To investigate the mechanical damage characteristics and energy evolution laws of primary coal–rock combinations with different coal–rock ratios, uniaxial compression tests were conducted. Combined with acoustic emission monitoring results, a comparative analysis was performed on the yield strength, elastic modulus, acoustic emission signal characteristics, failure characteristics, and energy accumulation–dissipation characteristics of five different coal–rock ratio specimens. The study reveals the following: (1) Defect structures and dimensions of coal bodies influence the strength of specimens, with the peak stress and elastic modulus of specimens showing a decreasing trend with an increasing coal–rock ratio. (2) The transitional zone of primary coal–rock composite exhibits distinct interface effects on coal and rock components. Coal-derived stresses manifest as horizontal tensile stresses, promoting deformation at the interface between coal and rock, whereas rock-derived stresses at the interface manifest as horizontal compressive stresses, restricting deformation at the coal–rock interface. Moreover, positions closer to the interface experience stronger limitations due to the interface effects. (3) Coal–rock interface cracks are more developed in the primary coal–rock composite, with multipoint cracking occurring at the interface during failure, and a feedback mechanism between coal and rock failure exacerbates the damage and dynamic manifestation intensity of coal bodies. (4) The acoustic emission signals from single-rock samples indicate shear failure of the specimens. Conversely, the acoustic emission signals from single-coal samples and combinations suggest that tensile failure is the primary destabilizing factor. Moreover, with an increase in the proportion of coal, specimens transition from tensile failure to tensile–shear composite failure.

The process of rock damage and failure is accompanied by the dissipation of energy and an increase in plastic strain. This study attempts to determine the relationship between dissipated energy and plastic strain in sandstone during the damage and failure process. A conventional triaxial cyclic loading and unloading test was conducted on sandstone samples to analyze the energy evolution and plastic strain characteristics of rock during the mechanical performance degradation and deformation failure process. The analysis results indicate that the evolution law of rock energy dissipation and plastic growth during the whole stress–strain process is highly consistent. Before the peak stress, dissipated energy and plastic strain increased linearly with input energy and axial strain, respectively. In the residual stress stage, there is an approximate linear evolution relationship between dissipated energy and plastic strain. Based on the correlation characteristics of energy dissipation and plastic growth, a modified damage model was established by characterizing plastic deformation by damage. In addition, a numerical program was developed using the Finite Volume Method (FVM) based on the damage model. The rock damage model has been validated by experimental results and numerical test. The research findings may provide valuable insights into the correlation mechanism between energy dissipation and plastic growth.

A quantitative characterization of the change in coal molecular structures with different cyclical microwave modification parameters and a better understanding of the reaction mechanism of the modification are of great significance for the commercial extraction of coal bed methane (CBM). Therefore, long-flame coal samples obtained from the Ordos Basin, China, were modified by microwave radiation with different times, and the long-flame coal molecular structure parameters were determined by solid-state ¹³C nuclear magnetic resonance (ss¹³C NMR), Fourier transform infrared (FTIR) spectrometry, and X-ray photoelectron spectrometry (XPS). Atomistic representations of the raw long-flame coal molecular model and modified long-flame coal molecular models were established. The temperature rise, pore volume increase, mineral removal, and functional group changes after the modification have a negative effect on methane adsorption. After the modification, the decrease in surface area of the micropores reduced the adsorption site of methane in coal. As a result, the methane adsorption amount decreased linearly with the decreasing surface area. The CH₄ adsorption isotherms of the long-flame models were dynamically simulated and analyzed. The results of this study can prove that after multiple cycles of microwave modifications, the functional groups in long-flame coal were fractured, and the number of micropores was reduced, which effectively decreased the methane adsorption performance in long-flame coal seams, thereby promoting methane extraction. Microwave modification is a promising method for enhancing CBM recovery.