Feasibility for High-Temperature Graphitization of Deformed Meager Coal

Abstract

The flattening of nanoscale pores is a crucial step in the process of anthracite graphitization, which primarily occurs due to lithostatic pressure or tectonic stress. Anthracite, a highly metamorphosed form of carbonaceous material, is a primary component of artificial graphite, but its availability is limited in China. Therefore, it is necessary to investigate the graphitization of low metamorphic coal. Note that the pores of deformed coal are substantially compressed, and the evolution of macromolecular structure is accelerated owing to strong tectonic extrusion and shearing action. Herein, the feasibility analysis of coal-based graphite at temperatures of 2200 and 2600 °C was investigated using mylonitized meager coal (YA-M; Ro,max = 2.34%), which exhibits the most intense deformation intensity, and it was compared with undeformed coal (YA-U; Ro,max = 1.87%) from the same coal seam. Results indicate that YA-U primarily consists of cylindrical pores with an average pore diameter of 10 nm, while YA-M exhibits extensive fine bottleneck and parallel plate pores with an average pore size of ∼4 nm. Additionally, interlayer spacing decreases from 0.3592 nm in YA-U to 0.3575 nm in YA-M, while crystallite height increases from 1.9633 nm in YA-U to 2.1424 nm in YA-M. The undeformed coal YA-U exhibits short and disordered aromatic fringes with a localized turbostratic structure that hinders graphitization. In contrast, YA-M exhibits more and longer localized fringe pairs arranged in parallel with improved structural ordering, indicating a favorable environment for graphitization. Ductile deformation narrows the pore diameter distribution, causing deformation pores on the mylonitic granules’ surface to close, shrink directionally, collapse, or connect, resulting in obvious flattening characteristics of nanoscale. Based on these results, the graphitization degree of the graphitized products YA-GM-2200 and YA-GM-2600 was 80.23% and 88.37%, respectively, with interlayer spacings of 0.3371 and 0.3364 nm, indicating that these products have entered the semigraphite (medium graphitization) and graphite range, respectively. In contrast, the graphitization degree of YA-GU-2200 and YA-GU-2600 was only 51.16% and 56.98%, respectively, indicating that they have only progressed from anthracite to meta-anthracite (the preliminary graphitization stage). These findings confirm the view that the shear stress generated by the geological age flattens the pores of mylonitized meager coal and optimizes the aromatic lattice arrangement and orientation, facilitating graphitization and lowering the graphitization temperature (from 2800 to 2200 °C). Deformed coal is soft and can be easily crushed into powdered form using the human hand, making it less suitable for transportation. Additionally, owing to the unfavorable separation of minerals and lower hydrocarbon-generating potential, deformed coal application potential and price are consistently lower than those of undeformed coal. However, deformed coal is widely distributed in various coal measures in China, making these research findings valuable in promoting the high value-added utilization of low-rank deformed coal resources and expanding the carbon source of coal-based graphite materials.