Effect of water content on adsorption/desorption of methane of different macroscopic lithotypes

In order to study the effect of different water content in low rank coal on methane adsorption/desorption of different macroscopic lithotypes,No. 4 coal samples from Yan’an formation of Dafosi Mine Field were collected and the bright coal and dull coal samples were separated, and liquid nitrogen was used respectively. Test methods such as adsorption, scanning electron microscopy, contact angle measurement and isothermal adsorption/desorption were used to analyze the coal sample material composition, pore structure characteristics, wettability characteristics, adsorption/desorption characteristics, and based on the calculation results of thermodynamic parameters such as equal adsorption heat and surface free energy, and the influence of the wettability of different macro-coal types in low-rank coal on the adsorption/desorption characteristics of methane can be analyzed from the energy point of view. The results show that: ① The content of ash, moisture and hydrogen, oxygen and nitrogen in bright coal is lower than that of dark coal, while the content of volatile matter and carbon and sulfur is higher than that in dark coal; the surface structure of bright coal is relatively simple, with the contact angle of 56.3°. The contact angle of dark coal is 51.7°, and the wettability of bright coal is worse than that of dark coal. ②During the boosting stage, the equivalent adsorption heat value of air-dried base sample is larger than that of equilibrium water sample, and the equivalent heat of adsorption of bright coal is greater than dark coal; during the decompression stage, the equal adsorption heat of the balanced water sample is less than that of the air-dried base sample, and the equal adsorption heat of the dim coal is greater than that of the bright coal. In addition, regardless of bright coal or dark coal, the equivalent adsorption heat in the depressurization stage is greater than that in the equivalent adsorption heat in the boosting stage, indicating that methane desorption also needs to absorb more energy from the external environment, and the depressurization cannot promote methane completely. There is a hysteresis in desorption and methane desorption. The essence is the difference in energy between the adsorption and desorption processes;③ Water molecules are easy to combine with the dangling bond on the surface of coal matrix and the hydrophilic functional group inside the coal matrix, which reduces the surface freedom of coal to some extent.It can make the methane-coal adsorption system reach an equilibrium and releases less heat. Moreover, the molecular interaction between water and coal is stronger than that of methane, which can occupy an effective adsorption site on the coal surface, making the coal′s ability to adsorb methane weaker.The research results can provide a theoretical basis for the subsequent efficient development of coalbed methane in the area.