Effect of moisture adsorption and air pre-oxidation on spontaneous combustion liability of upgraded lignite

The effect of moisture adsorption and air pre-oxidation on the spontaneous combustion liability of lignite upgraded was investigated for the purposes of obtaining the characteristics and mechanism of the spontaneous combustion liability during storage outside. By a testing apparatus for spontaneous combustion liability, the crossing-point temperature (CPT) for calculating the aggregative indicator FCC was measured for the fresh samples upgraded at the temperature ranging from 105℃ to 900℃ and for the upgraded samples suffering moisture adsorption and pre-oxidization. The changes of chemical structure were analyzed by Fourier transform infrared spectroscopy and energy dispersive spectrometer, and the evolution of pore structure was identified by physical adsorption experiments, and the difference of wetting heat for moisture absorption was determined by the micro calorimeter. The results show that the spontaneous combustion liability of fresh upgraded lignite is reduced with increasing upgrading temperature. But the adsorbing moisture and pre-oxidation make the spontaneous combustion liability of samples upgraded at 200-500℃ increase obviously comparing with the corresponding fresh upgraded samples. Pre-oxidation can increase the elemental oxygen content and the amount of oxidation active groups on the surface of samples. The oxidation active groups of aliphatic side chains, oxygen-containing functional groups, new free radicals from upgrading process, and the increase of specific surface area after deep upgrading all intensify the pre-oxidation and spontaneous combustion of the upgraded samples. The heat of wetting from adsorbing moisture not only further deepens the pre-oxidation, but also enhances the spontaneous combustion liability remarkably for the upgraded samples adsorbing more moisture.