Characteristics of microscopic pore structure and fractal dimension of bituminous coal by cyclic gas adsorption/desorption: An experimental study

The characteristics of micropore (0.32-2 nm), mesopores (2-50 nm) and fractal dimensions of bituminous coal during the process of cyclic gas adsorption/desorption were revealed by combining N-2 (77 K) and CO2 (273 K) adsorption experiments from microscopic aspect. The results indicate that the pore structure characterization in the coal matrix are changed, resulting in decreased mesopore volumes and increased micropore volumes. The mesopore volumes are mainly constituted by the pores of 10-20, and 20-30 nm, and it will increase at first and then decrease with the increasing pressures. The maximum change of micropore volume reaches 65.6%, indicating a great effect on the micropores influenced by pressures. In addition, the main micropore size range, major peak and the model diameter of coals all increase with the increasing pressures, the higher the adsorb pressure is, the higher swelling is. With the help of the conceptual models, we then analyzed the variation reasons, which may be result from transformation of mesopores and the connection of the inaccessible pores. D-1 and D-2 in #1 and D-2 in #2 all increase with the increasing adsorption pressures, enhancing the roughness of surface and complexity of structure, while D-1 in #2 shows an opposite property. The study of variations of microscopic pore structure by cyclic adsorption/desorption was aimed at providing new understanding for the exploration of the changes of diffusion and permeability.