JP2002336642A - Wet type waste gas desulfurizing method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform the high purification of gypsum recovered from a desulfurizing apparatus relatively simply and efficiently. SOLUTION: When gypsum is recovered as a by-product by bringing sulfur oxide in a waste combustion gas 1 into contact with an absorbing solution containing a calcium based absorbent in an absorbing tower 2 for absorption treatment and oxidizing the absorbing solution, in which the sulfur oxide is absorbed, in a circulation tank 7, high purification of the gypsum 14 is efficiently performed by supplying an acidic gypsum cleaning water 18 on a dehydrated cake layer, which consists essentially of the gypsum 14 and is formed in a certain degree. to dissolve and remove unreacted limestone or heavy metals existing as a solid in the gypsum 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wet exhaust gas desulfurization method and apparatus, and more particularly to a wet exhaust gas desulfurization method and apparatus suitable for dewatering gypsum as a by-product.

[0002]

2. Description of the Related Art As a device for removing sulfur oxides from combustion exhaust gas of a boiler or the like for preventing air pollution, wet limestone is used.
Gypsum desulfurization equipment has been widely put into practical use. FIG. 3 shows a conventional technique of this wet limestone-gypsum desulfurization apparatus.

[0003] Exhaust gas 1 containing sulfur oxides generated from a combustion device such as a thermal power plant is led to an absorption tower 2 of a desulfurization device. In the absorption tower 2, a spray header 3 having a large number of spray nozzles 4 is installed, and by contacting the exhaust liquid 1 with an absorbing liquid sprayed as fine droplets from the spray nozzle 4, sulfur oxides in the exhaust gas are absorbed. Chemically removed at the surface of the droplet. Small droplets accompanying the flow of exhaust gas in the absorption tower 2 are removed by a mist eliminator 5 installed on the upper part of the absorption tower 2, and the purified gas 6 that has been purified is fed to the downstream side of the absorption tower 2 as necessary. The temperature is raised by a reheating facility (not shown) installed and discharged from the chimney. Most of the droplets sprayed from the spray nozzle 4 absorb the sulfur oxides and then fall into the absorption tower circulation tank 7 provided at the lower part of the absorption tower 2. A part of the absorption liquid in the absorption tower circulation tank 7 is sent to the absorption tower 2 by the circulation pump 17, is sprayed from the spray nozzle 4 via the spray header 3, and comes into gas-liquid contact with the combustion exhaust gas again.

[0004] Sulfur oxide (SO ₂ ) absorbed in the absorbing solution
Reacts with limestone (CaCaSO ₃ ) contained in the absorption liquid, and is further oxidized by air 8 supplied to the absorption tower circulation tank 7 to form gypsum (CaSO ₄ .2H ₂ O).
This series of reactions is represented by the following formula. SO ₂ +2
_{H 2 O + CaCO 3 + 1} / 2O 2 → CaSO 4 · 2H 2
O + CO ₂

On the other hand, as for the limestone as the absorbent, the limestone component stored as limestone slurry in the limestone supply equipment 10 is supplied to the absorption tower circulation tank 7 by the pump 11. Further, in order to collect the gypsum generated in the absorption tower 2, a part of the absorption liquid in the absorption tower circulation tank 7 is extracted, and a pump 12
To the gypsum dewatering facility 13 to collect gypsum and dust contained in the absorbent as gypsum 14. Although different depending on the plant design conditions, the absorbent slurry mainly composed of gypsum 14 is led to the gypsum dewatering facility 13 with a gypsum content of about 10 to 30%. Generally, in a gypsum dewatering facility 13 using a centrifuge or a belt filter, the gypsum is collected as powdered gypsum containing about 10% or less of attached water.

The adhesion water in the gypsum is magnesium (M
g), chlorine (Cl), and metal ions such as sodium (Na) are dissolved, and these are not preferable for reuse of gypsum. Therefore, in the dehydration step in the gypsum dewatering equipment 13, gypsum washing with gypsum washing water 19 (industrial) Replacement with water).

[0007] The gypsum and dust dewatered filtrate 16 discharged from the gypsum dewatering facility 13 is once stored in the washing and recovery liquid tank 21 and then partially prevented from concentrating impurities in the system. Is discharged out of the system as wastewater, and a part of the remaining liquid is sent to the absorption tower circulation tank 7 of the absorption tower 2 as it is. A part of the liquid being sent to the absorption tower circulation tank 7 is sent to the limestone supply facility 10 and used as makeup water for limestone slurry production. The desulfurization wastewater extracted from the wastewater line 15 is sent to a wastewater treatment facility (not shown), and discharged after removing fluorine, heavy metals, and the like.

In recent years, due to the diversification of boiler fuels, even in the same plant, the range of change in sulfur oxide concentration in exhaust gas is large,
In addition, the operating load changes according to demand, but not only requires high desulfurization performance under all operating conditions, but also recovers gypsum of stable quality even if the amount of gypsum produced increases and decreases due to this. In particular, it is required that the gypsum has an attached water content of 10% or less. Recently, in order to utilize gypsum more effectively and to recover gypsum that does not cause secondary pollution from the desulfurization equipment even after disposal, to purify gypsum and produce high-grade gypsum containing as little as possible heavy metals Is strongly required.

[0009]

The gypsum recovered by the dewatering system using the wet exhaust gas desulfurization apparatus according to the prior art described above contains unreacted limestone. Pretreatment is required in the production of cement and gypsum board used, so not only manufacturers of cement and gypsum board etc. tighten the pickup conditions, but in the worst case plants with desulfurization facilities such as refusal to pick up This can be a very difficult situation for you.

[0010] Further, gypsum contains some heavy metals collected from exhaust gas and impurities from limestone which is an absorbent, which not only causes deterioration of the quality of the recovered gypsum, There is also a concern that problems such as secondary heavy metal contamination may occur when the gypsum that has been effectively used is discarded after being used as cement or gypsum board.
The gypsum washing recovered from the desulfurization unit using process water, which is currently being performed, is effective for replacing the filtrate that becomes water adhering to gypsum, but unreacted limestone and heavy metals that exist as solids in gypsum Is not always effective.

An object of the present invention is to purify gypsum recovered from a desulfurization apparatus relatively easily and rationally.

[0012]

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for absorbing sulfur oxides in a combustion exhaust gas by bringing the sulfur oxides into contact with an absorbent containing a calcium-based absorbent, and then absorbing the sulfur oxides. In a wet exhaust gas desulfurization method for recovering gypsum as a by-product by oxidizing treatment, an acidic washing water is supplied after a dehydrated cake layer mainly composed of gypsum is formed,
This is achieved by efficiently purifying gypsum by dissolving and removing unreacted limestone and heavy metals present as a solid in gypsum.

That is, according to the present invention, sulfur oxides in the combustion exhaust gas are brought into contact with an absorbent containing a calcium-based absorbent to perform an absorption treatment, and then the absorbent absorbing the sulfur oxides is oxidized to produce gypsum as a by-product. This is a wet exhaust gas desulfurization method in which gypsum produced as a by-product is washed with acid-containing water and then dewatered. By removing impurities such as heavy metals in the gypsum washing / recovery liquid during the gypsum dehydration treatment, the gypsum washing / recovery liquid can be purified.

Further, the amount of the absorbing liquid to be brought into contact with the exhaust gas in the absorption tower is changed according to the load of the boiler or the like.
Therefore, the generated gypsum amount also changes according to the load. Therefore, the acid content in the acid-containing water used as the gypsum washing water is changed so that the pH of the washing water before the acid addition becomes acidic, so that the generated gypsum is washed with an appropriate amount of the acid-containing water, and high-quality gypsum Can be recovered. In addition, while controlling the quality of the washing water based on the specific gravity or turbidity of the gypsum washing water, the pH of the gypsum washing water is controlled by controlling the water content and the acid content.
To make it possible to recover high quality gypsum. Further, a method of changing the acid content in the acid-containing water used as the gypsum washing water so that the pH of the gypsum washing water becomes acidic according to the amount of gypsum to be generated may be used.

Another object of the present invention is to provide an absorption tower for introducing a flue gas and bringing a sulfur oxide in the flue gas into contact with an absorbent containing a calcium-based absorbent to perform an absorption treatment;
The absorption liquid that has absorbed the sulfur oxides is recovered, the recovered liquid is oxidized to form gypsum, and a part of the recovered liquid is circulated in the absorption tower. The solution can be solved by a wet exhaust gas desulfurization apparatus equipped with a gypsum washing / dewatering tank for dewatering after washing with the contained water.

For this purpose, a measuring instrument for measuring at least one of pH, specific gravity, and turbidity of a gypsum washing / recovery liquid obtained from a gypsum washing / dewatering tank and a measured value of the at least one measuring instrument are used. A control device is provided for controlling the acid concentration of the gypsum washing water so that the pH of the gypsum washing water becomes acidic.

[0017]

In the gypsum washing / dewatering apparatus, a stable dewatered cake layer containing gypsum as a main component is formed during the dehydration of gypsum recovered from the desulfurization apparatus. By supplying acidic washing water to the upper part of the gypsum dehydrated cake layer, impurities in the gypsum dehydrated cake layer are dissolved and removed. Unreacted limestone, which is contained in a relatively large amount as compared with heavy metals, generates CO ₂ (carbon dioxide gas) when decomposed by acid, and this CO ₂ gas forms a new washing water flow path in the dewatered cake. Therefore, the effect of washing the gypsum is enhanced, and the purification of the gypsum can be efficiently performed.

If the recovered liquid after the acid washing of the gypsum dewatered cake layer is reused in a wet exhaust gas desulfurization apparatus, the specific gravity and turbidity of the recovered liquid are detected from the viewpoint of quality control. It is effective to control the amount of acid gypsum washing water and the amount of acid used for gypsum washing.

[0019]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an overall configuration diagram of a wet exhaust gas desulfurization apparatus of the present embodiment. The device of the present embodiment is shown in FIG.
And the exhaust gas 1 containing sulfur oxides generated from a combustion device such as a thermal power plant is guided to an absorption tower 2 of the desulfurization device. In the absorption tower 2, the absorbing liquid sprayed from the spray nozzles 4 of each of the spray headers 3 provided in a plurality of rows in a horizontal direction and a plurality of stages in a vertical direction comes into contact with the introduced exhaust gas 1 to absorb sulfur oxides in the exhaust gas. Chemically removed at the surface of the droplet. Fine droplets accompanying the flow of the exhaust gas are removed by a mist eliminator 5 installed above the absorption tower, and the purified gas 6 is heated and discharged from the chimney. Most of the droplets sprayed from the spray nozzle 4 absorb the sulfur oxides,
Falls into the absorption tower circulation tank 7 provided at the lower part of the tank. A part of the absorption liquid in the absorption tower circulation tank 7 is sprayed from the spray nozzle 4 via the spray header 3 using the circulation pump 17, and comes into gas-liquid contact with the combustion exhaust gas again. The sulfur oxides absorbed in the absorbing solution react with limestone contained in the absorbing solution, and are further oxidized by the air 8 supplied to the absorption tower circulation tank 7 to form gypsum.

On the other hand, limestone as an absorbent is supplied to the absorption tower circulation tank 7 by a limestone slurry pump 11 stored as limestone slurry in a limestone supply facility 10. The gypsum generated in the absorption tower 2 extracts a part of the absorption liquid in the absorption tower circulation tank 7 and is sent to the gypsum dewatering equipment 13 by the pump 12, and the gypsum and dust in the absorption liquid are removed from the gypsum 1.
Collected as 4.

The absorbent slurry containing gypsum 14 as a main component is introduced into the gypsum dewatering equipment 13 and collected in the gypsum dewatering equipment 13 as powdered gypsum having about 10% or less of adherent water. Further, the gypsum is washed with gypsum washing water 19 in order to wash and remove the attached water in which metal ions such as Mg, Cl, and Na are dissolved in the gypsum.

The dehydration filtrate 16 of the absorption tower gypsum and the dust discharged from the gypsum dewatering equipment 13 is temporarily washed and recovered in the tank 2.
Where the pH of the collected liquid is 22 and the specific gravity is 2
3. Measure various properties of the recovered liquid with the turbidimeter 24.

In order to prevent the concentration of impurities in the system, a part of the liquid is discharged out of the system through a drain line 15 as waste water, and a part of the remaining liquid is directly sent to the absorption tower circulation tank 7 of the absorption tower 2. It is sent. The remaining liquid is used by the limestone supply facility 10 as makeup water for limestone slurry production. Further, the desulfurization wastewater extracted from the wastewater line 15 is sent to a wastewater treatment facility (not shown), and discharged after removing fluorine, heavy metals, and the like.

Although a centrifugal separator can be used as the gypsum dewatering equipment 13, it is preferable to use a belt filter type equipment. Slurry concentration 1 extracted from absorption tower 2
The gypsum slurry of 0 to 30% is supplied to the gypsum dewatering equipment 13 via a gypsum tank (not shown) or directly, and the gypsum slurry is dehydrated so as to have a predetermined adhesion water amount or less, and the gypsum 14 is collected. .

The present embodiment is characterized in that an acidic water 18 for gypsum washing is prepared by adding an acid to the makeup water or industrial water. In the case of performing gypsum washing with the makeup water or industrial water, it is preferable to supply the acid gypsum washing water 18 after the supply of the gypsum slurry and before the gypsum washing with the gypsum washing water 19. The reason is that although it depends on the amount of acid to be supplied, even if the acid gypsum washing water 18 is supplied in a large excess, unreacted acid can be washed out by gypsum washing with the makeup water or industrial water.

The recovered liquid after the gypsum washing is recovered in the process and reused. The pH of the recovered solution after this washing is on the acidic side (pH <7) due to a slight excess of acid,
Acid gypsum washing water when impurities become more than supplied acid 1
There is a characteristic that the pH of 8 is on the alkaline side. The pH of the collected liquid after the gypsum washing is measured by a pH meter 22 provided in the collected liquid tank 21, and the amount of acid and water used as the acid gypsum washing water 18 is controlled by the control device 25 to control the amount of impurity reduction and Operation costs can be reduced.

Based on the specific gravity and / or turbidity of the recovered solution measured by a specific gravity meter 23 and / or a turbidity meter 24 in addition to the pH meter 22, the amount of acid and water (supplementary water or industrial water) Determine the quantity of 19).

Basically, the amount of acidic gypsum washing water 18 or the concentration of acid is controlled so that the pH of the recovered solution after gypsum washing is always acidic, but it is not necessary to completely remove impurities. In some cases, the pH of the recovered liquid may be set to the alkaline side.

The belt filter type gypsum dewatering equipment 13 will be described with reference to the schematic side view of FIG. The gypsum slurry supplied to the belt filter cloth 26 is subjected to formation and dewatering of the gypsum cake 14 ′ on the belt filter cloth 26 that has been absorbed and filtered by the vacuum pump 27.

The belt filter cloth 26 is continuously or batchwise moved or rotated in an endless belt conveyor system.
The structure enables continuous washing and dehydration. The belt filter cloth 26 and the acidic gypsum washing water 18 are collected through a filtrate receiver 28.

The gypsum cake 14 'formed in a filter cloth shape on the belt filter cloth 26 is mainly composed of gypsum containing coarse particles on the bottom surface and fine impurities on the surface. The acidic gypsum washing water 18 and the gypsum washing water 19 are supplied from the top of the gypsum cake 14 ', but some unreacted limestone has a particle size equivalent to the gypsum particle size, and there are many gypsum cakes in the gypsum cake 14' and at the bottom. I have. In addition, acid gypsum washing water 1
The supply of 8 may be one stage, but by making it multistage,
The gypsum 14 can be more efficiently washed.

The acid used for the acid gypsum washing water 18 is preferably sulfuric acid in view of handling gypsum slurry.
It is also possible to use other acids. When sulfuric acid is used, unreacted limestone is converted into gypsum 14 by the following reaction, which is particularly effective for purifying gypsum as compared with adding another acid. CaCO ₃ + H ₂ SO ₄ + H ₂ O → CaS
_{O 4 · 2H 2 O + CO} 2 ↑

In the operation of the dewatering system in the gypsum dewatering equipment 13, the amount of gypsum produced changes because the amount of exhaust gas and the concentration of SO ₂ in the exhaust gas at the inlet of the absorption tower 2 change due to a change in boiler load. To cope with this, it is effective from the viewpoint of economical operation to make the supply of acid to the gypsum dewatering equipment 13 variable.

The amount of gypsum produced can be detected by the amount of gypsum slurry supplied to the gypsum dewatering equipment 13, and the amount of acid washing water (acid gypsum washing water 18) may be increased or decreased based on this. Also, based on the pH value of the recovered washing water described above,
It is also possible to increase or decrease the amount of pickling water.

It is effective from the viewpoint of economical operation to collect and reuse the washing water after the gypsum washing in the gypsum dewatering facility 13. The recovered washing water contains impurities such as heavy metals contained in the gypsum and removed from the gypsum after acid washing, so it is possible to reuse it as it is. It is preferable to recycle the solution by sending it to the wastewater or adding a step of removing impurities in the recovered solution.

The impurities removed from the gypsum 14 dissolve and float in the acid washing and recovering liquid from the gypsum dewatering equipment 13. However, if the washing is insufficient, the turbidity and specific gravity increase. The specific gravity meter 23 and the turbidity meter 2 are used in the tank 21 (FIG. 1).
By monitoring this in step 4, the amount of water and the amount of acid necessary for washing the gypsum with acid are determined. In particular, in the case of a process for reusing the collected liquid after the gypsum acid washing, from the viewpoint of quality control, the specific gravity and turbidity of the collected liquid are detected, and the gypsum washing acidic water 18 used for the gypsum washing is used. It is effective to control the amount of water and the amount of acid.

The step of recovering the impurities removed from the gypsum includes a method such as ion exchange or coagulation and precipitation by adding an alkali and a flocculant, but the method of recovering the impurities is not limited to the present invention. .

As shown in FIG. 2, the gypsum component in the gypsum slurry supplied on the belt filter cloth 26 of the gypsum dewatering equipment 13 is a gypsum cake 14 mainly composed of gypsum formed on the surface of the belt filter cloth 21. 'Is the belt filter cloth 2
6 has a dehydration hole by receiving suction from the lower part of the same, and dehydration proceeds through this dehydration hole. Here, by supplying a new washing liquid containing acid (acid gypsum washing water 18), a decomposition reaction of unreacted limestone contained in the gypsum occurs, and CO ₂
Although gas is generated, new pores are further formed in the gypsum cake 14 ′, and it has been found for the first time in the present invention that the gas has an effect (pore generation effect) and a dehydration effect more than the mere removal of impurities.

It is also possible to adjust the concentration of the acid washing water (acid gypsum washing water 18) and omit the washing with the supplementary water or industrial water 19 in the latter stage. In other words, depending on the washing effectiveness of the supplied acid, it is possible to make the pH of the recovered gypsum excessively acidic by supplying washing water having an acid concentration equal to or less than the alkali content of unreacted limestone or heavy metals contained in the gypsum. It is because it disappears.

An acid is previously added to the gypsum slurry supplied to the gypsum dewatering equipment 13 to dissolve unreacted limestone and heavy metals in the acid.
There is a method of purifying the gypsum 14 by supplying unreacted limestone or heavy metals to the filtrate side by supplying the gypsum to the gypsum without supplying it to the gypsum.

However, in the present invention, as described above, gypsum 1
By adding an acid to the gypsum cake 14 ′ containing 4 as a main component, the unreacted limestone is decomposed to generate CO ₂ gas. There is an improvement in dewatering performance in equipment.

[0042]

According to the present invention, it is possible to configure a desulfurization system capable of reducing impurities in gypsum, achieving high purity, and operating a dehydrator stably in a wide range of operation.
Further, since the dewatering performance is improved, the dewatering equipment and the amount of gypsum adhering water can be reduced.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of a wet exhaust gas desulfurization apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic side view of the gypsum dewatering equipment of the belt filter type according to the present invention.

FIG. 3 is a diagram illustrating a conventional wet limestone-gypsum desulfurization apparatus.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Exhaust gas 2 Absorption tower 3 Spray header 4 Spray nozzle 5 Mist eliminator 6 Purification gas 7 Absorption tower circulation tank 8 Air 9 Limestone 10 Limestone supply equipment 11 Limestone slurry pump 12 Pump 13 Gypsum dewatering equipment 14 Gypsum 14 'Gypsum cake 15 Drainage line 16 Dehydration filter 17 Circulation pump 18 Acid gypsum washing water 19 Gypsum washing water 21 Washing and recovery liquid tank 22 pH meter 23 Specific gravity meter 24 Turbidimeter 25 Controller 26 Belt filter cloth 27 Vacuum pump 28 Filtrate receiver

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (reference) F23J 15/04 F23J 15/00 EF term (reference) 3K070 DA02 DA16 DA38 DA43 4D002 AA02 AA22 AA28 AC01 BA02 BA16 BA20 CA01 CA20 DA05 DA16 DA26 EA02 EA12 FA03 GA02 GA03 GB08 GB09 HA04 4G076 AA14 AB02 AB26 BA33 BC06 BD06 BH01 GA01

Claims (7)

[Claims] Claims: 1. Sulfur oxide in a combustion exhaust gas is brought into contact with an absorbent containing a calcium-based absorbent to perform an absorption treatment, and then the absorbent absorbing the sulfur oxide is oxidized to recover gypsum as a by-product. A wet exhaust gas desulfurization method comprising: washing gypsum produced as a by-product with acid-containing water, followed by dehydration treatment. 2. The wet exhaust gas desulfurization method according to claim 1, wherein impurities in the gypsum washing and recovery liquid after the gypsum dewatering treatment are removed. 3. The method according to claim 1, wherein the acid content in the acid-containing water used as the gypsum washing water is changed in accordance with the pH of the washing water before adding the acid, so that the pH of the gypsum washing water becomes acidic. Item 6. The wet exhaust gas desulfurization method according to Item 1. 4. The pH of the gypsum washing water is adjusted to be acidic by controlling the water content and the acid content of the acid-containing water used as the gypsum washing water based on the specific gravity or the turbidity of the gypsum washing recovery liquid. The wet exhaust gas desulfurization method according to claim 1, wherein: 5. The method according to claim 1, wherein the acid content in the acid-containing water used as the gypsum washing water is changed so that the pH of the gypsum washing water becomes acidic according to the amount of gypsum produced.
The wet exhaust gas desulfurization method as described in the above. 6. An absorption tower for introducing a flue gas and bringing a sulfur oxide in the flue gas into contact with an absorbent containing a calcium-based absorbent to perform an absorption treatment, and an absorption column for absorbing the sulfur oxide in the absorption tower. The liquid is collected, the recovered liquid is oxidized to form gypsum, and a part of the recovered liquid is circulated to the absorption tower. An absorption tower circulation tank, and gypsum produced as a by-product is washed with acid-containing water and then dehydrated. A wet exhaust gas desulfurization device comprising a gypsum washing / dewatering tank to be treated. 7. A measuring instrument for measuring at least one of pH, specific gravity, and turbidity of a gypsum washing / recovery liquid obtained from a gypsum washing / dehydrating tank, and a gypsum based on a measurement value of the at least one measuring instrument. 7. The wet exhaust gas desulfurization apparatus according to claim 6, further comprising a control device for controlling an acid concentration of the gypsum wash water so that the pH of the wash water becomes acidic.