Harvesting wood for fuel can take place during
timber harvesting, routine operations (e.g. early and late
thinning) or the removal of trees for a specific purpose.
During timber harvesting
Fuel products can be produced during the final harvesting of a
timber crop (clearfelling). A variety of different harvesting
techniques can be employed to maximise the wood fuel yield, for
example:
| Technique |
Tree part |
Process |
Comment |
| Terrain chipping |
Whole-tree |
Chipped directly at stump, in rack, at roadside |
Generally low-grade wood chip with high moisture content, high
bark and foliage content |
| Shortwood |
Stem, branch, crown |
- Felled, de-limbed and crosscut at stump, extracted to
roadside.
- Product assortment at stump and roadside.
|
If chipped immediately, chips will have a high moisture content
or left to air-dry to produce higher grade fuel. |
| Pole-length |
Stem |
- Felled, delimbed and extracted to roadside.
- Crosscut for product assortment and stacking at roadside -
fuelwood chipped on site or transported to chipping facility.
- Crown and branch residues may be extracted.
|
- Stemwood can produce higher grade fuel. If chipped immediately,
chips will have a high moisture content or can be left to air-dry
to produce higher grade fuel.
- Residues can be extracted for low-grade fuel.
|
| Part pole-length |
Part stem |
- Felled, delimbed and sawlogs removed at stump.
- Remaining stemwood and crown may be extracted.
|
Remaining stemwood and residues may produce a low-grade chip
with high bark and foliage content. |
| Whole-tree |
Whole-tree |
Felled and left in stand or extracted to roadside.
- Whole tree chipped
- Integrated harvesting to produce variety of products - fuelwood
chipped on site or transported to chipping facility
- Residue harvesting
|
- Stemwood can produce higher grade fuel. If chipped immediately,
chips will have a high moisture content or left to air-dry to
produce higher grade fuel.
- Residues can be extracted for low-grade fuel.
|
Brash harvesting
The Forestry Commission has carried out trials on the economic
feasibility and environmental implications of brash (foliage, small
branches etc) and stump harvesting.
Brash is produced during timber harvesting from and usually left
on site, often to provide a mat for harvesting machinery. A number
of systems are available, however, for the removal of brash based
on brash bailing or secondary extraction of brash mats to supply
woodfuel for heat and power generation.
Brash consists of the foliage, branches and crowns of the tree.
The amount of brash available will depend up tree species and site
conditions.
Trials have shown that there is some potential for brash
harvesting on selected sites under certain conditions. It is
advised that no more than 60% of brash should be removed from site
and that brash should be screened and crushed before chipping. It
is most suited to larger boilers with higher burn temperatures.
The removal of brash residues poses a number of hazards to the
forest environment that can threaten sustainable forest management.
The likelihood of damage depends on site sensitivity, however and
on many sites can be effectively controlled by good forest planning
and management. A full environmental analysis of the suitability of
each site for brash harvesting is required.
Brash economics
Haulage costs have a significant impact upon the economies of
using brash for woodfuel. Coupled with the amount of processing
required for this fuel this means that brash, may not, therefore,
be the most economical form of fuel. The economics can be improved
by mixing with other cheaper forms of fuel.
Stump harvesting
Trials on stump harvesting suggest the environmental risks and
economic costs likely outweigh any potential advantage of this
operation. Stumps will contain a higher ash content due to
ingrained and adhering soil and will attract a lower price than
roundwood, with higher harvesting costs than conventional forms of
woodfuel. The fuel is not suitable for domestic or commercial
boilers.
Thinning
Thinning is the process of removing selected trees from an
immature woodland to allow other trees to increase in diameter.
- Early thinnings: woodfuel can be produced from
very small diameter material, but requires efficient harvesting
such as terrain chipping or whole-tree harvesting.
- Later thinnings: may be able to produce a
variety of products, including woodfuel using shortwood or
pole-length wood.
Directly chipping the whole tree will increase productivity but
produce low quality chip with a high proportion of bark and
needles/leaves. Future growth of the stand may also benefit from
residues left on site to provide nutrients.
Selective tree removal for conservation and biodiversity
Woodland management for conservation and
biodiversity can produce material suitable for woodfuel. For
example, conifers may be removed selectively during an ancient
woodland restoration where the aim is to return the wood to the
species composition associated with ancient woodland.