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

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.