Investigation of the fractal characteristics of adsorption‐pores and their impact on the methane adsorption capacity of various rank coals via N₂ and H₂O adsorption methods

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 N2 adsorption and H2O 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 N2 adsorption analysis and H2O adsorption analysis are similar, especially for samples LH7 and WLH8. Fractal analysis indicates that D2(N2) (0.5 < P/P0 < 1) can more accurately characterize the fractal features of adsorption‐pores than D2(H2O), which may be a result of the significant coal‐H2O interaction. Moreover, D2(N2) has stronger correlations with the coal pore parameters. With the increase in D2(N2), 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 D2(N2) < 2.7‐2.8, the pore structure plays a key role, while for D2(N2) > 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 D2, 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.

Estimation of Adsorption Isotherm for Deep Coalbed Methane: A Monolayer-Filling Model Based on Pore Fractal Dimension

“…The FHH model has been extensively employed for calculating the fractal dimension of coal pores, , in which ln .25em V = B + A [ ln true( ln P 0 P true) ] where V is the volume of adsorbed gas molecules at the equilibrium pressure, cm 3 /g; P is the equilibrium pressure, cm 3 /g; P 0 is the saturation pressure of gas, MPa; and B and A are constants.…”

“…Samples. The FHH model has been extensively employed for calculating the fractal dimension of coal pores, 50,53 in which…”

A New Empirical Model to Predict Methane Adsorption Amount of Anthracite Considering Temperature Effect

“…Through the systematic analysis of pores using a series of experimental methods, such as low-pressure CO 2 adsorption (LCO 2 GA) and N 2 adsorption (LN 2 GA), coal has been proven to be a heterogeneous microporous material with a large specific surface area (SSA). , Generally, high rank coal with low volatile content (such as anthracite) has considerable SSA; for example, the obtained LCO 2 GA SSA of some anthracite samples even exceeded 100 m 2 /g. − As shown in Table , the micropores of the coal samples can contribute more than 75% of the N 2 -SSA, which can provide most adsorption sites and storage spaces for coal adsorbing gas. Moreover, combined with the LCO 2 GA method, the pore analysis of high rank coal indicates that micropore is mainly distributed in the size range of less than 1 nm. , …”

Exploring the Effect of Low-Temperature Oxidation on the Petrophysical Characteristics of Coal

“…The Frenkel− Halsey−Hill (FHH) model is chosen to explore the law between ln (V/V 0 ) and ln (ln (P 0 /P)), which can describe and quantify the roughness of the coal surface. 46,47 The general expression of its isotherm is given in eq 2…”