Study on the structural evolution and mechanical deterioration characteristics of heat-treated tar-rich coal

The high-temperature conditions of deep mining significantly affect the mechanical stability of tar-rich coal. The mechanical properties and energy evolution characteristics of heat-treated tar-rich coal are discussed in this study through experiments and corresponding analyses. The research object is tar-rich coal from the Shaanxi coalfield in China. Microstructural and static/dynamic mechanical tests were conducted on specimens heat-treated at different temperatures (25℃, 200℃, 400℃, and 600℃) to study their structural changes and mechanical behavior. The study shows that tar-rich coal undergoes rapid pyrolysis after 418℃. Therefore, the surface cracks of the coal sample treated at 600℃ deepen, leading to an eight-fold increase in porosity and a permeability as high as 87.1%. The size distribution range of pores and cracks expands, and multifractal characteristics become more pronounced. The carbon composition of the heat-treated tar-rich coal gradually changes from being predominantly aliphatic carbon to being predominantly aromatic carbon. Its surface structure undergoes an evolution from “dense” to “cellular” and then to “fracture-connected”. After static pressure, the failure mode shifts from brittle failure dominated by tension to ductile failure dominated by shear slip and plastic rheology. The energy evolution under different confining pressures exhibits an inherent consistency, with approximately 29% of the input energy dissipated irreversibly. The energy dissipation mechanism under dynamic compressive loading tends towards volumetric fragmentation and shear slip, while the energy in dynamic splitting tests is more concentrated in the generation of through-tensile cracks. This results in the energy absorbed during dynamic compressive failure being 2-3 times that absorbed during dynamic splitting under the same conditions. Overall, temperature significantly affects parameters directly reflecting mechanical properties, while impact pressure influences strength indicators by increasing energy absorption.