JPH07114918B2 - Wet Flue Gas Desulfurization Method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a wet flue gas desulfurization method, and in particular, it efficiently removes sulfur oxides in exhaust gas containing sulfur oxides, acid gases and dusts, and at the same time stabilizes them. The present invention relates to a wet flue gas desulfurization method suitable for producing quality gypsum as a by-product.

(Prior Art) Recently, due to the diversification of energy, coal is often used as a boiler fuel in power plants and factories. The flue gas of this coal contains dust mainly composed of hydrogen fluoride (HF), hydrogen chloride (HCl), and aluminum whose concentration is several ten times higher than that of heavy oil flue gas. The components have various adverse effects on desulfurization performance.
HCl reacts with the absorbent to form calcium chloride (CaCl ₂ ), which slows the dissolution rate of the absorbent, so that the excess ratio of the absorbent is increased or the slurry tank of the absorption tower is increased to increase the reaction time. It is necessary to increase the capacity of the part.

Further, when dust is mixed in the absorbent slurry, especially Al in the dust produces an extremely sparingly soluble compound (apatite) on the absorbent surface in the presence of HF in the exhaust gas absorbed in the slurry, It significantly reduces the solubility of the absorbent.

For the purpose of preventing a decrease in the dissolution rate of conventional absorbents, alkali was added to the absorbent slurry to raise the pH, and then sulfuric acid was added in a separate oxidation step to adjust the pH to a value suitable for oxidation.
It has been proposed to oxidize calcium sulfite (CaSO ₃ ) into gypsum.

Referring to FIG. 5, a conventional technique of using calcium carbonate (CaCO ₃ ) as an absorbent and producing and recovering gypsum (CaSO ₄ ) will be described.

Exhaust gas 1 from a boiler or the like is supplied to the absorption tower 2 after being removed and cooled by the dust removal tower 2. In the absorption tower 2, the sulfur oxide in the exhaust gas is absorbed and removed. The absorbent slurry used to absorb and remove the sulfur oxides in the exhaust gas is supplied to the absorption tower circulation tank 5, circulated as an absorption liquid slurry by the circulation pump 7, and sprayed into the tower from the spray means 25, The sulfur oxides in the exhaust gas are absorbed to form calcium sulfite, and a part thereof is oxidized by oxygen in the exhaust gas to become calcium sulfate (gypsum). A part of the absorbent slurry is air-oxidized, concentrated and dehydrated in the oxidation and gypsum recovery process, and recovered as gypsum 17.

In the above process, the absorbent is supplied so as to maintain the pH of the slurry in the absorption tower circulation tank 5 at a high value of 5.8 to 6.1 in order to improve the drainage performance. In addition, when dust containing acidic gas (HCl, HF) and metals is mixed in the absorbent slurry, such as coal-fired exhaust gas, the pH of the absorbent slurry is
In order to maintain the absorption rate of the absorbent, increase the excess rate of the absorbent, that is, increase the concentration of the absorbent limestone (CaCO ₃ ) in the absorbent slurry, and add an alkaline agent such as caustic soda (NaOH). The performance was maintained.

Calcium sulfite in the absorbing solution slurry produced by absorbing the SO ₂ in the process _{(CaSO 3 · 1 / 2H 2} ) is included in a large amount (10 to several hundred mmol / l), such slurry composition The desulfurization rate of absorbent slurry with
affected by pH. Therefore, it is understood that the absorption liquid pH must be increased in order to maintain high desulfurization performance in the above process.

(Problems to be Solved by the Invention) In order to maintain desulfurization performance in the above process, it is necessary to increase the pH of the absorbent slurry, that is, increase the concentration of the absorbent and increase the amount of the alkaline agent added. However, in such a method, CaCO ₃ which is an unreacted absorbent is increased in the absorbent slurry extracted in the oxidation and gypsum recovery step, and the purity of gypsum is lowered, so that a large amount of sulfuric acid (H ₂ S
O ₄ ) needs to be added. On the other hand, there is a problem that the consumption of not only the absorbent but also the alkali agent and sulfuric acid increases depending on the exhaust gas conditions.

An object of the present invention is to provide a wet flue gas desulfurization method capable of reducing the utility or consumption of an absorbent, an alkaline agent, sulfuric acid, and recovering high-quality gypsum.

(Means for Solving Problems) The present invention provides a combustion exhaust gas containing dust containing sulfur oxides, acidic gas (hydrogen fluoride, hydrogen chloride, etc.) and metals to an absorption tower, and the absorption tower Desulfurize by contacting with a slurry containing calcium-based absorbent (calcium carbonate, calcium hydroxide, etc.) supplied from the lower circulation tank, and at the same time supply air into the absorbent to oxidize calcium sulfite after desulfurization In the wet flue gas desulfurization method for recovering
After oxidizing calcium sulfite in the slurry in the upstream of the liquid flow in the circulation tank with air, while extracting it to the gypsum recovery system, adding a calcium-based absorbent and an alkaline agent to the slurry in the liquid flow downstream of the circulation tank, It is characterized in that the pH of the slurry supplied to the absorption tower is kept at 4.5 or higher.

In the present invention, in addition to the above steps, the gypsum-containing slurry after absorption and oxidation from the absorption tower is extracted, sulfuric acid is added thereto, and unreacted calcium carbonate or calcium hydroxide in the slurry is converted to gypsum. After that, add an alkaline agent,
The gypsum slurry is characterized by keeping the pH at 5.0 or higher.

(Operation) By supplying air for oxidation to the upper liquid part of the absorption tower circulation tank, calcium sulfite after absorbing the sulfur oxides becomes gypsum, while at the bottom of the absorption tower circulation tank, an alkaline agent and an absorbent By supplying an absorbent so that the excess ratio of is about 1.03 to 1.0, and maintaining the pH at 4.5 or higher, the calcium sulfite in the absorption tower circulating slurry liquid that newly absorbs sulfur oxides becomes almost zero. You can In addition, since the retention time of the absorbent in the absorption tower is short and the concentration of the absorbent is small, even in the low pH region of the absorption tower spray liquid of 4.5 to 5.3, acidic gases such as HCl and HF and metals such as Al can be removed. It is possible to prevent the formation of a sparingly soluble compound (apatite) on the surface of the absorbent due to the contained dust.

Furthermore, since the unreacted absorbent contained in the absorbent slurry is converted to gypsum by adding sulfuric acid after extracting the absorbent to the gypsum recovery step, the purity of the byproduct gypsum can be increased. In this case, the mother liquor after the gypsum recovery can be recovered and reused as make-up water for the absorption tower or for converting the absorbent into a slurry. In this case, by adding an alkali to the gypsum slurry and keeping the pH at 5.0 or higher, the metal ions can be precipitated as hydroxides and recovered together with the gypsum, and the concentration of metal ions in the absorbent slurry can be suppressed. It is possible to prevent deterioration of desulfurization performance.

Hereinafter, the present invention will be described in more detail with reference to the drawings.

FIG. 1 is a system diagram of a wet flue gas desulfurization method showing an embodiment of the present invention. In the figure, an exhaust gas 1 from a boiler or the like is guided to an absorption tower 2, comes into contact with a calcium-based absorption liquid slurry sprayed by a spraying means 25, and is cooled, dust-removed and desulfurized, and then entrained mist is removed by a demister 3. The treated gas 4 is discharged from the absorption tower 2. By supplying oxidizing air 6 to the upper part of the liquid storage section of the absorption tower circulation tank 5, sulfite ion (SO ₃ ²⁻ ) in the slurry having absorbed SO ₂ and increased SO ₂ partial pressure (SO ₃ ²⁻ ) Oxygen is oxidized to reduce the SO ₂ partial pressure to recover the SO ₂ absorption performance, and calcium sulfate (gypsum) is obtained.

When treating an exhaust gas containing HF that is an acidic gas and dust with a high content of Al as a metal, by adding alkali 11 to the absorbent slurry after oxidation, the recovery of the pH of the absorbent slurry becomes faster, The pH required for desulfurization without forming a sparingly soluble compound (apatite) on the absorbent surface
It can be recovered to 4.5 or higher (preferably 4.5 to 5.3).

The pH of the slurry suitable for the oxidation of sulfite ion is 4.0-4.5.
Therefore, the lower the concentration of CaCO ₃ that is the absorbent in the slurry, the better. Usually, it is preferably 15 mmol / l or less. Therefore, the absorbent slurry 10 should be supplied into the slurry after oxidation, that is, to the lower part of the storage part of the absorption tower circulation tank 5 (or the inlet part of the circulation pump 7) in proportion to the amount of sulfur oxide at the inlet. Is desirable. A stirring residue may be provided in the supply portion of the oxidizing air 6 as necessary to accelerate the oxidation.

After absorbing and oxidizing the sulfur oxides in the absorption tower 2, in order to increase the purity of the recovered gypsum 17, the amount of unreacted CaCO _{3 in} the absorbent slurry is extracted in the gypsum recovery process. However, for that purpose, it is preferable that the slurry after absorbing and oxidizing the sulfur oxide is extracted to the gypsum recovery step before supplying a new absorbent.

In the gypsum recovery step, sulfuric acid 13 is added in order to convert unreacted CaCO ₃ remaining in the slurry withdrawn from the absorbent into gypsum. In this case, the concentration of CaCO ₃ in the slurry is 1/2 to 1/5 that of the conventional one.
In addition, since the oxidation is performed in the absorption tower, sulfuric acid for adjusting the pH required for the oxidation is not required, so that the supply amount of sulfuric acid can be reduced.

Next, in order to reduce metal ions such as aluminum ions in the mother liquor 19 obtained in the gypsum recovery step, the gypsum slurry 20
Alkali 11 is added to the mixture to keep the pH at 5.0 or higher, and aluminum ions are precipitated to collect with gypsum. This makes it possible to suppress the concentration of aluminum ions in the absorbent slurry due to the recirculation of the recycled mother liquor 19.

According to the present invention, HCl, when treating the sulfur dioxide in the exhaust gas containing dust containing a metal such as an acidic gas consisting of HF and Al, in the slurry after absorbing the sulfur oxides in the absorption tower, By supplying the oxidizing air and adding the alkali after the oxidation, a high desulfurization rate can be obtained at a lower pH than in the conventional case, so that the utility of the absorbent and the alkaline agent can be minimized.

Further, if the slurry after absorption and oxidation is withdrawn from the absorption tower before supplying the absorbent, the amount of unreacted absorbent in the slurry can be reduced, so that the effect of reducing impurities in gypsum can be reduced. Not only is there an effect that the consumption of sulfuric acid added to convert the unreacted absorbent in gypsum into gypsum can be reduced.

(Example) Using the apparatus shown in FIG. 1, sulfurous acid gas 1000ppm, water content 9
-10%, O ₂ concentration 4-6%, HF30ppm, HCl80ppm, dust 2
A desulfurization experiment of coal combustion exhaust gas 1 containing 00 mg / Nm ³ was conducted. That is, the above-mentioned exhaust gas is absorbed by the absorption tower 2 at a flow rate of 580 Nm ³ / h.
To remove sulfur oxides in the exhaust gas, and to supply oxidizing air 6 to the absorption tower circulation tank 5 at a flow rate of ³ Nm ³ / h. The pH of the slurry after absorption and oxidation at this time was measured and found to be 4.5. The absorbent slurry 10 was supplied as 20% CaCO _{3 at} a rate of 11.3 kg / h to the bottom of the absorption tower circulation tank 5, and 0.05 kg / h as the alkaline agent 11 so as to keep the pH of the absorption tower circulation slurry at 5.0. Was supplied to the bottom of the absorption tower circulation tank 5.

The slurry after absorption and oxidation was extracted from the circulation tank 5 and then supplied to the reaction tank 12. The concentration of unreacted CaCO ₃ in this extracted slurry was 5 mmol / l. Sulfuric acid was added to the reaction tank 12 to adjust the pH to 4.8. CaCO ₃ was not detected in the slurry after addition of sulfuric acid.

Furthermore, this slurry is supplied to the pH adjusting tank 14, the pH is adjusted to 5.0 with a solution of sodium hydroxide 11, and then supplied to the thickener 15,
The gypsum 16 and the mother liquor 19 were separated, and the mother liquor 19 was supplied to the absorbent slurry tank 9 and mixed with the absorbent 8 to adjust the absorbent slurry concentration. A part of the mother liquor 19 was reused as make-up water for the absorption tower 2. The results are shown in Table 1, and it can be seen that a high desulfurization rate can be obtained by supplying oxidizing air to the absorption tower to simultaneously perform desulfurization and oxidation and adding an alkali agent.

In Example 1, the pH of the absorption tower spray liquid was changed and the desulfurization rate at that time was measured. The results are shown in A of FIG. In the process of adding the oxidizing air and the alkaline agent to the absorption tower, as shown in FIG. 3, it can be seen that the desulfurization rate has little dependency on pH, and high desulfurization performance can be obtained at low pH.

Further, FIG. 4 shows the relationship between the concentration of aluminum ion in the mother liquor (filtrate) and the pH in the test in which the alkaline agent was added to the gypsum slurry whose pH was adjusted by adding sulfuric acid. The mother liquor returns to the absorption system and is reused.

From the results shown in FIG. 4, it can be seen that when the pH is set to 5 or more, aluminum ions are precipitated and hardly exist in the liquid. Therefore, it is clear that the desulfurization performance is not hindered by using this liquid.

Comparative Example 1 A desulfurization experiment was conducted in the same manner as in Example 1 except that no alkali was added to the absorption tower. The results are shown in Table 1.

Comparative Example 2 A desulfurization experiment was conducted in the same manner as in Example 1 except that the oxidizing air was not supplied to the absorption tower and the oxidation tower 21 was separately arranged as shown in FIG. The results are shown in Table 1.

Comparative Example 3 A desulfurization experiment was performed in the same manner as in Comparative Example 1 except that alkali was not supplied to the absorption tower. The results are shown in Table 1.

(Effect of the invention) By supplying air into the absorbent slurry, making desulfurized calcium sulfite into gypsum in the absorption tower, and further adding an alkali, the pH of the absorbent slurry is desulfurized at a lower value than before. Performance can be obtained, and a high desulfurization rate can be achieved with low utility.

Furthermore, by extracting the absorbent slurry with a lower limestone excess rate than before, adding sulfuric acid, and further adding an alkaline agent to adjust the pH, high-purity gypsum with less utility than before is recovered. be able to.

[Brief description of drawings]

FIG. 1 is a diagram showing a flow sheet of the wet flue gas desulfurization method according to the present invention, FIG. 2 is a diagram showing a flow sheet of the wet flue gas desulfurization method used in a comparative example of the present invention, and FIG. , A diagram showing the relationship between the pH of the absorption tower spray liquid and the desulfurization rate, FIG. 4 is a diagram showing the relationship between the pH of the gypsum slurry and the aluminum ions in the filtrate,
FIG. 5 is a diagram showing a flow sheet of a conventional wet flue gas desulfurization method. 1 ... Exhaust gas, 2 ... Absorption tower, 3 ... Demister, 4 ...
Process gas, 5 ... Absorption tower circulation tank, 6 ... Oxidizing air, 7 ... Absorption tower circulation pump, 8 ... Absorbent, 9 ... Absorber slurry tank, 10 ... Absorber slurry, 11 ... Alkaline agent, 12 …… Reaction tank, 13 …… Sulfuric acid, 14 …… pH adjusting tank, 15
…… Thickener, 16 …… Centrifuge, 17 …… Gypsum, 18 ……
Mother liquor tank, 19 ... Mother liquor, 20 ... Gypsum slurry, B ... Separate oxidation tower type.

Front page continuation (72) Inventor Yasuki Hashimoto 6-9 Takaracho, Kure-shi, Hiroshima Babkotsu Hitachi Kure Factory (72) Inventor Hiromitsu Asano 6-9 Takaracho, Kure-shi, Hiroshima Babkotsu Hitachi Kure Factory (56) References JP-A-60-122029 (JP, A) JP-A-58-98126 (JP, A) JP-B-51-43039 (JP, B2)

Claims (2)

[Claims] 1. A combustion exhaust gas containing dust containing sulfur oxides, acid gases and metals is supplied to an absorption tower and brought into contact with a slurry containing a calcium-based absorbent supplied from a lower circulation tank of the absorption tower. At the same time as desulfurization, air is supplied into the slurry to oxidize calcium sulfite after desulfurization,
In a wet flue gas desulfurization method for recovering gypsum, after oxidizing calcium sulfite in the slurry in the liquid flow upstream of the circulation tank with air, the calcium sulfite is withdrawn to the gypsum recovery system, and calcium is added to the slurry in the liquid flow downstream of the circulation tank. A wet flue gas desulfurization method comprising adding a system absorbent and an alkali agent, and maintaining the pH of the slurry supplied to the absorption tower at 4.5 or higher. 2. The method according to claim 1, wherein sulfuric acid is added to the gypsum-containing slurry extracted to the gypsum recovery system, and unreacted calcium carbonate or calcium hydroxide in the slurry is converted to gypsum, and then alkali is added. A wet flue gas desulfurization method, characterized in that the pH of the gypsum slurry is maintained at 5.0 or higher by adding an agent.