JPH0122014B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for removing sulfur oxides from exhaust gas.

[Prior art]

Methods for removing sulfur oxides from exhaust gas using magnesium oxide, magnesium hydroxide, and magnesium carbonate as absorbents have long been known, and their use has increased in recent years due to their low construction and operating costs. There is a tendency.

Conventionally, gas-liquid countercurrent contact absorption towers such as spray towers and packed towers have been used because they have better contact efficiency with exhaust gas and removal efficiency of sulfur oxides than cocurrent contact. It was used.

An example of such a conventional countercurrent absorption tower is shown in FIG.

The exhaust gas 31 containing sulfur oxides is supplied from below to the absorption tower 32, and is brought into countercurrent contact with an absorption liquid 33 containing a magnesium compound supplied from above the absorption tower 32 to remove the sulfur oxides, and then removed from the system. is discharged.

The absorption liquid 34 staying in the lower part of the absorption tower is circulated again to the upper part of the absorption tower 32, while the other part of the absorption liquid 34 is led to the oxidation tank 35 outside the absorption tower and is converted into air 36.
Magnesium sulfite was oxidized by the blowing to form magnesium sulfate, and this magnesium sulfate aqueous solution 37 was discharged into public waters after dust separation.

In addition, a new absorption liquid 3 containing a magnesium compound is added to the absorption liquid 34 and the oxidation tank 35 at the bottom of the absorption tower.
8 and 39 were supplied, respectively.

However, in the conventional method using a magnesium compound as an absorbent, scale of magnesium sulfite may occur in the absorption tower due to the solubility of magnesium sulfite formed by absorption of sulfur dioxide gas. Lower the pH of the absorption liquid (make it more acidic),
The amount of absorption liquid introduced into the oxidation tank and the amount of water discharged from the oxidation tank were increased.

However, with these methods, it is difficult to eliminate all problems, and there are problems such as a decrease in desulfurization efficiency due to a decrease in the pH of the absorption liquid, and an increase in the amount of industrial water used and the amount of wastewater.

On the other hand, in the countercurrent contact method, the removal rate of soot and dust contained in the exhaust gas is generally low, and furthermore, the absorption liquid containing magnesium compounds tends to be entrained in the gas discharged after treatment in the absorption tower and become dust. In addition to the dust emitted from boilers and combustion furnaces, the absorption liquid generated from the desulfurization absorption tower due to entrainment is becoming an important problem as environmental standards have been strengthened in recent years. There is a strong demand for improved dust removal efficiency.

[Purpose of the invention]

The present invention has been made in order to eliminate the above-mentioned conventional drawbacks, and aims to improve the efficiency of dust and dirt through co-current contact, and to avoid a decrease in contact efficiency and desulfurization efficiency due to co-current contact. It is something.

[Structure of the invention]

The present invention achieves the above object by supplying an exhaust gas containing sulfur oxide and an absorption liquid containing a magnesium compound selected from magnesium oxide, magnesium hydroxide, and magnesium carbonate from above to below an absorption tower to carry out co-current contact. The new absorbing liquid is supplied to the lower retentate of the absorption tower, and a part of the lower retentate is circulated above the absorption tower and used for cocurrent contact with the exhaust gas, and the lower retentate is Another part of the liquid is supplied to an oxidation tank provided outside the absorption tower, and a new absorption liquid is supplied to the oxidation tank, air is blown into the oxidation tank, and part of the oxidation tank liquid is added to the oxidation tank. It is characterized in that it is circulated into the retentate to control the sulfite concentration in the retentate to 0.015 to 0.35 mol/and the pH to 5 to 7.5.

Hereinafter, the present invention will be explained based on the drawings.

FIG. 1 shows the present invention, in which a flue gas 1 containing sulfur compounds is supplied from the top of an absorption tower 2, while an absorption liquid 3 is also supplied from the top of the tower and is brought into contact with the flue gas 1 by cocurrent contact with the flue gas 1. Sulfur compounds are removed from the

As the absorption liquid 3, an aqueous solution containing magnesium oxide, magnesium hydroxide, or magnesium carbonate is used.

These magnesium compounds can be used either singly or in combination.

The exhaust gas 1' from which sulfur compounds have been removed by co-current contact with the absorption liquid 3 is discharged from the lower part of the absorption tower 2, and the absorption liquid after contact with the exhaust gas stays at the lower part of the absorption tower 2, and this stagnation occurs. A part of the absorption liquid 4 is circulated to the upper part of the absorption tower via the circulation pump 10 and used again for cocurrent contact with the exhaust gas.

On the other hand, the newly prepared absorption liquid 8 is supplied to the staying absorption liquid 4, and the staying absorption liquid 4 is supplied with the newly prepared absorption liquid 8.
The other part is sent to the oxidation tank 5 via the pump 11.

In the oxidation tank 5, air 7 is blown and a newly prepared absorption liquid 9 is supplied.

The pH of the oxidation tank liquid 14, which has been lowered by the sulfuric acid formed by air oxidation, is preferably adjusted to 5.8 to 8.6, which is within the drainage regulation value, and the formed magnesium sulfate aqueous solution 12 is discharged after dust separation. .

In addition, the gas that is supplied from the top of the absorption tower and comes into cocurrent contact with the absorption liquid collides with the absorption liquid that remains at the bottom of the absorption tower, reverses the flow, and is discharged from the bottom of the absorption tower, which is different from the conventional method. Compared to the flow contact method, the removal rate of soot and dust in exhaust gas is higher, and the amount of dust caused by entrainment can be reduced.

Further, in the present invention, a part of the absorption liquid 4 is sent to the oxidation tank 5 by the pump 11, and a part of the oxidation tank liquid 14 is supplied to the absorption liquid 4 via the pump 13, so that the oxidation tank liquid 14 and Absorption liquid 4 is circulated.

That is, in the present invention, by circulating the absorption liquid 4 staying in the absorption tower and the oxidation tank liquid 14, the PH of the absorption liquid 4 is adjusted to a level that has a good sulfur oxide desulfurization effect.
5.0 to 7.5, and the sulfite concentration in the absorption liquid 4 is usually adjusted to 0.015 to 0.35 mol/, preferably 0.025 to 0.20 mol/.

By controlling the sulfite concentration within this range, the desulfurization efficiency can be made equal to or higher than that of a countercurrent contact type absorption tower, and precipitation of magnesium sulfite can be completely eliminated. In particular, as described above, even when the amount of the absorbing liquid 9 added is reduced and the amount of the absorbing liquid 8 added is increased to make the pH of the absorbing liquid 4 on the alkaline side, precipitation of magnesium sulfite can be effectively prevented. Can be done.

If the sulfite concentration exceeds 0.35 mol/mol, magnesium sulfite, which is the main cause of cake generation in the absorption tower 2, will precipitate, the equilibrium partial pressure of sulfur dioxide gas will increase, and the desulfurization efficiency will also decrease.

Furthermore, if the amount is less than 0.015 mol/mol, the desulfurization efficiency will decrease.

Absorption liquid 4 by circulation of absorption liquid 4 and oxidation tank liquid 14
Control of the sulfite concentration in the absorbent is not affected by factors such as the content of sulfur compounds in the exhaust gas, the amount of the absorbent 4, and the amount of catalyst metal in the absorbent. It has the advantage of being easily controllable.

〔Effect of the invention〕

As described above, according to the present invention, by circulating the absorber retentate and the oxidation tank liquid, the sulfite concentration and PH in the absorber retentate can be simultaneously controlled with a simple operation, Desulfurization efficiency higher than conventional countercurrent contact can be achieved.

Furthermore, in the present invention, since the exhaust gas and the absorption liquid are in co-current contact, the efficiency of removing soot and dust from the exhaust gas is higher than in the case of conventional counter-current contact, and the removal of soot and dust caused by the entrainment of the absorption liquid is eliminated. It is possible to suppress the increase in dust.

In particular, even if the absorption tower is downsized and the gas hourly space velocity is increased, good dust removal efficiency can be maintained.

In particular, in the present invention, since a new absorption liquid is supplied to the lower retentate liquid and the oxidation tank, the PH difference between the lower retentate liquid and the oxidation tank liquid can be made small or equal.

As a result, pH control can be quickly and easily performed while controlling the sulfite concentration in the lower retentate within a desired range.

Therefore, according to the method of the present invention, the disadvantage of the parallel flow type in that the desulfurization rate is greatly restricted by the properties of the absorption liquid is avoided, and normal operation is possible.

Furthermore, since PH control becomes easier, it becomes possible to prevent scaling from occurring.

Furthermore, when discharging the oxidizing tank liquid, there is no need to adjust the pH again, and there is no need to further provide various facilities for adjusting the pH for discharging.

Examples of the present invention will be described below.

〔Example〕

The exhaust gas was absorbed and desulfurized according to the steps shown in FIG.

However, in this embodiment, the amount of the absorbing liquid 9 added in FIG.
The amount of the absorption liquid 8 added was controlled based on the pH of the aqueous magnesium sulfate solution 12 by increasing the amount of the absorption liquid 8 added.

The operating conditions were as follows.

Sulfur dioxide gas concentration in exhaust gas at absorption tower inlet
...1000 ppm PH of absorption liquid 4 ......~7.5 Sulfite concentration in absorption liquid 4 ...0.063 mol/PH of magnesium sulfate aqueous solution 12 ...5.8 As a result, the following results were obtained.

Exhaust gas desulfurization rate...95% Exhaust gas dust removal rate...94% Note that no scale trouble occurred.

Comparative Example According to the process shown in FIG. 2, an attempt was made to desulfurize exhaust gas using a magnesium salt aqueous solution using a conventional countercurrent contact absorption tower.

The operating conditions and results were as follows.

Sulfur dioxide gas concentration in exhaust gas at absorption tower inlet
…1000ppm Sulfite concentration in absorption liquid …0.31 to 0.44 mol/ PH of absorption liquid …5.9 Desulfurization rate …92% Dust removal rate …90% In addition, in this comparative example, PH fluctuation of absorption tower absorption liquid, Scale troubles occurred due to fluctuations in sulfur dioxide gas load and absorption liquid temperature.

[Brief explanation of drawings]

FIG. 1 is a process diagram showing the present invention, and FIG. 2 is a process diagram showing a conventional method. 1... Exhaust gas, 2... Absorption tower, 3, 4... Absorption liquid, 5... Oxidation tank, 6, 7... Air, 8, 9...
Newly prepared absorption liquid, 12...Magnesium sulfate aqueous solution.

Claims (1)

[Claims] 1. A flue gas containing sulfur oxide and an absorption liquid containing a magnesium compound selected from magnesium oxide, magnesium hydroxide, and magnesium carbonate are supplied from above to below the absorption tower to bring them into cocurrent contact,
The new absorbent liquid is supplied to the lower retentate of the absorption tower, and a part of the lower retentate is circulated above the absorption tower and used for cocurrent contact with the exhaust gas, and the lower retentate is The other part is supplied to an oxidation tank provided outside the absorption tower, and a new absorption liquid is supplied to the oxidation tank, air is blown into the oxidation tank, and a part of the oxidation tank liquid is transferred to the stagnant liquid. 0.015 to 0.35 mol/
, a method for treating sulfur oxides in exhaust gas, the method comprising controlling PH to 5 to 7.5.