Comparative High Temperature Shock Tube Ignition of C1− C4

Energy Fuels 2010, 24, 5834–5843 Published on Web 10/22/2010

: DOI:10.1021/ef1009692

Comparative High Temperature Shock Tube Ignition of C1-C4 Primary Alcohols Khalid Emilio Noorani, Benjamin Akih-Kumgeh, and Jeffrey M. Bergthorson* Department of Mechanical Engineering, McGill University, Montreal, Quebec, Canada Received July 27, 2010. Revised Manuscript Received October 1, 2010

The high temperature ignition of C1-C4 primary alcohols, methanol, ethanol, n-propanol, and n-butanol, is studied behind reflected shock waves. The experiments are carried out at pressures of 2, 10, and 12 atm with argon/oxygen ratios of 10, 15, and 20 under lean, φ = 0.5, stoichiometric, φ = 1, and rich, φ = 2, conditions between 1070 and 1760 K. It is observed that the ignition delay time data for ethanol, n-propanol, and n-butanol collapse under conditions of constant equivalence ratio, pressure, and dilution. The ignition delay times of methanol are comparable with the other alcohols but show a slightly lower activation energy than the other fuels. The observed collapse of the ignition delay times for the four alcohols under lean-to-stoichiometric conditions is comparable with recent observations by Veloo et al. (Veloo, P. S.; Wang, Y. L.; Egolfopoulos, F. N.; Westbrook, C. K. Combust. Flame 2010, 157, 1989-2004) that over a range of equivalence ratios less than one, methanol, ethanol, and n-butanol have similar laminar flame speeds. Measured ignition delay times for selected conditions are compared to simulated delay times using their corresponding chemical kinetic models developed in previous studies: methanol by Li et al. (Li, J.; Zhao, Z.; Kazakov, A.; Chaos, M.; Dryer, F.; Scire, J. Int. J. Chem. Kinet. 2007, 39, 109-136), ethanol by Marinov (Marinov, N. Int. J. Chem. Kinet. 1999, 31, 183-220), n-propanol by Johnson et al. (Johnson, M. V.; Goldsborough, S. S.; Serinyel, Z.; O’Toole, P.; Larkin, E.; Malley, G.; Curran, H. J. Energy Fuels 2009, 23, 5886-5898), and n-butanol by Sarathy et al. (Sarathy, S. M.; Thomson, M. J.; Togb
e, C.; Dagaut, P.; Halter, F.; Mounaim-Rousselle, C. Combust. Flame 2009, 156, 852-864). The agreement of the various mechanisms with experiment is reasonable; however, the lower temperature ignition delay times for n-propanol and n-butanol tend to be longer than measured. The closest agreement between experiment and model predictions is observed for ethanol with the Marinov mechanism. Ignition delay time correlations for the alcohols are obtained by linear regression of the experimental data. This correlation method is also applied to the chemical kinetic mechanisms to obtain simplified expressions for their ignition delay times that allow a general assessment of their performance relative to experiment. Although sensitivity and reaction pathway analyses indicate similar modeling approaches for the four alcohols, the models do not capture the quantitative similarity observed in the experiment. These results will be useful in the process of developing a generalized chemical kinetic model for C1-C4 primary alcohol combustion as well as reduced models for combustion engineering.

compared to those of relatively well-studied alkanes.6-10 The nature and importance of these alcohol-specific reactions may vary from one alcohol to the other. For instance, it has been observed that whereas H-abstraction in longer alcohols such as n-butanol5 lead to the formation of olefins (alkenes), in methanol the loss of the OH group is more significant.11 An overview of biofuel combustion, from ethanol to biodiesel, has been provided by Kohse-H€ oinghaus et al.,12 highlighting alcohol reaction pathways and possible oxygenated byproducts. Ignition is one of the combustion properties of a fuel central to the choice and subsequent control of an engine concept, such as spark ignition, compression ignition, or homogeneous charge compression ignition engines. Investigation of the ignition delay times of a fuel under various thermodynamic conditions and compositions therefore constitutes an important validation criterion for its associated chemical kinetic model. Empirical ignition correlations are also employed in industry for predicting the ignition behavior of fuels under conditions not previously

Introduction Production of biofuels from second and third generation biomass leads to diversification of final products. Among these biofuels are alcohols, where the increasing energy density with increasing carbon chain length (see Table 1) has prompted studies of the combustion properties of longer chain alcohols in addition to ethanol, which has seen widespread use as a transportation fuel or fuel additive. The oxygenated nature of primary alcohols results in additional chemical reaction pathways *To whom correspondence should be addressed. E-mail: jeff. [email protected]. (1) Veloo, P. S.; Wang, Y. L.; Egolfopoulos, F. N.; Westbrook, C. K. Combust. Flame 2010, 157, 1989–2004. (2) Li, J.; Zhao, Z.; Kazakov, A.; Chaos, M.; Dryer, F.; Scire, J. Int. J. Chem. Kinet. 2007, 39, 109–136. (3) Marinov, N. Int. J. Chem. Kinet. 1999, 31, 183–220. (4) Johnson, M. V.; Goldsborough, S. S.; Serinyel, Z.; O’Toole, P.; Larkin, E.; Malley, G.; Curran, H. J. Energy Fuels 2009, 23, 5886–5898. (5) Sarathy, S. M.; Thomson, M. J.; Togb
e, C.; Dagaut, P.; Halter, F.; Mounaim-Rousselle, C. Combust. Flame 2009, 156, 852–864. (6) Wiser, W. H.; Hill, G. R. Proc. Combust. Inst. 1955, 5, 553–558. (7) Fieweger, K.; Blumenthal, R.; Adomeit, G. Combust. Flame 1997, 109, 599–619. (8) Curran, H.; Gaffuri, P.; Pitz, W.; Westbrook, C. Combust. Flame 1998, 114, 149–177. (9) Shen, H.-P.; Steinberg, J.; Vanderover, J.; Oehlschlaeger, M. Energy Fuels 2009, 23, 2482–2489. r 2010 American Chemical Society

(10) Westbrook, C.; Pitz, W.; Herbinet, O.; Curran, H.; Silke, E. Combust. Flame 2009, 156, 181–199. (11) Held, T. J.; Dryer, F. L. Int. J. Chem. Kinet. 1998, 30, 805–830. (12) Kohse-H€ oinghaus, K.; Osswald, P.; Cool, T. A.; Kasper, T.; Hansen, N.; Qi, F.; Westbrook, C. K.; Westmoreland, P. R. Angew. Chem., Int. Ed. 2010, 49, 3572–3597.

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there are relatively fewer data sets at the higher pressure conditions investigated in the present study. The majority of existing detailed chemical kinetic mechanisms were developed based on the reaction classes employed by Curran et al.8 in the modeling of n-heptane.4,5,27,28 However, the assignment of rate parameters and the detail to which the oxidation of intermediate species is modeled varies in each case. For the purpose of comparing selected experiments with simulations, chemical kinetic mechanisms for the four alcohols have been chosen from diverse research groups. Experiments are compared to predictions by the chemical kinetic mechanism for methanol by Li et al.,2 ethanol by Marinov,3 npropanol by Johnson et al.,4 and n-butanol by Sarathy et al.5 This suite of models is expected to reveal the common features and relative performance of the various mechanisms in predicting high-temperature ignition delay times. As progress is made in the chemical kinetic modeling of longer-chain alkanes and oxygenated fuels, determining accurate rate parameters for the increasing number of intermediate reactions is an important problem that must be addressed. Estimations of rate parameters are made by reviewing kinetic databases, such as that compiled by Tsang,29 thermochemical methods, such as the group additivity methods of Benson,30 educated guessing, and ab initio quantum chemical calculations. For example, Galano et al.31 performed density functional theory calculations of the rate constants for H abstraction reactions from C1-C4 alcohols by OH radicals, which have been added to kinetic databases. Advances in computational chemistry now permit the study of more reaction classes from which more accurate estimation rules are being developed.32-36 It is hoped that by means of these more reliable rules, along with further exploration of additional possible reaction pathways, multicomponent kinetic models can be developed. Such comprehensive models will require systematic experimental studies for validation and further optimization. Previous alcohol ignition studies were performed individually with the aim of understanding the combustion properties, as well as validating a specific mechanism, without seeking to observe the effect of increasing carbon chain length on ignition delay times. However, such carbon chain length and structure effects have been investigated in other combustion experiments. Norton and Dryer37 compared the oxidation chemistries of C1 to C4 primary, secondary, and tertiary alcohols in a flow reactor. They found that primary alcohols react mostly by H abstraction to form more reactive aldehydes, while secondary alcohols react by both H abstraction to ketones and dehydration to alkenes. Tertiary alcohols were

Table 1. Energy Density of C1-C4 Alcohols and Gasoline fuel

HHV [MJ/kg]

methanol ethanol n-propanol n-butanol gasoline

23.613 29.713 33.613 36.013 44.414

studied. A comprehensive characterization of the combustion of alcohols requires in-depth attention to their ignition behavior. Many previous studies of alcohol combustion properties have been performed, and only those relevant to high-temperature ignition will be discussed here. The simplest alcohol, methanol, has been studied in shock tubes7,15-17 and other combustion experiments. The shock tube ignition studies were performed at equivalence ratios, φ, of 0.375-6 between 740 and 1800 K at 0.3-40 atm. The argon to oxygen ratio, D, was varied from 3.76 to 98 in the gas mixtures. These ignition studies, along with those in flow reactors and laminar flames, were used to validate mechanisms by Egolfopoulos et al.,18 Held and Dryer,11 and Li et al.2 Shock tube experiments have also been performed for ethanol across a wide range of temperatures, equivalence ratios, dilutions, and pressures.15,19-22 For instance, Dunphy and Simmie21 studied equivalence ratios between 0.25 and 2 with pressures from 1.8 to 4.6 atm and dilution levels ranging from 12 to 55. Ethanol ignition data, along with data from other combustion experiments, were then used by Marinov to validate a comprehensive chemical kinetic model.3 Other models of ethanol oxidation chemistry have since been proposed (e.g., Saxena and Williams23), using updated data sets, such as those of Li et al. in a variable pressure flow reactor.24 n-Propanol and its isomer, iso-propanol, were also studied behind reflected shock waves where n-propanol was found to be more reactive than iso-propanol due to the tendency of the latter to follow a slow dehydration breakdown to less reactive propene and water.4 Butanol isomers were studied25,26 at conditions similar to those of the other alcohols. Sarathy et al.5 developed a model which was validated against flame speeds and species evolution concentration profiles where it was noted that n-butanol was the most reactive of the four isomers. The majority of these earlier studies are at low pressures, 1-4 atm, and (13) Afeefy, H.; Liebman, J.; Stein, S. NIST Chemistry WebBook; http://webbook.nist.gov, 2009. (14) ORNL. Transportation Energy Data Book; Oak Ridge National Laboratory: Oak Ridge, TN, 2010; Appendix B, Table B.4. (15) Cooke, D. F.; Dodson, M. G.; Williams, A. Combust. Flame 1971, 16, 233–236. (16) Bowman, C. T. Combust. Flame 1975, 25, 343–354. (17) Cribb, P. H.; Dove, J. E.; Yamazaki, S. Combust. Flame 1992, 88, 186–200. (18) Egolfopoulos, F. N.; Du, D. X.; Law, C. K. Combust. Sci. Technol. 1992, 83, 33–75. (19) Curran, H. J.; Dunphy, M. P.; Simmie, J. M.; Westbrook, C. K.; Pitz, W. J. Proc. Combust. Inst. 1992, 24, 769–776. (20) Natarajan, K.; Bhaskaran, K. A. Proc. Combust. Inst. 1981, 13, 834–839. (21) Dunphy, M. P.; Simmie, J. M. J. Chem. Soc. Faraday Trans. 1991, 87, 1691–1696. (22) Akih-Kumgeh, B.; Bergthorson, J. SAE Technical Paper 201001-2113, 2010. (23) Saxena, P.; Williams, F. A. Proc. Combust. Inst. 2007, 31, 1149– 1156. (24) Li, J.; Kazakov, A.; Dryer, F. L. J. Phys. Chem. 2004, 38, 7671– 7680. (25) Moss, J. T.; Berkowitz, A. M.; Oehlschlaeger, M. A.; Biet, J.; Warth, V.; Glaude, P.-A.; Battin-Leclerc, F. J. Phys. Chem. 2008, 112, 10843–10855. (26) Black, G.; Curran, H.; Pichon, S.; Simmie, J.; Zhukov, V. Combust. Flame 2010, 157, 363–373.

(27) 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. Combust. Inst. 2009, 32, 221–228. (28) Herbinet, O.; Pitz, W.; Westbrook, C. Combust. Flame 2008, 154, 507–528. (29) Tsang, W. J. Phys. Chem. Ref. Data 1987, 16, 471. (30) Benson, S. W. Thermochemical Kinetics; John Wiley & Sons: 1976. (31) Galano, A.; Alvarez-Idaboy, J. R.; Bravo-P
erez, G.; Ruiz-Santoyo, M. E. Phys. Chem. Chem. Phys. 2002, 4, 4648–4662. (32) Sumathi, R.; Green, W. H., Jr. Theor. Chem. Acc. 2002, 108, 187– 213. (33) Sumathi, R.; Green, W. H., Jr. Phys. Chem. Chem. Phys. 2003, 5, 3402–3417. (34) Matheu, D. M.; Dean, A. M.; Grenda, J. M.; Green, W. H., Jr. J. Phys. Chem. 2003, 107, 8552–8565. (35) Carstensen, H.-H.; Dean, A. M. Proc. Combust. Inst. 2005, 30, 995–1003. (36) Carstensen, H.-H.; Dean, A. M.; Deutschmann, O. Proc. Combust. Inst. 2007, 31, 149–157. (37) Norton, T. S.; Dryer, F. L. Proc. Combust. Inst. 1991, 23, 179– 185.

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Figure 1. Chemical structures of C1-C4 primary alcohols.

observed to react predominantly by unimolecular dehydration to alkenes and water. More recently, methanol, ethanol, and n-butanol flame speeds, as well as those of their corresponding alkanes, were studied in a comparative manner,1 leading to the observation that methanol has much higher flame speeds under rich conditions than the other fuels, whose flame speeds collapse under similar conditions. Under lean conditions, methanol, ethanol, and n-butanol were found to have similar flame speeds. Systematic studies of structurereactivity effects based on high-temperature shock tube ignition data have been undertaken for alkanes. The seminal shock-tube ignition study of alkanes by Burcat et al.38 showed that methane had the longest delay times, while longer chain alkanes (butane, pentane) show similar reactivity as the temperature increases. For longer chain alkanes, Shen et al.9 showed that the ignition delay times are comparable within the experimental uncertainty, confirming model predictions using a comprehensive multifuel combustion chemical-kinetic model by Westbrook et al.10 Such observations on the relative oxidation characteristics of these fuels are important in the development of these comprehensive multifuel combustion models and subsequent generation of the reduced models needed for applied combustion. This study addresses the comparative high-temperature ignition behavior of another important class of oxygenated hydrocarbons, the primary C1-C4 alcohols: methanol, ethanol, n-propanol, and n-butanol (see Figure 1). The experiments are performed while constraining the argon to oxygen dilution ratio, D, equivalence ratio, φ, and average pressure, p, over a range of temperatures, T, as in the study by Burcat et al.38

Figure 2. End wall pressure and CH radical chemiluminescence measurements with corresponding ignition delay time, τ, for a nbutanol/O2/Ar mixture with φ = 1.0, D = 10, p = 10 atm, and T = 1173 K. The ignition delay time in this case is indicated by the star in the figure. Table 2. Composition of Gas Mixtures Studied fuel methanol

ethanol

n-propanol

n-butanol

mix

φ

D

Xfuel [%]

XO2 [%]

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

1.0 0.5 2.0 1.0 1.0 0.5 2.0 1.0 1.0 0.5 2.0 1.0 1.0 0.5 2.0 1.0

10 15 15 20 10 15 15 20 10 15 15 20 10 15 15 20

5.7 2.0 7.7 3.1 2.9 1.0 4.0 1.5 2.0 0.7 2.7 1.0 1.5 0.5 2.0 0.8

8.6 6.1 5.8 4.6 8.8 6.2 6.0 4.7 8.9 6.2 6.1 4.7 8.9 6.2 6.1 4.7

The experimental apparatus and its validation has been described previously.39 The shock tube has a circular cross-section of 5 cm and is divided into a 3 m driver section and a 4.2 m driven section. Postreflected shock pressures, p, of 2, 10, and 12 atm are studied. The ignition phenomenon is captured by end-wall pressure and CH-emission traces (see Figure 2). The ignition delay time, τ, is defined as the time interval between the shock reflection from the end wall and the intercept of the maximum slope of the CH-radical emission curve with the initial signal level. Mixtures of fuel, oxygen, and argon are prepared manometrically in a 90 L stainless steel tank at room temperature (298 K) and are allowed to mix by molecular diffusion for 18-24 h. The fuels are added to the mixing tank to a partial pressure below their

vapor pressure. Oxygen and argon are then added successively to reach the final static pressure. n-Butanol is the least volatile of the alcohols with a vapor pressure of 6.6 Torr as opposed to 125 Torr for methanol at 298 K.40 The fuel partial pressures for n-butanol were typically held below 85% of the vapor pressure, except in one mixture where the fuel partial pressure was 93% of the vapor pressure. For the other fuels, the fuel partial pressures were lower than 50% of their vapor pressures. Table 2 presents the mixture compositions in terms of the equivalence ratio, φ, argon to oxygen dilution ratio, D, and the molar fractions of fuel, Xfuel, and oxygen, XO2. Each experiment begins with the vacuuming of the two sections of the shock tube separated by a polycarbonate diaphragm. The driven (test) section is then filled with the combustible mixture from the mixing tank to a low pressure. The driver section is filled with pressurized helium until the resulting pressure difference across the diaphragm causes it to rupture, generating a shock wave from the sudden pressure discontinuity. This wave propagates down the driven section, heating and raising the pressure of the combustible mixture. Upon arrival at the end wall of the driven section, the shock wave is reflected, further raising the pressure and the temperature of the combustible mixture, which is considered to be chemically frozen until after the shock reflection. The postreflected shock temperature and pressure are determined

(38) Burcat, A.; Scheller, K.; Lifshitz, A. Combust. Flame 1971, 16, 29–33. (39) Akih-Kumgeh, B.; Bergthorson, J. M. Energy Fuels 2010, 24, 396–403.

(40) Yaws, C. L.; Narasimhan, P. K.; Gabbula, C. Handbook of Antoine Coefficients for Vapor Pressure, 2nd electronic ed.; http://knovel.com/web/ portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=1183& VerticalID=0:2009.

Experimental Technique

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from the one-dimensional shock equations and are used as initial conditions for the subsequent approximately constant-volume reaction process that leads to the ignition of the mixture. The temperature and pressure can be varied by varying the initial fill pressure in the driven section and the diaphragm thickness. The shock arrival times are recorded at four fast-response pressure transducers spaced near the end-wall of the shock tube. The incident shock velocity at the end-wall is obtained from the linear extrapolation of the three shock speeds calculated from the four transducer arrival times. The postreflected shock temperature, T, is calculated from the incident shock velocity at the endwall using the GasEq41 software package, assuming frozen chemistry between fuel, oxygen, and diluent and taking into account the variation of specific heats with temperature. The high signal-to-noise ratio of the emission signal, the fast response of the photodiode, and the use of a high-speed data acquisition system minimize error in the measured ignition delay time. This error is largest for the shortest ignition delay times, those on the order of tens of microseconds, and is negligible for longer ignition delay times. A detailed error propagation analysis based on the dependence of the ignition delay time on equivalence ratio, argon/ oxygen ratio, pressure, and temperature (see eq 2 below) reveals that typical uncertainties in composition and postreflected shock pressure have a negligible effect on the ignition delay time of the mixture. The postreflected shock temperature dominates the uncertainty in ignition delay time, and it is estimated that the average temperature uncertainty in this facility is between 10 and 25 K, as discussed in Akih-Kumgeh and Bergthorson.42 Simulations of ignition delay times are obtained using the CANTERA43 software package in a constant volume, adiabatic, zero-dimensional reactor. Ignition delay time is found from the simulation results as the time interval between the start of the reaction and the peak of the first time derivative of pressure in the reactor. This instant matches the maximum production of OH and CH radicals. This is verified by comparing results obtained with both methods. In this comparative study, the dilution ratio, equivalence ratio, and average pressure are constrained in each figure. The constraint on the dilution ratio naturally arises in applied combustion devices with air as the oxidizer.

Figure 3. Ignition delay times of C1-C4 alcohols at φ = 1, D = 10, p = 10 atm. (Mix 1 of each fuel from Table 2). Data: methanol (þ), ethanol (O), n-propanol (4), n-butanol (9).

Figure 4. Ignition delay times of C1-C4 alcohols at φ = 1, D = 10, p = 2 atm (mix 1 of each fuel from Table 2). Data: methanol (þ), ethanol (O), n-propanol (4), n-butanol (9).

Results and Discussions

The high-temperature ignition delay time curves for ethanol, n-propanol, and n-butanol collapse for these conditions. However, methanol ignition delay times are shorter, especially at low temperature, but get closer as the temperature increases. The temperature sensitivity, or apparent activation energy, of ethanol, n-propanol, and n-butanol are comparable, whereas that of methanol is slightly weaker. This similarity in ignition delay times is also observed in Figure 4, where the ignition delay times have been obtained at a lower pressure, 2 atm, thereby allowing higher temperatures to be accessed in the shock tube while maintaining resolution in the measured ignition delay times. Methanol ignition delay times are generally higher in this temperature range. The lower temperature sensitivity of methanol results in its ignition delay times being shorter at lower temperatures and comparable or higher than that of the other alcohols at higher temperatures. For temperatures lower than approximately 1480 K (1000/T = 0.68), the other three alcohols have comparable ignition delay times as previously observed in Figure 3. To optimize mechanisms for a wider range of compositions, it is important to study the effect of dilution on ignition. Increasing the ratio of argon/oxygen to 20, this relative behavior is investigated and the results are shown in Figure 5. Under these conditions, the alcohols again show comparable ignition behavior to that discussed above. The temperature range in Figure 5 is intermediate between the

Experimental results are presented in this section, followed by a comparison of selected data with predictions obtained with the chemical kinetic models for each fuel. Correlation of the experimental data and combustion model predictions is used to perform a general analysis of the trends observed. Reaction pathway and sensitivity analyses are then presented to highlight similarities and differences in the modeling approaches for the four alcohols. Experimental Results. Ignition results are presented here as plots of the ignition delay times, τ, versus reciprocal of temperature, T, at approximately constant pressure, p, equivalence ratios, φ, and argon/oxygen ratios, D. To account for variations in pressure, ignition delay times are scaled using a power law: ð1Þ τ µ pγ where γ is the pressure exponent obtained from linear regression of the experimental data as presented later in Table 3. Figure 3 shows ignition delay times of the four alcohols for stoichiometric mixtures at 10 atm and a dilution ratio of 10. (41) Morley, C. GASEQ: Chemical equilibria in perfect gases (version 0.7su); www.gaseq.com, 2005. (42) Akih-Kumgeh, B.; Bergthorson, J. Energy Fuels 2010, 24, 2439–2448. (43) Goodwin, D. An open-source, extensible software suite for CVD process simulation. In Proceedings of CVD XVI and EuroCVD Fourteen; 2003.

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Figure 5. Ignition delay times of C1-C4 alcohols at φ = 1, D = 20, p = 10 atm (mix 4 of each fuel from Table 2). Data: methanol (þ), ethanol (O), n-propanol (4), n-butanol (9).

Figure 7. Ignition delay times of C1-C4 alcohols at φ = 2, D = 15, p = 2 atm (mix 3 of each fuel from Table 2). Data: methanol (þ), ethanol (O), n-propanol (4), n-butanol (9).

Figure 6. Ignition delay times of C1-C4 alcohols at φ = 0.5, D = 15, p = 12 atm (mix 2 of each fuel from Table 2). Data: methanol (þ), ethanol (O), n-propanol (4), n-butanol (9).

Figure 8. Ignition delay times of C1-C4 alcohols at φ = 1, D = 10, p = 10 atm. Data: methanol (þ), ethanol (O), n-propanol (4), nbutanol (9). Simulations: methanol2 (dotted line), ethanol3 (dashed line), n-propanol4 (thin solid line), n-butanol5 (thick solid line).

range at 10 atm in Figure 3 and 2 atm in Figure 4 because of the lower oxygen content. The results consistently show a collapse in the ignition delay curves of ethanol, n-propanol, and n-butanol at lower temperatures, with deviations becoming apparent at higher temperatures. In addition to stoichiometric mixtures, experiments are also carried out at lean (φ = 0.5) and rich (φ = 2.0) conditions. The same trends as in the aforementioned cases are observed for both lean (see Figure 6) and rich (see Figure 7) mixtures. The collapse is very strong for these lean conditions where methanol delay times are in closer agreement with the other fuels; however, propanol data are generally lower at higher temperatures where the experimental scatter is largest due to the logarithmic y-axis and the increased uncertainty that accompanies the shortest ignition delay times. It is interesting to note that the comparable ignition behavior under lean conditions is similar to the results of flame speed measurements by Veloo et al.,1 which show that methanol, ethanol, and n-butanol have comparable flame speeds over a range of lean mixtures. For stoichiometric and richer flames, however, Veloo et al.1 report that methanol has higher flame speeds while ethanol and butanol have comparable flame speeds. The shock tube results (see Figure 7) show a reasonable collapse for all of the fuels under rich conditions, except for the lower delay times of propanol at higher temperatures. The difference between flame speed and ignition results under rich conditions is likely related to the higher diffusivity of

methanol, as well as different reaction pathways resulting in different effective reactivities under flame conditions. By varying the conditions of this study, it is thus observed that the high temperature ignition of the C1-C4 alcohols are comparable, with methanol showing a lower apparent activation energy. Comparison with Simulations. The experimental data presented in Figures 3, 6, and 7 are compared with ignition delay times predicted by the mechanisms for methanol by Li et al.,2 ethanol by Marinov,3 n-propanol by Johnson et al.,4 and nbutanol by Sarathy et al.5 Figure 8 is a comparison of the experimental data presented in Figure 3 with predicted ignition delay times for the four alcohols using their respective mechanisms. The curves for the ethanol, n-propanol, and n-butanol mechanisms do not collapse together as observed in the experiment; however, the n-propanol and n-butanol curves do follow each other closely. The methanol simulations are in close agreement with the methanol data in the higher temperature region and are higher than experiment by a factor of 2 in the lower temperature region (1000/T ≈ 0.9). There is generally good agreement between ethanol simulations and experiments, with a maximum deviation between simulation and experiment of a factor of 1.2 over this temperature range. Both n-propanol and n-butanol simulated ignition delay times are longer than measured by about a factor of 2 at 5838

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Figure 10. Ignition delay times of C1-C4 alcohols at φ = 2, D = 15, p = 2 atm. Data: methanol (þ), ethanol (O), n-propanol (4), nbutanol (9). Simulations: methanol2 (dotted line), ethanol3 (dashed line), n-propanol4 (thin solid line), n-butanol5 (thick solid line).

Figure 9. Ignition delay times of C1-C4 alcohols at φ = 0.5, D = 15, p = 12 atm. Data: methanol (þ), ethanol (O), n-propanol (4), nbutanol (9). Simulations: methanol2 (dotted line), ethanol3 (dashed line), n-propanol4 (thin solid line), n-butanol5 (thick solid line).

fuels and in order to specify constraints on the fuel-oxidizerinert mixtures, it is useful to normalize the fuel and oxygen concentrations and explicitly determine the pressure dependence. In this study, the fuel mole fraction in the mixture is normalized using the equivalence ratio, φ, and the oxygen mole fraction is described by the combination of the argon to oxygen ratio, D, and φ. As in a previous study,39 ignition delay time correlations can then be obtained in the format:

low temperatures (1000/T ≈ 0.9) and are in better agreement at higher temperatures (1000/T ≈ 0.7), especially n-butanol. The main cause in the difference between the simulations and experiment is that the temperature sensitivity predicted by the mechanisms is higher than that observed in the experimental data. This leads to a varying degree of agreement depending on the temperature region investigated. Comparison of experimental results with model predictions for the case with higher dilution, D = 20, reveals similar trends as discussed above; however, under these conditions the n-propanol simulations are in closer agreement with experiment than those for n-butanol. The factors discussed above and in the remainder of this section are obtained by taking the ratio of the correlation for the simulated ignition delay times to the correlation for the experimental data under the conditions corresponding to the specific figure in question. In Figure 9, simulated and measured ignition delay times are compared for the lean mixtures with an argon/oxygen ratio of D=15 and a pressure of 12 atm. Both n-propanol and n-butanol simulated ignition delay times are in close agreement with the measured delay times at higher temperatures, while at lower temperatures (1000/T > 0.75) the simulated values are approximately a factor of 2 higher than experimental data. On the other hand, methanol and ethanol show better agreement with their respective measured delay times at lower temperatures and have a maximum deviation of a factor of 1.5 throughout this temperature range. Figure 10 shows the comparison between simulated and measured ignition delay times under rich conditions at an argon/oxygen ratio of D=15 and a pressure of 2 atm. Methanol and ethanol simulations are in good agreement with their respective measurements, within an average factor of 1.5 and 1.2, respectively, and are also in close agreement with each other over this temperature range. While the simulated delay times of n-propanol and n-butanol are comparable with each other, they are longer than the measured delay times by a factor of 3.8 and 3.2 at a temperature of 1000/T ≈ 0.85 and by a factor of 2 at higher temperatures (1000/T ≈ 0.6). These observations show that a wide range of conditions is essential in the validation and development of comprehensive chemical kinetic mechanisms. Ignition Delay Time Correlations. Ignition delay times are known to be dependent on the concentrations of fuel and oxygen and the temperature. For the purpose of comparison of different

τ ¼ CφR Dβ pγ expðEa =RTÞ

ð2Þ

where τ is the ignition delay time in microseconds, C is a constant, φ is the equivalence ratio, D is the ratio of the argon mole fraction to the oxygen mole fraction, p is the pressure in atmospheres, Ea is the global activation energy in kilocalories/mole, and R = 1.986  10-3 kcal/(mol K) is the universal gas constant. Linear regression of this multivariable correlation is achieved by using its logarithmic form: lnðτÞ ¼ lnðCÞ þ R lnðφÞ þ β lnðDÞ þ γ lnð pÞ þ Ea =RT ð3Þ In matrix form, eq 3 can be written as y ¼ Xk

ð4Þ

where y is a column vector of the individual logarithmic ignition delay times, ln(τi), X is a matrix whose rows are vectors of the logarithms of the independent variables for each experimental realization, [1 ln(φi) ln(Di) ln(pi) 1/Ti], and k is a column vector of the correlation parameters, [ln(C), R,β,γ,Ea/R]. Least squares minimization of the error between the multivariable correlation and the data results in the following expression for the correlation parameters:44 k ¼ invðXT XÞXT y

ð5Þ

T

where inv(X X) is the inverse of the square matrix obtained by multiplying X by its transpose, XT. Standard linear leastsquares techniques also uniquely determine the uncertainty in the resulting correlation parameters and allow the prediction bound for the fit to be calculated.44 This correlation and regression method is also applied to simulated ignition delay times using the various mechanisms, as suggested by Akih-Kumgeh and Bergthorson,42 whereby the conditions are determined by a random number generator such (44) Brandt, S. Data Analysis: Statistical and Computational Methods for Scientists and Engineers, 3rd ed.; Springer-Verlag: New York, 1999.

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Noorani et al. Table 3. Correlation Parameters

variable

1

φ

D

p [atm]

T [K]

associated parameter

C

R

β

γ

Ea [kcal/mol]

methanol ethanol n-propanol n-butanol

2.9  10-3 1.7  10-4 5.5  10-5 1.6  10-4

Experiments -0.82 ( 0.07 -0.36 ( 0.08 -0.51 ( 0.14 -0.53 ( 0.14

1.10 ( 0.08 0.98 ( 0.09 1.50 ( 0.21 0.78 ( 0.07

-0.85 ( 0.04 -0.61 ( 0.04 -0.53 ( 0.09 -0.59 ( 0.07

25.3 ( 0.5 32.1 ( 0.7 30.9 ( 1.2 33.3 ( 1.3

methanol2 ethanol3 n-propanol4 n-butanol5

1.2  10-3 7.4  10-5 1.7  10-4 2.1  10-5

Mechanisms -0.63 ( 0.00 -0.34 ( 0.02 -0.33 ( 0.01 -0.27 ( 0.02

0.86 ( 0.01 0.81 ( 0.02 0.81 ( 0.02 1.10 ( 0.02

-0.93 ( 0.00 -0.71 ( 0.02 -0.75 ( 0.01 -0.60 ( 0.02

30.3 ( 0.1 35.9 ( 0.4 35.8 ( 0.3 38.3 ( 0.4

Figure 11. Calculated ignition delay times using respective correlations at φ = 0.7, D = 12, p = 11 atm. Methanol (dotted line) and its 95% correlation prediction bounds (dashed-dotted lines), ethanol (dashed line), n-propanol (thin solid line), and n-butanol (thick solid line).

Figure 12. Comparison of correlations derived in this study with literature data on alcohol ignition: methanol (φ=1.0, D=3.76, p=13.2 atm), data7 (þ), present correlation (dashed line); ethanol (φ=0.5, D= 12.2, p = 3.3 atm), data3 (O), present correlation (dotted line).

that only the high-temperature ignition region is considered. The equivalence ratio, φ, is varied over the range of 0.2-2.5, the argon/oxygen ratio, D, from 3.76 to 25, the pressure, p, from 1 to 20 atm, and the temperature, T, from 1150 to 1500 K. Table 3 is a summary of the correlation parameters as defined by eq 2, together with their associated uncertainties. It is observed that the apparent activation energies of both experiments and simulated delay times are generally lower than those of high-temperature alkane ignition. The main differences in correlation parameters leading to significant differences in ignition delay times are those associated with the constant, C, and the activation energy, Ea. The activation energies obtained from experiment are generally lower than those predicted by their respective mechanisms. However, the trend in activation energies is reflected, with methanol exhibiting the lowest activation energy in both the experimental and simulated ignition delay time correlations. Under conditions of constant argon/oxygen ratio, pressure, and temperature, ignition delay times decrease with increasing equivalence ratio in both the experimental and model correlations. Previous work based on mechanism analysis by Johnson et al.4 has shown that the exponent of φ is not constant over all conditions and tends to be negative at the higher pressures used in this study. The pressure sensitivity of methanol is observed to be higher than the other fuels in experiment and its model. Ethanol, n-propanol, and n-butanol show similar pressure sensitivities in the experiment, with values smaller than observed in the ethanol and n-propanol models but close to that in the n-butanol mechanism. The four experimental correlations are compared in Figure 11 under lean conditions at high pressure (φ=0.7, D=12, and p=11 atm). The ethanol, n-propanol, and n-butanol correlations collapse, as previously observed in the

data. The methanol correlation is close to that for the other alcohols, but it has a weaker temperature dependence that leads to shorter ignition delay times at lower temperatures and longer ignition delay times at higher temperatures. The 95% prediction bounds for the methanol correlation have been plotted to allow the uncertainty in the fit to be assessed. The weaker activation energy of methanol than the other alcohols can be seen to be significant compared to its uncertainty, consistent with a comparison of the Ea/R values and their uncertainties from Table 3 for the four alcohols. The ignition correlations obtained in this study were compared to a wide range of literature data for these fuels which were studied at different conditions and mostly at lower pressures. Typically, very good agreement is observed between the present correlations and previous data, except under conditions of very high or very low argon dilutions where deviations are observed. Selected comparisons for methanol and ethanol are presented in Figure 12 as well as for n-propanol and n-butanol in Figure 13. In Figure 12, calculated methanol ignition data using the present correlation are generally lower than data obtained previously by Fieweger et al.7 at φ = 1.0, D = 3.76, and p = 13.2 atm, close to the pressure of 10 atm investigated here. The data are obtained using air with a low nitrogen/oxygen ratio, D = 3.76, outside of the range studied here which could be a source of the deviation observed. The poor agreement with previous methanol data can be contrasted against the excellent agreement between the ethanol correlation and the previous data by Dunphy and Simmie21 for lean mixtures at 3.3 atm. For n-propanol and n-butanol, shown in Figure 13, while very good agreement between correlation predictions and n-butanol data by Black et al.26 is observed, the correlation 5840

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As rate estimation rules from quantum calculations are developed, such as those for oxygenated hydrocarbon species by Sumathi and Green,33 more accurate rates for such reactions can be determined. However, in the above discussion most of the fuel unimolecular reactions found in the mechanisms proceed by direct C-C or C-H bond cleavage involving activation barriers of the order of their respective bond dissociation enthalpies. As discussed by Benson30 and El-Nahas et al.,46 in relation to methyl butanoate and ethyl butanoate oxidation, concerted unimolecular reactions involving the formation and cleavage of multiple bonds can proceed with lower activation barriers compared to direct bond cleavage. Exploration of such pathways for oxygenated fuels, such as alcohols, could further contribute toward improved models. The npropanol mechanism by Johnson et al.4 was developed using the same approach as for the other three alcohols. However, concerted or complex unimolecular reactions are also included in this model. Some H abstraction reactions make use of the ethanol rate parameters, and the rest are estimated or found from the database. Sensitivity and reaction pathway analyses are complementary in elucidating the key reactions in combustion processes. Sensitivity analyses of the four mechanisms are carried out with D = 10, φ = 1.0 at 12 atm and 1150 and 1450 K. The frequency factor of the rate constant of each elementary reaction is perturbed to 10 times its model value, and the resulting change in ignition delay time was used to define a logarithmic sensitivity. It is observed that the 16 most important reactions in each case are made up of reactions from the C0-C1 oxidation systems, along with only a few fuel specific reactions. Fuel specific reactions are mostly H-abstractions and subsequent reactions of intermediate products. The sensitivity analyses at both temperatures yield similar results and the overall trends are demonstrated in Figure 14 for ethanol and Figure 15 for n-butanol. It should be noted that the n-butanol mechanism exhibits an extremely high sensitivity to the main chain branching reaction:

Figure 13. Comparison of correlations derived in this study with literature data on alcohol ignition: n-propanol (φ=1.0, D=43, p=1 atm), data4 (4), present correlation (thin solid line); n-butanol (φ=1.0, D= 26.6, p=8 atm), data26 (9), present correlation (thick solid line). Table 4. Details of Mechanisms in This Study mechanism

species

reactions

fuel reactions

fuel unimolecular reactions

methanol by Li et al.2 ethanol by Marinov3 propanol by Johnson et al.4 n-butanol by Sarathy et al.5

21 58 237

93 383 2624

18 19 100

3 4 12 (8 reversible)

117

884

66

6

for n-propanol is slightly shorter than the data by Johnson et al.,4 although the deviation is less than 50% which is within the uncertainty limits expected for the shock tube experiments and that of the correlation. These comparisons show that the present data and the associated correlations are consistent with previous ignition data under similar pressure and dilution conditions. The comparative approach used in this study and the new higher pressure data provide further insight into alcohol ignition properties. Analyses and Discussion of Chemical Kinetic Models. In this section, an analysis of the models with respect to their size and treatment of fuel-specific reactions, a sensitivity analysis and fuel reaction pathway study to determine controlling reactions, and a rate parameter comparison are carried out in order to further investigate the relative performance of the selected mechanisms. Table 4 gives information on the number of species, reactions, and fuel specific reactions included in the various mechanisms. As expected, an increase in the hydrocarbon chain length is accompanied by an increase in the mechanism size, except that the n-butanol mechanism by Sarathy et al.5 is much smaller than the shorter-chain n-propanol model. However, it should be noted that the propanol mechanism is developed for both isomers of propanol. The methanol kinetic database by Tsang29 was used by Marinov3 in conjunction with branching ratios of H-abstraction reactions to obtain key rates for the ethanol mechanism employed in this study. The same database and other experimental kinetic data were used by Held et al.11 in developing the methanol mechanism. The n-butanol mechanism by Sarathy et al.5 was developed based on a previous n-butanol mechanism by Dagaut et al.,45 itself based on earlier models for C1-C4 hydrocarbons. Some of the fuel-specific H-abstraction reactions were assigned rate parameters from the ethanol mechanism by Marinov.3

H þ O2 h OH þ O

ð6Þ

As expected, formation of stable molecules tend to reduce reactivity and increase the ignition delay times. The reaction pathway analysis shown in Figure 16 indicates that even though unimolecular fuel decompositions do not feature among the 16 most-sensitive reactions, they are crucial in the initiation process. Just after reaction onset, fuel decomposition by unimolecular complex reactions dominates H abstraction reactions. Although the activation energy barriers of the H abstraction reactions are generally lower than those of unimolecular reactions, the concentration of the abstracting radicals is very small at early times. This leads to an overall lower rate of fuel consumption through the H-abstraction channel until the radical pool builds up. In addition to the first three reactions shown in Figure 16, the remaining proportion of fuel is consumed at each state by several other H abstraction reactions by radicals such as H, CH3, and HO2 as well as through other unimolecular decomposition reactions. Although the same elementary reactions feature among the most important reactions in these sensitivity analyses and similar fuel consumption pathways are predicted by each (46) El-Nahas, A. M.; Navarro, M. V.; Simmie, J. M.; Bozzelli, J. W.; Curran, H. J.; Dooley, S.; Metcalfe, W. J. Phys. Chem. 2007, 111, 3727– 3739.

(45) Dagaut, P.; Sarathy, S.; Thomson, M. Proc. Combust. Inst. 2009, 32, 229–237.

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Figure 14. The 16 most important reactions from the sensitivity analysis of the ethanol mechanism by Marinov3 for a stoichiometric ethanol/ O2/Ar mixture at 12 atm with an argon/oxygen ratio, D, of 10 at a temperature of 1150 K. The unperturbed ignition delay time is 564 μs. The fuel-specific reactions are highlighted in white.

Figure 16. Decomposition pathways for ethanol at two different time intervals prior to ignition using the mechanism by Marinov3 for a stoichiometric ethanol/O2/Ar mixture at 12 atm with an argon/ oxygen ratio, D, of 10 at 1150 K. The ignition delay time is 564 μs.

Figure 15. The 16 most important reactions from the sensitivity analysis of the n-butanol mechanism by Sarathy et al.5 for a stoichiometric n-butanol/O2/Ar mixture at 12 atm with an argon/oxygen ratio, D, of 10 at a temperature of 1150 K. The unperturbed ignition delay time is 1024 μs, approximately twice that of ethanol under the same conditions. The fuel-specific reactions are highlighted in white.

and subsequently reduced chemical kinetic models can be greatly aided by using the ignition data obtained in this study and the observations resulting from the current mechanism analyses.

mechanism, variations in the assignment of thermodynamic and rate constants between the different models could explain why the models do not predict the collapse of alcohol ignition delays observed in experiment. Investigating the common reactions from the H and C1 submechanisms shows that these have different treatments and rate parameter assignments in the four models, which is expected due to the wide variability in experimentally measured and theoretically determined rate constants for typical hydrocarbon reactions. By systematic assignment of rate constants and use of transition state additivity for similar reaction classes, comprehensive mechanisms can be constructed that should be able to reflect the similarities observed in the experiments. The development of such comprehensive

Conclusions A comparative study of the high-temperature ignition of C1-C4 primary alcohols is carried out under conditions of constant argon/oxygen ratio, equivalence ratio, and pressure over a range of temperatures. This systematic approach to alcohol high temperature ignition reveals that the ignition delay time data for ethanol, n-propanol, and n-butanol collapse using these constraints. The collapse of ignition delay times can be compared to the findings of Veloo et al.,1 who showed that the flame speeds for methanol, ethanol, and n-butanol are comparable under lean to stoichiometric conditions. Chemical kinetic model predictions have been compared to the data for the four alcohols under selected conditions. The 5842

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best overall agreement between experiment and simulation is found for ethanol; however, discrepancies are observed between experiments and the model predictions for all fuels. These models do not predict the collapse of ignition delay curves that is seen in experiment, although the n-propanol and n-butanol models predict similar ignition delay times under these constraints. Ignition delay time correlations are obtained by linear regression of the experimental data for the four fuels, as well as from numerical simulation predictions over a random sample of reaction conditions. From these correlation parameters it is observed that the temperature sensitivity, or apparent activation energy, of methanol is lower than the other three alcohols which are comparable. The activation energies obtained from the experiment are lower than those from the mechanisms. Development and optimization of a single comprehensive chemical kinetic mechanism for these C1-C4 primary alcohols

should capture the comparable ignition behavior observed in experiment. Ignition delay times and flame speeds of less studied alcohols could be estimated from those of well studied alcohols by applying constraints on the equivalence ratio, diluent/oxygen ratio, pressure, and temperature. The observations on ignition delay time similarity will also be important in the development of reduced-order models needed for applied combustion engineering. Acknowledgment. The authors acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC), Le Fonds Qu
eb
ecois de la Recherche sur la Nature et les Technologies (FQRNT), and the AUTO21 Network of Centres of Excellence. Supporting Information Available: Ignition delay time data from this study are provided in Tables 5-20. This information is available free of charge via the Internet at http://pubs.acs.org/.

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