462
Energy & Fuels 1988,2,462-480
Soot Formation in Binary Hydrocarbon Mixturesf M. Frenklach,* T. Yuan, and M. K. Ramachandra Fuel Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received September 29, 1987. Revised Manuscript Received January 30, 1988
Soot formation in hydrogen-, allene-, and vinylacetylene-acetylene and acetylene-, allene-, vinylacetylene-, and 1,3-butadienebenzene argon-diluted mixtures was studied behind reflected shock waves by monitoring the attenuation of a 632.8-nm He-Ne laser beam. The experiments were performed at temperatures of 1500-2490 K, pressures of 1.3-3.1 bar, and total carbon atom concentrations of (2.0-6.8) X 1017atoms/cm3. The addition of molecular hydrogen to acetylene resulted in the decrease of soot production. Synergistic effects on soot formation were observed in the binary hydrocarbon mixtures. The experimental observations were interpreted by using detailed chemical kinetic modeling. The reaction mechanism used in the simulations was composed of 522 reversible reactions and 180 species. The computational results revealed that the mixture effects are caused primarily by the acceleration of acetylene-addition reactions.
Introduction effects by mixing benzene and 1-hexeneon soot formation in a laminar diffusion flame. Haynes et al.2and Harris and Interest in soot formation in binary hydrocarbon mixtures has increased in recent years.l-13 Besides the very (1)Haynes, B. S.; Wagner, H. G. Prog. Energy Combust. Sci.1981, 7 , practical aspects of such knowledge, the subject is of in229-273. terest from a fundamental point of view: to provide ad(2) Haynes, B. S.; Jander, H.; Matzing,H.; Wagner, H. G. Symp. (Znt.) Combust. [Proc.] 1983,19th, 1379-1385. ditional information for the elucidation of the soot for(3) Kent, J. H.; Wagner, H. Gg. Symp. (Znt.)Combust. [Proc.] 1985, mation mechanism. There is growing evidence, both ex20th, 1007-1015. perimenta111J4-21and t h e o r e t i ~ a l , ~that ~ * ~ the ~ - ~key ~ (4) Prado, G.; Lahaye, J. In Particulate Carbon: Formation During Combustion; Siegla, D. C., Smith, G. W., Eds.; Plenum: New York, 1983; chemical reactions leading to soot formation in hydropp 143-164. carbon systems are those between aromatic and acetylenic (5) Gill, R. J.: Olson, D. B. Combust. Sci. Technol. 1984,40.307-315. species. ( 6 ) Madronich, S. Combuat. Sci. Technol. 1985,42, 207-210. (7) Harris,S. J; Weiner, A. M. Combust. Sci. Technol. 1984,38,75-87. The importance of such reactions was suggested as early (8) Harris, S. J.; Weiner, A. M. Symp. (Znt.)Combust. [Proc.] 1985, as 1960 by Stehling et a1.,% who concluded that ‘the aro20th, 969-978. matic ring is evidently reacting with the acetylene or active (9) Colket, M. B., 111. Presented at the Eastern States Section of the Combustion Institute Meeting, Clearwater Beach, FL, December 3-5, radicals formed from it”. These authors pyrolyzed a series 1984. of hydrocarbons and their mixtures in a flow reactor at (10) Sidebotham, G. W.; Glassman, I. Presented at the Eastern States temperatures of 673-1173 K and reported that the pyroSection of the Combustion Institute Meeting, Clearwater Beach, FL, December 3-5, 1984. lysis of aromatic hydrocarbons, as judged by the rates of (11)Bauer, S. H.; Zhang, L. M. Proc. Int. Symp. . . Shock Tubes Waues formation of major products and disappearance of initial 1983; 14th, 654-661. reactants, is accelerated with the presence of acetylene. (12) Bauer, S. H.; Jeffers, P. M. Presented at the Symposium on Similar synergism was obtained for an acetylene-vinylAdvances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 31-September 4, 1987. acetylene mixture, including an observation that ‘the ex(13) Santoro, R. J. Presented at the Symposium on Advances in Soot haust from the reactor was brown when the mixture was Chemistry, 194th National Meeting of the American Chemical Society, used-indicating solid formation-as contrasted to a white New Orleans, LA, August 31-September 4, 1987. (14) Bittner, J. D.; Howard, J. B. Symp. (Znt.)Combust. [Proc.] 1981, fog of liquid for the individual reagents.” Scully and 18th, 1105-1116. DavieP reported a reduction in the yield of carbon black (15) Bittner, J. D.; Howard, J. B. In Soot in Combustion Systems and produced in flow-tube pyrolysis of benzene with the adIts Toric Properties; Lahaye, J., Prado, G., Eds.; Plenum: New York, 1983; pp 57-93. dition of cyclohexane. A reduction in soot yield was also (16) Bittner, J. D.; Howard, J. B.; Palmer, H. B. In Soot in Combusobserved in a recent study of shock-tube pyrolysis of tion Systems and Its Toric Properties; Lahaye, J., Prado, G., Eds.; chlorobenzene-acetylene mixtures.20 Prado and Lahaye4 Plenum: New York, 1983; pp 95-126. (17) Cole, J. A.; Bittner, J. D.; Longwell, J. P.; Howard, J. B. Combust. found that the soot volume fraction obtained in flow-tube 1984,56, 51-70. Flame pyrolysis of benzene-methane mixtures was strictly cu(18) Frenklach, M.; Taki, S.; Matula, R. A. Combust. Flame 1983,49, mulative, but the particle number density was not. The 275-282. observed additivity of individual topological i n d e x e ~ ~ * ~ $ * ~ (19) Frenklach, M.; Taki, S.;Durgaprasad, M. B.; Matula, R. A. Combust. Flame 1983,54, 81-101. implies the lack of synergism. In one case, Gill and Olson5 (20) Frenklach, M.; Ramachandra, M. K.; Matula, R. A. Symp. (Int.) reported a nonlinear dependence of the threshold sooting Combust. [Proc.] 1985, 20th, 871-878. index in premixed flames upon the fraction of fuel com(21) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 1067-1073. ponents; Madronich? however, described these results by (22) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S. E. nonlinear mixing rules assuming that the fuel components Symp. (Znt.) Combust. [Proc.] 1985,20th, 887-901. participate in the sooting process independently. Side(23) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S. E. Proc. Znt. Symp. Shock Waues Shock Tubes 1986, 15th, 295-301. botham and Glassmanlo found no chemical synergistic Presented at the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society. New Orleans, LA, August 31-September 4, 1987.
(24) Frenklach, M.; Warnatz, J. Combust. Sci. Technol. 1987, 51, 265-283. (25) Frenklach, M.; Clary, D. W.; Gardiner, W. C., Jr.; Stein, S. E. Symp. (Znt.) Combust. [Proc.] 1988,21st, 1067-1076. (26) Harris, S. J.; Weiner, A. M.; Blint, R. J. Combust. Flame 1988, 72, 91-109.
0887-062418812502-0462$0l.50/ 0 0 1988 American Chemical Society
Energy & Fuels, Vol. 2, No. 4, 1988 463
Soot Formation in Binary Hydrocarbon Mixtures
Table I. Summary of Experimental Conditions behind Reflected Shock Waves composition,
series A
B C
D E F
G H
I
J
% vol
in argon
4.65% CZHZ + 4.65% H2 0.73% C3H4 + 1.09% CzHz 0.54% C4H6 + 1.09% CzHz 0.31% C&3 + 0.54% CzHz 0.31% C& + 1.09% CzHz 0.31% C&3 + 2.72% CzHz 0.31% C&& + 0.54% C3H4 0.31% + 0.73% C3H4 0.31% C6He + 0.54% C4H4 0.31% C&3 + 0.54% C4Hs
10-17[carbon], atoms/cm3
T,K
P, bar
1744-2427
1.30-1.80
4.95-5.09
1533-2298 2.01-2.92
3.98-4.15
1631-2340
2.12-3.14
3.99-4.53
1571-2300
2.03-2.91
2.58-2.76
1471-2237
1.92-2.82
3.66-3.84
1593-2427
2.04-3.06
6.58-6.84
1510-2369
1.91-2.97
3.16-3.23
1506-2445
1.93-3.03
3.63-3.75
1568-2488
2.03-3.14
3.66-3.85
1537-2433
1.98-3.07
3.67-3.76
Weiner7p8determined that equal small increments of carbon introduced as methane, ethylene, or toluene in sooting premixed ethylene flames gave identical increments of soot volume fraction; however, methane was found2 to reduce the critical C/O ratio more effectively than ethylene. Thus, the experimental data on soot formation in binary hydrocarbon mixtures are rather scarce and conflicting, particularly with regard to the presence or absence of chemical synergism among the individual fuels. The objective of the present work was to investigate the formation of soot in a series of hydrocarbon mixtures under the conditions of shock-tube pyrolysis, i.e., under the conditions free of transport phenomena that may mask the chemical processes in flame environment. The results obtained in benzene-additive-where the additives were acetylene, allene, vinylacetylene, and 1,3-butadiene-and allene-acetylene, butadiene-acetylene, and acetylene-hydrogen mixtures are reported. A computer model, developed previously to explain the formation of polycyclic aromatic hydrocarbons (PAH) from the individual evencarbon-atom is applied to analyze the present results in acetylene-hydrogen, 1,3-butadiene-acetylene, benzene-acetylene, benzene-vinylacetylene, and benzene-l,&butadiene mixtures.
Experimental Section The experimental apparatusand procedures used in this study were similar to those described Briefly, the experiments were conducted behind reflected shock waves in a 7.62 cm i.d. stainless-steelshock tube. The test gas mixtures were prepared manometrically. The stated purities of the gases were as follows: argon, 99.998% (prepurifiedgrade, Matheson); hydrogen, 99.99% (Zero Gas grade, Matheson); acetylene, 99.6% (purified grade, Matheson); allene, 97% (Matheson); 1,3-buta(27) Colket, M. B.,111. Presented at the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orlenas, LA, August 31-September 4, 1987. (28) Stehling, F. C.; Frazee, J. D.; Anderson, R. C. Symp. (Znt.) Combust. [Proc.] 1962,8th, 774-784. (29) Scully, D. B.; Davies, R. A. Combust. Flame 1965, 9, 185-191. (30) Calcote, H. F.; Manos, D. M. Combust. Flame 1983,49,289-304. (31) Takahashi,F.; Glassman, I. Combust. Sci. Technol. 1984,37,1-19. (32) Harris, M. M.; King, G. B.; Laurendeau, N. M. Combust. Flame 1986,64,99-112; 1987,67,269-272. Takahashi, F.; Glassman, I. Combut. Flame 1987, 67, 267-268. (33) Hanson, M. P.; Rouvray, D. H. J. Phys. Chem. 1987, 91, 2981-2985. (34) Stournas, S.;Lois, E. Presented at the Symposium on Advances in Soot Chemistry, 194th National Meeting of the American Chemical Society, New Orleans, LA, August 31-September 4, 1987.
diene, 99% (CP grade, Matheson). These gases were used without further purification. The benzene (Spectranalyzed grade, Fisher) was purified by repeated freezing and evacuation. The vinylacetylene (technicalgrade, Wiley Organics)was supplied as a 50% solution by weight in xylene. From this solution vinylacetylene was distilled at 0 “C and was found by gas chromatographic analysis to contain less than 0.5% xylene. The shock tube was cleaned after every run. Ten different mixtures were tested during the course of this study. The experimental conditions, chosen so as to allow comparison with the previous results for the pyrolysis of individual fuel^,^^,^^ are summarized in Table I. The appearance of soot was monitored by the attenuation of a 632.8-nm HeNe laser beam located approximately 10 mm from the end plate of the shock tube. The term “soot”has been used in our experimental work as a lumped property meaning “species absorbing at wavelength of 632.8 nm”. The terms “soot yield” and ”soot amount”are used here as practical measures, providing a relative tendency of a given fuel or fuel mixture to form polycyclic aromatic hydrocarbons and soot particles. The latter measure, “soot amount”,is defined as the absolute amount of carbon atoms accumulated in “soot”and indicates the absolute rate of accumulation of carbon atoms in soot; the former measure, “soot yield”, is defined as the fraction of carbon atoms accumulated in “soot”and shows the conversion efficiency of a hydrocarbon to soot. For the comparison of soot formation from individual fuels both measures provide identical information; however, as shown below, it is important to distinguish these measures for the analysis of sooting tendencies in hydrocarbon mixtures. Soot yields and soot amounts were calculated in the usual assuming Rayleigh-limit light extinction, pmt = 1.8 g/cm3, and the complex refractive index m = 1.7 - 0.75i of Menna and D’Ale~sio.~~
Computer Model The computer model used in this study is principally the same as the one developed for the analysis of PAH formation in the pyrolysis of individual hydrocarbons,2s except for a few minor modifications, which will be discussed below. Since the details of the model could not be reported in full due to space limitation in the previous publication,2s they are reported here. The nomenclature of chemical species used in this work is that reported earlier.37 This information is not repeated here, and the reader is referred to ref 37 for the details on the naming rules as well as to Table I of that reference for the description of approximately 140 chemical species. Table I1 in this paper presents molecular structures and the corresponding names for additional chemical species considered in the present investigation. The inclusion of these new compounds necessitated the following extension of the naming rules: (a) when the name is longer than 12 characters, which occurred with some vinyl-containing species, “C2H3” in that name is replaced by a single letter “V”; (b) species containing n nonfused aromatic rings, like biphenyl, are specified by the term P,; (c) triphenylene and its derivatives are identified by the term Ds. The reaction mechanism has three components: a set of initiation reactions, a set of reactions describing the formation of small PAH, and a polymerization growth of aromatic rings. The initiation set, shown in Table 111, is composed of a modified m e c h a n i ~ mof~ Tanzawa ~ ~ ~ ~ ~and ~’ Gardiner38 augmented with’ decomposition reactions of ethylene, vinylacetylene, 1,3-butadiene, and benzene, whose rate coefficients were taken from Just et al.39and (35) Frenklach, M.; Taki, S.; Li Kwok Cheong, C. K.; Matula, R. A. Combust. Flame 1983,51, 37-43. (36) Menna, P.; D’Alessio, A. Symp. (Int.) Combust. [Proc.] 1982, I9th, 1421-1428. (37) Frenklach, M.; Clary, D. W.; Yuan, T.; Gardiner, W. C., Jr.; Stein, S . E. Combust. Sci. Technol. 1986,50, 79-115. (38) Tanzawa, T.; Gardiner, W. C., Jr. J. Phys. Chem. 1980, 84, 236-239. 1
Frenklach et al.
464 Energy & Fuels, Vol. 2, No. 4, 1988 Table 11. Reaction Swcies C4HSS AICZH3AIC6H4UX C6H3U AIC2H3' C6H4
H
- CE c-'c
AIC6H6U *C-C=CH
H
a
H,
H,C=C
