Exploring the ultramicropore structure evolution and the methane adsorption of tectonically deformed coals in molecular terms.

The mechanical deformation of coals occurring extensively during the geological period (tectonically deformed coals) can directly alter their pore structures and then the storage of coalbed methane. This study in-situ investigated the effects of different mechanical deformations on the ultramicropore structure and the methane adsorption of coal molecules using molecular simulations. The results show that the shear deformation (< 0.23 GPa) of coals was much easier than the compression (~ 20 GPa). Further, the shear deformation can increase the void fraction (200%) and the surface area (30%) of coal molecules, comparing to the reduction of them by the compressive deformation. Accordingly, compression is not benefited to the methane storage (only remaining 14-22% adsorption amount). While, the shear deformation of coals can increase the methane adsorption amount (reaching 42-50 mmol/g). The ~ 7.5 Å is a key pore size to evaluate the effect of the shear deformation on the methane adsorption amount. Also, the adsorption sites for methane depends on the deformation mode of coals (compression: heteroatoms; shear: C atoms). Overall, the strained Wiser (bituminous, medium-rank) coal shows relatively superiority in the methane storage, while the methane adsorption of Wender (lignite, low-rank) coal is much more sensitive to the mechanical strain.