JPH08243348A - Operation of wet desulfurizer - Google Patents

Abstract

PURPOSE: To prevent the supersaturation with gypsum and to obviate a decrease in the average grain diameter of the generated gypsum by detecting the sulfurous acid concn., etc., in an oxidation tank and controlling the amt. of air to be supplied to the oxidation tank. CONSTITUTION: The detection signal from the sulfurous acid detector 20, dissolved oxygen detector 27 and carbonic acid detector 21 arranged in a liq. absorbent extracting pipe 10 is sent to a computing element 19. The oxidation condition of the slurry absorbent in an oxidation tank 6 is calculated by the element 19 based on the signal, the signal is sent to a blower 14, and an optimum amt. of air is introduced into the oxidation tank 6. The sulfurous acid concn. or dissolved oxygen concn. or carbonic acid concn. in the slurry absorbent is detected in this way to control the amt. of air to be supplied into the oxidation tank, hence the oxidation reaction of sulfurous acid is suppressed, the oxidation condition of the slurry absorbent in the oxidation tank is optimized, and the supersaturation with gypsum is prevented. Accordingly, the diameter of the generated gypsum grain is not decreased even if the condition in the oxidation tank is changed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wet flue gas desulfurization apparatus for removing sulfur compounds in exhaust gas by using a slurry liquid containing calcium carbonate, and particularly to the amount of air supplied to an oxidation tank of the flue gas desulfurization apparatus. The present invention relates to a driving method for controlling the.

[0002]

2. Description of the Related Art Sulfur oxides, especially sulfur dioxide (SO ₂ ) in flue gas generated by burning fossil fuels in thermal power plants is a main cause of environmental problems such as air pollution and acid rain. one of. For this reason, research and development of a flue gas desulfurization method for removing SO ₂ from flue gas have become extremely important subjects.

As this desulfurization method, recently, a simple type dry desulfurization apparatus having a low cost and a simple system is being developed, but the desulfurization rate is as low as 70 to 80% at best, and therefore the wet method is still available. Is the mainstream.

The wet method includes a soda method using a soda compound as an absorbent, a calcium method using a calcium compound, and a magnesium method using a magnesium compound. Among these, the soda method is excellent in reactivity between the absorbent and SO ₂ , but the soda used is very expensive. For this reason, a method using a relatively inexpensive calcium compound such as calcium carbonate is most often used for a flue gas desulfurization device such as a large-scale boiler for power generation.

Desulfurization systems using this calcium compound as an absorbing liquid are roughly classified into three types, a spray system, a leak wall system, and a bubbling system, depending on the difference in gas-liquid contact method. Although each method has its own characteristics, the spray method, which has a proven track record and is highly reliable, is widely used worldwide.

As the spray type desulfurization system, a cooling tower for cooling and preventing dust of exhaust gas, an absorption tower for spraying an absorbing liquid to react with SO ₂ in the exhaust gas, and oxidizing calcium sulfite produced in the absorbing tower have been conventionally used. It consisted of three oxidation towers. However, in recent years, the development of a one-tower type desulfurization tower in which the absorption tower has functions of cooling and oxidation has advanced, and recently, the one-tower type desulfurization system has become the mainstream of the spray method.

FIG. 8 shows an example of a one-tower type desulfurization apparatus by a conventional spray method. The one-tower type desulfurization tower mainly includes a desulfurization tower main body 1, an inlet duct 2, an outlet duct 3, a spray nozzle 4, an absorption liquid circulation pump 5, an oxidation tank 6, an agitator 7, an air blowing pipe 8, a demister 9 and the like. Composed of. A plurality of spray nozzles 4 are installed in the horizontal direction and a plurality of stages in the height direction, and normally one circulation pump 5 is installed in each stage. Generally, the number of stages of the spray nozzle 4 is generally about 4 to 10, but in this figure, four stages are shown for simplification.

Further, the agitator 7 and the air blowing pipe 8 are installed in the oxidation tank 6 in the lower part of the desulfurization tower where the absorbing liquid stays,
The demister 9 is installed in the uppermost part of the absorption tower or in the outlet duct 3.

Exhaust gas 24 discharged from a boiler (not shown) is introduced into the desulfurization tower main body 1 through the inlet duct 2, and finally the treated gas 25 is discharged through the outlet duct 3.
During this time, the desulfurization tower is sprayed with the absorption liquid containing calcium carbonate sent from the circulation pump 5 from the spray nozzle 4 attached to the spray pipe 26 arranged in the desulfurization tower, and the absorption liquid and the exhaust gas are brought into contact with each other in gas-liquid contact. Is done.

At this time, the absorbing liquid selectively absorbs SO ₂ in the exhaust gas, and calcium bisulfite [Ca (HS
O ₃ ) ₂ ]. The fine-grained calcium carbonate (limestone CaCO ₃ ) that is the absorbent is supplied into the oxidation tank 6 by the limestone supply pipe 13. The absorption liquid that has generated calcium bisulfite is accumulated in the oxidation tank 6, and while being stirred by the stirrer 7, the air supplied from the air blowing pipe 8 forms the bubble group 23 in the absorption liquid, and the calcium bisulfite is oxidized. Produces gypsum (CaSO ₄ ). A part of the absorption liquid in the oxidation tank 6 in which calcium carbonate and gypsum coexist is sent to the spray nozzle 4 again by the circulation pump 5, and a part is sent from the absorption liquid extraction pipe 10 to the waste liquid treatment / gypsum recovery system. .

Of the absorbing liquid sprayed from the spray nozzle 4 and atomized, the absorbing liquid having a small droplet diameter is entrained in the exhaust gas, but is recovered by the demister 9 provided above the desulfurization tower. The desulfurization reaction and the reaction in the oxidation tank 6 described above are shown as follows.

(Spray part) SO ₂ + H ₂ O → H ₂ SO ₃ (1) CaCO ₃ + 2H ₂ SO ₃ → Ca (HSO ₃ ) ₂ + CO ₂ + H ₂ O ... (2) Ca (HSO ₃ ) ₂ + O ₂ + 2H ₂ O → CaSO ₄ · 2H ₂ O + H ₂ SO ₄ (part) …… (3) (oxidation tank) Ca (HSO ₃ ) ₂ + O ₂ + 2H ₂ O → CaSO ₄ · 2H ₂ O + H ₂ SO ₄ …. (3) CaCO ₃ + H ₂ SO ₄ + H ₂ O → CaSO ₄ · 2H ₂ O + CO ₂ (4) Also, the gypsum produced in the oxidation tank 6 is discharged as a slurry waste liquid from the absorption liquid extraction pipe 10 to recover the gypsum. The device 12 collects the gypsum.

When operating under normal conditions, even if a large amount of air is supplied to the oxidation tank 6, the oxidation tank 6
When the gypsum concentration in the inside is about 10 wt%, the particle size of the generated gypsum is several tens of μm and there is no particular problem. However, if the slurry concentration is tripled to 30 wt% and the thickener is omitted in the gypsum recovery system, the amount of gypsum particles in the slurry will be tripled and the growth rate of the gypsum particles will be slowed down. If the time is not increased, the particle size of the gypsum produced will be smaller.

Particularly, when the slurry concentration is increased, the particle size is decreased and the viscosity of the liquid is increased, so that the desulfurization rate is lowered. That is, it is very important to control the gypsum particle size in the oxidation tank 6 within a certain range in order to maintain the desulfurization performance.
Further, when the particle size of the generated gypsum is small, when the gypsum is collected, the wire mesh of the centrifuge is clogged with the gypsum.

Further, when the air supply amount is increased, the amount of dissolved oxygen in the slurry is increased. Therefore, the oxidation reaction of sulfurous acid in the above formula (3) is accelerated, and the generated gypsum (CaSO ₄ ) becomes supersaturated. Seed crystals are generated in the and the gypsum particle size shifts to the smaller one.

When a desulfurizing agent containing magnesium oxide (MgO) such as dolomite is used instead of limestone, the rate of dissolution of dolomite is low, so that the amount of air supplied should be increased as much as possible for rapid dissolution. It is important for improving desulfurization performance.

However, when the air is excessively supplied, the generated gypsum (CaSO ₄ ) becomes supersaturated by the reaction of the formula (3), new seed crystals are generated, and the gypsum particle size shifts to the smaller one. In the current operation, in order to make the concentration of sulfite in the gypsum recovery system as small as possible, air is supplied in a large excess and the gypsum particle size is not controlled to be large. Therefore, it is a great problem to be able to control the particle size of the generated gypsum even if the conditions in the tank change. On the other hand, if the air supply amount is too small, the gypsum purity will not increase.

[0018]

SUMMARY OF THE INVENTION As described above, in the above-mentioned conventional technique, in order to make the concentration of sulfite in the gypsum recovery system as small as possible, air is excessively supplied to control the gypsum particle size to be large. It has not been. For example, when the gypsum seed crystals are made into a high-concentration slurry and air is excessively supplied to increase the amount of dissolved oxygen in the slurry, the oxidation reaction of sulfurous acid in the formula (3) is accelerated, and the generated gypsum ( CaSO ₄ )
Becomes supersaturated. Under such conditions, new seed crystals are generated and the gypsum particle size shifts to the smaller one. That is, the amount of air supplied controls the particle size of the produced gypsum.

The present invention operates a wet desulfurization apparatus in which the amount of air supplied is controlled in order to always maintain high desulfurization performance so that the generated gypsum particle size does not become small even if the conditions in the tank change. It is intended to provide a method.

[0020]

[Means for Solving the Problems] The above-mentioned object is to provide a desulfurization tower for contacting exhaust gas containing sulfur oxides with a slurry absorbing solution containing calcium carbonate to absorb sulfur oxides in the exhaust gas, and a lower part of the desulfurization tower. A wet type provided with an oxidation tank for storing a slurry absorbing solution that has absorbed sulfur oxides and blowing air into the slurry absorbing solution, and a slurry circulation system for circulating the slurry absorbing solution from the oxidation tank to the desulfurization tower This is achieved by controlling the average particle size of the gypsum produced in the oxidation tank by adjusting the amount of air sent to the oxidation tank in the method of operating the desulfurizer.

[0021]

In the conventional method, in order to make the concentration of sulfite in the gypsum recovery system as small as possible, air is supplied in a large excess and the gypsum particle size is not controlled to be large.
On the other hand, if the present invention is carried out, it becomes possible to suppress the oxidation reaction of sulfurous acid by detecting the sulfurous acid concentration or the dissolved oxygen concentration of the slurry liquid and controlling the amount of air supplied into the oxidation tank. That is, by detecting the sulfurous acid concentration or dissolved oxygen concentration in the extracted slurry liquid and controlling the amount of air supplied to the oxidation tank, the slurry liquid in the tank is optimally oxidized so that the generated particle size of gypsum is not reduced. It is possible to prevent the supersaturated state of gypsum (CaSO ₄ ) because the condition is satisfied, that is, new seed crystals are not generated.

Further, when the concentration of sulfurous acid in the slurry liquid is within the range of 1 mmol / liter or less, the concentration of carbon dioxide in the liquid is detected and the control for optimizing the amount of air supplied to the oxidation tank is performed. Furthermore, if the method of detecting and controlling the concentration of sulfurous acid and carbonic acid is not sufficient, a pH adjusting tower is installed in the absorption liquid extraction line to supply air, and the difference in pH between the inlet liquid and the outlet liquid of the pH adjusting tower is detected. , Control the amount of air supplied to the oxidation tank. If sulfurous acid is present, it is oxidized by blowing air and becomes gypsum, so the pH increases. The concentration of sulfurous acid can be calculated by detecting this difference in pH.

Conventionally, a manual analysis by the JIS method or an ion chromatography method has been used as a method for detecting the concentration of sulfurous acid and carbonic acid, but the analysis time required 10 to 30 minutes. On the other hand, the flow injection method (F
With the IA method), analysis can be performed within 2 minutes, so there is no time delay due to analysis, and if the sulfurous acid or carbonic acid in the slurry liquid is detected quickly and the amount of air is controlled, the slurry liquid can be set to optimum oxidation conditions. . The ion electrode method may be used to detect the dissolved oxygen concentration.

[0024]

Embodiments of the present invention will be described with reference to the drawings. FIG.
Is a system diagram of the wet desulfurization apparatus according to the embodiment, and FIG. 2 is FIG. 1A-
It is sectional drawing on the A'line.

The combustion exhaust gas 24 containing SO ₂ enters the desulfurization tower main body 1 through an inlet duct 2 in the upper part of the tower. In the desulfurization tower, the absorption liquid containing calcium carbonate sent from the circulation pump 5 is sprayed from a plurality of spray nozzles 4. Due to the gas-liquid contact of the absorbing liquid and the exhaust gas sprayed here, SO ₂ gas is absorbed and falls into the oxidation tank 6.

Fine calcium carbonate (limestone) which is an absorbent is supplied into the oxidation tank 6 through a limestone supply pipe 13. The absorption liquid that has generated calcium bisulfite is stored in the oxidation tank 6, and while being sufficiently stirred by the stirrer 7, the calcium bisulfite in the absorption liquid is oxidized by the air supplied from the air blowing pipe 8 and gypsum (CaSO ₄ ) To generate. A part of the absorbing liquid in the oxidation tank 6 in which calcium carbonate (CaCO ₃ ) and gypsum (CaSO ₄ ) coexist is sent to the spray nozzle 4 again by the circulation pump 5, and a part of the absorbing liquid is recovered from the absorbing liquid extracting pipe 10. Sent to the system. Further, among the absorption liquids atomized by the spray nozzle 4 and atomized, those having a small droplet diameter are entrained in the exhaust gas, but are recovered by the demittus 9 provided in the outlet duct 3 of the desulfurization tower. These reaction formulas are as shown in the above formulas (1) to (4).

The detection signals from the sulfurous acid detector 20, the dissolved oxygen concentration detector 27 and the carbonic acid detector 21 installed in the absorbing liquid extraction pipe 10 are sent to the calculator 19. As a method for detecting these, it is optimal to use a flow injection method (FIA method) or an ion electrode method (method utilizing dissolved oxygen concentration), which has a fast analysis time of less than 2 minutes and is excellent in responsiveness. The analysis time is 10 to 10 in the conventional JIS method.
It takes a long time of 30 minutes and cannot cope with a sudden change in the conditions in the tank 6.

The computing unit 19 calculates the oxidizing condition of the slurry liquid in the tank 6 based on these signals, sends this signal to the air blower 14, and controls so as to introduce an optimum amount of air into the oxidizing tank 6. To do. Air blow pipe 8 at the bottom of tank 6
The air supplied from the air forms a bubble group 23 and the oxidation tank 6
Inside, it is absorbed by the slurry and becomes dissolved oxygen, which causes the oxidation reaction of sulfurous acid. Therefore, it is also possible to directly measure the dissolved oxygen concentration in order to detect the oxidizing conditions.

By detecting the sulfurous acid concentration or dissolved oxygen concentration of the slurry liquid and controlling the amount of air supplied to the oxidation tank 6, the oxidation reaction of sulfurous acid is suppressed, and the slurry liquid in the tank 6 is set to the optimum oxidation conditions. That is, it is possible to prevent the supersaturated state of gypsum (CaSO ₄ ), because new seed crystals are not generated. Therefore, it is possible to control the generated gypsum particle diameter so as not to become small even if the conditions inside the tank 6 change.

Further, when the concentration of sulfurous acid in the slurry liquid is within the range of 1 mmol / liter or less, the concentration of carbon dioxide in the liquid is detected, and control is performed to optimize the amount of air supplied to the oxidation tank 6. Further, when the method of detecting and controlling the concentration of sulfurous acid and carbonic acid is not sufficient, a pH adjusting tower 15 is provided in the absorption liquid extraction line to supply air from the air supply port 16 to adjust the pH.
By detecting the difference in pH between the inlet liquid and the outlet liquid of the adjusting tower 15,
The amount of air supplied to the oxidation tank 6 is controlled. If sulfurous acid is present, the pH will increase as it is oxidized by blowing air to gypsum. The concentration of sulfurous acid can be calculated by detecting this difference in pH.

Conventionally, a manual analysis by the JIS method or an ion chromatography method has been used as a method for detecting the concentration of sulfurous acid and carbonic acid, but the analysis time required 10 to 30 minutes. On the other hand, the flow injection method (F
With the IA method), analysis can be performed within 2 minutes, so there is no time delay due to analysis, and even when the conditions inside the tank 6 change rapidly, the amount of air can be quickly detected by detecting sulfurous acid or carbonic acid in the slurry liquid. If controlled, the slurry liquid can be set to the optimum oxidation conditions. In addition, if the ion electrode method is used for measuring the dissolved oxygen concentration, it can be detected usually within 1 minute.

In FIG. 1, 11 is a circulating fluid pipe and 1
Reference numeral 7 is an exhaust pipe, and 18 is a processing tower outlet pipe.

As shown in FIG. 2, a spray pipe 26 is disposed inside the desulfurization tower 1, and a spray nozzle 4 is attached to the tip end of the spray piping 26. SO ₂ is removed.

FIG. 3 is a characteristic diagram showing a comparison of desulfurization performance between the conventional method and the present invention. The amount of combustion exhaust gas is 15
The experimental results are shown under the condition that the SO ₂ concentration is 750 ppm and the calcium excess rate is 2% using a pilot device of 000 Nm ³ / h. The horizontal axis shows the ratio (molar ratio) of the amount of air required for the oxidation of sulfurous acid, and the vertical axis shows the concentration of sulfurous acid and dissolved oxygen in the liquid and the gypsum particle size ratio.

As shown in the figure, as the air ratio increases from 1 to 1, the dissolved oxygen amount increases and the sulfurous acid oxidation reaction (gypsum formation reaction) is accelerated, so the sulfurous acid concentration is almost zero. Becomes

In the conventional method having a very large air ratio, gypsum is produced in a short time, so that the gypsum is in a supersaturated state within a certain time and the gypsum particle size becomes small. On the contrary,
According to the present invention, sulfurous acid is detected and the air ratio is 0.8 to 1.
If the amount of supplied air is controlled so as to be 0, the gypsum particle size becomes about 1.4 times larger than that of the conventional method.

FIG. 4 is a characteristic diagram showing the relationship between the concentration of sulfurous acid in the liquid and the desulfurization rate. The desulfurization rate is around 75% under conditions where the sulfurous acid concentration is close to zero, but when the sulfurous acid concentration increases, p
H is lowered and the desulfurization rate becomes 50% or less. It is considered that this is because SO ₂ is less likely to be absorbed because the amount of sulfite increases in the liquid. When the sulfurous acid concentration is around 200 mmol / liter, the desulfurization rate increases because the SO ₂ absorption reaction by calcium sulfite occurs. However, when the concentration of calcium sulfite in the liquid increases, when collecting as gypsum,
Since the oxidation tower will be installed after that, the process becomes complicated. Therefore, it is desirable that the concentration of sulfurous acid is 50 mmol / liter or less.

FIG. 5 is a characteristic diagram showing the relationship between the concentration of sulfurous acid in the slurry liquid and the calcium excess ratio. Liquid-gas ratio (L
/ G) = 15 and pH = 5.3, when the desulfurization reaction is carried out, the pH value decreases and the desulfurization rate decreases when the concentration of sulfurous acid in the liquid increases, so excess limestone (CaCO ₃ ) is supplied and calcium is excessive. You need to increase the rate. When operating so that the desulfurization performance becomes constant in this way, under conditions where the sulfurous acid concentration becomes low and detection is difficult, the Ca excess rate, that is, the carbonic acid concentration is detected to control the amount of air in the tank 6 and to perform oxidation conditions. Can be controlled.

As described above, according to the present invention, the amount of air can be controlled to keep the oxidation conditions in the tank 6 constant and prevent the supersaturated state of gypsum. Therefore, even if the conditions in the tank 6 change. The gypsum particle size does not become too small.

FIG. 6 is a characteristic diagram showing the relationship between gypsum particle size and desulfurization performance. When the slurry concentration is 10 wt%, the desulfurization performance tends to decrease when the gypsum particle size is 15 μm or less, but when the slurry concentration is 30 wt%, the gypsum particle size is 30 μm
If it becomes below, the desulfurization performance sharply deteriorates. Therefore, a smaller gypsum particle size is not desirable from the viewpoint of desulfurization performance.

To clarify the reason, the relationship between the slurry concentration and the viscosity is shown in FIG. When the gypsum particle size is 30 μm, the viscosity of the slurry liquid hardly changes even when the slurry concentration is increased to 30 wt%. In contrast, the gypsum particle size is 5μ
When m is small, the viscosity rapidly increases as the slurry concentration is increased, so that the resistance on the liquid side is increased and the desulfurization performance is considered to be reduced. It is very important in the wet desulfurization method (wet limestone-gypsum method) to maintain the desulfurization performance in such a manner that the particle size of the produced gypsum is not reduced.

[0042]

According to the present invention, by detecting the sulfurous acid concentration or the dissolved oxygen concentration of the slurry liquid and controlling the amount of air supplied into the oxidation tank, the oxidation reaction of sulfurous acid can be suppressed as much as possible. In order to prevent the formation of crystals, the generated gypsum particle size does not become too small even if the conditions inside the tank change. Therefore, desulfurization performance can always be maintained high.

[Brief description of drawings]

FIG. 1 is a system diagram of a desulfurization apparatus according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along the line AA ′ of FIG.

FIG. 3 is a characteristic diagram showing the relationship between the air ratio of sulfite oxidation and gypsum particle size according to the present invention.

FIG. 4 is a characteristic diagram showing the relationship between the sulfurous acid concentration and the desulfurization rate.

FIG. 5 is a characteristic diagram showing the relationship between the concentration of sulfite and the excess calcium ratio.

FIG. 6 is a characteristic diagram showing the relationship between gypsum particle size and desulfurization performance.

FIG. 7 is a characteristic diagram showing a relationship between slurry concentration and viscosity.

FIG. 8 is a system diagram of a desulfurization device according to a conventional example.

[Explanation of symbols]

 1 Desulfurization Tower Main Body 4 Spray Nozzle 6 Oxidation Tank 8 Air Blow Pipe 12 Gypsum Recovery Device 15 pH Adjustment Tower 19 Operator 20 Sulfurous Acid Detector 21 Carbonic Acid Detector 22 pH Detector 27 Dissolved Oxygen Detector

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirofumi Yoshikawa 3-36 Takaracho, Kure-shi, Hiroshima Babkotsk Hitachi Ltd. Kure Laboratory

Claims (2)

[Claims] 1. A desulfurization tower that absorbs sulfur oxides in exhaust gas by bringing the exhaust gas containing sulfur oxides into contact with a slurry absorbing solution containing calcium carbonate, and a desulfurization tower provided below the desulfurization tower to absorb sulfur oxides. The method for operating a wet desulfurization apparatus, comprising: an oxidation tank for storing the slurry absorption liquid, and blowing air into the slurry absorption liquid; and a slurry circulation system for circulating the slurry absorption liquid from the oxidation tank to the desulfurization tower. A method for operating a wet desulfurization apparatus, wherein the average particle size of gypsum produced in the oxidation tank is controlled by adjusting the amount of air sent to the oxidation tank. 2. The method according to claim 1, wherein at least one of the concentration of sulfurous acid, the concentration of dissolved oxygen and the concentration of carbonic acid in the slurry absorbing liquid is detected, and the supply amount of the air is adjusted based on the detection result. A method for operating a wet desulfurization apparatus, which is characterized in that