Pellets made from wood sawdust are popular heating fuel in the Nordic countries. In pellet press, sawdust is compressed into a solid pellet, and it has experimentally been shown that temperature, moisture content and extractives content affect pellet properties. Mechanical durability is an important property since mechanically weak pellets tend to break during storage and transportation. Tree species behave differently in the process, and optimizing energy consumption and pellet quality requires detailed knowledge about the process taking place in the pellet press.
This study aims to characterize those manufacturing parameters that affect mechanical properties of pellets, and to study the behaviour of different tree species in the pelletizing process. We focus on density of pellets and flow conditions in the press channel. Mechanical strength of wood depends on density, and same is probably applicable to pellets. Several tree species are used in pellet manufacturing, and different tree species probably have different flow conditions in the press channel.
We have developed two single channel pellet presses in our laboratory. The first one contains a closed-end die and the second one an open-ended die. The latter one resembles more closely an industrial scale pellet press. Both experimental devices are electrically heated. Temperature and load can be recorded. Pellets were tested with tumbling and compression tests. Durability, strength, and density were reported. According to our results, density of a pellet increases first rapidly as compression force increases. At a certain load there is a threshold value, after which pellet density only slightly increases when load increases. Silver birch pellets were stronger than Norway spruce or Scots pine pellets at a given load. Mechanical strength of pellets depends on density: increase in density enhances strength. Production temperature also increases pellet strength and durability. In experiments with open-ended die, it was found that there are sharp load drops and oscillating flows in the press channel when sawdust is forced through the channel. Large load drops are problematic, since they may cause excessive wearing of machinery.
Keywords: Pelletizing, single channel, softwood, hardwood
Authors
Havimo M.
Department of Forest Sciences, University of Helsinki
Rikala J.
Department of Forest Sciences, University of Helsinki
Rissanen A.
Department of Forest Sciences, University of Helsinki
Sipi M.
Department of Forest Sciences, University of Helsinki
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