Fundamental Study of the Oxidation Characteristics and Pollutant

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Fundamental Study of the Oxidation Characteristics and Pollutant Emissions of Model Biodiesel Fuels Qiyao Feng,† Yang L. Wang,‡ Fokion N. Egolfopoulos,‡ and Theodore T. Tsotsis*,† Mork Family Department of Chemical Engineering and Material Science, and Department of Aerospace and Mechanical Engineering, UniVersity of Southern California, Los Angeles, California 90089

In this study, the oxidation characteristics of biodiesel fuels are investigated with the goal of contributing toward the fundamental understanding of their combustion characteristics and evaluating the effect of using these alternative fuels on engine performance as well as on the environment. The focus of the study is on pure fatty acid methyl-esters (FAME,) that can serve as surrogate compounds for real biodiesels. The experiments are conducted in the stagnation-flow configuration, which allows for the systematic evaluation of fundamental combustion and emission characteristics. In this paper, the focus is primarily on the pollutant emission characteristics of two C4 FAMEs, namely, methyl-butanoate and methyl-crotonate, whose behavior is compared with that of n-butane and n-pentane. To provide insight into the mechanisms of pollutant formation for these fuels, the experimental data are compared with computed results using a model with consistent C1-C4 oxidation and NOx formation kinetics. Introduction The world today faces the dual threats of a potentially crippling energy crisis and of serious, emerging environmental problems associated with the use of fossil fuels. Biodiesel is among the most viable and affordable solutions available today for reducing the world’s dependence on conventional petroleum resources and for diminishing the emissions of global-warming pollutants such as CO2. Biodiesel is a multicomponent mixture of long-chain fatty acid monoalkyl esters (known as FAMEs), derived from vegetable oils and/or animal fats. Biodiesel can be used either pure or as a blend with conventional petroleum diesel. It is thought to be a cleaner fuel than conventional diesel, resulting in lower emissions of CO, CO2, sulfur oxides, and particulate matter (PM). There have been studies in diesel engines of the emissions of PM and nitrogen oxides (NOx) resulting from the combustion of biodiesel and its blends. Most of these studies1-5 demonstrate that PM emissions of blends of biodiesel with conventional diesel decrease noticeably as the biodiesel content increases. However, contradictory conclusions were reached concerning the effect of biodiesel on NOx emissions. Some studies2,6-8 have shown an increase in such emissions when using biodiesel. Others9,10 found no difference between diesel and biodiesel fuels, while a third group of studies4,5,11 even found decreases in NOx emissions. Despite its potential as a practical fuel, the combustion characteristics of biodiesel are neither well characterized nor understood. Therefore, it is essential to advance further the understanding of biodiesel combustion and to evaluate the consequences of its use on both engine performance and the environment. At present, fundamental data and chemical kinetics models describing the behavior of actual biodiesels are not available. However, some studies have appeared on lower * To whom correspondence should be addressed. Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, 925 Bloom Walk, HED 210, Los Angeles, CA 90089-1211. Tel.: (213) 740-2069. Fax: (213) 740-8053. E-mail: [email protected]. † Mork Family Department of Chemical Engineering and Material Science. ‡ Department of Aerospace and Mechanical Engineering.

molecular weight (MW) FAME that are model surrogate compounds of real biodiesel fuels. Fisher et al.,12 for example, published a chemical model for methyl-butanoate (MB) using low-temperature oxidation data in an isothermal static reactor. Gail et al.13 subsequently updated the MB model and used it to simulate experimental results in jet-stirred reactors and in diffusion flames. Dooley et al.14 investigated the autoignition of MB in a shock-tube and produced a kinetic model of MB together with the compositional data in a flow and in a jet-stirred reactor and in an opposedflow diffusion flame. Huynh et al.,15 using theoretical approaches, proposed a kinetic model for MB, which was validated with measurements of CO2 time histories during the pyrolysis of MB in a shock-tube.16,17 More recently, Herbinet et al.18 developed a kinetic model for the oxidation of methyl-decanoate (MD), a longer chain FAME. The extinction and ignition characteristics of nonpremixed MD flames were studied by Seshadri et al.19 in the counterflow configuration, and the experimental data were simulated with a skeletal kinetic model that was deduced from the detailed mechanism of Herbinet et al.18 using the directed relation graph (DRG)20-22 reduction method. Lin et al.23 compared the oxidation pathways of MB and n-butane using an MB kinetic model16,17 combined with a detailed n-heptane mechanism.24,25 It was reported that the primary differences are due to the CO2 production from the ester group in MB through butanoic acid and methyl formate radicals. Metcalfe et al.26 studied the oxidation of MB and ethyl propanoate (EP) in shock-tube experiments and found that EP ignited faster than MB, particularly at lower temperatures. Walton et al.27 studied also the autoignition properties of MB and EP in a rapid compression facility. It was shown that MB consumption is dominated by the relatively slow bimolecular H-atom abstraction reactions, while ethyl-butanoate (EB) combustion is dominated by the faster unimolecular decomposition. Sarathy et al.28 studied the oxidation of MB and methyl crotonate (MC) in an opposed-flow diffusion flame and in a jet-stirred reactor. It was concluded that the unsaturated MC has a greater tendency to soot than the saturated MB. On the basis of the aforementioned considerations, the primary goal of the present work is to study systematically the

10.1021/ie100481q  2010 American Chemical Society Published on Web 06/18/2010

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Figure 1. Top figure: schematic of the experimental system. Bottom figure: photograph of the burners with a flat flame in between them.

fundamental combustion and emission characteristics of both real and model biodiesels and to provide insight into the kinetics differences that are at the root of their combustion performance. Results detailing the effects of carbon chain length and of the degree of saturation on the fundamental flame properties of these fuels will be published elsewhere29 and will be summarized only briefly here. In this paper, instead, the focus will be on the pollutant emission characteristics of two C4-type FAMEs, namely, MB and MC, whose behavior is compared with that of n-butane and n-pentane. To provide insight into the mechanisms of pollutant formation during the oxidation of these fuels, the experimental data are compared with simulations using a model with consistent C1-C4 oxidation and NOx formation kinetics. Experimental Approach The experiments are carried out in the stagnation-flow configuration,30,31 and the overall experimental system is shown schematically in Figure 1. The burner nozzle diameter and the nozzle separation distance are both 14 mm. Premixed flame studies entail counterflowing a pure N2 stream against an opposing fuel/air stream. Nonpremixed flame studies utilize an O2 jet counterflowing against a fuel/N2 jet; the choice of pure oxygen rather than air is because it makes it easier to establish a flame for low fuel/N2 molar ratios that are considered in this investigation to minimize the chances for potential condensation in the flow system. Sonic orifices are used to control the flow rates of air, N2, and O2. Liquid fuel vaporization is accomplished

through a vaporization system.32,33 It makes use of a highprecision syringe pump in combination with a quartz nebulizer with a flush capillary-lapped nozzle that introduces the fuel as a fine aerosol into the hot air or nitrogen that carries it into the vaporization chamber. This allows for complete vaporization to occur at a lower temperature and prevents unintended fuel cracking. To maintain the fuel in the gaseous phase, its partial pressure is always kept below its vapor pressure. The burner is heated using ceramic heating jackets, and the gas delivery line from the vaporization chamber to the burner is also heat-traced by wrapping it up with heating tape and insulating it to prevent fuel condensation. A K-type thermocouple is used to measure the temperature of the gas delivery line, and an R-type thermocouple is used to monitor the unburned gas temperature, Tu, at the center of the burner exit. The flame temperature was determined using an R-type thermocouple coated with a ceramic (a mixture of Y and Be oxides34,35) to prevent reactions from occurring on its surface. The diameter of the coated thermocouple wire was 50 µm, and its emissivity 0.3.35 Radiation corrections to the temperature measurements were made following the method of Peterson and Laurendeau,35 with gas properties estimated by solving for the flame structure and species concentrations using detailed reaction kinetics and transport (see Numerical Approach). The accuracy of the flame temperature measurement is estimated within (50 K. Flow-field measurements are performed using the digital particle image velocimetry technique.36,37 The axial velocity profile along the stagnation streamline is measured on the fuel

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side, and the absolute value of the maximum velocity gradient just upstream of the flame is defined as the local strain rate, K. For the NOx measurements, sampling is accomplished by continuously withdrawing gases from inside the flame using a quartz water-cooled microprobe with an orifice diameter of 100-150 µm.38,39 The sample is afterwards directed toward a Chemiluminescence NO-NO2-NOx Analyzer (model 42C, Thermo Environmental Instruments Inc., USA). The microprobe is mounted on a linear stage, thus allowing it to be vertically adjusted to determine the NOx concentration profile throughout the flame. The positioning system, which includes a Cathetometer, is capable of accurately locating the initial position of the probe within 25 µm, therefore minimizing the experimental uncertainty. The accuracy of the measured NOx concentration is estimated to be (5% based on the repeatability of experimental data. The soot volume fraction is measured using a laser-light extinction method, similarly to Wang et al.40 In the experiments, an argon-ion laser beam, directed by a series of mirrors and focused by a focal lens, is passed through the flame. Detection of the beam exiting the flame is accomplished with a photomultiplier whose output is first processed by a digital oscilloscope and is then interfaced to a computer. The ratio of light intensities with and without soot (I/I0) is then utilized to determine the soot volume fraction, fv, in the flame following the approach of Wang et al.40 The proper measure of the relative concentrations of fuel and oxygen in the premixed flames is the equivalence ratio, φ. Similarly to premixed flames, a fuel-oxygen ratio in the nonpremixed flame is defined as φ* ≡ (Yfuel/Yoxygen)*/(Yfuel/ Yoxygen)stoich., where (Yfuel/Yoxygen)* is the mass fraction ratio of fuel and oxidizer at the fuel and oxygen boundaries, respectively, and (Yfuel/Yoxygen)stoich. is the attendant ratio under stoichiometric conditions. Numerical Approach The experiments are simulated using the opposed-jet flow code,41 originally developed by Kee and co-workers.42 The code is coupled with the Sandia CHEMKIN43 and Transport44 subroutine libraries. In the simulations reported here on the pollutant formation in n-butane and MB flames, a model that combines the USC Mech II45 and the MB model of Huynh et al.15 has been used. The DRG method20-22 is first employed to reduce the detailed MB model, and the resulting skeletal submodel is then coupled with the USC Mech II model and validated against experimental Suo data of MB flames.29 This combined kinetic model enables the direct comparison of the combustion characteristics of MB and n-butane flames, as it incorporates consistent C1-C4 oxidation kinetics for the two fuels to differentiate the effects that are specific to a given fuel and those that relate to the underlying C1-C4 kinetic subset. To model NOx formation, two different sets of kinetics are tested after they are incorporated into the aforementioned combined model, namely, the one included in GRI 3.0,46 hereinafter referred to as Model 1, and that of Glarborg and co-workers,47 referred to as Model 2. To identify the relative importance of each NOx formation route, the technique of Smallwood and co-workers48,49 was utilized. Results and Discussion As noted in the Introduction, the focus of this study is on pure FAME that can serve as surrogates for real biodiesel fuels. Results on the effects of chain length, the degree of saturation,

Figure 2. NOx profiles for n-pentane/air, n-butane/air, MB/air, and MC/air premixed flames (φ ) 0.8; K ) 168 s-1; Tu ) 60 °C).

and the presence of oxygen in the structure on the fundamental flame properties of these fuels will be summarized only briefly here, with additional details being presented elsewhere.29 Measurements of laminar flame speeds, Suo, and extinction strain rates, Kext (defined as the local strain K at the point when the flame extincts), of premixed MB, MC, and n-butane flames have revealed that MC/air flames have slightly higher Suo and Kext values than MB/air flames for 0.7 e φ e 1.5.29 Compared to n-butane/air flames, the presence of the ester group appears to have a retarding effect on the overall mixture reactivity, as evidenced by the lower Suo and Kext values of the FAME flames. Measurements of n-decane/air and MD/air flames, on the other hand, demonstrate that the effect of the ester group on the overall reactivity diminishes as the carbon chain length increases, as these flames were found to have similar Suo and Kext. Studies of nonpremixed flames29 revealed similar behavior, with n-butane flames having the highest Kext values, followed by MC and MB flames. Nonpremixed n-decane and MD flames have similar resistance to extinction; due to their larger molecular weight and their lower diffusivities, their extinction strain rates were notably lower than those of the C4 FAME and n-butane flames. In the remainder of the paper, the focus will be on the pollutant emission characteristics of flames of FAME fuels. As with the basic combustion studies summarized above, two C4type FAMEs, namely, MB and MC, were investigated, and their behavior was compared with that of n-butane and n-pentane. Experimental NOx concentrations of premixed MB/air, MC/air, n-butane/air, and n-pentane/air flames under fuel-lean conditions (an equivalence ratio of 0.8) are shown in Figure 2. Simulations of the MB/air and n-butane/air flames using the combined C1-C4 kinetic model and the two different NOx submodels, as described previously, are also shown on the same figure. Efforts are currently under way to compile a kinetic mechanism appropriate for modeling the MC/air and n-pentane/air flames. Of the four flames, under fuel-lean conditions, the MC/air flame produces by far the highest NOx concentration, signifying the impact of the double bond in the molecular structure. The NOx profiles of the n-pentane/air and n-butane/air flames are close to each other but with the n-pentane flame producing slightly higher NOx concentrations. The MB/air flames produce the lowest concentration of NOx under these conditions. Simulations predict the right qualitative behavior; however, quantitative agreement is only fair as Models 1 and 2 overpredict and underpredict the data, respectively. Analysis of the computed flame structures using Models 1 and 2 indicates that comparing the two fuels the rates of NOx formation by the thermal, N2O,

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Figure 5. NOx profiles for n-pentane/air, n-butane/air, MB/air, and MC/air premixed flames (φ ) 1.2; K ) 168 s-1; Tu) 60 °C). Figure 3. Temperature measurements and simulations for n-butane/air and MB/air premixed flames (φ ) 0.8; K ) 168 s-1; Tu ) 60 °C).

Figure 4. NOx profiles for n-pentane/air, n-butane/air, MB/air, and MC/air premixed flames (φ ) 1.0; K ) 168 s-1; Tu) 60 °C).

and NNH routes46,47 are very similar. It is then the higher rates of formation via the prompt mechanism that result in the higher NOx concentrations in the n-butane/air flame. Figure 3 depicts the experimentally measured and simulated flame temperatures. The agreement between experiments and simulations in the hot flame region is very good, considering the experimental uncertainty for the measured temperatures. The substantial differences between theory and experiments in the preflame and postflame zone are due to the experimental difficulties with the intrusive temperature measurement technique, which are well-known and discussed in the prior literature.50 Simulations indicate that the maximum flame temperature, Tmax, for the n-butane flame is 1879 K, and it is slightly higher than that of the MB flame that is equal to 1835 K. Experiments fail to validate such differences, however, as they are obscured by the experimental uncertainties. Figure 4 depicts the NOx concentration profiles of the same four flames at stoichiometric conditions (φ ) 1.0). As expected, the NOx concentrations are significantly higher than those under fuel-lean conditions. Again, the MC/air flame produces significantly higher NOx concentrations than its saturated FAME counterpart. The two alkanes produce fairly similar amounts of NOx (the n-pentane producing again slightly higher NOx concentrations). Of all four fuels, the MB flame produces by far the lowest amounts of NOx. The computed NOx concentration profiles using Model 1 are in very close agreement with the

experimental data. Model 2 underpredicts the experimental data but predicts their qualitative trends well. A study of the relative contributions of the different routes to NOx formation using Model 1 indicates that the higher NOx concentrations in the n-butane/air flame result from the higher prompt formation rates, due to the higher CH concentrations. On the other hand, Model 2 predicts higher rates of formation for n-butane from all three (prompt, NNH, and N2O) routes. The NOx concentrations of the same flames are compared in Figure 5 under fuel-rich conditions (φ ) 1.2). Similarly to the results obtained at stoichiometric conditions, the NOx concentrations are significantly higher than those encountered under the fuel-lean conditions. The NOx profiles for the MC/air, n-butane/ air, and n-pentane/air flames are experimentally indistinguishable, while the MB/air flame produces again significantly lower NOx concentrations. Though both NOx models predict the correct trends, as with the simulations under fuel-lean conditions, Model 1 overpredicts the data while Model 2 underpredicts them. The simulations using both models indicate that under fuel-rich conditions the prompt route dominates NOx formation for both n-butane/air and MB/air flames, as expected. The higher NOx concentrations found in n-butane/air flames can then be attributed to the presence of a higher pool of CH radicals for the n-butane/air flame, as CH radicals initiate the prompt NOx formation route.51,52 Experimental NOx concentrations and simulations of MB and n-butane nonpremixed flames are shown in Figure 6 for K ) 70 s-1 and K ) 100 s-1. The maximum NOx concentrations under nonpremixed conditions are significantly higher than those under premixed conditions. The n-butane flame shows significantly (almost twice) higher NOx concentration than the MB flames. Model 2 satisfactorily predicts the NOx concentration profiles, whereas Model 1 overpredicts the data notably. Simulations using Model 1 reveal that the key contributor to NOx for both the MB and n-butane flames is the prompt route and that the higher NOx concentrations of n-butane flames are due to their higher rates of prompt NOx formation compared to the MB flames. Model 2 again predicts that the prompt route is the key contributor to NOx for the n-butane flames; however, for MB flames it predicts that the thermal route is the largest contributor to NOx formation. Model 2 also predicts that the prompt route rates for n-butane flames are higher than the thermal route for MB flames, and as a result n-butane flames produce overall more NOx than MB flames. Comparison of the results for K ) 70 s-1 and K ) 100 s-1 reveals that increasing

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Figure 6. NOx profiles for n-butane and MB nonpremixed flames (φ* ) 0.42; Tu ) 60 °C). Top figure, K ) 70 s-1; bottom figure, K ) 100 s-1.

the strain rate decreases the maximum NOx concentration in the flame due to the decreased residence time and temperature.53 Figure 7 depicts simulated temperature as well as fuel and oxygen molar fraction profiles for the premixed n-butane/air and MB/air flames, whose NOx profiles are shown in Figures 2 and 4. Figure 8 depicts the same profiles for the premixed flames in Figure 5 and for the nonpremixed flame in the top of Figure 6. For all flames, Tmax for n-butane flames is higher than that for MB flames. For example, for the three premixed flames the difference between the two Tmax (∆Tmax) is equal to 44 K for the fuel-lean flames, 47 K for the stoichiometric flames, and 42 K for the fuel-rich flames. For the nonpremixed flames, ∆Tmax ) 66 K. Studies are currently under way to try to compare NOx formation rates under conditions for which the temperature profiles are the same; this is to delineate whether the substantial differences in NOx production are simply due to accelerated rates of production in the n-butane flames that exhibit higher Tmax or instead due to the fundamental differences stemming from the different molecular structures of the two fuels. Looking at the fuel and oxygen mole fraction profiles, one notes that complete conversion occurs for both n-butane and MB, though for the fuel-rich flames substantial amounts of CO form also. As expected, n-butane reaches full conversion earlier than MB. Finally, as previously noted, experiments are currently under way to measure directly in the counterflow configuration soot particle concentrations, and the experimental results will be presented at the meeting. Simulations are also under way, and the preliminary findings shed light onto some of the experimental observations. Figure 9, for example, shows the numerically determined mole fractions of propargyl (C3H3) for n-butane

Figure 7. Simulated temperature and fuel and oxygen molar fraction profiles for premixed n-butane and MB flames (K ) 168 s-1; Tu ) 60 °C). Top figure, φ ) 0.8; bottom figure, φ ) 1.0.

and MB nonpremixed flames. The n-butane nonpremixed flame exhibits higher C3H3 molar concentrations than the MB nonpremixed flame. The C3H3 species is chosen here as an early indicator of soot formation, as it accounts54 for more than 60% of benzene formation, which is a key soot precursor. These preliminary results are consistent with the findings by Lin et al.23 and Westbrook et al.55 that oxygenated fuels reduce the production of soot precursor species during the oxidation process. Concluding Remarks In this study, the combustion and pollutant formation characteristics of model biodiesel fuels are investigated. NOx emissions of model biofuels in the stagnation-flow configuration are experimentally determined for both premixed and nonpremixed flames. The focus of the study currently is on fatty acid methyl-esters that serve as surrogate compounds for the real biodiesels. The experiments are conducted in the stagnationflow configuration, which allows for the systematic study of fundamental combustion and emission characteristics of these fuels and provides insight into the chemical differences that are at the root of their combustion performance. NOx concentration profiles are measured using a quartz microprobe followed by chemiluminescence analysis, while soot formation is studied using a laser-based, light-extinction technique. Several detailed chemical kinetic mechanisms are tested against the experimental data to provide insight into the high-temperature oxidation kinetics of the FAME considered. In this paper, the focus is primarily on the pollutant emission characteristics of flames of two C4-type FAMEs, namely,

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currently under way with larger molecular weight FAME fuels to assess whether these conclusions are also valid for these compounds. Studies of the combustion characteristics, such as laminar flame speeds and extinction limits of these larger FAMEs, briefly summarized here, indicate that the effect of the ester group tends to diminish as the chain size of the molecule increases. It would be interesting to see whether such conclusions also apply to the pollutant characteristics of these model biodiesel fuels. Acknowledgment The experimental aspects of this work were supported by NASA under contract NNC07CB45C and by the U.S. Department of Energy under contract DE-FC26-07NT43065. The computational and analytical aspects of this work were supported as part of the CEFRC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001198. The useful discussions with Dr. Ellen Meeks of Reaction Design are appreciated greatly. Literature Cited

Figure 8. Simulated temperature and fuel and oxygen molar fraction profiles for n-butane and MB flames. Top figure, premixed flame (K ) 168 s-1; Tu ) 60 °C, φ ) 1.2); bottom figure, nonpremixed flame (K ) 70 s-1, Tu ) 60 °C, φ* ) 0.42).

Figure 9. Numerically determined C3H3 species profiles for n-butane and MB nonpremixed flames (φ* ) 0.42; K ) 100 s-1; Tu ) 60 °C).

methyl-butanoate and methyl-crotonate, whose behavior is compared with that of n-butane and n-pentane flames. To provide insight into the mechanisms of pollutant formation for these fuels, the experimental data for MB and n-butane flames are compared with simulations using a mechanism with consistent C1-C4 oxidation and NOx kinetics. Methyl-butanoate/ air flames are shown to produce significantly less NOx than the two alkanes under all experimental conditions studied, which included premixed and nonpremixed flames. Preliminary simulations indicate that the same is true for the precursors leading to soot formation. These results indicate that the presence of oxygen in the fuel molecule can potentially result in lower NOx emissions and soot precursors. Experiments and simulations are

(1) Schumacher, L. G.; Borgelt, S. C.; Hires, W. G.; Fosseen, D.; Goetz, W. Fueling Diesel Engines with Blends of Methyl Ester Soybean Oil and Diesel Fuel. See (www.missouri.edu/∼pavt0689/ASAED94.htm) (Accessed November 10, 2009). (2) McCormick, B. Effects of Biodiesel on NOx Emissions. NREL/PR540-38296, 2005. (3) Graboski, M. S.; McCormick, R. L. Combustion of Fat and Vegetable Oil Derived Fuels in Diesel Engines. Prog. Energy Combust. Sci. 1998, 24, 125. (4) Lapuerta, M.; Armas, O.; Ballesteros, R.; Fernandez, J. Diesel Emissions from Biofuels Derived from Spanish Potential Vegetable Oils. Fuel 2005, 84, 773. (5) Lin, Y.; Lee, W.; Wu, T.; Wang, C. Comparison of PAH and Regulated Harmful Matter Emissions from Biodiesel Blends and Paraffinic Fuel Blends on Engine Accumulated Mileage Test. Fuel 2006, 85, 2516. (6) FEV Engine Technology, Inc. Emissions and Performance Characteristics of the Navistar T444E DI Diesel Engine Fueled with Blends of Biodiesel and Low Sulfur Diesel Fuel. Report 94-171F2, 1995. (7) Marshall, W.; Schumacher, L. G.; Howell, S. Engine Exhaust Emissions Evaluation of a Cummins L10E when Fueled with a Biodiesel Blend. SAE paper, 1995, 952363. (8) Lapuerta, M.; Armas, O.; Fernandez, J. R. Effect of Biodiesel Fuels on Diesel Engine Emissions. Prog. Energy Combust. Sci. 2008, 34, 198. (9) Durbin, T. D.; Norbeck, J. M. Effects of Biodiesel Blends and Arco EC-Diesel on Emissions from Light Heavy-Duty Diesel Vehicles. EnViron. Sci. Technol. 2002, 36, 1686. (10) Durbin, T. D.; Collins, J. R.; Norbeck, J. M.; Smith, M. R. Effects of Biodiesel, Biodiesel Blends, and a Synthetic Diesel on Emissions from Light Heavy-Duty Diesel Vehicles. EnViron. Sci. Technol. 2000, 34, 349. (11) Dorado, M. P.; Ballesteros, E.; Arnal, J. M.; Gomez, J.; Lopez, F. Exhaust Emissions from a Diesel Engine Fueled with Transesterified Waste Olive Oil. Fuel 2003, 82, 1311. (12) Fisher, E. M.; Pitz, W. J.; Curran, H. J.; Westbrook, C. K. Detailed Chemical Kinetic Mechanisms for Combustion of Oxygenated Fuels. Proc. Combust. Inst. 2000, 28, 1579. (13) Gail, S.; Thomson, M. J.; Sarathy, S. M.; Syed, S. A.; Dagaut, P.; Dievart, P.; Marchese, A. J.; Dryer, F. L. A Wide-ranging Kinetic Modeling Study of Methyl Butanoate Combustion. Proc. Combust. Inst. 2007, 31, 305. (14) Dooley, S.; Curran, H. J.; Simmie, J. M. Autoignition Measurements and a Validated Kinetic Model for the Biodiesel Surrogate, Methyl Butanoate. Combust. Flame 2008, 153, 2. (15) Huynh, L.K.; Violi, A. Thermal Decomposition of Methyl Butanoate: Ab Initio Study of a Biodiesel Fuel Surrogate. J. Org. Chem. 2008, 73, 94. (16) Huynh, L. K.; Lin, K. C.; Violi, A. Kinetic Modeling of Methyl Butanoate in Shock Tube. J. Phys. Chem. A 2008, 112, 13470. (17) Farooq, A.; Davidson, D. F.; Hanson, R. K.; Huynh, L. K.; Violi, A. An Experimental and Computational Study of Methyl Ester Decomposition Pathways using Shock Tubes. Proc. Combust. Inst. 2009, 32, 247.

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ReceiVed for reView March 3, 2010 ReVised manuscript receiVed May 24, 2010 Accepted June 1, 2010 IE100481Q