Effect of bituminous coal properties on carbon dioxide and methane high pressure sorption

High pressure sorption experiments with carbon dioxide and methane were carried out at a temperature of 45 °C and at pressures up to 15 MPa with three samples of methane-bearing, medium-rank coals in a moisture-equilibrated state using a manometric method. The samples were taken from selected positions of drill cores from exploration boreholes in the Bohemian part of the Upper Silesian Basin, and were characterized by a narrow range of degree of coalification and markedly different petrographic compositions, including a different mineral matter content. The total porosity of the coal samples was between 9% and 10%. A positive correlation was found between the equilibrium moisture in the coal samples and the total abundance of oxygen functional groups determined by FTIR. The excess sorption capacities ranged from 0.78 to 0.91 mmol g−1 for CO2 and from 0.45 to 0.52 mmol g−1 for CH4, and after recalculation to coal organic matter, the excess sorption capacities increased by up to 14% in the coal with the highest mineral fraction. The highest CO2/CH4 ratio was found in the sample that had the highest inertinite and liptinite content. The experimental isotherm data was fitted by modified Langmuir and Dubinin-Radushkevich sorption isotherms. The parameters obtained by these two methods were in good agreement for carbon dioxide. It was found that the sorption capacity of the organic matter in a coal sample with prevalence of inertinite (63.0 vol.%) was lower only by 14% for CO2 and by 18% for CH4 than the sorption capacity of the organic matter in a coal sample with prevalence of vitrinite (65.3 vol.%). This provided confirmation that the petrographic composition of a coal has an ambiguous effect.