Method and apparatus for decomposing hardly decomposable component in desulfurization wastewater

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a method and an apparatus for decomposing persistent components in desulfurized wastewater for decomposing persistent COD such as thioic acid contained in flue gas desulfurized wastewater.

【0002】

[Conventional technology]

Conventionally, the flue gas desulfurized wastewater after desulfurization of exhaust gas from boilers and the like may contain thioic acid (dithionic acid, etc.) which is a persistent COD (Chemical Oxiygen Demand). In many cases, the COD concentration as a discharge condition cannot be reduced to 10 ppm or less unless it is removed from the wastewater.

【0003】

Since this thionic acid is not easily oxidized by oxygen in the air, it is currently adsorbed and removed by a COD adsorption tower or the like.

【0004】

[Problems to be Solved by the Invention]

However, the COD adsorption tower has a problem that the construction cost is high and the operating cost is also high.

【0005】

Therefore, it is considered to decompose the flue gas desulfurized wastewater by hot water treatment, but in the decomposition by hot water, the COD is high unless the sulfite generated when nithionic acid is decomposed by hot water is oxidized. Therefore, this release is difficult.

【0006】

In this case, it is possible to reduce the COD by converting the generated sulfite with oxygen to form a plaster, but the wastewater contains selenate in addition to thioic acid, which is treated with hot water. It is reduced from hexavalent to tetravalent selenate, which is easily coagulated and precipitated, and if it is simply oxidized, it will be reoxidized to hexavalent selenic acid, which is difficult to separate by coagulation precipitation, and will fall below the discharge water quality standard. It becomes difficult to do.

【0007】

Therefore, an object of the present invention is to solve the above-mentioned problems, and to separate and remove thioic acid, which is a persistent COD contained in the flue gas desulfurized wastewater, and to remove selenate and the like in the desulfurized wastewater. It is an object of the present invention to provide a method and an apparatus for decomposing a persistent component.

【0008】

[Means for solving problems]

In order to achieve the above object, in the invention of claim 1, the exhaust gas from a boiler or the like is brought into contact with an absorbent slurry in an absorption tower to be desulfurized, and the sulfur content is separated and recovered from the desulfurized absorbent slurry. In the method of decomposing persistent components such as thionic acid contained in desulfurized effluent, the desulfurized effluent is treated with hot water to decompose thionic acid, and the exhaust gas from the boiler etc. is added to the decomposition treatment liquid after the hot water treatment. This is a method for decomposing persistent components in desulfurized wastewater by blowing in to dissipate sulfur dioxide gas generated by decomposition of thionic acid and returning the dissipated sulfur dioxide gas to the absorption tower for desulfurization.

【0009】

The invention of claim 2 is a method for decomposing persistent components in desulfurized wastewater according to claim 1, wherein the desulfurized wastewater is pressurized to 5 to 10 MPa and heated to a temperature of 200 to 320 ° C to perform hot water treatment. Is.

【0010】

The invention of claim 3 claims that the decomposition treatment liquid after hot water treatment is decompressed after heat recovery to bring the temperature to 50 to 100 ° C., and exhaust gas is introduced into the decomposition treatment liquid to dissipate sulfurous acid gas. 2 This is a method for decomposing persistent components in desulfurized wastewater.

【0011】

The invention of claim 4 is the method for decomposing a persistent component in desulfurized wastewater according to claim 3, which separates a precipitate containing tetravalent selenate from the decomposition treatment liquid after the emission of sulfur dioxide gas.

【0012】

In the invention of claim 5, the exhaust gas from a boiler or the like is brought into contact with an absorbent slurry in an absorption tower to be desulfurized, and sulfur content is separated and recovered from the desulfurized absorbent slurry. Thionic acid contained in the desulfurized wastewater. In a decomposition device for persistent components such as, a hot water reactor that treats desulfurized wastewater with hot water and a decomposition treatment liquid that has been hot water treated and decomposed by the hot water reactor are introduced, and from a boiler or the like. SO that blows exhaust gas into the decomposition treatment liquid to dissipate the sulfurous acid gas in the decomposition treatment liquid₂ Dispersion tank and its SO₂ It is a decomposition device for persistent components in desulfurized wastewater equipped with a sulfur dioxide gas return line that returns the sulfur dioxide gas emitted in the release tank to the absorption tower.

【0013】

According to the invention of claim 6, a heat recovery device for heat exchange between the desulfurized wastewater introduced into the hot water reactor and the decomposition treatment liquid decomposed by hot water treatment is connected to the upstream side of the hot water reactor. Item 5 is a decomposition apparatus for persistent components in desulfurized wastewater.

【0014】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

【0015】

In FIG. 1, reference numeral 10 denotes an absorption tower, which is connected to an exhaust gas line 11 such as a boiler, and the exhaust gas and CaCO.₃ SOx in the exhaust gas is absorbed and removed by gas-liquid contact with an absorbent slurry using the above as an absorbent.

【0016】

An exhaust line 12 for exhaust gas after desulfurization is connected to the top of the absorption tower 10, and the exhaust gas after desulfurization is exhausted to the atmosphere via a GGH heater or the like.

【0017】

The absorbent slurry that has absorbed SOx in the absorption tower 10 is oxidized by air or the like in the absorption tower 10 to be gypsumized, and is introduced into the gypsum separator 14 from the line 13 for solid-liquid separation.

【0018】

The desulfurized wastewater separated from the gypsum 15 by the gypsum separator 14 is returned to the absorption tower 10 as a replenisher for the absorbent slurry at the replenishment line 16, and the rest is hot water reaction from the line 17 via the heat recovery device 18. It is supplied to the vessel 19.

【0019】

Although the details of the hydrothermal reactor 19 are not shown, the desulfurized wastewater is boosted to a subcritical pressure of 5 to 10 MPa by a pressure pump and heated to a temperature of 200 to 320 ° C by a heat medium or the like. The thionic acid (H) contained in the desulfurized wastewater is hydrolyzed by the subcritical pressure water.₂ S_n O₆ , N 2), for example dithionic acid (H₂ S₂ O₆ ) Is decomposed as shown in the following equation.

【0020】

H₂ S₂ O₆ = H₂ SO₄ + SO₂ In addition, selenate (Na), which is hexavalent selenium contained in desulfurized wastewater.₂ SeO₄, CaSeO₄ ) Is decomposed to tetravalent selenous acid (Na)₂ SeO₃ , CaSeO₃ ).

【0021】

The decomposition treatment liquid decomposed by hot water in the hot water reactor 19 is supplied from the line 20 to the heat transfer tube 21 of the heat recovery device 18, and heat is recovered by exchanging heat with the desulfurized wastewater introduced into the heat recovery device 18. After that, the pressure is reduced to normal pressure by the pressure reducing valve 22, and it becomes a decomposition treatment liquid with a temperature of 50 to 100 ° C and SO.₂ Introduced in the dissipation tank 23.

【0022】

SO₂ The exhaust tank 23 is provided with an exhaust gas blowing pipe 24, and the exhaust gas blowing pipe 24 is connected to an exhaust gas supply line 25 branched from the exhaust gas line 11. Also, SO₂ A sulfur dioxide gas return line 26 for the emitted sulfur dioxide gas is connected to the top of the discharge tank 23, and the sulfur dioxide gas is returned to the exhaust gas line 11 or the absorption tower 10 from the return line 26.

【0023】

Also, SO₂ The decomposition treatment liquid after the sulfur dioxide gas is dissipated in the discharge tank 23 is supplied to the wastewater treatment apparatus (not shown) from the discharge line 27, and the precipitate is supplied to the sediment treatment apparatus 28.

【0024】

In the above, the desulfurized wastewater that has been desulfurized in the absorption tower 10 and separated in plaster by the plaster separator 14 is preheated by the heat recovery device 18 and pressurized and heated to the subcritical state of water by the hydrothermal reactor 19. , Dithionic acid, etc. in the desulfurization wastewater is decomposed, and the decomposition treatment liquid is heat-recovered through the heat recovery device 18 and reduced to the pressure reducing valve 22 to SO.₂ Introduced in the dissipation tank 23.

【0025】

On the other hand, the exhaust gas is blown into the decomposition treatment liquid from the exhaust gas blowing pipe 24 from the exhaust gas line 11 via the exhaust gas supply line 25.

【0026】

By this exhaust gas, the sulfur dioxide gas in the decomposition treatment liquid is dissipated together with the exhaust gas, returned from the sulfur dioxide gas return line 26 to the absorption tower 10 for desulfurization treatment, and SO from the discharge line 27.₂ The treatment liquid that does not contain the above will be drained. In this case, the temperature of the decomposition treatment liquid is set to 50 to 100 ° C, so SO₂ Can be efficiently dissipated.

【0027】

The oxygen concentration of the exhaust gas blown from the exhaust gas blowing pipe 24 is 4 to 6%. Therefore, even when the hexavalent selenate is reduced to the tetravalent selenate in the hydrothermal reactor 19, the exhaust gas is blown. Can be prevented from being oxidized to hexavalent. In addition, tetravalent selenate is usually in an aqueous solution state, but as a result of hydrothermal reaction using the actual liquid, it was confirmed that selenic acid was not detected in the liquid and became a solid content. It was. This is because ash and heavy metal components contained in the combustion exhaust gas are contained in the desulfurized wastewater, and it is recognized that tetravalent selenic acid combines with these heavy metals and the like to form a solid content precipitate.

【0028】

Therefore, SO₂ Selenic acid can also be separated and removed by supplying the precipitate from the dispersal tank 23 to the precipitate treatment device 28. The desulfurized wastewater used in the actual liquid is the desulfurized wastewater of a coal-fired boiler, and it is expected that the amount of heavy metals in the desulfurized wastewater is small in the heavy oil-fired boiler.₂ Iron (FeCl) chloride in the decomposition treatment liquid in the dissipative tank 23₃ ) And the like may be added to cause coagulation and precipitation.

【0029】

[Effect of the invention]

In short, according to the present invention, by blowing oxygen-free exhaust gas into the decomposition treatment liquid after hot water treatment of desulfurized wastewater and dissipating sulfur dioxide gas, tetravalent selenium simultaneously decomposed by hot water treatment is produced. Even if it is contained, sulfur dioxide gas can be dissipated without oxidizing it to hexavalent selenium. In addition, the emitted sulfur dioxide gas is returned to the absorption tower and desulfurized, so there is no problem of secondary pollution.

[Simple explanation of drawings]

[Figure 1]

It is a figure which shows one Embodiment of this invention.

[Explanation of symbols]

10 Absorption tower 11 Exhaust gas line 18 Heat recovery device 19 Hot water reactor 23 SO₂ Dissipation tank 25 Exhaust gas supply line