Influence of storage on properties of wood chip material

The use of bioenergy for district heating is usually seasonal with a high consumption during the cold periods. Therefore some type of the harvested material storage is necessary. Woody materials are usually reduced in size to chips and stored outdoors in piles or under cover. During storage the materials decompose as a result of chemical and biological processes resulting in dry matter loss. The degree and rate of decomposition primarily depend on material moisture content and temperature. In this study four piles of wood chips, each containing 240 t wet weight, were studied for moisture content and temperature development during 5.5 months of storage. Two piles were stacked normally and two compacted at 50 to 60 kPa pressures. Additionally, a ventilating tarp, TopTex, was used to test the effect of covering the material. Nylon net bags with the same chip material were placed at different positions in the piles for moisture content determination. The volume of three piles shrunk between 3 and 6% but the volume of the uncovered compacted pile shrunk almost 6%. The low shrinkage indicated that material losses in this study were small. The temperature development in all piles followed a similar pattern but with maximum temperatures at different positions, top for the uncompacted pile and innermost for the compacted one. The ventilation tarp on the piles had only a minor effect on the temperature development. Moisture content decreased but the results are uncertain due to problems with the scale precision. Net bag analyses showed that the lowest moisture loss occurred in the middle of the uncovered compacted pile but the values only refer to their specific position in the pile.