U.S. patent application number 12/608491 was filed with the patent office on 2010-05-06 for oxyfuel boiler system and method of controlling the same.
This patent application is currently assigned to Hitachi, Ltd.. Invention is credited to Yoshiharu Hayashi, Tsuyoshi SHIBATA, Masayuki Taniguchi, Akihiro Yamada.
Application Number | 20100107940 12/608491 |
Document ID | / |
Family ID | 41687542 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100107940 |
Kind Code |
A1 |
SHIBATA; Tsuyoshi ; et
al. |
May 6, 2010 |
Oxyfuel Boiler System and Method of Controlling the Same
Abstract
The oxyfuel boiler system comprises: an oxygen generator; a coal
mill; a burner to burn pulverized coal; an after-gas port to which
oxygen generated at the oxygen generator is supplied; a boiler
provided with the burner and the after-gas port on its wall; a flue
introducing combustion exhaust gas from the boiler to the outside;
a recirculation gas supply pipe having an exhaust gas tapping port
in the midway of the flue and supplying recirculation exhaust gas
to the coal mill, the burner, and the after gas port; and an oxygen
supply pipe supplying oxygen from the oxygen generator to the
burner and the after-gas port, wherein the exhaust gas tapping port
is disposed downstream of a dry dust-removing apparatus arranged in
the flue, and there is provided an oxygen controlling apparatus for
making a concentration of oxygen to be supplied to the after-gas
port lower than that of oxygen to be supplied to the burner.
Inventors: |
SHIBATA; Tsuyoshi;
(Hitachiota, JP) ; Taniguchi; Masayuki;
(Hitachinaka, JP) ; Yamada; Akihiro; (Tokai,
JP) ; Hayashi; Yoshiharu; (Hitachinaka, JP) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Hitachi, Ltd.
Tokyo
JP
|
Family ID: |
41687542 |
Appl. No.: |
12/608491 |
Filed: |
October 29, 2009 |
Current U.S.
Class: |
110/347 ;
110/205; 110/216; 110/234; 110/261; 110/297 |
Current CPC
Class: |
F23C 2202/20 20130101;
Y02E 20/344 20130101; Y02E 20/34 20130101; F23C 9/003 20130101;
F23L 7/007 20130101 |
Class at
Publication: |
110/347 ;
110/205; 110/234; 110/216; 110/261; 110/297 |
International
Class: |
F23C 9/00 20060101
F23C009/00; F23C 7/00 20060101 F23C007/00; F23J 3/00 20060101
F23J003/00; F23L 7/00 20060101 F23L007/00; F23L 9/00 20060101
F23L009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 31, 2008 |
JP |
2008-280675 |
Claims
1. An oxyfuel boiler system comprising: an oxygen generator to
separate oxygen from air; a coal mill to dry and pulverize coal; a
burner to burn the dried and pulverized coal with oxygen generated
at the oxygen generator; an after-gas port to which oxygen
generated at the oxygen generator is supplied; a boiler provided
with the burner and the after-gas port on its wall; a flue
introducing combustion exhaust gas from the boiler to the outside;
a recirculation gas supply pipe having an exhaust gas tapping port
disposed in the midway of the flue and supplying recirculation
exhaust gas to the coal mill, the burner, and the after-gas port;
and an oxygen supply pipe supplying oxygen from the oxygen
generator to the burner and the after-gas port, wherein the exhaust
gas tapping port is disposed downstream of a dry dust-removing
apparatus arranged in the flue, and there is provided an oxygen
controlling apparatus for making a concentration of oxygen to be
supplied to the after-gas port lower than that of oxygen to be
supplied to the burner.
2. An oxyfuel boiler system according to claim 1, wherein the
exhaust gas tapping port is disposed downstream of the dry
dust-removing apparatus and downstream of a moisture removing
cooler.
3. The oxyfuel boiler system according to claim 1, wherein the
exhaust gas tapping port is disposed downstream of the dry
dust-removing apparatus and upstream of a first moisture removing
cooler, and a second moisture removing cooler is provided in the
midway of the recirculation gas supply pipe.
4. An oxyfuel boiler system comprising: an oxygen generator to
separate oxygen from air; a coal mill to dry and pulverize coal; a
burner to burn the dried and pulverized coal with oxygen generated
at the oxygen generator; an after-gas port to which oxygen
generated at the oxygen generator is supplied; a boiler provided
with the burner and the after-gas port on its wall; a flue
introducing combustion exhaust gas from the boiler to the outside;
recirculation gas supply pipes having two exhaust gas tapping ports
disposed in the midway of the flue and supplying recirculation
exhaust gas taken in from the respective exhaust gas tapping ports
to the boiler; and an oxygen supply pipe supplying oxygen from the
oxygen generator to the burner and the after-gas port, wherein the
oxyfuel boiler system has: the first exhaust gas tapping port
arranged in the flue and disposed downstream of a dry dust-removing
apparatus and the second exhaust gas tapping port arranged in the
flue and disposed downstream of a moisture removing cooler; the
recirculation gas supply pipe including a first line supplying
exhaust gas taken in from the first exhaust gas tapping port to the
after-gas port and a second line supplying exhaust gas taken in
from the second exhaust gas tapping port to the burner; and an
oxygen controlling apparatus for oxidizer making a concentration of
oxygen to be supplied to the after-gas port lower than that of
oxygen supplied to the burner.
5. A method of controlling an oxyfuel boiler system, comprising: an
oxygen generator to separate oxygen from air; a coal mill to dry
and pulverize coal; a burner to burn the dried and pulverized coal
with oxygen generated at the oxygen generator; an after-gas port to
which oxygen generated at the oxygen generator is supplied; a
boiler provided with the burner and the after-gas port on its wall;
a flue introducing combustion exhaust gas from the boiler to the
outside; a recirculation gas supply pipe having an exhaust gas
tapping port disposed in the midway of the flue and supplying
recirculation exhaust gas to the coal mill, the burner and the
after-gas port; and an oxygen supply pipe supplying oxygen from the
oxygen generator to pipes supplying exhaust gas to the coal mill,
the burner and the after-gas port by the recirculation gas supply
pipe, the method comprising the steps of: taking exhaust gas in
from downstream of a dry dust-removing apparatus arranged in the
flue; and making a concentration of oxygen to be supplied to the
after-gas port lower than that of oxygen to be supplied to the
burner.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese patent
application serial No. 2008-280675, filed on Oct. 31, 2008, the
content of which is hereby incorporated by reference into this
application.
FIELD OF THE INVENTION
[0002] The present invention relates to an oxyfuel boiler system
and a method of controlling the same.
BACKGROUND OF THE INVENTION
[0003] A coal firing power generation system configured with a
pulverized coal firing boiler and a steam turbine electric
generator plays a significant role, given the recent years' price
increase of natural gas and the like resulting from oil supply
shortage and increase of natural gas demand.
[0004] As a means to greatly reduce CO.sub.2 emissions from the
coal firing power generation system, oxyfuel boiler systems have
been proposed.
[0005] Japanese Unexamined Patent Application Publication No.
2007-147162 discloses a technology for adjusting oxygen
concentration in the total gas introduced to a boiler main body by
controlling recirculation flow rate of combustion exhaust gas so
that the heat absorbing amount of the boiler main body becomes the
target heat absorbing amount.
[0006] However, in Japanese Unexamined Patent Application
Publication No. 2007-147162, no consideration is given on reduction
of fuel NO.sub.x.
[0007] In view of above, an object of the present invention is to
provide an oxyfuel boiler system and a method of controlling the
same capable of further reducing the forming amount of the fuel
NO.sub.x.
SUMMARY OF THE INVENTION
[0008] The present invention provides an oxyfuel boiler system in
which an exhaust gas tapping port is arranged downstream of a dry
dust-removing apparatus arranged in a flue, and there is provided
an oxygen controlling apparatus for making a concentration of
oxygen to be supplied to an after-gas port lower than that of
oxygen to be supplied to a burner. More specifically, the present
invention provides an oxyfuel boiler system comprising: an oxygen
generator; a coal mill; a burner burning pulverized coal; an
after-gas port to which oxygen generated at the oxygen generator is
supplied; a boiler provided with the burner and the after-gas port
on its wall; a flue introducing combustion exhaust gas from the
boiler to the outside; a recirculation gas supply pipe having an
exhaust gas tapping port disposed in the midway of the flue and
supplying recirculation exhaust gas to the coal mill, the burner,
and the after gas port; and an oxygen supply pipe supplying oxygen
from the oxygen generator to the burner and the after-gas port;
wherein the exhaust gas tapping port is disposed downstream of a
dry dust-removing apparatus arranged in the flue, and an oxygen
controlling apparatus for making a concentration of oxygen to be
supplied to the after-gas port lower than that of oxygen to be
supplied to the burner is provided.
BRIEF DESCRIPTION OF THE DRAWING
[0009] FIG. 1 illustrates configuration of an oxyfuel boiler system
according to a first embodiment;
[0010] FIG. 2 illustrates configuration of an oxyfuel boiler system
according to a second embodiment;
[0011] FIG. 3 illustrates configuration of an oxyfuel boiler system
according to a third embodiment;
[0012] FIG. 4 illustrates configuration of an oxyfuel boiler system
according to a fourth embodiment;
[0013] FIG. 5 illustrates an example of a calculation result of
reductive combustion zone temperature of a boiler in the first
embodiment; and
[0014] FIG. 6 illustrates an example of a calculation result of
oxidative combustion zone temperature of a boiler in the first
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Oxyfuel boiler systems according to respective embodiments
will be described below referring to the accompanying drawings.
However, the present invention is not limited to those
embodiments.
[0016] Meaning of each reference numeral in the description below
is as follows. [0017] 1: Boiler, [0018] 2: NO.sub.x removing
apparatus, [0019] 3: Heat exchanger, [0020] 4: Dry dust-removing
apparatus, [0021] 5: Wet desulfurization apparatus, [0022] 6: Wet
dust-removing apparatus, [0023] 7: Moisture removing cooler, [0024]
8: CO.sub.2 separation and liquefaction apparatus, [0025] 9:
Discharge stack, [0026] 10: Oxygen generator, [0027] 11: Coal mill,
[0028] 12: Burner, [0029] 13: After-gas port, [0030] 14:
Recirculation gas supply pipe, [0031] 14a: Primary gas pipe, [0032]
14b: Secondary gas pipe, [0033] 14c: After-gas pipe, [0034] 15:
Nitrogen gas transfer pipe, [0035] 16: Oxygen supply pipe, [0036]
17: Recirculation exhaust gas moisture removing cooler, [0037] 19:
Coal supply pipe, [0038] 20: Flue, [0039] 21, 21a, 21b:
Recirculation fan, [0040] 22, 22a, 22b: Exhaust gas tapping port,
[0041] 23: Oxygen and recirculation gas flow controller, [0042] 24:
Oxygen analyzer, and [0043] 25: Oxygen controlling apparatus for
oxidizer.
First Embodiment
[0044] FIG. 1 illustrates a configuration of an oxyfuel boiler
system. Fuel coal is supplied to a coal mill 11 via a coal transfer
device and is pulverized to a particle size suitable for pulverized
coal firing. The pulverized coal (powdered coal) is supplied to a
burner 12 through a coal supply pipe 19. The coal mill 11 is
connected with a primary gas pipe 14a which supplies recirculation
exhaust gas to the coal mill 11. An oxygen supply pipe 16 is
connected to a location along the coal supply pipe 19, where oxygen
is mixed as needed. Mixing of a proper amount of oxygen with the
recirculation exhaust gas in the coal supply pipe 19 has the effect
of enhancing the ignition performance of the coal in the burner
12.
[0045] An oxygen generator 10 separates oxygen from air and
supplies oxygen to the coal supply pipe 19 and the like through the
oxygen supply pipe 16. A great deal of nitrogen gas generated in
separation of oxygen is diffused from a discharge stack 9 by a
nitrogen gas transfer pipe 15.
[0046] The burner 12 is provided with a secondary gas pipe 14b,
which is a recirculation gas supply pipe, and the oxygen supply
pipe 16 is connected to the secondary gas pipe 14b. Also, the
burner 12 ejects gas mixture of the oxygen supplied from the oxygen
supply pipe 16 and the recirculation exhaust gas to a furnace.
Further, the burner 12 also ejects the pulverized coal supplied
from the coal supply pipe 19 to the furnace and forms a flame by
the pulverized coal and the gas mixture.
[0047] An after-gas port 13 is arranged downstream of the burner
12, and the recirculation gas supply pipe 14 is connected to the
after-gas port 13 as well through an after-gas pipe 14c. Also, the
oxygen supply pipe 16 is connected to the after-gas supply pipe 14c
as well. Further, independently of the burner 12, the after-gas
port 13 also feeds the gas mixture of the oxygen and the
recirculation gas to a boiler 1.
[0048] The after-gas port 13 has a function similar to that of the
after-air port of an air fired boiler. That is, by properly
regulating the flow amounts of the gas mixture and the oxygen
concentration supplied to the burner 12 and the after-gas port 13,
a burning zone of reductive atmosphere is formed in the boiler 1,
thus reducing the rate of conversion of the nitrogen in the coal to
NO.sub.x. Also, by the oxygen contained in the gas supplied from
the after-gas port 13, a burning zone of oxidative atmosphere is
formed in the upper part of the boiler. Further, the jet exiting
from the after-gas port 13 promotes gas mixing in the boiler and
reduces unburned portion remaining in the exhaust gas of the
boiler.
[0049] The flow rates of the recirculated exhaust gases supplied to
the coal mill 11, burner 12, and after-gas port 13 can each be
regulated independently by a flow rate regulator attached to the
primary gas pipe 14a, secondary gas pipe 14b, and after-gas pipe
14c respectively. Further, the oxygen supply pipe 16 also has
separate flow regulators in its branches for regulating the supply
amount to the branches. These three recirculation gas supply pipes
14 are connected with gas sampling pipes for measuring each oxygen
concentration.
[0050] An oxygen controlling apparatus for oxidizer 25 comprises an
oxygen and recirculation gas flow controller 23 and an oxygen
analyzer 24. The oxygen analyzer 24 can always measure the oxygen
concentration in the gas mixture in each pipe taken by a gas
sampling pipe. The oxygen and recirculation gas flow controller 23
can regulate the recirculation gas flow rate and the oxygen flow
rate of the three recirculation gas supply pipes 14 independently.
Also, the oxygen and recirculation gas flow controller 23 can
automatically control the oxygen concentration using the oxygen
concentration measured value from the oxygen analyzer 24 as an
input signal. That is, with this controller, for the gas mixture
supplied to the boiler 1 from the coal supply pipe 19, secondary
gas pipe 14b, after-gas pipe 14c, the distribution ratio of the
oxygen feeding amount and oxygen concentration can be set/regulated
independently.
[0051] High temperature and high pressure steam generated in the
boiler 1 is supplied to a steam turbine generating system and is
converted to electric power.
[0052] Exhaust gas generated in the boiler 1 is introduced to a
flue 20 and supplied to an NO.sub.x removing apparatus 2, and the
NO.sub.x component in the exhaust gas is reduced. In the flue 20,
an exhaust gas treatment system comprising a plurality of
apparatuses treating the exhaust gas is provided in addition to the
NO.sub.x removing apparatus 2. However, when the forming amount of
NO.sub.x can be sufficiently reduced by improvement of the
combustion method and the like, the NO.sub.x removing apparatus 2
may be omitted. The exhaust gas exited from the NO.sub.x removing
apparatus 2 is supplied to a heat exchanger 3 and the temperature
is reduced. Heat recovered from the exhaust gas by the heat
exchanger 3 is given to the recirculation exhaust gas supplied
similarly to the heat exchanger 3 for recirculating to the boiler 1
and inhibits deterioration of thermal efficiency of the plant. The
exhaust gas exited from the heat exchanger 3 is introduced to a dry
dust-removing apparatus 4, and 95% or more of the dust component is
removed.
[0053] An exhaust gas tapping port 22 is disposed downstream of the
dry dust-removing apparatus 4. A part of the exhaust gas taken in
from the exhaust gas tapping port 22 is induced to a recirculation
gas supply pipe 14 by a recirculation fan 21 and is heated by the
heat exchanger 3. The exhaust gas is thereafter supplied to the
coal mill 11, burner 12 and after-gas port 13 as described
above.
[0054] 95% or more of SO.sub.2 of the exhaust gas not recirculated
is removed by a wet desulfurization apparatus 5. Then, the exhaust
gas is removed of 98% or more of SO.sub.3 component by a wet
dust-removing apparatus 6, and the moisture content in the exhaust
gas is substantially reduced by a moisture removing cooler 7.
Further, in the present embodiment, with respect to the removing
apparatus for dust and sulfur compound in the exhaust gas such as
the dry dust-removing apparatus, wet desulfurization apparatus, and
wet dust-removing apparatus, decision on necessity or non-necessity
of installation and alteration of specification of the removal rate
may be made at a designer's discretion. Also, an apparatus with a
similar function such as a dry desulfurization apparatus maybe
installed as an alternative.
[0055] The CO.sub.2 concentration of the exhaust gas exiting from
the moisture removing cooler 7 becomes approximately 90% or more.
Therefore, a CO.sub.2 separation and liquefaction apparatus 8 can
separate and liquefy CO.sub.2 in the exhaust gas easily. Also, the
separated CO.sub.2 may be supplied to a user through a pipe line
and the like as high-pressure gas. The balance not liquefied by the
CO.sub.2 separation and liquefaction apparatus 8 is discharged as
off-gas. Main components of the off-gas are nitrogen and oxygen,
and minor components of NO.sub.x and the like and some amount of
CO.sub.2 are contained. The off-gas is mixed with a great deal of
nitrogen generated by the oxygen generator 10, and is diffused to
the atmosphere from the discharge stack 9.
[0056] Here, the basic principle of the oxyfuel boiler system will
be described. An ordinary coal firing boiler uses air as oxidizer
gas, whereas the oxyfuel boiler uses the gas mixture in which major
portion of the combustion exhaust gas is taken out from a location
along the flue and is mixed thereafter with high purity oxygen
generated by the oxygen generator for regulating the oxygen
concentration as the oxidizer gas. Thus, final exhaust gas flow
rate discharged from the plant is reduced to approximately 1/4th
compared with other ordinary systems. Further, because the CO.sub.2
concentration of the exhaust gas rises massively, CO.sub.2 can be
separated and recovered from the exhaust gas easily.
[0057] For CO.sub.2 emission-free coal-firing oxyfuel boiler
system, the main technical objectives are summarized as
follows:
[0058] (1) To suppress reduction in power generation efficiency
resulting from energy consumption in the oxygen generator and
CO.sub.2 separator.
[0059] (2) To establish a plant control method by which the plant
can stably respond to various conditions such as starting, stopping
and changing loads, and realize the stable operation by cooperation
with peripheral facilities (such as oxygen generator and a CO.sub.2
separator).
[0060] (3) To achieve stable burning performance and suppress the
formation of trace harmful substance when burning coal with an
oxidizer mixture gas of recirculation exhaust gas and oxygen.
[0061] (4) To prevent various problems arising from increased
concentrations of components contained in the various exhaust gas
resulting from introduction of a configuration in which a large
amount of the exhaust gas is recirculated.
[0062] The present embodiment relates mainly to the item (3) above,
and is objected to solve the problems related to inhibit the
forming amount of NO.sub.x which is the trace harmful substance.
Below, the features of the oxyfuel boiler system and the problems
related to NO.sub.x reduction will be described.
[0063] If CO.sub.2 recovery from exhaust gas is the only object,
burning coal exclusively with oxygen is an effective way. When coal
is burned exclusively with oxygen, the major components of the
exhaust gas are CO.sub.2 and H.sub.2O. Therefore, by separating and
removing the H.sub.2O from the exhaust gas, for example, by cooling
the exhaust gas, CO.sub.2 of a high concentration can be readily
collected. However, when coal is burned with oxygen only, the
temperature of the burning flame is higher than those in air fired
boiler system by 500 higher. Therefore, when this oxygen firing is
employed in a coal firing plant, expensive heat resistant steels
need to be used for metal materials constituting the boiler.
Another problem is that the oxidizer gas jet velocity in the burner
is relatively low, thus making it difficult to forma stable flame.
Also, the amount of the exhaust gas generated is less than 1/4th of
those in air firing, and therefore the velocity of the exhaust gas
flowing through the heat transfer tube of the boiler is extremely
slower. Accordingly, the thermal transfer efficiency degrades and
the thermal recovery becomes difficult.
[0064] To overcome above problems, the oxyfuel boiler system
employs an exhaust gas recirculation system in which a large amount
of the exhaust gas is recirculated, mixed with oxygen, and then
supplied to the boiler. Specifically, such system is designed so
that the flow rate of the oxidizer gas supplied to the burner and
the flow rate of the exhaust gas flowing through the boiler are not
less than approximately 70% of the flow rate of the air in an air
fired boiler. In this manner, high efficiency thermal recovery and
electric power generation can be stably achieved without greatly
modifying a conventional air fired boiler system.
[0065] In the oxyfuel boiler system with such basic configuration,
the problems related to NO.sub.x reduction are as below.
[0066] In the oxyfuel boiler system, nitrogen (N.sub.2)
concentration in the exhaust gas is maintained extremely low.
Therefore, thermal NO.sub.x formed by reaction of N.sub.2 and
oxygen (O.sub.2) in the high temperature zone in the upper part of
the boiler is suppressed greatly. As a result, the NO.sub.x forming
amount per unit supplied heat amount can be reduced compared with
those in the air fired boiler. Accordingly, in order to further
reduce the NO.sub.x forming amount in the oxyfuel boiler system,
the forming amount of fuel NO.sub.x generated by oxidation of N
portion in coal needs to be suppressed.
[0067] The reaction pathway that forms the fuel NO.sub.x from N
portion in coal is considered to be an oxidation reaction of
ammonia (NH.sub.3) and cyanogen (HCN) generated from the coal. The
mechanism of the oxidation reaction of NH.sub.3 and HCN is
different in the reductive combustion zone with extremely low
O.sub.2 concentration and in the oxidative combustion zone with
high O.sub.2 concentration. OH radical mainly contributes to
oxidation in the reductive combustion zone, whereas oxidation rests
upon O.sub.2 in the oxidative combustion zone. Accordingly, how
these oxidation reaction rates are suppressed according to the
atmosphere of the combustion zone is a key point for the fuel
NO.sub.x reduction.
[0068] Also, as a factor affecting NO.sub.x formation from NH.sub.3
and HCN, the temperature of the combustion zone is also important
in addition to the O.sub.2 concentration and the OH radical
concentration described above. In the reductive combustion zone, as
the combustion temperature becomes higher, the forming rate of the
fuel NO.sub.x lowers. The reason is considered that the reaction in
which the NO.sub.x generated in the reductive combustion zone
reacts with coal, NH.sub.3, HCN and the like again and is reduced
to N.sub.2 is promoted in a field with higher temperature. On the
other hand, in the oxidative combustion zone, as the combustion
temperature becomes lower, the forming rate of the fuel NO.sub.x
lowers. The reason is that, out of NH.sub.3, HCN formed in the
reductive combustion zone in the lower part of the boiler, those
remaining until the oxidative combustion zone in the upper part of
the boiler are oxidized in this zone, and the N portion contained
is converted to NO.sub.x or N.sub.2, where, when the combustion
temperature of the oxidative combustion zone is higher, conversion
rate to NO.sub.x rises, whereas when the combustion temperature is
lower, conversion rate to N.sub.2 rises.
[0069] To summarize above, in order to suppress formation of the
fuel NO.sub.x in the oxyfuel boiler system, it is necessary to
lower the OH radical concentration in the reductive combustion zone
to the most and to raise the combustion temperature. Also, it is
required to lower the O.sub.2 concentration in the oxidative
combustion zone while controlling the combustion temperature
lower.
[0070] In this connection, in the present embodiment, the oxygen
controlling apparatus for oxidizer 25 is provided with a function
of regulating the recirculation gas amount supplied to the coal
mill 11, which is a coal supply apparatus, burner 12, and after-gas
port 13 and setting the O.sub.2 concentration of the oxidizer gas
independently.
[0071] In the air fired type coal firing boiler, the air excess
ratio at the boiler outlet is made approximately 1.15, and the
distribution ratio of the combustion air amount supplied to the
burner and the combustion air amount supplied to the after-air port
is made approximately 0.8:0.35. Therefore, NO.sub.x and unburned
portion can be reduced with a good balance. Regulating air excess
ratio/distribution ratio of the combustion air amount is equivalent
with regulating oxygen excess ratio/distribution ratio of the
oxygen contained in the air. Therefore, in the oxyfuel boiler also,
it is preferable to set the oxygen amount supplied to the burner
and the oxygen amount supplied to the after-gas port as well as the
oxygen excess ratio at the furnace outlet using the condition
described above for the air firing case. That is, in the present
embodiment, the total oxygen amount supplied to the boiler is set
to approximately 1.15 times of the oxygen amount required for
complete combustion of coal. Also, the distribution ratio of the
oxygen amount supplied to the burner 12 and the oxygen amount
supplied to the after-gas port 13 is made approximately
0.8:0.35.
[0072] In addition, the oxygen controlling apparatus for oxidizer
25 also has a function of regulating the recirculation gas amount
supplied to the burner 12 and after-gas port 13. Furthermore, the
oxygen controlling apparatus for oxidizer 25 sets the O.sub.2
concentration of the oxidizer gas independently. Specifically, the
oxygen controlling apparatus for oxidizer 25 makes the
concentration of the oxygen supplied to the after-gas port lower
than the concentration of the oxygen supplied to the burner. With
respect to the regulation target for the O.sub.2 concentration,
following values are appropriate.
[0073] It is preferable to set the O.sub.2 concentration of the
oxidizer gas in the coal supply pipe 19 and the secondary gas pipe
14b connected to the burner between 32% and 36%. FIG. 5 illustrates
a calculation result of the combustion temperature in the reductive
combustion zone in the boiler 1 in accordance with the present
embodiment. In order to obtain the combustion temperature similar
to that of the air firing, the O.sub.2 concentration of
approximately 30% is needed. Also, it was found that the combustion
temperature rose with the increase of the O.sub.2 concentration of
the oxidizer gas. As described above, in the reductive combustion
zone, the forming rate of the fuel NO.sub.x lowers with the rise of
the combustion temperature. Therefore, in order to obtain a stable
high temperature flame, O.sub.2 concentration can be made 32% or
more.
[0074] On the other hand, when the combustion temperature rises by
150.degree. C. or higher if compared with the time of the air
firing, problems occur in the heat resistance performance of the
burner and boiler water tube material. Therefore, from this
viewpoint, the O.sub.2 concentration should be 36% or less. Based
on the above study, in order to reduce the forming amount of the
fuel NO.sub.x, the O.sub.2 concentration of the oxidizer gas
supplied to the burner can be set in the range between
approximately 32% and 36%.
[0075] Next, the O.sub.2 concentration of the oxidizer gas in the
after-gas pipe 14c connected to the after-gas port can be set
between 26% and 28%. FIG. 6 illustrates a calculation result of the
combustion temperature in the oxidative combustion zone in the
boiler 1 in accordance with the present embodiment. By making the
O.sub.2 concentration 30%, the combustion temperature approximately
similar to that in the air firing can be obtained.
[0076] As described above, in the oxidative combustion zone,
forming rate of the fuel NO.sub.x lowers with the lowering of the
combustion temperature. Therefore, in order to obtain a stable
temperature lowering effect, the O.sub.2 concentration can be made
28% or less.
[0077] On the other hand, when the combustion temperature lowers by
100.degree. C. or higher, there is a risk that the concentration of
the unburned portion at the boiler outlet rises exceeding an
allowable range. Therefore, the O.sub.2 concentration should be
made 26% or more. Based on the above study, in order to reduce the
forming amount of the fuel NO.sub.x, the O.sub.2 concentration of
the oxidizer gas supplied to the after-gas port can be set in the
range between 26% and 28%.
[0078] Because the CO.sub.2 emission-free oxyfuel boiler system in
accordance with the present embodiment can reduce the forming
amount of the fuel NO.sub.x generated in the combustion, NO.sub.x
reduction effect can be further enhanced. Also, the harmful
component emitted to the atmosphere can be reduced substantially.
Thus, the apparatuses related to NO.sub.x removal in an exhaust gas
treatment system can be substantially miniaturized and simplified,
and also the utilities cost of the ammonia solution and the like
required for their operation can be reduced.
[0079] Further, setting value of the appropriate O.sub.2
concentration changes also according to the distribution ratio of
the oxygen, oxygen excess ratio, boiler structure, kind of coal,
and the like. Therefore, the setting value should be decided after
each case is assessed, and the values of the appropriate O.sub.2
concentration setting range described above does not limit the
scope of the present embodiment.
Second Embodiment
[0080] FIG. 2 illustrates a schematic diagram of the oxyfuel boiler
system in accordance with the present embodiment.
[0081] Because the present embodiment comprises many sections
constituted of apparatuses having the similar actions as those of
the first embodiment, only the points different from the first
embodiment will be described below. The apparatuses not described
below have the similar action and effect as the first
embodiment.
[0082] The point of the present embodiment different from the first
embodiment is that the exhaust gas tapping port 22 is disposed
downstream of the moisture removing cooler 7, and the recirculation
exhaust gas removed with the dust and moisture can be supplied to
the boiler. The moisture removing cooler 7 serves as a moisture
removing apparatus. In the configuration in accordance with the
first embodiment, the moisture density in the recirculation exhaust
gas is approximately 30%. On the other hand, in the configuration
in accordance with the present embodiment, the moisture density in
the recirculation exhaust gas can be made 5% or less. Because the
moisture density in the recirculation exhaust gas lowers, the
moisture amount supplied from the burner also decreases. Therefore,
OH radical concentration derived from the moisture in the reductive
combustion zone greatly lowers. As described above, lowering of the
OH radical concentration in the reductive combustion zone is
effective in reduction of the fuel NO.sub.x.
[0083] Thus, with the configuration in accordance with the present
embodiment, the NO.sub.x reduction effect greater than that of the
first embodiment can be obtained.
Third Embodiment
[0084] FIG. 3 illustrates a schematic diagram of the oxyfuel boiler
system in accordance with the third embodiment.
[0085] Because the present embodiment comprises many sections
constituted of apparatuses having the similar actions as those of
the first embodiment and the second embodiment, only the points
different from the first embodiment and the second embodiment will
be described below. The apparatuses not described below have the
similar action and effect as the said embodiments.
[0086] The configuration of the present embodiment is similar to
the first embodiment in that the exhaust gas tapping port 22 is
disposed downstream of the dry dust-removing apparatus 4 and
upstream of the other exhaust gas treatment apparatuses. However,
the configuration is different in that the recirculation gas supply
pipe 14 branches to two lines before entering the heat exchanger 3.
Out of the recirculation gas supply pipes that branched, the first
line is connected to the primary gas pipe 14a and the secondary gas
pipe 14b through the heat exchanger 3 after the moisture in the gas
is removed through the recirculation exhaust gas moisture removing
cooler 17. Out of the recirculation gas supply pipes that branched,
the second line is connected to the after-gas pipe 14c through the
heat exchanger 3 without removing the moisture. With the
configuration in accordance with the present embodiment, similar to
the second embodiment, the OH radical concentration in the
reductive combustion zone is substantially reduced. Also, the
moisture in the recirculation exhaust gas supplied to the after-gas
port which does not affect the NO.sub.x reduction is not removed.
Therefore, the thermal loss of the recirculation exhaust gas
supplied to the after-gas port decreases, and the total capacity of
the moisture removing cooler 7 and the recirculation exhaust gas
moisture removing cooler 17 can be reduced. Also, when the wet
desulfurization apparatus and the wet dust-removing apparatus are
arranged like in the case of the present embodiment, the gas amount
flowing through these apparatuses becomes less than 1/4th of that
of the second embodiment, therefore the apparatuses can be
miniaturized.
[0087] Accordingly, with the present embodiment, the oxyfuel boiler
system having the NO.sub.x reduction effect greater than that of
the first embodiment and with the improved thermal efficiency and
with the miniaturized apparatuses when compared with the second
embodiment can be realized.
Fourth Embodiment
[0088] FIG. 4 illustrates a schematic diagram of the oxyfuel boiler
system in accordance with the fourth embodiment.
[0089] Because the present embodiment comprises many sections
constituted of apparatuses having the similar actions as those of
the third embodiment, only the points different from the third
embodiment will be described below. The apparatuses not described
below have the similar action and effect as the third
embodiment.
[0090] The point of the present embodiment different from the third
embodiment is that the exhaust gas tapping port for the
recirculation gas is arranged in two positions of an exhaust gas
tapping port 22a and an exhaust gas tapping port 22b. Similar to
the third embodiment, the exhaust gas tapping port 22a is disposed
immediately after the dry dust-removing apparatus. The exhaust gas
taken in from the exhaust gas tapping port 22a is sent out to the
boiler side by a recirculation fan 21a through the heat exchanger
3, and is supplied to the after-gas pipe 14c only. Also, similar to
the second embodiment, the exhaust gas tapping port 22b is disposed
immediately after the moisture removing cooler 7. The exhaust gas
taken in from the exhaust gas tapping port 22b is supplied to the
boiler side by a recirculation fan 21b through the heat exchanger
3, and is supplied to the primary gas pipe 14a and the secondary
gas pipe 14b only.
[0091] With these configurations, similar to the third embodiment,
the OH radical reduction effect in the reductive combustion zone
can be obtained without arranging the recirculation exhaust gas
moisture removing cooler. That is, because it is not necessary to
arrange the moisture removing apparatuses in two positions, layout
of equipment and piping is facilitated, therefore the apparatuses
can be made more simple and inexpensive.
[0092] According to the embodiments in accordance with the present
invention, with configuration of a CO.sub.2 emission-free oxyfuel
boiler system, a substantial reduction of the NO.sub.x discharge
can be realized by more simple apparatuses, therefore spread of
CO.sub.2 emission-free power generation can be promoted
contributing to suppressing global warming.
* * * * *