Drying characteristics of willow chips and stems

In supply chains of willow (Salix viminalis) biomass to energy plants, dryi ng is advisable in order to enable safe long-term storage, increase boiler efficiency and reduce gaseous emissions. To gain insight into the drying pr ocess, drying characteristics of willow chips and stems were investigated e xperimentally in a drying installation. The drying process was modelled wit h a diffusion equation. The effective water diffusivity D-eff was assumed t o be a simple algebraic function of the dimensionless moisture concentratio n m: D-eff = D-0 m(a), with D-0 being the initial diffusivity, and a an emp irical exponent. Drying of a chip and of a stem without bark could be succe ssfully described with a diffusion equation for a plane sheet and a cylinde r, respectively. Drying of a stem with bark could be successfully described as drying of a stem without bark surrounded by a thin layer (bark) with a much lower diffusivity. Compared to a chip, a stem without bark dried appro ximately 10 times slower from fresh state to equilibrium moisture content, mainly due to the larger diffusion distance of the stem. A stem with bark d ried approximately 10 times slower than a stem without bark due to the low diffusivity of the bark. (C) 2000 Silsoe Research Institute.