Study on pyrolysis reaction and mechanism of low-rank bituminous coal based on reactive molecular dynamics

Coal pyrolysis technology can not only achieve the clean and efficient utilization of coal resources, but its products can also alleviate the current supply and demand contradiction of fossil energy. In order to explore the pyrolysis product rules and reaction mechanism of low-rank bituminous coal, a molecular parallel simulator was used to simulate the coal pyrolysis process. The ReaxFF reactive molecular dynamics method was adopted to simulate different pyrolysis temperatures and heating rates, study the pyrolysis characteristics of low-rank bituminous coal, and analyze the coal pyrolysis reaction mechanism. The study found that around the temperature point of 2300 K, significant changes occurred in the yield and elemental proportion of semi-coke and tar; the reaction paths of typical pyrolysis gases changed: the formation of water molecules was related to hydroxyl groups at low temperatures and carboxyl groups at high temperatures, CO₂ mainly came from the detachment of carboxyl groups, and C₂H₂ and C₂H₄ were only generated at high temperatures, indicating that this temperature point is the transformation point of the isothermal pyrolysis reaction mechanism. Before 2300 K, the initial stage was dominated by the breaking of ether bonds in the coal molecular structure and the detachment of oxygen-containing functional group side chains, followed by the ring opening of naphthenes and the decomposition of coal molecules, but during this process, the groups and side chains connected to the aromatic structure did not detach; after 2300 K, the coal molecular structure decomposed rapidly in the initial stage of the reaction, the tar mainly underwent secondary reactions, and the yield of semi-coke increased due to the polycondensation of aromatic structures. The heating rate had a great influence on the pyrolysis products: a high heating rate would increase the molecular weight of semi-coke, while reducing the yields of tar and pyrolysis gas; the higher the heating rate, the higher the temperature required for product formation, which was not conducive to the occurrence of coal pyrolysis reactions; a high heating rate would also lead to the rapid accumulation of intramolecular energy, the rapid polycondensation of aromatic structures into semi-coke, and the disorder of the breaking rules of side chains and functional groups.