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The influence of gaseous additives on the formation of soot in premixed flames
Nineteenth Symposium(International) on Combustion/The Combustion Institute, 1982/pp. 1379-1385
THE INFLUENCE OF GASEOUS ADDITIVES ON THE FORMATION OF SOOT IN PREMIXED FLAMES B. S. HAYNES,t H. JANDER, H. MATZING, AND H. GG. WAGNER
Institut fur Physikalische Chemie Universitiit G6ttingen Federal Republic of Germany Laser-light scattering and absorption measurements of soot volume fractions and particle number densities in premixed ethylene-air flat flames at atmospheric pressure show that additions of up to 3%, by volume of the unburned gas, of N2, H20, Hz, NH3, CH~NH2, NO, H2S, SO2, SO~, HCI, CH 4, C~H4 or 02 do not influence the coagulation of soot particles although the soot volume fraction may be significantly affected. Pro-soot additives are CHA, C2H4 and CH~NH2 while additives which reduce the soot volume fraction are O2, NO, NH3, H2S, SO2 and SO3. Whatever their effects, these additives donot alter the specific surface growth rate for soot particle mass addition. The various additives retain their pro- or anti-soot tendencies when considered in terms of their effects on the critical C/O-ratio for the appearance of soot. Additions of H2 promote soot under these conditions. Special attention is paid to the effect of SO3--no evidence of any pre-soot effects at or beyond the critical C/O-ratio is found in aliphatic or aromatic flames. The mode of action of the additives is not generally understood, although the formal equivalence of the addition of one molecule of NH3 with removal of one fuel-C atom is consistent with the expected rapid formation of inert HCN in the flame and with the relatively weak effects of CH3NH2 addition.
Introduction Investigations of the effects of gaseous additives on soot formation have been widely reported--see, for example, [1-17]. Unfortunately, however, many of these reports have been concerned with the influence of the additive on the overall characteristics or general appearance of flames of often illdefined geometries. It is generally agreed that inert diluents such as N2 or Ar have little apparent effect other than a mild inhibition of the sooting tendency of both premixed and diffusion flames [4-7]. This is also the case for H20 and CO~ additions [4, 7, 8] although these additives are more effective than inerts added to the fuel side of a diffusion flame [6, 7]. Carbon monoxide, CO, promotes slightly the luminosity of flames [4]. The influence of Hz is complex in that additions of H 2 can alter completely the structure of some flames--for example, converting a polyhedral benzene-type flame to one of the acetylene type [9, 10]. Chemically, added hydrogen can be expected to compete for the availtCurrent address: CSIRO Division of Fossil Fuels, North Ryde, N.S.W. Australia.
able oxygen, merely making the flame richer and promoting the formation of soot [4]. Some other additives have more pronounced effectsl For example, ammonia added in concentrations of about 10% to low-pressure premixed flames completely suppresses soot formation [11]. Nitric oxide is reported to de-stabilize benzene flames [12] and weakly to inhibit soot formation in these and other premixed [13] and diffusion flames [14]. Additions of sulphur as H2S or SO2 apparently inhibit soot formation in both premixed [4, 15] and diffusion [7, 14, 16] flames, the reported magnitude of the effect varying from weak t o strong, depending on conditions. Sulphur trioxide, SO3, is reported to be an outstanding soot promoter at concentrations as low as 0.1% but its influence in diffusion flames is slight [16]. The objective of the work reported in this paper is to examine the influence of various gaseous additives on the overall process of soot formation in premixed flames. This process we view [3, 18, 19] phenomenologically as occurring in two stages: the formation of the first particles; and their subsequent growth by the effectively independent processes of (i) surface growth, which accounts for most of the soot mass; and (ii) coagulation, which brings about a reduction in number density as par-
1379
1380
SOOT AND PAH
ticles collide and fuse. In earlier work [20], we showed how traces (<1 ppm) of ionizing metal additives can drastically inhibit coagulation without much affecting the soot yield. By employing the same laser light-scattering and extinction techniques to measure soot volume fraction, particle size and number density in flames with various gaseous additives, the influence of these additives is found, by contrast, to lie in an inhibition of surface growth, coagulation rates remaining unaffected. Experimental
Procedure
Soot volume fraction f~, mean particle size d, and particle number density N are determined by laser light scattering and extinction measurements on flat, premixed flames of ethylene, or benzene, and air at atmospheric pressure. The theoretical bases for the measurements and the interpretation of results together with details of the experimental arrangement are described elsewhere [18, 20, 21]. Gaseous additives drawn from cylinders via calibrated rotameters are introduced into the fuel-air mixture well upstream of the burner. For addition of SO3, an argon stream is saturated in SO3 at known temperature and pressure by passage through a bath of oleum (65% SO3). The dilution effects caused by the argon are negligible. The amount of SO3 in the gases is determined by dissolving in water the SO3 from a known volume of main gas and titrating. Additions of H20 are made similarly by saturating part of the air supply with water vapour. The critical C/O ratios at the "soot limit" are determined visually in the darkened laboratory. The reproducibility of the measurements is typically better than 1% although this may be as poor as 2-3% in cases where additives colour the flame. For example, in the presence of sulphur species, a bluish-white emission makes determination of the critical C/O ratio uncertain beyond 1.5% of additive (HaS or SO2). For semi-quantitative gas analyses, samples are taken with a water-cooled quartz probe. The samples are drawn into a previously evacuated flask and analyzed mass-spectrometrically (Varian MAT). By this procedure an approximate survey of the stable sulphur-containing species present in the postflame gases of flames seeded with H2S or SOz is obtained. Measurements of the hydrocarbons will be reported and discussed elsewhere. Soot samples for elemental analysis are collected with a water-cooled quartz probe system maintained at about 0.I torr. The flame gases pass through a 0.3 mm diameter orifice and the soot particles are impinged onto an uncooled substrate of either stainless steel or quartz.
Flame temperatures are determined using the Kurlbaum method. The characteristic flame temperature is taken as that corresponding to the point at which the soot loading reaches half its ultimate value [19].
Results
(i) Nitrogen, Ne Dilution of the unburned gases with up to 1.5% N2 has no measurable effect on the critical C/Oratio for soot formation (Fig. 1). The soot yield at C/O = 0.76 is marginally diminished by this additive (Fig. 2) but the particle number density profiles throughout the flame are unaffected. No detectable temperature change is induced by these relatively low levels of N2-addition. (ii) Water Vapour, H20 No detectable changes in critical C/O-ratio, soot yield, particle number density profiles or flame temperature are brought about by additions of up to 1% HzO (Figs. 1 and 2).
o
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Additive Concentration ( mo[%) FIG. 1. Influence of various gaseous additives on the critical C/O-ratio for the onset of sooting in an ethylene/air flame, T - 1800 K. The additive concentration refers to the unburned gases. The curves for "C," "O," " - C , " and " - O " are calculated on the basis of notional "additions" of fuel and oxygen.
GASEOUS ADDITIVES ON FORMATION OF SOOT
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1381
The particle number density profiles in ammoniaseeded sooting flames are similar to those in the corresponding blank although the downstream number density is typically 1 to 2 times that in the blank flame. This effect can be ascribed in part to a slight, but definite, inhibiting effect of NH 3 in these flames so that the flame zone, and the appearance of the first soot, occurs further above the burner in the presence of the additive. In keeping with this observation, the temperatures in the NHaseeded flame of Fig. 3 are 10 to 20 K higher than those in the blank. (vi) Methylamine, CH3NH~ Methylamine additions tend to inhibit the flame in the same way as ammonia and also give rise to slightly higher final number densities. However, the soot loading is slightly increased (Figs. 2 and 3) and the critical C/O-ratio slightly reduced in the presence of this additive.
o e.
0.5
+ NH3
-- :. -'c 0
l
I 2
Additive Concentration (mo[%)
FIG. 2. Influence of gaseous additives on the downstream (t ~ 30 msee) soot yield in an ethylene/ air flame, C/O ~ 0.76, f~ - 1.5 • 10-7. The curves for 'C' etc refer to notional addition or removal of fuel or oxygen.
(vii) Nitric Oxide, NO Additions of NO corresponding to less than about 0.5% of the total flow have no apparent effects on either the critical C/O ratio (Fig. 1) or the soot yield in richer flames (Figs. 2 and 3). At concentrations beyond 0.5%, however, NO rapidly becomes effective in opposing soot formation as evi-
/
(iii) Hydrogen, H 2 Additions of hydrogen appear to promote the onset of soot formation in that they reduce the critical C/O-ratio (Fig. 1). Despite this influence, the actual soot yield in richer flames is not significantly affected by additions of up to 3% H2 to the unburned gases (Figs. 2 and 3). The soot volume fraction and particle number density profiles throughout H2-seeded flames are indistinguishable from those in the corresponding unseeded flames.
I.S
/ /
Addition of ammonia increases the critical C/O ratio for the onset of sooting (Fig. 1) and reduces the soot loading in richer mixtures (Figs. 2 and 3).
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/ / /
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(v) Ammonia, NH 3
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(iv) Methane, CH4 Methane additions, not surprisingly, encourage the onset of soot formation in leaner mixtures (Fig. 1) and enhance the formation of soot in already sooting flames (Fig. 2).
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~ 2"C-.
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2 Additive Concentration (mo[ %)
3
FIG. 3. As Fig. 2 except for ethylene/air flame, C/O - 0.86, f~ - 8 x 10-7.
1382
SOOT AND PAH
TABLE I Concentrations (mol. fraction) of SO~ and CS2 in the post-flame gases of premixed ethylene/air flames. Semi-quantitative gas analyses. (C/O = 0.76) seeded with H2S or SOz H2S Addition X,.tota] = 0.009 Time (msec) Xso2 Xcs2 8 26
0.002 0.001
0.002 0.004
SO~ Addition X,_tota] = 0.01 Xsoz Xcs~ 0.002 0.002
0.003 O.004
(Fig. 4) however, reveals an effect not seen with HzS, nor with any other gaseous additive used here--:-at higher levels of SO2 addition, the soot loading goes through a maximum downstream of the flame zone. At higher soot loadings, no such maximum is observed for up to 1.5% SO2 addition. As in the case of H~S additions, the dominant gaseous sulphur forms found are SO2 and CS 2 (Table I) although these species now only account for slightly more than 50% of the total added sulphur. Temperatures are markedly influenced by SO2 additions--l% SO 2 raises the average temperature of the C/O = 0.76 flame by about 40 K. (x) Sulphur Trioxide, SO2
denced by increases in the critical C/O-ratio (Fig. 1) and decreases in soot loading (Fig. 2). The partide number density profiles and flame temperature are not significantly affected by this additive. (viii) Hydrogen Sulphide, H2S Hydrogen sulphide inhibits strongly the formation of soot, both in allowing richer mixtures to be burnt without the onset of soot formation (Fig. 1) and in reducing significantly the yield of soot in sooting flames (Figs. 2 and 3). Temperatures in H~S-seeded flames are up to 10 K (0.5% H2S) higher than those in the corresponding blanks. An approximate distribution of the sulphur in the flame is given by mass-spectrometric analyses of batch samples withdrawn at various heights in the flame (Table 1). The dominant sulphur forms in the gaseous sample are SO 2 and CSz (50 to 80% of the total added sulphur). Traces (<1% of the total) of COS, SO3 or H2S are detected in the flame gas samples. For soot samples collected well downstream (t 26 msec) in the C/O = 0.76 ethylene/air flame seeded with 1.5% HzS, elemental analysis indicates 4,2 percent S by weight of the soot deposited; for 0.88% H~S addition this figure becomes 3.7%. These values, which are not affected by tempering of the samples at 100~ C for one hour, correspond to an atomic S/C ratio in the soot of approximately 0.015 so that less than 1% of the total sulphur is present in the soot. Soot samples collected on water-cooled substrates contain approximately four times as much sulphur as those collected on uncooled substrates.
(ix) Sulphur Dioxide, SO2 In terms of its ability to increase the critical C / O-ratio (Fig. 1) and reduce the downstream soot yield (Fig. 2), SO2 behaves very like, but more effectively than, H2S. A closer inspection of the soot volume fraction profiles in SO~-seeded flames
Additions of SO3 reduce the soot yield in sooting ethylene flames (Figs. 2 and 5) with an efficiency somewhat greater than that exhibited by SO 2 additions. Despite large reductions in the soot volume fraction brought about by SO3 additions, the particle number density profiles in the blank and seeded flames are indistinguishable. The same effects are observed in benzene/air flames. I n ethylene and propane flat flames operating just leaner than the critical C/O-ratio, additions of SO3 from 0.05 to 0.4% are incapable of making the flame soot. No visible changes in yellow-tipped bunsen burner flames of propane or methane are observed with 0.1% SO 3 additions. Humidification of the air stream to convert the SO3 to HzSOa does not affect any of these results. 20 SO2
Blon k
Addition
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FIC. 4. Profiles of soot volume fraction obtained for blank and SO2-seeded ethylene/air flame, C/ O ~ 0.76.
GASEOUS ADDITIVES ON FORMATION OF SOOT
Discussion
The critical C/O-ratio is known to be rather insensitive to temperature changes and dilution [3]. Therefore it is not surprising that the small Ng-additions in this work do not affect the critical C/O. The very slight decreases in soot loading observed in N~ addition may be the result of dilution, which is known to reduce the amount of soot formed at given C/O [3]. Apparently, H20 can also be treated as an inert diluent. Much higher (~10%) levels of H20 addition are required to influence the critical C/O ratio [8]. Like H20 and N~ addition of HC1 (up to 2%) did not effect the amount of soot. Before proceeding to discuss the action of the more effective additives, it is constructive to provide a scale by which to gauge the efficacy of these additives. The critical C/O-ratio for an unseeded ethylene/air flame is 0.600 +- 0.005. If part of the fuel supply is imagined to be an "additive," the critical C/O ratio of the remaining fuel-air mixture must decrease, as shown in Fig. 1 for "C" concentrations (as CgH4). Similarly as is also shown in Fig. 1, a curve for "O" additions (as O2) can be constructed as can curves for deficit carbon ( " - C ' ) and oxygen " - O." Beyond the critical C/O-ratio, the ultimate soot yield, f*, observed well downstream of the primary reaction zone, increases rapidly with increasing C/O-ratio [18], this relationship being correlated closely for ethylene flames as:
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Time [ms)
FIC. 5. Soot volume fraction (open symbols) and particle number density (closed symbols) profiles in blank and SO3-seeded ethylene/air flame, C/O 0.76.
1383
for 10-8 -< fv -< 10-6. The effect of additional, or decremental, fuel on the soot yield can thus be calculated, as shown in Fig. 2 (C/O = 0.76, f * 1.4 • 10-7) and Fig. 3 (C/O = 0.86, f * ~ 8 • 10-7) for the curves "C" and " - C . " Hydrogen can be expected to compete strongly for oxygen in the primary reaction zone. The addition of one H 2 has about the same effect as removal of one "O." In Fig. 1, the data for the critical C/O in the presence of up to 3% H 2 are well correlated by the line " - O " corresponding to removal of oxygen. In contrast to the success of the phenomenological equivalence "H2" = " - O " at the critical C/ O ratio, the virtual absence of effects due to hydrogen on the soot yield (Figs. 2, 3) is definitely at odds with the prediction. A soot-inhibiting effect of H2 has been observed in low-pressure flat flames [23] owing to reduced net diffusion of hydrogen into the unburned gas from the reaction zone--these two effects may to some extent be offsetting each other in the present case. Methane reduces the critical C/O-ratio slightly more strongly than the same amount of carbon as C2H4. On the basis of soot yield (Fig. 2) CH4 and C~H4 are indistinguishable. In these very rich flames, it can be expected [24] that recombination of methyl radicals to form C~-species will be the dominant route for CH 4 reaction, which may explain why changes brought about by perturbation due to CH4 and C2H4 additions are quantitively similar despite gross differences in their overall pure-fuel flame properties. In very fuel rich flames, NH 3 is converted practically quantitatively to HCN [11, 25] which, in the absence of oxidants, is practically inert. Thus we would expect a phenomenological equivalence "N" = " - C " to apply and inspection of Figs. 2, 3 and 4 confirms this to be approximately true. The relatively weak effects of CH3NHz are consistent with this picture in that they represent the cancellation of the effects of CH 4 and NH a. The influence of NO is unusual in that there appears to be a threshold concentration below which neither the critical C/O ratio nor the soot yields are affected. Some degree of conversion of NO to HCN can be expected as for NHa; the additional oxygen content of NO may also be significant in its anti-soot influence. Finally, NO is known [14] to catalyze the radical equilibrium H + OH ~ H~O and could cause significant alterations in the postflame conditions, depending on the state of the radicals in the unseeded flame. The most surprising result obtained in this work is that, in contrast to several earlier reports [4, 15, 17], SOa is found to inhibit soot formation and, indeed, strongly. In addition, we find no evidence of any attack of the fuel (benzene, ethylene, methane, or propane) in the burner chamber, such as
1384
SOOT AND PAH
was reported for benzene-containing fuels by Street and Thomas [4]. Thus the behaviour of SO3 in our experiments is simply consistent with a trend of increasing effectiveness with increasing oxygen content from H~S to SO2 to SO3. The mode of action of the sulphur-containing additives is not apparent. Clearly, they are far more effective than a phenomenological equivalence "S" = " - C " would predict, either at the critical C/O-ratio (Fig. 1) or beyond (Figs. 2 and 3), The possibility of some heterogeneous attack of sulphur species such as SOe on the soot or soot precursors is suggested by the observations of sulphur contained within soot samples, although the carbon in the observed yield of CS2 in seeded flames corresponds to somewhat more than is missing from the soot. Another possible mode of action for the sulphur additives lies in the ability of SO2 to catalyze the radical equilibrium and so perhaps to alter the post-flame conditions somewhat. The observation of the maximum in the soot loading in the presence of SO2 (assuming this not to be an artefact of changing absorption efficiency of the particles due to the presence of sulphur) is similar to that observed with additions of a few ppm of barium, a particularly efficient radical catalyst [20]. In fact, Cotton et al. [14] have already suggested that SOz, NO and Ba all tend to reduce soot formation by increasing the supply of OH radicals to consume soot. However, we do not observe a peak for NO additions, nor is one observed for H2S additions which produce flame SO2 concentrations similar to those produced by SOz itself. Here the observation that average temperatures in SOz-seeded flames are significantly higher than those in the blank, HzS-, and NO-seeded flames may be significant because higher temperatures of themselves reduce the soot yield [3, 19] and also allow greater equilibrium values of [OH] to be maintained by the catalyst,
monodisperse spheres [20, 27]. Allowance for the effects of a polydisperse particle size distribution on the light scattering results reduces the experimental values by a factor of approximately 2 [20, 27] while inclusion of the effects of dispersion forces on the particle collision cross-sections increases the theoretical values by a similar factor [27], with the net result being very close agreement between theory and experiment in all the blank and seeded flames studied. The failure of the gaseous additives to influence coagulation is to be contrasted with the ability of easily ionized metal additives such as Na, K, and Cs to inhibit strongly this process, even when present at concentrations below 1 ppm [20]. Clearly, the mode of action of the gaseous additives is not ionic. Whereas the metal additives do not appear to affect significantly either the rate or extent of the surface growth reactions which contribute most of the soot mass in these systems [19, 20], surface growth is definitely influenced by some of the gaseous additives. The kinetics of surface growth can be approximated as [19]. df ~-----~= k~g ( f * - f v) dt where f* is the ultimate soot yield and ks, is a rate constant which, in unseeded flames, depends only on temperature [19]. From Table II, none of the additives studied here (clearly the above expression is not applicable to the SO~ result) affects k,g significantly. Rather, their influence on surface growth is one of altering f * which, phenomenologically, is the amount of gas-phase material leaving the TABLE II Effects of gaseous additives on the soot coagulation rate constant and the phenomenological surface growth rate constant, ks~, in premixed ethylene/air flames, C/O - 0.76
Coagulation and Surface Growth The decrease in the number density of soot particles beyond the beginning of the soot-forming region (Fig. 5) is characteristic of all sooting flames [3, 18, 20, 21, 23, 26] and is a consequence of coagulation, i.e. the collision and sticking of particles. This phenomenon is not significantly affected by the additives used in this work. This can be seen clearly in Table II where the experimental coagulation rate constants for dN -
dt
keoagNz
are shown relative to the theoretical values for
Additive - -
Nz H20 H2 NH3 CH~NH~ NO H2S SO2 SO3
Concentration % - -
1.37 1.03 1.08 1.29 1.10 1.15 0.97 1.20 0.40
kc~g ktheo~ 4
5 3 4 4 2 2 5 5 5
k,a (sec-l) 60 60 60 60 60 60 60 70 -70
GASEOUS ADDITIVES ON FORMATION OF SOOT flame zone capable of being converted to soot. As we have seen, this quantity is a strong function of the effective C/O-ratio, additives such as CH 4 contributing one fuel-C per molecule and NH 3 removing one. The chemical effects of other additives NO, H2 and the sulphur species appear more complex but their net result is nevertheless one of altering fv* without influencing ksg. Conclusions
Gaseous additives containing C, H, O, N, and S do not influence the coagulation of soot particles when present in amounts up to a few percent by volume in the unburned gases. These additives can, however, modify the critical C/O-ratio and the soot yield in sooting flames. The influence of some additives can be understood in forms of simple chemical arguments. Thus: the conversion of NH 3 to HCN early in a flame makes the flame behave as if it were leaner by one C-atom for every N-atom added; on a carbon-atom basis, CH 4 promotes soot in ethylene flames just as effectively as additional C2H4; CH3NH 2 incorporates these opposing effects and has only moderate influence on soot formation. At the critical C / O-ratio, Hz addition is formally equivalent to the removal of 1/202 but this identity does not apply beyond the critical C/O-ratio where the yield of soot is largely unaffected by H2 additions. The most effective soot suppressors are the sulphur species, H2S < SO2 < SO3. The anti-soot tendencies of SO3, compared with the other sulphur compounds, are in keeping with its higher oxygen content.
Acknowledgments
4. STREET, J. C. AND THOMAS, A.: Fuel, 34, 4 (1955). Jost, W.: Naturwissenschaften, 31, 33 (1943). 5. WRIGHT, F. J.: Comb. Flame, 15, 217 (1970). 6. MCLINTOCK, I. S.: Comb. Flame, 12, 217 (1968). 7. SCHUG, K. P., MANHEIMER-TIMNAT,Y., YACCARINO, 1~ AND GLASSMAN,I.: Combust. Sci. Tech., 22, 235 (1980). 8. MULLER-DETHLEFS, K. AND SCHLADER, A. F., Comb. Flame, 27, 205 (1976). 9. BEHRENS, H.: Z. Phys. Chem., 196, 78 (1950). 10. HOMANN, K. H.: FVM Frankfurt, 327, 137 (1978). 11. RECK, R.: Dissertation, TH Darmstadt, 1977. 12. BEHRENS, H. AND ROSSLER, F.: Naturwissenschaften, 36, 218 (1949). 13. GAYDON,A. G. AND WOLFHARD, H. GG.: Proc. Roy. Soc. A, 201, 570 (1950). 14. COTTON, D. H., FRISWELL, N. J. AND JENKINS, D. R.: Comb. Flame, 17, 87 (1971). 15. GAYDON, A. G. AND WHITrlNGHAM, G.: Proc. Roy. Soc. A, 189, 313 (1947). 16. WOLFHARD, H. G. AND PARKER, W. G.: Fuel, 29, 235 (1950). 17. FENIMORE,C. P., JONES, G. W. AND MOORE, G. E.: Sixth Symposium (International) on Combustion, p. 242, Reinhold, New York, 1957. 18. HAYNES, B. S., JANOER, H. AND WAGNER, H. GG.: Ber. Bunsenges. Phys. Chem., 84, 585 (1980). 19. HAYNES, B. S. ANn WAGNER, H. GG.: "The surface Growth Phenomenon in Soot Formation," to appear. 20. HAYNES, B. S., JANDER, H. AND WAGNER, H. GG.: Seventeenth Symposium (International) on Combustion, p. 1365, The Combustion Institute, Pittsburgh, 1979. 21. D'ALESSIO,A., DILORENZO, A., SAROFIM,A. F., BERETrA, F., MASI, S. AND VENITOZZI, C.: Fif-
B.S.H. thanks the Alexander von Humboldt-Stiftung for the award of a stipend to carry out this work.
22.
REFERENCES 23. 1. PALMER, H. B. AND CULLIS, C. F.: Chemistry
and Physics of Carbon, Vol. 1 (Walker, P. L., Ed.), p. 265, Marcel Dekker, New York, 1965. 2. LAHAYE, J. ANn PRADO, G.: Chemistry and Physics of Carbon, Vol. 14 (Walker, P. L. and Thrower, P. A., ed.), p. 168, Marcel Dekker, New York, 1978. 3. HAYNES, B. S. AND WAGNER, H. GG.: Progr. Energy Combust. Sci., 7, 229.(1981).
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24.
25. 26. 27.
teenth Symposium (International) on Combustion, p. 1427, The Combustion Institute, Pittsburgh, 1975. GLASSMAN,I.: "Phenomenological Models of Soot Processes in Combustion Systems": Princeton University Dept. of Aerospace and Mechanical Engineering Report 1450, 1979. HOMANN, K. H., WAGNER, H. GG.: Ber. Bunsenges. Phys. Chem., 69, 20 (1965). WARNATZ, J.: Eighteenth Symposium (International) on Combustion, p. 369, The Combustion Institute, Pittsburgh, 1981. HAYNES, B. S.: Comb. Flame, 28, 113 (1977). GRAHAM, S. C., HOMER, J. B. AND ROSENFELD, J. L. J.: Proe. Roy. Soc. A, 344, 259 (1975). GRAHAM, S. C. ANn HOMER, J. B.: 8ymp. Faraday Soc., 7, 85 (1973).
THE INFLUENCE OF GASEOUS ADDITIVES ON THE FORMATION OF SOOT IN PREMIXED FLAMES B. S. HAYNES,t H. JANDER, H. MATZING, AND H. GG. WAGNER
Institut fur Physikalische Chemie Universitiit G6ttingen Federal Republic of Germany Laser-light scattering and absorption measurements of soot volume fractions and particle number densities in premixed ethylene-air flat flames at atmospheric pressure show that additions of up to 3%, by volume of the unburned gas, of N2, H20, Hz, NH3, CH~NH2, NO, H2S, SO2, SO~, HCI, CH 4, C~H4 or 02 do not influence the coagulation of soot particles although the soot volume fraction may be significantly affected. Pro-soot additives are CHA, C2H4 and CH~NH2 while additives which reduce the soot volume fraction are O2, NO, NH3, H2S, SO2 and SO3. Whatever their effects, these additives donot alter the specific surface growth rate for soot particle mass addition. The various additives retain their pro- or anti-soot tendencies when considered in terms of their effects on the critical C/O-ratio for the appearance of soot. Additions of H2 promote soot under these conditions. Special attention is paid to the effect of SO3--no evidence of any pre-soot effects at or beyond the critical C/O-ratio is found in aliphatic or aromatic flames. The mode of action of the additives is not generally understood, although the formal equivalence of the addition of one molecule of NH3 with removal of one fuel-C atom is consistent with the expected rapid formation of inert HCN in the flame and with the relatively weak effects of CH3NH2 addition.
Introduction Investigations of the effects of gaseous additives on soot formation have been widely reported--see, for example, [1-17]. Unfortunately, however, many of these reports have been concerned with the influence of the additive on the overall characteristics or general appearance of flames of often illdefined geometries. It is generally agreed that inert diluents such as N2 or Ar have little apparent effect other than a mild inhibition of the sooting tendency of both premixed and diffusion flames [4-7]. This is also the case for H20 and CO~ additions [4, 7, 8] although these additives are more effective than inerts added to the fuel side of a diffusion flame [6, 7]. Carbon monoxide, CO, promotes slightly the luminosity of flames [4]. The influence of Hz is complex in that additions of H 2 can alter completely the structure of some flames--for example, converting a polyhedral benzene-type flame to one of the acetylene type [9, 10]. Chemically, added hydrogen can be expected to compete for the availtCurrent address: CSIRO Division of Fossil Fuels, North Ryde, N.S.W. Australia.
able oxygen, merely making the flame richer and promoting the formation of soot [4]. Some other additives have more pronounced effectsl For example, ammonia added in concentrations of about 10% to low-pressure premixed flames completely suppresses soot formation [11]. Nitric oxide is reported to de-stabilize benzene flames [12] and weakly to inhibit soot formation in these and other premixed [13] and diffusion flames [14]. Additions of sulphur as H2S or SO2 apparently inhibit soot formation in both premixed [4, 15] and diffusion [7, 14, 16] flames, the reported magnitude of the effect varying from weak t o strong, depending on conditions. Sulphur trioxide, SO3, is reported to be an outstanding soot promoter at concentrations as low as 0.1% but its influence in diffusion flames is slight [16]. The objective of the work reported in this paper is to examine the influence of various gaseous additives on the overall process of soot formation in premixed flames. This process we view [3, 18, 19] phenomenologically as occurring in two stages: the formation of the first particles; and their subsequent growth by the effectively independent processes of (i) surface growth, which accounts for most of the soot mass; and (ii) coagulation, which brings about a reduction in number density as par-
1379
1380
SOOT AND PAH
ticles collide and fuse. In earlier work [20], we showed how traces (<1 ppm) of ionizing metal additives can drastically inhibit coagulation without much affecting the soot yield. By employing the same laser light-scattering and extinction techniques to measure soot volume fraction, particle size and number density in flames with various gaseous additives, the influence of these additives is found, by contrast, to lie in an inhibition of surface growth, coagulation rates remaining unaffected. Experimental
Procedure
Soot volume fraction f~, mean particle size d, and particle number density N are determined by laser light scattering and extinction measurements on flat, premixed flames of ethylene, or benzene, and air at atmospheric pressure. The theoretical bases for the measurements and the interpretation of results together with details of the experimental arrangement are described elsewhere [18, 20, 21]. Gaseous additives drawn from cylinders via calibrated rotameters are introduced into the fuel-air mixture well upstream of the burner. For addition of SO3, an argon stream is saturated in SO3 at known temperature and pressure by passage through a bath of oleum (65% SO3). The dilution effects caused by the argon are negligible. The amount of SO3 in the gases is determined by dissolving in water the SO3 from a known volume of main gas and titrating. Additions of H20 are made similarly by saturating part of the air supply with water vapour. The critical C/O ratios at the "soot limit" are determined visually in the darkened laboratory. The reproducibility of the measurements is typically better than 1% although this may be as poor as 2-3% in cases where additives colour the flame. For example, in the presence of sulphur species, a bluish-white emission makes determination of the critical C/O ratio uncertain beyond 1.5% of additive (HaS or SO2). For semi-quantitative gas analyses, samples are taken with a water-cooled quartz probe. The samples are drawn into a previously evacuated flask and analyzed mass-spectrometrically (Varian MAT). By this procedure an approximate survey of the stable sulphur-containing species present in the postflame gases of flames seeded with H2S or SOz is obtained. Measurements of the hydrocarbons will be reported and discussed elsewhere. Soot samples for elemental analysis are collected with a water-cooled quartz probe system maintained at about 0.I torr. The flame gases pass through a 0.3 mm diameter orifice and the soot particles are impinged onto an uncooled substrate of either stainless steel or quartz.
Flame temperatures are determined using the Kurlbaum method. The characteristic flame temperature is taken as that corresponding to the point at which the soot loading reaches half its ultimate value [19].
Results
(i) Nitrogen, Ne Dilution of the unburned gases with up to 1.5% N2 has no measurable effect on the critical C/Oratio for soot formation (Fig. 1). The soot yield at C/O = 0.76 is marginally diminished by this additive (Fig. 2) but the particle number density profiles throughout the flame are unaffected. No detectable temperature change is induced by these relatively low levels of N2-addition. (ii) Water Vapour, H20 No detectable changes in critical C/O-ratio, soot yield, particle number density profiles or flame temperature are brought about by additions of up to 1% HzO (Figs. 1 and 2).
o
S02
O3O / ~H2S
/
NH3 .. /
/" "C"
9-- 0.65
i
o
.... .o 0,60 ~"-1"'_ .2o ~
2'0" as O2
.N2 CH3Nfl;~
g. i
-
O.SS
"r-
O,SO
I 1
1 2
I 3
Additive Concentration ( mo[%) FIG. 1. Influence of various gaseous additives on the critical C/O-ratio for the onset of sooting in an ethylene/air flame, T - 1800 K. The additive concentration refers to the unburned gases. The curves for "C," "O," " - C , " and " - O " are calculated on the basis of notional "additions" of fuel and oxygen.
GASEOUS ADDITIVES ON FORMATION OF SOOT
CH4 " iI
/ / 9' / ' "-~
2.0
I / c
.o
// C as C2H~
I
/ / ~, /
1.5
/ r
I
/' /
_ CH3NH2
1381
The particle number density profiles in ammoniaseeded sooting flames are similar to those in the corresponding blank although the downstream number density is typically 1 to 2 times that in the blank flame. This effect can be ascribed in part to a slight, but definite, inhibiting effect of NH 3 in these flames so that the flame zone, and the appearance of the first soot, occurs further above the burner in the presence of the additive. In keeping with this observation, the temperatures in the NHaseeded flame of Fig. 3 are 10 to 20 K higher than those in the blank. (vi) Methylamine, CH3NH~ Methylamine additions tend to inhibit the flame in the same way as ammonia and also give rise to slightly higher final number densities. However, the soot loading is slightly increased (Figs. 2 and 3) and the critical C/O-ratio slightly reduced in the presence of this additive.
o e.
0.5
+ NH3
-- :. -'c 0
l
I 2
Additive Concentration (mo[%)
FIG. 2. Influence of gaseous additives on the downstream (t ~ 30 msee) soot yield in an ethylene/ air flame, C/O ~ 0.76, f~ - 1.5 • 10-7. The curves for 'C' etc refer to notional addition or removal of fuel or oxygen.
(vii) Nitric Oxide, NO Additions of NO corresponding to less than about 0.5% of the total flow have no apparent effects on either the critical C/O ratio (Fig. 1) or the soot yield in richer flames (Figs. 2 and 3). At concentrations beyond 0.5%, however, NO rapidly becomes effective in opposing soot formation as evi-
/
(iii) Hydrogen, H 2 Additions of hydrogen appear to promote the onset of soot formation in that they reduce the critical C/O-ratio (Fig. 1). Despite this influence, the actual soot yield in richer flames is not significantly affected by additions of up to 3% H2 to the unburned gases (Figs. 2 and 3). The soot volume fraction and particle number density profiles throughout H2-seeded flames are indistinguishable from those in the corresponding unseeded flames.
I.S
/ /
Addition of ammonia increases the critical C/O ratio for the onset of sooting (Fig. 1) and reduces the soot loading in richer mixtures (Figs. 2 and 3).
/
/ / /
J o
H2
1.0
O
o'I
0.5 0J
H2
O[
(v) Ammonia, NH 3
/
CH3NH2
/
(iv) Methane, CH4 Methane additions, not surprisingly, encourage the onset of soot formation in leaner mixtures (Fig. 1) and enhance the formation of soot in already sooting flames (Fig. 2).
/ "'C" ns C2H4
/
0
t I
~ 2"C-.
i
2 Additive Concentration (mo[ %)
3
FIG. 3. As Fig. 2 except for ethylene/air flame, C/O - 0.86, f~ - 8 x 10-7.
1382
SOOT AND PAH
TABLE I Concentrations (mol. fraction) of SO~ and CS2 in the post-flame gases of premixed ethylene/air flames. Semi-quantitative gas analyses. (C/O = 0.76) seeded with H2S or SOz H2S Addition X,.tota] = 0.009 Time (msec) Xso2 Xcs2 8 26
0.002 0.001
0.002 0.004
SO~ Addition X,_tota] = 0.01 Xsoz Xcs~ 0.002 0.002
0.003 O.004
(Fig. 4) however, reveals an effect not seen with HzS, nor with any other gaseous additive used here--:-at higher levels of SO2 addition, the soot loading goes through a maximum downstream of the flame zone. At higher soot loadings, no such maximum is observed for up to 1.5% SO2 addition. As in the case of H~S additions, the dominant gaseous sulphur forms found are SO2 and CS 2 (Table I) although these species now only account for slightly more than 50% of the total added sulphur. Temperatures are markedly influenced by SO2 additions--l% SO 2 raises the average temperature of the C/O = 0.76 flame by about 40 K. (x) Sulphur Trioxide, SO2
denced by increases in the critical C/O-ratio (Fig. 1) and decreases in soot loading (Fig. 2). The partide number density profiles and flame temperature are not significantly affected by this additive. (viii) Hydrogen Sulphide, H2S Hydrogen sulphide inhibits strongly the formation of soot, both in allowing richer mixtures to be burnt without the onset of soot formation (Fig. 1) and in reducing significantly the yield of soot in sooting flames (Figs. 2 and 3). Temperatures in H~S-seeded flames are up to 10 K (0.5% H2S) higher than those in the corresponding blanks. An approximate distribution of the sulphur in the flame is given by mass-spectrometric analyses of batch samples withdrawn at various heights in the flame (Table 1). The dominant sulphur forms in the gaseous sample are SO 2 and CSz (50 to 80% of the total added sulphur). Traces (<1% of the total) of COS, SO3 or H2S are detected in the flame gas samples. For soot samples collected well downstream (t 26 msec) in the C/O = 0.76 ethylene/air flame seeded with 1.5% HzS, elemental analysis indicates 4,2 percent S by weight of the soot deposited; for 0.88% H~S addition this figure becomes 3.7%. These values, which are not affected by tempering of the samples at 100~ C for one hour, correspond to an atomic S/C ratio in the soot of approximately 0.015 so that less than 1% of the total sulphur is present in the soot. Soot samples collected on water-cooled substrates contain approximately four times as much sulphur as those collected on uncooled substrates.
(ix) Sulphur Dioxide, SO2 In terms of its ability to increase the critical C / O-ratio (Fig. 1) and reduce the downstream soot yield (Fig. 2), SO2 behaves very like, but more effectively than, H2S. A closer inspection of the soot volume fraction profiles in SO~-seeded flames
Additions of SO3 reduce the soot yield in sooting ethylene flames (Figs. 2 and 5) with an efficiency somewhat greater than that exhibited by SO 2 additions. Despite large reductions in the soot volume fraction brought about by SO3 additions, the particle number density profiles in the blank and seeded flames are indistinguishable. The same effects are observed in benzene/air flames. I n ethylene and propane flat flames operating just leaner than the critical C/O-ratio, additions of SO3 from 0.05 to 0.4% are incapable of making the flame soot. No visible changes in yellow-tipped bunsen burner flames of propane or methane are observed with 0.1% SO 3 additions. Humidification of the air stream to convert the SO3 to HzSOa does not affect any of these results. 20 SO2
Blon k
Addition
eme e o e ~o
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>
5
0 0
I
i
10
20 Time
I 30
(ms)
FIC. 4. Profiles of soot volume fraction obtained for blank and SO2-seeded ethylene/air flame, C/ O ~ 0.76.
GASEOUS ADDITIVES ON FORMATION OF SOOT
Discussion
The critical C/O-ratio is known to be rather insensitive to temperature changes and dilution [3]. Therefore it is not surprising that the small Ng-additions in this work do not affect the critical C/O. The very slight decreases in soot loading observed in N~ addition may be the result of dilution, which is known to reduce the amount of soot formed at given C/O [3]. Apparently, H20 can also be treated as an inert diluent. Much higher (~10%) levels of H20 addition are required to influence the critical C/O ratio [8]. Like H20 and N~ addition of HC1 (up to 2%) did not effect the amount of soot. Before proceeding to discuss the action of the more effective additives, it is constructive to provide a scale by which to gauge the efficacy of these additives. The critical C/O-ratio for an unseeded ethylene/air flame is 0.600 +- 0.005. If part of the fuel supply is imagined to be an "additive," the critical C/O ratio of the remaining fuel-air mixture must decrease, as shown in Fig. 1 for "C" concentrations (as CgH4). Similarly as is also shown in Fig. 1, a curve for "O" additions (as O2) can be constructed as can curves for deficit carbon ( " - C ' ) and oxygen " - O." Beyond the critical C/O-ratio, the ultimate soot yield, f*, observed well downstream of the primary reaction zone, increases rapidly with increasing C/O-ratio [18], this relationship being correlated closely for ethylene flames as:
oeBIonk
,
VV. 0 . 2 % SO 3 9
15
txA* 0.4%
SO 3
1012 r~ o
,t >
A-~cm-
oO oo
."
~- 10
,
o
5
9
o
oo
o 9I o el
,"
ovVV
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o
.!!
|
m
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~176176
e~
_=
.=
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g
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~,,q|;-
vvv
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o V Ate& tx&~
~a a 0 0
i 10
I 20
I 30
10g 40
Time [ms)
FIC. 5. Soot volume fraction (open symbols) and particle number density (closed symbols) profiles in blank and SO3-seeded ethylene/air flame, C/O 0.76.
1383
for 10-8 -< fv -< 10-6. The effect of additional, or decremental, fuel on the soot yield can thus be calculated, as shown in Fig. 2 (C/O = 0.76, f * 1.4 • 10-7) and Fig. 3 (C/O = 0.86, f * ~ 8 • 10-7) for the curves "C" and " - C . " Hydrogen can be expected to compete strongly for oxygen in the primary reaction zone. The addition of one H 2 has about the same effect as removal of one "O." In Fig. 1, the data for the critical C/O in the presence of up to 3% H 2 are well correlated by the line " - O " corresponding to removal of oxygen. In contrast to the success of the phenomenological equivalence "H2" = " - O " at the critical C/ O ratio, the virtual absence of effects due to hydrogen on the soot yield (Figs. 2, 3) is definitely at odds with the prediction. A soot-inhibiting effect of H2 has been observed in low-pressure flat flames [23] owing to reduced net diffusion of hydrogen into the unburned gas from the reaction zone--these two effects may to some extent be offsetting each other in the present case. Methane reduces the critical C/O-ratio slightly more strongly than the same amount of carbon as C2H4. On the basis of soot yield (Fig. 2) CH4 and C~H4 are indistinguishable. In these very rich flames, it can be expected [24] that recombination of methyl radicals to form C~-species will be the dominant route for CH 4 reaction, which may explain why changes brought about by perturbation due to CH4 and C2H4 additions are quantitively similar despite gross differences in their overall pure-fuel flame properties. In very fuel rich flames, NH 3 is converted practically quantitatively to HCN [11, 25] which, in the absence of oxidants, is practically inert. Thus we would expect a phenomenological equivalence "N" = " - C " to apply and inspection of Figs. 2, 3 and 4 confirms this to be approximately true. The relatively weak effects of CH3NHz are consistent with this picture in that they represent the cancellation of the effects of CH 4 and NH a. The influence of NO is unusual in that there appears to be a threshold concentration below which neither the critical C/O ratio nor the soot yields are affected. Some degree of conversion of NO to HCN can be expected as for NHa; the additional oxygen content of NO may also be significant in its anti-soot influence. Finally, NO is known [14] to catalyze the radical equilibrium H + OH ~ H~O and could cause significant alterations in the postflame conditions, depending on the state of the radicals in the unseeded flame. The most surprising result obtained in this work is that, in contrast to several earlier reports [4, 15, 17], SOa is found to inhibit soot formation and, indeed, strongly. In addition, we find no evidence of any attack of the fuel (benzene, ethylene, methane, or propane) in the burner chamber, such as
1384
SOOT AND PAH
was reported for benzene-containing fuels by Street and Thomas [4]. Thus the behaviour of SO3 in our experiments is simply consistent with a trend of increasing effectiveness with increasing oxygen content from H~S to SO2 to SO3. The mode of action of the sulphur-containing additives is not apparent. Clearly, they are far more effective than a phenomenological equivalence "S" = " - C " would predict, either at the critical C/O-ratio (Fig. 1) or beyond (Figs. 2 and 3), The possibility of some heterogeneous attack of sulphur species such as SOe on the soot or soot precursors is suggested by the observations of sulphur contained within soot samples, although the carbon in the observed yield of CS2 in seeded flames corresponds to somewhat more than is missing from the soot. Another possible mode of action for the sulphur additives lies in the ability of SO2 to catalyze the radical equilibrium and so perhaps to alter the post-flame conditions somewhat. The observation of the maximum in the soot loading in the presence of SO2 (assuming this not to be an artefact of changing absorption efficiency of the particles due to the presence of sulphur) is similar to that observed with additions of a few ppm of barium, a particularly efficient radical catalyst [20]. In fact, Cotton et al. [14] have already suggested that SOz, NO and Ba all tend to reduce soot formation by increasing the supply of OH radicals to consume soot. However, we do not observe a peak for NO additions, nor is one observed for H2S additions which produce flame SO2 concentrations similar to those produced by SOz itself. Here the observation that average temperatures in SOz-seeded flames are significantly higher than those in the blank, HzS-, and NO-seeded flames may be significant because higher temperatures of themselves reduce the soot yield [3, 19] and also allow greater equilibrium values of [OH] to be maintained by the catalyst,
monodisperse spheres [20, 27]. Allowance for the effects of a polydisperse particle size distribution on the light scattering results reduces the experimental values by a factor of approximately 2 [20, 27] while inclusion of the effects of dispersion forces on the particle collision cross-sections increases the theoretical values by a similar factor [27], with the net result being very close agreement between theory and experiment in all the blank and seeded flames studied. The failure of the gaseous additives to influence coagulation is to be contrasted with the ability of easily ionized metal additives such as Na, K, and Cs to inhibit strongly this process, even when present at concentrations below 1 ppm [20]. Clearly, the mode of action of the gaseous additives is not ionic. Whereas the metal additives do not appear to affect significantly either the rate or extent of the surface growth reactions which contribute most of the soot mass in these systems [19, 20], surface growth is definitely influenced by some of the gaseous additives. The kinetics of surface growth can be approximated as [19]. df ~-----~= k~g ( f * - f v) dt where f* is the ultimate soot yield and ks, is a rate constant which, in unseeded flames, depends only on temperature [19]. From Table II, none of the additives studied here (clearly the above expression is not applicable to the SO~ result) affects k,g significantly. Rather, their influence on surface growth is one of altering f * which, phenomenologically, is the amount of gas-phase material leaving the TABLE II Effects of gaseous additives on the soot coagulation rate constant and the phenomenological surface growth rate constant, ks~, in premixed ethylene/air flames, C/O - 0.76
Coagulation and Surface Growth The decrease in the number density of soot particles beyond the beginning of the soot-forming region (Fig. 5) is characteristic of all sooting flames [3, 18, 20, 21, 23, 26] and is a consequence of coagulation, i.e. the collision and sticking of particles. This phenomenon is not significantly affected by the additives used in this work. This can be seen clearly in Table II where the experimental coagulation rate constants for dN -
dt
keoagNz
are shown relative to the theoretical values for
Additive - -
Nz H20 H2 NH3 CH~NH~ NO H2S SO2 SO3
Concentration % - -
1.37 1.03 1.08 1.29 1.10 1.15 0.97 1.20 0.40
kc~g ktheo~ 4
5 3 4 4 2 2 5 5 5
k,a (sec-l) 60 60 60 60 60 60 60 70 -70
GASEOUS ADDITIVES ON FORMATION OF SOOT flame zone capable of being converted to soot. As we have seen, this quantity is a strong function of the effective C/O-ratio, additives such as CH 4 contributing one fuel-C per molecule and NH 3 removing one. The chemical effects of other additives NO, H2 and the sulphur species appear more complex but their net result is nevertheless one of altering fv* without influencing ksg. Conclusions
Gaseous additives containing C, H, O, N, and S do not influence the coagulation of soot particles when present in amounts up to a few percent by volume in the unburned gases. These additives can, however, modify the critical C/O-ratio and the soot yield in sooting flames. The influence of some additives can be understood in forms of simple chemical arguments. Thus: the conversion of NH 3 to HCN early in a flame makes the flame behave as if it were leaner by one C-atom for every N-atom added; on a carbon-atom basis, CH 4 promotes soot in ethylene flames just as effectively as additional C2H4; CH3NH 2 incorporates these opposing effects and has only moderate influence on soot formation. At the critical C / O-ratio, Hz addition is formally equivalent to the removal of 1/202 but this identity does not apply beyond the critical C/O-ratio where the yield of soot is largely unaffected by H2 additions. The most effective soot suppressors are the sulphur species, H2S < SO2 < SO3. The anti-soot tendencies of SO3, compared with the other sulphur compounds, are in keeping with its higher oxygen content.
Acknowledgments
4. STREET, J. C. AND THOMAS, A.: Fuel, 34, 4 (1955). Jost, W.: Naturwissenschaften, 31, 33 (1943). 5. WRIGHT, F. J.: Comb. Flame, 15, 217 (1970). 6. MCLINTOCK, I. S.: Comb. Flame, 12, 217 (1968). 7. SCHUG, K. P., MANHEIMER-TIMNAT,Y., YACCARINO, 1~ AND GLASSMAN,I.: Combust. Sci. Tech., 22, 235 (1980). 8. MULLER-DETHLEFS, K. AND SCHLADER, A. F., Comb. Flame, 27, 205 (1976). 9. BEHRENS, H.: Z. Phys. Chem., 196, 78 (1950). 10. HOMANN, K. H.: FVM Frankfurt, 327, 137 (1978). 11. RECK, R.: Dissertation, TH Darmstadt, 1977. 12. BEHRENS, H. AND ROSSLER, F.: Naturwissenschaften, 36, 218 (1949). 13. GAYDON,A. G. AND WOLFHARD, H. GG.: Proc. Roy. Soc. A, 201, 570 (1950). 14. COTTON, D. H., FRISWELL, N. J. AND JENKINS, D. R.: Comb. Flame, 17, 87 (1971). 15. GAYDON, A. G. AND WHITrlNGHAM, G.: Proc. Roy. Soc. A, 189, 313 (1947). 16. WOLFHARD, H. G. AND PARKER, W. G.: Fuel, 29, 235 (1950). 17. FENIMORE,C. P., JONES, G. W. AND MOORE, G. E.: Sixth Symposium (International) on Combustion, p. 242, Reinhold, New York, 1957. 18. HAYNES, B. S., JANOER, H. AND WAGNER, H. GG.: Ber. Bunsenges. Phys. Chem., 84, 585 (1980). 19. HAYNES, B. S. ANn WAGNER, H. GG.: "The surface Growth Phenomenon in Soot Formation," to appear. 20. HAYNES, B. S., JANDER, H. AND WAGNER, H. GG.: Seventeenth Symposium (International) on Combustion, p. 1365, The Combustion Institute, Pittsburgh, 1979. 21. D'ALESSIO,A., DILORENZO, A., SAROFIM,A. F., BERETrA, F., MASI, S. AND VENITOZZI, C.: Fif-
B.S.H. thanks the Alexander von Humboldt-Stiftung for the award of a stipend to carry out this work.
22.
REFERENCES 23. 1. PALMER, H. B. AND CULLIS, C. F.: Chemistry
and Physics of Carbon, Vol. 1 (Walker, P. L., Ed.), p. 265, Marcel Dekker, New York, 1965. 2. LAHAYE, J. ANn PRADO, G.: Chemistry and Physics of Carbon, Vol. 14 (Walker, P. L. and Thrower, P. A., ed.), p. 168, Marcel Dekker, New York, 1978. 3. HAYNES, B. S. AND WAGNER, H. GG.: Progr. Energy Combust. Sci., 7, 229.(1981).
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24.
25. 26. 27.
teenth Symposium (International) on Combustion, p. 1427, The Combustion Institute, Pittsburgh, 1975. GLASSMAN,I.: "Phenomenological Models of Soot Processes in Combustion Systems": Princeton University Dept. of Aerospace and Mechanical Engineering Report 1450, 1979. HOMANN, K. H., WAGNER, H. GG.: Ber. Bunsenges. Phys. Chem., 69, 20 (1965). WARNATZ, J.: Eighteenth Symposium (International) on Combustion, p. 369, The Combustion Institute, Pittsburgh, 1981. HAYNES, B. S.: Comb. Flame, 28, 113 (1977). GRAHAM, S. C., HOMER, J. B. AND ROSENFELD, J. L. J.: Proe. Roy. Soc. A, 344, 259 (1975). GRAHAM, S. C. ANn HOMER, J. B.: 8ymp. Faraday Soc., 7, 85 (1973).