CO2-ECBM and CO2 sequestration in Polish coal seam – experimental study

Purpose
Methane recovery is interesting not only because of its clean combustion; it is also beneficial for the environment because of the reduction of the amount of methane emitted into the atmosphere, which is important because of methane’s significant impact on the greenhouse effect. However, desorption of methane is a slow process, significantly dependent on the coalification of coal, its porosity and petrographic composition. Injection of carbon dioxide into the coal bed under sufficient pressure might be a factor in stimulating the efficiency of this process, as - because of preferential sorption - carbon dioxide displaces methane molecules previously absorbed in the coal matrix.
Methods
The measurements were made for Polish low-rank coal used for the analysis of methane recovery from Polish coal mines. Coal samples were collected from sites used for geological, sorption and petrographic research, as well as for the assessment of the reservoir’s genetic origin CH₄ content. Experimental studies of sorption were performed with the use of the volumetric method at a lower and higher gas pressure.
Results
The methane isothermes show more than double the reduction of adsorption along with increasing temperature. The most significant changes of sorption capacity due to temperature variations can be seen when observing the difference in the course of the hysteresis of sorption / desorption of the gas as a function of temperature. In cases where there is a temperature of 323K, a temperature hysteresis loop might indicate larger quantities of methane trapped in the porous structure of coal. In cases of carbon dioxide as sorbate, a similar shape of sorption isotherms occurred at both temperatures, while the temperature increase caused approximately double the reduction of sorption capacity. Also the isotherm’s shape is similar for both temperatures of measurement, indicating no effect of temperature on the amount of gas within the structure of the tested coal.High-pressure isotherms of CO₂ and CH₄ are confirmed in the literature, proving that carbon dioxide is the gas that allows the best penetration of the internal structure of bituminous coal. The critical temperature of CO₂ (304.5 K) is so high, that sorption measurements can be performed at room temperatures (293, 298 K), where activated diffusion is relatively fast.
Practical implications
Understanding the sorption of gases is the primary issue, related to the exploitation of coal seams, when explaining the mechanism of gas deposition in coal seams and its relationship with outbursts of rocks and gases in mines.
Originality/ value
The results indicate successful sorption of carbon dioxide in each experiment. This provides the rationale to study the application of the coal tested to obtain methane genetic origin genetic methane with the use of the CO₂ injection.