Wood Fuel Pellets: Sawdust Drying in the Energy System

Abstract: In 2004, the amount of wood fuel pellets used in Sweden was 1 256 000 tons, it was mostly used in large scale burning for district heating. Over the last 10 years, a significant increase in pellet production has occurred and today the preferred raw materials for pellet production, i.e., dry sawdust and wood shavings, are insufficient. New raw materials could be used. However, the quality demanding residential customer that uses a pellet burner or a pellet stove with limited emission control prefers stem wood pellets. One solution could be to increase the drying of wet raw materials, such as wood chips.The most common technique in Sweden for the drying of sawdust is to use a directly fired co-current rotary dryer, although steam drying does occur when co-generation is possible. In the production chain for pellets, the drying process is the most energy consuming process and, together with the raw material, it is the main cost factor for the manufacturer. Thus, it is important to run the drying process as energy efficiently and environmentally friendly as possible. This thesis, therefore, discusses the effects of the drying of sawdust, which is the primary raw material for wood pellets.In the work described in Paper I, the moisture content and emission of terpenes during the drying and pelleting steps of the pellets production are examined. Furthermore, the effects on pellet quality are examined. In the work described in Paper II, the recirculation of drying gases in a rotary dryer is examined from the point of view of energy savings and dryer capacity.The drying process also affects the pellet quality. Drying of sawdust can be used to control quality properties, such as moisture content and emissions of Volatile Organic Compounds (VOC). Almost all of the monoterpenes are emitted during the drying and pelleting steps causing environmental and health related problems (see Paper I).Energy savings can be made using recirculation of the drying gases. Increased recirculation could mean higher dew points in the emitted drying gases, which increases the chances to use the energy from the emitted drying gases for secondary processes connected to the dryer system, such as district heating. However, there is a conflict between recirculation ratio/recovered energy and drying performance (see Paper II)