Note: Descriptions are shown in the official language in which they were submitted.
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A method for reducing nitrogen oxide emissions of a bubbling
fluidized bed boiler and an air distribution system of a bubbling
fluidized bed boiler
Field of the invention
The invention relates to a method for reducing nitrogen oxide
emissions of a bubbling fluidized bed boiler burning biofuel. The
invention also relates to an air distribution system of a biofuel-burning
bubbling fluidized bed boiler.
In this description biofuel refers to solid fuels, wherein the portion of
volatile matter in ash-free dry solids is over 60%. This type of fuels are,
for example, peat, bark, wood chips, sawdust, waste construction
timber, sludge created in process industry, and municipal solid waste.
Background of the invention
Bubbling fluidized bed boilers are generally used in energy production,
wherein the fuels include biofuels, such as, for example, peat and
wood chips. In the lower part of the furnace of a bubbling fluidized bed
boiler there is a fluidized bed, which is composed of a fine,
incombustible material, typically sand, which fluidizes over a grate
forming the bottom of the boiler. The fluidizing of the material is created
by feeding fluidizing gas through the grate to the fluidized bed. The
fluidizing gas can be composed solely of air, so-called primary air, or it
may be a gas mixture formed by primary air and inert gas, for example,
flue gas. In a bubbling fluidized bed boiler the flow rate of the fluidizing
gas supplied through the grate is set into such that the particles
forming the fluidized bed do not escape with air to the upper part of the
boiler, but they remain in the lower part of the furnace forming a
fluidized bed that is continuously moving and efficiently mixes the
fluidized bed material and the fuel supplied to it. The combustion air
needed for burning fuel is generally supplied stagewise and in several
portions to the furnace of the boiler in such a manner that a part of
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combustion air, i.e. fluidizing air, is formed by the primary air blown
through the grate with the fluidizing gas a part is formed by secondary
air supplied above the fluidized bed, and the rest of the combustion air
is supplied to the upper part of the furnace of the boiler as so-called
tertiary air. The boiler can also be divided into different air zones
according to the air supplied to the boiler: the area between the
fluidized bed and the secondary air supply is called the primary air
zone, and the area between the secondary air supply and the tertiary
air supply is called the secondary air zone.
Fuel is fed by means of so-called carrier air to the bubbling fluidized
bed boiler on the fluidized bed. In and on the fluidized bed takes place
the drying of fuel particles, the releasing of the volatile matter contained
in them (i.e. pyrolysis), and finally the combustion of the remaining
carbon residue. Drying and pyrolysis are very fast compared to the
total combustion time of fuel particles. The volatile matter released in
pyrolysis are mainly methane, CH4 and carbon monoxide CO, as well
as nitrogen oxide emissions causing ammonia NH3 and hydrogen
cyanide HCN. The volatile matter rises upwards and burns when
reaching an oxygenous area. In a boiler equipped with staged air
supply the combustion of volatile matter primarily is done by secondary
air and partly tertiary air, and the combustion of the carbon residue of
fuel particle is done by fluidizing, secondary and tertiary air.
By means of the staged air supply it is possible to reduce the formation
of nitrogen oxides. That is, when there is oxygen, NH3 and HCN react
into nitrogen monoxide NO. By staging the air supply, reducing,
substoichiometric areas are formed in the furnace of the bubbling
fluidized bed boiler. In these areas the NH3 and HCN formed of fuel are
reduced to molecular nitrogen in accordance with the following reaction
equations 1 and 2:
NH3 +OH ),NH2 +H ) NH +H >NI +No >N2 (1)
and
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HCN + >NCO +H >NH +" >N +NO >N2 (2)
In addition, nitrogen oxides are reduced by means of an internal
reburning reaction, wherein the hydrocarbon radicals formed in
pyrolysis take part in reducing nitrogen oxides. An example of this kind
of reaction is shown in reaction equation 3, wherein the hydrocarbon
radical is ¨CHi.
NO +CHi >HCN +0,+OH >HiNCO +H >NHi +NO >N2 (3)
Generally, the reducing areas are formed by adjusting the amount of
fluidizing and secondary air. The furnace is maintained
substoichiometric in relation to oxygen until the tertiary air supply, in
which case the delay time needed for reactions (1) and (2) is
maximized and the amount of NH3 and HCN is minimized before the
tertiary air level. The optimum total air coefficient before the tertiary air
supply in relation to NO emissions is slightly below 1 depending on the
combustion temperature. The air required for the burning out of volatile
matter and carbon residue is supplied to the furnace as tertiary air. The
residue NH3 and HCN in the flue gases are oxidized after the tertiary
air zone into nitrogen oxides.
With these procedures according to a conventional, staged air supply,
the nitrogen oxide emissions can be reduced approximately 30% in
comparison to a non-staged air supply. Still problematic are the fine
and light fuels, because most of the fuel particles do not end up in the
fluidized bed, but they are pulled with the fluidizing gas and secondary
air to the upper parts of the furnace. Thus, it is almost impossible for
the fuels to create fuel combustion conditions in the furnace that are
controlled and favourable to reducing nitrogen oxides.
It is known from patent Fl 108809 (corresponds to WO 01/96783) to
reduce the nitrogen oxide emissions formed in the fluidized bed
combustion by staging and directing the combustion air supply. The
secondary air supplied above the fluidized bed is supplied in a directed
manner to the furnace in such a manner that a cyclone is formed, in the
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middle of which there is a substoichiometric, hydrocarbon-radicals-containing
zone and an oxygen-rich zone on the edges. A reburning reaction takes place on
the interface of the zones. A purpose of the method is to slow down the
combustion of volatile matter on the secondary air level and thus limit the
rising of
the combustion temperature. However, this method is not efficient enough for
getting the nitrogen oxide emissions of the boiler in accordance with the
strict
emission standards.
Publication WO 02/090829 also discloses a nitrogen oxide reduction method
based on the staging of combustion air. In the method, recirculation gas
composed of flue gases is supplied between the supply points of secondary and
tertiary airs, in the elevation of the bubbling fluidized bed boiler. Thus,
the nitrogen
oxides contained by the recirculation gas take part in the final stage of the
above-
presented reduction reactions (1) and (2) and intensify the reaction of
nitrogen
compounds formed of the fuel into molecular nitrogen. The problem with this
solution is that it causes the amount of flue gases in the boiler to increase,
in
which case the size of the furnace must be increased, which in turn raises the
price of the boiler. In addition, the method is suitable mainly for dry fuels.
With wet
fuels the amount of heat needed for drying reduces the combustion temperature
in
the secondary stage too much, thus preventing the creation of conditions
favourable for reducing the nitrogen oxides.
Brief description of the invention
The purpose of the present invention is to provide a method for reducing
nitrogen
oxide emissions of a bubbling fluidized bed boiler burning biofuel, by means
of
which the above-mentioned drawbacks can be avoided. In addition, the purpose
of the invention is to create an air distribution system of a bubbling
fluidized bed
boiler.
Disclosed herein is a method for reducing nitrogen oxide emissions of a
bubbling
fluidized bed boiler burning biofuel. The method comprises supplying at least
primary air to a fluidized bed arranged in a lower part of a furnace of the
fluidized
bed boiler for fluidizing bed material forming the fluidized bed in the
furnace,
feeding fuel comprising volatile matter to the fluidized bed, which fuel dries
when
coming into contact with hot bed material and pyrolizes into pyrolysis gas
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comprising the volatile matter of the fuel, which pyrolysis gas rises upwards
in the
furnace and burns there, burning at least a part of a carbon residue from the
pyrolysis in the fluidized bed with the primary air, and supplying combustion
air of
volatile matter to the furnace in connection with the fuel feeding such that
the fuel
5 comprising volatile matter is forced substantially on the surface of the
fluidized
bed, and the fuel is pyrolyzed substantially entirely into the pyrolysis gas,
and
burning at least a part of the pyrolysis gas in a primary air zone, located
between
an upper part of the fluidized bed and a plurality of secondary air nozzles,
where
the air coefficient in relation to the volatile matter in the pyrolysis gas is
in the
substoichiometric area, wherein the air coefficient in relation to the
volatile matter
is 0.75 to 0.97, and wherein the total air coefficient in the primary air zone
is 0.5 to
0.8, supplying secondary air above the fluidized bed from the secondary air
nozzles, burning the pyrolysis gas in a substoichiometric secondary air zone,
located between the secondary air nozzles and a plurality of tertiary air
nozzles,
wherein the air coefficient in relation to the volatile matter is more than 1,
and
wherein the total air coefficient is 0.7 to 0.95, and supplying tertiary air
above the
secondary air nozzles from the tertiary air nozzles.
Also disclosed is an air distribution system of a biofuel-burning bubbling
fluidized
bed boiler. The system includes a plurality of nozzles arranged on a bottom of
a
furnace of the bubbling fluidized bed boiler for supplying at least primary
air for
fluidizing bed material forming the fluidized bed arranged in a lower part of
the
furnace, fuel feeding means arranged on walls of the furnace for feeding fuel
to
the fluidized bed, which fuel dries when coming into contact with hot bed
material
and pyrolizes into pyrolysis gas comprising volatile matter of the fuel, which
pyrolysis gas rises upwards in the furnace and burns there, wherein at least a
part
of a carbon residue from the pyrolysis is arranged to be burnt in the
fluidized bed
by means of the primary air, a plurality of secondary air nozzles arranged on
walls
of the furnace above the fluidized bed in order to supply secondary air into
the
furnace, and a plurality of tertiary air nozzles are arranged on the walls of
the
furnace above the secondary air nozzles in order to supply tertiary air into
the
furnace, wherein combustion air of volatile matter is arranged to be
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supplied to the furnace in connection with the fuel feeding such that the fuel
comprising volatile matter is forced substantially on the surface of the
fluidized
bed, and the fuel is pyrolyzed substantially entirely into the pyrolysis gas,
and
wherein at least a part of the pyrolysis gas is arranged to be burnt in a
primary air
zone, located between an upper part of the fluidized bed and the secondary air
nozzles, where the air coefficient in relation to the volatile matter in the
pyrolysis
gas is in the substoichiometric area, wherein the air coefficient in relation
to the
volatile matter is 0.75 to 0.97, and wherein the total air coefficient in the
primary
air zone is 0.5 to 0.8, and wherein the pyrolysis gas is arranged to be burnt
in a
substoichiometric secondary air zone, located between the secondary air
nozzles
and the tertiary air nozzles, wherein the air coefficient in relation to the
volatile
matter is more than 1, and wherein the total air coefficient is 0.7 to 0.95.
The invention is based on the idea that the nitrogen oxide emissions of a
bubbling
fluidized bed boiler are reduced by using a staged air supply in such a manner
that a part of primary air is supplied in connection with fuel supply, i.e.
with the fuel
or within the immediate vicinity of the fuel supply point in the same
direction as the
fuel itself. This part of primary air is in this application referred to as
the
combustion air of volatile matter. Thus, substantially all the fuel fed to the
furnace
is forced onto the surface of the fluidized bed for mixing it to the fluidized
bed and
for drying it quickly due to the effect of the hot fluidized bed material. The
pyrolysis
following the drying and the combustion of the volatile matter released from
the
fuel in the pyrolysis also takes place almost immediately after the fuel has
mixed
with the fluidized bed, because the fuel and the oxygen in the air supplied in
connection with it are mixed quickly. Due to the quick mixing of the fuel and
the
oxygen, most of the volatile matter released from the fuel can be burnt in the
upper part of the fluidized bed and on the fluidized bed, before the supply of
secondary air. The combustion of volatile matter creates a high temperature,
which maximizes the creation of hydrocarbon radicals formed of the fuel and
promotes the reduction of the released nitrogen oxides.
The amount of combustion air of volatile matter supplied in connection with
the
fuel supply is adjusted into such that the combustion of volatile matter
released in
pyrolysis from the fuel being burnt takes place in substoichiometric
conditions in
relation to the volatile matter. The air coefficient SR v in relation to the
volatile
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matter is thus as high as possible, however, below 1, advantageously between
0.75 to 0.97 and preferably between 0.90 to 0.95. The total air coefficient
SRtot on
the
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same level of the furnace varies between 0.50 to 0.80, advantageously
being 0.65.
Secondary air is supplied form the secondary air nozzles and tertiary
air is supplied from the tertiary air nozzles placed above the secondary
air nozzles. The task of the fluidizing gas supplied to the furnace
through its bottom is to maintain the fluidized bed bubbling and its
temperature suitable.
Thus, in the method according to the invention the combustion air
amounts supplied in different stages of the staged combustion are thus
adjusted, i.e. the total air coefficient SRtot needed for combustion and
further the air coefficient in relation to the volatile matter SR. This is
illustrated later in tables 1 and 2 of this description. Feeding the fuel
with air to the fluidized bed in such a manner that substantially all the
fuel particles are forced there enables controlling the combustion
substantially better than at present. Further, the amount of unburnt fuel
can be minimized, because the delay time of fuel in the furnace is
longer than in the solutions according to prior art.
Thus, in the method according to the invention, it is not the amount of
air supplied to the furnace nor the total air coefficient that is affected,
but how the air distribution is performed in order to have the air
coefficient in relation to the volatile matter of the fuel as high as
possible as low in the furnace as possible and yet as long as possible
before the secondary air nozzles.
Several advantages are reached by means of the invention. Because
combustion air is supplied together with the fuel or in the immediate
vicinity of the fuel feeding point in the same direction as the fuel itself,
the finely divided fuels, such as, for example peat, are forced on the
surface of the fluidized bed, in which case they come into immediate
contact with the hot fluidized bed material and do not escape to the
upper parts of the furnace. Thus, the fuel is made to dry and inflame
quickly in its entirety and the volatile matter released from it are burnt
as low in the furnace as possible. Most of the volatile matter can be
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burnt before the actual secondary air level. The high temperature
created in the combustion of volatile matter creates hydrocarbon
radicals, which breaks down the formed nitrogen monoxide into
molecular nitrogen before the secondary air zone.
Because the fine particles are forced by means of air on the surface of
the fluidized bed, their delay time in the furnace before the secondary
air supply is longer than in the solutions of prior art. Thus, the volatile
matter in the fuel have time to burn almost completely before they
travel upwards in the furnace.
The fact that the fuel particles are forced by means of air to the
fluidized bed also reduces the fouling of heat exchange surfaces of the
furnace. That is, the fuel particles escaping from the fuel supply and
burning in the upper parts of the furnace raise the temperature of the
fuel gases before they reach the heat surfaces and increase the fouling
caused by ash melting. Fouling of the heat exchange surfaces
diminishes the efficiency of the boiler and causes economical losses
through boiler shutdowns required by the possible cleaning. By means
of the invention fouling is minimized and through that, economic
advantage is gained.
By means of the invention it is also possible to build the furnace of the
boiler smaller in size and thus decrease the investment expenses of
the boiler, because the combustion reactions take place lower in the
boiler, in which case the heat transfer in the lower part of the boiler is
more efficient.
Brief description of the drawings
In the following, the invention will be described in more detail with
reference to the appended drawings, in which
Fig. 1 shows schematically a furnace of a bubbling fluidized bed
boiler in a side view,
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Fig. 2 shows schematically a furnace of a bubbling fluidized bed
boiler in a front view,
Fig. 3a shows fuel supply means seen from the inside of the
furnace,
Fig. 3b shows fuel supply means according to a second
embodiment seen from the inside of the furnace,
Fig. 3c shows fuel supply means according to a third embodiment
seen from the inside of the furnace,
Fig. 4 shows the amount of pyrolysis gas in a furnace, and
Fig. 5 shows temperature distribution in a furnace above the
surface of the bed.
Detailed description of the invention
Fig. 1 shows a side view of a furnace of a bubbling fluidized bed boiler
1. There is a fluidized bed 2 composed of bed material on the bottom of
the furnace. Fluidizing gas is supplied to the furnace 1 through nozzles
4 arranged on its bottom 3, which gas fluidizes the bed material. The
fluidizing gas can be solely air, or it may be a mixture of air and re-
circulating gas. The fuel is supplied to the fluidized bed 2 from fuel
supply means 5 arranged above the surface of the fluidized bed and
placed substantially on a mutually same level. In the figure there are
three fuel feeding means, but their number may vary depending on the
size of the furnace or other parameters of the boiler. Fuel feeding
means can also be arranged on the opposite side wall (not shown) of
the furnace substantially on the same level with the fuel feeding means
arranged on the side wall, as shown in the figure. They can also be
placed on the front and/or back walls of the furnace. The fuel is fed to
the fluidized bed by means of air. The amount of air used in connection
with fuel feeding is so large, that it prevents fuel particles from
escaping to the upper parts of the furnace by directing the fuel
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substantially on the surface of the fluidized bed. The additional air,
combustion air of the volatile matter supplied in connection with air
supply is a part of primary air. Secondary air is supplied from
secondary air nozzles 6 located above the fuel feeding means to above
the fluidized bed 2. Tertiary air is supplied to the furnace above the
secondary air nozzles 6 via tertiary air nozzles 7 arranged in the upper
part of the furnace.
The amount of air supplied to the furnace does not therefore increase
in comparison to a conventional solution, but it is distributed in a
different manner. The amount of primary air supplied as fluidizing gas
or as a part of it and the amount of tertiary air remain substantially
equal to conventional staged combustion. However, in connection with
fuel feeding, separate combustion air of volatile matter is supplied to
the furnace, which air correspondingly decreases the amount of
secondary air.
Fig. 2 shows a front view of a furnace 1 of a bubbling fluidized bed
boiler. Fuel is fed from a fuel supply means 5 to the fluidized bed 2.
Fuel feeding takes place by means of carrier air and the combustion air
of volatile matter, in which case the fuel-air-mixture 8 is forced all the
way to the surface 2a of the fluidized bed 2. Finely divided fuel supplied
to the fluidized bed dries immediately when coming into contact with
hot bed material and pyrolizes substantially entirely. In pyrolysis the
volatile matter released from the fuel burn on the surface 2a of the
fluidized bed 2 and above the surface 2a of the fluidized bed by means
of primary air forming a first, reducing primary air zone 9, which
extends from the upper part of the fluidized bed to the secondary air
nozzles 6. The combustion of volatile matter takes place in the primary
air zone 9 in substoichiometric conditions in relation to the air
coefficient SR v of volatile matter. Thus, also the total air coefficient
SRtot is naturally below 1. When the air coefficient SR v in relation to
volatile matter is as large as possible, but still a little below 1, the
volatile matter burns quickly and forms a high local temperature, and it
forms a maximum amount of hydrocarbon radicals, which are needed
in order to reduce nitrogen oxides formed from the fuel. The reducing
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secondary air zone 10 extends from the secondary air nozzles all the
way to the tertiary air nozzles arranged over them.
Reducing the nitrogen oxides formed from fuel into molecular nitrogen
5 is thus performed in two stages. In the first reducing stage, i.e. in the
primary air zone 9, most of the volatile matter released from the fuel
and a part of the carbon residue is burnt. This takes place in relation to
both the total air coefficient SRtot and the air coefficient SR v of the
volatile matter of the fuel in substoichiometric conditions, which results
10 in a large amount of hydrocarbon radicals. The primary air required in
this stage is brought to the furnace in connection with fuel supply and
at least as a part of the fluidizing gas. 75 to 95%, preferably 90% of the
air needed for combustion of pyrolysis gases is supplied as primary air.
In the second reducing stage, i.e. the secondary air zone 10 following
the first stage, combustion air is supplied to the furnace from
secondary air nozzles 6 arranged within a distance from the surface of
the fluidized bed in such a manner that the substoichiometric
conditions remain, i.e. the total air coefficient SRtot is still below 1. The
air coefficient SR v of volatile matter of the fuel rises in this zone above
1. Feeding of primary air in the manner described above in two stages,
as fluidizing air and as combustion air of volatile matter has the effect
that the temperatures in the lower parts of the furnace are higher than
in known boilers equipped with staged air distribution. By means of the
invention, the fuel is inflamed quickly and most of the volatile matter
can be burnt before the actual secondary air level.
In figure 2 there are three secondary and tertiary air nozzles 6 and 7
side by side on the front wall of the furnace. Their number and
placement may vary depending on the size of the boiler. The air
nozzles can also be placed on the side walls of the boiler.
The following tables 1 and 2 show two examples of applying the
invention in a bubbling fluidized bed boiler, whose firing rate is 300
MW. The examples show stagewise both the air distribution in a
bubbling fluidized bed boiler according to prior art and the air
distribution in the same boiler in accordance with the invention when
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the fuel and boiler load remain the same. The air amount (kg/s)
supplied to the boiler in each stage, the total air coefficient SRtot of the
stage in question and the air coefficient in relation to the volatile matter
SR v are shown. In the example of table 1 the fuel is peat and in the
example of table 2 the fuel is wood. When comparing the air amounts it
is to be taken into account that in air distribution according to prior art
air is supplied to the primary air zone only as fluidizing air along with
the fluidizing gas and the carrier air amount used in fuel supply is a
small part of the total amount of combustion air. In the method
according to the invention the combustion air of volatile matter supplied
in connection with fuel supply and the fluidizing air supplied together
with the fluidizing gas form the amount of primary air supplied to the
furnace. The small amounts of cooling air of the start-up burners have
not been taken into account in dimensioning the combustion air of
volatile matter, because they do not penetrate to the furnace and there-
fore they do not take part in the combustion in the primary air zone.
Table 1.
Fuel peat, full power of the boiler.
Air distribution according Air
distribution according
to prior art to the invention
Air (kg/s) SRtot SR v Air (kg/s) SRtot SRv
Fluidizing air 41 0,39 0,66 41 0,39 0,66
Carrier air 6 0,44 0,76 6 0,44 0,76
Combustion air
0 0,44 0,76 12 0,56 0,95
of volatile matter
Start-up burner
3,5 0,48 0,81 3,5 0,59 1,00
cooling
Secondary air 44 0,89 1,52 32 0,89 1,52
Load carrying
4 0,93 1,58 4 0,93 1,58
burner cooling
Tertiary air 23 1,14 1,95 23 1,14 1,95
Total 121,5 1,145 1,952 121,5 1,145
1,952
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The percentage of the volatile matter of the dry fuel flow is
approximately 70 mass-%, coke approximately 24.5 mass-% and ash
approximately 5.5 mass-%. The moisture content of the fuel is
46 mass-%.
Table 2.
Fuel wood, full power of the boiler.
Air distribution according Air
distribution according
to prior art to the invention
Air (kg/s) SRtot SR v Air (kg/s) SRtot SRv
Fluidizing air 38 0,36 0,48 38 0,36
0,48
Carrier air 6 0,42 0,55 6 0,42
0,55
Combustion air of
0 0,42 0,55 31,5 0,72
0,95
volatile matter
Start-up burner
3,5 0,45 0,60 3,5 0,75
0,99
cooling
Secondary air 46,5 0,89 1,18 15 0,89
1,18
Load carrying
4 0,93 1,23 4 0,93
1,23
burner cooling
Tertiary air 23 1,15 1,52 23 1,15
1,52
Total 121
1,147 1,520 121 1,147 1,520
The percentage of the volatile matter of the dry fuel flow is
approximately 85 mass-%, coke approximately 13 mass-% and ash
approximately 2 mass-%. The moisture content of the fuel is 46 mass-
%.
As can be seen in the examples, the new air distribution according to
the invention has an effect on the amounts of additional air and
secondary air supplied in connection with fuel supply. The other,
fluidizing air and tertiary air supplied together with the fluidizing gas, as
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well as the small air amounts used in cooling start-up burners and load
carrying burners remain the same as in air distribution according to
prior art. The amount of air supplied together with the fuel is
significantly larger than the amount of carrier air used for fuel supply in
prior art. The amount of air supplied with the fuel varies depending on
the fuel, because different fuels contain different amounts of volatile
matter and the purpose is to burn them in substoichiometric conditions
before the supply of secondary air.
The additional air fed in connection with fuel supply, i.e. the combustion
air of volatile matter, which is a part of primary air, can be supplied
either with the fuel, mixed into the carrier air of the fuel, or parallelly
with the fuel supply taking place by the carrier air. Figure 3a shows a
fuel supply means 5, i.e. a fuel feeding opening 5a seen from the inside
of the furnace, wherein the fuel and the combustion air supplied with it
are mixed right before they are fed to the furnace together. The feeding
opening 5a is rectangular, which is the most advantageous form for a
feeding opening, but otherwise shaped feeding openings may also be
applied. In the fuel feeding means 5 shown in figure 3b the fuel and
combustion air are fed parallel to the furnace. The fuel is fed to the
furnace by means of carrier air from the fuel feeding opening 5a, which
is on three sides surrounded by a uniform air channel 11 for supplying
combustion air of volatile matter. The air channel 11 is a uniform
channel and it surrounds the fuel feeding opening from its three sides
in such a manner that there is no air supply from below the feeding
opening 5a. Thus, when being fed, the fuel is forced in an "air tunnel"
formed by combustion air of volatile matter, which directs the fuel,
forces it onto the surface 2a of the fluidized bed 2 and thus prevents
the fuel particles from escaping to the upper parts of the furnace. Thus,
the momentum of combustion air is so high that the fuel cannot
escape. In the embodiments of figure 3c, the air channels 11a, llb and
11c are separate air channels in relation to the fuel feeding opening 5a,
which channels are placed within a small distance from the fuel feeding
opening 5a, on its three sides. The air channels 11a to 11c are also
placed in such a manner that there is no air channel under the fuel
feeding opening 5a. The design of the air channel and the placement
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of the air channels 11 a to 11c in relation to the fuel feeding opening
help in directing the fuel particles to be directed to the fluidized bed.
The invention is illustrated in figures 4 and 5 by means of graphs,
which are formed by modelling a bubbling fluidized bed boiler with a
fuel power of 300 MW with a full load, wherein the fuel has been a
mixture of peat (70%) and wood (30%). Each graph shows two curves:
curves for both the air distribution according to prior art and the air
distribution according to the invention. These different air distributions
are explained in connection with tables 1 and 2.
Figure 4 shows the portion of pyrolysis gas in a furnace above the
surface of the bed, wherein the portion of pyrolysis gases formed as a
result of air distribution according to prior art is illustrated by dashed
lines and the portion of pyrolysis gases reached by means of the
invention is illustrated with a solid line. The graphs show that by means
of the invention the releasing and combustion of pyrolysis gas mostly
take place before the secondary air supply. In a bubbling fluidized bed
boiler equipped with air distribution according to prior art the releasing
of pyrolysis gas significantly takes place before the secondary air level,
but because of lack of oxygen hardly any combustion takes place.
Because of the escaping fuel particles, the releasing of pyrolysis gases
takes place even on secondary and tertiary air levels.
Figure 5 shows the temperature distribution above the bed of a
furnace. The temperature distribution according to prior art is shown by
dashed lines and the temperature distributions reached by means of
the invention are shown by a solid line. It can be seen from the graphs
that by means of the invention the temperature of the furnace is higher
in the primary air zone than in a bubbling fluidized bed boiler equipped
with air distribution according to prior art. The temperature remains
higher almost to the tertiary level, after which it decreases to lower than
in prior art, because there is no more combustible matter left.
The invention is not intended to be limited to the embodiments
presented as examples above, but the invention is intended to be
CA 02597159 2007-08-02
WO 2006/084954 PCT/F12006/050056
applied widely within the scope of the inventive idea as defined in the
appended claims.
5
