Variation law of coal specific surface Gibbs function in gas adsorption process

The gas adsorption process of coal is an exothermic process and the desorption process is an endothermic process. The extent of temperature and energy changes is related to the degree of coal metamorphism,and the equilibri- um pressure of gas adsorption,etc. The data of gas adsorption were obtained by using isothermal adsorption experi- ments,and the influencing factors of coal adsorption gas was analyzed through its pore structure and interaction force. However,few scholars have explored the mechanism of coal adsorbing gas from the perspective of the change of surface Gibbs function. Therefore,the calculation formulas of relevant characteristic parameters of the change of Gibbs function of coal specific surface in gas adsorption process were studied. Isothermal adsorption experiments were carried out u- sing two coal samples from Fenghuangshan Mine and Qigou Mine in China. The data of isothermal adsorption experi- ment at four different temperatures (20,30,40 and 50 ℃ ) and six different pressures (0. 2,1. 2,1. 5,2. 5,3. 0 and 4. 0 MPa) were obtained. The specific surface Gibbs functions during the gas adsorption of different coal samples were calculated by using single-layer adsorption and multi-layer adsorption models. Studies showed that the specific surface Gibbs function of methane adsorbed on coal samples was calculated in two different ways,the change of Gibbs function of specific surface decreases with the increase of temperature and increases with the increase of pressure. The results of single-layer adsorption and multi-layer adsorption were quite different and this phenomenon was analyzed. The heat of the gas adsorption and desorption process was calculated. It can be seen from the calculation of heat that the specific surface Gibbs function obtained by using the multi-layer BET adsorption model is closer to the actual situ- ation. The mechanism of energy change in gas adsorption was discussed. The results show that the change of gas ad- sorption energy affects the change of adsorption capacity,and the change of energy is also affected by the microstruc- ture and internal chemical structure of coal samples. Therefore,the adsorption and desorption of coal samples is a com- plex and changeable process. To explore how to influence gas desorption can improve the gas extraction effect from the perspective of changing energy.