Wood energy

Wood has been used as a fuel since the dawn of time and is the main energy resource used of all the solid biomass available on earth. Once dry, it provides thermal energy by decomposing under the effect of heat. Wood energy is now available in various forms for both domestic use and highly specific industrial applications.

A bulldozer collects wood at the Vaxjo power plant in southern Sweden (march 2008).
© AFP / Soren Anderson

A thermal energy source

Wood is undoubtedly the first energy source harnessed by humans, as the use of fire dates back to prehistoric times. Renewable and abundant, this resource comes under the definition of traditional biomass and is widely used throughout the world. In southern countries, 2.5 billion people have access solely to the burning of wood and animal waste for heating, lighting and cooking¹.

The heating power of wood is lower than fossil fuels.

Burning wood generates thermal energy (heat) through a process that takes place in three successive stages:

   • Once heated, all wood, including very dry wood, first gives off humidity. Water accounts for 15-20% of its mass. This first stage of conversion, called drying, requires a lot of heat. This is why log fires are started by burning paper, kindling (twigs) or firelighters (paraffin, resin, wax, oil or alcohol).

   • When the temperature reaches 200°C, the wood starts to disintegrate under the effect of the heat - this is known as pyrolysis. The wood molecules are separated into lighter molecules, which generate gases such as methane. When these gases are burnt, they generate flames. The heat given off allows pyrolysis to continue.

  • Pyrolysis ends when all the combustible gases have been burnt off. Of the initial wood, all that's left are bits of almost pure carbon or charcoal. This continues to burn slowly without flames, giving off heat and red and infrared light - these are known as embers. Finally, when the embers have been fully burnt, the wood is reduced to ashes, there is nothing left to feed the fire and it goes out.

The heating power of wood is the amount of heat provided when it is burnt. This is expressed in kilojoules per kilogram of wood or kJ/kg (the joule is the universal unit of energy measurement).

The heating power of wood varies considerable from species to species. For example, burning hornbeam provides twice as much heat as poplar. Oak, ash and maple also have high heating power.

Preparing wood for use as fuel

Freshly cut wood contains 40-60% water. It is unsatisfactory for use as a fuel because:

   • Burning ‘green' wood releases thick smoke containing irritating substances that damages air quality and quickly clogs up chimneys, stoves and boilers

  • The heating power of humid wood is half that of dry wood².

This is why wood has to be dried before being burnt. Depending on the size of the logs and on whether it is left indoors or outdoors, it can take 1-2 years to dry wood naturally. This process reduces the humidity rate of the wood by 15-20%.

Wood can also be dried artificially by piping hot air over it or by dehumidifying the air in the drying shed. Although this drying method costs more it reduces the humidity rate of the wood by 8% in 7-15 times less time.

Any wood can be used in a fireplace or domestic boiler.

False. Chipboard, wood recovered from a demolition site or from old furniture should not be burnt because this wood has been treated and often retains residual glue, paint or varnish. If the wood was burnt, it would release corrosive or toxic pollutants.

   • Farm waste (stems and branches)

   • Waste from the wood industry (bark, sawdust, shavings and remnants from sawmills, paper mills, and pallet, furniture and timber product factories) 

   • Untreated used or scrap wood packaging (crates, cases and warehouse pallets).

The various uses of wood energy

Wood energy can be used in various forms depending on the required purpose. Similarly, some species are more suitable for some uses:

   • Charcoal can be used to cook food. In southern countries, much of the energy required to prepare meals comes from this fuel. It is often produced traditionally, by heating wood to a very high temperature in a closed space from which air is excluded. The dried wood is deprived of the oxygen it needs to burn completely and is instead turned into charcoal with 80% pure carbon content (it is charred). On a global scale, this is the main way biomass is used to produce energy. In northern countries, charcoal from industrial pyrolysis is used in barbecues. For the same mass, this fuel has twice as much heating power as dry wood. Transport costs between manufacturer and consumer are also reduced.

   • In conventional bread ovens, dry conifer wood (spruce, pine and fir) or softwood (larch, willow and poplar) is used. This wood burns quickly and gives off heat quickly and is therefore ideal for bakers and pizza chefs.

   • Wood is also used for heating purposes in open and closed fireplaces (inserts), stoves and boilers. Hardwood logs are used in these systems (such as oak, beech, ash, hornbeam, walnut and chestnut): this wood burns slowly and gives out constant heat. Stoves and boilers can also use chips (shredded branches) or pellets (compressed sawdust). Depending on the system, heat loss can occur and yield is variable - yield is the share of energy actually provided by burning the wood in relation to the maximum amount of energy it can provide in theory. Thus, open fireplaces have a yield of 10%, closed fireplaces 30-85% and stoves and boilers 40-95%. In France, in 2010, 6 million households used wood energy as a main or additional source of heat³.

   • In the industrial sector, wood energy has many applications, including firing ceramics, tiles or bricks, heating or drying products, producing hot water or steam (for example for dry cleaning). This fuel is also sometimes used to produce electricity. Water is heated in a large wood-fired boiler and the resulting steam drives a turbine that generates an electric current. Worldwide, wood is the main biomass resource used to produce electricity.

[1] Source: International Energy Agency (World Energy Outlook 2008).
[2] Source: ADEME, Le chauffage au bois , March 2010.
[3] Magazine Ademe et vous , February 2010.