Producing biomass fuels from forestry

Forestry is primarily focussed on high value sawlogs for timber, and this provides the primary economic driver, however both routine forest management and harvesting can be used to supply residues that can have a value as biomass fuel.

Harvesting, pre-preparation and transport options

Where residues for woodfuel are to be harvested alongside sawlogs, there are a number of harvesting options.  Optimum choice will depend on terrain, equipment available, tree size and species.

In addition, there are a number of pre-preparation alternatives to help minimise the moisture content  of the wood.  These include:

  • Ring barking
  • Chemical thinning
  • Sour felling
  • Use of dead and dying trees.

Once the produce has been felled and extracted from the forest there is the option of whether to process on site to the desired type of woodfuel, or to transport it in its harvested form.

Straight small roundwood

Wood chips can be conveniently transported, but the bulk density is lower than for SRW, and because airflow through a heap of chips is not good drying can be less efficient.  In addition there are potential problems with composting in heaps of chips that can lead to loss of biomass and raised temperatures that can cause spontaneous ignition.

It is therefore advised that heaps of chips not be constructed greater than 10 m high to prevent excessive heat build up.  If managed carefully, however, the heat from composting can help to drive moisture out from a pile of chips. 

Branches and brash

Branches and brash are the lowest density form, however they will give good drying rates.

Once dried to a suitable level they can be chipped. However there will be a high proportion of bark and leaves which are high in mineral (ash) content.

Equipment to bundle up brash into 'residue logs' has been developed in Sweden to assist the harvesting and handling of this low density material. However it is specialized equipment and is not widely available in the UK.

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