JP2738954B2 - Method for treating exhaust gas containing SO2 and HF - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for treating exhaust gas, and more particularly to a method for wet treatment of exhaust gas containing SO ₂ and HF such as coal combustion exhaust gas.

[Conventional technology]

When carrying out the flue gas desulfurization method by the commonly known wet lime method, HF is contained in the exhaust gas in addition to SOx as a harmful component.
May be included. As an example of each component present in the coal flue gas, SOx is about 1000 ppm and HF is about 40 ppm.

When such an exhaust gas is treated in a wet exhaust gas treatment tower using CaCO ₃ as a SO ₂ absorbent, the following reaction occurs.

CaCO ₃ + SO ₂ → CaSO ₃ + CO ₂ (1) CaCO ₃ + 2HF → CaF ₂ + CO ₂ + H ₂ O (2) [Problem to be Solved by the Invention] However, if the exhaust gas contains a lot of dust, The Al component contained in the dust dissolves and reacts with HF from the gas to produce a fluorine compound of Al (hereinafter referred to as AlFx)
It is known that this AlFx interferes with the dissolving action of limestone (CaCO ₃ ) (see JP-A-55-167023).

Further, for this purpose, a method for preventing a problem by adding a strong alkali such as a basic sodium salt is disclosed in the above-mentioned patent publication.

Therefore, when treating the exhaust gas containing SO ₂ and HF,
By adding a basic sodium salt corresponding to the amount of F, A
It can be seen that this method is effective as an exhaust gas treatment method free from problems caused by lFx.

However, this method has a drawback that it consumes a considerable amount of a strong alkali as an absorbent in addition to CaCO ₃ , which leads to an increase in running costs and an increase in injection equipment costs. Furthermore, since the supplied sodium is dissolved in the absorbing solution, the concentration of sodium in the absorbing solution increases when trying to reduce the amount of drainage, and when the gypsum is to be recovered, the purity of the recovered gypsum May decrease.

There is also known a method in which a strong alkali compound is added in a pulsed manner to temporarily increase the pH of the absorbing solution, thereby preventing a problem caused by AlFx. (JP 60-12202
No. 9).

However, in this case, the desulfurization performance may be reduced for a short time from when the desulfurization performance is reduced to when it is recovered.

[Means for solving the problem]

However, the present inventors conducted studies for the purpose of providing an exhaust gas treatment method free from the above-mentioned problems, and as a result, provided two slurry tanks holding an absorbing solution containing CaCO ₃ to be supplied to an exhaust gas absorption tower. The absorption liquid is supplied from the second slurry tank to the exhaust gas absorption tower, the absorption liquid is sent from the absorption tower to the first slurry tank, and the absorption liquid is circulated in a route from the first slurry tank to the second slurry tank. And sucking air into the absorbing liquid of the first slurry tank,
And, by supplying air to the second slurry tank, the pH of the absorbing solution in the second slurry tank is adjusted to 5.5.
The exhaust gas treatment according to the present invention, in which the carbon dioxide gas in the absorbing solution in the first slurry tank is dissipated to maintain the dissolution of CaCO ₃ by AlFx and high-purity gypsum can be recovered by maintaining the temperature at ~ 6.5 and dispersing the carbon dioxide gas in the absorbing solution in the first slurry tank. The way to come.

[Action]

In the present invention, as described above, CaCO ₃ is used as an absorbent, and the pH is controlled from 5.5 to 6.5 by supplying air to a second slurry tank that supplies the absorbing solution to the exhaust gas absorption tower. Air is blown into the absorbing solution of the first slurry tank to which the absorbing solution is sent to disperse the carbon gas in the absorbing solution, and the absorbing solution is discharged from the second slurry tank to the exhaust gas absorption tower and the first slurry tank. After that, it is circulated to the second slurry tank.

Conventionally, in order to decompose AlFx, various strong alkali compounds have been proposed in addition to the basic sodium salt. This is because Al, which is a component of AlFx, shifts to the solid phase due to alkali.

The present inventors have further studied the pH range in which AlFx is decomposed, and found that maintaining the pH at 5.5 to 6.5 is effective for decomposition. Furthermore, by dissolving the dissolved carbon dioxide gas in the slurry, the pH of the slurry with low AlFx concentration can be adjusted by using only CaCO ₃ as the alkali source.
Increased to the above-mentioned AlFx decomposition pH range.

It was also found that even if AlFx was once decomposed by increasing the pH, it was redissolved as AlFx when the pH was lowered, and CaCO ₃ activity was easily inhibited.

Based on the above facts, the present inventors as a result of intensive studies on the method of supplying the supply method and air carbon dioxide emission of CaCO _3, a slurry tank to hold an absorbent solution containing CaCO ₃ supplied to the exhaust gas absorption tower 2 The absorption liquid is supplied from the second slurry tank to the absorption tower, sent from the absorption tower to the first slurry tank, and circulated in a route of sending from the first slurry tank to the second slurry tank. Sending air for oxidation to the absorbent in the first slurry tank, and supplying air for carbon dioxide emission to the second slurry tank to raise the pH of the slurry in the second tank from 5.5 to 6.5. Found a way to control.

That is, in the second slurry tank, carbon dioxide contained in the absorbing solution is diffused as shown in the following formula to promote the dissolution of CaCO ₃ and raise the pH.

_{^{CaCO 3 + H + = Ca 2+}} + HCO 3 - ...... (3) HCO 3 - = OH - + CO 2 ↑ ...... (4) as the dissipation of CO ₂ is accelerated (3) (4) to the right side of the equation Although the reaction is promoted and the pH rises, excess air must be supplied to raise the pH of the absorbing solution to 6.5 or more, which is not advisable because it raises the running cost, and
It has no effect on the decomposition of AlFx. Also, absorbing liquid
If the pH is lower than 5.5, the decomposition of AlFx becomes insufficient. From the above, the pH of the absorbent in the second slurry tank was raised from 5.5.
It was found that stable CaCO ₃ reactivity can be maintained by controlling to 6.5.

Further, in the present invention, the air supply amount is adjusted so that the pH of the absorbing solution in the second slurry tank is always within a predetermined range even when the amount of the source of AlFx (ie, dust and HF) entrained in the exhaust gas varies. Although stable operation is possible by doing, if the source of AlFx greatly increases, or if the pH of the absorbent decreases due to some erroneous operation and AlFx increases in the absorbent,
It may be difficult to maintain the pH of the absorbing solution in a predetermined range only with air for CO ₂ emission. In such a case, the CO ₂ strong alkali such as in addition to the air for dissipation Ca (OH) ₂ was supplied intermittently or continuously to the second slurry tank to maintain the pH in a predetermined area Is valid.

Even in this case, the supply of air for carbon dioxide emission is
It has the effect of significantly reducing the amount of strong alkali added.

〔Example〕

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

The exhaust gas from a small pulverized coal burning (not shown), 200m ³ N
/ h, and processed by the exhaust gas treatment method according to the present invention shown in FIG. The temperature of the exhaust gas to be treated is adjusted to 110 ° C. by a heat exchanger (not shown), and the dust concentration is adjusted to about 200 mg / m ³ N by a bag filter (not shown).

The inlet SO ₂ concentration was 1200 ppm and the HF concentration was 53 ppm.

The exhaust gas to be treated is introduced into the absorption tower 5 through the line 3, and is discharged from the line 4 after SO ₂ and HF are removed from the gas. Below the absorption tower 5, there are provided two slurry tanks, a first slurry tank 1 and a second slurry tank 2, which hold the absorption liquid. The absorption liquid is supplied from the second slurry tank 2 by the absorption liquid pump 6. It is sent to the absorption tower 5, sent from the absorption tower 5 to the first slurry tank 1, and circulated in the route from the first slurry tank 1 to the second slurry tank 2.

The CaCO ₃ slurry as the absorbing liquid is supplied to the slurry tank 1 from the line 7, and the supply flow rate of the CaCO ₃ slurry at this time is such that the SO ₂ concentration in the gas of the line 4 discharged from the absorption tower is 50 ppm. Is automatically adjusted by the control valve 8 via the concentration controller 9 as described above.

On the other hand, the air for CO ₂ emission is supplied from the line 10 into the absorbing liquid in the second slurry tank 2, and the supply flow rate of the air is controlled by the amount of the absorbing liquid sent from the slurry tank 2 to the absorbing tower 5.
It is automatically adjusted by the control valve 11 via the pH controller 12 so that the pH becomes 5.7.

A certain amount of oxidizing air is introduced into the first slurry tank 1 through a line 13.

A part of the absorption liquid supplied to the absorption tower 5 by the absorption liquid pump 6 is withdrawn from the line 14.

Under the above conditions, when the steady operation was performed, it was possible to operate stably at 60 ppm of SO ₂ in the outlet gas, and the pH of the absorbent in the first slurry tank 1 was 5.2.

Also, the CaCO ₃ concentration in the absorbing solution was 0.06 mol / l,
The reactivity of CO ₃ was good, and the absence of calcium sulfite was also confirmed.

When the Al and F concentrations in this absorbing solution were measured,
1.3mg / l, trace amount of 20mg / l.

Supply amount ratio of oxidation and CO ₂ stripping air of this example was the CO ₂ emission for / {CO ₂ for dissipation + oxidation} = 0.20.

(Comparative Example) The operation was performed under the same apparatus and gas conditions as those of the above-mentioned example, with the supply of CO ₂ releasing air stopped.

That is, in FIG. 1, the air for dispersing CO ₂ supplied from the line 10 to the second slurry tank 2 was stopped.

In this comparative example, the slurry tank 1
And the pH of the absorbing solution in the slurry tank 2 was lowered, and finally the slurry tank 1
The pH of the absorbing solution of No. 2 and 4.5 was 4.5 and 4.8, respectively.

The CaCO ₃ concentration in the absorbing solution was 0.20 mol / l, and the reactivity of CaCO ₃ was remarkably reduced as compared with the above example. The concentrations of Al and F in the absorbent were 50 mg / l and 150 mg / l, respectively.

〔The invention's effect〕

As described above, the present invention, when processed by an exhaust gas absorption tower exhaust gas containing SO ₂ and HF by the absorption liquid containing CaCO _3, to exclude the adverse effects caused by the HF and dust in flue gas, the CaCO ₃ While maintaining the activity, it can effectively absorb SO ₂ and HF, and can eliminate or significantly reduce the amount of expensive strong alkali such as Ca (OH) ₂ Gypsum can be recovered.

[Brief description of the drawings]

FIG. 1 is a flowchart showing one embodiment of the present invention. 1 ... first slurry tank, 2 ... second slurry tank, 5 ... absorption tower, 6 ... absorption liquid pump, 7 ... absorption liquid line, 8 ... control valve, 9 ... concentration control 11 in total
Control valve, 12 …… pH controller, 10,13 …… Air line.

Claims (1)

(57) [Claims] In treating an exhaust gas containing SO ₂ and HF, an absorbent containing CaCO ₃ is used as an absorbent supplied to an exhaust gas absorber, and the absorbent is discharged from a second slurry tank. To the first slurry tank from the exhaust gas absorption tower, to the first slurry tank, to circulate in a route from the first slurry tank to the second slurry tank, and to supply air to the second slurry tank While maintaining the pH of the absorbing solution in the second slurry tank at 5.5 to 6.5, air is also blown into the absorbing solution in the first slurry tank to disperse carbon dioxide in the absorbing solution. A method for treating exhaust gas containing SO ₂ and HF.