Experimental kinetic study of the oxidation of p-xylene in a JSR and comprehensive detailed chemical kinetic modeling

Combustion and Flame 141 (2005) 281–297 www.elsevier.com/locate/combustflame

Experimental kinetic study of the oxidation of p-xylene in a JSR and comprehensive detailed chemical kinetic modeling Sandro Gaïl, Philippe Dagaut ∗ CNRS, Laboratoire de Combustion et Systèmes Réactif, 1C, Avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France Received 23 July 2004; received in revised form 17 December 2004; accepted 28 December 2004 Available online 4 February 2005

Abstract The oxidation of para-xylene was studied in a jet-stirred reactor at atmospheric pressure under dilute conditions. New experimental results were obtained over the high-temperature range 900–1300 K, and variable equivalence ratios (0.5
Φ
1.5). They consisted of concentration profiles of the reactants, stable intermediates, and final products, measured by sonic probe sampling followed by on-line GC-MS and off-line GC-TCD-FID and GC-MS analyses. The oxidation of para-xylene under these conditions was modeled using a detailed chemical kinetic reaction mechanism (160 species and 1175 reactions, most of them reversible) deriving from a previous scheme proposed for the ignition, oxidation, and combustion of simple aromatics (benzene, toluene, styrene, n-propylbenzene). The proposed kinetic scheme was also successfully tested against the ignition delays of pxylene–oxygen–argon mixtures, and the combustion of p-xylene in a low-pressure methane–oxygen–nitrogen flame doped with p-xylene, confirming its validity. Sensitivity analyses and reaction path analyses, based on rates of reaction, were used to interpret the results.  2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: para-Xylene; Aromatic; JSR; Kinetic modeling; Reaction mechanism

1. Introduction Aromatic hydrocarbons like xylenes represent an important fraction of commercial gasoline (ca. 14% in wt), and are also present in diesel fuel and kerosene [1]. Nevertheless, the oxidation and pyrolysis of aromatic hydrocarbons are far from being understood [2]. In order to reduce the emission of pollutants and toxic compounds, detailed modeling of combustion in engines is undertaken by engine makers. However, due to the high complexity of commer* Corresponding author. Fax: +33-238-69-60-04.

E-mail address: [email protected] (P. Dagaut).

cial fuels, these models need kinetic reaction mechanisms for the combustion of simple surrogate fuels including representative hydrocarbons [3,4]. Benzene and toluene have been studied extensively and kinetic models were proposed [5–7]. However, these fuels are not complex enough to represent the aromatics present in kerosene and diesel fuels. n-Propylbenzene, which is present in commercial gasoline, diesel fuel, and kerosene [1], could be included in the corresponding model fuels [7,8]. Xylenes are also good candidates to represent the poly-substituted aromatic fraction of commercial fuels such as gasoline. Several years ago, Emdee et al. [9] proposed a reaction scheme for the oxidation of m- and p-xylene

0010-2180/$ – see front matter  2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2004.12.020

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based on atmospheric flow-reactor experiments performed over the temperature range 1093 to 1199 K and for equivalence ratios ranging from 0.47 to 1.7. Gregory and Jackson [10] investigated the chemistry leading to the formation, in engine exhaust, of aromatic hydrocarbons from deuterium-labeled isomeric xylenes. Roubaud et al. [11] also studied the autoignition delays of xylenes in a rapid compression machine in the low-temperature oxidation regime (5 < P /bar < 25, 600–900 K). Dupont [12] studied the combustion of a methane flat flame seeded with pxylene under low-pressure conditions. In the present work, the oxidation of p-xylene was investigated experimentally over a wide range of conditions (900–1400 K at 1 atm/105 Pa and Φ = 0.5 to 1.5) using a fused-silica jet-stirred reactor (JSR). Concentration profiles of reactants, stable intermediates, and final products were measured by low-pressure sonic probe sampling followed by on-line and off-line gas chromatography analyses (GC-FID/TCD/MS). These results were interpreted in terms of a detailed chemical kinetic reaction mechanism.

2. The JSR experimental setup The JSR experiment used in this work is similar to that described earlier [13,14]. The reactor consisted of a small sphere of 40 mm diameter (30.5 cm3 ) made of fused silica (to minimize wall catalytic reactions), equipped with 4 nozzles of 1 mm i.d. for the admission of the gases which achieve the stirring. A nitrogen flow of 100 L/h was used to dilute the fuel and to avoid its pyrolysis before admission in the reactor. All the gases were preheated before injection in order to minimize temperature gradients inside the JSR. The reactants were diluted by nitrogen (<50 ppm of O2 ; <1000 ppm of Ar; <5 ppm of H2 ), and mixed at the entrance of the injectors [13]. High-purity reactants were used in these experiments: oxygen was 99.995% pure and p-xylene >99% pure. Furthermore, p-xylene was sonically degassed before use. A piston pump (Shimadzu LC-10AD VP) was used to deliver the fuel to an atomizer-vaporizer assembly maintained at 120 ◦ C. A good thermal homogeneity along the whole vertical axis of the reactor was observed for each experiment by thermocouple (0.1 mm Pt–Pt/Rh 10% located inside a thinwall silica tube) measurements (temperature change
5 K). The reacting mixtures were sampled via a low-pressure fused-silica sonic probe. The samples (30 Torr/4 kPa) were taken at steady temperature and residence time. They were analyzed online by a GC-MS or FID and off-line after collection and storage in 1 L Pyrex bulbs: Low vapor-pressure compounds were analyzed on-line whereas perma-

nent gases and high vapor-pressure species were analyzed off-line. All the products were analyzed by a chromatographic peak identified. The detection of aromatics (benzene, toluene, p-xylene) from the online and off-line analyses was used to calibrate the measurements performed on-line. These species were used as internal standards in the on-line analyses whereas relative response coefficients were derided from the injection of pure compounds (external standards) on both analytical systems. The present experiments were performed at steady state, at a constant mean residence time of 0.1 s, the reactants flowing continually in the reactor, varying stepwise the temperature of the gases inside the JSR. A high degree of dilution (0.1% vol of fuel) was used, reducing temperature gradients in the JSR and heat release (no flame occurred in the JSR). Several gas chromatographs (GC), equipped with capillary columns (Poraplot-U, Molecular Sieve-5A, DB-5ms, DB-624, Plot Al2 O3 /KCl, Carboplot-P7), thermal conductivity detector (TCD), and flame ionization detector (FID), were used for stable species measurements. Compound identifications were made through GC/MS analyses of the samples. Ion-trap detectors operating in electron impact ionization mode (GC/MS Varian Saturn 3) were used. CH2 O and CO2 were measured by FID after hydrogenation on a methanizer (Ni/H2 catalyst) connected to the exit of the Poraplot-U GC column. A good repeatability of the measurements and a reasonably good carbon balance (100 ± 10%) were obtained in this series of experiments.

3. Kinetic model The JSR modeling was performed using the PSR computer code [15] that computes species concentrations from the balance between the net rate of production of each species by chemical reaction and the difference between the input and output flow rates of species. The reaction mechanism (Table 1) used in this study is based on a comprehensive mechanism developed for the oxidation of natural gas blends to diesel fuel [5]. The reaction mechanism used here consisted of 1175 reversible reactions involving 160 species. It has a strong hierarchical structure. Since most of the present mechanism has been presented in detail in previous papers [6–8,20–24], only the reaction submechanism for the oxidation of p-xylene is presented here. The rates of reaction were computed from the kinetic reaction mechanism and the rate constants of the elementary reactions calculated at the experimental temperature, using the modified Arrhenius equation, k = AT b exp(−E/RT ). The rate constants for the reverse reactions were computed

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297

283

Table 1 The p-xylene oxidation submechanism used here (the rate constants, k = A exp(−E/RT ) are given in the units of s−1 , cm3 , mol−1 , cal) Reaction 963. 964. 965. 966. 967. 968. 969. 970. 971. 972. 973. 974. 975. 976. 977. 978. 979. 980. 981. 982. 983. 984. 985. 986. 987. 988. 989. 990. 991. 992. 993. 994. 995. 996. 997. 998. 999. 1000. 1001. 1002. 1003. 1004. 1005. 1006. 1007. 1008. 1009. 1010. 1011. 1012. 1013. 1014. 1015. 1016. 1017. 1018. 1019. 1020. 1021. 1022. 1023.

p-xylyl = CPD + C3H3 p-xylene + H = p-xylyl + H2 C6H5 + C6H5 = C12H10 C6H6 + C6H5 = C12H10 + H C3H3 + PhCH2 = NAPHT + 2H MePPMe + H = Styrene + C6H5 + H2 C6H4CH3 + C6H4CH3 = MePPMe p-xylene = C6H4CH3 + CH3 p-xylene = p-xylyl + H p-xylene + H = toluene + CH3 p-xylene + HO2 = p-xylyl + H2O2 p-xylene + CH3 = p-xylyl + CH4 p-xylene + O2 = p-xylyl + HO2 p-xylene + O = p-xylyl + OH p-xylene + OH = p-xylyl + H2O p-xylene + AC3H5 = p-xylyl + C3H6 p-xylene + C2H5 = p-xylyl + C2H6 p-xylene + C6H5 = p-xylyl + C6H6 p-xylene + C6H4CH3 = p-xylyl + toluene p-xylene + NC4H5 = p-xylyl + C4H6 p-xylene + C2H3 = p-xylyl + C2H4 p-xylyl = MeCPDY + C2H2 p-xylyl + HO2 = MePhCH2O + OH p-xylyl + OH = MePCH2OH p-xylyl + O2 = MePhHCO + OH p-xylyl + O = MePhHCO + H p-xylyl + O = C6H4CH3 + CH2O p-xylyl + C6H5OH = p-xylene + C6H5O p-xylyl + C3H3 = p-xylene + C3H2 p-xylyl + p-xylyl = DiMePCH2 C6H4CH3 + AMePhC2H4 = DiMePCH2 MePCH2OH + H = toluene + CH2OH MePhCH2O + OH = MePhHCO + H2O MePhCH2O = MePhHCO + H MePCH2OH + O2 ⇒ MePhHCO + HO2 + H MePCH2OH + OH ⇒ MePhHCO + H2O + H MePCH2OH + p-xylyl ⇒ MePhHCO + p-xylene + H MePCH2OH + C6H4CH3 ⇒ MePhHCO + toluene + H MePCH2OH + C6H5 ⇒ MePhHCO + C6H6 + H MePCH2OH + OH = MePhCHOH + H2O MePhCHOH = MePhHCO + H MePhCHOH + H = MePhHCO + H2 MePhCHOH + OH ⇒ MePhHCO + H2O p-xylyl + O2 = CH2C6H4CH2 + HO2 p-xylyl + HO2 = CH2C6H4CH2 + H2O2 p-xylyl = CH2C6H4CH2 + H CH2C6H4CH2 + O = CHOPhCH2 + H MePhHCO + H = MePhCO + H2 MePhHCO = MePhCO + H MePhHCO + H = PhHCO + CH3 MePhHCO + O = MePhCO + OH MePhHCO + OH = MePhCO + H2O MePhHCO + CH3 = MePhCO + CH4 MePhHCO + H = toluene + HCO MePhHCO + O2 = MePhCO + HO2 MePhHCO + HO2 = MePhCO + H2O2 MePhHCO + OH = MeC6H4OH + HCO MePhHCO + C6H4CH3 = MePhCO + toluene MePhHCO + C6H5 = MePhCO + C6H6 MePhHCO + C2H3 = MePhCO + C2H4 MePhCO + O2 = OC7H7 + CO2

A

b

1.00E+14 4.00E+14 5.01E+12 5.25E+12 6.02E+11 5.01E+13 2.51E+11 5.80E+15 2.57E+15 1.80E+14 7.94E+12 1.77E+12 3.00E+14 2.60E+13 2.00E+13 5.00E+12 1.01E+11 1.00E+13 2.10E+12 6.00E+12 4.00E+12 6.02E+13 3.17E+12 1.00E+13 6.31E+12 1.60E+13 1.00E+13 2.05E+11 1.00E+12 2.51E+11 2.51E+11 6.00E+12 1.43E+09 1.50E+14 2.00E+14 8.43E+12 2.11E+11 1.40E+12 1.40E+12 5.00E+09 1.00E+17 8.00E+10 5.00E+09 3.00E+15 1.00E+14 1.00E+16 1.50E+16 5.00E+13 3.98E+15 4.00E+13 9.04E+13 1.71E+09 2.77E+03 1.08E+14 1.02E+13 2.00E+12 1.20E+11 7.01E+11 7.01E+11 7.01E+11 3.00E+10

0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.4 0.0 1.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.2 2.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

E 71000.0 8370.0 500.0 8513.0 0.0 13019.0 0.0 90951.0 83360.0 8090.0 14069.0 8754.0 43062.0 3062.0 2180.0 14019.0 9514.0 4400.0 4400.0 7500.0 7500.0 70000.0 0.0 0.0 43000.0 0.0 0.0 9500.0 0.0 0.0 0.0 5155.5 1813.0 11100.0 40400.0 2583.0 9500.0 4400.0 4400.0 0.0 33040.0 8246.0 0.0 43062.0 7895.0 86124.0 4000.0 4928.0 83780.0 8090.0 3080.0 −447.0 5773.0 5156.0 39000.0 11665.0 5123.0 4400.0 4400.0 7500.0 2870.0

Note 1 2 1 3 1 1 4 2 2 5 5 6 7 8 6 9 6 4 4 10 10 1 11 9 12 11 4 4 13 4 4 9 14 12 4 4 4 4 4 15 9 9 15 7 7 2 18 9 9 5 9 4 4 9 16 17 17 4 4 4 18

(continued on next page)

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Table 1 (Continued) Reaction 1024. 1025. 1026. 1027. 1028. 1029. 1030.

1031. 1032. 1033. 1034. 1035. 1036. 1037. 1038. 1039. 1040. 1041. 1042. 1043. 1044. 1045. 1046. 1047. 1048. 1049. 1050. 1051. 1052. 1053. 1054. 1055. 1056. 1057. 1058. 1059. 1060. 1061. 1062. 1063. 1064. 1065. 1066. 1067. 1068. 1069. 1070. 1071. 1072. 1073. 1074. 1075. 1076. 1077. 1078. 1079. 1080. 1081. 1082. 1083. 1084.

MePhCO + HO2 ⇒ C6H4CH3 + CO2 + OH MePhCO = C6H4CH3 + CO C6H4CH3 + H = toluene C6H4CH3 + O2 = OC7H7 + O C6H4CH3 + HO2 = OC7H7 + OH C6H4CH3 + OH = OC7H7 + H OC7H7 + H (+M) = MeC6H4OH (+M) Low pressure limit: 0.10000E+95 −0.21840E+02 TROE centering: 0.43000E−01 0.30400E+03 MeC6H4OH + OH = OC7H7 + H2O MeC6H4OH + H = toluene + OH MeC6H4OH + H = OC7H7 + H2 MeC6H4OH + H = C6H5OH + CH3 MeC6H4OH + O = OC7H7 + OH MeC6H4OH + CH3 = OC7H7 + CH4 MeC6H4OH + C2H5 = OC7H7 + C2H6 MeC6H4OH + C2H3 = OC7H7 + C2H4 MeC6H4OH + NC4H5 = OC7H7 + C4H6 MeC6H4OH + IC4H5 = OC7H7 + C4H6 MeC6H4OH + MeCPDY = OC7H7 + MeCPD MeC6H4OH + C6H5 = OC7H7 + C6H6 MePhC2H5 = toluene + C2H4 MePhC2H5 = MY1P4Et + H MePhC2H5 = p-xylyl + CH3 MePhC2H5 = BMePhC2H4 + H MePhC2H5 = AMePhC2H4 + H MePhC2H5 + H = BMePhC2H4 + H2 MePhC2H5 + H = AMePhC2H4 + H2 MePhC2H5 + O = BMePhC2H4 + OH MePhC2H5 + O = AMePhC2H4 + OH MePhC2H5 + O2 = OY1P4Et + CH3O MePhC2H5 + OH = BMePhC2H4 + H2O MePhC2H5 + OH = AMePhC2H4 + H2O MePhC2H5 + H ⇒ PhC2H5 + CH3 MePhC2H5 + H = C6H4CH3 + C2H6 MePhC2H5 = C6H4CH3 + C2H5 MePhC2H5 + H = toluene + C2H5 MePhC2H5 + HO2 = BMePhC2H4 + H2O2 MePhC2H5 + HO2 = AMePhC2H4 + H2O2 MePhC2H5 + CH3 = BMePhC2H4 + CH4 MePhC2H5 + CH3 = AMePhC2H4 + CH4 MePhC2H5 + C2H3 = BMePhC2H4 + C2H4 MePhC2H5 + C2H3 = AMePhC2H4 + C2H4 MePhC2H5 + O2 = BMePhC2H4 + HO2 MePhC2H5 + O2 = AMePhC2H4 + HO2 AMePhC2H4 = C6H4CH3 + C2H4 BMePhC2H4 = MeStyrene + H BMePhC2H4 + OH = MeStyrene + H2O BMePhC2H4 + O = C6H4CH3 + CH3HCO BMePhC2H4 + O = MePhHCO + CH3 BMePhC2H4 + OH = MePhHCO + CH4 BMePhC2H4 + HO2 ⇒ MePhHCO + CH3 + OH MY1P4Et + O2 = EtPhHCO + OH MY1P4Et + O = EtPhHCO + H MY1P4Et + O = Y1P4Et + CH2O MY1P4Et + C6H5OH = MePhC2H5 + C6H5O MY1P4Et + MeC6H4OH = MePhC2H5 + OC7H7 MY1P4Et + C3H3 = MePhC2H5 + C3H2 MY1P4Et + C2H3 = MePhC2H5 + C2H2 MY1P4Et + C2H3 = MeStyrene + C2H4 MY1P4Et + CH3 = MeStyrene + CH4 MeStyrene + H ⇒ Styrene + CH3 C6H4CH3 + C2H4 = MeStyrene + H

A

b

2.00E+13 3.98E+14 7.80E+13 2.09E+12 5.00E+15 5.00E+15 1.00E+14 0.13880E+05 0.60000E+05 1.33E+12 2.21E+13 1.15E+14 1.15E+13 1.26E+13 3.62E+11 3.00E+02 3.00E+02 3.00E+02 3.00E+02 2.00E+12 3.00E+02 1.15E+09 1.00E+16 5.00E+14 2.51E+15 2.51E+15 1.42E+12 1.42E+12 1.10E+13 1.10E+13 6.10E+14 8.18E+12 8.18E+12 4.00E+15 1.20E+13 1.00E+16 6.00E+13 9.80E+10 9.80E+10 1.00E+11 1.00E+11 4.00E+12 4.00E+12 4.00E+13 4.00E+13 1.00E+14 5.00E+12 9.00E+12 1.60E+13 1.60E+13 1.60E+13 5.00E+12 6.31E+11 1.58E+13 1.00E+13 1.05E+11 1.05E+11 1.00E+12 4.00E+12 4.00E+12 4.00E+11 7.20E+13 1.00E+12

0.0 0.0 0.0 0.0 0.0 0.0 0.0

E

0.59000E+04 0.0 0.0 0.0 0.0 0.0 0.0 3.0 3.0 3.0 3.0 0.0 3.0 0.0 0.0 0.0 0.0 81260.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Note

0.0 29400.0 0.0 7470.0 1000.0 0.0 0.0

19 20 21 4 22 23 24

1870.0 7910.0 12400.0 8500.0 2891.0 8716.0 7650.0 7650.0 7650.0 7650.0 0.0 7650.0 51699.0 84850.0 71540.0 28 83263.0 0.0 1000.0 3660.0 4660.0 43000.0 0.0 1000.0 8090.0 −2000.0 100000.0 8090.0 12584.0 13584.0 8500.0 7500.0 7500.0 8500.0 35820.0 50620.0 35000.0 50670.0 0.0 0.0 0.0 0.0 0.0 43000.0 0.0 0.0 9500.0 9500.0 0.0 7500.0 7500.0 8500.0 5123.0 6206.0

25 4 4 19 5 19 26 26 26 26 19 26 9 5 27 28 28 28 5 5 19 29 29 19 9 19 17 16 16 19 19 19 10 30 30 19 30 9 19 19 19 19 12 11 4 4 4 13 10 19 19 17 9

(continued on next page)

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285

Table 1 (Continued) Reaction 1085. 1086. 1087. 1088. 1089. 1090. 1091. 1092. 1093. 1094. 1095. 1096. 1097. 1098. 1099. 1100. 1101. 1102. 1103. 1104. 1105. 1106. 1107. 1108. 1109. 1110. 1111. 1112. 1113. 1114. 1115. 1116. 1117. 1118. 1119. 1120. 1121. 1122. 1123. 1124. 1125. 1126. 1127. 1128. 1129. 1130. 1131. 1132. 1133. 1134. 1135. 1136. 1137. 1138. 1139. 1140. 1141. 1142. 1143. 1144. 1145. 1146. 1147. 1148.

MeStyrene = C6H4CH3 + C2H3 MeStyrene + H = toluene + C2H3 MeStyrene + O = C6H4CH3 + CH2HCO MeStyrene + O = p-xylyl + HCO MeStyrene = AMeStyryl + H MeStyrene = BMeStyryl + H MeStyrene + O2 = AMeStyryl + HO2 MeStyrene + O2 = BMeStyryl + HO2 MeStyrene + OH = AMeStyryl + H2O MeStyrene + H = AMeStyryl + H2 MeStyrene + HO2 = AMeStyryl + H2O2 MeStyrene + C6H5 = AMeStyryl + C6H6 MeStyrene + C2H3 = AMeStyryl + C2H4 MeStyrene + OC7H7 = AMeStyryl + MeC6H4OH MeStyrene + MeCPDY = AMeStyryl + MeCPD MeStyrene + C5H5 = AMeStyryl + CPD MeStyrene + OH = BMeStyryl + H2O MeStyrene + H = BMeStyryl + H2 MeStyrene + HO2 = BMeStyryl + H2O2 MeStyrene + C6H5 = BMeStyryl + C6H6 MeStyrene + C2H3 = BMeStyryl + C2H4 MeStyrene + OC7H7 = BMeStyryl + MeC6H4OH MeStyrene + C5H5 = BMeStyryl + CPD MeStyrene + MeCPDY = BMeStyryl + MeCPD AMeStyryl = C6H4CH3 + C2H2 AMeStyryl + O2 = MePhHCO + HCO BMeStyryl + O2 = MePhCO + CH2O BMeStyryl + O = C6H4CH3 + CH2CO CHOPCHO + O2 = CHOPCO + HO2 CHOPCHO + H = CHOPCO + H2 CHOPCHO + OH = CHOPCO + H2O CHOPCHO + O = CHOPCO + OH CHOPCHO + O = OY1P4CHO + HCO CHOPCHO = CHOPCO + H CHOPCHO + H = PhHCO + HCO CHOPCHO + C6H5 = CHOPCO + C6H6 CHOPCHO + C2H3 = CHOPCO + C2H4 CHOPCHO + HO2 = CHOPCO + H2O2 CHOPCHO + CH3 = CHOPCO + CH4 CHOPCO + H = PhHCO + CO CHOPhCH2 + O2 = CHOPCHO + OH CHOPhCH2 + O = CHOPCHO + H CHOPhCH2 + HO2 = CHOPCHO + OH + H CHOPhCH2 + O = Y1P4CHO + CH2O CHOPCO = Y1P4CHO + CO Y1P4CHO + O2 = OY1P4CHO + O Y1P4CHO + H = PhHCO EtPhHCO = EtPCO + H EtPhHCO + H = PhC2H5 + HCO EtPhHCO + CH3 = EtPCO + CH4 EtPhHCO + C6H5 = EtPCO + C6H6 EtPhHCO + C2H3 = EtPCO + C2H4 EtPhHCO + C6H4CH3 = EtPCO + toluene EtPhHCO + p-xylyl = EtPCO + p-xylene EtPhHCO + H = EtPCO + H2 EtPhHCO + OH = EtPCO + H2O EtPhHCO + O = EtPCO + OH EtPhHCO = CHOPhCH2 + CH3 EtPCO = BPhC2H4 + CO EtPCO = APhC2H4 + CO EtPhHCO + O2 = EtPCO + HO2 EtPhHCO + HO2 = EtPCO + H2O2 EtPhHCO = Y1P4Et + HCO EtPCO = Y1P4Et + CO

A

b

E

Note

5.00E+16 2.40E+13 6.00E+07 1.20E+08 5.00E+15 5.00E+15 4.00E+13 4.00E+13 5.00E+12 5.00E+13 7.50E+10 2.00E+11 2.00E+11 2.00E+11 2.00E+11 2.00E+11 5.00E+12 5.00E+13 7.50E+10 2.00E+11 2.00E+11 2.00E+11 2.00E+11 2.00E+11 1.00E+14 2.00E+11 2.00E+11 1.60E+13 1.02E+13 5.00E+13 1.71E+09 9.04E+13 1.75E+13 3.98E+15 7.96E+15 7.01E+11 7.01E+11 2.00E+12 2.77E+03 1.58E+13 6.31E+12 5.32E+11 6.31E+10 1.00E+10 3.98E+14 2.09E+12 3.98E+15 3.98E+15 1.20E+13 2.77E+03 7.01E+11 7.01E+11 7.01E+11 7.01E+11 5.00E+13 1.71E+09 9.04E+12 5.00E+14 1.58E+13 1.58E+13 1.02E+13 2.00E+12 3.98E+13 1.58E+13

0.0 0.0 1.4 1.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 2.8 0.0 0.0 0.0 0.0 0.0 1.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

100000.0 5123.0 530.0 530.0 109400.0 99200.0 59800.0 49600.0 6936.0 15009.0 14190.0 20000.0 20000.0 20000.0 20000.0 20000.0 5936.0 14009.0 13190.0 20000.0 19000.0 19000.0 19000.0 19000.0 36850.0 14000.0 14000.0 0.0 39000.0 4928.0 −447.0 3080.0 3080.0 83701.0 83701.0 4400.0 4400.0 11665.0 5773.0 17225.0 3000.0 0.0 15000.0 0.0 29400.0 7470.0 83701.0 83701.0 5148.0 5773.0 4400.0 4400.0 4400.0 4400.0 4928.0 −447.0 3080.0 74920.0 16225.0 17225.0 39000.0 11665.0 29401.0 17000.0

31 19 32 32 31 31 30 30 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 32 9 4 9 19 9 19 4 4 17 4 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19

(continued on next page)

286

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297

Table 1 (Continued) Reaction 1149. 1150. 1151. 1152. 1153.

1154. 1155. 1156. 1157. 1158. 1159. 1160. 1161. 1162. 1163. 1164. 1165. 1166. 1167. 1168. 1169. 1170. 1171. 1172. 1173. 1174. 1175.

A

b

E

Y1P4Et + O2 = OY1P4Et + O 2.09E+12 0.0 7470.0 Y1P4Et + H = PhC2H5 2.20E+14 0.0 0.0 OY1P4Et ⇒ C5H5 + C2H4 + CO 4.00E+12 0.0 0.0 OC7H7 ⇒ MeCPDY + CO 1.20E+17 0.0 81263.0 MeCPDY + H (+M) = MeCPD (+M) 1.00E+14 0.0 0.0 Low pressure limit: 0.44000E+81 −0.18280E+02 0.12994E+05 TROE centering: 0.68000E−01 0.40000E+03 0.41360E+04 0.55000E+04 MeCPD + O = MeCPDY + OH 1.81E+13 0.0 3080.0 MeCPD + H = MeCPDY + H2 1.29E+11 3.4 3122.0 MeCPD + H = CPD+CH3 2.50E+13 0.0 8090.0 MeCPDY + H = C5H5 + CH3 9.38E+06 2.0 7703.0 MeCPDY + O = C5H5 + CH2O 1.67E+12 0.0 7871.0 MeCPD + H = C5H5 + CH4 1.43E+09 1.1 1813.0 MeCPD + O = C5H5 + CH3O 1.20E+14 0.0 5127.0 MeCPD + O2 = MeCPDY + HO2 2.00E+13 0.0 25000.0 MeCPD + HO2 = MeCPDY + H2O2 2.00E+12 0.0 11660.0 MeCPD + OH = MeCPDY + H2O 3.43E+09 1.2 −447.0 MeCPD + CH3 = MeCPDY + CH4 3.11E+12 0.0 8500.0 MeCPD + C2H3 = MeCPDY + C2H4 4.11E+12 0.0 7500.0 MeCPD + C2H5 = MeCPDY + C2H6 4.11E+12 0.0 7500.0 MeCPD + NC4H5 = MeCPDY + C4H6 4.11E+12 0.0 7500.0 MeCPD + IC4H5 = MeCPDY + C4H6 4.11E+12 0.0 7500.0 MeCPD + C6H5 = MeCPDY + C6H6 4.11E+12 0.0 7500.0 MeCPDY ⇒ C6H6 + H 3.00E+13 0.0 0.0 OY1P4CHO = C5H5CHO + CO 3.98E+14 0.0 29400.0 C5H5CHO = CPDCO 1.00E+12 0.0 0.0 CPDCO = C5H5 + CO 2.00E+12 0.0 0.0 CPDCHO = C5H5CHO + H 3.98E+13 0.0 83780.0 CPDCHO = CPDCO + H 3.98E+15 0.0 83780.0

Note 19 19 19 9 24

4 4 19 14 14 14 9 4 4 4 19 19 19 19 19 19 19 19 19 19 19 19

Nomenclature: 5-methylcyclopenta-1,3-diene, MeCPD; 5-methyl-1,3-cyclopentadienyl, MeCDPY; 5-formylcyclopenta-1,3diene, CPDCHO; 5-carbonylcyclopenta-1,3-diene radical, CPDCO; 1-formylcyclopenta-2,4-diene-1-yl radical, C5H5CHO; 4-methyl-1-hydroxybenzene, MeC6H4OH; 4-methyl-1-hydroxybenzene radical, OC7H7; 4-methylphenyl radical, C6H4CH3; 4-formylphenyl radical, Y1P4CHO; 4-formyl-1-hydroxybenzene radical, OY1P4CHO; 1,4-dimethylbenzene, p-xylene; 4methylbenzyl radical, p-xylyl; p-xylylene or 3,6-dimethylenecyclohexa-1,4-diene, CH2C6H4CH2; 1-formyl-4-methylbenzene, MePhHCO; 1-carbonyl-4-methylbenzene radical, MePhCO; 4-methyl-1-hydroxymethylbenzene, MePCH2OH; 4-methyl1-hydroxydedydro-1-methylbenzene radical, MePhCHOH; 4-methyl-1-hydroxymethylbenzene radical, MePhCH2O; 1,4diformylbenzene, CHOPCHO; 1-carbonyl-4-formylbenzene radical, CHOPCO; 4-formylbenzyl radical, CHOPhCH2; 4ethylphenyl radical, Y1P4Et; 4-ethyl-1-hydroxybenzene radical, OY1P4Et; 4-ethyl-1-methylbenzene, MePhC2H5; 4-methyl1-ethylenebenzene radical, AMePhC2H4; 4-methyl-1-ethylidenebenzene radical, BMePhC2H4; 4-methyl-1-vinylbenzene, MeStyrene; 4-methyl-1-ethen-1-yl benzene radical, AMeStyryl; 4-methyl-1-ethen-2-yl benzene radical, BMeStyryl; 1-formyl4-ethylbenzene radical, EtPhHCO; 1-carbonyl-4-ethylbenzene radical, EtPCO; 4-ethylbenzyl radical, MY1P4Et; naphthalene, NAPHT; biphenyl, C12H10; 4,4 -dimethylbiphenyl, MePPMe; 4,4 -dimethylbibenzyl, DiMePCH2. Note: (1) Based on [32]; (2) H. Hippler et al. [28]; (3) Based on A. Fahr, S.E. Stein, Proc. Combust. Inst. 22 (1989) 1023; (4) Based on [35]; (5) Based on D.L. Baulch, C.J. Cobos, R.A. Cox, P. Frank, G. Hayman, Th. Just, J.A. Kerr, T. Murrells, M.J. Pilling, J. Troe, R.W. Walker, J. Warnatz, J. Phys. Chem. Ref. Data 23 (1994) 847; (6) Based on P. Dagaut, Phys. Chem. Chem. Phys. 4 (2002) 2079; (7) [28]; (8) H. Frerichs, V. Schliephake, M. Tappe, H.Gg. Wagner, Z. Phys. Chem. Neue Folge 165 (1989) 9; (9) [12]; (10) Based on A. Ristori, thesis, University of Orleans (2000); (11) J.L. Emdee, K. Brezinsky, I. Glassman, J. Phys. Chem. 95 (1991) 1626; (12) Based on K. Brezinsky, T.A. Litzinger, I. Glassman, Int. J. Chem. Kinet. 16 (1984) 1053; (13) Based on M. Braun-Unkhoff, P. Frank, Th. Just, Proc. Combust. Inst. 22 (1988) 1053; (14) Based on J. Warnatz, Proc. Combust. Inst. 24 (1992) 553–579; (15) H. Hippler, C. Reihs, J. Troe, Proc. Combust. Inst. 23 (1991) 37; (16) Based on C. Buchta, H. Frerichs, D.V. Stucken, M. Tappe, H.Gg. Wagner, Ber. Bunsenges. Phys. Chem. 97 (1993) 658; (17) Based on W. Tsang, R. F. Hampson, J. Phys. Chem. Ref. Data 15 (1986) 1087; (18) Based on R.S. Timonen, D. Gutman, J. Phys. Chem. 90 (1986) 2987; (19) This work; (20) Based on R.K. Solly, S.W. Benson, J. Am. Chem. Soc. 93 (1971) 2127; (21) I. Da Costa, R.A. Eng, A. Gebert, H. Hippler, Proc. Combust. Inst. 28 (2000) 1537; (22) Based on D.A. Bittker, Combust. Sci. Technol. 79 (1991) 49; (23) Based on J.A. Miller, C.F. Melius, Combust. Flame 91 (1992) 21; (24) Based on S.G. Davis, H. Wang, K. Brezinsky, C.K. Law, Proc. Combust. Inst. 26 (1996) 1025; (25) M. Semadeni, D.W. Stocker, J.A. Kerr, Int. J. Chem. Kinet. 27 (1995) 287; (26) Based on N.M. Marinov, W.J. Pitz, C.K. Westbrook, M.J. Castaldi, S.M. Senkan, Combust. Sci. Technol. 116–117 (1996) 211; (27) B.D. Barton, S.E. Stein, J. Phys. Chem. 84 (1980) 2141; (28) W. Muller-Markgraf, J. Troe, J. Phys. Chem. 92 (1988) 4914; (29) T. Ohta, T. Ohyama, Bull. Chem. Soc. Jpn. 58 (1985) 3029; (30) Based on R.W. Walker, in: R.G. Compton, Hancock (Eds.), Some Burning Problems in Combustion Chemistry, Research in Chemical Kinetics, Elsevier, Amsterdam, 1995; (31) Based on [30]; (32) G. Pengloan, thesis, University of Orleans (2001).

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287

Table 2 Thermochemical parameters calculated in this work, as used in this modeling, for a selected number of species pertaining to the p-xylene oxidation submechanism Species

∆Hf◦ (298)

∆Sf◦ (298)

Cp 300

Cp 400

Cp 500

Cp 600

Cp 800

Cp 1000

Cp 1500

MeC6H4OH CH2C6H4CH2 MeCPDY OC7H7 C12H10 AMeStyryl BMeStyryl MePhCO MePhHCO C6H4CH3 MePPMe p-Xylyl p-Xylene MeCPD EtPCO EtPhHCO CHOPCO CHOPCHO MePhCO CHOPhCH2 MePhCHOH MePhC2H5 MePCH2OH MeStyrene MePhCH2O DiMePCH2 MY1P4Et OY1P4CHO OY1P4Et Y1P4CHO Y1P4Et AMePhC2H4 BMePhC2H4 CPDCHO CPDCO C5H5CHO

−30.67 54.00 93.25 3.73 43.54 74.77 84.98 18.27 −16.53 70.68 27.91 40.10 4.30 26.65 13.05 −21.75 −2.67 −37.47 18.27 19.37 1.53 −0.81 −31.77 27.57 19.53 18.62 35.09 −17.21 −1.48 49.94 65.83 47.30 32.49 7.28 46.53 65.80

84.35 78.32 75.79 83.81 92.94 90.66 93.00 91.91 93.46 76.22 112.68 85.51 84.16 74.48 102.22 103.78 100.02 100.20 91.91 93.37 95.77 95.67 95.99 91.91 92.92 133.49 95.58 91.93 94.14 83.63 85.74 97.78 95.24 79.37 83.28 79.25

29.52 30.31 23.82 27.52 40.08 32.27 33.67 29.74 32.13 23.85 49.07 29.49 30.24 24.36 35.91 38.30 32.10 34.48 29.74 31.18 30.89 35.94 31.72 34.30 30.00 59.59 34.99 29.88 33.69 25.90 29.80 35.86 35.13 25.31 24.07 25.39

38.41 39.65 31.19 35.98 53.21 41.86 43.50 38.99 42.03 31.77 65.65 38.64 39.95 32.40 46.50 49.53 41.53 44.58 38.99 40.84 40.58 47.00 42.25 44.72 40.42 78.28 45.81 38.53 43.49 33.87 39.04 46.49 45.36 32.85 30.93 32.22

45.85 47.29 37.25 43.02 64.41 50.07 51.65 47.25 50.70 38.60 79.49 46.53 48.53 39.04 55.89 59.34 50.04 53.50 47.25 49.21 48.95 56.55 51.28 53.50 49.26 94.31 55.06 45.81 51.66 40.82 46.96 55.42 54.26 38.93 36.48 37.81

52.00 53.50 42.20 48.81 73.65 57.05 58.39 54.48 58.18 44.41 90.96 53.27 55.92 44.48 64.07 67.77 57.52 61.22 54.48 56.38 56.11 64.73 58.96 60.84 56.69 107.93 62.93 51.85 58.40 46.75 53.66 62.93 61.92 43.81 40.94 42.33

61.19 62.66 49.52 57.39 86.88 67.85 68.60 65.80 69.73 53.32 108.10 63.83 67.31 52.56 76.84 80.77 69.07 73.00 65.80 67.39 67.31 77.49 70.79 71.99 68.06 128.92 75.15 60.66 68.42 55.75 63.89 74.57 74.04 50.92 47.38 48.83

67.52 69.16 54.71 63.24 95.87 75.63 75.89 73.06 77.13 59.38 119.95 71.22 75.54 58.35 85.08 89.15 75.78 79.85 73.06 74.35 75.16 86.44 78.93 79.89 75.82 143.34 83.65 65.96 75.25 61.27 70.81 82.80 82.68 55.90 51.85 53.31

80.64 78.52 62.19 75.86 108.13 91.77 91.41 76.14 80.55 68.56 144.68 81.42 84.82 66.79 89.76 94.16 70.75 75.24 76.14 76.87 86.72 99.59 90.99 96.18 87.20 164.26 95.98 70.50 89.42 63.41 81.59 94.85 95.31 62.95 58.09 59.45

from the forward rate constants and the equilibrium constants (Kc = kforward /kreverse ) calculated using the appropriate thermochemical data [16–19], see Table 2. Table 3 gives the chemical structures of the species used in this modeling. The pressure dependencies of the unimolecular reactions and of some pressure-dependent bimolecular reactions were taken into account when information was available (i.e., k(P , T )). First-order local sensitivity analyses and reaction rates analyses were performed computing rates of consumption (ROC) and production (ROP) for every species. The present scheme was also used to successfully simulate the oxidation of benzene and toluene under conditions similar to those of the present study, as well as their ignition delays and flame speeds in air [5–8]. Figs. 1–3 compare the modeling results ob-

tained for the oxidation of p-xylene in a JSR, using the presently proposed scheme. To further assess the validity of the proposed kinetic scheme, the ignition of p-xylene–oxygen–argon mixtures [25] was modeled using the Senkin computer code [26]. The lowpressure (40 Torr) methane–oxygen–nitrogen flame seeded with p-xylene [12] was simulated using the Premix computer code [27].

4. Reactions of p-xylene The proposed detailed chemical kinetic scheme derives from that proposed for the ignition, oxidation, and combustion of toluene. It also follows the reaction paths delineated in [9]. In the presently proposed kinetic mechanism, the oxidation of p-xylene

288

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297 Table 3 Chemical species used in this modeling

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297

289

Fig. 1. p-Xylene oxidation in a JSR at 1 atm and Φ = 0.5. The initial conditions were: p-xylene, 0.1%; O2 , 2.10%; N2 , 97.80%; τ = 0.1 s. Experimental data (symbols) for CO, formaldehyde (CH2 O), oxygen, p-xylene, benzaldehyde, toluene, benzene (C6 H6 ), cyclopentadiene (CPD), styrene, methylethylbenzene (MePhC2H5), hydrogen, methane, acetylene, ethylene, ethane, vinylacetylene (C4 H4 ), allene (AC3 H4 ), propyne (PC3 H4 ), and 1,3-butadiene (C4 H6 ) are compared to computations (lines and small symbols).

initially proceeds via thermal decomposition: one reaction yields methylbenzyl radical (p-xylyl) and H, and the other one produces the methylphenyl radical (C6 H4 CH3 ) and CH3 :

Propagation reactions include H-atom abstraction by atoms and radicals yielding methylbenzyl: p-xylene + H ↔ p-xylyl + H2 ,

(964)

p-xylene + HO2 ↔ p-xylyl + H2 O2 ,

(973)

p-xylene ↔ C6 H4 CH3 + CH3 ,

(970)

p-xylene + O ↔ p-xylyl + OH,

(976)

p-xylene ↔ p-xylyl + H.

(971)

p-xylene + OH ↔ p-xylyl + H2 O,

(977)

p-xylene + AC3 H5 ↔ p-xylyl + C3 H6 ,

(978)

p-xylene + C2 H5 ↔ p-xylyl + C2 H6 ,

(979)

p-xylene + C6 H5 ↔ p-xylyl + C6 H6 ,

(980)

p-xylene + C6 H4 CH3 ↔ p-xylyl + toluene,

(981)

p-xylene + n-C4 H5 ↔ p-xylyl + C4 H6 ,

(982)

p-xylene + C2 H3 ↔ p-xylyl + C2 H4 .

(983)

The rate constants for these reactions were derived from the study of Hippler et al. [28] using their measured global rate constant and branching ratios (k970 /(k/970 + k971 ) = 0.15 at 1500 K and k970 / (k/970 + k971 ) = 0.25 at 2000 K). A reaction with molecular oxygen was also considered for the initiation; it yields the methylbenzyl radical and HO2 . The kinetics was taken from [29]. p-xylene + O2 ↔ p-xylyl + HO2 .

(975)

The kinetics of these reactions were estimated based on [6,30]. A displacement reaction of the

290

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297

Fig. 2. p-Xylene oxidation in a JSR at 1 atm and Φ = 1.0. The initial conditions were: p-xylene, 0.1%; O2 , 1.05%; N2 , 98.85%; τ = 0.1 s. Experimental data (symbols) for CO, formaldehyde (CH2 O), oxygen, p-xylene, benzaldehyde, toluene, benzene (C6 H6 ), cyclopentadiene (CPD), styrene, methylethylbenzene (MePhC2H5), hydrogen, methane, acetylene, ethylene, ethane, vinylacetylene (C4 H4 ), allene (AC3 H4 ), propyne (PC3 H4 ), and 1,3-butadiene (C4 H6 ) are compared to computations (lines and small symbols).

methyl group by an H atom, by analogy with the displacement reaction of toluene, yields toluene and methyl. The kinetics was assumed similar to that of toluene + H ↔ benzene + CH3 [31]. p-xylene + H ↔ toluene + CH3 .

(972)

The methylbenzyl radical (p-xylyl) reacts by thermal decomposition yielding p-xylylene (CH2 C6 H4 CH2 ) or p-quinodimethane (3,6-bis(methylene)-1,4cyclohexadiene), acetylene and methylcyclopentadienyl (CH3 C5 H4 or MeCPDY), the propargyl radical (C3 H3 ), and 1,3-cyclopentadiene (CPD). p-xylyl ↔ CPD + C3 H3 ,

(963)

p-xylyl ↔ CH3 C5 H4 + C2 H2 ,

(984)

p-xylyl ↔ CH2 C6 H4 CH2 + H.

(1008)

The rate constant for reaction (1008) was taken from [28] whereas that of reactions (963) and (984)

were derived from [32]. The reaction (984) was included by analogy with reaction (a). The authors estimated a preexponential factor with an uncertainty of 3. The rate constant used here is based on their estimate. Benzyl ↔ cyclopentadienyl + C2 H2

(a)

(984)

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297

291

Fig. 3. p-Xylene oxidation in a JSR at 1 atm and Φ = 1.5. The initial conditions were: p-xylene, 0.1%; O2 , 0.70%; N2 , 99.20%; τ = 0.1 s. Experimental data (symbols) for CO, formaldehyde (CH2 O), oxygen, p-xylene, benzaldehyde, toluene, benzene (C6 H6 ), cyclopentadiene (CPD), styrene, methylethylbenzene (MePhC2H5), hydrogen, methane, acetylene, ethylene, ethane, vinylacetylene (C4 H4 ), allene (AC3 H4 ), propyne (PC3 H4 ), and 1,3-butadiene (C4 H6 ) are compared on computations (lines and small symbols).

p-xylyl + HO2 ↔ CH2 C6 H4 CH2 + H2 O2 ,

Although this intermediate (CH2 C6 H4 CH2 ) is considered as a bi-radical, Pollack et al. [33] showed that the more stable form for this bi-radical is the quinoid form. The reactions of methylbenzyl with oxygen, O, OH, CH3 , and HO2 produce p-xylylene (CH2 C6 H4 CH2 ), methylbenzaldehyde (CH3 C6 H4 HCO), methylbenzyl alcohol (CH3 C6 H4 CH2 OH), methylbenzoxy radical (CH3 C6 H4 CH2 O), methylphenyl, methylethylbenzene (CH3 C6 H4 C2 H5 ), and formaldehyde.

Their kinetics were taken from the literature when available [29]. Otherwise, they were estimated by analogy with similar reactions in the toluene kinetic scheme [4,6,7]. Details are given in the notes of Table 1. The p-xylylene radical reacts via addition of O atoms to the double bond forming a benzylic radical with and aldehydic group (CHOC6 H4 CH2 ).

p-xylyl + HO2 ↔ CH3 C6 H4 CH2 O + OH,

(985)

CH2 C6 H4 CH2 + O ↔ CHOC6 H4 CH2 + H. (1009)

p-xylyl + OH ↔ CH3 C6 H4 CH2 OH,

(986)

p-xylyl + O2 ↔ CH3 C6 H4 HCO + OH,

(987)

p-xylyl + O ↔ CH3 C6 H4 HCO + H,

(988)

p-xylyl + O ↔ C6 H4 CH3 + CH2 O,

(989)

p-xylyl + O2 ↔ CH2 C6 H4 CH2 + HO2 ,

(1006)

p-xylyl + CH3 ↔ CH3 C6 H4 C2 H5 .

(1007) (-1045)

This benzylic radical then reacts similarly to methylbenzyl radical to form phthalaldehyde (CHO C6 H4 CHO) or ethylbenzaldehyde (C2 H5 C6 H4 HCO) CHOC6 H4 CH2 + O2 ↔ CHOC6 H4 CHO + OH, CHOC6 H4 CH2 + O ↔ CHOC6 H4 CHO + H,

(1125) (1126)

292

S. Gaïl, P. Dagaut / Combustion and Flame 141 (2005) 281–297

CHOC6 H4 CH2 + HO2 ↔ CHOC6 H4 CHO + OH + H,

(1127)

C2 H5 C6 H4 HCO ↔ CHOC6 H4 CH2 + CH3 . (1142) These aldehydes are consumed mainly through abstraction of an aldehydic H yielding the corresponding benzoylic radicals CHOC6 H4 CHO + X ↔ CHOC6 H4 CO + XH (X = O2 , H, OH, C6 H5 , C2 H3 , HO2 , CH3 ), (1113–1116, 1120–1123) C2 H5 C6 H4 HCO + X ↔ C2 H5 C6 H4 CO + XH (X = CH3 , C6 H5 , C2 H3 , C6 H4 CH3 , p-xylyl, H, OH, O, O2 , HO2 ). (1134–1140, 1145, 1146) In turn, these species decompose to phenylic radicals and CO CHOC6 H4 CO ↔ C6 H4 CHO + CO,

(1129)

C2 H5 C6 H4 CO ↔ C6 H4 C2 H5 + CO.

(1148)

In this model, the fate of the phenylic radicals is either through further oxidation, to form phenoxy type radicals, or through the formation of stable species via abstraction of a H atom from a stable hydrocarbon. Finally, the recombination of methylbenzyl radical yields 4,4-dimethylbibenzyl (CH3 (C6 H4 CH2 )2 CH3 ); its kinetics was taken from [34]. p-xylyl + p-xylyl ↔ CH3 (C6 H4 CH2 )2 CH3 .

(992)

5. Results and discussion 5.1. Oxidation in a JSR Molecular species concentration profiles were obtained from the oxidation of p-xylene in a JSR: O2 , CO, CO2 , CH2 O, CH4 , C2 H2 , C2 H4 , C2 H6 , allene, propyne, C3 H6 , acrolein, 1,3-C4 H6 , vinylacetylene, 1,3-cyclopentadiene, benzaldehyde, phenol, benzene, styrene, phenol, toluene, ethylbenzene, methylethylbenzene, p-methylbenzaldehyde, phthalaldehyde, ethylbenzaldehyde, methylstyrene, o-xylene, cresols, naphthalene, dibenzyl, and 4,4 dimethylbibenzyl were measured by sonic probe sampling and GC analyses. The products detected here and their relative abundances are in line with the experimental results obtained in a previous plug-flow reactor study [9], although acrolein, phenol, and dibenzyl were not reported in that previous work. These previous experiments were not modeled in this work since it is recognized that the oxidation in the flow reactor starts in the mixing section, complicating the kinetic modeling [35,36]. The present data were used to propose a detailed chemical kinetic reaction mechanism for the oxidation of p-xylene.

Figs. 1–3 present the results obtained at 1 atm for Φ = 0.5, 1, and 1.5, respectively. The experimental results show that besides CO, H2 , and CO2 , the major intermediate products were acetylene (C2 H2 ), methane (CH4 ), toluene, benzene, styrene, formaldehyde (CH2 O), 1,3-cyclopentadiene (CPD), phthalaldehyde, methylbenzaldehyde, ethylene (C2 H4 ), vinylacetylene (C4 H4 ) and propyne. The minor products were benzaldehyde, ethane, allene, acrolein, dibenzyl, formaldehyde, ethylbenzene, methylstyrene, methylethylbenzene, naphthalene, and 4,4 -dimethylbibenzyl. Only trace amounts of o-xylene, indene, methylnaphthalene, anthracene, phenantrene, phenylacetylene, ethyl benzaldehyde, methylbiphenyl, diphenylbutane were detected and are not shown. As can be seen from the Figs. 1–3, overall the model represents fairly well the experimental data. The reactivity of p-xylene and the formation of CO, CO2 , toluene, benzene, methane, formaldehyde, ethane, and allene (aC3 H4 ) are well predicted by the model. However, the model overpredicts the formation of acetylene (C2 H2 ), vinylacetylene (C4 H4 ), 1,3-butadiene, and propyne (pC3 H4 ). Under rich conditions the reactivity of p-xylene and the formation of CO, CO2 , toluene, benzene, methane, formaldehyde, and ethane are well predicted by the model. However, above 1300 K the model seems to underpredict the overall reactivity. Propyne, allene, and 1,3-butadiene (C4 H6 ) profiles are overpredicted. The benzaldehyde and styrene profiles are underpredicted. Sensitivity analyses and reaction path analyses, based on species reaction rate of production and rate of consumption computations, were used to elaborate the presently proposed mechanism and interpret the results. According to these computations under the present conditions, the kinetics of p-xylene oxidation is mainly sensitive to a limited number of reactions pertaining to its oxidation submechanism and to reactions of simple intermediates (Fig. 4). At 1300 K, under stoichiometric conditions and at 1 atm, the pxylene computed mole fractions are mostly sensitive to H + O2 ↔ OH + O

S = −0.143,

p-xylene ↔ p-xylyl + H

S = 0.248,

p-xylene + H ↔ toluene + CH3

S = −0.296,

p-xylyl ↔ CPD + C3 H3

S = −0.163,

p-xylene + H ⇒ p-xylyl + H2

S = −0.112,

p-xylyl ⇒ MeCPDY + C2 H2

S = −0.135,

where S represents the first order sensitivity coefficients computed for p-xylene. According to the proposed model, under the conditions of Fig. 2 at 1300 K,

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293

Fig. 4. The main reactions involved in the oxidation of p-xylene Φ = 1, P = 1 atm, T = 1300 K (conditions of Fig. 2).

Fig. 5. Sensitivity analysis results for p-xylene at Φ = 1, P = 1 atm, T = 1300 K (conditions of Fig. 2).

p-xylene mostly reacts through thermal decomposition and H-atom abstraction (Ci = normalized reaction rates, i = specie) (Fig. 5). p-xylyl + H ⇒ p-xylene C p-xylene = 0.988, p-xylene + H ⇒ p-xylyl + H2 C p-xylene = −0.446,

(-971) (964)

p-xylene ⇒ C6 H4 CH3 + CH3 C p-xylene = −0.031,

(970)

p-xylene + H ⇒ toluene + CH3 C p-xylene = −0.229,

(972)

p-xylene + CH3 ⇒ p-xylyl + CH4 C p-xylene = −0.036,

(974)

p-xylene + O2 ⇒ p-xylyl + HO2 C p-xylene = −0.011,

(975)

p-xylene + O ⇒ p-xylyl + OH C p-xylene = −0.055,

(976)

p-xylene + OH ⇒ p-xylyl + H2 O C p-xylene = −0.169,

(977)

p-xylene + C6 H5 ⇒ p-xylyl + C6 H6 C p-xylene = −0.020.

(980)

The p-xylyl radical partially reacts via (971), recycling p-xylene. The other reactions consuming the p-xylyl radical are reactions (963) and (984) yielding five-membered ring species, propargyl radicals and acetylene and oxidation routes yielding pmethylbenzaldehyde (MePhHCO), p-xylylene (CH2 C6 H4 CH2 ), methylethylbenzene (MePhC2 H5 ), and the p-methylbenzoxy radical (MePhCH2 O).

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p-xylyl ⇒ CPD + C3 H3 C p-xylyl = −0.156,

(963)

p-xylyl + H ⇒ p-xylene C p-xylyl = −0.477,

(-970)

p-xylyl ⇒ MeCPDY + C2 H2 C p-xylyl = −0.138,

(984)

p-xylyl + HO2 ⇒ MePhCH2 O + OH C p-xylyl = −0.021,

(985)

p-xylyl + O ⇒ MePhHCO + H C p-xylyl = −0.011,

(988)

The p-methylbenzoyl radical decomposes via (1025) MePhCO ⇒ C6 H4 CH3 + CO C MePhCO = −1.000,

(1025)

yielding the p-methylphenyl radical (C6 H4 CH3 ) and carbon monoxide. In turn, the p-methylphenyl radical is consumed via (1027)–(1029), yielding the p-methylphenoxy radical (OC7 H7 ). C6 H4 CH3 + O2 ⇒ OC7 H7 + O C C6 H4 CH3 = −0.266,

(1027)

p-xylyl + O2 ⇒ CH2 C6 H4 CH2 + HO2 C p-xylyl = −0.018,

(1006)

(1028)

p-xylyl + HO2 ⇒ CH2 C6 H4 CH2 + H2 O2 C p-xylyl = −0.032,

C6 H4 CH3 + HO2 ⇒ OC7 H7 + OH C C6 H4 CH3 = −0.504,

(1007)

C6 H4 CH3 + OH ⇒ OC7 H7 + H C C6 H4 CH3 = −0.207.

(1029)

p-xylyl ⇒ CH2 C6 H4 CH2 + H C p-xylyl = −0.045,

(1008)

The p-methylphenoxy radical decomposes yielding the methylcyclopentadienyl (MeCPDY) through

p-xylyl + CH3 ⇒ MePhC2 H5 C p-xylyl = −0.085.

(-1045)

The 1,3-cyclopentadiene (CPD) is consumed via the following reactions yielding the cyclopenta-1,3dienyl (C5 H5 ) radical:

OC7 H7 ⇒ MeCPDY + CO C OC7 H7 = −0.999.

(1152)

The methylcyclopentadienyl radical rearranges yielding benzene and hydrogen atoms: MeCPDY ⇒ C6 H6 + H C MeCPDY = −1.000.

CPD + O2 ⇒ C5 H5 + HO2 C CPD = −0.356,

(687)

CPD + OH ⇒ C5 H5 + H2 O C CPD = −0.191,

(689)

CPD + H ⇒ C5 H5 + H2 C CPD = −0.318.

(690)

MePhC2 H5 ⇒ BMePhC2 H4 + H C MePhC2 H5 = −0.086,

(1046)

The p-methylbenzoxy radical is mainly consumed through reaction (996).

MePhC2 H5 ⇒ AMePhC2 H4 + H C MePhC2 H5 = −0.045,

(1047)

MePhCH2 O ⇒ MePhHCO + H C MePhCH2 O = −1.000.

MePhC2 H5 + H ⇒ PhC2 H5 + CH3 C MePhC2 H5 = −0.699,

(1055)

MePhC2 H5 + H ⇒ C6 H4 CH3 + C2 H6 C MePhC2 H5 = −0.092,

(1056)

(996)

The p-methylbenzaldehyde decomposes (1010) and reacts with radicals yielding p-methylbenzoyl (MePhCO), benzaldehyde (PhHCO), and toluene: MePhHCO + H ⇒ MePhCO + H2 C MePhHCO = −0.130,

(1010)

MePhHCO ⇒ MePhCO + H C MePhHCO = −0.256,

(1011)

MePhHCO + H ⇒ PhHCO + CH3 C MePhHCO = −0.030,

(1012)

MePhHCO + O ⇒ MePhCO + OH C MePhHCO = −0.114,

(1013)

(1170)

Benzene reacts according to the scheme described earlier [5,27]. The p-methylethylbenzene (MePh C2 H5 ) is consumed via

yielding ethylbenzene (PhC2 H5 ), p-methylphenyl (C6 H4 CH3 ), and two radicals formed via the H-atom abstraction on the ethyl group in α-position (AMePh C2 H4 ) and in β-position (BMePhC2 H4 ). p-Methylstyrene (MeStyrene) is consumed via reactions (1083) and (1086) yielding styrene and toluene: MeStyrene + H ⇒ styrene + CH3 C MeStyrene = −0.661,

(1083)

MeStyrene + H ⇒ toluene + C2 H3 C MeStyrene = −0.216.

(1086)

MePhHCO + OH ⇒ MePhCO + H2 O C MePhHCO = −0.131,

(1014)

Whereas reaction (-1068) reforms p-methylstyrene.

MePhHCO + H ⇒ toluene + HCO C MePhHCO = −0.256.

(1016)

BMePhC2 H4 ⇒ MeStyrene + H C MeStyrene = 0.964.

(-1068)

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p-Xylylene (CH2 C6 H4 CH2 ) is depleted via (1007) producing the corresponding aldehyde that further reacts with oxygen yielding p-phthalaldehyde (CHOPCHO). CH2 C6 H4 CH2 + O ⇒ CHOPhCH2 + H C CH2 C6 H4 CH2 = −1.000,

(1009)

CHOPhCH2 + O2 ⇒ CHOPCHO + OH C CHOPhCH2 = −1.000.

(1125)

p-Phthalaldehyde reacts via CHOPCHO + H ⇒ CHOPCO + H2 C CHOPCHO = −0.219,

(1114)

CHOPCHO + O ⇒ CHOPCO + OH C CHOPCHO = −0.192,

(1116)

CHOPCHO ⇒ CHOPCO + H C CHOPCHO = −0.444,

(1118)

CHOPCHO + CH3 ⇒ CHOPCO + CH4 C CHOPCHO = −0.102.

(1123)

p-Benzoxyaldehyde (CHOPCO) decomposes forming p-formylphenyl (Y1P4CHO): CHOPCO ⇒ Y1P4CHO + CO C CHOPCO = −1.000.

(1129)

p-Phenoxyaldehyde (OY1P4CHO) and p-phenylaldehyde (Y1P4CHO) react via Y1P4CHO + O2 ⇒ OY1P4CHO + O C Y1P4CHO = −1.000,

(1130)

OY1P4CHO ⇒ C5 H5 CHO + CO C OY1P4CHO = −1.000.

(1170)

The 1-formylcyclopenta-2,4-dienyl radical (C5 H5 CHO) formed in (1171) isomerizes (1072) into 5formylcyclopenta-1,3-dienyl (CPDCO) that decomposes. C5 H5 CHO ⇒ CPDCO C C5 H5 CHO = −1.000,

(1172)

CPDCO ⇒ C5 H5 + CO C CPDCO = −1.000.

(1173)

Fig. 6. Comparison between experiment and computed ignition delays of p-xylene–oxygen–argon mixtures (initial conditions: Φ = 0.5, 0.045% mol of p-xylene; Φ = 1, 0.087% mol of p-xylene; Φ = 1.5, 0.125% mol of p-xylene; P5 = 1.2 ± 0.11 atm).

ignition delay times. The pressure behind the reflected shock wave, P5 , ranged from 1 to 1.4 atm and the temperature, T5 , ranged from 1450 to 1760 K. As can be seen from Fig. 6, the model predicts well the individual ignition delays in fuel-lean, stoichiometric, and fuel-rich conditions. Furthermore, the computed overall activation energy fits well the experiments. The combustion of p-xylene in a low-pressure premixed methane–oxygen–nitrogen flame doped with p-xylene [12] was also modeled. The data were obtained using a flat flame burner operating at 40 Torr. The mole fraction measurements were obtained by the MB-MS technique and MB-GC analyses. As can be seen from Fig. 7, the fuels (methane and p-xylene), the intermediate products (OH, CO, p-xylyl, toluene, and styrene), and a final product (H2 O) are fairly well simulated, further confirming the validity of the proposed scheme.

6. Conclusion

The cyclopenta-1,3-dienyl radical reacts according to the mechanism described before [5,28]. 5.2. Ignition and combustion To further test the validity of the proposed kinetic reaction mechanism, the ignition of p-xylene– oxygen–argon mixtures measured behind a reflected shock wave [25] were simulated. The data ranged from Φ = 0.5 to 1.5. The ignition delays were measured following the CH emission signal at 431 nm. The ignition delay corresponded to the maximum emission of CH radicals. In this modeling, the maximum concentration of CH was used to determine the

The two main objectives of this work were attained. The oxidation of p-xylene was studied in a JSR at atmospheric pressure over the temperature range 900–1300 K, for equivalence ratios ranging from 0.5 to 1.5. The concentration profiles of 30 molecular species were measured by sonic probe sampling and GC analyses (GC-MS, GC-TCD, and FID). For the first time, the kinetics of oxidation, ignition, and combustion of p-xylene was modeled using a detailed chemical kinetic reaction mechanism also validated for the oxidation of benzene and toluene over a wide range of conditions [5–8]. Overall, the kinetic model represents well the experimental results. The kinetic modeling was used to delineate the main reactions involved in the oxidation of p-xylene. Further

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Fig. 7. Mole fraction profiles (symbols, data from [12]; lines, this model) versus distance from the burner under flat flame conditions (initial mole fractions: O2 : 0.236; N2 : 0.710; CH4 : 0.039; p-xylene: 0.015; 0.05263 atm; total flow rate of 0.002384448 g cm−2 s−1 ).

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