Role of pore irregularity in methane desorption capacity of coking coal

Abstract

Gas desorption property of coal is closely related to pore distribution, and the accurate characterization of pore structures is of great significance for gas flow in coal seams. In this work, nine typical coking coal samples were collected in Xishan coalfield, North China. Both qualitative and quantitative analysis were made on these samples regarding the pore structure characteristics. Fractal theory was also used to evaluate the pore irregularity and its influence on methane desorption capacity of coal. Significant hysteresis loops are observed for all the LPNA isotherms due to the delay of gas desorption. Pore volume and pore specific surface area of these coking coals are changed in the wide ranges of 1.35–8.15 × 10−3 cm3/g and 0.87–2.59 m2/g, respectively. Micropores can provide tremendous surface area but have less pore volume, and the largest contribution of pore volume is made by macropores. Fractal dimensions are determined based on LPNA data. D1 fluctuates in the small range of 2.6036–2.7937, while D2 is changed in the wide range of 2.0898–2.9268. Pore structure has obvious impact on gas desorption of coking coal. Positive correlations have been recognized between gas desorption capacity and pore volume, pore specific surface area and the average pore diameter. Gas desorption behaviors within coking coal is sensitive to the change of D2, but exhibits little relationship with D1, indicating that the increased structure irregularity of macropores can hinder the gas flow. Besides, the increase of moisture content and f of coking coal can also inhibit gas rapid release from coal matrix. This work will provide theoretical support for gas extraction underground coalmine.