Mechanistic exploration of tar-rich coal pyrolysis through ReaxFF MD simulation coupled with experimental validation

Abstract

Pyrolysis is essential for the clean and efficient utilization of coal resources, but its inherent complexity and unpredictability pose significant challenges in elucidating the involved reaction mechanisms. Herein, we revealed the pyrolysis behavior of tar-rich coal through ReaxFF MD simulations combined with experimental validation. The simulation reveals that coal pyrolysis is a gradual process of removing volatile components in three main stages: activation, pyrolysis (primary and secondary), and condensation. The carboxyl group cleavage triggers coal pyrolysis, followed by successive decomposition processes including ether bond and side chain breakages, opening of hetero- and aliphatic rings, and cleavage of bridge bonds between aromatic rings, resulting in producing tar and gas products. Crucially, unsaturated fragments in the tar tend to condense into semi-coke or char, reducing the tar yield, while H• radicals can stabilize the tar, inhibit condensation, and improve its quality. The fixed-bed rapid pyrolysis experiments confirm the simulation results, validating their accuracy despite minor deviations due to neglected heat and mass transfer effects in the simulation. These findings provide atomic-scale insights into coal pyrolysis, offering valuable theoretical guidance for optimizing industrial pyrolysis processes and improving product control.