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Harvesting wood material

Harvesting wood for fuel

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 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 upon 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.