Structural Characteristics of Deformed Coals with Different Deformation Degrees and Their Effects on Gas Adsorption

To investigate the effect of structure characteristics of deformed coal on methane adsorption, four coal samples with different degrees of deformation were collected. Fourier transform infrared (FTIR) spectroscopy and low temperature N-2 adsorption experiment were applied to analyze the macromolecular structure and pore structure, respectively. Methane adsorption experiment was done to obtain the adsorption parameters with the Langmuir equation. The results indicate that coal structures with different deformation mechanisms have different evolution rules. Under the brittle deformation mechanism, with the increase in the degree of deformation, the aromaticity and condensation of coal noticeably increase, and the aliphatic side chains noticeably decrease. Coal's specific surface area (SSA) and total pore volume (TPV) slightly decrease while its average pore diameter (APD) increases. Under the ductile deformation mechanism, the aromaticity and condensation of coal further increase. The SSA and TPV of coal noticeably increase, and APD is noticeably reduced. Meanwhile, there exist different influential factors on methane adsorption under different deformation mechanisms. The main factor affecting methane adsorption capacity at the brittle deformation stage is the macromolecular structure of coal, while, at the ductile deformation stage, both macromolecular structure and pore structure affect methane adsorption capacity. This study expands our understanding of the influence of deformation mechanisms on coal structure and of the coupling relationship between coal structure and gas adsorption under the effect of tectonic stress. Simultaneously, it provides data support and a theoretical basis for the prevention and control of coal and gas outbursts and the exploitation of coalbed methane in enrichment areas of deformed coal.