The most commonly used types of woodfuel in biomass heating systems are logs, woodchip and pellets.
The focus of this guidance is woodfuel produced from virgin woody material generated through sustainable woodland management activities.
It is important to note that woodfuel is only one of a range of potential wood products and other markets make valuable use of timber. In most cases it is the lower quality wood which is used for woodfuel with higher quality wood being used for building and furniture.
Harvesting of wood should be done as part of a management plan that meets the UK Forestry Standard and the right wood should be sourced for the right markets - lower rather than higher quality wood should be used for woodfuel. Remember, most felling in the UK will also require a felling licence. See the section on woodland management for more information about applying for a Felling Licence and developing a UKFS compliant management plan.
It is important to note that when felled around half the weight of a piece of wood is actually made up of water. Wood must be dried, or ‘seasoned’, before use as woodfuel to improve burn efficiency by avoiding using energy in the wood to evaporate the water it contains. Wood should be seasoned for at least one year.
Turning wood into woodfuel - for more detail, including standards and quality control.
Logs are a popular form of woodfuel as they can be easily stored, dried and used in a variety of small to medium scale heating systems, from open fires and log stoves, to larger boilers.
Logs for fuel come from a variety of woodland activities including tree felling, coppicing, pruning and thinning.
Logs lend themselves to local production systems as their harvesting and processing requirements suit smaller scale operations and need less specialist expertise and equipment than other forms of woodfuel. However, logs are bulky and take up more storage space than other fuels and systems usually require manual fuel feed, which may not be appropriate for all users.
Almost any tree species can be used to produce logs but there are differences between logs from hardwood and softwood trees;
Hardwood logs from deciduous broadleaved trees tend to be denser than softwoods. This means that a tonne of hardwood logs will occupy a smaller space than a tonne of softwood logs. Dense wood will burn longer than less dense wood meaning that you will need to top up less to keep your log stove burning.
Softwood logs from evergreen coniferous trees can be less expensive than hardwoods and can be easier to light.
Logs should be seasoned for at least one year before use, reducing the moisture content to around 30%.
Open fires can be attractive and cosy but they are an inefficient method of heating as the uncontrolled airflow takes hot air from the fire up the chimney and draws warm air from the rest of the house in this updraught. This air is replaced by cold outside air drawn in from vents and windows in the house.
Log stoves provide radiant heat to a single room, and are more efficient than open fires, lowering fuel requirements. Many stoves are also available with a back boiler allowing the generation of hot water to heat radiators or for domestic use.
Log stoves (including those with back boilers) are not eligible under the Renewable Heat Incentive.
Modern log boilers provide an alternative to a fossil fuelled central heating and hot water system. The boiler, batch fed with logs, heats hot water for central heating and domestic use. These systems can be highly efficient, burning batches of logs in very high temperature environments. Typically they will require about 15 minutes per day of manual labour to manage; this may be longer for larger systems connected to district heat networks.
Relative to the other types of woodfuel, logs are easy to harvest and process, needing less specialist input. However, logs are bulky and take up more storage space than other fuels and systems usually require manual fuel feed, which may not be appropriate for all users.
Woodchip is widely used across mainland Europe as a source of wood heat. Systems using chip vary in size and scale from large homes to district heating networks supplying multiple buildings. Woodchip boilers are eligible under the Renewable heat Incentive
Woodchip can be produced from any tree type but the product is split into two broad categories, low grade and premium grade.
Low-grade chip often has high levels of moisture, variable size chips, large amounts of green leafy matter and potential contamination from dust and grit picked up in the course of harvesting, processing and handling. Low-grade chip can be used with appropriately designed boilers but the value of the material can be very low and it will be largely unsuitable for small to medium size heating systems.
Premium grade chip is produced to quality standards, with low moisture content, a consistent range of particle sizes, and is free from contamination. Careful attention to quality standards in the harvesting, processing and handling of woodchip ensures that the resulting fuel is matched to modern highly efficient boilers. Most community heating projects are likely to require premium grade chip. It is important to note that modern woodfuel boilers are designed for particular fuel grades, using the wrong grade of fuel will result in costly breakdowns.
Woodchip boilers vary hugely in scale and size. A typical woodchip heating system will have the following elements:
- boiler and flue/chimney
- accumulator tank – large highly insulated water tank which stores heat
- fuel store and chip feed mechanism close to the boiler to allow for easy transfer of fuel to the boiler
- hot water pipework – highly insulated to transport heat, within buildings and between buildings in the case of district heating networks
Woodchip heating systems typically need more space than fossil fuel equivalents as boilers are larger and a fuel store is needed to hold enough wood chip to supply the boiler. Space is also needed to allow for delivery of chip to the fuel store which is often delivered by lorry and tipped in to the store. Woodchip boiler flues are also typically larger and taller than fossil fuel equivalents as more gases are produced from the burning of chip. It is vital to ensure that woodchip systems are matched carefully to the available fuel type and delivery mechanism to ensure a successful outcome.
Woodchip is a good option for district heating projects, providing automatic boiler feed options not available for log systems and a cheaper more local fuel than pellet.
Woodchip can be produced from a wide range of woody material. It requires more specialist equipment such as chippers, to process and tight quality controls to meet fuel standards. Woodchip boilers are usually automatically fed. The fuel is cheaper than pellet but you will need a larger woodfuel store.
Wood pellets are a refined type of woodfuel. They are made by compressing clean dry wood (usually sawdust) under very high pressure to form small pellets. They are made from low quality wood and/or material left over from wood processing industries such as sawmills and furniture manufacturers. There are a number of pellet suppliers across the UK.
Pellets can be used in both stoves and boilers. Pellet stoves are similar to log stoves and can be small units (1kW) providing warm air for space heating in a single room, or larger units with back boilers combining space and hot water heating. Pellet stoves with back boilers are eligible for the RHI however pellet stoves without back boilers are not.
Pellet boilers range in size and can be used for small domestic systems in energy efficient homes or in larger scale district heating systems. Like woodchip systems, pellet systems will require a boiler, pellet store, pellet feed mechanism and a flue. Pellet systems are usually smaller in size than equivalent woodchip systems as a smaller volume of the highly energy dense pellets are needed to produce the same heat output as woodchip.
Wood pellets have very consistent characteristics, which means that the systems that run on them are engineered to operate very efficiently. They also have a lower moisture content than chip or logs (around 10%), they are more energy dense and require smaller woodfuel stores.
|Logs||- Easy to use
- Least mechanised harvesting and processing
- Better heat output when well-seasoned
- More input needed to operate heating systems
|- Most commonly used in small scale systems (<50kW) require daily input to load system
- Stoves and back boilers not eligible for RHI. Log boilers are eligible
|Wood chip||- More processing needed than logs
- Low quality wood from local woodland management can be chipped
- Systems usually have automatic fuel feed so less manual input required.
- Cheaper fuel than pellet
|- Typical fuel for most automated biomass systems (50kW –multi-MW)
- Eligible for RHI.
|Pellet||- Highly processed fuel type
- Most energy dense woodfuel
- Flexible fuel for use in small or large applications
- More expensive per kWh than chip
|- Most commonly used in smaller systems and where space is constrained e.g. urban applications, (<150kW) as their greater energy density allows for smaller storage space
- Pellet stoves not eligible for the RHI. Pellet back boilers and standard boilers are eligible for RHI.