Microstructural characteristics of Shengli lignite during low-temperature oxidation and promotion effect of iron species

It is important to understand low-temperature oxidation to inhibit the spontaneous combustion of lignite for its safe transportation and efficient utilization. The study investigates the effect of temperature on microstructure and mass change during low-temperature oxidation of Shengli lignite (SL) and further explores the promoting effect of iron species. In this work, inherent minerals of Shengli lignite were first demineralized by HCl leaching (SL+), Fe3+ were then loaded by impregnation method (SL+-Fe), and low-temperature (200–300 °C) oxidation was carried out in laboratory-scale fixed bed reactor. The structures of low-temperature oxidation treated coal samples were characterized by SEM-EDS, FT-IR, XPS, XRD and Raman spectroscopy. The low-temperature oxidation experiment results showed that coal displayed noticeable jump in mass change at particular oxidation temperatures (called jump temperature). The jump temperature of SL+-Fe was approximately 20 °C lower than that of SL+. The FT-IR analysis indicated presence of analogous quinone structure that was originating from SL+-Fe after low-temperature oxidation at 200–210 °C, which was approximately 20–30 °C lower than that of SL+. XRD and XPS results revealed that iron species changed from highly dispersed or unknown chemical state to α-Fe2O3 before and after jump temperature. It is deduced that the jump phenomenon of coal samples maybe attributed to the oxidation decomposition of analogous quinone structure, and highly dispersed iron species caused shifts in jump temperature of SL+-Fe to lower values.