Investigations into the drying kinetics of biomass in a fluidized bed dryer using electrostatic sensing and digital imaging techniques

Investigations into drying kinetics of biomass in fluidized bed dryers are essential for the control of drying processes, enhancing productivity and reducing energy consumption. However, there is limited research on drying characteristics of biomass at different bubble locations due to complex hydrodynamics around bubbles in the bed. In this paper, a new method is proposed by combining electrostatic sensing and digital imaging techniques to obtain moisture contents, drying models, moisture diffusivities, activation energies and mass transfer coefficients of biomass at different bubble locations. Experimental tests were conducted on a laboratory-scale fluidized bed dryer for different air velocities at a range of air temperatures. Five mathematical drying models are evaluated in the paper. It is found that the Page drying model is most suitable for describing the drying process of biomass in the fluidized bed. The results also show that the mass transfer coefficient of biomass at the interior and boundary of the bubble is higher than that at the exterior of the bubble. In addition, although the mass transfer coefficient increases with the air temperature and air velocity, the highest air temperature and highest air velocity are not optimal conditions. For example, a bubble flow turns into a slug flow or plug flow at an air velocity of 0.56 m/s and an air temperature of 75°C.