Influence mechanism of ammonia mixing on NO formation characteristics of pulverized coal combustion and N oxidation in ammonia-N/coal-N

Abstract

Through ammonia-coal co-firing, ammonia as a carbon-free fuel can be used to effectively reduce CO2 emissions in coal-fired power generation. However, ammonia can also act as an N source that allows NOx to be generated through more reaction paths. Therefore, it is highly necessary to explore the N oxidation mechanism in the ammonia-coal co-firing process to achieve low nitrogen combustion. In this study, the N oxidation mechanism in the ammonia-coal co-firing process was explored through an experiment with a high-temperature furnace, XPS testing, and calculations based on quantum chemical theory. The experimental results show that both temperature and ammonia mixing ratios have a significant impact on NO formation. When the combustion temperature is 1200℃, the NO yield increases gradually as the ammonia mixing ratio increases from 20% to 40% and then to 60%; when the combustion temperature is higher than 1300℃, the NO yield increases first and then decreases as the ammonia mixing ratio increases. In addition, the XPS testing demonstrates that the content of char-N resulting from ammonia-coal co-firing at 1200℃ is higher than that resulting from pure coal combustion at 1200℃. This indicates that the N atoms in ammonia can be adsorbed or embedded in char. The theoretical calculations at the molecular level further reveal that the amino groups in the ammonia-coal co-firing system can be oxidized to produce NO, NO2, and HNO via different paths. Both the experimental data and the theoretical calculations confirmed that NO is the main oxidation product. Moreover, the amino groups can stably exist on the char surface under the effects of different adsorption mechanisms, which is consistent with the characterization test. This work lays a foundation for deepening the understanding of the N migration and transformation mechanism in ammonia-coal co-firing. It provides theoretical support for developing methods to ensure low nitrogen combustion in ammonia-coal co-firing.