Forestry Commission Scotland logo
Wood chips

Wood chip specifications



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 or leaves and bark than small roundwood, which may affect the ash content.

Size of chip

Many smaller wood chip boilers use a screw auger feed mechanisms which requires a small (usually  less than 5 cm), consistent particle size to feed efficiently. Large-scale industrial facilities will have more flexible feed mechanisms such as walking floors.

Specification: Particle size (P16/P45/P63/P100)

Moisture content

Wood chips, when produced from freshly felled trees, can be up to 70% moisture. Larger boilers over 400kW output may be able to burn wet woodchips. Smaller boilers need chips of a moisture content of 30% or less.

Moisture content will vary depending on when material is felled, how long it is left to dry before chipping, 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.

Specification: Moisture content (M20/M30/M40/M55/M65)


Wood is composed primarily of cellulose and lignin, along with small quantities of other chemical constituents. 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.

Earth, stones and other inert matter can seriously damage a wood fuel system, particularly those with an automatic feed. So, keep wood chip on a clean hard standing and ensure that equipment used to move wood chip, such as front loaders, are clear of stones and dirt.

Specification: Ash content (A0.7/A1.5/A3.0/A6.0/A10.0)

Chemically handled wood or used wood should also give: Nitrogen (N0.5/N1.0/N3.0/N3.0+)

Other specifications

Informative specifications include:

  • Net energy content (lower heating value) as MJ/kg or kWh/m3 loose
  • Bulk density in kg/m3 loose
  • Chlorine content (Cl0.03/Cl0.07/Cl0.10/Cl0.10+)
  • Nitrogen (N0.5/N1.0/N3.0/N3.0+)

Many other properties may also be specified, including concentrations of many other elements and volatile matter and ash melting behaviour. Different specifications are required for different fuels, and for pellets and briquettes these include mechanical durability and particle density.

Sampling and testing

The CEN/TC 355 standards include detailed specifications for sampling and testing biomass fuel.

CEN/TS 355 standards can be downloaded from the Biomass Energy Centre website, as well as further information on standards.