Swelling of coal induced by cyclic sorption/desorption of gas: Experimental observations indicating changes in coal structure due to sorption of CO2 and CH4

Cyclic gas sorption/desorption experiments were made on two medium-rank coals from Lower and Upper Silesia Basins, Poland, bituminous A and bituminous C. Measurements were made during two sets of experiments, in three successive cycles of sorption/desorption of gas on cylindrical coal specimens (diameter 22 mm, height 44 mm), cut with the long axis perpendicular to the bedding plane. Methane was used in the first group of tests, in the second, the coal specimen was in contact with CH4 in the first and third cycles while during the middle one, the coal was subjected to sorption/desorption of CO2. Experiments were conducted at 298 K, for a gas pressure of 4.0 MPa and changes in strain were monitored throughout. Sorption of CO2 and CH4 affected the two coals differently and there could be a connection between changes in the coal structure due to sorption of the gases and the character of the induced strain. The study of swelling induced in coal by sorption of CO2/CH4 was aimed at improving the understanding of coal behavior, for practical reasons, as a part of an ECBM project.

Research highlights
• The amount and character of volumetric strain induced in coal by gas sorption depend upon the type of coal, its previous history and the kind of sorbate used.
• Sorption of either CO2 or CH4 appears to lead to different, irreversible changes, probably associated with structural rearrangements in the samples examined.
• The strain data demonstrated that during each cycle of gas sorption–desorption the measurements were made on a material modified by the previous cycles.