JP2002224533A - Method and apparatus for flue gas desulfurization - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and apparatus for flue gas desulfurization whereby the reliability of apparatus is secured at a low equipment cost without increasing the chlorine concentration, i.e., a cause of troubles of a desulfurization apparatus. SOLUTION: In this method, after a flue gas 1 discharged from a boiler is contacted with an absorbent liquid containing limestone, etc., in an absorption tower 2 to treat sulfur oxides in the flue gas 1, gypsum as a by-product is recovered from the absorbent liquid. The chlorine concentration in the absorbent liquid can be kept at a specified value or lower by adjusting the water content of gypsum recovered from the absorbent liquid to the amount of chlorine absorbed by the absorbent liquid by adjusting the number of revolutions of a gypsum dehydrator 12. The amount of chlorine absorbed by the absorbent liquid is calculated in advance from a boiler fuel signal 24 changing with the difference in load on a combustion apparatus, the kind of used fuel, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a sulfur oxide (hereinafter referred to as SO) in exhaust gas discharged from a combustion device such as a boiler.
₂ )), particularly when operating a non-drainage type desulfurization unit that does not discharge wastewater to the outside of the system, to prevent the chlorine concentration in the system from increasing more than a certain level. The present invention relates to a flue gas desulfurization method and apparatus.

[0002]

2. Description of the Related Art To prevent air pollution, SO ₂ in exhaust gas is used.
As a device for removing limestone, a wet limestone-gypsum desulfurization device has been widely put into practical use.

FIG. 3 shows an example of a conventional flue gas desulfurization apparatus. The flue gas desulfurization apparatus includes an absorption tower 2, a circulating liquid pipe 3, a spray nozzle 4, a mist eliminator 5, an absorption tower liquid reservoir 7, a circulation pump 8, an oxidizing stirrer 9, an extraction pump 11, a gypsum dewatering machine 12, and a filtrate tank 13. , Filtrate pump 14, limestone slurry tank 15, limestone slurry pump 16, limestone equipment 17, flow meter 18, control valve 19, flow regulator 20, and control device 2.
1 as a subject.

The exhaust gas 1 from the boiler is pressurized by a desulfurization fan (not shown), introduced into the absorption tower 2, and the absorption liquid containing calcium carbonate (CaCO ₃ ) sent from the circulation pump 8 is injected from the spray nozzle 4. Thus, gas-liquid contact between the absorbing liquid and the exhaust gas is performed. At this time, the absorbing liquid selectively absorbs SO ₂ in the exhaust gas, reacts with calcium carbonate, and further oxidizes with oxygen in the air 10 supplied to the absorption tower liquid reservoir 7 while being stirred by the oxidizing stirrer 9. by generating the gypsum _{(CaSO 4 · 2H 2 O)} . This series of reactions is represented by the following reaction formula. SO ₂ + CaCO ₃ + ／ O ₂ + 2H ₂ O → CaS
_{O 4 · 2H 2 0 + CO} 2

A part of the absorption liquid in the absorption tower liquid reservoir 7 containing gypsum is sent again to the spray nozzle 4 by the circulation pump 8, and a part is sent to the gypsum dehydrator 12 from the extraction pump 11. The absorbing solution sent to the gypsum dewatering machine 12 is separated into a solid gypsum and a filtrate by a centrifuge such as a decanter type, the gypsum is discharged out of the system, and the filtrate is stored in the filtrate tank 13. After that, a part is directly supplied to the absorption tower liquid reservoir 7 by the filtrate pump 14, and the rest is supplied to the limestone slurry tank 15, mixed with the limestone powder supplied from the limestone equipment 17, and becomes a limestone slurry as a limestone slurry. The liquid is supplied to the absorption tower liquid reservoir 7 by the pump 16.

[0006] Among the finely divided absorbents ejected from the spray nozzle 4, those having a small droplet diameter are entrained by the exhaust gas, but are collected by the mist eliminator 5 provided at the outlet of the absorption tower 2. Is done.

[0007]

In the prior art shown in FIG. 3, the amount of the absorbent extracted to the gypsum dewatering machine 12 is as follows.
For example, the control device 21 calculates the amount of absorbed SO _2, determines a withdrawal amount corresponding to the amount, and sets the flow meter 18 and the control valve 1
9 Gypsum separated by the gypsum dewatering machine 12 and discharged out of the system usually contains about 10% of moisture (adhered water), and chlorine contained in the exhaust gas 1 is dissolved in the adhering water. Therefore, the chlorine in the exhaust gas 1 absorbed in the absorbing solution is discharged out of the system together with the gypsum-adhering water. In some cases, the chlorine concentration in the system will increase. Normally, the chlorine concentration in the absorbing solution is 5,000 to 10,000 ppm. However, if the chlorine concentration increases beyond this range, equipment that comes into contact with the absorbing solution, such as a pump, is corroded and causes a serious failure. There is a risk. FIG. 4 shows a conceptual diagram of the relationship between the absorbed chlorine amount and the gypsum moisture content and the chlorine concentration in the absorbing solution in the prior art. In the case where the gypsum moisture content is constant, the absorption solution in the absorbing solution increases as the absorbed chlorine amount increases. Chlorine concentration increases.

In order not to increase the chlorine concentration in the system, a part of the filtrate separated by the gypsum dewatering machine 12 is discharged out of the system, and the water is replenished in the system by that amount to dilute the filtrate. There is a method for preventing an increase in chlorine concentration, but for that purpose, it is necessary to treat the filtrate discharged out of the system with a wastewater treatment device, which increases the cost of the entire plant equipment such as the installation cost of the wastewater treatment equipment.

In the prior art shown in FIG. 3, the increase in the chlorine concentration of the absorbing solution is not sufficiently considered, and there is a problem that the durability of the equipment is reduced or the equipment cost is high. .

[0010] An object of the present invention is to provide a wet flue gas desulfurization method and apparatus which can ensure the reliability of equipment without increasing the chlorine concentration in the absorbing solution which causes troubles in the desulfurization apparatus and which have low equipment costs. To provide.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to treat an exhaust gas discharged from a combustion device with an absorbent containing limestone and the like to treat sulfur oxides in the exhaust gas, and then to form a gypsum as a by-product. In the wet flue gas desulfurization method for recovering water from the absorbent, the gypsum recovered from the absorbent according to the amount of chlorine absorbed in the absorbent calculated in advance according to the load of the combustion device, the type of fuel used, and the properties of the exhaust gas By adjusting the water content of the solution, the problem can be solved by a wet flue gas desulfurization method in which the chlorine concentration in the absorbing solution is suppressed to a predetermined value or less.

At this time, when gypsum is recovered from the absorbent using a gypsum dehydrator, the number of revolutions of the gypsum dehydrator is absorbed into the absorbent which is calculated in advance according to the load of the combustion device and the type of fuel used. By controlling according to the amount of chlorine
Adjust the moisture content in the gypsum recovered from the absorbent.

[0013] Further, it is possible to provide a non-drainage flue gas desulfurization method in which the absorbent after gypsum separation is used for absorbing sulfur oxides in exhaust gas and is not discharged out of the system.

Further, the object of the present invention is to provide an absorption tower for treating sulfur oxides in exhaust gas and a gypsum recovery apparatus for collecting gypsum by bringing exhaust gas discharged from a combustion device into contact with an absorbent containing limestone and the like. In the wet flue gas desulfurization device provided with the above, the problem is solved by a wet flue gas desulfurization device provided with a device for varying the moisture content in gypsum collected by a gypsum collecting device such as a gypsum dehydrator.

The gypsum dehydrator rotation speed control device controls the load on the combustion device and the amount of chlorine absorbed in the absorption liquid calculated in advance according to the type of fuel used.

Further, a circulation channel for circulating the absorbent recovered by the gypsum dewatering machine to the absorption tower may be provided to provide a non-drainage flue gas desulfurization apparatus.

[0017]

The wet flue gas desulfurization device needs to keep the solid concentration (slurry concentration) in the absorption liquid constant, and thus replenishes the system with the same amount of water as the water discharged outside the system. Therefore, if the amount of water discharged out of the system is constant, the properties of the absorbing liquid can be kept constant. However, when the amount of water discharged out of the system decreases, the amount of water to be replenished in the system also needs to be correspondingly reduced, thereby increasing the chlorine concentration in the absorbing solution. Metal materials used in pumps, stirrers, pipes, and other devices that come into contact with the absorbent are usually acid-resistant materials that can support a chlorine concentration of 5,000 to 10,000 ppm. Corrosion cannot be prevented if the chlorine concentration in the water in the system is high, and if high-grade materials having high acid resistance are used, the equipment cost becomes extremely high.

If a part of the filtrate separated by the gypsum dewatering machine is discharged out of the system as waste water, the same amount of water discharged out of the system can be replenished into the system. However, since the wastewater contains trace amounts of solids and heavy metals, it is necessary to install a wastewater treatment facility for removing these, which increases the equipment costs and operating costs.
Therefore, it is desirable that the desulfurization device be a non-drainage system that does not discharge wastewater out of the system.

Water (outflow water) discharged from the desulfurization unit
Are three types of evaporating water, gypsum crystal water and gypsum-adhering water.
As for the evaporating water, the exhaust gas at 120 to 200 ° C comes into contact with the absorbing liquid.
When the exhaust gas is cooled and humidified to the saturated moisture temperature
Moisture in the absorbing solution evaporates and is discharged out of the system together with the processing gas
Is done. In addition, gypsum crystal water is contained in SO gas in exhaust gas.₂Is absorbing liquid
Is absorbed into the limestone, and finally neutralized by the neutralization reaction with limestone.
Plaster (CaSO₄・ 2H ₂O) when the water of crystallization
Is the moisture taken into the gypsum. Gypsum adhesion water is stone
Moisture remaining on the solid side without being completely separated by the plaster dehydrator
(Usually about 8 to 10%).

Of these effluents, evaporating water and gypsum crystal water do not contain chlorine, and only gypsum-adhering water contains chlorine. Therefore, the chlorine concentration in the absorbing solution is determined by the amount of gypsum-adhering water. Since the amount of gypsum adhering water is determined by the characteristics of the gypsum dewatering machine, it is relatively easy to change the amount of adhering water. For example, in a centrifuge such as a decanter type, the number of rotations for generating the centrifugal force is usually constant, and if the amount of the supplied slurry liquid is within the planned range, the water content of the gypsum due to the adhered water of the collected gypsum is It is constant. Therefore, in order to change the water content of the gypsum due to the water adhering to the gypsum, the centrifugal force may be changed by controlling the rotation speed of the decanter.

Since the amount of chlorine in the flue gas is determined in advance in accordance with the fuel condition (coal type, load, etc.), it is necessary to determine the amount of chlorine absorbed by the absorbing solution upon contact with the flue gas and the overall water balance. The gypsum moisture content (varies depending on the amount of gypsum adhering water) is calculated in advance so that the chlorine concentration in the absorbing solution does not exceed the upper limit based on the chlorine amount and the water amount. The rotational speed of the gypsum dewatering machine may be controlled according to the relationship between the rates.

Therefore, by controlling the dewatering performance of the gypsum dewatering machine to vary the amount of gypsum adhering water, it is possible to prevent an increase in the chlorine concentration in the absorbing solution with relatively simple equipment.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration diagram of a wet flue gas desulfurization apparatus according to an embodiment of the present invention. The flue gas desulfurization apparatus shown in FIG. 1 is the same as the wet flue gas desulfurization apparatus shown in FIG. 3, and has an absorption tower 2, a circulating liquid pipe 3, a spray nozzle 4, a mist eliminator 5, an absorption tower liquid reservoir 7, a circulation pump 8, and an oxidation pump. Stirrer 9, extraction pump 11, gypsum dewatering machine 12, filtrate tank 13, filtrate pump 14, limestone slurry tank 15, limestone slurry pump 16,
A limestone facility 17, a flow meter 18, a control valve 19, a flow regulator 20, and a control device 21 are mainly constituted, and a rotation speed control device 23 and a boiler combustion signal 24 shown in FIG. Different from the configuration of the smoke desulfurization unit.

In the present embodiment, a centrifugal separator such as a decanter type is used as the gypsum dewatering machine 12. The absorption liquid extracted from the absorption tower liquid reservoir 7 by the extraction pump 11 is supplied to a gypsum dewatering machine 12 and separated into a solid gypsum 22 and a filtrate using centrifugal force. Usually, the supply amount of the absorbing liquid supplied to the gypsum dewatering machine 12 is determined in accordance with the amount of SO ₂ absorbed and removed in the absorption tower 2, and the flow meter 1
8. The flow rate is adjusted by the control valve 19 and the flow rate regulator 20. If the absorption liquid supply flow rate is within the processing capacity range of the gypsum dewatering machine 12, the water content of the gypsum by the attached water of the separated gypsum is constant.

Here, assuming that the boiler (not shown) is switched to a fuel having a high chlorine concentration in the fuel, the exhaust gas 1 introduced into the absorption tower 2 based on the boiler combustion signal 24.
The controller 21 calculates the amount of chlorine absorbed in the absorbent from the gypsum, calculates the amount of gypsum in the amount of absorbent supplied to the gypsum dewatering machine 12, and sets the chlorine concentration in the gypsum-adhered water based on these. Calculate the gypsum moisture content below the value. Further, the rotation speed of the gypsum dewatering machine 12 is controlled by the rotation speed control device 23 so that the calculated gypsum moisture content is obtained.

If the amount of gypsum adhering water increases and the water content of the gypsum increases, the amount of water flowing out of the system increases. Therefore, it is necessary to increase the amount of makeup water to maintain the water balance in the system. The amount of water is also determined by calculating the overall water balance by the control device 21.

However, in addition, for example, the absorption tower liquid reservoir 7
The liquid level may be measured in advance, and if the supply water is insufficient, the liquid level decreases. Therefore, control may be performed such that water is supplied to the absorption tower liquid reservoir 7 so as to keep the liquid level constant. By controlling the supply amount of make-up water so as to keep the liquid level constant while observing the liquid level in the absorption tower liquid storage section 7, which is an actual phenomenon, accurate control of make-up water can be performed.

If the water content of the gypsum due to the water adhering to the gypsum is usually 15% or more, the gypsum becomes muddy and hinders transportation after recovery. Therefore, the water content is set to a maximum of 15%.
When it exceeds 15%, it is preferable to evaporate excess gypsum adhering water by a drying device.

In the present embodiment, the gypsum dewatering machine 12 has been described as a centrifugal separator. However, for example, a gypsum dewatering machine of a belt filter type may be used. In the case of using a gypsum dewatering machine of a belt filter type, it is possible to change the water content of gypsum due to gypsum-adhered water by controlling the belt conveyance speed.

[0030]

According to the present invention, it is possible to prevent an increase in the chlorine concentration in the absorbent by changing the water content in the gypsum, thereby preventing a significant increase in equipment costs and operating costs.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a conceptual diagram showing the relationship between the amount of absorbed chlorine, the gypsum moisture content and the chlorine concentration in the absorbing solution in the present invention.

FIG. 3 is a diagram showing an embodiment of the related art.

FIG. 4 is a conceptual diagram showing the relationship between the amount of absorbed chlorine, the water content of gypsum, and the concentration of chlorine in the absorbing solution in the prior art.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Exhaust gas 2 Absorption tower 3 Circulating fluid pipe 4 Spray nozzle 5 Mist eliminator 6 Processing gas 7 Absorption tower liquid reservoir 8 Circulation pump 9 Oxidation stirrer 10 Oxidation air 11 Extraction pump 12 Gypsum dehydrator 13 Filtrate tank 14 Filtrate pump 15 Limestone slurry tank 16 Limestone slurry pump 17 Limestone equipment 18 Flow meter 19 Control valve 20 Flow regulator 21 Control device 22 Gypsum 23 Speed control device 24 Boiler fuel signal

Continued on the front page F-term (reference) 4D002 AA02 BA02 CA01 DA05 DA16 EA07 FA03 GA03 GB20 HA04 4G076 AA14 AB27 BA33 BC06 BH01 GA02

Claims (7)

[Claims] 1. A wet flue gas desulfurization method for contacting an exhaust gas discharged from a combustion device with an absorbent containing limestone and the like to treat sulfur oxides in the exhaust gas and recovering gypsum from the absorbent as a by-product. The chlorine concentration in the absorbent is adjusted by adjusting the water content in the gypsum recovered from the absorbent according to the amount of chlorine absorbed in the absorbent, which is calculated in advance according to the load on the combustion device and the type of fuel used. Is reduced to a predetermined value or less. 2. Gypsum is recovered from the absorbent using a gypsum dehydrator, and the number of chlorine absorbed in the absorbent is calculated in advance according to the load of the combustion device and the type of fuel used. 2. The wet flue gas desulfurization method according to claim 1, wherein the moisture content in the gypsum recovered from the absorbent is adjusted by controlling according to the following conditions. 3. The wet flue gas desulfurization method according to claim 1, wherein the absorbent after the separation of the gypsum is used for absorbing sulfur oxides in the exhaust gas and is not discharged out of the system. 4. A wet process comprising an absorption tower for treating sulfur oxides in exhaust gas by bringing exhaust gas discharged from the combustion device into contact with an absorbent containing limestone and the like, and a gypsum recovery device for recovering gypsum from the absorbent. A wet flue gas desulfurization device comprising a device for varying the water content in gypsum collected by a gypsum recovery device. 5. The gypsum collecting device is a gypsum dewatering device for dehydrating adhering water in gypsum by centrifugal force, and the device for changing the moisture content in the gypsum to be recovered is a rotation speed control device of the gypsum dewatering device. The wet flue gas desulfurization device according to claim 4, characterized in that: 6. The gypsum dehydrator rotation speed control device is controlled in accordance with the amount of chlorine absorbed in the absorption liquid which is calculated in advance according to the load on the combustion device and the type of fuel used. Item 5. A wet flue gas desulfurization device according to Item 4. 7. A circulation flow path for circulating an absorbent recovered by a gypsum dewatering machine to an absorption tower is provided.
The wet flue gas desulfurization apparatus according to the above.