustionefficiency .Experimental data used in the studyof Becker
andYamazaki (1978) withpropane fuel .Estimate of gases flared in
the United States ...Experimental parameters and fuel costs for
pilot scaleflaretests. . . . .. . . . .xiii2-112-122-142-15.
2-162-182-232-28. . 2-332-352-372-472-562-632-983-24-9LIST OF
TABLES (Continued)Table4-27-17-27-37-4Basicflare test matrixFlare
head dimensions .Fuel supplyspecificationsTracer usage (ft3/hr
S02atInput flowrates .....1%volume in_fuel), .Page4-197-57-77-10.
7-137-5 Time toquench the rate of reactions to 10%of the
rateat15000K (Chedai11e and Braud, 1972) . 7-307-6 Sample gas
measurements . . . . . . . 7-327-7 Dewpoint (OF) of
combustionproducts for propanewith steaminjectionat 11b steam/1b
propane 7-347-8 Response of FIDtocarbonatoms in compounds (Beckman,
1970). 7-447-9 Comparisonof techniques tomeasure velocity 7-477-10
Controlled parameters. . . . . 7-487-11 Dependent output parameters
7-59A-1 Comparisonof commercial flare designs (Brzustowski, 1976)
A-aA-2 Ground level flares CBrzustowski, 1976) . . . . . . A-13A-3
Suppliers of flare equipment (Brzustowski, 1976) A-15C-1 Flare
input andoutput parameters to bemeasured .. C-3C-2 Continuous gas
analysis instruments . . . . .. . C-5C-3 Tentative goals for
precisionand accuracyof measurements . C-7D-1 Fl are emi ss i on
factors . D-1Compositionof flare gas and heat content of
individualcomponents . . . . . . . . . . . . . . . . .
.D-2D-3Divisionof gas streams . .. . .". 0-20-3xivVariable.
SymbolAbCECsCLccpCvoDEOFdEFFRGFRf.GgHHVHChICBFKKTLMNTABLE OF
NOMENCLATUREMeaningCross-sectional areaCharacteristicjet
radiusLocal entrainment
coefficientSpreadcoefficientFlammabilitylean
limitConcentrationSpecific heat, constant pressureSpecific heat,
constant volumeDistanceLocal fuel destructionefficiencyDi 1ution
factorDiameter.Global emission, E = l-UFractionof heat released
inaflare that is radiatingFederal Republicof GermanyFroude
numberEddy generation frequencyJet momentumfluxGravitational
constantHeightLowHeatingvalueof gasHydrocarbon (expressed as
equivalent methane)Height of stackIncompletelyburned fuelRadiation
heat fluxTracer concentration ratioLengthMol ecul ar wei
ghtMassMass flowrateRatioof mass flowratesNumber of
eddiesxvi--VariableSymbolN.nopQRReRirSSRTtUuvVw\xyzTABLE OF
NOMENCLATURE (Cont1d)MeaningNumber of mol esNumber of atoms per
moleculeGlobal efficiencybasedonoxygenconsumptionPressureRadiation
intensityper unit volumeof productGas constantReynol ds
numberRichardson numberRadius or intermediate variableDimensionless
coordinateSti rY'ed reactorTemperatureTimeGlobal
efficiencyVelocityVolume. Volumetric flowrateVerti cal veloci ty
(ref. Brzustowski)Dimensionless downwind coordinateRadial distance
fromcenterlineMass fractionAxial distance
fromnozzlexviSymbolynEll.lp'" poTABLE OF NOMENCLATURE lCont
ld)GREEKSYMBOLSMeaningLocal combustionefficiencybased on
CO2Empirical entrainment coefficientEmpirical factor used to
predict the amount of steamrequired tosuppres s sootBurning rate
parameterEntrainment coefficientThi ckness of jetLocal
efficiencybased on pollutantsJet spreading angleKolmogorov lenqth
scaleVi scos ityDimensionless streamline coordinateDens i tyMixing
cup densityStandarddeviation;-0- Standarderror of xX Stoichiometric
coefficientQ Intermittencefactor-fractionof the time theis
presentat radial distance
XxviiSubscriptSymbolaadCceFfjLopsstunhcwoverbar[JTABLE OF
NOMENCLATURE (Cont'd)SUBSCRIPTSMeaningAirAdiabaticCarbonCenterline
valuesEddyFlameFuelJetFlame 1engthConditions at
nozzleProductsSonicSteamUnburned hydrocarbonWindAmbient
conditionsAmbient conditionsConcentrationsxviiil; ,-1.0 SUMMARYThe
U.S. Environmental Protection Agency has contractedwith
EnergyandEnvironmental Research Corporation toconduct aresearch
programwhich willresult in the quantificationof emissions fromand
efficiencies of industrialflares. The studyis being conducted
infour phases:I - Experimental DesignII/- Design of Test
FacilitiesIII - Development of Test FacilitiesIV- Data
Collection'andAnalysisThis report provides the results of Phases I
and II of the study.The report summarizes the technical literature
on the use of flares andreviews available emissionestimates.
Technical critiques of past flareefficiencystudies are provided.
Mathematical models of flame behavior areexplored, and
recommendations for flare flamemodels are made.The parameters
affectingflare efficiencyare evaluatedandadetailedexperimental test
plan is developed. The designof aflare test facility isprovided,
includingdetails on theflare tips, fuel and steamsupply,
flowcontrol andmeasurement, emissions sampling and analysis, and
data acquisi-tionand processing. AQualityAssurance/QualityControl
Programis alsodescribed. R ~ s u l t s of the testing
program(Phases III and IV) will be pro-videdinthe project's final
report to be issuedat alater date.1.1 Pollutant Emissions
fromFlaresA flare is adevice which allows the economic safe
disposal of waste gases,.by combusting them. The waste gases are
injected into the openair through atipwhich is designed to promote
entrainment of the ambient air and provide astable flame withawide
range of throughputs in high cross-winds. In order toreduce flame
radiation at ground level, the flare tipmust be elevatedand
itsheight will be dependent upon flame size (i.e., flare
throughput). If thewaste gas has too lowaheating value tosustain "a
flame. auxiliary fuel may beadded. Small flares may utilize fans to
provide some air premixing beforeinjection, but most large flares
are natural draft with optional steaminjectionto promote fuel air
mixing. Flares are usedextensively to burn purged and1-1,-waste
products fromrefineries, excess productionfromoil wells, vented
fromblast furnaces, unusedgas fromcoke ovens and gaseous wastes
fromthechemical industry.An estimated 16Mtons/yr of gasbe flared in
the U.S. The amountis difficult toestimate because throughputs
fluctuate widelywith time andare seldommeasured. The normal,
time-average throughput is in the range ofzero to5percent of design
capacity, which is exceededonlyduring emergenciesor upsets. The
flared gases fall intothree categories (Klett &Galeski,
1976);Lowheating value gas produced in blast furnaces which account
for60 percent of the.weight and 19 percent of the heating value of
theestimated annual flared gases;Mediumheating value gases produced
in coke ovens and in thechemical industry;High heating value gases
flared in refineries which account for 18percent of theweight and
32 percent of the heating value of theestimatedannual flared
gases.Pollutant emissions fromflares result fromafailure to
completelycombustthe flared gases. ' The pollutant species are
normallycarbonmonoxide, hydro':'carbon and soot, and total
emissions are assessed based upon an estimate of.flare efficiency.
The efficiencyof combustionof aflare, which is ameasureof its
abilityto destroy the flaredgas,is difficult tomeasure, and
consequentlyestimates of pollutant emission indices vary. Estimates
of flare efficienciesvarywidely, some are very high, in excess of
99 percent, whereas others rangeas lowas 70 percent (T.A. - Luft
1974) leading to the conclusion that emissionfactors are unknown.
If flares were 90 percent efficient, then emissions
ofcarbonmonoxide and hydrocarbons would be approximately 12 percent
of thoseemitted by all stationarysources. More important are the
contributions offlares as localized sources because of their
concentration in refineries andsteel plants where theycould be
among the'mostsources ofpollutants if the efficiencies are
relatively low.Ii can only be.concludedthat pollutant emissions
fromelevatedflaresare unknown. This is due to acombinationof
uncertainties in the quantityof gases beingflared and their
composition together with the uncertainties1-2\-inflare efficiency.
Before adecision can be made whether pollutant emissionsfromflares
are of concern, an accurate assessment of flare must bemade.
Theoretical estimates of flare efficiencycannot be made,
emissionmeasurements fromoperating flares are difficult and
previous pilot scale studiesare contradictoryor incomplete. Thus,
there is aneed for astudytoaccuratelyassess flare efficiencyas a
function of:flared gas composition;throughput;flare
design"and(steaminjection, etc.);ambient conditions;scale.Data
fromthis study canthen be used to provide an accurate assessment
ofpollutant emissions fromflares.1.2 Deficiencies in Previous Flare
EmissionStudiesThere have been relativelyfewinvestigations reported
in the open litera-ture concernedwi th pollutant emi ss i on or
effi ci encY'of fl ares.Table 1-1 summarizes the most recent, known
studies, eachofwhichaddressed one or more of thefollowing
topics:the emissions of incompletely burnedmaterial;the distance
required toburn theflared gases;the impact of steaminjection on
pollutant emissions;the effect of ambient conditions on pollutant
emissions.Although these studies have made valuable contributions
to the knowledge offlare performance, none allowan accurate
detennination of pollutant emissionsnor do they provide adequate
information on the effects of scale or flared gascomposition.A
reviewof the previous studies indicates that data acquisition
andmanipulation arecommon problems which prevent an accurate
assessment offlare efficiency. These problems are discussed belowin
fourmainareas,namely:1-3r..:-",--'J~TABLE1-1.PREVIOUSFLAREEMISSIONSTUDIESThroughputFlareEfficiencyInvestigatorFlareTipDesignFlaredGasMBtu/hr%Palmer(1972).0.5"dia.Ethylene0.4-2.1>97.8Lee&Whipple(1981)DiscreteHolesin2"Propane0.396-100dia.cap.Siegel(1980)CommercialDesign"50%H249-178I97->99(27.6"dia.steam)pluslighthydro-carbonsHowesetal(1981)CommercialDesignPropane4491-100(6"dla.airassist).CommercialDesignH.P.NaturalGas28(pertip)>99(3tips@4"dla.).'.
'0 Inabilitytocloseamaterial balance. Measurement of soot
concentration. Difficulties caused by flare "intermittency". Lackof
scalingmethodology.Closureof Material BalanceThe global (overall)
efficiencyof aflare flame can be calculated if.theinlet fuel
compositionand mass flux is known together withmass flux of
allIhydrogenand carboncontaining species of flaredmaterial at some
height abovethe flame whereall reaction has ceased. There is more
interest in that frac-tion of the fuel flux that becomes air
polluting species rather than harmlessCO2and H20. It is usual
toconcentrate on the carbon inthe fuel becauseall'of the
ultimateair polluting species containit CO, HxCy' soot). Ifthe
carbonfraction of all product gas flux species is summed, the
result"should equal the carbonfraction of fuel mass flux. This is
the usual massbalance concept and is anaccounting checkon the
pollutant species measurements.It is easyto state but rather
difficult to implement. Of the studies inTable1-1, onlySiegel
attempted toclose the mass balance. Generally he was onlyable
toaccount for approximatelyhalf of the fuel carbon in the off-gas
flux.Siegel statedthat the largest errors were associatedwith the
velocitymeasure-ments needed to.determine themass flux. Siegel
circumvents the needfor furthermass balance by using "l ocal
burnout II efficiency, and showing that errors in the global
efficiencyvalues areminor.Amaterial balance requires time
averagedconcentration, velocity andtemperaturemeasurements at some
plane normal to the mean direction of flow.These measurements are
made above the flame when total emissions are beingassessedwhich
requires an integration of the species flux across the total
jet.The major errors which prevent adequatematerial balance closure
are:Material escapesundetected, becauseat the flame
extremitiesdilution lowers itsconcentration
belowthedetectabilitylimit ofthe analytical equipment;All the
species are not measured;The time average velocityis difficult
tomeasure inand nearturbulent flames.1-5A tracer inthe fuel can be
used toaid in obtaining amass balance byyielding adouble check on
the dilution factor in the product gases. However,the useof atracer
do'es not eliminate the need for velocitymeasurements
indeterminationof mass flux. More details on tracers will be
discussed inSec-tion7.7.2.Measurement of Soot Concentration. Soot
represents uncombustedfuel carbonwhich should be included in
flareflame efficiencycalculations. Siegel (1980) meisured soot
concentrations be-. tween 20 and 80 mg/m3in an intentionally
smoking flame, estimating that atthose dilution conditions this
reducedflare efficiency by 3to4percentagepoints. More recently,
Howes, et al (1981) performedsoot measurements in asmoking propane
flame. Usingadilutionfactor obtained fromthe CO2concen-tration,
the18mg/m3. of soot measured represented adecrease.
incombustioneffi-ciencyof 0.4 percent. It should be noted that
these local efficiencies are notequivalent toglobal efficiency,
since theywere samples collectedat one samplingpoint.Flare
IntermittencyThe term"intermittency" essentiallymeans that at one
fixed point abovethe flare, the flame is not present all of the
time. Even incalmwind condi-tions, the turbulence induced by the
combustion processcauses the flame toundulate andappear unsteady.
This usuallycauses corresponding fluctuationsinmeasuredquantitites
at fixed points above the flame. Using sufficientsampling times
provides one means of time averaging data to avoid this inter-mi
ttencyeffect. As di scussed inSecti ons 1and 8, one objecti ve of
the proposed. experimental plan is todetermine sampling times so
that the characteristicsof the flame aremeasured, unmasked by
intermittency.ScalingMethodology'The studies listed inTable
1-1didnot provide amethodologywhereby thedata fromthese pilot scale
orsmall, plant scaleflares could be used to assessthe emissions
fromthe total populationof flares. Amethodology is requiredwhich
will allowdata to be obtained economicallyat pilot scale and used
todetermine performance of full scale systems. The current
state-of-the-artof turbulent flame structure precludes the use of
predictive models. Thus the1-6pilotIt must6-experimental planmust
provide data whichwill allowthe effects of scale to bedetenninedand
in conjuctionwith developing theories of turbulent flame struc-ture
will allowextrapolation to full scale.1.3 Technical ApproachAs will
be shown in the BackgroundSection, current information on
flarecombustion is fragmentary and inconclusive. This
programattempts toanswerthesequestions: What are the
combustionefficiencies of small flare flames? Howare these
efficiencies influenced byoperational parameters, flaredesign, fuel
composition and scale? What are the mechanisms of these influences?
Howcan the efficiencies of large industrial flares be estimated?A
research programwithemphasis inexperimental measurements on ascale
flare is themost cost-effective way toapproach these
questions.fulfill these requirements: Representativeness - The
hardware and operational conditions mustrelate to full scale
practice. Data Accuracy- The measurement methods must be developed
and verifiedsatisfactorilytoeliminate the uncertainties that
plagued p r ~ v i o u sexperiments. Basic Understanding -
Experiments must be designed to bring under-standingon the
underlying controlling processes that take place inflare flames.
Extrapolation - Informationmust be generatedtoextend the
applicabi-lityof the small scale data tofull scaleflares.The
designof validexperiments on flares must consider the factthat
flare flames are different fromother combustion processes, such
asenclosedboil'er flames, in that theyare buoyancy dominated, are
affectedbyambient air movements, and lose heat toamuch colder
environment.It is commonly accepted that ifsufficient air is
mixedwith the fueland if the resultant mixture is kept above the
reaction temperature,combustionwill go to near 100%completion.
However, these two conditions1-7.;....are not necessarily true
inflare combustionsystems, particularlyfor the fueleddies that
arefromthe mainflame body .. Because of the geometryof the eddies,
they tend to be quenchedat ahigher rate than themain flamebody and
hence are more likelyto be extinguished beforeall the fuel is
burned.The presence of oxygen next to the fuel ;s essential for
continuationofcombustion. In aflare flame, air may be entrained
into the fuel jet bynatural convection and assisted by forced
convection throughair- or stearn-assist. The effectiveness of these
mixing processes directlyaffects thecombustion.reaction. If the
mixing is not completed before the burning fuelelements are
quenched belowthe reaction temperature, the flame will be
extin-guished. Therefore, the research programmust develop the
basic understandingof the mixing and eddy behavior of flare flames.
This may be aided by modelingwhichwill be discussed in more detail
later on in this report.The emphasis of the research programwill be
themeasurements andcharacterization of emissions and flame
structures. It will include thesecons i derat ions:Four flare sizes
3, 6and 12 inches indiameter) will be linearlyscaled replicas of
each other andwill include features of
commercialflares.Detailedmeasurements will be made throughout and
beyond the visible-flame envelope todetermine profiles of
temperature and speciesconcentration.Tracers will be injectedand
measured toassess air entrainment.Photographywill . recordflame
structuresThe experiments will start with the smaller flares
todevelopand verifythemeasurement methods. Once the baseline flare
behavior is defined, the effectsof operational parameters and
scalewill be studied.The experimental test programcan be
logicallydivided intofour tasks.The Task 1 objective is generation
of adata baseof gross flame parame-ters as afunction of the
complete range of all input parameters. This will bea
rapidscreening process on all flareto assess themajor effects of
fuelrate, wind level, steamrate, andgas composition. The output
measurements will1-8; ..:' ." _1be limitedtovisual and photographic
observations of fJame length, formandstructure, and sooting
tendency. Video recordings can also supPlement the photo-graphic
technique. The utilityof this task lies in its identificationof
thoseregimes of the original test planthat need greater emphasis in
the succeedingtasks.Task 2will be concernedwithdevelopment and
verificationof all measure-ment techniques. This canmost
effectively be done using the smaller flare sizes.The
measurementswill consist of species concentrationmeasurements inand
nearthe flame, including atracer. Development of an integrating
hood will be in-cluded. Amajor objectiveof this task is
verificationof an adequate carbonmass balance to provide confidence
in the succeeding task.Task 3will be concernedwith the
detailedmeasurements according to thetest planas revised by Task 1.
The major effort will be on the smaller sizes,with the knowledge
gained indicating the most important test conditions to beused'for
the limited number of large size tests.Task 4is ageneral
categoryrelated tocontinuous evaluationof test data,development of
modeling and scalingparameters anddocumentation. Amore de-. ' :
~tailed breakdownof these tasks is found inSection4.1.4 Report
OrganizationThe report describes the background relatedtoflare
design, characteris-tics and emissions inSection2. Sections 3and
4discuss the need for futherwork and a technical approach to
carrythis work out. Section5gives a reviewof the potential test
facilities which may be consideredfor theexperiments.The
designobjectives, afacilitydescriptionand the measurement
techniques tobe used in assessingflare characteristics aredescribed
inSections 6and 7.Section8discusses data analysis and applicationof
the information generatedin the experimental program.1-9., .,-2.0
BACKGROUNDThe primaryuse of flares by industry is the safe
ventingand combustionof process gases during emergencyor "upset"
conditions. Theyare alsoavail-able todispose of much lower
flowrate$ of waste gas that occur during normalprocess operation.
It is this latter conditionthat prevails most of the timeand is
thus of major importance in determination of flare efficiency.
Indus-trial flares encompass awide varietyof conditions; both in
terms of the typeof installationand operatingconditions. Important
factors are gas composi-tion, heatingvalue and percent dilution by
inert gases, flowrate, ambientconditions and combustionsuchas
steamor forced draft air.These depend on the type of plant and its
location suchthat flare designstend to be sitespecific.
Consequently, there is awealth6f
hardengineeringexperiencewithrespect toflare design and ,adaptation
todifferent conditions,but due to the varied nature of the flaring
process, there is alackof compre-hensive information in the open
literaturewith regard to specificoperationaldetails as theyaffect
thewaste gas combustionprocess. There is onlymeagerand often
contradictory informationpublished on the potentiallyharmful
mate-rials issuing fromindustrial flares; thus there exists an
information gapthat must be closed inorder toassess the
environmental impact of flaresystems.Withinthis framework, the
objective of this present programis todefinean experimental
planwhichwill both improve the understandingof flare combus-tion"
and provide ameans of estimatingemissions fromflares of various
sizesand characteristics. There is agreat deal ofbackground
informationwhich isrelevant tothis task, andthe purpose of this
section is to review'the impor-tant available literature inorder
todefine the scope of the required experi-mental, program. Of
particular interest is informationconcerningflare useand the range
of gas compositions encountered in the di'fferent industries,flare
designs and the range of operatingconditions, and experimental data
andmethods available for themodeling and scalingof flare type
combustion.In recent years, a number of surveys of flare use have
been carriedout,both in the UnitedStates and abroad, in an attempt
todefine the'significanceof flare emissions. Results fromsuch
studies providebasis for the estima-tion of total gas quantities
flared, the range of gas
compositions_encountered,2-1\"\..I;....and, by inference, possible
emission factors for the different industries.Information is
alsoavailable concerning flare types and designmethods,althoughthe
nature of the flare combustion process, and the lackof
appro-priatemeasurement methods, has precluded the kindof detailed
studythatwould permit adefinition of combustionefficiencies or
allowquantificationof theeffect of different flare design
parameters. Anumber of small andpilot scale studies have, however,
been carried out, and these are reviewedto provide insight into the
relevance of small scaleexperiments and thepossibilities for data
extrapolation. Much of the available data shows, how-ever, that our
knowledge of basic combustionphenomena is lacking incertainareas,
and that direct transfer of experimental data fromsmall tofull
scaleis usuallynot possible. To this end, asimplemathematical
modeling approachis requiredtodirectionfor the designof small scale
experimentsand ameans of defining scalingcriteria bywhich the data
can be extrapolated.Availablemodeling approaches and basic
information concerningthe character-istics of large turbulent
diffusionflames are reviewed inthis light.2.1 Use of Industrial
FlaresMuch of the informationdesign, andoperationof flares has
beenreviewed by Klett and Galeski (1976). More recentlythe German
SocietyforPetroleumSciences and Coal Chemistry, DGMK,
(Program135-01) analyzed the- /' "results of a "questionnaire sent
to 31 German refineries. Unpublished resultsfromasent out to
California refiners by the California AirResources Board (Metzger
andVincent, 1980) has also beenmade available.Most of the
informationwhich follows is based on these studies.Flares are
designed for the maximumanticipatedgas release caused byprocess
upset or emergencyshutdown. These conditions occur
infrequentlyandare of relativelyshort duration. A lower level
continuous or semi-continuousrelease is caused by leaks
inequipment, necessaryventingof aprocess, andpurging of gases
duringstart up and shutdown. These flows, while of
muchreducedmagnitude compared toemergency use, occur most of the
time. Thus, aflaremust be a veryflexible device, capableof high
throughput, and sustain-ed operationat a high turn down ratio. For
instance, an instantaneous flowrate of 100MSCF/hr may be
demandedwhile sustained normal operationoccurs at1/1000of this
value.2-2\..-This section reviews the iridustries inwhich" flares.
are used, character-izes the gases flared, and estimates the amount
of gas flared in thesetries. Unfortunately, the use of flares is
largelyuncontrolled and, hence,the flowrates of gases are
infrequentlymonitored. Flowrates are onlyoccasionallymeasured
sothat the amount of steamrequired to suppress smokingcan be
regulated. Rough estimates have been made of the amount of gas
flaredin four major industrial operations: oil refining, blast
furnaces, coke ovens,and ethylenemanufacturing for 1974. Here we
extrapolate this estimate to1980and estimate the amount of gas
flared in petroleumproduction and thechemical industry.2.1.1
PetroleumRefiningThe petroleumindustryflares largequantities of gas
fromrefineryoperations and productionwells.Table 2-1 has been
constructed fromthe data of CaliforniaAir ResourcesBoard Survey.
Althoughthe number of initial questionnaires sent out is un-known,
the 63 flares referenced by the 21 respondents showverysimilar
char-acteristics. Although "emergency" is the primaryuse,
continuous use is alsoassumed. Steamis themost universal means of
smoke suppression. None ofthe flares are equipped tomeasure
flowrate of the flared gas, soannualamounts are onlyestimates. The
compositionand heatingvalue varywidely.These ,results are similar
to the results fromGerman refinerysurvey(DGMK, 1981) wherein
thirty-one West German refineries responded. Figure 2-1shows the
flare gas densityvariationfromone of these refineries
duringtheperiod in 1978of Siegel's researchthere. The flowrate is
that of Siegel'sside streamand not the main flare.Generally, the
gas flared inrefineries is not measured and isdifficult toestimate.
However, gases flared inrefineries contribute signif-icantly to the
total amount of gas flared in the UnitedStates. Therefore,the
amount of gas flared inpetroleummusi be estimated, if the
totalemissions of incompletelyburnedfuel is to be estimated.In
1974, a survey (Table 2-2) of 11 of the 288 reflneries in the
UnitedStates showed that from0.039to2.8 percent of the
refinerythroughput wasflared. The average amount flared excluding
the highest number was about 0.22-3TABLE 2-1. SURVEY OF
CALIFORNIAOIL REFINERY FLARES(CALIFORNIAAIR RESOURCE BOARD,
1980)FlareSmokeAnnualSteamRefineryFlareDiameterService Flowrafe
Fuel~Type(i n)Suppressionscf/yr1 Elevated. 30 Steam Emergency ---
---...a. 351 Elevated 24 SteamEmergency--- ---...a. 351 Elevated 24
Steam Emergency-- ---...a. 352 Elevated---Steam Continuous--- --
---3 Elevated 30 Steam Emergency---H2' CO. N2 ---C1-C33 Elevated 30
Steam . Emergency--- ------ .3 Elevated 8 Steam Emergency---
------4 Ground---Steam Emergency --- ---0.404 Elevated 30 Steam
Emergency------0.384 Elevated 8 Steam Emergency---Cl C ~ , S 0.304
Elevated 10 SteamEmergency---LPG---5 Ground---SteamEmergency
180M--- ---5 Elevated---Steam Emergency------ ---5 Elevated 16
Steam Emergency--- ------5 Elevated 20 Steam Emergency--- --- ---5
Elevated 10 Steam Emergency--- ------6 Elevated 30 Steam Emergency
36M--- ---7 Ground--Venturi Emergency--- ------8 Elevated 8
SteamCanI t &Erner.--- ---...a.59 Elevated 8 Steam.. Ernergency
SOH H'! . Ct -C{i , 1.7H109 Elevated Il SteUl Emergency 3.5M
H2C1-C6 , 1.7H2O10 Elevated--Steam Emergency 0.9M---0.5- ..11
Elevated 36 Steam Emergency -------...a. 311 Elevated 36 Steam
Emergency--- ---....0.311 Elevated 36 Steam Emergency---
---....0.311 Elevated 10 Steam Emergency--- ---....0.312 Elevated
18 -Steam Emergency 547M HC, H2S, 1J.3 ->1.RSR13 Elevated 31
Steam Emergency---HC, H2S, 0.33RSR14 Elevated 6 Steam Can't
&Erner. 3.9M C I - C 5 ~ H20.4315 Elevated 48 Steam Emergency
111---p.3-0.415 Elevated 48 Steam Emergency 283---p.3-0.415--3D
ForcedDraft .Emergency 1.2--- ---15 Elevated 16 Steam Emergency
27.6---p.2-0.3516 Elevated 36 Steam Emergency---Cl .C2, H2
....0-2.H2, H2. 02.CO,16 Elevated 36 Steam Emergency--- Cl.C2. H2'
....0-2.H2 H2. 02.CO16 Ground---Steam Emergency---Cl,C2. H2,
....0-2.N2. H2,. 02.CO216 Elevated 36 Steam Emergency---Cl ,C2' H2
"'0-2.N2. H2. 02, CO216 Elevated 36 Steam Emergency---Cl,C2, H2
....0-2.Nz H2. 02, COz2-4TABLE 2-1. SURVEY OF CALIFORNIAOIL
REFINERY FLARES (CONTINUED)(CALIFORNIAAIR RESOURCE BOARD, 1980).
FlareFlareSmokeAnnualSteamRefineryTypeDiameterSuppressionService
Flowrate Fuel(in)scf/yrruel16 Elevated 42 Steam Emergency---ci.C2
H2."-0-2.N2 H2. 02. CO216 Elevated 42 SteamEmergency---CloC2.
H2.'\00-2.N2H2. 02.C0216 Elevated 48 SteamEmergency---Cl.C2.
H2'I.oQ-2.N2.H2. 2.C0216 Elevated---SteamEmergency ---Cl .C2.
H2'\00-2.N2 H2.. 02.C0216 Elevated 70 Steam Emergency---Cl.C2 H2
'\00-2.N2.H2. 2.C0216 Elevated--Steam Emergency--- CloC2.
H2.'\00-2.~ N2.H2. 2,C0216 Elevated Steam Emergency Cl .C2.
H2.\'\00-2.--- ---N2.H2. 2.C0217 Ground---Steam Emergency ,---
CitC2. H2 '\00-2.\N2 H2. 02.C0217 Elevated 42/100 Steam
Emergency---Cl .C2. H2'\00-2.N2 H2. 02. .C0217 Elevated 36 Steam
Emergency---Cl .C2 H2 '\00-2.N2 .H2. 2.C0217 Elevated---Steam
Emergency---Cl,C2 H2. "-0-2.N2.H2. 02.C0217 Elevated 48/72 Steam
Emergency--- CloC2. H2.'\00-2.,N2 H2. 02.C0217 Elevated 12 Steam
Emergency---ci .C2. H2 'I.oQ-2.N2 H2. 02. C0217Elevated 12 Steam
Emergency---Cl.C2 H2 'I.oQ-2.N2 H2. 02.C0218 Elevated---Steam
Emergency}Cl-Cs.NH3.C02'\00.318 Elevated---Steam Emergency10.740H2S
0.319 Elevated 42 Steam Emergency--- ---'\00.319 Elevated 36 Steam
Emergency--- --'I.oQ.319 Ground--- ---Emergency--- ---020
Ground---Steam Emergency--- ---20 Elevated---Steam Emergellcy ---
---20 Elevated---Steam Emergency------20 Elevated---Steam Emergency
--- ---20 Elevated---Venturi Emergency --- ---21 Ground---Self-
Emergency 0.25M Cl -C 30Inspiration2-5r.."L.I:.......'"Q)~2-a
.....QJ~1-G::~G::-,.'IJI(Y)2I11II~I1III~IIIII>,I1II.~1.5:1II1(a)0I'1I1I:i1IIIIIQJIIII1~IIIII"-0.5I1II3IIIIII.1II"II,IIIIIIIIIIIIIIIItII1IIIITest~o.~~;~1~6117NI0\(b)NI0)Figure2-1.(a)VariationingasdensityflaredfromaGermanRefinery;(b)ActualflowrateoftestflareusedbySiegel(1980).t..:'TABLE2-2.GASFLAREDINU.S.REFINERIES(KLETTANDGALESKI,1976)NI-....JTotalRefinerytComposition(t)Hydro-CompositionRefineryThroughputFlaredClC2C3[4[5AromatlcOleffnsPilraflnscarbonH2112SINH3Otherbbl/cdlb/cd.154,4370.17001.21.02.40005.45.409.4085.12167,6580.55449.414.512.99.311.1010.387.097.31.90.8003213,0000.1432.35.170.530.540.01l.529.557.498.41.60.000473,7000.14547.914.914.212.28.9-010.387.798.10.60.9005106.0640.05911.126.01.82.82.5012.731.444.2,.043.81.39.86255,0000.0397.832.429.214.36.81.217.775.093.83.22.90(j7239,4000.0568.58.434.441.74.509.187.191.51.41.1008369,5000.2101.39.153.65.38.8010.275.185.30.42.1012.29112,6520.60420.917.834.911.58.6019.774.093.11.61.303.310162,9080.14222.932.118.11.28.9011.877.489.14.66.300II145,0600.18924.213.267.31.14.20094.394.30.40.504.7Total1,899,4190.1923.314.431.611.411.01.012.976.390.21.52.405.812306,5902.788.36.548.433.6.3.100100.0100.00000percent.
This number is the same as that estimated by Siegel (1980) for
Ger-man refineries. About 12.3x106BBL/cd (cd=calendar day) of
petroleumwasrefined in 1974. Therefore, petroleumrefineries flared
approximately7.4x106lb/cd of gas based on barrels refined. The
figures for 1980are 18x106BBL/cd refined and 10.8x106lb/cdof gas
flared.The compositionof gas flared in refineries varies widely,
bothwithin arefinery (Figure 2-1 and Figure 2-2 andTable.2-3)
andrefineries.(Table 2-1 and 2-2). The amount of gas flared
inaGerman refineryvaried byafactor of 22, the density by afactor of
3.4, and the compositionof somespecies by afactor of 5. However,
most of the refinerygases flares arelight paraffinic hydrocarbons
with large amounts of C3and C4compounds. Anaverage composition for
arefinerygas is shown inTable 2-4.2.1. 2 PetroleumProductionGas
flared during production of petroleumalsocontributes to the
totalamount of gas flared in the UnitedStates. In the past, large
amounts of lowmolecular weight gases have been flared fromoil
producingwells. This prac-tice has been reduced recently, since the
gas product is nowvaluable and muchof it can be sold.The amount' of
gas flared in petroleumproduction has not been previouslyestimated,
andit is difficult tomake such an estimate. In one report
approx-imately0.7percent of theoil productionwas flared (Minkkinen,
1981). Assum-ing aratioof 0.5 percent gas flared toallproduction
inthe UnitedStates, the 10produced in the UnitedStates in
1980resulted in the of approximately3 Mtons/yr of gases fromoil
production.The compositionof gases flared during productionof
petroleumis thesame asnatural gas. These gases aremostlymethanewith
small quantitiesof other light hydrocarbongases and inert gases
(Table"2-5).2.1.3 Blast Furnaces. \Another major use of flares is
todispose of waste gas fromthe blastfurnaces used in the ironand
steel industries. As inrefineries, blast fur-nace gftses are flared
intermittentlytocontrol process pressures. Gases.flared fromblast
furnaces account for approximately60 percent of
theweight2-8),-(a)C4HlOC H1ZCH4CZH6CZH4CZHZC3HaC3H6n n3I
IIIIIIIIIIIIIII IIIIII-- II IZI.IIIII
97.8Siegel(1980)27.6ReflnllrybVarIable49-178-15290-6401VV13-32V.JVVVVVV97->99Gas\lowes(1981)6C3"840-604413-17(1)VV20.J.J.J.JV.JVNO92-100"owes(1981)3x4"C"480B48(2);NONONRVVVVV.JVNO~99.9~I~(1)AirAssIsted(2)"IghPressureaNotReportedb45-69percentHydrogen,balanceCI-C4hydrocarbons(Table2.3)The
characteristics of the flame depend both on fuel and
operating.con-ditions, and considerablework has been carriedout
todefine flame length,spread angle, andentrainment rate as a f u n
t t i ~ n of thesevariables. However,verylittlework is available on
the movement of the flame, and the formationand transport of
eddies.Flame lengths have been defined in several ways, visible
flame lengthbeing the most common. However, it does not represent
the end of chemicalreactions which may continue beyond the
visibleflame envelope. Therefore,amore precise definitionof flame
length is requiredand should ideally bedefinedas the point where
the oxidationof the fuel and intermediates stop.However, inthe
experiments describedlater, flame lengthusuallyrefers tothe visible
flame envelope ... Unburnedfuel, partiallyoxidizedfuel, cracked and
polymerizedproducts,and soot may all potentiallybe emitted
fromflares. All are products of in-complete combustion. The extent
of incompletecombustion depends largelyonthe rate and extent of
fuel-air mixing and the flame temperature achieved
andmaintained.Jet diffusion flames are not steadystateprocesses.
The flame structuremoves aroundas aresult of combustion reactions,
buoyancyforces, and combus-tion inducedturbulence. Eddies are
formed inthe shear layers of the flameand may breakaway fromthemain
body. In some instances aflame eddymay bequenched belowthe reaction
temperature and extinguished. This can thenresult inthe
productionof incompletelyburnedmaterial whichescapes aspollutants
..2.4.2 Characteristics of Previous Experimental Studies on FlaresA
number of previous studies have contrjbutedtothe current
stateofknowledge of flare flames. This section reviews the
experimental flaresystems andoperatingconditions used in the
previous studies.Siegel (1980) made the onlycomprehensivestudyof
acommercial flaresystem. He studiedburningof refinerygas
onacommercial flare head (typeFS-6-anti-pollutant) manufactured by
Flaregas Co. Table 2-3shows thecompo-. .sitions of" the flare gases
used which consistedprimarilyof hydrogen (45.4to 69.3 percent by
volume) and the light paraffins (methane to butane).2-48 Traces of
H2S were also present in some runs. The f1ar.e was operated
from0.13 to2.9metric tons/hour (287to 6393 1b/hr). Hence,
themaximumheatrelease rate was approximately 235 x106Btu/hr.
However, most of the exper-iments were conducted between 49 x106and
178x106Btu/hr.Palmer (1972) experimentedwithaone-half inch IDflare
head, the tipof which was locatedfour feet fromthe ground. Ethylene
was flaredat 50to 250ft/sec at the exit (0:4x106to 2.1 x106Btu/hr).
Heliumwas addedto the ethyleneas atracer at 1to3volume percent and
theeffect of steaminjection was investigated in some
experiments.Lee and Whipple (1981) studieda bench-scale
propaneflare. 'The flarehead was two inches in diameter withone
13/16inchcenter hole surrounded bytwo rings of sixteen
1/8-inchholes, and two rings of sixteen 3/16-inch holes.This
configuration had an openarea of 57.1 percent. One hundred and
thirty-one (131) CFHof'propane,with 12 CFHof helium, was fired
through theflare head. The velocitythroughthe head was
approximately3 ft/sec, andthe, heating rate was 0.3MBtu/hr. The
effects of steamand crosswindwerenot investigated in this
study.Howes, et al,studiedflares producedon two ,types ,of
commercial flareheads at John Zink's test facility. The commercial
flare heads were an LHair assisted head and an LRGO (Linear Relief
Gas,Oxidizer) headmanufacturedby John Zink Co. Since bothdesigns
are proprietary, detailed configurationsof the flare heads are
unavailable. The LH flare burned 23001b/hr of commer-cial propane
on a6-inchdiameter gas pipe. The exit gas velocity based onthe pipe
diameter was 27 ft/secwithout air assist and the firing ratewas44
x106Btu/hr withair assist, and the combinedvelocityranged from40
to60ft/sec. The LRGOflare consistedof th;ee burner heads 3 feet
apart. Thecombined three burners fired 42001bs/hr of natural gas.
This corresponds toafiring rate of 83.7xl06Btu/hr. Steamwas not
usedfor either flare, butthe LH flare headwas in some trials
assisted byaforced draft fan.2.4.3 The Structureof Flare
FlamesDestructionefficiencyand pollutant formation in flares is
dependentupon their structure. Inmany previous studies, physical
structure was oftenused to flare flames. However, these studies
were interested2-49only in the gross properties of the flame, e.g.,
flame length, flame spread,and the rateofair entrainment. Although
these properties are important, amore complete characterizationis
required if themechanisms of pollutant for-Imationare to be
understood and emissions fromflares assessed accurately.An accurate
characterizationof flare flames requires aknowledge of thespatial
distributionof temperature, velocity, and species concentration.The
time-averagedvalue of these parameters is affected
by.theflamestructure since their" instantaneous values fluctuate
withtime: The instan-taneous temperature profile influences the
rate of destructionofmaterial atanyposition inthe flame. The
concentrationprofiledefines the local aver-age species
concentrations., the average rateof destructionor
productionofspecies, and the geometric limits of burning. The
instantaneous velocityprofile describes the flowrate and hence the
flux of material in the flame.While some previous investigators
have studiedthe temperature, concentrationand velocityprofiles
inflares, mosthave concentratedonlyonobservations of overal.l flame
length, formand appearance. Observers reportthat the flare flame is
typicallynot stationary, and 'that slight changes in,for example,
thewind speedanddirectioncancause the flame toshift
posi-tionandalter its shape. Many authors also report the
presenceof largecoherent structures within such flames, where
eddies developalong the lengthof flames and seriesof detached flame
pockets results (Figure 2-7a). Eddieswhich are formed on theedges
of the flame could be quenched and extinguishedIas aresult of rapid
heat transfer to the surroundings and thus be responsiblefor
producingmuch of the incompletely burnedfuel inflares. Yet the
forma-tion, separation, and quenchingof eddies has been
inadequatelyaddressed inprevious studies. However, some studies
have recognizedthat
quenchededdiescouldcontributesignificantlytoemissions fromflares.
Gunther and Lenze(1972) have estimatedthat quenchededdies might
result inas much as onepercent of the fuel remaining unburned.
Howes, et al, notedthat more eddieswere shed in highwinds and that
these eddies contained burningfuel. Leeand Whipple (1981)
concludedthat quenchingof eddies was themajor sourceof emissions
fromtheir flares. Incontrast, Siegel identifiededdies sepa-rating
fromhis flare but concludedtheywere an unimportant source of
emis-sions.2-50J tFigure 2-9shows thepotential emissions
fromincompletelycombustededdies as afunction of the rateof eddy
generation and efficiencyof eddycombustionfor the flare presentedas
an example inSection2.3.1. Forexample, the inefficiencyof the flame
would be on the order of 1-5percentif the eddy generation rate is
5-20per second, andif the combustioneffici-ency is 50 percent.
(Theseestimates are consistent withthose made byGUnther and Lenze,
1972.) These levels cannot be dismissed as small becauseeven one
percent inefficiencyis significant whenmost studies have
concludedthat flares are greater than 99 percent efficient.The
amount of unburnedmaterial is predicted to increaseas the
diameterof the nozzle increases, provided the frequencyof
generationremains con-stant. However, the frequency of generationis
probablysmaller for alargernozzle. Therefore, theseeffects of
nozzle size are expectedtocancel par-tially, but not completely.
The effects of, nozzle sizewill be discussed inmore detail
inSection 2.5.2.4.3.1 Temperature Profiles in FlaresThe spatial
distributionof temperature inaflare flame is affected bywind,
steaminjection, and the heating value of the flared gas. With
lowwind velocities, the temperature profile inaplane above the
flame appearsto be normallydistributed (Figure 2-10and Siegel,
1980). However, recentevidence producedat Caltech (Oimotakis, 1981)
indicates that the profiles inaturbulent flame are relativelyflat
except near the edges. The normalradial distributionof properties,
usuallymeasured inturbulent flames, isan artifact caused by
fluctuations of the flame about the axis, i.e., theflame isless
frequentlyat large radial distances at the centerline.
Consequently, the time-averaged temperature appears
todecreasewith,increasing radius.The time-averaged temperatures
measured in such systems are not usefulfor making estimates of
combustionrates. Inmany instances, the averagemeasured temperatures
are less than 600Cwhich is near the ignitionture of most
hydrocarbon fuels (Table 2-13). Hence, much of the fuel wouldbe
quenched before being burnedcompletelyif the average temperature
wererepresentative of the combustionprocess. these temperatures
arenot representative of the heat release zones, but. rather the
average tempera-2-51EddySize ~ FlareTipDiameter16 in. Diameter
Flare25%EfficiencyofEddy CombustionuQ)III--%unburnedfuel
emittedFigure 2-9. Estimates of flare emissions due to
incompletecombustion of eddies.2-522-53tureat apoint where hot
combustingeddies move throughat some frequencyfollowed
bycoldeddies. Thus, while the instantaneous temperatures of
burn-ingeddies can be used to predict combustionefficiency, the
averagemeasuredtemperature cannot.The averagemeasured temperature
profiles in aflame help todefine theboundaryof combustion. Measured
temperatures which approachambient onesindicate that fewhot eddies
are passing themeasuring locationand, therefore,the location is
outside the nominal edge of the f l ~ m e . Axial temperature
pro-files in Figure 2-10for twodifferent flowrates reported by
Siegel (1980) in-dicate that increasing the flowrate lengthened the
flame and raised the temper-ature at the same distance above the
nozzle. The radial temperature profiles,showthat at apositionof
6.5meters above anozzle firing 3800 lb/hr of fuelthe fl arne is
appr,oximately6 meters wi de. Thi s fl ame is much wi der than
pre-dicted byjet theory. However, Siegel used adivergent flare head
which wouldinduce more rapid spread of thejet than a normal head.
However, others (Leeand Whipple, 1980) who have used normal flare
tips have also found more rapidspread of flare flames than
predicted byjet theory.2.4.3.2 ConcentrationProfiles in FlaresThe
major components inaflare flame are O2, CO, CO2, hydrocarbons,H20,
and carbonaceous particulates (soot). ' Since the flare entrains
ambientair along its flowpath, the concentrationof
combustionproducts becomesincreasinglydilute. Thus,
lowconcentrations of incompletelyburned fuelspecies are not
necessarily indicative of anefficient flare. Rather,
theefficiencyof the flare is the product of the instantaneous
concentrationand the instantaneous mass' flux surrmed over an
envelope enclosing the flare.Inaddition, the valuesmust be
averagedover asufficient time period toensure that the emission
ratedoes not varywithtime. This has not beendone in previous
studies and is impossible if conventional sampling tech-niques are
used.Themeasured normal distributionof concentrations inaflare
flameappear to be the result of flame fluctuations. However, the
averaged con-centrationprofiles of the flame aremore valuable than
the average tempera-ture profiles. The average concentration
profiles can be used tocalculatelocal destruction efficiencies,
identifythe regions of average production2-54or destruction of
aspecies, specifythe concentrations of incompletelyburnedspecies,
as well as to indicate the flame envelope.Concentrationmeasurements
inflare flames have beenmade by Palmer(197i), Siegel (1980), Lee
andWhipple (1981), and Howes, et al (1981). Table2-16 shows the
species and concentrations measured in these studies. Palmermade
onlysingle-point measurements of heliumandethylene. For inlet
helium/concentrations of about 1percent, the concentrationof
heliumat the end ofthe flame was between 21 and 206 ppm.' The
measured range for hydrocarbons wasfrom0to 104 ppm. Lee and
Whipplemeasured CO, total hydrocarbons, propane,and
heliumconcentrations inapropane flame dopedwith8percent helium.
Be-,tween 50 and 81 inches above the flare nozzle, the
concentration ranges of thespecies were: CO, 7to 1000 ppm; total
hydrocarbons, 0.3 to 270 (arbitraryscale); propane, 1to6000 ppm;
helium, 130to 2700 ppm.Measurement of lowconcentrations of, species
does not ensure that emis-sions fromflares are insignificant. The
concentrationof species escapingthe flame are diluted byair andexit
the flame regionat highflowratesthrough alarge area. Figure 2-11
estimates the relation betweenmeasuredconcentrationof 'propane
andefficiencyat v a r i o ~ s dilution levels and Figure,2-12
estimates theeffect for emissions of carbonmonoxide. At the
estimateddilution of 1000, 30 ppmpropane or 10 ppmcarbonmonoxide
represent aonepercent loss of efficiency.Siegel has takensufficient
data to showthe variation in average concen-trationdistributions
withdistance under no-windconditions. Figure 2-13shows themeasured
O2, CO2and gaseous hydrocarbons along the flare axis fortwo firing
rates. These concentrations conformtoone's expectation. Fuelburnout
is displaceddownstreamfor the flare withthe higher firing rate.For
boththroughputs, the concentrationof fuel decreases more
rapidlythanthe CO concentrationwhich indicates that the
concentrationof fuel is decrea-sing byconsumptionas well as
bydilution. However, the instrument used bySiegel tomeasure COwas
not sufficientlysensitivetodetermine the COcon-centrations tothe
required accuracy. Extrapolation of the fuel concentra-tions
tozeroyields areasonable estimateof flame lengths. The 780
kg/hrflame is 6 meters long and the 1100kg/hr flame is 8 meters
long.2-55~TABLE2-16.RANGEOFCONCENTRATIONSMEASUREDINFLARESSTUDIESCONCENTRATIONPPMVTracerHCCOC3H8CO2.02SootAuthor~v-!!!.v!!!.v!!!.v%%mg/m3Palmer21-2060-104Lee&Whipple130-2700a7-10001.5-60000.5-1.019.8-20.3Siegel-0-12000-2000-0.5-316-2023-81NIHowes0->10000-50000->1000.0.1-7.29.6-20,u
~.~88u'r-It-864-w~ o84'r-N.fJIIIICJ1:::s82........0 E
0u8078767472101001000Concentrationascarbon,ppmv10,000100,000Figure2-11.Effectofpropaneemissionsoncombustionefficiency.Propaneasfue1~r10098969492........;,.,g--.....-90>.u
c88(lJ'r-U'r-4-864-wN~84IU1'r-oo+.lIII~82..0.E
0u80787674721.0D=DilutionFactor'10100100010,000Concentrationascarbon.ppmvFigure2-12.EffectofCOemissionsoncombustionefficiency.COasfuel.Siegelo
780 kg/hr.1100 kg/hr no steam541o20o700900800300200.1,000. 100600
12 3::=::, Q..Q..~500 10 CO2%~:r:xu400 8 22 3 4 5 6 7 8Height Above
FlareTip, MFigure 2-13. The effect of throughput on concentration
profilesin two flare flames. (Siegel,1980).2-59Radial profiles of
concentrationare also useful for definingthe flameenvelope.
Profiles reported by Siegel (Figure 2-14) showsome unusual
charac-teristics. First, the profiles are asymmetric. The more
rapiddecrease offuel, the larger concentration CO2andmore gradual
increase in O2concentra-tion on the right side of the center
lineall indicate that on the averagethe flame burns more to the
right than to the left side of the center line.This could be caused
by.distortionof the flare off-axis by the nozzle orwind, or by
sampling for atime periodwhich was insufficient toaverage thenormal
fluctuations. The concentrationmeasurements also
showconsiderablescatter on the left sideof the center linewhich may
be caused by intermit--tent sheddingof eddies. Such ,phenomena are
not accounted for in the samplingprocess.Finally, fuel does not
asymptoticallyapproachzeroconcentrationat theedges of the flame.
Instead, the fuel concentrationapproaches an asymptoticvalue of 10
ppm. This concentrationof fuel might contributeas much as 1percent
toflare inefficiency. Inaddition, the concentrationof fuel
shouldreach an asymptotic value of zeroif transport of fuel
occurred by diffusionalone. The finite asymptoticvalue implies that
fuel is being transportedfromthe center line by some
mechanisminaddition todiffusion. Once again,themost
likelyexplanationis transport of fuel in large eddies that
havebroken away fromthe flame envelope, aphenomenon noted bySiegel,
Howes, eta1, and Lee and Whipple. Siegel
concludedemissionfromquenchededdies wasunimportant, while Whipple
concludedthat itwas. Althoughthe species con-centrationprofiles of
flare flames normallydefine the envelope of the heatrelease zone,
interpretationof time-averagedprofile concentrationto produceflame
envelopes is made difficult becauseof the fluctuating nature of
buoyantturbulent flames.2.4.4 Flare EfficiencyAmajor objective of
many experiments on flares has been to determinetheir
combustionefficiency. The efficiencyof combustion is difficult
tomeasure directlyand, consequently, various methods have been
proposed toprovide pollutant emission indices for flares.2-6050
,.....---..,...----r---............oGas = 1720
Kg/hrSteam=0Kg/hrp=0.59 kg/m3Z =6.5m0.2o..... -+- +-__-+ -+-
+-__
20. 0L..L__--L__ __ -3 -2 0 1 2Radial Position,
M20.5E0.0.>,::I:XUNou
. NoFigure 2-14. Radial concentration profiles in aflarefl arne.
(Siege1, 1980) .2-61Fivemethods have beenused tocalculateflare
efficiency inpreviousstudies (Table 2-17). Degree of
carbonconversion (e.g., Siegel). Extent of destructionof flaredgas
(e.g., Lee, Palmer). Amounts of final combustionproducts formed.
Emissionof undesirable products/intermediates. Extent of the
oxidation,process.In Siegel's study, efficiencywas based upon local
carbonconversion, U,defined in Table 2-17. This requires that
measurements be made of the localgas velocityand concentrationof
all carbon containing species in order tospecifyan overall
efficiency. Siegel attempted toclose a material balance,but he was
unable to account for approximately 50percent of the input
carbon.This was attributedto errors in thematerial balances caused
by inaccuraciesin themeasuredvelocities. While
inaccuratelymeasuredvelocities cancon-tribute toerrors in
thematerjal balance, other factors must be considered,such as:
Improper use of the average velocityand concentration. Errors in
the average concentrationcaused by short sampling times.
Incompletely burnedmaterial whichescapedthe flare undetected. Use
of inappropriate techniques tointegrate the mass fluxes.Howes, et
al, studiedthemeasurement methods that may be applicable toflares.
Theydetermined species concentrations by analyzingsamples
with-drawn fromthe flame usinga heated probe. The extractedsamples
were anal-yzedcontinuouslyfor CO2, CO, 02 and THC. The probe was
also used toextractgrab samples whichwere analyzed byamodified SASS
trainfor particulates andorganicmolecules.Howes, et al, reported
local destructionefficiencies between 92 and 100percent, measured2
m above the flame. The lower values were for sootingflares, and the
higher values were for flares which suppressed soot. Thestudydid
not: (a) report global efficiencies, nor (b) confirmthe accuracyof
local efficiencies bydifferent techniques.2-62TABLE 2-17.
EXPERIMENTAL'MEASURES OF COMBUSTION EFFICIENCY1. Used bySiegel(gl
oba 1)u = _m=.c,_. __ . m1n(CH)c mn, 2. Used by Lee_[ fD] xDF]x
100DE - 1- [C H- U(1 oca 1) . 38 0.where DF=[He] in sample[He] in
flare gas3. Used by Palmer DE=(1 oca 1)where DF=1_xDF]
x100[C2H4]0He] in sample )[He] assumi ng 75% reacti on13.254.COUsed
by Howes! DE = - Mc2( 1oca 1) MC02+MCO+MHCc c c=DE= 1where [HC]
isthe concentrationof hydrocarbons as methane.-outmuhc-inmuhc5.
Proposed:(global)6. Proposed:(gl oba 1) .-outF =m of final
productsmout assuming 100%conversion8. Proposed:(gl oba1)E = 7.
Proposed:(global)r-outd . bl .me . un eSlra espec,es- inmca
consumedo = 2Theoretical O2required for 100%conversion2-63K=Grams
tracer in feedT Grams carbon in feedTABLE 2-17.9. Proposed:(loca
1)EXPERIMENTAL MEASURES OF COMBUSTION EFFICIENCY (CONCLUDED),12KT
[C02J ppma = [T] ppm~10. Proposed:(1 oca1)11= 1-(36 [C3Ha]ppm+12
[CO] ppm+ (:&)[TJ ppmM,-where RT/PNconverts to volume units.
msootis grams soot insample. c11. Proposed: =12.wheremcdenotes
carbon fromthat species.then aI -+ Howes1 DE (# )muhc+
mCO+msootProposed: 111= 1_ c c c(local) mCO2+ to+ uhc + sootc mcmc,
mcNote that ifmsoot::::: 0 ,CCOMMENTS:(a) Equations (2) and (3) are
incorrect because theyrelate to propaneand
ethylenedestructionefficiencyonly, and not tooverall
com-bustionefficiency. Soot, CO and lower hydrocarbons mayalso
bepresent as uncombusted species.(b) Equation (4) is subject
toexperimental error.(c) Equations (5) -'(a) are unworkable
incurrent format without ahood,as is Equation (1)(d) Equations (11)
and (12) relyon assumed mass balance.SUGGESTI ONS :(a) Use Eq. (9)
until mass balance is obtained,then use Eq. (10) toquantifylocal
efficiencies.(b) Given 1 0 ~ a l efficierrcies, weight,
themaccording toarea andvelocitytogive global efficiencyand check
this result withthat obtained by using the hood and Eq. (1) or
(7).2-64Palmer (1972) measured the local efficiencyof an ethylene
flare. Hisdata are tabulated inTable 2-16. Palmerwithdrewgrab
samples into2.25liter bags withaprobe fromaposition two feet
downstreamof the "flametip.: The samples were analyzedfor
helium(used as a tracer) by gas chroma-tographyand mass
spectrometryandfor ethylene (fuel) bygas chromatographyand flame
ionizationdetection.The.heliumtracer technique used by Palmer
toobtain local combustionefficiencyis questionable. Heliumis apoor
tracer because the diffusionvelocity is much higher than themajor
components in the flare. Consequently"heliumwill diffusemore
rapidly away fromthe flame region and produce dilu-tion factors
that are erroneouslyhigh. The sampling time used by Palmer,one
minute, is probablyinsufficienttoaccuratelydetermine the species
con-centrations of afluctuatlng flame. In addition, Palmer sampled
onlyon theaxis and did not attempt tosample or integrate the
concentrations toobtainaglobal combustionefficiency. The sampling
used by Palmer could notaccuratelydetermine the concentration of
high-boiling-point hydrocarbonswhichwere observed. Also, the
heliumanalyses, whichwere the basis for themass balance
calculations, onlyaccurate to6.7percent. Consequently,Palmer's data
are of limitedvalue inassessing theemissionof
incompletelyburnedmaterial fromflares.Lee andWhipple (1981) have
also reported local efficiencies measuredon apilot-scale
flare'using aheliumtracer
todeterminedilutionfactors.Destructionprofiles of propane,
hydrocarbons, andcarbonmonoxide are shownin Figure 2-15. Although
no soot concentrationmeasurements were attempted,the data can be
utilizedtodetermine minimumcarbonemissions by consideringthe sumof
propani and COonly (Figure 2-16). Theyalsoreported
combustionefficiencyof 96 to 100percent. However, the data
contained the same poten-tial errors as the other data on flares.
Only local concentrations weredetermined. The tracer usedwill
potentiallyyield too highadilutionfactorbecause of the
highdiffusivity. The sampling timewas minutes, but ascwill be shown
later; still too ,short toachieve the accuracyrequired. Itshould be
notedthat escape of 0;001 percent of the fuel represents about aone
percent loss in efficiency"and this was
ignored.2-65C'lo.....Jco::I:MU1.02.03.04.04 04.0rax +.J.r::.#of
pOints=Nav.g. (ArittiC'l. ~OJ.r::.m1 n
~ttlC1Jc..U::I:1.0C'l0.....J0(HC)12(CO)-Ec..c..0uC'l1.0.....J0(50)
(61) (72) (81)60 70 80Distance Above Burner Tip, Z(in)Figure 2-15.
Species centerline concentrations as afunction ofheight above
burner. Fuel 92 percent propane,remainder heliumtracer. Calmwind,
sootyflame.Effective nozzle diameter 1.51 in.(Lee and Whipple,
1981).2-66CV=0.02Grab Sample 1.5ft.Off AxisN=l JGrab SampleN=3,
CV=0.417'N=Number of data pointsCV = Coefficient
ofVariation\ON=l\\(U= 0.9990)(U=0.9900).(U= 0. 9000 )..
0-..... COVl::I:Vl ('t").... WEl.LJ OW..0-
l'OWQJ
l'O,....U.:::>I-IIl.LJ.......I ....L.- """ ---l50 60. 70
80Distance Above Burner Z (in)Figure 2-16. Summary of flare
emission, excluding soot, asafunction of height above burner tip.
Sameconditions as Figure 2-15. (Lee &Whipple,
1980).2-67Concentrationof species has beenmeasuredabove and
inflares. How-ever, interpretationof these results isdifficult
because of the intermit-tent nature of flare flames. Estimates
reveal that in order todeterminethe efficiencyof aflare,
concentrations must be measured down toabout 10ppm. Instruments
used by Howes, et a1, a n ~ Lee and Whipple were sensitiveenough so
that emissions could have beendetermined. However,
instrumentsavailable to Siegel were not.2.4.5 Productionof Soot in
FlaresThe amount of soot produced inflares canmateriallyaffect the
flareefficiency. Figure 2-17 shows that aconcentrationof 15 mg/m3of
soot atadilution factor of 1000corresponds 'toa loss of one percent
in flareefficiency. However, fewstudies have measuredthe
concentrationof soot.This is so partly: because themeasurements are
time consuming., Sometimes,soot measurements are
consideredunnecessary because some flares burn non-sootingfuels,
while others inject steamor air to suppress soot
formationduringmost operatingconditions.Howes, etal, and Siegel
made limited soot concentrationmeasurements.However, their
measurements were neither systematic nor complete. Howesmeasured
soot concentrations ranging fromless than 0.1 mg/m3for naturalgas
flared on a LRGd head, to 18.3mg/m3for C3H8flared on an ~ H head.
Theefficiency loss caused byasoot concentrationof 18.3mg/m3at
adilutionfactor of 1000is about 1.5percent. Siegel measured soot
concentrationranging from20to 80mg/m3in asootingflare. He
estimatedthat productionof this amount of soot resulted
inanefficiencyloss of 2to 4percent.Figure 2-17would indicate an
efficiencyloss of about 1.5to 5.5percent atadilution factor of
1000. The .observedconcentrationof soot fromflareflames can,
therefore, substantiallyalter combustionefficiency. The
con-centrationof soot should be measured infuture studies to levels
below10mg/m3.2.4.6 The Effect of' Wind on the Performanceof
FlaresElevatedflares areexposedto uncertainweather condttions
includingchanges inwind velocities. In acalmatmosphere, the flare
flame primarilyfluctuates about the center lineof the flare head.
As the
crosswindvelocity2-68,.10098969492..-...90(j-Q.-..J.>,.88u c
QJ......u86......If--If--Nw84IcC"I0~......~82Vl';:'IDF=DilutionFactor..0E800u7876747210/100SootConcentration(mg/m3)100010,000Figure2~17.Effectofsootconcentrationoncombustionefficiency.Propaneasfuel.increases,
the flame axis is bent inthe directionof the wind. As the
windpromotes penetrationof air intothe flame, combustion is
enhancedand theflame is shortened. When thewind velocity increases
further, thewindvelocitydominates and the flame becomes longer, and
the possibilityforstripping unburnedmaterial fromthe flame edges
increases.The effect of crosswind on the efficiencyof flames was
studied toalimitedextent by bothSiegel and Howes, et al. Howes, et
al, studied flamessubjected towinds from8to 17 mph, while Siegel
studied flares subjectedtowinds up to 15 mph. Acrosswindmakes
measurements and the closing of mate-rial balances in flames
verydifficult. Thus, reportedmeasurements are oflimited
value.Global efficiencies have not beenmeasured for flares in the
presenceofwind, but some local efficiencies have been reported. For
aflare in a6.7m/s wind, Siegel reports that local
combustionefficiencyon the center-lineof the flare approaches 97
percent, compared to better than 99 percent at acomparable point
inquiescent conditions.2.4.7 The Effect of SteamInjection/Forced
Draft on the Performanceof FlaresIn practice several methods are
used to reduce soot production and.improve the efficiencyand visual
appearance of flares. Thesemethods nor-mallyserve to improveair
induction andmixingat the root of the flame.Air can be induced by
the use of separate steamjets injected intothe baseof the flame, by
aerodynamicmeans suchas in the Coanda flare, bypremixing,~ or bythe
use of forced draft fans. -The inductionof additional air intothe
flame typicallyincreases combustion intensity, shortens the flame,
andimproves the efficiency.Howes, et al, studiedaJohn Zink LH flare
which used aforced-draft fantoassist airmixing, and a John Zink
LRGOflare which burned highpressuregas. The estimatedlocal
efficiencyof the LRGOflare was slightlybetterthan that of the
LHflare., But since the LRGOflare burnedmethane and theLH flare
burnedpropane, comparisons are difficult. In one case, Howes, etal,
estimatedthe local efficiencyof the LH flarewithout using the
forced-draft fan. In this test, soot concentration increased by
afactor of 20, andthe estimated local efficiencydropped fromgreater
than 99 percent to 922-70percent. Use of forced-draft fans
definitely improves the efficiencyofflares. However, forceddraft
fans are impractical on the very large ele-vatedflares which
potentiallycancontribute significantlyto the emissionsof
incompletelyburned fuels fromflares.Inductionof air through the
injectionof steamis the most commonmethod of improving the
efficiency o ~ large elevatedflares, and both PalmerandSiegel have
studied theeffect of steaminjection into flares.Palmer estimated
the efficiencyof a flare with andwithout injectionof steam. He
reported that the flame lengthdid not stronglydepend on exitgas
velocities for velocities between 50and 250ft/sec. However,
injectionof steamchanged the physical characteristics of the flame.
Without steaminjection, the flame was betweenfour and one-half and
five and one-half feetlong, "lazy," reddish, and smoky. When 0.3
lbs of steamwas injectedforeach pound of ethylene, the flame became
turbulent, shortened to two feetIandemitted no smoke.Siegel
studiedtheeffect of steaminjection on the performanceof flaresmore
systematically. He demonstrated that injectionof steamchanges
thephysical characterist)cs of the flare and that the changes are
reflected inthe concentratiqnprofiles of theflare and the local
combustionefficiencies.An optimumlevel of steaminjectionexists,
which depends on the characteris-tics of the gas being
flared.Steaminjectionchanges the physical characteristics of the
flare in amanner which suggests that the intensityof combustion is
increased becausethe flame is shortened, its luminosityis reduced,
and soot formation issuppressed. Figure 2-18shows that the flames
are shortened byabout 50percent at asteam/gas ratioof
approximately0.1. Without steam, the flamewas verysooty.
Steaminjectionchanges the color of the flame fromorangetoyellow.
When the steam/gas ratio .reaches about 0.25, soot
productionwassuppressed. At steam/gas ratios of about 0.55, the
flame develops ablueinner cone and ayellowouter cone. When the
steam/gas ratiowas above 0.55,the blue inner region spread
totheentire flame andit resembles afullypremixedflame. Increasing
steam/gas ratio past 1.35produces awhite innerconewhichcontains
steamand water.2-71.,-~Q).+.JQ)::::-,-'=+-'01t:Q)-JQ)E' 0Ricou and
Spalding (1961) continued tothat theoreticalscaling basedon the
Froude number, Fr, led to good predictions for thedifferent flames.
The Froude number is defined as:where:Un2( )
.!::Se.PPogdoCT======specific heat of gas at constant
pressuretemperature (absolute)gravitational accelerationmass of
fuel in injected gasheat of combustion of fuelvelocityat
nozzle-.,...... -Air entrainment by natural convectionoccurs faster
than forced convection(but at differentFigure 2-30shows the
assymptotic limits offorced and natural convection. Figure 2-31
shows the correlation betweentheoretical and experimental values
scaled by Froude number.A studyof aturbulent flame in acrossflowwas
done by Escudier (1972).Escudier proposedthat fluid is entrained
intothe plume at arate propor-tional to the velocitydifference in
boththehorizontal and vertical direc-tions. Thus, referring to
Figure 2-32, the mass conservationequationmaybe written:2-89, ,1'-)
Asymtope for pure, natural convection,'I'II /1 //Y ,
/1'/for pureforced convectiono 1"'-__.L' __ __ __--"'__1 0 10
:100,xI- - F--do Po r'10_INI 10E.......E-Figure 2-30. Theoretical
predictionof entrainment inbuoyant jets. '(Ricou&Spalding,
1961;reprintedwith permissionof CambridgeUniversity
Press).2-90,-105210 I(;,5
0E:-002E:-001(;,05()'02heo. .I 1.1I I. ) retialcurve" (figure2 I
IYYih's dam_-' /r I7"'":--,."I IJ"'i'/Qe:ves' md Bociter's-. /dam V
./.
, 6/ aJ"o\6
" . .......;;i/!"-r-mF,-t.o-32 Z(1'1) i F.-I- 1" - -
1-oJmOPor-
.r-1/O{) 1 002 ()oQ 5 Qol 0'2 ()05 10 2:0 G:) 5 10Figure 2-31.
Entrainment by buyoant jets and flames.+, unburnt propanejet; 0,
unburnt hydrogenjet;0, pre-mixedair-hydrogenflame;
hydrogendiffusionflame; e, propane diffusionflame(RicouandSpalding,
1961; reprintedwithpermissionof Cambridge
UniversityPress).2-91zUcow.J~ gr...,w.-----------__ x, .:
,-.Figure2-32. Definitions of x, z, the curvilinear coordinate ~
,horizontal and vertical velocitycomponents u and w.velocity U, and
flame radius o. The gas jet dischargesat velocity Wj with densityPj
fromanozzle of diameter d.intoacross streamof density P at velocity
u. Jco co2-92"-.where y,are both. empirical constants. Brzustowski,
et al (1975) noticedthat for Escudier's equationto reduce to Ricou
and Spalding's without cross-wind and buoyancy, ymust have
aspecific value. Thus, we get=2'Pmo[0.08 ( P: )"(u- +"Ewhich
assumes that we canextendapplication of the equation back to the
nozzle, .=0) withminor errors.Brzustowski has also studi edjets
in'across wi nd. He defi nes aburni ngrate parameter Bby(4)M )
=-0.233e pp dm Mo0where Mp' Moare molecular weights of products and
reaftants,arestoichiometric coefficients.Bvaries from0for.a coldjet
to 1forajet whose oxidationrate is limited by the entrainment rate.
The generalsolutionof these equations cannot be expressed inclosed
form. However,closed formsolutions can be obtained for the
casewhere (lE=0; i.e., buoy-ancy is negligiblysmall, and
entrainment is given by the Ricou and Spaldingequation. These
solutions can be qualitativelycomparedwith real flames topredict
that high entrainment will causea flare to bend over earlier.
Thelimiting solutions are:m*m/mk: -(a) = = 1+0.32 (p""I po) 2=
1+0.32
0(b) Yp=-.0746(4)pMpl4>oMo)S =mass fraction of products1 +
0.32t(c) Yf=1Ym*p2-93whereE,; =
(:: -d-oand(d) x::X(:: )'=
.10.32Nel'lr tI":e nozzle, this may be appr.cximatedbyz = Define
the flame lengthas the location beyondwhich mass flowof combus-tion
products stays constant, thenfL =13.4 +,sinceat that point Yf=O.
Note that at this locationm*,(fL)=1+ 10.233Under the above
assumptions, this gives the dilutionfactor for acase when nofuel is
burned. This model is then consistent when obtainedfromthe
flame-lengthequation, leads totheobserveddilutionfactor inthe
equationfor en-trainment.As an example, consider the combustionof
propanewhichoccurs via'In this case, the following values are
specified,where'M is the averagemolecularweight of the products.
For propaneatp ,ambient temperature and pressure, is 1.24and leads
toaflamelengthpredictionof 60/Sregardless of crossf1ow. The
lengthof a turbu-lent propanediffusionflame has been found to be
about 300dj, implying2-94s = 0.2 (Hottel and Hawthorne, 1949). This
derivation is applicable onlywhen the Froude number is large enough
to neglect buoyancyeffects.In addition to presenting the
limitedcase of aflame in the absenceof buoyancy, Brzustowski
presentedanumerical solutionto the 13differen-tial equations
describingthe general diffusion flame system. The parameterSwas
evaluatedfor the no-crosswindconditions
bymatchingpredictedandobserved flame length andwas found to be
about 0.15. The empiri.cal entrain-ment coefficient, a, was
obtainedwith = 0.05at nozzle velocities of15m/sec and 57m/sec.
Avalue of a = 0.13matchedthe data quitewell.Compositionprofiles are
presented in Figure 2-33. Note that fuel andoxygen can exist
together inthis model since S does not have to be unity.The
13equations solvedare: continuityof fuel, intermediates,
oxygen,products, and nitrogen; energy balance; horizontal and
vertical momentumbalance; total continuity; ideal gas equationof
state; defin-itionof S; and acalculation' of residence time. The
method used for solvingthe systemwas not given.Becker and Yamazaki
(1978) presentedtheir data for turbulent flames inadifferent form.
They used the Richardson number which corresponded to thereciprocal
of the Froude number.. Theyalso .definedanear-fieldcoordinateand
afar-field coordinate.Richardson number: Ri =9do/Uo2Near-field
coordinate:=Ril/3z/dFar-fieldcoordinate: .
="Ril/2Theyalsodefinedacharacteristicjet area with radius (b)
suchthat at theradial position2pu, .': ,-.whereis the average
vertical yelocityat b and values with subscript carecenter
linevalues. Thejet radius, b, may be expressedas afunction ofaxial
distance (z) fromthe nozzlewithaspreadcoefficient, Cs.b = C
zs2-95.........a......>-25 50p75 100 1 ~ 5 200 300 400~ L Fi
gure 2-33. Computed prof; lesof compos i ti on and dens ityfor d
=0.005m, w= 22.1m/sec, and u ~ = 2.55m/sec.(Note scale changeat the
end of the flame).2-96They found that up toof about C is constant
at about 0.071. Beyonds . Csperhaps due tofree convection buoyancy.
Other deriva-tions implied a Cslimit of 0.07 for forced convection
and 0.10for free con-vection. It is.interestingt6notethat the flame
jet appearedtoemanatefromavirtual originabout five diameters above
the nozzle tips,whilesimplejettheorywould predict avirtual.originof
about three diametersbelowthe nozzle This may be due to
boundarylayer effects of the sim-ple tube used in the study.In
contrast with Ricou andBecker and Yamazaki define adif-ferent
entrainment coefficient CE such thatdm= C (7TpG/4) 1/2dz Ewhere
mandGare thejet momentumfluxesp is the II mixing CUpli density
(stirredreactor).It was found that forof 2and pof the entrainment
was 0.36. This value is different fromthe 0.16found by Ricou and
Spalding(Figure 2-31).the correct model is somewhat uncertain.
Table 2-18sunvnarizes the data used by Becker and Yamazaki. Further
discrepancies areevident when the free convection limit of 1.84is
comparedwithexperimentalresults shown in Figure 2-31 which suggest
acoefficient of about 0.60.The studies discussed previouslydealt
withthe aerodynamics and reac-tions of fuel jets.(1980) developeda
systemof ninedifferentialequations to describe soot formation. Some
of his results are presentedforan ethylene flame in Figure 2-34.
Magnussen developed three kinetic rateexpressions based on and
final products and used the onewhichwould limit the reactions under
the specific conditions. Without adefinite numerical solutionwould
have onlylimited use and is probablybeyond the scopeof astudyof
flares.Inresults fromthe past modeling studies of burningjetsarenot
consistent. The most recent work (Becker) contradicts
theearlyresultsof Ricou and Spaldingand does not suggest amethod of
reconciling the dif-ference. the limited case solutions of
Brzustowski 1977) may be used to predict the approximate nature of
the
flare.2-97...,,~.NIU)ex>TABLE2-18.EXPERIMENTALDATAUSEDINTHESTUDYOFBEKCERANDYAMAZAKI(1978).WITHPROPANEFUEL(REPRINTEDWITHPERMISSIONOFTHECOMBUSTIONINSTITUTEMassFraction--Far-FieldNozzleNozzleGasofSource105RiLengthSteamFlameMeterVelocityMaterialNozzleNozzleDia.CoordinateEnddoReo0LIdo~LReLu.w0mmm/s'5.633.50.0602131044312821.0-53005.637.80.0602291090.416015.59400.5.6717.50.0597657018.119511.0175005.6927.10.0594102007.612189.24243005.7142.00.0590158003.192467.80353005.7259.20.0588223001.612656.69437003.2576.90.0582164000.55.2784.91306007.9191.40.0602482000.912896.03860001.0..----......----,.----...,..------.oo~~
M E---s::. ~ 0.5+-'ltls..+-'s::OJus::O'u.pooVlox- axial distanced-
nozzle diameter100x/d"-Figure 2,,:,34. Experimental Soot
Concentrations on the Axis of the C2H2Diffusion Flame (Re=7000)
Compared with Predictions(Magnussen, 1980).2-992.5.3 Solutions of
theTransport EquationsAgeneral method of obtaining numerical
solutions to the various partialdifferential equations (POE's)
governing the flare systemwould require pro-hibitiveamounts of
computer time. Amathematical model for predictingemis-sion of
pollutants due to incomplete combustionmust include the
followingsubmodels: A two- or three-dimensional fluid dynamic code
which accountsfor the effects of backmixing, buoyancy, and
densityfluctuationsof bulkflow. Aturbulence model withcoefficients
that depend on local Reynoldsnumber andwhich takes intoaccount
themean densitygradients andfluctuations. The turbulencemodel under
consideration requiresfully-developed turbulent flowwith Reynolds
numbers larger than10,000. However, typical Reynolds numbers
incontinuous flareflames are less than 20,000and the model will
averageall thedynamic responses of aflame.
Afinite,reactionratemechanismincludingamodel of soot forma-tion and
burnout. An approximationwhichdescribes theeffects of turbulence
on'mean reaction rates based on species and temperature
fluctuations. A formulation of simultaneous gas and soot radiation.
Amodel of radiation self-absorptionwhichconsiders the
inter-mittencyof the turbulent flame.For the most part, versions of
eachof the submodels are available,though integrating themfor
solution to the problems of flares usingiter-ative POEmethods is
difficult. Buildupof an overall flare model withthesame levelof
sophisticationas the submodels mayyieldasystembeyond
thehandlingcapacityof present-daycomputers. The POEmethods
available forsolutionto turbulent jet flames (Hutchinson, et al,
1976; Janicka, 1979;and Pope, 1981), usuallyneglect or simplifythe
chemistryand heat transfer.Solutions found by these
complicatedmethods explaindata no better thanthose obtainedfrommuch
simpler approaches.2-100-Solutions generated by POE are expensive
and require agreat dealof interpretation. For instance, just one
POE submodel developed by Dr. Wolf-gang Richter of EER
requ1res.one-halfan hour of runtime on a CDC 7076 andcosts several
thousand dollars. Output fromthese models must then be con-verted
to common engineering parameters for use indesig