Depending on its source, the moisture content of raw wood furnish used in pellet production ranges from about 15 percent to 60 percent when it enters the mill. In most cases, that has to be reduced to between 8 and 10 percent for pelletizing. The choice of dryer system can impact productivity, energy use and the end quality of the product.
Dirk Koltze, executive vice-president, Siempelkamp and Büttner, sees a logical link between his company’s expertise in drying systems for the particleboard industry and the growing pellet industry. “The front process of a pellet mill is a copy of the typical particleboard mill. Everything - chipping, drying, screening - is the same, and instead of using a continuous press the mill uses a pellet press. A typical pellet dryer is very similar to a particleboard dryer with different options available.”
One option is a direct heated dryer system which uses flue gases from the furnace to heat the product. The Büttner direct heated dryer system can be designed to output from 5 to 50 tons/hour of product. Currently seven of these systems are installed in North American particleboard mills including Tafisa, Flakeboard and Unibord. Another 27 dryers are installed in OSB mills.
However, says Koltze, he sees “a tendency in the pellet side to go to indirect heated dryers which Büttner has been supplying in Europe very successfully since 1995.”
The indirect dryer uses an internal bundle of heating pipes which are heated by either steam, thermal oil or hot water. Instead of using air to dry the particles the hot surface of the pipes inside the drum dries up the material. The wood chips touch only the surface of the pipes.
“This is a fairly sophisticated dryer,” Koltze says, “but the pellet industry is most concerned with ash content within the pellet itself and there are certain regulations that diverge between commercial and residential pellet use. On the commercial side they allow ash content of about 3 percent, on the residential side, 1 percent at most, because ash means more combustion ash and particles in the oven so the end user has to clean the oven much more often, which makes it less attractive to use. These indirect heated dryers don’t add any foreign ash or any foreign material to the pellet raw product. This is a dryer we provide that is very successful here. It makes perfect sense for the pellet side.”
The Büttner indirect heated dryer system has a water evaporation capacity of up to 7 tons per hour for each set of double bundles utilized. The heat source can be steam or thermal oil. The main advantage is no contamination of the material by airflow. The system also offers the benefit of low emissions and minimized exhaust air volume and is an efficient alternative to belt dryer systems. Nineteen installations are currently operating.
