Laboratory investigation on pore characteristics of coals with consideration of various tectonic deformations

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Coal structure is highly related to paleo-stress variation and the pore structure of coals can be modified through tectonic activities. This study characterizes the pore structure variations for various coal samples with different tectonic deformation intensities. The low temperature N2 and pressure CO2 adsorption analyses were conducted to quantify the pore structures, and high pressure CH4 adsorption measurements were carried out for the sorption analysis. The pores were classified into ultramicropores (<2 nm), micropores (2–10 nm), mesopores (10–100 nm) and macropores (>100 nm). The results show that with the increase of coal deformation intensity, the proportion of pore size of 50–300 nm decreased, showing that more macropores and mesopores were deformed to the smaller pores (<50 nm). Micropores (2–10 nm) in granulated and mylonitic coals obviously increased. The predominant ultramicropore in four kinds of coal structures were distributed at the range of 0.45–0.6 nm and 0.80–1.0 nm. Tectonic coals formed more ultramicropores lower than 0.65 nm. The adsorption of CH4, N2 and CO2 increased as the coal deformation degree increased, following the order: mylonitic coal > granulated coal > cataclastic coal > intact coal. Fractal dimensions show that tectonic coals characterized the rougher pore surface (higher D1) and more homogeneous pore structures (lower D2), which led to the higher adsorption capacity. The development of micropores was positive with Langmuir VL, but the ultramicropore SSAs were significantly larger than the contribution to SSAs of micropores and mesopores. Thus, ultramicropore provided the main adsorption sites for CH4. The variations of adsorption potential increased with the coal deformation intensity increased, and the smaller the adsorption space volume was, the larger adsorption potential would be, illustrating that adsorption in micropores was higher than mesopores and macropores. Tectonic coals have the higher reduced rate of cumulative surface free energy, which shows that tectonic damages promote the adsorption potential and surface free energy in coals.