ARTICLE pubs.acs.org/EF
A Detailed Kinetic Modeling Study of Benzene Oxidation and Combustion in Premixed Flames and Ideal Reactors G. Vourliotakis,* G. Skevis, and M. A. Founti Laboratory of Heterogeneous Mixtures and Combustion Systems, Thermal Engineering Section, School of Mechanical Engineering, National Technical University of Athens, Heroon Polytechniou 9, Polytechnioupoli-Zografou, Athens 15780, Greece
bS Supporting Information ABSTRACT: The pyrolysis and oxidation of benzene occupies a critical role in the combustion chemistry of practical fuels. Despite numerous experimental and numerical investigations, uncertainties still exist regarding even major benzene combustion features. Recent benzene premixed flame data sets offer a unique possibility for the judicious evaluation of mechanisms developed solely on the basis of a single flame. In this context, a validated detailed kinetic mechanism for benzene oxidation and combustion has been further developed and assessed against recently available premixed flame data and data from shock tubes and stirred and flow reactors. Speciation data from phenol and benzoquinone pyrolysis and oxidation are additionally used as validation targets. This approach provides the opportunity for a more systematic evaluation of uncertainties associated with experimental data obtained under similar conditions. A re-evaluation of the phenyl radical oxidation, phenol/phenoxy chemistry, and a cyclopentadiene submechanism is proposed in view of both new rate data and validation targets. Benzene oxidation is shown to be largely controlled by oxidation of phenoxy and cyclopentadienyl radicals, with C5 and C6 linearization reactions also being crucial. A notable exception is benzyne, which is predominantly consumed to a linear isomer. The mechanism is shown to successfully reproduce benzene experimental data covering a wide range of operating conditions. Phenol and benzoquinone pyrolysis and oxidation are also adequately captured. Further, the chemistry of C1C4 small hydrocarbons is satisfactorily reproduced. Uncertainties related to both kinetic and thermodynamic data are evaluated. Finally, the study identifies aspects of benzene combustion chemistry where further work is required, most notably the rate and product distribution of the C6H5 þ O2 reaction.
1. INTRODUCTION Uncertainties regarding fuel prices and concerns about climate change and environmental pollution have sparked a renewed interest in combustion process optimization and the combustion chemistry of conventional and alternative fuels. Incomplete combustion results in reduced efficiency and formation of pollutants such as oxygenated species (fuel-lean), polycyclic aromatic hydrocarbons (PAHs; fuel-rich), and eventually soot, with deleterious consequences for both engine operation and human health.1,2 This is even more pronounced in the case of practical transportation fuels having a high aromatic content. Additionally, advanced engine technologies, utilizing low temperatures and lean fuel/air mixtures, present favorable conditions for the formation of oxygenated aromatic compounds, which, in turn, may lead to the formation of dioxins and furans.3 Cyclic oxygenates, such as benzoquinones, are also present in the exhaust of conventional automotive engines,4 and toxic aromatic compounds are a byproduct of municipal waste incinerators.5 Though still largely unregulated, such species are of great importance, especially in the case of a wider use of secondgeneration biofuels, produced from lignocellulosic feedstock and built up from C4C6 oxygenates.6 The rate-limiting step for PAH and soot in combustion systems is the formation of the first aromatic ring.1,2 The oxidation of the first aromatic ring would lead to the formation of oxygenated toxic species, such as phenol, quinones, and catechol, and persistent organic radicals, such as phenoxy.7 A thorough understanding of benzene chemistry under both low- and high-temperature conditions is thus crucial. r 2011 American Chemical Society
The combustion chemistry of benzene has been the subject of numerous studies. In their pioneering work, Bittner and Howard8 were the first to measure a rich (equivalence ratio φ = 1.8), nearsooting, low-pressure (20 Torr) premixed C6H6/O2/Ar flame using molecular beam mass spectroscopy (MBMS). Renewed interest in the combustion of aromatic compounds has resulted in the emergence of new experimental data sets. Defoeux et al.9 studied the structure of a rich (φ = 2.0) low-pressure (p = 50 mbar) C6H6/O2/Ar flame, also using MBMS. A comprehensive experimental study of a similar (φ = 1.78; p = 40 mbar) flame has been performed by Yang et al.10 using tunable synchrotron photoionization and MBMS. The above experimental technique is particularly useful because it allows isomer identification. Detilleux and Vandooren11,12 reported MBMS data from low-pressure (p = 45 mbar), lean (φ = 0.7), stoichiometric (φ = 1.0), and rich (φ = 2.0) C6H6/O2/Ar flames. The latter study is of importance because it provides the opportunity for a direct comparison of the effect of stoichiometry on the benzene combustion pathways and flame structure. Benzene oxidation has also been studied in shock tubes and flow and stirred reactors. Shock tubes have mainly been used for the determination of the induction time of benzene/oxygen mixtures, including the early work of Burcat et al.13 and Thyagarajan and Bhaskaran.14 More recently, da Costa et al.15 performed ignition Received: December 16, 2010 Revised: March 26, 2011 Published: March 28, 2011 1950
dx.doi.org/10.1021/ef101712p | Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Table 1. Experimental Conditions for Benzene Laminar Premixed Flames Computed in the Present Worka flame A flame B flame C flame D flame E flame F ref
8
Table 4. Experimental Conditions for Phenol Flow Reactor Pyrolysis and Oxidation Experiments Considered in the Present Worka PFR 1
PFR 2
PFR 3
9
10
11
11
11
φ 1.8 C6H6 (mole fraction) 0.135
2.0 0.118
1.78 0.095
0.7 0.030
1.0 0.030
2.0 0.120
ref
20
21
φ
pyrolysis
1/1274
pyrolysis
O2 (mole fraction)
0.565
0.442
0.405
0.290
0.200
0.440
C6H5OH (%)
0.2016
0.0028
0.005
Ar (mole fraction) · m(mg cm2 s1)
0.300
0.440
u0(cm s1)
50
35
P (mbar)
26.7
50
21
0.500
0.680
0.770
0.440
O2 (%)
25
2.1
2.931
2.984
3.102
H2O (%) Tin (K)
1162
4.7 9001500
2.0 9001500
P (atm)
1
1.036
1.036
τres (s)
0.18
115/T [K]
165/T [K]
40
45
45
45
a
Information is exclusively based on the explicit data given in original references. In all flames, the measured temperature profile was imposed on computations.
a
Table 2. Experimental Conditions for Benzene Stirred Reactors Computed in the Present Worka
Table 5. Experimental Conditions for Ignition Time Delay Experiments Considered in the Present Worka
JSR
Information is exclusively based on the explicit data given in original references. In all cases, nitrogen was the dilution gas.
τign 1
PSR
τign 2
ref
23
22
ref
13
13
φ
1.5
1.02
φ
1.0
2.0
C6H6 (mole fraction)
0.15
0.0051
O2 (mole fraction) N2 (mole fraction)
0.75 99.12
0.2013
C6H5OH (%) O2 (%)
1.69 12.675
1.35 5.09
T (K)
12501650
13001850
P5 (atm)
2.043.01
2.053.02
Ar (mole fraction)
0.7936
T (K)
11001350
9001300
P (atm)
1
0.46
τres (s)
0.07
0.05
V (cm3)
30.5
0.011
a
Information is exclusively based on the explicit data given in original references.
a
Information is exclusively based on the explicit data given in original references. In all cases, argon was the dilution gas.
Table 6. Experimental Conditions for p-Benzoquinone Flow Reactor Pyrolysis and Oxidation Experiments Considered in the Present Worka PFR 1
Table 3. Experimental Conditions for Benzene Flow Reactors Computed in the Present Worka
ref
PFR ref
19
φ C6H6 (%)
0.91 0.1495
O2 (%)
1.230
N2 (%)
98.6204
Tin (K)
1096
P (atm)
1
τres (s)
0.15
a
Information is exclusively based on the explicit data given in original references.
time delay experiments of C6H6/O2/Ar mixtures (0.5 < φ < 3.0) in the temperature and pressure ranges of 12301970 K and 7.39.5 atm, respectively. Additionally, shock tubes have also been utilized for the study of benzene pyrolytic reactions16 and also for measurement of the temperature dependence of the rate coefficients of several reactions crucial to benzene oxidation, such as benzene þ H and phenyl þ H,17 and benzyne thermal dissociation.18 Furthermore, the intermediate temperature oxidation of benzene has been studied under plug-flow environments. Early experimental work was mainly carried out in the Princeton plug-flow reactor.19,20
55
PFR 2
PFR 3
PFR 4
PFR 5
55
55
55
55
p-C6H4O2 (%) 0.0052
0.0108
0.0081
0.0064
0.004
O2 (%)
0.005
0.005
0.1
4.1
3.8
H2O (%)
2.4
5.1
1.8
1.8
1.6
CO(%)
0.014
Tin (K)
6001400 6001400 6001400 6001400 6001400
P (atm)
1.03
τres (s)
210/T [K] 202/T [K] 240/T [K] 197/T [K] 220/T [K]
1.03
1.03
1.03
1.03
a
Information is exclusively based on the explicit data given in original references. In all cases, nitrogen was the dilution gas.
Experiments included fuel-lean (φ = 0.65) to fuel-rich (φ = 1.38) mixtures and were conducted at a temperature of around 1100 K and at atmpospheric pressure. Reported data include phenol, cyclopentadiene, and carbon monoxide (CO) profiles. Alzueta et al.21 have measured exhaust CO and CO2 concentrations in lean and stoichiometric benzene plug-flow oxidation in the temperature range of 9001450 K under atmospheric pressure conditions and a residence time on the order of 150 ms. On the other hand, well-stirred reactor data are available from the work of Chai and Pfefferle,22 who also studied lean and stoichiometric benzene oxidation at 350 Torr (467 mbar), in the temperature range of 9001300 K and for a 50 ms mean residence time. The study provides the only available benzoquinone measurements in a benzene oxidation experiment. More recently, benzene oxidation in 1951
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Figure 1. Major reaction carbon pathways up to and including acetylene formation in (a) the stoichiometric (φ = 1.00), low-pressure (p = 45 mbar) laminar premixed benzene flame of Detilleux and Vandooren11 (plain numbers) and in (b) the rich (φ = 1.78), low-pressure (p = 40 mbar) laminar premixed benzene flame of Yang et al.10 (numbers in parentheses). Dashed lines correspond to key C1 and C2 species formation routes (see text).
Figure 2. Computed (lines) and experimental (symbols) benzene profiles in the rich flames AC.810 and F.11
well-stirred reactor environments has been reported by Ristori et al.23 and da Costa et al.15 Both studies were carried out under atmospheric pressure conditions, with the former considering lean to moderately rich mixtures (0.3 < φ < 1.5) in the temperature range of 9501300 K and a residence time of 70 ms and the latter focusing on very rich mixtures (1.9 < φ < 3.6) and long residence times (almost 10 s) at 923 K. Several detailed kinetic models of benzene oxidation exist in the literature.15,19,2329 Most of these, however, have been developed on the basis of the flame data of Bittner and Howard.8 Lindstedt and Skevis24 proposed a detailed kinetic scheme to adequately predict major and intermediate species of benzene combustion in premixed flames. Later efforts include the mechanisms of Tan and Frank,25 the modified Zhang and McKinnon26 model of Shandross
et al.,27 and the modified EBG (Emdee-Brezinsky-Glassman) scheme19 of Davis et al.28 Finally, Richter and Howard,29 also on the basis of the work of Shandross et al.,27 proposed a detailed kinetic model for the formation and consumption of single-ring aromatic hydrocarbons in premixed acetylene, ethylene, and benzene flames. Detailed kinetic models for benzene oxidation, such as those proposed by Ristori et al.23 and da Costa et al.15 and largely developed on the basis of stirred reactor data, have also been shown to be capable of adequately simulating the Bittner and Howard flame. Despite the numerous experimental and modeling efforts, significant uncertainties still exist regarding even major benzene combustion features, particularly under lean and stoichiometric conditions.11 These are mainly related to the initial stages of benzene oxidation and include the chemistry of the phenyl radical and the fate of the oxygenated C6 and C5 species. In this context, a detailed kinetic mechanism for benzene oxidation and combustion is further developed and validated against all of the above premixed flame data and data from shock tubes and stirred and flow reactors. The initial and boundary conditions for all validation targets are summarized in Tables 16. A systematic approach is adopted and aimed at addressing existing uncertainties. The presentation follows the chemistry of the fuel from its initial breakdown to small hydrocarbon species formation. Particularly, the benzene and phenyl radical chemistry is primarily assessed, followed by a reevaluation of the phenol/phenoxy chemistry and the chemistry of key C5 species. Secondary paths to small hydrocarbons are also discussed.
2. DETAILED KINETIC MECHANISM AND NUMERICAL METHODOLOGY A comprehensive mechanism3034 for the combustion of smalland medium-sized hydrocarbons has been used as a starting point 1952
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
Figure 3. Computed (lines) and experimental (symbols) phenyl radical profiles in flames AC810 and in the rich (φ = 1.8) flame of Richter et al.46
in the present work. The mechanism has been extensively validated against experimental speciation data from counterflow and premixed flames, including laminar flame speeds, shock tubes, including ignition time delays, and perfectly stirred and plug-flow reactors, all under a wide range of temperatures, pressures, and stoichiometries. In particular, an early version of the mechanism has been shown to successfully reproduce the chemistry of rich premixed benzene flames.30 Subsequent versions of the mechanism have resulted in very accurate descriptions of benzene formation and destruction chemistry in flames and reactors of both conventional (e.g., CH4, C2H2, C2H4, C3H4)3035 and alternative (e.g., CH3OH, C2H5OH) C1C4 fuels.33,34 The current version of the mechanism consists of 135 species and 820 reversible elementary reactions and is available as Supporting Information, together with thermodynamic and transport data. The latter have mostly been adopted form from the work of Lindstedt and Skevis30,31 and the database of Burcat et al.,36 as stated in the Supporting Information. The benzene reaction sub-mechanism also appears in Table 7. All computations have been performed using the CHEMKIN software.37 Ignition time delay modeling is realized through transient calculations in a closed, constant-pressure, adiabatic, perfectly stirred reactor (PSR). The adequacy of this procedure has been demonstrated by Davidson and Hanson,38 and further explored by Chaos and Dryer.39 In the present study, the ignition event is defined as the time corresponding to the maximum gradient of the temperature rise. In short, such comparisons can provide a good understanding of the underlying chemistry because the zero-dimensional computations are free from transport effects. Plug-flow computations are performed under isothermal or adiabatic conditions, depending on the simulated experiment. In this work, all plug-flow computations refer to atmospheric pressure conditions. For the premixed burnerstabilized flames, the experimentally determined temperature profiles have been imposed to calculations, so that heat losses to the burner are explicitly taken into account. Following a common, widely tested and validated practice,40 the temperature profile of fuel-rich premixed flames is translated away from the burner surface (on the order of few millimeters) and reported experimental values are slightly lowered (about a 100 K) in order to take into consideration the cooling effect caused by the sampling probe, when necessary. Table 1 presents the experimental conditions of the simulated premixed benzene flames.
ARTICLE
Figure 4. Computed (lines) and experimental (symbols) phenol and phenoxy radical profiles in flames A8 and C.10 Solid gray lines correspond to computations obtained using the provisional C6H5 þ O2 product distribution based on the work of Tokmakov et al.48 Dashed gray lines correspond to C6H5 þ O2 rate sensitivity computations using the total rate of Emdee et al.19 with the branching ratio of Frank et al.47 (see text).
The multicomponent diffusion model was mostly used, and grid adaptation parameters are chosen on a case-by-case basis, ensuring high grid resolution and grid-independent solutions.
3. BENZENE AND PHENYL RADICAL CHEMISTRY A snapshot of benzene destruction paths corresponding to the rich benzene flame of Yang et al. (flame C, Table 1)10 and to the stoichiometric benzene flame of Detilleux and Vandooren (flame E, Table 1)11 is depicted in Figure 1. The graph presents major carbon paths up to C2H2 formation (solid lines), while only the formation routes of key C1 and C2 species are shown (dashed lines). Presented rate-of-production analyses for flames C and E have been performed at 50% of the fuel consumption (0.53 and 0.85 cm above the burner surface, respectively). Figure 2 presents a comparison between the experimental data and numerical results for benzene consumption profiles in the four rich flames computed in the present work. Note that, for the rich flames A and F, temperature profiles have been shifted for 1 mm. The initial steps of the benzene breakdown process appear relatively well established.29 A crucial issue is the branching between abstraction (phenyl) and addition reactions, leading to oxygenated (phenol/ phenoxy) and/or higher C6 (cyclohexane) species. C6 H6 þ H ¼ C6 H5 þ H2
ð657Þ
C6 H6 þ OH ¼ C6 H5 þ H2 O
ð661Þ
C6 H6 þ OH ¼ C6 H5 OH þ H
ð662Þ
C6 H6 þ O ¼ C6 H5 O þ H
ð660Þ
Reactions with H and OH radicals predominantly lead to phenyl radical formation. Reaction (657) is the dominant phenyl formation path in all rich flames, while reaction (661) dominates under stoichiometric and lean (both flame and reactor) environments. The experimentally determined rate constant of Kiefer et al.41 (shock tube experiments with the laser-schlieren technique and MBMS), also in agreement with the more recent theoretical estimation (calculations with a modified G2M scheme) of Mebel 1953
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Figure 6. Comparison between computations (lines) and measurements (symbols) in the phenol flow reactor of Lovell et al.23
Figure 5. Comparison between computations (lines) and measurements (symbols) in (a) the rich (φ = 1.5) atmospheric pressure JSR of Ristori et al.23 and (b) the lean (φ = 0.91) atmospheric pressure PFR of Emdee et al.19
et al.,42 was retained for reaction (657). The branching ratio between the OH abstraction and addition paths is of the order of 20:1. The preferred rates of Baulch et al.,43 which are in close agreement with the recent evaluation of Seta et al.,44 have been adopted for reactions (661) and (662). On the other hand, the addition path (660) dominates for the C6H6 þ O reaction, even at combustion temperatures, as was also supported by theoretical calculations.45 With the present choice of rates, 70% of benzene consumption in rich environments, e.g., both flame C and the PSR of Ristori et al.,23 goes through the phenyl radical, whereas the remaining amount is channeled primarily via the phenoxy radical. Under leaner flame conditions, flame D (φ = 0.7), the relative ratio for C6H5/C6H5O formation from the initial fuel breakdown becomes 0.55:0.45, while in the lean (φ = 0.91) plug-flow reactor of Emdee et al.,19 benzene is equally consumed to both phenyl and phenoxy radicals. The molecular decomposition of C6H6 [reaction (666)] does not contribute to benzene consumption but instead proceeds in the reverse direction, forming benzene and accounting for a modest [less than 7% even in the rich (φ = 2.0) hightemperature premixed flame of Detilleux and Vandooren11] fraction of the total phenyl radical consumption rate. Figure 3 presents computed phenyl radical profiles in flames AC and in the rich (φ = 1.80) flame of Richter et al.46 In all cases, there is more than satisfactory agreement, suggesting that the overall phenyl radical chemistry is adequately captured. In more detail, in the case of Bittner and Howard,8,46 both the peak levels and location are excellently captured, while the profile shape is broader. In the case
of Defoeux et al.,9 agreement with peak levels and profile width is very good, although the predicted peak location is slightly shifted to the right. Note that Yang et al.10 provide only a single peak value, which is also very well predicted. All of the above may also suggest uncertainties in the thermodynamics of the equilibrated C6H5/ C6H5O system in the post-flame regime. The complexity of phenyl radical destruction chemistry is well documented.1,29,49,50 In the present kinetic scheme, the phenyl radical is assumed to undergo isomerization reactions to linear C6H5 species (1,3-hexadien-5-yn-1-yl, l-C6H5), abstraction reactions to cyclic C6H4 species (o-benzyne, c-C6H4), and reactions with HO2 and O2, leading to oxygenated compounds. Particularly, and as rate-of-production analyses indicate, the reaction with molecular oxygen is crucial. The reaction has been extensively studied both experimentally47 and theoretically.48,49 However, there is relatively little direct kinetic information available for the C6H5 þ O2 reaction. There are limited experimental determinations,51,52 all at low temperatures, and there is no general consensus, on both the product distribution and branching ratios as well as its pressure dependence.53,54 Generally, the reaction is thought to proceed through formation of the phenyl peroxy radical (C6H5OO) intermediate, with he latter decomposing fast to form stable products [see reactions (673), (674), (I), and (II)].18 The phenoxy path is found to be dominant under all temperature and pressure conditions relevant to this work. Frank et al.47 assumed p-benzoquinone to be the product of reaction (674). However, there is evidence suggesting that o-benzoquinone is the most likely product,21,29 while the para isomer is more thermally stable.47,55 Additional paths are also possible, leading, for example, to cyclopentadiene and pyranyl, as was also suggested by ab initio G2M calculations of Tokmakov et al.,48 which were further experimentally investigated by Gu et al.52 C6 H5 þ O2 ¼ C6 H5 O þ O
ð673Þ
C6 H5 þ O2 ¼ o-C6 H4 O2 þ H
ð674Þ
C6 H5 þ O2 ¼ C5 H5 þ CO2
ðIÞ
C6 H5 þ O2 ¼ C5 H5 O þ CO
ðIIÞ
Earlier modeling work assumed phenoxy as the sole reaction product,24,29 leading to severe overpredictions of C6H5O concentrations in flames. Here, and in the absence of any direct rate 1954
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
Figure 7. Computed (lines) and experimental (symbols) exhaust CO and CO2 values in p-C6H4O2 oxidation in a PFR.55
constant determination for the C5 pathways [reactions (I) and (II)], the reaction products are specified as phenoxy and o-benzoquinone, with the rate constant of Frank et al.47 adopted for both channels. This results in a branching ratio of 3:1 in favor of reaction (673), leading to overpredictions of more than a factor of 2.5 for both phenol and phenoxy radicals in flame C (Figure 4) and to pbenzoquinone levels almost 10 times higher than the experimental data of Chai and Pfefferle.22 On the other hand, computations in the lean (φ = 0.91) and rich (φ = 1.5) atmospheric pressure reactors of Emdee et al.19 and Ristori et al.,23 respectively, result in very good agreement for phenol (Figure 5). The addition channel (673) is indeed dominant at lower temperatures and relatively high pressures. However, for high temperatures and lower pressures, adduct stabilization may not be easily achieved. Indications from similar reactions,55 e.g., C2H3 þ O2, suggest possible β-scission processes, leading to ring opening. In order to assess such a possibility, a very tentative computation has been performed including paths (I) and (II). The total rate of Frank et al.47 has been retained, and an activation energy of 26.5 kJ mol1 (which is the apparent activation energy of the total reaction rate) has been assigned to both reactions (I) and (II). Also, on the basis of the theoretical work of Tokmakov et al.,48 a relative branching ratio of 50:30:10:10 for reactions (673):(674):(I):(II) has been assumed. Such a product distribution leads to a branching ratio of 80:20 in favor of C5 paths and results in a perfect matching of the phenol and phenoxy levels in flames A and C (Figure 4). Further, the total rate for reactions (673) and (674) given by Frank et al.47 is more than 20 times faster than the rate of Emdee et al.19 An alternative computation using the total rate of the latter with the product distribution of the former resulted in excellent agreement for both phenol and phenoxy in flame C but more than 5 times overpredictions for phenyl. The above discussion is summarized in Figure 4, where solid gray lines correspond to computations obtained using the provisional C6H5 þ O2 product distribution based on the work of Tokmakov et al.48 and dashed gray lines correspond to C6H5 þ O2 rate sensitivity computations, using the total rate of Emdee et al.19 with the branching ratio of Frank et al.47 Clearly, as was also suggested,21 the rate of Frank et al.47 is rather fast, at least for the present conditions (note that originally the rate had been experimentally derived for a pressure of about 2 bar), and it can be argued that a somewhat slower rate coupled with a product distribution in favor of C5 species will accommodate the early part of the flame. Although there is no evidence to solidly
ARTICLE
Figure 8. Comparison between computed (lines) and experimentally determined (symbols) C6H4 profiles in flames A8 and B.9
support such a choice, it is clear that a further investigation of the C6H5 þ O2 reaction is needed. Finally, note that the above augmented kinetic scheme [i.e., including reactions (I) and (II)] for the C6H5 þ O2 reaction also results in slightly superior agreement between computed and measured c-C5H6 levels in the flame of Yang et al.10 With the current choice of rates, the mechanism accurately captures the phenol-to-phenoxy ratio and their relative peak locations. This indicates that their destruction chemistry can be at least adequately described. The results obtained for phenol pyrolysis20 and oxidation21 (Figure 6) further support this argument. In particular, both CO and C5H6 levels are well captured, except for very early residence times, where plug-flow conditions are questionable. On the other hand, some discrepancies are observed for benzene levels at longer residence times. Note that reaction path analysis indicates that benzene is exclusively formed directly from phenol through reaction (662), with other formation paths (e.g., C3 þ C3, C2 þ C4, C1 þ C5) being negligible (less than 1% of the total C6H6 formation rate for a residence time of 0.1 s). In any case, numerical results are within a factor of 2 of the measured values. Generally, phenol is exclusively consumed to the phenoxy radical; however, slightly different pathways predominate under specific conditions. Particularly, phenol pyrolysis is initiated by reaction (676), followed by phenoxy decomposition in reaction (675). C6 H6 þ OH þ ¼ C6 H5 OH þ H
ð662Þ
C6 H5 O ¼ C5 H5 þ CO
ð675Þ
C6 H5 O þ H þ M ¼ C6 H5 OH þ M
ð676Þ
The fall-off curves of Wang and Frenklach56 have been adopted for reaction (676). Further downstream, C5H5 reacts with phenol to produce phenoxy radicals, thus sustaining a chain mechanism. Under rich flame conditions, phenoxy is formed by H and OH radical abstraction reactions and is consumed by reaction (677), as suggested by Tan and Frank.25 Abstraction reactions57 are generally unimportant as phenoxy destruction paths. C6 H5 O þ H ¼ c-C5 H6 þ CO
ð677Þ
It has also been suggested that a possible phenol destruction path may involve unimolecular decomposition to cyclopentadiene and early CO formation. However, computations involving the rate of Xu 1955
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Figure 9. Comparison between computed (lines) and experimentally determined (symbols) (a) CO, (b) CO2, (c) H2, and (d) H2O profiles in flames DF.11
major importance, while no significant changes were made to any of the other targets (particularly in rich flames10 and JSR oxidation23), and thus the above path is not included in the present mechanism. The second major product of the C6H5 þ O2 reaction is obenzoquinone, as discussed above. Benzoquinone destruction chemistry is generally unclear. Alzueta et al.21,55 proposed two possible channels for the initial breakdown of p-benzoquinone (p-C6H4O2), leading to 2,4-cyclopentadien-1-one (C5H4O) and 1,4-pentadiyne (l-C5H4).
Figure 10. Comparison between computed (lines) and experimentally determined (symbols) H and OH radical profiles in flame A.8
and Lin58 have led to unacceptable overpredictions of both CO and c-C5H6 levels in the data set of Lovell et al.,20 where this reaction is of
p-C6 H4 O2 ¼ C5 H4 O þ CO
ð689Þ
p-C6 H4 O2 ¼ l-C5 H4 þ CO2
ðIIIÞ
o-C6 H4 O2 ¼ C5 H4 O þ CO
ð690Þ
Initial computations using both product paths with the rate constants of Alzueta et al. resulted in a factor of 20 overpredictions of l-C5H4 levels in flames A and C. Assuming C5H4O as the sole product of both p- and o-benzoquinone decomposition leads 1956
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
Figure 11. (a) Ignition time delays for C6H6/O2/Ar mixtures.13 (b) Temperature gradient sensitivity coefficients for the φ = 2.0 case.
to excellent agreement for C5H4 levels in both flames. Using the above choice of rates, decomposition of o-benzoquinone [reaction (690)] is more than two orders of magnitude greater than the decomposition of its para isomer [reaction (689)] at the location of the maximum temperature gradient in flame C,10 and the mechanism correctly reproduces CO and CO2 exhaust levels in the p-benzoquinone pyrolysis and oxidation experiments of Alzueta et al.,55 as is indicatively shown in Figure 7. However, the sharp increase in the CO2 concentration is predicted to occur ca. 100 K higher than that in the experiments. The C6H5 þ O2 reaction is the dominant phenyl radical consumption route even in rich flames. Reaction path analyses indicate that phenyl linearization reactions are of secondary importance and that the hydrogen abstraction reaction to cC6H4 (o-benzyne) is the second most important phenyl consumption path (up to 25% of the total rate in flame C). Benzyne subsequently isomerizes to 1,3-dien-6-yne, initiating the linear C6 chain. In Figure 8, data from flames A and B are compared to the computed sum of cyclic and linear C6H4 isomers, while both isomers are shown for flame C. In all three flames, linear C6H4 is predicted to be dominant by up to a factor of 3. This is in contrast to earlier modeling work,29 where the more thermodynamically stable benzyne was reported to dominate. It should be noted that both Lindstedt and Skevis24 and Richter and Howard29 predict an earlier ring opening, through phenyl radical isomerization to the 1,3-dien-6-ynyl radical, leading to almost a factor of 8 overprediction of the overall C6H4 levels in flame A.29 Improvement in the quality of the predictions can partly be attributed to
ARTICLE
Figure 12. Computed (lines) and experimental (symbols) (a) cyclopentadiene and (b) cyclopentadienyl radical profiles in flames AC.810
updated benzyne thermodynamics.18,36 Specifically, the heat of formation of benzyne has been changed from a value of 115 kcal mol1 31 to the most recent estimate of 110.21 kcal mol1.36 This resulted in a 5-fold increase in benzyne, bringing it in line with available experimental data.
4. MAJOR BENZENE COMBUSTION FEATURES AND CYCLOPENTADIENE CHEMISTRY Major benzene combustion features are generally satisfactorily predicted by the present mechanism. As a typical example, major species mole fraction profiles in flames DF are presented in Figure 9ad. Changes in the absolute values and peak positions are excellently captured as a function of stoichiometry. Additionally, the model predictions for H and OH radicals in flame A (Figure 10) are also exceptionally good. Unfortunately, there are no experimental data for the above radicals in any of the more recent benzene flames. Ignition time delays for benzene mixtures of similar stoichiometries and at moderate pressures are also accurately reproduced. Typical results are shown in Figure 11a for stoichiometric (φ = 1.0) and rich (φ = 2.0) C6H6/O2/Ar mixtures at a pressure of about 2.5 bar.23 Sensitivity analysis for the peak temperature gradient in the case of the rich mixture at a temperature of 1400 K is presented in Figure 11b. Note that an increase in the rate constant of a reaction with positive logarithmic sensitivity coefficient [the sensitivity coefficient is defined as ∂F/∂R, where F is a target value (here, the peak temperature) and R is the preexponential factor in the Arrhenius rate constant expression; 1957
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Figure 13. Computed sensitivity coefficients for the peak (a) cyclopentadiene and (b) cyclopentadienyl radical levels in the rich flame C of Yang et al.10
sensitivity coefficients are directly computed by the CHEMKIN code37] would result in a decrease in the ignition delay times. Apart from the main chain branching step, H þ O2 = OH þ O, reaction sequences involving phenoxy and cyclopentadienyl radicals appear to control the system reactivity. Particularly, phenoxy decomposition to C5H5 and oxidation of the latter through ring opening to C4 species enhance the overall reactivity. Competing with the above sequence is the chain termination reaction (606), which depletes the two most active radicals in the benzene oxidation process. A large negative effect is also observed for the similar H radical addition to phenoxy [reaction (676)]. Cyclopentadienyl radical levels tend to be overpredicted, with cyclopentadiene underpredicted. Path analysis in flame C shows that C5H5 mainly forms c-C5H6 through reaction (606), while the linearization reaction is less important (approximately 20% of the total rate). C5 H5 þ H ¼ c-C5 H6
ð606Þ
There are uncertainties regarding the rate and fall-off curves of reaction (606). The use of the rate constants of Tokmakov et al.53 for the dissociation reaction resulted in an unacceptable cC5H6/C5H5 ratio. Roy et al.59 proposed a temperature-dependent rate constant for the addition reaction with a peak value of about 3 1014 cm3 mol1 s1. This value is in agreement with the high-pressure limit rate of both studies53,59 for the dissociation reaction. Here, a rate constant of 5 1014 cm3 mol1 s1 for the addition reaction is used as a conceivable upper limit.
Figure 14. Comparison between computed (lines) and experimentally determined (symbols) (a) C4H3, (b) C4H4, and (c) C4H5 species profiles in flames AC.810
Cyclopentadiene is mainly consumed by reactions (595) and (598), at a 3:2 branching ratio. c-C5 H6 þ H ¼ a-C3 H5 þ C2 H2
ð-595Þ
c-C5 H6 þ H ¼ C5 H5 þ H2
ð598Þ
59
The rate of Roy et al., which is more than an order of magnitude slower than that of Emdee et al.,19 has been assigned to reaction (598), but the rate of Richter and Howard29 has been retained for reaction (595). The use of the rate of Roy et al.59 for the latter 1958
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
Figure 15. Comparison between computations (lines) and experimentally determined (symbols) C3H4 species profiles in flame C.10
Figure 16. Comparison between computations (lines) and experimentally determined (symbols) C3H4 species profiles in flames A8 and B.9
reaction results in unacceptable C5H5 overpredictions in all flames. Cyclopentadienyl levels are generally well predicted for flame A but are overpredicted by a factor of 3 (in line with the generally observed phenoxy level overprediction) in flame C (Figure 12b). Furthermore, peak cyclopentadiene levels are very well predicted in the flame of Bittner and Howard8 and in stirred reactors (Figure 5) but are underpredicted by 50% in flames B and C (see Figure 12a). In order to gain more insight in the C5H5/c-C5H6 system, sensitivity analyses were performed at their peak locations for the case of the rich flame C of Yang et al.10 The results are presented in Figure 13. Sensitivity analysis clearly reveals that the reactions that are responsible for the development of both species profiles are identical. This indicates that the chemistry of both species is closely linked and decreasing the levels of one (e.g., C5H5) will increase the levels of the other (e.g., c-C5H6). This, at least in principle, can be achieved in three distinct ways. The first involves a decrease in the phenoxy radical levels, which, in turn, can be achieved by channelling the products of the C6H5 þ O2 reaction away from phenoxy and more into either benzoquinone (as indicated by sensitivity analysis) or C5 species (as discussed earlier in the text). Alternatively, decreases in the phenoxy levels are also expected by assuming c-C5H6 to be a primary product of the C6H6 þ O reaction (as suggested by the recent work of
ARTICLE
Figure 17. Comparison between computations (lines) and experimentally determined (symbols) propargyl radical profiles in flames AC.810
Figure 18. Comparison between computed (line) and experimentally determined (symbols) C2 species profiles in flames AC.810
Taatjes et al.60). A second option would be to increase the rate of cyclopentadienyl radical linearization through, e.g., decreasing the heat of formation of the linear C5H5 isomer. A final option would be to alter the relative thermodynamic stability of the C5H5/c-C5H6 system. However, the heats of formation of both species appear relatively well established, and computations performed by using a higher heat of formation value that has recently appeared in the literature61 for C5H5 did not result in any appreciable changes in its predicted levels. Given the above uncertainties and the level of agreement for C3 and C4 species that are direct products of C5H5 destruction chemistry, the current choice of rate constants and thermochemistry appears adequate.
5. SECONDARY PATHS IN BENZENE PREMIXED FLAMES The destruction chemistry of C5 species largely determines the levels of C4 and C3 intermediates in rich benzene flames. The C4 chain is initiated by reactions (607) and (630), leading to 1,3butadienyl radical formation.
1959
C5 H5 þ O ¼ 1, 3-C4 H5 þ CO
ð607Þ
C5 H4 O þ H ¼ 1, 3-C4 H5 þ CO
ð630Þ
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Figure 19. Computed (lines) and experimental (symbols) acetylene profiles in flame A8 and in flames DF.11
The latter is almost equally consumed either to the dominant 1,2C4H5 radical (through isomerization reaction) or to the vinyl radical and acetylene (via decomposition reaction), thus initiating the C2 chain. Total C4H5 levels are very well captured in flame C, as presented in Figure 14c. Vinylacetylene is the dominant C4 species in all flames and is also a major product of 1,2-C4H5 destruction. Vinylacetylene levels for flames A and B shown in Figure 14b are very well predicted; however, the agreement in flame C is rather poor (a factor of 2.5 underprediction). Note that measured values in the latter flame are an order of magnitude higher than that in the other two flames, well above the reported experimental uncertainty (∼40%) expected in such a measurement,10 while computations naturally indicate very similar values for all flames. Vinylacetylene is almost exclusively consumed to the n-C4H3 radical, which, in turn, isomerizes to the dominant i-C4H3 radical. The agreement for C4H3 levels in flame C is very satisfactory, as shown in Figure 14a. The C3 chain is initiated by reaction (595), and allyl radical levels are acceptably predicted (within a factor of 2 overprediction in flame C10). Subsequently, the allyl radical is consumed to a-C3H4 and C2H2 through unimolecular decomposition reactions (324) and (321), respectively, at a 3:2 ratio. a-C3 H4 þ H ¼ a-C3 H5
ð324Þ
C2 H2 þ CH3 ¼ a-C3 H5
ð321Þ
c-C3 H4 ¼ a-C3 H4
ð335Þ
c-C3 H4 ¼ p-C3 H4
ð336Þ
Allene then isomerizes to propyne through the cyclic C3H4 intermediate [reactions (335) and (336)], as was discussed in more detail in previous publications.30,31 Note that recent experimental investigations of C3H4 isomers in flames40 provide significant insight in unraveling the pressure dependency and isomerization patterns of the C3H4/C3H3 system and are the subject of another publication by the authors.35 With the current choice of rates for the reactions (335) and (336), excellent agreement is observed for both the total C3H4 peak values and profile shape for the data set of Yang et al.,10 as presented in Figure 15. Additionally, computations reproduce the 3:2 propyne/allene peak level ratio quoted in ref 10. Defoeux et al.9 and Bittner and Howard8 do not distinguish between C3H4 isomers.
Figure 20. Computed (line) and experimental (symbols) methane profiles in flames A8 and C.10
However, Defoeux et al.9 explicitly state that experimental data can be ascribed to either allene or propyne. As shown in Figure 16, the agreement is very good if measured values are assigned to allene, but a factor of 2 overprediction is observed if values are assigned to propyne. The same argument holds true for the data of Bittner and Howard; see also Figure 16. Such an overprediction can be attributed to the rate and product distribution on the H radical attack on cyclopentadiene. It is possible that either the rate of reaction (595) is too fast or competing reactions leading, for example, to C5H7 species are too slow. In any case and given the overall overprediction of the phenoxy radical, the level of agreement of C3 species is satisfactory. The propargyl radical is also mainly a product of C5 chemistry, through isomerization of cyclopentadienyl to the 1-pentyn-3-en5-yl radical and unimolecular decomposition of the latter. A not insignificant path, on the order of 25% of the total C3H3 formation rate in the flame of Defouex et al.,9 comes directly from benzene isomerization reactions. On the other hand, the propargyl radical is mainly consumed to propynal, which quantitatively decomposes to acetylene. Predictions of the propargyl levels are in excellent agreement with experimental data in flame A and a factor of 2 lower in flame B. On the contrary, the model fails to capture the propargyl data of flame C. Nevertheless, the measured C3H3 values of Yang et al.10 are significantly higher than the corresponding experimental values8,9 by factors of 10 and 4, respectively. Such a discrepancy is surprising, given the fact that all flames operate at almost identical conditions. The above discussion is summarized in Figure 17. Finally, note here that, for the benzene-fueled premixed flames examined in the present work, the chemistry of C3 species is significantly different from that in the cases of other fuels, where, for example, C3H3 chemistry largely determines the levels of C6H6 (as in intermediate/product species). Vinyl radical chemistry is also coupled to the C4 chain, with reactions (509) (C2H2 þ C2H3 = 1,3-C4H5) and (464) (C2H2 þ C2H3 = C4H4 þ H) being the dominant C2H3 formation paths. Vinyl radical predictions in flame C are excellent (Figure 18). Ethylene, as also shown in Figure 18, is very well predicted in both flames A and B. In both cases, C2H4 is a product of C1 species recombination, either directly [e.g., 3CH2 þ CH3 = C2H4 þ H, reaction (75)] or indirectly through methyl radical recombination to ethane and subsequent abstraction reactions 1960
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Table 7. Benzene Reaction Submechanisma reaction no.
A
reaction
n
Ea
ref
657
C6H6 þ H = C6H5 þ H2
2.50 1014
0.00
66.940
30
658
C6H6 þ H = c-C6H7
4.00 10
13
0.00
18.042
30
659
C6H6 þ O = C6H5 þ OH
2.00 10
13
0.00
61.520
30
660
C6H6 þ O = C6H5O þ H
2.40 10
13
0.00
19.539
30
661
C6H6 þ OH = C6H5 þ H2O
1.63 10
8
1.42
6.100
30
662
C6H6 þ OH = C6H5OH þ H
1.32 10
0.00
44.315
30
5.00
51.497
30
13
4
663
C6H6 þ CH3 = C6H5 þ CH4
4.37 10
664 665
C6H6 þ HO2 = C6H5 þ H2O2 C6H6 þ O2 = C6H5 þ HO2
5.50 10 6.30 1013
0.00 0.00
120.918 251.040
43
666
C6H5 þ H = C6H6
7.83 10
0.00
0.000
30
667
C6H5 = 1,3-C6H5
4.00 10
0.00
303.400
30
668
C6H5 = c-C6H4 þ H
13
3.00 10
0.00
372.376
30
669
C6H5 þ H = c-C6H4 þ H2
1.50 10
14
0.00
0.000
30
670
C6H5 þ O = c-C6H4 þ OH
2.00 1013
0.00
0.000
30
671
C6H5 þ OH = c-C6H4 þ H2O
2.00 1013
0.00
0.000
30
672 673
C6H5 þ HO2 = C6H5O þ OH C6H5 þ O2 = C6H5O þ O
5.00 1013 2.60 1013
0.00 0.00
4.186 25.610
30
674
C6H5 þ O2 = o-C6H4O2 þ H
3.00 1013
0.00
37.577
47
675
C6H5O = C5H5 þ CO
4.50 1011
0.00
183.680
30
676
C6H5O þ H (þM) = C6H5OH (þM) b,c k¥
2.50 1014
0.00
0.000
56
k0
1.00 1094
21.84
58.158
12
13 13
30
47
677
C6H5O þ H = c-C5H6 þ CO
1.10 1053
10.70
173.278
678
C6H5O þ O = p-C6H4O2 þ H
2.55 1013
0.00
0.000
57,62
679 680
C6H5O þ O = o-C6H4O2 þ H C6H5O þ O = C5H5 þ CO2
2.55 10 1.70 1013
0.00 0.00
0.000 0.000
57,62
681
C6H5OH þ H = C6H5O þ H2
14
1.15 10
0.00
51.920
30
682
C6H5OH þ O = C6H5O þ OH
2.81 10
13
0.00
30.780
30
683
C6H5OH þ OH = C6H5O þ H2O
6.00 10
12
0.00
0.000
30
684
C6H5OH þ HO2 = C6H5O þ H2O2
1.00 10
12
0.00
4.184
21
685
C6H5OH þ CH3 = C6H5O þ CH4
1.80 10
11
0.00
32.317
686
C6H5OH þ O2 = C6H5O þ HO2
1.13 10
13
0.00
158.992
21
687 688
C6H5OH þ C5H5 = C6H5O þ c-C5H6 C6H5OH þ C6H5 = C6H5O þ C6H6
2.67 1014 4.90 1012
0.00 0.00
105.620 18.410
30
689
p-C6H4O2 = C5H4O þ CO
3.70 10
11
0.00
246.856
21
690
o-C6H4O2 = C5H4O þ CO
1.00 10
12
0.00
167.360
21
598
c-C5H6 þ H = C5H5 þ H2
2.19 10
1.77
12.550
30
599
c-C5H6 þ H = C5H7
73
2.40 10
17.85
131.796
21
600
c-C5H6 þ O = C5H5 þ OH
1.81 10
13
0.00
12.870
30
601
c-C5H6 þ O = C5H5O þ H
8.90 10
12
0.15
2.469
21
602
duplicate c-C5H6 þ O = C5H5O þ H
5.60 1012
0.06
0.839
13
8
25
30
est,30
21
duplicate 603
c-C5H6 þ OH = C5H5 þ H2O
3.43 109
1.18
1.870
30
604
c-C5H6 þ HO2 = C5H5 þ H2O2
2.00 10
12
0.00
48.780
30
605
c-C5H6 þ O2 = C5H5 þ HO2
2.00 10
13
0.00
104.600
30
606
C5H5 þ H = c-C5H6
5.00 10
14
0.00
0.000
pw,d30
607
C5H5 þ O = 1,3-C4H5 þ CO
1.00 10
14
0.00
0.000
30
608 609
C5H5 þ O = C5H5O C5H5 þ OH = C5H4OH þ H
1.00 1013 3.50 1057
0.00 12.18
0.000 202.296
30
610
C5H5 þ OH = C5H5OH
14
6.50 10
0.85
11.422
21
1.10 1043
8.76
78.366
1.10 1059
13.08
139.955
21
duplicate 611
C5H5 þ OH = C5H5OH duplicate
612
C5H5 þ OH = C5H5OH
1961
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels
ARTICLE
Table 7. Continued reaction no.
A
reaction
n
Ea
ref
duplicate
a
613
C5H5 þ HO2 = C5H5O þ OH
6.30 1029
4.69
48.744
21
614
C5H5 þ O2 = C4H4O þ HCO
1.20 10
19
2.48
45.898
21
615
C5H5O = 1,3-C4H5 þ CO
4.50 10
11
0.00
183.680
30
616
C5H5O = C5H4O þ H
2.90 10
32
6.50
88.784
21
n
Rate constants in the form of k = AT exp(Ea/RT); units are mol, cm, s, K, and kJ. Reaction numbering refers to the original mechanism numbering (see also the Supporting Information). b Troe fall-off parameters: Fc = 0.957 exp(T/304) þ 0.043 exp(T/60000) þ exp(5896/T). c Enhanced collision efficiencies: H2O = 6.00, H2 = 2.00, CO = 1.50, CO2 = 2.00, CH4 = 2.0, C2H6 = 3.0, and Ar = 0.7. d pw = present work.
to C2H5 and unimolecular decomposition of the latter. The same pathways are also responsible for ethylene formation in flame C. However, the measured C2H4 values in the latter flame are almost a factor of 3 higher than those reported in flame B, while the computed values are in close agreement with the other two similar flames and lie almost on the average of the two data sets. Given the fact that flames A and B are almost identical, variation in the measured profiles is an indication of the experimental uncertainty. Further downstream in the carbon path, acetylene formation is primarly due to C5 species destruction chemistry, with reactions (595) and (629) accounting for about 40% (with a relative ratio of 1) of the total formation rate. Unimolecular decomposition of propynal [reaction (413)] provides another 25% of the C2H2 formation rate, with other C2C4 paths being of less importance. The agreement for acetylene is very good in all flames considered, as shown in Figure 19. Particularly, the mechanism captures both the acetylene rise in the early part of the flames and its peak value across a wide stoichiometry spectrum (φ = 0.72.0) and a wide range of values (spanning almost 2 orders of magnitude). C3 H2 O ¼ C2 H2 þ CO
ð413Þ
C5 H4 O ¼ C2 H2 þ C2 H2 þ CO
ð629Þ
a-C3 H5 þ C2 H2 ¼ c-C5 H6 þ H
ð595Þ
Finally, the present model results in good agreement with the methane profile of Bittner and Howard8 and the point measurement of Defoeux et al.,9 as shown in Figure 20.
6. CONCLUSIONS A direct comparison of a single, thoroughly validated, detailed kinetic mechanism against recent experimental data from a total of six laminar premixed benzene flames, as well as data from shock tubes and stirred and flow reactors, has been performed, providing the opportunity for a critical assessment of benzene oxidation and combustion chemistry under a broad range of operating conditions, in terms of the temperature, pressure, and stoichiometry. This approach provides the opportunity of a more systematic evaluation of uncertainties associated with experimental data obtained under similar conditions. Phenyl radical oxidation chemistry has been thoroughly evaluated. It has been shown that a provisional product distribution in favor of C5 species for the C6H5 þ O2 reaction leads to a superior description of the early benzene oxidation steps. Further work is required regarding both the rate and product distribution of the
above reaction. An updated cyclopentadiene submechanism has been assembled in view of both new rate data and validation targets. Phenol and benzoquinone pyrolysis and oxidation are also adequately captured. However, further work would naturally involve the development of a detailed and validated kinetic scheme for the combustion and oxidation of cyclic oxygenates such as quinones and polyphenols. The linearization reaction of C5 and C6 species largely determines ring rupture, leading to C1C4 hydrocarbon formation that is generally successfully reproduced by the model.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed chemical kinetic mechanism, together with thermodynamic and transport data. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ30 210 772 3664. Fax: þ30 210 772 3527. E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank Anna Gazi for assistance with some of the computations. The present work has been partially performed in the context of the COST Action CM0901 (Detailed Chemical Kinetic Models for Cleaner Combustion). ’ REFERENCES (1) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565–608. (2) Frenklach, M. Phys. Chem. Chem. Phys. 2002, 4, 2028–2037. (3) Altarawneh, M.; Dlugogorski, B. Z.; Kennedy, E. M.; Mackie, J. C. Prog. Energy Combust. Sci. 2009, 35, 245–274. (4) Jakober, C. A.; Riddle, S. G.; Robert, M. A.; Destaillats, H.; Charles, M. J.; Green, P. G.; Kleeman, M. J. Environ. Sci. Technol. 2007, 41, 4548–4554. (5) Sch€obel-Ostertag, A.; Braun-Unkhoff, M.; Wahl, C.; Krebs, L. Combust. Flame 2005, 140, 359–370. (6) Boot, M.; Frijters, P.; Luijten, C.; Somers, B.; Baert, R.; Donkerbroek, A.; Klein-Douwel, R. L. H.; Dam, N. Energy Fuels 2009, 23, 1808–1817. (7) Lemieux, P. M.; Lutes, C. C.; Santoianni, D. A. Prog. Energy Combust. Sci. 2004, 30, 1–32. (8) Bittner, J. D.; Howard, J. B. Proc. Combust. Inst. 1981, 28, 1105–1116. (9) Defoeux, F.; Dias, V.; Renard, C.; Van Tiggelen, P. J.; Vandooren, J. Proc. Combust. Inst. 2005, 30, 1407–1415. 1962
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
Energy & Fuels (10) Yang, B.; Li, Y.; Wei, L.; Huang, C.; Wang, J.; Tian, Z.; Yang, R.; Sheng, L.; Zhang, Y.; Qi, F. Proc. Combust. Inst. 2007, 31, 555–563. (11) Detilleux, V.; Vandooren, J. Combust. Explos. Shock Waves 2009, 45, 392–403. (12) Detilleux, V.; Vandooren, J. Combust. Sci. Technol. 2008, 180, 1347–1369. (13) Burcat A.; Snyder C.; Brabbs T. Ignition delay times of benzene and toluene with oxygen in argon mixtures. NASA TM-87312, 1986. (14) Thyagarajan, K.; Bhaskaran, K. A. Int. J. Energy Res. 1991, 15, 235–248. (15) da Costa, I.; Fournet, R.; Billaud, F.; Battin-Leclerc, F. Int. J. Chem. Kinet. 2003, 35, 503–524. (16) Sivaramakrishnan, R.; Brezinsky, K.; Vasudevan, H.; Tranter, R. S. Combust. Sci. Technol. 2006, 178, 285–305. (17) Giri, B. R.; Bentz, T.; Hippler, H.; Olzmann, M. Z. Phys. Chem. 2009, 223, 539–549. (18) Xu, C.; Braun-Unkhoff, M.; Naumann, C.; Frank, P. Proc. Combust. Inst. 2007, 31, 231–239. (19) Emdee, J. L.; Brezinsky, K.; Glassman, I. J. Phys. Chem. 1992, 96, 2151–2161. (20) Lovell, A. B.; Brezinsky, K.; Glassman, I. Int. J. Chem. Kinet. 1989, 21, 547–560. (21) Alzueta, M.; Glarborg, P.; Dam-Johansen, K. Int. J. Chem. Kinet. 2000, 32, 498–522. (22) Chai, Y.; Pfefferle, L. D. Fuel 1998, 77, 313–320. (23) Ristori, A.; Dagaut, P.; El Bakali, A.; Pengloan, G.; Cathonnet, M. Combust. Sci. Technol. 2001, 167, 223–256. (24) Lindstedt, R. P.; Skevis, G. Combust. Flame 1994, 99, 551–561. (25) Tan, Y.; Frank, P. Proc. Combust. Inst. 1996, 26, 677–684. (26) Zhang, H. Y.; McKinnon, J. T. Combust. Sci. Technol. 1995, 107, 261–300. (27) Shandross, R. A.; Longwell, J. P.; Howard, J. B. Proc. Combust. Inst. 1996, 26, 711–719. (28) Davis, S. G.; Wang, H.; Brezinsky, K.; Law, C. K. Proc. Combust. Inst. 1996, 26, 1025–1033. (29) Richter, H.; Howard, J. B. Phys. Chem. Chem. Phys. 2002, 4, 2038–2055. (30) Lindstedt, R. P.; Skevis, G. Combust. Sci. Technol. 1997, 125, 73–137. (31) Lindstedt, R. P.; Skevis, G. Proc. Combust. Inst. 2000, 28, 1801–1807. (32) Vourliotakis, G.; Skevis, G.; Founti, M. A.; Al-Hamamre, Z.; Trimis, D. Int. J. Hydrogen Energy 2008, 33, 2816–2825. (33) Vourliotakis, G.; Skevis, G.; Founti, M. A. Proc. Combust. Inst. 2008, 32; Poster paper 1P12. (34) Vourliotakis, G.; Skevis, G.; Founti, M. A. Int. J. Hydrogen Energy 2009, 34, 7626–7637. (35) (a) Gazi, A.; Vourliotakis, G.; Skevis, G.; Founti, M. A. A modelling study of allene and propyne combustion in flames. Fifth European Combustion Meeting ECM2011, Cardiff, U.K., 2011. (b) Gazi, A.; Vourliotakis, G.; Skevis, G.; Founti, M. A. Assessment of appropriate chemical markers for heat release rate correlation in laminar premixed flames. Fifth European Combustion Meeting ECM2011, Cardiff, U.K., 2011. (36) Goos, E.; Burcat, A.; Ruscic, B. Ideal Gas Thermochemical Database with updates from Active Thermochemical Tables available at ftp://ftp.technion.ac.il/pub/supported/aetdd/thermodynamics, accessed March 1, 2011. (37) CHEMKIN Collection, release 4.1; Reaction Design, Inc.: San Diego, CA, 2006. (38) Davidson, D. F.; Hanson, R. K. Int. J. Chem. Kinet. 2004, 36, 510–523. (39) Chaos, M.; Dryer, F. L. Int. J. Chem. Kinet. 2010, 42, 143–150. (40) Hansen, N.; Cool, T. A.; Westmoreland, P. R.; K€ohse-Hoinghaus, K. Prog. Energy Combust. Sci. 2009, 35, 168–191. (41) Kiefer, J. H.; Mizerka, L. J.; Patel, M. R.; Wei, H. C. J. Phys. Chem. 1985, 89, 2013–2019. (42) Mebel, A. M.; Lin, M. C.; Yu, T.; Morokuma, K. J. Phys Chem. A 1997, 101, 3189–3196.
ARTICLE
(43) Baulch, D. L.; Bowman, C. T.; Cobos, C. J.; Cox, R. A.; Just, T.; Kerr, J. A.; Pilling, M. J.; Stocker, D.; Troe, J.; Tsang, W.; Walker, R. W.; Warnatz, J. J. Phys. Chem. Ref. Data 2005, 34, 757–1359. (44) Seta, T.; Nakajima, M.; Miyoshi, A. J. Phys. Chem. A 2006, 110, 5081–5090. (45) Nguyen, T. L.; Peeters, J.; Vereecken, L. J. Phys. Chem. A 2007, 111, 3836–3849. (46) Richter, H.; Benish, T. G.; Mazyar, O. A.; Green, W. H.; Howard, J. B. Proc. Combust. Inst. 2000, 28, 2609–2618. (47) Frank, P.; Herzler, J.; Just, Th.; Wahl, C. Proc. Combust. Inst. 1994, 25, 833–840. (48) Tokmakov, I. V.; Kim, G.-S.; Kislov, V. V.; Mebel, A. M.; Lin, M. C. J. Phys. Chem. A 2005, 109, 6114–6127. (49) Sebbar, N.; Bockhorn, H.; Bozelli, J. W. Combust. Sci. Technol. 2008, 180, 959–974. (50) Wang, H.; Laskin, A.; Moriarty, N. W.; Frenklach, M. Proc. Combust. Inst. 2000, 28, 1545–1555. (51) Yu, T.; Lin, M. C. J. Am. Chem. Soc. 1994, 116, 9571–9576. (52) Gu, X.; Zhang, F.; Kaiser, R. I. Chem. Phys. Lett. 2007, 448, 7–10. (53) Tokmakov, I. V.; Moskaleva, L. V.; Lin, M. C. Int. J. Chem. Kinet. 2004, 36, 139–151. (54) da Silva, G.; Bozzelli, J. W. J. Phys. Chem. A 2008, 112, 3566–3575. (55) Alzueta, M. U.; Oliva, M.; Glarborg, P. Int. J. Chem. Kinet. 1998, 30, 683–697. (56) Wang, H.; Frenklach, M. Combust. Flame 1997, 110, 173–221. (57) Buth, R.; Hoyermann, K.; Seeba, J. Proc. Combust. Inst. 1994, 25, 841–849. (58) Xu, Z. F.; Lin, M. C. J. Phys. Chem. A 2006, 110, 1672–1677. (59) Roy, K.; Braun-Unkhoff, M.; Frank, P.; Just, Th. Int. J. Chem. Kinet. 2001, 33, 821–833. (60) Taatjes, C. A.; Osborn, D. L.; Selby, T. M.; Meloni, G.; Trevitt, A. J.; Epifanovsky, E.; Krylov, A. I.; Sirjean, B.; Dames, E.; Wang, H. J. Phys. Chem. A 2010, 114, 3355–3370. (61) da Silva, G.; Cole, J. A.; Bozzelli, J. W. J. Phys. Chem. A 2010, 114, 2275–2283. (62) Lin, M. C.; Mebel, A. M. J. Phys. Org. Chem. 1995, 8, 407–420.
1963
dx.doi.org/10.1021/ef101712p |Energy Fuels 2011, 25, 1950–1963
