Isothermal adsorption characteristics of various phases of CO₂ and CH₄ in different rank coals

ABSTRACT

Using the remaining coal seams of closed coal mines for CO₂ geological storage can effectively slow down the greenhouse effect and realize the redevelopment and utilization of closed coal mines. The burial depth of coal seams in closed coal mines in China covers an extensive range, and CO₂ and CH₄ could exist in various phases in the coal. The existing experimental objects for CO₂ adsorption in coal are mainly associated with the supercritical and gaseous phases, and a research gap comparison between the CO₂ adsorption characteristics in the liquid phase and those of other phases is very obvious. Herein, the isotherm adsorption experiments of CO₂ and CH₄ in different phases on different rank coal samples are methodically conducted in the pressure range of 0-12MPa. The experimental results indicate that the excess adsorption capacity of gaseous CO₂ and CH₄ (gas pressure <4MPa) rapidly raises with the growth of the adsorption pressure; however, a significant difference for the case of the medium- and high-pressure stage (>4MPa) is detectable. With the growth of the adsorption pressure, the excess adsorption capacity of the supercritical CH₄ gradually reaches the maximum value and then remains stable, where the maximum experimental value is 1.009 mmol/g. While the excess adsorption capacity of CO₂ near the critical pressure of liquid and supercritical state rapidly reduces and then tends to be stable. The maximum adsorption capacity is capable of touching 2.096 mmol/g, which is stable around 0.5 mmol/g. The analysis indicates that the change in the excess adsorption amount of CO₂ near the critical pressure corresponds to the change in its density. The absolute adsorption capacity of CO₂ and CH₄ monotonically grows in the pressure range of 0-12MPa, and the absolute adsorption capacity of the same coal sample follows the following order: liquid CO₂ > supercritical CO₂ > CH₄. The maximum adsorption capacity values for these substances in order are 2.493, 2.345, and 1.296 mmol/g. With the growth of the coal rank, the Gibbs free energy, specific surface area, and adsorption capacity of the understudied coal samples exhibit a U-shape relationship as a function of their coal ranks.

KEYWORDS:

• CO₂ underground storage
• coalbed methane mining
• abandoned coal mines
• coal rock adsorption
• storage approach

Acknowledgements

This study was funded by National Natural Science Foundation of China (Nos. 52074045) and Fundamental Research Funds for the Central Universities (Nos.2022CDJQY-008).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Kai Deng

Kai Deng is currently a PhD candidate in the School of Resources and Safety Engineering, Chongqing University. He is mainly engaged in the development and utilization of the remaining resources of abandoned mines.

Zhaolong Ge

Zhaolong Ge received his PhD in Safety science and Engineering from Chongqing University in 2011. He is an expert on water jets theory and its application in coalbed methance (CBM) development.

Hongwei Zhang

Hongwei Zhang is a master's student at Chongqing University. His research direction is the application of high-pressure water jet technology in coal mines.

Shihui Gong

Shihui Gong is currently a PhD candidate in the School of Resources and Safety Engineering, Chongqing University. His areas of interest are CBM development, CO2 sequestration.

Hui Zhang

Hui Zhang is currently pursuing a master's degree at Chongqing University in China. His research interests include hydraulic fracturing and unconventional gas development.