Wood energy is derived from woody biomass, the organic material left over from trees, shrubs, and wood processing residues. This bioenergy is considered a renewable resource because wood originates from plants that continuously capture solar energy through photosynthesis. Plants convert radiant energy, carbon dioxide, and water into chemical energy stored in complex molecules like cellulose, hemicellulose, and lignin. When the wood is used for energy, this stored potential is released, completing a natural energy cycle.
Preparing Wood Biomass for Energy Use
Before woody material can be efficiently converted into energy, it must undergo preparation to standardize its form and reduce moisture content. Wood is processed into uniform formats to optimize handling and energy output. These prepared forms include wood chips, which are irregularly shaped pieces used widely in larger-scale heating and power generation facilities.
Wood pellets are another widely used format, created by compressing finely ground wood fibers into small, dense, and uniformly sized cylinders. This densification increases the energy content per unit volume, making the pellets more economical to transport and store. Processed firewood, typically split logs, is primarily used for residential and small-scale heating applications.
A major preparatory step involves reducing the moisture content. Freshly harvested “green” wood can contain 50% moisture or more, and when burned, much of the potential heat is consumed evaporating this water. Drying the biomass, either naturally or mechanically, is necessary to increase its net calorific value and improve combustion efficiency. For instance, wood at 60% moisture content has nearly 60% less usable energy than wood dried to 10% moisture.
Direct Thermal Conversion (Combustion)
Direct thermal conversion, or combustion, is the most common method for releasing the energy stored in wood. This process involves burning wood in the presence of excess oxygen inside a controlled chamber, such as a furnace or boiler, releasing the stored chemical energy primarily as heat.
The heat generated can be used directly for space heating in residential wood stoves or for industrial process heat. For utility-scale power generation, the hot combustion gases are directed to a boiler containing water. This heat converts the water into high-pressure steam, which acts as the mechanical energy carrier.
The high-pressure steam then flows through and spins a turbine connected to an electric generator. The generator converts the mechanical energy into electrical energy distributed across the power grid. In Combined Heat and Power (CHP) systems, the exhaust steam is captured and used for secondary purposes, such as heating nearby buildings or supporting industrial processes, significantly raising the overall energy efficiency.
Thermochemical Conversion Methods
Wood can be converted into more versatile energy carriers through advanced thermochemical methods, which use heat to break down the biomass structure in controlled environments. Gasification converts solid wood into a combustible gas called synthesis gas, or syngas. This is achieved by heating the biomass to high temperatures, typically between 700°C and 1600°C, with a limited amount of oxygen, steam, or air.
The controlled, partial oxidation breaks the wood down into a gas mixture predominantly composed of hydrogen (H2) and carbon monoxide (CO). This syngas can be burned to produce heat and electricity, or it can be cleaned and conditioned to be used as a chemical feedstock for synthesizing liquid fuels. The specific composition of the syngas is influenced by the moisture content of the wood and the operating temperature.
Pyrolysis involves heating wood to temperatures, often around 500°C, in the complete absence of oxygen. This thermal decomposition avoids combustion and yields three primary products: a dark, viscous liquid known as bio-oil, a solid carbonaceous residue called biochar, and non-condensable gases. Fast pyrolysis, which uses rapid heating rates, is optimized to produce the highest yield of bio-oil, sometimes reaching 60–75% of the initial mass.
Bio-oil is not a direct substitute for crude oil due to its high oxygen content, but it can be stored and transported before being upgraded through refining processes like hydrotreating to create liquid transportation fuels. Biochar can be used as a soil amendment or a solid fuel. The non-condensable gases, such as methane and hydrogen, can be used to provide the heat required to power the pyrolysis process itself.
Utilizing Wood Energy
The energy released from wood finds a wide range of applications across different sectors. The most direct application is thermal use, including residential heating and providing high-temperature process heat for industrial facilities.
Electrical use is a major application, with dedicated biomass power plants generating electricity for the grid. Wood chips or pellets are also mixed and burned alongside coal in existing power plants (co-firing), offering a renewable component to the fuel mix.
Advanced conversion methods like gasification and pyrolysis enable wood energy use for transportation. Syngas can be chemically synthesized into liquid biofuels, and bio-oil can be upgraded into hydrocarbon fuels suitable for vehicles.