Wood is probably the most important renewable energy source known to man.
The world’s total above ground timber biomass reserve to be 420 (109) tonnes, whilst the FAO (2000) estimated the global production of wood fuel to be approximately 1,634 (106) tonnes, however 90% of this was produced in developing countries. In contrast to the production of fuel wood, the world production of round wood (1,515 (106) m3) was dominated by developed countries. However, of the 1,515 (106) m3 about 40% is classed as primary or secondary processing residues and suitable for composites or energy production.
Wood fuel can be bought in variety of forms ranging from logs, through billets, chip to processed pellets. The form of the fuel will depend on the size and style of the combustion chamber.
When wood burns it released energy, however the amount of energy will partly depend on the species of the wood but is more influenced by the moisture content. A common hardwood, oak, has an energy content (Heat value) of 14.9 megajoules per kilogram and 10.4 megajoules recoverable if burned at 70% efficiency. Whilst the ultimate energy content of the timber does not change as the moisture content of the timber increases the amount of usable energy falls, this is due to energy being used to drive off the water before the energy can be used.
The graph below (taken from the Forestry Commissions ‘Wood as a Fuel fact sheet’) shows the reduction of calorific value with the increasing moisture content. It is therefore imperative that the fuel is dried to the correct moisture contents before being burnt.
Pellets are usually manufactured from waste wood or off cuts. They have consistent quality, with low moisture content, high energy density and a consistent size and shape, this lends them to being feed in to the burns using automatic systems.. The manufacture of pellets is more closely regulated than fuel wood, and an EN standard defines the parameters for manufacture (BS EN 14961).
Pellets are used at both industrial and domestic scale, with the domestic market becoming more popular, they are easier to maintain and can be easily automated making them work more akin to gas boiler than other wood fuel systems