Biomass is organic material, chiefly plant material such as logs, straw, and wood pellets. When this is burnt to provide energy, as in a wood-burning stove, it acts as a biofuel. 
Non-woody biomass, such as crop residues (e.g. straw), food waste and animal waste, can also be used as a biofuel, as can certain byproducts of industrial processes, notably waste wood.
Biofuels are, in theory, carbon neutral. The carbon dioxide released when a biofuel is burned equals that which the plant absorbed from the atmosphere when it was growing. Hence, there is no net increase in carbon dioxide, unlike when fossil fuels such as coal are burned.
However, if a biofuel crop requires fuel for cultivation, harvest or transportation, there will be some net carbon release, for example from the diesel or electricity used in those processes. Local sources of biofuel are generally the most environmentally friendly because transport is cut to a minimum.
Types of biomass
Biomass as used in the UK can be described under several categories:
- Wood - including logs, wood chips, wood pellets, briquettes, sawdust and brash. These are produced, unsurprisingly, mainly by the forestry and wood processing industries, although some derive from tree surgery and prunings, e.g. in maintaining parks and gardens.
- Energy crops -these are crops grown specially as biofuels, and include short-rotation forestry and coppiced timber, and fast-growing grasses, such as Miscanthus and hemp
- Crop residues - chiefly straw from wheat and barley, and corn stover (stalk and leaf residues) from maize crops
For small-scale domestic biomass boilers, the biofuel is usually in the form of wood chips, wood pellets, or logs.
Wood pellets
These are made from compressed sawdust or wood shavings that are extruded through a die in the presence of steam. The resulting pellets range from 6 to 12 mm in diameter, depending on the application. Pellets for small-scale domestic biomass boilers are typically 6-8 mm. Briquettes have a similar composition to pellets, but are larger.
Suitable for small/medium-scale biomass systems, including domestic. More expensive than wood chips or logs.
Good-quality pellets are a clean, energy-dense biofuel, able to flow well in automated feed systems and pack closely. They require less storage space than wood chips or logs. Many modern biomass boilers are designed specifically to burn pellets.
Although they are a renewable energy source, pellets have a greater embodied energy than, say, logs or wood chips, because of the energy used in grinding the wood into sawdust and compressing the sawdust. Transport distances also tend to be greater, because production involves large regional industrial-scale plants.
Wood pellets should be dry, clean and produced to a consistent quality standard. Poor-quality pellets will be less mechanically robust, and tend to disintegrate into sawdust. The standard should also define the energy density, ash content, moisture content, and levels of any contaminants, such as heavy metals or sulphur.
In the UK, quality standards are variable, whereas in other parts of Europe wood pellets are big business and produced to strict quality standards. These ensure that boilers function reliably and consistently.
Wood chips
These are chunks of wood, typically about the length of a thumb, produced by passing material through a mechanical chipper. This creates a relatively uniform fuel that can be handled mechanically and is easier to transport than, say, logs.
As a biofuel, wood chips are more suitable for larger biomass systems, because of their more variable quality compared with pellets.
Chips can be produced from various material, including forestry residues and brash, sawmill or timber processing residues, and small-diameter round timber grown in coppiced woodland or short-rotation forestry. Usually produced locally, they have less embodied energy than pellets.
Characteristics such as size of chip, moisture content, and absence of wood slivers and sawdust are important is assessing suitability as a biofuel.
Need greater storage area than pellets; should be stored under cover.
Logs
Logs for fuel are obtained from a wide variety of timber, but most often small round branches and trunks, sawn and split. They are suitable for certain domestic biomass systems, whether wood-burning stoves, open fires or boilers, that are designed to accommodate logs and the plentiful ash that results. Logs are generally sourced locally, often from managed woodland, and their use helps to encourage sustainable cycles of biomass production.
Handling is easy, but generally the stove or burner must be loaded manually every day. Proper stacking in a log pile will accelerate drying.
Points to consider
- Biofuels are not piped but must be delivered when needed
- Ensure continuity and quality of supply
- Try to source fuel as locally as possible to reduce transport
- A dry and secure storage area is essential
- Biomass boilers tend to be larger and more expensive than fossil fuel boilers
- Wood burning can be a pollution issue
- Burning chemically treated timber can produce toxic flue gases and ash
- Flues need regular cleaning