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Microcrystalline Characterization and Morphological Structure of Tectonic Anthracite Using XRD, Liquid Nitrogen Adsorption, Mercury Porosimetry, and Micro-CT

Tectonic deformation acts as an indispensable factor affecting the crystalline structure, but its relationship with the evolution mechanism of the morphological structure is seldom considered. In this work, the characterization of microcrystalline and morphological structures in tectonic coal was comprehensively explored using X-ray diffraction (XRD), pore structure analyzers (liquid nitrogen and mercury intrusion methods), and micro-computed tomography (micro-CT). The XRD results indicate that a transformation of the chemical composition was observed in the coal body after continuous tectonic deformation. Meanwhile, the variation in microcrystalline parameters, such as the interlayer spacing (d), stacking height (L), effective stacking layer number (N), aromaticity (f), and graphitization degree (g), reveal that the microcrystalline structure of tectonic coal has experienced a positive evolution with graphitization and aromatization. Pore morphological analyses demonstrate that pore volume distributions of micropores, mesopores, and macropores were totally enhanced by tectonism while the pore shape potentially changes slightly. Simultaneously, the incremental noneffective pores of tectonic coal may promote gas accumulation and storage in the pore space. Fracture morphological analyses utilizing micro-CT lay emphasis on the orderly distributed cleats with a uniform scale in original coal, whereas the reconstruction of microstructures in tectonic coal illustrates a scattered distribution with good connectivity. In addition, the evolution of the macromolecular structure has an influence on the growth of nanoscale pores since the lattice defects may generate from the interlayer of the aromatic ring or the internal space. This research is of an enlightening significance for understanding the influence of tectonic stress on the chemical structure evolution of coals and its relationship with the nanoscale morphological structure development in tectonic coal.