JPH10118668A - Selenium reducing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a selenium reducing apparatus by reducing hexavalent selenium (Se<6+> ) containing in desulfurization waste water discharged from the absorbing tower of wet desulfurization equipment to tetravalent selenium (Se<4+> ) to facilitate the removal of selenium in later waste water treatment. SOLUTION: When exhaust gas generated after combustion of coal is desulfurized by using an absorbing soln. in an absorbing column 1, hexavalent selenium discharged in a state mixed with desulfurized waste water is reduced to tetravalent selenium in a selenium reducing apparatus 6. In this apparatus, a reducing agent injection device 2 injects the reducing agent into desulfurized waste water, and the selenium reducing apparatus 6 composed of a controller 4 requlating the amt. of hexavalent selenium injected by the reducing agent injection device 2 corresponding to the amt. of hexavalent selenium in desulfurized exhaust water is provided so as to be connected to the rear stage of the absorbing column 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing the amount of hexavalent selenium discharged from desulfurization wastewater into tetravalent selenium when desulfurizing exhaust gas generated after burning coal using an absorption liquid in an absorption tower. The present invention relates to a selenium reduction device, and more particularly to a selenium reduction device for reducing hexavalent selenium to tetravalent selenium by injecting a reducing agent such as hydrochloric acid into desulfurization wastewater while controlling the same.

[0002]

2. Description of the Related Art As one of devices for removing sulfur oxides from exhaust gas generated after burning coal in a boiler, an exhaust gas is introduced into an absorption tower, and an absorption liquid containing an absorbent (lime or the like) is introduced into the absorption tower. There is a wet desulfurizer for absorbing sulfur oxides.

[0003]

Coal contains harmful substance selenium (Se). In the case of conventional wet desulfurization equipment, this selenium (more precisely, selenium oxide) is used.
Is transported by the exhaust gas to the absorption tower, where it is mixed with the absorption liquid. When entering the absorption tower, hexavalent selenium (SeO)
The ratio of trivalent selenium (present as ₃ ) to tetravalent selenium (present as SeO ₂ ) is about half.

However, in the absorption tower, oxidizing air is fed to forcibly oxidize generated sulfites (CaSO _{3 and the} like), and the oxidizing air oxidizes most of tetravalent selenium to hexavalent selenium. You.

[0005] Therefore, eventually, most of selenium is hexavalent selenium (Se).
⁶⁺ ) enters the desulfurization wastewater and is discharged. However, Se ⁶⁺ cannot be subjected to wastewater treatment by sedimentation or the like (Se ⁴⁺ can be treated by sedimentation or the like), and the emission regulation values of selenium and its compounds revised on February 1, 1994 (0 .1
(mg / liter).

Accordingly, an object of the present invention is to reduce hexavalent selenium (Se ⁶⁺ ) contained in desulfurization effluent discharged from an absorption tower of a wet desulfurization apparatus to tetravalent selenium (Se ⁴⁺ ), thereby ^obtaining To provide a selenium reduction device that facilitates selenium removal in wastewater treatment of wastewater.

[0007]

Means for Solving the Problems In order to achieve the above object, the invention according to claim 1 is characterized in that when desulfurizing exhaust gas generated after burning coal using an absorbing liquid in an absorption tower, the exhaust gas is mixed with the desulfurized wastewater and discharged. A selenium reduction device for reducing hexavalent selenium to tetravalent selenium, which is used for injecting a reducing agent into desulfurization wastewater, and reducing the amount of reducing agent injected by the reducing agent injection device into the desulfurization wastewater. A selenium reduction device comprising a controller that adjusts according to the amount of valent selenium is provided connected to the latter stage of the absorption tower.

According to a second aspect of the present invention, the reducing agent is hydrochloric acid, and the controller detects the amount of hexavalent selenium in the desulfurization effluent from the amount of the withdrawn liquid to control the amount of hydrochloric acid injected. After the reduction of the selenium, a neutralization tank for neutralizing excess hydrochloric acid with an alkaline solution is connected to the subsequent stage of the selenium reduction device.

[0009]

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

[0010] As one of the devices for desulfurizing exhaust gas containing sulfur oxides, the exhaust gas is introduced into an absorption tower, and the absorption liquid containing an absorbent such as lime is sprayed on the exhaust gas in the absorption tower to form an absorption liquid. There is a wet type desulfurization device that absorbs sulfur oxides and desulfurizes them.

FIG. 1 shows an absorption tower 1 of such a wet type desulfurization apparatus. A combustion means (not shown) such as a boiler for burning coal is connected to the upstream side of the absorption tower 1, while another exhaust gas treatment device (not shown) is connected to the downstream side of the absorption tower 1. Is done.

A spraying means 10 for spraying the absorbing liquid into the exhaust gas is provided above the absorption tower 1, and an eliminator 11 for preventing the sprayed absorbing liquid from flowing out is provided immediately above the spraying means 10. In addition, a liquid reservoir 1a is formed below the absorption tower 1. The spraying means 10 and the liquid reservoir 1a are connected by the absorbent circulating means 12, and the absorbing liquid in the liquid reservoir 1a is returned to the spraying means 10 and sprayed again.

The liquid reservoir 1a is provided with an oxidizing air supply means (blower or the like) 13 for supplying oxidizing air for forcibly oxidizing a product (CaSO ₃ or the like) requiring forced oxidation. Absorbing liquid supply means 14 for supplying the absorbing liquid is connected to the liquid reservoir 1a.

A liquid reservoir 1 a at the lower part of the absorption tower 1 is further provided with an absorption liquid extraction pump 7 for extracting the absorption liquid from the absorption tower 1.
Is connected. Downstream of the absorbent pump 7, a reducing agent (for reducing Se ⁶⁺ to Se ⁴⁺ ) is injected into the desulfurization wastewater extracted from the absorption tower 1 by the absorbent pump 7. The reducing agent injection device 2 is connected. In the present embodiment, hydrochloric acid is used as a reducing agent.

On the downstream side of the reducing agent injection device 2, when hydrochloric acid as a reducing agent is excessively injected and the desulfurization effluent becomes acidic, it is neutralized with an alkaline solution (such as NaOH). 8 are provided. On the downstream side of the neutralization tank 8, a dehydrator 9 for dewatering the neutralized desulfurization wastewater to separate gypsum and the like is connected, and on the downstream side of the dehydrator 9, other wastewater treatment equipment (not shown) is provided. Can be

The reducing agent injection device 2 is provided with a valve 3 for adjusting the injection amount of the reducing agent (hydrochloric acid) into the desulfurization wastewater. Then, the controller 4 that controls the valve 3 so that an appropriate amount of the reducing agent is injected into the desulfurization wastewater,
Is provided in connection with. Detecting means 5 for detecting information necessary for the controller 4 to determine the amount of the reducing agent to be injected is provided to be connected to the controller 4 and the absorbent extraction pump 7. In the present embodiment, the detection means 5 is configured as a flow rate detection controller (FIC) for detecting the flow rate of the desulfurization wastewater extracted from the absorption tower 1.

The above-described reducing agent injection means 2, valve 3, controller 4, and detection means 5 constitute a selenium reduction device 6 of the present invention.

Combustion means (boiler, etc.) not shown for coal
To generate exhaust gas containing sulfur oxides, and this exhaust gas is introduced into the lower part of the absorption tower 1. Exhaust gas is in absorption tower 1
At this time, the absorbing solution is sprayed by the spraying means 10 and the sulfur oxides (SO ₂ and SO _3, etc.) contained in the exhaust gas are absorbed by the absorbing solution (that is, the sulfur oxides absorb lime and the like). ) And desulfurization is performed as shown below.

SO ₂ + CaCO ₃ → CaSO ₃ + CO ₂ (2) SO ₃ + CaCO ₃ → CaSO ₄ + CO ₂ (3) As a result of this desulfurization reaction, gypsum and sulfite are generated.
The desulfurized exhaust gas passes through the eliminator 11 and passes through the absorption tower 1
After being discharged from the top and treated by another exhaust gas treatment device (not shown), and then released to the atmosphere.

After the sprayed absorbing liquid falls into the liquid reservoir 1a, the absorbing liquid circulating means 12 re-sprays the spraying means 1a.
It is returned to 0 and sprayed, and this process is repeated thereafter. In addition, the absorbing liquid is appropriately supplied to the liquid reservoir 1a by the absorbing liquid supplying means 14. On the other hand, oxidizing air is blown into the liquid reservoir 1a by the oxidizing air means 13 to forcibly oxidize the sulfite (CaSO ₃ or the like) generated by the above reaction formula (2). After that, a part of the absorbing liquid in the liquid reservoir 1a is appropriately extracted by the absorbing liquid extracting pump 7, discharged as desulfurization waste water, and guided to the selenium reduction device 6 of the present invention.

Now, coal contains harmful substance selenium (S
e), but this selenium, after burning coal,
It becomes selenium oxide and is transported together with the exhaust gas to the absorption tower 1 (at this time, the ratio of SeO ₃ and SeO ₂ is about half as described above), and is mixed into the absorption liquid in the absorption tower 1. Then, most of the tetravalent selenium (present as SeO ₂ ) is oxidized to hexavalent selenium (present as SeO ₃ ) due to the oxidizing air blown into the liquid reservoir 1 a by the oxidizing air supply means 13 (reaction). Equation (1)), absorption liquid extraction pump 7
As a result, most of the selenium contained in the desulfurization effluent extracted from the liquid reservoir 1a) becomes hexavalent selenium (Se ⁶⁺ ) (about 75% or more).

As described above, the desulfurization wastewater containing more hexavalent selenium than tetravalent selenium is extracted by the absorption liquid extraction pump 7. Then, the flow rate is detected by the flow rate detection controller 5 as a detecting means, and the information is sent to the controller 4. The controller 4 detects the amount of Se ⁶⁺ contained in the desulfurization effluent based on this information (Se ⁶⁺ in the desulfurization effluent).
Is almost proportional to the amount of the withdrawn liquid from the absorption tower 1),
The valve 3 is appropriately adjusted so that a sufficient amount of hydrochloric acid for reducing Se ⁶⁺ to Se ⁴⁺ is injected into the desulfurization wastewater by the reducing agent injection means 2.

With hydrochloric acid as a reducing agent injected, S
e ⁶⁺ is reduced to Se ^{4+ as} in the following formula.

[0024] (SeO ₃ + 2HCl → SeO ₂ + Cl ₂
+ H ₂ O) The desulfurization effluent thus reduced from Se ⁶⁺ to Se ⁴⁺ is then introduced into the neutralization tank 8. In the neutralization tank 8, Se ⁶⁺
To neutralize the excess hydrochloric acid remaining without reacting with
An appropriate amount of an alkaline solution (such as NaOH) is injected.

The desulfurized wastewater in which the excess hydrochloric acid has been neutralized is then sent to a dehydrator 9 to remove gypsum and the like, and the wastewater from which gypsum and the like have been removed is sent to another wastewater treatment facility for treatment. You.
The treatment includes a treatment for removing Se ⁴⁺ by precipitation with ferric chloride or the like. In this way, selenium contained in the desulfurization effluent is removed to a level below the emission regulation value (0.1 mg / liter).

In summary, according to the selenium reduction apparatus of the present invention, Se ⁶⁺ contained in the desulfurization effluent discharged from the absorption tower of the wet type desulfurization apparatus is reduced to Se ^4+. Of selenium can be removed by a precipitation treatment or the like, so that the selenium emission regulation value can be easily cleared.

[0027]

As described above, in short, according to the selenium reduction apparatus of the present invention, Se ⁶⁺ contained in the desulfurization wastewater discharged from the absorption tower of the wet type desulfurization apparatus is reduced to Se ⁴⁺ , Selenium in the desulfurization effluent can be removed by sedimentation treatment or the like, so that the selenium discharge regulation value can be easily cleared.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a selenium reduction device of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Absorption tower 2 Reducing agent injection device 4 Controller 6 Selenium reduction device

Claims (2)

[Claims] In a selenium reduction device for reducing hexavalent selenium mixed with desulfurization wastewater to tetravalent selenium when exhaust gas generated after burning coal is desulfurized in an absorption tower using an absorbing solution, A selenium reducing device comprising a reducing agent injection device for injecting a reducing agent into the wastewater, and a controller for adjusting the amount of the reducing agent injected by the reducing agent injection device according to the amount of hexavalent selenium in the desulfurization wastewater, A selenium reduction device, which is provided connected to a stage subsequent to the absorption tower. 2. The method according to claim 1, wherein the reducing agent is hydrochloric acid, the controller detects the amount of hexavalent selenium in the desulfurization effluent from the amount of the withdrawn liquid and controls the amount of hydrochloric acid injected. 2. The selenium reduction device according to claim 1, wherein a neutralization tank for neutralizing the hydrochloric acid with an alkali solution is provided downstream of the selenium reduction device.