John Gillis monitors the flow of fibre and the spark detection systems that have been strategically placed throughout the pellet mill. in from a long distance even if they may be cost competitive. Sometimes the pine’s better because it has a lower moisture content,” Bennett says. Wet sawdust can require as much as a third of its volume to get the other two-thirds dry enough to pelletize. And besides the economic advantage of using the dead, dry pine, there are social ben-efits to keep the residuals away from the match. Nobody likes to think of all that wood going up in smoke. RUNNING THE MILL It takes about 40 employees to keep the pellet plant running 24 hours a day, seven days a week, but half the crew is on mainte-nance duties. In fact, the mill could run on as few as two people: one loader operator to put the raw material into the plant and somebody to operate it. In reality, however, on top of the two essential operators, there are a couple loader operators, a quality con-trol crew, a crew loading the finished pellets into rail cars and those maintaining the ma-chinery and keeping the plant clean. The forest residuals go through a Brunette Biosizer to reduce them to the required size before joining the saw-dust and shavings on the conveyor belt. It goes through a metal detector, which aborts the material by reversing the belt if metal is detected. The mill has an old hammermill that isn’t very efficient but can be used if it’s needed to grind extra forest residuals. When it isn’t needed, the material is diverted around the piece of equipment to conserve power. The material passes through a SolaGen single pass dryer that isn’t slowed by the wide range of moisture content. “The the-ory is the lighter, smaller particles will go through the dryer very quickly, the larger and heavier particles will take longer to move through,” Bennett says. The dryer uses what is called “flight-ing” to lift the fibre and cascade it through the heated gas stream. It creates suction that draws the material through the dryer and into a cyclone which then separates the air from the fibre, drops the fibre into an airlock and the air flows through a PPC Air Pollution Control Systems elec-trostatic precipitator. A lot of the fire explosion risk is within the drying system itself, but if the dryer is tightly sealed, the material is processed in a hot, steamy but oxygen-free environ-ment. Operators need to follow proce-dures carefully at start-up and shut-down to avoid introducing oxygen into the sys-tem, but Bennett stresses the system is perfectly safe in operation. The system is well monitored to detect any defects or explosion risks (with 40 zones of extinguishment throughout the mill that are all computer-monitored) and there are aborts built in to safely redirect any explosions. There is a burner cham-ber that runs on wood dust and residual heat is recycled within the system. Partic-ulates from the process are scrubbed out of the exhaust before it is released. Bennett is well versed on how the par-ticulates are kept out of the atmosphere. “That water attaches itself to the particles and there’s an electric charge that draws the particle with the water to the sides of the tubes. There’s a whole series of tubes and it essentially runs down and flushes JANUARY/FEBRUARY 2015 16 Canadian BIOMASS
Canadian Biomass January-February 2015: Page 16
