High-pressure methane and carbon dioxide adsorption on dry and moisture-equilibrated Pennsylvanian coals

In the context of research on the reduction of CO2 emissions and the production of coalbed methane (CBM), high pressure adsorption measurements of CH4 and CO2 have been performed on dry and moisture-equilibrated Pennsylvanian coals of different rank (0.72, 1.19 and 1.56% VRr). Adsorption isotherms of the two gases were measured up to pressures of 20 MPa (200 bar), at 40, 60 and 80 degreesC using a volumetric method. Total excess sorption capacities for methane on dry coals ranged between I I and 14 Std. cm(3)/g coal. The 40 degreesC sorption isotherms showed a saturation behavior while the 60 and 80 degreesC isotherms exhibited a monotonous increase over the entire experimental pressure range (up to 20 MPa). Methane sorption capacities of moisture-equilibrated coals were lower by similar to 20-25% than those for dry coals and ranged between 7 and I I Std. cm(3)/g coal. No distinct maturity effect was discernible for methane adsorption on the three samples studied, neither in the dry nor in the moist state. CO2 adsorption isotherms for dry and moist coals showed substantial differences. For dry coals the highest CO2 excess sorption capacities were observed at 40 degreesC with maximum values of 70 Std. cm(3)/g within limited pressure ranges. Carbon dioxide excess sorption for the moisture-equilibrated coals was usually lower than for the dry samples in the low pressure range. All high-pressure CO2 adsorption isotherms for moist samples were bimodal with distinct minima and even negative excess sorption values in the 8-10 MPa (80-100 bar) range. Beyond this range CO2 adsorption capacity increased with increasing pressure. High-temperature (80 degreesC) sorption capacities for CO2 were very low (< 5 Std. cm(3)/g) in the low-pressure range but reached much higher levels (25-50 Std. cm(3)/g) above 12 MPa. The strong bimodal character of the CO2 excess isotherms on moist coals is interpreted as the result of a swelling effect caused by supercritical CO2 and enhanced by water. Some extent of swelling was also inferred for dry coals. Absolute sorption isotherms for CO2 were calculated assuming a sorbed-phase density of 1028 kg/m(3) and compared with literature data. Like the excess isotherms, the absolute isotherms show a distinct decline in the 8-10 MPa pressure interval. At higher pressures, however, they increase monotonously. (C) 2002 Elsevier Science B.V. All rights reserved.