GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 24, PAGES 3603-3606, DECEMBER 1, 1996
Aircraft
exhaust
sulfur
emissions
R.C. Brown, M.R. Anderson,R.C. Miake-Lye, and C.E. Kolb AerodyneResearch,Inc., Billerica, MA
A.A.
Sorokin
and Y.Y.
Buriko
Scientific Research Center "Ecolen", Moscow, Russia
surementsfor the Concordewake, Fahey et al., [1995] estimated (assumingl0 nm sulfuricacid aerosols)between12% and 45%
Abstract. The conversion of fuel sulfurto S(VI) (SO3 -4-H2SO4) in supersonicand subsonicaircraftenginesis estimatednumerically. Model resultsindicatebetween2% and 10% of the fuel sulfuris emittedas S(VI). It is alsoshownthat, for a high sulfur massloading,conversionin the turbineis kineticallylimited by the level of atomicoxygen. This resultsin a higheroxidationefficiencyat lower sulfurloadings.SO3 is the primaryS(VI) oxidationproductand calculatedH2SO4 emission levels were less
of fuel sulfur was in aerosols after l0 to 30 minutes at 16 km.
Schumann et al. [1996] and Gierens and Schumann [1996] re-
portedan increasein particlenumberdensityanda changein the number of activated soot particles with sulfur content for the ATFAS within the first 100 m. Additionally, resultsfrom the SUCCESS (SUbsonicAircraft: Contrail and Cloud EffectsSpecial Study) program [NASA, 1996] indicate a large numberof small volatile aerosolswhose number density varies with fuel sulfur[Anderson.,personalcommunication,1996]. Theserecent observationsare qualitatively consistentwith the Concorde
than 1% of the total fuel sulfur. This sourceof S(VI) can exceed
the S(V!) sourcedueto gasphaseoxidationin the exhaustwake. Introduction
aerosol measurements
Aircraft aerosolemissionscan affect the atmosphereby in-
and with the sulfur conversion to aerosol
creasing sulfateaerosollevels,therebyenhancing heterogeneous in an SR-71 plumeat 23 km [Hofmannand Rosen, 1978]. The discrepancybetweennumericalmodelsand in-flight obprocessing of nitrogenoxideandhalogenreservoir gases,andby servations suggestthat currentemissionscenariosfor sulfurspenucleating persistent contrailsandcirruscloudswhichaffectatmospheric photochemistry andradiativetransport [WMO, 1995]. ciationdo not accuratelyrepresentsulfurchemistryin the engine Measurements[Hofrnannand Rosen,1978;Faheyet al., 1995] and/or exhaustwake. The presentstudyconsidersthe percent conversionof fuel sulfurto S(VI) in the engine. The goalsare to andmodelingstudies[Miake-Lyeet al., 1993, 1994;K•ircheret (1) help guideflight testexperimentswith "low" and "high"sula/., 1995, 1996, Zhao and Turco, 1995, Brown et al., 1996] indicate that, in additionto exhaustsootand water vapor, sulfuric fur fuels by providingestimatesfor S(VI) at the exhaustexit and (2) betterdefine the extentto which gasphaseor heterogeneous acidformedby the oxidationandhydrolysisof fuel sulfuris a oxidation mechanisms in the wake arerequiredto accountfor the criticalcomponentof exhaustaerosols. percent conversion estimated from in-flight measurements. To date,manynumericalmodelsfor thecoupledfluid dynam-
icsandgasphasechemistryin aircraftexhaustplumeshaveassumedthatfuel sulfuris emittedat the exhaustexit as SO2. SO2 is thenconvenedto S(VI) (SO3+ H2SO4>accordingto
$0 2 + OH + M = HSOs+ M
(1)
HSO3 + 0 2 = SO3+ HO2
(2)
so.• + 2H20 -- H2S04 + H20
(3)
Sulfur Oxidation in Aircraft Engines This analysistreatsthe ConcordeandATFAS engines.Engine parametersfor the Concordewere suppliedby Rolls Royce. The fuel compositionwas setbasedon an analysisof the fuel burned duringthe 1994 flight [Fahey et. a/, 1995]. For the ATFAS, runs were performedfor four fuel sulfur massloadings(2, 166, 260, and 5400 ppmm) used in recent in-flight contrail observations [Busen and Schumann,1995; Schumannet al., 1996]. Engine and fuel parametersfor the ATFAS were providedby Schumann andDeidewig [personalcommunication,1995].
where reaction 3 is secondorder in water vapor [Kolb et al.,
1994]. ReactionI is ratelimitingand,consequently, SO,_oxida-
tionis indirectly coupled to NOy,CO, andHOxreactions that competefor OH. FrenzelandArnold[1994] estimatedthat> 0.4% of the fuel sulfuris convertedto gaseousH,_SO4 basedon
Equilibrium SOx Speciation
massspectrometric measurements of gaseous negativeionsin a jet engineexhaustat groundlevel20 m behindtheengine.Modelingstudies indicatetheconversion of fuelsulfurto sulfuricacid by reactions1-3 is < 1%-2% [Miake-Lyeet al., 1993, 1994;
The equilibrium trendsfor sulfur oxidation by oxygen have beendiscussedby Hunter [1982]. SO,_is the dominantproduct at typicalflame temperatures for fuel-to-airmassratioslessthan or equal to that required for stoichiometriccombustion. The Karcher et al., 1996; Brown et al., 1996]. equilibriumshiftstowardsSO3 asthetemperature decreases.BeHowever,in-flightaerosolmeasurements in jet engineexhaust low approximately700 K, in wet environments,the equilibrium wakeshave resultedin much higherestimatesfor the percent is againshiftedfrom SO3 to the acidvapor,H2SO4. conversion of fuel sulfur to sulfuric acid. Based on aerosol meaFigure I illustratesthe equilibrium conversionto SO3 as a functionof temperatureand fuel-to-air massratio (F/A). The Copyright1996by theAmericanGeophysical Union. carbon,hydrogenand sulfurfuel massfractionscorrespond to the Concorde fuel composition during its 1994 flight [Fahey et al., Papernumber96GL03339. 1995]. The fuel and air mass flow rates for the Concorde would
0094-8534/96/96GL-03339505.00
3603
3604
BROWN ET AL: AIRCRAFT EXHAUST SULFUR EMISSIONS Table
-o ([)=0.22 -e(!)= 0.7 -z•- (D= 0.9 -a,- (!)= 1.0
Mechanism
reaction
rate constant
(mole-cm-sec-K-cal)
u3 60_ •
•
1. Sulfur Oxidation
• \•.,(I) =fuel-to-air
9.64x10+13 4.64x 10+14e-82468'8/RT 4.36x10+12e-3301/RT 1.08x10+13 e-2980/Rt 4.70x10+12e-2901/Rt 1.44x10+11e-4740/RT 1.20x10+ø9 8.91x10+12 5.06x10+13 2.90x10+16e-117180/RT 6.3 lx10+12e-27176/RT 3.16xl 0+15e-63759/RT 4.40x10+14e-6326/RT 6.50x10+14e-10800/RT 9.03x10+11 1.63x10+25x t '3'3 7.83xl 0+11e-660/RT 7.23x 10+ø8
SH+O=SO+H
H2S+M=SH+H+M H2S+O=OH+SH H2S+H=SH+H2 H2S+OH---SH+H20 SO+O2=SO2+O SO+SO3=SO2+SO2 SO+NO2---SO2+NO SO+OH=SO2+H SO2+M=SO+O+M SO2+NO2=SO3+NO SO3+M=SO2+O+M SO2+O+M=SO3+M SO3+O+M=SO2+O2+M SO2+OH+M=HSO3+M low pressurelimit: HSO3+O2=HO2+SO 3 SO3+H20=H2SO 4
'.•40\ •ti•equivalence ratio 800
1200
1600
2000
temperature (K) Figure 1. Equilibriumconversion to SO3 for theConcorde (229 ppmmsulfur)combustor pressure(8900 hPa)as a functionof temperature andfuel-to-airequivalence ratio. resultin a (F/A) that is 22% of the ratiorequiredfor stoichiometric combustion.The temperaturerangevariesfrom below the temperature at the Concordecombustorinlet (869 K) to 400 K abovethe meanmixturetemperature (1400 K)at the combustor exit. The profilesshowan increase in theequilibriumconversion to SO3asthe temperature andF/A ratiodecrease.The time re-
by Westleyet al. [1983]. For the reactionbetweenSO2 and atomicoxygen,the hightemperaturerate reportedby Smithet al. [1982] was used. Sincehigh temperaturereactionrate data has not been reportedfor the reactionbetweenSO2 and hydroxyl radical, the recommendedrate for stratosphericmodeling quiredto achieveequilibriumhasbeenestimated throughkinetic [DeMore et al., 1994] wasused. Sincethis low temperature rate calculations to generallyexceed5 to 6 ms,whiletypicalcombusis expectedto be too large, additional calculationswere pertor residence times are on the order of 1 to 3 ms. formed using a rate constantthat rangedfrom 1 to 2 ordersof magnitude lower to assessthe affectof the low temperaturerate Kinetic ConversionIn Hydrocarbon/Sulfur-Air Flames constant in Table 1.
For the combustor,one dimensional flame models were used
to characterizesulfuroxidationin hydrocarbon/air flamesand to
estimatethe partitioningof SOx speciesat the combustor exit. The initial pyrolized fuel reactantswere approximatedby CH4/C2H2/H2 S (thedominantequilibriumproducts fromthepyrolysisof CHriS fuels)andC4Hi0/Smixtures.Botha premixed
Numerical resultsusing the premixed laminar flame model indicated1% conversionto SO3. The nonequilibriumflamelet model resulted in approximately 2% conversion in the flamefront. In bothmodels,oxidationwasdrivenby the reaction with atomic oxygen and the higher percentconversionin the flameletmodelwasdue to a higherlevel of atomicoxygen. The
laminar flame model and a turbulent flamelet model were used.
flamelet
Neithermodeltreatsthe initial vaporizationandoxidativepyrolysisof fuel dropletsor the threedimensionalfluid dynamicsin real combustors.However, the modelsare usefulin identifying the dominantgas phasekinetic oxidationpathwaysand the flameletmodelalsoprovidesan estimatefor the nonequilibrium partitioning of SOx species at thecombustor exit. The mostimportantparameterusedin the flameletmodelis thescalardissipation whichcharacterizes thediffusivefluxesof
towards the combustor
species to thereactionzoneandis a measure of the intensityof turbulentmixing [Burikoet al., 1994]. In real flows,the scalar dissipation hasa distribution alongthecombustor chamber.For thecalculations presented here,thescalardissipation rangedfrom
model, however,
indicated
additional
conversion
exit due to an increased contribution
of
the hydroxyl radical reaction. However, as noted, the high temperature reactionrate for the latterreactionis not known,but is expectedto be slower then the rate listed in Table 1. Thus, while the flameletmodelpredicted2% - 6% conversionfor both the Concorde and ATTAS combustors,parametric studies indicatethe lowervalueis probablyrealistic. SOn Oxidation in Turbine Flow The turbine flow was modeled as a one dimensional, two
stream(film-coolingair + core exhaust)flow. The temperature 0.01s-l to 3.6 s-•. Thehighervaluecharacterizes theflamefront and pressureprofile along the turbine axis was set by a cycle and the lower value characterizes the flow near the combustor deck calculationwhich usesmass,momentumand energy balexit. anceto determinegasdynamic propertiesfor the flow. To assess in the speciationat the combustorexit, turbinecalThe gasphaseoxidationkineticswasmodeledwith an exten- uncertainties sivekineticmechanism forCHx,HOx,COx,NOy,SHx,andSOx culations were performed with initial conditionsset by (1) kinetics(describedbelow) chemistry. Thereaction mechanism describing CHx,HOx,NOy chemicalequilibrium,(2) steady-state andCOx kineticswasformulated usingselected reactions and and (3) the combustionspeciationpredictedusingthe turbulent rate constantsfor methane combustion [Tsang and Hampson, flamelet model. The same basic trends were found in each case.
1986]with additional NOyoxidation reactions adopted from Tsangand Herron [1991]. Sulfuroxidationreactionsand rate constants(summarizedin Table 1) were selectedfrom the review
As anexample,Figure2 showsSOx,O, OH, andtemperature profilesfor the Concordeturbineandnozzlewith the combustor exit speciation setby theresultsof theflameletmodel. Although
BROWN ET AL: AIRCRAFT EXHAUST SULFUR EMISSIONS
10'$
x
ß --2 ppmm
1400
S02
3605
(a)
I I
1200_ (13
_= 50
1000'•
.• 4-
•t
.o 10'7
temperature
'r:
800
"- ,:
600
,/
c '"
;•
I I
>
I
c• 10'a x
I
•'
o)
3
9 --166 ppmm
• .--•. 260 ppmm
-
ß 2 E
-
H2S04 1-
!
ß
0
5400 ppmm
2
4
6
8
10
'
I
0
12
4000
2000
time (ms) Figure 2. SOxmolefractionsfor theConcordeturbine.The fuel sulfurcontentis 229 ppmm. The air-to-fuelmassratio is 67.8.
(b)
10. $
.--"-)•
-.--""'SO2
o 10-6 both O and OH decreasealong the flow, a reactionflux analysis indicatedthat the reactionof SO:Zwith atomic oxygen was the dominantmechanismfor the formationof SO3. The dropin OH
• 10 '7
wasprimarily dueto NOyreactions withlittlecontribution from
'• 'E= 1
SO:z,exceptfor thenotableincreasein H:zSO 4 duringsupersonic expansionthroughthe divergentnozzle(9-11 ms in Figure2). Figure 3 showsSOx speciesprofiles for the ATTAS engine burninga fuel with a sulfurmassloadingof 5400 ppmm. These calculationswere initialized by assumingthat bimolecularreactions quickly oxidize pyrolysisproductssuch as SH2 to SO2, while the subsequent conversionto S(VI) throughthird bodyreactionsis a slowerkineticprocess.Thus,a restricted(sulfurspeciationlimited to SO:) steadystatemixturein the combustorwas first determined.
A kinetic calculation for this mixture was then
S03
_.
H2 s04
10'9
10.40 I
0
'-'
I
2000
'
'
I
'
4000
fuel sulfurcontent (ppmm) Figure 4. (a) Percent conversionof fuel sulfur to S(VI) by
volumeand (b) SO:, SO3, H:SO4 and O mixingratiosas a function of fuel sulfur content for the ATTAS.
performedfor combustorresidencetimesof 2-4 ms. The results indicated that between 0.5 % and 1.0 % of the sulfur exits the
combustoras SO3. Usingtheseresultsas initial conditionsfor An interestingconsequenceis that the percentconversionto theturbinecalculationprovidesan estimatefor the partitioningof SO3doesnot scalelinearlywith the fuel sulfurcontent. Figure4 sulfuroxidesat the exhaustexit planeassuminglittle conversion showsthe percentconversionto S(VI) by volume(Figure4a) and in the combustor. As seenin Figure 3, atomic oxygen is com-
pletelydepleted.Thus,the profilesin Figure3 indicatethat SO3 formation is kinetically limited, as the fuel sulfur content is increases,by the level of atomicoxygenat thecombustorexit.
10-5
• •
10-6
-e- SO2
10-7
•
H202
'-eSO 3 • H• 04
•10• 10• 10-m 0.0
1.0
2.0
3.0
time (ms) Figure 3. HOx and SOxmixing ratiosfor the ATTAS turbine. The fuel sulfurcontentis 5400 ppmmand the air-to-fuelmass ratio is 68.2. Temperatureand pressurevary from 1137 K and 5677 hPa at the inlet to 578.1 K and 2879 hPa at the exit.
SO:Z, SO3,H:ZSO4 andO mixingratios(Figure4b) at theATTAS turbineexit for fuelswith 2, 166,260 and5400 ppmmsulfur. As seen in Figure 4a, the percentconversiondecreasesas the fuel sulfurcontentincreases.The rangeextendsfrom 6% for a fuel sulfurcontentof 2 ppmmto 1% for 5400 ppmm. As seenin Figure 4b, SO3 increasesby approximatelya factor of 5 when the fuel sulfurcontentis greaterthanabout100 ppmm. Additionally, while the SO3 mixing ratio decreaseswhen the fuel sulfur is reducedto 2 ppmm,it is still nearthe predictedvalue for a typical aircraftfuel sulfurcontent(e.g. 300-500 ppmm) assumingonly hydroxylradicaloxidationin the plume. Conclusions
Kinetic modelswere usedto studysulfuroxidationin modem jet aircraft engines. Resultsfor the Concordeand ATTAS enginesindicated2% and 10% conversionto S(VI). SO3 was the primary S(VI) product,with < 1% conversionto H2SO4. The range in theseestimatesreflectsthe resultsof parametricvariations in the turbulentmixing in the combustor,sulfurmassloading, andsulfuroxidationrates. Althoughthereis no quantitative validationof theseresultsfor jet aircraftengines,they are consistentwith measuredvaluesfor hydrocarbon-air flames[Merryman and Levy, 1971] andgasturbines[Hunter, 1982; Harris, 1990]. Due to the complicatedkinetic and fluid dynamicalissuesin modelingreal combustorsandturbines,the numericalresultspre-
3606
BROWN ET AL: AIRCRAFT EXHAUST SULFUR EMISSIONS
sented here are not conclusive. Since the reaction with atomic
Gardiner,W.C., Jr., ed., CombustionChemist•, Springer-VerlagNew
oxygenis the dominantmechanism for S(VI) formation,conversionis dependentuponthe reactionkineticsandfluid dynamics governingthe superequilibriumlevel of atomicoxygenin the combustor.Hydrocarbon combustion chemistryhasanextensive literature[seefor example,Gardiner, 1984] andreactionrateparametersare reasonablywell characterized for manyof the key
York, !nc., 1984. Gierens, K. and U. Schumann, Colors of contrails from fuels with
reactions. However, rate constantsfor sulfur oxidation reactions,
aswell asthird bodyefficienciesfor the termolecular reactionof
SO2 with atomicoxygenandhydroxylradical,havenot been measuredover the temperatureandpressurerangeappropriate to theengine. Also,the presentstudyhasnotconsidered heterogeneoussulfur oxidationon turbine bladesor particulatesin the coreflow, whichcouldincreasethe percentconversion.Neither hasthe studyconsideredheterogeneous reactionswhich might decreasethe level of atomicoxygenand, consequently, the percentconversionto S(VI) products. Basedon theseresults,oxidation in the engineis predictedto at leastequal, and mostlikely exceed,the percentconversion fromgasphaseoxidationby OH in the wake. Furthermore, oxidationin the enginewas foundto be kineticallylimited by the level of atomicoxygen. Thus,SO3emissions may not scalelinearlywith fuel sulfurandthe percentconversion may decrease as fuel sulfuris increased.Therefore,quantitativeinterpretation of in-flightmeasurements on the effectof "low"and"high"sulfur fuels on contrail formationor volatile aerosolemissionsrequire accuratespeciated sulfuroxideemissionindices.
differentsulfurcontents,J. Geophys.Res. 101, 16,731-16,736,1996. Harris, B.W., Conversionof sulfur dioxide to sulfur trioxide in gas turbine exhaust,J. Engin.for Gas Turbinesand Power 12, 585-589, 1990.
Hofmann,D.J. and J.M. Rosen,BalloonObservationsof a ParticleLayer Injectedby StratosphericAircraft at 23 km, Geophys.Res. Lett. 5, 511-574, 1978.
HunterS.C.,Formationof SO3 in gasturbines,J. Eng.Power 104,44-51, 1982.
Kfircher,
B., Th.
Peter and R. Ottmann,
Contrail
formation:
Homogeneous nucleationof H2SOn/H20droplets,Geophys. Res.Lett. 22, 1501-1504, !995.
K•ircher, B., M.M. Hirschberg and P. Fabian, Small-scale chemical evolutionof aircraftexhaustspeciesat cruisingaltitudes,J. Geophys. Res. 101, 15,169-15,190, 1996.
Kolb, C.E., J.T. Jayne,D.R. Worsnop,M.J. Molina, R.F. Meadsand A.A. Viggiano,GasPhaseReactionof SulfurTrioxidewith WaterVapor,J. Am. Chem. Soc. 116, 10,3!4-10,315,
1994.
Merryman,E.L. andA. Levy, Sulfurtrioxideflamechemistry- H2Sand COS flames,ThirteenthSymposium(International)on Combustion, The CombustionInstitute,p. 427-436, 1971. Miake-Lye, R.C., M. Martinez-SanchezR.C. Brown, and C.E. Kolb, Plumeandwakedynamics,mixingandchemistrybehinda high speed civil transportaircraft,J. Aircraft 30, 467-479, 1993. Miake-Lye, R.C., M.R. Anderson, R.C. Brown, and C.E. Kolb, Calculationsof condensationand chemistryin an aircraft contrail, in DLR-Mitteilung,94-06, pp. 274-279, DLR, D-51140, Koln, 1994. NASA, SUbsonicAircraft: Contrail and Cloud Effects Special Study, ProjectOffice Report, Ames ResearchCenter, National Aeronautics and SpaceAdministration,March, 1996. Schumann, U., J. Str6m, R. Busen, R. Baumann, K. Gierens, M.
Acknowledgments. The authorswish to acknowledgeNASA for the supportof this work underthe Atmospheric Effectsof AviationSubsonic Assessmentthrough contract NAS 1-20273. The authorsalso wish to thank Terry Jordanof Rolls Royce and Ulrich Schumannand Frank Deidewig of DLR for providing engine parametersfor the Concorde Olympusand ATLAS, respectively.
679, 1982.
Tsang,W. and R.F. Hampson,Chemicalkineticdatabasefor combustion chemistry.Part I. Methane and related compounds,J. Phys. Chem. Ref. Data 15, 1087,1986. Tsang,W. and J.T. Herron, Chemical kinetic data basefor propellant
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Aviamotornayastr., 111250,Moscow, Russia (ReceivedJuly 17, 1996; revisedOctober4, 1996; accepted,October16, 1996.)
