Forest residues for bioenergy
New Zealand Tree
Grower November 2007
Peter Hall
 After
forest harvesting there is inevitably material left behind that is not
suitable for selling as logs. Rather than leave this to rot, there are
a number of ways in which these residues can be used, depending on the
volumes available.
The amount of material that is left as residue varies with a number of
factors such as crop quality, terrain, harvesting system, logmaking
system and log markets. The material includes branches and sometimes
deformed sections of stem wood, as well as the off-cuts created during
the process of cutting stem lengths into logs.
Collecting the residues
It is usually left in two main locations − at the landing where the
stems are cut into logs, and on the cutover where the trees are felled
and often de-limbed. Some of this material is more accessible than
others and therefore more useable. The most likely material to be easy
accessible is what is left on or around the logging landings.
Landing residues can lead to problems with the stability of the landing
edge if they, and the water run-off from the landing, are not managed
well. The results can be significant earth movement, usually towards
waterways at the bottom of the slope. Therefore one option to consider
is to collect the material and use it for something worthwhile, instead
of pushing the waste wood material over the edge of the landing.
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| Material left at the landing where the
stems are cut into logs |
Amount of residues available
It is possible to predict to some extent the amount of residue that can
be expected from a harvesting operation, and once the amount is known,
it is possible to start planning what might be viable in terms of
managing or using this material.
The amounts of residue that are likely to occur have been studied in a
number of production forest situations. The volumes from woodlot
operations are likely to be similar, and possibly higher, depending on
the nature of the block. For example, if there are a lot of boundaries
and edge trees the branch residue volumes will increase.
Residues are estimated as a percentage of the extracted volume.
Typically the total extracted log volume is likely to be in the range
of 500 to 700 cubic metres per hectare, but less than 500 for areas
such as Canterbury and Westland. The volume to be extracted should be
known before the operation starts.
The percentages of waste likely to be produced from forest harvesting
are presented in the table at the bottom of the page.
| Residues
available for conversion into biomass fuel |
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Residue uses
There are a number of uses for residues, assuming that there is
sufficient to warrant doing something with them. These can be as simple
as domestic firewood, or as complicated as ending up as fuel for an
electricity generation plant in a rural distributed power generation
system.
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| Hoggers come in a range of sizes
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Grinding or chipping the wood
There are also non fuel uses to consider. For example, if the material
is chipped or ground up using a hogger it can be used as animal
bedding. Hoggers come in a range of sizes, and while some can go from
landing to landing quite easily, others are better suited to having the
residue brought to them. The size of the machine required will be
dictated by the composition of the material as well as the size of the
pile. Smaller machines are not suitable for large chunks of wood such
as stumps or large diameter off-cuts.
Larger machines capable of processing all of the residues need a
significant amount of wood before it is economically viable for them to
be brought in. The larger machines can process 25 to 30 tonnes an hour,
and need at least a week’s work at a site. However, this can be made up
of several smaller lots of material from a variety of sites that are
trucked to the hogger at a central point.
Transport
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| Transport in specialist chip trucks
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Once hogged, the material needs to be taken to a point of use or sale.
Transport is best done in specialist chip trucks that have sufficient
volume for the truck to maximise its payload. Transport costs can
contribute a significant part of the delivered cost of a fuel, and
making sure that the site is easily accessible for large heavy vehicles
is essential to avoid expensive delays associated with trucks getting
stuck.
Depending on the site and the residues it is possible to produce a
hogged residue at a cost of $24 to $26 per green tonne with
approximately 55% moisture content. This cost is made up of a variety
of machine costs, typically an excavator to gather and pile the
residues, the cost of the hogger and then the loader which is
required to get the residues on to the truck.
Depending on the nature of the site, there may have to be a two-stage
operation, involving trucking the residues a short distance to the
hogger. Hoggers are often quite large, up to 30 tonnes, and need a
solid surface and room to work in, with space for piles of unprocessed
material at one end and a pile of processed material at the other.
Prices
If the hogged logging residue is clean with a minimum of dirt
contamination it is suitable for use as a bio-energy fuel. Its value to
a user will be dictated by local supply and demand and the transport
distance to the point of use.
Delivered costs assuming a transport distance of 70km could be in the
range of $38 to $42 a tonne. However, if the material is to be used as
a fuel, its value lies its energy not its weight, and the energy
content of the green wood is likely to be in the order of 8.5
gigajoules a tonne at 50% moisture content. This would make the fuel
cost $4.50 to $4.90 per gigajoule, which is competitive with current
coal prices.
One way to improve the energy value of the fuel is to leave it in its
unprocessed form for the summer months, during which time it will dry
out. This leaves less tonnage, but the energy content of the remaining
tonnage is higher as it has lower moisture content. The residues can
dry from 60% moisture to 30% or 35% moisture content in six months. The
energy content of the wood then improves to 11 to 12
gigajoules per tonne. It is important that if this approach is taken
that the material is paid for on its energy content, not its weight.
Likely users of this material are wood processing plants such as pulp
mills, saw mills, and timber drying plants. In some cases large users
of coal such as cement works or power plants are considering the
use of biomass to substitute part of their fuel requirements to offset
carbon emissions.
Volume of residues available
Nationally it is estimated that there are 800,000 to 900,000 tonnes a
year of landing residues produced. Currently about 100,000 tonnes a
year of landing residues are collected, hogged and used as fuel. Most
of this is happening in the Central North Island, but there are also
some landing recovery operations in Hawkes Bay and Nelson.
A further 1.1 million tonnes of residues are on cutover that has been
logged with ground based systems, where the recovery of the residues is
at least technically possible, but more expensive. In some cases,
removing and using the cutover residues could alleviate
re-establishment problems on sites where residue levels are high.
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| Wood chips available now |
For those who live in remote locations, the biomass from forests may be
a potential source of heat and electricity. One of the options for
distributed generation is small scale wood gasifiers coupled to an
engine or generator.
Residues are available now
As energy demand and the price of gas rise there will be interest from
gas users in other fuels, and biomass is attractive as it is a
renewable fuel. Government policy on carbon emissions may also have an
influence in the next two to three years, driving up the interest in
renewable, carbon neutral fuels. One of the advantages that forest
residues have over other potential biomass fuels is that harvest
residues can be accessed now, without having to establish a crop or
compete for land, as the resource is already there.
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For further information contact Peter
Hall, Energy Group, Scion phone 07 343 5899, or visit www.bioenergy-gateway.org.nz
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