Characteristics of carbon dioxide sorption in coal and gas shale - The effect of particle size

Gas retention mechanisms in shales are to some extent similar to that of coal. The gas is adsorbed in organic matter (mostly organic carbon) and clay minerals whereas transport of gas occurs in fractures. In the study two materials were analyzed coal from the Upper Silesia Coal Basin and shale sample from Baltic Basin. The coal selected for experiments was a bituminous steam coal with 3.8%wt ash content. The shale sample was characterized by rather low TOC (1.1%) but high clay minerals content. The purpose of the study was to compare the high pressure CO2 sorption characteristics of coal and gas shale and relate it to the particle size of samples subjected to tests. The size of the adsorbate has an impact on the sorption equilibrium time and reaching thermodynamic equilibrium occurs much faster in fine grained fractions. On the contrary, the particle size must reflect natural in-situ conditions and accessibility to the nano- and micropores where sorption occurs. In this study both sorbents were crushed and sieved into three particle size fractions and the CO2 sorption was measured. The measurements were performed at the constant temperature of 55 degrees C and up to the pressure of 15-16 MPa. To model the sorption behavior a three parameter Langmuir model was fitted to experimental values. Results of the sorption tests show that the particle size of the sorbent has an impact on the obtained sorption isotherm. In case of coal, the difference in the sorption capacity could'be related to the ash and inertinite content which are passing to fine particle size (<0.1 mm) whereas in shale it could be related to the area of exposed surface and extended time of sorption equilibrium. (C) 2015 Elsevier B.V. All rights reserved.