JP3092095B2 - Wet flue gas desulfurization method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flue gas desulfurization apparatus, and more particularly to a flue gas desulfurization apparatus suitable for removing sulfur oxides from exhaust gas and recovering gypsum having a large particle size.

[0002]

2. Description of the Related Art Conventionally, dust and acid gases (eg, HCl and HF) in exhaust gas have been removed by a dust removing tower, and sulfur oxides in the exhaust gas have been removed by contact with a calcium compound slurry in an absorption tower. _2. Description of the Related Art A wet flue gas desulfurization apparatus is known which blows air into a bottom circulation tank to oxidize calcium sulfite generated by absorption of sulfur oxides (hereinafter referred to as SO ₂ ). The schematic system is shown in FIG.
In this apparatus, flue gas 1 from a boiler or the like is introduced into a dust removal tower 2, where dust and acid gas in the flue gas are removed by gas-liquid contact with circulating water supplied by a circulating pump 20. The exhaust gas from which dust and acid gas have been removed passes through the sump mist eliminator 3 to remove mist, and then is guided to the absorption tower 4. The exhaust gas introduced into the absorption tower 4 is gas-liquid contacted with a calcium-based absorbent slurry supplied by a circulation pump 8 to absorb and remove SO ₂ in the exhaust gas. After that, accompanying mist is removed by a demister 5 and a clean gas 6 is removed. Is discharged. Here, the absorbent slurry in the circulation tank 7 absorbs SO ₂ in the exhaust gas and oxidizes generated calcium sulfite and the like to form gypsum.
6 supplies air. The oxidizing air is supplied to the vicinity of the blade of the stirrer 18 installed in the circulation tank 7 to be converted into fine bubbles to be used for oxidation. The gypsum obtained by oxidizing calcium sulfite is sent to the thickener 11 through the extraction pipe 9. The slurry is concentrated to a slurry concentration suitable for gypsum separation with a thickener 11 and the gypsum separator 1 is passed through a tank 12.
The gypsum is then dewatered by the gypsum separator 14 into gypsum with 10% or less of adhering water, and gypsum 15 is collected.

[0003]

In the above conventional apparatus, the oxidizing air supplied to the circulation tank 7 in the vicinity of the stirrer 18 is always supplied in a constant amount. A method has been adopted in which the supply of oxidizing air is increased when the load is high and the load is high. The reason is that the sulfite ion concentration of the absorbent slurry in the circulation tank 7 greatly affects the desulfurization performance, and if the sulfite ion concentration in the absorbent slurry fluctuates, the desired desulfurization rate cannot be obtained or the gypsum purity decreases. Will happen. Therefore, for example, when the load is high, the sulfite ion concentration becomes high. Therefore, the sulfite ion having the increased concentration is oxidized, and the supply amount of the oxidizing air is increased to lower the concentration.

[0004] However, in the above-mentioned conventional flue gas desulfurization apparatus, if the operation is continued for a long time in a state where the SO ₂ concentration at the inlet of the absorption tower is high, the vibration of the dehydrator in the gypsum separator becomes large, and the trip may eventually occur. there were.

Therefore, as a result of various investigations and investigations on the cause, when the amount of SO _{2 at the} inlet of the absorption tower is low, the amount of calcium sulfite oxidized at the gas-liquid contact portion of the absorption tower 4 becomes oxidized in the circulation tank 7. Larger than the amount to be
_The higher the amount, the opposite was found. FIG. 8 shows the relationship.

In general, since the oxidation rate of calcium sulfite is proportional to the oxygen concentration, the oxidation reaction at the gas-liquid contact portion in the absorption tower 4 occurs in an atmosphere having an oxygen concentration of about 5 to 7%. Is slower than the oxidation reaction that occurs in
On the other hand, the gypsum particle diameter tends to increase as the oxidation rate decreases, and the gypsum particles increase as the proportion of the oxidation amount of calcium sulfite at the gas-liquid contact portion in the absorption tower 4 having a slow oxidation rate to the total oxidation amount increases. The diameter tends to be large.

When the supply amount of the oxidizing air 10 is kept constant or increases when the SO ₂ amount at the inlet of the absorption tower is increased, the oxidation ratio of calcium sulfite in the circulation tank 7 is increased. It is larger than the gas-liquid contact part inside. As a result, the oxidation rate of calcium sulfite becomes too fast, and a large amount of fine crystals are generated.
It was found that vibration occurred and stable operation became impossible.

[0008] Therefore, an object of the present invention is to provide an SO ₂ inlet SO ₂
It is an object of the present invention to provide a wet flue gas desulfurization method capable of securing a stable absorbent slurry composition and recovering gypsum having a large particle diameter even when the amount fluctuates.

[0009]

In order to achieve the above object, the present invention employs the following constitution. That is, the calcium-based compound slurry in the circulation tank provided at the lower part of the absorption tower is sprayed into the absorption tower to make contact with the exhaust gas to remove sulfur oxides in the exhaust gas, and air is supplied to the calcium-based compound slurry in the circulation tank. In the wet flue gas desulfurization method of oxidizing calcium sulfite generated by the reaction between the sulfur oxide and the calcium compound in the blown exhaust gas and sending a part of the calcium compound slurry to a gypsum recovery system to recover gypsum,
This is a wet flue gas desulfurization method in which the supply amount of oxidizing air is reduced as the amount or concentration of sulfur oxide at the inlet of the absorption tower becomes higher or the gypsum separator vibrates.

[0010]

According to the present invention, the calcium compound slurry stored in the circulation tank installed at the lower part of the absorption tower is sprayed into the absorption tower and brought into contact with the exhaust gas to remove sulfur oxides contained in the exhaust gas. On the other hand, air is blown into the calcium compound slurry in the circulation tank to oxidize calcium sulfite generated by the absorption of sulfur oxides by the calcium compound.

The amount of oxidizing air blown into the slurry in the circulation tank is reduced when the amount or concentration of SO _{2 at the} inlet of the absorption tower exceeds a certain value or when the gypsum separator vibrates. By reducing the supply amount of the oxidizing air, a stable absorbent slurry composition in the absorption tower is secured, and gypsum having a large particle size can be recovered.

Hereinafter, the reaction in the wet flue gas desulfurization apparatus to which the present invention is applied and the growth of the gypsum particle size, which is one of the reasons why the present invention is effective in relation thereto, will be described. 1 or 7, SO _{2 in} the exhaust gas introduced into the absorption tower 4 is subjected to the following reaction formula (1) by gas-liquid contact with limestone in the absorbent slurry supplied by the circulation pump 8;
The absorption reaction of (2) and part of O ₂ contained in exhaust gas
A solid gypsum is produced by the oxidation reaction of the reaction formula (3) according to the following, followed by the crystallization reaction of the reaction formula (4).

Absorption reaction SO ₂ + H ₂ O → H ₂ SO ₃ (1) CaCO ₃ + 2H ₂ SO ₃ → Ca (HSO ₃ ) ₂ (2) Oxidation reaction Ca (HSO ₃ ) ₂ + O ₂ → CaSO ₄ + H ₂ SO ₄ … (3) Crystallization reaction CaSO ₄ + 2H ₂ O → CaSO ₄ .2H ₂ O… (4)

Next, the calcium sulfite generated by absorbing SO ₂ and remaining without being oxidized reacts with the oxidizing air in the circulation tank, and undergoes an oxidation reaction (formula is the same as in formula (3)) and a crystallization reaction (formula (3)). Is the same as (4)), thereby producing solid gypsum.

As described above, since the oxidation rate of calcium sulfite is proportional to the oxygen concentration, the oxidation reaction at the gas-liquid contact portion in the absorption tower 4 is slower than the oxidation reaction occurring in the circulation tank 7. On the other hand, the gypsum particle size tends to increase as the proportion of the oxidation amount of calcium sulfite in the gas-liquid contact portion in the absorption tower having a low oxidation rate to the total oxidation amount increases.

However, a high calcium sulfite concentration in the absorbent slurry in the circulation tank adversely affects the desulfurization performance. Pilot plant (processing gas amount 600Nm ³ /
FIG. 4 shows a conceptual diagram of the result of examining the relationship between calcium sulfite in the absorbent slurry and the desulfurization performance in h). That is, in order to secure a required desulfurization rate, the calcium sulfite concentration needs to be lower than a certain value.

The rate of oxidation of calcium sulfite in the absorbent slurry tends to decrease as the calcium sulfite concentration decreases. FIG. 5 is a conceptual diagram showing the results obtained in a pilot plant. That is, the absorption tower inlet SO ₂
Even if the amount becomes low and the oxidation ratio at the gas-liquid contact part of the absorption tower (the amount oxidized at the gas-liquid contact part / the amount that must be oxidized) increases, the oxidizing air will remain at the concentration of calcium sulfite in the slurry. Needs to be supplied in a predetermined amount in order to maintain the oxidation rate below a certain value and to maintain the oxidation rate. Also, when the absorption tower inlet SO ₂ amount is low, the absolute amount of the resulting gypsum is reduced, a tendency of the gypsum purity decreases, the supply of oxidizing air to do this is needed.

When the SO ₂ amount at the inlet of the absorption tower becomes high, the amount of calcium sulfite that needs to be oxidized in the circulation tank increases, and the oxidation rate increases in the circulation tank due to the high concentration of oxidizing air. Therefore, if the amount of air supplied to the circulation tank is large, the gypsum particle size becomes fine. When this fine gypsum is supplied to the gypsum separator, the gypsum passes through the mesh of the basket (cage) of the gypsum separator, does not fill the basket, and the gypsum does not uniformly adhere in the basket. Therefore, when the basket is in an unbalanced state and dewatering is performed in that state, vibration of the separator rotating at a high speed (rotating at 700 to 800 rpm) occurs, and the gypsum separator trips and continuous operation cannot be performed. Therefore, if the amount of the absorption tower inlet SO ₂ is increased, reducing the oxidizing air amount supplied to the circulation tank, slows the oxidation rate of the circulating tank is approximately equal speed as the crystallization reaction rate if possible. In this way, gypsum with a large particle size can be collected,
In addition, the calcium sulfite in the absorbent slurry can be maintained at a certain value or less, the required desulfurization rate can be obtained, and the vibration of the gypsum separator can be prevented. Less than,
This will be described more specifically with reference to examples of the present invention.

[0019]

FIG. 1 is a schematic system diagram of a wet flue gas desulfurization apparatus according to one embodiment of the present invention. The difference from FIG. 7 is that an air control valve 21 is installed in the air pipe 10 that supplies air to the circulation tank 7. Hereinafter, a method of controlling the air amount will be described.

This plant is a flue gas desulfurization unit with an output of 175 MW, operates at 750 Nm ³ / h of oxidizing air when the SO ₂ concentration at the absorption tower inlet is 200 ppm, the gypsum particle size is 100 μm on average, and the circulation tank The calcium sulfite concentration therein was also 2 mmol / l or less, and the desulfurization rate was 85%. The sulfur content in the coal used rises and the SO
_{2 The} concentration became 450 ppm,
The operation was performed at 0 Nm ³ / h. After several days, the desulfurization performance was maintained without any problem, but trips occurred frequently due to abnormal vibration of the gypsum separator. Therefore, based on the relationship between the gypsum particle size and the air supply amount for oxidation shown in FIG.
The amount of the oxidizing air from the oxidizing air blower 16 was set to 600 m ³ N so that the gypsum particle diameter of about 90 μm at which the vibration of Step 4 was eliminated.
/ H and the operation was continued. As a result, the gypsum particle size grew to about 140 to 160 μm, and there was no trouble due to the vibration of the dehydrator in the gypsum separator 14 and the desulfurization performance could be maintained.

A control method for recovering gypsum having a large particle size will be described below. FIG. 2 shows an air control system for oxidation. An inlet SO ₂ concentration signal 34 obtained from the absorption tower inlet SO ₂ concentration meter 31 and an exhaust gas flow signal 35 obtained from the exhaust gas flow meter 32 are output to a multiplier 37,
In a multiplier 37, a total SO ₂ amount signal 38 in the exhaust gas is obtained. The required air amount signal is obtained by the function generator 39 based on the function data shown in FIG. 3 by the total SO ₂ amount signal 38. On the other hand, the actual oxidizing air flow rate signal 36 measured by the oxidizing air flow meter 33 is compared and adjusted by the controller 41, and the control valve 21 provided in the air pipe 10 is operated to reduce the oxidizing air amount. Increase or decrease.

This makes it possible to prevent the rate of oxidation of calcium sulfite in the circulation tank 7 from being excessively increased, and to recover gypsum having a large particle size. Absorption tower input S
When the amount of O ₂ is low, by operating the control valve 21 to increase the amount of air, oxidation can be completely performed, and high-purity gypsum can be recovered.

In a plant where the boiler load does not change much, the amount of oxidizing air can be controlled only by the concentration of SO _{2 at the} inlet of the absorption tower, so that the control system can be simplified and the same effect can be obtained.

Further, an abnormal vibration of a plurality of gypsum separators is detected to control the supply amount of the oxidizing air, but the same effect can be obtained.

In the above embodiment, the supply amount of oxidizing air is controlled by the control valve 21. However, when a plurality of oxidizing air blowers 16 are installed, the number of oxidizing air blowers 16 is changed by changing the number of operating units. It is also possible to change the amount of working air.

It is also possible to provide an apparatus for detecting the gypsum particle size in a gypsum extraction piping (not shown) of the gypsum recovery system, and adjust the supply amount of the oxidizing air based on the signal.

[0027]

According to the present invention, gypsum having a large particle size is recovered by controlling the supply amount of oxidizing air based on the total SO ₂ in the exhaust gas or the abnormal vibration of the gypsum separator. A wet flue gas desulfurization device capable of stable operation without generating vibrations or the like in the separator is obtained.

[Brief description of the drawings]

FIG. 1 is a system diagram of a wet-type flue gas desulfurization apparatus used in the present invention.

FIG. 2 is a control system diagram according to the present invention.

FIG. 3 is a diagram showing the relationship between the amount of SO _{2 at the} inlet of the absorption tower and the required amount of air.

FIG. 4 is a conceptual diagram showing the relationship between calcium sulfite concentration and desulfurization rate.

FIG. 5 is a conceptual diagram showing a relationship between calcium sulfite and an oxidation rate.

FIG. 6 is a diagram of experimental data showing the relationship between the gypsum particle size and the amount of air for oxidation.

FIG. 7 is a diagram showing a system of a conventional wet flue gas desulfurization device.

FIG. 8 is a graph showing the relationship between the SO ₂ amount at the inlet of the absorption tower and the oxidation ratio.

[Explanation of symbols]

 Reference Signs List 4 absorption tower 7 circulation tank 14 gypsum separator 15 gypsum 16 air blower for oxidation 21 control valve

Continued on the front page (72) Inventor Terutsugu Watanabe No. 1 Maejima, Daigahama, Nanigahama-cho, Miyagi-gun, Miyagi Prefecture Tohoku Electric Power Co., Inc. No. 1, Hama-ji Maejima Tohoku Electric Power Co., Inc.Sendai Thermal Power Station (72) Inventor Ikuei Ohno No. 1, Mashima, Shichigahama-cho, Daiichihama-cho, Miyagi-gun, Miyagi Pref. No. 1, Maegima, Shichigahama-machi, Daiichihama, Miyagi-gun, Tochihoku Electric Power Industry Co., Ltd. No. 6-9, Takaracho, Kure City, Hiroshima Prefecture Inside the Kure factory of Babcock Hitachi Ltd. (56) References JP-A-62-262728 (JP, A) JP-A-62-250931 (JP, A) JP-A-63-49231 (JP, A) JP-A-60-58230 (JP, A) JP-A-63-65936 (JP, A) JP-A-63-197626 (JP , U) (58) Field surveyed (Int. Cl. ⁷ , DB name) B01D 53/50 B01D 53/34 B01D 53/77

Claims (4)

(57) [Claims] 1. A calcium-based compound slurry in a circulation tank, wherein a calcium-based compound slurry in a circulation tank provided in a lower part of an absorption tower is sprayed into an absorption tower to make contact with exhaust gas to remove sulfur oxides in the exhaust gas. Wet flue gas desulfurization in which air is blown into air to oxidize calcium sulfite generated by the reaction between sulfur oxides and calcium compounds in the exhaust gas and send part of the calcium compound slurry to the gypsum recovery system to recover gypsum A wet flue gas desulfurization method, wherein the amount of air supplied to a circulation tank is reduced as the amount or concentration of sulfur oxide at the inlet of an absorption tower increases. 2. The wet flue gas desulfurization method according to claim 1, wherein the supply amount of the oxidizing air is controlled by controlling the opening and closing of a control valve or controlling the number of operating oxidizing air blowers. 3. The calcium-based compound slurry in the circulation tank, wherein the calcium-based compound slurry in the circulation tank provided in the lower part of the absorption tower is sprayed into the absorption tower and brought into contact with the exhaust gas to remove sulfur oxides in the exhaust gas. Wet flue gas desulfurization in which air is blown into air to oxidize calcium sulfite generated by the reaction between sulfur oxides and calcium compounds in the exhaust gas and send part of the calcium compound slurry to the gypsum recovery system to recover gypsum A method for wet flue gas desulfurization, wherein the supply amount of oxidizing air is adjusted by generating vibration of a gypsum separator. 4. The wet flue gas desulfurization method according to claim 1, wherein an apparatus for detecting the gypsum particle size is provided in a gypsum extraction pipe of the gypsum recovery system, and the signal for reducing the amount of oxidizing air is used. .