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What´s ash?
Waste or utility?
Ash is a solid, particulate, inorganic combustion residue.
The ash content varies greatly between different fuels. Of
forest fuels, ash content varies between different
components, stemwood 0,4-0,6%, stem bark 2-5% and 1-2%
branches. The ash content is highest in those parts of the
tree where growth occurs.
In the leaves, needles and
branches and leading shoot varied between 2 and 6%. As a
mean value of forest fuels can be expected 1-2% ash content.
All values are calculated on the fuel solids. Ash from the
wood fuel contains nutrients which the tree raised,
including important trace elements. Nitrogen (N) is missing
because it largely leaves in gaseous combustion.
Since trees take up heavy
metals and radioactive substances from soil and air, are
also those substances in the ash. Generally, the ash is
between 10% and 30% of calcium (Ca). The content of
potassium (K) and magnesium (Mg) is usually a few percent,
while the phosphorus (P) represents approximately one
percent of the total content.
Most of these
substances have been lime-acting nature and occurs both as
soluble salts, as well as more insoluble compounds in the
form of oxides, hydroxides, carbonates and silicates. Ashes
from the incineration of wood generally have a pH between 9
and 13.
The lime effect
and nutrients in the ash makes it suitable to use to
compensate for losses of such substances in the woodland.
The law consider ash as a waste. That is why we must have a
permit to store it.
Combustion and
gasification ash
Boilers in the heating plants are designed based on
different technologies. They are either incineration or
gasification plants. When combustion air supply is so
great that complete combustion takes place directly in
the forehead.
The simplest
combustion boiler which most resembles a wood burning
boiler in a typical house, is
grate boiler.
A grate boiler
is a grid where the combustion of fuel takes place. The
simplest form of the roster is flat and solid and are
primarily in smaller boiler centers. Sloping grate boiler or
multistage grate boiler
is an inclined or step format grids where the fuel
burned while sliding down along the grate.
source:
bioenergiportalen.se
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A variant, Wander grate, consists of a carpet of links that enter the fuel in the
boiler. In
grate boilder
combustion temperature is 1000-1150 ºC.
Some plants use powder fired boilers. A powder fired
boiler is like a oil burner. Instead of oil, a
fine wood, peat or in carbon dust sprays into the boiler.
The combustion temperature c:a 1200 ºC.
source:
energiochmiljo.se
Another common boiler
at large facilities are circulating fluidized bed (CFB),
where fuel and bed material swirls around in the
combustion. Inpatient and fuel mixture behaves then as a
liquid, a "fluid". Bedding material consists mostly of
sand. A similar design is the bubbling fluidized bed (BFB),
where the bed material does not circulate but 'hovers'
over the boiler bottom.
source:
bioenergiportalen.se
Fluid bed boilers are
especially suitable for combustion of moist fuels such
as forest fuels. The combustion temperature of 850-900 ºC.
In a gasification plant heated the fuel in oxygen-free
conditions so that a combustible gas formed burned in a
later step.
Dry and wet output
ash
How to remove the ash from the boiler is of
significance to the further use. The ash can either be
charged as it is, dry output, or in
connection with the output, wetted with water, wet
output .
Dry output
ash is difficult to control, because of the dust, and
that may cause environmental problems and it can damage skin and eyes.
Dry output ash with high carbon content
may also easily catch
fire at the wrong output or storage .
It can only be brought back to the woods after it has
become wet,
then it is in contact with water the ash hardens.
Wet output
ash is easier to handle, but if the curing starts
spontaneously, it is difficult to produce a
uniform product. Therefore, it is best that the ashes
are not stored in the container, but preferably falls
down in a concrete. |
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