Effects of ionic liquids on the chemical structure and exothermic properties of lignite

The chemical structure and morphological of lignite treated with four imidazole ionic liquids (ILs), namely [BMIM][BF4],[BMIM][NO3], [BMIM][I], and [EMIM][BF4], was characterized through scanning electron microscopy and X-ray diffraction. Furthermore, the exothermic properties of the oxidation process at low temperatures (<200.0 degrees C) were investigated using a C80 microcalorimeter. The results revealed that the IL partially dissolved minerals and side chains of surface molecules in lignite, resulting in reduction of the unit stacking degree, ductility, and number of effective piles, which ultimately led to a lower coalification degree of the treated coal sample. The initial exothermic temperature of the treated coal sample increased and that more thermal energy was absorbed and less thermal energy was released in the endothermic and exothermic phases, respectively, as a result of the destruction of the molecular structure of coal by IL. The exotherm and the exothermic heat release rates from treated coal samples were consistent and in the following order: IL-untc > [BMIM][NO3]-tc> [BMIM][I]-tc > [EMIM][BF4]-tc> [BMIM][BF4]-tc. The IL destroys the chemical structure of the coal, resulting in a decrease in the activity of the coal, which is manifested by an increase in apparent activation energy. (C) 2020 Elsevier B.V. All rights reserved.