The processing of wood into a suitable fuel involves a series of interlinking processes and stages, all of which need to be taken into consideration in order to tailor the fuel specification to the desired end use:
- Pre-treatment, storage and quality assurance
- The characteristics of the starting wood and the place where any processing of the material will occur
- The type of combustion system and boiler/burner to be employed.
A variety of pre-treatment options exist to reduce the moisture content of the biomass prior to harvesting:
- Sour felling: an old method used to dry out timber. Trees are felled and left with branches intact so that transpiration (water loss) through the leaves will assist the drying of the stem
- Ring barking: a section of bark is either totally or partially removed from the circumference of the stem, effectively killing the tree over a prolonged period. Chemical thinning/treatment: chemicals are used to kill the tree
- Use of dead and dying material: the timber is harvested as a source of woodfuel rather than being left to decay in the forest.
Timings of pre-preparation and harvesting can vary.
Storage: roundwood and chip
Within forest storage as roundwood, essential considerations include:
- Storage space
- Storage conditions
- Environmental aspects
- Health and safety
- Visual impact
For long-term chip storage (up to one year) there are several methods. One well-established model is the Danish model where chipped material is stacked to a maximum height of 10 m and covered to prevent wetting and spontaneous combustion problems.
Quality assurance and long-term confidence in supply
Several organisations including British BioGen have specified standards for woodfuel.
The delivered cost of woodfuel is greatly affected by the method of transport employed and the haulage distance involved. In most UK circumstances woodfuel haulage will be by road and the cost is high; consequently the distance should be minimised.
The species and quality of trees used for woodfuel production, and in particular wood chips, are the primary determinants of the overall quality of the fuel. In many instances, woodland and tree management determine which trees are to be removed and therefore directly affect quality.
It is important to recognise that woodfuel containing a proportion of needles/leaves, bark, twigs, slivers, wood dust or contaminants will affect both the processing and burning and will have a lower calorific value. Wood with twisted grain or a high proportion of knots requires an increased amount of energy to split into logwood or convert into wood chips.
Chip specification and standards
It is important both for the woodfuel producers and users that a consistent size and quality of woodfuel is achieved. To ensure that the type of woodfuel meets the required standard (in terms of size, quality and moisture content) for the boiler/burner, a simple common standard for end users is required. Such a specification/standard for woodfuel should aim to guarantee a predetermined level of quality, in terms of size and moisture content. It provides reassurance to purchasers and end users that a consignment of woodfuel of a given specification will be of consistent quality. This enables a producer/supplier to decide on the species and size of trees and processing machinery that is best suited to produce the specified woodfuel. Similarly, the buyer can more accurately assess the amount of woodfuel needed to produce the required heat and power.
Wood pellets are a compacted form of wood fibre with low moisture content and high energy density. Pellets are typically more expensive to produce than alternative woodfuel sources, including chip and logs, but they have the benefit of being easier to handle, cheaper to transport and are ideally suited to automated burning systems. The convenience of woodfuel pellets is better suited to small-scale domestic heating requirements.
Woodfuel may not be pure wood and may contain a proportion of other materials such as soil. The method of woodfuel harvesting and the care involved in handling the product will determine the proportion of contaminants present in woodfuel. Non-combustible contaminants and chemical constituents of the wood remain after burning.