Lawrence Livermore National Laboratory

Dimethyl carbonate (DMC) has been of interest as an oxygenate additive to diesel fuel because of its high oxygen content. In this study, a chemical kinetic mechanism for DMC was developed for the first time and used to understand its combustion under conditions in an opposed flow diffusion flame. Computed results were compared to previously published experimental results from an opposed flow diffusion flame. It was found that the decomposition rate DMC => H3COC(=O)O. + CH3 in the flame was much slower than originally thought because resonance stabilization in the H3COC(=O)O. radical was less than expected. Also, a new molecular elimination path for DMC is proposed and its rate calculated by quantum chemical methods. In the simulations of DMC in the flame, it was determined that much of the oxygen in dimethyl carbonate goes directly to CO2. This characteristic reduces the effectiveness of DMC for soot reduction in diesel engines. In an ideal oxygenate additive for diesel fuel, each oxygen atom stays bonded to one carbon atom in the products thereby preventing the formation of carbon-carbon bonds that can lead to soot. When CO2 is formed directly, two oxygen atoms are bonded to one carbon atom thereby wasting one oxygen atom in the oxygenate additive. To determine how much CO2 is formed directly, the branching ratio of the key reaction, CH3OC.=O going to the products CH3 + CO2 or CH3O + CO was determined by ab initio methods. The A-factors of the rate constant of this reaction were found to be about 10-20 times higher than previous estimates. The new reaction rate constants obtained can be used as reaction rate rules for all oxygenates that contain the ester moiety including biodiesel.

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Glaude, P. A., W. J. Pitz, and M. J. Thomson, "Chemical Kinetic Modeling of Dimethyl Carbonate in an Opposed-Flow Diffusion Flame," Proceedings of the Combustion Institute 30, pp. 1095-1102 (2004); Lawrence Livermore National Laboratory, Livermore, CA, UCRL-JC-201358. (The reaction rate references are given here).