Ammonia composite combustion with alcohol/ether: A systematic review from engine applications to combustion enhancement and emission mechanisms

Abstract

Ammonia, as a zero-carbon fuel and efficient hydrogen carrier, has great potential for decarbonization in engineering applications, transportation and power generation, emerging as a powerful candidate for alternative energy in the context of global carbon neutrality. However, the problems of difficult ignition, slow combustion and poor stability of ammonia limit its application as the pure fuel in energy conversion machinery. Combining ammonia with active alcohol/ether fuels is a feasible solution aimed at improving ammonia combustion characteristics while reducing carbon emissions. This work systematically reviewed the research progress of ammonia composite combustion with various alcohols/ethers, including ammonia with methanol, ethanol, butanol, dimethyl ether, diethyl ether, dimethoxymethane, and polyoxymethylene. This review covers macro-level engine applications, performance and emission characteristics, fundamental combustion characteristics (ignition, flame propagation and species distribution), and micro-level reaction kinetics (combustion enhancement and emission mechanisms). Based on these findings, future directions for further exploration are proposed. Ammonia-alcohol/ether combination fuels can be successfully applied in SI and CI engines through mixed-fuel, dual-fuel, and jet-controlled compound ignition modes. They have shown potential to outperform gasoline/diesel, but their effectiveness is limited by factors such as alcohol/ether type, energy ratio, engine operating conditions, ignition timing, and injection strategy. Ammonia-alcohol/ether typically reduce C-based emissions but increase N-based emissions. Both NO_x emissions, resulting from the competition between fuel-NO_x and thermal-NO_x, and soot emissions, influenced by the competition between increased carbon content and improved combustion, show nonlinear trends with alcohol/ether ratio. At the fundamental combustion level, the addition of alcohol/ether significantly shortens the ignition delay of ammonia, accelerates its burning velocity, and enhances its combustion stability. However, the promotion efficiency shows a nonlinear relationship with the alcohol/ether blending ratio and a considerable dependence on molecular structure, temperature, and pressure. At the kinetics level, the discussion focuses on the key reaction pathways of alcohol/ether enhanced ammonia ignition, the interactions between C-N components leading to promotion, synergy and inhibition mechanisms, and the coupled mechanisms for the formation/inhibition of NOx and soot emissions. The research results of this work contribute to a comprehensive understanding of the key technologies of ammonia-alcohol/ether engines and reaction mechanisms of ammonia composite combustion, providing important theoretical references for achieving the integration of ammonia and alcohol/ether fuels and near zero emissions.