Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons and Soot

Article pubs.acs.org/EF

Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons and Soot Formation in Acetylene Pyrolysis Chiara Saggese,*,† Nazly E. Sánchez,‡ Alessio Frassoldati,† Alberto Cuoci,† Tiziano Faravelli,† María U. Alzueta,‡ and Eliseo Ranzi† †

Dipartimento di Chimica, Materiali ed Ingegneria Chimica “G. Natta” Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy ‡ Aragón Institute of Engineering Research (I3A), Department of chemical and environmental engineering, University of Zaragoza, Río Ebro Campus, 50018 Zaragoza, Spain ABSTRACT: The aim of this kinetic work is a critical and detailed analysis of the acetylene pyrolysis in a wide range of conditions, especially from 900 to 2500 K, in order to further validate and refine the kinetic mechanism of C2−C4 acetylenic species. In particular, the successive reactions of the intermediate products, such as C4 species and polyynes, were especially investigated to better understand the primary C2H2 kinetics. The subsystem of acetylene pyrolysis reactions clearly constitutes an important portion in the overall kinetic scheme and also a crucial topic in the further extension of the model toward the formation of polycyclic aromatic hydrocarbons (PAHs) and soot particles. The analysis of acetylene experiments, often with the formation of large amount of soot, requires the coupling of the gas phase kinetic scheme with a soot kinetic model. The gas-phase kinetics involves the formation of the first polycyclic aromatic hydrocarbons up to the first components of the soot kinetic mechanism (C20H16 and C20H10). The soot kinetic model is based on a discrete sectional method with an extensive use of lumping rules. Analogy and similarity rules with gas phase kinetics are used to extend the soot mechanism up to the formation of species with more than 107 carbon atoms. The lumped approach, extensively applied in the kinetic modeling of large hydrocarbon species, becomes a necessity in treating chemical reacting systems of these dimensions. The analysis of the experimental data of acetylene pyrolysis obtained in a wide range of conditions permits us not only to highlight the competition between the molecular and the free radical pathways but also to analyze the successive condensation reactions for the formation of PAHs and the growing of soot particles.

1. INTRODUCTION The formation of polycyclic aromatic hydrocarbons (PAHs) and soot particles in combustion processes is a topic of continuous research interest because of their environmental and health issues. In particular, diffusion flames, rich combustion, and pyrolysis conditions represent the most favored reacting systems in which soot formation occurs.1−3 All these conditions also highlight the key role of acetylene as an important precursor to PAHs and soot nucleation and formation through the H-abstraction and C2H2-addition (HACA) mechanism.4 An additional reason for the rising interest in the characterization of soot composition and morphology is also due to a large concern for the possible toxicity of soot particles from biofuel combustion.5 Because of all these reasons, the pyrolysis of different hydrocarbon fuels, rich flames with different configurations and several sooting conditions have been extensively studied during the last decades by many research groups, both experimentally and theoretically, as partially summarized in a couple of workshops on soot formation in combustion.6,7 As was clearly highlighted in several pioneering works,8−11 acetylene pyrolysis constitutes the first important reaction step toward the initial formation of vinylacetylene (C4H4) and diacetylene or 1,3-butadiyne (C4H2). Successive reactions form polycyclic aromatic hydrocarbons,12 well-defined intermediates, and soot precursors. Thus, it is widely accepted that this mechanism is largely attained through the well-known HACA © 2013 American Chemical Society

mechanism, which progressively increases the size of PAHs with the subsequent inception and formation of soot particles.13 Therefore, it is quite evident that there is a close relationship among C2H2, PAHs, and soot. The first experimental data on acetylene pyrolysis were measured in shock tube reactors in order to study the high temperature mechanism of acetylene pyrolysis and oxidation. They are very useful and permit a broad analysis of the first steps of acetylene decomposition, exploring different operating conditions. First, Wu et al.14 studied high temperature conditions (2000−2500 K) where the free radical pathway is favored by the formation of the very stable polyynes (C4H2, C6H2, etc.). PAHs and successive soot formation through molecular reaction paths were experimentally and kinetically investigated by Frenklach et al.11 and Colket et al.15,16 The experimental and kinetic study of Hidaka et al.,17 always investigating the pyrolysis and oxidation of acetylene behind reflected shock waves, summarized the previous works and also analyzed the reaction products, which were mainly C4H4, C4H2, C6H2 (1,3,5- hexatriyne), and C8H2 (1,3,5,7-octatetrayne) together with hydrogen. The product distribution was similar and confirmed the one obtained in previous works.14,16 Soot Received: October 14, 2013 Revised: December 24, 2013 Published: December 26, 2013 1489

dx.doi.org/10.1021/ef402048q | Energy Fuels 2014, 28, 1489−1501

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intermediates. This mechanism is indeed characterized by a competition between molecular and radical reaction paths.

formation was not reported and scarcely observed in these conditions. More recent studies on the pyrolysis of acetylene, both in flow reactors with relevant formation of soot and carbonaceous deposits, were reported by Norinaga et al.18,19 and by Alzueta and co-workers.20−22 These works report data on several compounds in great detail, from hydrogen and methane up to heavy PAHs, such as dibenzo(a,h)anthracene (C22H14), benzo(g,h,i)perylene (C22H 12), and coronene (C24H12). The influence of residence time, pressure, and fuel concentration on acetylene conversion and soot formation was also investigated. Therefore, the aim of this kinetic work is a critical and detailed analysis of all these pyrolysis data in order to refine and validate the acetylene pyrolysis reactions inside the POLIMI detailed kinetic mechanism.23 In particular, the relative low temperature conditions were not deeply investigated yet either experimentally or from the modeling point of view. The subsystem of acetylene pyrolysis, together with the recently revised high temperature reactions of benzene,24 clearly constitute not only a further step toward a more complete validation of the whole kinetic mechanism but also the necessary preliminary step in the kinetic analysis of PAHs and soot particle formation. The high severity conditions experimentally analyzed also involve the formation of large amount of soot and little amount of pyrolitic carbon. The proper analysis of these data, in which even more than 50% of the feed is transformed into soot, requires the use of a kinetic scheme able to predict the formation of both heavy PAHs and soot. For this reason, the POLIMI gas phase kinetic scheme, coupled with a soot kinetic model, is applied in this work. The gas phase kinetics involve the formation of the first PAHs, including phenanthrene (C14H10) and pyrene (C16H10) up to the first components of the soot kinetic mechanism (BIN1A: C20H16 and BIN1B: C20H10). The lumped approach, largely applied in the kinetic modeling of large hydrocarbon species,25,26 becomes not only useful but also a suitable way to treat chemical reacting systems of these dimensions. The soot kinetic model is based on a discrete sectional mechanism.27,28 Thus, the extensive use of lumped species allows us to extend the kinetic analysis and to describe the formation of soot particles with more than 107 carbon atoms. The reaction mechanism is largely developed by using analogy and similarity rules with the gas phase kinetics. The reference kinetic parameters of the different reaction classes involved in the soot kinetic mechanism are derived from the analogous reactions involved in the gas phase kinetics. As schematically shown in Figure 1, the pyrolysis mechanism of acetylene conversion first moves through successive addition and condensation reactions. Vinylacetylene and diacetylene, together with C2H, C2H3, and C4H3 radicals, are the first

2. KINETIC MODEL AND NUMERICAL METHODS The hierarchical and modular detailed kinetic scheme of hydrocarbon pyrolysis and oxidation here adopted29 consists of ∼10 000 reactions and more than 350 species. Thermochemical data for most species were obtained from thermodynamic databases.30,31 For those species whose thermodynamic data are not available in the literature, the group additive method was used to estimate these properties.32 In particular, the thermodynamic data of the polyynes (C4H2,C6H2, and C8H2) that are very stable species formed in the acetylene pyrolysis were obtained from databases.30,31 It was verified that those of C4H2 and C6H2 are consistent with the thermochemistry analysis, whereas C8H2 was modified and estimated on the basis of Benson’s method. Table 1 reports the thermodynamic data of the main species. Table 1. Thermodynamic Data of the Main Species: Enthalpy and Entropy at 300 K and Specific Heat at Different Temperatures ΔH [kcal/mol]

ΔS [cal/mol/K]

species

300 K

300 K

300 K

1000 K

1500 K

C2H2 C2H C4H2 C4H3 C4H4 C4H5 C6H2 C6H3 C8H2

54.2 135.2 111.7 129.8 67.9 86.1 169.6 173.2 226.1

48.1 51.0 59.8 69.1 66.6 73.1 71.0 78.5 76.1

10.6 10.0 17.7 18.0 17.5 19.4 24.6 24.4 29.5

16.3 12.6 26.6 30.4 33.7 38.9 36.8 39.7 46.9

18.3 14.1 28.9 33.4 37.9 43.1 39.9 43.7 50.9

Cp [cal/mol/K]

Because of their relevant role in acetylene pyrolysis, Table 2 reports the primary and successive reactions of acetylene, together with major reactions of vinylacetylene and diacetylene, which were discussed in this work. Additionally, the overall kinetic model, with thermo and transport properties, is available in CHEMKIN format from: http://creckmodeling.chem. polimi.it/. Similarly, Table 3 reports and summarizes the experimental conditions of pyrolysis of acetylene, together with the conditions of vinylacetylene and diacetylene pyrolysis. The results from these experimental conditions are critically analyzed and compared with model predictions in the next paragraphs. All numerical simulations were performed using OpenSMOKE code, an upgraded version and extension of the welltested DSMOKE code.33,34 The BzzMath 6.0 numerical library was adopted.35,36 The wide range of operating conditions of the experimental data of Table 3 allows us to highlight the relative importance of primary and successive reactions toward PAH and soot. Although several data are only affected by the primary reactions of acetylene pyrolysis, other data also include soot formation. This is partially true for the data of Colket et al.15 and becomes progressively more important in the conditions studied by Norinaga et al.18,19 and by Sánchez et al.21,22 For this reason, it is more convenient to first analyze the data involving the primary decomposition of the feed with negligible soot formation, and only then to analyze the data relating to more

Figure 1. Acetylene pyrolysis: major radical (filled arrows) and molecular paths (empty arrows). 1490


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Table 2. Major Reactions of Acetylene, Vinylacetylene, and Diacetylene Pyrolysis k = ATn exp(−Ea /RT)a

reactions 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 a

A

Acetylene Reactions C2H2 + C2H2 ↔ C4H4 1.5 × 109 C2H2 + C2H2 ↔ C4H2 + H2 1.0 × 1013 C4H4 ↔ C4H2 + H2 3.5 × 1011 2.0 × 1013 C2H2 + C2H2 ↔ C4H3 + H C4H4 ↔ C4H3 + H 3.682 × 1019 C2H3 + C2H ↔ C2H2 + C2H2 3.3 × 109 H + C2H2 + M ↔ C2H3+ M k∞ = 5.9 × 109 C2H2 + H ↔ C2H + H2 5.7 × 105 1.5 × 1010 C2H + C2H2 ↔ C4H3 C2H + C4H2 ↔ C6H2 + H 7.5 × 109 C2H + C6H2 ↔ C8H2 + H 5.0 × 109 C2H2 + C4H4 → C6H6 6.0 × 109 8.0 × 108 C2H2 + C2H3 ↔ C4H4 + H C2H2 + C4H5 ↔ C6H6 + H 5.0 × 108 C2H2 + C6H5 ↔ C8H6 + H 2.0 × 109 1.0 × 109 C2H2 + C8H5 → C10H7 Vinylacetylene and Diacetylene Reactions C4H4 ↔ C2H + C2H3 1.0 × 1016 C4H4 + C4H4 ↔ C4H3 + C4H5 5.0 × 1013 C4H4 + C4H4 → C8H8 1.2 × 1010 C4H4 + C4H4 → C8H6 + H2 1.0 × 1011 C4H4 + C4H4 → C6H6 + C2H2 2.5 × 1011 C4H5 ↔ H + C4H4 5.0 × 1012 H + C4H4 ↔ C2H4 + C2H 2.0 × 1010 H + C4H3 ↔ C4H2 + H2 1.0 × 1011 H + C4H2 ↔ C4H3 2.5 × 1011 5.5 × 109 C6H3 + C2H ↔ C4H2 + C4H2 C4H2 + C4H2 ↔ C6H2 + C2H2 2.0 × 1013 C4H2 + C4H2 ↔ C8H2 + H2 2.5 × 1011 C4H2 + C6H2 ↔ C8H2 + C2H2 6.0 × 1012 C4H2 + C2H2 ↔ C6H2 + H2 1.0 × 1013 C2H2 + C6H2 ↔ C8H2 + H2 5.0 × 1010

n

Ea

reference

0 0 0 0 −1 0 0 1.9 0 0 0 0 0 0 0 0

37 400 68 200 66 000 81 500 116 282 −1 980 2 770 30 259 0 0 0 37 400 5 000 5 000 8 000 5 000

15b 15 15 15, 41, 42 15 43 44 39 45b 45 this workb 15 46 46 24 24

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

105 000 81 500 37 400 44 000 44 000 44 000 2 000 0 3 016 0 54 000 44 000 44 000 68 125 44 000

16 this workb 15b this workb this workb 29 this work this work 47 this workb this work 39b this work this workb 39

Units are: mole, L, s, K, and cal. bAdjusted to maintain the similarities and analogies among similar reactions.

Table 3. Experimental Data of Acetylene, Vinylacetylene, and Diacetylene Pyrolysis in Shock Tubes (ST) and Flow Reactor (FR) Conditions fuel

reactor

temperature (K)

pressure (atm)

residence time

feed composition

reference

C2H2

ST ST ST FR FR ST ST ST ST ST ST ST ST

1300−2200 2032−2534 1100−2400 873−1473 1000−1400 1100−2400 1500−2000 1882−1993 2158 1987 2067 1300−2000 1300−2000

1.1−2.6 0.3−0.5 8 1 0.08 8 0.2−0.5 0.3−0.4 0.4 0.34 0.45 1.1−2.6 1.1−2.6

0.8−2.5 ms 0.75 ms 0.7 ms 1.5 ± 0.3−3.8 ± 1 s 0.5−2 s 0.7 ms 0.75 ms 0.75 ms 0.75 ms 0.75 ms 0.75 ms 1.6−2.5 ms 1.6−2.5 ms

2.5% C2H2 in Ar 3.2% C2H2 in Ne/Ar 3.7% C2H2 in Ar 1−3% C2H2 in N2 C2H2 1% C4H4 in Ar 2% C4H4 in Ne 1% C4H2 in Ne 1% C4H2/1% C2H2 in Ne 1% C4H2/1% H2 in Ne 3% C4H2/7.5% H2 in Ne 1% C4H2 in Ar 1% C4H2/1−4% H2 in Ar

17 14 15 21, 22 18, 19 15 48 49 49 49 39 50 50

C4H4 C4H2

severe time, temperature, and pressure conditions, in order to study first the primary reactions of the feed and then the successive addition and condensation reactions to form heavy PAHs and soot. The core mechanism of acetylene pyrolysis discussed by Kiefer et al.37 relies on five major reaction steps, where C4H4 and C4H3 are key intermediates 1491

C2H 2 + C2H 2 ↔ C4 H4

(R1)

C2H 2 + C2H 2 ↔ C4 H 2 + H 2

(R2)

C4 H4 ↔ C4 H 2 + H 2

(R3)

C2H 2 + C2H 2 ↔ C4 H3 + H

(R4)


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Figure 2. Shock tube pyrolysis of three argon diluted mixtures of 1% C4H2 and with 1% or 4% H2.50 Comparisons of experimental (symbols: triangles for 1% C4H2 in Ar; circles for 1% C4H2 /1% H2 mixture in Ar; squares for 1% C4H2 /4% H2 mixture in Ar) and predicted results (lines with small symbols).

C4 H4 ↔ C4 H3 + H

3. COMPARISONS BETWEEN MODEL PREDICTIONS AND EXPERIMENTAL MEASUREMENTS As already mentioned, the comparisons between model predictions and experimental measurements here are discussed, initially analyzing diacetylene pyrolysis and only then the data relating to vinylacetylene and acetylene pyrolysis. Inside these three classes of experimental data, the kinetic study will move from the analysis of the experiments at lower severity toward data in which PAHs and soot become more important products. 3.1. Diacetylene Pyrolysis. 3.1.1. Shock Tube Experiments of Hidaka Et Al.50 Hidaka et al.50 analyzed the argondiluted pyrolysis of diacetylene in a shock tube device of 4.1 cm inner diameter. This work also includes experimental data on the pyrolysis of 1% diacetylene mixtures with 1 and 4% H2 diluted in argon. Further data relating to the pyrolysis and oxidation of acetylene from Hidaka and co-workers will be discussed in Section 3.3.1. Figure 2 shows a comparison between experimental and predicted data, at temperatures in the range of 1500−1900 K, for diacetylene pyrolysis in Ar and also for diacetylene mixtures with hydrogen at 1% and 4%. At the lower temperatures, C4H2 conversion as well as C2H2 formation are slightly overpredicted. On the contrary, the model underpredicts vinylacetylene formation and hydrogen only at high temperatures. This deviation is not easily explained on the basis of the hydrogen balance. Considering the heavier species, the model predicts a relevant formation of C6H2 not reported and not discussed in the paper of Hidaka et al.50 In fact, at low temperatures, the rate of production analysis (ROPA) shows the molecular reaction R27 as the most important in diacetylene pyrolysis

(R5)

Successive molecular polymerization reactions explain the formation of heavier species, whereas secondary radical processes allow the acetylene pyrolysis mechanism to complete. According to the kinetic analysis of Kiefer et al.,37 acetylene pyrolysis follows a molecular polymerization path at temperatures lower than 1100 K and chain radical reactions at temperatures higher than 1800 K, where H and C2H drive the polymerization path to form polyynes. Vinylacetylene (C4H4), benzene, and phenylacetylene are the major products of a second order molecular path. At high temperatures, the acetylene pyrolysis becomes of lower order and C4H2 together with C6H2 are the more stable intermediates, with only minor traces of C3H4 and butadiene. Because of the high activation energies of the radical chain initiation reactions R4 and R5, the high temperature mechanism is mainly a free radical process.38,39 At temperatures lower than 1200 K, the molecular reaction path is the prevailing one, and reaction R1 is the major one responsible for acetylene conversion. In these conditions, the successive addition reaction of C2H2 on vinylacetylene forms the first aromatic ring of benzene C2H 2 + C4 H4 → C6H6

(R12)

At the higher temperatures of the shock tube experiments, the chain radical mechanism prevails, together with the HACA mechanism to form aromatics and PAHs (R15, R16, in Table 2). As will be better discussed later, the mechanism below 1500 K is still the subject of sharp controversy and is influenced by the impurities of the feed.40 Figure 1 shows the importance of the molecular paths forming vinylacetylene and diacetylene. Thus, it seems convenient to analyze and compare model predictions with experimental data moving initially from C4H2 and C4H4, and only then to investigate the primary reactions of acetylene pyrolysis, when successive reactions of intermediate products are better defined.

C4 H 2 + C4 H 2 ↔ C2H 2 + C6H 2

(R27)

Similarly, the dimerization of diacetylene (R28) is the main reaction responsible of the formation of C8H2. C4 H 2 + C4 H 2 ↔ C8H 2 + H 2

(R28)

50

According to the authors, C6H2 and C8H2 were not measured using gas chromatography (GC) because of their low amount or because they were not gaseous products. 1492


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model, as shown in Figure 2. The C4H radical proposed by Hidaka to explain and support the radical path is not considered in the model. 3.1.2. Shock Tube Experiments of Kern Et Al.39,49 Kern et al.49 studied the pyrolysis of acetylene and diacetylene in a stainless steel shock tube reactor (330 cm length and 2.54 cm internal diameter). Neon diluted pyrolysis of 1% diacetylene, neat and in mixtures with 1.0% C2H2 and 1.0% H2 are presented in Figures 4−6, which show the comparisons between experimental and predicted results as a function of the residence time. Feed mixture and reaction product characterization were analyzed by using time-of-flight mass spectrometer.14,49 The experimental data refer to C 4H2 conversion and the formation of C2H2 and C6H2. Again, the dimerization reaction R27 is the most important reaction in diacetylene conversion. Diacetylene conversion is underpredicted at high temperatures, as also observed in the previous comparisons. These results highlight the importance of a specific investigation of the rate constants of these molecular reactions in the whole temperature range. Figures 5 and 6 show the effect of hydrogen and acetylene addition to C4H2 pyrolysis. The model properly predicts the increase of diacetylene conversion and acetylene formation promoted by the presence of H2. As previously observed,50 H2 addition favors the formation of C4H3 (R24, R25 in Table 2) and the successive radical paths. On the contrary, the C2H2 addition to the mixture limits C4H2 conversion, mainly because of the H abstraction reaction with the formation of C2H radical

For this reason, Figure 2 simply refers to the predicted values of C6H2 and C8H2. These two new molecular reactions are included in the model to account for the low temperature reactivity. Their rate constants are estimated in analogy with other bimolecular reactions like R31 in Table 2. Moreover, the experimental mass balance for the carbon atom in C4H2 pyrolysis decreased with increasing temperature, down to about 45% at 1900 K. This large mass deficit at higher temperatures may be related to the GC analyses, which were not able to identify C6H2 and the soot deposited on the inside wall of the shock tube. In H2 addition mixtures, the observed mass balance for the carbon atom was higher, up to about 65% at 1900 K, but still with a significant deviation. Figure 3 shows the ROPA of diacetylene at 1600 K in Ar in panel A and for the mixture with 4% of hydrogen in panel B.

C2 H 2 + H ↔ C2 H + H 2

(R8)

Successive addition reaction of C2H with acetylene forms C4H3 (R9 in Table 2), which forms diacetylene through the dehydrogenation reaction R25 with the effect of a reduction of conversion, as schematically shown in Figure 7. Figure 8 reports data related to a mixture of 3% C4H2 and 7.5% H2 in neon and studied experimentally by Kiefer et al.39 This is the only case in which the products up to C8H2 are detected and it is clear that the model properly captures the diacetylene reactivity but underpredicts the C8H2 formation, even though the total amount is very low and then uncertainties higher. 3.2. Vinylacetylene Pyrolysis. 3.2.1. Shock Tube Experiments of Kiefer Et Al.48 Kiefer and Manson51 and Kiefer et al.48 analyzed the vinylacetylene pyrolysis in the shock tube device. Figure 9 compares the experimental and predicted results in the temperature range 1492−2035 K. Acetylene is the major product and the model properly accounts for the dehydrogenation path to form diacetylene. The molecular paths of R1 and R3 dominate at 1492 K and still prevail at 2035 K, where they are responsible of ∼70% of C4H4 conversion, leading to the two

Figure 3. Rate of production analysis of diacetylene pyrolysis at 1600 K in the shock tube of Hidaka et al.50 Panel A: 1% C4H2/Ar mixture. Panel B: 1%C4H2 /4%H2/Ar mixture. The thickness of the arrows is proportional to the rate of reaction.

Indeed, H2 addition increases the formation of acetylene and vinylacetylene, whereas the formation of C6H2 decreases. Especially at high temperatures, the radical path through the H addition reaction to form C4H3 (R25 in Table 2) becomes more important in the C4H2 consumption, and its kinetics is modified taking into account the ab initio calculations of Klippenstein et al.47 The reaction between C4H2 and H2 (R2 in Table 2) increases the acetylene formation. Similarly, vinylacetylene production increases through R3, which has the kinetic constant proposed by Colket et al.15 The high concentrations of C4H4 and H2 justify the formation of a significant amount of ethylene, properly predicted by the

Figure 4. Shock tube pyrolysis of 1% diacetylene diluted with neon.49 Comparisons of experimental (symbols) and predicted results (lines: solid, T = 1882 K and dashed, T = 1993 K). 1493


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Figure 5. Shock tube pyrolysis of a neon-diluted mixture of 1% diacetylene and 1% H2.49 Comparisons of experimental (symbols) and predicted results (lines).

Figure 6. Shock tube pyrolysis of a neon-diluted mixture of 1% diacetylene and 1% C2H2.49 Comparisons of experimental (symbols) and predicted results (lines).

al.,15 whereas R3 is directly taken from Colket’s suggestion. Diacetylene is also formed through the reaction R25 from C4H3, which is formed by vinylacetylene and by the reaction of addition of C2H on acetylene (R9 in Table 2), whose kinetic constant is derived from that proposed by Wang et al.45 Carbon selectivity toward benzene and heavier species is limited to a low percentage. 3.2.2. Shock Tube Experiments of Colket Et Al.15 Colket at al.15 studied the pyrolysis of 1% vinylacetylene in a shock tube device (285 cm in length and of 3.8 cm internal diameter), operating at 8 atm, in a wide range of temperatures, with residence times lower than 1 ms. They provided quantitative detection of hydrocarbons up to C10. Figure 10 shows the fair comparisons between model predictions and experimental measurements as a function of the temperature. At lower

Figure 7. Rate of production analysis of diacetylene pyrolysis with C2H2 at 2158 K in the shock tube of Kern et al.49 The thickness of the arrows is proportional to the rate of reaction.

major products C2H2 and C4H2. The rate constant of reaction R1 was assumed as 75% of the value proposed by Colket et

Figure 8. Shock tube pyrolysis of a neon-diluted mixture of 3% diacetylene and 7.5% H2.39 Comparisons of experimental (symbols) and predicted results (lines). 1494


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Figure 9. Shock tube pyrolysis of vinylacetylene.48 Comparisons of experimental (symbols) and predicted results (lines: solid, T = 1492 K; dotted, T = 1688 K; dashed, T = 1877 K; and dot-dashed, T = 2035 K).

Figure 10. Shock tube pyrolysis of vinylacetylene.15 Comparisons of experimental (symbols) and predicted results (lines).

Figure 11 reports the rate of production analysis of vinylacetylene at 1700 K in order to investigate the naphthalene

temperatures, vinylacetylene pyrolysis is mainly ruled by the molecular reactions R1 and R19−R21, whereas chain radical reactions prevail only above 2000 K. The predicted C4H4 conversion is lower in these conditions in respect to the previous ones of Kiefer et al.48 and the model prediction results intermediate between the two sets of experimental data. In order to investigate this behavior, a rate of production analysis and a sensitivity analysis have been performed and they have shown that the relevant reactions in both conditions are the same. There is a slight overprediction of C4H2 at high temperatures, due to the reaction R25 in Table 2. The peaks of styrene, phenylacetylene, and benzene at about 1500 K are due to the molecular reactions R19−R21, respectively: C4 H4 + C4 H4 → C8H8

(R19)

C4 H4 + C4 H4 → C8H6 + H 2

(R20)

C4 H4 + C4 H4 → C6H6 + C2H 2

(R21)

Figure 11. Rate of production analysis of vinylacetylene pyrolysis at 1700 K in the shock tube of Colket et al.15 The thickness of the arrows is proportional to the rate of reaction.

formation. The pathways from naphthyl radical account for 75% of the naphthalene formation instead of those from benzene and phenylacetylene, which account for the remaining 25%. The naphthyl radical is formed mainly through the wellknown HACA mechanism with the reaction R16 and also considering the addition of C4 species to the ring. These reactions with the vinylacetylene dimerization appear in the sensitivity analysis favoring C10H8 formation, whereas the formation of acenaphthylene is a competitive path. 3.3. Acetylene Pyrolysis. 3.3.1. Shock Tube Experiments of Hidaka Et Al.17 As already mentioned in Section 3.1.1, Hidaka et al.17 studied the argon diluted acetylene pyrolysis in a shock tube between 1300 and 2200 K and pressure range 1.1− 2.6 atm. The acetylene feed was carefully purified in order to remove the possible presence of acetone.

The kinetics of reactions R20 and R21 are similar to the Kiefer’s kinetics, proposed for the reaction R31, in order to maintain analogies among similar reactions. The model agrees well with experimental data, except for the predicted peaks of styrene and naphthalene, which are shifted at higher temperature. The model prediction takes into account the progressive growth of aromatic species, whereas experimentally, the styrene formation is even anticipated in respect of its precursors, C6H6 and C8H6. The kinetic constant of R19 is already doubled in respect of the value of Colket et al.15 in order to take into account the primary formation of styrene. 1495


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Figure 12. Shock tube pyrolysis of acetylene.17 Comparisons of experimental (symbols) and predicted results (lines).

Figure 13. Low pressure shock tube pyrolysis of acetylene at 2032−2147 and 2534 K.14 Comparisons of experimental (symbols) and predicted results (lines: solid, T = 2032 K; dotted, T = 2147 K; and dashed, T = 2534 K).

Figure 14. Shock tube pyrolysis of acetylene.15 Comparisons of experimental (symbols) and predicted results (lines).

3.3.2. Shock Tube Experiments of Wu Et Al.14 As described by Wu et al.,14 the neon-diluted pyrolysis of 3.2% of acetylene was studied in the temperature range 2032−2534 K at 0.3−0.5 atm. Figure 13 compares experimental data and model predictions in terms of acetylene conversion and diacetylene and triacetylene formation. Chain radical reactions prevail in these conditions, and the molecular paths account for less than 20% of acetylene decomposition at 2032 K. Despite the low pressure and the limited reaction times, the formation of heavier species, lower than 5% in terms of carbon selectivity at 2032 K, becomes higher than 40% at 2534 K. 3.3.3. Shock Tube Experiments of Colket Et Al.15 An argon mixture containing 3.7% acetylene was used for studying acetylene pyrolysis at temperatures between 1100 and 2300 K by Colket et al.15 The shock tube device is the same used and is already discussed in Section 3.2.2. Quantitative measurements of hydrocarbons up to C10 are provided. The authors report that despite of the efforts for purifying feed, the final acetylene feed still contains 0.1 to 0.2% of acetone. Colket et al.16 discussed the role of these acetone impurities in the feed in explaining the low temperature chain radical process. This fact was further analyzed by Hidaka et al.17 and by

These conditions allow us to focus the different importance of molecular and free radical pathways. At 1300 K, the decomposition of acetylene mainly follows the molecular decomposition path with the prevailing formation of C4H4 (R1) and a minor role of the dehydrogenation reaction R2. At high temperatures (T > 1600 K), the free radical decomposition reactions rise in the importance and, above 2000 K, dominate the acetylene decomposition through the Habstraction reaction to form C2H (reaction R8 in Table 2) and the successive addition reaction to form C4H3 (R9 in Table 2). At temperatures higher than 2000 K, the model predicts 10−20% of carbon selectivity toward heavy PAHs and soot inside the reactor, as experimentally highlighted. Figure 12 shows the comparisons between model predictions and experimental measurements. It is noteworthy that the intermediate product (C4H4) is still underpredicted by the model and it comes not only from reaction R1 but also from reaction R13 in Table 2. The contribution of reaction R13 is negligible in these conditions, but it increases considering the presence of acetone at low temperatures, as it will be presented in the comparisons with the experimental data of acetylene pyrolysis of Colket et al.15 1496


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Xu and Pacey.40 Manufacturers often specify acetylene to be only 99.6% pure, containing up to 30 000 ppm of acetone. For this reason, the acetylene feed is usually passed through a dry acetone cold trap packed with glass beads to remove acetone contamination to less than 100−150 ppm of acetone. Figure 14 shows the comparisons between model predictions and experimental measurements at the different reactor temperatures. The acetylene reactivity is well predicted, but diacetylene is overpredicted at high temperatures, as in the case of vinylacetylene pyrolysis. This is probably due to both experimental uncertainties and theoretical poor knowledge (wrong rate constants or missing kinetic paths). Therefore, there is a need for fundamental studies and larger sets of data on polyynes formation especially at high temperature and high pressure to help modelers to improve mechanisms. Moreover, Figure 15 reports a comparison of rate of production analysis of acetylene consumption in percentage

scheme here proposed was not sensitive to the vinylidene mechanism. The introduction of CH2C:, assuming the kinetic constants suggested by Wang et al.,45 did not modify the model predictions. Therefore, this path is not included. This result is not completely surprising; as a matter of fact, the lifetime for vinylidene in a vacuum was determined to be 3.5 μs by Levin et al.52 Moreover, Colket et al.16 discussed the possible role of vinylidene in acetylene decomposition, and they observed that vinylidene has short lifetimes with respect to the isomerization to CH2C:, and its role is not compatible with the experimental observation of induction periods in the pyrolysis. Furthermore, vinylidene does not describe the variety of products that have been observed experimentally. 3.3.4. Flow Reactor Data of Norinaga Et Al.18,19 The acetylene pyrolysis was studied by Norinaga et al.18,19 in a vertical flow reactor at 8 kPa and over a temperature range 1073−1373 K. The total reactor length is 44 cm and a 4-cmlong quasi-isothermal zone was placed at the center of the reactor, into which a channel structure of cordierite is fitted in an alumina ceramic tube with 22 mm width and 40 mm length. According to the indications of Norinaga, an isothermal plugflow reactor was used to simulate these data. A large number of compounds were identified and analyzed, ranging from gases to PAHs up to coronene (C24H12). Because of the wide density variations and the nonisothermal temperature profile along the reactor, there is a relevant uncertainty on the real residence time. For this reason, Figure 16 compares the experimental data with the model predictions at 0.7 s (nominal contact time), as well as with the predictions at 2 s, in order to describe the influence of this possible uncertainty. HACA mechanism well explains the successive formation of the different PAHs. It is important to note that C14H10 and C16H10 are lumped species and they represent phenanthrene/anthracene and pyrene and his isomers, respectively. Model predictions fairly agree with experimental data, even if C14H10 and C16H10 are overpredicted. At the highest severity (i.e., 1400 K and 2 s) model predicts a quasi-complete acetylene conversion with a carbon selectivity to soot higher than 80%. In this case, vinylacetylene is overpredicted and the simulations lie between the different measurements, thus inside the experimental uncertainties. 3.3.5. Flow Reactor Data of Sánchez Et Al.21,22 Sánchez et 21,22 studied acetylene pyrolysis in a quartz flow reactor of 4.5 al. cm inside diameter and 80 cm in length placed inside an electric furnace. The reactor inlet and outlet can be cooled by means of an air flow that allows the control of temperature

Figure 15. Shock tube pyrolysis of acetylene.15 Rate of acetylene consumption at 1300, 1600 and 2000K.

contribution, performed at 1300, 1600, and 2000 K. This figure clearly shows three different conditions. At lower temperatures, acetylene decomposition mainly follows a radical reaction path, due to the presence of acetone impurities, so that the reactions R7 and R13 in Table 2 have an important role. In particular, the attack of methyl radical, derived by acetone decomposition, on acetylene forms propyne and the H radical, which promotes the reactivity of the system. Molecular reactions, such as R1 and R2, prevail at intermediate temperature, whereas radical reaction paths supported by H and C2H radicals become dominant at temperatures higher than 2000 K. It is well known from literature that the dimerization reaction R1 could proceed through C2H2 isomerization and the formation of vinylidene (CH2C:). Despite this, the kinetic

Figure 16. Pyrolysis of acetylene in a flow reactor at 8 kPa.18,19 Comparisons of experimental (symbols) and predicted results at 0.7 s (solid lines) and 2 s (dashed lines). 1497


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Figure 17. Nitrogen-diluted pyrolysis of 10 000 ppm of acetylene in a flow reactor at atmospheric pressure21,22 without acetone (solid line) and with 50 ppm (red dashed line) and 200 ppm of acetone (black dashed line). Comparisons of experimental (symbols) and predicted results at 1.5 ± 0.3 s (lines).

As already discussed elsewhere,56 no appreciable acetone is present in the acetylene feed, and the possible traces are expected to be limited to a few parts per million. Feed analysis during some experiments confirmed that acetone was measured up to 50 ppm only when the bottle was about to be finished. Anyway, following the kinetic study of Colket et al.15 and only to analyze the possible acetone effect, the model predictions in Figure 17−19 are reported both without acetone in the feed and with 50 ppm of acetone. To further stress this effect, 200 ppm of acetone in the case of 10 000 ppm of acetylene and 600 ppm in the case of 30 000 ppm of acetylene are also considered. The model predictions show that the influence of acetone concentration is not significantly important. Molecular reaction paths of acetylene conversion are the dominant ones in these conditions. In agreement with the experiments, the carbon selectivity toward soot is lower than 10% in the first series of experiments (Figure 17) and becomes higher than 70% at the highest severity conditions of Figure 19. The conversion of acetylene is generally well predicted together with the formation of the main final products, hydrogen and soot. In these simulations, C14H10 and C16H10 are lumped species and so is C20H12, which is lumped between the first two pseudospecies BINs.

inside the reactor. Additionally, the reactor outlet is mobile and, thus, can be adjusted to vary the residence time. A full description of the experimental installation and specific characteristics can be found, for example, in Ruiz et al.53,54 Temperature inside the reactor has been measured with a ceramic S thermocouple, so the temperature profile data are available and are used in the calculations. The outlet of the reactor was linked to a quartz filter for soot collection and a resin trap for capturing the PAH in the gas phase. Thus, suitable methods were used to analyze light gases, PAH, and soot.22,55 These data are interesting because of the very severe conditions tested and relative low temperatures, as well as the accurate details on intermediate PAHs, and also because these experiments are carried out under sooting conditions. Figures 17−19 compare the experimental data and model predictions for three different series of pyrolysis experiments with different C2H2 concentration and residence times (10 000 ppm and 1.5 ± 0.3 s; 30 000 ppm and 1.5 ± 0.3 s; 30 000 ppm and 3.8 ± 1 s) as a function of the reaction temperatures. The experiments were carried out with reaction temperatures between 873 and 1473 K and large amount of soot was formed, mainly at the highest temperatures. 1498


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Figure 18. Nitrogen-diluted pyrolysis of 30 000 ppm of acetylene in a flow reactor at atmospheric pressure21,22 without acetone (solid line) and with 50 ppm (red dashed line) and with 600 ppm of acetone (black dashed line). Comparisons of experimental (symbols) and predicted results at 1.5 ± 0.3 s (lines).

According to the kinetic model, the impurity of acetone seems to justify a higher reactivity at low temperatures and a larger formation of methane and fluorene because of the increased methyl radical concentration. Acetone enhances benzene formation and PAHs odd growth pathways. In fact, the rate of production analysis and sensitivity analysis of acetylene pyrolysis show that the first ring is formed not only through the molecular reaction R12 in Table 2 but also through C3 pathways, such as propargyl recombination and toluene decomposition. Propargyl radical is mainly formed by propyne, which derives again from the methyl attack on acetylene, increased in quantity by acetone presence. The benzyl radical, which then evolves into toluene, is formed by the propargyl radical and the acetylene addition on cyclopentadienyl radical. The general agreement is fair, especially in the case of benzene, though C14H10 is overpredicted as already observed in the data of Norinaga et al.18,19 This fact, mainly if further supported by different experimental data, seems to indicate that the reaction path to form heavier species from phenanthrene could be increased. Moreover, the dimerization of the heavy PAHs, such as pyrene and the first pseudospecies BINs, can be considered of lower importance, in line with the fact that the

collision efficiency decreases reducing the mass of colliding particles, as also suggested by Sabbah et al.57 and Wang.3 It is important to note that these experiments are particularly challenging, both from the experimental and from the modeling point of view, because of the severe conditions tested and the great amount of soot formed.

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CONCLUSIONS

In this work, a vast amount of experimental data on acetylene pyrolysis, reported in literature from 900 to 2500 K, is collected and reviewed to develop an in-depth study of the kinetic mechanism of C2−C4 acetylenic species inside the kinetic scheme of Politecnico di Milano, in order to further confirm and extend its comprehensiveness, looking forward to moving toward the modeling of PAHs and soot formation. Studying first the C4H2 and C4H4 reactivity resulted in useful information to better understand and investigate the primary reactions of acetylene pyrolysis, in which successive reactions of intermediate products play an important role. This is a first attempt to discuss in a more comprehensive way the reactivity of these species, understanding also their reciprocal interactions. The broad validation spans different operating 1499


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Figure 19. Nitrogen-diluted pyrolysis of 30 000 ppm of acetylene in a flow reactor at atmospheric pressure21,22 without acetone (solid line) and with 50 ppm (red dashed lines) and 600 ppm of acetone (dashed line). Comparisons of experimental (symbols) and predicted results at 3.8 ± 1 s (lines).

Notes

conditions with increasing severity, making the coupling of a gas phase kinetic model and a soot kinetic model important. The mechanism presented here shows quite a good agreement with the experimental data, especially when the measured uncertainties are considered; nevertheless, some deviations are still present and require additional investigation. The competition between the molecular pathways and the wellknown radical reactions requires a better analysis. Indeed, the kinetics of the molecular reactions of diacetylene and vinylacetylene, important for the low temperature reactivity, should be studied more specifically using both ab initio calculations and experimental activity. These kinetic studies aimed also at investigating the primary products in different conditions also allow us to approach and to analyze the successive aromatization and condensation reactions toward the formation of PAHs and soot. The comparisons of model predictions with the experimental data, especially at low temperatures, resulted in an important tool to investigate the different paths of PAHs formation in conditions where there is still a lack of knowledge.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the useful comments and suggestions of Prof. Mario Dente. Marı ́a U. Alzueta and Nazly E. Sánchez acknowledge the Aragón Government, the European Social Fund (ESF), and MINECO (Project CTQ201234423/PPQ) for financial support. Nazly E. Sánchez acknowledges the Banco Santander Central Hispano, Zaragoza University, and the Colombian Institute for the Development of Science and Technology (COLCIENCIAS) for her financial award and the European Union, specially to the COST Action CM0901, for the Short Term Scientific Mission (STMS) given.

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REFERENCES

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*C. Saggese. E-mail: [email protected]. 1500


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Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons and Soot

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