Chipping equipment
Chippers vary in size from the small hand-fed chipper for
dealing with arboricultural waste to machines which can chip large
diameter roundwood with outputs of over 100 tonnes an hour. Buying
a chipper is a considerable investment and the characteristics,
outputs and costs need to be considered before purchase.
Should it not be economically feasible for suppliers to purchase
individual chippers themselves, chippers capable of processing up
to 200 tonnes of woodfuel per hour can be hired from various
sources across Scotland. For further information upon this
matter please see your local Usewoodfuel Officer.
Chipping can be carried out at any stage from terrain-chipping
at stump to a centralised chipping facility at a fixed location, so
it will be important to consider what type of machine will suit
your supply chain. In terms of transport costs, due to the
favourable bulk density of wood in the round compared to woodchip
it is more economical to transport whole wood compared to
woodchip.
Each type of chipper will have its own strengths and
weaknesses. The quality of chips varies between different
makes of chippers and it is important to select a machine that will
make good chips from the material at hand. Principally, chippers
are used to reduce the size of wood residues to fit boiler feed
systems. There are 3 main types of chippers, categorised by the
device used to produce the chips:
- Disc chipper, consisting of a heavy rotating
disc with usually 2, 3 or 4 blades mounted on the face of the disc.
Material to be chipped is fed in, towards the blades. The rotating
knives cut woody material into chips as they pass an anvil or fixed
knife. Blower paddles on the back of the disc accelerate the chips
up a spout where they are discharged.
- Drum chipper: A rotating drum with 2-4 blades
inserted on its circumference. Wood to be chipped is fed in and cut
against an anvil. Chip size can be adjusted. Drum chippers
require a separate blowing provision, produced either by the
airflow of the blades themselves or a separate fan and can be fed
either via gravity or a conveyor belt.
- Screw cone chipper: The screw cone chipper
consists of a conical screw with a sharp edge that cuts against the
inside of the housing. When rotated, the screw pulls the tree into
the chipper and cuts it into large chips or chunks, there is,
therefore, no separate mechanical feed mechanism required. The base
of the blade generally includes a disk to generate the airflow to
propel the chips outwards and may also incorporate a sliver
breaker. Chip size is determined by the pitch of the screw and can
be varied by replacing the blade with one of a different
pitch. This type of chipper is unsuitable for twiggy
material.
To chip for fuel use it is critical to have consistency in
particle size. Chippers are available with a special grading device
to restrict chips to a suitable size at a small extra
cost.
Chippers are very sensitive to metal and stones. Blades
will have a relatively fast wear and will require changing at
regular intervals to avoid the slowing of production and increased
use of fuel for operation.
For many chippers, the principle aim is volume reduction and to
assist handling and transport, and the quality of the chip produced
is unimportant. For many wood fuel applications, however,
components such as the chip transport/feed mechanism (especially
smaller scale auger feeds and underfed stoker grates), there are
certain physical requirements of the chips if smooth operation is
to be ensured and blockages avoided.
As part of the CEN 335 European standards
and HETAS soild biomass assurance scheme for
solid biofuels, there are standards for wood
chips, which include the distribution of chip dimensions for a
given chip size. If a chipper is to be used for wood fuel
systems, it should be able to produce chips within this
distribution.
Of possibly greatest importance for many wood fuel systems is
the absence of slivers within the wood fuel, these long pieces can
cause bridging within fuel feed systems, potentially causing a
blockage.
Anyone using a wood chipper is required to undergo appropriate
training. Free leaflets on using chippers and on training and
certification are published by the Agricultural and Forestry
Advisory Group and are available from the Health & Safety
Executive.
LOG SPLITTERS
The majority of log splitters are simple hydraulic devices used
for splitting firewood from softwood or hardwood logs that have
been pre-cut into sections (rounds). Powered either through the
electric mains or a small petrol engine the hydraulic device pushes
the log splitter against a sharp axe like blade which cuts the log
in two. They are often rated by the tons of pressure they can
generate, the higher the pressure rating, the greater the thickness
or length of the rounds that can be split. Most log splitter models
for home use have a rating around 10 tons, but professional
hydraulic models may exert 25 tons of pressure or more.
There are also manual log splitters, which use mechanical
leverage to force logs through a sharpened blade assembly and screw
or 'corkscrew' types that are driven directly from an agricultural
tractor's power take-off shaft where the splitter is mounted on the
three point linkage.
PELLETISING PLANTS
Most pellet production is by large-scale industrial pellet
manufacture, integrated with a large sawmill and/ or heat user.
This is to make the project economics viable. For commercial
ventures to make pelletisation plants economically feasible the
cost involved will likely exceed £300,000 upwards.
If you have a supply of sawdust that is already dry, small-scale
pellet machines are available with an output of around 200-300kg
per hour. These pellet production systems are available at a cost
of anywhere from £50,000 to £150,000, depending on level of quality
required and the level of automation needed for operation.
These smaller scale pellet plants are unlikely to be economically
feasible unless you have to pay to get rid of waste wood
anyway.
Smaller systems are also available and it is also sometimes
possible to convert an existing animal grass-feed mill to make wood
pellets, as the equipment needed is similar and many feed factories
have facilities for drying the feedstock as well. Such an
arrangement could bring economic benefits to the feed mill,
enabling production to continue throughout the year especially over
the winter when grass is not growing. The production would be
considerably less than for a dedicated plant, but the capital costs
would be much lower.
Shredders and grinders
A range of grinders and shredders are available to reduce
different types of woodfuel material to small pieces for
convenience of handling, volume reduction or as a suitable feed for
conversion processes. These are used specifically for the
production of hogfuel, which is of lower quality
than standard wood chip and is unlikely to be suitable for many
small and medium scale combustion systems that require an even
sized, good quality wood chip. However use of a screen can help to
remove oversize pieces.
Shredders, grinders and mills can be high power, high throughput
units, suitable for handling low quality, mixed material,
particularly if it might be contaminated with stones or soil.
Types of Shredders and grinders include:
- Shredders- Have rotating axle(s) which have
teeth that tear apart wood material through a screen.
- Stump grinders- Consist of a fast rotating
tools which can pivot around an axle when too much resistance is
met. Grinders are used to grind down large tree stumps such as
during land clearance operations.
- Hammermill- material is forced through holes
in a screen using hammers. Can be used to reduce woody
biomass to small pieces prior to forming pellets
- Tub grinder - general purpose grinder for use
on a wide range of waste, including construction and demolition
waste