Fuel quality of stored spruce bark - Influence of semi-permeable covering material

Bark is a by-product of the sawmilling and pulp industries that is produced year-round. It is mainly used as a fuel, making its storage unavoidable due to seasonal variation in energy demand. Storing bark can lead to high energy losses but can also increase its fuel quality, e.g. by reducing its moisture content (M) and increasing its net calorific value (Q). Covering wood chip stacks with semi-permeable fabric sheets can improve fuel quality and reduce dry matter losses (DML) during storage. The objective of this study was, therefore, to evaluate the effect of similar coverage during storage on the fuel quality and energy recovery of stored bark. Two bark stacks of 1000 and 1400 m(3) were established, with half of each stack being covered with a water-resistant, vapour-permeable fabric. The temperature, M, and Q were measured in each stack immediately after construction and after three and seven months of storage. Additionally, the DML, total accessible energy, and economic value of each stack were calculated at the same time points. The combined changes in fuel quality and DML reduced the amount of accessible energy by 13.7% in the uncovered parts but increased that in the covered parts by 0.6%. The economic value of the covered parts was unchanged after storage whereas that of the uncovered parts was reduced by 14%. Covering bark stacks with semi-permeable fabric during storage could thus increase the profitability of bark fuel supply chains.