Study on coal/ammonia co-combustion and NO formation characteristics: Quantitative analysis using online mass spectrometry in fluidized bed

Coal/ammonia co-combustion represents a key pathway toward the low-carbon transition of coal-fired power generation. In this study, a micro-fluidized bed reactor coupled with online mass spectrometry (MFBR-MS) was employed for quantitative analysis to systematically investigate the co-combustion characteristics of coal char and ammonia, as well as the NO formation mechanisms. Using the equivalent characteristic spectrum analysis method, transient quantitative measurements of CO, H2, H2O, and NO were performed, based on which the effects of NH3 flow rate, temperature, and oxygen supply on product release were clarified, and the evolution of the dominant pathways governing NO formation was revealed. The results showed that coal char had a stronger competitive affinity for O2 than NH3. As the NH3 flow rate increased, combustion time decreased, and CO peak flow initially rose and then declined, while H2O and NO production continued to increase. At lower NH3 flow rates, NH3 and coal char displayed a synergistic reduction effect on NO. Temperature significantly regulated the reaction pathways. At 800 °C, H2 production increased with NH3 flow, while at 900 °C, the trend reversed, as NH3 and its intermediates tended to react with O/OH radicals rather than thermally decompose. NO production was high at 800 °C. As the temperature increased, NH3 was more readily converted into reducing intermediates, and the heterogeneous reduction of NO by coal char was enhanced, thereby effectively suppressing NO formation. In addition, high oxygen concentrations accelerated NH3 oxidation to NO, thereby increasing NO production under co-combustion conditions.