Energy Fuels 2011, 25, 60–71 Published on Web 12/16/2010
: DOI:10.1021/ef101335q
Development and Validation of a Reduced Chemical Kinetic Model for Methanol Oxidation S.-Y. Liao,*,†,‡ H.-M. Li,† L. Mi,‡ X.-H. Shi,‡ G. Wang,† Q. Cheng,† and C. Yuan† † Department of Power Engineering, College of Chongqing Communication, Chongqing 400035, PR China, and Key Laboratory of Manufacture and Test Techniques for Automobile Parts, Ministry of Education, Chongqing 400050, PR China
‡
Received June 3, 2010. Revised Manuscript Received November 22, 2010
Following the analysis of the reactive routine of methanol oxidation, a new reduced chemical kinetic mechanism has been developed for investigation of methanol oxidation. The reduced model involves 17 species undergoing 40 reactions and has been validated against a series of experimental measurements. Experimental data from shock tubes, flow reactors, and static reactors showed that, when the temperature is between 823 and 2180 K, the pressure is between 0.005 and 2.0 MPa, and the equivalence ratio is between 0.2 and 2.6, the proposed mechanism can predict the methanol oxidation process quite well. The premixed laminar flame speeds and ignition delay times computed by this mechanism have demonstrated good agreement with the experimental data as well. Moreover, the reactive intermediates and radicals in static reactors, flow reactors, and premixed laminar flames can also been predicted very well, using this reduced mechanism. Compared with other comprehensive mechanisms, the reduced model is validated by more experimental measurements and a large number of reaction steps involved in the base mechanism have been markedly simplified, while its essential features remain. important characteristic of methanol is that it is undoubtedly the cheapest liquid alternative fuel per calorific unit, which is now considered a renewable energy source, because it can be produced from a mass of fossil raw materials, including coal, natural gas, and biosubstances. Moreover, methanol can also be used in engines with lower NOx, CO, and nonsooting emissions, because of its high octane number and oxygen content. It is well-known that fundamental research is a key way to develop new combustion technology with high efficiency and ultralow emissions. Thus, such studies concerning methanol had obviously been promoted with the growing application of methanol in the combustion field. Liao et al.,5,6 Metghalchi and Keck,7 G€ ulder,8 and Saeed and Stone9 measured methanol laminar burning velocities using the constant volume bomb method. Zhang et al.10,11 conducted a detailed
1. Introduction The chemical kinetic mechanism is a useful engineering tool for understanding the fuel combustion reaction, which permits the exploration of microscopic chemical processes that underlie and sometimes control the macroscopic physical processes.1 Generally, the chemical reaction mechanism is also one of the most required schemes for computational fluid dynamics (CFD) research on engine combustion. When a detailed or comprehensive chemical kinetics mechanism is used in CFD modeling, especially in multidimensional dynamic computational simulation, the complexity of the computing process could be enhanced, and the computational time would be increased severalfold, compared to that of a reduced mechanism, because of the large number of reactions involved. With the growing importance of engine CFD simulation, the development of reduced models for fuel oxidation has been promoted in recent years; however, most attention is given to hydrogen and small hydrocarbon fuels, such as methane, ethane, propane, etc.,2-4 and some satisfying reduced oxidation models for these fuels had been proposed and validated over broad ranges of fuel/oxide mixture conditions. Methanol has been demonstrated to be one of the most promising alternative engine fuels in recent years. The most
(5) Liao, S. Y.; Jiang, D. M.; Huang, Z. H. Laminar burning velocities for mixtures of methanol and air at elevated temperatures. Energy Convers. Manage. 2007, 48 (3), 857–863. (6) Liao, S. Y.; Jiang, D. M.; Huang, Z. H.; Zeng, K. Characterization of laminar premixed methanol-air flames. Fuel 2006, 85 (10-11), 1346– 1353. (7) Metghalchi, M.; Keck, J. C. Burning velocities of mixtures of air with methanol, isooctane, and indolene at high pressure and temperature. Combust. Flame 1982, 48, 191–210. (8) G€ ulder, O. L. Laminar burning velocities of methanol, ethanol and isooctane-air mixtures. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; Vol. 19, Issue 1, pp 275-281. (9) Saeed, K.; Stone, C. R. Measurements of the laminar burning velocity for mixtures of methanol and air from a constant-volume vessel using a multizone model. Combust. Flame 2004, 139 (1-2), 152–166. (10) Zhang, Z. Y.; Huang, Z. H.; Wang, X. G.; Xiang, J.; Wang, X. B.; Miao, H. Y. Measurements of laminar burning velocities and Markstein lengths for methanol-air-nitrogen mixtures at elevated pressures and temperatures. Combust. Flame 2008, 155 (3), 358–368. (11) Zhang, Z. Y.; Huang, Z. H.; Wang, X. G.; Xiang, J.; Wang, X. B.; Miao, H. Y.; Zheng, J. J.; Miao, H. Y.; Wang, X. B. Combustion characteristics of methanol-air-diluent premixed mixtures at elevated temperatures and pressures. Appl. Therm. Eng. 2009, 29 (13), 2680–2688.
*To whom correspondence should be addressed. E-mail: shyliao@ yahoo.com.cn. (1) Held, T. J.; Dryer, F. L. A comprehensive mechanism for methanol oxidation. Int. J. Chem. Kinet. 1998, 30 (11), 805–830. (2) Lee, K. Y.; Puri, I. K. A reduced kinetic mechanism for premixed CH3Cl/CH4/air flames. Combust. Flame 1993, 94 (1-2), 191–204. (3) Peter, G.; Lilleheie, N. I.; Byggstøy, S.; Magnussen, B. F.; Kilpinen, P.; Hupa, M. A reduced mechanism for nitrogen chemistry in methane combustion. Twenty-fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; Vol. 24, Issue 1, pp 889898. (4) Ting, D. S.-K.; Reader, G. T. Hydrogen peroxide for improving premixed methane-air combustion. Energy 2005, 30 (2-4), 313–322. r 2010 American Chemical Society
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: DOI:10.1021/ef101335q
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Table 1. Overview of the Methanol Oxidation Mechanism authors a
Bowman Aronowitzb Westbrook and Dryera,b Tsuboi and Hashimotoa Natarajan and Bhaskarana Yuno and Itoc Norton and Dryerb Chend Egolfopoulosa,b,e,f Grotheerc,f Held and Dryera,b,e,f Lindstedt and Meyera,b,f Lia,b,f
no. of species
no. of reactions
temperature (K)
pressure (MPa)
equivalence ratio
time (ref)
14 18 26 22 17 24 26 7 30 43 22 17 18
28 37 84 57 35 94 84 5 171 414 89 19 84
1545-2180
0.18-0.46 0.1 0.1-0.5 0.25-0.45
0.375-6.0 0.03-3.16 0.05-3.0 0.2-2.0 0.5-1.5
0.1
0.6-1.6
820-2180
0.005-0.47
g0.05
633-2050
0.026-2
0.05-2.6
300-2200
0.1-2
0.05-6.0
197516 197812 197918 198119 198115 198320 198922 199113 199223 199224 19981 200214 200727
Data from the shock tube. Data from the flow reactor. Data from the engine. Data from the stirred reactor and diffusion flame. e Data from the static reactor. f Data from the premixed laminar flame. a
b
1000-2180 1200-1800 1300-1700 700-1000 1025-1090
c
d
study of methanol laminar burning characteristics, and the combustion characteristics, laminar burning velocities, and Markstein lengths for methanol/air/nitrogen mixtures at elevated pressures and temperatures had also been determined. The detailed methanol oxidation mechanisms had been well-established, and a few simplified models have been proposed.12-16 However, as mentioned above, to improve engine CFD modeling efficiency, more accurate and reduced methanol oxidation schemes, in which a small number of reaction steps involved, are still demanded. The primary objective of this work is to present a new reduced methanol mechanism, as well as to develop and validate it with a series of experimental data sets obtained under a variety of temperature, pressure, and equivalence ratio conditions. The developed methanol oxidation mechanism consists of 40 reactions and 17 active species. Compared with other mechanisms, a large number of reaction steps involved in the detailed mechanism have been markedly simplified on the basis of the analysis of the main reactive pathway of methanol oxidation, while its essential features remain.
the combustion field. In general, there are 13 mainstream reaction mechanisms for methanol oxidation, developed by Bowman, Dryer, Aronowitz, Egolfopoulos, Grotheer, and their co-workers, as listed in Table 1. Bowman can be considered one of the pioneers in the development of the reduced methanol mechanism. In 1975, he proposed the first simplified reaction kinetic mechanism for methanol oxidation,16 which consists of 14 active species and 28 reaction routines, on the basis of experimental measurements of ignition delay times in shock tubes. This mechanism could predict the methanol ignition delay under relatively high temperature conditions, to some extent. However, the modeling accuracy would decrease significantly when the flame temperature was below 1800 K. Aronowitz and his co-workers12 conducted a flow reactor measurement on the intermediate profiles during methanol oxidation under normal pressure. Consequently, a reduced mechanism involving 18 species and 28 reactions was compiled, and the validation against experimental data sets was implemented as well. The first comprehensive methanol kinetic mechanism was developed by Westbrook and Dryer.18 Their comprehensive model had been validated under relatively wide mixture conditions, which covered temperatures from 1000 to 2180 K and pressures from 0.1 to 0.5 MPa. Using this comprehensive model, a series of flow reactor and shock tube experimental data had been predicted successfully under both high- and intermediate-temperature conditions. However, because of the lack of information for some elementary reactions relevant to the methoxy radical (CH3O), which is a very important intermediate during methanol oxidation, this mechanism had demonstrated an incomplete reactive pathway. The models developed by Tsuboi,19 Natarajan,15 Yuno,20 and their co-workers were verified by only shock tube experiments; some in-depth investigations are still demanded for their validation. In 1987, Tsang21 conducted a systematic analysis of chemical kinetic data for methanol reaction routing. Thereafter, an improved chemical mechanism was proposed by Norton and Dryer22 on the basis of the model of Westbrook and Dryer.18 This updated mechanism had been compared to a new set of atmospheric-pressure flow reactor data, and
2. Overview of Methanol Mechanisms Studies relevant to methanol oxidation have been the subject of a number of experimental and numerical investigations. The first study of methanol kinetics was that by Wiser and Hill in the 1950s.17 However, as shown in the literature reported previously, the first systematic study of the reaction kinetic mechanism for methanol oxidation was initiated by Bowman16 in 1975. Subsequently, systematic and in-depth developments of methanol mechanisms have been rapidly promoted since the 1990s, with the growing interest of methanol utilization in (12) Aronowitz, D.; Santoro, R. J.; Dryer, F. L.; Glassman, I. Kinetics of the oxidation of methanol: Experimental results, semi-global modeling, and mechanistic concepts. Seventeenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; Vol. 17, pp 633-644. (13) Chen, J. Y. Reduced reaction mechanisms for methanol-air diffusion flames. Combust. Sci. Technol. 1991, 78 (1), 127–145. (14) Lindstedt, R. P.; Meyer, M. P. A dimensionally reduced reaction mechanism for methanol oxidation. Proceedings of the Combustion Institute; The Combustion Institute: Pittsburgh, PA, 2002; Vol. 29, pp 1395-1402. (15) Natarajan, K.; Bhaskaran, K. A. An experimental and analytical study of methanol ignition behind shock waves. Combust. Flame 1981, 43, 35–49. (16) Bowman, C. T. A shock-tube investigation of the high-temperature oxidation of methanol. Combust. Flame 1975, 25, 343–354. (17) Wiser, W. H.; Hill, G. R. A kinetic comparison of the combustion of methyl alcohol and methane. Fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1955; p 553.
(18) Westbrook, C. K.; Dryer, F. L. A comprehensive mechanism for methanol oxidation. Combust. Sci. Technol. 1979, 20, 125–140. (19) Tsuboi, T.; Hashimoto, K. Shock tube study on homogeneous thermal oxidation of methanol. Combust. Flame 1981, 42, 61–67. (20) Yano, T.; Ito, K. Behavior of methanol and formaldehyde in burned gas from methanol combustion. Bull. JSME 1983, 26, 94–101. (21) Tsang, W. Chemical kinetic data base for combustion chemistry (Part 2) methanol. J. Phys. Chem. Ref. Data 1987, 16, 471–620. (22) Norton, T. S.; Dryer, F. L. Some new observations on methanol oxidation chemistry. Combust. Sci. Technol. 1989, 63 (1), 107–129.
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excellent agreement was achieved. Egolfopoulos et al. proposed a detailed comprehensive mechanism based on the measurements of premixed laminar flame speeds, and agreements attained for both the laminar flame and atmosphericpressure flow reactor data. Unfortunately, obvious scatterings could be observed in the predictions against Bowman’s shock tube measurements. The model of Grotheer et al.24 includes 43 reaction species, and 414 reaction routines, in which some reactions with a strong influence on laminar flame speeds had been introduced, by means of a sensitivity analysis; as a result, this model could exhibit good performance in the calculation of laminar flame speeds, as well as the prediction of a spontaneous combustion phenomenon in a spark ignition engine. The skeletal model proposed by Chen13 in which only five global reactions are involved is fairly simple. Results showed that this five-step mechanism would lead to relatively high flame temperature prediction for diffusion flames, and a great uncertainty in predicting flame flammability limits could be observed.14 Until now, the model of Held and Dryer1 could be considered as one of the most perfect comprehensive methanol oxidation mechanisms, in which 22 species and 89 reactions were involved. This mechanism had been verified by a wide variety of experimental measurements, such as experimental measurements of the static reactor, flow reactor, shock tube, and premixed laminar flame. The methanol mechanism of Lindstedt and Meyer14 is a skeletal mechanism as well. It was simplified on the basis of the kinetic mechanism of low-alkane oxidation proposed by Lindstedt and Skevis,25,26 in which 19 reaction steps and 17 active species were involved. Using this model, acceptable agreements were attained in the comparisons of laminar flame speeds and ignition delay times between experimental measurements and simulations. The disadvantage of this mechanism is its failure to predict the concentrations of some intermediate species, such as CH3 and HCO. Li et al.27 proposed a comprehensive methanol oxidation mechanism on the basis of the model of Held and Dryer, in which some reaction rate constants and thermochemical data for OH, HO2, and CH2OH had been updated. Compared to the model of Held and Dryer, this mechanism improved the reproduction of experimental results significantly, over a wide temperature range.
Figure 1. Schematic pathways of methanol oxidation. The dashed line indicates the subordinate reaction routine.
secondary products, such CH2OH, CH3O, and CH3, can be transformed to CH2O, following their self-thermal decomposition pathway and dehydrogenation reactions with oxygen. CH2O is the most important intermediate product during methanol oxidation. Almost all of the C atoms in methanol would lead to the production of CH2O. This generation of CH2O dominates the oxidation routine for methanol decomposition, while very small amounts of C2H6, C2H5, and CH4 could be produced from some secondary reaction routines. CH2O might react with radicals H, OH, and O and absolutely lead to HCO. The consumption of HCO is mainly through its thermal decomposition pathway and reaction with O2 and would result in CO. Finally, oxidation reactions occur between CO and OH until the ultimate product CO2 appears. For the methanol reduction mechanism in this study, the mechanisms of Held and Dryer1 and Li et al.27 are selected as the starting points for simplification because of their relative comprehensiveness and proven applicability. Note that the mechanism reduction process should have started with a more detailed model. However, the process would have produced a similar reduced mechanism that included the same essential species and reaction pathways. Here, a five-step method has been used to implement this reduction. The detailed information about this five-step methodology can be found in the literature,28 while a concise depiction appears here: (1) identifying the most essential species and reaction steps of the base model by means of the analysis of reaction paths, (2) eliminating unimportant species and reactions from the base mechanism to compile a new model, and (3) running the SENKIN solution29 to obtain the change rates of some important species concentration histories like those predicted by the model of Held and Dryer under shock tube experimental conditions. Herein, radicals O, OH, and H2O2 have been taken into account. Subsequently, adding or removing reactions from the new mechanism limits only 5% of fluctuations in the rate of change for O, OH, and H2O2 concentration histories. The final step is using a microgenetic algorithm (uGA) optimization technique28,30 to optimize the reaction rate constants to match the ignition delay time of the reduced
3. Developments of the Reduced Mechanism The understanding of reaction routines is of crucial importance for the compilation of a reaction mechanism. Shown in Figure 1 is a skeletal summary of methanol oxidation pathways. The initiation of methanol decomposition can be marked by the occurrence of a dehydrogenation reaction. During this stage, methanol is attached by some radical species, such as H, O, OH, and HO2, and ∼70-75% would be converted to hydroxymethyl radical (CH2OH) and only ∼25-30% to methoxy radical (CH3O). Subsequently, the (23) Egolfopoulos, F. N.; Du, D. X.; Law, C. K. A comprehensive study of methanol kinetics in freely-propagating and burner-stabilized flames, flow and static reactors, and shock tubes. Combust. Sci. Technol. 1992, 83 (1), 33–75. (24) Grotheer, H. H.; Kelm, S.; Driver, H. S. T. Elementary reactions in the methanol oxidation system. Phys. Chem. Chem. Phys. 1992, 96, 1360. (25) Lindstedt, R. P.; Skevis, G. Chemistry of acetylene flames. Combust. Sci. Technol. 1997, 125, 73–137. (26) Lindstedt, R. P.; Skevis, G. Molecular growth and oxygenated species formation in laminar ethylene flames. Twenty-eighth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 2000; Vol. 28, pp 1801-1807.
(27) Li, J.; Zhao, Z. W.; Kazakov, A. A comprehensive kinetic mechanism for CO, CH2O, and CH3OH combustion. Int. J. Chem. Kinet. 2007, 39 (3), 109–136. (28) Patel, A.; Kong, S. C.; Reitz, R. D. Development and validation of a reduced reaction mechanism for HCCI engine simulations. SAE Technical Paper 2004-01-0558, 2004. (29) Kee, R. J.; Rupley, F. M.; et al. Chemkin Collection, release 3.6; Reaction Design Inc.: San Diego, 2000. (30) Krishnakumar, K. SPIE 1196, intelligent control and adaptive systems, 1989.
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model with those of base mechanism with an accuracy of 95%. The fifth step discussed in ref 27, assimilating two or more reactions into a single reaction, is not implemented in this work, as some petit reactions had been integrated into the base mechanism. Generally, the first four steps are still simple processes, which can be implemented by means of comparative analysis for changes of some radicals combining the SENKIN solution. The most important step for optimization methodology is the last step, using a microgenetic algorithm to determine the kinetic parameters to be adjusted in our model. The uGA has been extensively used to optimize model constants in many studies.29,30 Its application process starts with an initial set of five citizens (i.e., each with a specified set of reaction rate constants). By comparison of their agreements obtained by the objective function, the most fit citizen carried directly to the next generation while the next four citizens are used to generate new citizens through a tournament strategy and crossover operation until the operating characteristics of the citizens converge. Then, the four citizens of secondary generation are randomly selected, and the optimization process must be repeated to provide a model more accurate than the previous one. In this study, the SENKIN solution is integrated into the uGA code to provide the change rates of some important species concentration histories. The improvement is quantified in terms of an objective function that is defined as28 N X τR - τH&L f ¼ ð1Þ ð-100Þ τR i¼1
CH2OH is a primary product when methanol is oxidized, and its main reaction pathway is the self-thermal decomposition, R26 (CH2OH þ M = CH2O þ H þ M), and the dehydrogenation reaction with oxygen, R27 (CH2OH þ O2 = CH2O þ HO2). Reactions R26 and R27 can result in the same intermediate product of CH2O. However, the consumption paths for CH3O are drastically different from those of CH2OH. The methoxy radicals either react with H, R30 (CH3O þ H = CH3 þ OH), or thermally selfdecompose, R29 (CH3O þ M = CH2O þ H þ M), in which CH3 or CH2O, respectively, is produced. In these reaction routes described above, CH3 exists at a low concentration because it is produced only by the partial consumption of CH3O, whose concentration is significantly lower than that of CH2OH. In the reduced model, three reactive pathways are introduced to present CH3 consumption, i.e., CH3 reacts with O and HO2 and leads to CH2O, CH3O, and CH4. These reactions are R22 (CH3 þ O =CH2O þ H), R23 (CH3 þ HO2=CH3O þ OH), and R24 (CH3 þ H þ M= CH4 þ M), while elementary reactions of H and CH3, which lead to C2 hydrocarbon, have been neglected because their oxidation rates are very slow in methanol flames.23 3.2. CH2O Submechanism. Formaldehyde (CH2O) oxidation kinetics is of great importance for the oxidation of large hydrocarbon and oxygenated hydrocarbon fuels. Under most circumstances, nearly all of the carbons in these species would be oxidized through a route involving formaldehyde. Egolfopoulos et al.23 studied the pathway of CH2O consumption in methanol flames, and results showed that CH2O reacts primarily with OH, H, and O; all reactions lead to HCO production, especially for lean methanol and air flame, and ∼80% CH2O would be converted to HCO by means of reaction R20 (CH2O þ OH=HCO þ H2O). It was reported that the HO2 concentration presented in a methanol flame remains at a very high level as well.33 However, formaldehyde oxidation is also sensitive to abstraction reaction R21 (CH2O þ HO2=HCO þ H2O2) under flow reactor conditions.34 Thus, eight elementary reactions relevant to CH2O oxidation in the previous model are substituted by the following reactions in the mechanism of Li.27,31 Those are R18 (CH2O þ H =HCO þ H2), R19 (CH2O þ O = HCO þ OH), R20 (CH2O þ OH = HCO þ H2O), and R21 (CH2O þ HO2 = HCO þ H2O2). 3.3. CO/H2/O2 Submechanism. The carbon monoxide/ hydrogen/oxygen reaction kinetics model used in this study is mainly taken from the reports of Li et al.,27,33 Yetter et al.,34 and others.35-39 In this submodel, the reactions related to
where τR values are the ignition delays predicted by the reduced mechanism and τH&L are the values computed by the model of Held and Dryer or Li et al. N is the number of different initial conditions for which the optimization code is executed. The objective function provides a measure of the closeness within the ignition delay computations and is used by the uGA code to direct its optimization process. Following the procedures described above, a reduced kinetic mechanism, containing 40 reactions and 17 active species, can be obtained. As mentioned above, methanol oxidation routes are mostly similar to those of large hydrocarbon and other oxygenated hydrocarbon fuels. That is to say, methanol oxidation can be considered as an aggregation of a series of standard consecutive programs, as described below. 3.1. CH3OH Submechanism. In this study, the submodel for CH3OH decomposition is mainly established on the basis of the models of Li et al.27,31 and Held.32 Generally, there are three main routines to describe the initialization of methanol oxidation. When methanol is attached by radical species, such as H, O, OH, and HO2, some important species in the reduced mechanism, such as hydroxymethyl radical (CH2OH) and methoxy radical (CH3O), are introduced, while some minor intermediates, such as formic acid (HCOOH) and 1, 2-ethanedioyl (ethylene glycol, HOC 2H4OH), have been removed from the reduced model, because of their small amounts and weak influence on oxidation progress. In this developed model, only seven reactions are used to present the occurrence of methanol decomposition (R34-R40), and the reaction rate constants for reactions relevant to the generation of CH2OH from CH3OH attacked by radicals of H and O (R34 and R36) are updated to the latest reported data.27,31
(33) Li, J.; Zhao, Z.; Kazakov, A.; Dryer, F. L. An updated comprehensive kinetic model of hydrogen combustion. Int. J. Chem. Kinet. 2004, 36, 566–575. (34) Yetter, R. A.; Dryer, F. L.; Rabitz, H. A comprehensive reaction mechanism for carbon monoxide/hydrogen/oxygen kinetics. Combust. Sci. Technol. 1991, 79 (1), 97–128. (35) Michael, J. V.; Sutherland, J. W. Rate constants for the reactions of hydrogen atom with water and hydroxyl with hydrogen by the flash photolysis-shock tube technique over the temperature range 1246-2297 K. J. Phys. Chem. 1988, 92, 3853–3857. (36) Cribb, P. H.; Dove, J. E.; Yamazaki, S. A kinetic study of the pyrolysis of methanol using shock tube and computer simulation techniques. Combust. Flame 1992, 88 (2), 169–185. (37) Hessler, J. P. Calculation of reactive cross sections and microcanonical rates from kinetic and thermochemical data. J. Phys. Chem. A 1998, 102, 4517–4526. (38) Cobos, C. J.; Hippler, H.; Troe, J. High-pressure falloff curves and specific rate constants for the reactions atomic hydrogen molecular oxygen hydroxyl atomic oxygen. J. Phys. Chem. 1985, 89, 342–349. (39) Tsang, W. Chemical kinetic data base for combustion chemistry (Part 1) methane and related compounds. J. Phys. Chem. Ref. Data 1986, 15 (3), 1087–1279.
(31) Li, J. Ph.D Thesis, Princeton University, Princeton, NJ, 2004. (32) Held, T. J. Ph.D. Thesis, Princeton University, Princeton, NJ, 1993.
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Table 2. Reduced Mechanism for Methanol Oxidationa reaction
E
ref
3.547 10 -0.406 16599.0 33 5.01 104 2.67 6290.0 32 2.16 108 1.51 3430.0 30 3.80 1022 -2.0 0.0 34 enhanced third-body efficiencies: 2.5:12:1.9:3.8:0.38 H2/H2O/CO/CO2/AR 1.475 1012 0.60 0.0 35 R5 H þ O2 (þM) = HO2 (þM) -1.72 524.8 31 low-pressure limit 6.366 1020 TF parameter: a = 0.8, T*** = 1 10-30, T** = 1 1030; enhanced third-body efficiencies: 2.0:11:0.78:1.9:3.8 H2/H2O/O2/CO/CO2 R6 HO2 þ H = H2 þ O2 6.63 1013 0.0 2130.0 36 7.079 1013 0.0 295.0 37 R7 HO2 þ H = OH þ OH 1.74 1013 0.0 -400 32 R8 HO2 þ O = O2 þ OH 1.45 1016 -1.0 0.0 32 R9 HO2 þ OH = H2O þ O2 12 3.02 10 0.0 1390.0 3 R10 HO2 þ HO2 = H2O2 þ O2 1.20 1017 0.0 45500.0 32 R11 H2O2 þ M = OH þ OH þ M 2.41 1013 0.0 3970.0 36 R12 H2O2 þ H = H2O þ OH 7.08 1012 0.0 1430.0 32 R13 H2O2 þ OH = HO2 þ H2O 5 2.229 10 1.89 -1158.7 27 R14 CO þ OH = CO2 þ H 11 0.7 14874.0 27 R15 HCO þ M = H þ CO þ M 4.75 10 enhanced third-body efficiencies: 2.5:12:1.9:3.8 H2/H2O/CO/CO2 R16 HCO þ O2 = CO þ HO2 1.42 1013 0.0 410.0 23 9.03 1013 0.0 0.0 41 R17 HCO þ H = CO þ H2 14 1.00 10 0.0 4928 39 R18 CH2O þ H = HCO þ H2 1.81 1013 0.0 3080.0 39 R19 CH2O þ O = HCO þ OH 3.43 109 1.2 -447.0 42 R20 CH2O þ OH = HCO þ H2O 4 4.11 10 2.5 10210.0 43 R21 CH2O þ HO2 = HCO þ H2O2 7.83 1013 0.0 0.0 39 R22 CH3 þ O = CH2O þ H 2.41 1010 0.8 -2325.0 31 R23 CH3 þ HO2 = CH3O þ OH 1.27 1016 -0.6 383.0 27 R24 CH3 þ H (þM) = CH4 (þM) 33 -4.760 2440.00 27 low pressure limit 1.477 10 TF parameter: a = 0.7830, T*** = 74.00, T* = 2941.00, T** = 6964.00; enhanced third-body efficiencies: 2.0:6.0:2.0:1.5:2.0:0.7 H2/H2O/CH4/CO/ CO2/AR 5.72 106 2.0 2639.0 44 R25 CH4 þ OH = CH3 þ H2O 1.00 1014 0.0 25100.0 31 R26 CH2OH þ M = CH2O þ H þ M 14 2.41 10 0.0 5017.0 45 R27 CH2OH þ O2 = CH2O þ HO2 9.635 1013 0.0 0.0 39 R28 CH2OH þ H = CH3 þ OH 8.30 1017 -1.2 15500 46 R29 CH3O þ M = CH2O þ H þ M 3.20 1013 0.0 0.0 47 R30 CH3O þ H = CH3 þ OH 13 9.03 10 0.0 11980.0 47 R31 CH3O þ O2 = CH2O þ HO2 4.68 102 3.2 5380.0 47 R32 CH3O þ CO = CH3 þ CO2 2.79 1018 -1.4 1330.0 27 R33 OH þ CH3 (þM) = CH3OH (þM) -5.920 3140.00 27 low pressure limit 4.00 1036 TF parameter: a = 0.4120, T*** = 195.0, T* = 5900.00, T** = 6394.00; enhanced third-body efficiencies: 2.00:6.00:1.50:2.00 H2/H2O/CO/CO2 R34 CH3OH þ H = CH2OH þ H2 3.20 1013 0.0 6095.0 27 8.00 1012 0.0 6095.0 27 R35 CH3OH þ H = CH3O þ H2 3.88 105 2.5 3080.0 31 R36 CH3OH þ O = CH2OH þ OH 6 1.00 10 2.1 496.7 32 R37 CH3OH þ OH = CH3O þ H2O 6 7.10 10 1.8 -596.0 32 R38 CH3OH þ OH = CH2OH þ H2O 2.05 1013 0.0 44900.0 31 R39 CH3OH þ O2 = CH2OH þ HO2 3.98 1013 0.0 19400.0 42 R40 CH3OH þ HO2 = CH2OH þ H2O2 R1 R2 R3 R4
a
H þ O2 = O þ OH O þ H2 = H þ OH H2 þ OH = H2O þ H H þ OH þ M = H2O þ M
β
A 15
Where k = ATβ exp(-E/RT), with A in units of moles per cubic centimeter per second per kelvin and E in units of calories per mole.
coefficients and corresponding references.23,30-47 Compared to the base mechanisms of Held and Dryer1 and Li et al.,27
species of Ar and He have been neglected and reaction rate constants for R3-R6 are modified according to the study of Li.31 Generally, four different routes are possible for carbon monoxide (CO) consumption. CO might react with radicals of O, O2, OH, and HO2 and ultimately lead to CO2 generation. Reaction R14 (CO þ OH = CO2 þ H) is the main CO oxidation reaction, producing heat and H radicals. Egolfopoulos et al.23 reported that reaction R1 (H þ O2 = O þ OH) has a very strong positive influence on methanol oxidation for lean methanol flames, because of its chain branching nature. It was reported that R14 is a dominated reaction as well,32 no matter under lean or rich methanol/ oxygen mixture conditions. As a result, CO can be assumed to be an abundant “final product” species, and its rate of oxidation is no longer a key factor .Therefore, reaction of CO with OH had been considered as the only route of CO oxidation in the proposed model, to minimize the model size. The detailed information of this developed methanol oxidation mechanism can be found in Table 2, with the rate
(40) Mueller, M. A.; Kim, T. J.; Yetter, R. A.; Dryer, F. L. Flow reactor studies and kinetic modeling of the H2/O2 reaction. Int. J. Chem. Kinet. 1999, 31 (2), 113–125. (41) Norton, T. S.; Dryer, F. L. Toward a comprehensive mechanism for methanol pyrolysis. Int. J. Chem. Kinet. 1990, 22, 219–241. (42) Vasudevan, V.; Davidson, D. F.; Hanson, R. K. Direct measurements of the reaction OH þ CH2O f HCO þ H2O at high temperatures. Int. J. Chem. Kinet. 2005, 37 (2), 98–109. (43) Friedrichs, G.; Davidson, D. F.; Hanson, R. K. Validation of a thermal decomposition mechanism of formaldehyde by detection of CH2O and HCO behind shock waves. Int. J. Chem. Kinet. 2004, 36 (3), 157–169. (44) Felder, W.; Madronich, S. High temperature photochemistry: Kinetics and mechanism studies of elementary combustion reactions over 300-1700 K. Combust. Sci. Technol. 1986, 50, 135–150. (45) Grotheer, H. H.; Riekert, G.; Walter, D.; Just, T. Non-Arrhenius behavior of the reaction of hydroxymethyl radicals with molecular oxygen. J. Phys. Chem. 1988, 92, 4028–4030. (46) Page, M.; Lin, M. C.; He, Y.; Choudhury, T. K. Kinetics of the methoxy radical decomposition reaction: Theory and experiment. J. Phys. Chem. 1989, 93, 4404–4408.
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Figure 2. Normalized first-order sensitivity coefficients of the most important reactions to the laminar flame speed of lean, stoichiometric, and rich methanol/air flames at 318 K and 0.1 MPa.
the number of reactions and the number reactive species have been sizably reduced from 89 and 22 to 40 and 17, respectively. 4. Validation of the Reduced Kinetic Mechanism In this section, a series of experimental data, obtained from flow reactors, static reactors, shock tubes, and laminar flames, are selected to verify the applicability of the reduced kinetic mechanism over broad mixture conditions. The characteristic parameters, such as laminar flame speeds, ignition delay times, and intermediate product concentration profiles in flow reactors, static reactors, and premixed laminar flames, are predicted using the present reduced model, to make comparisons against experimental measurements and calculations of other mechanisms reported previously. The simulations are conducted by using the Chemkin package program,29 which solves the conversation equations of mass, momentum, energy, and each chemical species, and this code has been widely used in a range of computational studies of combustion in laminar flames, flow reactors, static reactors, and shock tubes. The detailed information for fundamental modeling assumptions for these experiments can be found elsewhere,1,23,29,31 while the outlines can be summarized as follows. Static reactor experiments are generally used to study low-temperature oxidation chemistry, in which constant-volume and spatially homogeneous mixtures are prepared for reaction and the reaction time is assumed to be much longer than the characteristic thermal and mass diffusion times to the reactor walls. Flow reactor experiments are generally implemented in the temperature range of 800-1200 K. The assumptions of flow reactor experiments are as follows: constant pressure, adiabatic, and zero-dimensional mixture conditions, in which the flow is assumed to be essentially an entry-region flow and axial diffusion is negligible where the characteristic diffusion length was much greater than the convective length. Reactortype experiments described above generally provide information about the low and intermediate temperature ranges, while shock tube studies complement these experiments by operating in the higher temperature range. For the simulation of
Figure 3. Computed and measured laminar flame speeds vs mixture equivalence ratio φ, where highlighted portions indicate the zones of (5 cm/s burning velocity vs the predictions of the reduced model.
static reactors, flow reactors, and shock tubes, the SENKIN29 code in the Chemkin software package was used to predict these homogeneous gas phase chemical kinetics by integrating the time-dependent conservation equations of species and energy in this study. While the freely propagating, one-dimensional, constant-pressure adiabatic flame version of PREMIX29 is used to predict the premixed laminar flame speed, laminar flame species profiles are determined by using PREMIX with
(47) Wantuck, P. J.; Oldenborg, R. C.; Baughcum, S. L.; Winn, K. R. Removal rate constant measurements for methoxy radical by oxygen over the 298-973 K range. J. Phys. Chem. 1987, 91, 4653–4655.
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Figure 4. Shock tube ignition delay calculations under the conditions described by Bowman.16
the burner-stabilized flames option. Detailed information about the use of these codes can be found in the literature,29 and comments about the validity of the reduced mechanism are given here. 4.1. Modeling Laminar Flame Speed in Premixed Flames. The laminar flame speed is a characteristic response of a given
combustible mixture and embodies the fundamental diffusive, reactive, and exothermic mixture properties. The flame speed is dependent on a large number of kinetic parameters. Many studies1,23 have reported that the radical H is crucially important for flame propagation, and the H þ O2 branch tunnel (R1) is one of the controlling reactions. Plotted in 66
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Figure 5. Normalized first-order sensitivity coefficients of the most important reactions on the ignition delay time for mixture 1 in the Bowman experiment16 at different temperatures.
Figure 2 are the first-order sensitivity coefficients of the most important reactions on the laminar flame speeds under lean, stoichiometric, and rich mixture conditions. Clearly, those reactions, such as R7, R14, R15, and R37, show a stronger positive sensitivity in flame propagation as well. Reaction R14 (CO þ OH = CO2 þ H) is the main CO oxidation reaction, producing heat and H radical, leading to an enhancement of reaction R1. For lean flames, its great sensitivity to flame speed dominates over that of R1 because there is enough O2 to oxidize CO to CO2, while its importance gradually declines when the mixture stoichiometry becomes richer because of the decreasing concentrations of O2.23 Formyl radical decomposition reaction R15 (HCO þ M = H þ CO þ M) presents another important source of H-atoms to the premixed methanol flame system, aside from R14 (CO þ OH = CO2 þ H). Its preference for flame propagation is due to its significant product of radical H as well. For R37 (CH3OH þ OH=CH3O þ H2O), we can see that relatively small positive sensitivity coefficients have been observed for all stoichiometric methanol mixtures. Reaction R38 (CH3OH þ OH=CH2OH þ H2O), however, produces a contrary behavior. CH3O, the product of R37, produces H atoms, following thermal decomposition reaction R29 (CH3O þ M=CH2O þ H þ M), while CH2OH, the product of R38, may react with O2 (R28, CH2OH þ O2 = CH2O þ HO2), leading to CH2O. R28 can retard propagation as compared to the CH3O path (R29), which leads directly to the more reactive CH2O. Metghalchi and Keck,7 Saeed and Stone,9 Egolfopoulos et al.,23 and Liao et al.5 conducted laminar flame speed measurements for methanol flames at various equivalence ratios, temperatures, and pressures. Figure 3a presents a comparison of these experimental data with computed results for one-dimensional laminar premixed methanol flames using the current scheme. Obviously, acceptable agreements have been attained for nearly all experimental conditions. This indicates that the reduced model can well predict the effects of initial temperature and mixture equivalence ratio on laminar flame speed. When the mixture equivalence ratio is ∼1.15, the predicted laminar flame speed reaches its peak value, which is nearly consistent with the experimental results. It
was also worth noting that better agreement between experimental and numerical data can be observed at a low temperature, i.e., 318 K, while at the higher initial temperatures, such as 368 and 480 K, slightly larger discrepancies are found; however, overall errors still remain at a low level. Shown in Figure 3b is the comparison of the reduced mechanism with some classical models in predicting laminar burning velocities. To illustrate the difference of dominant models in simulating laminar flame propagation, some experimental measurements have been plotted as well. In this figure, there is some obvious scatterings in the experimental data, because of the uncertainties associated with the experimental technique used.6 However, we can see that, whether for a lean or rich mixture, the reduced mechanism produces an acceptable agreement with the predictions of other models, and a majority of calculated values are close to those of experimental measurements within the discrepancy of a (5 cm/s burning velocity, as indicated by highlighted portions of Figure 3b. Certainly, a spot of data, obtained by Metghalchi and Keck7 and Saeed and Stone9 under rich mixture conditions, are exceptions. That is to say, more information about the temperature dependencies of some key reactions should be needed to access their causes. Despite the laminar flame speed being only a global property of the flame, the agreements of the modified modeling versus most experiments can still be considered as a validation of the flame kinetics skeleton proposed here. It also should be noted that reactions relevant to C2 hydrocarbons have been ignored in this model, to minimize the reaction matrix as much as possible. Luckily, Figure 3b shows that this simplified procedure has no obvious effect on the flame speed propagation as well, compared to other mechanisms including the C2 submodel, such as the model of Held and Dryer1 or the model of Li et al.27 This reveals that C2 hydrocarbon pathway is far from methanol mainstream reaction channels and C2 hydrocarbons are fairly rare species in methanol flames, which quantitatively demonstrates the lower propensity of methanol to soot. 4.2. Modeling Ignition Delays in Shock Tubes. Bowman16 investigated methanol ignition behavior in a shock tube in an initial temperature range of 1545-2180 K, in a pressure 67
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range of 0.18-0.46 MPa, and in an equivalence ratio range of 0.375-6.0. In his study, the ignition delay time was defined as the elapsed time between the time when the shock wave compresses the mixture to the time at which the peak CO and O concentrations are attained. Given in Figure 4 is a comparison of calculated ignition delays versus Bowman’s measured results, where three different methanol/air mixtures are simulated, and the predictions based on the mechanism of Held and Dryer and the scheme of Li et al. have been plotted as well. Obviously, the temperature has a great influence on methanol autoignition. The mixture ignition delay time remarkably decreases with the increase in initial temperature. From the comparison, it can be said that the calculated results of the reduced model show excellent agreement with those of Held and Dryer, as shown in Figure 4, and the comprehensive mechanism of Egolfopoulos et al. has illustrated the best performance in predicting methanol autoignition versus experimental measurements. However, note that there is no large difference in the calculated results among these kinetic models, and all discrepancies are completely within the acceptable level. That is to say, the developed model can predict methanol autoignition behavior as well as be used to simulate the ignition process of methanol at high temperatures. Detailed sensitivity analysis is conducted to discuss the most likely reactions that might be controlling reactions in determining methanol autoignition. Plotted in Figure 5 are the sensitivity coefficients of the most important reactions. The results are nearly consistent with the importance of HO2 and H2O2 during the early oxidation of methanol.1,18,22 Generally, the five most sensitive reactions are as follows: H þ O2 =O þ OH (-) (R1), HO2 þ H=H2 þ O2 (þ) (R6), HO2 þ HO2 = H2O2 þ O2 (þ) (R10), H2O2 þ M = OH þ OH þ M (-) (R11), and CH3OH þ HO2=CH2OH þ H2O2 (-) (R40) (the plus or minus sign indicates the reaction might present a positive or negative influence, respectively, on the increase in the methanol ignition delay time). The initial temperature plays an important role in controlling reaction branching. As shown in Figure 5, the sensitivity coefficients of the five reactions vary with the variation in initial temperature, which demonstrates that some different reaction routines might exist during methanol oxidation at different temperatures. The abstraction reaction with HO2 (R40) does not significantly contribute directly to the fuel destruction; the fuel concentration, however, is most sensitive to mixture autoignition, because it is a main source of hydrogen peroxide (H2O2). Most hydrogen peroxide is consumed by means of thermal decomposition reaction R11 (H2O2 þ M = OH þ OH þ M). Reaction R10 (HO2 þ HO2 = H2O2 þ O2) is hence a competitive reaction of R40 as its snatching HO2. At a relative low initial temperature, abstraction reactions with OH, R37 (CH3OH þ OH = CH3O þ H2O) and R38 (CH3OH þ OH=CH2OH þ H2O), are the dominant reactions of methanol consumption; it is hence easy to understand the decrease in the ignition delay with the promotion of R11 and R40 and the positive sensitivity of R10. When the initial temperature is above 1400 K, the branching reactions of methanol oxidation with O and H, R34 (CH3OH þ H = CH2OH þ H2), R35 (CH3OH þ H=CH3O þ H2), and R36 (CH3OH þ O=CH2OH þ OH), can be strengthened, and then the roles of reactions relevant to radical H and O, such as R1 and R6, become obvious. 4.3. Modeling Species Profile in Static Reactors. Static reactors are typically used to study low-temperature oxidation
Figure 6. Species profiles for static reactor experiments of Cathonnet et al.,48 where symbols denote experimental measurements and curves modeling results.
chemistry. To verify the accuracy of the current mechanism in describing the methanol low-temperature oxidation process, a set of experimental data reported by Cathonnet et al.48 have been referenced to make a comparison with the modeling simulation. The experiments were conducted at 823 K and an initial pressure of 0.0263 MPa, with mixture equivalence ratios from 0.5 to 2.0. Figure 6 plots the comparison of the experimental measurements with these modeling results. From this figure, we can see that the reduced model can correctly predict methanol consumption rates and intermediate product concentrations in static reactors. It is (48) Cathonnet, M.; Boettner, J. C.; James, H. Etude de loxydation et de l auto-inflammation du methanol dans le domaine de temperatures 500-600 °C. J. Chim. Phys. Phys.-Chim. Biol. 1982, 79, 475.
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Figure 8. Reaction profiles of CH3OH/air mixtures in a flow reactor, where symbols represent the experimental data of Held and Dryer49 and curves are predictions of the reduced model.
Figure 7. Reaction profiles of CH3OH/air mixtures in a flow reactor, where symbols represent the experimental data of Norton and Dryer27 and curves are predictions of the reduced model.
are still realized in predictions of H2, CO2, and CH2 O. Because Egolfopoulos et al.23 mentioned that there were some uncertainties associated with the measurements, further attempts to assess the cause of these discrepancies are not made herein. 4.4. Modeling Species Profile in Flow Reactors. Experimental measurements conducted in a flow reactor are commonly used to study fuel chemical kinetics in temperature range from 800 to 1200 K, which essentially bridges the temperature gap between static reactor and shock tube experiments. Norton and Dryer22 implemented an experiment at atmospheric pressure in which both rich and lean methanol mixtures were tested. Afterward, Held and Dryer49 extended the previous experimental pressure conditions and decreased the reactive temperature range from 752 to 1043 K. These experimental measurements yield detailed information about stable species concentration profiles and temperature behavior for methanol oxidation in a flow reactor. In this study, the reduced mechanism calculation is conducted for the flow reactor to validate these experiments. The comparisons of the experimental data of Norton and Dryer against the numerically computed species concentrations on a mole basis are plotted for O2, CO, CH3OH, CO2, and CH2O in Figure 7. In general, the modeling results show good agreement with the experimental measurements for both lean and rich methanol/air mixtures. The predicted results can be considered as the replications of experimental
known that the overall low-temperature reaction mechanism for methanol is quite simple. The low-temperature oxidation of methanol is initiated from reaction R39 (CH3OH þ O2 = CH2OH þ HO2). The propagation reactions include reactions R27 (CH2OH þ O2 =CH2O þ HO2), R38 (CH3OH þ OH=CH2OH þ H2O), R21 (CH2O þ HO2=HCO þ H2O2), R16 (HCO þ O2 = CO þ HO2), R37 (CH3OH þ OH = CH3O þ H2O), and R20 (CH2O þ OH = HCO þ H2O). In the previous studies,1,27 comparable results were obtained in the modeling of methanol oxidation in static reactors, because the same set of rate constants have been used in their low-temperature reaction mechanisms. In this reduced model, HCO sub-branching reactions are simplified by three steps of skeleton reactions of R15-R17, instead of 10 reactions in the studies of Li et al.27 and Held and Dryer.1 Besides, rate constants of R16 are modified on the basis of the study of Egolfopoulos et al.23 Its Arrhenius number changes from 7.58 1012 to 1.42 1013, with an increased amplitude of ∼1.873. Simulations based on the mechanism of Held and Dryer and that of Egolfopoulos et al. are plotted in Figure 6. Obviously, the reduced model presents a relatively fast rate of methanol consumption, primarily because of the rapid reaction rate of R16. This can lead to a relatively higher concentration of HO2 and consequently promote the overall reaction process. These calculations also demonstrate some discrepancies in the late stage versus experimental data, especially for CO species, because the isothermal hypothesis in simulations is not available experimentally. However, it is noted that Figure 6c illustrates that the reduced model shows the more obvious discrepancies in modeling methanol consumption and CO production, whereas good agreements
(49) Held, T. J.; Dryer, F. L. An experimental and computational study of methanol oxidation in the intermediate-and high-temperature regimes. Twenty-fifth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1994; Vol. 25, pp 901-908.
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Figure 9. Calculated and measured species profiles for premixed laminar flame experiments of Vandooren and Van Tiggelen,50 where symbols represent the experimental data and curves are predictions of the reduced model. HAB means height above burner.
measurements at the early stage of methanol oxidation to some extent, while at the late stage of methanol oxidation, the predicted results show a slight deviation versus the experimental data, especially for the temperature profiles. Such discrepancies are found in the comparisons between
experiments and predictions of the mechanism of Held and Dryer1 and the model of Egolfopoulos et al.23 These are possibly due to the difficulty of the flow reactor experiment being conducted under a completely adiabatic condition like that of the theoretical model. The enhanced heat losses make 70
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the experimental temperature decrease at the late stages of oxidation, subsequently leading to an experimental deceleration of methanol oxidation rate and consequently the relatively large deviations of T, CH3OH, and CO2 within the measurements and predictions. However, in the study of Held and Dryer,49 experimental data have been reproduced quite well by the model, as shown in Figure 8, because the experimental uncertainty in temperature has been well-restricted. Figure 8a illustrates some important intermediate products versus the residual time delay at a pressure of 0.25 MPa and a mixture equivalence ratio of 0.83. We can see that the calculated consumption rates for CH3OH and O2 are fully consistent with the experimental data. Even if there were any deviations for other predicted products, those are also within experimental uncertainty. Similarly, at an elevated pressure of 1.5 MPa, close agreement is realized for many species as well, as shown in Figure 8b, but it should be noted that relatively large deviations remain in predicted CH2 O consumption at the early stage of reaction. The analysis of the reactive pathway has demonstrated that CH2O reacts primarily with OH, H, and O, and all lead to HCO production. In this model, four elementary reactions relevant to CH2O oxidation (R18-R21) are compiled to replace the traditional eight reactions in the model of Held and Dryer1 and to simplify the overall reaction mechanism to a great extent, while elementary reactions relevant to CH2O decomposition and attacked by O2 and CH3 have been neglected. The selected rate constants for reactions R18-R21 are appropriately fitted ones reported by Tsang,39 Mueller et al.,40 and Vasudevan et al.42 Rate constants in the model of Held and Dryer1 are used to conduct attempts to obtain better agreements for the CH2O profile as well. However, just as mentioned by Held and Dryer,1 obvious discrepancies can be seen in Figure 8 as well. Figure 8 also reports that, with the delay in the residual time in flow reactor experiments, these deviations are diminished gradually. This indicates that the possible imperfection of this scheme in simulating flow reactor experiments is the submodel relevant to its low-temperature oxidation. As there are not much more experimental data available and these discrepancies are still within acceptable ranges, further attempts to assess the cause of these discrepancies have not been made here. It should be noted that previous studies had reported that the classical mechanisms, such as the model of Held and Dryer, the model of Egolfopoulos et al., and the model of Li et al., had showed similar discrepancies.1,23,27 Therefore, the predictions of more mechanisms have not been plotted in the figures mentioned above for comparison, for the sake of clarity, and Figure 9 is plotted in the same way as well. 4.5. Modeling Flame Structure of Premixed Flames. Laminar flame speed is a global property of flame; its sensitivity to certain aspects of flame kinetics can be quite limited. A more stringent test should be used to conduct predictions and
comparisons of flame structures, especially the stable and radical species profiles. Vandooren and Van Tiggelen50 investigated methanol laminar flame structures at mixture equivalence ratios of 0.89, 0.36, and 0.21, by means of molecular beam sampling and mass spectrometry detection techniques. Shown in Figure 9 are predicted concentrations for some important radicals, such as H, O, and OH, and quasi-stable species, such as CH2O, CO, CO2, CH3OH, H2, etc., versus those of experimental measurements. The results show that the reduced model can simulate reactive species for flames studied very well, as close agreements have been observed in the comparison of predictions and experiments. Among these comparisons, the H2 profile demonstrates a slightly larger difference. This phenomenon had been reported by Held and Dryer49, primarily because of the small amounts of H2 and H2 being nonstable product under flame conditions, which is more susceptible to decomposition when an intrusive probing technique is used. Frankly speaking, we find it is very difficult to achieve good agreements between simulations and experiments in premixed flame structure modeling compared to those in a shock tube and flow reactor, no matter what mechanism is used, because the reactive intermediates have greater activity under flame conditions and generally intrusive sampling techniques used to detect species concentrations have some influence on the experimental results. Fortunately, the reduced mechanism can accurately predict reactive radicals H, O, and OH for flames 1 and 2, while for flame 3, an acceptable discrepancy appears, as observed in the prediction of the mechanism of Egolfopoulos.23 5. Conclusions In this investigation, a summary of the current comprehensive methanol oxidation mechanisms is conducted, and the advantages and disadvantages of prevalent models are discussed. From the analysis of the main reaction pathway of methanol oxidation, the crucial reactions are investigated emphatically, and then a new reduced chemical kinetic model is developed to describe methanol oxidation on the basis of some of the previous comprehensive mechanisms. The validation of the developed model is conducted via evaluation of laminar flame speeds, ignition delay times, and reactive species profiles in static reactors, flow reactors, and premixed flames, over much more broader mixture conditions. The results of the optimized reduced model are generally in good agreement with the experimental measurements, and compared with comprehensive mechanisms, the reduced model has many fewer reaction routines involved and gives comparable results in predicting laminar flame speeds, ignition delay times, and reactive species profiles in static reactors, flow reactors, and premixed flames. Acknowledgment. This study was supported by the National Natural Science Foundation of China (50706058), the Natural Science Foundation of Chongqing (CSTC2008BB6147), and the Open Fund from the Key Laboratory of Manufacture and Test Techniques for Automobile Parts (2009KLMT10).
(50) Vandooren, J.; Van Tiggelen, P. J. Experimental investigation of methanol oxidation in flames: mechanisms and rate constants of elementary step. Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; Vol. 18, pp 473-483.
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