Comprehensive technical review for fundamental characteristics and application of NH3 co-firing with coal

•Fundamental investigation of NH3 combustion chemistry and NH3 co-firing technologies are reviewed.•NOx emission normally increases with increasing the NH3 co-firing ratio except under specific conditions.•Advanced technologies to reduce high NOx emissions caused by NH3 co-firing are discussed.•Effects of NH3 co-firing on performance, ash deposition, soot, H2S formation and economic analysis are discussed. Owing to the global demand to prevent global warming and achieve carbon neutrality by reducing greenhouse gas emissions, ammonia (NH3) has attracted attention as a promising carbon-free fuel for replacing fossil fuels, especially in coal-fired boilers. Because NH3 combustion is mostly performed by adding fuel to the existing system, understanding NH3 co-firing behavior to efficiently utilize NH3 combustion in the existing system is necessary. Therefore, in this study, we conducted a comprehensive review of the latest research on NH3 co-firing and NOx reduction strategies for coal-fired boilers and a fundamental investigation of the chemical reactions and NOx mechanisms. The flame propagation speed of NH3 co-firing with coal varies depending on the ambient conditions (fuel lean/rich) and coal type (high/low fuel ratio). NOx emission has been reported to increase during NH3 co-firing, except under specific conditions. Advanced technologies such as air-staged combustion, secondary fuel injection, MILD combustion, and porous medium applications have been discussed to reduce the high NOx emissions caused by NH3 co-firing. Moreover, the effects of NH3 co-firing on performance, ash deposition, soot, H2S formation, and economic analysis are discussed. This study contributes to finding the optimal conditions by analyzing the characteristics of NH3 co-firing and presents technical challenges and future research prospects for NH3 co-firing with low NOx emissions and better performance.