Wood pellets are pellets made from fine wood particles. They are cylindrical in shape, typically 6 or 8mm wide (diameter), and 3-45mm long.
Because of their consistent shape and characteristics, they flow more easily and burn more consistently than other forms of wood, which makes it easier to design automated heating systems that need little more intervention than gas- or oil-fired boilers.
Wood pellets are produced by compressing dried wood fibre through a pellet press. The low moisture content and compression mean that wood pellets require a lot less space for a given amount of energy, compared to other forms of wood fuel. For example, wood chip will typically require three to four times as much space for the same amount of energy as wood pellets.
This makes wood pellets the efficient form of wood fuel where the fuel is not consumed locally to where it was growing (say, within around 50 miles of the forest). As highly-forested parts of the world tend to have few people to use the wood, and regions with high population density tend to have few trees, pelletisation is the technology that allows the sustainable resource to be brought efficiently to where it is required.
Wood pellets can be made from softwood or hardwood, but are more often made of softwood (e.g. pine, spruce, larch, cedar, etc.).
Wood pellets can be made from virgin fibre or from recovered wood. High quality wood pellets are made from de-barked virgin fibre.
Virgin-fibre wood pellets can be produced from forestry by-products, such as tops and thinnings, or from sawmill by-products, such as sawdust, or from whole log. However, other uses of wood, such as for timber, are much more valuable than for energy. Wood is normally only used for pelletisation where there are no other higher-value uses for the wood in the area. That may be because the quality of the wood is not good enough for other uses, or because there is insufficient demand for other purposes in the area to use all the mature growth.