TECHNICAL FIELD
The present invention relates to a boiler provided with a NOx remover that removes NOx in boiler exhaust gas generated in a furnace.

BACKGROUND ART
Known boilers provided with NOx removers  which remove NOx in boiler exhaust gas generated in a furnace  include  for example  the one disclosed in PTL 1.
Citation List
Patent Literature
PTL 1 Japanese Unexamined Patent Application  Publication No. 2009-166013

SUMMARY OF INVENTION
Technical Problem
However  when the boiler disclosed in PTL 1 is installed in  for example  an iron mill  and by-product gas (blast furnace gas  converter gas  and so forth) generated during the production process in the iron mill is introduced to a furnace of the boiler as fuel for mixed-fuel combustion  the amount of boiler exhaust gas generated in the furnace increases as the mixed-fuel burning ratio increases. Therefore  the rotational speed (output) of an induced-draft fan  which induces (sucks) the boiler exhaust gas and sends (discharges) it to a stack  needs to be increased as the mixed-fuel burning ratio increases; as a result  the draft pressure at the inlet of the induced-draft fan is reduced and falls below the lower operational limit of an electrostatic precipitator  posing the risk that operation (running) of the electrostatic precipitator is not possible.
In addition  there is also a problem in that the production costs are increased because the pressure-withstanding strengths of an air pre-heater  an electrostatic precipitator  a duct  and so forth arranged downstream of a NOx remover need to be increased so as to be able to withstand the drop in the draft pressure at the inlet of the induced-draft fan.
The present invention has been conceived in light of the above-described circumstances and provides a boiler in which  even when by-product gas generated in the production process in an iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio is increased  an electrostatic precipitator arranged downstream of a NOx remover can be operated  and an increase in production costs can be suppressed because the pressure-withstanding strengths of an air pre-heater  a duct  and so forth arranged downstream of the NOx remover do not need to be increased.

Solution to Problem
In order to solve the problems described above  the present invention employs the following solutions.
A first aspect of the present invention is a boiler that is provided with a furnace  an economizer  a NOx remover  and an air pre-heater  and that is configured to be capable of introducing by-product gas generated in a production process in an iron mill to the furnace as fuel for mixed-fuel combustion  including: a bypass duct that bypasses the NOx remover; a bypass damper that is provided at an intermediate position in the bypass duct and that opens/closes a flow channel in the bypass duct; and a control device that closes the bypass damper fully when a mixed-fuel burning ratio is lower than a prescribed value  and that opens the bypass damper fully when the mixed-fuel burning ratio is higher than a prescribed value.
According to the boiler of the first aspect of the present invention  when the mixed-fuel burning ratio is lower than a prescribed value  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct but is passed through the NOx remover  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is higher than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct with low channel (flow channel) resistance.
By doing so  even when the by-product gas generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan positioned downstream of the air pre-heater at a prescribed value or more; it is possible to operate the electrostatic precipitator arranged downstream of the air pre-heater; and it is possible to suppress an increase in production costs because the air pre-heater  the duct  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
A thermal power plant according to a second aspect of the present invention is a thermal power plant comprising: the above-described boiler; an electrostatic precipitator; an induced-draft fan; a steam turbine; a generator; and a stack.
According to the thermal power plant of the second aspect of the present invention  when the mixed-fuel burning ratio is lower than a prescribed value  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct but is passed through the NOx remover  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is higher than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct with low channel (flow channel) resistance.
By doing so  even when the by-product gas generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan positioned downstream of the air pre-heater at a prescribed value or more; it is possible to operate the electrostatic precipitator arranged downstream of the air pre-heater; and it is possible to suppress an increase in production costs because the air pre-heater  the electrostatic precipitator  the duct  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
A third aspect of the present invention is a method for running a boiler that is provided with a furnace  an economizer  a NOx remover  an air pre-heater  a bypass duct that bypasses the NOx remover  and a bypass damper that is provided at an intermediate position in the bypass duct and that opens/closes a flow channel in the bypass duct  and that is configured to be capable of introducing by-product gas generated in a production process in an iron mill to the furnace as fuel for mixed-fuel combustion  comprising closing the bypass damper fully when a mixed-fuel burning ratio is lower than a prescribed value  and opening the bypass damper fully when the mixed-fuel burning ratio is higher than a prescribed value.
According to the method for running a boiler of the third aspect of the present invention mentioned above  when the mixed-fuel burning ratio is lower than a prescribed value  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct but is passed through the NOx remover  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is higher than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct with low channel (flow channel) resistance.
By doing so  even when the by-product gas generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan positioned downstream of the air pre-heater at a prescribed value or more; it is possible to operate the electrostatic precipitator arranged downstream of the air pre-heater; and it is possible to suppress an increase in production costs because the air pre-heater  the duct  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
A fourth aspect of the present invention is a method for running a thermal power plant that is provided with a boiler including a furnace  an economizer  a NOx remover  an air pre-heater  a bypass duct that bypasses the NOx remover  and a bypass damper that is provided at an intermediate position in the bypass duct and that opens/closes a flow channel in the bypass duct  and which is configured to be capable of introducing by-product gas generated in a production process in an iron mill to the furnace as fuel for mixed-fuel combustion; an electrostatic precipitator; an induced-draft fan; a steam turbine; a generator; and a stack  the method comprising closing the bypass damper fully when a mixed-fuel burning ratio is lower than a prescribed value  and opening the bypass damper fully when the mixed-fuel burning ratio is higher than a prescribed value.
According to the method for running a thermal power plant of the fourth aspect of the present invention mentioned above  when the mixed-fuel burning ratio is lower than a prescribed value  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct but is passed through the NOx remover  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is higher than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct with low channel (flow channel) resistance.
By doing so  even when the by-product gas generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan positioned downstream of the air pre-heater at a prescribed value or more; it is possible to operate the electrostatic precipitator arranged downstream of the air pre-heater; and it is possible to suppress an increase in production costs because the air pre-heater  the electrostatic precipitator  the duct  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
Advantageous Effects of Invention
According to a boiler of the present invention  even when by-product gas generated in a production process in an iron mill is used as fuel for mixed-fuel combustion and when a mixed-fuel burning ratio is increased  an electrostatic precipitator arranged downstream of a NOx remover can be operated  and an increase in production costs can be suppressed because pressure-withstanding strengths of an air pre-heater  a duct  and so forth arranged downstream of the NOx remover do not need to be increased.
Brief Description of Drawings
FIG. 1 is a system diagram showing  in outline  a thermal power plant including a boiler according to one embodiment of the present invention.
FIG. 2 is a chart showing the relationship between an open/closed state of the bypass damper shown in Fig. 1  the draft pressure at the inlet of an induced-draft fan  and the mixed-fuel burning ratio.
FIG. 3 is a chart showing the relationship between an open/closed state of the bypass damper shown in Fig. 1  the NOx concentration  and the mixed-fuel burning ratio.

DESCRIPTION OF EMBODIMENTS
A boiler according to an embodiment of the present invention will be described below with reference to Fig. 1. Fig. 1 is a system diagram showing  in outline  a thermal power plant including a boiler according to this embodiment.
As shown in Fig. 1  a boiler 1 according to this embodiment is provided with a furnace 2  an economizer (economizer) 3  a NOx remover 4  and an air pre-heater (air pre-heater) 5. In addition  a thermal power plant 20 according to this embodiment is provided with the boiler 1  an electrostatic precipitator (electrostatic precipitator) 6  an induced-draft fan (induced draft fan) 7  a steam turbine (not shown)  a generator (not shown)  and a stack 8.
The NOx remover 4 is a device that mixes ammonia into the boiler exhaust gas containing NOx  which has been generated in the furnace 2 and sent from the economizer 3  and causes the mixture to pass through a catalyst layer  thereby decomposing the NOx into harmless nitrogen and water.
Reference numerals 9 and 10 in Fig. 1 are each a NOx meter that determines the NOx concentration in the exhaust gas.
In addition  the furnace 2  the economizer 3  the air pre-heater 5  the electrostatic precipitator 6  the induced-draft fan 7  the steam turbine  the generator  and the stack 8 are of the same type as those of known types  and descriptions thereof will be omitted.
The boiler 1 according to this embodiment is provided with a bypass duct 11 that bypasses (avoids) the NOx remover 4  and a bypass damper 12 that opens/closes the flow channel in the bypass duct 11 is provided at an intermediate position in the bypass duct 11. The bypass damper 12 is controlled by a control device 13  and the control device 13 is operated based on BFG (blast furnace gas) mixed-fuel burning ratio (see Figs. 2 and 3).
In addition  a duct 16 that guides  to the furnace 2  part of the boiler exhaust gas sent out from the economizer 3 is provided at an intermediate position in a duct 14  which guides  to the NOx remover 4  the boiler exhaust gas sent out from the economizer 3  at the downstream side of the NOx meter 9 and at the upstream side of a junction 15 to which the upstream end of the bypass duct 11 is connected. A GMF (gas mixing fan: gas mixing fan) (not shown) is provided at an intermediate position in the duct 16  and the boiler exhaust gas sent from the GMF is re-introduced into the furnace 2 together with combustion air.
As shown in Fig. 2  when the mixed-fuel burning ratio with BFG is increased by switching the combustion in the furnace 2 from single-fuel combustion with heavy oil (the mixed-fuel burning ratio with BFG equals 0%) to the mixed-fuel combustion with heavy oil and BFG (fuel gas generated in a blast furnace of an iron mill (blast furnace gas))  the bypass damper 12 is kept fully closed by the control device 13 until the draft pressure at the inlet of the induced-draft fan 7 reaches  for example  the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa (until the mixed-fuel burning ratio with BFG reaches 32% in this embodiment)  and the bypass damper 12 is fully opened by the control device 13 when the draft pressure reaches the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa.
On the other hand  when the combustion in the furnace 2 is switched from the mixed-fuel combustion with heavy oil and BFG to the single-fuel combustion with heavy oil  the bypass damper 12 is kept fully opened by the control device 13 until a draft pressure at the inlet of the induced-draft fan 7 equal to  for example  the lower operational limit of the electrostatic precipitator 6 plus 0.8 kPa (until the mixed-fuel burning ratio with BFG reaches 29% in this embodiment) can be ensured with the fully closed bypass damper 12 as the mixed-fuel burning ratio with BFG is lowered  and the bypass damper 12 is fully opened by the control device 13 when a draft pressure equal to the lower operational limit of the electrostatic precipitator 6 plus 0.8 kPa can be ensured with the fully closed bypass damper 12.
In addition  when the bypass damper 12 is fully closed  because all of the boiler exhaust gas sent out from the economizer 3 and reaching the junction 15 is passed through the NOx remover 4  the NOx remover 4 is operated (run) as shown in Fig. 3. In other words  ammonia is sprayed in the NOx remover 4.
On the other hand  when the bypass damper 12 is fully opened  because the mixed-fuel burning ratio with BFG is high and the NOx concentration is relatively low (the NOx concentration is about 35 ppm lower than the regulation value (for example  130 ppm))  as shown in Fig. 3  the running of the NOx remover 4 is stopped. In other words  the spraying of ammonia in the NOx remover 4 is stopped.
In the above  the mixed-fuel burning ratio with BFG is the proportion of the BFG in the fuel (heavy oil and BFG) introduced into the furnace 2 and is calculated on the basis of calorific value. In addition  the calorific value of the BFG is about 800 kcal/Nm3.
According to the boiler 1 of this embodiment  when the mixed-fuel burning ratio is lower than a prescribed value (for example  it is 32% when the single-fuel combustion with heavy oil is switched to the mixed-fuel combustion with heavy oil and BFG  thus increasing the mixed-fuel burning ratio with BFG  and it is 29% when the mixed-fuel combustion with heavy oil and BFG is switched to the single-fuel combustion with heavy oil  thus lowering the mixed-fuel burning ratio with BFG)  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct 11 but is passed through the NOx remover 4  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is greater than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct 11 with low channel (flow channel) resistance.
By doing so  even when the BFG generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan 7 positioned downstream of the air pre-heater 5 at a prescribed value (for example  the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa) or more; it is possible to operate the electrostatic precipitator 6 arranged downstream of the air pre-heater 5; and it is possible to suppress an increase in production costs because the air pre-heater 5  the duct 14  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
According to the thermal power plant 20 of the present invention of this embodiment  when the mixed-fuel burning ratio is lower than a prescribed value (for example  it is 32% when the single-fuel combustion with heavy oil is switched to the mixed-fuel combustion with heavy oil and BFG  thus increasing the mixed-fuel burning ratio with BFG  and it is 29% when the mixed-fuel combustion with heavy oil and BFG is switched to the single-fuel combustion with heavy oil  thus lowering the mixed-fuel burning ratio with BFG)  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct 11 but is passed through the NOx remover 4  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is greater than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct 11 with low channel (flow channel) resistance.
By doing so  even when the by-product gas (for example  BFG) generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan 7 positioned downstream of the air pre-heater 5 at a prescribed value (for example  the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa) or more; it is possible to operate the electrostatic precipitator 6 arranged downstream of the air pre-heater 5; and it is possible to suppress an increase in production costs because the air pre-heater 5  the electrostatic precipitator 6  the duct 14  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
In addition  according to the thermal power plant 20 of this embodiment  when the combustion in the furnace 2 is switched from the mixed-fuel combustion with heavy oil and BFG to the single-fuel combustion with heavy oil  thus lowering the mixed-fuel burning ratio with BFG  the draft pressure at the inlet of the induced-draft fan 7 is set such that a pressure equal to  for example  the lower operational limit of the electrostatic precipitator 6 plus 0.8 kPa can be ensured even if the bypass damper 12 is fully closed; in other words  the draft pressure is set such that the pressure is greater than a prescribed value (the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa) that is set for fully opening the fully closed bypass damper 12 when the single-fuel combustion with heavy oil is switched to the mixed-fuel combustion with heavy oil and BFG  thus increasing the mixed-fuel burning ratio with BFG.
By doing so  it is possible to reliably prevent the draft pressure at the inlet of the induced-draft fan 7 from falling below the lower operational limit of the electrostatic precipitator 6 and to improve the safety of the whole plant.
Furthermore  according to the thermal power plant 20 of this embodiment  by opening the bypass duct 11  the boiler exhaust gas is guided downstream through the bypass duct 11 with low channel (flow channel) resistance.
By doing so  the induced-draft fan 7 having the same level of output as that conventionally required can be employed  an increase in the size of the induced-draft fan 7 can be prevented  and an increase in the production costs can be suppressed.
According to the method for running the boiler 1 of this embodiment  when the mixed-fuel burning ratio is lower than a prescribed value (for example  it is 32% when the single-fuel combustion with heavy oil is switched to the mixed-fuel combustion with heavy oil and BFG  thus increasing the mixed-fuel burning ratio with BFG  and it is 29% when the mixed-fuel combustion with heavy oil and BFG is switched to the single-fuel combustion with heavy oil  thus lowering the mixed-fuel burning ratio with BFG)  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct 11 but is passed through the NOx remover 4  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is greater than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct 11 with low channel (flow channel) resistance.
By doing so  even when the BFG generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan 7 positioned downstream of the air pre-heater 5 at a prescribed value (for example  the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa) or more; it is possible to operate the electrostatic precipitator 6 arranged downstream of the air pre-heater 5; and it is possible to suppress an increase in production costs because the air pre-heater 5  the duct 14  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
According to the method for running the thermal power plant 20 of this embodiment  when the mixed-fuel burning ratio is lower than a prescribed value (for example  it is 32% when the single-fuel combustion with heavy oil is switched to the mixed-fuel combustion with heavy oil and BFG  thus increasing the mixed-fuel burning ratio with BFG  and it is 29% when the mixed-fuel combustion with heavy oil and BFG is switched to the single-fuel combustion with heavy oil  thus lowering the mixed-fuel burning ratio with BFG)  in other words  when the NOx concentration is high  the boiler exhaust gas is not passed through the bypass duct 11 but is passed through the NOx remover 4  where it is subjected to NOx removal  and when the mixed-fuel burning ratio is greater than a prescribed value  in other words  when the NOx concentration is low  the boiler exhaust gas is guided towards the downstream side through the bypass duct 11 with low channel (flow channel) resistance.
By doing so  even when the by-product gas (for example  BFG) generated in the production process in the iron mill is used as the fuel for mixed-fuel combustion  thus increasing the mixed-fuel burning ratio  it is possible to maintain the draft pressure at the inlet of the induced-draft fan 7 positioned downstream of the air pre-heater 5 at a prescribed value (for example  the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa) or more; it is possible to operate the electrostatic precipitator 6 arranged downstream of the air pre-heater 5; and it is possible to suppress an increase in production costs because the air pre-heater 5  the electrostatic precipitator 6  the duct 14  and so forth having the same level of pressure-withstanding strength as that conventionally required can be employed.
In addition  according to the method for running the thermal power plant 20 of this embodiment  when the combustion in the furnace 2 is switched from the mixed-fuel combustion with heavy oil and BFG to the single-fuel combustion with heavy oil  thus lowering the mixed-fuel burning ratio with BFG  the draft pressure at the inlet of the induced-draft fan 7 is set such that the pressure of  for example  the lower operational limit of the electrostatic precipitator 6 plus 0.8 kPa can be ensured even if the bypass damper 12 is fully closed; in other words  the draft pressure is set such that the pressure is greater than a prescribed value (the lower operational limit of the electrostatic precipitator 6 plus 0.5 kPa) that is set for fully opening the fully closed bypass damper 12 when the single-fuel combustion with heavy oil is switched to the mixed-fuel combustion with heavy oil and BFG  thus increasing the mixed-fuel burning ratio with BFG.
By doing so  it is possible to reliably prevent the draft pressure at the inlet of the induced-draft fan 7 from falling below the lower operational limit of the electrostatic precipitator 6 and to improve the safety of the whole plant.
Furthermore  according to the method for running the thermal power plant 20 of this embodiment  by opening the bypass duct 11  the boiler exhaust gas is guided downstream through the bypass duct 11 with low channel (flow channel) resistance.
By doing so  the induced-draft fan 7 having the same level of output as that conventionally required can be employed  an increase in the size of the induced-draft fan 7 can be prevented  and an increase in the production costs can be suppressed.
The present invention is not restricted to the embodiments described above. Various alterations and modifications are possible within a range that does not depart from the spirit of the present invention.
For example  in the embodiments mentioned above  the mixed-fuel combustion with heavy oil and BFG has been described as a specific example; however  the present invention is not limited thereto  and it is possible to use gaseous fuel formed by gasifying coal  natural gas  or coke oven gas (COG; coke oven gas) instead of heavy oil  and to use LDG (converter gas (fuel gas) generated in a steel converter in an iron mill) instead of the BFG. Note that the calorific value of the LDG is about 2000 kcal/Nm3.
In addition  it is more suitable to provide a damper (not shown) that closes the flow channel in the duct 14 when the bypass damper 12 is fully opened and that opens the flow channel in the duct 14 when the bypass damper 12 is fully closed on the duct 14 on the downstream side of the junction 15 at the upstream side of the NOx remover 4.
By doing so  when the running of the NOx remover 4 is stopped  the boiler exhaust gas is reliably prevented from flowing into the NOx remover 4  which makes it possible to prevent clogging of the catalyst layer with ash contained in the boiler exhaust gas.
Furthermore  the bypass damper 12 and a damper provided at an intermediate position in the duct 14 may be used not only at the fully opened and the fully closed positions  but also at an intermediate position between the fully opened and fully closed positions  as appropriate.

Reference Signs List
1 boiler
2 furnace
3 economizer
4 NOx remover
5 air pre-heater
6 electrostatic precipitator
7 induced-draft fan
8 stack
11 bypass duct
12 bypass damper
13 control device
20 thermal power plant

WE CLAIM:

1. A boiler that is provided with a furnace  an economizer  a NOx remover  and an air pre-heater  and that is configured to be capable of introducing by-product gas generated in a production process in an iron mill to the furnace as fuel for mixed-fuel combustion  comprising:
a bypass duct that bypasses the NOx remover;
a bypass damper that is provided at an intermediate position in the bypass duct and that opens/closes a flow channel in the bypass duct; and
a control device that closes the bypass damper fully when a mixed-fuel burning ratio is lower than a prescribed value  and that opens the bypass damper fully when the mixed-fuel burning ratio is higher than a prescribed value.

2. A thermal power plant comprising: a boiler according to Claim 1; an electrostatic precipitator; an induced-draft fan; a steam turbine; a generator; and a stack.

3. A method for running a boiler that is provided with a furnace  an economizer  a NOx remover  an air pre-heater  a bypass duct that bypasses the NOx remover  and a bypass damper that is provided at an intermediate position in the bypass duct and that opens/closes a flow channel in the bypass duct  and that is configured to be capable of introducing by-product gas generated in a production process in an iron mill to the furnace as fuel for mixed-fuel combustion  comprising
closing the bypass damper fully when a mixed-fuel burning ratio is lower than a prescribed value  and opening the bypass damper fully when the mixed-fuel burning ratio is higher than a prescribed value.

4. A method for running a thermal power plant that is provided with a boiler including a furnace  an economizer  a NOx remover  an air pre-heater  a bypass duct that bypasses the NOx remover  and a bypass damper that is provided at an intermediate position in the bypass duct and that opens/closes a flow channel in the bypass duct  and which is configured to be capable of introducing by-product gas generated in a production process in an iron mill to the furnace as fuel for mixed-fuel combustion; an electrostatic precipitator; an induced-draft fan; a steam turbine; a generator; and a stack  comprising
closing the bypass damper fully when a mixed-fuel burning ratio is lower than a prescribed value  and opening the bypass damper fully when the mixed-fuel burning ratio is higher than a prescribed value.

Dated this 28th day of November  2011

Aparna Kareer
Of Obhan & Associates
Agent for the Applicant
Patent Agent No. 1359