Brash and arboricultural arisings

This is the form of a large quantity of biomass in the UK that is of low value and potentially destined for, at best, composting, or being left in the wood (where it returns nutrients and organic matter to the soil and CO2 to the atmosphere).

As harvested, brash and arboricultural arisings (that material that is removed as part of tree surgery, management of municipal parks and verges of roads and railways) are typically very low density, including many small diameter branches, twigs, leaves etc.

Environmental issue with burning or landfill

Alternatively it is likely to be consigned to a bonfire (where all the carbon is converted to CO2) or to landfill, where the carbon is converted to methane (CH4). Although methane might be collected as landfill gas, it is a far (21 times) more potent greenhouse gas than CO2, and much will inevitably escape to the atmosphere.

Uses for brash and arboricultural arisings

During conventional forestry harvesting brash may be laid on the ground to provide protection from harvesting and extraction machinery.  If used in this way it is likely that the brash will be significantly contaminated with soil and therefore unlikely to be suitable for fuel.  On certain sites it is also preferable to leave some or all of the brash on site to return nutrients and organic matter to the soil.

The Forestry Commission has issued Guidance on site selection for brash removal to provide advice on under what circumstances it is appropriate to remove brash from a harvesting site.

If they are not to be left at the harvest site, brash and arboricultural arisings are typically chipped using a general purpose chipper to reduce bulk and assist handling.  The chips produced will not generally meet the specifications for most domestic and small to medium scale heating and boiler systems and may contain a large proportion of slivers.

Chipping is also likely to be performed with the material green and so will have a high moisture content.  These properties make it will suited for composting, and there is a growing number of municipal composting sites to keep such material from landfill, however is not well suited for most energy applications, although there are boilers that are designed to burn green material.

Harvesting for energy applications

Such equipment is not, however, widely available in the UK.

If brash is to be harvested in quantity for energy applications, equipment has been developed (in Sweden) to gather up large quantities of brash and compact it into bales for convenience of handling and transport.  These are known as 'residue logs'.