Biomass Heating Factsheet
What is biomass?
Energy from biomass is produced from organic matter of recent origin. It does not include fossil fuels, which have taken millions of years to form. Although there are many different forms of biomass, the focus here is wood fuel as the most common fuel option for heat production.
How is it carbon neutral?
As the wood is burned, CO2 is released, but this will be equivalent to the amount absorbed by the plant when it was growing. There are emissions associated with the production and transportation of wood fuel, but if transportation distances are short (no more than 25 miles), the use of wood to generate heat is generally regarded as being carbon neutral.
To also be sustainable, the rate of use must be the same as or less than the rate of natural replenishment. It is therefore important to ensure that fuel supply is from a renewable source.
Types of fuel
There are three main types of wood fuel and each has different energy content, storage space requirements, availability and plant applicability. The characteristics are listed below.
Logs - obtained directly from trees with minimum processing involved. 'Seasoned' logs (those that have been left to dry) will have a higher energy value, providing more heat and less smoke. Heating plant will require manual input and storage space required will reflect local availability.
Pellets - derived from wood by-products of saw mills and wood manufacturing which may otherwise have gone to landfill. Pellets are a dense, dry fuel (approx. 10% moisture content) that contain more energy per kg and require less storage space than logs or wood chips. A UK production and distribution market is now functioning, but much of this fuel is still imported from Europe. This fuel suits both small and large scale applications.
Chips - short rotation coppice, recycled wood waste, 'lop and top' and forestry thinning are all good sources of wood chip and are often locally available, reducing transport requirements. More storage space is generally required than for pellets, due to the lower calorific value of this fuel. Chips also need to reach a certain moisture content (typically 30%) and be of uniform size. Suitable boiler sizes for this fuel are 30kW and above, so it is not generally appropriate for an individual property.
There is a range of methods for using biomass to heat a building(s) and these are outlined below.
Stoves - the traditional stove or room heater can range from 2 - 15kW, have efficiencies of up to 90% and burn either logs or pellets. They are normally utilised for individual room applications and tend to work in conjunction with conventional heating systems, although in a well-insulated house there is no reason not to utilise two or three well placed stoves for all the heating requirements. They can also be fitted with a back boiler to also provide hot water.
Temperature output in log stoves can be controlled by internal fans. Pellet stoves have microprocessors that regulate fuel flow, which also serves to control heat output.
Log boilers - loaded by hand every one to three days. They are normally located in a separate boiler room and range in output rating from 5 ? 50kW. Integrating hot water storage (buffer tank) allows the boiler to operate at higher loads, which will improve its efficiency.
Pellet boilers - have an integral pellet hopper, or (for larger models) will be screw-fed from a silo or other form of storage facility (bunker or pit being common options). Storage capabilities are a few days' fuel in the case of an integral hopper, which will then require refilling (in this instance sacks of pellets will probably have been delivered and stored). For silos and other options, the amount of storage is dictated by availability of space and given the automated nature of larger systems, refilling need not occur often, depending on heat requirements.
Boiler sizes range in output size from 5 - 100kW and as with log boilers a buffer tank is often a good addition to a system. Controls are similar to those of conventional fossil fuel boilers, allowing for times and outputs to be set by the end user.
Chip boilers - these do not have an integral hopper, so will need a fuel store, such as an adjacent pit, hopper or silo. This will be larger than for a corresponding sized pellet boiler. Chip boilers have a minimum size of 30kW heat output, rising to 1,000kW, so they are not suitable for most domestic properties. They are also at their best where there is a constant heat demand as they are not as responsive as pellet boilers, although a buffer tank can provide more flexibility. Like pellet boilers, they are fully automated.
Combination boilers - are an increasingly common option, allowing for two or more of the fuel choices outlined to be utilised.
Community/District Heating - can run off pellet and chip boilers, although fuel supply costs normally favour the latter. As chip systems are at their best when a constant heat demand is required, a district heating system might well be set up for the base-load heat to come from woodchips, with oil or gas boilers supplying peak-load and back-up. This may become a common solution for larger new-build developments as planning regulations require higher carbon reduction standards.
Designing the right system
Biomass heating systems can, if correctly sized and installed, provide all the heating and hot water needs of a building. However, they do require more space than a conventional fuel heating system. Fuel needs to be stored and, to ensure maximum efficiency, it is often a good idea to install a large heat storage tank. This improves the performance of the boiler against fluctuating heat demands.
Typical wood chip systems will have the fuel store adjacent to the boiler house to allow for automated feed of fuel from the store to the boiler grate. This can also be the case for a pellet system. Storage could be in the form of a pit, bunker or silo and this must be easily accessible for delivery vehicles.
Pellet systems have the additional option of storing fuel remote to the boiler, but this means manual transportation and feeding to an integral boiler store or an attached hopper. Space is still needed somewhere to store the fuel.
Biomass boilers can link into new and most existing heat distribution systems, from traditional high-temperature radiators, to low-temperature underfloor heating networks.
Operation, maintenance and fuel supply
Wood pellet boilers can be turned on and off manually, or via traditional controls, as and when required. Wood chip boilers work best when heat demand is constant, so they may either be left on through the heating season (turned down to low settings when not required), or will work in conjunction with a large heat store, which will release heat when required (this too can be completely automated).
During the summer, if hot water is required only in limited quantities (such as in a house for washing and bathing), it may be more cost-effective to have an LPG or electric heater, or more preferably a solar thermal water heating system.
As with all boilers, wood pellet and chip boilers need an annual maintenance check and most installers can undertake this service. Stoves and room heaters should be swept regularly to remove ash. The only other ongoing operation issue will be to periodically empty the ash tray and this can disposed of into the garden or general household waste. Quantities will be minimal, as under 1% of input comes out as ash (the image shows three days' worth of ash from a medium-sized wood chip boiler).
Fuel supply for wood pellets and chips is a rapidly expanding industry and TV Bioenergy can supply both ? contact on 01635 817420.
Approximate system and fuel costs as of May 2011 below:
|Fuel||Wood chip (30%) moisture content||Wood pellet|
|Cost £||£90 per tonne||£185 per tonne|
|kWh per unit||3,500 kWh/t||4,800 kWh/t|
|Pence per Kw||2.6p/kWh||3.9p/kWh|
|Wood chip-Small (90 Kw)||£380|
|Wood chip-Medium (up to 500 Kw)||£180|
|Wood chip-Large (Over 500 Kw)||£150|
Prices above includes boiler, fuel feed mechanism and buffer tank installed & commissioned. It would cost extra for power connection, mains water connection, heating pump, flue, or fuel store construction.
Pellet boilers often have cheaper starting prices (For example a 42 kW boiler would around £5,900 installed).This is a reflection of the fact these boilers tend to be smaller than their wood chip equivalents.
The Government is expected to introduce the Renewable Heat Incentive (RHI) from June 2011. Energy users that generate heat from renewable technologies will be paid at a level set for each technology and range of size.
There may be issues around size and design of fuel store, and height of flue. These issues are more likely if the building is listed, in a Conservation Zone, or within an Area of Outstanding Natural Beauty (AONB), although these are not difficult to resolve.
Emission levels are comparable to gas and oil boilers and are tightly controlled under EU legislation. Manufacturers are capable of providing the necessary certification for this. In addition, boilers rated at under 400kW heat output are not subject to planning controls unless located in a smoke-free zone, as stipulated under the Clean Air Act (1956). If located in a smoke-free zone, damp wood cannot be burned unless the system is 'smoke free'. Most modern wood boilers are smoke-free when fuelled with dry wood (wood must have a moisture content of no more than 25-30%). At over 400kW, the host local authority would need to issue an emission authorisation regardless of the location.
Vehicular movements associated with a large installation depend on fuel storage area available and rate of burn. It is possible to work these figures out and they are normally significantly below common perceptions.
- TV Bioenergy – www.tvbioenergy.co.uk – Suppliers of wood fuels and biomass scheme consultants and general information on suppliers and installers of stoves and boilers
- REA – www.r-e-a.net – The voice of the renewables industry in the UK
- BEC - www.biomassenergycentre.org.uk - A 'one stop shop' to provide information to anyone in the UK with an interest in biomass derived solid, liquid and gaseous fuels and associated conversion technologies.