Förbränning av pellets framställda av defibrerad granråvara

Abstract: Fuel pellets are in Sweden mainly produced from sawdust and cutter shavings. Before the pelletising process the raw material is dried to attain consistent moisture content and ground up in a hammer mill. In the densification process (pelletising) the ground material is forced through holes in a rotating die. This heats up the wood particles which cause them to soften and self bond. After the process the pellets are cooled, sieved and bagged. The milling process creates a wide range of particle sizes. How the particle size distribution influences the characteristics of pellets are still rather unclear although it is generally assumed that a mixture of particle sizes will increase pellet durability. The influence of particle size distribution of a pellet on burning characteristics is not known. In pulp mills the woody raw material is usually converted into chips which thereafter are treated in a defibrator into fibre bundles. Such a material is physically different from a mill ground material. One of the main aims of this work was to investigate the burning characteristics of pellets produced from defibrated spruce wood and to compare these pellets with pellets made from mill ground saw dust. The influence of saw dust particle size distribution on burning characteristics was also evaluated for industrially produced and laboratory made pellets. It was found that the char combustion time of pellets made from defibrated wood was somewhat longer than that of saw dust pellets. Anyhow, the differences were not statistically significant. The particle size distribution of the saw dust did affect the burning characteristics e.g. pellets made of a coarse fraction had faster char combustion than pellets made from a fine fraction. The investigation also revealed that during the pelletising (densification) process the saw dust was grinded to smaller particle sizes.