Energy from Forests

The most obvious, and visible sources of woodfuel in Scotland are its woodlands, forests and parks. The existing Scottish forestry and timber industry produces a wide range of valuable products.  High quality logwood would be expected to go to sawmills, and the timber processing industry for example for construction or furniture, where the greatest added value and carbon benefits can be achieved.  However,  there is the capacity to use additional material as woodfuel which otherwise would not have a market. This added value has the potential to:

• improve the quality of existing timber resources through thinning and maintenace;
• support other management objectives, such as biodiversity and access/recreation;
• encourage new woodland creation.

Woodfuel can be produced from a wide range of forestry material which is not suitable for other markets , this includes:

• Distorted wood
• Small round wood (SRW)
• Stumps
• First thinnings
• Uneconomic thinnings
• Lop and top
• Brash
• Branches

Offcuts, chips and sawdust produced by timber processors are also a very useful source of woodfuel.

Production of wood fuel through forest management practices

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 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:

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.

Management of undermanaged woodlands

An increasing number of rural estates, farms and rural businesses are using wood fuel to heat their buildings, so there is a growing demand for locally produced wood fuel. This provides a market for the lower-quality produce, making the management of woodland more economically viable.  Bringing under-managed woods back into management for wood-chip production, will result in healthier faster growing woodlands and increased biodiversity. Forestry Commission Scotland is currently working to promote the production of woodfuel from farm woodlands, for further information please farm woodlands for woodfuel production.

Wood Residues

Harvesting of residue in the form of brash and stumps can increase the volume produced from a site by up to half. It can also make site preparation easier and cheaper:

Brash Harvesting

Brash is produced during timber harvesting from the  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: 

Spruces produce as much as 30-40% brash whereas Scots Pine can produce as little as 20-25% brash.

• Site conditions:

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).

  The removal of brash residues poses a number of hazards to the forest environment that can threaten sustainable forest management.  The propensity for damage depends on site sensitivity, however and on many sites can be effectively controlled by good forest planning and management. The ground conditions and soil type should be considered as well as nutrients for future crops.  Brash harvesting should operate on mineral rich soils only, avoiding organic rich soils.

It is advised that no more than 60% of brash should be removed from site, material will require to be screened and crushed prior to chipping. 

Suitability as woodfuel

Brash material left on the ground is an efficient absorber of water with moisture content sometimes in order of 50% plus, it is therefore more suited to larger boilers with higher burn temperatures.  The ash content and therefore particulate emissions when burning is also higher relative to round wood due to the large percentage of bark in the material.

 Brash economics

Haulage costs have a significant impact upon the economies of using brash for wood fuel. 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.

For further information on brash harvesting go to the Biomass Energy Centre

Stump Harvesting

In plantation forests in parts of Europe, the stumps left after felling are now sometimes pulled out of the ground to supply wood fuel for biomass power stations since they contain a significant proportion of the woody biomass on a site.  Stumps can also be removed during road widening activities, where trees have been removed to make way for urban developments or where forest areas are converted to other uses.

As with brash harvesting, stump removal should only be carried out on mineral rich soils, avoiding organic rich soils and no more than 60% of stumps should be removed. Stumps will contain a higher ash content due to ingrained and adhering soil and will attract a lower price than roundwood, the economics of harvesting may make it unviable.  The fuel is not suitable for domestic or commercial boilers.

Stump harvesting poses a number of risks to the forest environment that can threaten both sustainable forest management and the wider environment. Where these can be reduced to a low level on suitable sites, the benefits of increasing the use of biomass, as well as other potential advantages (such as easier site conditions for subsequent replanting), are expected to outweigh the potential disadvantages.