A Study on the Mechanism of Methane Adsorption–Desorption Hysteresis in Coal from an Energy Decoupling Perspective

Abstract

Methane adsorption–desorption hysteresis in coal, widely documented across extensive experimental observations, continues to elude a unified mechanistic understanding. In response to issues in existing research─limited applicability and the lack of quantitative characterization of desorption energy barriers─this study introduces an energetic decoupling approach grounded in desorption energy barrier heterogeneity and surface diffusion theory. The programmed cooling adsorption–heating desorption cycle experiments were designed and conducted. Quantitative calculations of potential well depths at distinct adsorption sites, methane molecular kinetic energy, and desorption energy barriers were achieved through analyses grounded in heat of adsorption and adsorption potential theories. The results demonstrate that the heat released during methane adsorption comprises potential energy changes, kinetic energy changes, and volumetric work, with kinetic energy accounting for 6%–10% of the total heat. Calculations based on energy conservation reveal that methane molecules at high-affinity adsorption sites possess greater potential well depths and lower molecular kinetic energy, whereas those at low-affinity adsorption sites exhibit the opposite behavior. Furthermore, the average desorption energy barriers for newly adsorbed methane molecules during two consecutive cooling experiments (333.15 K–318.15 K–303.15 K) are 3.118 × 10–20 J and 1.507 × 10–20 J, respectively. These findings indicate pronounced heterogeneity in potential well depth, molecular kinetic energy, and desorption energy barrier across distinct adsorption sites. Finally, a new perspective on the hysteresis mechanism is proposed based on desorption energy barrier heterogeneity across adsorption sites and surface diffusion theory. This perspective posits that desorption energy barrier heterogeneity induces surface diffusion of methane molecules along the coal surface during desorption, causing methane retention at low-affinity adsorption sites and facilitating desorption initiation. The energy required for molecular surface diffusion and the energy dissipated during this process may constitute the primary cause of hysteresis.