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

Abstract

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 002), stacking height (L c), effective stacking layer number (N ave), aromaticity (f a), 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.

The Influence Mechanism of Pore Structure of Tectonically Deformed Coal on the Adsorption and Desorption Hysteresis

“…This is not the case with medium-rank fat coal. Similar occurrences have been reported in the literature (Nie et al, 2015;Zhang et al, 2019).…”

“…N 2 was injected under pressure at a low temperature of 77 K after the coal sample had cooled (Niu et al, 2019). Finally, the instrument supporting software can be used to calculate the PSD, PV, and SSA of the sample for various pore sizes using the BJH, BET, and DFT methods (Li et al, 2019b;Zhang et al, 2019).…”

Coal Pores: Methods, Types, and Characteristics

“…104 Different pore structures of different coal ranks are closely related to the evolution of the macromolecular coal structure in different stages of coal formation. The development of lattice defects in the interlayer and internal space of the aromatic ring is one of its manifestations, 117 as confirmed by the results of Liu et al 78 Low-rank coal is in the early stage of coalification. The coal structure is loose and porous and mainly plant tissue pores are developed.…”

Thermal Evolution Characteristics of the Pore Structure in Coal and Its Dominant Factor Conversion

“…As shown in Figure a, various macromolecular structure units in coal are composed of aromatic rings and side chains, while the decrease in volatile content with increasing heat treatment temperature means that more side chains and small molecular compounds are lost. , The number of micropores increased due to the escape of small molecular compounds. However, pore size distributions show that the pore with a size below 0.4 nm was unchanged under the vacuum condition.…”

“…A coal matrix is mainly constituted of complex macromolecular polymers, and the volatile matters are mainly side chains and functional groups of coal macromolecules . In the macromolecular models of coal, micropores are formed by internal defects of macromolecules, and the mesopores and macropores are considered as the spaces between the different macromolecular units. , The increase in temperature will reduce the moisture and volatile matter, and the changes in the chemical structure of coal can affect the evolution of nanopores. , …”