Changes in the physicochemical characteristics and spontaneous combustion propensity of Ximeng lignite after hydrothermal dewatering

Spontaneous combustion behaviour and physicochemical characteristics of Ximeng lignite dewatered by hydrothermal dewatering (HTD) were investigated. In addition, effect of upgrading temperature, as well as the mechanism for evolution of spontaneous combustion propensity was discussed. The results showed that after HTD, fixed carbon content of lignite increased, whereas equilibrium moisture content and volatile content decreased; aromatic carbons increased relatively at the expense of oxygen-containing functional groups and aliphatic hydrocarbons. Pore structures of lignite were developed by different extents after HTD. Spontaneous combustion propensity of upgraded lignite was evaluated using the crossing-point temperature method. The results indicated that the spontaneous combustion propensity of HTD-upgraded lignite significantly depended on upgrading temperature. When upgrading temperature was lower than 230 degrees C, spontaneous combustion propensity was raised due to development in pore structure. However, when upgrading temperature exceeded 230 degrees C, spontaneous combustion propensity was suppressed because active sites involved in coal oxidation were extensively removed. In conclusion, a higher upgrading temperature is recommended to extend the upgrading effect and to further suppress the spontaneous combustion propensity of lignite.