Characteristics of methane adsorption/desorption heat and energy with respect to coal rank

Abstract

Changes of heat and energy during methane adsorption and desorption are essential factors affecting the production of coalbed methane. To characterize adsorption and desorption behaviors, low-temperature nitrogen adsorption experiments and methane isothermal adsorption/desorption experiments were carried out on coals with different rank. The Brunauer–Emmett–Teller (BET) specific surface area (SSA) of the coal samples studied range from 0.29 m2 g−1 to 2.31 m2 g−1, and Barrett–Joyner–Halenda (BJH) pore volume ranges from 0.95 × 10−3 cm3 g−1 to 6.14 × 10−3 cm3 g−1. It is observed that the BET SSA and BJH pore volume decrease first and then increase with the increase of coal rank. The isometric heat in the adsorption process is less than that in the desorption process. There is an energy difference between adsorption and desorption, which makes the sorption process irreversible. The larger the Langmuir volume, the higher the adsorption capacity, making the external energy required for the adsorption process smaller and the limit isosteric heat more minor. At different temperatures, the Gibbs free energy of samples BD-1, LL-1 and HC-1 changes between 2.26–2.98 kJ mol−1, 2.05–3.24 kJ mol−1 and 1.91–3.08 kJ mol−1, respectively, indicating that the process of methane desorption on coal is a spontaneous reaction. The cumulative reduction of surface free energy (Δγ) and the reduction of surface free energy at each pressure point (Δγp) show that the adsorption capacity of the low- and medium-rank coal are more easily affected by temperature, while that of the high-rank coal is less affected by temperature. Moreover, the smaller the BET SSA, the greater Δγ and Δγp.