Morphological Characterization of the Microcrystalline Structure of Tectonic Coal and Its Intrinsic Connection with Ultra-micropore Evolution

A systematic knowledge of the physical and chemical microstructure of coal is indispensable for better understanding gas storage and migration, and the variations of coal microcrystalline morphological structures have significant impacts on the ultra-micropore development at the molecular scale. However, the response mechanism of metamorphism and tectonism on ultra-microporous evolution in the macromolecular structure still needs further study. In this work, the microcrystalline morphological structure of original and tectonic coals and its essential relation with ultra-micropore evolution were investigated via XRD, Raman spectroscopy, and CO2 (273 K) adsorption. The results demonstrate that the XRD patterns of high-rank tectonic coals have sharper and right-shifted 002 bands compared to those of lower-rank original coals, and tectonic deformation factors may affect the development characteristics of coal microcrystalline structures. The relative content of kaolinite in high-rank tectonic coals increases, whereas that of quartz decreases; tectonism has a great influence on the composition of silicate minerals. With the enhancement of metamorphism and tectonism, the aromatic interlayer spacing d(002 )decreases from 0.3651 to 0.3482 nm, while the stacking height L-c and the aromatic slices N-ave increase by 60.7 and 68.6%, respectively, promoting the degree of aromatization f(a) and degree of graphitization g. Tectonism further advances the evolution of the above structural parameters. In addition, the peak position difference G - D-1 of Raman spectrum has a sensitive response to metamorphism and tectonism. The peak intensity ratio I-D1/I-G is directly negatively correlated with metamorphism, and tectonism brings about a further decrease. In this case, the degree of crystallinity in the basic structural units increases, and the degree of large-scale defects in the crystal decreases, contributing to the well-developed microporous structure of original and tectonic coals with different ranks. Metamorphism facilitates the ordered macromolecular structural evolution on aromatization and condensation, and subsequently, tectonism results in the intensive recombination, stacking, condensation, and aromatization of microcrystalline structures in advance. Thus, this combination may lead to the macromolecular structure evolution from disordering to ordering and then essentially alter the chemical structure of tectonic coal. The aforementioned results have guiding significance for revealing the gas sorption and diffusion behaviors as well as the mechanism of coalbed methane generation, storage, and migration.

Automated summary

A systematic understanding of the physical and chemical microstructure of coal is crucial for gas storage and migration. The variations in coal microcrystalline morphological structures have significant effects on the development of ultra-micropores. This study investigated the microcrystalline structures of original and tectonic coals and their relation to ultra-micropore evolution. The results showed that tectonic deformation factors affected the development of coal microcrystalline structures and influenced the composition of silicate minerals. Metamorphism and tectonism also had significant impacts on the evolution of structural parameters. The findings of this study are important for understanding gas sorption and diffusion behaviors in coal and the mechanism of coalbed methane generation, storage, and migration.