Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

Abstract

In order to study the evolution of the mechanical properties and macromolecular structures in different macerals of tectonically deformed coal (TDC), vitrinite and inertinite samples were handpicked from six block TDCs in the same coal seam with an increasing deformation degree (unaltered, cataclastic, porphyroclast, scaly and powdery coal). The micro mechanical properties were tested by the nanoindentation experiment and the macromolecular structures were measured using 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the range of hardness and elastic modulus of inertinite is 0.373–1.517 GPa and 4.339–12.158 GPa, respectively, which is significantly higher than that of vitrinite with values of 0.278–0.456 GPa and 4.857–7.810 GPa, respectively. From unaltered coal to powdery coal, the hardness of vitrinite and inertinite gradually decreases, with the difference between these macerals becomes smaller and the elastic modulus of vitrinite shows an increasing trend, while that of inertinite was more variable. Both the NMR and FITR results reveal that the macromolecular structure of inertinite has similar structural transitions as vitrinite. As the degree of deformation increases, the aliphatic side chains become shorter and the aromaticity is increasing. Macromolecular alterations caused by tectonic stress is expected to produce defects in the TDCs, therefore there should be more interspacing among the macromolecular groups for the extrusion of macromolecules caused by the indenter of the nanoindentation experiment, thereby reducing the hardness. The elastic modulus of coal is believed to be related to intermolecular forces, which are positively correlated to the dipole moment. By calculating the dipole moments of the typical aromatic molecular structures with aliphatic side chains, the detachment of the aliphatic side chains and the growth of benzene rings can both increase the dipole moment, which can promote elastic modulus. In addition, the increasing number of benzene rings can create more π-π bonds between the molecules, which can lead to an increase in the intermolecular forces, further increasing the elastic modulus.

Comparison of Nanopore Structure Evolution in Vitrinite and Inertinite of Tectonically Deformed Coals: A Case Study in the Wutongzhuang Coal Mine of Hebei Province, North China

“…When coal is subjected to strong deformation by tectonic movements, it exhibits a gradual decrease in hardness on the macroscopic scale (Wang et al, 2020a). On the microscopic scale, the aliphatic carbon in the basic structural unit (BSU) of the macromolecular structure in coal decreases and the aromatic carbon increases.…”

“…Guo et al (2017) revealed that the specific surface area and total pore volume of TDCs slightly decreased under the brittle deformation mechanism and then noticeably increased under ductile deformation. Wang et al (2020a) believed that as the deformation degree of TDCs increased, the pore volume and specific surface area increased at the deformation stages of cataclastic and ganulitic coals and then remained stable at the stage of crumple coal. Those conclusions have deepened the understanding of pore structure evolution in TDCs; however, further discussions related to how the pore structure of the maceral in a single coal evolves in TDCs need to be held.…”

“…Most research studies on the pore structure of TDCs were conducted on whole coal samples that have not been stripped. However, the physical and chemical properties of different macerals in coal are different; for example, the hardness of inertinite is generally greater than that of vitrinite (Wang et al, 2020a). Under tectonic stress compression, the pore structure evolution of different macerals is bound to be different.…”

Molecular Dynamics Simulation of the Nanoindentation of Coal Vitrinite

“…Furthermore, its deformation behavior is believed to be influenced by the distribution of various maceral groups with different mechanical properties, as has been demonstrated in recent studies (Anmin Hou et al, 2020). The aforementioned studies on the properties of coal macerals utilized depth-sensing nanoindentation technology (DSNI), which allows the mechanical properties of coal maceral groups to be precisely measured (Kossovich et al, 2019;Anmin Wang et al, 2020;Hou et al, 2020). In contrast to traditional centimeter-scale tests of coal mechanical properties (for example, uniaxial compression, triaxial compression, and tension tests etc.…”

Thermal simulation experimental study on the difference of molecular structure evolution between vitrinite and inertinite in low-rank coal

“…Vitrinite and inertinite are the main macerals in coal. They have different chemical structures and physical properties due to their different genesis (Wang et al, 2018c;Wang et al, 2020a). In recent years, due to the extensive distribution and industrial importance of vitrinite-rich coals, the research mainly focused on the characterization and evolution of vitrinite molecular structure.…”