Wood pellets are a clean, free flowing form of woodfuel that can be delivered by tanker through a hose and allow a fully automatic heating system.
What are wood pellets
Wood pellets are made from dry sawdust compressed under high pressure and extruded through a die. They may include a low level of added binder, such as starch, but many use nothing other than steam.
They come in a range of sizes: for domestic and relatively small scale systems 6 or 8 mm is typical, while for larger systems 10 or 12 mm are common.
Wood pellets should be dry, clean, mechanically robust and have an ash content defined by the appropriate standard (see below) to which they have been made, which may also define other contaminants such as chlorine content.
They should also flow freely and can be delivered via a pneumatic system.
The combination of low moisture content (8-10%) with a compressed, dense pellet (typically >1,000 kg/m3, i.e. greater than the original wood) and the ability to flow and pack closely, gives a fuel with a high bulk energy density. This then requires less storage space than lower density fuels such as wood chips.
Using as an alternative to coal
Wood pellets also provide an alternative, renewable and low carbon alternative to fuel old coal boilers.
Commercial conversion kits are available, but some systems are being run with few modifications other than to the fuel feed which needs to be higher owing to the lower energy density of wood pellets.
Where coal is in use storage and handling infrastructure for a solid fuel will already be in place. Wood pellets provide a fuel that, in addition to its environmental benefits, is cleaner, more pleasant to handle, is less aggressive to fuel handling components such as augers, and produces considerably less ash.
A coal storage bunker will hold enough pellets for operation for around one half to a third of the time possible with the same volume of coal.
Standardized fuel specifications are critical to the efficient and reliable operation of pellet boilers and stoves. BS EN 14961-1 sets out the general specifications and classes of solid biofuels. BS EN 14961-2 sets out a specific set of specifications and classes for non-industial wood pellets. This includes parameters such as ash content and mechanical durability. Pellets defined according to this are described as ENPlus pellets.
Wood pellet manufacture
Wood pellets are a commodity fuel, traded internationally. Their manufacture cannot be realistically undertaken on a small scale or DIY basis. Self-supply is therefore not a realistic option.
Wood pellets have a higher embedded energy than woodchips, owing to the energy needed to grind the wood to fine sawdust (typically in a hammer mill) and press it through a die at high pressure. The energy (and financial) cost of active drying is not usually justified, however they do allow low value waste wood residues, such as sawdust, to be made use of.
A very low moisture content (typically <14%) is required of the sawdust feedstock, so many pellet production facilities are established where there is an existing supply of very dry material, such as were processing of wood that is already kiln dried takes place. For example:
- Joinery workshops
- Wood recycling sites
- Sawmills processing dried wood.
This also removes the need to transport the very low density sawdust. Where a supply of bone dry sawdust is available, this may be supplemented with sawdust of higher moisture content provided the overall moisture content does not become too high. The precise embedded energy figure for a batch of pellets will depend on the details of production (plus that of the feedstock), especially drying, and transport etc.
Non-wood biomass pellets
Fuel pellets can also be made from a number of other forms of biomass, including Miscanthus, straw, and various forms of biomass residues. Although these are all perfectly acceptable types of biomass fuel pellet, they have different properties, especially combustion properties, and require suitable, specialist combustion equipment to burn them safely and efficiently. If burned in inappropriate equipment, not designed for this fuel, they can cause damage that is not likely to be covered by the warranty.
In suitable equipment however, they can be burned entirely satisfactorily, and are likely to be cheaper than conventional wood pellets.