Reaction pathways for the reduction of NO by nitrogen-containing char

Density functional theory was used to investigate the reaction pathways for the reduction of NO by nitrogen-containing char with char-bound nitrogen, viz., char(N); the reaction paths of heterogeneous reduction of NO by char(N) were analyzed from the thermodynamic and kinetic point of view. The results show that the energy barrier of the rate-determining step via the indirect NO reduction path by char(N), viz., first producing NH₂ intermediate and then reducing NO, is 183.76 kJ/mol higher than that via the direct heterogeneous NO reduction path, suggesting that the later direct heterogeneous NO reduction path is more favorable. Thermodynamically, the direct heterogeneous NO reduction by char(N) is a spontaneous and exothermic process in the coal combustion system. Kinetically, the rate constant of the rate-determining step in indirect NO reduction path by char(N) is at least 10 orders of magnitude lower than that in direct heterogeneous reduction, also illustrating the superiority of the direct heterogeneous NO reduction path by char(N).