Energy from forests
Silviculture
Production of fuel can be from routine silvicultural operations such as early and late thinning, or it can take place during final harvesting operations.
The present economic situation of forestry in Scotland is depressed and a consequence of this is that thinning operations are not usually carried out in commercial plantations, to the detriment of timber quality. Being able to sell the products of thinning as fuel would allow this work to be carried out economically, with positive silvicultural benefits.
Woodfuel harvesting can have environmental and amenity benefits. Thinning of dense conifer stands can improve their biodiversity by allowing more light to reach the ground and encouraging the growth of an herbaceous layer. This in turn makes the habitat suitable for a wider range of invertebrate species.
The selective removal of conifers for fuel can be used during the restoration of Plantations on Ancient Woodland Sites (PAWS), where there is a desire to return the wood to the species composition associated with ancient woodland.
Harvesting as part of standard operations.
Fuel products can be produced during the final harvesting of a timber crop and a variety of different harvesting techniques can be employed to maximise the yield of biomass for energy such as:
Terrain chipping
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Whole-tree
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Chipped directly at stump, in rack, at roadside
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Generally low-grade wood chip with high moisture content, high bark and foliage content
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Shortwood
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Stem, branch, crown
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Felled, delimbed and crosscut at stump, extracted to roadside.
Product assortment at stump and roadside.
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If chipped immediately, chips will have a high moisture content or left to air-dry to produce higher grade fuel.
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Pole-length
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Stem
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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.
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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.
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Part pole-length
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Part stem
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Felled, delimbed and sawlogs removed at stump.
Remaining stemwood and crown may be extracted.
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Remaining stemwood and residues may produce a low-grade chip with high bark and foliage content.
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Whole-tree
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Whole-tree
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Felled and left in stand or extracted to roadside.
a) whole tree chipped
b) integrated harvesting to produce variety of products – fuelwood chipped on site or transported to chipping facility
c) residue harvesting
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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.
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Early thinnings
Thinnings are a silvicultural practice which can be beneficial to the overall quality of a stand. Woodfuel can be produced from very small diameter material, but requires efficient harvesting methods 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. 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.
Clearfelling
Final harvesting of a stand can potentially generate several products: sawlogs, pulpwood, chipwood, pallets, fencing, and woodfuel. Where a variety of products is available, shortwood , pole-length or whole-tree harvesting are likely to be the preferred systems. Part-pole length will be suitable in some situations, generally broadleaf stands. Effective integration of harvesting will allow efficient product assortment and maximise productivity. The potential for residue harvesting will depend on the economics of extraction, site conditions and availability of markets.
Residues
Harvesting of residue can increase the volume produce from a site by up to half. It can also make site preparation easier and cheaper. The ground conditions and soil type should be considered as well as nutrients for future crops.
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The foliage, branches and crowns of the tree. The amount of brash available can depend on a number of factors. The species of tree affects the amount of brash available: Spruces produce as much as 30-40% brash whereas Scots Pine can produce as little as 20-25% brash. The site conditions can dictate the success of these systems and in some cases it may be necessary to leave the brash on site to allow for the running of machinery over wet ground (brash-mats). Stumps can be extracted on some sites and can generate a substantial amount of wood energy. Some species such as pine are unsuitable as they have deep tap roots. Soil disturbance will also be a big consideration and the fuel will contain contaminants such as soil, stones and humus so will only be suitable for large industrial facilities.