Correlation of swelling and sorption properties of block coal sample

The swelling and sorption experiments were carried out simultaneously for two block coal samples under CO2 and CH4 pressure (up to 16 MPa) at three temperatures (25, 45 and 65 °C). The absolute sorption isotherms present type-II pattern and can be described by a modified BET model. The swelling isotherms present L-type pattern and can be described by a modified Langmuir model. The relationship between swelling ratio and absolute sorption amount presents S-type pattern and can be described by a modified Gompertz function (a logistic function). It is interesting to find that the correlations of normalized properties have a single relationship that is independent of experimental conditions, coals and gases. When the sorption amount approaches to the monolayer sorption coverage (na/nm = 1), the swelling ratio reaches ∼93% of the maximum swelling ratio. It implies the importance of the mutual contact between coal and sorbate. The differential characteristic of swelling process shows that the swelling rate with respect to sorption amount varies with the sorbate coverage; the swelling rate peaks at na/nm ≈ 0.37 and approches zero at na/nm ≈ 1.5. In addition, a theoretical model correlating swelling and sorption was established. The concept of reduction of surface free energy by sorption was taken into account. It is encouraged that the experiment data match of model is generally acceptable.