Dispersive diffusion of gases in coals. Part II: An assessment of previously proposed physical mechanisms of diffusion in coal

Carbon dioxide (CO2) and methane (CH4) vary in their diffusion behaviour in coal, with CO2 being sorbed more extensively and diffusing faster than CH4. Several different mechanisms have been proposed to explain this behaviour. To test these mechanisms, we describe here experiments investigating the rates of gas sorption in an Australian, high-volatile, bituminous coal as a function of particle size, temperature and gas type on sorption kinetics. One explanation for greater CO2 uptake proposes that CO2 can penetrate finer pore throats than CH4. If this were true, we would expect that the CH4 sorption capacity in crushed coal would approach that of CO2. However, this was not found: crushing slightly increased both CO2 and CH4 sorption capacity, as well as the helium density. Another theory proposed that methane in coal has a greater activation energy associated with its diffusion; however, we found that the temperature dependence of both CO2 and CH4 sorption rates were similar. Experiments measuring the diffusion of other gases (ethane, argon, nitrogen and krypton) showed a relationship between the critical temperature and molecular diameter of the gas and its diffusion rate in coal. We suggest that absorption occurs once gas condenses on the coal surface, with the absorption rate depending on the size and shape of the molecule. The experiments discussed in this paper confirm the dispersive nature of gas diffusion in coal, and provide evidence to explain much of the variation in diffusion rates.