Expansion of Hard Coal Accompanying the Sorption of Methane and Carbon Dioxide in Isothermal and Non-isothermal Processes

Abstract

Coal swelling because of methane and carbon dioxide was measured by the volumetric method in isothermal and non-isothermal conditions. These investigations are of key importance in the context of potential CO2 sequestration in deep unmined coalbeds. Changes of the temperature underground may disturb the adsorption balance, leading to volumetric processes in the coal strata (swelling or shrinking), which can give rise to leaks and gas desorption toward the ground surface. The isothermal results show that the strain exhibited by a coal sample during CO2 sorption is about twice that of CH4. The liner strain kinetics also show that the swelling of the sample when exposed to both gases is anisotropic and greater in the direction perpendicular to the bedding plane than parallel to it. In the case of the non-isothermal process, the pattern of dilatometric processes seems to be different. The temperature increase gives rise to the sample swelling when exposed to methane, yet the presence of CO2 leads to sample contraction, which can be attributed to the different mechanisms involved in CO2 deposition. CO2 accumulated in pores undergoes a rapid phase transition as a result of capillary condensation, leading to rapid desorption and, in consequence, shrinking of the coal sample.

Kinetics of methane and carbon dioxide sorption and sorption–induced expansion of coal – kinetic equations assessment

“…Primary experiments were performed using the apparatus and test procedures described by Baran et al [11]. The experimental program uses a purpose-built apparatus, enabling the concurrent measurements of sorption and dilatometric parameters of cuboid-shaped coal samples.…”

“…Coal sample deformations were measured with a strain gauge engineered at the Strata Mechanics Research Institute of the Polish Academy of Sciences. Linear strains are measured with an electric resistant wire strain gauge incorporating a resistance (bridge) transducer [11]. For the purpose of measurements, a cuboid samples (18 × 18 × 40 mm) were cut from the basic coal fragments, obtained from the Pniówek mine and Budryk mine.…”

“…The results of analysis performed on the basic coal samples are shown in Table 1 and Table 2. Experiments on coal Pniówek followed test procedures described by Baran et al [11]. Experiment on coal…”

“…The values of the sorption progress (V [NTPcm 3 /g]) were then calculated. The data on kinetics of linear strains of the coal sample were stored simultaneously in the internal memory of the strain meter [11]. …”

“…Gas sorption gives an opposite effect of induced expansion of microporous regions and narrowing of transport pores and the in situ a decrease of system permeability [9][10]. The majority of research in the field shows that the swelling of coal is approximately proportional to the amount of sorbed gas implicating a linear relation between sorption and expansion [6,[11][12][13][14][15][16]. Studies that suggest non linear dependence are a minority [9,17].…”