Wood chips are a uniform fuel that can flow and can be fed to a boiler, gasifier or other conversion system as a steady flow using an auger feed or a conveyor. With a large surface area to volume ratio they can also be burned very efficiently.

Choose from the following or scroll down the page to read all of the sections:

• Producing wood chip
• Burning wood chip
• Wood chip burners
• Buying wood chip

Producing wood chip

The characteristics of wood chips will depend both on the chipper and the material from which they are made. They can be divided into the following groups:

• Forest chips - including:
  o Log chips - from delimbed stem wood
  o Whole tree chips - from all the above-ground biomass of a tree
  o Logging residue chips - from branches, brash, etc
  o Stump chips - from stumps

• Wood residue chips - from untreated wood residues, recycled wood and off-cuts

• Sawing residue chips - from sawmill residues

• Short rotation coppice / short rotation forestry chips - from the respective energy crops

Depending on the equipment they are to be used with, wood chips typically have a longest dimension from 20-50 mm, though larger chips (known as hog fuel), and chunks can be 100 mm or more. Long thin pieces (slivers) amongst the chips should be avoided as they can cause bridging and blockages in a chip feed system.

Wood chips should be stored under cover to prevent wetting, however good airflow is necessary to disperse water vapour and minimize the chance of composting and mould formation.

In addition, stack height should be kept below 10 m to prevent heat build up from composting and spontaneous combustion.

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Burning wood chip

Wood chips may have a bulk energy density of about 50% of that of the solid wood.

Wood chips for energy applications should meet an appropriate quality standard if they are to be used reliably in combustion equipment, especially small scale and domestic equipment.

Physical parameters, such as maximum size and absence of slivers or fines (sawdust), and maximum moisture content are important to allow reliable operation and prevent feed blockages. Also levels of contaminants and ash content will have an impact on emissions and maintenance schedules.

The upcoming European standard for solid biomass, including wood chips, is being drafted by CEN/TC 335 that allows all of these parameters, and acceptable ranges, to be defined.

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Wood chip burners

As with wood pellets modern wood chip boilers can provide a high level of automation and convenience for wood fuelled space heating. Wood chip systems generally have an output of greater than 20 kW (suitable for a large farmhouse or larger) and are not cost effective or appropriate for typical domestic scale applications.

Extensive fuel handling systems and fuel storage facilities are required for automated operation. It is important to be able to source a steady supply of woodchip with a consistent size and moisture content suitable for burning in a boiler, as not all chips are suitable for burning.

For details of installers of wood chip installers see the Woodlots directory.

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Buying wood chip

Wood chip is the cheapest form of wood fuel suitable for automated boilers. The cost of heating with wood chip is typically between 2-3p per kWh; to see how this compares with other fuels see fuel costs.

There are a growing number of suppliers of fuel grade wood chip in the UK, including several based in Surrey. See the Surrey Hills Wood Fuel directory for local suppliers and the Woodlots directory for South East based suppliers.