New Insights into the Heterogeneous Reduction Reaction between NO and Char-Bound Nitrogen

New insights into the heterogeneous reduction reaction between NO and char-bound nitrogen [char(N)] were obtained by a combination of density functional theory (DFT) and conventional transition state theory (TST). A detailed description of the nature of NO chemisorption is reported based on the HOMO and LUMO, Mulliken atomic charges, and spin densities. It is suggested that, during chemisorption, char(N) contributes electrons to NO. The seven most stable structures (I-VII) resulting from NO chemisorption were identified, and the exothermicity was found to increase in the order III < V < IV < < VII < II < I. This finding is reasonable considering the fact that the HOMO of char(N) is predominantly reflected in the active C(2) atom and the LUMO of NO is mainly concentrated on the N(8) atom. Three stepwise reactions leading to N-2 formation have been characterized with low energetic penalty acceptable for occurring at the practical heterogeneous combustion temperature. The highest energy penalty was calculated to be 270.794 kJ/mol. A kinetic similarity over the temperature range of 850-1000 K between the rate-limiting step and char gasification was found (10(-3)-10(0)) compared to 10(-4)-10(-1) s(-1), respectively). By comparison with previous experiments, the calculated results were validated, and on the basis of these results, the reburning of superfine bituminous coal is recommended.