Relationships between the Critical Properties of Gases and Their High Pressure Sorption Behavior on Coals

Abstract

Sequestration of carbon dioxide in coal seams is a potential method of reducing atmospheric emissions of carbon dioxide. If carbon dioxide can be sequestered in coal seams and it simultaneously results in enhanced coal bed methane (ECBM) production, some of the sequestration costs can be recovered in the value of the methane produced. This requires knowledge of both the carbon dioxide and methane sorption behavior of coal at high pressures. However, the relationship between methane and carbon dioxide sorption at high gas pressure is not well understood. To elucidate their relationship, we investigated the sorption of carbon dioxide, methane, ethane, nitrogen, argon, krypton, xenon, carbon tetrafluoride, and sulfur hexafluoride by dry coals at 55 °C at pressures up to 20 MPa; all of these gases have critical temperatures below 55 °C. Sorption isotherms for the different gases were very different but all could be fitted by a modified Dubinin−Radushkevich model to within about 1% of their calculated maximum adsorption capacity. We found that the maximum adsorption capacity, determined from the isotherms, of the coals investigated for a supercritical gas increases linearly with the critical temperature of the gas when the maximum adsorption capacity is expressed on a (van der Waals) volume basis, except for gases that could not penetrate the coal as effectively as the other gases: carbon tetrafluoride and sulfur hexafluoride. This behavior is consistent with the idea that, in sorption of supercritical gases, the surface phase is not condensed but acts as a compressed gas. This provides a simple explanation of why the molar maximum sorption capacity at temperatures near ambient decreases in the order carbon dioxide > methane > nitrogen: their critical temperature decreases in the same order. The heats of sorption of different gases on a given coal, calculated from the isotherm, were closely related to their van der Waals attraction constant and were similar to those reported for sorption of these gases onto graphite at low pressure. The calculated heats of sorption for xenon and ethane on coal were higher than that for carbon dioxide on the corresponding coal. For the three bituminous coals examined, carbon tetrafluoride and sulfur hexafluoride did not penetrate the coal as completely as the other gases. About 3−5% (by volume) of each coal was calculated to be inaccessible to these two gases that were accessible by the other gases, which we attribute to their greater molecular diameter. Except for these two gases, the volume of each coal accessible by each gas was found to be similar (to within 1.5% of the coal volume).