Pellet and briquette manufacture complements other wood processing industries as it makes use of co-products or waste that would otherwise be costly to remove. In countries with developed pellet markets, pellet mills are often sited alongside other timber industries to reduce the transport of raw material. This is usually sawdust, grinding dust, shavings and small off-cuts.
For pellets, the raw materials are screened to remove stones, metals and other impurities. They are then dried to achieve consistently low moisture content. After drying, the material is pulverized in a hammer mill and refined to a fine dust. This is sieved and conveyed to a pellet press which is composed of a perforated matrix with a rotating cylindrical roller. The roller forces the dust through the round holes of the matrix where a knife slices off the pellets at the required length. Production of briquettes is similar.
The compression process heats the pellets/briquettes helping the natural lignin to bind the dust. Some manufacturers add a small proportion of lignin or vegetable starch to aid the binding. After milling, the pellets are cooled, sieved and weighed, then conveyed to storage. Bulk pellet trucks are filled using blower conveyors and transport the pellets to bulk users or to suppliers for bagging and sale to domestic customers.
• Pellets are commonly available in 6mm diameter and 10-20mm length, similar in appearance to chicken feed pellets! Pellet size is critical to the smooth operation of appliances.
• They can be used in stoves and boilers, flow easily for automatic feeding, are easy to handle and to ignite.
• Pellets have low moisture content (<10%), a high and consistent energy density compared with logs and chip and produce little ash.
• The recognised QUALITY certification for pellets is ‘ENplus’, see HETAS. This scheme has been adopted by the European Pellet Council.
• For bulk users with fully-automated boiler feed systems,
pellets can be delivered by tanker and blown into a storage hopper. Their high density means that less deliveries will be necessary compared to wood chips and logs.
• For smaller scale users who have single room heaters, pellets can be purchased in small bags weighing around 10-20kgs.
• Avoid poor quality pellets that fall apart easily and create dust. Instead, choose pellets of a consistent length with a smooth finish and certified to ENplus standards.
• Using dry raw materials, the pellet production process uses about 2% of their energy content. This rises to about 10% where the raw material is unseasoned timber co-product and needs pre-drying.
• Pellets take less storage space than other wood fuels, but require about three times that needed for heating oil containing the same energy.