Detailed chemical kinetic analysis of flame speed using surrogates of low-carbon gasoline fuels

Low-carbon gasoline fuels are one of the most promising approaches to reduce the carbon footprint of the transportation sector. Despite the fact that drop-in fuels can be produced by methods such as methanol-to-gasoline (MTG), ethanol-to-gasoline (ETG) or biomass hydrothermal processing, it is unclear how the composition of the fuel may affect engine performance and emissions. In fact, differences between low-carbon and petroleum gasoline have been observed experimentally and reported in the literature, including differences in deflagration speed and burning rate. However, to the best of the authors’ knowledge, the chemical kinetic mechanisms responsible for these differences have not been studied with detailed chemistry before. This is in part caused by the computational cost of solving tens of thousands of conservation of species and diffusion equations in 1-D flame simulations, which prevents detailed chemical kinetic mechanisms from being used in these simulations. Thus, reduced models are used in laminar flame speed simulations, leading to chemical analyses that may be incomplete, since the chemistry in reduced mechanisms is heavily simplified.

To solve this issue, a novel approach to study fuel decomposition, rate of production and chemical sensitivity at laminar flame speed like conditions with detailed chemistry was developed in this work. The approach consists of imposing the temperature profile and residence time across the flame in a plug flow reactor to simulate fuel decomposition at flame conditions (including residence time of each fuel molecule at each pressure-temperature conditions within the flame). This approach showed a very low computational cost even with highly detailed chemistry (tens of thousands of reactions), allowing in-depth analyses of the chemistry within the flame. Rate of production and sensitivity analyses were performed for multiple single-component fuels typically found in gasoline composition, for regular-grade E10 (10%vol ethanol) petroleum gasoline, MTG, ETG and biomass-derived renewable gasoline surrogates following this approach. Simulations at engine-like conditions showed that the flame accelerates instants before knock occurs due to a combination of energy and radicals released by the low-temperature chemistry of the end gas, with radical production in the reactants upstream the flame being the dominant factor. Sensitivity analyses showed that flame deflagration is controlled by reactions that generate atomic hydrogen, whereas reactions that act as a sink of atomic hydrogen (such as methyl radical recombination) are detrimental for flame speed. Species sensitivity showed that laminar flame speed is strongly sensitivity to the production of vinyl radical within the flame because vinyl radicals rapidly decompose and release atomic H and atomic O in the reaction zone of the flame. Finally, metrics to correlate the flame speed of a fuel with the formation of key species within the flame are explored.