STEAM DRYING OF WOOD FUELS

The objective of this work ir, to investigate some of the important aspects in the design of a steam band dryer for wood fuels. For this purpose the drying of the material in a bench-scale fixed bed dryer has been studied. Drying times and thermal efficiencies for experiments conducted under different conditions are compared. The investigated materials are soft- and hardwood chips and softwood bark. The thermal efficiency, the part of the sensible heat which is used during one passage of the steam through the bed, increases with increasing mass load (mass of dry matter per unit area) and with decreasing steam mass flux. At a mass load of 30 kg/m2 the thermal efficiency is about 0.85 even at steam mass fluxes as high as 0.6 kg/m2 s (1.2 m/s). The thermal efficiency proves to be almost independent of pressure and temperature of the steam. Due to the very inhomogeneous materials the steam mass flow distribution was uneven. This causes a decreased thermal efficiency. When bark was dried the bed of material shrank and this also causes a decreased thermal efficiency. A model of the drying is used to predict the influence of an uneven steam mass flow distribution.