Quality

Suppliers must ensure quality control in the suppy chain, in order to guarantee that fuel meets customers' specifications. The quality of the fuel required will depend on the boiler equipment installed by the user and this can vary considerably. The key variables are:

origin
particle size
moisture content
ash content.

The quality of the wood chip that you produce will depend firstly on the origin of the material - for example, woodfuel from brash or arboricultural arising will contain more needles / leaves than small roundwood, which may affect the ash content. Particle size depends mainly on the equipment used for processing and screening of wood chip to remove large pieces and fines. A lot of smaller wood chip boilers use a screw auger feed mechanisms which requires a small (usually <5cm), consistent particle size to feed efficiently. Large-scale industrial facilities will have more flexible feed mechanisms such as walking floors. Moisture content will vary depending on when material is felled, how long it is left to dry and whether additional processing by forced air drying is used - it is essential that wood chip is supplied at an appropriate moisture content for the combustion equipment, and at a reasonably consistent content, whether that is around 25% or around 50%.

Standards

It is vitally important for customer confidence to have fuel which is fit for purpose and delivered to a quality standard and specification. This has been demonstrated time and time again internationally. The EU has now published clear technical specifications – CEN TC335 for solid biofuels and CEN TC343 for solid recovered fuels - which are available from BSi. More information on the standards is available from the Biomass Energy Centre. (Simplied standards and guidance on testing are currently in production and will be available soon from the Forestry Commission).

The British Pellet Club is also seeking to develop a Wood Pellet Accreditation Scheme based on the CEN technical specifications.

Specification

Generally small (<100kW) boilers require relatively high quality fuel, usually with a moisture content around 25-35%, with a consistent particle size. Larger boilers may be more tolerant in terms of moisture content and industrial scale facilities (5 MWth+) may be able to take a wide range of fuel types as a mix of supply.

A specification / standard for wood fuel provides reassurance to end users that a consignment of wood fuel of a given specification will be of consistent quality. This enables a supplier to decide on the most suitable raw material and processing machinery to produce the specified wood fuel. This also allows the buyer to assess more accurately how much wood fuel they need to produce the required amount of heat energy.

Numerous wood fuel processing machines and boiler/burner appliances are available. The chip producers should verify that any particular chipper with an appropriate set up and raw material type can meet the required wood chip specification.

Monitoring

Sampling of the wood fuel is required; this can either take place in the forest as roundwood or as chips. Some methods may include oven dry weight sampling or moisture probing. The CEN Standards include detailed specifications for testing biomass fuel and there is an increasing number of accredited testing bodies. For more information, please contact us.

Contaminants

Wood is composed primarily of cellulose and lignin, along with small quantities of other chemical constituents, and there may also be a proportion of other materials such as soil. Factors such as the method of wood fuel harvesting and the care of handling of the product will determine the proportion of contaminants present in wood fuel. The ash produced from burning woodfuel in a modern efficient systems is about 0.5 to 2.5% of the oven dry weight of the wood fuel.

References and further reading

The Biomass Energy Centre

BSi

The British Pellet Club

European Pellet Centre