Investigation of pollutant generation and the NO /burnout trade-off in NH3-coal co-firing under various combustion organization conditions

The integration of green NH 3 as a renewable energy carrier with coal-fired boilers has emerged as a promising pathway to support deep decarbonization. However, the inconsistent dependence of NO x emission on the NH 3 fraction reported in previous studies suggests that the mechanisms of fuel-N migration and conversion in NH 3 -coal co-firing remain unclear. The influence of combustion organization and mixing methods under air-staged conditions has also received insufficient investigation. This study examined the effects of co-firing modes within an air-staged framework by simulating the main combustion and overfire air (OFA) zones in a two-stage drop tube furnace, and further integrating a fuel-staged strategy to optimize NO x control and clarify fuel-nitrogen transformation behavior. The results demonstrated that combustion characteristics were strongly dependent on co-firing mode. When NH 3 and pulverized coal were introduced together into the same main combustion zone, three distinct NO x evolution patterns emerged with increasing NH 3 fraction: continuous increase in premixed combustion, initial rise followed by decline and subsequent growth in incompletely premixed co-firing, and slight initial decline followed by steady increase in pure NH 3 complete diffusion co-firing. Fuel-staged co-firing strategy, wherein the furnace was divided into a main combustion zone ( α > 1), fuel-staged zone ( α < 1), and OFA zone ( α > 1), suppressed NO formation across a 20%-80%. An OFA ratio near 30%-35% minimized NO x emissions while maintaining acceptable burnout performance. Air distribution in the fuel-staged zone was critical. The introduction of pure NH 3 into the fuel-staged zone or sustaining a slightly oxygen-rich atmosphere in the main combustion zone effectively suppressed NO formation without severely deteriorating burnout. These findings demonstrate that fuel-staged co-firing can effectively reduce NO x emissions, thereby informing strategies for the industrial implementation of NH 3 -coal co-firing. • NO x trends depended strongly on fuel mixing mode and combustion organization. • Diffusion co-firing suppressed NO x more effectively than premixed modes. • Fuel-staged combustion achieved the lowest NO x across NH 3 -fuel ratios of 20-80%. • An overfire air ratio of 30-35% reduced NO x emission with notable NH 3 slip.