Pore and chemical structure variation of tectonically deformed coal and their influences on methane adsorption

Coal deformation involves the transformation of coal from its initial geometric state to its final state, encompassing rigid body translation, rotation, strain, distortion, and volume changes driven by factors such as embrittlement, fracturing, or ductile deformation. This study selected three types of tectonically deformed coal (TDC) from the Sanjiang-Mulinghe Basin Group—cataclastic, granulated, and mylonized—characterized by progressively increasing degrees of deformation, to evaluate their methane adsorption capacities and elucidate the underlying mechanisms driving the observed differences in adsorption capacity. The adsorption characteristics were examined using an isothermal adsorption apparatus employing the volumetric method. Nitrogen adsorption-desorption and CO₂ adsorption techniques were utilized to analyze the mesoporous and microporous structure. X-ray diffraction and Fourier-transform infrared spectroscopy were performed to determine the basic structural units and organic molecular structure parameters in coal. The findings indicated that methane adsorption capacity progressively enhances across the cataclastic → granulated → mylonized deformation sequence in the TDCs of the Sanjiang-Mulinghe Basin Group, which is attributed to the deformation-induced expansion of available adsorption spaces and the strengthening interactions between coal macromolecules and methane molecules. The deformation of coal prompts the polycondensation and depolymerization of its macromolecular structure, leading to an increase in both pore volume and specific surface area of TDCs. This enhancement is particularly significant with higher degrees of coal deformation, offering additional space and adsorption sites for methane, thereby improving methane adsorption capacity. Concurrently, parameters such as crystallite size and aromaticity were found to increase with more intense coal deformation. The increase in crystallite size is due to the growth in the number of crystallites per stack, the height of the stacks, and the diameter of crystallites within a stack, accompanied by a decrease in the distance between aromatic layers. The study enhances comprehension regarding the gas-bearing characteristics of tectonically deformed coals.