Dielectric properties and microwave upgrading characteristics of Zhaotong lignite

The dielectric properties of lignite were studied by using the resonant cavity perturbation method. The effect of temperature, particle size, moisture content, and microwave frequency were investigated. Results showed that the variation of dielectric parameters could be divided into four stages: Stage I (25–200°C), Stage II (200–550°C), Stage III (550–800°C), and Stage IV (800–1000°C). The dielectric properties increased with particle size due to its high void ratio. The higher moisture content leads to the higher dielectric properties. The frequency of 915 MHz was more appropriate to dispose the tonnage material in high power industrial than that of 2450 MHz. Additionally, the micro-structure of lignite was analyzed by the scanning electron microscope (SEM) and the fourier transform infrared spectroscopy (FT-IR). The characterization analyses demonstrated that a large quantity of pores and cracks were generated in lignite surface because of the microwave pumping effect. The polar substances in sample selectively absorbed and transferred microwave into heat and then evaporated water to steam, causing an instantaneous increase in internal pressure. There was a uni-directional mass and heat transfer process from the interior to the surface under the steam driving force, which caused a great number of pores and cracks emergence to substantially facilitate the removal of moisture content and accelerate heat dissipation. And the apparent aromaticity of lignite dried by microwave could be increased and the capability of potential re-absorption could be decreased. Meanwhile, the mechanisms of microwave heating and generation of cracks and pores were analyzed in detail. These results could provide a theoretical basis for microwave processing of lignite.