Micropores manipulation of medium-rank coal through aggregate structure evolution during coalification: A study based on HRTEM analysis

Micropores in coal, which can be manipulated by aggregate structure, have been widely considered to be a key factor affecting the gas adsorption of coal. In this study, to explore the aggregate structure evolution and its influence on micropores during coalification, a well characterized sample set of medium-rank coals from Shanxi province, covering a vitrinite reflectance (Ro) interval from 0.68% to 1.98%, was selected. High resolution transmission electron microscopy (HRTEM) and low-pressure CO2 adsorption experiments (LPCO2) were employed to quantify the evolution of aggregate structure and micropores in coal during coalification. The re-sults show that the two methods give similar results for the micropore distribution. With the increase of coali-fication, four obvious stages are found for fringe length and distribution of micropores, with turning points appear near Ro = 1.20% and 1.50%, respectively, indicating that the coalification process are combination of continuity and jumping and related to the coalification jumps. The dominant fringes are short fringes (0.3 -0.54 nm) and intermediate fringes (0.55 -1.14 nm) in all coals, which contribute most to the curvature of c = 1.0 -1.04 and c greater than 1.04, separately, suggesting the existence of non-six-membered rings. The cur-vature of c = 1.0 - 1.04 decreases while the curvature of c = 1.04 -1.15 increases with increasing Ro, indicating that the fringes mainly tend to bend in medium-rank coals. For micropores, most pores in coal were found to be in the range of 0.5 -0.8 nm based on the result of HRTEM images, which mostly appeared in the disorder regions featured with intermediate fringes. However, the pores of 0.8 -1.2 nm and 0.4 -0.5 nm are always found in the regions without the basic structural units (BSUs) in lower-rank coals and the edge regions stacked with long fringes in the higher-rank coals, respectively. This work provides a strategy to manipulate micropores of medium-rank coal through aggregate structure evolution during coalification, which is expected to provide theoretical basis for further research on gas adsorption of coal.