Wood chips offer the advantage over logs that they can be fed automatically into a boiler. This makes them suitable for larger boilers, however they are a relatively bulky fuel so require a large fuel store.
Advantages over 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 are not suited to automated handling and feeding. Also, the relatively small surface area to volume ratio makes efficient combustion or gasification difficult to achieve.
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.
Producing 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.
To produce chips suitable for most boilers requires a specialist chipper designed for woodfuel use to produce chips of even size that will flow well and ensure there are no long slivers that can cause jams or bridging. A general purpose chipper is not suitable.
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 the potential for spontaneous combustion.
Using in energy applications
Wood chips may have a bulk energy density of about 40-50% 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 British standards for solid biomass, including wood chips, allow all of these parameters, and acceptable ranges, to be defined.
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.