JPS62193629A - Method for removing sulfur oxide contained in exhaust gas - Google Patents

Abstract

PURPOSE:To enhance both degree of desulfurization for exhaust gas and coefficient of utilization for lime by adding a fresh calcium absorbent to absorbing slurry drawn out from a circulation tank and bringing it into contact with sulfur oxide contained in exhaust gas in the inside of an absorption tower. CONSTITUTION:Exhaust gas 101 sent from a boiler or the like is introduced into an absorption tower 102 and ascended in the tower while it is countercurrently brought into contact with calcium slurry 105 which is drawn out via a pump 104 from a circulation tank 103 in the bottom part of the tower and sprayed to the uppermost part, cooled and dust-removed and furthermore desulfurized. The carrier mists of exhaust gas are removed by a demister 106 and purified gas 107 is introduced into a flue via a duct 108. Calcium slurry used in dust removal and desulfurization falls into the circulation tank 103 and air is fed into the circulation tank and thereby sulfite generated by absorption of sulfur oxide contained in exhaust gas is oxidized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for treating sulfur oxides in exhaust gas,
In particular, the present invention relates to a method for treating sulfur oxides in exhaust gas, in which gypsum is recovered by oxidizing an absorption liquid that has absorbed sulfur oxides without using a dedicated oxidation tower.

(Conventional technology and problems to be solved) In wet flue gas desulfurization, which treats sulfur oxides in flue gas with liquid, alkali metals, alkaline earth metals, hydroxides such as ammonia, carbonates, sulfites, or A common method is to absorb and remove sulfur oxides from exhaust gas using an oxide solution or suspension, and recover stable sulfate as a by-product.

A conventional technique for recovering calcium sulfate (gypsum) using a calcium-based absorbent will be explained with reference to FIG. Exhaust gas 201 from a boiler etc. is guided from a dust removal tower inlet duct 202 to a dust removal tower 203, where it is transferred to a dust removal tower circulation tank 204.
The slurry is sprayed to remove dust and cooled, and after the scattered mist is removed by a demister 205, it is sent to an absorption tower 206. Inside the absorption tower, the water is extracted from the absorption tower circulation tank 207, and the absorption tower circulation pump 208
The calcium-based absorbent slurry supplied from the pipe 209 is sprayed through the nozzle 210, and sulfur oxides in the exhaust gas are absorbed and removed. The entrained mist in the exhaust gas is removed by a demister 211, and the green clean gas 212 is guided to the flue via a duct 213. On the other hand, the circulating liquid slurry containing the calcium-based absorbent that has absorbed sulfur oxides becomes calcium sulfite in the absorption tower 206 and the absorption tower circulation tank 207, but a part of this is oxidized by oxygen in the exhaust gas in the absorption tower. It becomes plaster.

This absorbent slurry is passed through the absorption tower circulation pump 208 into the absorption tower from the pipe line 209 or into the absorption tower through the bleed pump 214.
The dust is supplied to the dust removal tower circulation tank 204 via the dust removal tower circulation tank 204. The slurry in the dust removal tower circulation tank comes into contact with the exhaust gas in the dust removal tower 203, and by removing sulfur oxides from the exhaust gas, the amount of unreacted limestone in the slurry is reduced, and the slurry is extracted to the by-product gypsum recovery system.

That is, the sulfuric acid is first extracted to the reaction tank 215, where sulfuric acid
Unreacted CaCO contained by adding 16
3 is converted to gypsum and the pH is adjusted to suit the oxidation of calcium sulfite. This slurry is fed by an oxidizer feed pump 217 to an oxidizer 218 where the calcium sulfite is oxidized to gypsum by air 219. The obtained gypsum slurry is led to a plaster 220, separated into solid and liquid, and then dehydrated with a centrifugal separator 221 or the like, and gypsum 222 is recovered. The filtered water 223 during solid-liquid separation and dehydration is reused for making limestone slurry, etc.

Note that the limestone slurry, which is an absorbent for sulfur oxides, is stored in a limestone slurry tank 224 with limestone 225 and filtered water 2.
23 and make-up water 226, and is supplied into the absorption tower circulation tank 207 by a bleed pump 227.

As described above, in conventional methods, it is difficult to completely convert sulfur oxides into gypsum during the absorption process, and a method has been adopted in which sulfites, which are normally generated in the absorption system, are converted into gypsum in a separate oxidation tower. However, in recent years, many methods have been proposed in which the oxidation tower is omitted and the oxidation of sulfur compounds to sulfates is proceeded in the absorption section. Examples include a method using an oxidation catalyst (Japanese Patent Publication No. 58-36619), a method of blowing air into an absorption tower circulation tank or a separate reaction tank (Japanese Patent Publication No. 55-116423, No. 55-116424, Japanese Patent Publication No. 55-116424, Showa 58-98126, Showa 5B-92452, Showa 58-95
543, 58-104619, Utility Model 58-9521
8) or a two-stage desulfurization method (JP-A-58-74126).

However, methods using catalysts are not economically viable unless they are recovered at a high rate, and air blowing methods cannot be used in conventional oxidation towers unless a large amount of air is supplied as fine bubbles. It is not possible to oxidize sulfite fast enough to replace it.

In response, the present inventors proposed a new process with the following features for the purpose of rationalizing the wet flue gas desulfurization method (
Patent application No. 59-028764). (1) Dust removal, which has traditionally been carried out with the Henchuri type, is incorporated into the lower part of the absorption tower as a spray method. (2) The bottom of the tower is used as a circulation tank for dust removal, and the slurry used for dust removal is dropped directly into the circulation tank.
3) The exhaust gas after dust removal is supplied to the absorption section at the top of the tower, and is brought into countercurrent contact with the sprayed calcium-based slurry to remove the contained sulfur compounds. (4) The slurry in the absorption section is collected by the collector. (5) Supplying air into the circulation tank of the dust removal section to generate gypsum and omitting the dedicated oxidation tower; (6) Limestone slurry as an absorbent is returned to the absorption section circulation tank, which is provided separately. The slurry is supplied to a circulation tank, and a part of the slurry in the tank is extracted and supplied to the circulation tank of the dust removal section.

In the above process, the circulation tanks for the dust removal section and the absorption section are installed separately, so these can be combined to completely remove dust.
It is desired to develop a process in which absorption and oxidation reactions are carried out in the same column. However, if the circulation tanks for the dust removal (sub-oxidation) section and the absorption section are placed separately as dedicated tanks, the slurry pH can be maintained at their respective appropriate values, but the complete Rakuhoku process as described above In this case, reactions from dust removal to oxidation must be carried out in the same slurry and therefore at the same pH. Furthermore, even if CaSO3 is converted into gypsum with a sufficient yield in the circulation tank, unreacted CaCO3 must not remain in the slurry that is extracted from the tank and sent to the gypsum recovery system. , no process has been proposed to date that satisfies these conditions.

(Means for Solving the Problems) The present invention is a one-tower wet flue gas desulfurization method that does not have a dust removal tower or an oxidation tower other than an absorption tower, and in which combustion flue gas is brought into contact with a calcium-based compound slurry. In order to remove the sulfur oxides contained in the slurry, a calcium-based compound slurry as an absorbent is mixed with the slurry extracted from the circulation tank and then sprayed into the tower, and near the blades of the agitator installed in the tank. By supplying air, dust removal, absorption, and oxidation reactions are carried out in the same column, making it possible to achieve high desulfurization efficiency and high limestone utilization.

That is, the present invention provides a method for removing sulfur oxides in exhaust gas by bringing them into contact with a calcium-based absorbent slurry extracted from an absorption tower circulation tank in an absorption tower. A new calcium-based absorbent is added to the agent slurry and brought into contact with sulfur oxides in the exhaust gas in the absorption tower.The absorbent slurry after the contact is stored in the circulation tank, and oxygen is added to the slurry in the tank. The method is characterized in that a gas containing gas is supplied to cause a reaction, and the slurry after the reaction is introduced into a separation device to separate gypsum from the slurry.

Next, the present invention will be explained with reference to FIG. The present invention was made for the purpose of streamlining wet flue gas desulfurization equipment,
Unlike conventional methods, it is implemented as a process that omits dedicated dust removal and oxidation towers.

Exhaust gas 101 from a boiler etc. is guided to an absorption tower 102,
Here, it is extracted from the circulation tank 103 at the bottom of the tower via the pump 104, comes into countercurrent contact with the calcium-based slurry 105 sprayed at the top of the tower, and ascends through the tower while being cooled, dusted, and desulfurized.

The entrained mist of the exhaust gas is removed by a demister 106, and the clean gas 107 is led to the flue via a duct 108.

The calcium-based slurry subjected to dust removal and desulfurization falls into a circulation tank, but in the method of the present invention, air is supplied into the circulation tank, and sulfites produced by absorption of sulfur oxides in the exhaust gas are oxidized. Its characteristic is that In the air oxidation method of sulfite in a circulation tank, microbubbles are generated in the tank, and the amount of sulfite in the slurry reaches the desired value (when recovering gypsum, the sulfite content is usually Any method may be used as long as it can be suppressed to 0.5% or less. Specifically, a method of supplying air 110 near the blades of an agitator 109 installed in a circulation tank 103 as shown in FIG. 1 provides an oxidation rate that meets this purpose. In this method, the supplied air is atomized by stirring blades, so that the calcium sulfite in the circulation tank is oxidized to gypsum. In addition, in order to achieve complete oxidation of sulfite in the circulation tank using this method, the shape of the stirring blade, stirring speed, air supply amount,
It is necessary to optimize the slurry pH, etc. Regarding the shape of the stirring blade, the oxidation rate compared at the same stirring speed is paddle type, inclined paddle type, and propeller type.
Comparing the oxidation rate per unit power light, propeller type
Inclined paddle>paddle type, and the propeller type has been found to be the most efficient stirring blade (Figure 2). However, the stirring blade may have any shape as long as it can simultaneously prevent slurry sedimentation and oxidize sulfite.

The stirring speed and required air supply amount are determined by the operating conditions of the plant, but as long as propeller-shaped blades are used,
It is sufficient to provide a stirring speed that is sufficient to prevent sedimentation of the slurry, and the amount of air supplied should be approximately 3 to 10 times the theoretical amount of air required to oxidize the sulfite absorbed and removed in the column. This objective is achieved.

On the other hand, as described above, the pH of the slurry must be such that absorption of sulfur oxides and oxidation of generated sulfites proceed at a desired rate. The optimum pH for absorption of sulfur oxides and oxidation of calcium sulfite is usually 5.
．． 0 to 6.5 and 4.0 to 5.5, therefore, the appropriate pH common to both reactions is limited to around 5.0 to 5.5. That is, if the pH of the slurry in the circulation tank is higher than this value, the solubility of calcium sulfite decreases, and its oxidation rate decreases, making it impossible to recover it as gypsum unless a separate oxidation treatment is performed.

Furthermore, when the pH of the slurry becomes 5 or less, the absorption rate of sulfur dioxide gas decreases, so it becomes necessary to take measures such as increasing the amount of slurry to be sprayed. The present invention was achieved through intensive research into a method that satisfies these contradictory characteristics. In other words, whereas limestone slurry, which is an absorbent, was conventionally supplied to the absorption tower circulation tank, in the method of the present invention, it is directly sprayed into the absorption tower without passing through the absorption tower circulation tank. It is one of the characteristics.

By the way, when fresh absorbent slurry is supplied into a circulation tank as in the conventional method, the absorbent slurry is diluted by a large amount of slurry in the circulation tank. In contrast, according to the method of the present invention, the absorbent slurry is mixed with only a small amount of slurry taken out from the circulation tank, so the slurry pH after mixing is clearly determined. In addition, as the slurry OpH decreases by absorbing sulfur oxides in the tower, the pH of the slurry at the time it falls into the circulation tank at the bottom of the tower is adjusted to the appropriate f for calcium sulfite oxidation. ) can be brought to the H value. In addition, it is more efficient to supply limestone to the circulation line from the viewpoint of limestone utilization. That is, a portion of the slurry in the circulation tank is continuously withdrawn for gypsum recovery, but when adding absorbent slurry to the circulation tank as in the conventional method, this fresh absorbent (limestone ) is sent to the gypsum recovery system as a short bath, so the utilization rate of limestone is usually around 98% at most. On the other hand, in the method of the present invention, the fresh absorbent first reacts with sulfur oxide in the absorption tower and then falls into the circulation tank, so it takes a short pass and heads to the extraction port, where it is immediately extracted from the tank. The amount of limestone produced is less than that of the conventional method.

By the way, in the conventional general wet flue gas desulfurization method, limestone slurry as an absorbent is supplied to an absorption tower circulation tank, but in this case, the slurry extracted from the tank and sprayed is CaSO3- as a solid substance. ! 4H,,0 and Ca
It contains three components: CaSO4/2H20, which is produced when SO3/%H20 is oxidized by the residual 0□ in the exhaust gas, and CaCO3, which remains unreacted in the circulation tank, but when sprayed, Those that react with sulfur oxides are Ca5O:l .%Hz○ and CaCO3.

Although 5032- in the solution also participates in the absorption of sulfur oxides, this contribution is small.

According to the results of basic experiments, when H2S03 is added to a slurry in which CaSO3.'A H20 and CaCO3 coexist, CaCO3 reacts with it first. Third
The figure demonstrates this phenomenon. That is, CaC
O3100mmon and H2SO350mmo+i! The pH, which was initially about 8.3, immediately dropped to around 5, and the solution became Ca (H3O) with a concentration of 25 mmoA.
3) It becomes two solutions. Hereafter Ca 24: all-so3”
- The Ca2+ and total 5032- concentration in the solution decreases while maintaining the molar ratio #1:2. These results are explained by the following reaction equations (1) and (2). That is, when H2SO3 water is added to limestone slurry, it quickly becomes (1
) reaction occurs, producing Ca(H3C)+)2 in the solution, and the pH drops to about 5.1. CaCO3, which exists in an unreacted state at this stage, then reacts with Ca (H3O3)2 produced by the reaction of formula (1), and (formula (2))
, CaSO3·'AHzO crystals are precipitated, so that the Ca 2+ and total 5o32− concentrations in the liquid decrease, and the pH of the liquid gradually increases accordingly.

Ca CO3+ 2 H2S 03- Ca (H3O3)2 +CO2→-H20(1)Ca
(H3O3)2 +CaCO3=2CaSO3・%H
20+CO2 (2) This makes CaCO350mm
It becomes a mixed slurry of o #
was added, the solution again became 25 mmoβ of Ca(H3C
)+)2, and H2SO3 is Ca5O3.
%H20 (see equation (3)) preferentially reacts with CaCO3 (formula (1)). In addition, if the reaction of formula (3) occurs at this time, the Ca(H3O3)2 concentration will be approximately 5Qmm.
ojl! must become. As a result, Ca (
It can be seen that H3O3)2 is produced and then the reaction of formula (2) proceeds.

40 minutes after the first addition of H2SO3, the pH increases to around 7.7 due to the presence of unreacted CaCO3 in the slurry, whereas after the second addition of H2SO3 water, the pH increases to around 7.7. : The molar ratio of H2SO3 (based on the amount added) is 1:1, so the liquid does not become alkaline.

Ca SO3・%H20+H2SO3=Ca (H3
O3)2 +%H20(3) In the circulation tank, (
The reaction of formula (2) will mainly proceed, but according to the above results, the reaction of formula (2) will proceed to convert CaCO3 to C.
Even if it is converted to aSO3.%H20, the reactivity with H2S03 and therefore with S02 will not be improved at all.

In other words, as long as the purpose is to absorb SO□, C
This means that it is sufficient to directly supply aCo3 into the absorption tower.

In addition, as a method of directly supplying fresh absorbent to the absorption tower, there is a method of supplying it to the first or last stage of the absorption zone of a conventional spray type absorption tower that requires the installation of a dedicated oxidation tower (especially 1986-61818) is known. However, in this method, no special oxidizing device is provided in the circulation tank, so the slurry is mainly composed of calcium sulfite. In contrast,
In the method of the present invention, since air is blown into the circulation tank, the main component of the slurry is gypsum, which is mixed with limestone slurry and then supplied into the absorption tower. If the concentration of sulfite in the slurry is high, even if limestone is added due to its pH buffering effect, the pH of the slurry will not increase much, but in the method of the present invention, sulfite is oxidized in the air and converted to sulfate (gypsum). Because of this, the degree of pH increase due to the addition of limestone is greater than in the above case, and the desulfurization reaction progresses more easily.

As mentioned above, the present invention is based on the inventor's analysis of the basic phenomenon that the role of conventional circulation tanks in terms of absorption efficiency for sulfur oxides is surprisingly small. It is characterized in that sulfur oxide is directly supplied to the absorption system, but at the same time, another major feature is that calcium sulfite produced by absorbing sulfur oxide in exhaust gas is quickly oxidized to gypsum in a circulation tank.

In other words, only in combination with this oxidation treatment can the reactions of dust removal, absorption, and oxidation be carried out in the same column. That is, in the conventional method, as mentioned above, the reaction of formula (2) mainly proceeds in the absorption tower circulation tank, but at this time, the Ca CO3 particles gradually become finer, and on the other hand, the pH increases as the reaction progresses. For CaCO
The dissolution reaction rate of 3 decreases. In addition, CaS 03 %H20 crystals generated by the reaction cover the surface of the CaCO3 particles, so-called "blinding" (bj2indi
ng) phenomenon occurs, and from this point of view as well, it is impossible to completely eliminate CaCO3 in the circulating tank.On the other hand, in the method of the present invention, the slurry is circulated after absorbing sulfur oxides. If you fall into the tank, (
Since the gypsum production reaction represented by formula (4) occurs before the reaction of formula 2) proceeds, the blinding phenomenon due to caso3·'AH20 is avoided.

Ca (H3O3)2 +02 +2H20=CaS
O4-2H20+H2SO4 (4) Also, as this reaction progresses, free H2SO4 is generated and unreacted CaCO
Even if 3 is present, it will be easily decomposed by this H2SO (formula (5)).

Ca CO3 + H2SO4 + H20 = Ca so, H2H20 + CO2 (5) This H2SO4
This is originally caused by sulfur oxides in the exhaust gas, not newly added sulfuric acid. Therefore, by promoting the oxidation of sulfite in the circulation tank by the method of the present invention, it is possible to improve the utilization rate of CaCO3 and reduce the amount of H2SO4 added to decompose unreacted CaCO3.

A method of supplying fresh absorbent to slurry extracted from a circulation tank and oxidizing the slurry (Utility Model No. 58)
-95218) has already been proposed. In this method, a liquid mixing container divided into two, different from the circulation tank, is provided, and air is supplied into both tanks. That is, in one tank (circulation tank) air is blown in and the absorbent is added, while in the other tank (gypsum tank) H2SO
4 and air are supplied to proceed with decomposition of unreacted CaCO3 and oxidation of calcium sulfite. In contrast, in the method of the invention proposed this time, a liquid mixing vessel different from the circulation tank is not installed, and therefore the air oxidation of sulfite is carried out in the circulation tank placed at the bottom of the absorption tower. By blowing air into the circulation tank in this way, the sulfite produced by absorbing sulfur oxides will be immediately oxidized when it falls into the circulation tank.

By the way, the present inventors have revealed that calcium sulfite is relatively easy to form a supersaturated solution, and this property is also one of the principles of the present invention. In other words, Fig. 3 shows C
When H2SO3 water is added to aCo3 slurry, the pH
It shows that the value immediately drops to around 5.2 and then gradually rises, but the small peak seen within a few minutes of adding H2S03 is due to CaS from the solution.
Precipitation of O3.'A H20 crystals indicates that the solution was supersaturated with respect to calcium sulfite.

The reaction at this time is shown by equation (6).

Ca (H3O3) 2 + ! 4H20=CaS○3
・%H20+H2303(6)Ca SO3・'A
When H20 crystals precipitate, H2S03 is liberated and p
This shows that H decreases. In this way Ca S
The low precipitation rate of O3.%HzO indicates that a significant portion of the calcium sulfite is present in solution when the sulfur oxide-absorbed slurry falls into the circulation tank. Of course, sulfite is oxidized more quickly when it exists as a solution than as a solid, so from this point of view, air is supplied into the circulation tank to immediately remove the sulfite that falls from the absorption section. Oxidation results in a higher oxidation rate, and this is also a clear difference between the present invention and the above-mentioned invention (Utility Model Application Publication No. 58-95218).

In addition, if calcium sulfite is oxidized in a supersaturated state, it will reach a pH of 5, which was previously thought to be the upper limit of the appropriate pH.
, p) (values higher than 5 also allow oxidation,
Therefore, the appropriate pH range for both absorption of sulfur oxides and oxidation of calcium sulfite is expanded.

In addition, in the method of supplying air into the liquid mixing container, crystal precipitation of caso3 ・'AH20 progresses in the circulation tank, and the oxidation rate of calcium sulfite decreases because the crystals cover the surface of unreacted CaC03 particles. Not only is it smaller, but the utilization rate of CaCO3 is also smaller compared to the method of the present invention.

The phenomena that form the principle of the present invention have been described in detail above, but now let us consider the function of the circulation tank in the conventional method. If H3O3- is present in the liquid, it is difficult to absorb new sulfur oxides, so this is reacted with CaCO3 in a circulation tank, resulting in a slurry pH of usually 5.0 to 6.5. However, CaCO3 and CaS
Since the solubility of O3/%HzO decreases as the pH increases, it is generally used to "dissolve limestone in a circulation tank".
However, Ca21 and Ca32- in the solution
This does not mean that the concentration of In other words, the function of the circulation tank in the conventional method is to improve the desulfurization performance of the slurry by eliminating H3O3- produced by the absorption of sulfur oxides through the reaction of equation (2), and at the same time to promote the decomposition of unreacted CaCO3. , it is believed that the purpose is to reduce the amount of H2SO4 added before oxidation of calcium sulfite.

On the other hand, in the method of the present invention, the circulation tank should rather be viewed as an oxidizing device, and therefore its size can be determined based on the oxidation rate of calcium sulfite. As mentioned above, the oxidation of Ca(H3O3)2 ((4)
(2SO4 is produced by the formula), but this H2SO4
Since the reaction between CaCo3 and CaCo3 (formula (5)) proceeds much more rapidly than the reaction of formula (2), the amount of unreacted CaCO3 becomes extremely small as long as the oxidation of H3O- proceeds. Furthermore, experimental results (Fig. 4) show that if air is supplied near the blades of the agitator installed in the circulation tank, the oxidation rate (approximately 40 mmo I!/j2-h), which is considered necessary to omit the oxidation tower, can be obtained. ) is more obvious.

As mentioned above, in the method of the present invention, the contents of the circulation tank are made into a slurry mainly composed of gypsum by blowing air into the circulation tank and directly introducing the calcium-based compound slurry, which is an absorbent, into the upper part of the absorption tower. I can do it. A part of the slurry extracted from the circulation tank is transferred to the pipe 111 to the sysofuna 1.
After being guided to 12 and subjected to solid-liquid separation, it is dehydrated by a centrifuge 113 or the like, and gypsum 114 is recovered. The filtrate 115 from solid-liquid separation and dehydration is reused for preparing limestone slurry and the like. Note that limestone 116, which is an absorbent for sulfur oxides, is added to a limestone slurry preparation tank 117, and the filtered water 1
15 and makeup water 118 to form limestone slurry 11
9 and is led to the absorption tower. As a method of supplying this calcium-based compound slurry, which is typified by limestone, into the absorption tower, it is possible to directly spray only this slurry, but in order to smoothly proceed with the desulfurization reaction, it is usually necessary to Considering that the liquid-gas ratio (L-liquid/Nrrr-gas) is required, it is preferable to mix it with the slurry 105 extracted from the circulation tank and spray it into the column at an increased flow rate. . The mixing location can also be selected at the outlet of the circulation pump 104 located at the outlet of the circulation tank so that the slurry sprayed into the absorption tower contains all fresh CaCO3 (feed line 120);
It is also possible to mix in fresh limestone slurry (feed line 121) only to specific spray stages.

(Effect of the invention) In the wet treatment method of the present invention for treating sulfur oxides in exhaust gas, new absorbent (limestone) is mixed in a circulation tank, and the mixed liquid is taken out and sprayed from the upper part of the absorption tower. Unlike the conventional method, in which new limestone is mixed with a small amount of slurry extracted from the circulation tank and sprayed at the top of the absorption tower to absorb and remove sulfur oxides, the pH of the absorbent slurry is high. High ability to absorb sulfur oxides.

In addition, in the circulation tank, only the absorbent that has absorbed sulfur oxides in the upper part of the absorption tower is oxidized, so the purity of the gypsum obtained is high, and the newly replenished limestone is turned into gypsum without absorbing it. There is no need to put it away.

Next, the present invention will be explained in detail based on examples.

Example 1 A gypsum slurry with a concentration of 530 mmo7 was placed in the circulation tank of an exhaust gas desulfurization equipment, in which the absorption tower had a diameter of 0.3 m and a height of 7.1 m, and the circulation tank at the bottom had a diameter of 1.0 m and a height of 1.2 m.
/7! I prepared it so that it would be. In addition to this, the SO2 concentration is 740
ppm, 02 concentration 6% coal combustion exhaust gas flow rate 58ON
The slurry was supplied at a rate of rrr/h, circulated in the circulation tank so that L/G was 15 in the absorption tower, and was sprayed from a 2-inch nozzle (4 h). pH of slurry in circulation tank
20% limestone slurry was added using the feed line 120 shown in Figure 1 so that the ratio of
11. Rotation speed 10100Orp and air flow rate 2Nnr/
When sprayed at h, the composition of the slurry in the circulation tank was total Ca 493mmoff/j2, sulfite 8.0mm
ail/l, CaCO33, 5 mmoA/4, almost an equilibrium state was reached, and the oxidation rate was 99%. The desulfurization rate at this time was 92%.

Example 2 In Example 1, the amount of CaCO3 slurry supplied was adjusted so that the pH of the circulation tank was 6.0.

The desulfurization rate was 94%, and the oxidation rate determined from the slurry composition in the circulation tank was 98.5%.

Reference Example 1 In Example 1, when only the desulfurization operation was performed with the air supply to the circulation tank stopped, the desulfurization rate was 91.
The oxidation rate (natural oxidation rate) was 53%.

Reference Example 2 Slurry obtained in Example 1 of the present invention (pH 5,5
) and 500 mj2 of slurry (pH 5,85) obtained from the absorption tower circulation tank of the conventional method shown in FIG. 5 were placed in a beaker, stirred at 30°C, and the pH of the slurry was measured. 1.2.4.6 and 10 minutes p are 5.70 (6,00), 5.81 (6,06), respectively
6.01 (6,15), 6.17 (6,19)
and 6.36 (6, 24), indicating that the slurry obtained by desulfurization while oxidizing dissolves the limestone contained therein faster. Note that the values in parentheses are the pH for the conventional absorber circulation tank slurry.

[Brief explanation of drawings]

Figure 1 shows a system diagram for explaining the method of treating sulfur oxides in exhaust gas according to the present invention, and Figure 2 shows the shape of the circulation tank stirring blade and the oxidation of sulfites related to the present invention. A drawing showing the relationship between speeds, Figure 3, shows limestone (CaCO3
) and sulfite, FIG. 4 is a diagram showing the oxidation rate of sulfite according to the present invention, and FIG. 5 is a diagram illustrating a conventional method for treating sulfur oxides in exhaust gas. This is a drawing. 101... Exhaust gas, 102... Absorption tower, 103...
・Circulation tank, 104...Circulation pump, 105...
Absorbent slurry, 109... Stirrer, 1] 0... Air, 111... Gypsum slurry extraction pipe, 112... Shisofuna, 113... Centrifugal separator, 114... Plaster, 11
7... Limestone adjustment tank, 120... Limestone slurry supply pipe, 122... Spray nozzle.

Claims (1)

[Claims] In a method for removing sulfur oxides in flue gas by bringing them into contact in an absorption tower with a calcium-based absorbent slurry extracted from an absorption tower circulation tank, fresh calcium is added to the absorbent slurry extracted from the circulation tank. Adding a system absorbent and bringing it into contact with sulfur oxides in the exhaust gas in an absorption tower, storing the absorbent slurry after the contact in the circulation tank, and supplying oxygen-containing gas to the slurry in the tank. A method for removing sulfur oxides from exhaust gas, which comprises performing a reaction and introducing the slurry after the reaction into a separation device to separate gypsum from the slurry.