Wood is a form of biomass energy derived from forest materials, agricultural residues, and wood processing byproducts. This fuel source is renewable because forests can be replanted, allowing for carbon neutrality when sourced sustainably. The chemical energy stored within the wood’s organic matter is released through thermal conversion processes to produce usable heat or electricity. Wood utilization spans small-scale domestic heating to large industrial power generation.
Preparation and Forms of Wood Fuel
Before wood can be efficiently converted into energy, it must be processed into standardized fuel forms. The most basic form is cordwood, primarily used in residential settings for direct combustion. Industrial applications require forms with lower moisture content and higher energy density.
Wood chips are produced by mechanically chipping raw wood materials, creating a uniform size easily handled by commercial and industrial boilers. Chips are cost-effective and readily available but have low energy density and high moisture content, limiting transport distance and energy output.
Wood pellets are a highly refined fuel form, created by grinding wood into sawdust and then compressing it under high pressure. This process reduces moisture content below 10% and significantly increases bulk density, improving transport and storage efficiency.
Direct Thermal Energy Generation
Direct thermal energy generation relies on combustion to release heat. This application is typically confined to residential homes and small commercial buildings for space heating and hot water production. Wood is burned in appliances like wood stoves, fireplaces, and small biomass boilers, converting chemical energy directly into thermal energy.
In a modern, enclosed wood stove, heat transfers to the surrounding space through radiation and convection. Radiant heat is emitted from the hot metal surface, warming objects nearby. Convection currents are created as the stove heats the surrounding air, causing warm air to circulate throughout the room.
Small-scale biomass boilers operate similarly to fossil fuel boilers, burning wood chips or pellets to heat water circulated through radiators or domestic hot water systems. This method achieves higher efficiency than open fireplaces because the combustion process is tightly controlled. The heat generated can also be used in district heating systems, where a central boiler provides warmth to multiple buildings through underground pipes.
Large-Scale Electricity Production
Wood is used on an industrial scale to generate electricity in two primary ways: dedicated biomass power plants and co-firing in existing coal facilities. Dedicated biomass plants burn wood chips or pellets in large, specialized boilers to produce high-pressure steam. This steam turns a turbine connected to a generator to produce electricity, following the conventional thermodynamic cycle. These facilities manage the unique characteristics of wood ash, which differs chemically from coal ash.
Co-firing involves mixing wood biomass, usually pellets or finely ground wood, with coal in a conventional pulverized coal boiler. This method allows existing power plants to partially substitute a renewable fuel without requiring a complete overhaul. Co-firing ratios are typically low to avoid issues such as corrosion or excessive ash fouling.
Utilizing the established infrastructure of coal plants, co-firing offers a fast and low-cost pathway to introduce bioenergy into the electricity grid. Wood typically contains much higher volatile matter and lower sulfur content than coal. This difference means the wood ignites and burns more rapidly, influencing the overall combustion dynamics within the furnace.
Advanced Thermochemical Conversion
Beyond direct burning, wood can be converted into more versatile, non-solid fuels through advanced thermochemical processes: gasification and pyrolysis. Gasification heats the wood material to high temperatures in a controlled environment with limited oxygen or a gasifying agent like steam. This partial oxidation converts the solid wood into a combustible gas mixture known as syngas, composed mainly of hydrogen, carbon monoxide, and methane.
The resulting syngas can be cleaned and used directly as a fuel for internal combustion engines, gas turbines, or as a chemical building block for synthetic fuels. Pyrolysis involves heating the wood biomass in a completely oxygen-free atmosphere, usually at lower temperatures. This thermal decomposition breaks down the wood into three distinct products: bio-oil (a dense liquid fuel), a non-condensable gas, and a solid residue called biochar.
Bio-oil, which is a complex mixture of organic compounds, can be upgraded and refined into transportation fuels or used in industrial burners. Biochar, the solid carbonaceous residue, is similar to charcoal and can be used as a solid fuel or for soil amendment. Both processes create fuels that are easier to transport and store than raw wood, allowing utilization in a wider variety of applications.