Why measure moisture in biomass pellets?
The moisture level in woody material in pellet production is critial to meet end-product quality and profit goals. Pellets that are too wet can result in incomplete combustion, leading to heating instability. Additionally, stored pellets with a high moisture content can cause microbiological activity, which can lead to sponstaneous combustion in silos. Overdried pellets are also vulnerable because they become fragile, compromising integrity and finished product quality. Unmonitored drying leads to increased energy consumption.
TYPICAL PELLET PRODUCTION PROCESS
Raw Material Delivery – Large batches of raw materials varying from sawdust, wood chips, hog fuel, micro chips, lumber and trees unsuitable for lumber. This feedstock may be freshly cut, partially dry, or kiln dried. Some manufacturers use sifters or separators to remove non-wood materials such as metal, plastics or rocks.
Chipper – A chipping machine breaks down large non-uniform wood pieces into smaller wood pieces.
Hammermill – Hammermills grind wood pieces into consistent small pieces. These micro chips (3mm) could have wide and inconsistent moisture content ranges.
Dryer – Large high-heat dryer drums, convection ovens, or air blowers dry the sawdust to a consistent moisture level, usually less than 15 percent.
Pellet Mill – The dried sawdust is pressed through heated pellet dies at high pressure. The naturally present lignin, a complex organic polymer, acts as a glue to hold the pellet together.
Cooling – Now hot and soft, pellets must be cooled and dried, usually with blown air. The finished materials moisture content should be no higher than eight percent. Storage or Packaging – Cooled, dry pellets are stored in silos for bulk shipping or bagged for immediate distribution.
*Typical measurements, subject to change per manufacturer’s requirement