Mechanistic insights into coal pyrolysis in hydrogen-rich atmospheres via ReaxFF MD

Hydrogen-rich atmospheres offer significant potential to enhance coal pyrolysis efficiency and selectivity, yet studies on the influence of atmospheric ratios and underlying mechanisms remain limited. Here, ReaxFF MD simulations were used to investigate the effects of different hydrogen-rich atmospheres (CH₄, H₂O, NH₃) on coal pyrolysis, focusing on product distribution and hydrogen supply mechanisms. The results show that CH₄ and H₂O promote pyrolysis, suppress char formation, and increase tar yields, with the effect strengthening at higher atmospheric ratios. Conversely, NH₃ exhibits a mild inhibitory effect, leading to higher char yields and lower tar production, although it still facilitates gas generation. Compared to pyrolysis with non-atmosphere, the highest yields of H-tar, L-tar, and gas were increased by 10.30%, 7.24%, and 6.06%, respectively. Chemical bond and atomic composition analyses reveal that hydrogen-donating radicals (•H, •CH_X, and •OH) from CH₄ and H₂O accelerate pyrolysis, while •NH_X radicals in NH₃ form nitrogen-containing compounds that inhibit free radical chain reactions, slowing coal decomposition. Atom-tracking shows that a substantial portion of H, C, and O atoms in tar originated from CH₄ and H₂O, confirming the active participation of the atmospheres in tar formation. The tar generated under CH₄ has higher carbon and hydrogen content, suitable for fuel processing, while H₂O generates oxygenated tar, ideal for chemical conversion. These findings provide valuable insights for optimizing coal pyrolysis processes and advancing the sustainable development of coal resources.