The influence factors for gas adsorption with different ranks of coals

In this paper, scanning electron microscopy, low-temperature N-2 adsorption and CH4 isothermal adsorption experiments were performed on 11 coal samples with R-o,R-max between 0.98 and 3.07%. The pore structure characteristics of coals (specific surface area, total volume distribution) were studied to assess the gas adsorption capacity. The results indicate that there is significant heterogeneity on coal surface, containing numerous channel-like pores, bottle-shaped pores and wedge-shaped pores. Both Langmuir volume (V-L) and Langmuir pressure (P-L) show a stage change trend with the increase of coalification degree. For different coalification stages, there exist different factors influencing the V-L and P-L values. For low-rank coals (R-o,R-max < 1.1%), the increase of V-L values and decrease of P-L values are mainly due to the abundant primary pore and fracture within coal. For middle-rank coals (1.1% < R-o,R-max < 2.1%), the moisture content, vitrinite content and total pore volume are all the factors influencing V-L, and the reduction of P-L is mainly attributed to the decrease of moisture content and inertinite content. Meanwhile, this result is also closely related to the pore shape. For high-rank coals (R-o,R-max > 2.1%), V-L values gradually increase and reach the maximum. When the coal has evolved into anthracite, liquid hydrocarbon within pore begins pyrolysis and gradually disappears, and a large number of macropores are converted into micropores, leading to the increase of specific surface area and total pore volume, corresponding to the increase of V-L. In addition, the increase of vitrinite content within coal also contributes to the increase of V-L. P-L, reaches the minimum, indicating that the adsorption rate reaches the largest at the low pressure stage. The result is mainly controlled by the specific surface area and total pore volume of coal samples. This research results will provide a clearer insight into the relationship between adsorption parameters and coal rank, moisture content, maceral composition and pore structure, and it is of great significance for better assessing the gas adsorption capacity.