Wood chips can form a much more uniform fuel that 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.
Depending on the boiler and feeding system, wood chips typically have a maximum dimension of 20-50mm, though larger chips (known as hog fuel) can be 100mm 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, although good airflow is necessary to disperse water vapour and minimize the chance of composting and mould formation. Stack height should be kept below 10m to prevent heat build-up from composting and spontaneous combustion.
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 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