CALU press release – 12/06
Slash-and-burn forestry welsh-style: forest residue
harvesting for energy production
There is a
rapid increase in the production of energy from woodfuel
biomass in the UK, due to recent dramatic rises in energy
costs and concern about the climate change impacts of traditional non-renewable
energy sources. Recently, a
purpose-built Forest Residue Bundler has begun operation in north Wales,
capable of removing in excess of 50% of the residues from clearfell
sites, and producing neatly packaged ‘bundles’ weighing around 500kg and
representing about 1MWh of energy. These are left to
dry out and for the needles to drop, before collection and transport for
combustion in a biomass boiler. One
machine can produce 20-30 bales per hour, equivalent to 30,000 tons of bundles
per year. Sales of these residues present
an interesting additional source of revenue for forest owners.
However,
without proper management these forest resources may not be sustainable. For example, more intensive harvesting may lead to declines
in nutrient reserves and acidification within forest ecosystems, such as the
acidic, nutrient poor upland forests found across much of Wales. Residue harvesting typically removes between
1.5 and 5 times the amount of nutrients removed through traditional forest
harvesting.
A
by-product of biomass energy generation is the production of nutrient rich alkaline
ash. Typically, this represents
approximately 1% by weight of fuel input, this
percentage increases when bark, twigs and branches are included in the fuel. Recycling this wood ash to forests has been
proposed as a means of replacing nutrients removed during forest residue
harvesting, as a means of combating acidification and as a way of reducing
landfill burden.
Current
work at CALU partner, the University of Wales Bangor, is focussing on the effects on soil
chemistry and nutrient cycling resulting from the recycling of wood ash in
loose, pellet and co-composted form to two 2nd rotation Sitka Spruce
sites near Bala.
Initial findings include:
- Loose wood ash is difficult to handle – it is
very dusty, a large proportion of it is typically very fine material. It is also corrosive and abrasive and
highly soluble.
- Application of wood ash to forest soils may
raise the pH of the organic upper soil horizon, reducing acidity, but at
the same time actually reduce the pH of the lower mineral horizon. This is thought to occur as a result of
the displacement of aluminium ions from upper soil horizons by base cations which are present in large quantities in wood
ash.
- After 6 months, the effects of wood ash on soil
pH, electrical conductivity, calcium, potassium and sodium are much more
noticeable with treatments of loose wood ash and co-composted wood ash compared
to treatments of hardened wood ash.
This suggests that processing wood ash into pellet form might be an
effective method of producing a slow-release nitrogen-free forest
fertiliser.
James Walmsley
School of the
Environment and Natural Resources,
University of Wales, Bangor
Bangor
Gwynedd
LL57 2UW
Wales
afp050@bangor.ac.uk
+44 7828 743724