Unsteady-state diffusion of gas in coals and its relationship with coal pore structure
Coalbed methane (CBM) is under consideration as a potential energy resource because of its global abundance. The exploitation and development of CBM depends on the correct characterization of coal structure and gas migration properties. In this paper, four coal samples with different degrees of metamorphism were collected from the northern China mining area. The gas desorption properties of these samples were studied using a modified gas desorption experimental setup. A nonconstant diffusion coefficient (non-CDC) model was introduced to analyze the gas diffusion properties. In addition, both mercury intrusion porosimetry (MIP) and low-pressure nitrogen gas adsorption (LP-N2GA) were used to investigate the coal pore structure. The results indicate that gas desorption and diffusion vary significantly in coal samples with different degrees of metamorphism and that the non-CDC model could accurately describe gas diffusion in coal. In bituminous and anthracite coal, gas desorption and diffusion abilities increased with the increasing degree of metamorphism, but both properties were greater in lignite coal. Comparing the pore structure characteristics and the gas desorption and diffusion properties showed that lignite's particular pore structure resulted in a higher gas adsorption capacity than for high-volatility bituminous coal. The initial desorption and diffusion in lignite were also greater than in medium-volatility bituminous coal or anthracite coal. These results suggest that lignite has significant potential for CBM exploitation and development.