Note: Descriptions are shown in the official language in which they were submitted.
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Afterburner for gas from gasification plant
The present invention relates to an afterburner for gas from a gasification
plant. The
afterburner provides an optimal mixture of combustible gas and air, permitting
an
optimal reaction between the 02 in the air and the gas and creating a mixture
ratio
that enables the load on a burner to be regulated without altering the mixture
ratio
between air and gas. This offers the possibility of optimal combustion even
during
regulation of the burner, over the whole of the relevant regulating range. The
result
is extremely good combustion and reduced emissions of 02, CO and NOx.
During combustion of gas it is important that air and gas should be mixed to
form a
homogeneous mass and that the combustible gas is permitted to react fully with
the
02 content in the air. This creates an optimal mixture of combustible gas and
02
which is crucial for achieving clean, good and efficient combustion of the
gas. This
in turn provides a high level of utilisation of the combustible gas and a low
level of
emission of noxious gases and soot.
Furthermore, for a plant for combustion of gas, whether it stems from wood
fuel or
oil (atomized oil), it is important to be able to vary the load on the burner
unit over
an appropriate load range in order to obtain a flexible plant. Relevant
examples are
combustion of gas from wood for production of steam which in turn is employed
in
a steam turbine for production of electricity or combustion for heating oil
which is
circulated in a plant for heating and/or drying of, for example, wood. When
the load
on, for example, the electric generator increases, the energy supply to the
steam
turbine has to be increased and consequently the heating of the steam has to
increase. This is accomplished through the supply of air and thereby also
fuel. The
air volume is traditionally regulated by regulating the air flow to the burner
in step
with the load. This causes the air velocity and turbulence in the mixing zone
to be
reduced correspondingly which in turn leads to a less efficient mixture of air
and
gas.
Amongst the known solutions are several for mixing combustible gas and air and
a
common solution is the supply of air in connection with a constriction or
venturi
where gas from smouldering wood pulp is mixed with air and combusted.
The fact which is particularly important and which forms the basis for the
present
invention is that the air is brought together with the combustible gas at high
velocity
and thereby with high turbulence. It is also important for the velocity of the
air to be
maintained. This is particularly important when the burner installation has to
be
regulated as indicated above and it is important for the velocity to be
maintained
over the entire regulating range. It will therefore be possible to regulate
the volume
of air in the same way as in the combustion process, but in the mixing phase
the air
velocity and turbulence are constant over the entire regulating range. In this
way the
good mixture of air and combustible gas is maintained and the reaction between
the
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combustible gas and the air's 02 remains optimal at all regulating stages
within the
regulating range. This in turn leads to good, clean combustion and good
utilisation
of the calorific value in the combustible gas. This is also crucial for
keeping down
the costs per produced power unit.
On this basis, according to the present invention an afterburner is provided
for
mixing combustible gas and air, which afterburner comprises a substantially
circular
mixing chamber with open ends where the combustible gas is introduced into the
mixing chamber at the first end. The afterburner is characterised in that air
is added
to the mixing chamber along the mixing chamber's circumference through one or
more openings in the wall of the mixing chamber from an air supply chamber so
that the combustible gas and the air are mixed in the mixing chamber and where
the
mixture of the combustible gas and the air are discharged from the other
opening in
the mixing chamber and where the air is introduced into the mixing chamber
from
the air supply chamber substantially tangentially to the interior of the
mixing
chamber and has a velocity generated by a fan in connection with the air
supply
chamber. In this way a swirl of air and combustible gas is created where the
air
spins in a rotating motion through the afterburner. According to the so-called
spin
rate, the angular velocity w multiplied by the radius of the mixing chamber
will be
constant. In a preferred embodiment of the invention the diameter D1 of the
mixing
chamber's air inlet and the diameter D2 of the mixing chamber's outlet are
different
and the diameter Dl of the mixing chamber's air inlet is preferably larger
than the
diameter D2 of the mixing chamber's outlet. Since the spin rate is constant,
the
angular velocity will increase when the diameter is reduced.
Furthermore, the air supply chamber may surround the part of the mixing
chamber
where the air inlet openings in the mixing chamber's walls are provided. This
enables the air to be easily passed from the fan for supplying air to the
afterburner
according to the invention.
In a further embodiment an overflow chamber may be connected to the air supply
chamber, which overflow chamber is provided with an outlet and a damper in
connection with the outlet. By adjusting the damper the volume of air passing
from
the air supply chamber to the overflow chamber will be regulated. The volume
of
air which is not supplied to the mixing chamber will thereby be regulated
since it
bypasses the inlet. In a further embodiment the overflow chamber may surround
the
whole or parts of the mixing chamber and is connected to the air supply
chamber. In
order to regulate the air to the mixing chamber, the position of the damper
can
therefore be varied with the result that the damper varies the air flow out of
the
overflow chamber.
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In an embodiment the connection between the air supply chamber and the
overflow
chamber is substantially at the openings between the air supply chamber and
the
mixing chamber.
In different embodiments a cone may be provided at the inlet to the mixing
chamber, forcing the combustible gas out towards the air inlet openings in the
mixing chamber and assisting in creating swirling in the mixing chamber. In a
further embodiment thereof, in the mixing chamber at the inlet to the mixing
chamber, a cone may be provided whose pointed end points towards the inlet to
the
chamber. At this cone, moreover, oil may be supplied which is atomized or is
already atomized.
The ingoing air to the mixing chamber has a velocity direction which is
substantially tangential and the combustible gas has a velocity vector which
is
substantially axial. The tangential velocity vector is determined by the air
supply
fan's capacity and pressure (combustion air). The axial velocity vector is
determined by the area in the burner where the air flows, which in turn is
determined by the ratio between Dl and D2 as indicated above. The resulting
velocity vector has a direction with an axial and a tangential component.
Through
regulation of supplied air the resulting velocity vector will be altered by
reducing
the axial velocity while the tangential velocity is increased. The air
velocity will
therefore be varying with little variation and will be approximately constant
over
the regulating range, giving a higher velocity where the velocity would
normally be
reduced with known solutions, thereby producing the highly favourable
combustion
possibilities provided by the invention.
The invention is further explained with reference to the attached figures, in
which:
Figure 1 is a cross sectional view from the side of an embodiment of the
invention
with mixing chamber together with chambers for air inlet and overflow.
Figure 2 illustrates the air inlet chamber in section A-A from figure 1.
Figure 3 illustrates the overflow chamber in section B-B from figure 1.
Figure 4 is a graphic presentation of air and gas velocity together with
resulting
velocity with full load on the burner.
Figure 5 is a graphic presentation of air and gas velocity together with
resulting
velocity with regulated load on the burner.
Figures 6 and 7 illustrate alternative embodiments of the inlet to the
afterburner
according to the present invention.
Figure 1 is a cross sectional view from the side of an afterburner according
to the
present invention with a mixing chamber 1, an inlet 2 for combustible gas and
an
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outlet 3 for combustible gas mixed with air. Also illustrated is an air supply
chamber 6 surrounding the mixing chamber with connections 4 to the mixing
chamber 1. The air supplied to the air supply chamber 6 comes from a fan which
gives the air a velocity and a pressure. An overflow chamber 7 is further
illustrated
where excess air can be discharged and this is controlled by a damper 9 (fig.
3) at
the outlet of the overflow chamber 7.
The combustible gas enters the mixing chamber through the opening 2 and air is
supplied through the openings 4 from the air supply chamber 6. If the load on
the
burner is reduced, the supply of air is regulated by letting some air pass
through the
openings 5 and on out into the overflow chamber. This is regulated by the
damper 9
(fig. 3) in the overflow chamber.
It is further illustrated in figure 1 that the inlet for combustible gas has a
diameter
D 1 while the outlet of mixed gas and air has a diameter D2. D2 is smaller
than D I
and this difference gives increased velocity to the air axially through the
mixing
chamber.
Furthermore, in cross section A-A from figure 1, figure 2 is a view from below
(from the inlet side) of an air supply chamber 6. It shows that this has an
inlet where
the air is supplied by a fan. Moreover it is apparent that the air rotates in
the
chamber 6 and is admitted to the mixing chamber through the openings 8 with
the
result that the air's direction is substantially tangential in the chamber 1
where the
air meets the combustible gas and is mixed. The air here has a high velocity
and
high turbulence and the mixture with the combustible gas is highly effective
and the
gas essentially reacts fully with the air. This has been proved by means of
experiments and the following measurements of 02, CO and NOx have been made
showing that the combustion gives values that are bordering on theoretical
without
smoke being observed from the chimney. This applied, furthermore, over the
whole
load range.
Furthermore, in figure 3 the overflow chamber 7 is illustrated where excess
air can
escape instead of being mixed into the mixing chamber 1. It also shows that
the
chamber 7 has an outlet with a damper 9 which is adjusted in order to remove
air
from the mixing chamber 1. If the damper 9 is completely closed, all the air
goes to
the mixing chamber while if the damper 9 is fully open, a substantial part of
the air
goes outside the mixing chamber 1.
Figure 4 further illustrates in a diagram the ratio between axial and
tangential air
velocity and the resulting air velocity and direction through the mixing
chamber.
Figure 5 further illustrates a corresponding diagram where the axial velocity
is
reduced as a result of less air supply (a greater proportion to the overflow
chamber
7). Since the velocity ratio is constant, the angle of the resultant flow (the
vector)
will be constant and the velocity will also be constant.
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Furthermore, in figures 6 and 7 alternative embodiments are illustrated where
a
cone 10 is mounted at the inlet 2 for combustible gas, forcing the gas out
towards
the peripheral edge of the chamber 1 where the gas meets the air (at high
velocity)
and is mixed. Figure 7 further illustrates that the cone 11 may contain an
outlet 12
5 for supplying oil which has been or is being atomized.
