Investigation of N Transfer during Coal Char Oxidation by Reactive Molecular Dynamics

This work employs large-scale simulations of reactive molecular dynamics to investigate influences of coal char structures, O2 concentration, and temperature on NOx formation during coal char oxidation processes, which are facilitated by combining high-performance computing and cheminformatics-based reaction analysis. The char property models are constructed by using a new strategy on the basis of nascent structures taken from different pyrolysis stages individually from a large coal model. Analysis results of the char model structures show that nitrogen atoms would transform into N-containing six-membered ring groups from coal to a matured char structure at the high temperature of the late pyrolysis stage. Analysis of the simulation trajectories reveals that fuel/O2 ratio and temperature play important roles in NOx emission, while the N-gas generation in the oxidation of char is not sensitive to the nascent coal char structure. NO formed in char oxidation tends to be reduced to N2 at high temperature for a fuel/O2 ratio of 1.0, which is an optimal condition for NO reduction in decoupling combustion. The reaction pathways revealed for the transformation of N-containing functionalities and generation of NO/N2 show the vital roles of NH3, HCN, char-CNO, and char-NCO in NOx emission. Particularly, the intermediate structures of CNO at coal char provide the very first N to react with NO, leading to N2 generation. It should be noted that the N2 in the air would not affect the conclusion regarding fuel-N transfer in the pure oxidation system of O2 obtained.