Title: Combustion method and apparatus for NOx reduction
Abstract: Combustion method and apparatus for NO.sub.x reduction and CO reduction which are capable of easily achieving super NO.sub.x reduction with the value of exhaust NO.sub.x under 10 ppm. The combustion method for NO.sub.x reduction by controlling the temperature of combustion gas derived from a burner includes in combination the steps of suppressing combustion gas temperature by heat absorbers; suppressing the combustion gas temperature by recirculating burning-completed gas to a combustion-gas burning reaction zone; and suppressing the combustion gas temperature by adding water or steam to combustion-use air of the burner, whereby the temperature of the combustion gas derived from the burner is suppressed.
Patent Number: 6,875,009 Issued on 04/05/2005 to Kayahara, et al.
| Inventors:
|
Kayahara; Toshihiro (Matsuyama, JP);
Takubo; Noboru (Matsuyama, JP)
|
| Assignee:
|
Miura Co., Ltd. (Ehime-ken, JP)
|
| Appl. No.:
|
622489 |
| Filed:
|
July 21, 2003 |
Foreign Application Priority Data
| Jul 29, 2002[JP] | 2002-219397 |
| May 20, 2003[JP] | 2003-141253 |
| Current U.S. Class: |
431/9; 431/12; 431/115 |
| Intern'l Class: |
F23M 003//00 |
| Field of Search: |
431/8,9,10,115,116,350,351,354,181,174,187,12
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Yeung; James C.
Attorney, Agent or Firm: Birch, Stewart, Kolasch & Birch, LLP
Parent Case Text
This nonprovisional application claims priority under 35 U.S.C. .sctn.119
(a) on Patent Application No.(s). 2002-219397 filed in Japan on Jul. 29,
2002 and 2003-141253 filed in Japan on May 20, 2003, which is (are) herein
incorporated by reference.
Claims
What is claimed is:
1. A combustion method for NO.sub.x reduction by controlling temperature of
combustion gas derived from a burner, comprising in combination the steps
of: suppressing combustion gas temperature by heat absorbers; suppressing
combustion gas temperature by recirculating burning-completed gas to a
combustion-gas burning reaction zone; and suppressing combustion gas
temperature by adding water or steam to combustion-use air of the burner,
whereby the combustion gas temperature is suppressed.
2. A combustion method for NO.sub.x reduction as claimed in claim 1,
further comprising in combination the step of suppressing combustion gas
temperature by burning the burner as a fully-premixing type burner at a
high excess air ratio.
3. The method of claim 2 including the additional step of maintaining the
high excess air ratio at a substantially constant level independent of an
outside air temperature.
4. The method of claim 1 comprising the additional step of providing a
blower supplying combustion-use air to the burner and wherein said step of
suppressing combustion gas temperature by adding water or steam to
combustion-use air of the burner comprises the step of adding water or
steam upstream of the blower.
5. The method of claim 1 wherein said step of adding water or steam to
combustion-use air of the burner comprises the step of adding water or
steam to recirculating burning-completed gas.
6. The method of claim 1 including the additional steps of providing a
blower blowing combustion use-air and recirculating burning-completed gas
into a burner wherein said step of adding water or steam to combustion-use
air of the burner comprises the step of adding water or steam to
recirculating burning-completed gas upstream of the blower.
7. The method of claim 1 including in combination the step of suppressing
combustion gas temperature by burning the burner as a fully-premixing type
burner at high excess air ratio whereby NOx emissions are maintained at a
level of 10 ppm or less, at 0% O.sub.2 in an exhaust gas, dry basis.
8. The method of claim 1 whereby NOx emissions are maintained at a level of
10 ppm or less, at 0% O.sub.2 in an exhaust gas, dry basis.
9. A combustion apparatus for NO.sub.x reduction by controlling temperature
of combustion gas derived from a burner, comprising: first suppression
means for suppressing combustion gas temperature by heat absorbers
provided in a burning reaction zone; second suppression means for
suppressing combustion gas temperature by recirculating burning-completed
gas to the combustion-gas burning reaction zone; and third suppression
means for suppressing combustion gas temperature by adding water or steam
to combustion-use air of the burner.
10. A combustion apparatus for NO.sub.x reduction as claimed in claim 9,
further comprising, in combination, fourth suppression means for
suppressing combustion gas temperature by burning the burner as a
fully-premixing type burner at a high excess air ratio.
11. A combustion apparatus for NO.sub.x reduction by controlling
temperature of combustion gas derived from a burner having a burning
reaction zone and an exhaust gas passage, comprising: heat absorbers
provided in the burning reaction zone for suppressing combustion gas
temperature; an exhaust gas recirculation passage connected to the exhaust
gas passage for recirculating burning-completed gas to an air supply
passage, and a line feeding water or steam to the exhaust gas
recirculation passage upstream of the burner.
12. The combustion apparatus of claim 11 including a blower providing
combustion-use air and recirculating burning-completed gas to the burning
reaction zone and wherein said line feeds water or steam into the exhaust
gas recirculation passage upstream of said blower.
13. A combustion method for NO.sub.x reduction by controlling temperature
of combustion gas derived from a burner, comprising the steps of:
suppressing combustion gas temperature by heat absorbers;
recirculating burning-completed gas to a combustion-gas burning reaction
zone;
adding water or steam to combustion-use air of the burner; and
burning the burner as a fully-premixing type burner at high excess air
ratio;
whereby NOx emissions are maintained at a level of 10 ppm or less, at 0%
O.sub.2 in an exhaust gas, dry basis.
14. A combustion method comprising the steps of:
burning fuel to produce gasses and exhaust gasses; and
maintaining a NOx level in the exhaust gasses at no more that 10 ppm or
less, at 0% O.sub.2, dry basis;
wherein said step of maintaining a NOx level in the combustion gasses at no
more that 10 ppm comprises the steps of:
suppressing combustion gas temperature by heat absorbers;
recirculating burning-completed gas to a combustion-gas burning reaction
zone; and
adding water or steam to combustion-use air of the burner.
15. The method of claim 14 wherein said step of maintaining a NOx level in
the combustion gasses at no more that 10 ppm includes the additional step
of burning the burner as a fully-premixing type burner at high excess air
ratio.
16. The method of claim 14 wherein said step of adding water or steam to
combustion-use air of the burner comprises the steps of adding water or
steam to recirculating burning-completed gas and mixing the
burning-completed gas with the combustion-use air.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a combustion method for NO.sub.x
reduction, as well as an apparatus therefor, to be applied to water-tube
boilers, reheaters of absorption refrigerators, or the like.
Generally, as the principle of suppression of NO.sub.x generation, there
have been known (1) suppressing the temperature of flame (combustion gas),
(2) reduction of residence time of high-temperature combustion gas, and
(3) lowering the oxygen partial pressure. Then, various NO.sub.x reduction
techniques to which these principles are applied are available. Examples
that have been proposed and developed into practical use include the
two-stage combustion method, the thick and thin fuel combustion method,
the exhaust gas recirculate combustion method, the water addition
combustion method, the steam jet combustion method, the flame cooling
combustion method with water-tube groups, and the like.
With the progress of times, NO.sub.x generation sources even of relatively
small capacity such as water-tube boilers have been coming under
increasingly stricter regulation of exhaust gas, and so further reduction
of NO.sub.x are demanded therefor. The present applicant proposed a
NO.sub.x reduction technique for these demands by Japanese Patent
Laid-Open Publication HEI 11-132404 (Specification of U.S. Pat. No.
6,029,614).
This prior art technique is intended to achieve NO.sub.x reduction by a
combination of suppression of combustion gas temperature with water tubes
and suppression of combustion gas temperature with exhaust gas
recirculation. However, the technique was capable of NO.sub.x reduction up
to only about 25 ppm, other than one that allows NO.sub.x reduction to
below 10 ppm to be achieved. It is noted that NO.sub.x reduction with the
value of NO.sub.x generation being not more than 10 ppm will hereinafter
be referred to as super NO.sub.x reduction.
In this prior art technique, it is conceivable to enhance the function of
combustion-gas-temperature suppression with water tubes with the aim of
achieving the super NO.sub.x reduction. This functional enhancement is to
provide water tubes in contact with a burner or to increase the heat
transfer surface of water tubes. However, excessive fulfilment of this
functional enhancement would cause an increase in pressure loss or an
unstable combustion such as oscillating combustion.
Further, it is also conceivable to enhance the function of
combustion-gas-temperature suppression with exhaust gas recirculation to
achieve the super NO.sub.x reduction. This functional enhancement is to
increase the exhaust-gas recirculation quantity. However, this functional
enhancement would cause an amplification of unstable characteristics of
exhaust gas recirculation. That is, the exhaust gas recirculation has a
characteristic that exhaust-gas flow rate or temperature changes due to
changes in combustion quantity or changes in load. Increasing the
exhaust-gas recirculation rate would cause these unstable characteristics
to be amplified, so that stable NO.sub.x reduction could not be achieved.
Furthermore, the functional enhancement for exhaust gas recirculation would
cause the combustion reaction to be suppressed, which would lead to an
increase in emission of CO and unburnt components as well as to an
increase in thermal loss. Also, increasing the exhaust gas recirculation
rate would cause the blower load to increase. Excessive suppression of
burning reaction would lead to an increase in emission of CO and unburnt
contents, as well as to an increase in thermal loss.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a combustion method for
NO.sub.x reduction, as well as an apparatus therefor, capable of easily
achieving NO.sub.x reduction with the value of exhaust NO.sub.x under 10
ppm.
The present invention having been accomplished to solve the above object,
in a first aspect of the invention, there is provided a NO.sub.x reduction
combustion method for fulfilling NO.sub.x reduction by controlling
temperature of combustion gas derived from a burner, comprising in
combination the steps of: suppressing combustion gas temperature by heat
absorbers; suppressing combustion gas temperature by recirculating
burning-completed gas to a combustion-gas burning reaction zone; and
suppressing combustion gas temperature by adding water or steam to
combustion-use air of the burner, whereby the combustion gas temperature
is suppressed.
In one embodiment, there is provided a NO.sub.x reduction combustion method
as described in the first aspect, further comprising in combination the
step of suppressing combustion gas temperature by burning the burner as a
fully-premixing type burner at a high excess air ratio.
In a second aspect of the invention, there is provided a combustion
apparatus for NO.sub.x reduction for fulfilling NO.sub.x reduction by
controlling temperature of combustion gas derived from a burner,
comprising: first suppression means for suppressing combustion gas
temperature by heat absorbers provided in a burning reaction zone; second
suppression means for suppressing combustion gas temperature by
recirculating burning-completed gas to the combustion-gas burning reaction
zone; and third suppression means for suppressing combustion gas
temperature by adding water or steam to combustion-use air of the burner.
Further, in one embodiment, there is provided a combustion apparatus for
NO.sub.x reduction as described in the second aspect, further comprising,
in combination, fourth suppression means for suppressing combustion gas
temperature by burning the burner as a fully-premixing type burner at a
high excess air ratio.
Before the description of the embodiments of the present invention, terms
used herein and the drawings are explained. The combustion gas includes
burning-reaction ongoing (under-combustion-process) combustion gas, and
combustion gas that has completed burning reaction. Then, the
burning-reaction ongoing gas refers to combustion gas that is under
burning reaction, and the burning-completed gas refers to combustion gas
that has completely burning-reacted. The burning-reaction ongoing gas is
indeed a concept of substance, but can also be referred to as flame as a
concept of state because it generally includes a visible flame so as to be
in a flame state. Therefore, herein, the burning-reaction ongoing gas is
referred to also as flame or burning flame from time to time. Further, the
exhaust gas (flue gas) refers to burning-completed gas that has decreased
in temperature under an effect of endothermic action by heat transfer
tubes or the like.
Also, the combustion gas temperature, unless otherwise specified, means the
temperature of burning-reaction ongoing gas, equivalent to combustion
temperature or combustion flame temperature. Further, the suppression of
combustion gas temperature refers to suppressing the maximum value of
combustion gas (combustion flame) temperature to a low one. In addition,
normally, burning reaction is continuing although in a trace amount even
in the burning-completed gas, and so the combustion completion does not
mean a 100% completion of burning reaction.
Further, the excess air ratio, which is expressed as (actual amount of
combustion air)/(theoretical amount of combustion air), corresponds in a
specified relationship to exhaust-gas O.sub.2 (%) (oxygen concentration in
exhaust gas), therefore being expressed in exhaust-gas O.sub.2 (%). Also,
the value of NO.sub.x shows a value at 0% O.sub.2 in the exhaust gas, dry
basis, while the value of CO shows not an equivalent value but a reading
value.
Next, as a detailed description of the foregoing characteristics of the
present invention, embodiments of the present invention are described. The
present invention is applied to thermal equipment (or combustion
equipment) such as small-size once-through boilers or other water-tube
boilers, water heaters, reheaters of absorption refrigerators or the like.
The thermal equipment has a burner and a group of heat absorbers to be
heated by combustion gas derived from the burner.
An embodiment of the method according to the present invention is a
NO.sub.x reduction combustion method for fulfilling NO.sub.x reduction by
suppressing temperature of combustion gas derived from a burner by a
NO.sub.x reduction means implemented by a combination of: suppression
means for suppressing combustion gas temperature by heat absorbers
(hereinafter, referred to as "first suppression means"); suppression means
for suppressing combustion gas temperature by recirculating
burning-completed gas to a combustion-gas burning reaction zone
(hereinafter, referred to as "second suppression means"); and suppression
means for suppressing combustion gas temperature by adding water or steam
(hereinafter, referred to as "water/steam addition") to combustion-use air
of the burner (hereinafter, referred to as "third suppression means"). The
NO.sub.x reduction means is so designed as to reduce the generated
NO.sub.x value to not more than 10 ppm, which is a NO.sub.x reduction
target value, at not less than a specified excess air ratio.
The first suppression means forming part of the NO.sub.x reduction means is
based on the following principle. That is, the NO.sub.x value is reduced
by suppressing the combustion gas temperature by a cooling effect of heat
absorbers implemented by arranging a multiplicity of heat absorbers in the
burning-reaction ongoing gas derived from the burner, i.e., in the burning
reaction zone. This first suppression means is implemented by arranging
the heat absorbers to cool the burning-reaction ongoing gas, hence a
nonuniform cooling. There are also sites where the burning is ongoing
actively in the gaps between the heat absorbers of the burning reaction
zone. Particularly in the downstream of the heat absorbers, eddy currents
are formed so that the combustion flame is stabilized by the heat transfer
tubes. The heat absorbers are implemented by heat absorbers such as water
tubes, but this is not limitative.
The arrangement configuration as to how the heat absorbers are arranged
with respect to the flow of the burning-reaction ongoing gas, includes the
following two modes. One of those arrangement configurations is that a
combustion gas passage is formed so as to allow combustion gas to flow
generally linearly therethrough from the burner to the exhaust gas outlet,
and moreover the heat absorbers are arranged so as to cross the
burning-reaction ongoing gas derived from the burner with gaps present
among the heat absorbers to allow the combustion gas to flow therethrough.
The other arrangement configuration is that heat absorbers are arrayed in
an annular state with gaps present thereamong to allow the combustion gas
to flow therethrough, so that the combustion gas derived from the burner
flows radially from the inside of the annular heat absorbers toward the
heat absorbers, where the heat absorbers are arranged in the
burning-reaction ongoing gas derived from the burner. The latter
configuration is described in detail in Japanese Patent Laid-Open
Publication HEI 11-132404 (U.S. Pat. No. 6,029,614), the disclosure of
which is hereby incorporated by reference.
The second suppression means is what is called exhaust-gas recirculation
combustion method. Exhaust gas which has decreased in temperature through
endothermic action by the heat absorbers and is then to be emitted to the
atmosphere is partly mixed with combustion-use air via an exhaust-gas
recirculation passage. The combustion gas temperature is suppressed by a
cooling effect of the mixed exhaust gas, by which NO.sub.x value is
reduced. This second suppression means exerts uniform cooling of
combustion gas.
The third suppression means is water/steam addition to the burning reaction
zone. By this water/steam addition, the burning-reaction ongoing gas is
cooled, so that the combustion gas temperature is suppressed and the
NO.sub.x value is reduced. This third suppression means also exerts
uniform cooling of the combustion gas. The water/steam addition may be
carried out in the exhaust-gas recirculation passage in another
embodiment. Besides, in an embodiment in which the burner is provided as a
fully-premixing type gas burner and mixed gas of combustion-use air and
exhaust gas is fed to the burner by a blower, it is possible to perform
the steam addition between the burner and the blower. For the water
addition, water is added in the form of mist.
Working effects by the combination of the first to third suppression means
are as follows. Enhancing the combustion-gas-temperature suppression
functions of the first suppression means and the second suppression means
would cause drawbacks of the respective suppression means to matter.
However, combining the three suppression means makes it possible to
achieve super NO.sub.x reduction relatively easily without causing the
emergence of those drawbacks. In particular, by combining the third
suppression means, unstable characteristics of the second suppression
means can be alleviated, producing a working effect that stable super
NO.sub.x reduction can be achieved.
In this embodiment, preferably, suppression of combustion gas temperature
by burning the burner as a fully-premixing type burner at a high excess
air ratio (hereinafter, referred to as fourth suppression means) may be
combined. The fourth suppression means is based on the following
principle. That is, when the burner is burned at a high excess air ratio,
the combustion gas temperature is suppressed so that the NO.sub.x value
decreases. The high excess air ratio in this case is 5% O.sub.2 or more
contained in exhaust gas, preferably, not less than 5.5% O.sub.2. This
suppression effect acts generally uniformly on the entire burning reaction
zone formed by the burner.
By combining this fourth suppression means, the problems due to the
functional enhancement of the foregoing individual suppression means can
be further alleviated.
Furthermore, in the foregoing embodiment, preferably, an excess-air-ratio
control means for controlling the excess air ratio to a specified high
excess air ratio is additionally provided. More specifically, an oxygen
concentration detection means for detecting the oxygen concentration in
exhaust gas is provided, and the rotational speed of the blower for
blowing combustion-use air to the burner is controlled so that the oxygen
concentration detected by the oxygen concentration detection means becomes
a set value corresponding to the specified high excess air ratio. The
specified high excess air ratio is determined in the following manner.
Given a NO.sub.x reduction target value of 10 ppm, an excess air ratio
corresponding to the target value is determined under the condition of the
excess air ratio versus NO.sub.x characteristic of the NO.sub.x reduction
means, and then the excess air ratio determined in this way or a value
higher than the excess air ratio is taken as a specified high excess air
ratio. Finally, the specified high excess air ratio corresponds to the
NO.sub.x reduction target value.
Further, the excess-air-ratio control means includes the following
modifications. The foregoing excess-air-ratio control means is designed to
control the rotational speed of the blower. Instead, the excess-air-ratio
control means may be designed to control the opening of a
combustion-use-air flow rate adjusting means such as a damper or a valve
provided downstream or upstream of the blower so that the excess air ratio
is controlled constant. Further, in another embodiment, it is also
possible that an outside-air temperature detection means for detecting
outside-air temperature is provided in place of the oxygen concentration
detection means, where the blower or the flow rate adjusting mechanism is
controlled by this outside-air temperature detection means so that the
excess air ratio is controlled constant.
Next, embodiments of the apparatus of the present invention are described.
The present invention includes the following embodiments (1) to (2) of the
apparatus corresponding to the foregoing embodiments.
Embodiment (1): A combustion apparatus for NO.sub.x reduction by
controlling temperature of combustion gas derived from a burner, wherein
NO.sub.x reduction means is made up of the first suppression means, the
second suppression means and the third suppression means.
Embodiment (2): A combustion apparatus for NO.sub.x reduction, in which the
NO.sub.x reduction means further includes the fourth suppression means.
Furthermore, the embodiments of the apparatus further include the following
embodiments (3) to (7).
Embodiment (3): A combustion apparatus for NO.sub.x reduction as defined in
the first embodiment (1), comprising: NO.sub.x reduction means having an
excess air ratio versus NO.sub.x characteristic that generated NO.sub.x
value decreases with increasing excess air ratio of the burner, as well as
an excess air ratio versus CO characteristic that exhaust CO value
increases with increasing excess air ratio; and excess-air-ratio control
means for controlling the excess air ratio of the burner to a specified
high excess air ratio, wherein the specified excess air ratio is
determined from the excess air ratio versus NO.sub.x characteristic and a
NO.sub.x reduction target value.
Embodiment (4): A combustion apparatus for NO.sub.x reduction as defined in
the embodiment (2), comprising: NO.sub.x reduction means having an excess
air ratio versus NO.sub.x characteristic that generated NO.sub.x value
decreases with increasing excess air ratio of the burner, as well as an
excess air ratio versus CO characteristic that exhaust CO value increases
with increasing excess air ratio; and excess-air-ratio control means for
controlling the excess air ratio of the burner to a specified high excess
air ratio, wherein the specified excess air ratio is determined from the
excess air ratio versus NO.sub.x characteristic and a NO.sub.x reduction
target value.
According to the foregoing embodiments (3) to (4), a stable super NO.sub.x
reduction can be achieved by the control of the excess air ratio even with
the outside-air temperature varied.
Embodiment (5): A combustion apparatus for NO.sub.x reduction and CO
reduction as defined in the foregoing embodiment (1), wherein the burner
is switchable between high combustion and low combustion, and wherein
combustion gas temperature is suppressed by the first suppression means,
the second suppression means and the third suppression means in both high
combustion state and low combustion state, and the exhaust-gas
recirculation quantity by the second suppression means as well as the
water/steam addition quantity by the third suppression means are
controlled between the low combustion state and the high combustion state.
According to this embodiment (5), since the exhaust-gas recirculation
quantity and the water/steam addition quantity are controlled in
accordance with increases or decreases of combustion quantity, there can
be provided a working effect that the problems that would be caused by
enhancing the function of only either one of the second suppression means
or the third suppression means can be solved or alleviated.
Embodiment (6): A combustion apparatus for NO.sub.x reduction and CO
reduction as defined in the foregoing embodiment (1), wherein the burner
is switchable between high combustion and low combustion, and wherein
combustion gas temperature is suppressed by the first suppression means
and the second suppression means in the low combustion state, and
combustion gas temperature is suppressed by the first suppression means,
the second suppression means and the third suppression means in the high
combustion state, and wherein exhaust-gas recirculation quantity by the
second suppression means is kept unchanged between the low combustion
state and the high combustion state.
Embodiment (7): A combustion apparatus for NO.sub.x reduction and CO
reduction as defined in the foregoing embodiment (1), wherein the burner
is switchable between high combustion and low combustion, and wherein
combustion gas temperature is suppressed by the first suppression means
and the second suppression means in both low combustion state and high
combustion state, and wherein exhaust-gas recirculation quantity by the
second suppression means is kept unchanged between the low combustion
state and the high combustion state while water/steam addition quantity by
the third suppression means in the high combustion state is set larger
than that of the low combustion state.
These embodiments (6) and (7) are so constituted that the exhaust-gas
recirculation quantity are kept unchanged between low combustion state and
high combustion state, that is, the exhaust-gas recirculation quantity is
not controlled but kept unchanged therebetween, while the water/steam
addition quantity is controlled between high combustion state and low
combustion state. As a result, there can be provided working effects that
the exhaust-gas recirculation quantity adjusting means that would
otherwise be involved for the switching between high combustion and low
combustion is no longer necessary, and that unstable characteristics of
exhaust gas recirculation upon increasing the exhaust-gas recirculation
quantity can be alleviated.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view of a longitudinal section of a steam boiler
of an embodiment of the present invention;
FIG. 2 is a sectional explanatory view of the same embodiment taken along
the line II--II of FIG. 1;
FIG. 3 is a cross-sectional explanatory view of the same embodiment taken
along the line III--III of FIG. 2;
FIG. 4 is a chart showing excess air ratio versus NO.sub.x characteristic
(NO.sub.x emission characteristic) curves, and excess air ratio versus CO
characteristic (CO emission characteristic) curves in high combustion
state of the same embodiment;
FIG. 5 is a chart showing excess air ratio versus NO.sub.x characteristic
curves, and excess air ratio versus CO characteristic curves in low
combustion state of the same embodiment;
FIG. 6 is a main-part control circuit diagram of the same embodiment;
FIG. 7 is a front view showing a main-part constitution of a CO oxidation
catalyst member in the same embodiment;
FIG. 8 is an explanatory view of a longitudinal section of another
embodiment of the present invention which is equipped with another fourth
suppression means;
FIG. 9 is an explanatory view of a longitudinal section of another
embodiment of the present invention which is equipped with another fourth
suppression means;
FIG. 10 is an explanatory view of a longitudinal section of another
embodiment of the present invention which is equipped with another
excess-air-ratio control means;
FIG. 11 is a main-part control circuit diagram of another excess-air-ratio
control means of another embodiment of the present invention; and
FIG. 12 is a sectional explanatory view of another embodiment of the
present invention, corresponding to FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinbelow, working examples in which the NO.sub.x reduction combustion
method and apparatus of the present invention are applied to a
once-through steam boiler, which is one type of water-tube boilers, are
described in accordance with the accompanying drawings. FIG. 1 is an
explanatory view of a longitudinal section of a steam boiler to which an
embodiment of the present invention is applied, FIG. 2 is a sectional view
taken along the line II--II of FIG. 1, FIG. 3 is a cross-sectional view
taken along the line III--III of FIG. 1, FIGS. 4 and 5 are charts showing
excess air ratio versus NO.sub.x characteristic as well as excess air
ratio versus CO characteristic in high combustion state and low combustion
state, respectively, in the same embodiment, FIG. 6 is a main-part control
circuit diagram of the same embodiment, and FIG. 7 is a view showing a
main-part constitution of a CO oxidation catalyst member in the same
embodiment, as viewed along the direction of the exhaust gas flow.
Now the overall construction of the boiler according to this embodiment is
explained below, and then the construction of its characteristic parts is
explained. The characteristic parts include: NO.sub.x reduction means made
up by a combination of a combustion-gas-temperature suppression for doing
the suppression by a multiplicity of heat transfer tubes (first
suppression means), a combustion-gas-temperature suppression means for
doing the suppression by recirculating burning-completed gas to a burning
reaction zone (second suppression means), a combustion-gas-temperature
suppression means for doing the suppression by addition of steam to the
burning reaction zone (third suppression means), and a
combustion-gas-temperature suppression means for doing the suppression by
burning a fully-premixing type burner at a high excess air ratio (fourth
suppression means); and an excess-air-ratio control means for controlling
the excess air ratio of the burner to maintain it at a specified high
excess air ratio.
First, the overall construction of the steam boiler is explained. This
steam boiler is switchable between operations at high combustion and low
combustion. Then, the steam boiler comprises: a boiler body 3 having a
fully-premixing type burner 1 having a planar burning surface (jet-out
surface for premixed gas and a multiplicity of endothermic-use heat
transfer tubes 2, 2, . . . ; a blower 4 and an air supply passage 5 for
feeding combustion-use air to the burner 1; a gas fuel supply tube 6; an
exhaust gas passage (normally referred to as flue) 7 for discharging
exhaust gas exhausted from the boiler body 3; an exhaust-gas recirculation
passage 8 for mixing, into the combustion-use air, part of the exhaust gas
that is circulating along the exhaust gas passage 7 to feed it to the
burner 1; and a steam addition tube 9 (see FIG. 3) for adding steam to the
combustion-use air. It is noted that the outer diameter of each of the
heat transfer tubes 2 is 60.5 mm.
The boiler body 3 is provided with an upper header 10 and a lower header
11, and has a plurality of the heat transfer tubes 2 arranged between the
two headers 10, 11. Referring to FIG. 2, a pair of water walls 14, 14
formed by coupling outer heat transfer tubes 12, 12, . . . to one another
with coupling members 13, 13, . . . are provided on lengthwise both sides
of the boiler body 3, so that a combustion gas passage 15 that allows
burning-reaction ongoing gas and burning-completed gas derived from the
burner 1 to pass generally linearly therethrough is formed between the two
water walls 14, 14 and the upper header 10 and the lower header 11.
Next, conjunction relationships among the foregoing individual elements are
explained. As shown in FIG. 1, the burner 1 is provided at one end of the
combustion gas passage 15, and the exhaust gas passage 7 is connected to
an exhaust gas outlet 16 located at the other end. The air supply passage
5 is connected to the burner 1, and the gas fuel supply tube 6 is
connected to the air supply passage 5 so that fuel gas is jetted out into
the air supply passage 5. The gas fuel supply tube 6 is provided with a
first valve 17 as a fuel flow adjusting means for adjusting the fuel flow
between high combustion and low combustion. On the air supply passage 5 is
provided a throttle portion (not shown), which is so called venturi, for
enhancing the mixability of the fuel gas and the combustion-use air, but
the throttle portion may be omitted for reduction of pressure loss in
another embodiment.
Further, as shown in FIG. 3, an air inlet passage 19 is connected to an
inlet port 18 of the blower 4, and the exhaust-gas recirculation passage 8
is connected between the air inlet passage 19 and the exhaust gas passage
7. The steam addition tube 9 is inserted in the air inlet passage 19.
Operation of this steam boiler based on the above-described constitution is
outlined below. In the air supply passage 5, combustion-use air (outside
air) fed through the air inlet passage 19 is premixed with fuel gas fed
through the gas fuel supply tube 6, and the resulting premixed gas is
jetted out from the burner 1 into the boiler body 3. The premixed gas is
ignited by an ignition means (not shown), thus burning. Burning-reaction
ongoing gas generated along with this burning crosses with upstream-side
heat transfer tubes 2 so as to be cooled, resulting in burning-completed
gas, which exchanges heat with downstream-side heat transfer tubes 2 so
that its heat is absorbed, thus resulting in exhaust gas. The resultant
exhaust gas is discharged into the atmosphere through the exhaust gas
passage 7. Then, part of the exhaust gas is fed to the burner 1 through
the exhaust-gas recirculation passage 8, and used for suppression of
combustion gas temperature.
Water in the individual heat transfer tubes 2 is heated by the heat
exchange with the combustion gas, thereby changed into steam. This steam
is fed from a steam extraction means (not shown), which is connected to
the upper header 10, to steam-utilizing equipment (not shown), while part
of the steam is fed to the steam addition tube 9 so as to be used for the
cooling of the burning-reaction ongoing gas.
Next, the above-noted characteristic parts of this embodiment are
explained. The NO.sub.x reduction means reduces the value of NO.sub.x
generation to not more than 10 ppm at a specified excess air ratio or
more. The first suppression means forming part of the NO.sub.x reduction
means is explained. This first suppression means is so structured that a
multiplicity of the heat transfer tubes 2 are arranged generally all over
the burning reaction zone (a zone where the combustion gas temperature is
not less than about 900.degree. C.) 20 formed by the burner 1, with gaps
present thereamong to allow the combustion gas to flow therethrough. The
burning-reaction ongoing gas derived from the burner 1 is cooled by these
heat transfer tubes 2. As a result of this cooling, the combustion gas
temperature is suppressed, so that the value of NO.sub.x is lowered. The
arrangement pitch of the heat transfer tubes 2, which affects the degree
of cooling of the combustion gas, is determined in consideration of the
amount of combustion per time, pressure loss and the like.
The second suppression means is an exhaust-gas recirculating means composed
of the exhaust gas passage 7, the exhaust-gas recirculation passage 8, the
air supply passage 5 and the burner 1. At a proper place within the
exhaust-gas recirculation passage 8 is provided a first damper 21 as a gas
flow rate adjusting means for adjusting the exhaust-gas recirculation
quantity to a specified quantity. Mixing exhaust gas with the premixed gas
fed to the burner 1 causes the combustion gas temperature to be
suppressed, so that the value of NO.sub.x lowers. The ratio of the
quantity of exhaust gas to be recirculated (exhaust-gas recirculation
quantity) to the combustion-use air quantity (actual combustion air
quantity) is adjusted by the first damper 21 so as to be unchanged between
high state and low combustion state.
The third suppression means, as shown in FIG. 3, is composed of the steam
addition tube 9, the air inlet passage 19, the blower 4, the air supply
passage 5 and the burner 1. A counter-addition-side end of the steam
addition tube 9 is connected to the upper header 10 via a second valve 22
serving as a steam flow rate adjusting means for adjusting the quantity of
steam addition, so that steam generated by the steam boiler is utilized as
it is. Between the second valve 22 and the upper header 10 is provided an
orifice or other pressure reducing mechanism (not shown). The steam is
mixed uniformly into the combustion-use air fed to the burner 1, and
jetted out into the boiler body 3 generally uniformly from a multiplicity
of premixed-gas nozzles (not shown) of the burner 1. As a result, an
effective cooling of the expandedly formed premixed combustion flame is
achieved.
Further, the fourth suppression means is so structured that the
fully-premixing type burner 1 is burned at a high excess air ratio. When
the burner 1 is burned at a high excess air ratio, the combustion gas
temperature lowers, so that the value of NO.sub.x reduction lowers. The
burner 1 is a longitudinally 60 cm, laterally 18 cm sized
rectangular-shaped burner with the premixed-gas jet nozzles formed
generally uniformly therein. Then, the burner 1 is implemented by a known
one made up by alternately stacking a multiplicity of flat plates and wave
plates (not shown either), for example.
The steam boiler of this working example, as stated before, is switchable
between operations at high combustion and low combustion. Then, the
NO.sub.x reduction means of this steam boiler has the excess air ratio
versus NO.sub.x characteristics and the excess air ratio versus CO
characteristics in high combustion state and low combustion state shown in
FIGS. 4 and 5. These excess air ratio versus NO.sub.x characteristics and
excess air ratio versus CO characteristics are explained below.
First, the excess air ratio versus NO.sub.x characteristic and the excess
air ratio versus CO characteristic in the high combustion state are
determined as shown by a curve A and a curve B, respectively, of FIG. 4
with the excess air ratio varied under the continued-combustion condition.
These operating conditions are a fuel of LPG, a combustion rate of the
burner 1 of 50 Nm.sup.3 /h (combustion rate of the steam boiler at high
combustion), an exhaust-gas recirculation rate of 4% (exhaust-gas
recirculation quantity/actual combustion air quantity), and a steam
addition amount of 17 kg/h. Then, the actual combustion air quantity and
the exhaust-gas recirculation quantity at the exhaust-gas recirculation
rate of 4% are 1669 Nm.sup.3 /h and 67 Nm.sup.3 /h, respectively, at 6%
O.sub.2, for instance.
Varying the excess air ratio is implemented by varying the actual
combustion air quantity. Varying the actual combustion air quantity is
implemented by controlling the rotational speed of an electric motor 24
(see FIG. 3) that drives a fan 23 of the blower 4. It is noted that a
curve C and a curve D in FIG. 4 represent an excess air ratio versus
NO.sub.x characteristic and an excess air ratio versus CO characteristic
of comparative examples in which the cooling by the second suppression
means and the third suppression means is not performed, given for contrast
to the curve A and the curve B of this working example.
The excess air ratio versus NO.sub.x characteristic in the high combustion
state of the NO.sub.x reduction means is, as shown by the curve A, one
that the NO.sub.x value decreases with increasing excess air ratio. Also,
the excess air ratio versus CO characteristic is, as shown by the curve B,
one that the exhaust CO value increases with increasing excess air ratio,
in particular, the exhaust CO value abruptly increases at 5% O.sub.2 or
more. It is noted that the curve C and the curve D in FIG. 4 represent an
excess air ratio versus NO.sub.x characteristic and an excess air ratio
versus CO characteristic of comparative examples in which the suppressions
of combustion gas temperature by the second suppression means and the
third suppression means are not performed, given for contrast to the curve
A and the curve B of this working example.
Next, the excess air ratio versus NO.sub.x characteristics and the excess
air ratio versus CO characteristic in the low combustion state of the
NO.sub.x reduction means are explained below. These characteristics are
determined as shown by a curve E and a curve F, respectively, of FIG. 5 as
in the case of the high combustion state. The operating conditions in the
low combustion state are a fuel of LPG, a combustion rate of the burner of
25 Nm.sup.3 /h (combustion rate of the steam boiler at low combustion), an
exhaust-gas recirculation rate of 4% (exhaust-gas recirculation
quantity/actual combustion air quantity), and a steam addition amount of
8.5 kg/h. Then, the actual combustion air quantity and the exhaust-gas
recirculation quantity at the exhaust-gas recirculation rate of 4% are 834
Nm.sup.3 /h and 33 Nm.sup.3 /h, respectively, at 6% O.sub.2, for instance.
The excess air ratio versus NO.sub.x characteristic in the low combustion
state of the NO.sub.x reduction means is, as shown by the curve E, also
one that the NO.sub.x value decreases with increasing excess air ratio.
Further, the excess air ratio versus CO characteristic is, as shown by the
curve F, one that the exhaust CO value increases with increasing excess
air ratio, in particular, the exhaust CO value abruptly increases at 5.5%
O.sub.2 or more. It is noted that a curve G and a curve H, in FIG. 5
represent an excess air ratio versus NO.sub.x characteristic and an excess
air ratio versus CO characteristic of comparative examples in which the
suppressions of combustion gas temperature by the second suppression means
and the third suppression means are not performed.
The excess-air-ratio control means, as shown in FIG. 6, is composed of an
oxygen concentration sensor 25 (see FIG. 1) provided on the exhaust gas
passage 7 and serving as the oxygen concentration detection means, and a
control circuit 26 to which an output of the oxygen concentration sensor
25 is inputted and which controls the rotational speed of the electric
motor 24. The electric motor 24 is so designed as to be controllable in
rotational speed by inverter control. By controlling the rotational speed
of the fan 23 so that the excess air ratio of the burner 1 becomes a
specified high excess air ratio (specified value), a specified NO.sub.x
reduction effect is maintained against changes in outside air temperature.
In this working example, given a NO.sub.x reduction target value of 10 ppm,
the specified value can be determined as 5.8% O.sub.2 in the high
combustion state from the curve A of FIG. 4 and the value of 10 ppm. Of
course, an O.sub.2 ratio of higher than 5.8% satisfies the reduction
target value, and so the specified value may be set to, for example, 6%.
For the low combustion state, the specified value can be determined as
6.25% O.sub.2 from the curve E of FIG. 5 and the value of 10 ppm.
In this working example, there is provided a CO reduction means for
reducing CO, which is emitted from the NO.sub.x reduction, to not more
than a CO reduction target value. This CO reduction means oxidizes CO
emitted from the NO.sub.x reduction means to achieve CO reduction below a
CO reduction target value. The CO reduction means of the working example
is implemented by a CO oxidation catalyst member 27 that reduces the CO
value to about 1/10. CO reduction characteristic by this CO oxidation
catalyst member 27 is shown by a curve M of FIG. 4 and a curve N of FIG.
5. CO quantities in the exhaust gas shown by the curve D and the curve E
are finally reduced as shown by the curve M and the curve N, respectively.
This CO oxidation catalyst member 27, having such a structure shown in FIG.
7, is formed in the following manner, for example. With a flat plate 28
and a wave plate 29 as base materials, both of which are made of
stainless, a multiplicity of minute pits and bumps are formed on their
surfaces, and oxidation catalyst is applied on top of the surfaces. Then,
the flat plate 28 and the wave plate 29 are cut into a specified elongate
shape and laid on each other and spirally rolled into a roll state. This
roll is surrounded and fixed by a side plate 30. In this way, the CO
oxidation catalyst member 27 as shown in FIG. 7 is formed. Platinum is
used as the oxidation catalyst. It is noted that FIG. 7 shows only part of
the flat plate 28 and the wave plate 29.
The CO oxidation catalyst member 27, as shown in FIG. 1, is removably
fitted to the exhaust gas outlet 16 portion. Size and processing capacity
of this CO oxidation catalyst member 27 are designed in consideration of
the performance of the oxidation catalyst, the quantity of CO to be
oxidized, and the pressure loss occurring when the exhaust gas flows
through the CO oxidation catalyst member 27.
Further, the NO.sub.x reduction means, as shown in FIG. 2, includes another
CO reduction means. This CO reduction means is a heat-transfer-tube
removal space 31 called heat insulating space formed by eliminating some
of the heat absorbers. Then, as shown in FIG. 2, part of the heat transfer
tubes 2, i.e., four heat transfer tubes 2 in this working example are
removed so that the heat-transfer-tube removal space 31 where the
combustion gas temperature falls within a range not more than 1400.degree.
C. and not less than 900.degree. C. is formed.
The heat-transfer-tube removal space 31 falls generally within the
aforementioned temperature range in the high combustion state, while it
involves a shorter combustion flame, i.e., a narrower burning reaction
zone in the low combustion state so as to no longer fall within the
temperature range. Accordingly, the CO oxidation catalyst member 27 and
the heat-transfer-tube removal space 31 serve as CO reduction means in the
high combustion state, while the heat-transfer-tube removal space 31 does
not serve as CO reduction means and the CO oxidation catalyst member 27
serves as CO reduction means in the low combustion state.
Operations and actions of the working example of the above-described
constitution are explained below. Burning-reaction ongoing gas derived
from the burner 1 is subjected to a NO.sub.x reduction action, i.e.,
combustion-gas-temperature suppression actions by the first to fourth
suppression means, at the same time, and still also subjected to such
constant excess-air-ratio control that O.sub.2 (%) is held at 5.8 in the
high combustion state and at 6.25 in the low combustion state by the
excess-air-ratio control means.
By such excess-air-ratio control, the excess air ratio is maintained at a
generally constant excess air ratio at all times even with the outside-air
temperature varied, so that the value of NO.sub.x generation is suppressed
to 10 ppm. That is, as a result of the combustion-gas-temperature
suppression action by the NO.sub.x reduction means, the combustion gas
temperature is lowered by about 100.degree. C. on an average, compared
with the comparative example in which the burning-reaction ongoing gas is
not subjected to the actions by the second suppression means and the third
suppression means. As a result, the NO.sub.x value in the combustion gas
flowing out from the upstream-side heat transfer tubes 2 is suppressed to
about 10 ppm as shown by the curve A and curve E of FIGS. 4 and 5,
respectively.
Also, by the foregoing excess-air-ratio control, the value of exhaust CO
derived from the NO.sub.x reduction means is also controlled to a
specified value. The value of exhaust CO in the exhaust gas at the exhaust
gas outlet 16 is about 400 ppm in the high combustion state and about 100
ppm in the low combustion state as shown by the characteristic curve B and
curve F of FIGS. 4 and 5, respectively.
CO generated in the NO.sub.x reduction shown above is reduced in the
following manner. The generated CO is, first, partly oxidized at the
heat-transfer-tube removal space 31 in the high combustion state, and
scarcely oxidized in the low combustion state, then reaching the exhaust
gas outlet 16 as exhaust gas. CO remaining in this exhaust gas is oxidized
by the CO oxidation catalyst member 27 so that the CO value is reduced to
about 1/10, as shown by the characteristic curve M and curve N of FIGS. 4
and 5.
According to this working example, since the NO.sub.x reduction means is
implemented by a combination of the first suppression means to the fourth
suppression means, the following working effects are produced. Whereas
enhancing the functions of the individual suppression means singly would
cause drawbacks of the respective suppression means to matter, combining
the four suppression means makes it possible to achieve super NO.sub.x
reduction relatively easily without causing the emergence of those
drawbacks. In particular, later-described unstable characteristics of the
fourth suppression means are alleviated, so that stable super NO.sub.x
reduction can be achieved. This will be detailed below.
It is noted that the functional enhancement of the first suppression means
(heat-absorber cooling) is the provision of the heat transfer tubes 2 in
contact with the burner 1 or the increasing of the heat-transfer-surface
density of the heat transfer tubes 2. Due to this functional enhancement,
there would occur an increase in pressure loss or an unstable combustion
such as oscillating combustion.
Also, the functional enhancement of the second suppression means (exhaust
gas recirculation) is to increase the exhaust-gas circulation quantity.
Due to this functional enhancement, there would occur an amplification of
the unstable characteristics of the second suppression means. That is, the
exhaust gas recirculation has a characteristic that the exhaust-gas flow
rate or temperature changes with changes in combustion quantity or changes
in load. An increase in the exhaust-gas recirculation quantity would cause
these unstable characteristics to be amplified, making it impossible to
achieve a stable NO.sub.x reduction. Also, due to the functional
enhancement of the second suppression means, burning reaction would be
suppressed, causing an emission increase of CO and unburned components as
well as an increase in thermal loss. Further, increasing the exhaust-gas
recirculation quantity would cause the blower load to increase.
Also, the functional enhancement of the third suppression means
(water/steam addition) is to increase the quantity of water to be added.
Due to this functional enhancement, the quantity of condensations would
increase with increasing thermal loss, where, particularly in boilers
having a feed water preheater for preheating the water fed to the heat
transfer tubes 2 by exhaust gas, there would matter corrosion of the feed
water preheater due to the condensations.
Further, the functional enhancement of the fourth suppression means
(premixing high excess-air-ratio combustion) is to increase the excess air
ratio. Due to this functional enhancement, there would occur a halt of
burning reaction and an unstable combustion of the burner 1.
In contrast, according to this embodiment, since the first to fourth
suppression means are combined together, the problems that would otherwise
emerge upon enhancing the functions of the individual suppression means
each singly can be prevented from becoming issues.
Also, according to this working example, the following working effects are
produced. Since the excess air ratio can be controlled to a generally
constant high excess air ratio by the excess-air-ratio control means, a
stable NO.sub.x reduction effect can be obtained even with outside air
temperature varied. As a result, the NO.sub.x reduction target value can
be met over a wide range of operating points on the day and year bases.
Further, the exhaust CO value from the NO.sub.x reduction means is also
controlled to a constant one by the constant constant excess-air-ratio
control. As a result, the possibility that the exhaust CO value increases
due to changes in excess air ratio beyond the processing capacity of the
CO oxidation catalyst member 27 is eliminated, thus producing an effect
that a stable CO reduction can be achieved. In particular, for a NO.sub.x
reduction means of which the NO.sub.x reduction target value is not more
than 10 ppm, involving an abrupt increase of the exhaust CO value at
around 10 ppm, the constant excess-air-ratio control produces quite a
large effect in terms of the achievement of a CO reduction target value
and the facilitation of the capacity design of the CO oxidation catalyst
member 27.
The facilitation of the capacity description of the CO oxidation catalyst
member 27 is further explained. The CO oxidation catalyst member 27, in
which pressure loss increases with increasing capacity, is so designed
that the CO reduction target value can be satisfied just at the very
limit. Without the constant excess-air-ratio control, there would arise a
need for designing the processing capacity of the CO oxidation catalyst
member 27 with a margin. Meanwhile, with the processing capacity
increased, the pressure loss would increase. As a result, the pressure
loss of the steam boiler itself would increase, giving rise to a need for
redesigning the blower 4 or the boiler body 3. Performing the constant
excess-air-ratio control produces, as in this working example, has an
effect of solving these problems.
Further, according to this working example, both the NO.sub.x reduction for
reducing the generated NO.sub.x value to not more than 10 ppm as well as
the CO reduction can be achieved at the same time, greatly contributing to
air pollution control. Besides, in the low combustion state, although the
heat-transfer-tube removal space 31 does not function effectively as CO
reduction means, yet CO is oxidized by the CO oxidation catalyst member
27, so that CO reduction can be fulfilled regardless of whether it is in
the high combustion state or the low combustion state.
It is noted that the present invention is not limited to the
above-described working example, and includes the following modified
example. Although the heat transfer tubes 2 of the first suppression means
are implemented by vertical water tubes in the foregoing working example,
yet the heat transfer tubes 2 may also be implemented by water tubes which
are positioned horizontal or tilted. Further, the shape of the heat
transfer tubes 2 is also not limited to a perfect circle of the foregoing
working example, and may be shaped into elliptical or other shapes in
another embodiment.
Also, the heat transfer tubes 2 of the first suppression means are provided
as bare tubes in the foregoing working example. However, it is also
possible that some of the heat transfer tubes 2 in the downstream of the
heat-transfer-tube removal space 31 may be fitted with horizontal
fillet-like fins or full-peripheral fins (not shown either) so that the
heat recovery rate can be enhanced, in another embodiment.
Also, steam of the steam addition tube 9 of the third suppression means
