Supercritical Pure Methane and CO₂ Adsorption on Various Rank Coals of China: Experiments and Modeling

Abstract

Carbon dioxide sequestration in coal seams with enhanced coal-bed methane recovery (CO2-ECBM) is considered as a promising option for permanent carbon dioxide storage. As one of the most important factors to the success of the CO2-ECBM process, adsorption of methane and CO2 on coal helps to assess the amount of recoverable methane as well as the storage capacity of CO2 of the targeted coal seam. In this work, the methane and CO2 adsorption isotherms were measured with a volumetric technique at temperatures of 35, 50, and 65 °C and pressures up to 16 and 12 MPa (CO2 adsorption at 35 °C is limited below 6 MPa), respectively. Four coals of various rank exploited from four main coal seams in China were tested. The isotherms fit well to the Ono-Kondo lattice model, which confirms the applicability of this model in describing the adsorption behaviors of methane and CO2 on coal under the supercritical conditions. In addition, the experimental results show that the excess adsorption of CO2 reaches the maximum level between 7 and 9 MPa, while the excess adsorption of methane exhibits a less pronounced maximum. The maximum adsorption capacities of the coals for methane and CO2 decrease slightly with temperature increase. Additionally, the maximum adsorption capacities of methane and CO2 are also dependent on coal rank (indicated by vitrinite reflectance coefficient, R o max) and present a U-shaped trend with coal rank. The preferential adsorption ratio of CO2 to methane on a basis of absolute adsorption obtained from Ono-Kondo lattice model is in the range of 1.13−3.52 under test conditions. This ratio of bituminous coals (R o max ranging from 0.47% to 1.35%) decreases significantly with increasing pressure; however, the pressure dependence of the preferential adsorption ratio is less pronounced for the anthracite (R o max = 4.06%). The preferential adsorption ratio decreases with an increase in coal rank with the only exception of anthracite.

Influence of CO2 Exposure on High-Pressure Methane and CO2 Adsorption on Various Rank Coals: Implications for CO2 Sequestration in Coal Seams

“…6 The experimentally measured variable, called Gibbsian surface excess (GSE, mmol g −1 ), was calculated by 26,29 …”

Supercritical CO2 adsorption in a simulated deep coal reservoir environment, implications for geological storage of CO2 in deep coals in the southern Qinshui Basin, China

“…14,57,58 Although, this rank dependence does not always facilitate an increase in CO 2 adsorption capacity. This may be a discrepancy in our regular recognitions related to coal rank effect on adsorption capacity.…”

“…It should be noted that the temperature and pressure of unmineable coals suitable for CO 2 sequestration are commonly above the CO 2 critical point 13,14 ; therefore, assessment of deep coal CO 2 sequestration potential should be based on observations of supercritical CO 2 adsorption. It should be noted that the temperature and pressure of unmineable coals suitable for CO 2 sequestration are commonly above the CO 2 critical point 13,14 ; therefore, assessment of deep coal CO 2 sequestration potential should be based on observations of supercritical CO 2 adsorption.…”

Applicability of various adsorption isotherm models on adsorption of methane and CO2 on Indian shales

“…Later Ono-Kondo model was applied on studies related to gas adsorption on microporous solids [30][31][32][33][34]. Among all the AI models, Ono-Kondo model has advantages which include its ability to describe adsorption data for a wide range of temperature and pressure, calculation of data accurately, and predicts adsorption behavior to DR model but for extended range of conditions [28][29][30][31][32][33][34][35]. The Ono-Kondo model has been mostly used for porous adsorbents such as activated carbons.…”