Moisture content, weight loss and potential of energy wood in South and Central Ostrobothnia regions in western Finland

Abstract

The aim of this thesis was to improve the quality of energy wood and therefore increase the potential of forest energy. The role of moisture content in energy wood was crucial in this study and the data concerning it was collected at various stages in the operational energy wood supply chain. About half of the mass of a freshly-felled tree consists of water. From the point of view of energy generation this water is unwelcome. There are two main ways to dry energy wood; these are artificial drying and drying naturally. The Norway spruce (Picea abies L. Karst.) stump wood dries fairly quickly in favourable natural conditions. The average moisture content (wet basis) of a stump was about 31 % one month after stump harvesting. Spruce stump wood also retains its dryness well in storage all year round; providing the stumps are dried well one time after harvesting. Small-sized whole trees did not dry well at roadside storage sites under natural conditions. About one year after harvesting the moisture content of a small-sized whole tree was still about 43 %. However, during storing a remarkable weight loss of 37 % was detected between the forest and the heating plant. The most effective and the fastest drying method found in this study was the continuous compression drying method. The lowest moisture content of 30 % was achieved for Downy birch (Betula pubescens Ehrh.) by continuous pressing using 38 MPa and with a pressing time of 30 seconds. Correspondingly, the moisture content of softwood was about 35 % under the same pressing conditions. The energy consumption for compression drying is very low compared to the energy required to vaporise water in thermal drying. The techno-economic forest energy potential of the study area was 1.6 TWh/y and it could be even greater (2.7 TWh/y) if the Scots pine (Pinus sylvestris L.) stumps were also fully utilised for energy recovery. The forest energy potential calculations were made using the heating value of fresh wood and therefore the real potential will be greater when using dried energy wood. For absolutely dry wood the potential was about 1.9 TWh/y. The properties of energy wood vary widely depending on its assortment, storage conditions, as well as the weather conditions and the origin of the energy wood. However, a better understanding of energy wood properties will increase forest energy’s potential and the use of renewable energy and thus help mitigate climate change globally.