Ultra micropores in macromolecular structure of subbituminous coal vitrinite

Pores in coal provide surface area and volume for coalbed methane storage. In this study, a 3 dimensions (3D) macromolecular structure of subbituminous coal vitrinite was constructed, which was demonstrated to be consistent with the experimental Nuclear Magnetic Resonance (NMR) data, density data and pore volume data, to study ultra micropores formed in the macromolecule. The results showed that both accessible pores and inaccessible pores existed in the macromolecular structure and that all pores were smaller than 0.62 nm. Most of the accessible pores were formed by aliphatic chains and most of the atoms that directly touched the pore surface were hydrogen atoms. The accessible pores showed obvious fractal features, and the fractal dimension of pores detected by probes smaller than 0.5 nm is 2.73. We also found that the volume of pores seen by helium is considerably larger than that of pores seen by methane. This would result in underestimation of methane adsorption capacity in a volumetric methane adsorption experiment, where the void volume is tested by helium. For the coal sample Y-1, the methane adsorption capacity would be underestimated by 2.7 cm3/g at 10 MPa.