The CARBOROBOT biomass boilers use the standards for solid
biofuels in Europe EN14961, which include specifications for wood
Usable woodchips in CARBOROBOT Biomass boilers are P16A,
P16B and P45A
fuel using possible only with limitations.
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" and "medium" grades
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.
affecting the properties of wood as a fuel are: heating value, chemical
composition, moisture content, density, hardness, the amount of
volatile matters, the amount of solid carbon, ash content and composition,
the melting behaviour of ash, the slagging behaviour of ash and
the amount of impurities, dust and fungi spores. n all cases chips
should be stored away from work areas, to reduce contact with spores
and the possibility of allergic reactions.
Wood fuel chips, for instance, are often
made of various tree species with various proportions of wood, bark,
foliage, branches, buds, and even cones. This causes variation in
the fuel properties.
Approximately one half of fresh,
just fallen tree is water. The other half consists of dry matter
of wood, approximately 85% of which consists of volatile matters,
14,5% of solid carbon and 0,5% of ash. When wood is combusted, its
components will change into steam of water (H2O), carbon dioxide
(CO2), nitrogen oxides (NO2), sulphur oxides (SO2) and ash. Wood
has practically no sulphur at all, as its share in wood is 0,05%
at the highest.
The wood chips technology
most important equipment to harvest woody biomass is undoubtedly
the wood chipper, a machine that cuts the wood into small pieces
with sharp tools like knives. The market is dominated by the disc
and drum chippers. There are very few screw chippersbeing produced.
In a disc chipper, the knives are mounted in slots in a disc. The
wood is cut against an anvil and once cut, passes through the disc,
where casting plates will throw the chips out of the chute. The
casting wings also generate a generous air stream, helping in blowing
the chips out of the spout.
The quality of the wood chips from a disc chipper is usually good.
The particles are rather uniform in length and shape because the
angle of attack of the blade towards the wood is more or less the
same all the time. Because of the rotary motion of the disc, the
wood is awlays pushed to the outside of the disc. This concentrates
the wear on the blade to the outside 1/3 of the knife. The angle
of attack is shown in the drawing below. The angle of attack of
a disc chipper is typically around 37 degrees. At that angle experience
has shown that the energy consumption of the chipper is at its lowest.
The more oblique the chipper is cutting towards the fibre direction
of the wood, the higher the energy consumption, see picture in link
In a drum chipper the knives are mounted on theperifery of a large
steel drum. In front of the knife there is typically a pocket, where
the chips that have been cut are stored until they can be released
behind or below the drum.
A drum chipper does not have fan blades on the drum and therefore
an extra fan has to be added to blow the chips out of the spout,
or the chips can be dropped onto a conveyor belt.
A drum chipper produces irregular chips because the angle of attack
of the blades towards the wood varies with the diameter of the wood.
In a thick piece of wood, the angle of attack when the knives first
hit the wood is 34 degrees, while at the bottom it might be 79 degrees.
The top chips are nice and blocky, while the bottom chips are long
and sliver like. This is shown in the next link.
The size of the chips can be adjusted by adjusting the knives and
anvil or by inserting a screen with larger or smaller diameter holes
in the exhaust opening of the chipper. Particles which are too large
to pass through the screen aretaken around once more and cut against
the anvil. The internal working of the drum chipper can be seen
on the picture in the link below.
Overview of chippers
Woodchip - chipped woody biomass in the
form of pieces with a defined particle size, produced by chopping
with sharp mechanical cutters. (EN14961_4:2011).