JPH07256050A - Non-drainage method of wet exhaust gas desulfurizer - Google Patents

Abstract

PURPOSE:To eliminate the drainage of water in a wet exhaust gas desulfurizer by using a liquid used in the extraction of dust as the washing water of gypsum separated in a solid-liquid separation process before circulating and reuse the same for the purpose of the extraction of dust. CONSTITUTION:After combustion exhaust gas is pretreated, the pretreated gas is brought into contact with the liquid circulated in a dust extraction tower 1 to be subjected to the extraction of dust while cooled and humidified. Next, the exhaust gas is sent to an absorbing tower 4 to be brought into contact with an absorbing soln. being a calcium type slurry to absorb and remove sulfur oxide. Thereafter, the formed gypsum-containing slurry is guided to a solid-liquid separator 7 to be separated into a gypsum cake and a mother liquid. The mother liquid is sent to a mother liquid tank 12 and a part of the mother liquid is reused as the supply water to the dust extraction tower 1 while a part thereof is refluxed to the absorbing tower 2 and the remainder thereof is used in the slurrying of a calcium compd. By this constitution, this desulfurizer is stably and continuously operated in a non-drainage state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for eliminating drainage of a wet flue gas desulfurization apparatus.

[0002]

2. Description of the Related Art Harmful sulfur oxides contained in combustion exhaust gas from boilers, various heating furnaces, incinerators, etc. Mainly sulfurous acid gas is produced by contacting with an absorbing liquid containing a calcium compound as a gas-liquid contact. Absorbed and removed in the absorbent,
Next, gypsum, which is detoxified and simultaneously produced as a by-product by a wet flue gas desulfurization method of oxidizing, fixing and collecting as gypsum, is also industrially used.

However, in addition to sulfur oxides, trace components such as dust, heavy metals, chlorine compounds, fluorine compounds, ammonium salts, etc. are also contained in the exhaust gas as impurities, and these impurities, particularly chlorine compounds and fluorine compounds, etc. If it flows into the absorption tower, various problems such as a decrease in the desulfurization rate due to a decrease in the dissolution rate of the calcium-based compound that is the absorbent and corrosion of the apparatus may occur.

Therefore, in the wet flue gas desulfurization method, a dust removal tower (also referred to as a cooling tower) for cooling and dust removal is usually installed before the exhaust gas is brought into gas-liquid contact with the calcium compound-containing absorption liquid in the absorption tower. The exhaust gas is contacted with an aqueous liquid to cool the exhaust gas, and at the same time, most of these harmful trace components are captured and removed. Since the liquid used in this dust removal tower is normally circulated, the impurities are accumulated in the circulating liquid, which causes problems such as dust accumulation, salt precipitation, pH increase, and further device corrosion. Normally, about 10 to 30% of makeup water is drained, and industrial water is supplemented by that amount to continue the operation.

However, the wastewater thus discharged contains the above-mentioned impurities and causes environmental problems such as water pollution, so that it cannot be discharged as it is from the viewpoint of pollution prevention, and in order to remove or reduce these harmful impurities, for example, It was discharged after neutralization of low pH liquid with NaOH, coagulation sedimentation, ion exchange resin treatment, pH adjustment, and other wastewater treatment.

However, in recent years, due to environmental problems, reduction of water resources, etc., not only drainage from a dust removal tower, but also a so-called closed or no drainage method of operating without discharging drainage from the flue gas desulfurization system itself is required. , And various methods have been proposed.
For example, the wastewater is concentrated by adjusting the pH or concentrated and then evaporated to dryness to remove, or the solid matter obtained by spraying in the exhaust gas is collected and removed, or the wastewater is concentrated and then mixed with lime ash or the like. , A method of removing by drying (Japanese Patent Publication No. 54-1922)
No. 6, JP-B-63-63248, JP-A-3-
No. 8411, JP-A-3-249919, JP-B-3-59730), a method of removing wastewater by heating and drying gypsum (JP-A-51-124669), and after neutralizing the wastewater. A method in which a concentrated liquid phase is mixed with the solid-liquid separated and the solid phase is heated and dried, and removed (Japanese Patent Publication No.
5-33927), a method of mixing a chlorine-containing wastewater obtained by applying a chlorine-containing wastewater to an electrodialysis device into lime or gypsum to dry and solidify (JP-A-4-83586), and the like.

All of these methods require heat energy to concentrate or dry the wastewater. Even if the heat of the exhaust gas flue is used as a heat energy source, a device for that purpose is required. In addition to expenses such as drug costs for neutralization, facility costs for electrodialysis equipment, etc., operating costs for these are required. As a method that does not require concentration, Japanese Patent Publication No. 61-16490 discloses a method of eliminating drainage by installing two absorption towers. However, by installing an additional absorption tower, equipment costs are increased. The site area is required, and operating costs and maintenance costs are extra.

[0008] As described above, the conventional method without drainage requires new equipment and the like even if the wastewater treatment device itself is not required, and therefore, industrially, no great merit can be expected.

[0009]

Therefore, there is no need for concentrating the waste water or spray-drying it back to the flue, no chemical costs, and no new equipment for draining. If a method can be developed, it is expected to make a great contribution not only in terms of industry but also in terms of environment and water resource conservation.

The inventors of the present invention focused on the bleeding liquid from the dust removing tower, conducted various studies on the possibility of circulating the liquid in the desulfurization system by devising the operating method, and found the drawbacks of the above-mentioned conventional method. Research was conducted to develop a drainage-free method that does not have the As described above, in the wet flue gas desulfurization device, dust is removed in the preceding stage of the absorption tower for the purpose of capturing harmful trace components that are impurities in the exhaust gas that cause various problems when flowing into the absorption tower and simultaneously cooling the exhaust gas. A tower (also called a cooling tower) is installed. Since a relatively large amount of these harmful components accumulates in the liquid circulating in the dust removal tower,
About 30% of the water is drained and the operation is continued by replenishing industrial water, etc., and the wastewater is discharged after removing harmful components by installing wastewater treatment equipment.

On the other hand, the gypsum produced in the absorption tower is sent from the absorption tower as a gypsum slurry to a solid-liquid separation step, where it is separated into a gypsum cake and a mother liquor by dehydration treatment. About 10% by weight of mother liquor adheres to this gypsum cake.
If the gypsum cake is washed with a bleed solution containing a relatively large amount of harmful trace components from the dust removal tower, the adhered water, which accounts for about 10% by weight of the gypsum cake, is almost replaced with the waste water. I found that. By this replacement, salts and the like contained in the bleeding liquid from the dust removing tower are discharged out of the system together with gypsum.

This usually produces the same effect as that of supplying 10% to 30% of makeup water to the outside of the system and replenishing industrial water etc. accordingly. The filtrate after washing the gypsum cake can be returned to the dust removing tower and repeatedly used repeatedly. This allows stable and continuous operation without draining to the outside of the desulfurization system. The present invention has been completed based on these findings.

[0013]

Means for Solving the Problems That is, the present invention is directed to a step of removing flue gas containing sulfur oxides and dust with an aqueous liquid component to remove dust, and then a liquid absorption containing a calcium compound and gas-liquid contact. In the wet flue gas desulfurization treatment including a step of absorbing and removing sulfur oxides from the combustion exhaust gas and oxidizing it, and a solid-liquid separation step of separating the produced solid mainly composed of gypsum, it is used for the dust removal. The purpose of the present invention is to provide a method for eliminating the drainage of a wet flue gas desulfurization apparatus by using the obtained liquid as washing water for the gypsum separated in the solid-liquid separation process and then reusing it in the dust removal process.

According to the present invention, the bleed liquid from the dust removing tower can be circulated and reused, so that the conventional waste water treatment facility for treating the waste water from the dust removing tower is not required. No drug costs are required. It is sufficient to install simple additional equipment to the existing flue gas desulfurization equipment without installing new large equipment. Therefore, it can be said that it is an excellent method in terms of equipment cost, space aspect, operation aspect, drug expense and environmental aspect.

The present invention will be described in detail below with reference to the drawings. The flow sheet of the flue gas desulfurization process of the drainage-free method by circulating the dust removal water of the present invention is shown in FIG.
Combustion exhaust gas G from a heavy oil boiler, a coal boiler, a sintering plant, a metal refining plant, etc. is subjected to a pretreatment such as denitration and electrostatic dust collection, and then a dust removal tower 1 for cooling and dust removal 1
And is brought into contact with the liquid circulated by the pump 2 here, and is cooled and humidified at the same time as dust is removed. The dust removing tower is usually a spray type.

The exhaust gas thus dedusted and cooled and humidified is sent to the absorption tower 4 through the pipe line 3. In the absorption tower 4, the sulfur oxide is absorbed and removed by making gas-liquid contact with the absorption liquid containing the calcium compound slurry. The gas-liquid contact method in the absorption tower is a gas bubbling method in which a gas is blown into the absorption liquid, a spray method in which the absorption liquid is sprayed in the absorption tower, and the like.
Any method can be adopted. The sulfur oxide absorbed in the absorption tower is oxidized by the oxidizing air introduced into the absorption tower, and then neutralized by the calcium-based compound introduced into the absorption tower to be fixed as gypsum. The oxidation may be performed there by providing an oxidation tower separately from the absorption tower.

Examples of the calcium compound usable in the present invention include calcium carbonate, calcium hydroxide, calcium oxide and the like, but calcium carbonate is preferably used because it is inexpensive and available in large quantities. The gypsum-containing slurry is supplied from a pump 5 through a pipe 6 to a solid-liquid separator 7 having a cleaning function, where it is solid-liquid separated by dehydration and separated into a gypsum cake and a mother liquor. As the solid-liquid separation device 7, it is necessary to use one having a cleaning function.

The mother liquor is conveyed to the mother liquor tank 10 through the pipe 9, from which it is reused by the pump 11 through the pipe 12 as a make-up water for the dust removing tower 1 and a part is returned to the absorption tower 2. The other part is used for slurrying the calcium compound (not shown). In the present invention,
By washing the gypsum cake remaining in the apparatus after separation by the solid-liquid separation apparatus 7 with the liquid introduced from the dust removal tower 1 through the pipe line 17, the adhered water occupying about 10% by weight of the gypsum cake is thoroughly washed. Replaced by water. As a result, wastewater containing harmful trace components in the exhaust gas adheres to the gypsum 8 and is discharged together with the gypsum 8. The filtrate after washing the gypsum cake is sent to the washing filtrate tank 14 through the pipe 13, and the pump 15
Thus, it is returned to the dust removing tower 1 through the pipe line 16 and circulated and reused.

On the other hand, the exhaust gas from which the sulfur oxides have been removed is discharged out of the system through the pipe 18. In the dedusting tower 1, since the water evaporates after the gas-liquid contact and goes out together with the exhaust gas, the liquid circulated in the dedusting tower is gradually concentrated, but after the operation of the desulfurization treatment apparatus starts, for example, about 500 hours, The concentration of impurities in the liquid circulating in the dust removal tower becomes almost constant and does not rise any further. This is because a balance is established between the amount of impurities in the exhaust gas removed in the dust removal tower and the amount of impurities attached to gypsum and discharged outside the system. In this way, the desulfurizer can be stably operated continuously without draining water, and is not drained to the outside of the system.

[0020]

【Example】

Example 1 A spray type dust removing tower was used to perform wet flue gas desulfurization treatment under the following exhaust gas composition and reaction conditions by a soot separation type desulfurization device (Chiyoda Thoroughbred CT-121 type) by a gas bubbling method (air introduction). The gypsum cake was washed with the waste water from the dust removing tower, and the filtrate was returned to the dust removing tower and reused by circulation. Exhaust gas composition: Coal combustion exhaust gas: 400 NL / h Inlet gas temperature: 150 ℃ Inlet SO ₂ concentration: 1000 ppm-D Soot dust: 115 mg / m ³ N Of which Cl concentration: 14 ppm HF concentration: 35 ppm pH: 4 Calcium-based compound: 200 90% mesh pass, 95% purity
Calcium carbonate Drainage: None Dust removal tower liquid gas ratio: 2 L / m ³ Dust removal tower liquid retention time: 5 minutes (relative to the circulating liquid flow rate) About 500 hours after the start of operation, the concentration of impurities in the liquid circulating in the dust removal tower Is a Cl concentration of 2.6% by weight and an F concentration of 3.
It became almost constant at 5% by weight, and did not increase further thereafter. It was possible to operate continuously with a desulfurization rate of about 96% without draining outside the system.

[0021]

According to the present invention, by devising the operation method of the bleed liquid from the dust removal tower, the concentration of waste water required for the conventional non-drainage method, equipment therefor, other drug cost, electrodialysis device No drainage is required without incurring equipment costs, operating costs, etc. According to the present invention, it is not necessary to install a new large-scale device for drainage-free, and it is only necessary to provide a simple additional equipment to the present flue gas desulfurization equipment, and the equipment cost, space aspect, operation aspect, and environment. This is a very advantageous method.

[Brief description of drawings]

FIG. 1 shows a flow sheet of a flue gas desulfurization process of the drainage-free system of the present invention.

[Explanation of symbols]

1 Dust removal tower 4 Absorption tower 7 Solid-liquid separator
8 plaster

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Claims (1)

[Claims] 1. A step of contacting a combustion exhaust gas containing sulfur oxides and dust with an aqueous liquid component to remove dust, and then gas-liquid contacting with a calcium compound-containing absorbing liquid to thereby perform sulfur oxidation from the combustion exhaust gas. In a wet flue gas desulfurization process including a step of absorbing and removing a substance and oxidizing, and a solid-liquid separation step of separating a generated gypsum-based solid matter, a liquid used for the dust removal in the solid-liquid separation step. A method for eliminating the drainage of a wet flue gas desulfurization device, which comprises using the separated gypsum as washing water and then recycling it in a dust removal process.