Investigation of the fractal characteristics of adsorption‐pores and their impact on the methane adsorption capacity of various rank coals via N<sub>2</sub> and H<sub>2</sub>O adsorption methods
Ming‐yi Chen, Ya‐pu Yang, Caihong Gao, Yuanping Cheng, Jing‐chun Wang, Ning Wang
Abstract
Abstract Coal seam is a sedimentary body with complex pore system. The gas adsorption property of coal is greatly determined by the adsorption‐pores (<100 nm), and the fractal dimension can be used an index to estimate the impact of the adsorption‐pores on the methane adsorption capacity of various rank coals. In this paper, low‐temperature N 2 adsorption and H 2 O adsorption methods were used to study the adsorption‐pores structure and its fractal features. The results show that the development of adsorption‐pores closely depends on the coalification, and the degree of pore development exhibits a U‐shaped trend with increasing coal rank. Additionally, the pore size distributions of coal samples from N 2 adsorption analysis and H 2 O adsorption analysis are similar, especially for samples LH7 and WLH8. Fractal analysis indicates that D 2 (N 2 ) (0.5 < P / P 0 < 1) can more accurately characterize the fractal features of adsorption‐pores than D 2 (H 2 O), which may be a result of the significant coal‐H 2 O interaction. Moreover, D 2 (N 2 ) has stronger correlations with the coal pore parameters. With the increase in D 2 (N 2 ), the Langmuir volume first decreases and then increases, which is probably associated with the competition effect of the pore structure and surface irregularity of coal. When D 2 (N 2 ) < 2.7‐2.8, the pore structure plays a key role, while for D 2 (N 2 ) > 2.7‐2.8, the influence of the specific surface area is more prominent. The equilibrium moisture content of the coal samples also has a positive correlation with D 2 , except for low‐rank coal sample YZG2 due to the presence of a large amount of oxygen‐containing functional groups, which increases its water‐holding capacity.