Pelletising
Pellets and other reconstituted wood fuels such as briquettes, are usually formed from the by-products of the wood processing industry such as sawdust or planer and moulder shavings. Because a fine, dry feedstock is required to make pellets it is more energy efficient to utilise such by-products rather than to grind up and dry freshly felled timber. Wood pellets have a number of desirable characteristics:
- Consistent density and heat content (around 19 GJ/tonne)
- Low moisture content 8 – 10 %
- Consistent size
- A clean fuel free from dust, bark and slivers.
These characteristics make pellets easier to handle, cheaper to transport and more suited to automated handling systems than other types of woodfuel. Pellets, however cost more to produce than other wood-based fuels, and require a high capital investment in plant and machinery.
Pellet production generally requires a supply of dry sawdust of a consistent moisture content and a uniform particle size. This processed sawdust is then forced or pressed through holes in a rotating die. The pressure causes the wood to heat up, briefly liquefying the lignins and resin in the wood which act as a glue and bind the pellets together. The extruded pellets are cut to length as they emerge from the die. They are then allowed to cool before being stored in bulk or bagged for delivery.
Most pellet production is by large-scale industrial pellet manufacture, integrated with a large sawmill and/ or heat user. This is to make the project economics viable. If you have a supply of sawdust that is already dry, small-scale pellet machines are available with an output of around 200-300kg per hour. These pellet production systems are available at a cost of anywhere from £50,000 to £150,000, depending on level of quality required and the level of automation needed for operation. Smaller systems are also available and it is also sometimes possible to convert an existing animal grass-feed mill to make wood pellets, as the equipment needed is similar and many feed factories have facilities for drying the feedstock as well. Such an arrangement could bring economic benefits to the feed mill, enabling production to continue throughout the year especially over the winter when grass is not growing. The production would be considerably less than for a dedicated plant, but the capital costs would be much lower.
The production of pellets is more energy-intensive than the production of other woodfuels, and it is estimated that the energy needed is approximately 10% of the pellet's own energy content in the case of fresh, wet sawdust. If already dry sawdust is available then the energy requirement falls to around 2% of the pellet's energy content.
Useful Links
European Pellet Forum
European Pellet Centre