The classic Dubinin-Radushkevich (DR) equation, which is used to describe the adsorption of gases, requires a "saturation pressure" in its calculation, which is not defined for supercritical conditions. By using gas density instead of gas pressure (and adsorbed phase density rather than saturation pressure) to describe the sorption of the gas onto the surface, the DR equation can be applied in supercritical conditions. A proportionality term (k times the gas density) was added to this modified DR equation to account for possibilities such as Henry's law dissolution by the coal, errors in cell volume and helium density, and differences in the accessibility to coal between helium and the other gases. The sorption characteristics of three dry Australian bituminous coals when exposed to carbon dioxide, methane, and nitrogen were measured over the range 0-20 MPa at 53 °C. The data were fitted by the modified DR equation to within 1% of the sorption capacity across the isotherm in nearly all cases, substantially better than any of the alternatives tested. Without the proportionality term, sorption at high pressures was substantially underpredicted.

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If CO 2 can be sequestered in coal seams while simultaneously displacing coalbed methane [enhanced coalbed methane (ECBM)], some of the sequestration costs can be recovered through the production of methane. One potential difficulty with ECBM is that CO 2 is known to swell coal, which may reduce its permeability. Coals also swell in other gases, although not to the same extent. Here, we report on the swelling of sub-bituminous and bituminous coals in CO 2 , CH 4 , N 2 , CF 4 , ethane, and various noble gases. Helium and Ne induced negligible swelling; all other gases swelled the coals to varying degrees. The maximum swelling was proportional to the critical temperature of the gas, except for CF 4 , which is attributed to its greater size, preventing it from penetrating the coal as completely as the other gases. This indicates that swelling of these coals by all of these gases has a similar basic mechanism; CO 2 is only different in the extent to which it swells coal. All coals swelled more in the direction perpendicular to the bedding plane than parallel to it, with the ratio of the swelling in each direction independent of pressure or gas type. Gas sorption and swelling in coal were found to be related according to a simple quadratic polynomial expression. The same relationship held for all of the coals and all gases investigated here. This means that swelling can be accurately predicted from the condensed volume of the gas adsorbed, regardless of the type of coal or gas.