Experiments on bituminous Coal/NH₃ Co-combustion in a 0.3 MW pilot-scale facility: effects of inner-to-outer secondary air ratio

Based on an MW-scale coal-NH₃ co-combustion pilot facility, this study systematically investigated the effects of four inner-to-outer secondary air ratios (R_SA, defined as the mass flow ratio of inner secondary air to outer secondary air) on the in-furnace temperature field, key species distributions (O₂, CO, NH₃, NO_x), and final emissions. The results indicate that R_SA is a critical parameter controlling the atmosphere structure in the main combustion zone and the fuel nitrogen conversion pathways. As R_SA increased from 0.11 to 0.57, the preferential oxygen consumption by pulverized coal was enhanced, and the main combustion zone gradually shifted from a relatively oxidizing to a strongly reducing environment, effectively suppressing fuel nitrogen conversion to NO_x. Under the R_SA = 0.57 condition, NO_x emissions decreased to 432 mg/m³, approximately 14.5% lower than those at R_SA = 0.11, while the tail-end CO concentration remained low at 14 ppm, and the combustible content in fly ash was only 5.01% (achieving a burnout rate of 99.53%). These findings demonstrate that appropriately configuring the radial secondary air ratio can balance “high combustion efficiency” and “ultra-low emissions” in coal-NH₃ co-combustion, providing an experimental basis for the optimized design of coal-NH₃ burners.