Kinetic analysis on the microwave drying of different forms of water in lignite

Abstract

A Chinese raw lignite with total water (RC) and molecular water (MWC) were prepared. Those samples were dewatered by a microwave drying equipment using constant power selected from 450–600 W. The first order reaction kinetic equation and ten thin-layer drying models were used to analyze the dewatering processes of MWC and RC respectively. Moreover, the effective diffusion coefficient of water, the activation energy of the water removal and the drying rate constant (k) were calculated. The first order reaction kinetic equation and the Two-term exponential model provided a better fit for the drying processes of MWC and RC respectively. The effective diffusion coefficient of water obtained by Fick's second law varies from 0.3708 × 10−8 to 1.6715 × 10−8 m2·min−1 for MWC and from 0.5099 × 10−8 to 3.3169 × 10−8 m2·min−1 for RC. The activation energy for the removal of the molecular water is 28.59 kJ·mol−1, and that for the total water is 24.25 kJ·mol−1. The k of MWC and RC are in the range of 0.0894–0.4364 min−1 and 0.2737–1.4740 min−1 respectively. From the energy consumption aspects, it is suggested the microwave power higher than 550 W should be used to remove the free water and capillary water, and 525 W used to remove the molecular water.