Investigation on the methane adsorption capacity in coals: considerations from nanopores by multifractal analysis

Methane adsorption properties of coal are essential for coalbed methane (CBM) extraction and clean energy utilization. However, the effect of nanopores on CH4 adsorption in high volatile bituminous coal and anthracite remains to be revealed. In this work, the multi-dimensional description of nanopores was established using gas adsorption and FIB-SEM experiments. And the heterogeneous features at different sizes were finely quantified by multifractal analysis. Results show that the pores with size smaller than 100 nm, as storage section, are isolated in the space. The nanopores of the sample LHG have stronger heterogeneity in multiple dimensions. The pore size distributions with apparent aggregation are composed of 0.45-0.70 nm (from CO2 adsorption), 2-50 nm (from N2 adsorption) and 10-50 nm (from FIB-SEM). Nanopore structure affects the adsorption capacity of CH4 mainly in micropore structure, pore morphology and heterogeneity. The well-developed micropore structure is conducive to methane enrichment. The ink-bottle pores with the higher specific surface area are beneficial to gas storage, whereas plate-like/slit-like pores will facilitate the desorption and diffusion of gas. The more robust pore heterogeneity in the range of 0.45-0.70 nm and 2-50 nm significantly contribute to methane adsorption. This work may allow significant insights into the interaction of coal with gases during enhancing CBM recovery.