Sourcing fuel
Wood waste fuel for sawmills comes from on-site production waste and external sources, including other wood processing plants and forest residue recovery. There is also the option of utilising wood-waste in combination with other fuels (link to duel fuels below).
A typical breakdown of sawmill wood waste is: 5% bark, 10% green
sawdust, 5% dry shavings and 2% off-cuts. For a quick assessment of the
energy content of your on-site waste try our On-site resource assessment tool.
The
main issues with on-site production waste are the different combustion
characteristics of different wood waste streams, and fuel contamination.
Combustion characteristics
Different
wood waste streams have different combustion characteristics. If
handled correctly, sawdust is a good clean fuel with very low ash
content (about 1%). Wet sawdust has a very high moisture content which
reduces its net calorific value and will require different combustion
techniques.
Bark has a lower moisture content, which increases its net calorific
value. However, bark tends to be highly contaminated (ie. dirty) and
this produces high ash levels that can cause problems in the combustion
stage.
Dry wood shavings have the highest fuel value at around 20 GJ/tonne
(net value) and are usually a very clean low-ash fuel. Their very low
bulk density requires conveying systems, and large storage facilities
to be designed specifically for this attribute which can be costly.
Note: Emission standards do not permit the combustion of some treated timbers.
For more information about fuel value or calorific value try our Biomass calorific value calculator.
For more information about moisture content see our Wet/dry basis converter.
Fuel contamination
An important issue in using
wood waste is fuel contamination, mainly soil. This contamination often
occurs when fuel is stored and handled on unsealed surfaces.
Contamination not only reduces the calorific value of the fuel, it
increases wear and tear on equipment.
Radiata pine has 0.5% non-combustible material that ends up as ash.
Excessive ash is the most tell-tail sign of fuel contamination.
When on-site production waste is not sufficient for heat demand,
some sawmills source wood waste streams from other wood processing
plants. However, there is often a lack of quality control on the
sourced fuel, sometimes due to the increased handling involved.
Often the cost of this fuel is determined purely by transport distance.
A commercial entity focused on the distribution of wood waste for fuel is Living Energy.
For
larger energy plants there is the option of sourcing wood waste from
forestry landings or skid sites. In general, the quality of the fuel
from landing sites is significantly lower than on-site fuel. The wood
waste is exposed to the elements and therefore has a high moisture
content. It is often sitting on unsealed bare ground and is handled
without concern for fuel quality and is therefore often contaminated
with soil and other foreign bodies.
A modern forest residue recovery operation involves a loader, mobile
hogger, excavator and fuel transport trucks. There are two New Zealand
based hogger manufacturers: Screening & Crushing Systems Ltd (SCS)
Christchurch, manufacture the Ripper and Woodweta Manufacture the Woodweta.
Modern biomass processing equipment sometimes incorporate fuel
screening components that enable the separation of soil form fuel, see Woodweta.
Read
an article in Forestry News about a number of forest residue recovery
operations carried out by Central Wood Recyclers, based in Tokoroa. Forestry News is a publication of the Forestry Association of New Zealand.
Read about a possible scenario for the potential supply and cost of forest residues to a bioenergy plant [PDF 112 ].
A commercial entity focused on the distribution of wood waste for fuel is Living Energy.
The Bioenergy Association of New Zealand has more information about recovering forest residues for fuel.
Several operations have been made to successfully run on a
combination of biomass and other fuels, such as coal. The possibility
of this type of conversion needs to be assessed on a case-by-case basis
and depends critically on the fuel ratios.
For more information on co-firing including a international database of co-firing installations see the IEA Bioenergy Co-combustion site.