Stumps as fuel

Wood fuels make a key contribution to renewable energy sources in the Nordic countries. The growing demand for forest biomass can be partly met by utilising energy-rich stumps left after clear cutting. As fuel, stump wood is commonly associated with a high presence of contaminants, resulting in high ash content. The concentration of contaminants depends on factors such as soil type, weather conditions, harvesting method, storage duration and other handling operations. The high ash content is a major drawback when using stump wood as fuel. Handling methods that could improve fuel quality are therefore highly desirable. To produce a fuel of acceptable quality, i.e. with low moisture and ash content and high energy value, the above-mentioned factors require evaluation and quantification. In this thesis, various handling methods within the supply chain for Norway spruce stump and their effects on the biomass as fuel are evaluated. The following handling processes are dealt with: harvesting technique, pre-treatments including stump splitting/fractionation and cleaning using vibration or sieving, storage methods and storage duration. Choice of stump harvesting head proved to have an impact on fuel quality, as splitting of stumps during extraction allowed better drying during storage. Such stumps could be stored directly in windrows, since no difference could be established between this method and storage in heaps prior to windrow storage. In general, storage improved fuel quality. Changes in fuel quality parameters do not reflect the whole picture, however, since dry matter losses occur during storage, and therefore the storage period should be kept as short as possible. During winter months, when demands for fuel are highest, stump ash content can be high, since frozen contaminants are difficult to detach from stumps during transport. To achieve acceptable fuel quality during high demand periods, long storage duration or a pre-treatment involving additional stump cleaning is required. Fuel quality was clearly improved by vibration-based cleaning before the comminution of stumps or by sieving stump wood after crushing. These methods can allow stump wood fuel of acceptable quality to be supplied within a harvesting season.