Manufacturers of pellet equipment has asked for that the question of pellets cooling are investigated. The purpose is to examine how the temperature, cooling time and the velocity of the cooling air affects the durability of the pellets and the amount of fines during production. Wood fuel pellets have been produced at Karlstad University in a Pilot pellet production unit. Two cooling devices have been used, a custom built cooling tower and smaller cooling boxes where temperature, humidity, fan speed etc. are measured. Reference samples were cooled in a bucket over night and scattered in a large box on a perforated plate. All samples were cooled to ambient air temperature.
The best results for the cooling tower on the durability and bulk density was the pellets scattered in a box and left overnight. Additionally, there were a minimal amount of fine fractions. Pellets cooled with the lowest fan effect also showed good results on durability. Pellets cooled in a bucket over night got very humid, and pellet cooled at maximum fan power iniciated cracks in the pellets due to a cool outside and hot inside of the pellets. The results for the smaller cooling boxes showed similair results but also showed that a lower bed hight of the pellets in the boxes gave increrased durability and hardness. A high fan speed gave poorer results on durability, mostly depending on that the air transport through the boxes became uneven. Further tests in an industrial environment measuring the electrycity should be done.
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