An alternative to a rotary dryer for biomass is a bed dryer. In this type of dryer, the wood particles are stationary, and the air passes upward through them. Compared with rotary dryers, bed dryers may be more economical to operate on a small scale. The present work investigated drying biomass in a fixed bed dryer.
The drying characteristics of wood biomass were measured using a thin-layer drying technique at inlet gas conditions of 50°C to 200°C and 0.3 to 0.9 m/s. The drying behavior was modeled using a one-parameter Newton model. The thin-layer model was then used in a deep bed model to predict drying times and moisture profiles in the bed. Drying times based on the model at depths up to 23 cm were within −22 to +12 percent, and typically within ±4 percent, of the experimental results.
A drying zone ranging from 0.13 to 0.18 m in depth moves up the bed as drying occurs. The biomass is wet above and dry below this zone, resulting in considerable variability in moisture content within the bed. The material in an industrial dryer would need to be mixed or a dryer would need to be designed with stages if a uniform moisture content among the particles was an important criterion.