Millisecond-resolved gas sorption kinetics and time-dependent diffusivity of coal

We explore the dynamics of gas sorption and diffusion in coal in impacting key mechanisms controlling coal and gas outburst phenomena. We apply a unique time-resolved technique to examine millisecond-resolved gas transport kinetics of CO₂ and CH₄. In particular, we define gas transport response for two distinct coals: one outburst-prone (JH) and the other non-outburst-prone (KINGII). Equilibrium gas sorption and dynamic sorption characteristics are compared between the two coals as proxies for their different outburst responses. Outburst-prone (JH) coal more readily pulverizes during mechanical comminution suggesting ready disintegration compared to non-outburst-prone coals. Sorption capacities of both coals are independent of particle size but influenced by the internal structure and the external geometry of the coal matrix. Cryomilling to reduce particle size enhances gas diffusivity. CH₄-infiltrated coals showed a marked increase in diffusivity with increasing pressure, while CO₂-infiltrated displayed similar trends at higher pressures. A critical finding of the study was the temporal variability in diffusivity. Over time, small time window measurements show that diffusivity decreases with elapsed time, suggesting that only early time-resolved measurements should be used in defining gas outburst potential. Mass fraction methods reveal that early-time diffusion is driven under a steady pressure gradient, shifting later to a transient response—allowing straightforward analysis at early time. This shift is especially significant in understanding outburst-prone coals with complex pore structures. The sorption kinetics analysis confirms that rapid gas transfer predominantly occurs in the early phases of the sorption process when a pseudo-second-order adsorption rate might potentially serve as a predictive index for evaluating the risk of outbursts with the outburst-prone (JH) coal returning a higher rate compared to the non-outburst-prone (KINGII). The findings from this study provide an improved understanding of rapid and dynamic gas transport in the coal matrix with implications for better understanding, characterizing, predicting and mitigating hazardous gas outbursts in coal.