Effects of pore structure and methane adsorption in coal with alkaline treatment

•The SEM and LTNA are used to study the distribution of pores in coal.•Fractal theory and adsorption model are used to analyze the changing of micropore after alkaline treatment.•A new method combining CO2 injection and alkaline treatment is proposed. Coal bed methane (CBM) is an important type of clean energy, but it can also result in coal mining disasters. Increasingly more technologies have been applied to improve the effectiveness of gas extraction. Coal contains a large number of organic minerals and inorganic minerals, and some inorganic minerals fill in pores and block the gas seepage. To improve the connectivity of coal pores, chemical methods can be used to change the distribution of the minerals in coal. In this paper, representative bituminous (TY) and anthracite (QC) are used, and an alkaline solution is also used to treat the coal samples. The SEM results show that some minerals in coal can be dissolved in alkaline solution and removed from the pores, thereby changing the distribution of the minerals. According to the LTNA test, the micropore content of coal is changed, especially in the pore size range of 2–5 nm. With the alkaline treatment, the average aperture decreases and the specific surface area increases. Fractal theory was used to analyze the data with relative pressure (P/P0) less than 0.5. The results showed that the fractal dimension of the coal sample increased after the alkaline treatment, which indicated that the micropore content of the coal sample increased due to alkaline erosion. To verify the pores changes, adsorption experiments were carried out to analyse the adsorption capacity of the coal samples. After the alkaline treatment, the maximum sorption capacity and Langmuir-like pressure both increased. CO2 injection is an effective technology to displace CBM, but it easy to leave a great deal of CO2 in coal. The residual CO2 could be reduced by injecting an alkaline solution, and the coal would also be eroded. Based on the results, a new method combining CO2 injection and an alkaline treatment was proposed to improve the effectiveness of the gas control.