Investigation on micro-surface adhesion of coals and implications for gas occurrence and coal and gas outburst mechanism

The adhesion of coal micro-surface has been proved to be a powerful phenomenon in understanding the micromechanism of coal seam gas occurrence, migration, and outburst. With this motive, an attempt has been made in the present study to investigate micro-sruface adhesion of outburst-prone and non-prone coal. The micro-surface force curves of three outburst-prone (OP) coals and two non-outburst-prone (NOP) coals have been measured with atomic force microscope (AFM), then obtained the micro-surface adhesion and the action distance corresponding to the adhesion. Results show that the adhesion of OP coal can reach hundreds to thousands of nanonewtons, while NOP coal acheives approximately tens of nanonewtons. The action distance of the OP coal is slightly higher than that of NOP coal, and the adhesion of the OP coal has a positive linear correlation (R-2 > 0.73) with action distance. The calculation shows that the action distance of the OP coal can reach up to 30 nm, with an average of 6-8 nm. Moreover, the mechanism of physical adsorption of the coal seam gas was analyzed, and the concept of critical velocity V-cr for gas adsorption is proposed. It is inferred that only gas molecules having motion velocity less than V-cr can be adsorbed on the near-surface of the coal body. Finally, the coal and gas outburst was divided into four stages, and the micro chain reaction mechanism was then raised. This study may provide valuable basis for the worldwide research on occurrence of coal seam gas and revealing the mechanism of coal and gas outburst.