Methane capacities of Bowen Basin coals related to coal properties

Abstract

To help in assessing coalbed methane resources, methane adsorption capacities of Permian coals from the Bowen Basin of Queensland were investigated and related to other coal properties. Maximum methane capacities of moisture-equilibrated coals, normalized to a pressure of 5 MPa and 30°C, showed a continuously increasing (and reasonably linear) trend with increasing rank over the range 80–92 wt% total carbon. Such a linear trend was not observed for methane adsorption calculated on a dry basis. Methane adsorption capacity decreased with increasing temperature by ∼0.12 mL g −1K −1. Methane capacity also decreased as moisture content increased, by ∼4.2 mL g −1 coal for each 1 wt% increase in moisture. Coal surface area (CO 2, Dubinin-Radushkevich) showed reasonable correspondence with methane capacity, although not precise enough to provide a reliable estimate of capacity. Comparison of Langmuir adsorption isotherms for nitrogen and carbon dioxide on Bowen Basin coals with the corresponding methane isotherms showed that, with a knowledge of the methane isotherm alone, nitrogen and carbon dioxide isotherms could be reliably constructed. Volumetric and gravimetric methane isotherms measured on the same coal were identical, confirming the accuracy of the procedures and calculations used.