ABSTRACT

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.

摘要

采用谐振腔微扰法研究了褐煤的介电性能。研究了温度、粒度、水分含量和微波频率的影响。结果表明，介电参数的变化可分为四个阶段：阶段I（25-200°C），阶段II（200-550°C），阶段III（550-800°C）和阶段IV（800 –1000°C）。由于其高空隙率，介电性能随着粒径的增加而增加。较高的水分含量导致较高的介电性能。915MHz的频率比2450MHz的频率更适合大功率工业中吨位材料的处理。此外，通过扫描电子显微镜（SEM）和傅里叶变换红外光谱（FT-IR）分析了褐煤的微观结构。表征分析表明，由于微波泵浦效应，褐煤表面产生了大量的孔隙和裂纹。样品中的极性物质选择性地吸收微波并将其转化为热量，然后将水蒸发成蒸汽，导致内部压力瞬间升高。在蒸汽驱动下，从内部到表面发生了单向的传质和传热过程，导致大量孔隙和裂缝出现，大大促进了水分的去除，加速了散热。微波干燥褐煤的表观芳香性增加，潜在的再吸收能力降低。同时，详细分析了微波加热和产生裂纹和气孔的机理。这些结果可为褐煤的微波加工提供理论依据。