Reaction mechanisms of coal gasification in supercritical water with CO₂ transport agent: A ReaxFF molecular dynamics study

Supercritical water gasification (SCWG) is a promising technology for efficient and low-carbon coal utilization. As a potential transport agent, introducing CO₂ into SCWG offers a prospect for optimizing the gasification environment and reducing carbon emissions. In this study, a novel coal SCWG process using CO₂ as the transport agent is investigated through reactive force field molecular dynamics (ReaxFF MD) method. The reaction mechanisms of coal fragment decomposition and gaseous product generation, along with the effects of key operating parameters, are revealed at the molecular level. The initial coal breakage primarily occurs at C-O bonds, followed by ring-opening and carbon chain cleavage reactions. Three primary ring-opening routes involving direct cleavage, contracted-ring cleavage, and expanded intermediate cleavage are identified. These processes are significantly facilitated by active O and OH radicals generated from CO₂ and water dissociation, which effectively weaken C–C bonds within the carbon structure. H radicals derived from water serve as key intermediates for H₂ formation through radical recombination. CO₂ contributes to CO production both through direct decomposition and by providing reactive O radicals that oxidize coal fragments. CO₂ addition increases the CO/H₂ molar ratio and enhances the decomposition of coal macromolecules. An appropriate CO₂ range of 100–300 molecules further improves carbon conversion efficiency. Moreover, higher reaction temperatures and higher water concentrations are beneficial to promote hydrogen production and carbon conversion.