A kinetic study of microwave and fluidized-bed drying of a Chinese lignite

• Chinese lignite was dried in nitrogen and steam fluidized-bed and microwave.
• Ten empirical drying models were employed to simulate the drying kinetic data.
• Midilli–Kucuk model best fitted the drying data in conventional drying.
• Page model described best the microwave drying behavior of lignite.
• Drying mechanism is different in conventional and microwave drying methods.

The drying kinetics of Chinese lignite in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave were investigated. The changes in the mass as a function of drying time were measured under various drying conditions. The variations of moisture ratio with time were used to test ten different thin-layer empirical drying models given in the literature. In studying the consistency of all the models, some statistical tests, such as χ2, residual sum of squares (RSS) and F-value were also used as well as coefficient of determination R2. In nitrogen fluidized-bed and superheated steam fluidized-bed, the Midilli–Kucuk model best described the lignite drying process. Drying data in microwave were best described by the Page model, indicative of a difference in kinetics between the two drying methods. This difference was attributed to different heat transfer mechanisms under conventional and microwave drying conditions. The effects of drying parameters in nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying on the constants and coefficients of the selected models were studied by multiple regression analysis. The apparent diffusion coefficient of moisture in samples was obtained from the kinetics data and the apparent activation energies under nitrogen fluidized-bed, superheated steam fluidized-bed and microwave drying were found to be rather similar.