Relationships between the sorption behaviour of methane, carbon dioxide, nitrogen and ethane on coals

Sequestration of carbon dioxide in coal seams can reduce emissions of carbon dioxide to the atmosphere. If such sequestration simultaneously results in enhanced coal bed methane (ECBM) production, some of the sequestration costs can be recovered by the value of the methane produced. This requires knowledge of both the carbon dioxide and methane sorption behaviour of coal at high pressures. In order to elucidate the connection between them, we investigated the sorption of carbon dioxide, methane, ethane and nitrogen at 55 degrees C at pressures up to 20 MPa for a number of coals. Sorption isotherms were fitted by a modified Dubinin-Radushkevich model. The maximum sorption capacities of the coals for the different gases were found to be highly correlated. The relationship between maximum sorption capacity of a coal for a gas and its critical temperature was approximately proportional. The relationship between methane and nitrogen maximum sorption capacity was particularly close: on a volume basis, the maximum sorption capacity of all coals examined for methane was twice that of nitrogen. The ratio of maximum sorption capacity of carbon dioxide and methane decreased linearly with increasing carbon content. The ethane/methane sorption ratio also tended to decrease with increasing rank though to a smaller extent, indicating that the proportionally greater sorption at low rank coals is not unique to carbon dioxide. The heat of sorption tends to increase with increasing vitrinite reflectance; this may reflect the greater polarisability of higher rank coals (which also determines their reflectance). (C) 2012 Elsevier Ltd. All rights reserved.