Mechanisms Governing Ammonia–Coal Cofiring in a Fluidized Bed

To facilitate the low-carbon transition of the power sector, this study investigates carbon and nitrogen conversion during ammonia–coal cofiring in a bubbling fluidized bed. Experiments were conducted in a laboratory-scale two-dimensional bubbling fluidized bed to investigate the effects of the ammonia cofiring ratio, excess air ratio, and bed temperature at a constant thermal input. With increasing ammonia cofiring ratio, CO formation increased, indicating inhibited deep oxidation due to radical competition. NO emissions exhibited a U-shaped trend with the ammonia cofiring ratio, with the minimum marking the transition from a shift between reduction-dominated and oxidation-dominated regimes. At low NH 3 input, NO reduction to N 2 was promoted by NH 2 -type intermediates synergized with char-surface active sites; at higher NH 3 input, the overall reducing capacity appeared to weaken, and oxidation pathways became increasingly dominant, leading to an increased net conversion of fuel-N to NO. Concurrently, NH 3 emissions increased by over 2 orders of magnitude. Further examination of the excess air ratio and bed temperature confirmed a thermochemical trade-off: while a higher excess air ratio or bed temperature enhanced NH 3 conversion and thus reduced NH 3 emissions, both conditions simultaneously promoted the oxidation of fuel-N, resulting in higher NO emissions. These results highlight the challenge of regulating the dual functional role of NH 3 as both a reductant and a nitrogen-containing fuel in fluidized beds.