Characteristics of coal water solid liquid contact in gas atmosphere

In order to reveal the solid liquid contact mechanism of coal and water in gas atmosphere, the wetting angles for two kinds of coal samples were tested under the different gas pressures with methane or helium by a solid liquid contact angle test system in gas atmosphere. Combining with the coal surface morphology test, low pressure CO2 adsorption experiments and XRD experiments, the change laws of wetting angle in gas atmosphere were analyzed. Results showed that the wetting angle increases with the growing gas pressure. However, in the gas atmosphere with adsorptive gas, the increase of coal wetting angle is more obvious. The wetting angle of the CZ sample increases from 72.99°to 106.90° in the range from 0 MPa to 2 MPa, and the wettability changes from hydrophilic wetting to neutral wetting, and finally to hydrophobic wetting. The wetting angle of the DW sample increases from 61.23° to 85.14° in the range of 0-2 MPa, and the wettability keeps in the hydrophilic wetting. The adsorption capacity of coal to methane is the main reason affecting the wetting angle of coal. After coal adsorbs methane, a gas film is formed on the coal surface leading to the poor wettability. Especially under the high pressure conditions, the free gas in pore may reduce the contact for coal and water. Moreover, the free energy of coal surface decreases after coal adsorbs methane, making the surface is more stable and thus the poor wettability. In addition, the competition adsorption also reduces the wettability. Furthermore, the surface of anthracite is rougher and has a more developed microporous structure, resulting in higher surface free energy and stronger adsorption capacity, which makes a remarkable change of wetting angle for anthracite in methane atmosphere. Finally, the minerals in coal can affect the gas adsorption capacity and surface wetting characteristics of coal, and it is also an important factor affecting the wetting angle of gas bearing coal.