Investigation of Stoichiometric Methane/Air/Benzene (1.5%) and Methane/Air Low Pressure ﬂames

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Combustion and Flame 135 (2003) 171Ð183 Investigation of stoichiometric methane/air/benzene (1.5%) and methane/air low pressure ﬂames

Laurent Duponta, Abderrahman El Bakalia, Jean-Franc¸ois Pauwelsa, Isabelle Da Costab, Philippe Meunierb, Henning Richterc,*

aUniversite´ des Sciences et Technologies de Lille, UMR 8522 “Physicochimie des Processus de Combustion”, Centre d’Etudes et de Recherches Lasers et Applications, F-59655 Villeneuve d’Ascq, France bGaz de France, Direction de la Recherche, B.P. 33, F-93211 Saint Denis la Plaine Cedex, France cDepartment of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA Received 28 October 2002

Abstract Benzene depletion in a laminar premixed flat stoichiometric low-pressure methane/air/benzene (1.5%) flame was investigated using a recently developed kinetic model that has been tested for low-pressure combustion of acetylene, ethylene, and benzene. Experimental flame structures of two stoichiometric methane/air flames (v ϭ34.2 cm sϪ1, 5.33 kPa) with and without the addition of 1.5% of benzene were measured previously by means of molecular beam sampling using mass spectrometry as well as gas chromatography coupled to mass spectrometry as analytic tools. Model computations were conducted using the Premix code within the Chemkin software package. Experimental temperature profiles were used as input. The analysis of rates of production of selected species allowed for the identification of major formation and depletion pathways. The predictive capacility of the model was assessed in both flames. Good to excellent agreements between predictions and measured mole fraction profiles were obtained for reactants, intermediates, and products such as methane, O2, methyl, H, OH, CO, CO2. and H2O. Benzene depletion and the formation and consumption of intermediates such as cyclopentadiene were predicted correctly. According to the model, methyl is exclusively formed by hydrogen abstraction from methane and subsequently oxidized by reaction ϩ u ϩ ϩ u ϩ with O to formyl, (1) CH3 O HCO H2, and formaldehyde, (2) CH3 O CH2O H, the latter channel being the dominant one. Benzene consumption occurred mainly by hydrogen abstraction with OH and H as reactant but also the contribution of its oxidation by O to phenoxy was significant. Phenol and phenoxy chemistries are strongly coupled, unimolecular decay of phenoxy to cyclopentadienyl radicals and CO is the dominant consumption route. Small PAH are predicted to be formed in the reaction zone, followed by complete depletion in the postflame region. ϩ u The pressure dependence of the dominant dimethylether formation (32) CH3 CH3O CH3OCH3 was found to be significant and assessed by means of Quantum Rice-Ramsperger-Kassel (QRRK) analysis. © 2003 The Combustion Institute. All rights reserved.

Keywords: Methane; Benzene; PAH

1. Introduction

The presence of ﬁne particles generated by in- complete combustion processes in atmospheric aero- * Corresponding author. Tel.: ϩ1-617-253-6536; fax: sols is of signiﬁcant health concern due to their as- ϩ1-617-258-5042. sociation with lung cancer and cardiopulmonary E-mail address: [email protected] (H. Richter). disease shown in epidemiological studies [1,2]. Poly-

Table 1 Flame parameters. a) Flame I: laminar premixed stoichiometric methane/air ﬂame b) Flame II: laminar premixed stoichiometric methane/air/benzene (1.5%) ﬂame

Flame Cold gas velocity v, Pressure XCH4 Xbenzene XO2 XN2 25 ¡C (cm sϪ1) (kPa) Flame I 34.2 5.33 0.111 0.00 0.222 0.667 Flame II 34.2 5.33 0.060 0.015 0.230 0.695

cyclic aromatic hydrocarbons (PAH) are thought to while soot and PAH remaining in the exhaust gas be key precursors of soot [3]. PAH have been quan- represent a major environmental concern. Complex tified in atmospheric aerosol samples [4] and many of mixing patterns in practical systems can lead to a them have been found to be mutagenic [5]. The large range of different local conditions, for instance minimization of pollutants such as soot and PAH in equivalence ratios and temperatures. Before the at- the exhaust of combustion devices requires the con- tempt of describing such complex systems, a detailed trol and therefore a detailed understanding of chem- understanding of the chemical processes is required. ical reaction pathways. The presence of PAH and For this purpose, the combustion of model com- soot in the exhaust of hydrocarbon combustion is the pounds, representative for practical fuels, under well- overall result of competing processes leading to the defined conditions and setups has to be investigated. growth of PAH of increasing size and ultimately to For instance, a quantitative understanding of the ox- soot as well as their depletion. Depending on the idation of small aromatic species such as benzene is local conditions, PAH, soot and their precursors can needed in order to assess the competition between the be oxidized within the flame before release. Kinetic depletion of such compounds and their growth to modeling by means of complex reaction networks PAH of increasing size and ultimately to soot. allows for the assessment of chemical processes in- In the present work, a recently developed kinetic herent to hydrocarbon oxidation and pyrolysis. De- model has been used for the analysis of benzene tailed reaction mechanisms have been developed and depletion in a laminar premixed stoichiometric meth- tested for premixed combustion of different fuels ane/air low-pressure flame. The model has been such as methane [6], acetylene [7,8], ethylene tested for premixed low- pressure combustion of [7,9,10], ethane [6], propane [11], 1,3-butadiene acetylene, ethylene and benzene [19]. A good predic- [12,13] and benzene [14Ð18] by comparison to ex- tive capability was observed for reactants, products perimental flame structure data. The quality of a and intermediates, including radical species [19]. The kinetic model can be assessed by its capability to oxidative depletion of all three fuels under fuel-rich predict correctly experimental data, particularly the evolution of concentrations of reactants, products and conditions was investigated and, in addition, a lean intermediates, over a large range of experimental ethylene flame was analyzed. Also, the formation of conditions such as fuel type and equivalence ratio. single-ring aromatics was assessed quantitatively in Thus, Lindstedt and Skevis [8] tested their kinetic the case of aliphatic fuels, ie, acetylene and ethylene mechanism for six lean (␾ ϭ0.12) to sooting (␾ [19]. ϭ2.50) laminar, low-pressure acetylene flames. The reaction network used in the present work has been developed and tested by comparison of model 2. Approach predictions with experimental mole fraction profiles of stable and radical species measured in four acet- Flame structures of two laminar premixed flat ylene (␾ ϭ2.40), ethylene (␾ ϭ0.75, ␾ ϭ1.90) and stoichiometric methane/air (Flame I) and methane/ benzene (␾ ϭ1.80) premixed flat low-pressure flames air/benzene (Flame II) low-pressure flames (v ϭ34.2 Ϫ1 using molecular beam sampling coupled to mass cm s , 5.33 kPa) were measured recently [20]. In spectrometry (MBMS) [19]. the latter case, 1.5% of benzene was added to the cold In the present work, the depletion of benzene in a gas mixture taking into account the contribution of stoichiometric methane/air/benzene (1.5%) flame benzene to the equivalence ratio, ie, the overall ␾ was was investigated. Combustion of methane, a major kept at unity. Therefore, the mole fraction of methane compound of natural gas, is of significant practical in the initial mixture had to be reduced from 11.1% in importance. Due to enhanced radiative heat release, the reference flame to 6% in the seeded flame. The the presence of soot particles can be desirable in parameters of both flames are summarized in Table 1. practical devices before their release from the flame Sampling was conducted by molecular beam with L. Dupont et al. / Combustion and Flame 135 (2003) 171–183 173

Fig. 1. Experimental temperature profiles [20]. — stoichiometric methane/air flame (Flame I). ... stoichioimetric methane/air/ benzene (1.5%) flame (Flame II).

subsequent online analysis by mass spectrometry chemically activated reactions [19,23]. The reaction (MBMS) and, alternatively, by gas chromatography mechanism and the corresponding thermodynamic using thermal conductivity and flame ionization de- property data, both carefully documented, are pro- tectors (GC-TCD-FID) and a mass spectrometric de- vided in the supplementary information of [19] and tector (GC-MS). The use of two complementary an- are available electronically [19,24]. All model com- alytical techniques allowed for the identification of putations were conducted with the Premix code radical intermediates (by MBMS), essential for a within the Chemkin software package [25] using ex- detailed understanding of the combustion chemistry, perimental temperature profiles (Fig. 1). The associ- and of species of identical mass such as cyclic and ated postprocessor was used for the determination of linear C5H6 or C6H6 species (by GC-MS). Detection rates of production of selected species, ie, the contri- limits of both techniques were at mole fractions of bution of specific reactions to their formation and Ϸ ϫ Ϫ7 5 10 for stable species. Due to the necessity to consumption. The model attempts the description of operate the MBMS system at reduced ionization po- the studied combustion systems using the best avail- tentials to avoid fragmentation of parent molecules, able thermodynamic and kinetic property data for detection limits of radical species are higher but de- each species and individual reaction step; therefore pend on the specific conditions. Radical mole frac- Ϫ no adjustments were made in the present work. How- tions of close to 1 ϫ 10 5 could be detected in the ever, pressure dependence of dimethylether forma- present work. Temperature profiles were measured tion, not investigated previously, has been taken into by means of a coated Pt/Rh6%-Pt/Rh30% thermo- account in the present work. couple located 0.2 mm upstream the tip of the sampling probe. Radiative heat losses were taken into Quantitative measurements, particularly of spe- account by electrical compensation. Details of the cies with concentrations close to the detection limit experimental procedure were described previously and of radicals, are challenging. Despite the excellent [21,22]. reproducibility found in the present work, uncertain- A recently developed kinetic model [19] was used ties of up to 30% for some species, such as hydrogen for the investigation of reaction pathways in methane radicals, cannot be excluded. Experimental tempera- combustion and benzene depletion under such con- ture profiles were used for all model calculations, ditions. Thermodynamic and kinetic property data therefore, the impact of possible uncertainties was were updated using most recent literature, density assessed. For instance, even an increase of 100 K of functional theory (DFT) and ab initio computations all temperatures led only to an insignificant increase on a CBS-Q and CBS-RAD level. Quantum Rice- of predicted maximum mole fractions while most Ramsperger-Kassel (QRRK) analysis was conducted profiles were shifted by 1 to 1.5 mm toward the to determine pressure-dependent rate constants of burner. 174 L. Dupont et al. / Combustion and Flame 135 (2003) 171–183

3. Results to play an important role. The good agreement between prediction and experiment for molecular hy- 3.1. Methane combustion drogen (Fig. 3) provides evidence of the correct thermodynamic description of these hydrogen abstraction Meaningful investigation of benzene depletion in reactions because of the non-negligible contribution a seeded premixed methane/air ﬂame necessitates the of the reverse reaction under certain conditions. accurate description of methane oxidation chemistry. Measured and computed mole fraction proﬁles of

A high level of predictive capability for methane propane (C3H8) are given in Fig. 4 in order to illus- combustion is required from kinetic models due to trate the formation and consumption of larger hydro- the practical importance of methane and its formation carbons in the case of stoichiometric methane com- ϩ u as intermediate in the case of other fuels. Model bustion. (4) C2H5 CH3 C3H8 was found to be predictions for reactants, products, and intermediates the only major propane formation pathway. Ethyl were compared to mole fraction profiles measured in (C2H5) is formed by hydrogen abstraction from the unseeded reference flame. Measured and com- ethane (C2H6) by H, OH and O radicals. Self-com- ϩ u puted mole fractions profiles of methane, O2, methyl, bination of methyl, ie, (5) CH3 CH3 C2H6, was H, H2, OH, CO, CO2, and propane (C3H8) are shown identified as dominant ethane formation route. The in Fig. 2 to 4. Original experimental data points depletion of propane occurs by hydrogen abstraction subsequent to calibration are given. No fitting was with H and OH to n- and i-propyl. Unimolecular performed in order to allow the assessment of the decay of both propyl isomers to methyl and ethylene reproducibility of the experimental procedure. Nearly (C2H4) and of i-propyl to propene (C3H6) are domi- complete methane consumption beyond approxima- nant subsequent reactions. tively 0.9 cm from the burner was correctly predicted by the model and a computed oxygen mole fraction 3.2. Benzene depletion of about 0.02 remaining in the postflame zone is in excellent agreement with the experimental findings. Consumption pathways of benzene were investi- The determination of rates of production showed gated in a premixed low-pressure methane/air/ben- hydrogen abstraction to methyl to be the only signif- zene (1.5%) flame. The fresh gas velocity was 34.2 icant methane consumption route. Relative contribu- cm sϪ1, the pressure 5.33 kPa, and the overall equiv- tions decrease from reactions with OH and H (both alence ratio was ␾ ϭ1.0. Agreements between model Ϸ40% of the maximum consumption rate) to that predictions and experimental mole fractions for key with O. Shape, peak location and value of the methyl species of methane oxidation such as CH4,O2,H,H2, mole fraction profile was well predicted (Fig. 2). Its O, OH, CH3,H2O, CO, and CO2 are, similar to the rates of production showed its exclusive formation above discussed reference flame, good to excellent. ϩ u ϩ from methane while (1) CH3 O HCO H2, and Comparisons between predicted mole fraction pro- ϩ u ϩ (2) CH3 O CH2O H were found to be the files and experimental data for methane, methyl and only significant consumption pathways. Using ki- O radicals are shown in Fig. 5. Due the identical mass netic data based on the total rate constant measured of methane and O, ie, 16 amu, the oxygen profile was by Lim and Michael [26] and the branching ratio only available after complete methane depletion. The determined by Marcy et al. [27], formation of form- height above the burner where methane was totally aldehyde (CH2O) was identified as dominant product consumed was determined by means of the gas chro- channel. Subsequently, formaldehyde is depleted by matographic measurement of its profile as well as the hydrogen abstraction to HCO via reaction with H use of an ionization potential below the appearance (Ϸ75%) and OH (Ϸ20%). Unimolecular hydrogen potential of O, in the case of MBMS. In should be loss and hydrogen abstraction by H, O2, OH, and pointed out that despite the small benzene mole frac- CH3 to CO are the only significant HCO consumption of only 1.5% in the cold gas mixture, the per- tion and CO formation routes. CO consumption and turbation of the methane/air system was significant.

CO2 formation were found to occur exclusively via In fact, 60% of all carbon present in the flame came ϩ u ϩ (3) CO OH CO2 H. The equilibrium constant from the initial benzene feed. Benzene consumption of reaction (3) depends strongly on the temperature. was correctly reproduced by the model and is com- Consistent with the encouraging quality of the com- pleted at Ϸ0.9 cm from the burner (Fig. 6). Major puted H and OH profiles (Fig. 3), the good agree- reactions involved in benzene depletion are summa- ments for both CO and CO2 model predictions with rized in Table 2. Phenol and cyclopentadiene (C5H6) experimental data (Fig. 4), indicate the accuracy of are intermediates in the benzene depletion process, the temperature profile used as input for the model both experimental and computed mole fraction pro- computations. Hydrogen abstraction reactions with H files are plotted in Fig. 6. Gas chromatography cou- radicals from methane, CH2O and HCO were found pled to mass spectrometry (GC-MS) allowed for the L. Dupont et al. / Combustion and Flame 135 (2003) 171–183 175

Fig. 2. Comparison between experimental mole fraction proﬁles and mode predictions in a stoichioimetric methane/air ﬂame (Flame I). F CH4 : (experiment [20]), — (prediction) ■ O2 : (experiment [20]), –––(prediction) Œ CH3 : (experiment [20], right scale), ---- (prediction, right scale).

inambiguous identiﬁcation of cyclopentadiene (and computed C5H6 mole fraction proﬁle with the exper- not linear C5H6) as dominant C5H6 species. Simi- imental one is excellent while the phenol peak mole larly, the presence of measurable amounts of C6H6 fraction was about two-fold underpredicted. The rates isomers other than benzene, ie, fulvene or linear of production of benzene (Fig. 7) showed hydrogen species could be excluded by means of GC-MS. The abstraction by OH, ie, (6) benzene ϩ OH u phenyl ϩ agreement of shape, peak location, and value of the H2O, to be the most important single benzene

Fig. 3. Comparison between experimental mole fraction proﬁles and model predictions in a stoichoimetric methane/air ﬂame (Flame I). H:F (experiment [20], left scale), — (prediction, left scale) OH : ■ (experiment [20], left scale), –––(prediction, left scale) Œ H2 : (experiment [20], right scale), ---- (prediction, right scale). 176 L. Dupont et al. / Combustion and Flame 135 (2003) 171–183

Fig. 4. Comparison between experimental mole fraction proﬁles and model predictions in a stoichoimetric methane/air ﬂame (Flame I). CO : F (experiment [20], left scale), — (prediction, left scale) ■ CO2 : (experiment [20], left scale), –––(prediction, left scale) Œ C3H8 : (experiment [20], right scale), ---- (prediction, right scale).

ϩ u consumption reaction. In addition, reaction with routes. In addition, also (15) i-C4H3 C2H3 atomic oxygen leading to phenoxy, (7) benzene ϩ O phenyl ϩH, suggested by Pope and Miller [38] and u phenoxy ϩ H, and hydrogen abstraction with H reaction with H to benzene, (9), contribute to phenyl ϩ u ϩ radicals, (8) benzene H phenyl H2, were ϩH, consumption. found to be significant. While the resulting rate of benzene production was negative along the whole 3.3. Phenol and phenoxy axis of the flame, (9) phenyl ϩ H u benzene and the reverse reaction of (10) benzene ϩ OH u phenol ϩ The rates of production of phenol show (16) phe- H were benzene forming pathways. Consistent with noxy ϩ H u phenol to be the only significant for- the analysis of benzene consumption, the rates of mation pathway. Hydrogen abstraction by OH and in production of phenyl showed reactions (6) and (8) to a smaller extent by H, leading to phenoxy, (17) phe- be its only significant formations routes. Phenyl ox- nol ϩ OH u phenoxy ϩ H O and (18) phenol ϩ H ϩ 2 idation by molecular oxygen, i.e., (11) phenyl O2 u phenoxy ϩ H are the dominant consumption u phenoxy ϩ O was identified as the most important 2 routes. This observation indicates the strong coupling depletion reaction. QRRK analysis was performed in between phenol and phenoxy chemistries. Consistent order to account for the pressure dependence of re- with the above discussed benzene formation, also the action (11) [19,23] using the input parameters sug- reverse of reaction (10) benzene ϩ OH u phenol ϩ gested by Dinaro et al. [34]. Phenyl oxidation to ϩ u H contributes significantly to phenol consumption. o-benzoquinone via reaction (12) phenyl O2 o-benzoquinone ϩ H, suggested by Alzueta et al. Another non-negligible phenol depletion route con- [35], was found to contribute significantly to phenyl sists in its unimolecular decay to cyclopentadiene u ϩ depletion. However, in other studies the role of ben- (C5H6), (19) phenol C5H6 CO. zoquinones has been found to remain questionable In agreement with their significant contributions [19]. No benzoquinone could be identified experi- to the consumption of benzene and phenyl, (7) ben- ϩ u ϩ ϩ u mentally [20] while peak mole fractions of Ϸ1.9 ϫ zene O phenoxy H, (11) phenyl O2 10Ϫ5 and Ϸ1.7 ϫ 10Ϫ4 were predicted by the model phenoxy ϩ O and (13) phenyl ϩ OH u phenoxy ϩ for o- and p-benzoquinone, respectively. Oxidation to H were found to be major phenoxy formation path- phenoxy by reaction with OH, (13) phenyl ϩ OH u ways. Phenoxy consumption occurs via reaction with phenoxy ϩ H and depletion to cyclopentadienyl by hydrogen radicals to phenol (16) and unimolecular ϩ u ϩ u reaction with O, (14) phenyl O c-C5H5 CO decay to cyclopentadienyl, (20) phenoxy c-C5H5 [19,37] are also non-negligible phenyl depletion ϩ CO, confirming previous findings [19,32]. L. Dupont et al. / Combustion and Flame 135 (2003) 171–183 177

Fig. 5. Comparison between experimental mole fraction proﬁles and model predictions in a stoichoimetric methane/air/benzene (1.5%) ﬂame (Flame II). F CH4 : (experiment [20], left scale), — (prediction, left scale) ■ CH3 : (experiment [20], right scale), –––(prediction, right scale) O:Œ (experiment [20], right scale), ---- (prediction, right scale).

3.4. Cyclopentadienyl (c-C5H5) and a major cyclopentadienyl formation route while (21) ϩ u cyclopentadiene (c-C5H6) c-C5H5 H c-C5H6 was the most important cyclopentadienyl consumption and cyclopentadiene The analysis of the rates of production of cyclo- formation reaction. In addition and consistent with its u pentadienyl (Fig. 8) and cyclopentadiene (Fig. 9) role in phenol depletion, also (19) phenol c-C5H6 showed unimolecular decay of phenoxy via (20) to be ϩ CO contributes signiﬁcantly the cyclopentadiene

Fig. 6. Comparison between experimental mole fraction proﬁles and model predictions in a stoichoimetric methane/air/benzene (1.5%) ﬂame (Flame II). benzene : F (experiment [20], left scale), — (prediction, left scale) phenol : ■ (experiment [20], left scale), –––(prediction, left scale) cyclopentadiene : Œ (experiment [20], right scale), ---- (prediction, right scale). 178 L. Dupont et al. / Combustion and Flame 135 (2003) 171–183

Table 2 ϭ n Ϫ Major reactions involved in the depletion of benzene. Rate constants are given as k AT exp( Ea/RT). Concentrations are expressed in mole cmϪ3, i.e., A-factors of bimolecular reactions are given in cm3 moleϪ1 sϪ1.

Reaction A n Ea [cal] Ref. ϩ % ϩ ϫ 13 Benzene OH phenyl H2O 2.11 10 0.0 4571 [28] Benzene ϩ O % phenoxy ϩ H 2.40 ϫ 1013 0.0 4668 [29] ϩ % ϩ ϫ 7 Benzene H phenyl H2 3.23 10 2.095 15842 [30]* Phenyl ϩ H % benzene 8.02 ϫ 1019 Ϫ2.011 1968 [31] Benzene ϩ OH % phenol ϩ H 1.59 ϫ 1019 Ϫ1.82 12800 [32]** ϩ % ϩ ϫ 21 Ϫ † Phenyl O2 phenoxy O 2.39 10 2.62 4400 [34] ϩ % ϩ ϫ 13 Phenyl O2 o-benzoquinone H 3.00 10 0.0 9000 [35] Phenyl ϩ OH % phenoxy ϩ H 5.00 ϫ 1013 0.0 0 [36] ϩ % ϩ ϫ 13 Phenyl O c-C5H5 CO 9.00 10 0.0 0 [37] ϩ % ϩ ϫ 12 i-C4H3 C2H3 phenyl H 6.00 10 0.0 0 [38] Phenoxy ϩ H % phenol 4.43 ϫ 1060 Ϫ13.232 30010 [39]‡ ϩ % ϩ ϫ 8 Ϫ Phenol OH phenoxy H2O 1.39 10 1.43 962 [32] ϩ % ϩ ϫ 14 Phenol H phenoxy H2 1.15 10 0.0 12400 [40] % ϩ ϫ 12 Phenol c-C5H6 CO 1.00 10 0.0 60802 [41] % ϩ ϫ 11 Phenoxy c-C5H5 CO 2.51 10 0.0 43900 [42] ϩ % ϫ 63 Ϫ ¤ c-C5H5 H c-C5H6 2.71 10 14.79 21050 [43] ϩ % ϩ ϫ 13 c-C5H6 H c-C5H5 H2 2.80 10 0.0 2259 [44] ϩ % ϩ ϫ 4 c-C5H6 O c-C5H5 OH 4.77 10 2.71 1106 [43] ϩ % ϩ ϫ 6 c-C5H6 OH c-C5H5 H2O 3.08 10 2.00 0 [43] ϩ % ϩ ϫ 13 Ϫ ¤ c-C5H5 O c-C5H4O H 6.71 10 0.03 40 [43] ϩ % ϩ ϫ 61 Ϫ c-C5H4O H n-C4H5 CO 2.10 10 13.27 40810 [35] ϩ % ϩ ϫ 13 c-C5H5 OH c-C5H4 H2O 2.11 10 0.0 4571 [28]*** ϩ % ϩ ϫ 7 †† c-C5H5 H c-C5H4 H2 3.23 10 2.095 15842 [30] ϩ % ϩ ϫ 13 Ϫ ¤ c-C5H5 O n-C4H5 CO 7.27 10 0.28 470 [43] ϩ % ϩ ϫ 14 c-C5H6 H C3H5 C2H2 6.60 10 0.0 12345 [44] % ϩ ϫ 12 o-C6H4O2 c-C5H4O CO 1.00 10 0.0 40000 [35] ϩ % ϩ * deduced by via the equilibrium constant of Benzene H phenyl H2 using the rate constant determined by Mebel et al. [30]; ** Quantum Rice-Ramsperger-Kassel (QRRK) analysis at 20 torr based on Shandross et al. [32] using improved treatment allowing for three frequencies as input [23]. The rate constant at the high-pressure limit of the benzene ϩ OH reaction, i.e., 1.39 ϫ 1012 exp(Ϫ0.38 kcal/RT) cm3 moleϪ1 sϪ1 was taken from Witte et al. [33]; † QRRK analysis [23] at 20 torr using the input parameters suggested by Dinaro et al. [34]; ‡ QRRK analysis [23] at 20 torr using 2.50 ϫ 1014 cm3 moleϪ1 sϪ1 [39] as high pressure limit. No additional exit channel, i.e., only stabilization to phenol and the decay to the reactants (phenoxy ϩ H) are taken into account; ¤ QRRK analysis [23] at 20 torr using the input parameters provided by Zhong and Bozzelli [43]; ϩ % ϩ †† ϩ % ϩ *** rate constant for Benzene OH phenyl H2O; rate constant for Benzene H phenyl H2.

ϩ u ϩ formation. The chemistries of cyclopentadiene and tion, (26) c-C5H4O H n-C4H5 CO, as its cyclopentadienyl are strongly coupled, hydrogen ab- dominant consumption pathway. Other, secondary, ϩ u stractions from cyclopentadiene (22) c-C5H6 H cyclopentadienyl consumption pathways are hydro- ϩ ϩ u ϩ c-C5H5 H2, (23) c-C5H6 O c-C5H5 OH gen abstraction to cyclopentatriene (c-C5H4), (27) ϩ u ϩ ϩ u ϩ ϩ and (24) c-C5H6 OH c-C5H5 H2O play a c-C5H5 OH c-C5H4 H2O and (28) c-C5H5 u ϩ significant role in c-C5H6 consumption and c-C5H5 H c-C5H4 H2 as well as its oxidation to n- ϩ u ϩ formation. In agreement with its role in phenyl con- butadienyl (29) c-C5H5 O n-C4H5 CO. Be- ϩ u ϩ sumption, also (14) phenyl O c-C5H5 CO sides hydrogen abstraction to cyclopentadienyl, (22) [19,37] contributes to the formation of cyclopentadi- Ð (24), decomposition of cyclcopentadiene to propyl ϩ u ϩ enyl. Oxidation of cyclopentadienyl to cyclopentadi- (C3H5) and acetylene, (30) c-C5H6 H C3H5 ϩ u ϩ enone (c-C5H4O), (25) c-C5H5 O c-C5H4O H C2H2 is its major depletion pathway. Despite a pre- Ϸ ϫ Ϫ4 was found to be a major c-C5H5 consumption path- dicted peak mole fraction of 2 10 , no exper- way. The rate constant of (25) [19,24] was deter- imental cyclopentadienyl profile was available in the mined by means of a QRRK computation [23] using present work but the excellent agreement between the the input parameters given by Zhong and Bozzelli computed and experimental cyclopentadiene profiles

[43]. Analysis of the rates of production of c-C5H4O (Fig. 6) indicates a realistic description of the C5 allowed for the identiﬁcation of n-butadienyl forma- chemistry by the kinetic model [19,24]. This ﬁnding L. Dupont et al. / Combustion and Flame 135 (2003) 171–183 179

Fig. 7. Rates of production of benzene in a stoichoimetric methane/air/benzene (1.5%) flame (Flame II). is consistent with the previous modeling of premixed and peak location of both species were correctly low-pressure acetylene [19] and benzene [18,19] predicted, but a two-fold overprediction of the peak flames. value occurred for dimethylether if the rate constant However, open questions remain concerning the k ϭ 1.21 ϫ 1013 cm3 moleϪ1 sϪ1, recommended by ϩ link between cyclic C6 and C5 species. For instance, Tsang and Hampson [45], was used for (32) CH3 u the overpredictions of phenoxy and cyclopentadien- CH3O CH3OCH3. The analysis of rates of pro- one described recently [19] need to be addressed in duction showed reaction of methoxy (CH3O) with future work. Unimolecular decay of o-benzoquinone methyl (CH3), (32), to be the only dimethylether u ϩ (o-C6H4O2), (31) o-C6H4O2 c-C5H4O CO, sug- formation pathway. The reverse reaction is the most gested by Alzueta et al. [35], was found to be a major important single dimethylether consumption reac- cyclopentadienone formation pathway and additional tion, the rate of production of (32) is negative at 6.5 work on benzoquinone formation and consumption is mm from the burner and beyond. In addition, hydro- required. gen abstraction with H, O and OH plays a significant It is worthwhile to mention that even at stoichio- role in dimethylether consumption. The resulting metric conditions, as investigated in the present CH3OCH2 decays subsequently to methyl and form- work, non-negligible amounts of small PAH can be u ϩ aldehyde (CH2O), (33) CH3OCH2 CH3 CH2O. formed in the flame front. Naphthalene (C10H8) and The two-fold overprediction of dimethylether is phenanthrene (C H ) were predicted to be formed 14 10 likely to be related to a decrease of k32 at higher Ϫ6 with peak mole fractions of Ϸ1 ϫ 10 and Ϸ1 ϫ temperature due to chemical activation and an in- Ϫ7 10 , respectively, followed by complete depletion creasing contribution of the reverse reaction, in par- in the postflame zone. ticular at low-pressure conditions. A similar behavior was observed for the self-combination of phenyl to 3.5. Aliphatic oxygenated species: acetaldehyde, biphenyl [18] and, similar to this case, a QRRK dimethylether, propanal and 2-propenal computation [23] for reaction (32) was performed in ϭ 58 -14.14 the present work. k32 7.69 10 T exp(-17.84 The use of gas chromatography coupled to mass kcal/RT) cm3 moleϪ1 sϪ1 at 5.33 kPa resulted from spectrometry (GC-MS) allowed for the unambiguous the QRRK treatment exhibiting a pronounced de- identification of oxygen containing species. Mole crease with increasing temperature. Its use in the fraction profiles of dimethylether (CH3OCH3) and kinetic model led to a significant reduction of the acetaldehyde (CH3CHO) measured in the benzene- peak mole fraction (Fig. 10) but did not effect the seeded stoichiometric methane/air flame are plotted prediction of methyl, in excellent agreement with the in Fig. 10 and compared to model predictions. Shape experimental data. 180 L. Dupont et al. / Combustion and Flame 135 (2003) 171–183

Fig. 8. Rates of production of cyclopentadienyl radicals in a stoichoimetric methane/air/benzene (1.5%) ﬂame (Flame II).

ϩ Oxidation of vinyl (C2H3) by OH, (33) C2H3 stractions from the aldehyde group by H, O and OH u OH CH3CHO was found to be a major formation were identified as dominant consumption routes. The route of acetaldehyde (CH3CHO). The reaction of resulting CH3CO decays readily via (35) CH3CHO u ϩ methyl with formyl (HCO) radicals, ie, the reverse of CH3 HCO, a peak mole fraction of only 1.5 ϩ u Ϫ10 (34) C2H3 OH CH3 CHO, contributed signifi- 10 was predicted by the model. cantly to acetaldehyde formation in the first part of Mole fraction profiles of two three carbon atom the reaction zone but became a consumption pathway containing aldehydes, propanal (C2H5CHO) and beyond about 7 mm from the burner. Hydrogen ab- 2-propenal (CH2CHCHO, acrolein), were measured

Fig. 9. Rates of production of cyclopentadiene in a stoichoimetric methane/air/benzene (1.5%) ﬂame (Flame II). L. Dupont et al. / Combustion and Flame 135 (2003) 171–183 181

Fig. 10. Comparison between experimental mole fraction profiles and model predictions in a stoichioimetric methane/air/ benzene (1.5%) flame (Flame II). F dimethylether (CH3OCH3): (experiment [20], left scale), — (prediction, using ϭ ϫ 13 3 Ϫ1 Ϫ1 k32 1.21 10 cm mole s [45], left scale), Ϫ . Ϫ . Ϫ ϭ ϫ 58 -14.14 (prediction, using k32 7.69 10 T exp(-17.84 kcal/RT) cm3 moleϪ1 sϪ1, left scale) ■ acetaldehyde (CH3CHO) : (experiment [20], left scale), –––(prediction, left scale) Œ propanal (C2H5CHO): (experiment [20], right scale), ---- (prediction, right scale). with GC-MS. Shape and peak values of both species laminar premixed flat stoichiometric methane/air low were well predicted but a shift of the computed pro- pressure flames with and without the addition of files relative to the experimental ones toward the 1.5% of benzene. Mole fraction profiles for stable postflame zone by 1 to 2 mm occurred. Both exper- and unstable species including radicals were mea- imental and predicted profiles of propanal are given sured previously by molecular beam sampling cou- in Fig. 10. Reaction of propyl (C3H5) with OH, (36) pled to mass spectrometry and allowed for a thorough u ϩ CH3 CO CH3 CO was found to be the dominant testing of the kinetic model. Mole fraction profiles propanal formation pathway. Its consumption occurs established by means of gas chromatographic analy- through unimolecular decay to ethyl (C2H5) and sis (GC-TCD-FID, GC-MS) were available and made formyl (HCO), ie, the reverse of reaction (37) C3H5 the inambiguous identification of structural isomers, ϩ u OH C2H5CHO, and hydrogen abstraction by e.g., cyclic and aliphatic C6H6 or C5H5, possible. ϩ u OH from the aldehyde group, (38) C2H5 HCO Good to excellent agreements between predictions C2H5CHO. The resulting radical decays quantita- and measured mole fraction profiles were obtained tively to ethyl and CO. Its rates of production show for reactants, intermediates and products such as

2-propenal (CH2CHCHO) to be formed by oxidation methane, O2, methyl, H, OH, CO, CO2, and H2O. of propyl and propargyl (C3H3) by reaction with O Benzene depletion as well as the formation and con- ϩ % ϩ and OH, ie, (39) C2H5 CHO OH C2H5CO sumption of intermediates such as cyclopentadiene ϩ u ϩ H2O and (40) C3H5 O CH2CHCHO H. were predicted correctly. Reaction pathways respon- Hydrogen abstraction from the aldyhyde group and sible for the formation and consumption of selected ϩ unimolecular decay to vinyl and formyl, (42) C2H3 species were identiﬁed by means of the analysis of u HCO CH2CHCHO, are its major consumption their rates of production. Methane consumption was pathways. found to be initiated by hydrogen abstraction with H, OH and O while the resulting methyl radicals were ϩ oxidized by reaction with O to formyl, (1) CH3 O u ϩ ϩ u 4. Conclusions HCO H2, and formaldehyde, (2) CH3 O ϩ CH2O H. Subsequent hydrogen abstractions by H

A recently developed kinetic model was used for and OH from CH2O and HCO as well as unimolecu- the assessment of major reaction pathways in two lar hydrogen loss from HCO led ﬁnally to CO, par- 182 L. Dupont et al. / Combustion and Flame 135 (2003) 171–183

tially oxidized by OH to CO2. Benzene consumption ler, J.-C. Vergnaud, H. Sommer, Lancet 356 (2000) occurred mainly by hydrogen abstraction with OH 795Ð801. and H as reactant but also the contribution of its [3] H. Richter, J.B. Howard, Prog. Energy Combust. Sci. oxidation by O to phenoxy is significant. Reaction of 26 (2000) 565Ð608. phenyl with O was identified to be a major phenyl [4] J.O. Allen, N.M. Dookeran, K.A. Smith, A.F. Sarofim, 2 K. Taghizadeh, A.L. Lafleur, Environ. Sci. Technol. consumption and phenoxy formation pathway. Phe- 30 (1996) 1023Ð1031. nol and phenoxy chemistries are strongly coupled, [5] J.L. Durant, W.F. Busby, A.L. Lafleur, B.W. Penman, unimolecular decay of phenoxy to cyclopentadienyl C.L. Crespi, Mutation Research 371 (1996) 123Ð157. radicals and CO is the dominant consumption route. [6] N.M. Marinov, W.J. Pitz, C.K. Westbrook, M.J. Cyclopentadiene was formed from cyclopentadienyl Castaldi, S M. Senkan, Combust. Sci. Technol. 116Ð but also by phenol decomposition, (19) phenol u 117 (1996) 211Ð287. ϩ C5H6 CO. Naphthalene (C10H8) and phenanthrene [7] H. Wang, M. Frenklach, Combust. Flame 110 (1997) (C14H10) were predicted to be formed in the reaction 173Ð221. zone, followed by complete depletion in the post- [8] R.P. Lindstedt, G. Skevis, Combust. Sci. Technol. 125 flame region. The formation and depletion of the (1997) 73Ð137. aliphatic oxygenated species acetaldehyde, dimethyl- [9] A.D’Anna, A. Violi, Twenty-Seventh Symposium (In- ternational) on Combustion, The Combustion Institute, ether, propanal and 2-propenal were correctly repro- Pittsburgh, 1998, p. 425Ð433. duced by the model. The pressure dependence of the [10] T. Faravelli, A. Goldaniga, E. Ranzi, Twenty-Seventh dominant dimethylether formation pathway (32) CH3 Symposium (International) on Combustion, The Com- ϩ u CH3O CH3OCH3 was found to be significant bustion Institute, Pittsburgh, 1998, p. 1489Ð1495. and was assessed by means of a Quantum Rice- [11] N.M. Marinov, M.J. Castaldi, C.F. Melius, W. Tsang, Ramsperger-Kassel (QRRK) analysis. Combust. Sci. Technol. 128 (1997) 295Ð342. [12] R.P. Lindstedt, G. Skevis, Twenty-Sixth Symposium (International) on Combustion, The Combustion Insti- tute, Pittsburgh, 1996, p. 703Ð709. Acknowledgments [13] A. Goldaniga, T. Faravelli, E. Ranzi, Combust. Flame 122 (2000) 350Ð358. In the USA, this research was supported by the [14] R.P. Lindstedt, G. Skevis, Combust. Flame 99 (1994) Chemical Sciences Division, Office of Basic Energy 551Ð561. Sciences, Office of Energy Research, U.S. Depart- [15] H.-Y. Zhang, J.T. McKinnon, Combust. Sci. Technol. ment of Energy under Grant DE-FGO2 to 107 (1995) 261Ð300. 84ER13282. Prof. J. B. Howard is acknowledged for [16] Y. Tan, P. Frank, Twenty-Sixth Symposium (Interna- many helpful discussions. Prof. W. H. Green, Dr. O. tional) on Combustion, The Combustion Institute, Mazyar and Dr. R. Sumathi are thanked for their Pittsburgh, 1996, p. 677Ð684. contributions to the development of the thermody- [17] H. Richter, W.J. Grieco, J.B. Howard, Combust. namic and kinetic property data sets. The authors Flame 119 (1999) 1Ð22. thank Prof. J. W. Bozzelli for providing software and [18] H. Richter, T.G. Benish, O.A. Mazyar, W.H. Green, J.B. Howard, Proc. Combust. Inst. 28 (2000) 2609Ð valuable advice. One of the authors (H.R.) thanks the 2618. Universite« des Sciences et Technologies de Lille for [19] H. Richter, J.B. Howard, Phys. Chem. Chem. Phys. 4 its generous invitation. (2002) 2038Ð2055. In France, this research was supported by Gaz de [20] L. Dupont, Etude expe«rimentale et mode«lisation cine«- France. 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