Effects of oxygenated biofuel additives on soot formation: A comprehensive review of laboratory-scale studies

Oxygenated fuels have been widely used in combustion engines to tailor fuel properties, improve combustion performance and reduce harmful emissions such as soot particles. While an overall effectiveness in soot reduction can be observed, there has been no consensus as to the mechanism by which oxygenated additives inhibit soot formation. This is due partly to the complex physicochemical processes involved in engine combustion that make it challenging to isolate the fuel molecular effects of the oxygenated additives. To address this issue, fundamental studies have been performed in well-controlled laboratory-scale flames and reactors, aiming to decouple the convoluted factors and obtain a mechanistic understanding. This review strives to summarize recent laboratory studies on sooting characteristics of neat or blended oxygenated fuels. An overview of latest understanding of the sooting mechanisms is firstly presented to provide prerequisite knowledge for interpreting the reviewed soot results. The effects of the addition of various oxygenated fuels (e.g., alcohols, ethers, esters, or real biodiesel) on soot formation in combustion of conventional hydrocarbons are then discussed in detail. The dependence of oxygenated fuels' soot-reducing potential on the type of oxygenated functional groups, the carbon skeletons of the baseline hydrocarbons and the flame configurations were covered and supplemented by detailed analysis of the underlying mechanisms. Special focus was put on the chemical cross-linking effects (or soot synergistic effects) occurring in the oxygenated/hydrocarbon fuel mixtures. This is followed by a discussion on the variations of soot morphology, nanostructure and reactivity induced by oxygenated additives. The potential implications of the laboratory-scale results for understanding and predicting engine-level sooting behavior was also presented. This review concluded with a summary of the major conclusions drawn from the literature works, wherein future research works are suggested for more clean and efficient utilization of oxygenated fuels.

Automated summary

This review summarizes recent laboratory studies on the sooting characteristics of oxygenated fuels. It provides an overview of the latest understanding of soot formation mechanisms and discusses the effects of different oxygenated fuels on soot formation in conventional hydrocarbon combustion. The review emphasizes the chemical cross-linking effects and variations in soot morphology, nanostructure, and reactivity induced by oxygenated additives. It also highlights the potential implications of laboratory-scale results for understanding and predicting sooting behavior at the engine level.