The effect of moisture on the sorption process of CO₂ on coal

The effect of moisture on both the sorption capacity and the kinetics of CO₂ sorption was investigated on three coal samples originating from the Czech Republic. One sample was Darkov bituminous coal from the Czech part of the Upper Silesian Basin, while the two other samples were enriched maceral fractions of vitrinite (Fučík coal, Upper Silesian Basin) and huminite (Bílina coal, North Bohemian Basin), which differ in the degree of coalification and petrographic composition. Sorption measurements were performed up to pressures of 8bar and 1bar at Darkov coal and temperatures of 45°C (moisturized coal) and 55°C (dry coal). The modified Dubinin–Radushkevich equation was used to fit the sorption isotherm data, and the kinetic data was fitted by the linear driving force model. In all cases, the moisturized coal showed lower sorption capacity than the dry coal and the reduction in sorption capacity decreases with the increasing of carbon content. For the Darkov sample, the heat of sorption decreases with moisture, which is attributable to preferential attraction of water to high-energy sorption sites. Both moisturized enriched maceral fractions showed an increase in heat of sorption induced by the shift of the modus of radii towards smaller values. The sorption rates of all samples decreased with increasing pressure. It is assumed that the reason for this behavior is especially intensive gas-molecule collision. Both enriched maceral fractions showed a reduction in the sorption rates on the moisturized samples. This finding is in agreement with previously published results and is attributed to accumulation of water in the coal structure, with a resulting reduction in the pore radii and gas diffusion rates. The Darkov sample showed different behavior with an increase in the sorption rates of the moisturized samples. This behavior is connected with the fact that higher energy polar sites are preferentially occupied by water and are therefore not available for the uptake of CO₂.