High-pressure sorption of nitrogen, carbon dioxide, and their mixtures on Argonne Premium Coals

Gas sorption isotherms have been measured for carbon dioxide and nitrogen and their binary mixture (N₂/CO₂ -80/20) on three different moisture-equilibrated coals from the Argonne Premium Coal Sample Program by the U.S. Department of Energy, varying in rank from 0.25 to 1.68% vitrinite reflectance (VR_r). The measurements were conducted at 55 °C and at pressures up to 27 MPa for the pure gases and up to 10 MPa for the gas mixture. The effects of the large differences in equilibrium moisture contents (0.8 to 32.2%) on sorption capacity were estimated on the basis of the aqueous solubility of CO₂ and N₂ at experimental conditions. Especially for the Beulah-Zap coal with an equilibrium moisture content of ∼32%, the amount of dissolved CO₂ contributes significantly to the overall storage capacity, whereas the amounts of N₂ dissolved in the moisture water are low and can be neglected. Sorption measurements with nitrogen/carbon dioxide mixtures showed very low capacities for N₂. For Illinois coal, these excess sorption values were even slightly negative, probably due to small volumetric effects (changes in condensed phase volume). The evolution of the composition of the free gas phase in contact with the coal sample has been monitored continuously during each pressure step of the sorption tests. This composition changed strongly over time. Apparently, CO₂ reaches sorption sites very quickly initially and is subsequently partly replaced by N₂ molecules until concentration equilibration is reached.