Effects of nano-pore system characteristics on CH4 adsorption capacity in anthracite

Abstract

This study aims to determine the effects of nanoscale pores system characteristics on CH4 adsorption capacity in anthracite. A total of 24 coal samples from the southern Sichuan Basin, China, were examined systemically using coal maceral analysis, vitrinite reflectance tests, proximate analysis, ultimate analysis, low-temperature N2 adsorption–desorption experiments, nuclear magnetic resonance (NMR) analysis, and CH4 isotherm adsorption experiments. Results show that nano-pores are divided into four types on the basis of pore size ranges: super micropores ( 100 nm). Super micropores, micropores, and mesopores make up the bulk of coal porosity, providing extremely large adsorption space with large internal surface area. This leads us to the conclusion that the threshold of pore diameter between adsorption pores and seepage pores is 100 nm. The “ink bottle” pores have the largest CH4 adsorption capacity, followed by semi-opened pores, whereas opened pores have the smallest CH4 adsorption capacity which indicates that anthracite pores with more irregular shapes possess higher CH4 adsorption capacity. CH4 adsorption capacity increased with the increase in NMR porosity and the bound water saturation. Moreover, CH4 adsorption capacity is positively correlated with NMR permeability when NMR permeability is less than 8 × 10–3 md. By contrast, the two factors are negatively correlated when NMR permeability is greater than 8 × 10–3 md.