Wood is the most widely available solid biomass resource. Once dry, it provides thermal energy when it is burned, through a thermal decomposition reaction.
The Three Stages of Wood Combustion
Burning wood generates heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... — thermal energy — through a three-stage process:
- Moisture is driven off when wood, even very dry wood, is heated. This water accounts for 15 to 20% of its mass. This first stage of conversion, known as heating and evaporating, requires considerable amounts of heat.
- When the temperature reaches 200°C, the wood starts to decompose under the effect of the heat. This heat-producing stage is known as pyrolysisThermochemical decomposition of organic material at elevated temperatures... . The wood molecules (cellulose, lignin and hemicellulose) are separated into lighter molecules that generate combustible gas, such as methane (ch4)The main component of natural gas deposits and oil deposit gas caps. Methane is produced naturally by landfills... and volatile matter. Pyrolysis ends when all that is left of the initial wood is chunks of charcoalCharcoal is carbon produced by the pyrolysis of wood in the absence of oxygen... containing 80% of pure carbon. This process is known as carbonization.
- In the presence of oxygen, the charcoal or embers, which have a surface temperature of up to 800°C, continue to burn slowly. When the embers have been fully burned, nothing is left but ash, representing about 1% of the mass of the initial wood.
800°C, the surface temperature of embers
Pyrolysis, the second stage of the transformation of the wood, takes place in the absence of oxygen or in an atmosphere with low oxygen content and does not produce flames. A fire deprived of air will go out. Oxygen is necessary for complete combustion.
The Heating Value of Wood
The heating value of wood, also known as its heating powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... , is the amount of heat provided when it is burned. This is expressed in kilojoules per kilogram of wood or kJ/kg (the jouleA derived unit of energy, work or amount of heat in the International System of Units. Its symbol is J... is the universal unit of energy measurement.) The calorific valueThe amount of heat released by the complete combustion of a fuel, expressed in kilowatt-hours (kWh) or in megajoules (MJ) per cubic meter. of wood varies considerably from species to species. For example, burning hornbeam provides twice as much heat as poplar. Oak, ash and maple also have high heating values.
Preparing Wood for Use as Fuel
Freshly cut wood contains 40-60% water. It is a poor fuelFuel is any solid, liquid or gaseous substance or material that can be combined with an oxidant... . When burned, “green” wood releases thick smoke containing irritants, pollutes the air and quickly clogs up chimneys, stoves and boilers. In addition, its heating value is half that of dry wood1.
This is why wood has to be dried before being burned. Depending on the size of the logs and whether the wood is stored indoors or outdoors, it can take one to two years to dry naturally. This process reduces the moisture content of the wood to 15 to 20%.
Water still represents 15 to 20% of the mass of very dry wood.
Wood can also be dried artificially by pipingAn instability phenomenon that occurs when seepage through a dam is not properly filtered and soil particles continue to progress... hot air over it or by dehumidifying the air in the drying shed. Although this drying method costs more, it reduces the moisture content of the wood to 8%, 7 to 15 times faster.
Once prepared, wood has many uses (See Close-Up: "The Different Usage of Wood Energy").
(1) French Environment and Energy Management Agency (ADEME) - (in French only)