A hierarchical methane adsorption characterization through a multiscale approach by considering the macromolecular structure and pore size distribution

Pore structure of coal is known to be strongly heterogeneous in terms of size, shape and occurrence. The underlying sorption mechanisms are expected to be different depending on the type and size of pores..In this study, both experiments and numerical simulation were used to study sorption behavior of coal. Vitrinite maceral was chosen as the study material and the macerals were separated from Yilan subbituminous coals. Pores in vitrinite were categorized into two different types depending on the occurrence mechanisms and they are termed as elemental particle pore (EP pore) and molecular structure pores (MS pore). According to the pore structure and macromolecular structure, pore models of the two types of pores were established and used to numerically estimate the methane adsorption capacity. It was found that gas sorption varies significantly for different types of pores. Methane adsorption capacity of MS micropores is determined by the pore volume, and while that of EP pores is determined by the internal surface area due to the different sorption mechanisms. Methane adsorption in MS pores showed pore volume filling mechanism, and the methane adsorption results can be well modeled by Dubinin and Astakhov (D-A) model. On the contrary, two distinguishable adsorbed layers can be identified for in EP pore sorption, and the methane density of the first layer was much larger than that of the second layer, which is consistent with the BET model. Of the total gas adsorption amount in coal, the amount of absorbed methane in MS pores contributed a relatively large proportion compared to EP pores. With elevated gas pressure, the difference between these two mechanisms decreased, and when the pressure was 10 MPa, the proportion of methane adsorption in EP pores was similar to 40%. The overall measured gas adsorption isotherm is a sum of methane adsorption in different types of pores. The combination of simulation and experimental methods can provide more accurate and detailed information and help understand methane adsorption in coal.