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Wood chips
Advantage vs. logs

Although logs can be stored and transported conveniently when stacked, and the ease of air passage through a log pile allows good drying, they may not always be the most convenient form for automated handling and feeding.  Also, the relatively small surface area to volume ratio is not ideal for efficient combustion or gasification.

Wood chips can form a much more 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.

Creating wood chips
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.
More about chipping
Storing wood chips

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.

More about storage
Using in energy applications

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.

Characteristics

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

  • Forest chips - including:
    • Log chips - from delimbed stem wood
    • Whole tree chips - from all the above-ground biomass of a tree
    • Logging residue chips - from branches, brash, etc
    • 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.
More about chippers
More about short rotation energy crops

Other physical forms of woodfuelOther physical forms of woodfuel

Wood chips

Wood chips being stacked by front loader
Resources
Map of woodfuel chippers in South East England
From the Wood Heat Solutions Project