ReaxFF-based molecular dynamics simulation of the initial pyrolysis mechanism of lignite

In this paper, a series of ReaxFF molecular dynamics (ReaxFF-MD) simulations were employed to explore the characteristics of pyrolysis products, transformation behavior of major elements, and thermal decomposition mechanism of lignite. The results suggest that the pyrolysis of lignite mainly undergoes decomposition of macromolecular structure and breakage of bridge bonds at lower temperatures. At relatively high temperatures, further cracking of tar fragments and condensation of aromatic structures occur. The relationship between the main pyrolysis gases and the structural characteristics of lignite has been studied. The results show that the formation of H2O, CO2, and C2H4 is associated with the typical structure of lignite i.e., hydroxyl groups, carboxyl groups, and methylene carbon, respectively. Unlike bituminous coal and subbituminous coal, light tar is the main component of pyrolysis tar, which is due to large amounts of monocyclic and bicyclic structures in lignite. The pyrolysis mechanism of lignite was further analyzed through element migration behavior, and the simulation results are consistent with the above reaction mechanism. This work provides in-depth insight into the initial reaction mechanism of lignite pyrolysis and may be useful for the industrialization of lignite clean utilization.