Ammonia utilization technology for thermal power generation: A review

Hydrogen is a clean secondary energy source; however, effective hydrogen storage is demanded as it has a very low volumetric density. Ammonia is promising to store hydrogen effectively due to its high gravimetric and volumetric hydrogen densities, carbon-free, established infrastructure, and possibility for direct utilization without decomposition. However, ammonia utilization faces several challenges, including low combustion performance and the possibility of NOx emission. This work reviews the characteristics of ammonia during combustion, ammonia combustion technologies, and efforts made to answer the problems in ammonia combustion. Ammonia adoption as fuel is promising, although some challenges must be solved toward broader and more efficient combustion. The low flammability of ammonia is mainly determined by the low flame speed of ammonia and high ignition energy, and catalytic combustion is considered a promising technology to overcome this problem. Moreover, ammonia combustion and NOx formation include complicated reactions, and the mechanisms have been studied intensively. Some methods to mitigate NOx formation include improvements in reaction and kinetics and adoptions of swirl burners, two-stage combustion, gas-turbine-like combustor, and dedicated low-NOx ammonia gas turbine combustor. Some realistic options to introduce ammonia combustion are ammonia co-firing in the existing coal power plant and retrofitting the natural gas in the gas turbine using the premix fuel of ammonia and methane. However, in the future, it is expected that dedicated combustion of ammonia can be adopted toward the realization of a zero-carbon society.