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| The wood chips |
Bad quality wood chip with too long
pieces and fibre |
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Good quality wood chips
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Woodchip can be sourced from a
range of forest sources, ranging from round wood to forest
thinnings. Wood greater than 2.5cms in diameter will generally
be acceptable as chipping material. The residues from thinning,
and the removal of side branches (snedding) would generally
be left in situ for preliminary drying before chipping.
The main quality criteria for woodchips are:
-- chip size: only the "fine" (smaller than 30
mm) and "medium" grades (below 50 mm) are suitable
for small-scale installations;
-- water content: this determines the energy content of
the fuel on the one hand and its storability on the other;
-- bulk density: this indicates the weight per cubic metre
(bulk volume) and depends on wood type, particle shape,
degree of compaction and water content.
The CARBOROBOT boilers use the G30 , W20-W30 type
woodchips by the Austrian
and DIN standard.
The upcoming standards for solid
biofuels in Europe are still under consideration by the
Technical Committee CEN/TC
335, which include specifications for wood chips, including:
At some stage of the processing the wood chip
need to be dried. Different technologies require different
moisture contents and these can range between 15%
wb and 45% wb. Freshly harvested green wood can, depending
on species, have a moisture content of up to 59%. As a guide,
for efficient combustion a woodchip should generally have
a moisture content of 25% wet basis. This produces a fuel
which can be burnt efficiently, is not too dusty and stores
without excessive self-heating or loss of dry matter.
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Disk Chippers.
A newer chipper design employs a steel disk with knives
mounted upon it as the chipping mechanism. In this design,
(usually) reversible hydraulically powered wheels draw the
material from the hopper towards the disk, which is mounted
on a perpendicular angle to the incoming material. As the
disk spins, the knives cut the material into chips. They
are thrown out the chute by flanges on the drum. This design
is not as energy-efficient as the drum-style design, but
produces chips of more uniform shape and size. Most chippers
currently used by commercial tree care companies are of
this type. Disk-style chippers usually have a material diameter
capacity of 6 to 24 inches.
Drum Chippers
The chipping mechanism in a drum style chipper is large
steel drum powered by the motor, usually by a belt. It is
mounted parallel to the hopper and spins towards the output
chute. The drum also serves as the feed mechanism, drawing
the material through as it chips it. This caused it to be
colloquially known as a "chuck-and-duck" chipper,
because material would start moving through the chipper
very quickly as soon as it made contact with the drum.
Newer models have reversible hydraulic feed wheels and muffling
systems. The reversible feed system allows the newer style
drum chippers to handle larger diameter materials. Modern
Drum-style chippers usually have a material capacity of
6 to 19 inches. 
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Drum Chipper
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| Quality of wood chips
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This is the Austrian and DIN standard
CEN/TC 335 biomass
standards
CEN/TC 335 is the technical committee developing
the draft standard to describe all forms of solid
biofuels within Europe, including wood chips, wood
pellets and briquettes, logs, sawdust and straw
bales.
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| Fuel storage |
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Stacking wood chips
Stacked wood chips do not allow air to flow easily
through the stack. Unless the biomass is very dry
this tends to lead to composting and the growth of
moulds. Composting leads to loss of biomass and can
present a fire hazard owing to the high temperatures
created in the core of a large stack. Advice varies
between different sources as to the safe maximum size
for stacks of wood chips to minimize the risk of fire,
with figures typically around 8-10 m high. The high
core temperature in a stack of wood chips can, however,
be used to assist drying as moisture is driven out
from the core to the periphery. Spores from moulds
growing on piles of stored wood chips have been shown
to cause health problems from particle irritation,
type I and type II allergic responses, potentially
toxic volatile organic compounds (VOCs) and mycotoxins,
and inflammatory reactions to cell wall components
when inhaled. A P3 filter dust mask should be worn
when working in close proximity with stores or deliveries
of stored chips. For a primary central heating system
for a domestic house a fuel store of at least 5-6
m3 is recommended, preferably larger for wood chips.
This may require filling once or twice a year for
pellets, or four to six times for wood chips. Wood
chip is widely use in other European countries as
a fuel for heating and in combined heat and power
systems, particularly in countries with a strong forestry
tradition such as Scandinavia and Austria.
The advantages of wood chip are:
• it is cheap, often as cheap as logs and now (June
2006) cheaper than mains gas;
• it is a standardised product with Europe wide quality
standards in place;
• fully automated, highly efficient boiler systems
are available;
• it can be made from a wide range of raw materials,
often sourced locally.
Its drawbacks are:
• fuel quality is often variable, since suppliers
are still inexperienced;
• it can be difficult to find supplies without assistance;
• it is a bulky fuel, leading to storage problems
in some sites, particularly at the domestic scale.
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