Investigation of N behavior during coal pyrolysis and oxidation using ReaxFF molecular dynamics

This paper presents ReaxFF molecular dynamics simulations of pyrolysis and oxidation processes at 500-2500 K and 1000-2800 K performed using the GPU-enabled ReaxFF MD code of GMD-Reax and reaction analysis code of VARxMD. The simulation results of a large-scale coal model with varied types of N-containing groups shows that more than 65% N still remains in C40+ fragments and about 25% N migrates into C5-C40 fragments after pyrolysis stage. The evolution tendencies of light gases (NH3, HCN, HNCO, NO, N2 and NO2) and important radicals (CN, NH2/NH, NCO and HNO) with temperature were observed from the slow heat-up coal oxidation simulation at fuel lean condition. The detailed structures and conversion reactions associated with light nitrogen species were obtained. The NCO-containing intermediates are found playing a significant role for NO emission that more than 50% of the NO formation is through NCO-containing intermediates, some of which comes from the oxidation of CN radicals and HNCO. The hydrogen abstraction of N-H in pyrrole groups by O2 will create an initial unstable environment, followed with O insertion reactions into N-containing heterocyclic ring structures to promote the activation of N-containing groups in coal structure.