Wood Fuels

Wood fuel needs to be refined like other fuel sources such as coal and oil, but in different ways. It requires drying to reduce moisture content, and reduced to a size that makes handling easier. Wood fuel can come in many different forms all of which will have different combustion properties, behave differently in terms of handling, and are suited to particular types of boilers. Wood fuels are divided into three main types:

• Log wood
  
• Densified fuels such as pellets
  
• Wood chips

Wood fuel characteristics

The Forestry Commission has published a Woodfuel Strategy for England, providing useful background to the sustainable supply of woodfuels. The Bio Energy Group has published a Woodfuel Pamphlet describing the environmental benefits of using woodfuels.

The critical elements in describing a particular wood type are moisture content, particle size and bulk density of wood. If wood fuel is sold by weight the critical parameter is the moisture content.

The energy density of 1 tonne of well seasoned logs is considerably higher than 1 tonne of green logs. For seasoned logs you are likely to be buying about 3/4 tonne of dry wood and 1/4 tonne of water. With green logs the figures are more likely to be 1/2 tonne of dry wood and 1/2 tonne of water. This has very important implications for transporting fuel - there is a considerable cost associated with transporting quantities of water!

There is also a considerable loss of energy in evaporating the excess moisture when burning the fuel, so using high moisture content fuels will mean lower efficiency and greater cost and has other negative effects. The right wood fuel for a particular situation will depend on a few key properties:

• The physical size and shape of the material.
  Logs must be the right size to fit in the stove or boiler and wood-chips must pass through the fuel handling machinery.
  
• The species of tree.
  Hardwoods are generally denser than softwoods so a hardwood log will give off more heat than a softwood log of the same size. However, the heat output per weight (unit mass) is roughly the same for all woods at the same moisture content. So the volume of wood (but not the weight) required for a particular power output will vary depending on the species.
  
• The moisture content of the wood.
  Damp wood is heavier and gives off less heat than dry wood. Freshly felled wood may contain over 50% moisture compared with 'dry' wood such as building timbers that have a moisture content of 10-15%. Wood chips for heating is usually supplied at 25-30% moisture or less.