Heterogeneous mechanical and sorption characteristics induced interaction among different components in coal: Experiment and simulation

Coal formation necessitates a long geological age and intricate physical, chemical, and biological processes. Throughout this process, variations in the raw materials and external conditions contribute to coal formation, resulting in the final mined coal being a heterogeneous mixture of multiple components. This study takes bituminous coal as the research object to investigate its heterogeneous sorption and mechanical characteristics and their effects. Firstly, various non-destructive techniques employing CT, SEM, and EDS were conducted to elucidate coal’s structure. It is found that in the CT and SEM images, the mineral components contribute more white color, while organic components contribute more dark color. As organic and mineral components mixed in various forms in the observations, the coal has a heterogeneous structure characteristic on various scales. Based on the heterogeneous structure, indentation experiments were conducted on different areas in coal. The indentation results demonstrated the relationship between the mechanical modulus and the structure of various components. It was found that the area with more mineral matter had a higher mechanical modulus, implying heterogeneous mechanical characteristics in coal considering its heterogeneous structure. Simultaneously, sorption deformation kinetics experiments were performed in various areas dominated by different coal components. It was found that the CO${}_2$-induced sorption deformation is faster to reach equilibrium in the high-density area than in the low-density area. The difference in sorption deformation rate is able to result in the mechanical interaction among components. That can be reflected in a specific kinetics of sorption deformation where the sorption strain firstly increases and then decreases throughout the sorption process, revealing the heterogeneous local sorption deformation characteristics in coal. According to experimental findings and theoretical analysis, a multi-component contact mechanics model involving the heterogeneous coal structure was constructed for simulations using a self-developed nonlinear contact finite element program. Considering heterogeneous mechanical and sorption deformation characteristics from the experimental part, the program simulated the mechanical response of coal components. In alignment with the experimental results, the strain that increases first and then decreases can be obtained, confirming that the mechanical interaction among components may be induced by the heterogeneous sorption and mechanical characteristics of coal. The findings derived from the current work can provide a deeper understanding of the mechanical behavior of coal bodies in the context of solid-gas coupling and establish a foundation for practical coal-gas engineering applications, such as predicting the geomechanical performance of coal and mitigating potential geohazards (e.g., gas outbursts) associated with heterogeneous coals.