Energy Fuels 2010, 24, 396–403 Published on Web 11/09/2009
: DOI:10.1021/ef900899g
Shock Tube Study of Methyl Formate Ignition Benjamin Akih-Kumgeh and Jeffrey M. Bergthorson* Department of Mechanical Engineering, McGill University, 817 Sherbrooke West, Montreal, Quebec, H3A 2K6, Canada Received August 18, 2009. Revised Manuscript Received October 5, 2009
The autoignition of methyl formate mixed with oxygen/argon and oxygen/nitrogen behind reflected shock waves is studied. Experiments are carried out at average pressures of 2, 4, and 10 atm over a temperature range of 1053-1561 K. The effects of equivalence ratio, dilution, and the nature of the bath gas are investigated. An empirical correlation for the ignition delay is proposed. Using this correlation, methyl formate ignition is compared to published methane and ethane ignition data. It is found that methyl formate ignites more readily than methane but less readily than ethane. The experimental data is also compared with ignition delays predicted by two different methyl formate kinetic mechanisms. It is observed that both mechanisms agree with measured ignition delays at 10 atm but deviate significantly from experimental data at lower pressure. In order to obtain closer agreement between the models and experiment at lower pressures, further analysis of possible fuel consumption pathways and improved estimation of (pressure-dependent) reaction rates should be investigated.
developed with the aid of laminar burning velocity data from a number of combustion laboratories. Ignition studies of these alkyl esters have not yet been reported. Methyl formate (MF or CH3OCHO) is the simplest methyl ester of alkanoic acids. Because of its short chain, it is hoped that its detailed study will reveal the effect of the methyl ester structure on reactivity. In low temperature combustion, this may be characterized by significant alkyl and alkyl ester peroxide chemistry. While there are few studies of the combustion properties of methyl formate, its formation and decomposition in the atmosphere have been more extensively addressed. Wallington et al.8 studied the chlorine-initiated oxidation of methyl formate in a smog chamber and found that it undergoes Habstractions from both the carbonyl and the methoxy sites with comparable ratios. Good et al.9 studied the oxidation of methyl formate as an alternative pathway in atmospheric decomposition of dimethyl ether. In a later study, Good et al.10 investigated hydrogen abstraction reactions from methyl formate and observed that abstractions by H and OH radicals dominate in the presence of oxygen. Francisco11 performed an ab initio mechanistic study of the decomposition pathways of methyl formate, observing that in addition to the previously suggested decomposition of methyl formate into CH3OH and CO, other pathways, such as decomposition to 2CH2O or to CH4 and CO2, have comparable activation barriers and may be important in the gas phase chemistry. In addition to providing insight into methyl ester combustion, methyl formate has also been found to be an important intermediate in the oxidation of other oxygenated hydrocarbons. Japar et al.12 and Liu et al.13 observed that in the
Introduction Current efforts to address challenges in the energy economy include the use of biofuels. Biodiesel is increasingly used in diesel engines because it can be employed in conventional engines with little or no modifications. In order to assess the net emission reduction potential and other combustion properties of biodiesel, detailed fundamental studies are necessary. Practical fuels are generally a mixture of various hydrocarbons, and in the case of biodiesel the components are usually complex long-chain saturated and unsaturated methyl esters. Analogous to the method adopted for alkanes, chemical kinetic studies are carried out by means of fuel surrogates. Small-chain methyl esters such as methyl formate and methyl butanoate as well as moderately long chain esters such as methyl decanoate have been proposed as surrogates for biodiesel.1-3 Experimental studies have been carried out to validate the methyl butanoate mechanism.4-7 Although good agreement has been observed in some cases, discrepancies between predictions and experiments have shown that the combustion chemistry of methyl esters is still not well established. Westbrook et al.2 have proposed a mechanism for the combustion of four small alkyl esters: methyl formate, methyl acetate, ethyl formate, and ethyl acetate. This mechanism was *To whom correspondence should be addressed. E-mail: jeff.
[email protected]. (1) Fisher, E.; Pitz, W.; Curran, H.; Westbrook, C. Proc. Comb. Inst. 2000, 28, 1579–1586. (2) Westbrook, C.; Pitz, W.; Westmoreland, P.; Dryer, F.; Chaos, M.; Osswald, P.; Kohse-H€ oinghaus, K.; Cool, T.; Wang, J.; Yang, B.; Hansen, N.; Kasper, T. Proc. Comb. Inst. 2009, 32, 221–228. (3) Herbinet, O.; Pitz, W.; Westbrook, C. Combust. Flame 2008, 154, 507–528. (4) Metcalfe, W.; Dooley, S.; Curran, H.; Simmie, J.; El-Nahas, A.; Navarro, M. J. Phys. Chem. A 2007, 111, 4001–4014. (5) Dooley, S.; Curran, H.; Simmie, J. Combust. Flame 2008, 153, 2–32. (6) Gaı¨ l, S.; Thomson, M. J.; Sarathy, S. M.; Syed, S. A.; Dagaut, P.; Dievart, P.; Marchese, A. J.; Dryer, F. L. Proc. Comb. Inst. 2007, 31, 305–311. (7) Sarathy, S. M.; Gaı¨ l, S.; Syed, S. A; Thomson, M. J.; Dagaut, P. Proc. Comb. Inst. 2007, 31, 1015–1022. r 2009 American Chemical Society
(8) Wallington, T.; Hurley, M.; Maurer, T.; Barnes, I.; Becker, K.; Tyndall, G.; Orlando, J.; Pimentel, A. S.; Bilde, M. J. Phys. Chem. A 2001, 105, 5146–5154. (9) Good, D.; Francisco, J. J. Phys. Chem. A 2000, 104, 1171–1185. (10) Good, D.; Francisco, J. J. Phys. Chem. A 2002, 106, 1733–1738. (11) Francisco, J. J. Am. Chem. Soc. 2003, 125, 10475–10480. (12) Japar, S. M.; Wallington, T. J.; Richert, J. F. O.; Ball, J. C. Int. J. Chem. Kinetics 1990, 22, 1257–1269. (13) Liu, I.; Cant, N.; Bromly, J.; Barnes, F.; Nelson, P.; Haynes, B. Chemosphere 2001, 42, 583–589.
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presence of NO, methyl formate is formed in the low-temperature oxidation of dimethyl ether. With increasing focus on the reduction of volatile organic compound emissions from combustion processes, it is important to develop reliable submodels for stable intermediate species such as methyl formate. We report here shock tube ignition data of methyl formate in oxygen/argon and oxygen/nitrogen mixtures. This work seeks to investigate the effects of pressure, equivalence ratio, and dilution on ignition of methyl formate. The results are compared with simulations using the mechanisms proposed by Fisher et al.1 and Westbrook et al.2 Sensitivity and reaction pathway analyses are performed to gain insight into the controlling chemical reactions. An empirical correlation is proposed for methyl formate ignition under conditions similar to those investigated in this work. The correlation thus obtained is compared to methane and ethane ignition delays at their experimental conditions. Experimental Technique
Figure 1. End wall pressure and chemiluminescence of the CH radical (430 nm) histories and resulting ignition delay time for mixture A at 4.2 atm and 1222 K.
Experiments are carried out in a 5.0 cm inner diameter stainless steel shock tube. The driver and driven sections are 3.0 and 4.2 m long, respectively. Mixtures are prepared in a 90 L mixing tank and allowed to mix for at least 24 h. The mixing tank and the shock tube are evacuated with an Edwards RV12 vacuum pump rated to 2 10-3 mbar prior to preparing each mixture and each experiment. Each experiment is preceded by flushing the driven section with the combustible mixture to avoid contamination by any residual gases. For the purpose of preparing mixtures, a 100 torr Barocel pressure transducer and a 1000 torr MKS Baratron transducer are used to accurately measure fuel, oxygen, and inert gas partial pressures. The liquid fuel is injected into the evacuated mixing tank by means of a gastight syringe. Methyl formate has a vapor pressure of 476 torr at 20 °C, much higher than the maximum fuel partial pressure (60 torr) employed in the mixture preparation in this work, thus reducing the risk of fuel condensation. The methyl formate is of 98% purity, whereas the gases employed in this study are oxygen (99.995%), argon (99.995%), and nitrogen (99.998%), with helium (99.995%) used as a driver gas. The shock tube is helium driven, with the driven and driver sections separated by mylar diaphragms. The incident shock velocity is determined by means of shock arrival times measured by four fast-response pressure transducers (PCB models 113A24 and 113A26) mounted 50 cm apart. The endwall shock velocity is obtained by linear extrapolation of the decaying shock velocity to the endwall. The shock attenuation ranges from 1%/m to 3.5%/m of the incident shock velocity. The ignition delay is obtained from light emission at the endwall with the shock arrival time determined from the endwall pressure trace. Light emission is observed using a photodiode equipped with a filter centered at 430 nm with a 10 nm narrow bandwidth, corresponding to a chemiluminescence emission band of the CH radical. Data acquisition is by means of a 12 bit, 60 MHz digitizer (National Instruments PCI 5105). In the present work, data acquisition is carried out at 10 MS/s. Figure 1 shows that the rise in CH emission and pressure occur at the same time, so that either signal can be used to determine the ignition delay. The ignition delay, τ, is obtained in this study by the intersection of the line of maximum slope of the CH signal with the average photodiode signal prior to ignition onset. The postreflected shock temperature, T, was calculated from the one-dimensional shock equations with frozen chemistry using the equilibrium software GasEq.14 The thermodynamic data for methyl formate were taken from Westbrook et al.,2 and those for
Figure 2. Comparison of shock ignition data to previous data for n-heptane [4% O2, φ = 1.0 and p = 2 atm: this study (b), Horning et al.16 (O)] and iso-octane [20.66% O2, φ = 1.0 and p = 10 atm: this study (9), Shen et al.17 (0)].
(14) Morley, C. A Chemical Equilibrium Program for Windows, http:// www.arcl02.dsl.pipex.com/; 2009 (Accessed 22 June, 2009).
(15) Goodwin, D. Proceedings of CVD XVI and EuroCVD Fourteen, Electrochemical Society 2003, 14, 155–162.
the bath gases and oxygen were taken from the thermodynamic database in GasEq. Uncertainty in the calculated temperature results from the uncertainties in the shock velocity and composition of the combustible gas. It is estimated that the average temperature uncertainty in this work is between 10 and 25 K. For the purpose of comparison with simulations, the shock tube ignition process is modeled as a constant volume adiabatic reactor using the CANTERA software package.15 At ignition, the reactor pressure increases rapidly. This increase in pressure correlates with a simultaneous increase in the concentration of radicals like CH and OH. The ignition delay is obtained as the time from the beginning of the simulation to the time of peak CH concentration. Because of the very rapid rise in CH concentration, the ignition delay thus obtained is practically the same as would be obtained by extrapolating the line of maximum slope to the preignition CH concentration level. In the shock tube experiment, however, the CH signal is broader because observation is through the endwall. Chemiluminescence for successive ignition processes further from the endwall are also captured by the photodiode. The shock tube facility has been characterized and
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: DOI:10.1021/ef900899g
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Table 1. List of Methyl Formate/O2/Inert Mixtures mixture
φ
% fuel
% O2
% inert
inert
inert/O2 ratio, D
A B C D E F
1.0 2.0 0.5 1.0 1.0 1.0
4.1 7.8 2.1 9.4 9.5 2.0
8.2 7.9 8.4 19.0 19.0 4.0
87.7 84.3 89.5 71.6 71.5 94.0
Ar Ar Ar Ar N2 Ar
10.7 10.7 10.7 3.77 3.77 23.5
found to perform well by comparing n-heptane and iso-octane ignition data with those found in the literature. Figure 2 shows the validation against data for n-heptane by Horning et al.16 and isooctane by Shen et al.17 Table 1 lists the composition of the mixtures employed in this study. The effect of pressure on ignition is studied at average pressures of 2, 4, and 10 atm. The influence of equivalence ratio is also studied at 10 atm using lean, rich, and stoichiometric mixtures at an average argon/oxygen ratio of 10.7. To investigate the effect of dilution, experiments are also carried out at an argon/ oxygen ratio of 3.76, corresponding to synthetic air with a relative argon and oxygen composition of 79 and 21%, respectively. Experiments are also performed to compare the results of synthetic air (argon/oxygen) to actual air (nitrogen/oxygen).
Figure 4. Comparison of experimental data with simulations using mechanism by Fisher et al.1 for mixture A. 2 atm: data (1), sim. (dash-dot line); 4 atm: data (b), sim. (dash line); 10 atm: data (9), sim. (solid line).
Results and Discussion Figure 3 shows the effect of pressure on ignition delay time, τ, for a stoichiometric mixture as a function of the postreflected shock temperature, T. The experimental data have been fitted with exponential functions. It is observed that ignition delays decrease with increasing postreflected shock pressure, as is commonly observed for high temperature hydrocarbon ignition. Pressure variations from the average pressure being studied have been corrected with a power law, as is common in ignition delay correlations, with the exponent determined by regression as presented later in this paper in eqs 2 and 3: τµp -0:97
ð1Þ
In Figures 4 and 5, experimental results are compared with simulations based on the mechanisms by Fisher et al.1 and Westbrook et al.2 It is observed that both mechanisms predict shorter ignition delays, a higher temperature sensitivity (steeper slope), and a weaker dependence of ignition time on pressure than observed. At 10 atm, the Fisher et al. mechanism
Figure 5. Comparison of experimental data with simulations using mechanism by Westbrook et al.2 for mixture A. 2 atm: data (1), sim. (dash-dot line); 4 atm: data (b), sim. (dash line); 10 atm: data (9), sim. (solid line).
is in closer accord with experiment. Representative experimental data from this study are provided in the Appendix for reference. Investigating the effect of equivalence ratio at a constrained Ar/O2 ratio, D, of 10.7, it is found that ignition delays decrease with increasing equivalence ratio as shown in Figure 6. However, the ignition delays for the rich and stoichiometric mixtures are comparable within the limit of experimental uncertainty. This trend is in close agreement with that predicted by the mechanism, whereby ignition delays decrease with increasing equivalence ratio. Another important factor affecting the ignition delay is the level of dilution of fuel and oxygen with an inert gas, typically argon to minimize nonideal gas effects or nitrogen in order to simulate air. The presence of these inert gases has three effects on the reactor. The heat capacity and the sound speed of the (16) Horning, D. C.; Davidson, D. F.; Hanson, R. K. J. Propul. Power 2002, 18, 363–371. (17) Shen, H.-P. S.; Vanderover, J.; Oehschlaeger, M. A. Combust. Flame 2008, 155, 739–755.
Figure 3. Effect of pressure on ignition delays for mixture A. 2 atm: data (1), fit (dash-dot line); 4 atm: data (b), fit (dash line); 10 atm: data (9), fit (solid line).
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Figure 6. Effect of equivalence ratio on ignition delay at 10 atm and an Ar/O2 ratio of D = 10.7. Simulations use mechanism by Westbrook et al.2 φ = 0.5: data (1), sim. (dash-dot line); φ = 1.0: data (9), sim. (solid line); φ = 2.0: data (b), sim. (dash line).
Figure 8. Effect of inert gas dilution on ignition delay time at 2 atm and φ = 1.0. Simulations use mechanism by Westbrook et al.2 D = 10.7: data (9), sim. (solid line); D = 3.77: data (b), sim. (dash line).
Figure 7. Effect of inert gas dilution on ignition delay time at 10 atm and φ = 1.0. Simulations use mechanism by Westbrook et al.2 D = 10.7: data (9), sim. (solid line); D = 3.77: data (b), sim. (dash line).
Figure 9. Effect of inert gas type: ignition delay time versus temperature at 10 atm, φ = 1.0, and D = 3.77. Simulations use mechanism by Westbrook et al.2 Argon: data (9), sim. (solid line); nitrogen: data (b), sim. (dash line).
mixture are functions of the mixture composition, and these are altered by different levels of dilution. Second, reaction rates are subject to the law of mass action, so that the presence of a third gas affects the effective concentration of the fuel and oxidizer. Third, some reactions, such as unimolecular decompositions, require collision partners for energy transfer to the decomposing molecule. The rates of these reactions are influenced by the concentrations of the gases in the system, weighted by their respective collision efficiencies. It is therefore important to test kinetic models under varying levels of dilution (for the same equivalence ratio) in order to improve the model performance in predicting actual combustion processes. Figures 7 and 8 show the effect of dilution on ignition delay time for stoichiometric MF/O2 mixtures with argon. As expected, the ignition delay time becomes longer with higher levels of argon dilution. Compared to the simulation using the Westbrook et al. mechanism,2 there is fairly good agreement at 10 atm, but a significant deviation at 2 atm is observed. However, the deviation at 2 atm is linked to the aforemen-
tioned weaker pressure dependence and higher temperature sensitivity of the model compared to the data. In Figures 9 and 10, the effect of the type of inert gas is shown. Based on the temperature behind the reflected shock wave, it is observed that the ignition delay times in both cases are comparable. This behavior is also observed in simulations using the mechanism by Westbrook et al.2 This behavior is understandable since the bath gases do not change the chemical reactivity significantly. Reactions involving inert collision partners do depend on the concentration of the bath gases. However, the collision efficiencies of argon and nitrogen are not significantly different in order to effect observable changes in ignition delay times. On the other hand, the type of bath gas employed changes the gas dynamic performance of the shock tube. It requires much higher incident shock speeds for mixtures with nitrogen to establish the same temperature behind the reflected shock wave as for mixtures with argon (see Figure 10). This is due mainly to differences between diatomic nitrogen and monatomic argon with respect to their molar masses and specific heats. 399
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: DOI:10.1021/ef900899g
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Figure 10. Effect of inert gas type: ignition delay time versus incident shock speed at 10 atm, φ = 1.0, and D = 3.77. Argon data (9), nitrogen data (b).
Figure 12. Correlation tested against mixture F with φ = 1.0 and D = 23.5. 2 atm: data (1), correlation (solid line); 4 atm: data (b), correlation (dash line); 10 atm: data (9), correlation (dash-dot line).
Figure 11. Experimental data (9) scaled to 10 atm, φ = 1.0, and D = 10.7 using correlation. Correlation (solid line) and 2σ error bounds (dashed lines) are plotted.
Figure 13. Comparison of methyl formate and methane ignition at 2 atm, φ = 1.0, and Ar/O2 of D = 3.99 (conditions from ref 20). Methane data20 (O) and MF correlation (solid line).
In order to develop a correlation for ignition delay times based on the present results, the following form is adopted:
structure. It is expected that this correlation will be useful for determining high-temperature ignition delay times for mixtures of MF with equivalence ratios between 0.5 and 2.0, argon/oxygen ratios, D, between 3 and 24 (corresponding to the range of 25% O2:75% Ar to 4% O2:96% Ar), and average pressures between 1 and 10 atm. Using the above correlation, all the experimental data in this study have been scaled to 10 atm for a stoichiometric mixture with an argon/oxygen ratio of D = 10.7, and the plot is shown in Figure 11. Experimental data obtained using mixture F from Table 1 with an argon/oxygen ratio of D = 23.5 were not used in the development of the correlation but are used to test its applicability. Figure 12 shows that both the correlation and experimental data are in good agreement. At 2 and 4 atm, however, measured ignition delay times tend to be slightly shorter than those obtained from the correlation for this highly diluted mixture.
τ ¼ C φR Dβ pγ expðEa =RTÞ
ð2Þ
where τ is the ignition delay time in μs, C is a constant, φ is the equivalence ratio, D is the ratio of the argon or nitrogen concentration to the oxygen concentration, p is the pressure in atm, Ea is the global activation energy in kcal/mol, and R = 1.986 10-3 kcal/(mol K) is the universal gas constant. The resulting correlation is τ ¼ ð1:1 ( 0:3Þ 10 -3 φð -0:31 ( 0:04Þ Dð0:74 ( 0:04Þ pð -0:97 ( 0:03Þ expð31:5 ( 0:6=RTÞ
ð3Þ
with a goodness of fit of 98%. The correlation parameters and their uncertainties are calculated using standard linear regression techniques after taking the logarithm of eq 2 and the resulting normalized root-mean-square deviation from the fit is 16.3%. The global activation energy of Ea = 31.5 kcal/mol is lower than the typical activation energies of hydrocarbons.18 This could be due to the stronger influence of the ester group in methyl formate, which has the shortest alkane
(18) Davidson, D. F.; Hanson, R. K. Int. J. Chem. Kinetics 2004, 39, 510–523. (19) de Vries, J.; Hall, J. M.; Simmons, S. L.; Rickard, M. J. A.; Kalitan, D. M.; Petersen, E. L. Combust. Flame 2007, 150, 137–150. (20) Seery, D. J.; Bowman, C. T. Combust. Flame 1970, 14, 37–48.
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Figure 15. Sensitivity analysis for a stoichiometric mixture at 1200 K and pressures of 2 atm (open bars) and 10 atm (filled bars), with Ar/O2 ratio of D = 10.7.
Figure 14. Comparison of methyl formate and ethane ignition at 2 atm, φ = 1.0, and Ar/O2 of D = 13 (conditions from ref 19). Ethane data19 (0) and MF correlation (solid line).
To assess the relative ignition properties of methyl formate, the correlation developed in this study is compared to selected ignition data for methane by Seery and Bowman20 and ethane by de Vries et al.19 In Figure 13, ignition data for a stoichiometric mixture with a composition of 9.1% CH4, 18.2% O2, and 72.7% Ar (Ar/O2 ratio of D = 3.99) from ref 18 have been scaled to 2 atm and compared to MF ignition delay times calculated using the present correlation with p = 2 atm, φ = 1.0, and D = 3.99. It is found that methyl formate ignites more readily than methane. This is most likely due to the more reactive sites and oxygenated nature of methyl formate as compared to methane with all primary C-H bonds. In Figure 14, ethane ignition data from de Vries et al.19 are compared to the MF correlation scaled to the conditions in their ethane experiments (p = 2 atm, φ = 1.0, and D = 13). It is observed that ethane has shorter ignition delay times than methyl formate. Figures 4-8 show that further improvement of the detailed kinetic modeling of methyl formate combustion is required. Optimization of the model for ignition could be achieved by reducing the rate constants of reactions which tend to enhance oxidation or by increasing the rates of inhibiting reactions within the uncertainty bands for these reactions. Pressure dependent reaction rates could be further improved to obtain better agreement both at low and moderate pressures. In order to identify important reactions relevant to ignition, sensitivity analysis has been performed to guide model development. The logarithmic sensitivity, LS, employed here is defined as: ! Δτ Δkj ÷ ð4Þ LS ¼ τ0 kj
Figure 16. Decomposition path of methyl formate at 10 μs after reaction onset. Ignition delay time at these conditions is 402 μs.
reaction for ignition. The rate constants of this and other fuel decomposition reactions in the mechanism are given in the high pressure limit, thus no pressure dependence is included in their treatment. These reactions are possibly in the falloff region for the lower pressures (2 and 4 atm) studied here, therefore a pressure-dependent treatment of their reaction rate constants could improve the model performance at these lower pressures. Other fuel-specific reactions feature among the 20 reactions in addition to known reactions of the hydrogen and methane mechanisms. Two reactions formerly among the 20 most important at 10 atm cease to be among the 20 most important reactions at 2 atm:
where τ0 is the ignition delay time obtained with unchanged rate parameters, Δτ is the change in ignition delay time resulting from increasing the rate constant of the jth reaction, kj, by Δkj. Negative LS values indicate shorter ignition delay times and therefore increased overall reaction rates. Sensitivity analyses of the mechanism at 1200 K, and pressures of 2 and 10 atm have been carried out. Figure 15 shows the normalized change in the ignition delay time of the most important reactions as their rates are doubled. It is observed that the unimolecular decomposition of methyl formate (CH3OCHO) into CH3 and OCHO is the most sensitive
CH3 OCHO þ CH3 f CH2 OCHO þ CH4
ð5Þ
CH3 þ O2 f CH2 O þ OH
ð6Þ
On the other hand, the 2 atm results show an enhanced sensitivity to the following reactions: CH3 OCHO f CH3 OH þ CO 401
ð7Þ
Energy Fuels 2010, 24, 396–403
: DOI:10.1021/ef900899g
Akih-Kumgeh and Bergthorson
Figure 19. Methanol (MeOH) concentration (dash line) and consumption rate (solid line) for ignition at 10 atm and 1200 K using mechanism by Westbrook et al.2 Table 2. Representative experimental data Figure 17. Decomposition path of methyl formate at 392 μs after reaction onset,10 μs prior to ignition.
Figure 18. Methyl formate concentration (dash line) and consumption rate (solid line) for ignition at 10 atm and 1200 K using mechanism by Westbrook et al.2
CH2 O þ OH f HCO þ H2 O
ð8Þ
The change in the relative importance of different MF decomposition pathways as pressure varies could also partially explain the discrepancies observed in Figures 5 and 6. In addition to sensitivity analysis, reaction pathway analysis provides another means of determining the main reaction channels for species of specific interest. For the ignition of a stoichiometric mixture of MF/O2/Ar at a pressure of 10 atm and temperature of 1200 K, the consumption rate of methyl formate was analyzed at 10 μs after reaction onset (shock reflection) and at 392 μs, 10 μs before ignition onset. As seen in Figure 16, methyl formate is consumed mostly by the reaction leading to methanol (MeOH or CH3OH) and CO formation. Methanol is then consumed by H-abstraction reactions, involving H, OH, CH3, and HO2 radicals. The unimolecular decomposition of methyl formate to methanol and CO remains the most important path until just prior to ignition, although to a lesser degree (see Figure 17). In Figure 18, the fuel concentration, XMF, and consumption
p[atm]
T[K]
τ [μs]
p[atm]
T[K]
τ [μs]
11.4 11.3 10.7 9.5 9.5 4.3 4.2 3.8 3.7 3.5 2.6 2.3 2.1 1.9 1.8
Mix A 1144 1202 1245 1253 1274 1184 1222 1282 1376 1470 1282 1301 1421 1452 1561
650 320 197 288 177 964 658 381 165 91 575 600 209 183 118
11.5 10.8 10.4 10.6 9.2 2.6 2.3 2.2 2.2 1.8
Mix D 1053 1081 1123 1193 1226 1189 1291 1326 1392 1524
1051 624 449 143 118 833 297 218 130 60
Mix E 1079 1109 1143 1152 1172
813 719 374 384 372
11.1 11.2 10.7 10.2 9.5 3.2 3.0 2.2 2.3 2.1
Mix B 1098 1157 1198 1225 1297 1179 1239 1313 1403 1559
8.8 9.2 8.6 8.3 8.2
950 442 327 190 106 849 689 374 212 110
10.6 10.7 10.5 10.7 10.6 2.3 2.3 2.4 2.0 2.3
Mix C 1171 1189 1248 1380 1427 1246 1355 1361 1449 1527
843 600 319 75 57 945 308 385 189 117
11.0 9.7 9.8 9.6 9.9 4.3 4.1 3.9 3.7 3.7 2.7 2.5 2.3 2.3 2.1
Mix F 1162 1286 1367 1390 1462 1250 1288 1335 1432 1524 1362 1390 1462 1591 1649
1081 262 142 103 66 844 503 400 180 97 405 419 243 106 82
rate, dXMF/dt, are plotted as a function of time. Ignition occurs when the methyl formate has been fully consumed, and the maximum consumption rate occurs just prior to ignition. The methanol produced by methyl formate decomposition is consumed slowly by H-abstraction reactions. The net result as shown in Figure 19 is a build up of methanol that is rapidly consumed just before ignition onset. This perhaps explains 402
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why the methanol reaction does not feature as one of the 20 most sensitive reactions to changes in its rate constant at 10 atm. When the reaction rate is increased as in the sensitivity analysis, more methanol is produced, which in turn competes with the methyl formate and other reactions for H-abstraction radicals such as H, OH, HO2, and CH3. Once the methyl formate is depleted, methanol is then rapidly consumed primarily through H-abstractions by these radicals. Analysis of the Fisher et al. mechanism1 reveals that fuel consumption is modeled primarily through H-abstraction reactions by H and OH radicals from the methoxy and carbonyl sites. The resulting radicals then undergo O2 addition reactions to form peroxy radicals, which further lead to the production of smaller radicals. In the study by Francisco,11 unimolecular reactions feature as the main methyl formate consumption pathways. However, only the decomposition into CH3OH and CO is common to both the Westbrook et al.2 and Francisco11 mechanisms. Francisco further considers the decompositions into CH4 þ CO2, 2CH2O, and CH2O þ HCOH. Including these reactions in the mechanism by Westbrook et al.2 may further improve the performance of the model in predicting methyl formate oxidation. It could be possible that unimolecular decomposition of the methyl esters is more important in smaller esters such as methyl formate and H-abstraction becomes more important in longer chain methyl esters. The transition states considered by Fransisco et al.11 seem to be more favored in methyl esters with very short alkyl chains.
a decrease in ignition delay time with increasing equivalence ratio at 10 atm with a constrained argon/oxygen ratio. Sensitivity and reaction pathway analyses using the mechanism by Westbrook et al.2 indicate that methyl formate consumption proceeds mainly through decomposition to CO and methanol, which itself is later consumed through Habstraction reactions. In contrast, the Fisher et al.1 mechanism models methyl formate consumption primarily through H-abstractions by radicals such as H, OH, and HO2. In order to obtain closer agreement between the models and the present data, further analysis of possible fuel consumption pathways and improved estimation of pressure-dependent reaction rates should be investigated. A correlation is proposed for methyl formate ignition under conditions similar to those in this work. Using this correlation, methyl formate ignition has been compared to selected ignition data for methane and ethane from the literature. It is found that methyl formate ignites more readily than methane but less readily than ethane. In the case of methane, the increased reactivity of methyl formate is thought to be related to the presence of the carbonyl and methoxy groups in the molecule as opposed to the four primary C-H bonds in methane that result in reduced reactivity. The higher reactivity of ethane is attributed to the different chain initiation and branching mechanisms involved in its oxidation. Although methyl formate is the simplest alkanoic acid methyl ester, the observed influence of its ester group on ignition behavior relative to alkanes can contribute toward improved understanding of the ignition of longer chain methyl esters that make up biodiesel.
Conclusions The autoignition of methyl formate mixtures with oxygen/argon and oxygen/nitrogen has been studied behind reflected shock waves. The effects of pressure, equivalence ratio, and dilution have been investigated and the experimental data have been compared with simulations using two different methyl formate combustion models. Although both mechanisms predict ignition delay times in accord with experiment at 10 atm, deviations between the models and data are observed at lower pressures. The temperature sensitivity, or global activation energy, predicted by the simulations is higher than that of the experimental data, indicating that the methyl ester structure influences reactivity to a greater degree than is accounted for in the models. Both experiments and simulations show
Acknowledgment. Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Further support by the National Science and Engineering Research Council of Canada is acknowledged. The authors thank Professor Andrew Higgins and other members of our Shock Wave Physics Group at McGill University for useful discussions, as well as Dr. Charles Westbrook for providing the mechanism in ref 2.
Appendix Representative experimental data are listed in Table 2.
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