A comprehensive review on the effects of additives on fundamental combustion characteristics and pollutant formation of biodiesel and ethanol

An introduction of additives to liquid biofuels is known as one of the best approaches to improve combustion performance. Efforts to improve the combustion performance and the consequent pollutant emissions from combustion applications such as engines and gas turbines rely on the basic understanding of the physiochemical pathways, flame structure and burning characteristics, which result from the interaction between the additives and the fuel species in various laboratory-scale configurations. In this review, various published articles have been assembled to cover the studies conducted in different combustion configurations including coflow diffusion flame, counterflow diffusion flame, single droplet combustion, spray combustion and wick generated flame. As such, the effects of the biofuels additives were discussed in terms of the flame temperature, flame liftoff, heat release, ignition delay and other fundamental combustion characteristics. The role of the biofuels additives on the pollutant formation, especially soot evolution, was described in detail. The additives reviewed here were categorised into four main groups: metal-based additives, oxygenated additives, non-metal-based additives which consist of boron-based and carbon-based additives, and water additives. The pros and cons of the introduction of additives, particularly the addition of water and the dosing of nanoparticles, were summarised and discussed. This review concludes with a summary of the findings gathered from the research efforts, outlines the possible challenges, and provides a guideline for future research in the impact of the additives. In addition, a detailed list of summaries of all related published works in the last two decades was provided in the Appendix A section.