The most commonly used and available form of biomass is wood. Here the Building Cost Information Service discusses this source of fuel, and the technology used to burn it

The main types of fuel are logs, wood chip and wood pellets. Logs are normally used in open fires, stoves and manually fed boilers for space heating, water heating and cooking. They can cost between £20 and 60 per m3 depending on the quality and quantity of logs required.

Which fuel?

Wood chips should comply with the specification for each boiler type. The particle size and moisture content cannot be compromised or the user is likely to encounter problems. Normally, fuel-quality chip will need to have dimensions of about 30mm and be of less than 30% moisture content. The cost of wood chip can range between £45 and £67 a tonne depending on the quantity required and the location of the boiler.

Wood pellets can be used at any level, from domestic pellet stoves to very large boilers. They are a compressed form of wood with a low moisture content (not more than 8%) and good flow properties, so they are easy to deliver and store. They are usually made from compressed sawdust and can be 6, 8, or 10mm in diameter, the smaller pellets being used for smaller boilers. They can cost between £140 and £200 a tonne depending on the method of delivery.

Although wood chip is cheaper than pellets, wood chip boilers are normally more expensive, owing to the different means of delivery of the fuel to the boiler from storage. Further, the means of delivery of fuel for wood chip boilers is normally more restrictive in that access straight of the back of a lorry, for tipping into a storage room, is required, whereas pellets can be delivered in bags and blown through pipes and tubes. Although wood chip can be delivered through pipes and tubes, the delivery truck will need to be specially adapted to do this.

For many heat users, heat supply contracts will be the most convenient way to use wood fuel. Rather than paying for fuel by the load or by weight, a heat meter is connected to the boiler that records its output. This way, the user pays for the heat delivered every month.

Getting the most from biomass

To get the maximum benefit from the use of the system, the following factors should be considered:

  • Space: Biomass boilers are larger than fossil fuel boilers and should ideally be situated in an outhouse or large garage. Simple room stoves do not require as much space.
  • Fuel: It is important that you have storage space for the fuel, appropriate access to the boiler for loading, and a local fuel supplier that can deliver the fuel required and agree the method of unloading it.
  • Flue: Solid fuel appliances require special flues and there must be sufficient air movement for proper operation of the stove. Chimneys can fitted with a lined flue.
  • Regulations: The installation must comply with all safety and building regulations.
  • Smokeless zone: Wood can only be burned on exempted appliances, under the Clean Air Act; this mainly applies to domestic appliances.
  • Planning: If the building is listed, or in an area of outstanding natural beauty, you will need to check with your local authority planning department before a flue is fitted.

Types of boiler

For domestic systems, a 5-60kW capacity is usual. At the domestic scale, biomass boilers start with simple log-and-pellet stoves to provide heat to single rooms. Stoves can be fitted with back boilers to provide hot water.

Log-and-pellet-fired boilers that can meet all a home’s space heating and hot water needs are also available, if there is space to accommodate them.

A chip boiler may be appropriate for large homes but space will be needed to store wood chip.

Commercial systems generally have a 50-500kW capacity. Above domestic scale, the use of automated wood chip or wood pellet boilers are more common. Logs become a problem since greater heat output means frequent refuelling, and automated log boilers above 100kW are rare.

Other reasons why automated boilers become more common at this scale is that they are generally self-cleaning and provide automatic means for the removal of ash.

If there is sufficient space available for storing wood chip it will offer greater savings on running costs than pellets.

Pellet boilers have their own advantages. Fuel delivery is easier, less space is required for fuel storage and they have a better modulating ability than chip boilers.

Large commercial systems are between 500kW and 20MW. Very large biomass boilers exist. At this scale, more robust supply chains need to be put in place and careful consideration given to details such as traffic management to enable deliveries of fuel to be maintained.

Costs of boilers

The costs of boilers and their accessories are difficult to determine as each case will be different. Therefore, it is not safe to compare the costs of similar installations as each will have different feed systems, storage considerations and civil works.

The following ballpark figures are quoted as follows, including boiler, feed system, design, installation and commissioning, but excluding civil works or other mechanical accessories:

  • Boiler capacity 10-100kW, pellet only, £250-500 per kW.
  • Boiler capacity 10-70kW, log only, £250-400 per kW.
  • Boiler capacity 30-500kW, chip or pellet, £150-350 per kW
  • Boiler capacity 500kW-10MW, chip boilers, £100-150 per kW.

Cost per kW/h of fuel

How do the costs of producing heat using biomass boilers compare with using fuel and alternative technologies? A quick survey has found the following costs:

Fuel — Cost per kW/h (p)

Fuel oil — from 2.1p to 6.8p
Gas — from 2.4p to 4p
Wood chip — from 1.6p to 2.5p
Wood pellet — from 3p to 3.5p