Study of methane and carbon dioxide adsorption-desorption hysteresis in coals from Sydney Basin: A theoretical and experimental approach

Methane (CH₄) and carbon dioxide (CO₂) are primary components of coal seam gas (CSG). Understanding their adsorption-desorption hysteresis characteristics, along with the fundamental mechanism, is crucial for CSG exploitation and related hazards mitigation. This research focused on the representative Bulli coal seam in the Sydney Basin, Australia. Through the purpose-built indirect gravimetric high-pressure isothermal adsorption-desorption hysteresis experiment, a novel Langmuir-based desorption model, incorporating hysteresis effect and residual gas, was proposed. Quantitative characterization of the adsorption-desorption hysteresis degrees of CO₂ and CH₄ in coal particles of various sizes and in Φ50 mm × 100 mm intact coal samples were achieved using the improved hysteresis index (IHI). The experimental findings validated that the proposed desorption model accurately describes the desorption behavior of CO₂ and CH₄ in coal (R²>0.99). Based on the adsorption-desorption properties of ink-bottle-shaped micropores and pore deformation caused by gas adsorption-induced coal expansion, the occurrence mechanism of adsorption–desorption hysteresis and the fundamental reasons for the presence of residual gas were elucidated. Furthermore, the study explored the impact of CO₂ and CH₄ adsorption-desorption hysteresis effects on coal and gas outbursts, suggesting that coal seams rich in CO₂ do not have a higher propensity for outbursts than those rich in CH₄.