Wood energy efficiency
Heat release
The amount of heat released by wood combustion depends on a number of factors, such as wood species, moisture content, wood density and combustion efficiency. The heat released by wood combustion is generally expressed in calorific value units, such as megajoules per kilogram (MJ/kg) or British Thermal Units (BTU) per pound.
The calorific value of wood generally varies between 15 and 21 MJ/kg for hardwoods (such as oak or beech) and between 18 and 20 MJ/kg for softwoods (such as pine or spruce). Hardwoods generally have a higher density and release more heat per unit volume than softwoods. The moisture content of the wood also has a significant influence on the amount of heat produced during combustion. Dry wood (with a moisture content of less than 20%) releases more heat than wet wood, as part of the combustion energy is used to evaporate the water contained in the wood.
Yield per hectare
Here are a few examples of average yields for different tree species, taking into account their dry weight:
- Poplar (Populus spp.): 10 to 20 cubic meters per hectare per year. Poplars grow rapidly and are often used for short-rotation timber production. The dry weight of poplar wood is around 400 to 500 kg per cubic meter, equivalent to 4 to 5 t/ha/year.
- Eucalyptus (Eucalyptus spp.): 10 to 40 cubic meters per hectare per year. Eucalyptus trees are fast-growing and can be harvested in 10 to 15 years. The dry weight of eucalyptus wood varies from 500 to 900 kg per cubic meter, depending on the species and growing conditions. It yields 5 to 36 t/ha/year.
- Pine (Pinus spp.): 5 to 20 cubic meters per hectare per year. Pines are commonly used for lumber and pulp production. The dry weight of pine wood generally varies between 350 and 700 kg per cubic meter. This gives between 1.75t and 14t/ha/year.
- Oak (Quercus spp.): 2 to 6 cubic meters per hectare per year. Oaks are slow-growing trees that produce high-quality wood for construction and cabinet-making. The dry weight of oak wood is around 600 to 900 kg per cubic meter. This gives 1.2 to 5.4 t/ha/year.
This shows that some woods are more suitable than others for the production of firewood. These values are indicative and may vary according to growing conditions, tree age and forest management practices.
Wood energy applications
Wood can be used as energy in a number of ways, the main ones being in biomass power plants or in residential heating, notably in wood-fired boilers or pellet stoves. However, another possibility would be to pyrolyze it to produce a hydrogen-rich syngas.
Biomass power plants
Direct combustion is the most common method of producing energy from wood in biomass power plants. Wood is burnt in a boiler to produce steam, which then drives a turbine connected to an electricity generator. Waste heat can be recovered and used for heating or other industrial applications. This is known as cogeneration, or combined heat and power (CHP).
Domestic heating
Domestic wood heating is a popular and environmentally-friendly heating solution. It uses wood as a fuel to produce heat, which can be achieved via wood-burning stoves, fireplaces and wood-fired boilers. These systems can be stand-alone or connected to a central heating system. One problem is the emission of fine particles into the home. To avoid this, modern wood-burning systems have high efficiencies and reduced particle emissions, thanks to advanced combustion technologies. However, it is essential to maintain and ventilate these systems properly to avoid indoor air quality problems.
Pyrolysis or pyrogasification
By slowly heating wood in a specific way, it is possible to extract its molecules, leaving most of the carbon in solid form (known as biochar). This is an extremely difficult process, which has yet to be implemented industrially and commercially. We presented this process in the section on hydrogen.
Wood combustion emissions
Wood combustion is a complex process involving the thermal decomposition of wood components into gases, liquids and solids. The main components of wood are cellulose, hemicellulose and lignin, which make up around 90% of the dry weight of wood. There are also other, smaller components, such as extracts (resins, oils, etc.) and minerals (ash).
The process releases not only carbon dioxide (CO2), but also carbon monoxide (CO), nitrogen oxides (NOx), fine particles and other volatile organic compounds (VOCs). Emissions of these substances depend on wood quality and combustion conditions. Complete and efficient combustion minimizes the production of harmful pollutants such as CO, NOx and fine particles. It is therefore important to use modern, well-maintained combustion equipment and proper combustion practices to ensure clean, efficient wood combustion.
Using wood as energy around the world
The following is a list of the 10 countries with the highest per capita wood energy consumption in kWh. These figures are estimates and may vary according to data sources and annual fluctuations.
- Finland: approx. 10,000 kWh/capita – Finland is a world leader in wood heating, with a large proportion of the population using wood-burning stoves for domestic heating. The country also has several biomass power plants that use wood to produce electricity and heat.
- Sweden: approx. 8,000 kWh/capita – Sweden has invested in the development of biomass for energy production, including wood. Wood-burning stoves and pellet boilers are popular for domestic heating, and biomass is also used in cogeneration plants to produce electricity and heat.
- Canada: approx. 6,500 kWh/capita – Canada has extensive forest resources and uses wood as an energy source for residential heating, particularly in rural areas. The country also boasts a number of biomass power plants.
- Estonia: approx. 6,000 kWh/capita – Estonia has a tradition of using wood for domestic heating. The country has also invested in biomass power plants for electricity and heat production.
- Latvia: approx. 5,500 kWh/capita – Latvia mainly uses wood for domestic heating and biomass power generation. The country has extensive forestry resources.
- Austria: approx. 5,000 kWh/capita – Austria is a country where wood heating is commonplace, thanks in particular to pellet stoves and wood-chip boilers. Wood is also used in cogeneration plants to produce electricity and heat.
- Switzerland: approx. 4,800 kWh/capita – Switzerland uses wood for domestic heating, mainly in the form of logs and pellets. The country also has a number of biomass power plants.
- Norway: approx. 4,500 kWh/capita – Norway mainly uses wood for domestic heating, with a preference for wood-burning stoves. The country has also invested in biomass power plants for electricity and heat production.
- Denmark: approx. 4,000 kWh/capita – Denmark uses wood for domestic heating and biomass power generation. The country has invested in research and development of biomass technology.
- Slovenia: approx. 3,800 kWh/capita – Slovenia has a tradition of using wood for domestic heating, and the country has also invested in biomass power plants for electricity and heat production.