Energy & Fuels, Vol.35, No.3, 1890-1917, 2021
Higher Alcohol and Ether Biofuels for Compression-Ignition Engine Application: A Review with Emphasis on Combustion Kinetics
High molecular weight alcohol and ether fuels with their advanced autoignition propensities and oxygenated molecular structures are promising future fuel candidates for compression-ignition engine application, because they can provide improved combustion efficiencies and reduced pollutant emissions. In addition, their production from lignocellulosic biomass as second-generation biofuels offers an improved CO2 balance and avoids the adverse impact of the first-generation biofuel production on the food supply. This review aims to summarize the recent research progress on the combustion of long-chain alcohols and ethers with more than four carbon atoms, putting a particular emphasis on their fundamental combustion kinetics. The article starts with aspects related to their production routes, physical and chemical properties, and practical applications, highlighting the resulting consequences for their potentials as renewable fuels in comparison with conventional diesel fuels. This is followed by a comprehensive evaluation of their fundamental ignition and combustion characteristics. The existing chemical mechanisms for the oxidation of higher alcohols and ethers are introduced in conjunction with discussions of their development approaches. Their reaction kinetics are further explored to understand the dependence of their combustion behaviors on the molecule's structural features, such as functional groups and carbon chain length. In particular, the different influences of the molecule size on the cetane numbers of longer alcohols and ethers are analyzed. The review finishes with a discussion suggesting directions for future experimental and numerical investigations, which will allow deepening of the understanding of the combustion kinetics of higher alcohol and ether fuels.