Drying kinetics and upgrading characteristics analysis of Zhaotong lignite with microwave deep drying

Abstract

Dewatering is considered as extremely significant and primary step to enhance the quality of lignite. A great number of research focus on conventional fluidized-bed drying technology or dynamics. In this paper, the microwave deep drying kinetics of Zhaotong lignite under different power levels and with different sample thicknesses were investigated. The drying kinetics of microwave deep drying was fitted by 13 different drying models given in the literature. Experimental results indicated that the microwave deep drying process exists three segments: increasing-rate, falling-rate and brief constant-rate segment. The drying time decreased and drying rate increased with microwave output power level increase, while the microwave penetration depth in lignite was affected by sample thickness, hence there is an irregular relationship between lignite sample thickness and microwave drying time or drying rate. Based on non-linear regression analysis, the Page model was considered adequate to describe the microwave deep drying behavior of Zhaotong lignite under various conditions, and the model could be used for residual moisture water content prediction at any time during the microwave deep drying process. Effective diffusion coefficient increased along with microwave output power level increase. The activation energy of the moisture emission from the lignite under different power levels was estimated at 10.2202 W/g using modified Arrhenius equation. The pore structure damage was observed in the lignite samples after microwave deep drying by SEM analysis. The surface of upgraded lignite had visible cracks, defects and small fragments compared with the samples by conventional drying.