JPH0938454A - Method and apparatus for supplying oxidizing air to flue gas desulfurization apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deactivation of the nitrifying bacteria of a waste water treating device and to obviate the deterioration of the treating device. SOLUTION: An absorbing solution 2 is brought into contact with a waste gas G to absorb and remove the SOx in the waste gas G in an absorption tower 5, and oxidizing air A is blown into the solution 2 after contact with the waste gas G from an oxidizing air feed passage 26 through a controller 42. Gypsum 19 is recovered from the solution 2 after contact with the waste gas, and the nitrogen compd. contained in the solution 2 is decomposed in a waste water treating device 24 furnished to the flue gas desulfurizer. The oxidation-reduction potential of the absorbent 2 is detected, and, when the detected potential 46 exceeds a set value, the supply of the oxidizing air A is reduced by the controller 42.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for supplying oxidizing air to a flue gas desulfurization apparatus.

[0002]

2. Description of the Related Art Conventionally, lime CaO (lime) has been used as an absorbent.
Flue gas desulfurization equipment using stone, slaked lime or quick lime is generally
As shown in FIG. 4, the liquid pool portion 1 formed in the lower portion
The absorption liquid 2 of the above is disposed on the upper part by the operation of the circulation pump 3.
When sprayed from the spray nozzle 4 and circulated,
Is externally supplied via the exhaust gas introduction duct 5a.
Absorption of the exhaust gas G sprayed from the spray nozzle 4
After contact with the liquid 2, through the exhaust gas discharge duct 5b
The absorption tower 5 for discharging is provided, and the liquid in the absorption tower 5
In the reservoir 1, lime CaO of the absorbing liquid 2 and sulfur in the exhaust gas G
Reactant CaSO with S _ThreeTo oxidize CaSO_FourStone
Oxidation with a compressor 6 supplying oxidizing air A for plaster oxidation
The air supply flow path 26 is connected. Furthermore, in the absorption tower 5,
A stirrer 7 for stirring the absorbing liquid 2 in the liquid reservoir 1 is provided.
You.

Further, an absorbent 23 supplied from a mother liquor tank 25 described later and lime 9 supplied from a silo 8 are kneaded to produce an absorbent slurry 10, and the absorbent slurry 10 is mixed with the absorbent tower 5. An absorbent slurry pit 11 for supplying the liquid to the liquid reservoir 1 is provided.

Part of the absorbent 2 in the liquid reservoir 1 of the absorption tower 5 is supplied, and part of the neutralizer 12 such as caustic soda supplied to the liquid reservoir 1 of the absorption tower 5 is supplied. A neutralization tank 13 for mixing and stirring the absorbing liquid 2 and the neutralizing agent 12 is provided, and a thickener 15 for concentrating the absorbing liquid 14 extracted in the neutralizing tank 13 is provided.
A dehydrator supply tank 17 for supplying the absorbent 16 concentrated in 5 and stirring the absorbent 16 is provided, and the absorbent 16 extracted from the dehydrator supply tank 17 is dehydrated to produce gypsum 19. A machine 20 is provided.

On the other hand, a filtrate pit 22 for supplying the water 21 dehydrated by the dehydrator 20 to supply a part of the water 21 to the thickener 15 is further provided, and the thickener 15 is further provided.
The supernatant absorption liquid 23 is supplied to the mother liquor tank 25, a part of the absorption liquid 23 of the mother liquor tank 25 is supplied to the liquid reservoir 1 of the absorption tower 5 and the absorbent slurry pit 11, and the rest is supplied. It has a configuration in which it is sent to the wastewater treatment device 24. In addition, reference numeral 18 in FIG. 4 denotes makeup water that is appropriately supplemented to the absorption tower 5.

Further, an inlet SOx detector 27 for detecting the concentration of sulfur oxides is installed in the exhaust gas introducing duct 5a, and an exhaust gas flow meter 28 for measuring the flow rate of the exhaust gas G.
And an outlet S is installed in the exhaust gas discharge duct 5b.
An Ox detector 29 is installed, and the SOx detection values 30 and 31 of the inlet SOx detector 27 and the outlet SOx detector 29 and the exhaust gas flow rate detection value 32 from the exhaust gas flow meter 28 are input to output a desulfurization load signal 33. A desulfurization load calculator 34 for calculating and outputting is installed, and a desulfurization load signal 33 from the desulfurization load calculator 34 is input to generate a function corresponding to a load-corresponding air flow rate command signal 35 corresponding to the desulfurization load signal 33. The vessel 36 is installed. As shown in FIG. 5, the function generator 36 outputs a load-corresponding air flow rate command signal 35 that is proportional to the desulfurization load signal 33, for example.

Further, the load-corresponding air flow rate command signal 35
And a subtraction unit 40 for inputting and subtracting the oxidation air flow rate detection value 38 from the oxidation air flow rate detector 37 installed in the oxidation air supply flow path 26 and outputting a difference signal 39. The flow rate of the oxidizing air A supplied to the liquid reservoir 1 is adjusted by adjusting the controller 42 that adjusts the opening degree of the inlet vane 41 of the compressor 6 so that the difference signal 39 from the container 40 becomes zero. ,
A PI adjuster 43 is provided to control so as to match the load-corresponding air flow rate command signal 35.

In the case of the flue gas desulfurization apparatus as described above, the absorbing liquid 2
Is circulated by the operation of the circulation pump 3, and the exhaust gas G sent from the exhaust gas introduction duct 5a to the absorption tower 5 comes into contact with the absorbing liquid 2 sprayed from the spray nozzle 4 to absorb and remove sulfur oxides. After being discharged, it is discharged to the outside by the exhaust gas discharge duct 5b.

On the other hand, a part of the absorption liquid 2 which has absorbed the sulfur oxides from the exhaust gas G is supplied to the neutralization tank 13 from the bottom of the liquid pool portion 1 of the absorption tower 5 and neutralized in the neutralization tank 13. The agent 12 is mixed and stirred, and the mixed and stirred absorption liquid 14 is sent to the thickener 15, concentrated in the thickener 15, and the concentrated absorption liquid 16 is sent to the dehydrator 20 via the dehydrator supply tank 17. The water is removed in the dehydrator 20 to produce the gypsum 19.

The water 21 dehydrated by the dehydrator 20 is returned to the thickener 15 through the filtrate pit 22, and the supernatant absorbing liquid 23 that appears when the absorbing liquid 14 is concentrated in the thickener 15 is a mother liquor tank. It is supplied to the liquid reservoir 1 and the absorbent slurry pit 11 of the absorption tower 5 via 25 and is sent to the waste water treatment device 24.

The absorbent 23 supplied to the absorbent slurry pit 11 is kneaded with the lime 9 supplied from the silo 8 in the absorbent slurry pit 11 to form an absorbent slurry 10 in the liquid reservoir of the absorption tower 5. 1 is supplied.

Absorbing liquid 23 sent to wastewater treatment device 24
Removes nitrogen compounds in the absorption liquid 23, recovers the chemical oxygen demand COD, and performs other treatments. The nitrogen compounds are removed by decomposing using nitrifying bacteria by anaerobic bacteria. The treated wastewater is discharged to the outside.

Further, the desulfurization load calculator 34 which inputs the respective SOx detection values 30 and 31 of the inlet SOx detector 27 and the outlet SOx detector 29 and the exhaust gas flow rate detected value 32 from the exhaust gas flow meter 28 is desulfurized. Calculate the load signal 33,
The desulfurization load signal 33 is output to the function generator 36 and converted into a load-corresponding air flow rate command signal 35. The load-corresponding air flow rate command signal 35 is input to the subtractor 40, and
The subtraction device 40 is subtracted by inputting the oxidation air flow rate detection value 38 from the oxidation air flow rate detector 37 installed in the oxidation air supply flow path 26, and a subtraction signal 39 from the subtraction device 40 is obtained. The PI controller 43 adjusts the controller 42 so that it becomes zero.
Is adjusted so that the flow rate of the oxidizing air A supplied to the liquid reservoir 1 is controlled to match the load-corresponding air flow rate command signal 35.

[0014]

However, in the flue gas desulfurization apparatus as described above, the flow rate of the oxidizing air A supplied from the compressor 6 to the absorbing liquid 2 of the liquid pool portion 1 is simply based on the desulfurization load signal 33. Since it is controlled by the load-corresponding air flow rate command signal 35, the amount of the oxidizing air A supplied may be too large due to a detection error of the SOx detectors 27 and 29, a change in the property of the exhaust gas G, or the like. However, in such a case, the chloride (HCl
Etc.) is changed to a substance such as chlorine (Cl ₂ ) and the absorption liquid 2
The concentration of oxidizing substances such as residual chlorine contained in the
The redox potential of the absorption liquid 2 is increased, and the activity of nitrifying bacteria (bacteria) in the wastewater treatment device 24 is decreased,
Alternatively, there is a problem that the nitrifying bacteria are killed and the function of the wastewater treatment device 24 is significantly reduced.

In view of the above situation, the present invention provides a method and an apparatus for supplying oxidant air to a flue gas desulfurization device, which can prevent the activity of nitrifying bacteria in the wastewater treatment device from decreasing and the function of the wastewater processing device from decreasing. Is what you are trying to do.

[0016]

The method for supplying oxidizing air to a flue gas desulfurization apparatus according to the present invention absorbs sulfur oxides in the exhaust gas G by bringing the absorbent 2 using lime as an absorbent and the exhaust gas G into contact with each other. The absorption tower 5 provided with the oxidizing air supply flow path 26 for blowing the oxidizing air A through the controller 42 to the absorbent 2 brought into contact with the exhaust gas G to be removed is provided, and from the absorbent 2 brought into contact with the exhaust gas G Absorbing liquid 2 after collecting gypsum 19
Wastewater treatment equipment 2 for decomposing nitrogen compounds contained in
A method for supplying oxidative air to a flue gas desulfurization apparatus provided with 4,
It is characterized in that the redox potential of the absorbing liquid 2 is detected, and the controller 42 is controlled so as to decrease the supply amount of the oxidizing air A when the redox potential detection value 46 increases above a set value 47.

The oxidizing air supply apparatus for the flue gas desulfurization apparatus of the present invention comprises the exhaust gas G for absorbing and removing the sulfur oxides in the exhaust gas G by bringing the absorbent 2 using lime as an absorbent and the exhaust gas G into contact with each other. After recovering the gypsum 19 from the absorption tower 5 provided with the oxidizing air supply flow path 26 for injecting the oxidizing air A into the contacted absorbent 2 through the controller 42, and the absorbent 2 contacted with the exhaust gas G An oxidizing air supply device for a flue gas desulfurization device, which comprises a wastewater treatment device 24 for decomposing nitrogen compounds contained in the absorbing liquid 2, and outputs a load-corresponding air flow rate command signal 35 based on a desulfurization load signal 33. Function generator 36, redox potential detector 45 for detecting the redox potential of absorbing liquid 2, redox potential detection value 46 and set value 47 of redox potential detector 45, and subtraction The difference signal 48 A subtracter 49 for force, the oxidation-reduction potential detected value 46 based on the signal 48 of the difference is a set value 4
A PI controller 56 that outputs an air flow rate command signal 50 corresponding to a redox potential that reduces the flow rate of the oxidizing air A when it is greater than 7.
A low signal selector 53 for inputting the load-corresponding air flow rate command signal 35 and the oxidation-reduction potential-corresponding air flow rate command signal 50 to select and output the smaller signal, and selection from the low signal selector 53 A subtracter 55 for inputting and subtracting the signal 52 and the oxidizing air flow rate detection value 38 from the oxidizing air flow rate detector 37 provided in the oxidizing air supply flow path 26 and outputting a difference signal 54, and the subtractor 55. And a PI controller 56 that adjusts the controller 42 so that the difference signal 54 from the calculator 55 becomes zero.

Further, the PI controller 51 is provided with a shake-off prevention circuit 58 for limiting the upper limit of the air flow rate command signal 50 corresponding to the oxidation-reduction potential by a value obtained by adding a predetermined addition value 57 to the selection signal 52. It has a feature.

Here, the controller 42 is for adjusting the opening of the inlet vane 41 of the compressor 6 or for adjusting the opening of the valve provided in the oxidizing air supply passage 26. May be.

In the above means, the redox potential of the absorbing liquid 2 is detected, and when the redox potential detection value 46 increases above the set value 47, the supply amount of the oxidizing air A is decreased. It is possible to prevent the oxidation-reduction potential of No. 2 from rising above the set value 47, to prevent the activity of the nitrifying bacteria of the waste water treatment device 24 from decreasing, and to prevent the function of the waste water processing device 24 from decreasing.

Further, the PI controller 51 constituting the control circuit 44 of the oxidizing air supply device limits the upper limit of the oxidation reduction potential corresponding air flow rate command signal 50 by a value obtained by adding a predetermined addition value 57 to the selection signal 52. Since the shake-off prevention circuit 58 is provided, when the redox potential corresponding air flow rate command signal 50 for reducing the flow rate of the oxidizing air A is issued, the redox potential corresponding air flow rate command signal 50 immediately outputs the low signal selector 53. As described above, the effect of reducing the flow rate of the oxidizing air A can be responsively performed.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of an embodiment of the present invention applied to the conventional flue gas desulfurization apparatus shown in FIG. 4, and its basic construction is the same as that of the conventional apparatus shown in FIG. However, a feature of this embodiment is that the control circuit 44 shown in FIG. 1 is provided.

The inventors of the present application can reduce the residual chlorine concentration affecting the nitrifying bacteria in the waste water treatment device 24 by reducing the supply amount of the oxidizing air A supplied to the liquid pool portion 1 of the absorption tower 5. It became clear by the experiment of 1., and the residual chlorine concentration is a factor of the change of the redox potential.
It was revealed that the activity reduction of the nitrifying bacteria in the wastewater treatment device 24 can be prevented by detecting the redox potential of No. 2 and increasing / decreasing the amount of the oxidizing air A supplied to the absorbing solution 2 according to the redox potential. .

Therefore, as shown in FIGS. 1 and 2, a redox potential detector 45 for detecting the redox potential of the absorption liquid 2 in the liquid reservoir 1 of the absorption tower 5 or the absorption liquid 2 in the circulation path.
Is provided, and a subtracter 49 for inputting and subtracting the redox potential detection value 46 and the set value 47 of the redox potential detector 45 to output a difference signal 48 is provided. Based on the difference signal 48, when the redox potential detection value 46 is larger than a set value 47, a PI controller 51 that outputs a redox potential corresponding air flow rate command signal 50 to reduce the air flow rate.
Is provided.

On the other hand, as in FIG. 4, the inlet SOx detector 2
From the desulfurization load calculator 34 which inputs the SOx detection value 30 of 7 and the SOx detection value 31 of the outlet SOx detector 29 and the exhaust gas flow rate detection value 32 from the exhaust gas flow meter 28 to calculate the desulfurization load signal 33. Load corresponding air flow rate command signal 3 obtained by inputting desulfurization load signal 33 of
5 and the air flow rate command signal 50 corresponding to the oxidation-reduction potential are input, a low signal selector 53 for selecting the signal on the smaller side and outputting it as the selection signal 52 is provided.

Further, the selection signal 5 from the low signal selector 53
2, and a subtractor 55 that inputs and subtracts the oxidizing air flow rate detection value 38 from the oxidizing air flow rate detector 37 provided in the oxidizing air supply flow path 26 and outputs a difference signal 54,
A PI adjuster 56 is provided which adjusts the adjuster 42 to zero the difference signal 54 from the subtractor 55.

Next, the operation of the above embodiment will be described.

Desulfurization load signal 3 from the desulfurization load calculator 34
3 is input to the function generator 36, and the load-corresponding air flow rate command signal 35 obtained through the low signal selector 53 is added to the subtracter 5
5 and the oxidizing air flow rate detection value 38 from the oxidizing air flow rate detector 37 provided in the oxidizing air supply flow path 26.
Is input to a subtractor 55 and subtracted to obtain a difference signal 54, and P is set so that the difference signal 54 becomes zero.
The I adjuster 56 adjusts the adjuster 42 so that the flow rate of the oxidizing air A supplied to the liquid reservoir 1 is controlled to match the load-corresponding air flow rate command signal 35.

On the other hand, when the redox potential of the absorbing liquid 2 in the liquid reservoir 1 rises, the redox potential detection value 46 of the redox potential detector 45 detecting the redox potential is detected.
Is increased and the redox potential detection value 46 becomes larger than the set value 47, the redox potential detection value 46 is subtracted from the subtracter 49.
Since it is input to the minus (-) side of, the difference signal 4
8 becomes a negative value, so that the PI controller 51 outputs to the low signal selector 53 the air flow rate command signal 50 corresponding to the redox potential in the direction of decreasing so as to eliminate the signal 48 of the negative difference. Corresponding air flow rate command signal 3
Since the air flow rate command signal 50 corresponding to the redox potential is smaller than that of 5, the air flow rate command signal 50 corresponding to the redox potential is selected and output to the subtractor 55 as the selection signal 52. The flow rate of the oxidizing air A in the oxidizing air supply channel 26 is the air flow rate command signal 5 corresponding to the redox potential.
Reduced to match zero.

Further, when the flow rate of the oxidizing air A is decreased and the redox potential detection value 46 becomes lower than the set value 47, the difference signal 48 becomes a positive value, so that the PI controller 51 outputs a positive difference signal 48. The air flow rate command signal 50 corresponding to the redox potential in the increasing direction so as to eliminate
3 to output the air flow rate command signal 50 corresponding to the oxidation-reduction potential to the air flow rate command signal 35 corresponding to the load.
When it becomes larger, the low signal selector 53 in turn selects the load-corresponding air flow rate command signal 35 and outputs it to the subtractor 55.

Therefore, normally, the flow rate of the oxidizing air A is controlled by the load-corresponding air flow rate command signal 35, and when the oxidation reduction potential detection value 46 exceeds the set value 47, the oxidizing air A is detected.
Is controlled by the air flow rate command signal 50 corresponding to the oxidation-reduction potential, which prevents the oxidation-reduction potential of the absorbing liquid 2 in the liquid reservoir 1 from rising above a predetermined value, and nitrification of the wastewater treatment device 24. It is possible to prevent the activity of the bacterium from decreasing.

In addition, the upper limit of the air flow rate command signal 50 corresponding to the oxidation-reduction potential is set to the PI controller 51 by a value obtained by adding a predetermined addition value 57 (for example, several percent) to the selection signal 52 from the low signal selector 53. A shake-off prevention circuit 58 (reset-windup prevention circuit) for limiting is provided.

The PI regulator 51 corresponds to the redox potential which increases or decreases between 0% and 100% as shown in FIG. 3 so as to eliminate the difference signal 48 from the subtractor 49. The air flow rate command signal 50 is output to the low signal selector 53. That is, when the load-corresponding air flow rate command signal 35 from the function generator 36 is, for example, 50%, the redox potential-corresponding air flow rate command signal 50 gradually increases from 0% (the difference (When the signal 48 is positive), the low signal selector 53 selects the small redox potential corresponding air flow rate command signal 50 to increase the flow rate of the oxidizing air A, and the redox potential corresponding air flow rate command signal 50. Exceeds 50%, the load-corresponding air flow rate command signal 35 becomes smaller, so that the load-corresponding air flow rate command signal 35 is switched to be selected. This time,
The air flow rate command signal 50 corresponding to the oxidation-reduction potential continues to increase and finally reaches 100% as shown by the broken line 50 '.

On the other hand, when the redox potential of the liquid reservoir 1 becomes larger than the set value 47 and the difference signal 48 becomes negative, the PI controller 51 responds to the redox potential in the decreasing direction from the point B at 100%. Although the air flow rate command signal 50 is output, it takes time for the air flow rate command signal 50 corresponding to the oxidation-reduction potential to decrease to 50%, and during this period, the load-corresponding air flow rate command signal 35 is the low signal selector. 53
Therefore, a large time delay H occurs until the air flow rate command signal 50 corresponding to the redox potential is selected and the flow rate of the oxidizing air A actually supplied to the liquid reservoir 1 is reduced. There's a problem.

For this purpose, the PI adjuster 51 is provided with a swing-out prevention circuit 58 so that a predetermined addition value 57 (several percent) is added to the output of the selection signal 52, for example, 50% + several percent. Since the upper limit is set to (for example, 3%) = 53%, for example, as shown in FIG. 3, when the air flow rate command signal 50 corresponding to the redox potential of the PI controller increases and reaches 53%, It will keep its value without going up further, so PI
When a signal for reducing the air flow rate command signal 50 corresponding to the redox potential of the controller 51 is output, the air flow rate command signal 50 decreases immediately from the value of 53%, and the time delay is minimized to H '. be able to.

[0037]

As described above, according to the oxidizing air supply method and device of the flue gas desulfurization apparatus of the present invention, the redox potential of the absorbing liquid 2 is detected, and the redox potential detection value 46 is the set value 47. Since the supply amount of the oxidizing air A is decreased when the amount is increased above, the oxidation-reduction potential of the absorbing liquid 2 is prevented from rising above the set value 47, and the wastewater treatment device 24
It is possible to prevent a decrease in the activity of the nitrifying bacteria and prevent a decrease in the function of the wastewater treatment device 24.

Further, the PI controller 51 constituting the control circuit 44 of the oxidizing air supply device limits the upper limit of the air flow rate command signal 50 corresponding to the oxidation reduction potential by the value obtained by adding the predetermined addition value 57 to the selection signal 52. Since the shake-off prevention circuit 58 is provided, when the redox potential corresponding air flow rate command signal 50 for reducing the flow rate of the oxidizing air A is issued, the redox potential corresponding air flow rate command signal 50 immediately outputs the low signal selector 53. Thus, the action of reducing the flow rate of the oxidizing air A can be performed with good responsiveness.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a control circuit.

FIG. 3 is a diagram showing a change in an air flow rate command signal corresponding to a redox potential by a shake-off prevention circuit.

FIG. 4 is a schematic diagram of a conventional example.

FIG. 5 is a diagram showing a relationship between a desulfurization load signal of a function generator and a load-corresponding air flow rate command signal.

[Explanation of symbols]

 2 Absorbing liquid 5 Absorption tower 19 Gypsum 24 Waste water treatment device 26 Oxidizing air supply flow path 33 Desulfurization load signal 35 Load corresponding air flow rate command signal 36 Function generator 37 Oxidizing air flow rate detector 38 Oxidizing air flow rate detection value 42 Regulator 45 Oxidation Reduction potential detector 46 Oxidation reduction potential detection value 47 Set value 48 Difference signal 49 Subtractor 50 Redox potential corresponding air flow rate command signal 51 PI controller 52 Selection signal 53 Low signal selector 54 Difference signal 55 Subtraction device 56 PI controller 57 Addition value 58 Shake-off prevention circuit A Oxidizing air G Exhaust gas

Claims (3)

[Claims] 1. An absorbing liquid using lime as an absorbent (2)
And the exhaust gas (G) are brought into contact with each other to absorb and remove the sulfur oxides in the exhaust gas (G). The absorbing liquid (2) brought into contact with the exhaust gas (G) is oxidized with air (A) through the controller (42). )
The absorption liquid (2) after recovering the gypsum (19) from the absorption liquid (2) brought into contact with the exhaust gas (G) by providing the absorption tower (5) provided with the oxidizing air supply flow path (26) for blowing in Wastewater treatment equipment for decomposing nitrogen compounds contained in
4) A method for supplying oxidizing air to a flue gas desulfurization apparatus, wherein when the redox potential of the absorbing liquid (2) is detected and the redox potential detection value (46) increases above a set value (47). A method for supplying oxidizing air to a flue gas desulfurization device, characterized in that the controller (42) is controlled so as to reduce the supply amount of oxidizing air (A). 2. An absorbing liquid using lime as an absorbing agent (2)
And the exhaust gas (G) are brought into contact with each other to absorb and remove the sulfur oxides in the exhaust gas (G). The absorbing liquid (2) brought into contact with the exhaust gas (G) is oxidized with air (A) through the controller (42). )
The absorption tower (5) provided with an oxidizing air supply flow path (26) for blowing the gas into the absorption liquid (2) after recovering the gypsum (19) from the absorption liquid (2) brought into contact with the exhaust gas (G). Wastewater treatment equipment for decomposing nitrogen compounds contained (2
And a function generator (36) for outputting a load-corresponding air flow rate command signal (35) based on a desulfurization load signal (33), and an absorbing liquid. The redox potential detector (45) for detecting the redox potential of (2), the redox potential detection value (46) and the set value (47) of the redox potential detector (45) are input and subtracted. A subtractor (49) that outputs a difference signal (48) by
An air flow rate corresponding air flow rate command signal (50) for reducing the flow rate of oxidizing air (A) is output when the redox potential detection value (46) is larger than a set value (47) based on the difference signal (48). And a low signal selector (56) that inputs the load-corresponding air flow rate command signal (35) and the oxidation-reduction potential-corresponding air flow rate command signal (50) and selects and outputs the smaller signal ( 53), the selection signal (52) from the low signal selector (53), and the oxidizing air flow rate detection value (38) from the oxidizing air flow rate detector (37) provided in the oxidizing air supply flow path (26). And a subtracter (55) that subtracts and outputs a difference signal (54), and the subtractor (5)
And a PI controller (56) for adjusting the controller (42) so that the signal (54) of the difference from (5) becomes zero. . 3. A PI controller (51) is provided with a selection signal (5).
2) A flue gas desulfurization characterized by comprising a shake-off prevention circuit (58) for limiting the upper limit of the redox potential corresponding air flow rate command signal (50) by a value obtained by adding a predetermined addition value (57) Oxidizing air supply device of equipment.