Molecular simulation study on generation and adsorption laws of C₂H₂/C₂H₆ during coal spontaneous combustion

C₂H₂ and C₂H₆ are common indicator gases for spontaneous combustion, which are widely used to assist the judgment and early warning of residual coal spontaneous combustion in goaf, and also serve as important criteria for identifying the accelerated oxidation stage of coal spontaneous combustion. To clarify the generation laws and adsorption characteristics of C₂H₂ and C₂H₆ during coal spontaneous combustion and remedy the insufficient microscopic research on this issue in previous studies, three macromolecular coal models with different metamorphic degrees (YM anthracite, Wiser bituminous, and Wolfrum lignite) were constructed. The generation laws, adsorption capacities, and characteristics of dominant adsorption functional groups of C₂H₂ and C₂H₆ during the oxidation and pyrolysis of coal molecules were investigated using ReaxFF reactive force field, molecular dynamics (MD), and grand canonical Monte Carlo (GCMC) methods. The results show that the generation amount of C₂H₂ during spontaneous combustion pyrolysis is obviously higher and its generation time is obviously earlier than that during the oxidation process. Under the same conditions, the gas generation amount follows the order: lignite > bituminous coal > anthracite. The generation amount of C₂H₆ is very small under both oxidation and pyrolysis conditions, indicating that the release of C₂H₆ in the early stage of spontaneous combustion mostly originates from the primary existing gas in coal. The limiting adsorption heats of C₂H₂ and C₂H₆ in all models are less than 42 kJ/mol, belonging to physical adsorption. The saturated adsorption capacity ranges of C₂H₂ and C₂H₆ in competitive adsorption are 1.1-2.7 mmol/g and 0.4-1.1 mmol/g, respectively. C₂H₂ has an absolute competitive adsorption advantage over C₂H₆, and the anthracite model exhibits stronger gas adsorption capacity than other low-rank coal models. The main adsorption sites of C₂H₂ and C₂H₆ are distributed in the intermolecular cavities of coal molecules inside the unit cell, close to oxygen-containing functional group structures. C₂H₂ has high interaction energy with carboxyl and carbonyl groups, while C₂H₆ has higher interaction energy with carbonyl and alcoholic hydroxyl groups, and also shows strong adsorption binding ability to phenol structures.