JPH0691938B2 - Method for removing sulfur oxides in exhaust gas - Google Patents

Description

TECHNICAL 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 by recovering gypsum by oxidizing an absorbing liquid that has absorbed sulfur oxides without using a dedicated oxidation tower.

(Problems to be solved with conventional technology) In wet flue gas desulfurization in which sulfur oxides in exhaust gas are treated with a liquid, alkali metals, alkaline earth metals, hydroxides such as ammonia, carbonates, sulfites or A method of absorbing and removing sulfur oxides in exhaust gas using an oxide solution or suspension and recovering stable sulfate as a by-product is common.

A conventional technique for recovering calcium sulfate (gypsum) using a calcium-based absorbent will be described with reference to FIG. Exhaust gas 201 from the boiler or the like is guided from the dust removal tower inlet duct 202 to the dust removal tower 203 where it is dusted and cooled by spraying the slurry from the dust removal tower circulation tank 204, and then the scattered mist is removed by the demister 205. After absorption tower 206
Sent to. In the absorption tower, the calcium-based absorbent slurry extracted from the absorption tower circulation tank 207 and supplied from the pipe 209 through the absorption tower circulation pump 208 is sprayed from the nozzle 210 to absorb and remove the sulfur oxides in the exhaust gas. To be done. The entrained mist in the exhaust gas is removed by the demister 211, and the clean gas 212 is guided to the flue through the duct 213. on the other hand,
The circulating liquid slurry containing the calcium-based absorbent that has absorbed the sulfur oxides becomes calcium sulfite in the absorption tower 206 and the absorption tower circulation tank 207, part of which is oxidized by oxygen in the exhaust gas in the absorption tower to produce gypsum. become. This absorbent slurry is supplied from the conduit 209 into the absorption tower via the absorption tower circulation pump 208 or into the dust removal tower circulation tank 204 via the bleed pump 214. The slurry in the dedusting tower circulation tank comes into contact with the exhaust gas in the dedusting tower 203, reduces the amount of unreacted limestone in the slurry by removing the sulfur oxides in the exhaust gas, and is extracted to the byproduct gypsum recovery system. . That is,
First, it is extracted into a reaction tank 215, where unreacted CaCO ₃ contained therein is converted into gypsum by adding sulfuric acid 216, and the pH is adjusted to be suitable for the oxidation of calcium sulfite. This slurry is supplied to the oxidation tower 218 by the oxidation tower supply pump 217.
Where calcium sulphite is oxidized to gypsum by air 219. The gypsum slurry obtained is Thickener 22.
The gypsum 222 is guided to 0, separated into solid and liquid, and then dehydrated by the centrifuge 221 or the like to recover the gypsum 222. The filtered water 223 at the time of solid-liquid separation and dehydration is reused for preparation of limestone slurry and the like. The limestone slurry, which is a sulfur oxide absorbent, is prepared from limestone 225, filtered water 223 and makeup water 226 in a limestone slurry tank 224, and is supplied into the absorption tower circulation tank 207 by a bleed pump 227.

As described above, in the conventional method, it is difficult to form gypsum completely in the process of absorbing sulfur oxides, and a method of forming gypsum in an oxidation tower which is usually provided with a sulfite generated in the absorption system has been adopted. However, in recent years, many methods have been proposed in which the oxidation tower is omitted and the oxidation of the sulfur compound to the sulfate is promoted in the absorption section. Examples thereof include a method using an oxidation catalyst (Japanese Patent Publication No. 36619/1983) and a method in which air is blown into an absorption tower circulation tank or a reaction tank provided separately (Japanese Patent Laid-Open No. 55-11).
6423, 55-116424, JP-A-58-98126, 58-92452,
No. 58-95543, No. 58-104619, and Japanese Utility Model Laid-Open No. 58-95218) or a two-stage desulfurization method (JP-A-58-74126).

However, the method using a catalyst is not economically viable unless it is recovered at a high rate, and in the air blowing method, unless a large amount of air is supplied as fine bubbles, the conventional oxidation tower is used. It is not possible to oxidize sulfite at a rate that alters.

On the other hand, the present inventors have proposed a new process characterized by the following points for the purpose of rationalizing the wet flue gas desulfurization method.
(Japanese Patent Application No. Sho 59-028764). (1) The dust removal conventionally performed by the venturi type is installed in the lower part of the absorption tower by a spray method, (2) the bottom of the tower is used as a dust removal circulation tank, and the slurry used for dust removal is dropped directly into the circulation tank ( 3)
The dust-removed exhaust gas is supplied to the absorption section at the upper part of the tower to countercurrently contact the sprayed calcium-based slurry to remove the sulfur compounds contained therein. (4) The absorption section slurry is collected by a collector and separately Return to the provided absorption tank circulation tank,
(5) Air is supplied into the dust removal circulation tank to generate gypsum, and the dedicated oxidation tower is omitted. (6) Limestone slurry that is the absorbent is supplied to the absorption circulation tank, and one of the slurry in the tank is supplied. Part is taken out and supplied to the dust removal part circulation tank.

In the above process, since the circulation tanks for the dust removal section and the absorption section are installed separately, they are combined to completely remove dust,
It is desirable to develop a process in which the absorption and oxidation reactions are carried out in the same column. However, when the circulation tanks for the dust removal (and oxidation) section and the absorption section are separately installed as dedicated tanks, the slurry pH can be maintained at their respective appropriate values. In the same slurry, therefore, the reaction from dust removal to oxidation must be carried out at the same pH. Further, even if CaSO ₃ is converted to gypsum in the circulation tank with a sufficient yield, unreacted CaCO ₃ remains in the slurry extracted from the tank and sent to the gypsum recovery system. However, no process that satisfies these conditions has been proposed so far.

(Means for Solving Problems) The present invention is a one-column wet flue gas desulfurization method having neither a dust removal tower nor an oxidation tower outside an absorption tower, in which combustion exhaust gas is contacted with a calcium-based compound slurry. When removing the contained sulfur oxides, the calcium compound slurry that is the absorbent is mixed with the slurry extracted from the circulation tank and then sprayed in the tower, and near the blade of the stirrer installed in the tank. By supplying air, dust removal, absorption, and oxidation reactions are carried out in the same tower, and it is possible to achieve a high desulfurization rate and a high limestone utilization rate.

That is, the present invention, in the method of removing the sulfur oxides in the exhaust gas by contacting with the calcium-based absorbent slurry extracted from the absorption tower circulation tank in the absorption tower, the absorption extracted from the circulation tank A new calcium-based absorbent is added to the agent slurry to bring it into contact with the sulfur oxide in the exhaust gas in the absorption tower, and the absorbent slurry after the contact is stored in the circulation tank, and oxygen is added to the tank slurry. It is characterized in that the gas containing is supplied to cause a reaction, and the slurry after the reaction is introduced into a separator to separate gypsum from the slurry.

Next, the present invention will be described with reference to FIG. The present invention has been made for the purpose of rationalizing a wet flue gas desulfurization apparatus,
Unlike the conventional method, it is carried out as a process in which a dedicated removal and oxidation tower is omitted.

Exhaust gas 101 from a boiler or the like is guided to an absorption tower 102, where it is withdrawn from a circulation tank 103 at the bottom of the tower via a pump 104 and cooled by countercurrent contact with a calcium-based slurry 105 sprayed at the top of the tower, Dust removal and desulfurization ascend in the tower. In the exhaust gas, entrained mist is removed by the demister 106, and the clean gas 107 is guided to the flue through the duct 108.

The calcium-based slurry that has been subjected to dust removal and desulfurization falls into the circulation tank, but in the method of the present invention, air is supplied into the circulation tank and the sulfite generated by absorption of sulfur oxides in the exhaust gas is oxidized. The feature is that. As an air oxidation method of sulfite in the circulation tank, fine bubbles are generated in the tank, and the amount of sulfite in the slurry is a desired value (when gypsum is recovered, the sulfite content is usually Any method may be used as long as it is suppressed to 0.5% or less). Specifically, as shown in FIG. 1, the method of supplying the air 110 to the vicinity of the blades of the stirrer 109 installed in the circulation tank 103 provides an oxidation rate that matches this purpose. In this method, the supplied air is atomized by the stirring blades, so that the calcium sulfite in the circulation tank is oxidized into gypsum. In order to achieve complete oxidation of sulfite in the circulation tank by this method, it is necessary to optimize the shape of stirring blade, stirring speed, air supply amount, slurry pH and the like. Among them, regarding the shape of the stirring blade, the oxidation speeds compared at the same stirring speed are paddle type> inclined paddle type> propeller type, but when compared in terms of oxidation rate per unit power, propeller type> inclined paddle>
The result is that the propeller type is the paddle type, and the propeller type is the most efficient stirring blade (Fig. 2). However, a stirring blade of any shape may be used as long as it can simultaneously prevent the sedimentation of the slurry and oxidize the sulfite. The agitation speed and the required air supply amount are determined by the operating conditions of the plant, but as long as the propeller blades are used, it is sufficient that the agitation speed is such that the sedimentation of the slurry is prevented.
The object is achieved if the amount of air supplied is about 3 to 10 times the theoretical amount of air required for the oxidation of sulfite absorbed and removed in the tower.

On the other hand, the slurry pH must be such that the absorption of sulfur oxides and the oxidation of the sulfite produced proceed at the desired rate as described above. The optimum pH for absorption of sulfur oxides and oxidation of calcium sulfite is usually 5.0-
It is said to be 6.5 and 4.0 to 5.5, and therefore, the appropriate pH common to both reactions is limited to around 5.0 to 5.5.
That is, if the slurry pH in the circulation tank is higher than this value, the solubility of calcium sulfite is lowered, and the oxidation rate thereof is lowered, and gypsum cannot be recovered unless a separate oxidation treatment is performed. Further, when the slurry pH becomes 5 or less, the absorption rate of sulfurous acid gas decreases, so that it is necessary to take measures such as increasing the amount of slurry to be sprayed. The inventors have earnestly studied the method for satisfying the contradictory characteristics, and arrived at the present invention. That is, while the limestone slurry that is the absorbent was conventionally supplied to the absorption tower circulation tank, in the method of the present invention, it is directly supplied into the absorption tower without passing through the absorption tower circulation tank. One feature.

By the way, when a fresh absorbent slurry is supplied into the circulation tank as in the conventional method, the absorbent slurry is diluted by a large amount of the slurry in the circulation tank, so that the slurry
The pH will not change much before and after mixing the absorbent slurry. On the other hand, according to the method of the present invention, since the absorbent slurry is mixed with only a small amount of the slurry extracted from the circulation tank, the slurry pH after mixing is clearly increased. Since the pH of the slurry is lowered by absorbing the sulfur oxides in the tower, the pH of the slurry at the time of dropping into the circulation tank at the bottom of the tower can be brought to an appropriate pH value for the oxidation of calcium sulfite. In addition, it is more efficient to supply limestone to the circulation line in terms of utilization rate of limestone. That is, part of the slurry in the circulation tank is continuously extracted for gypsum recovery, but when the absorbent slurry is added to the circulation tank as in the conventional method, this fresh absorbent (limestone ) Is partly called so-called short pass and is sent to the gypsum recovery system, the utilization rate of limestone is usually at most about 98%. On the other hand, in the method of the present invention, the fresh absorbent first reacts with the sulfur oxides in the absorption tower and then drops into the circulation tank. It is assumed that the amount of limestone produced is smaller than in the conventional method.

By the way, in the conventional general wet flue gas desulfurization method, limestone slurry as an absorbent is supplied into the absorption tower circulation tank, but in this case, the slurry extracted and sprayed from the tank is CaSO ₃ as a solid matter.・ 1 / 2H ₂ O is residual O _{2 in} exhaust gas
It contains _three of CaSO ₄ · 2H ₂ O which is oxidized by and the remaining CaCO ₃ which remains unreacted in the circulation tank. Of these, it reacts with sulfur oxide when sprayed. Is CaSO ₃ 1 / 2H ₂ O and CaCO ₃ . SO ₃ ²⁻ in the solution also participates in the absorption of sulfur oxides, but this contribution is small.

According to the results of basic experiments, if the CaSO ₃ · 1 / 2H ₂ O and CaCO ₃ were added H ₂ SO ₃ to the slurry to coexist, it is CaCO ₃ to react first with this. FIG. 3 demonstrates this phenomenon. That is, when 50 mmol of H ₂ SO ₃ was added to 100 mmol of CaCO ₃ , the pH, which was initially about 8.3, immediately dropped to around 5, and the solution became a Ca (HSO ₃ ) ₂ solution having a concentration of 25 mmol.
After that, Ca ⁺² : Total -SO ₃ ^2-
Ca ²⁺ and total SO ₃ ²⁻ concentrations are reduced. These results are explained by the following reaction equations (1) and (2). That is, when H ₂ SO ₃ water is added to the limestone slurry, the reaction of formula (1) immediately occurs, Ca (HSO ₃ ) ₂ is generated in the liquid, and the pH is
Drops to about 5.1. CaCO ₃ existing in an unreacted state at this stage is the Ca (HS) produced by the reaction of the formula (1).
It reacts with O ₃ ) ₂ (formula (2)) and CaSO ₃ · 1 / 2H ₂ O crystals are precipitated, so that the Ca ⁺² and total SO _{3 2} ^{-concentrations} in the liquid decrease,
Along with this, the pH of the liquid gradually rises.

CaCO ₃ + 2H ₂ SO ₃ = Ca (HSO ₃ ) ₂ + CO ₂ + H ₂ O (1) Ca (HSO ₃ ) ₂ + CaCO ₃ = 2CaSO ₃ · 1 / 2H ₂ O + CO ₂ (2) CaCO ₃ 50mmol , CaSO ₃ · 1 / 2H ₂ O 50 mmol mixed slurry, but when H ₂ SO ₃ 50 mmol was added again to this, the liquid again contained 25 mmol of Ca (HSO ₃ ) ₂ and H ₂ SO ₃ Is more CaCO than CaSO ₃ 1 / 2H ₂ O (see equation (3))
Reacts with ₃ preferentially (equation (1)). If the reaction of formula (3) occurs at this time, the Ca (HSO ₃ ) ₂ concentration is about 50 m.
Must be mol. As a result, it is found that Ca (HSO ₃ ) ₂ is produced and then the reaction of the formula (2) proceeds.

After 40 minutes from the first addition of H ₂ SO ₃ , the pH increased to around 7.7 due to the presence of unreacted CaCO ₃ in the slurry, while the second addition of H ₂ SO ₃ water Later on, CaCO ₃ :
Since the molar ratio of H ₂ SO ₃ (based on the amount added) is 1: 1, the liquid does not become alkaline.

CaSO ₃ · 1 / 2H ₂ O + H ₂ SO ₃ = Ca (HSO ₃ ) ₂ + 1 / 2H ₂ O (3) The reaction of the formula (2) mainly proceeds in the circulation tank. According to the above, even if CaCO ₃ is converted to CaSO ₃ · 1 / 2H ₂ O by advancing the reaction of the formula (2), H ₂ SO ₃ ,
Therefore, the reactivity with SO ₂ is not improved at all. That is, as long as the purpose is to absorb SO ₂ , C
It is sufficient to supply aCO ₃ directly into the absorption tower. As a method of directly supplying the fresh absorbent to the absorption tower in this way, a method of supplying it to the first stage or the final stage of the absorption zone of the conventional spray absorption tower, which is premised on the installation of a dedicated oxidation tower (special Kaisho 58-61818) is known. However, in this method, since a special oxidizing device is not provided in the circulation tank, the slurry is mainly composed of calcium sulfite. On the other hand, in the method of the present invention, the main component of the slurry is gypsum because air is blown into the circulation tank, and the limestone slurry is mixed with the gypsum and then supplied to the absorption tower. When the concentration of sulfite in the slurry is high, the pH increase of the slurry is small even if limestone is added due to its pH buffering action, but in the method of the present invention, sulfite is air-oxidized to form sulfate (gypsum). Therefore, the degree of pH increase due to the addition of limestone is larger than that in the above case, and the desulfurization reaction easily proceeds.

The present invention is based on the analysis result of the basic phenomenon of the present inventor that the role of the conventional circulation tank is surprisingly small in the point of the absorption efficiency for sulfate as described above, and CaCO ₃ which is an absorbent. It is characterized in that the slurry is directly supplied to the absorption system, but at the same time, it is also greatly characterized in that calcium sulfite produced by absorbing the sulfur oxides in the exhaust gas is rapidly oxidized into gypsum in the circulation tank.
In other words, the dust removal, absorption and oxidation reactions can be carried out in the same column only after being combined with this oxidation treatment. That is, in the conventional method, the reaction of the formula (2) mainly proceeds in the absorption tower circulation tank as described above,
The CaCO ₃ particles gradually become finer, while the pH of the CaCO ₃ particles increases with the progress of the reaction, so that the dissolution reaction rate of CaCO ₃ decreases. In addition, CaCO ₃ 1/2 generated by the reaction
A so-called “blinding phenomenon” occurs in which H ₂ O crystals cover the surface of CaCO ₃ particles, and from this viewpoint also, it is impossible to completely eliminate CaCO _{3 in the} circulation tank. On the other hand, in the method of the present invention, when the slurry absorbs the sulfur oxide and then falls into the circulation tank, the gypsum-forming reaction represented by the formula (4) occurs before the reaction of the formula (2) proceeds. Therefore, the blinding phenomenon caused by CaCO ₃ 1 / 2H ₂ O is avoided.

Ca (HSO ₃ ) ₂ ＋ O ₂ ＋ 2H ₂ O ＝ CaSO ₄・ 2H ₂ O ＋ H ₂ SO ₄ (4) When this reaction proceeds, free H ₂ SO ₄ is produced and unreacted CaCO ₃ exists. Is also easily decomposed by this H ₂ SO ₄ (equation (5)).

CaCO ₃ + H ₂ SO ₄ + H ₂ O = CaSO ₄ · 2H ₂ O + CO ₂ (5) This H ₂ SO ₄ originates from the sulfur oxides in the exhaust gas,
Not freshly added sulfuric acid. Therefore, by promoting the oxidation of sulfite in the circulation tank by the method of the present invention, as a result, the utilization rate of CaCO ₃ is improved and the amount of H ₂ SO ₄ added for decomposing unreacted CaCO ₃ is reduced. It will be.

A method in which a fresh absorbent is supplied to the slurry extracted from the circulation tank and the slurry is subjected to an oxidation treatment.
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), H ₂ SO ₄ and air are supplied and unreacted. The decomposition of CaCO ₃ and the oxidation of calcium sulfite proceed. On the other hand, in the method of the present invention proposed this time, a liquid mixing container different from the circulation tank is not installed, so that the air oxidation of sulfite is performed in the circulation tank placed at the bottom of the absorption tower. When air is blown into the circulation tank in this way, the sulfite produced by absorbing the sulfur oxides is 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 characteristic is also one of the principles of the present invention. That is, Fig. 3 shows Ca
When H ₂ SO ₃ water was added to the CO ₃ slurry, the pH immediately changed to 5.
Although it shows that after decreasing to around 2, it gradually increases, but a small peak is seen within a few minutes of H ₂ SO ₃ addition because the precipitation of CaSO ₃ 1 / 2H ₂ O crystals from the solution. , That is, the solution was supersaturated with respect to calcium sulfite. The reaction at this time is shown by the equation (6).

Ca (HSO ₃ ) ₂ + 1 / 2H ₂ O = CaSO ₃ · 1 / 2H ₂ O + H ₂ SO ₃ (6) When CaSO ₃ · 1 / 2H ₂ O crystals precipitate, H ₂ SO ₃ is released and pH
Is shown to decrease. Thus CaSO ₃ 1 / 2H ₂
The low deposition rate of O indicates that a considerable portion of calcium sulfite exists as a solution when the slurry absorbing sulfur oxides falls into the circulation tank. Of course, sulfite is oxidized more rapidly when it exists as a solution than as a solid.From this point as well, air is supplied to the circulation tank to immediately oxidize sulfite that falls from the absorption section. A higher oxidation rate can be obtained by doing so, and there is also a clear difference between the present invention and the above-mentioned invention (Actual Development 58-95218). In addition, if calcium sulfite is oxidized in a supersaturated state in this manner, it becomes possible to oxidize even at a pH value higher than 5.5, which was conventionally considered to be the upper limit of the proper pH, and therefore, absorption of sulfur oxide and calcium sulfite The appropriate pH range for both oxidations will be expanded. Incidentally, in the method of supplying air into the liquid mixing container, crystal precipitation of CaSO ₃ 1 / 2H ₂ O proceeds in the circulation tank, and this crystal covers the surface of the unreacted CaCO ₃ particles, so that sulfite is added. Not only is the rate of oxidation of calcium reduced, but the utilization of CaCO ₃ is also low compared to the method of the invention.

The phenomenon that is the principle of the present invention has been described above in detail. Now, let us consider the function of the circulation tank in the conventional method. If HSO ₃ ^- is present in the liquid, the absorption of new sulfur oxides is difficult to proceed.
Although the pH of the slurry is usually adjusted to 5.0 to 6.5 by reacting with CO ₃ , the solubility of CaCO ₃ and CaSO ₃ 1 / 2H ₂ O decreases as the pH increases. Although it is said to dissolve limestone, "it does not increase the concentration of Ca ²⁺ or CO ₃ ^{2- in} the solution. In other words, the function of the circulation tank in the conventional method is to improve the desulfurization performance of the slurry by eliminating HSO ₃ ⁻ generated by the absorption of sulfur oxide by the reaction of the formula (2), and at the same time, to decompose the unreacted CaCO ₃ H ₂ SO to be added before the oxidation of calcium sulfite
It is thought to be to reduce the amount of ₄ . In contrast, in the process of the present invention, the circulation tank should be regarded as an oxidizer rather, so that its size may be determined from the oxidation rate of calcium sulfite. As described above, the oxidation of Ca (HSO ₃ ) ₂ (Equation (4)) causes H ₂ SO ₄
However, since the reaction between H ₂ SO ₄ and CaCO ₃ (Equation (5)) proceeds much faster than the reaction of Equation (2), unreacted as long as HSO ⁻ oxidation progresses. The amount of CaCO ₃ is extremely low. It should be noted that from the experimental results (Fig. 4), it is possible to obtain the oxidation rate (about 40 mmol / l · h) that is considered necessary for omitting the oxidation tower by supplying air near the blades of the stirrer installed in the circulation tank. it is obvious.

As described above, in the method of the present invention, the content of the circulation tank is made into a gypsum-based slurry by injecting air into the circulation tank and directly charging the calcium compound slurry as the absorbent to the upper part of the absorption tower. You can A part of the slurry extracted from the circulation tank is guided to a thickener 112 through a pipe line 111, and solid-liquid separated, and then dehydrated by a centrifuge 113 or the like to recover gypsum 114. The filtrate 115 at the time of solid-liquid separation and dehydration is reused for preparation of limestone slurry and the like. In addition,
Limestone 116, which is a sulfur oxide absorbent, was added to the limestone slurry preparation tank 117, and filtered water 115 and makeup water 118 were added.
It is mixed with limestone slurry 119 and guided to the absorption tower. As a method of supplying the calcium-based compound slurry represented by this limestone into the absorption tower, a method of directly spraying only this can be carried out, but in order to smoothly proceed the desulfurization reaction, a liquid of about 10 to 15 is usually used. Considering that the gas ratio (l-liquid / Nm ³ -gas) is required, it is preferable to mix the slurry 105 with the slurry 105 taken out from the circulation tank and increase the flow rate to spray-supply the mixture into the column. . As the mixing position, select the outlet of the circulation pump 104 placed at the outlet of the circulation tank so that the slurry sprayed to the absorption tower is all fresh.
It may also contain CaCO ₃ (supply line 12
It is also possible to mix fresh limestone slurry only in a specific spray stage (supply line 121).

(Effect of the invention) In the wet treatment method of the present invention for treating sulfur oxides in exhaust gas, a new absorbent (limestone) is charged and mixed in a circulation tank, and the mixed liquid is extracted and sprayed from the upper part of the absorption tower. Unlike the conventional method, the small amount of slurry extracted from the circulation tank is mixed with new limestone, and the sulfur oxide is absorbed and removed by spraying at the upper part of the absorption tower, so the pH of the absorbent slurry is high, High absorption capacity for sulfur oxides.

Also, in the circulation tank, since only the absorbent that has absorbed sulfur oxides is oxidized in the upper part of the absorption tower, the obtained gypsum has a high purity, and the newly replenished limestone becomes gypsum without absorbing action. There is no need to waste it.

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

Example 1 The absorption tower has a diameter of 0.3 m and a height of 7.1 m, and the circulation tank at the bottom has a diameter of
A gypsum slurry was charged into a circulation tank of an exhaust gas desulfurization device having a height of 1.0 m and a height of 1.2 m so that the concentration was 530 mmol / l. Coal combustion exhaust gas with SO ₂ concentration of 740 ppm and O ₂ concentration of 6% was supplied to this at a flow rate of 580 Nm ³ / h, and the slurry in the circulation tank was circulated so that L / G was 15 in the absorption tower. 2 inch nozzle (4
h) sprayed. The 20% limestone slurry was adjusted so that the pH of the slurry in the circulation tank would be 5.5.
Was added by the method of using the
Agitator blade (propeller type φ12 cm, rotation speed 1000 rpm)
When air was blown onto the tank at a flow rate of ² Nm ² / h, the composition of the slurry in the circulation tank was: total Ca 493 mmol / l, sulfite 8.0 mmol / h.
l, CaCO ₃ 3.5 mmol / l, almost equilibrium state, oxidation rate 99
It became%. At this time, the desulfurization rate was 92%.

Example 2 In Example 1, C was adjusted so that the pH of the circulation tank was 6.0.
The supply of aCO ₃ slurry was adjusted. The desulfurization rate was 94%, and the oxidation rate calculated from the slurry composition in the circulation tank was 98.5%.

Reference Example 1 In Example 1, when only desulfurization operation was performed with the air supply to the circulation tank stopped, the desulfurization rate was 91.5.
%, The oxidation rate (natural oxidation rate) was 53%.

Reference Example 2 The slurry (pH 5.5) obtained in Example 1 of the present invention and 500 ml of the slurry (pH 5.85) obtained from the conventional absorption tower circulation tank shown in FIG. 5 were placed in a beaker at 30 ° C. The pH of the slurry was measured with stirring. P after 1, 2, 4, 6 and 10 minutes was 5.70 (6.00), 5.81 (6.06),
It is 6.01 (6.15), 6.17 (6.19) and 6.36 (6.24), and it can be seen that the limestone contained therein is dissolved faster in the slurry obtained by desulfurization while oxidizing. The value in parentheses is the pH for the conventional absorption tower circulation tank slurry.

[Brief description of drawings]

FIG. 1 is a system diagram for explaining a method for treating sulfur oxides in exhaust gas according to the present invention, and FIG. 2 is a shape of a circulation tank stirring blade and oxidation of sulfite relating to the present invention. Drawing showing the relationship of velocity, Fig. 3 is a drawing showing the experimental results on the reaction of limestone (CaCO ₃ ) and sulfite, and Fig. 4 is
FIG. 5 is a drawing showing the oxidation rate of sulfite according to the present invention, and FIG. 5 is a drawing explaining a conventional method for treating sulfur oxides in exhaust gas. 101 ... Exhaust gas, 102 ... Absorption tower, 103 ... Circulation tank, 104 ... Circulation pump, 105 ... Absorber slurry, 109 ... Stirrer, 110 ... Air, 111 ... Gypsum slurry extraction tube, 112 ... Thickener, 113 ... Centrifuge , 114 ... Gypsum, 117 ... Limestone adjustment tank, 120 ...
Limestone slurry supply pipe, 122… Spray nozzle.

Front Page Continuation (72) Inventor Satoshi Nishimura 6-9 Takara-cho, Kure City, Hiroshima Prefecture Babkotuku Hitachi Ltd. Kure Factory (56) Reference JP-A-52-19175 (JP, A) JP-A-59-13624 ( JP, A) Actual development Sho 58-95216 (JP, U)

Claims (1)

[Claims] 1. A method for removing sulfur oxides in exhaust gas by contacting with a calcium-based absorbent slurry extracted from an absorption tower circulation tank in an absorption tower, wherein the absorbent extracted from said circulation tank. A new calcium-based absorbent is added to the slurry to bring it into contact with the sulfur oxides in the exhaust gas in the absorption tower, and the absorbent slurry after the contact is stored in the circulation tank, and the slurry in the tank contains oxygen. A method for removing sulfur oxides in exhaust gas, which comprises supplying gas to cause a reaction and introducing the reacted slurry into a separator to separate gypsum from the slurry.