JP2015068597A - Exhaust gas boiler with denitrification device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent overshooting a target concentration in increasing a concentration of nitrogen oxide to the target concentration in an exhaust gas boiler with a denitrification device.SOLUTION: A reducing agent injection amount by a reducing agent injector is controlled to keep a concentration of nitrogen oxide by a nitrogen oxide measuring device to a set concentration. In increasing the set concentration from a second target concentration B to a first target concentration A, the set concentration is changed to be increased with a lapse of time so that the concentration reaches the first target concentration A by a set return time Y. For example, the set return time Y when the set concentration is stepwisely increased from the second target concentration B to the first target concentration A, is the time Z or more necessary for the nitrogen oxide concentration by the nitrogen oxide measuring device to exceed the first target concentration A first, and then return to the first target concentration A.

Description

  The present invention relates to an exhaust gas boiler equipped with a denitration device for reducing nitrogen oxide (NOx).

  Conventionally, as disclosed in Patent Document 1 below, a denitration device that reduces the nitrogen oxide concentration by injecting a nitrogen oxide reducing agent into exhaust gas from combustion equipment and passing it through a denitration catalyst is known. Yes. In this denitration device, the nitrogen oxide concentration of the exhaust gas after passing through the denitration catalyst is monitored by the nitrogen oxide measuring device, and the reducing agent injection amount is adjusted so as to maintain the nitrogen oxide concentration at the set concentration.

  It is preferable that the set density serving as the adjustment target value can be changed as desired. For example, depending on the local government, it is required to switch to an operation in which the nitrogen oxide concentration is further reduced by 40% from the normal time when the oxidant alarm is issued. Therefore, it is necessary to lower the set concentration when the oxidant alarm is issued. When the oxidant alarm is canceled, the set concentration needs to be restored.

JP 2007-330835 A (Claim 1, Claim 2)

  However, when switching in the direction of increasing the set concentration, simply switching to a step shape results in the nitrogen oxide concentration overshooting the target concentration.

  Therefore, the problem to be solved by the present invention is to provide an exhaust gas boiler with a denitration device that can suppress overshooting of the target concentration when the nitrogen oxide concentration is raised to the target concentration.

  The present invention has been made to solve the above problems, and the invention according to claim 1 is directed to a boiler using exhaust gas as a heat source, and a reducing agent injector for injecting a nitrogen oxide reducing agent into the exhaust gas. A denitration catalyst for reducing the nitrogen oxides in the exhaust gas through which the exhaust gas into which the reducing agent has been injected is passed; a nitrogen oxide measuring device for detecting the nitrogen oxide concentration of the exhaust gas after passing through the denitration catalyst; Control means for controlling the reducing agent injection amount by the reducing agent injector so as to maintain the nitrogen oxide concentration by the nitrogen oxide measuring device at a set concentration, and when the set concentration is increased, the set concentration is An exhaust gas boiler with a denitration device, which is changed so as to rise with time.

  According to the first aspect of the present invention, the reducing agent injection amount is adjusted so that the nitrogen oxide concentration measured by the nitrogen oxide measuring device is maintained at the set concentration. By changing the value so that the nitrogen oxide concentration increases, the nitrogen oxide concentration can be prevented from overshooting the target concentration.

  According to a second aspect of the present invention, when the set density is increased from the second target density to the first target density, the set density is changed so as to increase with time so that the first target density is reached in the set return time. When the set concentration is raised from the second target concentration to the first target concentration stepwise, the nitrogen oxide concentration measured by the nitrogen oxide measuring device is changed from the time point to the first target concentration. More than the time until it first returns to the first target concentration, or more than the time until the nitrogen oxide concentration by the nitrogen oxide measuring device falls within the range of the first target concentration ± tolerance. It is an exhaust gas boiler with a denitration device according to claim 1 characterized by things.

  According to the second aspect of the present invention, when the set density is raised from the second target density to the first target density, it can be controlled to reach the first target density in the set return time. In addition, when the set concentration is increased from the second target concentration to the first target concentration in steps, the set return time is the time from that point until the first target concentration overshoots and returns to the first target concentration. Alternatively, it is easy to suppress the nitrogen oxide concentration from overshooting the first target concentration by setting the time until stabilization at the first target concentration or longer.

According to a third aspect of the present invention, when the set density is raised from the second target density to the first target density, the change rate set by the following equation is used so as to reach the first target density in a set return time. The exhaust gas boiler with a denitration device according to claim 1 or 2, wherein the set concentration is changed.
Rate of change = (first target concentration-second target concentration) / setting return time

  According to the third aspect of the present invention, when the set density is increased from the second target density to the first target density, it can be controlled to reach the first target density within the set return time. In addition, the nitrogen oxide concentration can be prevented from overshooting the first target concentration by proportionally changing the set concentration.

  In the invention according to claim 4, the set concentration is switched to the second target concentration reduced by 40% from the first target concentration based on the issuance of the oxidant alarm, and based on the release of the oxidant alarm, the set concentration is The exhaust gas boiler with a denitration device according to claim 2 or 3, wherein the exhaust gas boiler is gradually changed so as to be returned from the second target concentration to the first target concentration over a set return time.

  According to invention of Claim 4, based on the issuing of an oxidant warning, it can switch to the driving | running which reduced the nitrogen oxide density | concentration further 40% from normal time, and it returns to a normal driving | operation based on cancellation | release of an oxidant warning Can do. In addition, in that case, the nitrogen oxide concentration can be prevented from overshooting the target concentration by gradually returning over a set return time.

  The invention according to claim 5 is the exhaust gas boiler with a denitration device according to any one of claims 1 to 4, wherein the rate of change is changed while the set concentration is being increased. .

  According to the fifth aspect of the present invention, the rate of change can be changed while the set concentration is being increased. Therefore, the nitrogen oxide concentration can be raised to the target concentration at a change rate according to the situation.

  Furthermore, the invention according to claim 6 determines the actual change rate of the nitrogen oxide concentration by the nitrogen oxide measuring device while raising the set concentration, and sets the set value with a gradient according to the change rate. The exhaust gas boiler with a denitration device according to claim 5, wherein the concentration is changed.

  According to the sixth aspect of the present invention, it is ensured that the actual nitrogen oxide concentration overshoots the target concentration by changing the set concentration with a gradient that matches the actual change rate of the nitrogen oxide concentration. Can be prevented.

  According to the present invention, when the nitrogen oxide concentration is raised to the target concentration in the exhaust gas boiler with a denitration device, overshooting of the target concentration can be suppressed.

It is the schematic which shows one Example of the exhaust gas boiler with a denitration apparatus of this invention. It is the schematic which shows the state at the time of switching a setting density | concentration from the 1st target density | concentration A to the 2nd target density | concentration B based on the announcement of an oxidant warning, and shows the relationship between reducing agent injection amount and nitrogen oxide density | concentration, and elapsed time. Show. It is a figure which shows an example of the test result of switching control from normal driving | operation to high denitration driving | operation. It is the schematic which shows the state at the time of switching a setting density | concentration from the 2nd target density | concentration B to the 1st target density | concentration A based on cancellation | release of an oxidant warning, and has shown the relationship between target density | concentration and NOx density | concentration, and elapsed time. It is a figure which shows an example of the test result of the switching control from high denitration operation to normal operation.

Hereinafter, specific embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a schematic view showing an embodiment of an exhaust gas boiler 1 with a denitration apparatus of the present invention.

  The exhaust gas boiler 1 of the present embodiment includes a boiler 2, a reducing agent injector 3, and a denitration catalyst 4, and further includes an economizer 5 as desired. The reducing agent injector 3 and the denitration catalyst 4 constitute a denitration device.

  The reducing agent injector 3 injects a nitrogen oxide reducing agent into the exhaust gas upstream of the denitration catalyst 4. On the other hand, the economizer 5 is provided downstream of the exhaust gas flow from the boiler 2 and preheats the water supplied to the boiler 2.

  The arrangement relationship between the “reducing agent injector 3 and the denitration catalyst 4” and the “boiler 2 and the economizer 5” is not particularly limited, but in the illustrated example, the boiler 2, the denitration catalyst 4 and the exhaust gas passage 6 from the combustion equipment are connected to the exhaust gas passage 6. The economizer 5 is installed in order, and the exhaust gas from the combustion equipment is passed through the boiler 2, the denitration catalyst 4 and the economizer 5 in order. Then, a reducing agent injector 3 installed upstream of the denitration catalyst 4 (upstream of the boiler 2 in the illustrated example) injects a nitrogen oxide reducing agent into the exhaust gas. In addition, as exhaust gas from a combustion apparatus, the exhaust gas from the gas engine of a generator can be used, for example.

  The boiler 2 is a boiler that uses exhaust gas as a heat source. More specifically, in this embodiment, it is a steam boiler that heats feed water and vaporizes it. The steam from the boiler 2 is sent to various steam-using facilities.

The reducing agent injector 3 injects a nitrogen oxide reducing agent into the exhaust gas passage 6 (in other words, in the exhaust gas). In this embodiment, the reducing agent injector 3 includes a pump (not shown) and a spray nozzle 7, and controls the pump to adjust the injection amount (flow rate) of the reducing agent into the exhaust gas passage 6. Can do. In addition, various conventionally well-known things can be used as a reducing agent, For example, although ammonia water may be used, urea water is used in a present Example. The urea water sprayed in the exhaust gas is decomposed by the heat of the exhaust gas and becomes ammonia (NH ₃ ).

  The denitration catalyst 4 is passed through the exhaust gas into which the reducing agent has been injected by the reducing agent injector 3 to reduce the nitrogen oxides in the exhaust gas. In this embodiment, when the exhaust gas mixed with ammonia passes through the denitration catalyst 4, the denitration catalyst 4 promotes the reduction reaction of nitrogen oxides by ammonia, and the nitrogen oxides are decomposed into nitrogen and water. Reduction of nitrogen oxides in exhaust gas is achieved.

  The economizer 5 preheats the water supplied to the boiler 2 using the residual heat of the exhaust gas after passing through the boiler 2. However, installation of the economizer 5 can be omitted depending on circumstances.

  The exhaust gas passage 6 is provided with a nitrogen oxide measuring device 8 for detecting the nitrogen oxide concentration in the exhaust gas downstream of the exhaust gas flow from the denitration catalyst 4 (in the illustrated example, downstream of the exhaust gas flow from the economizer 5). This nitrogen oxide measuring device 8 can monitor the nitrogen oxide concentration after denitration.

  The exhaust gas boiler 1 further includes control means (not shown). This control means is a controller connected to the reducing agent injector 3 and the nitrogen oxide measuring device 8. The controller controls the reducing agent injector 3 (more specifically, the pump) so as to maintain the nitrogen oxide concentration by the nitrogen oxide measuring device 8 at a set concentration, and the reducing agent injector 3 performs the reduction. Control the dose of agent.

  The set concentration, which is the target value of the nitrogen oxide concentration, can be changed. Here, the case where the set concentration is changed by issuing or canceling the oxidant alarm will be described, but the present invention is also applicable when the set concentration is changed at a desired timing regardless of whether the oxidant alarm is started or stopped. The same applies.

  In normal times when the oxidant alarm is not issued, the set concentration is set to the first target concentration (for example, 150 ppm). When the oxidant alarm is issued, the operation is switched to the operation in which the nitrogen oxide concentration is further reduced by a predetermined rate (40% here) from the normal time. That is, when the oxidant alarm is issued, the set concentration is switched to the second target concentration (for example, 90 ppm) reduced by 40% from the first target concentration. Thereafter, when the oxidant alarm is canceled, the set concentration is returned from the second target concentration to the first target concentration.

  The controller receives the set density switching signal. For example, a changeover switch between a first target concentration and a second target concentration is provided, and is switched by this changeover switch based on the on / off of an oxidant alarm. Or the information regarding an oxidant alarm and the cancellation | release may be received and the said setting density | concentration may be switched based on it. Each target density may be changeable by a setting device.

  FIG. 2 is a schematic diagram showing a state when the set concentration is switched from the first target concentration A to the second target concentration B based on the issuance of the oxidant alarm. The reducing agent injection amount, the nitrogen oxide concentration, and the elapsed time Shows the relationship. That is, in the figure, the injection amount indicates the injection amount of the reducing agent by the reducing agent injector 3, and the NOx concentration indicates the nitrogen oxide concentration detected by the nitrogen oxide measuring device 8.

  Now, at time t1, the set density is switched from the first target density A to the second target density B. As described above, the second target concentration B is lower than the first target concentration A. The theoretical injection amount required for the second target concentration B is already known. That is, the theoretical injection amount required for the second target concentration B is known in advance theoretically or experimentally.

  Suppose that the reducing agent injection amount is merely increased from the injection amount C required for the first target concentration A to the theoretical injection amount D required for the second target concentration B, as indicated by a two-dot chain line. After that, it is assumed that the reducing agent injection amount is controlled so that the nitrogen oxide concentration by the nitrogen oxide measuring device 8 is maintained at the second target concentration B. In this case, it takes a relatively long time until the nitrogen oxide concentration falls to the second target concentration B.

  Therefore, in this embodiment, as shown by the solid line, a reducing agent that is larger than the theoretical injection amount D required for the second target concentration B is excessively injected for a set time X, and then nitrogen is measured by the nitrogen oxide measuring device 8. The reducing agent injection amount is controlled so as to maintain the oxide concentration at the second target concentration B.

  Excess injection is an injection amount E that is twice or more the theoretical injection amount D required for the second target concentration B, and an allowable limit value (for example, 8 ppm) of leaked ammonia (reducing agent leakage downstream from the denitration catalyst 4). It is better to perform the set time that does not exceed. By injecting the reducing agent with an injection amount E that is twice or more the theoretical injection amount D required for the second target concentration B, the nitrogen oxide concentration can be quickly lowered to the second target concentration B. Moreover, leak ammonia outside the exhaust gas boiler 1 can be suppressed by performing such excessive injection for a set time during which the leak ammonia concentration does not exceed the allowable limit value.

  For example, the excessive injection is performed at an injection amount that is two to three times the theoretical injection amount D required for the second target concentration B, and in a time that is half or less of the target time for reaching the second target concentration B. This is because if the excessive injection amount is too large, leaked ammonia is generated outside the exhaust gas boiler 1, while if the excessive injection amount is too small, it takes time to lower the nitrogen oxide concentration. Although it is preferable that the target arrival time is short, the target is actually within 10 minutes. Then, by performing excess injection in a time that is half or less of the target arrival time, the nitrogen oxide concentration by the nitrogen oxide measuring device 8 is set to the second target concentration B at least in the second half of the target arrival time to the second target concentration B. Therefore, the amount of the reducing agent injected is controlled so as to maintain the nitrogen oxide concentration, so that the nitrogen oxide concentration can be reliably lowered to the second target concentration B within the target time.

  FIG. 3 is a diagram illustrating an example of a test result of switching control from normal operation to high denitration operation. That is, the test results for reducing the nitrogen oxide concentration from 150 ppm to 90 ppm are shown. In the figure, the alternate long and short dash line indicates a one-minute excess spray, the two-dot chain line indicates a two-minute excessive spray, the broken line and the solid line indicate a three-minute excessive spray, the broken line is the first test, and the solid line is the second test The results are shown. In the figure, each curve in the upper half indicates the nitrogen oxide concentration (left vertical axis), and each curve in the lower half indicates the ammonia concentration (right vertical axis).

The excessive spray amount was fixed at 360 spm (the injection amount was 46 ml / min, three times the theoretical injection amount) of the pump of the reducing agent injector 3. And the overspray time was changed and the proper time was grasped. After excessive spraying, PID control was performed using the outlet NOx value (nitrogen oxide concentration by the nitrogen oxide measuring device 8) as a 90 ppm target value. The test was started when the NOx value was stabilized at 150 ppm (time 0). The NOx value is an O ₂ = 0% conversion value.

As can be seen from the figure, a change was observed in the measuring instrument about one minute after switching to the high denitration operation. All the test results drew a similar curve, and the outlet NOx value decreased. However, a difference appeared in the lower limit due to the difference in the overspray time. In the case of 1 minute excess spraying, 90 ppm was not reached, and in the case of 2 minute excess spraying, once it reached 90 ppm, it rose again. In the case of overspraying for 3 minutes, 90 ppm was reached in 4 to 6 minutes, and the denitration rate increased from 54% to 74%. The difference in the arrival time of 90 ppm between the first and second three-minute sprays is thought to be due to the increase in the denitration catalyst inlet temperature promoting the reaction. The leak NH ₃ took a value of 7.8 ppm at the maximum when sprayed for 3 minutes, and stabilized at 5 to 6 ppm. In addition, it is thought that the value of NH ₃ is higher in the case of 1-minute overspraying than in the case of 3-minute overspraying because calibration was performed during that time.

  From this test result, it can be seen that it is better to overspray for 3 minutes with an injection amount three times the theoretical injection amount required for the second target concentration B. Thereby, it is possible to switch from the first target concentration 150 ppm to the second target concentration 90 ppm within 10 minutes. This type of test sets the overfill volume and spray time.

  FIG. 4 is a schematic diagram showing a state when the set concentration is switched from the second target concentration B to the first target concentration A based on the cancellation of the oxidant alarm, and shows the relationship between the target concentration and NOx concentration and the elapsed time. Show. That is, in the figure, the target concentration indicates the set concentration as the control target concentration by the controller, and the NOx concentration indicates the actual nitrogen oxide concentration detected by the nitrogen oxide measuring device 8.

  Now, at time t2, the set density is switched from the second target density B to the first target density A. However, as will be described later, in the present invention, it can be said that, at the time t2, the set density is not changed from the second target density B to the first target density A all at once, but strictly, the set density is changed. As described above, the first target concentration A is higher than the second target concentration B.

  As shown by a two-dot chain line, if the target concentration is switched from the second target concentration B to the first target concentration A stepwise, the nitrogen oxide concentration will overshoot.

  Therefore, in this embodiment, as shown by the solid line, when the set density is increased, the set density is changed with time. More specifically, when the set density is increased from the second target density B to the first target density A, the set density is changed so as to increase with time so as to reach the first target density A in the set return time Y. To do.

  Here, when the set concentration is raised in steps from the second target concentration B to the first target concentration A, the nitrogen oxide concentration by the nitrogen oxide measuring device 8 is the first target when the set concentration is raised from the second target concentration B to the first target concentration A. More than the time Z until the concentration A first exceeds the first target concentration A or until the nitrogen oxide concentration by the nitrogen oxide measuring device 8 falls within the range of the first target concentration A ± allowable value. Set over time. In other words, when the set concentration is raised from the second target concentration B to the first target concentration A in a stepped manner, the set return time Y starts from the instant when the set concentration is raised to the first target concentration A. The time Z until the concentration overshoots the first target concentration A and returns to the first target concentration A, or the nitrogen oxide concentration by the nitrogen oxide measuring device 8 is stable at the first target concentration A after a further time has passed. It is set to the time until or longer than these.

  In any case, if the set density is stepped up from the second target density B to the first target density A, the time Z until the first target density A is reached after overshooting or the first By securing a minimum time until stabilization at the target concentration A and gradually changing the set concentration, it becomes easy to suppress the nitrogen oxide concentration from overshooting the first target concentration A. In other words, the overshoot can be suppressed by increasing the set concentration over the time Z until the overshoot (maximum overshoot) at the time of the step response is completed or more than the time until the first set concentration A is stabilized. Becomes easy.

  By the way, when the set density is increased from the second target density B to the first target density A, the set density is set at a change rate set by the following equation so that the first target density A is reached in the set return time Y. It may be changed. As a result, the set density is gradually changed from the second target density B to the first target density A over the set return time Y. Since the setting return time Y is not too different from the step-like control if it is too short, it is set from the characteristics at the time of the step response described above, for example, 60 minutes or longer (90 to 150 minutes).

  Rate of change = (first target density A−second target density B) / setting return time Y

  Further, the rate of change may be changed while the set concentration is being increased. In this case, the nitrogen oxide concentration can be raised to the target concentration at a change rate according to the situation.

  Further, while the set concentration is being increased (set timing such as a predetermined cycle), the nitrogen oxide measuring device 8 obtains the actual change rate of the nitrogen oxide concentration, and the set concentration is set to a gradient according to the change rate. May be changed. By changing the set concentration with a gradient that matches the actual change rate of the nitrogen oxide concentration, it is possible to reliably prevent the actual nitrogen oxide concentration from overshooting the target concentration.

  FIG. 5 is a diagram illustrating an example of a test result of switching control from high denitration operation to normal operation. That is, the test results for increasing the nitrogen oxide concentration from 90 ppm to 150 ppm are shown. In the figure, the alternate long and short dash line and the broken line indicate the case where the set concentration is switched in a step shape, and the alternate long and two short dashes line and the solid line indicate the case where the set concentration is switched with a proportional time gradient according to the above theory and formula. ing. Note that the alternate long and short dash line indicates the NOx target value, and the broken and solid lines indicate the actual measured NOx value.

  As indicated by the alternate long and short dash line, when the NOx target value was switched in steps from 90 ppm to 150 ppm, it reached 150 ppm in about 13 minutes as shown by the broken line, but overshooted to 175 ppm. This is presumably because PID control of the number of strokes of the pump of the reducing agent injector 3 reacted excessively due to a large deviation between the NOx value and the target value. After that, it took about 1 hour to stabilize at 150 ppm by adjustment by PID control.

  On the other hand, as a measure against overshoot, the NOx target value was switched from 90 ppm to 150 ppm with a time gradient of 60 ppm / 120 min as indicated by a two-dot chain line. As a result, as indicated by the solid line, the NOx value does not greatly exceed 150 ppm because the deviation between the NOx value and the target value has decreased. For switching from high denitration operation to normal operation, that is, for controlling the NOx concentration to increase, even if the arrival time up to 150 ppm is somewhat longer, only the time on the high denitration side is longer, There are no environmental problems.

  From this test result, it can be seen that when returning from the second target concentration B to the first target concentration A, overshoot can be prevented by switching the target NOx value with a time gradient of 60 ppm / 120 min. This type of test sets the rate of change of the target NOx value.

  According to the exhaust gas boiler 1 with the denitration device of the present embodiment, when the nitrogen oxide concentration is lowered from the first target concentration A to the second target concentration B, the reduction is larger than the theoretical injection amount required for the second target concentration B. By injecting the agent, the nitrogen oxide concentration can be rapidly lowered to the second target concentration B. Moreover, leak ammonia can be suppressed by performing such excessive injection only for the set time X.

  On the other hand, when the nitrogen oxide concentration is increased from the second target concentration B to the first target concentration A, the set concentration is gradually increased so as to reach the first target concentration A in the set return time Y. It is possible to suppress the concentration from overshooting the first target concentration A.

  The exhaust gas boiler 1 with a denitration device of the present invention is not limited to the configuration of the above embodiment, and can be changed as appropriate. In particular, when the set concentration is increased in the exhaust gas boiler 1 with a denitration device that controls the reducing agent injection amount by the reducing agent injector 3 so as to maintain the nitrogen oxide concentration by the nitrogen oxide measuring device 8 at the set concentration, If the concentration is changed so as to increase with time, other configurations can be appropriately changed.

DESCRIPTION OF SYMBOLS 1 Exhaust gas boiler with a denitration apparatus 2 Boiler 3 Reductant injector 4 Denitration catalyst 5 Economizer 6 Exhaust gas path 7 Spray nozzle 8 Nitrogen oxide measuring device t1 Timing to switch the set concentration from the first target concentration A to the second target concentration B t2 setting Timing for switching the concentration from the second target concentration B to the first target concentration A A First target concentration B Second target concentration C Injection amount of the first target concentration D Injection amount of the second target concentration E Excess injection amount X Setting time ( Excess injection time)
Y Set return time Z When the set concentration is raised in steps, the time from when the nitrogen oxide concentration by the nitrogen oxide measuring device first exceeds the first target concentration to return to the first target concentration

Claims (6)

A boiler using exhaust gas as a heat source;
A reducing agent injector for injecting a nitrogen oxide reducing agent into the exhaust gas;
A denitration catalyst for reducing the nitrogen oxides in the exhaust gas through which the exhaust gas into which the reducing agent has been injected is passed;
A nitrogen oxide measuring device for detecting the nitrogen oxide concentration of the exhaust gas after passing through the denitration catalyst,
Control means for controlling the reducing agent injection amount by the reducing agent injector so as to maintain the nitrogen oxide concentration by the nitrogen oxide measuring device at a set concentration;
An exhaust gas boiler with a denitration device, wherein when the set concentration is increased, the set concentration is changed over time. When the set concentration is increased from the second target concentration to the first target concentration, the set concentration is changed so as to increase with time so as to reach the first target concentration with a set return time.
When the set concentration is raised in steps from the second target concentration to the first target concentration, the nitrogen oxide concentration measured by the nitrogen oxide measuring device starts with the first target concentration. More than the time until it returns to the first target concentration, or more than the time until the nitrogen oxide concentration by the nitrogen oxide measuring device falls within the range of the first target concentration ± allowable value. The exhaust gas boiler with a denitration device according to claim 1. When the set density is increased from the second target density to the first target density, the set density is changed at a change rate set by the following equation so that the first target density is reached with a set return time. An exhaust gas boiler with a denitration device according to claim 1 or 2.
Rate of change = (first target concentration-second target concentration) / setting return time Based on the announcement of the oxidant warning, the set concentration is switched to the second target concentration reduced by 40% from the first target concentration,
The said set density | concentration is gradually changed so that a setting return time may be returned from a 2nd target density | concentration over time based on cancellation | release of an oxidant warning. Exhaust gas boiler with denitration equipment. The exhaust gas boiler with a denitration device according to any one of claims 1 to 4, wherein the rate of change is changed while the set concentration is being increased. In the middle of raising the set concentration, an actual change rate of the nitrogen oxide concentration is obtained by the nitrogen oxide measuring device, and the set concentration is changed with a gradient according to the change rate. 5. An exhaust gas boiler with a denitration device according to 5.

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