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PAH growth and soot formation in the pyrolysis of acetylene and benzene at high temperatures and pressures: Modeling and experiment
Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 1605–1612
PAH GROWTH AND SOOT FORMATION IN THE PYROLYSIS OF ACETYLENE AND BENZENE AT HIGH TEMPERATURES AND PRESSURES: MODELING AND EXPERIMENT ¨ HM,1 H. JANDER,2 and D. TANKE2 H. BO 1Physikalische
Chemie I Universita¨t Bielefeld Universita¨tststr. 25 33615 Bielefeld, Germany 2Physikalische Chemie Universita¨t Go¨ttingen Tammannstr. 6 37077 Go¨ttingen, Germany
The growth of high molecular polycyclic aromatic hydrocarbons (PAHs) before soot inception is modeled during C2H2 and C6H6 pyrolysis at temperatures between 1600 and 2400 K, p 4 60 bar, and C-atom concentrations of (3.8–4.0) 2 1016 mol/cm3. The formation of high molecular PAH is mainly computed by two different reaction pathways: (1) successive H-abstraction C2H2 additions and (2) a combinative ring-ring condensation of aromatics. The calculations are compared with measurements obtained from C2H2 and C6H6 pyrolysis by the shock wave method. From reaction flux analysis, it is deduced that the combinative route plays a major role in forming PAHs, especially at early reaction times. The calculated induction times of higher PAHs reasonably reflect the experimental trends of soot inception. The slope of the computed and measured induction times in dependence on the temperature resulted in a similar activation energy for high molecular PAH and soot formation. It is concluded that soot mass growth rates during the pyrolysis of C2H2 and C6H6 are strongly related to PAH formation. Thus, the decrease of the soot mass growth rates during C2H2 pyrolysis for T . 2000 K seems to be substantially influenced by a change in the nucleation process of soot precursors.
Introduction In practical high-pressure combustion, soot can be an unwanted product in the exhaust gases. Thus, a thorough investigation of soot formation at high pressure and temperature is required. Shock tube methods and premixed high-pressure flames are suitable tools to perform such measurements. In the past, many experiments have addressed soot formation at atmospheric and elevated pressures [1– 12]. The main experimental results relating to soot formation at atmospheric and elevated pressures are summarized next. During shock wave pyrolysis studies, the activation energy of the induction period for soot formation of aliphatic and aromatic hydrocarbons has been found to vary between 215 and 229 kJ/mol. Despite the similar activation energies for all studied fuels, the inception of soot formation differs considerably. Soot formation during benzene pyrolysis starts, for example, at a given temperature and carbon concentration much earlier than during acetylene pyrolysis [6,13]. Generally, the soot yield for all aliphatic and aromatic hydrocarbons shows a near Gaussian shape
distribution as a function of temperature. These “bell-shaped” curves have been found in pyrolysis as well as in flame experiments at normal and high pressures [1,2,4–6,8,9]. In flames, the soot yield peaks in the range between 1600 and 1700 K for 0.1 , p [MPa] , 10 and during pyrolysis between 1800 and 2000 K [5,6,8,9]. The amount of soot formed during pyrolysis and in flames depends on temperature and on the carbon concentration of the fuels. Deduced from measurements, the soot yield in shock waves shows a less-pronounced dependence on pressure [6] than that in flames. Concerning the mass growth of soot particles in the pyrolysis at higher pressures, two temperature ranges have to be distinguished: T , 2000 K and T . 2000 K. For T , 2000 K, the rate of soot mass growth in the pyrolysis of benzene is distinctly higher than the rate in the pyrolysis of aliphatics [5,6,14]. For T . 2000 K, the growth of soot particles in the pyrolysis of aliphatic hydrocarbons slows down with increasing temperatures [5,6]. The firstorder rate constant for soot mass growth, kf , in shock waves can be normalized to the carbon concentration of the hydrocarbons (kf /[C]). For T , 2000 K, the formal activation energy (AE) of kf /[C] in the
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shock wave pyrolysis of all studied hydrocarbons with the exception of acetylene is approximately about 200 kJ/mol. For acetylene, AE ' 100 kJ/mol [6]. However, the experimental results on the investigation of soot formation at high pressure leave open questions on several details of this process. For example, the shock tube method shows a decline of soot formation rates at high temperatures but fails to explain the phenomenon directly. Furthermore, shock tube methods cannot contribute to investigations on reactions causing the decrease of the soot yield at high temperatures or on details of the decline of the formal rate constant of soot mass growth for aliphatic fuels at T . 2000 K. Modeling of soot precursors and soot formation can contribute to the understanding of combustion processes if reliable thermodynamic and kinetic data are available and can be used to construct a realistic reaction mechanism. During the last decades, many instructive experimental results from low-pressure sooting hydrocarbon flames were obtained [15–19]. Therefore, the understanding of, for example, the formation of the first aromatic ring, benzene, and the growth of the actual soot precursors, the polycyclic aromatic hydrocarbons (PAHs), has improved considerably [16,17,20,21]. Recent experimental results from the group of Homann [16,22] indicate an important impact of biaryl reactions on the formation of PAH (“combinative growth”), especially for aromatic fuels. In view of this fact and of earlier studies in shock waves [6], our main emphasis is on the investigation of the efficiency of a combinative reaction pathway in comparison to “successive growth” of PAH via acetylene addition [23]. We present results on modeling of PAH growth, from benzene to PAH species consisting of about 700 amu. The formation of the PAH is modeled with three different growth mechanisms: a successive mechanism relying on the addition of acetylene to aromatics [23], a combinative growth resulting from ring-ring condensation, and a reaction pathway including cyclopentadienyl [24,25]. The focus of this work is the comparison of the characteristic trends of calculated induction times s and formation rates of high molecular weight PAH as a function of temperature, carbon atom concentration, and pressure, with experimental data obtained by measurements during pyrolysis using the shock tube method. In simulations and experiments, both aliphatic acetylene and aromatic benzene fuels are considered. The temperature varied between 1560 and 2400 K, and the pressure range was 5.7– 6.0 MPa. The range of the carbon atom concentration in the fuel was (3.8–4.0) mol/m3. Experimental The experimental setup has been described elsewhere [6,13]; therefore, only the main details are
given. The experiments were conducted behind reflected shock waves in a 70-mm-i.d. steel shock tube with a 4.5-m-long driven section and a 3.5-m-long driver section. Shock speed and the pressure time profile were measured with piezo-electric pressure transducers. Shock parameters were computed following the standard procedure [26,27] using the measured incident shock speed. The conversion of hydrocarbon to soot was measured via the attenuation of the light beam from a 15-mW He–Ne laser, operated at k 4 632.8 nm. The light extinction profiles I(t) were converted into soot yield profiles SY(t) using Beer’s law [28], a refractive index of m 4 1:57 1 0.56 i, a soot density of 1.86 g/cm3, and the atomic weight of carbon. The test gas mixtures were prepared manometrically and mixed by convection. The gases C2H2 (.99.6%), H2 (.99.998%), and Ar (.99.9%) were used without further purification, whereas benzene (.99.9%) was purified by distillation.
The Model Detailed chemical kinetic calculations have been performed using the Homrun program [29]. The basis of the reaction model relies on the work of Warnatz and coworkers [30–32]. It includes the C4H3 and C4H5 reaction channels to benzene following Frenklach [23,33] and the C3H3 recombination forming benzene postulated by Homann and coworkers [16,34] and Miller et al. [35]. Further benzene reactions as proposed, for example, by Zhang et al. [36] and Lindstedt et al. [37] were also taken into account. For PAH formation, three reaction pathways are incorporated into the mechanism: 1. The alternating H-abstraction C2H2 addition (HACA) route, resulting in a successive growth of PAH as suggested by Frenklach et al. [23]. As an example for this route, the acetylene addition to naphthalene (C10H8) forming phenanthrene (C14H10, outer ring) and pyrene (C16H10, inner ring) is demonstrated in Fig. 1 (1). 2. The combination reactions of phenyl with C6H6 as reported, for example, in the work of Zhang et al. [36]. Details are given in Fig. 1 (2). 3. The cyclopentadienyl recombination [24,25] that is deduced from experimental results [38,39] showing that naphthalene may be formed bypassing the benzene route, see Fig. 1 (3). To account for the successive growth, we extended the HACA route, Fig. 1 (1), to circumcoronene (C54H18). In our reaction sequence, as many inner rings as possible were postulated to minimize the more time-consuming formation of outer rings. The combinative reaction model of PAH growth developed here is mainly driven by biaryl reactions. It follows the reaction pathways of Colket and Seery
ACETYLENE AND BENZENE PYROLYSIS
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Thermochemical data from readily available sources were used [46–50], when possible. Otherwise, thermodynamic estimates of species were made using techniques such as group additivity [51–53]. Of particular concern during this study was the careful determination of the heat of formation of the higher aromatics in order to minimize the errors in the proposed rate constants. The detailed reaction mechanism will be reported elsewhere [54]. Examples for the PAH reactions are given in Table 1.
Modeling and Experimental Results Fig. 1. Reaction pathways of PAH formation. (1) Successive growth including outer and inner-ring formation, (2) Combination reaction including inner-ring formation (4), (3) Cyclopentadienyl recombination.
[40] and Zhang and McKinnon [36] for biphenyl production and extended the reaction type (2) up to PAH molecules of roughly 700 amu. In addition, the biaryls are allowed to react with acetylene, forming inner rings. An example for this pathway is shown for the formation of phenanthrene (C14H10) in Fig. 1 (2) and (4). The mechanism also includes five-membered rings with respect to the assumption that such nonplanar PAH structures could provide three-dimensional soot precursors [41]. Pressure-dependent reaction rate coefficients were calculated [42–45].
In the calculations it was found that during C2H2 pyrolysis, most of the benzene is formed by C3H3 recombination, whereas the C2H2 addition to C4H3 and C4H5 radicals producing benzene is less important. In general, the computed concentrations of aromatics in benzene pyrolysis were approximately two orders of magnitude higher than in acetylene pyrolysis. For the formation of PAH containing 36 C-atoms and a number of H-atoms: 14 # x # 22, C36Hx, two different reaction types of PAH growth were distinguished and found to relate to each other: (a) the successive C2H2 addition (HACA) [23,33], and (b) the ring-ring condensation including inner-ring formation. Figure 2 presents the contribution to PAH formation predicted by the HACA model (succ. growth), and by the combinative ring-ring reactions
TABLE 1 Selected PAH reactions for benzene and acetylene pyrolysis Reaction A ` H 4 A* ` H2 A* ` C2H2 4 AC2H* 2 A* ` C2H2 4 AC2H ` H A ` C2H 4 AC2H ` H ACH3 4 A* ` CH3 ACH2 ` C2H2 4 AZP ` H AC2H* 3 ` CH3 4 AZP ` H ` H AZP* ` ZP* 4 A`2 ` H ` H A* ` A 4 AA ` H AA* ` C2H2 4 A`2 ` H AC2H* 2 ` C2H2 4 A`1 ` H AC2H* ` C2H2 4 A* `1 A* ` C3H3 4 AZP A* ` B 4 AB ` H AB 4 AB`1 ` H2 A* ` B* 4 AB
k8
n
EA
Ref.
2.5 1014 4.0 107 1.6 1014 1.0 1013 1.4 1016 3.2 1011 5.0 1013 1.0 1013 1.1 1023 4.6 106 1.6 1016 5.1 1048 1.0 1013 1.0 1012 1.0 108 3.1 1012
0 1.56 10.19 0 0 0 0 0 12.92 1.97 11.33 110.53 0 0 0 0
66.9 15.9 83.7 0.0 417.7 29.3 0.0 16.7 66.5 30.5 22.61 117.2 0.0 20.9 133.9 0.0
[33] [33] [33] [23] [24] [24] [24] [24] [33,36] [33] [33] [33] Est. [40] [40] [20]
A, B, AA, AB: PAH, six-membered rings; AZP; PAH containing a cyclopentadiene ring; ZP: cyclopentadiene; *: notation for radical, `1, `2: formation of additional PAH ring(s). k 4 k8 * Tn * exp(1EA/RT); Units for k8: cm3, mol11, s11, units for EA: kJ, mol11.
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Fig. 2. Modeled (C36Hx) in C2H2 and C6H6 pyrolysis by two different growth mechanisms at p 4 6 [MPa], C-atom concentration 4 4.0 [mol/m3]. ——: C6H6 pyrolysis; - - -: C2H2 pyrolysis.
Fig. 3. Induction time s. (a) calculated, averaged value for the blocks of PAH C36Hx to C48Hx, ——, and (b) measured data, - - -, in the C2H2 and C6H6 pyrolysis as a function of 1/T[K11]. C2H2 pyrolysis: p 4 5.7 [MPa]; C-atom conc. 4 3.8 [mol/m3]; C6H6 pyrolysis: p 4 6.0 [MPa]; Catom conc. 4 4.0 [mol/m3].
(comb. growth). To determine the importance of the ring-ring condensation (b) in comparison to the successive growth (a), two kinds of calculations were performed. The first case included both types of growth mechanisms, whereas for the second case all ring-ring condensation reactions were removed from the mechanism. As an example, the formation pathway of the block of PAH containing 36 C-atoms (C36Hx) is analyzed in the C2H2 and C6H6 pyrolysis.
The percentage of the formation of C36Hx resulting from these two channels is displayed as a function of the normalized reaction times, s(t)/scalc, where scalc is the calculated induction time for the C6H6 pyrolysis and for the C2H2 pyrolysis, respectively (see the following). During benzene pyrolysis, C36Hx formation starts very early. Compared with the ring-ring condensation, the successive HACA reaction channel is inefficient in the benzene pyrolysis. Even after 2.5 times the induction period, it amounts to less than 1.0%. During acetylene pyrolysis, the formation of C36Hx is also dominated by the combinative PAH growth, especially at early reaction times. With increasing reaction times, the influence of the combinative channel decreases in favor of the successive pathway. These results demonstrate the importance of the combinative growth mechanism for the modeling of high molecular weight PAHs, which are the direct precursors of the soot particles. In the following, the calculated induction periods of PAHs, and in particular those of high molecular mass, are compared with measured soot induction times in different hydrocarbon pyrolysis systems. The induction time in pyrolysis obtained by the shock tube method is defined by the intersection of the tangent of the maximum slope to the measured soot yield curve with the time axis, and in the calculations by that of the tangent of the modeled PAH curve with the time axis [6,13]. Our experimental results on the induction time demonstrate that, for example, during benzene pyrolysis, the induction time is much smaller than that found during acetylene pyrolysis for nearly the same C-atom concentration. These results indicate that soot formation starts much earlier in the benzene pyrolysis than in the acetylene system, although the activation energies for the induction time for both pyrolysis system are similar [AE 4 228 kJ/mol (C2H2) and 229 kJ/mol (C6H6)] [6,13]. The experimentally obtained induction time reflects the rate of all reactions from the first building blocks of the aliphatic radicals to the first aromatic ring system and also for the growing of the PAH compounds up to the first soot particles. Because the measured induction time sexp is an expression composed of many chemical reactions with very different reaction rates, and the required size of PAH essential for soot precursor formation is assumed to be 400– 800 amu [55–57], the induction times for the blocks of PAH of C36Hx to C48Hx were calculated and averaged. In Fig. 3, these averaged calculated induction times and the experimentally determined induction times are displayed as a function of temperature. The model correctly predicts the longer induction period of acetylene pyrolysis in comparison to benzene pyrolysis. In acetylene pyrolysis, the averaged calculated induction periods for the blocks of PAH of C36Hx to C48Hx are roughly
ACETYLENE AND BENZENE PYROLYSIS
Fig. 4. Experimentally determined relative soot mass growth rate constants kf /[C], ——, and calculated relative formation rates for PAH C48Hx, - - -, as a function of 1/T [K11].
two times the measured ones. In benzene pyrolysis, the induction period is approximately 1.5 times the experimental one. The slightly longer calculated induction periods of these high molecular weight PAHs might be caused by an additional reaction channel forming soot precursors not considered in the present model. This supplementary route could prevent very high PAHs from growing exclusively via the successive and/or combinative pathway to attain the required size of nuclei, causing shorter induction times of the soot precursors. Additional calculations will be performed to clarify this point. The slope of the calculated and measured curves are nearly identical, resulting in a similar activation energy of AE 4 192 kJ/mol for acetylene and AE 4 201 kJ/mol for benzene, in comparison to the measured activation energies of AE 4 228 kJ/mol (acetylene) and AE 4 229 kJ/mol (benzene) [6,13]. As an additional check for the accuracy of the used pressure-dependent rate constants at 6 MPa, we also calculated the induction time for the high molecular weight PAHs at lower, near atmospheric pressure and found no pressure dependency of the induction periods. These results are in agreement with experiments [5–7]. The experimental results obtained by the shock tube method for the soot mass growth rate in aliphatic and benzene pyrolysis differ considerably. In the entire temperature range, covered by the measurements, the soot mass growth rates were always faster in benzene pyrolysis than in the pyrolysis of aliphatics, including n-C6H14, C2H4, C2H2 and CH4 [5,6,14]. The measured activation energy for the soot
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mass growth of benzene was AE 4 205 kJ/mol for 1600 , T [K] , 2400 [6]. In the C2H2 pyrolysis, the rate of soot mass growth increases up to 2000 K and then decreases toward higher temperatures. The resulting activation energy is AE 4 101 kJ/mol for T , 2000 K [6]. The different behavior of the soot mass growth rates for the fuels C2H2 and C6H6 may be caused by different factors. The decrease of the soot mass growth rate in the C2H2 pyrolysis for T . 2000 K can result from (1) a change in the nucleation of soot precursors (i.e., PAH) or in the soot particle inception process, or (2) a change in the soot surface growth processes, such as a lack of additive species, or tempering. Some hints for the observed temperature dependence in the C2H2 pyrolysis can be obtained by calculating higher PAH concentrations as a function of temperature for the fuels C6H6 and C2H2. This has been done for the block of PAH C48Hx using the reaction mechanism specified earlier. In Fig. 4, the relative rate constants for the soot mass growth kf/[C] [cm3 mol11 s11)], measured in the shock waves experiments are presented along with the relative formation rates kf of C48Hx calculated for the pyrolysis of C2H2 and C6H6. The curves of the computed relative formation rates of C48Hx have a similar shape to those of the relatives kf/[C], the rate constants of soot mass growth. This result holds for C6H6 as well as for C2H2. Although for the pyrolysis of C2H2, the maximum of the calculated formation rates of C48Hx is reached at '50 K lower temperature, the similarity of computed and measured curves is clearly seen. Thus, the decrease of the soot mass growth rates during acetylene pyrolysis for T . 2000 K may be caused by a change in the kinetics involving PAH. This result strongly indicates that soot growth is substantially influenced by the kinetics of the soot precursors. The modeling of C48Hx also shows a faster formation rate for the pyrolysis of C6H6 than for the pyrolysis of C2H2 in the entire temperature regime, supporting the conclusion that the faster growth rates of the soot particles are related to the faster formation rates of the PAH.
Discussion In both C2H2 and in C6H6 pyrolysis it is shown that the biaryl reaction pathway is important for the formation of PAH. The increasing importance of the successive growth to longer reaction times in C2H2 pyrolysis in comparison to C6H6 pyrolysis is caused by the higher acetylene concentrations. In acetylene pyrolysis, the most sensitive reaction for the PAH growth is the formation of the first aromatic ring; in this work and at the conditions used
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here, the C3H3 recombination to benzene predominates. Comparing pyrolytic results from both systems, it is concluded that PAH inception is mainly influenced by the benzene concentration. In contrast to benzene, acetylene attains substantial influence on PAH growth at longer reaction times where the benzene concentration is decreasing. These results indicate that a maximum PAH growth, followed by soot growth, will take place when these two species benzene (responsible for PAH inception) and acetylene (responsible for PAH/soot growth at longer reaction times) work “hand in hand” [6]. The calculations also show the importance of the combinative PAH pathway in the model. Removing these reactions and taking only the HACA mechanism into account causes up to 10 times longer induction periods for the calculated PAH. Even variations of the thermodynamic data in the error range given in Ref. [53] and an increase of the reaction constants of the H-abstraction and C2H2 addition in the HACA route could not significantly raise the induction periods predicted by this model. In the HACA route, in particular, the H-abstraction reaction and the formation of the outer rings (see Fig. 1 (1)) slow down the induction periods of the aromatics. Therefore, we have minimized the formation of outer rings in the reaction scheme. However, outer rings contribute significantly to the growth of smaller aromatics. The larger the PAH, the smaller is the ratio of the outer to inner rings that have to be formed in the reaction sequence to obtain high condensed PAH such as circumcoronene. But even a HACA model including as few outer ring formation steps as possible could not produce the required size of PAH nuclei of 400–800 amu, which are essential for soot precursor formation [55–57]. These results confirm those of earlier studies of McKinnon and Howard [58], Homann and coworkers [59], and Alexiou and Williams [60]. Based on a comparison of the formation rates of PAH and kf the soot mass growth rate constant, the conclusion is drawn that in aromatic, as well as in aliphatic pyrolysis, mainly the same PAH growth mechanism takes place up to temperatures of 1900– 2000 K. Above this temperature in acetylene pyrolysis, the benzene concentration is too small to initiate a combinative PAH growth. The PAH growth only due to a successive acetylene addition is too unefficient and too slow, especially at early reaction times (see Fig. 2). In benzene pyrolysis, the biaryl concentration is up to two orders of magnitude higher than in the pyrolysis of aliphatic hydrocarbons. Therefore, the formation rate curve of PAH above 2000 K increases further when the one of acetylene pyrolysis is already decreasing. It is worthwhile to mention that including a third reaction pathway leading to phenanthrene [24,25] in the mechanism
indene ` cyclopentadienyl → anthracene ` H ` H anthracene ↔ phenanthrene
may further reduce the influence of the HACA route. Apparently the cyclopentadienyl channel is also more successful in producing phenanthrene than the successive acetylene addition to benzene. In future work, we shall also extend this mechanism to the formation of higher aromatics. Conclusions Shock tube data for the pyrolysis of benzene and acetylene have been compared with computations. The data have been used to examine three chemical kinetic pathways for PAH mass growth: 1. the H-abstraction acetylene addition (HACA) route, 2. the ring-ring condensation of aromatics, and 3. the cyclopentadienyl pathway leading to anthracene and phenanthrene. The calculated induction times of higher aromatics reasonably reflect the experimental trends of soot induction. The investigation of the reaction mechanism for PAH growth show that ring-ring condensation is more efficient than the HACA route in producing high aromatics at short reaction times. These results hold even during acetylene pyrolysis. With increasing reaction times, the HACA reactions became more prominent for PAH growth. The cyclopentadienyl pathway is also an important route in forming PAHs, especially in acetylene pyrolysis. Soot mass growth rates and the calculated formation rates of PAHs show the same dependence on temperature. These observations are a further indication that soot mass growth is strongly related to PAH and soot precursor formation. Acknowledgments H. Bo¨hm appreciates Prof. K. Kohse-Ho¨inghaus for valuable discussions on elementary reaction kinetics and her interest in the present work. We are indebted to Prof. H. Gg. Wagner for his support and suggestions throughout this research. We thank Prof. U. Maas for providing the Homrun source code to perform the model calculations. Computing time support from HLRZ, Forschungszentrum Ju¨lich and the Institute of Theoretische Physik, Universita¨t Bielefeld, is gratefully acknowledged.
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ACETYLENE AND BENZENE PYROLYSIS 3. Frenklach, M. and Wang, H., in Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1559–1566. 4. Mu¨ller, A. and Wittig, S., in Soot Formation in Combustion (H. Bockhorn, ed.), Springer Series in Chemical Physics 59, Springer-Verlag, Berlin/Heidelberg, 1994, pp. 350–370. 5. Bauerle, St., Karasevich, Y., Slovov, St., Tanke, D., Tappe, M., Thienel, Th., and Wagner, H. Gg., in Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 627–634. 6. Knorre, V. G., Tanke, D., Thienel, Th., and Wagner, H. Gg., in Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 2303–2310. 7. Hwang, S. M., Vlasov, P., Wagner, H. Gg., and Wolff, Th., Z. Phys. Chem. Neue Folge 173:129–139 (1991). 8. Bo¨hm, H., Feldermann, Ch., Heidermann, Th., Jander, H., Lu¨ers, B., and Wagner, H. Gg., in TwentyFourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 991– 998. 9. Bo¨nig, M., Feldermann, Ch., Jander, H., Lu¨ers, B., Rudolph, G., and Wagner, H. Gg., in Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1581–1587. 10. Hanisch, S., Jander, H., Pape, Th., and Wagner, H. Gg., in Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 577–584. 11. von Gersum, S. and Roth. P., in Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 999–1006. 12. D’Alessio, A., D’Anna, A., D’Orsi, A., Minutolo, P., Barbarella, R., and Ciajolo, A., in Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 973–980. 13. Tanke, D., “Russbildung in der Kohlen wasserstoffpyrolyse hinter Stosswellen,” Ph.D. thesis, Go¨ttingen, 1995. 14. Geck, C., “Untersuchung der Bildungsgeschwindigkeit von Russ bei der Pyrolyse von Ethylen und Acetylen in Stosswellen,”Ph.D. thesis, Go¨ttingen, 1975. 15. Homann, K.-H., in Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 857–870. 16. Kovacs, R., “Multi-Photonen-Ionisations-Massenspektrometrie an aromatischen Kohlenwasserstoffen und Fullerenen in Flammen,” Ph.D. thesis, Darmstadt, 1996. 17. Bachmann, M., “Bildung und Wachstum von aromatischen Kohlenwasserstoffen und Fullerenen,” Ph.D. thesis, Darmstadt, 1997. 18. Doute´, C., Delfau, J.-L., and Vovelle, C., Combust. Sci. Technol. 103:153–173 (1994). 19. Howard, J. B. and Bittner, J. D., in Eighteenth Symposium (International) on Combustion, The Combustion Institute, 1980, pp. 1105–1126.
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20. Sarofim, A. F. and Masonjones, M. C., Chem. Phys. Proc. Combust. 1:265–268 (1995). 21. Westmoreland, P. R., Dean, A. M., Howard, J. B., and Longwell, J. P., J. Phys. Chem. 93:8171–8180 (1996). 22. Griesheimer, J., “Aromaten und ihre Radikale in Naphthalinflammen-Mechanismus des PAH Wachstums,” Ph.D. dissertation, Darmstadt, 1996. 23. Frenklach, M., Clary, D., Gardiner, W. C. Jr., and Stein, S. E., in Twentieth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, pp. 887–901. 24. Marinov, N. M., Pitz, W. J., Westbrook, C. K., Castaldi, M. J., and Senkan, S. M., Combust. Sci. Technol. 116:211–287 (1996). 25. Melius, C. F., Colvin, M. E., Marinov, N. M., Pitz, W. J., and Senkan, S. M., in Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 685–692. 26. Greene, E. F. and Toennies, J. P., Chemische Reaktionen in Stosswellen. Dr. Dietrich Steinkopff Verlag, Darmstadt, 1959. 27. Gardiner, W. C., Walker, B. F., and Wakeguild, C. B., in Shock Waves in Chemistry (A. Lifshitz, ed.), Marcel Dekker, New York, 1981, pp. 319–329. 28. Graham, S. C., Homer, J. B., and Rosenfeld, J. L., Proc. R. Soc. London, Ser. A 344:259–379 (1975). 29. Esser, C., Maas, U., and Warnatz, J., in Proc. Int. Symp. on Diagnostics and Modelling of Combustion in Reciprocating Engines, The Japanese Society of Mechanical Engineers, Tokyo, 1985, pp. 335–402. 30. Warnatz, J. and Maas, U., Technische Verbrennung, Springer-Verlag, Berlin/Heidelberg, 1996. 31. Chevalier, C., “Entwicklung eines detaillierten Reaktionsmechanismus zur Modellierung der Verbrennungsprozesse von Kohlenwasserstoffen bei Hochund Niedertemperaturbedingungen,” Ph.D. thesis, Stuttgart, 1993. 32. Klaus, P., “Entwicklung eines detaillierten Reaktionsmechanismus Zur Modellierung der Bildung von stickoxiden in Flammenfronten,” Ph.D. thesis, Heidelberg, 1997. 33. Wang, H. and Frenklach, M., Combust. Flame 110:173–221 (1997). 34. Alkemade, U. and Homann, K.-H., Z. Phys. Chemie. Neue Folge 161:19–34 (1989). 35. Miller, J. A. and Melius, C. F., Combust. Flame 91:21– 39 (1992). 36. Zhang, H.-Y. and McKinnon, J. T., Combust. Sci. Technol. 107:261–300 (1995). 37. Lindstedt, R. P. and Skevis, G., Combust. Flame 99:551–561 (1994). 38. Manion, J. A. and Louw, R., J. Phys. Chem. 93:3563– 3573 (1989). 39. Spielmann, R. and Cramers, C. A., Chromatographia 5:295–301 (1972). 40. Colket, M. B. and Seery, D. J., in Twenty-Fifth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 883–891. 41. Gerhardt, Ph., Lo¨ffler, S., and Homann, K.-H., in
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Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 395–401. Troe, J., J. Chem. Phys. 75:226–237 (1981). Warnatz, J., in Combustion Chemistry (W. C. Gardiner, ed.), Springer-Verlag, New York, 1984, pp. 197– 360. Westmoreland, P., “Experimental and Theoretical Analysis of Oxidation and Growth Chemistry in a FuelRich Acetylene Flame,” Ph.D. thesis, MIT, 1988. Wang, H. and Frenklach, M., J. Phys. Chem. 98:11465–11489 (1994). Alberty, R. A. and Reif, A. K., J. Phys. Chem. Ref. Data 17:242–265 (1988). Alberty, R. A., Chung, M. B., and Reif, A. K., J. Phys. Chem. Ref. Data 18:77–109 (1989); 19:349–370 (1990). Stull, D. R. and Prophet, H., Thermochemical Tables. National Bureau of Standards, Washington, DC, 1971. Burcat, A. and McBride, B., Technion report TAE 697, Haifa, 1993. Kee, R. J., Rupley, F. M., and Miller, J. A., The CHEMKIN Thermochemical Data Base. Sandia report SAND87-8215.UC4. Benson, S. W., Thermochemical Kinetics, Wiley, New York, 1976.
52. Stein, S. E. and Fahr, A., J. Phys. Chem. 89:3714–3725 (1985). 53. Wang, H. and Frenklach, M., J. Phys. Chem. 97:3867– 3874 (1993). 54. Bo¨hm, H., Jander, H., Tanke, D., and Thienel, T., sent for presentation on the EUROTHERM seminar no. 61, ’s-Hertogenbosch, October 1998. 55. Baum, Th., “Grosse Ionen in Ethin-Sauerstoff-Flammen,” Ph.D. thesis, Darmstadt, 1996. 56. Homann, K.-H., Lo¨ffler, S., Lo¨ffler, Ph., and Weilmu¨nster, P., in Soot Formation in Combustion (H. Bockhorn, ed.), Springer Series in Chemical Physics 59, Springer-Verlag, Berlin/Heidelberg, 1994, pp. 66– 82. 57. Miller, J. A., in Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 91–98. 58. McKinnon, J. T. and Howard, J. B., in Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 965–971. 59. Guthier, K., Hebgen, P., Loock, H. P., Homann, K.-H., Hoffmann, J., and Zimmermann, G., Ber. Bunsen-Ges. Phys. Chem. 97:140–142 (1993). 60. Alexiou, A. and Williams, A., Combust. Flame 104:51– 65 (1996).
COMMENTS Assa Lifshitz, The Hebrew University of Jerusalem, Israel. Have you considered the possibility of varying the concentrations of C2H2 and C6H6 in order to examine its effect on the induction time for appearance of soot? Also, would the model predict such an effect? Author’s Reply. In an additional poster presented at this Symposium [1], we determined the influence of acetylene and benzene mixtures on the induction time by varying the composition of these mixtures. The measured increase of the induction time of soot formation with decreasing contents of benzene is in line with the predictions of the model for the induction time of PAH formation.
REFERENCE 1. Jander, H., Tanke, D., Thienel, T., and Bo¨hm, H., in Twenty-Seventh International Symposium on Combustion, Abstracts of Work-in-Progress Posters, 1998, p. 413.
● P. Cadman, University of Wales, Aberystwth, UK. Can your mechanisms predict the relative amounts of small to large PAH(s) with time, P, and T? You did not mention the modeling results for the soot mass growth, etc., for benzene/C2H2 mixtures. Can you comment on these? Author’s Reply. The relative amount of low-molecularweight PAH to high-molecular-weight PAH can be predicted by the model in dependence on the reaction time. So far, in the calculations, we have mainly determined the influence of the temperature on the PAH species at pressures of 57 to 60 bar. We did a few calculations and experiments at 6 bar and found that the measured induction periods of soot as well as the calculated ones for PAH formation do not differ much to the ones obtained at 60 bar. In addition, we observed that in benzene/hydrogen mixtures, the induction time of soot formation is increased in comparison to benzene pyrolysis. The same trend for PAH formation is calculated by the model.
PAH GROWTH AND SOOT FORMATION IN THE PYROLYSIS OF ACETYLENE AND BENZENE AT HIGH TEMPERATURES AND PRESSURES: MODELING AND EXPERIMENT ¨ HM,1 H. JANDER,2 and D. TANKE2 H. BO 1Physikalische
Chemie I Universita¨t Bielefeld Universita¨tststr. 25 33615 Bielefeld, Germany 2Physikalische Chemie Universita¨t Go¨ttingen Tammannstr. 6 37077 Go¨ttingen, Germany
The growth of high molecular polycyclic aromatic hydrocarbons (PAHs) before soot inception is modeled during C2H2 and C6H6 pyrolysis at temperatures between 1600 and 2400 K, p 4 60 bar, and C-atom concentrations of (3.8–4.0) 2 1016 mol/cm3. The formation of high molecular PAH is mainly computed by two different reaction pathways: (1) successive H-abstraction C2H2 additions and (2) a combinative ring-ring condensation of aromatics. The calculations are compared with measurements obtained from C2H2 and C6H6 pyrolysis by the shock wave method. From reaction flux analysis, it is deduced that the combinative route plays a major role in forming PAHs, especially at early reaction times. The calculated induction times of higher PAHs reasonably reflect the experimental trends of soot inception. The slope of the computed and measured induction times in dependence on the temperature resulted in a similar activation energy for high molecular PAH and soot formation. It is concluded that soot mass growth rates during the pyrolysis of C2H2 and C6H6 are strongly related to PAH formation. Thus, the decrease of the soot mass growth rates during C2H2 pyrolysis for T . 2000 K seems to be substantially influenced by a change in the nucleation process of soot precursors.
Introduction In practical high-pressure combustion, soot can be an unwanted product in the exhaust gases. Thus, a thorough investigation of soot formation at high pressure and temperature is required. Shock tube methods and premixed high-pressure flames are suitable tools to perform such measurements. In the past, many experiments have addressed soot formation at atmospheric and elevated pressures [1– 12]. The main experimental results relating to soot formation at atmospheric and elevated pressures are summarized next. During shock wave pyrolysis studies, the activation energy of the induction period for soot formation of aliphatic and aromatic hydrocarbons has been found to vary between 215 and 229 kJ/mol. Despite the similar activation energies for all studied fuels, the inception of soot formation differs considerably. Soot formation during benzene pyrolysis starts, for example, at a given temperature and carbon concentration much earlier than during acetylene pyrolysis [6,13]. Generally, the soot yield for all aliphatic and aromatic hydrocarbons shows a near Gaussian shape
distribution as a function of temperature. These “bell-shaped” curves have been found in pyrolysis as well as in flame experiments at normal and high pressures [1,2,4–6,8,9]. In flames, the soot yield peaks in the range between 1600 and 1700 K for 0.1 , p [MPa] , 10 and during pyrolysis between 1800 and 2000 K [5,6,8,9]. The amount of soot formed during pyrolysis and in flames depends on temperature and on the carbon concentration of the fuels. Deduced from measurements, the soot yield in shock waves shows a less-pronounced dependence on pressure [6] than that in flames. Concerning the mass growth of soot particles in the pyrolysis at higher pressures, two temperature ranges have to be distinguished: T , 2000 K and T . 2000 K. For T , 2000 K, the rate of soot mass growth in the pyrolysis of benzene is distinctly higher than the rate in the pyrolysis of aliphatics [5,6,14]. For T . 2000 K, the growth of soot particles in the pyrolysis of aliphatic hydrocarbons slows down with increasing temperatures [5,6]. The firstorder rate constant for soot mass growth, kf , in shock waves can be normalized to the carbon concentration of the hydrocarbons (kf /[C]). For T , 2000 K, the formal activation energy (AE) of kf /[C] in the
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SOOT FORMATION AND DESTRUCTION
shock wave pyrolysis of all studied hydrocarbons with the exception of acetylene is approximately about 200 kJ/mol. For acetylene, AE ' 100 kJ/mol [6]. However, the experimental results on the investigation of soot formation at high pressure leave open questions on several details of this process. For example, the shock tube method shows a decline of soot formation rates at high temperatures but fails to explain the phenomenon directly. Furthermore, shock tube methods cannot contribute to investigations on reactions causing the decrease of the soot yield at high temperatures or on details of the decline of the formal rate constant of soot mass growth for aliphatic fuels at T . 2000 K. Modeling of soot precursors and soot formation can contribute to the understanding of combustion processes if reliable thermodynamic and kinetic data are available and can be used to construct a realistic reaction mechanism. During the last decades, many instructive experimental results from low-pressure sooting hydrocarbon flames were obtained [15–19]. Therefore, the understanding of, for example, the formation of the first aromatic ring, benzene, and the growth of the actual soot precursors, the polycyclic aromatic hydrocarbons (PAHs), has improved considerably [16,17,20,21]. Recent experimental results from the group of Homann [16,22] indicate an important impact of biaryl reactions on the formation of PAH (“combinative growth”), especially for aromatic fuels. In view of this fact and of earlier studies in shock waves [6], our main emphasis is on the investigation of the efficiency of a combinative reaction pathway in comparison to “successive growth” of PAH via acetylene addition [23]. We present results on modeling of PAH growth, from benzene to PAH species consisting of about 700 amu. The formation of the PAH is modeled with three different growth mechanisms: a successive mechanism relying on the addition of acetylene to aromatics [23], a combinative growth resulting from ring-ring condensation, and a reaction pathway including cyclopentadienyl [24,25]. The focus of this work is the comparison of the characteristic trends of calculated induction times s and formation rates of high molecular weight PAH as a function of temperature, carbon atom concentration, and pressure, with experimental data obtained by measurements during pyrolysis using the shock tube method. In simulations and experiments, both aliphatic acetylene and aromatic benzene fuels are considered. The temperature varied between 1560 and 2400 K, and the pressure range was 5.7– 6.0 MPa. The range of the carbon atom concentration in the fuel was (3.8–4.0) mol/m3. Experimental The experimental setup has been described elsewhere [6,13]; therefore, only the main details are
given. The experiments were conducted behind reflected shock waves in a 70-mm-i.d. steel shock tube with a 4.5-m-long driven section and a 3.5-m-long driver section. Shock speed and the pressure time profile were measured with piezo-electric pressure transducers. Shock parameters were computed following the standard procedure [26,27] using the measured incident shock speed. The conversion of hydrocarbon to soot was measured via the attenuation of the light beam from a 15-mW He–Ne laser, operated at k 4 632.8 nm. The light extinction profiles I(t) were converted into soot yield profiles SY(t) using Beer’s law [28], a refractive index of m 4 1:57 1 0.56 i, a soot density of 1.86 g/cm3, and the atomic weight of carbon. The test gas mixtures were prepared manometrically and mixed by convection. The gases C2H2 (.99.6%), H2 (.99.998%), and Ar (.99.9%) were used without further purification, whereas benzene (.99.9%) was purified by distillation.
The Model Detailed chemical kinetic calculations have been performed using the Homrun program [29]. The basis of the reaction model relies on the work of Warnatz and coworkers [30–32]. It includes the C4H3 and C4H5 reaction channels to benzene following Frenklach [23,33] and the C3H3 recombination forming benzene postulated by Homann and coworkers [16,34] and Miller et al. [35]. Further benzene reactions as proposed, for example, by Zhang et al. [36] and Lindstedt et al. [37] were also taken into account. For PAH formation, three reaction pathways are incorporated into the mechanism: 1. The alternating H-abstraction C2H2 addition (HACA) route, resulting in a successive growth of PAH as suggested by Frenklach et al. [23]. As an example for this route, the acetylene addition to naphthalene (C10H8) forming phenanthrene (C14H10, outer ring) and pyrene (C16H10, inner ring) is demonstrated in Fig. 1 (1). 2. The combination reactions of phenyl with C6H6 as reported, for example, in the work of Zhang et al. [36]. Details are given in Fig. 1 (2). 3. The cyclopentadienyl recombination [24,25] that is deduced from experimental results [38,39] showing that naphthalene may be formed bypassing the benzene route, see Fig. 1 (3). To account for the successive growth, we extended the HACA route, Fig. 1 (1), to circumcoronene (C54H18). In our reaction sequence, as many inner rings as possible were postulated to minimize the more time-consuming formation of outer rings. The combinative reaction model of PAH growth developed here is mainly driven by biaryl reactions. It follows the reaction pathways of Colket and Seery
ACETYLENE AND BENZENE PYROLYSIS
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Thermochemical data from readily available sources were used [46–50], when possible. Otherwise, thermodynamic estimates of species were made using techniques such as group additivity [51–53]. Of particular concern during this study was the careful determination of the heat of formation of the higher aromatics in order to minimize the errors in the proposed rate constants. The detailed reaction mechanism will be reported elsewhere [54]. Examples for the PAH reactions are given in Table 1.
Modeling and Experimental Results Fig. 1. Reaction pathways of PAH formation. (1) Successive growth including outer and inner-ring formation, (2) Combination reaction including inner-ring formation (4), (3) Cyclopentadienyl recombination.
[40] and Zhang and McKinnon [36] for biphenyl production and extended the reaction type (2) up to PAH molecules of roughly 700 amu. In addition, the biaryls are allowed to react with acetylene, forming inner rings. An example for this pathway is shown for the formation of phenanthrene (C14H10) in Fig. 1 (2) and (4). The mechanism also includes five-membered rings with respect to the assumption that such nonplanar PAH structures could provide three-dimensional soot precursors [41]. Pressure-dependent reaction rate coefficients were calculated [42–45].
In the calculations it was found that during C2H2 pyrolysis, most of the benzene is formed by C3H3 recombination, whereas the C2H2 addition to C4H3 and C4H5 radicals producing benzene is less important. In general, the computed concentrations of aromatics in benzene pyrolysis were approximately two orders of magnitude higher than in acetylene pyrolysis. For the formation of PAH containing 36 C-atoms and a number of H-atoms: 14 # x # 22, C36Hx, two different reaction types of PAH growth were distinguished and found to relate to each other: (a) the successive C2H2 addition (HACA) [23,33], and (b) the ring-ring condensation including inner-ring formation. Figure 2 presents the contribution to PAH formation predicted by the HACA model (succ. growth), and by the combinative ring-ring reactions
TABLE 1 Selected PAH reactions for benzene and acetylene pyrolysis Reaction A ` H 4 A* ` H2 A* ` C2H2 4 AC2H* 2 A* ` C2H2 4 AC2H ` H A ` C2H 4 AC2H ` H ACH3 4 A* ` CH3 ACH2 ` C2H2 4 AZP ` H AC2H* 3 ` CH3 4 AZP ` H ` H AZP* ` ZP* 4 A`2 ` H ` H A* ` A 4 AA ` H AA* ` C2H2 4 A`2 ` H AC2H* 2 ` C2H2 4 A`1 ` H AC2H* ` C2H2 4 A* `1 A* ` C3H3 4 AZP A* ` B 4 AB ` H AB 4 AB`1 ` H2 A* ` B* 4 AB
k8
n
EA
Ref.
2.5 1014 4.0 107 1.6 1014 1.0 1013 1.4 1016 3.2 1011 5.0 1013 1.0 1013 1.1 1023 4.6 106 1.6 1016 5.1 1048 1.0 1013 1.0 1012 1.0 108 3.1 1012
0 1.56 10.19 0 0 0 0 0 12.92 1.97 11.33 110.53 0 0 0 0
66.9 15.9 83.7 0.0 417.7 29.3 0.0 16.7 66.5 30.5 22.61 117.2 0.0 20.9 133.9 0.0
[33] [33] [33] [23] [24] [24] [24] [24] [33,36] [33] [33] [33] Est. [40] [40] [20]
A, B, AA, AB: PAH, six-membered rings; AZP; PAH containing a cyclopentadiene ring; ZP: cyclopentadiene; *: notation for radical, `1, `2: formation of additional PAH ring(s). k 4 k8 * Tn * exp(1EA/RT); Units for k8: cm3, mol11, s11, units for EA: kJ, mol11.
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SOOT FORMATION AND DESTRUCTION
Fig. 2. Modeled (C36Hx) in C2H2 and C6H6 pyrolysis by two different growth mechanisms at p 4 6 [MPa], C-atom concentration 4 4.0 [mol/m3]. ——: C6H6 pyrolysis; - - -: C2H2 pyrolysis.
Fig. 3. Induction time s. (a) calculated, averaged value for the blocks of PAH C36Hx to C48Hx, ——, and (b) measured data, - - -, in the C2H2 and C6H6 pyrolysis as a function of 1/T[K11]. C2H2 pyrolysis: p 4 5.7 [MPa]; C-atom conc. 4 3.8 [mol/m3]; C6H6 pyrolysis: p 4 6.0 [MPa]; Catom conc. 4 4.0 [mol/m3].
(comb. growth). To determine the importance of the ring-ring condensation (b) in comparison to the successive growth (a), two kinds of calculations were performed. The first case included both types of growth mechanisms, whereas for the second case all ring-ring condensation reactions were removed from the mechanism. As an example, the formation pathway of the block of PAH containing 36 C-atoms (C36Hx) is analyzed in the C2H2 and C6H6 pyrolysis.
The percentage of the formation of C36Hx resulting from these two channels is displayed as a function of the normalized reaction times, s(t)/scalc, where scalc is the calculated induction time for the C6H6 pyrolysis and for the C2H2 pyrolysis, respectively (see the following). During benzene pyrolysis, C36Hx formation starts very early. Compared with the ring-ring condensation, the successive HACA reaction channel is inefficient in the benzene pyrolysis. Even after 2.5 times the induction period, it amounts to less than 1.0%. During acetylene pyrolysis, the formation of C36Hx is also dominated by the combinative PAH growth, especially at early reaction times. With increasing reaction times, the influence of the combinative channel decreases in favor of the successive pathway. These results demonstrate the importance of the combinative growth mechanism for the modeling of high molecular weight PAHs, which are the direct precursors of the soot particles. In the following, the calculated induction periods of PAHs, and in particular those of high molecular mass, are compared with measured soot induction times in different hydrocarbon pyrolysis systems. The induction time in pyrolysis obtained by the shock tube method is defined by the intersection of the tangent of the maximum slope to the measured soot yield curve with the time axis, and in the calculations by that of the tangent of the modeled PAH curve with the time axis [6,13]. Our experimental results on the induction time demonstrate that, for example, during benzene pyrolysis, the induction time is much smaller than that found during acetylene pyrolysis for nearly the same C-atom concentration. These results indicate that soot formation starts much earlier in the benzene pyrolysis than in the acetylene system, although the activation energies for the induction time for both pyrolysis system are similar [AE 4 228 kJ/mol (C2H2) and 229 kJ/mol (C6H6)] [6,13]. The experimentally obtained induction time reflects the rate of all reactions from the first building blocks of the aliphatic radicals to the first aromatic ring system and also for the growing of the PAH compounds up to the first soot particles. Because the measured induction time sexp is an expression composed of many chemical reactions with very different reaction rates, and the required size of PAH essential for soot precursor formation is assumed to be 400– 800 amu [55–57], the induction times for the blocks of PAH of C36Hx to C48Hx were calculated and averaged. In Fig. 3, these averaged calculated induction times and the experimentally determined induction times are displayed as a function of temperature. The model correctly predicts the longer induction period of acetylene pyrolysis in comparison to benzene pyrolysis. In acetylene pyrolysis, the averaged calculated induction periods for the blocks of PAH of C36Hx to C48Hx are roughly
ACETYLENE AND BENZENE PYROLYSIS
Fig. 4. Experimentally determined relative soot mass growth rate constants kf /[C], ——, and calculated relative formation rates for PAH C48Hx, - - -, as a function of 1/T [K11].
two times the measured ones. In benzene pyrolysis, the induction period is approximately 1.5 times the experimental one. The slightly longer calculated induction periods of these high molecular weight PAHs might be caused by an additional reaction channel forming soot precursors not considered in the present model. This supplementary route could prevent very high PAHs from growing exclusively via the successive and/or combinative pathway to attain the required size of nuclei, causing shorter induction times of the soot precursors. Additional calculations will be performed to clarify this point. The slope of the calculated and measured curves are nearly identical, resulting in a similar activation energy of AE 4 192 kJ/mol for acetylene and AE 4 201 kJ/mol for benzene, in comparison to the measured activation energies of AE 4 228 kJ/mol (acetylene) and AE 4 229 kJ/mol (benzene) [6,13]. As an additional check for the accuracy of the used pressure-dependent rate constants at 6 MPa, we also calculated the induction time for the high molecular weight PAHs at lower, near atmospheric pressure and found no pressure dependency of the induction periods. These results are in agreement with experiments [5–7]. The experimental results obtained by the shock tube method for the soot mass growth rate in aliphatic and benzene pyrolysis differ considerably. In the entire temperature range, covered by the measurements, the soot mass growth rates were always faster in benzene pyrolysis than in the pyrolysis of aliphatics, including n-C6H14, C2H4, C2H2 and CH4 [5,6,14]. The measured activation energy for the soot
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mass growth of benzene was AE 4 205 kJ/mol for 1600 , T [K] , 2400 [6]. In the C2H2 pyrolysis, the rate of soot mass growth increases up to 2000 K and then decreases toward higher temperatures. The resulting activation energy is AE 4 101 kJ/mol for T , 2000 K [6]. The different behavior of the soot mass growth rates for the fuels C2H2 and C6H6 may be caused by different factors. The decrease of the soot mass growth rate in the C2H2 pyrolysis for T . 2000 K can result from (1) a change in the nucleation of soot precursors (i.e., PAH) or in the soot particle inception process, or (2) a change in the soot surface growth processes, such as a lack of additive species, or tempering. Some hints for the observed temperature dependence in the C2H2 pyrolysis can be obtained by calculating higher PAH concentrations as a function of temperature for the fuels C6H6 and C2H2. This has been done for the block of PAH C48Hx using the reaction mechanism specified earlier. In Fig. 4, the relative rate constants for the soot mass growth kf/[C] [cm3 mol11 s11)], measured in the shock waves experiments are presented along with the relative formation rates kf of C48Hx calculated for the pyrolysis of C2H2 and C6H6. The curves of the computed relative formation rates of C48Hx have a similar shape to those of the relatives kf/[C], the rate constants of soot mass growth. This result holds for C6H6 as well as for C2H2. Although for the pyrolysis of C2H2, the maximum of the calculated formation rates of C48Hx is reached at '50 K lower temperature, the similarity of computed and measured curves is clearly seen. Thus, the decrease of the soot mass growth rates during acetylene pyrolysis for T . 2000 K may be caused by a change in the kinetics involving PAH. This result strongly indicates that soot growth is substantially influenced by the kinetics of the soot precursors. The modeling of C48Hx also shows a faster formation rate for the pyrolysis of C6H6 than for the pyrolysis of C2H2 in the entire temperature regime, supporting the conclusion that the faster growth rates of the soot particles are related to the faster formation rates of the PAH.
Discussion In both C2H2 and in C6H6 pyrolysis it is shown that the biaryl reaction pathway is important for the formation of PAH. The increasing importance of the successive growth to longer reaction times in C2H2 pyrolysis in comparison to C6H6 pyrolysis is caused by the higher acetylene concentrations. In acetylene pyrolysis, the most sensitive reaction for the PAH growth is the formation of the first aromatic ring; in this work and at the conditions used
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here, the C3H3 recombination to benzene predominates. Comparing pyrolytic results from both systems, it is concluded that PAH inception is mainly influenced by the benzene concentration. In contrast to benzene, acetylene attains substantial influence on PAH growth at longer reaction times where the benzene concentration is decreasing. These results indicate that a maximum PAH growth, followed by soot growth, will take place when these two species benzene (responsible for PAH inception) and acetylene (responsible for PAH/soot growth at longer reaction times) work “hand in hand” [6]. The calculations also show the importance of the combinative PAH pathway in the model. Removing these reactions and taking only the HACA mechanism into account causes up to 10 times longer induction periods for the calculated PAH. Even variations of the thermodynamic data in the error range given in Ref. [53] and an increase of the reaction constants of the H-abstraction and C2H2 addition in the HACA route could not significantly raise the induction periods predicted by this model. In the HACA route, in particular, the H-abstraction reaction and the formation of the outer rings (see Fig. 1 (1)) slow down the induction periods of the aromatics. Therefore, we have minimized the formation of outer rings in the reaction scheme. However, outer rings contribute significantly to the growth of smaller aromatics. The larger the PAH, the smaller is the ratio of the outer to inner rings that have to be formed in the reaction sequence to obtain high condensed PAH such as circumcoronene. But even a HACA model including as few outer ring formation steps as possible could not produce the required size of PAH nuclei of 400–800 amu, which are essential for soot precursor formation [55–57]. These results confirm those of earlier studies of McKinnon and Howard [58], Homann and coworkers [59], and Alexiou and Williams [60]. Based on a comparison of the formation rates of PAH and kf the soot mass growth rate constant, the conclusion is drawn that in aromatic, as well as in aliphatic pyrolysis, mainly the same PAH growth mechanism takes place up to temperatures of 1900– 2000 K. Above this temperature in acetylene pyrolysis, the benzene concentration is too small to initiate a combinative PAH growth. The PAH growth only due to a successive acetylene addition is too unefficient and too slow, especially at early reaction times (see Fig. 2). In benzene pyrolysis, the biaryl concentration is up to two orders of magnitude higher than in the pyrolysis of aliphatic hydrocarbons. Therefore, the formation rate curve of PAH above 2000 K increases further when the one of acetylene pyrolysis is already decreasing. It is worthwhile to mention that including a third reaction pathway leading to phenanthrene [24,25] in the mechanism
indene ` cyclopentadienyl → anthracene ` H ` H anthracene ↔ phenanthrene
may further reduce the influence of the HACA route. Apparently the cyclopentadienyl channel is also more successful in producing phenanthrene than the successive acetylene addition to benzene. In future work, we shall also extend this mechanism to the formation of higher aromatics. Conclusions Shock tube data for the pyrolysis of benzene and acetylene have been compared with computations. The data have been used to examine three chemical kinetic pathways for PAH mass growth: 1. the H-abstraction acetylene addition (HACA) route, 2. the ring-ring condensation of aromatics, and 3. the cyclopentadienyl pathway leading to anthracene and phenanthrene. The calculated induction times of higher aromatics reasonably reflect the experimental trends of soot induction. The investigation of the reaction mechanism for PAH growth show that ring-ring condensation is more efficient than the HACA route in producing high aromatics at short reaction times. These results hold even during acetylene pyrolysis. With increasing reaction times, the HACA reactions became more prominent for PAH growth. The cyclopentadienyl pathway is also an important route in forming PAHs, especially in acetylene pyrolysis. Soot mass growth rates and the calculated formation rates of PAHs show the same dependence on temperature. These observations are a further indication that soot mass growth is strongly related to PAH and soot precursor formation. Acknowledgments H. Bo¨hm appreciates Prof. K. Kohse-Ho¨inghaus for valuable discussions on elementary reaction kinetics and her interest in the present work. We are indebted to Prof. H. Gg. Wagner for his support and suggestions throughout this research. We thank Prof. U. Maas for providing the Homrun source code to perform the model calculations. Computing time support from HLRZ, Forschungszentrum Ju¨lich and the Institute of Theoretische Physik, Universita¨t Bielefeld, is gratefully acknowledged.
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COMMENTS Assa Lifshitz, The Hebrew University of Jerusalem, Israel. Have you considered the possibility of varying the concentrations of C2H2 and C6H6 in order to examine its effect on the induction time for appearance of soot? Also, would the model predict such an effect? Author’s Reply. In an additional poster presented at this Symposium [1], we determined the influence of acetylene and benzene mixtures on the induction time by varying the composition of these mixtures. The measured increase of the induction time of soot formation with decreasing contents of benzene is in line with the predictions of the model for the induction time of PAH formation.
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● P. Cadman, University of Wales, Aberystwth, UK. Can your mechanisms predict the relative amounts of small to large PAH(s) with time, P, and T? You did not mention the modeling results for the soot mass growth, etc., for benzene/C2H2 mixtures. Can you comment on these? Author’s Reply. The relative amount of low-molecularweight PAH to high-molecular-weight PAH can be predicted by the model in dependence on the reaction time. So far, in the calculations, we have mainly determined the influence of the temperature on the PAH species at pressures of 57 to 60 bar. We did a few calculations and experiments at 6 bar and found that the measured induction periods of soot as well as the calculated ones for PAH formation do not differ much to the ones obtained at 60 bar. In addition, we observed that in benzene/hydrogen mixtures, the induction time of soot formation is increased in comparison to benzene pyrolysis. The same trend for PAH formation is calculated by the model.