The kinetic modeling of soot precursors in ethylene flames

Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 1489–1495

THE KINETIC MODELING OF SOOT PRECURSORS IN ETHYLENE FLAMES TIZIANO FARAVELLI, ALESSANDRO GOLDANIGA and ELISEO RANZI CIIC Department, Politecnico di Milano Piazza Leonardo da Vinci 32 20133 Milan, Italy

A comprehensive, semidetailed kinetic scheme describing hydrocarbon oxidation is applied to the simulation of premixed, rich, sooting, ethylene laminar flames. The main goal of this work is to investigate the soot precursor and aromatic pathways under different operative conditions in terms of temperatures and feed composition. The modeling computations are in good agreement with the experimental data and are also comparable with predictions of different kinetic schemes present in the literature. The recombination reactions of resonantly stabilized radicals (such as propargyl) and C2H2 addition on linear dehydrogenated molecules (C4Hx) are taken into account to explain formation of the first aromatic ring. Two major channels of benzene and polycyclic aromatic hydrocarbon (PAH) formation are observed in the conditions under analysis. The former, which is not included in previous literature schemes, is faster and occurs first (where the conversion is still low). It is governed by ethylene and vinyl radical, which, through butadiene and butenyl radicals, explain the formation of cyclopentadiene and through further successive additions give rise to benzene and styrene. This mechanism should be the starting point for the initial formation of heavy highly hydrogenated compounds. Acetylene and resonantly stabilized radicals are mainly responsible for the successive aromatic growth. The study of such pathways is also important for the analysis of low NOx burners and new process alternatives, such as recirculating flue gases, where pollutant emission reductions are pursued by the use of low-temperature flames. The comparisons with experimental data for pure ethylene pyrolysis at lower temperatures (1100 K) confirms the validity of the assumed mechanism.

Introduction Much work has gone into understanding the main pathways through soot formation in hydrocarbon flames in the recent years. In particular, experimental and theoretical studies have been performed to increase the knowledge of the detailed reaction kinetics of soot precursors. Due to its fundamental importance, the mechanism of benzene and first aromatic formation, growth, and oxidation during hydrocarbon combustion has been investigated in depth, especially at high temperatures. Despite this, several uncertainties remain regarding the formation paths of benzene in flames. The role of acetylene and C4 species or the effect of the propargyl radical are still being debated and have not yet been completely clarified. Nevertheless, detailed models that predict benzene production with a reasonable degree of accuracy are referred to in the literature. In particular, Frenklach and coworkers proposed a detailed kinetic scheme for the combustion of hydrocarbon and the consequent formation of polycyclic aromatic hydrocarbon (PAH) and soot. This mechanism, which will be referred to as WF in this context, was successively revised and tuned [1]. More recently, Marinov et al. [2] presented a new and different kinetic model (indicated as MAR here). In the

MAR scheme, the key reaction path toward aromatics and PAHs involves the interactions of resonantly stabilized radicals, for example, propargyl and 1methylallenyl. WF, on the contrary, assumes that reactions between C4 species and acetylene are of the same importance in benzene formation. Moreover, according to WF, the successive mass growth of aromatic compounds is governed by the two-step HACA (Hydrogen Abstraction followed by aCetylene Addition) mechanism, whereas MAR proposes a relevant role played by cyclopentadienyl and species with C5 ring structures. Taking advantage of these kinetic schemes, we extended the reliability range of an already available detailed kinetic model for the simulation of hydrocarbon oxidation and combustion. We introduce the previous channels into our scheme in a simplified way that reduces the number of total species by lumping some of the isomers with rate parameters that were initially taken directly from those mechanisms and sometimes partially modified according to analogy rules by comparison with similar reactions. It is worthwhile mentioning that, due to our previous and well-established experience of pyrolysis reactions, vinyl, allyl, methylallenyl (a4C4H7) cyclopentadienyl radicals, as well as cyclopentadiene, are involved in a relevant subset of reactions presented in

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the past [3]. The extension of the model’s reliability to lower temperature conditions has shown that this reaction subset is important for the proper prediction of the conversion, the initial reactivity of the system, and, above all, the correct reproduction of the formation of primary species such as butadiene, cyclopentadiene, and, finally, the first aromatic ring. The p-C4H7 radical is formed via vinyl addition to ethylene as well as ethyl radical addition to C2H2. This radical can isomerize to a-C4H7, and both of these C4 radicals decompose to butadiene. The more stable a-C4H7 can also react with C2H4 to give cyclopentene (cy-C5H8) and methyl-cyclopentene. Successive dehydrogenation and dealkylation reactions explain the formation of cyclopentadiene. cyC5H6 is also formed by the addition of C2H3 to C4H6. This reaction, which is very important in butadiene disappearance, competes with the possibility of benzene formation. C6H6 also comes from the cyclo-addition of ethylene to cy-C5H6. In short, both C4H6 and cy-C5H6 are important benzene intermediates. To validate the kinetic scheme at high temperatures, the hierarchical approach suggests the preliminary study of simpler species. That is why we analyze and compare model predictions for three different ethylene sooting flames obtained at different temperature and pressure conditions. As already mentioned, the study of new process alternatives, the analysis of staged combustion, and/ or low NOx burners in order to reduce the pollutant emissions tends toward flames operating at lower temperatures. Furthermore, the difficulties in the complete mixing of process streams can emphasize features of a subset of the ethylene pyrolysis reactions. Computational Model and Kinetic Scheme The computational model used in this work is a slightly modified version of the Sandia code of laminar, one-dimensional premixed flame (PREMIX) [4]. The modifications introduced allow the management of lumped reactions with several products and, above all, the use of H abstraction reactions in a simplified form that assumes analogy and similarity rules for the estimation of the kinetic parameters [5]. The kinetic mechanism (SOX) is based on a general approach for the development, validation and extension of a detailed oxidation scheme for alkanes [6]. Due to the hierarchical modularity of the mechanistic scheme, this model is based on a detailed submechanism of C1-C4 species. Assuming analogy rules for similar reactions, only a few fundamental kinetic parameters are needed for the progressive extension of the scheme toward heavier species [6,7]. These parameters define the main classes of primary oxidation reactions appropriate to the temperature ranges. At higher temperatures (1000–3000

K), the decomposition of alkyl radicals as well as molecular decomposition reactions prevail for different reaction paths. The resulting kinetic model of hydrocarbon oxidation from methane up to iso-octane consists of about 200 species and 3000 reactions. Aromatic oxidation at low temperature has also been investigated with particular attention to combustion engine conditions [8]. The peculiarity of corresponding comprehensive schemes is the need to simultaneously cover hydrocarbon pyrolysis and oxidation in both high- and low-temperature regions. The investigation of soot precursors is then also extended to relatively lower temperatures (1000–1200 K) where experimental data for ethylene pyrolysis are available. Due to lack of space, only a few reactions that specifically refer to the investigated operating conditions will be discussed. Their kinetic parameters are reported in Table 1. Nevertheless, the different subsets of the complete scheme are discussed elsewhere [9–11] and available upon request. Thermochemical information was primarily obtained from the CHEMKIN thermodynamic database [12]. Unavailable thermodynamics were estimated by group additivity and difference methods [13]. The transport parameters were obtained from the CHEMKIN transport database [14] or using the method described in Wang and Frenklach [15]. The development of a scheme that is reliable for a very wide range of operative conditions requires comparison with different experimental data carried out under different conditions in terms of temperature, dilution, and stoichiometry.

Comparison of Modeling Results In the light of what has already been said about the general validity of the kinetic scheme, we compared our calculations with three different premixed ethylene laminar flames. Ethylene oxidation can be a useful and simple starting point in the understanding of soot precursor formation. The conditions are quite different both in terms of temperature (see Fig. 1) and dilution (from more than 50% inert to pure ethylene–oxygen flame). The stoichiometry, on the other hand, is quite similar under all three conditions. The key chemical reactions that produce the principal products under investigation, discussed in the following section, were identified through a sensitivity study based mainly on the reaction flux analysis. High-Temperature Ethylene Flame Data The modeling results are first compared with experimental results of two ethylene sooting flames at high temperatures [16,17]. The two flames are fuel rich (f 4 2.76 and f 4 2.36, respectively) and at

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TABLE 1 Rate constants of intermediate temperature mechanism reactions

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 R22

Reaction

A

Ea

C2H3 ` C4H6 ⇒ H2 ` H ` C6H6 C2H3 ` C4H6 ⇒ CH3 ` cy-C5H6 C2H3 ` C4H4 ⇒ C6H6 ` H C2H3 ` C4H2 ⇒ C6H5 ` H C2H2 ` C2H5 ⇒ p-C4H7 C2H2 ` C2H3 ⇒ C4H4 ` H C2H2 ` CH2s ⇒ C3H3 ` H C2H4 ` C2H4 ⇔ 1-C4H8 C2H4 ` cy-C5H6 ⇒ C6H6 ` H ` CH3 C3H3 ` C3H3 ⇒ C6H6 C4H4 ` C4H5 ⇒ styrene ` H p-C4H5 ⇔ C4H4 ` H p-C4H5 ` C2H4 ⇒ C6H6 ` H ` H2 C4H6 ⇔ C2H3 ` C2H3 p-C4H7 ⇔ C2H4 ` C2H3 p-C4H7 ⇔ a-C4H7 a-C4H7 ⇔ C4H6 ` H p-C4H7 ⇔ C4H6 ` H a-C4H7 ` C2H4 ⇒ CH3 ` H2 ` cy-C5H6 1-C4H8 ⇔ a-C3H5 ` CH3 C5H5 ⇔ C3H3 ` C2H2 cy-C5H6 ⇔ C5H5 ` H

0.30 109 0.60 109 0.63 109 0.50 109 0.30 109 0.10 1010 0.40 1012 0.10 108 0.30 109 0.20 1010 0.40 1011 0.10 1014 0.40 109 0.25 1016 0.25 1014 0.25 1013 0.15 1015 0.50 1013 0.15 109 0.25 1017 0.30 1016 0.50 1016

4000 4000 3105 6000 7600 5000 0 40000 30000 0 3000 44000 6000 107000 38000 36000 51000 38000 13000 77000 74000 83000

Units are kmol, m3, s, K

Fig. 1. Experimental temperature profiles obtained in laminar ethylene flames [16,17,19].

atmospheric pressure. The first is diluted in Ar (65.6%) and the maximum temperature is about 1600 K, and the second flame is undiluted with a peak temperature of about 1800 K. The two kinetic schemes referred to have been assumed as the starting point in analyzing these

flames. WF model is well tested and consolidated under these conditions and has already been compared with the flame of Harris et al. [16]. Recently, the MAR model was applied to methane and ethane flames under similar conditions in terms of maximum temperature and equivalence ratio. Results from this model suggested a key role of resonantly stabilized radicals and cyclo-C5 species. These key species and reactions have been already discussed and proposed by other authors [3,18]. The SOX model takes into account the experience of both of these works and is thus able to match these high temperature experiments quite well. For example, Figs. 2 and 3 show the experimental and predicted mole fraction profiles of some species in both of the flames. The general good agreement of the model can be observed. It is not only the ethylene conversion and main products, such as CO, CO2, that are correctly predicted; minor species behaviors such as CH4, C4H2 or C6H6 are also modeled in an accurate way. The same level of accuracy (with some differences, however) can be observed using both the MAR and WF schemes. The main SOX characteristic lies in the presence of an initial maximum of the heavier hydrocarbon species (C6`), followed by a decrease and then by a secondary rise. This behavior was also experimentally observed by Ciajolo

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et al. [17]. Fig. 4 shows the matching between experiments and calculations for the three mechanisms. The total amount of the C6` species is always underestimated close to the burner, whereas SOX slightly overestimates under the final conditions. SOX and MAR reproduce the initial trend, but in SOX this first rapid formation is more evident. WF is more in line with SOX in the second part of the flame. A sensitivity analysis of the main mechanisms involved in this behavior shows that the first peak is initially governed by the ethylene and vinyl radical. In particular, benzene and cyclopentadiene are formed by Fig. 2. Ethylene laminar flame [16]. Comparison of model predictions with experimental profiles. Symbols represent the experimental measurements, and curves represent the model predictions. Reactant composition: C2H4 16.5%, O2 17.9%, Ar 65.6%.

C2H3 ` C4H6 4 H2 ` H ` C6H6

(R1)

C2H3 ` C4H6 4 CH3 ` cy-C5H6

(R2)

C2H3 ` C4H4 4 C6H6 ` H

(R3)

Once formed, benzene can partially add C2H3 to form styrene, which is also produced by direct interaction between C4H4 and C4H5: p-C4H5 ` C4H4 4 C8H8 ` H

(R11)

This hydrogenated aromatic growth could explain the formation of the tarlike materials experimentally revealed in the soot preinception region of the flame. Only successively do C3H3 and C2H2 mechanisms become important in benzene formation and explain the second PAH growth. Intermediate-Temperature Ethylene Flame Data

Fig. 3. Ethylene laminar flame [17]. Comparison of model predictions with experimental profiles. Symbols represent the experimental measurements and curves represent the model predictions. Reactant composition: C2H4 44%, O2 56%.

Fig. 4. Ethylene laminar flame [17]. C6` mass fraction comparison between experimental data and model predictions (SOX, WF, and MAR).

Castaldi et al. [19] have presented experimental results of a premixed, rich (f 4 3.06) sooting, ethylene–oxygen–argon burner-stabilized flame. The main difference is due to the lower temperature investigated, that is, a maximum value of about 1450 K. Under these conditions, the subset of reactions previously discussed becomes more significant and can justify the formation of some intermediate species. Fig. 5 shows that the SOX model correctly reproduces the ethylene profile up to the almost complete conversion. Predictions and measures are in good agreement for other low-molecular-weight species, too. The interactions between C2H3 and O2, that give CH2CO and HCO ` CH2O, are very important reactions for the oxidation path. CH2CO plays also an important role in CH4 formation. Fig. 6 makes it possible to analyze heavier species. cy-C5H6 formation is correctly reproduced with the presence of a maximum amount, slightly underestimated, which is initially sustained by the vinyl addition on butadiene and the successive cyclization and methyl detachment reaction (R2). Butadiene must therefore be necessarily well estimated. Sensitivity analysis shows that major pathways toward C4H6 are C4H7 radical decomposition and C2H3 ` C2H3 termination reaction. A correct

SOOT FORMATION IN ETHYLENE FLAMES

Fig. 5. Ethylene laminar flame [19]. Comparison of model predictions with experimental profiles. Symbols represent the experimental measurements and curves represent the model predictions. Reactant composition: C2H4 21.3%, O2 20.9%, Ar 57.8%.

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Fig. 7. Ethylene laminar flame [19]. Comparison of model predictions with experimental profiles. Symbols represent the experimental measurements, and curves represent the model predictions for styrene, toluene, ethylbenzene, xylene.

been yet introduced into the model, and a specific study will address this issue. Low-Temperature Ethylene Pyrolysis

Fig. 6. Ethylene laminar flame [19]. Comparison of model predictions with experimental profiles. Symbols represent the experimental measurements, and curves represent the model predictions for C4H6, cyC5H6, C6H6, phenyl-acetylene, pyrene.

match of benzene inception can be observed, though final levels are significantly overpredicted. Other species comparisons are reasonable as shown in Fig. 7. Toluene, phenyl acetylene, indene, and acenaphthylene predictions agree with measured values. However, for the sake of clarity and space, only some of these compounds have been reported. Xylenes, ethylbenzene, and pyrene species, present in small amounts, are also quite well predicted. Particular interest should again be devoted to styrene, which is initially overestimated. Two possible competitive paths can be assumed. On the one hand, styrene dehydrogenation leads to acenaphthylene, and the successive fate to PAH formation is already defined. Alternatively, styrene could undergo successive interactions also with ethylene and vinyl leading to a polymerization and the formation of hydrogenated PAH. This class of reactions has not

At even lower temperatures no flame data are available. Nevertheless, some experimental results of ethylene pyrolysis can be used to emphasize the role and validity of the intermediate temperature mechanism. The pyrolysis reactions of alkenes and, typically, ethylene and propylene have been extensively studied both experimentally [20] and kinetically [3] due to their importance in the characterization of effluents from steam cracking units. As already suggested, reactions featuring these species are responsible for the initial formation of cyclopentadiene and benzene in the present flames. Experimental data on the pyrolysis of pure ethylene have been obtained by Kunugi et al. [20] in the temperature range 970–1127 K. Fig. 8 shows a comparison between experimental and predicted ethylene decomposition at two different temperatures. The presence of an induction period is explained by a dimerization of ethylene to form 1-butene reaction (R8), via a concerted four-center molecular reaction [13]. The successive decomposition of 1-butene forms methyl and allyl radicals and initiates the radical reaction chain. C2H4 ` C2H4 4 1-C4H8

(R8)

1-C4H8 4 a-C3H5 ` CH3

(R20)

As soon as the first radicals are formed, the decomposition proceeds through the addition of the vinyl radical to ethylene, leading to the formation of the primary p-C4H7 radical, which can isomerize to form

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the more stable methylallenyl radicals (aC4H7) or dehydrogenate leading to butadiene. p-C4H7 4 C2H4 ` C2H3

(R15)

p-C4H7 4 a-C4H7

(R16)

C4H7 4 C4H6 ` H

Fig. 8. Low temperature ethylene pyrolysis in an atmospheric PFR [20]. Comparison between experimental data (points) and model predictions (lines) at different temperatures.

(R17–R18)

Fig. 9a shows that the predicted selectivity of butadiene agrees quite well with the experimental data. The same level of accuracy is obtained for the other C4 species, which are extremely important in ethylene pyrolysis. As already discussed for higher temperature conditions, successive formation of cyclopentadiene comes both via the addition of vinyl radical to butadiene and via the addition of methylallenyl radicals to ethylene. C2H3 ` C4H6 4 cy-C5H6 ` CH3

(R2)

a-C4H7 ` C2H4 4 cy-C5H6 ` H2 ` CH3

(R19)

Both of these addition reactions are global reactions. As a matter of fact, the resulting intermediate unstable C6 radicals can isomerize with the formation of the methyl-cyclo C5 radical, which decomposes to form methyl (hydrogen) and cyclopentadiene. At low ethylene decomposition, cyclopentadiene becomes one of the main causes of the successive production of radicals through the following addition/initiation reaction: C2H4 ` cy-C5H6 4 (norbornene) 4 Benzene ` H ` CH3

Fig. 9. (a) Low-temperature ethylene pyrolysis in atmospheric PFR [20]. Mole selectivity vs. ethylene conversion at 1026 K. Symbols represent the experimental measurements, and curves represent the model predictions. (b) Low-temperature ethylene pyrolysis in atmospheric PFR [20]. Mole selectivity vs. ethylene conversion at 1127 K. Symbols represent the experimental measurements, and curves represent the model predictions.

(R9)

where norbornene is a very reactive intermediate that, through skeletal isomerization to methyl-cyclohexadiene, rapidly decomposes to form benzene, hydrogen, and methyl radicals [3]. This formation of the first aromatic ring from ethylene decomposition is evident in Fig. 9 where benzene selectivity is reported. Fig. 9 also shows the model’s capacity to predict a temperature shift of about 100 K: computed results again correctly agree with the experimental ones. Attention should also be paid to the good predictions of benzene and cyclopentadiene. For this case, both the MAR and WF mechanisms are not able to take into account the formation of these species due to the lack of intermediate-temperature reaction paths.

Conclusion A detailed kinetic mechanism of aromatic formation, growth, and oxidation in laminar premixed sooting ethylene flames has been presented. The predicted profiles agree quite well with experimental

SOOT FORMATION IN ETHYLENE FLAMES

data obtained under different operative conditions in terms of temperature and dilution. The observed results confirm that the intermediate temperature reactions of ethylene and vinyl radicals play a significant role and that butadiene and cyclopentadiene are important intermediates for benzene formation in sooting flames. The extension of the validation at lower temperatures, where ethylene pyrolysis data are available, emphasizes the importance of this subset of reactions and has allowed an extended assessment of the reliability of the overall model. Acknowledgments We are indebted to Mario Dente for his many suggestions and to Andrea D’Anna for the technical discussions. We gratefully thank Michael Frenklach and Nick Marinov for having promptly given us their kinetic schemes. We would also like to thank Michele Aina and Stefano Rocco for their helpful work. This project was supported by ENEL, Polo Termico di Pisa, under contract no. UT 021.

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