Methane Adsorption Interpreting with Adsorption Potential and Its Controlling Factors in Various Rank Coals

Abstract

Water content, metamorphism (coal rank) particle size, and especially pore structure, strongly influence the adsorption capacity of coal to methane. To understand the mechanism of methane adsorption in different rank coals, and its controlling factors, isothermal adsorption experiments with different coal ranks, moisture contents and particle sizes at the temperature of 303.15 K were conducted. In addition, the pore structures of coals were investigated through N2 adsorption/desorption experiments at the low-temperature of 77 K for selected coals from the Junggar Basin of NW China, Qinshui Basin and Ordos Basin of north China. Moreover, the adsorption potential of methane on the surface of the coal matrix was calculated, the controlling factors of which were discussed. The obtained methane isothermal adsorption result shows that the Langmuir volume (VL) of coal is independent of the particle size, and decreases with the increase of moisture content, which decreases first and then increases when the coal rank increases. Combined with the pore structure by the N2 adsorption at 77 K, VL increases with the increase of pore surface area and pore volume of coal pores. Besides, the adsorption potential of all selected coals decreased with the increase of the methane adsorption volume, showing a negative relationship. The interesting phenomena was found that the surface adsorption potential of the coal matrix decreases with the increase of moisture content, and increases with the decrease of particle size at the same pressure. With the same adsorption amount, the adsorption potential on the surface of coal matrix decreases first, and then increases with the increase of coal rank, reaching a minimum at Ro,m of 1.38%, and enlarging with the increase of pore surface area and the pore volume of coal pores. These findings may have significant implications for discovering CBM accumulation areas and enhancing CBM recovery.

Experimental Study of Adsorption Characteristics and Deformation of Coal for Different Gases

“…17 Some researchers analyzed the macroscopic and microscopic differences between medium- and high-grade coals 18 , 19 and described their adsorption/desorption mechanisms. 20 − 22 …”

Critical pore size for micropore filling in coal samples with different rank coals

“…Coal is a complex porous media material with a relatively developed microporous structure, which is closely related to factors such as the degree of deterioration, moisture, temperature, confining pressure, mineral composition, and micro-organisms. Absorbed and packed methane accounts for 80%-90% of the methane in coal reservoirs [22,23]. Yakovlev et al [24] selected microporous adsorbents for CO 2 adsorption experiments and found that the adsorption heat is closely related to the temperature in the high microporous filling area or at high temperatures, the microporous adsorbents expand, and the adsorption heat in the low filling area gradually stabilizes.…”

Influence of H2O on Adsorbed CH4 on Coal Displaced by CO2 Injection: Implication for CO2 Sequestration in Coal Seam with Enhanced CH4 Recovery (CO2-ECBM)

“…Besides, H 2 O adsorbed on oxygenic functional groups of coal tends to form H 2 O clusters. Then, it could occupy micropores and induce capillary condensation in mesopores of coal, thereby weakening CO 2 and CH 4 diffusion. , Moreover, other types of physicochemical force between H 2 O and the coal matrix, including the van der Waals force and the electrostatic force, also hinder CO 2 and CH 4 adsorption in coal micropores. , Unlike physicochemical force, physicomechanical force between free H 2 O and fractures of coal belongs to weak-intermolecular force. , Thus, it mainly affects CO 2 and CH 4 flow in macropores and fractures of coal.…”

“…14,15 Moreover, other types of physicochemical force between H 2 O and the coal matrix, including the van der Waals force and the electrostatic force, also hinder CO CH 4 adsorption in coal micropores. 16,17 Unlike physicochemical force, physicomechanical force between free H 2 O and fractures of coal belongs to weak-intermolecular force. 18,19 Thus, it mainly affects CO 2 and CH 4 flow in macropores and fractures of coal.…”