Adsorption of pure carbon dioxide and methane on dry coal from the sulcis coal province (SW Sardinia, Italy)

Abstract

Enhanced coalbed methane recovery (ECBM) increases the recovery of the methane present in a coal seam by injecting CO 2 at high pressure. It is attractive from two perspectives, the valuable methane recovered and the storage of the greenhouse gas CO 2 for geological times. In the framework of a feasibility study for the Sulcis Coal Province in Italy, the adsorption of pure CO 2 and CH 4 on dry coal has been measured at 45 and 608C, using a magnetic suspension balance with in situ density measurement. The results show that the CO 2 adsorption isotherms on coal are similar to those for other standard adsorbents such as silica gel and activated carbon. From the excess adsorption isotherms, the absolute adsorption is calculated using the assumption of constant volume of the adsorbed phase. As expected, CO 2 get adsorbed more than CH 4 in all cases. The Sulcis coal can uptake CO 2 at the reservoir conditions in an amount of about 10% of its mass.

Competitive adsorption of methane and carbon dioxide at nanometer level

“…Studies have found the concentration of CH 4 in free phase is increasing in the process of adsorption and desorption of CH 4 and CO 2 mixed gas, while the CO 2 concentration decreases continuously, which fully proves that coals absorb more CO 2 than other gases, providing theoretical basis for coal seam storage of CO 2 and enhanced CH 4 output. The study of adsorption-desorption characteristics of multicomponent gases can predict the change trend of output gas composition and provide key basis for correct economic evaluation of coal bed methane development.…”

“…[2]. The results show that the adsorption capacity of mixed gas coal is between that of pure CH 4 with that of CO 2 , and the adsorption of CH 4 and CO 2 mixed gas by coals show different characteristics; coal adsorbs CO 2 first, when pressure increases, the coal adsorbs more CO 2 and less CH 4 [3]. With the increased proportion of carbon dioxide in the gas, the curve is more similar to the adsorption curve of carbon dioxide [4].…”

“…b is a parameter related to temperature and absorbed gas, and demonstrates the change in absorption capacity at different temperatures. In summary, the changes in the adsorption capacity of CH 4 and CO 2 can be reflected by value a and b. Figure. 2 Relationship between CH 4 pressure and CH 4 adsorption capacity Based on previous research results, the adsorption capacity of methane by coals is in line with Henry's law under low pressure and reaches saturation limit under high pressure, therefore, Langmuir equation can demonstrate the adsorption of gas by coals.…”

“…2.2 Adsorption capacity of CH 4 and CO 2 mixed gas by coals Adsorption-desorption characteristic curve obtained from pure gas and carbon dioxide adsorption isotherm experiment could not demonstrate true adsorption situation of CH 4 and CO 2 mixed gas in coal seam. So it is necessary to study the adsorption-desorption characteristics of mixed gas by coal seam when there are other gases existing in coal seam, which is important for the correct evaluation of the development potential of coal bed gas.…”

“…When gas or liquid contacts coal surface, residual force field is generated due to the difference in stress on surface molecules and internal molecules of coal cracks and pores, and thereby generating surface potential, to increase the concentration of gas molecules in pore walls of coals and thus giving rise to adsorption phenomenon and releasing adsorption heat [1]. The adsorption experiment of single component gas showed that the absorption of CO 2 was higher than that of CH 4 by coals, finding that the adsorption capacity of CO 2 by coal surface was higher than that of CH 4 , and the absorption capacity ratio of CO 2 /CH 4 changed with pressure and coal rank, but the pressure was not directly proportional to absorption capacity ratio of CO 2 /CH 4 . With the increase of pressure, adsorption ratio of CO 2 /CH 4 decreases gradually.…”