Mechanisms of the N2O formation and decomposition over coal char surface

Abstract

N2O is an important gaseous pollutant in fluidized bed combustion. This work first presents a comprehensive thermodynamics study of the formation of N2O and its simultaneous destruction using the density functional theory (DFT). In addition, the kinetics is reported in a 0-D batch reactor using the rate parameters computed based on the transition state theory (TST). The armchair configuration with pyridine nitrogen is selected as the char(N) model. This work emphasizes the effect of chemisorption mode, temperature, and oxygen form on N2O evolution during the NO+char(N) reaction. There are two modes (NN-OC and NO-NC) for the chemisorption of NO on char surface leading to the generation of N2O. By comparison, N2O formation is more favorable under the NO-NC mode, which can avoid the largest barrier caused by the NO antibonding node. The involvement of ether oxygen promotes low-temperature generations of N2O (<673 K), while that of ketone oxygen facilitates moderate-temperature generations of N2O (673–873 K). In oxygen-free conditions, N2O is released at relatively high temperatures (973–1073 K). Once the temperature is higher than 1073 K, a large amount of N2O will be decomposed to gaseous N2 and CO. The results can explain why the available N2O formation mechanisms from char combustion considering the involvement of oxygen species, how different types of oxygen affect the N2O formation, and the well-known temperature inflexion point in N2O evolution. These findings will advance our understanding of the reaction and help develop a more accurate kinetics model to predict N2O emission.