Study of the NO formation characteristics and ammonia-N/coal-N transformation mechanism of ammonia-coal co-combustion in O2/CO2 atmosphere

Exploring the characteristics of NO formation during ammonia-coal co-combustion is crucial for achieving clean combustion. O₂/CO₂ atmosphere is an effective way to achieve low nitrogen combustion of pulverized coal, but the effect of O₂/CO₂ atmosphere on NO formation characteristics of ammonia-coal co-firing is still unclear. This work used high-temperature furnace experiments to investigate the conversion characteristics of fuel-N in ammonia-coal co-combustion under CO₂ atmosphere and conducted quantum chemistry calculations to investigate the molecular pathways of fuel-N oxidation. The experimental results showed that the higher temperature and oxygen concentration results in earlier occurrence of the peak and completion of NO in ammonia-coal co-combustion. Compared to N₂ atmosphere, CO₂ has an inhibitory effect on NO generation in ammonia-coal co-firing and the degree of CO₂ inhibition on the formation of NO per unit mass gradually decreases with increasing the temperature. The theoretical calculation results showed that CO₂ reduces the energy barrier of the rate determining step for ammonia-N oxidation by about 133.81 kJ/mol, promoting the conversion of ammonia-N to nitrogen oxides. However, CO₂ also has a certain degree of inhibitory effect on the coal-N oxidation, increasing the energy barrier of the rate determining step for NO generation from coal-N oxidation. The kinetic results showed that when the temperature is higher than 1673 K, CO₂ has a more obvious promotion effect on ammonia-N oxidation and inhibition effect on coal-N oxidation, but a more significant inhibitory effect on coal-N oxidation, so that the CO₂ atmosphere can inhibit the generation of nitrogen-containing products in ammonia-coal co-combustion to a certain extent.