Macromolecular and pore structures of Chinese tectonically deformed coal studied by atomic force microscopy

In this study, the macromolecular and nanoscale pores of different types of tectonically deformed coals were characterized by AFM. The nanostructures of coal differed in terms of the metamorphism degree, deformation degree, and deformation properties. The macromolecular grid-like structure of low-rank bituminous coal was loosely reticular, and the round or elliptical pores between chains were largely isolated. The macromolecular reticular grid-like structure of high-rank bituminous coal was more compact than that of low-rank bituminous coal. The large molecules of anthracite exhibited enhanced polymerization with significant reductions in the spacing between macromolecular layers. The reticular grid-like structure of this coal was fairly compact, and the pores between the chains were squeezed flat. Flat micropores were the predominant pore morphology in this type of coal. Metamorphism generally increased the quantity of micropores in the coal. The macromolecular reticular grid-like structures and pore structures of coal varied according to their properties and deformation degree. Under brittle deformation, the ratio of the transitional pores and the interconnectivity of the pores increased with increasing deformation degree. This phenomenon was beneficial for gas diffusion. Under ductile deformation, the coal surface exhibited creep flow features, the surface pore morphology of the coal was irregularly developed, and the ratios of micropores and transitional pores in the coal increased.