A review on ammonia, ammonia-hydrogen and ammonia-methane fuels

Combustion of fuels to generate energy is integral to various human activities, both domestic and industrial. However, the predominance of hydrocarbon fuel usage produces emissions containing pollutants that cause multiple environmental complications and risks to human health. Therefore, replacement of conventional fuels to achieve zero carbon emission is of utmost importance. In terms of carbon-free fuel, ammonia offers several advantages over hydrogen. However, its low burning velocity and high fuel NO_x emissions inhibit large-scale usage. Hence, hydrogen and methane have been studied in this review as possible secondary fuels to aid ammonia combustion and address the aforementioned issues. This review starts from the suitability of ammonia fuel as energy vector in terms of physicochemical and combustion characteristics, moving through the kinetics and mechanisms of ammonia-based and ammonia-fuel combustion. The impacts and limitations of each system are also addressed, thus providing a comparison on each system. Particularly, this review assesses and discusses the advantages and mechanisms involved with secondary fuel addition to the ammonia combustion, presenting the role of key reaction differences and the change in key reaction mechanism under different conditions at the level of reaction mechanisms. Finally, this review covers future perspectives and challenges on the usage and development of ammonia-based fuels, emphasizing the maturity of ammonia-based and ammonia-fuel combustion kinetics. Herein, this work summarizes the principles of the combustion reactions of ammonia-based and ammonia-fuel systematically and serves as a theoretical reference of ammonia-fuel combustion kinetics for transitioning into future practical applications where ammonia is an important energy vector.