TECHNICAL FIELD
The present invention relates to a combustion system, and
more particularly relates to removal of nitrogen oxide in
exhaust gas.
BACKGROUND ART
Coal is generally used as fuel in thermal power plants
and the like since the resource volume of coal is abundant.
However, coal is high in carbon contents in fuel compared with
oil and gas. When coal is combusted by an air combustion
boiler system, the emission amount of carbon dioxide is
increased.
In order to increase concentrations of carbon dioxide for
easy recovery, an oxygen combustion boiler system 101 as shown
in Fig. 5 is used. The oxygen combustion boiler system 101
includes a coal pulverizer 103 for pulverizing coal, an oxygen
combustion boiler 102 for combusting the coal pulverized by
the coal pulverizer 103 and discharging exhaust gas, a
denitration device 104 for removing nitrogen oxide in the
exhaust gas discharged from the oxygen combustion boiler 102,
a dust removal device 105 for removing dust and the like in
the exhaust gas, a desulfurization device 106 for removing
sulfur oxide in the exhaust gas, and a gas cooler 107 for
cooling the exhaust gas to remove moisture in the exhaust gas.
Carrier gas is introduced into the coal pulverizer 103
for drying the pulverized coal and conveying the pulverized
coal from the coal pulverizer 103 to the oxygen combustion
boiler 102. As the carrier gas, exhaust gas (hereinafter
referred to as "primary recirculation gas") discharged from
the desulfurization device 106 and traveling through the gas
cooler 107 is used. The primary recirculation gas is heated
by an air heater 108 provided between the denitration device
104 and the dust removal device 105 for drying the coal. The
air heater 108 performs heat exchange between high-temperature
2
1xhaust gas discharged from the denitration device 104 and
low-temperature exhaust gas which has passed the gas cooler
107, and thereby heats the primary recirculation gas
introduced to the coal pulverizer 103.
A boiler capable of conducting denitration inside a
furnace (not shown) of the oxygen combustion boiler 102 by
two-stage combustion (e. g., Patent Literature 1) is used as
the oxygen combustion boiler 102. The oxygen combustion
boiler 102 includes a burner unit 102a for supplying oxygen
introduced from a combustion oxygen supply system, laterdescribed
secondary recirculation gas, and coal as fuel into
the oxygen combustion boiler 102. The oxygen combustion
boiler 102 also includes an additional air port (hereinafter
referred to as "AA port") 102b provided on the downstream side
of the burner unit 102a for supplying oxygen introduced from
the combustion oxygen supply system and later-described
secondary recirculation gas into the oxygen combustion boiler
102.
The oxygen supplied into the oxygen combustion boiler 102
through the burner unit 102a and the AA port 102b contains
part of the exhaust gas (hereinafter referred to as "secondary
recirculation gas") which has been introduced from the
downstream of the gas cooler 107 and mixed therein as dilution
gas. The secondary recirculation gas is heated by the air
heater 108 and is mixed into the oxygen which is introduced to
the burner unit 102a and the AA port 102b.
The oxygen supplied to the burner unit 102a is adjusted
so that the amount of oxygen introduced from the combustion
oxygen supply system is not larger than 1.0 times the
theoretical combustion oxygen amount of the coal supplied from
the coal pulverizer 103. A remaining amount of the oxygen
introduced from the combustion oxygen supply system is
supplied to the AA port 102b. Accordingly, a zone between the
burner unit 102a and the AA port 102b in the oxygen combustion
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~oiler 102 is in an oxygen-poor state.
Since oxygen is poor in between the burner unit 102a and
the AA port 102b, the zone between the burner unit 102a and
the AA port 102b is made to have a reducing atmosphere. The
fuel charged into the oxygen combustion boiler 102 through the
burner unit 102a combusts and generates exhaust gas. Nitrogen
oxide (NOx) contained in the generated exhaust gas is
partially reduced when passing the reducing atmosphere present
between the burner unit 102a and the AA port 102b. This makes
it possible to decrease nitrogen oxide within the oxygen
combustion boiler 102.
Patent Literature 2 and Patent Literature 3 disclose
oxygen combustion boilers for receiving part of exhaust gas
introduced as secondary recirculation gas which has passed a
denitration device, an air heater, a dust removal device, and
a desulfurization device.
CITATION LIST
Patent Literature
PTL 1
The Publication of Japanese Patent No. 3068888
PTL 2
Japanese Unexamined Patent Application, Publication No.
Hei6-94212
PTL 3
Japanese Unexamined Patent Application, Publication No.
Sho59-195013
SUMMARY OF INVENTION
Technical Problem
However, in the oxygen combustion boiler of the invention
disclosed in Patent Literature 3 as well as in the oxygen
combustion boiler system 101 shown in Fig. 5, the exhaust
having a high nitrogen oxide concentration discharged from the
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~xygen combustion boiler 102 was recirculated as secondary
recirculation gas and was recharged into the oxygen combustion
boiler 102. As a consequence, the denitration device 104
provided on the downstream siae of the oxygen combustion
boiler 102 had a heavy treatment burden.
Moreover, since the concentration of nitrogen oxide
treated by the denitration device 104 was high, the
consumption of ammonia sprayed to the exhaust gas passing the
denitration device 104 increased.
In view of the above-stated problems, an object of the
present invention is to provide a combustion system capable of
decreasing nitrogen oxide discharged from exhaust gas.
Solution to Problem
A combustion system of the present invention employs the
following solutions to solve the foregoing problems.
A combustion system according to the present invention
includes: a combustion furnace having a burner unit for
supplying fuel and combustion oxygen to an inside of the
furnace, a reduction zone formed on a downstream side of the
burner unit for combusting the fuel, and a combustion oxygen
supply port for supplying combustion oxygen so that unburned
fuel which has passed the reduction zone completely combusts;
and a smoke removal device for removing smoke in the exhaust
gas discharged from the combustion furnace, wherein part of
exhaust gas diverging from between the combustion furnace and
the smoke removal device is introduced to the burner unit,
while part of exhaust gas diverging from a downstream side of
the smoke removal device is introduced to the combustion
oxygen supply port.
In the combustion system according to the present
invention, part of exhaust gas diverging from between the
combustion furnace and the smoke removal device is resupplied
to the inside of the combustion furnace through the burner
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1nit. A zone under reducing atmosphere (reduction zone) is
formed between the burner unit of the combustion furnace and
the combustion oxygen supply port. Consequently, the exhaust
gas diverging from between the combustion furnace and the
smoke removal device can be reduced in the zone under reducing
atmosphere formed in the combustion furnace and then be
discharged. This makes it possible to decrease the flow rate
of the exhaust gas introduced from the combustion furnace to
the smoke removal device and to decrease the amount of smoke.
Therefore, the capacity of the smoke removal device can be
reduced.
Moreover, part of exhaust gas diverging from the
downstream side of the smoke removal device is the exhaust gas
having the concentration of nitrogen oxide, which is smoke,
decreased by the smoke removal device. The exhaust gas having
a decreased nitrogen oxide concentration is introduced to the
combustion oxygen supply port and is used for promoting
complete combustion of the unburned fuel contained in the
exhaust gas which has partially been reduced by passing the
reducing atmosphere inside the combustion furnace. Therefore,
the exhaust gas can be discharged out of the combustion
furnace while the concentration of nitrogen oxide contained
therein is maintained low. The exhaust gas having a decreased
nitrogen oxide concentration is recirculated to the combustion
furnace and the smoke removal device, so that increase in the
concentration of nitrogen oxide in an outlet of the combustion
furnace can be suppressed.
Further, in the combustion system according to the
present invention, the smoke removal device includes: a
denitration unit for removing nitrogen oxide in the exhaust
gas discharged from the combustion furnace; a heat exchange
unit for conducting heat exchange between exhaust gas which
has passed the denitration unit and exhaust gas which is
introduced to the combustion oxygen supply port; a dust
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~emoval unit for removing dust in exhaust gas which has passed
the heat exchange unit; a desulfurization unit for removing
sulfur oxide in exhaust gas which has passed the dust removal
unit; and a cooling unit for cooling exhaust gas which has
passed the desulfurization unit, wherein part of exhaust gas
diverging from between the desulfurization unit and the
cooling unit is introduced to the combustion oxygen supply
port.
In the combustion system according to the present
invention, part of the exhaust gas diverging from between the
desulfurization unit and the cooling unit is introduced to the
combustion oxygen supply port of the combustion furnace. This
makes it possible to decrease the flow rate of nitrogen oxide
in the exhaust gas introduced to the denitration unit as well
as to decrease the flow rate of the exhaust gas introduced to
the cooling unit. Therefore, it becomes possible to downsize
the denitration unit and to reduce the capacity of the cooling
unit.
Further, in the combustion system according to the
present invention, part of exhaust gas diverging from between
the dust removal unit and the desulfurization unit is
introduced to the combustion oxygen supply port.
In the combustion system according to the present
invention, part of the exhaust gas diverging from between the
dust removal unit and the desulfurization unit is introduced
to the combustion oxygen supply port. This makes it possible
to decrease the flow rate of nitrogen oxide in the exhaust gas
introduced to the denitration unit as well as to decrease the
flow rate of the exhaust gas introduced to the desulfurization
unit and the cooling unit. Therefore, it becomes possible to
downsize the denitration unit and to reduce the capacity of
the desulfurization unit and the cooling unit.
Further, in the combustion system according to the
present invention, part of exhaust gas diverging from between
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~he denitration unit and the dust removal unit is introduced
to the combustion oxygen supply port.
In the combustion system according to the present
invention, part of the exhaust gas diverging from between the
denitration unit and the dust removal unit is introduced to
the combustion oxygen supply port. This makes it possible to
decrease the flow rate of nitrogen oxide in the exhaust gas
introduced to the denitration unit as well as to decrease the
flow rate of the exhaust gas introduced to the heat exchange
unit, the dust removal unit, the desulfurization unit and the
cooling unit. Therefore, it becomes possible to downsize the
denitration unit and to reduce the capacity of the heat
exchange unit, the dust removal unit, the desulfurization unit
and the cooling unit.
Further, in the combustion system according to the
present invention, the denitration unit includes: an ammonia
supply unit for supplying ammonia into exhaust gas; and a
catalyst unit for allowing exhaust gas supplied by the ammonia
supply unit to pass therethrough.
In the combustion system according to the present
invention, the exhaust gas having a decreased flow rate of
nitrogen oxide is introduced to the denitration unit.
Therefore, the amount of ammonia to be supplied can be reduced
as compared with the case where the exhaust gas whose flow
rate of nitrogen oxide is not decreased.
Advantageous Effects of Invention
In the present invention, part of exhaust gas diverging
from between the combustion furnace and the smoke removal
device is resupplied to the inside of the combustion furnace
through the burner unit. A zone under reducing atmosphere is
formed between the burner unit of the combustion furnace and
the combustion oxygen supply port. Consequently, the exhaust
gas diverging from between the combustion furnace and the
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~noke removal device can be reduced in the zone under reducing
atmosphere formed in the combustion furnace before being
discharged. This makes it possible to decrease the flow rate
of the exhaust gas introduced from the combustion furnace to
the smoke removal device and to decrease the flow rate of
smoke. Therefore, the capacity of the smoke removal device
can be reduced.
Moreover, part of exhaust gas diverging from the
downstream side of the smoke removal device is the exhaust gas
having the concentration of nitrogen oxide, which is smoke,
decreased by the smoke removal device. The exhaust gas having
a decreased nitrogen oxide concentration is introduced to the
combustion oxygen supply port and is used for promoting
complete combustion of the unburned fuel contained in the
exhaust gas which has partially been reduced by passing the
reducing atmosphere inside the combustion furnace. Therefore,
the exhaust gas can be discharged out of the combustion
furnace while the concentration of nitrogen oxide contained
therein is maintained low. The exhaust gas having a decreased
nitrogen oxide concentration is recirculated to the combustion
furnace and the smoke removal device, so that increase in the
concentration of nitrogen oxide in the outlet of the
combustion furnace can be suppressed.
BRIEF DESCRIPTION OF DRAWINGS
Fig. 1 is a schematic structure view of a combustion
system according to a first embodiment of the present
invention.
Fig. 2 is a schematic structure view
system according to a second embodiment
invention.
of a combustion
of the present
Fig. 3 is a
system according
invention.
schematic structure view of
to a third embodiment of
9
a combustion
the present
Fig. 4 is a
system according
invention.
schematic structure view
to a fourth embodiment
of
of
a combustion
the present
Fig. 5 is a schematic structure view of a conventional
oxygen combustion boiler system.
of a combustion
of the present
schematic structure view
to the first embodiment
DESCRIPTION OF EMBODIMENTS
First Embod~ent
Fig. 1 is a
system according
invention.
The combustion system 1 includes a coal fired boiler
(combustion furnace) 2, a coal pulverizer 3 for pulverizing
coal supplied to the coal fired boiler 2, and a smoke removal
device 9.
The coal fired boiler 2 is an oxygen combustion boiler
which can conduct denitration inside the furnace (not shown)
by two-stage combustion. The coal fired boiler 2 includes a
furnace inside for combusting fuel, a burner unit 2a, and an
additional air port (hereinafter referred to as "AA unit") 2b.
Coal as fuel supplied from the coal pulverizer 3, oxygen
(combustion oxygen) introduced from a combustion oxygen supply
system 21, and later-described secondary recirculation gas for
burner unit 22 are introduced to the burner unit 2a.
Remaining part of the oxygen introduced from the combustion
oxygen supply system 21 to the burner unit 2a and laterdescribed
secondary recirculation gas for AA unit 23 are
introduced to the AA unit (combustion oxygen supply port) 2b.
The coal pulverizer 3 is for pulverizing the coal, which
is supplied to the coal fired boiler 2, into fine powder of a
size of several pm to hundreds of pm. Part of exhaust gas
(hereinafter referred to as "primary recirculation gas") 24
discharged from the smoke removal device 9 is introduced to
the coal pulverizer 3 as high-temperature carrier gas for
10
the combustion oxygen supply
gas (hereinafter referred to as