(557f) Design of Exergy Recuperative Fluidized Bed Drying System for Biomass

An innovative drying process based on self-heat recuperation technology utilizing both latent and sensible heat has been developed aiming for drying-energy saving of biomass particles. Drying processes are divided into three main subsequent stages, which are pre-heating, evaporation and superheating. A state of the art of this technology is on the exergy elevation and its recuperation through heat exchange and heat pairing for each sensible and latent heat. Hence, intensification of heat and mass transfer inside the drier becomes one of the most crucial matters that must be paid attention to obtain the optimum drying performance and efficiency. Furthermore, fluidized bed drier with internal heat exchanger is selected as the main drier conducting the second stage of drying which is evaporation because of its characteristics on high heat and mass transfer and good solids contact between particles leading to shorter drying time and its uniformity. In this system, the exhausted vapor from the biomass is once superheated and subsequently it is compressed by the compressor to certain temperature rise required for heat exchange. Next, the compressed vapor is re-circulated and utilized as the heat source for superheating, evaporation and pre-heating subsequently. The current study is focused mainly to the experimental study on the main drying process which is evaporation. The objective of this study is observing the optimal drying condition for biomass particles inside the fluidized bed drier applying self-heat recuperation technology. On the other hand, the biomass samples, which are wood chip and paddy straw, generally have a characteristic of peculiar shape resulting in difficult fluidization. Hence, a fine and inert particle, which is silica sand, is inserted inside the fluidized bed to enhance the fluidization performance of the biomass particles and improve the heat transfer and its uniformity across the bed. The effects of inert particle diameter, biomass to inert particle mass fraction, fluidization velocity and bed temperature corresponding to vapor pressure elevation to the drying performance of biomass particles are observed. From the experimental results, it is shown that decrease of inert particle diameter leads to increase of heat transfer across the bed resulting in lower moisture content of the dried products. Furthermore, higher pressure elevation of the exhausted vapor also brings to better drying performance as the temperature gradient increases leading to higher drying rate of the biomass samples. In addition, there is no significant effect of the fluidization velocity to the drying performance and energy efficiency in dynamic fluidization regimes.