Characteristics of pore size distribution of coal and its impacts on gas adsorption

By using N2 and CO2 adsorption/desorption tests, the pore size distribution of samples from 6 coal mines was obtained and differences of pore shape were analyzed according to the adsorption/desorption isotherms. Meanwhile, gas adsorption tests were also conducted to research the relationship between adsorption pressure and adsorption volume. Relationships between adsorption capacity and pore size distribution were analyzed. Based on the results of proximate and petrographic analysis, influences of coal rank and moisture on gas adsorption were researched. The results show that the adsorption volume of gas mainly concentrates in micropores and it is influenced by mesopores at the same time. The Langmuir volume is determined by both micropores and mesopores while the Langmuir pressure is only related to micropores. Moisture content has negative effect on methane adsorption due to the competitive adsorption between H2O and CH4. The relationship between Langmuir volume and coal rank is characterized by a 'U-shaped' curve, whereas the Langmuir pressure decreases with an increase of coal rank. In the mining process, measures should be taken to magnify the pores in coal, and reduce the proportion of micropores and then accelerate the desorption of gas.