Erosion effect on microstructure change and oxidation behavior of long-flame coal under different pH conditions

In order to study the effect of aqueous solutions with different pH values on coal oxidation behavior, coal samples were eroded in aqueous solution of various pH values (4, 5, 7 and 8) for 30 days and to study its microstructure change and oxidation behaviors. The pore structure and functional groups were characterized by low-temperature N-2 adsorption and Fourier transform infrared spectroscopy. The oxidation characteristics of coal were investigated by temperature-programmed experiments. Compared with raw coal, larger average pore diameter and pore volume, more active functional groups, such as methyl, methylene, hydroxy and ether, were found in the eroded coal. Among them, the microstructure of coal eroded in acid water changes most obviously. Eroded coal samples have more pores with diameter > 20 nm and an obvious increase in particle surface roughness, which facilitates the adsorption and transportation of O-2 in coal body. The O-2 consumption and CO production of dried eroded coal, especially eroded in aqueous solution with pH = 4, are more than those of raw coal, and its apparent activation energy decreases both in the slow oxidation stage and the fast oxidation stage, which indicates that the eroded coal is more prone to spontaneous combustion.