Contribution of thermal expansion on gas adsorption to coal sorption-induced swelling

Adsorption of gases such as methane and carbon dioxide in coal is an exothermic process. Under isentropic conditions (fully insulated condition), the ejected heat results in an additive transient thermal expansion in addition to the sorption-induced swelling. The magnitude of thermal expansion and its feedback on methane adsorption remains ill-defined. We explore this response in high volatile bituminous coal by measuring the exothermal release of heat (integral heat of adsorption) via temperature change during methane adsorption and use these calorimetric data to define the thermodynamic response. These data link thermal expansion directly to adsorption and adsorption swelling. The results show that methane adsorption can lead to the elevation of coal temperature by more than 10 °C. The resulting thermal expansion is consistent with adsorption swelling and related to adsorption capacity and pressure. The decline of surface potential energy resulting from adsorption is usually considered to be the reason for adsorption swelling. However, the results of this study show that thermal expansion due to the heat of adsorption may account for up to 35% of the total adsorption strain under ideal isentropic conditions. In non-isentropic laboratory experiments of adsorption swelling (approaching isothermal conditions), thermal expansion during adsorption cannot be measured accurately due to the rapid heat loss. However, in-situ within coalbed reservoirs, reduced heat dissipation may retain thermal deformation for longer and impact short-term permeability evolution.