Study on fuel-N conversion mechanism of ammonia-coal co-firing at different combustion stages

The co-combustion of ammonia and pulverized coal can effectively reduce the carbon emission of thermal power generation. However, ammonia, as a carbon-free fuel, is rich in large amounts of nitrogen, which increases the risk of high NOx emissions. Therefore, it is important to clarify the influence mechanism of ammonia on the NO formation of coal volatile-N and coal char-N in the co-combustion, and to reveal the N oxidation pathway in different combustion stages.

In this study, simulations were carried out on the CHEMKIN software to investigate the generation characteristics of NO and the transformation mechanism of fuel-N at different combustion stages of ammonia-coal co-firing. The study showed that ammonia-blending combustion promoted the release of coal volatiles and the oxidation to NO. In the total NO generation during the ammonia-coal co-firing, the proportion of NO produced by ammonia-coupled coal char combustion was very low. Compared with ammonia-coupled coal combustion, the amount of NO produced in ammonia-coupled coal volatile combustion was significantly reduced. Sensitivity analysis and rate of production (ROP) analysis indicated that the increase of H, OH, and O free radicals promoted the NO formation, and that NH_i free radicals played an important role in the NO reduction. By analyzing the elementary path of NO generated from ammonia-coupled pulverized coal, coal volatiles and coal char combustion at 1400 °C and 10 % ammonia ratio, it can be seen that the main path of NO formation during ammonia-coupled coal volatiles combustion is VOL→HCN→NCO→NO, CHAR→NO, NH₂→HNO→NO, compared with ammonia-coupled coal combustion. The proportion of NH₂→NH→NO reaction paths decreased, while the proportion of NH₂→N₂, NCN→NCO→N₂, and NH₂→NNH→N₂ reaction paths increased respectively, indicating that separation combustion promoted the reduction of NO by NH_i free radicals while inhibiting the oxidation of N-containing components.