U.S. patent application number 11/883887 was filed with the patent office on 2008-06-26 for method for reducing nitrogen oxide emissions of a bubbling fluidized bed boiler and an air distribution system of a bubbling fluidized bed boiler.
This patent application is currently assigned to Metso Power OY. Invention is credited to Pauli Dernjatin, Marko Fabritius, Jukka Roppanen.
Application Number | 20080149012 11/883887 |
Document ID | / |
Family ID | 34224273 |
Filed Date | 2008-06-26 |
United States Patent
Application |
20080149012 |
Kind Code |
A1 |
Dernjatin; Pauli ; et
al. |
June 26, 2008 |
Method For Reducing Nitrogen Oxide Emissions of a Bubbling
Fluidized Bed Boiler and an Air Distribution System of a Bubbling
Fluidized Bed Boiler
Abstract
A method for reducing nitrogen oxide emissions of a bubbling
fluidized bed boiler burning biofuel and an air distribution system
for a bubbling fluidized bed boiler biofuel. A fluidized bed is
arranged in a lower part of a furnace of the boiler. The bed is
fluidized by fluidizing gas that includes primary air. Fuel is fed
to the fluidized bed, which dries and pyrolizes into pyrolysis gas
including volatile matter of the fuel. The gas rises upwards in the
furnace and burns. Secondary air is supplied above the fluidized
bed from secondary air nozzles, and tertiary air is supplied above
the secondary air nozzles. A part of primary air is supplied in
connection with fuel feeding such that the fuel is forced
substantially on the surface of the fluidized bed, thus pyrolizing
entirely, and at least a part of the pyrolysis gases formed in the
pyrolysis is burned by primary air such that the air coefficient in
relation to the volatile matter of fuel in the pyrolysis gases is
in the substoichiometric area.
Inventors: |
Dernjatin; Pauli; (Helsinki,
FI) ; Fabritius; Marko; (Vantaa, FI) ;
Roppanen; Jukka; (Vantaa, FI) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20043-9998
US
|
Assignee: |
Metso Power OY
Tampere
FI
|
Family ID: |
34224273 |
Appl. No.: |
11/883887 |
Filed: |
February 9, 2006 |
PCT Filed: |
February 9, 2006 |
PCT NO: |
PCT/FI2006/050056 |
371 Date: |
August 8, 2007 |
Current U.S.
Class: |
110/245 ;
110/251 |
Current CPC
Class: |
F23G 5/30 20130101; F23C
2201/101 20130101; F23G 7/10 20130101; F23C 10/00 20130101 |
Class at
Publication: |
110/245 ;
110/251 |
International
Class: |
F23G 7/00 20060101
F23G007/00; F23C 10/22 20060101 F23C010/22 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 11, 2005 |
FI |
20055063 |
Claims
1-25. (canceled)
26. A method for reducing nitrogen oxide emissions of a bubbling
fluidized bed boiler burning biofuel, the method comprising:
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
to the fluidized bed, which dries when coming into contact with hot
bed material and pyrolizes into pyrolysis gas comprising volatile
matter of fuel, which gas rises upwards in the furnace and burns
there; burning at least a part of the carbon residue from the
pyrolysis in the fluidized bed with primary air; supplying
secondary air above the fluidized bed from secondary air nozzles;
supplying tertiary air above the secondary air nozzles; and
supplying a part of primary air to the furnace in connection with
fuel feeding such that the fuel is forced substantially on the
surface of the fluidized bed, and the fuel is pyrolyzed
substantially entirely, and at least a part of the pyrolysis gases
formed in the pyrolysis is burned in an atmosphere where the air
coefficient in relation to the volatile matter of fuel in the
pyrolysis gas is in the substoichiometric area.
27. The method according to claim 26, wherein combustion air of
volatile matter is supplied to the furnace in connection with fuel
feeding in such a manner that at least a part of the pyrolysis
gases are burnt before secondary air is supplied to the
furnace.
28. The method according to claim 26, wherein a part of the
pyrolysis gases formed in the pyrolysis is burned by primary air
supplied to the furnace.
29. The method according to claim 26, wherein fuel is forced onto
the surface of the fluidized bed by the momentum of the primary air
supplied in connection with fuel feeding.
30. The method according to claim 26, wherein a primary air zone,
comprising a great deal of hydrocarbon radicals which reduce
nitrogen oxides, is formed between the upper part of the fluidized
bed and the secondary air nozzles.
31. The method according to claim 26, wherein primary air is
supplied to the furnace in connection with fuel feeding such that
at least a part of the pyrolysis gases formed in the pyrolysis are
burned in a manner such that an air coefficient in relation to
volatile matter of fuel is 0.75 to -0.97.
32. The method according to claim 31, wherein the air coefficient
in relation to volatile matter of fuel is 0.90 to 0.95.
33. The method according to claim 31, wherein primary air is
supplied to the furnace in connection with fuel feeding such that
at least a part of the pyrolysis gases formed in the pyrolysis are
burned in a manner such that the total air coefficient is 0.5 to
0.8.
34. The method according to claim 33, wherein the total air
coefficient is 0.65.
35. The method according to claim 26, wherein secondary air is
supplied to the furnace from the secondary air nozzles such that a
substoichiometric, nitrogen-oxides-reducing secondary air zone is
formed between the secondary air nozzles and the tertiary air
nozzles, in which zone the total air coefficient is 0.7 to
0.95.
36. The method according to claim 35, wherein the total air
coefficient is 0.85 to 0.9.
37. The method according to claim 26, wherein the part of the
primary air is supplied to the furnace together with the fuel or
parallel with the fuel feeding.
38. The method according to claim 37, wherein a part of primary air
is supplied to the furnace around the fuel feeding opening.
39. The method according to claim 26, wherein recirculating gas is
supplied to the fluidized bed in order to fluidize the bed material
forming the fluidized bed in the furnace.
40. An air distribution system of a biofuel-burning bubbling
fluidized bed boiler, comprising: 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 gas rises upwards in the furnace and bums
there, wherein at least a part of the carbon residue from the
pyrolysis is arranged to be burnt in the fluidized bed by means of
primary air; secondary air nozzles arranged on the walls of the
furnace above the fluidized bed in order to supply secondary air
into the furnace; and tertiary air nozzles arranged on the walls of
the furnace above the secondary air nozzles in order to supply
tertiary air into the furnace, wherein a part of the primary air is
arranged to be supplied to the furnace in connection with fuel
feeding such that the fuel is forced substantially on the surface
of the fluidized bed, and the fuel is pyrolyzed substantially
entirely, and at least a part of the pyrolysis gases formed in the
pyrolysis is arranged to be burnt in an atmosphere where the air
coefficient in relation to the volatile matter of fuel in the
pyrolysis gas is in the substoichiometric area.
41. The air distribution system according to claim 40, wherein
combustion air of volatile matter is arranged to be supplied to the
furnace in connection with fuel feeding in such a manner that at
least a part of the pyrolysis gases are burnt before secondary air
is supplied to the furnace.
42. The air distribution system according to claim 40, wherein a
part of the pyrolysis gases formed in the pyrolysis is arranged to
be burnt by the primary air supplied to the furnace.
43. The air distribution system according to claim 40, wherein the
momentum of the primary air supplied in connection with fuel
feeding forces the fuel onto the surface of the fluidized bed.
44. The air distribution system according to claim 40, wherein a
primary air zone, comprising a great deal of hydrocarbon radicals
which reduce nitrogen oxides, is arranged to be formed between the
upper part of the fluidized bed and the secondary air nozzles.
45. The air distribution system according to claim 40, wherein at
least a part of the pyrolysis gases formed in the pyrolysis is
arranged to be burnt by means of the primary air supplied to the
furnace in connection with fuel feeding in such a manner that the
air coefficient in relation to volatile matter of fuel is 0.75 to
0.97.
46. The air distribution system according to claim 45, wherein the
air coefficient in relation to volatile matter of fuel is 0.90 to
0.95.
47. The air distribution system according to claim 45, wherein at
least a part of the pyrolysis gases formed in the pyrolysis is
arranged to be burnt by means of the primary air supplied to the
furnace in connection with fuel feeding in such a manner that the
total air coefficient is 0.5 o 0.8.
48. The air distribution system according to claim 47, wherein the
total air coefficient is 0.65.
49. The air distribution system according to claim 40, wherein
secondary air is arranged to be supplied to the furnace from the
secondary air nozzles in such a manner that a substoichiometric,
nitrogen-oxides-reducing secondary air zone is formed between the
secondary air nozzles and the tertiary air nozzles, in which zone
the total air coefficient is 0.7 to 0.95.
50. The air distribution system according to claim 49, wherein the
total air coefficient in the zone is 0.85 to 0.9.
51. The air distribution system according to claim 40, wherein the
primary air supplied in connection with fuel feeding is arranged to
be supplied to the furnace together with the fuel or parallel with
the fuel feeding.
52. The air distribution system according to claim 51, wherein the
fuel feeding means comprises a fuel feeding opening and in its
immediate vicinity at least one air supply opening, and wherein the
primary air supplied in connection with fuel feeding is arranged to
be supplied to the furnace around the fuel feeding opening parallel
with the fuel fed from the fuel feeding opening.
53. The air distribution system according to claim 52, wherein an
air supply opening is arranged around the fuel feeding opening in
order to supply the primary air supplied in connection with fuel
feeding.
54. The air distribution system according to claim 40, wherein the
nozzles are arranged to supply recirculation gas to the fluidized
bed in order to fluidize the bed material forming the fluidized bed
in the furnace.
Description
FIELD OF THE INVENTION
[0001] The invention relates to a method for reducing nitrogen
oxide emissions of a bubbling fluidized bed boiler burning biofuel
according to the preamble of the appended claim 1. The invention
also relates to an air distribution system of a biofuel-burning
bubbling fluidized bed boiler in accordance with the preamble of
the appended claim 13.
[0002] 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
[0003] 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 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.
[0004] 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, CH.sub.4 and carbon monoxide CO, as
well as nitrogen oxide emissions causing ammonia NH.sub.3 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.
[0005] By means of the staged air supply it is possible to reduce
the formation of nitrogen oxides. That is, when there is oxygen,
NH.sub.3 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
NH.sub.3 and HCN formed of fuel are reduced to molecular nitrogen
in accordance with the following reaction equations 1 and 2:
##STR00001##
[0006] 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.
##STR00002##
[0007] 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 NH.sub.3 and HCN is minimized
before the tertiary air level. The optimum total air coefficient
before the tertiary air supply in relation to NO.sub.x 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 NH.sub.3
and HCN in the flue gases are oxidized after the tertiary air zone
into nitrogen oxides.
[0008] 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.
[0009] It is known from patent FI 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 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.
[0010] 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
[0011] 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.
[0012] To attain this purpose, the method according to the
invention is primarily characterized in what will be presented in
the characterizing part of the independent claim 1.
[0013] The air distribution system of a bubbling fluidized bed
boiler according to the invention, in turn, is primarily
characterized in what will be presented in the characterizing part
of the independent claim 13.
[0014] The other, dependent claims will present some preferred
embodiments of the invention.
[0015] 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.
[0016] 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.sub.v in relation to
the volatile 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 SR.sub.tot on the same level of the
furnace varies between 0.50 to 0.80, advantageously being 0.65.
[0017] 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.
[0018] 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
SR.sub.tot needed for combustion and further the air coefficient in
relation to the volatile matter SR.sub.v. 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.
[0019] 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.
[0020] 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 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.
[0021] 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.
[0022] 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.
[0023] 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
[0024] In the following, the invention will be described in more
detail with reference to the appended drawings, in which
[0025] FIG. 1 shows schematically a furnace of a bubbling fluidized
bed boiler in a side view,
[0026] FIG. 2 shows schematically a furnace of a bubbling fluidized
bed boiler in a front view,
[0027] FIG. 3a shows fuel supply means seen from the inside of the
furnace,
[0028] FIG. 3b shows fuel supply means according to a second
embodiment seen from the inside of the furnace,
[0029] FIG. 3c shows fuel supply means according to a third
embodiment seen from the inside of the furnace,
[0030] FIG. 4 shows the amount of pyrolysis gas in a furnace,
and
[0031] FIG. 5 shows temperature distribution in a furnace above the
surface of the bed.
DETAILED DESCRIPTION OF THE INVENTION
[0032] 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 recirculating 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 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.
[0033] 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.
[0034] 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.sub.v of volatile matter. Thus,
also the total air coefficient SR.sub.tot is naturally below 1.
When the air coefficient SR.sub.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
secondary air zone 10 extends from the secondary air nozzles all
the way to the tertiary air nozzles arranged over them.
[0035] Reducing the nitrogen oxides formed from fuel into molecular
nitrogen 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
SR.sub.tot and the air coefficient SR.sub.v of the volatile matter
of the fuel in substoichiometric conditions, which results 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 SR.sub.tot is still below 1. The air coefficient
SR.sub.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.
[0036] In FIG. 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.
[0037] 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 the fuel and boiler load remain the same. The air amount
(kg/s) supplied to the boiler in each stage, the total air
coefficient SR.sub.tot of the stage in question and the air
coefficient in relation to the volatile matter SR.sub.v are shown.
In the example of table 1 the fuel is peat and in the example of
table 2 he 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 therefore they do
not take part in the combustion in the primary air zone.
TABLE-US-00001 TABLE 1 Fuel peat, full power of the boiler. Air
distribution Air distribution according to according to prior art
the invention Air (kg/s) SR.sub.tot SR.sub.V Air (kg/s) SR.sub.tot
SR.sub.V 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
[0038] 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-US-00002 TABLE 2 Fuel wood, full power of the boiler. Air
distribution Air distribution according to according to prior art
the invention Air (kg/s) SR.sub.tot SR.sub.V Air (kg/s) SR.sub.tot
SR.sub.V 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
[0039] 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-%.
[0040] 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 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.
[0041] 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. FIG. 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 FIG. 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 FIG. 3c, the air channels 11a, 11b 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 of the air channels 11a to 11c in relation to the fuel
feeding opening help in directing the fuel particles to be directed
to the fluidized bed.
[0042] The invention is illustrated in FIGS. 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.
[0043] FIG. 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.
[0044] FIG. 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.
[0045] The invention is not intended to be limited to the
embodiments presented as examples above, but the invention is
intended to be applied widely within the scope of the inventive
idea as defined in the appended claims.
* * * * *