Quantitative study of the macromolecular structures of tectonically deformed coal using high-resolution transmission electron microscopy

Tectonic deformations damage the macrostructure and impact the macromolecular structures of coal. In this study, high-resolution transmission electron microscopy (HRTEM) was applied to observe the macromolecular structures of six different types of tectonically deformed coal, and quantitative analyses were conducted by using image analysis algorithms to extract the images of lattice fringes to obtain the following three structural parameters: fringe length, fringe tortuosity and fringe separation. The results show that coal rank has an important influence on the macromolecular structures of coal, and this effect varies with different stages of metamorphism. For weak tectonically deformed coal, increases in coal rank indicate an increase in fringe length and decrease in fringe separation and fringe tortuosity. The increments in the fringe separation and fringe length first increase and then decrease, and the maximum values occur during the evolution from mid metamorphic stage to high metamorphic stage. Fringe tortuosity shows a gradually decreasing trend. Tectonic deformation also has an important influence on the macromolecular structure of coal, and this effect varies with different stages of coal rank. With decreases of tectonic deformation, fringe length is increased and fringe separation and tortuosity are decreased; however, the impact of tectonic deformation on the macromolecular parameters of low-rank coal is stronger than that of the mid-rank coal. The impacts of tectonic deformation on the nanoscale pores, coal methane adsorption capacity and coalbed methane contents of low-rank coal are higher than those of the mid-rank coal.