Effects of Ammonia and Coal Interactions on NO and CO Formation Mechanisms during Coal–Ammonia Co-combustion

In the context of dual-carbon initiatives, transitioning coal-fired power plants to co-firing with low-carbon ammonia has become a critical strategy. As a nitrogenous fuel, ammonia can undergo incomplete combustion during co-firing with coal, leading to the formation of pollutants, such as NOx and CO. This study investigates the effects of overfire air on the formation mechanisms of NO and CO during coal–ammonia co-combustion, using a 50 kW entrained-flow reactor and simulations with Chemkin software. With overfire air injection, the results show that NO concentrations during coal–ammonia co-firing are substantially higher than those observed in pure coal or ammonia combustion or the theoretically calculated sum of the two. This suggests that coal promotes the chain dehydrogenation of ammonia, primarily attributed to increased H/OH/O radical concentrations and enhanced NH2/NH generation within the system. After overfire air is supplied, the interaction of coal with ammonia becomes more intense than that without overfire. The introduction of overfire air modifies the primary reaction pathways in all three combustion scenarios. During the initial combustion stage, a competitive relationship emerges between the formation of CO and the formation of NO. H/OH radicals primarily govern CO generation, whereas the OH/OH radicals dominate NO formation. The rate of CO formation is substantially higher than that of NO, thereby consuming most of the available H/OH radicals.