NOx Emission and Control in Ammonia Combustion: State-of-the-Art Review and Future Perspectives

Global warming as a result of greenhouse gas emissions generated by anthropogenic activities (mainly the combustion of fossil fuels) has become an increasingly more severe challenge for all mankind. As a promising zero-carbon energy carrier toward the transition to a carbon-neutral society, ammonia has drawn broad attention from academia, industry, and government bodies. However, widespread applications of ammonia as a primary fuel in combustion devices require solutions for multiple technical challenges, among which the possibly very high nitrogen oxide (NOx) emission in the flue gas is the most concerned one. In this regard, extensive research efforts have been devoted over recent years to a deep understanding of the NOx formation mechanisms, which, in turn, result in a number of NOx-inhibiting technologies successfully developed; these include staged combustion, ultra-lean combustion, moderate or intense low oxygen dilution combustion, plasma-assisted combustion, etc. In this review, a brief literature review has been performed on NOx emission from ammonia flames, with the emphasis put on discussions of advances in understanding chemical kinetics related to NOx formation and reduction, effective technologies for NOx abatement, and applications of these technologies in practical ammonia-fired combustion devices. This review can serve as a reference for future studies on NOx emission and control in ammonia flames and the promotion of ammonia as a zero-carbon fuel in practical applications.