Stress degradation mechanism of coal macromolecular structure: Insights from molecular dynamics simulation and quantum chemistry calculations

The shear action caused by tectonic stresses deforms and destroys the coal macrostructure and significantly affects the coal macromolecular structure. To study the degradation mechanism of the macromolecular structure of low-rank coal during shearing, the macromolecular structure model of Wender coal was selected in this study. Forty molecules of this structure were used to construct a polymer cell. Shearing simulations were performed using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software, and bond strength was determined using Gaussian software. Two chemical bond breakage types were observed: tensile break and shear break. The former is controlled by the bond's position and strength, while the latter is controlled by the bond's position and angle strength. The mechanism of tensile break is different from that of shear break. The former tends to occur when the bond rotates clockwise around the point of action, towards the shear action direction (X axis positive direction), and the rotation angle is between 0 degrees and 90 degrees. The latter easily occurs when the bond rotates clockwise around the point of action, towards the shear action direction (X axis positive direction), and the rotation angle is between 90 degrees and 180 degrees. Small molecules, such as CH4, are produced by the shear action, partially revealing the excess coalbed methane eruption mechanism of coal and gas outburst. The shear action increases the order degree and stratification of the coal molecular structure by breaking and cutting side chains and bridge bonds, promoting the evolution of the coal rank.