JP2000176242A - Wet flue gas desulfurization process and equipment therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To secure stabilized dehydration performance of gypsum even in conditions that a slurry concentration varies at the inlet of a dehydrator. SOLUTION: Exhaust gas discharged from a combustion device and an adsorption liquid slurry using a calcium-based adsorbent as adsorbent, are brought into gas-liquid contact in an adsorption tower 2 to adsorb sulfur oxide in the exhaust gas into the adsorption liquid slurry, and air is fed to the obtained adsorption slurry in a circulation tank 7 to form gypsum. Successively, when the liquid containing the gypsum is dehydrated in a primary dehydrator 13a and a secondary dehydrator 13b to recover the gypsum, one portion of the slurry fed to the primary dehydrator 13a is bypassed, and by finally stabilizing the concentration and the flow rate at the inlet of the secondary dehydrator 13b, stabilized dehydration performance of the gypsum can be secured even in the conditions that the concentration of the adsorption liquid slurry varies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art A wet limestone-gypsum desulfurization apparatus has been widely put into practical use as an apparatus for removing sulfur oxides in exhaust gas for preventing air pollution. FIG. 4 shows a conventional technique of this wet limestone-gypsum desulfurization apparatus.

[0003] Exhaust gas 1 containing sulfur oxides generated from a thermal power plant or the like is led to an absorption tower 2 of a desulfurizer. Absorption tower 2
Inside, a spray header 3 having a large number of spray nozzles 4 is installed, and an absorbing liquid slurry sprayed as fine droplets from the spray nozzle 4 (using a calcium-based absorbent (limestone, lime, etc.) as an absorbent) and By bringing the exhaust gas 1 into contact, the sulfur oxides in the exhaust gas are chemically removed on the surface of the absorbent slurry drops. The minute droplets entrained in the exhaust gas 1 are removed by a mist eliminator 5 installed on the upper part of the absorption tower 2, and the purified gas 6 is re-heated (not shown) installed on the downstream side of the absorption tower 2 if necessary. Heated by the equipment and discharged from the chimney. Most of the droplets sprayed from the spray nozzle 4 absorb sulfur oxide (SO ₂ ), and then fall into an absorption tower circulation tank 7 provided below the absorption tower 2. The absorption liquid slurry in the circulation tank 7 is supplied to the circulation pump 1
At 7, it is sent to the spray header and sprayed again from the spray nozzle 4 toward the exhaust gas 1.

[0004] The SO ₂ absorbed in the absorbing solution slurry reacts with limestone (CaCO ₃ ) contained in the absorbing solution slurry,
Further, it is oxidized by the 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 + 2H ₂ O + CaCO ₃ + ／ O ₂ → CaSO
₄ · _2H 2 O + CO

On the other hand, limestone 9 as an absorbent is stored as limestone slurry in a limestone supply facility 10 and supplied to an absorption tower circulation tank 7 from a limestone slurry pump 11. Further, in order to collect the gypsum generated in the absorption tower 2, a part of the absorption liquid slurry in the absorption tower circulation tank 7 is extracted and a pump 1 is used.
The liquid is sent to the gypsum dewatering equipment 13a and 13b in 2, and the gypsum and the dust contained in the absorbent slurry are collected as the gypsum 14. About 10 to 30% of the absorbent slurry containing gypsum 14 as a main component (this value varies depending on plant design conditions) is led to the dewatering equipment 13 and the primary dewatering device 13a
(Generally thickener and hydrocyclone), the slurry is concentrated and separated by a fine particle size,
After being concentrated to the range of 0%, the secondary dewatering device 13b (generally, a centrifugal separator, a belt filter) finally recovers the powdered gypsum with the amount of adhering water of about 10% or less.

A part of the dehydrated filtrate 16 of gypsum and dust is discharged out of the system as drainage 15 in order to prevent impurities from being concentrated in the system, and a part of the remaining liquid is discharged by the limestone supply system 10 to limestone. Primary dehydrator 13 as makeup water for slurry production
The remaining part of the liquid used in a is sent to the absorption tower circulation tank 7 of the absorption tower 2. Here, the desulfurization wastewater extracted from the wastewater line 15 is sent to a wastewater treatment facility (not shown), and is discharged after removing iron, 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 the concentration of sulfur oxides in the exhaust gas is large, and the operating load also changes in accordance with the demand. Not only performance is required, but also stable increase / decrease of gypsum production, and stable recovery of by-products, especially gypsum with low content of adhering water (generally 10%) Is required.

[0008]

In the above prior art, since the boiler plant is operated in a wide operating range with various fuels or loads, it is necessary to change the concentration of the slurry of the absorbent. Due to the change in the absorption liquid slurry concentration, the inlet slurry concentration changes in the dehydration facility 13, and as a result, there is a problem that the dehydration performance changes and becomes unstable.

In particular, when the secondary dewatering device 13b is composed of a belt filter, if the absorbent slurry concentration is lower than the design conditions, the dewatering performance deteriorates, and gypsum containing a large amount of adhering water is obtained. In the case of the concentration, a longer dehydration time is required. In this case, by changing the belt speed by the belt speed control (dewatering time adjustment) of the belt filter, it is possible to cope with a certain change in the slurry concentration at the inlet of the dewatering equipment 13.

However, when the slurry concentration of the absorbing solution is too high from the design conditions of the secondary dewatering device 13b, the fluidity of the slurry is deteriorated, so that it is difficult to uniformly disperse the slurry over the entire surface of the filter, and there is no cake. Also, the vacuum may come off from a thin part, which again causes a significant decrease in dewatering performance. In this case, there is a problem that the dehydration performance does not increase even if the dehydration time is slightly increased.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the conventional wet flue gas desulfurization apparatus, and to ensure stable dehydration performance of gypsum even under the condition where the concentration of the slurry of the absorbent at the inlet of the dehydrator fluctuates. Is to do.

[0012]

An object of the present invention is to provide an absorbent slurry after exhaust gas desulfurization when supplying the absorbent slurry after exhaust gas desulfurization to a dehydration facility equipped with a primary and secondary two-stage dehydrator. This is achieved by bypassing the primary dehydrator and finally stabilizing the concentration and flow rate at the secondary dehydrator inlet.

That is, by mixing a slurry having a relatively low concentration and a high flow rate with the slurry after the primary dehydration having a high concentration and a low flow rate, the slurry is adjusted to a slurry suitable for the secondary dehydration. Stable operation becomes possible.

According to the present invention, the exhaust gas discharged from the combustion device is brought into gas-liquid contact with an absorbent slurry using a calcium-based absorbent as an absorbent to absorb sulfur oxides in the exhaust gas into the absorbent slurry. In the wet flue gas desulfurization method of supplying gypsum by supplying air to the absorbed liquid slurry and dehydrating the gypsum-containing liquid to recover gypsum, the gypsum-containing liquid is separated into two stages of primary dehydration and secondary dehydration. Perform dehydration and mix a part of the absorbent slurry with a relatively low concentration and high flow rate before the primary dehydration with the high-concentration, low-flow rate absorbent slurry without primary dehydration. By this, wet flue gas desulfurization method to adjust the slurry concentration and flow rate suitable for secondary dehydration, or gas-liquid contact between the exhaust gas discharged from the combustion device and the absorbent slurry using a calcium-based absorbent as the absorbent Letting suck A tower, an absorbing solution slurry circulating tank for supplying gypsum by supplying air to an absorbing solution slurry in which sulfur oxide in exhaust gas is absorbed by the absorbing solution slurry, and an absorbing solution containing gypsum in the absorbing solution slurry circulating tank In a wet flue gas desulfurization apparatus equipped with a dewatering device for dewatering the slurry liquid and collecting gypsum, the dewatering device has a primary dewatering device and a secondary dewatering device for secondary dewatering the absorbent slurry dehydrated by the primary dewatering device. And a bypass flue gas desulfurization apparatus provided with a bypass system for supplying an absorbent slurry containing gypsum in the absorbent slurry circulation tank directly to a secondary dehydrator bypassing the primary dehydrator.

[0015]

FIG. 1 shows an embodiment of the present invention. The outline of the system of the flue gas desulfurization device is the same as that shown in FIG. 4, and the exhaust gas 1 containing sulfur oxides, the absorption tower 2 of the desulfurization device, the spray header 3, the spray nozzle 4, the mist eliminator 5, the purified gas 6, the absorption tower It is composed of a circulation tank 7, and includes air 8, limestone 9, limestone supply equipment 10, limestone slurry pump 11, withdrawal pump 12, primary gypsum dewatering equipment 13a, secondary gypsum dewatering equipment 13b, gypsum 14, and drainage 1.
5, a dehydration filtrate 16 and a circulation pump 17 are provided.

In the flow shown in FIG. 1, a primary dewatering bypass line 18 which bypasses the secondary gypsum dewatering equipment 13b is provided in addition to the configuration of the flow shown in FIG. And the outlet slurry of the primary dehydrator 13a can be supplied to the secondary dehydrator 13b.

The bypass amount of the absorbent slurry flowing through the primary dewatering bypass line 18 is achieved by bypassing about 12% of the absorbent slurry when the inlet concentration of the secondary dewatering device 13b is set to 50%.

In actual operation, since the flow rate of exhaust gas and the concentration of SO _{2 at the} inlet of the exhaust gas of the exhaust gas change due to the change in boiler load, the amount of gypsum produced changes. In order to cope with this, there are two cases: a case where the supply of the absorbent slurry to the dehydrating equipment 13 is performed in a batch system, and a case where the absorbent slurry is continuously supplied even if the flow rate and / or concentration of the absorbent slurry in the dehydrator 13 changes. ,
This choice is determined based on design specifications.

In the case of the batch method, the dehydration equipment 1 is basically used.
3 and the flow rate of the absorbing slurry was constant, and the operation (ON-O
FF) Dehydration is performed according to the actual amount of gypsum produced by increasing or decreasing the time. However, when the temperature is not constant, adjustment can be performed by the present invention.

On the other hand, in the case of continuous operation, the flow rate and concentration of the absorbent slurry change due to an increase or decrease in the amount of gypsum produced (the amount of gypsum varies depending on the boiler load, the concentration of SO _{2 in} the exhaust gas, etc.). The entry conditions to 13 are constantly changing. In this case, the adjustment according to the present invention can mitigate the change in the inlet condition. Preferably, the primary dewatering bypass line 18 for the lowest expected gypsum production
It is effective to design in advance such that the amount of bypass flowing through the container is minimized, and to increase the amount of bypass with an increase in the amount of gypsum produced.

As a method of using the present invention, a change in flow rate can be handled by controlling the speed of the belt filter when the secondary dewatering device 13b is provided with a belt filter (not shown). It is particularly important to keep the concentration of the inlet slurry of the dewatering equipment 13 constant, and therefore, based on the measurement results of the flow rate and the specific gravity of the inlet slurry (the specific gravity of the absorbent slurry in the absorbent slurry tank 7 may be used). By controlling the amount of bypass flowing through the primary dewatering bypass line 18, the concentration of the slurry at the inlet of the secondary dewatering device 13b can be controlled. In this case, although depending on the degree of the bypass flow rate, the primary dehydrator 13a actually composed of a plurality of hydrocyclones is used.
By increasing or decreasing the number of hydrocyclones operated in accordance with the bypass amount, the operating conditions of each hydrocyclone can be kept constant, and a stable primary dewatering effect can be obtained.

FIG. 2 shows an image diagram of a distribution cross section of the gypsum cake 21 on the belt filter 20 in a good condition, and FIG. 3 shows that the concentration of the slurry of the absorbent supplied to the secondary dewatering device 13b is too high. A cross-sectional image diagram of the non-uniform distribution of the gypsum cake 21 is shown.

The supplied slurry is supplied by a disperser having shelves, vanes and the like so that the gypsum cake 2 on the belt filter becomes uniform. However, when the concentration is too high, the fluidity of the slurry becomes poor. Therefore, the dispersion becomes non-uniform, and the vacuum 22 is not uniformly applied. In particular, when the cake thickness at the edge of the belt filter 20 is reduced, the vacuum 22 may come off 23 and vacuum dehydration may not be performed. In such a case, as shown in FIG. 3, by blending the absorbent slurry before the primary dehydration,
Appropriate fluidity can be given, and good dehydration can be performed.

(Experiment) A hydrocyclone was used as the primary dehydrator 13a, and a primary dehydration bypass line was installed in a dehydration facility composed of a vacuum belt filter as the secondary dehydrator 13b, and a test was performed by changing the bypass amount. Table 1 shows an example of the results.

[Table 1]

From the test results in Table 1, it was confirmed that good dehydration performance can be obtained by setting the bypass amount appropriately.
ADVANTAGE OF THE INVENTION According to this invention, the operation | movement of a stable dehydration apparatus can be performed in the wide operating range of a flue gas desulfurization apparatus, and the gypsum of a stable property can be collect | recovered.

Further, according to the embodiment of the present invention, even if the slurry concentration at the outlet of the hydrocyclone is designed at 50%, the actual gypsum particle diameter is different and the initial concentration becomes higher than the designed one. Even in the case of a temporary trouble, the outlet concentration is adjusted to the design value of 5 by the slurry that bypasses the primary dehydrator.
It became possible to adjust it to 0%.

[0027]

According to the present invention, a stable operation of the dewatering device can be performed in a wide range of operation of the flue gas desulfurization device, and gypsum having a stable property can be recovered. According to the embodiment of the present invention, for example, the slurry concentration at the hydrocyclone outlet is 5
Even if the gypsum is designed at 0%, the actual gypsum particle diameter is different, and even in the case of elementary troubles such as a higher outlet concentration than designed, the outlet concentration is reduced to 50% of the designed value by the slurry bypassing the primary dehydrator. It is also possible to adjust.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram according to the embodiment of the present invention.

FIG. 3 is a conceptual explanatory diagram of a problem in the embodiment of the present invention.

FIG. 4 is an explanatory diagram in the related art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Exhaust gas 2 Absorption tower of a desulfurization device 3 Spray header 4 Spray nozzle 5 Mist eliminator 6 Purified gas 7 Absorption tower circulation tank 8 Air 9 Limestone 10 Limestone supply equipment 11 Limestone slurry pump 12 Extraction pump 13a Primary gypsum dewatering equipment 13b
Secondary gypsum dewatering equipment 14 Gypsum 15 Drainage 16 Dehydration filtrate 17 Circulation pump 18 Primary dehydration bypass line 20 Belt filter 21 Gypsum cake

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Claims (4)

[Claims] An exhaust gas discharged from a combustion device is brought into gas-liquid contact with an absorbent slurry using a calcium-based absorbent as an absorbent to absorb sulfur oxides in the exhaust gas by the absorbent slurry. In a wet flue gas desulfurization method in which gypsum is generated by supplying air to the absorbent slurry and dehydrating the gypsum-containing liquid to recover gypsum, the gypsum-containing liquid is dehydrated in two stages, primary dehydration and secondary dehydration. Doing, and, without primary dehydration, a relatively low concentration of the slurry before the primary dehydration, and mixing with the absorbent slurry after the primary dehydration with a high concentration and a small flow rate without performing the primary dehydration A wet flue gas desulfurization method comprising adjusting the slurry concentration and the flow rate suitable for secondary dehydration. 2. An absorption tower for bringing an exhaust gas discharged from a combustion device into gas-liquid contact with an absorbent slurry using a calcium-based absorbent as an absorbent, and the absorbent slurry absorbs sulfur oxides in the exhaust gas. Wet draining system including an absorbing solution slurry circulating tank for supplying air to the absorbing solution slurry to produce gypsum, and a dehydrating device for dewatering the absorbing solution slurry solution containing gypsum in the absorbing solution slurry circulating tank to collect gypsum. In the smoke desulfurization device, the dehydration device includes a primary dehydration device and a secondary dehydration device for secondary dehydration of the absorbent slurry dehydrated by the primary dehydration device,
A wet flue gas desulfurization device comprising a bypass system for directly supplying an absorbent slurry containing gypsum in the absorbent slurry circulation tank to a secondary dewatering device, bypassing the primary dewatering device. 3. The wet flue gas desulfurization apparatus according to claim 2, wherein the flow rate of the bypass system is variable. 4. The wet flue gas desulfurization apparatus according to claim 2, further comprising means for detecting the flow rate and specific gravity of the absorbent slurry at the inlet of the primary dehydrator, and controlling the flow rate of the bypass system.