Drying of willow biomass in supply chains

The drying process of willow ( Salix viminalis ) in biomass supply chains to energy plants is quantitatively described. Drying at particle level was modelled for chips and stems by a diffusion equation linked to the mass transfer of moisture to the air. Drying at bulk level is described by a deep bed model, which accounted for the moisture and temperature gradients of wood and air therein. Experimental validation showed that the deep bed model adequately described forced convective drying (with ambient air) of a willow chip bed, and natural wind drying of willow stems in large piles.

The technical possibilities and costs of drying of chips, using farm facilities for potatoes, were assessed. March to September turned out to be the most suitable period. Compared to harvest costs, forced convective drying was a considerable cost factor. For stems, statistical analysis of experimental data showed that a pile dried uniformly, except for the top layer. Pile coverage had no long term effect on the moisture content. Within a single stem, the moisture content was largely uniform. During open storage from harvest (December-April) until August, the moisture content could be reduced to close to that of the equilibrium moisture content.

Experiments and model calculations showed that chunks could be dried relatively quickly by forced convective drying, and very cheaply by natural wind drying. A model was developed to optimize supply chains by Dynamic Programming, which proved to be useful in deriving supply strategies. The natural wind drying time, required to achieve the correct moisture content, specified by the energy plant for chunks and stems, was a decisive factor in the design of supply chains.