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REDUCED CHEMICAL KINETIC MECHANISM FOR METHYL PENTANOATE COMBUSTION Ilya E. Gerasimov, Tatyana A. Bolshova, Ivan A. Zaev, Alexander V. Lebedev, Boris V. Potapkin, Andrey G. Shmakov, and Oleg P. Korobeinichev Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01907 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017
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REDUCED CHEMICAL KINETIC MECHANISM FOR METHYL PENTANOATE COMBUSTION I.E. Gerasimov1, T.A. Bolshova1, I.A. Zaev2, A.V. Lebedev2, B.V. Potapkin2, A.G. Shmakov1,3, O.P. Korobeinichev1 1
Voevodsky Institute of Chemical Kinetics and Combustion, Novosibirsk, Russia 2 Kintech Lab Ltd., Moscow, Russia 3 Novosibirsk State University, Novosibirsk, Russia
ABSTRACT A reduced mechanism for combustion of methyl pentanoate (MPe), consisting of 330 elementary reactions involving 92 species, has been developed based on the previously proposed combustion mechanism for MPe using the Mechanism Workbench software. The reduced model has been validated against experimental data on the structure of burner-stabilized stoichiometric and fuel-rich MPe/O2/Ar flames at pressures of 20 Torr and 1 atm. The modeling results for the full and reduced mechanisms are in good agreement for major flame species and for most of the intermediates, including hydrogen, methane, methyl radical, ethylene, acetylene, propyne, butadiene, methyl propenoate, and other intermediates. The proposed kinetic model also was validated against experimental data on MPe/air flame propagation velocities and extinction strain rates at atmospheric pressure, as well as autoignition delay times of stoichiometric MPe/air mixtures at T = 815 K and pressures p=10−18 bar. Key words: methyl pentanoate, chemical kinetic mechanism, flame speed, reduced mechanism.
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INTRODUCTION Recently, there has been enormous interest in renewable fuels. Their use in transport and power system reduces the consumption of oil and net emissions of greenhouse gases and other harmful substances into the atmosphere. The major components of biodiesel fuel derived from vegetable oils and animal fats are fatty acid methyl esters.1 The most important feature of this fuel is that its physical properties such as density, viscosity, and cetane number are close to the characteristics of conventional diesel fuels. This allows it to be used both as an individual fuel and in mixtures with conventional diesel fuel without substantially modifying the design of existing engines. It has also been established that even partial replacement of conventional diesel fuel with biodiesel fuel significantly reduces the formation of CO, NOx, and soot particles in internal combustion engines and other combustion processes.2,3 The components and intermediate combustion products of biodiesel fuels have a high molecular weight and a complex structure. The current combustion mechanisms for complex hydrocarbons and oxygen-containing compounds contain hundreds of components and thousands of reactions and are, thus, extremely large and require substantial computing power for implementation.4 This necessitates the optimization of kinetic models used in combustion calculations for real engines. In most cases, not all of the fuel conversion pathways included in kinetic models occur in practice. The development of a skeletal mechanism makes it possible to optimize the kinetic scheme of fuel conversion, identify the main pathways and the main reactions involved in this conversion, and analyze the rate constants of key reactions. Methyl pentanoate (C6H12O2, MPe) is a light model biodiesel fuel whose oxidation mechanism can also be used to explore the oxidation kinetics of biofuels based on valerates (ethers of pentanoic acid) derived from vegetable oils and cellulose.5 A mechanism for the oxidation of MPe was first proposed by Dayma et al.,6 and was developed as part of the oxidation mechanisms of methyl hexanoate,7 methyl heptanoate,8 and ethyl valerate9 based on experimental data on the oxidation of MPe in a jet-stirred reactor at a pressure of 10 atm. Thus, the development of a reduced model for MPe will make it possible not only to perform calculations for actual gas-dynamic systems, but also to revise the mechanisms of heavier component of biofuels. The combustion of MPe and its difference from other hydrocarbon fuels have been investigated in a number of experimental and theoretical studies. In particular, the low-temperature oxidation of MPe was investigated by HadjAli et al.,10 who presented data on its ignition delays in a rapid compression machine at a temperature of 815 K and pressures from 10 to 17 bar. Lowtemperature processes were also studied by quantum chemical methods.11 In that work the kinetic parameters of MPe oxidation by various possible pathways were calculated and some features of peroxide formation in ester flames were identified. Diévart et al.12 measured extinction strain rates for opposed-flow diffusion flames and studied the reactivity of various methyl esters from methyl formate to methyl decanoate. They also developed a reduced model for studied esters, which was validated with obtained data. This work has revealed that the smaller methyl esters (C2 to C4) exhibit unique behavior while methyl esters inclusive and larger than methyl butanoate exhibit similar global reactivity to that of the n-alkanes. Data on the flame structure of two esters ― MPe and methyl hexanoate ― at pressures of 20 Torr and 1 atm are presented in ref 13. The investigations were carried out using molecular beam mass spectrometry and microthermocouples. A new detailed mechanism was proposed that satisfactorily described the concentration profiles for most species in the flames, including measured concentrations of various intermediates. The effect of MPe on soot formation was also examined,14 using the same methods as in the previous study. Investigation of the structure of atmospheric flames showed that substitution part of
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the initial fuel mixture in fuel-rich n-heptane/toluene flame (φ = 1.75) by methyl pentanoate decreased the concentration of naphthalene, which is a typical representative of soot precursors. Thus, sufficient experimental data are available in the literature that can be used to validate the reduced mechanism of MPe combustion. Therefore, the objectives of this study were to test a new approach to the reduction of detailed chemical kinetic mechanisms, develop a reduced kinetic model of MPe combustion, and validate the reduced mechanism against the experimental data available in the literature. MECHANISM DEVELOPMENT The reduced kinetic mechanism presented in this paper is based on the mechanism proposed by Dayma, et al.6 This mechanism of chemical reactions has already been used15 to model the structure of burner-stabilized flat flames at low (20 Torr) and atmospheric pressures and calculate the burning velocity of atmospheric MPe/air flames. Calculations have shown a good overall agreement with experimental data and sufficiently less discrepancies with flame speed measurements than other models of MPe combustion. The original kinetic model consisted of 1630 reactions and 215 species, which contain oxygen-containing compounds and hydrocarbons up to C5 inclusive, methyl pentanoate, and its conversion products, but did not include low-temperature chemistry like O2 addition to MPe radicals and subsequent reactions. To develop a reduced model, we used the Mechanism Workbench integrated software (Kintech Lab),16 which allows kinetic models to be automatically reduced based on parameters specified by the user, such as the type of modeled process, process conditions, and target characteristics. The mechanism reduction algorithm is iterative and implements an optimal sequence of application of the following mechanisms reduction techniques: Directed Relation Graph (DRG), Rate of Production (ROP), and Computational Singular Perturbation (CSP). Detailed implementations of these techniques are described in ref 17. During the iterations, the algorithm compares the values of the variables defined by the user as reduction targets in calculations using the full and reduced models. Based on the results of the comparison, selection of the reduction parameters or switching between the indicated methods occurs. Also, the algorithm allows one to simultaneously set one or more types of processes for modeling and choose their corresponding reactor models: 0-dimensional models, burner-stabilized flames, and freely propagating premixed flames (one-dimensional models). To reduce the mechanism, we used an adiabatic calorimetric bomb reactor model which describes autoignition processes. Its use as a model for reduction allows to describe all the main steps in the chain of reactions, from initiation and branching to the reactions of the pool of accumulated radicals with fuel molecules. The fuel was a MPe/O2/Ar mixture with different equivalence ratios. The initial temperature was varied from 1100 K to 1700 K in increments of 50 K. To obtain the reduced mechanism, several reduction targets were set. The induction period (ignition delay) was set as the first target. The second target was the criterion of similarity of temperature profiles with an absolute deviation of no more than 50 K scaled to the induction period. During the reduction, we also controlled the maximum concentrations of H and OH radicals and some important intermediate compounds: C2H2, H2, CH4, and methyl propenoate (C4H6O2). The reduced models obtained for different mixtures and conditions were combined together to provide adequate operation of the obtained reduced mechanism over the entire range of conditions. The reduced mechanism was manually checked for any inconsistencies which could remain after automatic reduction procedure with several reduction targets. For example, several species involved in only one reaction and, hence, playing no significant role in the conversion of the fuel and its decomposition products, were removed. Transport properties of MPe species were updated with data from ref 12, to achieve a better agreement with extinction strain rate measurements (see ACS Paragon Plus Environment
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corresponding part). The resulting kinetic scheme describing the combustion of methyl pentanoate mixtures consists of 330 elementary reactions for 92 species. 129 reactions which describe the conversions of methyl pentanoate and the primary products of its decomposition summarized in Table 1 (full reduced mechanism is available in supplemental material). Table 2 shows the designations of the compounds used in the mechanism, the formulas of these compounds, and thermodynamic data for them. Table 1. Reduced methyl pentanoate submechanism (units: cm3, mole, 1/s, cal). No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
reaction mpe+O2=mpe5j+HO2 mpe+O2=mpe4j+HO2 mpe+O2=mpe3j+HO2 mpe+O2=mpe2j+HO2 mpe+O2=mpemj+HO2 mpe+H=mpe5j+H2 mpe+H=mpe4j+H2 mpe+H=mpe3j+H2 mpe+H=mpe2j+H2 mpe+H=mpemj+H2 mpe+O=mpe5j+OH mpe+O=mpe4j+OH mpe+O=mpe2j+OH mpe+O=mpemj+OH mpe+OH=mpe5j+H2O mpe+OH=mpe4j+H2O mpe+OH=mpe3j+H2O mpe+OH=mpe2j+H2O mpe+OH=mpemj+H2O mpe+HO2=mpe5j+H2O2 mpe+HO2=mpe4j+H2O2 mpe+HO2=mpe3j+H2O2 mpe+HO2=mpe2j+H2O2 mpe+HO2=mpemj+H2O2 mpe=mpe4j+H mpe=mpe3j+H mpe2j+H=mpe nC4H9CO+CH3O=mpe mpe=mb4j+CH3 mpe=mp3j+C2H5 mpe=me2j+nC3H7 mpe=CH3OCO+pC4H9 mpe+CH3=mpe5j+CH4 mpe+CH3=mpe4j+CH4 mpe+CH3=mpe3j+CH4 mpe+CH3=mpe2j+CH4 mpe+CH3=mpemj+CH4 mpe+CH3O=mpe3j+CH3OH ACS Paragon Plus Environment
A 2.00E+13 4.00E+13 4.00E+13 4.00E+13 2.05E+13 9.40E+04 1.30E+06 1.30E+06 5.40E+04 1.44E+13 9.65E+04 4.77E+04 8.80E+10 9.65E+04 5.25E+09 4.68E+07 4.68E+07 3.00E+06 7.10E+06 8.40E+12 5.60E+12 5.60E+12 6.40E+03 9.64E+10 5.00E+15 5.00E+15 1.00E+14 1.50E+13 7.90E+22 1.58E+17 1.58E+17 1.13E+16 4.52E-01 2.70E+04 2.70E+04 1.00E+11 3.57E+11 1.10E+11
n 0 0 0 0 0 2.8 2.4 2.4 2.5 0 2.7 2.7 0.7 2.7 1 1.6 1.6 2 1.8 0 0 0 2.6 0 0 0 0 0 -1.8 0 0 0 3.6 2.3 2.3 0 0 0
E 50870 47690 47690 45200 44910 6280 4471 4471 -1900 6095 3716 2106 3250 3716 1590 -35 -35 -1520 -596 20440 17690 17690 12400 12580 94990 94990 0 0 88630 87040 87040 81700 7154 7287 7287 7300 8663 5000
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mpe+CH3O=mpe2j+CH3OH mpe+CH3O=mpemj+CH3OH mpe+CH3O2=mpe5j+CH3O2H mpe+CH3O2=mpe4j+CH3O2H mpe+CH3O2=mpe3j+CH3O2H mpe+CH3O2=mpe2j+CH3O2H mpe+CH3O2=mpemj+CH3O2H mpe+C2H5=mpe4j+C2H6 mpe2j=C2H5+mp2d mpe2j=mpe5j mpe2j=>mpemj mpe3j=>1-C4H8+CH3OCO mpe3j=CH3+mb3d mpemj=>CH2O+nC4H9CO mpe3j=H+mpe3d mpe3j=H+mpe2d mpe3j=>mpemj mpe3j+O2=mpe3d+HO2 mpe3j+O2=mpe2d+HO2 mpe4j=H+mpe4d mpe4j=H+mpe3d mpe4j=mpemj mpe4j+O2=mpe4d+HO2 mpe4j+O2=mpe3d+HO2 mpe4j=>C3H6+me2j mpe5j=C2H4+mp3j mpe5j+O2=mpe4d+HO2 memj=>CH2O+CH3CO me2j=memj HCO+CH3OCO=me2*O mp3j=>C2H4+CH3OCO mpmj=>CH2O+C2H5CO mp2d+H=>mp3j mp3j+O2=mp2d+HO2 mp3j=>mpmj mpmj=>mp3j mp2d=C2H3CO2+CH3 mp2d+H=mp2dmj+H2 mp2d+O=mp2dmj+OH mp2d+OH=mp2dmj+H2O mp2d+HO2=mp2dmj+H2O2 mp2d+CH3=mp2dmj+CH4 mp2d+OH=mp2d3j+H2O mp2d3j=C2H2+CH3OCO mp2d3j=mp2dmj mp2d3j+O2=HCO+me2*O mp2d+O=CH3OCO+CH2CHO ACS Paragon Plus Environment
1.78E+12 3.01E+11 8.40E+12 5.60E+12 5.60E+12 6.40E+03 8.40E+12 5.00E+10 2.00E+13 1.50E+08 1.50E+08 4.53E+12 2.00E+13 1.23E+13 3.00E+13 3.20E+13 2.50E+07 1.95E+12 2.60E+11 3.00E+13 3.00E+13 4.35E+06 8.07E+11 1.95E+12 5.25E+11 2.00E+13 1.95E+12 1.23E+13 1.50E+08 1.00E+13 3.03E+13 1.23E+13 4.18E+08 2.60E+11 2.50E+07 2.50E+07 3.16E+16 1.44E+13 9.65E+04 7.10E+06 9.64E+10 4.52E-01 2.20E+06 2.00E+13 2.50E+07 4.60E+16 5.01E+07
0 0 0 0 0 2.6 0 0 0 1 1 0.3 0 0.4 0 0 1 0 0 0 0 1 0 0 0.5 0 0 0.4 1 0 0.3 0.4 1.6 0 1 1 0 0 2.7 1.8 0 3.6 2 0 1 -1.4 1.8
1200 4070 20440 17690 17690 12400 20440 10400 30700 19800 19800 34269 32000 36714 38000 34800 14500 5000 2500 39000 38000 19900 5000 5000 26591 28700 5000 36714 19800 0 34667 36714 1697 2500 14500 15500 83070 6095 3716 -596 12580 7154 2780 45000 14900 1010 76
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86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129
mb4j=>C2H4+me2j mb4j=H+mb3d mb4j+O2=mb3d+HO2 mb3d=CH3OCO+aC3H5 mb3d+OH=mb3d2j+H2O mb3d+HO2=mb3d2j+H2O2 mb3d+HO2=mb3dmj+H2O2 mb3d2j=mb3dmj mb3dmj=>CH2O+aC3H5CO mpemj=>mpe3j mpemj=>mpe2j mpe3d+O2=mpe3d2j+HO2 mpe3d+H=mpe4d3j+H2 mpe3d+H=mpe3d2j+H2 mpe3d+OH=mpe4d3j+H2O mpe3d+OH=mpe3d2j+H2O mpe2d+OH=mpe3d2j+H2O mpe3d+CH3O=mpe3d2j+CH3OH mpe2d+CH3O=mpe3d2j+CH3OH mpe4d3j=CH3OCO+C4H6 mp2d=mp2dmj+H mp2d+O=me2j+HCO mib3j=C3H6+CH3OCO mp2d+CH3=mib3j mb3d+H=mb3d2j+H2 mb3d+H=mb3dmj+H2 C2H4+me2j=>mb4j mpe2d+O2=mpe3d2j+HO2 mpe2d+H=mpe3d2j+H2 mb3d+CH3O=mb3d2j+CH3OH mb3d+O=mp3j+HCO mpe3d+O=mpe4d3j+OH mpe3d+O=mpe3d2j+OH mpe2d+O=mpe3d2j+OH mpe3d+CH3O=mpe4d3j+CH3OH mpe4d3j=mpe4d2d+H mpe4d2j=mpe4d2d+H mpe4d+H=mpe4d3j+H2 mpe4d+OH=mpe4d2j+H2O mpe3d+HO2=mpe4d3j+H2O2 mp2dmj=>C2H3CO+CH2O mp2j=>mpmj mp2j
