Overview of fundamental kinetic mechanisms and emission mitigation in ammonia combustion

With the global need to achieve carbon neutrality and to stop global warming by reducing greenhouse gas emissions, ammonia is being growingly recognized as a promising renewable-energy-sourced energy carrier to replace conventional fossil fuels in propulsion and power generation systems. Unfortunately, unlike conventional fuels, ammonia combustion features some unfavorable characteristics like high nitrogen oxide (NOx) emissions, significantly hindering its practical industrial utilization. The present work overviews the state-of-the-art advances in ammonia combustion, that summarize the reaction mechanisms, NOx generation mechanism and mitigation strategies, and how ammonia addition affects soot formation. Fundamental kinetic studies of ammonia pyrolysis and oxidation mechanisms are first reviewed, followed by a discussion of different effective strategies for abating nitrogen oxides. Then, ammonia addition to soot formation in hydrocarbon fuels-based combustion is discussed, and the underpinning mechanisms are described. Finally, it is concluded with a discussion of technical challenges and future research prospects on ammonia combustion with lower NOx emissions.