This is undoubtedly the oldest source of energy used by man. The use of fire dates back to prehistoric times.
Burning wood seems quite simple, but in fact it is a three-stage process:
- First, the wood gets rid of its humidity (even when well dried, it still contains 20% humidity). This stage consumes heat: that is why a lot of twigs or fire-lighters are required to get a log fire started;
- When the temperature reaches 200°C, the wood begins to decompose under the impact of heat (pyrolisis), giving off combustible gases. These are the gases which burn to give the fire its flames;
- By the end of the pyrolisis stage, the wood has been transformed into almost pure carbon (charcoal). The fire is reduced to glowing embers, which are consumed very slowly without flames, giving off a red glow and infrared radiation.
The calorific content of perfectly dry wood is inferior to that of fossil energy sources: a kilo of wood supplies 55% of the energy of a kilo of coal, and 45% of a kilo of heating fuel. And the damper the wood, the lower its calorific content. Freshly cut wood contains between 40 and 60% of water. It therefore has to be well dried before use. Natural drying, enabling humidity to be reduced to 15 to 20%, takes about two years. The wood can also be transformed into charcoal, which doubles its calorific capacity and consequently improves the economics of transportation. Wood used as a
fuel
comes from forests, off-cuts from the wood industry, the paper industry and miscellaneous scrap (palettes, crates and packing cases, wood from demolition activities, stakes, posts …). Some types of trees are specifically planted to produce high-energy wood for burning (eucalyptus, willow, poplar …).
Wood is used:
As a way of cooking food (75% of the energy requirements in third world countries);
As a source of heating the surrounding air by means of fireplaces, stoves or inserts, or of heating water in wood-fired boilers. Fireplaces have a yield 15% lower than
stoves
(60%) and boilers (70%);
To produce electricity. This option is above all used in large sawmills and paper mills where wood waste is abundant. In sawmills, the co-generation of electricity and heat is extremely useful:
electricity is used to power machines and the heat is used to dry wood.
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