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

Abstract

In the context of research on the reduction of CO 2 emissions and the production of coalbed methane (CBM), high pressure adsorption measurements of CH 4 and CO 2 have been performed on dry and moisture-equilibrated Pennsylvanian coals of different rank (0.72, 1.19 and 1.56% VR r). Adsorption isotherms of the two gases were measured up to pressures of 20 MPa (200 bar), at 40, 60 and 80 °C using a volumetric method. Total excess sorption capacities for methane on dry coals ranged between 11 and 14 Std. cm 3/g coal. The 40 °C sorption isotherms showed a saturation behavior while the 60 and 80 °C 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 ∼20–25% than those for dry coals and ranged between 7 and 11 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. CO 2 adsorption isotherms for dry and moist coals showed substantial differences. For dry coals the highest CO 2 excess sorption capacities were observed at 40 °C 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 CO 2 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 CO 2 adsorption capacity increased with increasing pressure. High-temperature (80 °C) sorption capacities for CO 2 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 CO 2 excess isotherms on moist coals is interpreted as the result of a swelling effect caused by supercritical CO 2 and enhanced by water. Some extent of swelling was also inferred for dry coals. Absolute sorption isotherms for CO 2 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.