Specifications
Typically wood chips used in domestic boilers should possess the following characteristics:
A uniform size 8-15 mm
Maximum 30% moisture content (the dryer is better)
High energy and low ash content
Free of dust and fungal spores
Wood chips are generally more common for non-domestic heating applications.
Another consideration regarding wood chip storage is the feeding mechanism for bringing the wood chips
from the fuel store to the boiler. A mechanical stirrer is required in the store to avoid bridging (whereby chip
outlet is blocked by larger pieces) of the chips as they are not as uniform in terms of shape. The minimum
storage volume also needs to be larger when storing wood chips because the energy density of the chip is
less than that of pellets, and the bulk density is also smaller (approximately 500 kg of chips per cubic metre
(m
3
) storage space compared with 650 kg of wood pellets per m
3
storage space).
Wood logs / re wood
These are generally manufactured from forestry by products. If you have a source of clean wood, cut and
store the wood (to season and dry the fuel) and manually feed into the boiler as required.
With regard to wood burning technology, boilers can be divided into two main categories:
Overre boilers - these are the most common boilers, wood combustion in these boilers occurs
throughout the entire fuel batch from the bottom up for example like an open re
Gasication boilers or “downdraught” log boilers - in these boilers, the wood pile is “smouldered” at the
very bottom by pushing air down through the wood batch. The smouldering wood is then converted to
wood gas as a result of this process. The gases are then ignited in a secondary chamber to create a high
temperature wood gas “jet” ame. Wood gasication boilers oer very high eciencies.
Wood chipper in action