Higher Alcohol and Ether Biofuels for Compression-Ignition Engine Application: A Review with Emphasis on Combustion Kinetics

Abstract

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.

Symmetric Ethers as Bioderived Fuels: Reactivity with OH Radicals

“…The data are compiled from Luo, 40 Zhou et al, 29 Shang et al, 41 Yang et al, 42 Serinyel et al, 10 comparable branching ratios but with a slight T dependence. In line with Cai et al, 5 we observed that the contribution of the α(2°) channel decreases with increasing alkyl chain length in dialkyl ethers and thus an increase in the number of sites where hydrogen atoms can be abstracted. For the DPE + OH reaction, the α(2°) sites contribute ∼58% while β(2°) and γ(1°) hydrogen abstraction share ∼20% each (see Figure 7b).…”

“…This review article considers the production routes, physical and chemical properties, and combustion characteristics of higher alcohols and ethers as potential biofuels with respect to their applicability in CI engines. Higher di- n -alkyl ethers have significantly high cetane numbers (CNs > 110) and thus very short ignition delay times. They can be blended with less reactive fuels, such as n -alcohols, to mimic the combustion properties of conventional diesel fuel.…”

“…They can be blended with less reactive fuels, such as n -alcohols, to mimic the combustion properties of conventional diesel fuel. For instance, a blend of 50% di- n -butyl ether/50% n -octanol by volume was found to exhibit similar reactivity as a conventional diesel …”

“…Among the biofuels currently available, alcohols and ethers derived from lignocellulosic biomass have recently attracted significant interest from the combustion community. The use of short-chain alcohols and ethers, such as ethanol and dimethyl ether, in spark-ignition and compression-ignition engines, respectively, has shown promising results in terms of improved thermal efficiency and reduced pollutant emissions (see the recent review by Cai et al 5 and the references cited therein). This review article considers the production routes, physical and chemical properties, and combustion characteristics of higher alcohols and ethers as potential biofuels with respect to their applicability in CI engines.…”

“…For instance, a blend of 50% di-n-butyl ether/50% n-octanol by volume was found to exhibit similar reactivity as a conventional diesel. 5 Hydroxyl (OH) oxidation of fuel molecules is one of the most important initiation steps in both atmospheric and combustion processes. In the atmosphere, volatile organic compounds (VOCs) predominantly undergo oxidation via reaction with OH radicals.…”