JPS58174223A - Treatment of waste gas - Google Patents

Abstract

PURPOSE:To prevent the generation of gypsum scale by exhibiting the effect of preventing oxidation only in the absorption column and to prevent the oxidation of sulfite to gypsym from being inhibited in an oxidation stage by adding sulfide or thiosulfate to the absorbing liquid in a lime gypsum method. CONSTITUTION:The waste gas 2 of a boiler 1 flows into an absorption column 8 through denitrating, dust collecting and cooling devices. The slurry contg. the unreacted CaCO3 and CaSO3 formed by absorption of SO2 in a liquid wall 14 is circulated and is brought into contact with the waste gas so as to absorb SO2. Sulfide, such as H2S or Na2S, or Na2S2O3 having the effect of suppressing oxidation reaction is added to the gas through a line 16 in this stage. Then, the generation of the scale trouble occuring in the crystallization of a gypsum crystal in the absorption tower is obviated. A part of the circulating slurry is fed to an oxidation tank 18, where the air 20 for oxidation is passed through the slurry under controlling pH with a sulfuric acid 19 to oxidize CaSO3 to gypsum dihydrate. The sulfide or thiosulfate that has exhibited the effect of suppressing oxidation in the column 8 is made acidic under the oxidation condition in the tank 18 and if the temp. is maintained at >=55 deg.C, the sulfide or thiosulfate is decomposed and the effect of suppressing oxidation is lost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is based on 80. The present invention relates to a method for desulfurizing exhaust gas containing T-containing gas, and particularly aims to provide an exhaust gas treatment method that does not cause scale troubles in an absorption tower.

In the absorption tower of the wet lime method ζ smoke desulfurization equipment, gypsum crystals with low solubility precipitate as a result of the SO2 absorption reaction, but some of these crystals become scales and stick to the equipment material surface, impeding the flow of gas and liquid. It is easy to cause troubles such as narrowing and even blockage of the road.

The main cause of gypsum scale generation in the absorption tower is zero in the exhaust gas.
As a result of 2 occurring simultaneously with the absorption of 802, an oxidation reaction progresses in the absorption liquid to produce sulfuric acid, which combines with calcium to crystallize dihydrate gypsum crystals, some of which are deposited on the surface of the equipment material. This is due to sticking.

Therefore, in order to prevent gypsum scale from adhering, it is sufficient to suppress the oxidation reaction within the absorption tower, and by using an effective antioxidant, it is possible to prevent dihydrate gypsum crystals from crystallizing.

In such a case, only the absorbent liquid discharged from the absorption tower will contain
It is desired to use dihydrate gypsum as a by-product, which contains sulfite produced by the SO2 absorption reaction and can currently be used in large quantities for boards and cement. It is necessary to oxidize to

However, the antioxidant, which is effective for preventing gypsum scale in the absorption tower, becomes an obstacle to oxidation in the oxidation process, making it difficult to recover dihydrate gypsum.

In order to solve this problem, the present invention has been developed as a result of research on additives that exhibit an antioxidant effect in the absorption tower and suppress the generation of gypsum scale, but whose oxidation inhibitory effect is no longer observed in the oxidation process. A friend of mine discovered that thiosulfates exhibit excellent effects, and when the exhaust gas containing SO is brought into contact with an absorption solution containing calcium compounds in an absorption tower, sulfides are added to the absorption solution. Alternatively, the present invention relates to an exhaust gas treatment method characterized in that thiosulfate is supplied.

One embodiment of the present invention will be described with reference to the accompanying drawings.

Exhaust gas 2 generated from a boiler 1 that burns sulfur-containing fuel passes through a denitrification device 3, an air preheater 4, an electrostatic precipitator 5, and a heat exchanger 6, and then enters a cooling tower 7 of a wet lime flue gas desulfurization device. The exhaust gas sufficiently cooled by the cooling water spray in the cooling tower 7 enters the absorption tower 8, where sol 1 is absorbed and removed, and then it is discharged from the chimney 10 via the reheater 9. On the other hand, crushed limestone (calcium carbonate) 11 as an absorbent is slurried in a raw material tank 12 and supplied from a line 15 in proportion to the amount of SO2 absorbed.

In the liquid reservoir 14 of the absorption tower 8, unreacted calcium carbonate and 8
A slurry containing calcium sulfite produced by the 02 absorption is circulated and sprayed by the absorption tower circulation pump 15, and SO is absorbed by coming into gas-liquid contact with the exhaust gas.

The exhaust gas contains 02, but 02 absorption occurs at the same time as So absorption, and sulfuric acid is produced by the oxidation reaction.
82 is soluble in water or a slightly acidic solution, which has the effect of suppressing oxidation reactions, to prevent problems such as binding with calcium and crystallization of dihydrate gypsum, part of which becomes gypsum scale and sticks to equipment materials. Add sulfides such as Na2S or sodium thiosulfate from Li/16. The amount of water-soluble sulfide added may be such that the sulfide concentration in the absorption tower circulation slurry is approximately: 1 mmol/l.

In the case of sodium thiosulfate, a similar concentration may be used. As a result, the oxidation reaction is suppressed to the point where dihydrate gypsum crystals do not crystallize in the absorption tower circulation slurry. The oxidation inhibitory action of sulfide or sodium thiosulfate is catalytic. That is, S.O.
, has an oxidation-inhibiting effect when added in an amount equivalent to about 1/1000 mole of the absorbed amount.

A part of the absorption tower circulation slurry containing unreacted calcium carbonate and calcium sulfite is sent from the liquid reservoir 14 of the absorption tower 8 to the oxidation tank 18 via the cylinder line 17t'' according to the material balance. Sulfuric acid is added from the line 19 and acidic conditions are applied. Oxidizing air is passed through line 20 into the slurry to oxidize the sulfite to gypsum while adjusting the pH to .

The sulfides that showed an oxidation inhibiting effect in the absorption tower 8 were oxidized in the oxidation tank 1B by making the slurry acidic and raising the temperature to 55°C.
If the temperature is maintained above, the oxidation inhibitory effect will disappear and calcium sulfite will be oxidized to dihydrate gypsum.

The dihydrate gypsum slurry discharged from the oxidation tank 18 is then sent to a centrifugal separator 21 and separated into dihydrate gypsum crystals 22 (i-), while the separated liquid is sent to the raw material tank 12 via a line 23 to prepare a calcium carbonate slurry. used in circulation as

According to the method of the present invention, H2B' is
Approximately 1 m of sulfide or thiosulfate such as p Na28
The oxidation reaction can be suppressed by simply reducing the gypsum to a level of mot/7, and the gypsum can be brought to a so-called unsaturated state where no dihydrate gypsum exists. It can eliminate the drawbacks of smoke desulfurization equipment.

Moreover, this oxidation suppressing effect is due to the oxidation strip Ft-t- in the oxidation tank.
As mentioned above, the acidity and the temperature of 55° C. or higher eliminate the calcium sulfite, which has the advantage of turning calcium sulfite into dihydrate gypsum, which has high utility value. Quinone, which is commonly used in the past,
Organic compound oxidation inhibitors such as amines and phenols are
The process of recovering dihydrate gypsum as a by-product is unsuitable because its oxidation-inhibiting effect does not diminish even in an oxidation tank, and there are measures to prevent secondary pollution due to drainage of liquid containing such organic compounds.
It is also unsuitable from the viewpoint of pollution prevention equipment such as organic matter adhesion and contamination of by-products.

On the other hand, the sulfide or thiosulfate used in the present invention decomposes into sulfur oxide in the oxidation tank, and the amount thereof is very small, which is extremely convenient for the wet calcareous flue gas desulfurization process. be.

Experimental Example 1 Distilled water 101 was placed in a glass vented reaction tank equipped with a rotary atomizer, and while heavy oil combustion exhaust gas containing So, = 1000 ppm K was vented through the rotary atomizer, calcium carbonate powder was added to adjust the pH of the liquid to 5. 5, and the production ratio of calcium sulfite and calcium sulfate in the reaction solution was determined.

The temperature was maintained at 50°C. If the oxidation rate is defined as the ratio of the sulfate concentration to the total amount of sulfite concentration and sulfate concentration, the oxidation rate is approximately 40 molt, and dihydrate gypsum crystals are observed. Next, an aqueous solution containing 1 mmol// of Na2S was first put into the aeration reaction tank, and then the experiment was conducted in exactly the same way.
No hydrogypsum crystals were observed, and the gypsum was found to be unsaturated.

The oxidation rate was also significantly lowered to 6%, and it was confirmed that the sulfate ions produced by oxidation were incorporated into the calcium sulfite crystals.

Experimental Example 2 Sulfuric acid was added to the calcium sulfite slurry obtained by adding 28ft1 mmol/l of Na in Experimental Example 11C, and air was aerated from a rotary atomizer while maintaining the slurry pH at around 4. Temperatures range from 55°C to 8
Although the temperature was changed to 0°C, all calcium sulfite was converted to dihydrate gypsum.

Experimental Example 3 An experiment was conducted under exactly the same conditions as in Experimental Example 1 except that H2El'i was used instead of Na2B, but the oxidation rate was almost zero, and the oxidation suppressing effect was remarkable. Further, when air oxidation was performed in the same manner as in Experimental Example 2, dihydrate gypsum was obtained.

Example 1 2000? similar to the flow sheet shown in FIG. FL”l
The effectiveness of the method of the present invention was confirmed using a wet lime-gypsum flue gas desulfurization pilot plant that treats heavy oil combustion flue gas from J/1.

The SO2 concentration in the absorption tower artificial exhaust gas is 1200 Ppm%
"The Oz concentration was 10%, and the O2 concentration was 2q6. A grid-packed absorption tower was used, and the liquid-gas ratio was 2017 m3N.
The pH of the circulating slurry in the absorption tower was adjusted to 5.5 using calcium carbonate passed through a 325 mesh sieve as an absorbent. Na2B is 1 mmol/j in the absorption tower circulating slurry
The addition was continued so that the amount was ~5 mmol/7.

A desulfurization rate of 95% was obtained, and 2
Hydrogypsum crystals were not crystallized, and the oxidation rate was almost 0%.

A portion of the absorption tower circulation slurry was continuously sent to a pressurized oxidation tank equipped with a rotary atomizer, and the slurry IJ-pH was adjusted to 2 to 5 with sulfuric acid while passing compressed air. The oxidation tank temperature was 55-80°C. The slurry extracted from the oxidation tank contains only dihydrate gypsum crystals, calcium carbonate crystals,
No calcium sulfite crystals were observed, and highly pure dihydrate gypsum was recovered as a by-product by centrifugation.

Continuous operation was carried out for about 120 hours in a steady state, but there was no gypsum scale in the grid-filled part of the absorption tower.

Example 2 The operation was carried out under the same conditions as in Example 1 except that Na2S206 (sodium thiosulfate) was used instead of Na2S. No dihydrate gypsum crystals were precipitated in the circulating slurry of the absorption tower, and gypsum was removed. There was no scale at all. In addition, high-purity dihydrate gypsum was recovered as a by-product. Na282
Although it is well known that 06 has an oxidation inhibiting effect, it is not known that this effect disappears in an oxidation tower and that highly pure dihydrate gypsum can be obtained.

[Brief explanation of the drawing]

The attached drawings show one embodiment of the method of the present invention. exchanger. 7... Cooling tower, 8... Absorption tower, 9... Reheater. 10... Chimney, 11... Calcium carbonate, 12φ...
Raw material tank, 14...Liquid reservoir, 15...Absorption tower circulation pump. 18... Oxidation tank, 21... Centrifugal separator, respectively.

Claims (1)

[Claims] Exhaust gas containing SO2 is desulfurized by contacting it with an absorption liquid containing a calcium compound, sulfide or thiosulfate is supplied to the absorption liquid, and the absorption liquid extracted from the absorption process is led to an oxidation tower where 2 water is removed. Exhaust gas treatment method characterized by recovering gypsum