JPH06142447A - Treatment of gypsum slurry from wet stack gas desulfurizer - Google Patents

Abstract

PURPOSE:To save energy for the device and to economize resources by drawing off a gypsum slurry from a wet stack gas desulfurizer, spraying the slurry into a high-temp. waste gas on the upstream side of the desulfurizer to dry the gypsum slurry, recovering the gypsum as the powdery and granular body to eliminate the waste water. CONSTITUTION:The SOx in the waste gas are brought into contact with a liq. absorbent contg. CaCO3 as the absorbent in the absroption tower of a wet stack gas desulfurizer 8 to form CaSO3 which is oxidized with air and deposited as a gypsum crystal, and the crystal is drawn off as a gypsum slurry 11. The slurry 11 is sent to a spray drier 6 furnished on the upstream side of the desulfurizer 8 and sprayed into the waste gas at about 140 deg.C. The contact time with waste gas is controlled to about 10 sec, the water is vaporized, and the gypsum 12 is drawn off from the bottom. The powdery and granular by-product gypsum is recovered in such a manner without using a centrifugal separator. Since the filtrate is not left, the filtrate recycling and waste water treating equipment can be eliminated and an absorbent slurry preparing equipment is not needed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating gypsum slurry from a wet flue gas desulfurization apparatus. More specifically, it is a by-product obtained by wet flue gas desulfurization of sulfur compounds (hereinafter abbreviated as SOx) in exhaust gas. The present invention relates to a method of recovering gypsum as an energy-efficient and efficient method, and not producing any drainage.

[0002]

2. Description of the Related Art Conventionally, many wet flue gas desulfurization methods have been put to practical use. Among them, the wet lime / gypsum method which is the mainstream as a large flue gas desulfurization apparatus for commercial thermal power plants is well known. There is. {Junpei Ando: "World's flue gas purification technology"
Coal Research Institute (1990), P.99-121}
This technique is disclosed, for example, in Japanese Patent No. 899234 (Japanese Patent Publication No.
As disclosed in Japanese Patent No. 17318), CaCO
₃ and Ca (OH) ₂ are used as an absorbent to form a slurry suspended in water, which is supplied to an absorption tower equipped with a slurry dispersion means for flue gas desulfurization, and SOx is stored in a slurry receiving tank installed at the bottom of the absorption tower. The calcium sulfite generated by absorption is oxidized and converted into gypsum, and the slurry containing this gypsum is guided to a centrifuge to separate the gypsum in the form of granules, and the separated filtrate is discharged. The separated gypsum is put into a gypsum storage via a belt conveyor and is carried out to a gypsum board or a cement factory via a reclaimer or the like.

[0003]

By the way, in such wet lime / gypsum flue gas desulfurization, the gypsum slurry was once sent to a centrifuge to collect the gypsum in the form of granules and to separate the separated filtrate. It was something to be discharged.

However, the conventional method has the following drawbacks. (1) Gypsum recovered as a by-product is used in gypsum board and cement factories, but it is handled as powder and granules for efficient transportation and storage, so a large amount of gypsum slurry in wet flue gas desulfurization equipment is used. Large-scale centrifuge equipment was required to separate the granular gypsum from the ground, and the power cost for that was high. (2) Since the discharged separated filtrate contains fine gypsum, it is necessary to discharge the separated filtrate through the wastewater treatment facility in order to comply with the strict control of wastewater quality, which increases the facility operating cost.

At the present time when the problems of the global environment and energy are being highlighted, it is particularly important to save energy and resources. The present invention solves the drawbacks of the above item 2 and saves energy of a wet flue gas desulfurization apparatus, An object of the present invention is to provide a method for treating gypsum slurry from a wet flue gas desulfurization device, which can contribute to resource saving.

[0006]

The present invention provides a method for recovering gypsum as a by-product by treating sulfur oxides contained in exhaust gas with a wet flue gas desulfurization apparatus to obtain gypsum slurry from the wet flue gas desulfurization apparatus. Extracted, the gypsum slurry is sprayed in the high temperature exhaust gas in the upstream of the wet flue gas desulfurization apparatus to evaporate and dry to recover the gypsum as powder particles, and the waste flue gas desulfurization apparatus is characterized by no drainage. It is a method of treating gypsum slurry.

The present invention has the above structure,
The gypsum slurry extracted from the wet flue gas desulfurization unit is concentrated through a cyclone and condensed to an appropriate concentration, for example 5 to 50 wt%.
The gypsum slurry is returned to the absorption tower of the wet flue gas desulfurization device, and the concentrated gypsum slurry is, for example, in the high temperature exhaust gas in the spray dryer installed downstream of the electrostatic precipitator and upstream of the wet flue gas desulfurization device. The gypsum powder, especially gypsum particles with a large particle size, is separated and recovered from the exhaust gas by the cyclone effect of the bottom of the spray dryer, and the exhaust gas alone or the exhaust gas containing fine gypsum particles is subjected to wet flue gas desulfurization. It is preferable to be introduced into the device.

[0008]

[Operation] Although spray drying is generally known as a means for obtaining solids from a slurry, it has not been possible to use it for wet flue gas desulfurization equipment until now. It is thought that this is because the spray drying was not focused on for use.

However, as in the method of the present invention.
In addition, a large amount of stones is contained in the exhaust gas introduced into the wet flue gas desulfurization equipment.
By spraying the plaster slurry, the exhaust gas temperature decreases.
Therefore, the water content of the wet flue gas desulfurization equipment required by the conventional method is
In addition to reducing the amount of make-up water by evaporation, the method of the present invention
Says that the absorbent is CaCO₃Or Ca (OH)
₂Directly supplying powder to the absorption tower as is (this method
Has the advantage that the equipment is cheap and the power cost is low)
It As in the past, after making the absorbent into a slurry,
The complicated absorbent supply method for supplying
Spray drying is necessary because it requires water supply and a large amount of gypsum separation filtrate.
It was something that could not be used.

Exhaust gas containing SOx, for example, boiler exhaust gas from a coal-fired power plant, undergoes dry denitration treatment, heat recovery in a heat exchanger, fly ash collection in a dry electrostatic precipitator, and wet lime / gypsum method flue gas desulfurization. In the present invention, all or part of the exhaust gas from the dry electrostatic precipitator outlet is passed through a spray dryer and then introduced into a wet lime / gypsum flue gas desulfurization device for treatment.

In the absorption tower of the wet lime / gypsum flue gas desulfurization apparatus, SOx is absorbed by contacting it with an absorbent containing CaCO ₃ as an absorbent. Been SOx absorbed in the absorbing liquid generates a CaSO ₃ immediately reacts with CaCO ₃ but CaSO ₄ by an absorbing liquid to air oxidation. Crystallized as 2H ₂ O gypsum crystals. The absorption liquid containing gypsum is extracted from the absorption tower as so-called gypsum slurry corresponding to the SO ₂ absorption amount, but since the concentrated gypsum slurry is extracted through the cyclone, the gypsum slurry is 5 to 50 wt%.

This gypsum slurry is sent to a spray dryer and sprayed into exhaust gas at about 140 ° C. The contact time with the exhaust gas is about 10 seconds, the water evaporates, and the gypsum becomes powder. The temperature of the exhaust gas changes depending on the amount of evaporated water, but since it can be evaporated and dried normally to around 60 ° C, the gypsum slurry by-produced by desulfurization of SO ₂ concentration in the exhaust gas up to about 2500 ppm should be treated without external heating. You can

At the bottom of the spray dryer, the gypsum powder and granules are separated from the exhaust gas by the cyclone effect, while the low temperature exhaust gas at about 60 ° C. is guided to the wet lime / gypsum flue gas desulfurization system. With this wet desulfurization device, there is almost no evaporation of water into the exhaust gas, so make-up water due to evaporation loss is unnecessary, and the water balance is achieved by the wash water of the mist eliminator and the seal water of the pump. Moreover, since there is no gypsum separation liquid as in the past, and the method of directly supplying the absorbent CaCO ₃ to the absorption tower in the form of powder or granules can be adopted, the conventional complicated absorbent slurry adjusting and supplying equipment is omitted, and the equipment is simple. And power costs can be reduced. Further, since excess water for cleaning water and sealing water is extracted as gypsum slurry and spray-dried, it has a characteristic that no drainage is generated.

As described above, the present invention has an effect that the centrifugal separation equipment, the separated liquid treatment equipment, the wastewater discharge treatment equipment, and the absorbent slurry adjusting equipment, which require high power costs, can be omitted.

[0015]

EXAMPLE One embodiment of the method of the present invention will be described with reference to FIG. Exhaust gas from the boiler 1 of the coal-fired power plant is guided to the spray dryer 6 through the denitration 2, the air heater 3, the electrostatic precipitator 4, and the fan 5. The exhaust gas at the inlet of the spray dryer 6 has a temperature of about 140 ° C. and SO ₂ gas of about 800 pp
It contains m. A rotary atomizer 7 is installed above the spray dryer 6, and the rotary atomizer 7 is provided.
After that, the gypsum slurry 11 is sprayed into the exhaust gas. The gypsum slurry 11 is extracted from the desulfurization device 8 of the wet lime / gypsum method through a cyclone (not shown), and is about 10 wt.
% Water slurry containing gypsum crystal particles. The gypsum slurry concentration is 5 to 50w depending on the SO ₂ concentration in the exhaust gas.
Adjusted to t%. The gypsum slurry sprayed in the exhaust gas at about 140 ° C. immediately evaporates water to form gypsum powder,
The plaster 12 is extracted from the bottom of the spray dryer 6. Here, the exhaust gas outlet of the spray dryer is made into a cyclone shape or a cyclone is separately provided so that the gypsum powder and granules can be easily separated from the exhaust gas.

Although the temperature of the exhaust gas discharged from the spray dryer becomes about 60 ° C. due to the evaporation of water, the amount of water to be evaporated is adjusted in consideration of the gypsum slurry concentration so as to be higher than the water dew point temperature of 52 ° C. of the outlet exhaust gas. 800 ppm SO ₂
Exhaust gas temperature after spray-drying 10 wt% gypsum slurry obtained by desulfurization is 140 ℃ to 60 ℃, but the outlet exhaust gas temperature when spraying after concentrating the gypsum slurry concentration to 15 wt% is 90 ℃ Become. On the contrary, if the gypsum slurry concentration is reduced and the amount of evaporated water is increased, the exhaust gas gypsum powder eventually becomes wet and cannot be handled as a granular material.Therefore, make sure that the exhaust gas temperature is higher than the water dew point temperature. It is necessary to manage the amount of water.

Exhaust from the spray dryer 6 at about 60 ° C.
The gas is sent to the wet lime / gypsum method flue gas desulfurization device 8. This
Wet lime and gypsum method flue gas desulfurization equipment 8 of
Contact and SO₂Absorption tower to absorb and remove
Absorbing liquid tank, limestone powder that is the absorbent in the absorbing liquid tank
(CaCO₃) Equipment for supplying 10 as powder, absorption liquid
Circulation pump to circulate and disperse
SO₂Is CaCO ₃CaSO generated by reacting with₃
To oxidize CaSO_Four・ 2H₂O to make gypsum crystals
The absorbing liquid mist is discharged from the oxidizing air sparger and absorption tower.
Mist Elimine that prevents you from dissipating with you
Water cleaning equipment for mist eliminator, absorption tank
Is it a facility for extracting gypsum slurry from a liquid cyclone through a liquid cyclone?
It is composed of

In the absorption tower, the exhaust gas is washed with the absorption liquid, about 90% of 800 ppm of SO ₂ is desulfurized, and the exhaust gas temperature becomes 52 ° C. The amount of evaporated water in the exhaust gas here is about 1 / the amount of evaporated water in the spray dryer 6.
It is as small as 10. This evaporated water amount is extracted and the total amount of water in the gypsum slurry 11 is in equilibrium with the amount of water that flows in due to the water washing of the mist eliminator.

Wet lime / gypsum method The purified exhaust gas discharged from the flue gas desulfurization device 8 is heated by a reheating device if necessary,
It diffuses from the chimney 9 into the atmosphere.

Hereinafter, the coal combustion exhaust gas will be described with reference to a concrete test example to which the method of the present invention is applied according to the flow of FIG. Exhaust gas 200 m ³ N / h generated from a combustion furnace with a coal combustion rate of 25 kg / h is led to a dry denitration device by an ammonia catalytic reduction method and then passed through a heat exchanger to 1
After cooling to 40 ° C. and treating with a dry dust collector, as shown in FIG. 1 through an exhaust gas fan, a spray dryer constituted by the method of the present invention and a test apparatus for a wet lime / gypsum method flue gas desulfurization apparatus were used. The exhaust gas of 18 m ³ N / h was sampled and guided.

○ Spray dryer: 200 mmφ × 1500 mmH ○ Inlet exhaust gas flow rate: 18 m ³ N / h Inlet exhaust gas temperature: 140 ° C. Inlet exhaust gas moisture: 8 vol% Inlet exhaust gas SO ₂ concentration: 800 ppm (dry standard) ○ Spray gypsum slurry concentration: 10 wt % ○ Spray gypsum slurry supply rate: 870 ml / h ○ Outlet exhaust gas temperature: 60 ° C

Flue gas desulfurization device: Absorption tower: 20 mmφ
× 4000mmH Absorbent tank: 10 liters Absorbent slurry circulation flow rate: 270 liters / h Inlet gas flow rate: 19m ³ N / h Inlet gas temperature: 60 ° C ○ Inlet gas SO ₂ concentration: 790ppm (dry basis) Outlet gas SO ₂ concentration : 80 ppm (dry basis) withdrawn gypsum slurry concentration: 10 wt% withdrawal gypsum slurry flow rate: 870 ml / h extraction gypsum contained in the gypsum slurry (CaSO ₄ · 2H ₂
O): 9.5 wt% Unreacted limestone (CaC contained in the extracted gypsum slurry)
O ₃ ): 0.5 wt%

The above numerical values represent typical data when the test system was continuously operated for about 100 hours and the entire system was maintained in steady state operation.

The composition of the spray-dried product obtained by separating from the cyclone at the bottom of the spray dryer has a water content of 5 wt.
%, Gypsum (CaSO ₄ .2H ₂ O): 95 wt% (dry basis), limestone (CaCO ₃ ): 5 wt%, and it could be handled as a powder or granular material.

The slurry extracted from the wet lime / gypsum flue gas desulfurization unit is sent to a spray dryer to collect the gypsum as powder particles as described above, while the water evaporates. Driving was achieved that did not need to be put out. The gypsum powder obtained in this example has the following two features. One of them is that high-purity gypsum containing no fly ash was obtained. This is because the fly ash (dust) generated from the combustion furnace is removed by the dry dust collector in the downstream of the dry dust collector and by providing the spray dryer in the upstream of the wet flue gas desulfurization device. . Two of them are that the average particle size of the gypsum powder is larger than the average particle size of the gypsum particles contained in the absorbent slurry. That is, the number of small gypsum powder particles that have a small particle diameter is small. Since gypsum having a small particle size is not preferred because it easily scatters as dust and lowers the market value, the gypsum obtained by the method of the present invention has a high market value.

In the industrial field where a spray dryer is normally used, a precision dust collector such as a dry electrostatic precipitator or a bag filter is provided downstream of the spray dryer in order to sufficiently recover the dry solid matter obtained by the spray dryer. However, in the present invention, this common sense is not adopted, and only a coarse-grained gypsum is collected by providing a cyclone at the bottom of the spray dryer, and fine-grained gypsum is guided to the wet lime / gypsum method flue gas desulfurization device that is entrained in the exhaust gas. It was That is, since the precision dust collector that has been conventionally required is not provided in the downstream of the spray dryer, the fine-grained gypsum is collected again together with SO _{2 in the} absorption tower.

This fine-grained gypsum can be utilized as seed crystals of gypsum produced one after another by a chemical reaction in the absorption tower, and grows into large gypsum particles. By this cycle, coarse gypsum could be continuously recovered from the spray dryer bottom cyclone by the method of the present invention. Furthermore, as a seed crystal effect of fine-grained gypsum, it is possible to keep the gypsum supersaturation degree of the absorption liquid low, and gypsum scale does not adhere to the inner walls of the absorption tower and piping, and the effect of preventing obstacles to operation due to scale clogging troubles is also possible. It was confirmed that they could be obtained together.

[0028]

The present invention has the following effects. (1) By-product gypsum can be collected as powder and granules by omitting the centrifugal separation equipment, which was conventionally required for wet lime / gypsum flue gas desulfurization equipment, and spray-drying the gypsum slurry in the exhaust gas. (2) Since the gypsum separation filtrate does not come out, it is possible to omit the filtrate sending / recirculating facility and the wastewater treatment facility. (3) Limestone powder can be directly supplied to the absorbent tank without the need for absorbent slurry adjustment equipment.

[Brief description of drawings]

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

Claims (1)

[Claims] 1. A method for recovering gypsum as a by-product by treating a sulfur oxide contained in exhaust gas with a wet flue gas desulfurization device to extract gypsum slurry from the wet flue gas desulfurization device and wet the gypsum slurry. A method for treating gypsum slurry from a wet flue gas desulfurization apparatus, characterized in that gypsum is recovered as a granular material by being sprayed in high-temperature exhaust gas in the upstream of the flue gas desulfurization apparatus and evaporated to dryness to eliminate drainage.