Fuel types

There are many potential sources of wood from which wood fuel can be produced.  These include:

- Thinnings from woodlands

- Timber harvesting residues

- Previously under-managed woodlands and farm woodlands

- Short Rotation Coppice (SRC)

- Sawmill co-products including dry wood dust

Wood fuel considerations

Commercial wood supplies have three crucial characteristics which define its suitability to be used in wood heating, which are moisture content, particle size and energy density (expressed as megajoules per kilogram or kWh per cubic metre). 

The amount of water contained in freshly felled wood varies dependant upon the species.  Generally there is more water in softwoods than hardwoods.  The impact of moisture content on wood fuel is important.  The higher the moisture content the lower the calorific value and useable energy as more energy is required to evaporate the water from the wood before it can burn efficiently.  This lowers the efficiency of the boiler system and increases the cost of heat delivered from the boiler to the heating system. 

Particle size is important in that all boilers have feed systems with tolerances which need to be respected in order to allow a continuous, smooth transfer of wood fuel.  The fuel needs to flow easily from storage to boiler.  This is most important for wood chip boilers, as the wood is chipped into an approximate maximum size with different sizes being suitable for different types of boiler. 

Energy density in simple terms is the amount of energy contained per unit of weight or cubic metre.  Generally energy density in wood chips is around 980 kWh per cubic metre, whereas for wood pellets the energy density is around 3300 kWh per cubic metre.