Structural characteristics of adsorption pores in low-rank coals and their effects on methane adsorption performance

The study of the CH₄ adsorption characteristics of low-rank coals is of great significance for gas content prediction, gas drainage, and hazard prevention. Therefore, this study selected six typical low-rank coal samples from coal mines in Shaanxi Province, on which low-temperature N₂ adsorption experiments, low-pressure CO₂ adsorption experiments, and CH₄ isothermal adsorption experiments were carried out. As a result, the structural characteristics of adsorption pores in low-rank coals were obtained. Moreover, this study quantitatively characterized the relationships between the parameters of CH₄ adsorption characteristics and those of adsorption pore structure using the micropore filling and monolayer adsorption theories, determining the CH₄ adsorption mechanism of adsorption pores. The results are as follows: (1) The specific surface area of the adsorption pores was mainly contributed by micropores; the CH₄ adsorption capacity primarily depended on the pore volume of the adsorption pores; micropores contributed 74.71%‒88.97% of the total pore volume of the adsorption pores; (2) The ultimate CH₄ adsorption capacity of the adsorption pores was negatively linearly correlated with their average pore size and was positively linearly correlated with their pore volume and specific surface area. The Langmuir pressure constant fluctuated only in a small range with an increase in the average pore size, pore volume, and specific surface area of the adsorption pores, with no strong linear correlation between the former and the latter three elements; (3) The six low-rank coal samples exhibited significant fractal characteristics, with comprehensive fractal dimensions of 2.573‒2.720 (average: 2.647), indicating that the adsorption pores of low-rank coals had had strong heterogeneity. The ultimate CH4 adsorption capacity first increased and then decreased with an increase in the fractal dimension, exhibiting an overall upward trend; (4) The relationships between the structure and CH₄ adsorption capacity of adsorption pores can be quantitatively characterized using the micropore filling and monolayer adsorption theories. There were small relative errors between the calculated values and the experimental values of the ultimate CH₄ adsorption capacity, which were 4.47%‒6.65% for long-flame coal and 13.77%‒16.02% for non-caking coal. The results of this study can provide theoretical guidance for the subsequent quantification of the relationships between CH₄ adsorption characteristics and adsorption pore structure, as well as the accurate prediction of the gas content in coal seams.