Molecular dynamics simulations of Zhundong coal pyrolysis using reactive force field

•Zhundong coal pyrolysis was studied using ReaxFF-MD method.•Secondary reactions of tar occur at high temperatures.•The pathways of secondary reactions of tar were revealed.•Higher heating rate leads to higher tar yield and lower gas yield.•The thermal cracking of tar is followed by the polymerization of tar. In this paper, the pyrolysis process of a typical Zhundong coal was studied using the reactive molecular dynamics simulations (ReaxFF-MD) to investigate the secondary reactions of tar which has a great influence on coal conversion behavior. The coal molecular model was built based on a combination of experiments and classical coal model. First, simulations were carried out at 1400–3000K for 250ps to investigate the effect of temperature on Zhundong coal pyrolysis process. The results show that the secondary reactions of tar occur at temperature higher than 2600K, contributing to an increase of char yield and production of large amount of H2 and CO. The secondary reactions of tar need more energy compared to the initial thermal cracking of coal. Then, the secondary reaction mechanisms of tar were revealed by analyzing the evolution of pyrolysis species at 3000K. Ethenone (C2H2O) is found as one of the main intermediates from thermal cracking of tar. There are two pathways of the secondary reaction of tar leading to char formation: (1) one tar fragment reacts with another leading to the generation of char; (2) the radicals from tar decomposition attack the char leading to the growth of char fragment accompanied by the release of H2 and CO. Finally, the effect of heating rate on Zhundong coal pyrolysis was studied. The results show that lower heating rate leads to higher gas yield and lower tar yield due to the secondary reactions of tar which mainly undergoes two stages: thermal cracking resulting in gas formation and polymerization leading to char formation.