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Effects of Flame Temperature and Fuel Composition onSoot Formation in Gas Turbine CombustorsDavid W. Naegeli a , Lee G. Dodge a & Clifford A. Moses aa Southwest Research Institute , P.O. Drawer 28510, San Antonio Texas, 78284Published online: 09 Jun 2010.

To cite this article: David W. Naegeli , Lee G. Dodge & Clifford A. Moses (1983) Effects of Flame Temperature and FuelComposition on Soot Formation in Gas Turbine Combustors, Combustion Science and Technology, 35:1-4, 117-131, DOI:10.1080/00102208308923706

To link to this article: http://dx.doi.org/10.1080/00102208308923706

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Combustion Science and Technology, 1983, Vol. 35, pp. 117-131OOI0-2202/83/3504-0117S 18.50{0

© 1983 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

Effects of Flame Temperature and Fuel Composition on SootFormation in Gas Turbine Combustors

DAVID W. NAEGELI.t LEE G. DODGE and CLIFFORD A. MOSESSouthwest Research Institute. PO. Drawer 28510. San Antonio Texas 78284

(Received November 4, 1982; ill filial form May 19, 1983)

Abstract-The dependence of relative soot concentration on flame temperature and fuel compo­sition was measured in a small-scale research combustor. The purpose was to gain a better under­standing of the correlation of soot formation with H/C ratio. First, the effect of flame temperatureon soot concentration was determined by varying the burner inlet temperature. Then, 10 fuelswith H/C ratios in the range of 1.98 to 1.55 were used in an experiment 10 determine the effectsof both flame temperature and fuel composition on relative soot concentration. Flame tempera­tures were calculated and measured optically by the Kurlbaum technique. Flame opacitymeasurements were used to determine relative soot concentration. The results showed that whilesoot concentration increased significantly as flame temperature increased, the ncrease in sootwith fuels of lower HIe ratio was much stronger than could be attributed to associated increasesin the tlame temperature.

INTRODUCTION

Gaining a more basic understanding of soot formation in gas turbine combustorshas become increasingly important because future fuels may vary significantly incomposition, and specifications may need to be changed to insure an adequate fuelsupply. Aside from the environmental aspects and possible signature problems onmilitary aircraft, the greatest concern about soot in gas turbine engines is the effect ithas on combustor liner durability.

The radiative component of combustor liner temperature has been found tocorrelate most consistently with fuel hydrogen content. For a typical engine such asthe J79, increased liner temperature due to a reduction in fuel hydrogen content from14.5 to 12 percent will reduce the cycle life by 65 percent (Gleason and Bahr, 1980).This has provided a basic inecntive for continued research on the nature of sootformation in gas turbine combustors.

It is well acecptcd that the sooting tendency in gas turbine combustors correlatesconsistently with the hydrogen/carbon (H/C) ratio of the fuel. However, there isconsiderable controversy over the basic chemical and/or physical mechanisms of thesoot-forming process, and the applicability of results obtained in sm:111 laboratorygaseous-fueled laminar flames to large-scale turbulent liquid-fueled combustors. Onemechanism is based On the observation that the sooting tendency of a laminar dif­fusion flame increases as thc flame temperature is raised. Since it is generally foundthat lowering thc H/C ratio of a kerosene-type fuel increases its flame temperature,it might appear that the correlation of sooting tendency with H/C ratio is simplycaused by changes in flame temperature.

t Author to whom correspondence should be addressed.

117

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118 D. W. NAEGELI, L. G. DODGE AND C. A. MOSES

The purpose of this investigation was to compare the effects of changing both fuelcomposition (i.e., HjC ratio) and flame temperature on the soot concentration in aresearch combustor. This combustor was operated on kerosene-type fuels and pro­vided a highly turbulent flame zone more representative of gas turbine combustorsthan small laminar flames. Basically, two experiments were performed which involvedchanging the flame temperature and measuring the effect on relative soot concen­tration. In the first experiment, the flame temperature was changed by varying theburner inlet conditions, while in the second experiment it was varied by changingonly the HjC ratio of the fuel. In each experiment, the flame temperatures were bothcalculated and measured non-intrusively by the Kurlbaum method, and the relativesoot concentrations were measured as flame opacity in the primary zone of the burner.

BACKGROUND

Since the early work by Schirmer and colleagues at Phillips Petroleum (Schirmer,1962, 1965, 1972; Quigg, 1972; and Street, 1959) and McClelland (1963), a numberof investigators have found a good correlation between fuel hydrogen content andsoot formation (usually measured by flame radiation or liner temperature) in gasturbine combustors (Blazowski, 1976, 1979; Blazowski and Jackson, 1978; Butzeand Ehlers, 1975; Butze and Smith, 1977; Friswell, 1979; Gleason et al., 1979;Gleason and Bahr, 1980; Gleason and Martone, 1980; Horstman and Jackson, 1962;Jackson and Blazowski, 1977, 1979; Martel and Angello, 1973; Moses and Naegeli,1978, 1979; Naegeli and Moses, 1978; and Naegeli 1'1 al., 1978), and smoke point(Schirmer, 1972). The correlation of soot formation with hydrogen content, or HjCatom ratio, has recently been extended to inelude not only petroleum-based fuels,but also fuels derived from oil shale, coal. tar sands, emulsions containing bothwater and alcohols, and solutions of alcohols and jet fuels (Naegeli and Moses,1978; and Naegeli 1'1 a!., 1980). These results showed that the correlation with HjCratio was distinctly more consistent than that with aromatic content.

The effects of physical properties on soot formation have also been investigated.Increases in viscosity and boiling-point distribution increase droplet lifetime which,in some bench-type experiments, has been shown to increase sooting (Sjorgren, 1973).However, tests in actual gas-turbine combustors and engines have shown that reason­able variations in viscosity and boiling point distribution do not increase the tendencyof fuels to soot any more than would be expected from their hydrogenjcarbon ratio(Blazowski and Jackson, 1978: Gleason and Bahr, 1980; Moses and Naegeli, 1979;Naegeli and Moses, 19n).

Glassman and Yaccarino (1978) and Schug 1'1 al. (I nO) have demonstrated theimportance of flame temperature on soot formation in laminar diffusion flames[although the measured flame temperatures of Schug 1'1 al. (1980) seem to show muchless variation with dilution than the predicted temperatures of Glassman andYaccarino (1978)]. They found that soot is formed more readily at higher flametemperatures because the pyrolysis reactions which make soot precursors have agreater temperature dependence than the oxidation reactions that destroy virgin sootmaterial. In view of the rather marked effect of flame temperature on soot productionin laminar diffusion flames, they have suggested that the correlation of soot concen­tration with HjC ratio is simply an artifact caused by variations in flame temperaturewith fuel composition. Note that the adiabatic flame temperature usually increasesas HjC ratio of liquid fuels is reduced and this is in accordance with the dependence

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SOOT FORMATION IN TURBINE COMBUSTORS 119

of soot production on flame temperature in laminar diffusion flames. This argumentis examined in the present study.

EXPERIMENTAL APPARATUS

The Phillips 2-inch Combustor

Figure I is a schematic of the Phillips 2-inch combustor and the optical systememployed in this study. Basically, this combustor is a straight-through cylindricaltype, with fuel atomization by a simplex pressure atomizer (Moses and Naegeli,1978). The combustor liner is constructed from 2-inch, Schedule 40 Inconcl pipe.Film cooling of surfaces exposed to the flame is accomplished by internal deflectorrings. The combustor rig inlet air supply system is capable of operating at burnerinlet temperatures and pressures up to 1030 K and 1.8 MPa, respectively.

Optical Temperature Measurement

Because of the importance of flame temperature to the results of this study, tem­peratures were both calculated from an equilibrium code (assuming an equivalenceratio of one) and measured using a non-intrusive line-of-sight optical technique.Measurements in a highly turbulent flame are difficult to interpret occausc of thespatial and temporal variations, but the relative change of measured temperatureswith changes in the burner inlet temperature or fuel properties can add significantconfidence to the calculated temperatures if the percentage changes are similar.

The Kurlbaum technique, or soot reversal method, was used to measure a tem­perature in the primary reaction zone of the combustor. It is a well-establishedtechnique and straightforward to calibrate, requiring much simpler apparatus thanother non-intrusive optical techniques such as CARS. This method is similar tosodium D-line-reversal except that broader-band soot emission and absorption areused rather than a spectral line. The Kurlbaum method is described in Gaydon andWolfhard (1970), and the details of the mathematics are presented by Tourin (1966).Precautions such as those suggested by Thomas (1968) for line-reversal methods havebeen observed.

The optical setup depicted in Figure I is conventional. The dependence of detectorsignal on lamp current is first measured without the flame in the optical path; thenthe same measurements are repeated with the flame present. The region where thesetwo curves of signal versus lamp current cross is the point of reversal. The spectralbrightness temperature of the tungsten strip lamp at the reversal point is equal to theflame temperature after allowing for window and lens losses on the lamp side, andcorrecting the brightness temperature for the difference in pyrometer wavelength(T=0.65 pm) and flame measurement wavelength (T=0.5 pm). A silicon photodiodewith a 0.5 pm filter was used as a detector and a lock-in amplifier was used to amplifythe chopped signal. Careful usc of apertures, as suggested by Thomas (1968), wasnecessary to assure proper calibration, although only relative temperatures were ofsignificance for this work.

It is worthwhile to consider what is actually measured with this technique, becausethe soot concentration and temperature gradients complicate the interpretation ofthe data. This method responds only to absorption and emission by soot and isunaffected by hot or cool gases not containing soot because there are no significant

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12Q

AIR

FLOW

FUELFLOW

AIRFLOW

D. w. NAEGELI, L. G. DODGE AND C. A. MOSES

TUNGSTEN STRIP LAMP

CHOPPER (OPACITY MEASUREMENTI

LENS

oAPERTURE

LENS

CHOPPER (REVERSAL MEASUREMENT)

0.5-,..rn FILTER (REVERSAL MEASUREMENT)0.9-,..rn FILTER (OPACITY MEASUREMENT)

Si DETECTOR

FIG URE I Experimental apparatus for optical measurements.

EXHAUST

gaseous absorbers at 0.5 p.m. Thus, the temperature is measured only where sootexists along the line-of-sight and the measured temperature is weighted by a factorapproximately proportional to the soot concentration. A second weighting factor isdue to the temperature gradients and tends to cause the measured temperature to beheavily weighted toward the highest temperatures where soot exists along the line-of­sight. Millikan (1961) has shown that the soot particle temperature equals the gastemperature within the measurement uncertainty, and Kuhn and Tankin (1968)

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SOOT FORMATION tN TURBINE COMBUSTORS J21

showed equivalenee of gas and particle temperatures exeept at initial earbon for­mation, where the earbon temperature was somewhat lower, and during particleburnout. where the carbon temperature was higher.

The temperatures of importance for this study are those on the fuel-rich side ofthe flame zone where soot precursors are formed and oxidized and in the leaner partsof the flame where the soot particles are oxidized. It would seem reasonable toassume that these temperatures would change proportional to the calculated flametemperatures and the measured soot particle temperature. Thus, the relative changein calculated or measured flame temperature with changes in burner inlet temperaturesor fuel properties should be representative of the relative change of the temperaturefield throughout the flame where soot precursors and soot are formed and oxidized.

Although there are significant uncertainties associated with temperature measure­ments in this type of flame, the calculated and measured flame temperature generallycorresponded and the conclusions presented may be derived from either the measuredor calculated temperatures alone.

Opacity Measurements

The same optical apparatus was used for opacity measurements except that thechopper was moved to the Jamp side of the combustor to remove flame radiationfrom the signal, and a 0.9 fLm band pass filter was used to discriminate againstabsorption not related to soot particles. The soot concentration was assumed toattenuate the light from the background lamp according to the Beer-Lambert law,i.e., Illo=exp( -sl), where s is the soot concentration and I is the path length. Thus,the soot concentration was assumed to be proportional to In(/o/I). The maximumattenuation was 60 percent.

RESULTS

Two experiments were performed to determine the effects of temperature and fuelhydrogen content on soot formation. The first experiment determined the effect ofburner-inlet-temperature on soot formation without any variation in fuel properties;in the second experiment, the fuel properties were varied and the operating con­ditions were hcld constant.

Experiment 1

The burner inlet temperature was varied from 533 to 922 K while the gas density andmass flow rates of the reference fuel (Jet A) and inlet air were held constant.

TABLE 1

Combustor operating conditions

Operating condition

Burner inlet temperature, KBurner inlet pressure, kPa

533786

2

589862

3

644938

4

7281082

5

8t21200

6

9221344

Reference velocity and burner inlet gas density were held constant at 30.5 m/sec and 5.26 kg/rn'',respectively.

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122 D. W. NAEGELI, L. G. DODGE AND C. A. MOSES

Table I shows the burner inlet temperature and corresponding burner inlet pressuresrequired to maintain constant gas density for the six operating conditions examined.These tests were repeated for three different overall stoichiometries, i.e., fuel/airratios of 0.0096,0.0108, and 0.0136. The primary zone fuel/air ratios were approxi­mutely 0.0255, 0.030, and 0.039. These fuel/air ratios, which are relatively low com­pared to actual combustors, were selected to provide reasonable opacities for the sootconcentration and temperature measurements. The flame temperature (soot tem­perature by the Kurlbaum technique) and the relative soot concentration (flameopacity) were measured in the primary zone of the combustor at each operating con­dition. Figure 2 shows the effect of burner inlet temperature on flame temperature.[Lines through the data are based on a least-squares fit for this and all other figures.The correlation coefficient (cc) is equivalent to the statistical parameter r.] Thechange in flame temperature with the burner inlet temperature is in good agreementwith adiabatic flame temperature calculations. An increase in burner inlet tem­perature from 533 to 922 K changes the calculated stoichiometric adiabatic flametemperature from 2400 to 2590 K or 7.9 percent. The measured flame temperaturesincrease from about 2104 to 2297 K or 7.3 percent. The difference of about 300 Kbetween the measured and calculated adiabatic flame temperatures is partly due toconvective and radiative heat losses; gradients in temperature and soot concentrationwould account for the rest. The measured flame temperatures were essentially inde­pendent of fuel/air ratio, in agreement with the concept that diffusion flames establisha stoichiometric flame zone. The slight decrease in flame temperature with increasedfuel/air ratio may have been caused by the increased flame opacity resulting in aslight shift of the temperature weighting factor out from the center of the flametoward the cooler aliter parts of the flame. Figure 3 shows the effect of flame tern­perature (measured) on the relative soot concentration, Clearly, the soot concen­tration increases with both increased flame temperature and increased fuel/air ratio.This is consistent with diffusion flame behavior.

900800700600

•

• F/A=0.0096

• F/A = 0.0108

A F/A =0.0136

~2400

u.ia::=>~ 2300a::w0...~w 2200t-w:::?:«...JI.L. 2100vi«w::!:

2000500

BURNER INLET TEMPERATURE, K

FIG URE 2 The effect of burner inlet temperature on flame temperature as measured by theKurlbaum technique.

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SOOT FORMATION IN TURBINE COMBUSTORS 123

.......

I) F/A,,; 0.0096

II FIA,,; 0.0108

• F/A,,; 0.0136

••I

2100 2120 2140 2160 2180 2200 2220 2240 2260 2230 2300 2320

MEASURED FLAME TEMPERATURE, K

0.8

1.2

0.4

1.6

2.0

2.4zo~c::I­ZwuzoUI­oo(J)

w:>

~...JWc::

FIGURE 3 The effect of the measured flame temperature on soot concentration.

Experiment 11

The purpose of the second experiment was to determine the effect of Hie ratio onflame temperature and soot formation. Ten test fuels with hydrogen contents rangingfrom 1l.5 to 14.5 percent were burned at operating condition number 3 (see Table I)at a fuel/air ratio of 0.0098. The fuels shown in Table II were a blend of the ref­erence fuel, Jet A, a mixture of alkyl benzenes, and a mixture of alkyl naphthalenes.These fuels had been blended originally for a program to study the effects of polycyclicaromatics on soot formation, but their wide range of hydrogen content was alsovaluable in this work.

TABLE II

Fuel characterist ics

Fuel No. Description

Hydrogencontent(wl.%)

ReferenceI23456789

Jet A81.5% Jet A + 18.5% BTX88.8 % Jet A + 11.2 % Methyl naphthalene58.0% Jet A + 42.0% BTX66.3% Jet A + 21.0% BTX + 12.7% Meth. nap.74.5 % Jet A + 25.5 % Methyl naphthalene34.5% Jet A + 65.5% BTX51.6% Jet A + 22.0% BTX + 26.4% Meth. nap.43.4% Jet A + 43.0% BTX + 13.6% Meth. nap.60.3 %Jet A + 39.7% Methyl naphthalene

14.3] 3.513.512.512.512.511.511.511.511.5

BTX= High boiling alkyl benzene derivatives.Meth nap. = Methyl naphthalenes and other polycyclic aromatics.

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124 D. W. NAEGELI, L. G. DODGE AND C. A. MOSES

TABLE III

Measured and calculated adiabatic stoichiometric flame temperaturesand measured relative soot concentration

Hydrogen Gross heat of Flame temperature (K) Relativecontent combustion ------------- SOOt

Fuel No. (wt. %) tkJjkg) Calculated Measured concentration----------------------------------------Reference 14.3 46,622 2475 2193 1.00

1 13.5 45,897 2475 2208 1.022 13.5 45,874 2475 2200 1.163 12.5 44,996 2477 2212 1.184 12.5 44,970 2476 2216 1.295 12.5 44,935 2475 2210 1.166 11.5 44,099 2478 2218 1.827 11.5 44,034 2477 2223 2.708 11.5 44,066 2477 2227 2.419 11.5 44,006 2476 2228 2.55

The effect of hydrogen/carbon ratio on flame temperature was determined boththeoretically and experimentally. The calculated flame temperatures presented inTable III assume adiabatic stoichiometric conditions and show negligible changewith H/e ratio. If flame temperatures were calculated for fuels of equal heat contentand varying H/e ratio, the fuels with lower H/e ratio would have higher temperaturesdue to the shift in equilibrium products. However, for these fuels, the temperatureincrease was very small because the heat of combustion decreased more than usualas the H/e ratio decreased. As a general ruel, lowering the hydrogen content of ajet fuel one percent will raise the stoichiometric flame temperature 10 K or less. Forthe fuels used here the calculated flame temperatures were approximately constant,but the measured soot concentrations varied significantly, as shown in Table III,indicating that the effect of H/e ratio on soot formation is not explained by differencesin flame temperature. Because there might be some question about the accuracy ofcomputed flame temperatures, the temperatures were also measured with theKurlbaum technique. It was found that a decrease in H/e ratio from 1.98 to 1.55increased the measured flame temperature 35 K.

Both the calculated and measured flame temperatures seem to indicate too small atemperature increase with decreasing H/e ratio to account for the factor of ~ 2.4increase in soot concentration. If the above arguments are correct, the temperaturedependence of soot concentration based on data from experiment I (changing inlettemperature), should be significantly different from that based on data from exper­iment II (changing fuel composition), On the other hand, if the effect of H/e ratioon soot concentration is simply due to temperature effects alone, the temperaturedependencies of soot concentration should be the same in both experiments.

Using data from experiment I (changing inlet temperature), a favorable correlationof relative soot concentration, S, with fuel/air ratio, F/A, and flame temperature, T,was obtained with

S = A(F/A)a cxp( -biT) (I)

where A is an arbitrary pre-exponential factor, a is a constant expressing the depen­dence on fuel/air ratio and b is an Arrhenius type temperature coefficient. The cor-

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12.4

12.0

~..,~--u,

--(/)

I:::~

11.6

11.2

SOOT FORMATION IN TURBINE COMBUSTORS

•.~ CHANGE IN FUEL TypF.Y b '" 325,216

cc '" 0.87

CHANGE IN BURNER........---INLET TEMPERATURE

". b '" 29,101cc > 0.97

125

4.2

l()4/T, K-1

4.4 4.6

FIGURE 4 Arrhenius correlation of relative soot concentration with the average name tem­perature (7;ue". + T,·nlc.)/2. (a) Change in burner inlet temperature, (b) change in fuel composition.

relation showed that the relative soot concentration was strongly dependent onfuel/air ratio, a=3.425, and flame temperature, 11=29,101 (equivalent to an activationenergy of 58 kcalnnolc). It is interesting to note that Khan and Grccvcs (1974) alsofound that soot formation in a diesel engine depended on the equivalence ratio raisedto the third power. For experiment II (changing fuel composition), the fuel/air ratiowas constant so only the temperature coefficient h was evaluated using (1= 3.425from experiment I. Figure 4 shows that there is a strikingly big difference betweenthe temperature coefficients of experiments I and II. The flame temperature used inthese correlations is an average of the measured and calculated stoichiometric flametemperatures. Clearly, the temperature coefficient based on data from experiment II

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J26 D. W. NAEGELI, L. G. DODGE AND C. A. MOSES

is unreasonably high; it is equivalent to an activation energy of 646 kcaljmole, ascompared with 58 kcaJjmoic from experiment I. This latter value compares favorablywith the results of Glassman and Yaccarino (1978), Tesner et al. (197Ia) and Tesneret al. (1971 b) when account is taken for the activation energy of soot oxidation,which is assumed to compete with the soot formation reactions in the turbulent flamestudied here. That is, if it is assumed that the measured value of 58 kcaljmole is theresult of an activation energy for formation minus a competing activation energy of39 kcaljmole for soot oxidation (from Lee et 01.• 1962), then the soot formation stepwould have an activation energy of 97 kcal/molc.

Glassman and Yaccarino found high activation energies ranging from 80 to118 kealjmole for the formation of soot in laminar dilfusion flames. Tesner et al.(1971a) and Tesner et al. (1971 b) measured soot particle formation rates in diffusionflames of several hydrocarbons including aromatics and determined activation energiesranging from 67 to 180 kcaljmole for soot formation. The overall temperaturedependence of soot concentration in a turbulent flame such as the one studied hereis much less than what would be predicted from the activation energy for soot for­mation alone because of the temperature dependence of the competing soot oxidationreactions. This is in contrast with laminar dilTusion flames where the oxidationreactions arc assumed to be unimportant in affecting the temperature dependence ofthe sooting process, either due to the longer residence time in the pyrolysis zone orto the measurement technique (Glassman and Yaccarino, 1978). The result fromexperiment II data (£=646 kealjmole) shows that changes in flame temperaturecannot explain the strong correlation of 0'001 concentration with the H'[C ratio, andthat other fuel chemistry effects must be significant in soot formation. This is in con­flict with the interpretation by several workers. of the results obtained by Schug et al.(InO) and Glassman and Yaccarino (1978) that the HjC correlation is an artifactof the change in flame temperature.

Other supporting evidence for the above conclusion can be seen in the dependenceof NO, on temperature. Sawyer 1'1 al. (1973) observed that the NO,-emissions datafrom gas turbine combustors could be presented in standard Arrhenius form similarto the plot of Ln(NO, E.1.)/Tfo.5 verses IjTf in Figure 5, where Tf is an average ofthe measured and the calculated adiabatic flame temperatures. This assumes that theZcl'dovich mechanism governs thermal NO, formation. In Figure 5 the NO, emissionsfrom experiment I arc compared with those from experiment II. Clearly. the NO,emissions index is dependent only on flame temperature and not fuel chemistrybecause correlations of the data from both experiments I and II have essentially thesame slopes. Here the effect of changing the HjC ratio of the fuel is only to change theflame temperatures, whereas in the case of the soot correlations shown in Figure 4,the elTeetof HjC ratio cannot be accounted for on the basis of flame temperature alone.

Results of Full-Scale Combustor Tests

The purpose or this research was not to understand soot formation in a Phillipsresearch combustor, but rather in gas turbine combustors. The authors feel that thehigh pressure liquid-fueled turbulent flame in the Phillips combustor is more rep­resentative of a gas turbine combustor than atmospheric pressure laminar gas-fueledburners. However, these results arc meaningful only if they arc representative ofpractical combustors. Experimental data of the type reported here for experiment IIarc available for practical combustors in the literature, in the form of flame radiationmeasurements as a function of fuel properties. Calculated flame temperatures are

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SOOT FORMATION IN TURBINE COMBUSTORS 127

CHANGE IN BURNER INLET TEMPERATURE

-4.730 . 1()4LnIE.I.IVTl = T + 18.94

EA ;;; 94 kcal/molecc =0.98(V,., =30 m/Sl

1.0

0.6

0.8

~0.4

--....:ui"0

CHANGE IN FUEL TYPEz

Ln(E.I./VT) =-3.965 . 1()4

0.2T

EA ;;; 79 kcal/molecc = 0.91(V,.,= 61 m/SI

0.14.0 4.1 4.2

104fT, K-1

4.3 4.4 4.5

FIG U RE 5 Flame temperature dependence of NOx emissions index for (a) change in burnerinlet temperature, and (b) change in fuel composition.

also available. These data permit an analysis such as the one used with Eq. (I) togenerate the line labeled "Change in Fuel Type" in Figure 4. Corresponding datafor soot concentration versus inlet temperature, labeled "Change in Burner InletTemperature" in Figure 4, arc not available for full-scale combustors. However.NO" formation is known to be a very temperature dependent process and NO"concentration data arc available at the same conditions as the soot radiation measure­men ts. Therefore t he soot concentration for vari ou s fuel types may be plotted as afunction of flame temperature and the slope compared with either the NO" data orthe temperature dependence for soot determined in the Phillips combustor inexperiment r.

For example, consider flame radiation and NO" data from the J79-17A engine(Gleason ct al., 1979) which show apparent increases in soot formation and NO"emissions as the H/C ratio of the test fuel is decreased. However, the changes insoot production with H/C ratio were significantly greater than that for the NOx

emissions index. An analysis of the data similar to that carried out above for theexperimental results presented in this paper shows that the temperature dependenceof the NO" emissions at the "Takeoff" condition is similar to the curve (fuel changeonly) shown in Figure S. The Ln(NOx E.I.) was plotted against the reciprocal of thecalculated flame temperature and the slope corresponded to an acn vat ion energyof 79 kcal/rnole. (Note, the Zel'dovich theory predicts an activation energy of

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t28 D. W. NAEGELI, L. G. DODGE AND C. A. MOSES

135 kcaljmole.) When the soot concentrations were correlated using Eq. (I) andplotted in the same way as that shown in Figure 4, the temperature coefficient wasagain unusually high, b = 185,513, eorresponding to a global activation energy of369 kcaljmolc. These results were determined using the calculated flame temperaturesof Gleason et al. (1979) and assumed that the flame radiation was proportional tothe relative soot concentration in the primary zone of the burner.

The change in NO x concentration with changing fuel type can be attributed to thechange in flame temperature, indicating the calculated flame temperatures are reason­able. However, if the increased sooting with low HjC fuels is attributed solely toname temperature, the temperature dependence is much too large when comparedwith either the NO x temperature dependence or the soot temperature dependencefrom experiment I. This supports the extension of the results obtained on the Phillipsresearch combustor to practical combustors, i.e., the eorrelation of soot concen­tration with HjC ratio is not explained simply in terms of flame temperature.

DISCUSSION

For laminar diffusion flames, Schug et al. (1980) have shown that CjH ratio is not adominant parameter in the sooting tendency of a fuel. They conclude that the sootingbehavior of a laminar difTusion flame is controlled by the initial fuel pyrolysis whichis dominated by flame temperature. It was also realized that fuel structure was amajor parameter determining the propensity to soot. Calcote and Manos (1982)developed a "threshold soot index" which thcy used to examine the correlation ofvarious fuel properties with sooting tendeney. They place more emphasis on molecu­lar structure than flame temperature, but comply with Schug et al. (1980) on thesignificance of HjC ratio. The experimental results from the Phillips research com­bustor show that increases in flame temperature increase the sooting tendency whichis typical of diffusion flames, but contrarily the fuel property effects tend to correlatemost favorably with thc HjC ratio. For gas turbine combustors, the HjC ratio isa very good indicator of a fuel's tendency to soot. The correlation has been found tobe almost universal for all turbine fuels (Dodge et al., 1980) except those containinghigh concentrations of polycyclic aromatics. In a few engines, the sooting tendenciesof fuels containing polycyclic aromatics are higher than other fuels with the sameHjC ratio.

In a recent study (Naegeli et al., 1982) on the efTects of fuel composition on sootingtendency, the results suggested that HjC ratio may be more important to soot oxi­dation than fuel pyrolysis. In that study, a normalized sensitivity of sooting tendencyto HjC ratio was defined as [dRjd(HjC)]jR, where R was the measured flame radiationin the primary zone of the Phillips research combustor. It was found that the sensi­tivity of sooting tendency to HjC ratio was strongly dependent on the burner inletconditions of the combustor. In particular, the sensitivity increased as the referencevelocity increased and as the burner inlet temperature decreased. This indicated thatthe sensitivity to HjC ratio increased as thc fuel/air mixing rate increased and as therate of fuel pyrolysis decreased.

When this is compared with the results of experiment II, it becomes evident thatthe unusually high temperature dependence shown in Figure 4 could have, in fact,been very different including, b=O, depending on the operating conditions of thecombustor. At the operating condition of experiment II, the sooting tendency justhappened to be highly sensitive to HjC ratio. It is clear that the effect of HjC ratio

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SOOT FORMATION IN TURBINE COMBUSTORS 129

on sooting tendency is not the result of fuel composition-related changes 111 flametemperature because if that were the case, the sensitivity of sooting tendency to HICratio would not be dependent on parameters such as reference velocity which havelittle effect on flame temperature,

Although the flame in the Phillips' combustor had properties of a turbulent dif­fusion flame, increasing the mixing rate and reducing the fuel pyrolysis rate wouldtend to give the flame more premixed character. Glassman and Yaccarino (1978)showed that fuel pyrolysis is the dominant temperature dependent process in laminardiffusion flames, while Millikan (1962) found that the soot precursor oxidationreactions involving the OH radical had the strongest temperature dependence inpremixed flames, From this view it appears that HIC ratio may play its most import­ant role in the oxidation of soot precursors.

CONCLUSIONS

Both flame temperature and fuel composition are important in determining thesooting tendency of fuels in turbulent diffusion flames such as found in gas turbinecombustors. The correlation of soot with HIC ratio in gas turbine engines cannot beattributed to flame temperature alone. rn many cases, the increase in flame tempera­ture with lower HIC ratio fuels is insignificant relative to the several-fold increase insoot concentration,

Conelusions about soot formation drawn from the Phillips high pressure turbulentresearch combustor have been substantiated by full-scale, practical combustor tests.If results from atmospheric pressure gaseous-fueled laminar diffusion flames contra­dict these results, it raises questions about the extrapolation of those results (laminarflames) to practical combustors.

In turbulent diffusion flames typical of gas turbine combustors, the net temperaturedependence for soot formation is less than for laminar diffusion flames, presumablydue to the increased importance of competing soot and soot precursor oxidation.

ACKNOWLEDGEMENTS

This work was performed at the U.S. Army Fuels and Lubricants Research Laboratory under thesponsorship of the U.S. Army Mobility Equipment Research and Development Command(MERADCOM), Fort Belvoir, Virginia. The authors would like to acknowledge the excellenttechnical help of Messrs. R. C. Hauflcr and Frank Lessing.

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