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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.

Abstract The gas content is crucial for evaluating coal and gas outburst potential in underground coal mining. This study focuses on investigating the in-situ coal seam gas content and gas sorption capacity in a representative coal seam with multiple sections (A1, A2, and A3) in the Sydney basin, where the CO2 composition exceeds 90%. The fast direct desorption method and associated devices were described in detail and employed to measure the in-situ gas components (Q 1, Q 2, and Q 3) of the coal seam. The results show that in-situ total gas content (Q T) ranges from 9.48 m3/t for the A2 section to 14.80 m3/t for the A3 section, surpassing the Level 2 outburst threshold limit value, thereby necessitating gas drainage measures. Among the gas components, Q 2 demonstrates the highest contribution to Q T, ranging between 55% and 70%. Furthermore, high-pressure isothermal gas sorption experiments were conducted on coal samples from each seam section to explore their gas sorption capacity. The Langmuir model accurately characterizes CO2 sorption behavior, with fit coefficients (R 2) greater than 0.99. Strong positive correlations are observed between in-situ gas content and Langmuir volume, as well as between residual gas content (Q 3) and sorption hysteresis. Notably, the A3 seam section is proved to have a higher outburst propensity due to its higher Q 1 and Q 2 gas contents, lower sorption hysteresis, and reduced coal toughness f value. The insights derived from the study can contribute to the development of effective gas management strategies and enhance the safety and efficiency of coal mining operations.