JPH0521005B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention uses NaHSO ₃ , Na ₂ SO ₃ , CaSO ₃・1/2
_H2O , Ca( _HSO3 ) ₂ , _MgSO3 , Mg( _HSO3 ) ₂ ,
This relates to a method for controlling the concentration of sulfites and bisulfites (hereinafter collectively referred to as sulfites, including sulfites) such as (NH ₄ ) ₂ SO ₃ and NH ₄ HSO _3. For example, when oxygen and SO ₂ In a wet flue gas sulfur system that cleans flue gas containing sulfites with an SO ₂ cleaning solution containing sulfites, the use of antioxidants is necessary to suppress sulfites contained in the SO ₂ cleaning solution and maintain the desired sulfite concentration. This provides an extremely effective method for adjusting the supply amount. [Conventional technology] Dilute combustion exhaust gas generated from a boiler
So-called flue gas desulfurization equipment, which removes SO ₂ with cleaning fluid, is widely used in practical use, and one of them is
Hydroxides such as NaOH, NH ₄ OH, Mg(OH) ₂ and Ca(OH) ₂ and carbonates such as Na ₂ CO ₃ and CaCO ₃ are converted to SO ₂
There is a method of removing SO ₂ by cleaning the exhaust gas with an aqueous solution (hereinafter collectively referred to as aqueous solution, including slurry) containing it as an absorbent. For example, if NaOH is used as an SO ₂ absorbent, 2NaOH + SO ₂ → Na ₂ SO ₃ + H ₂ O Na ₂ SO ₃ + SO ₂ + H ₂ O → 2NaHSO ₃ , and the cleaning solution that has absorbed and removed SO ₂ is Na ₂ SO ₃ and
This results in an aqueous solution containing NaHSO ₃ as a product. However, exhaust gas contains O ₂ resulting from excess air combustion, and at the same time as SO ₂ is absorbed, O ₂
Oxidation reactions occur due to absorption. 2NaHSO ₃ +O ₂ →Na ₂ SO ₄ +H ₂ SO ₄ Na ₂ SO ₃ +1/2O ₂ →Na ₂ SO _4This sulfate is absorbed when the O ₂ absorption rate increases due to an increase in the O ₂ concentration in the exhaust gas, etc. , the molar ratio of sulfate to sulfite (hereinafter referred to as oxidation rate) increases. In pulp production using sulfite as a by-product in flue gas desulfurization equipment, if the oxidation rate increases, there is a problem in that sufficient sulfite solution cannot be obtained as a wood cooking liquor. In order to deal with this problem, the use of antioxidants can be considered as a conventional method. Conventionally, it has been known to add antioxidants to the SO ₂ cleaning solution of flue gas desulfurization equipment to control the oxidation of sulfites. Phenols and amines are known, such as diamine (Japanese Patent Publication No. 48-1594), and more specifically, ethylediaminetetraacetic acid (Japanese Patent Publication No. 49-40357, Patent Publication No. 57).
-205314), aminopolycarboxylic acid (Special Publication No. 1973)
-14077), sulfide (JP-A-48-21694), L-
Ascorbic acid (Japanese Patent Publication No. 55-109210), alkyldithiocarbamic acid (Japanese Patent Publication No. 51-45554), nonylphenol (Japanese Patent Publication No. 57-61300, 59-
1202), trialkyltrithiophosphite (Japanese Patent Publication No. 58-58283, Japanese Patent Publication No. 59-1203), sorbitol (Japanese Patent Publication No. 52-21994), and thiobisphenol compounds (Japanese Patent Publication No. 58-58282). However, in the conventional method of industrially using these antioxidants, the oxidation rate of sulfite is experimentally determined and the amount of antioxidant added is adjusted accordingly to control the sulfite concentration. The method used was to take a small sample of the liquid, determine the sulfite concentration by manual analysis in accordance with JISK0102, and check whether the antioxidant was in the appropriate amount. Therefore, when operating a flue gas desulfurization equipment continuously, where the exhaust gas properties and exhaust gas processing amount fluctuate from time to time due to changes in the boiler load and fluctuations in the S content in the fuel, SO In order to control the sulfite concentration in the cleaning solution in _{step 2} , it required the effort of adjusting the amount of antioxidant supplied and a lot of manual analysis, which had the drawback of requiring manpower and time. Furthermore, sulfites in the cleaning fluid of flue gas desulfurization equipment
When manually analyzing according to JISK0102, there were problems with interference from antioxidants and interference from iron ions, which caused problems in controlling the concentration of sulfites. Therefore, the conventional method has the drawback of continuously supplying an excessive amount of antioxidant, thereby half-blindly suppressing the oxidation rate. Although there is a need to control the sulfite concentration to a desired level without oversupplying expensive antioxidants, there are limits to controlling it by manual analysis, and this need has not been met. [Problems to be solved by the invention] The present invention has been made in view of the current situation, and
The object of the present invention is to provide a method for constantly and quickly supplying an appropriate amount of antioxidant without waste even in response to load fluctuations, and for adjusting the sulfite concentration in a cleaning liquid to a desired value. [Means to solve the problem] Exhaust gas containing oxygen and SO ₂ containing sulfites
In flue gas desulfurization equipment that is cleaned with SO ₂ cleaning liquid,
The supply amount of the oxidation inhibitor for suppressing the oxidation of sulfite in the SO ₂ cleaning solution is adjusted based on the signal that detects the sulfite concentration of the SO ₂ cleaning solution and the deviation signal from the sulfite concentration setting value. This paper proposes a method for controlling sulfite concentration. Hereinafter, an embodiment in which the method of the present invention is applied to a wet soda method flue gas desulfurization apparatus will be described with reference to FIG. It should be noted that the present invention was achieved by developing a method for continuously and instantaneously detecting sulfites in liquid online without relying on manual analysis, so this is one method for detecting sulfites. will be specifically explained with reference to FIG. FIG. 2 is an explanatory diagram showing the configuration of the sulfite detector 112 in FIG. 1 used in the present invention. In FIG. 2, A is a sample liquid, B is a carrier gas such as air or nitrogen, and C is a carrier gas such as air or nitrogen. An acid that decomposes sulfites such as hydrochloric acid or sulfuric acid, D is waste liquid, E is SO ₂
The extracted gas containing , F and G are exhaust, and H is drain. Furthermore, 1 is a metering pump, 2 is a heater, 3 is a temperature controller, 4 is a temperature detector, and 5 is an acid decomposition container, which is a stirring type continuous acid decomposition container that is isolated from the outside air. 6 is a retained liquid, 7 is an agitator, 8 is a carrier gas blowing pipe, 9 is a sealing material, 10 is a motor, 11 is a flow rate regulator, 12 is a micro-volume pump, 1
3 is a liquid seal, 14 is a carrier gas, 15 is a flow rate indicator, 16 is a gas pump, 17 is an SO ₂ analyzer, 1
8 is a sulfite concentration calculator, 19 is a sulfite concentration indicator, 20 is an overflow pipe, 21 is a valve, 22
23 is a dehumidifier, 23 is a calculation setting device, 24 is a sulfite concentration signal, 25 is a calculation signal for acid supply amount control,
*1 indicates the carrier gas flow rate signal, and *2 indicates the sample liquid A sampling flow rate signal. A sample solution A containing sulfite is collected using a metering pump 1, and the temperature of the retained solution 6 in the acid decomposition container 5 is 50°C.
It is fed through the heater 2 so that the temperature is as above. When acid C, which decomposes sulfites such as hydrochloric acid, is injected into the retained liquid 6 through the carrier gas blowing pipe 8, the sulfites in the sample liquid A react with the acid as shown in equations (1) and (2) below. Generates _SO2 . (When adding sulfuric acid) 2NaHSO ₃ +H ₂ SO ₄ →Na ₂ SO ₄ +2H ₂ O+2SO ₂ ↑ (1) (When adding hydrochloric acid) NaHSO ₃ +HCl→NaCl+H ₂ O+SO ₂ ↑ (2) Note (1), (2) ) Formula shows the case where the sulfite is NaHSO ₃ , but the same applies to sulfites (including bisulfites) of Na, NH ₄ , Ca, Mg, and K. The generated SO ₂ is used as a carrier gas by controlling part or all of air or nitrogen, which has been adjusted to the required flow rate with the flow rate controller 11, by operating the distribution valve 21, and combining it with the gas blown into the liquid through the flow rate indicator 15. The air is vented as extraction gas E. On the other hand, the remaining carrier gas 14 used for blowing is combined with the extracted gas E, and the combined gas is used for the SO ₂ analyzer 17, which will be described later. It is also released from the SO ₂ analyzer 17 as exhaust G. The signal from the SO ₂ analyzer 17 is sent to the sulfite concentration calculator 18, and the following theoretical calculations are performed using the carrier gas flow rate signal *1 and sample liquid A sampling flow rate signal *2 that are input at the same time. The sulfite concentration contained in liquid A is calculated and displayed on the sulfite concentration indicator 19. (Calculation of sulfite concentration) Sulfite concentration [mol/] = SO ₂ [%] / 100−SO ₂ [%]
× Carrier gas [Nl/min] / 22.4 [Nl/mol] × Sample liquid flow rate [/min] Furthermore, input the sulfite concentration signal 24 and sample liquid A sampling flow rate signal *2 to the calculation setting device 23, and use the following calculation formula. Perform calculations according to . a・b・x+b・d=F where a: constant [-] b: sampling flow rate of sample solution A [/h] x: sulfite concentration [mol/] d: constant [mol/] F: acid supply amount [mol/h] This calculated value F controls the amount of acid C as the acid supply amount control calculation signal 25. Here, the constant a defines the amount of acid required to completely decompose sulfite, and the constant d defines the concentration of acid required to keep the retentate 6 acidic. shows. To adjust the supply amount of antioxidant, the sulfite concentration of the SO ₂ cleaning solution is detected online using a sulfite detector as described above, and then the sulfite concentration signal from the sulfite detector is sent to a sulfite concentration controller. by sending the sulfite concentration set value and deviation signal to the antioxidant flow controller, and supplying the appropriate amount of antioxidant-containing liquid to the SO ₂ cleaning solution through the antioxidant flow meter and the adjustment valve. conduct. [Example] Hereinafter, the method of the present invention will be explained in detail with reference to Examples. The used apparatus is shown in FIG. 1. Exhaust gas 100 containing SO ₂ and O ₂ generated from a small coal combustion furnace is introduced into an absorption tower main body 101, brought into contact with an SO ₂ cleaning liquid, and then discharged as purified gas 102. Exhaust gas 10
The flow rate at 0 is 20Nm ³ /h, and the composition of the exhaust gas is
SO ₂ = 600 ppm dry base, O ₂ = 10 vol.% dry base. A grid packed tower was used as the absorption tower. A reservoir for the SO cleaning solution was provided at the bottom of the absorption tower, and the reservoir capacity was 8, and the SO ₂ cleaning solution was sprayed from the column section of the absorption tower 101 via the circulation pump 103 to clean the exhaust gas. A 0.4 mol/aqueous solution 110 of NaOH, which is an absorbent for SO ₂ , is used and is supplied to the liquid reservoir of the absorption tower via an absorbent flow meter 108 and a regulating valve 109 . A stirrer 104 is provided in the liquid reservoir for stirring, and a PH detector 105 measures the pH in order to adjust the SO ₂ absorption capacity of the SO ₂ cleaning solution.
was detected, and a deviation signal from the PH setting value from the PH controller 106 was sent by the flow controller 107 to adjust the supply amount of the NaOH aqueous solution. _The SO ₂ cleaning solution contains sulfites (Na ₂ SO ₃ ,
NaHSO ₃ ) was completely oxidized by O ₂ in the exhaust gas, and only sulfate (Na ₂ SO ₄ ) was contained. According to the mass balance commensurate with the amount of SO ₂ absorbed
A portion of the SO ₂ cleaning solution was withdrawn from withdrawal line 111. The sulfite concentration in the SO ₂ cleaning solution in the extraction line 111 is oxidized by O ₂ in the exhaust gas, so it is hardly detected in steady state, and the reason why it was all sulfate is (1) A grid-packed tower with good contact efficiency was adopted as the absorption tower, (2) the SO ₂ concentration was
(3) The O ₂ concentration is as high as 10 vol.%. (4) The Na ⁺ concentration is as low as 0.4 mol/%. (5) The dust contained in the coal combustion exhaust gas is trapped in the SO ₂ cleaning solution. For example, impurities such as manganese and iron that promote oxidation have been mixed in. The sulfite concentration in the SO ₂ wash solution was detected by a sulfite detector 112 having exactly the same configuration as that described in connection with FIG. Next, the sulfite concentration signal from the sulfite detector is sent to the sulfite concentration controller 113, the deviation signal from the sulfite concentration setting value is sent to the flow rate controller 114, and the antioxidant flow meter 1
15 and the antioxidant-containing liquid 1 through the regulating valve 116.
17 can be supplied to the SO ₂ cleaning solution. Hydroquinone was used as the antioxidant, and the sulfite concentration in the SO ₂ cleaning solution was adjusted to a desired value. As mentioned above, when no antioxidant is supplied, the absorbed
Sulfite in the SO ₂ cleaning solution was not detected because all SO ₂ was oxidized by oxygen in the exhaust gas, but the setting value of the sulfite concentration controller 113 was
The supply amount of the hydroquinone-containing liquid 117 was adjusted based on the deviation signal from the signal of the sulfite detector 112, and the composition of the SO ₂ cleaning liquid was investigated when a steady state was reached. Set value of sulfite concentration
Table 1 shows the results for 0.19, 0.25, and 0.
Shown below.

〔Effect of the invention〕

According to the present invention, sulfites in the SO ₂ cleaning solution can be detected continuously and instantaneously online without manual analysis, and the detection signal can be used to constantly and quickly determine the appropriate amount of oxidation inhibitor even when the load fluctuates. Supply without waste,
It has the unique effect of being able to adjust the sulfite concentration in the SO ₂ cleaning solution to a predetermined value.

[Brief explanation of the drawing]

FIG. 1 is an illustrative diagram of an embodiment of the present invention, and FIG. 2 is an explanatory diagram showing the configuration of a sulfite detector used in the present invention. Explanations of the symbols in FIG. 1 are given in FIG. 1, and explanations of the symbols in FIG. 2 are as follows. 1... Metering pump, 2... Heater, 3... Temperature controller, 4... Temperature detector, 5... Acid decomposition container,
6... Remaining liquid, 7... Stirrer, 8... Carrier gas blowing pipe, 9... Seal material, 10... Motor, 11... Flow rate controller, 12... Trace pump,
13...liquid seal, 14...carrier gas, 15
...Flow rate indicator, 16...Gas pump, 17...
SO ₂ analyzer, 18...Sulfite concentration calculator, 19
... Sulfite concentration indicator, 20 ... Overflow pipe, 21 ... Valve, 22 ... Dehumidifier, 23 ... Calculation setting device, 24 ... Sulfite concentration signal, 25 ...
Acid supply amount control calculation signal, *1...Carrier gas flow rate signal, *2...Sample liquid A sampling flow rate signal,
A...Sample liquid, B...Carrier gas, C...
Acid, D... Waste liquid, E... Exhaust gas, F... Exhaust, G... Exhaust, H... Drain.

Claims (1)

[Claims] 1. In a flue gas desulfurization equipment that cleans flue gas containing oxygen and SO ₂ with an SO ₂ cleaning solution containing sulfites,
a signal detecting the sulfite concentration of the SO ₂ cleaning solution;
Based on the deviation signal from the sulfite concentration setting value, the SO ₂
A method for controlling sulfite concentration, the method comprising adjusting the supply amount of an oxidation inhibitor for suppressing oxidation of sulfite in a cleaning solution.