Drying Characteristics of Willow Chips and Stems

In supply chains of willow (Salix viminalis) biomass to energy plants, drying is advisable in order to enable safe long-term storage, increase boiler efficiency and reduce gaseous emissions. To gain insight into the drying process, drying characteristics of willow chips and stems were investigated experimentally in a drying installation. The drying process was modelled with a diffusion equation. The effective water diffusivity Deffwas assumed to be a simple algebraic function of the dimensionless moisture concentration m: Deff=D0ma, with D0being the initial diffusivity, and a an empirical exponent. Drying of a chip and of a stem without bark could be successfully described with a diffusion equation for a plane sheet and a cylinder, 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 approximately 10 times slower from fresh state to equilibrium moisture content, mainly due to the larger diffusion distance of the stem. A stem with bark dried approximately 10 times slower than a stem without bark due to the low diffusivity of the bark.