Ab initio combustion kinetics of biodiesel surrogates: hydrogen abstraction from methyl esters and subsequent unimolecular radical reactions

Abstract

Combustion of renewable biofuels, including energy-dense biodiesel, is expected to contribute significantly toward meeting future energy demands. It is important to elucidate the elementary reaction kinetics of biodiesel surrogates, e.g., small-to-medium size methyl esters, to appropriately model the combustion and emission characteristics of real biodiesel molecules before they can be reliably used in engines. Hydrogen atom abstractions from fuels and subsequent unimolecular reactions of fuel radicals formed from hydrogen abstraction are expected to dominate the consumption of fuels at relativity high temperatures and therefore play a crucial role in determining the distinct combustion properties of different fuels. However, most kinetics parameters for these reactions implemented in current combustion models are estimated, leading to high uncertainties in simulations. Consequently, we assess the reaction kinetics of three methyl esters, methyl formate, methyl acetate, and methyl propanoate by employing high-level ab initio quantum chemistry methods and state-of-the-art kinetics techniques for the two types of reactions. High-pressure-limit and pressure-dependent rate constants are computed and provided in modified Arrhenius expressions for the bimolecular and unimolecular reactions, respectively.

The analysis shows that knowledge of accurate rate coefficients and branching fractions is crucial to accurately elucidate unique combustion properties of different fuels. The predicted rate coefficients are also used to develop combustion models for these fuels. The improved agreement between simulations and experimental results demonstrates the success of our theoretical predictions and that these newly developed oxidation models can be used to accurately describe combustion and emission characteristics of the methyl esters.