Storage effects on pelletised sawdust, logging residues and bark

Abstract

Storage of wood pellets is substantially different from the storage of unprocessed raw materials, one of the most obvious differences being the low moisture content of pellets. However, temperature development and fungal growth in the pellet piles have been observed at some plants. Documented knowledge of the storage of pellets is limited. In this pilot study, nine pellet assortments, made on a large scale of fresh and stored sawdust, bark and logging residues (a mixture of Norway spruce and Scots pine) were tested for changes in moisture content, heating value and ash content. Dimensions, bulk density, density of individual pellets and durability were also determined. The process variables were constant during pelleting. The five months storage has done in plastic bags (1.3 m 3) in an unheated barn. The storage led to negative effects on durability, especially on pellets made of fresh materials. Pellets made out of fresh logging residues were found to have lowest durability after storage. The average length was decreased due to breakage during storage. Microbial growth was noticed in some of the pellet assortments. Water absorption tests showed the largest hygroscopicity of the pellets from fresh materials. In general, the changes in pellet quality during storage in large bags were not large, but notable. The tendency to reach the equilibrium with the ambient moisture content should be taken into consideration during production if the pellets are to be stored.