Mechanisms of N2 Formation from Armchair Configurations with Different Dinitrogen Active Sites During Coal Pyrolysis

The generation of N2 during coal pyrolysis has the potential to significantly reduce NOx emissions during coal combustion. Understanding the mechanism of N2 formation is critical for NOx reduction. In this paper, two dinitrogen-containing armchair model coals were selected: one with two N atoms numbered 2 and 7 on the carbon surface (R1), and another with two N atoms numbered 2 and 6 on the carbon surface (R2), to investigate the effect of different active sites on N2 production during coal pyrolysis. The density functional theory (DFT) was used to investigate the formation paths of N2 at the microscopic level. The findings indicate that the different dinitrogen active sites of the armchair structure mainly change the electronic properties of the carbonaceous surface, which results in a slight difference in the energy barrier required for the fracture of the C-N bond in the structure. The energy barriers for stripping the first N of R1 and R2 were 196.59 kJ/mol and 188.99 kJ/mol, and for stripping the second N atom were 167.21 kJ/mol and 179.31 kJ/mol. However, different active sites have no effect on the main pyrolysis paths of the armchair structure. The rate-determining steps of the two structures are a dinitrogen six-membered ring converted to a dinitrogen four-membered ring and the same intermediate (IM2 in R1 pyrolysis process and IM4 in R2 pyrolysis process) was formed after the stripping of two N atoms. Furthermore, the reaction process of IM2 → P1 (product 1) in R1 pyrolysis is consistent with IM4 → P2 (product 2) in R2 pyrolysis.