JP2000084351A - Desulfurization of waste gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remarkably reduce the ratio of the circulating quantity of a liquid absorbent to the quantity of gas to be treated by constructing a desulfurizing column to be vertical, dividing the inside of the desulfurizing column into plural divisions with trays and supplying a part of a treated liquid in an oxidation process for treating the treated liquid after dasulfurizing process with a gas containing oxygen to any of the divisions. SOLUTION: The desulfurizing column liquid absorbent containing magnesium hydroxide flows down and is circulated in the desulfurizing column and is brought into contact with a waste gas containing sulfur oxide in parallel flow to absorb and fix the sulfur oxide as magnesium sulfite or the like. The desulfurizing column is divided into plural divisions 11:14 with the trays, which enable to pass and make the liquid absorbent to flow down. The liquid absorbent containing magnesium hydroxide is sprayed in the division 11, flows down and is circulated and the waste gas containing sulfurous acid gas is introduced from the upper part of the desulfurizing column and is brought into contact with the liquid absorbent in parallel flow. A part of the liquid absorbent in the desulfurizing column is taken out to an oxidation vessel to oxidize COD in the liquid absorbent into nearly zero by blowing air to obtain an oxidized liquid mainly containing magnesium sulfate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a desulfurization technique for various kinds of exhaust gas containing sulfur oxides such as combustion exhaust gas of heavy oil and coal.

[0002]

2. Description of the Related Art The following two methods are generally known as desulfurization methods using magnesium as a desulfurizing agent.

(1) An exhaust gas containing sulfur oxides is brought into contact with an absorbing solution containing a magnesium-based desulfurizing agent in a desulfurization tower to absorb and remove sulfur oxides contained in the exhaust gas as magnesium sulfite or acidic magnesium sulfite, and further oxidize. A method of oxidizing in a tank, converting magnesium sulfite or acidic magnesium sulfite having low solubility in water into magnesium sulfate having high solubility, and discharging the same. in this way,
Both the desulfurizing agent and the sulfur oxides in the absorbed exhaust gas are discarded.

(2) In order to recycle the magnesium salt used as a desulfurizing agent, the above magnesium sulfate is reacted with a basic calcium compound to convert the magnesium sulfate into magnesium hydroxide to regenerate and to utilize magnesium resources effectively. A method of recovering and utilizing absorbed sulfur oxides as gypsum.

The present applicant has proposed the following method in Japanese Patent Application No. 9-167469 as one of the methods (2). That is, "a desulfurization step of contacting an exhaust gas containing sulfur oxides with an absorbent containing a magnesium-based desulfurizing agent to absorb and remove sulfur oxides contained in the exhaust gas, and treating the treatment liquid after the desulfurization step with a gas containing oxygen. A pre-process consisting of an oxidation process and a metathesis process in which the treatment solution after the oxidation process is reacted with a basic calcium compound, and the slurry after the metathesis process containing magnesium hydroxide regenerated in the metathesis process is converted to gypsum dihydrate. In a desulfurization method including a gypsum separation step of returning gypsum from the treatment liquid after the desulfurization step and / or after the oxidation step, the pH of the desulfurization step absorbent is adjusted to 5.5 to 7.0. And the chemical oxygen demand (CO
D) A method for desulfurizing exhaust gas in which D) does not exceed the upper limit determined by the concentration of magnesium sulfate in the absorbent.

The above-mentioned method is a desulfurization method in which gypsum and magnesium hydroxide produced by the metathesis reaction are returned to the desulfurization tower without being separated, while the production of gypsum is suppressed and the quality of by-product gypsum is improved. This is an improved desulfurization method that facilitates separation of gypsum and enables stable operation.

[0007]

In the treatment of exhaust gas having a high concentration of sulfurous acid gas, if the absorbing solution absorbs a large amount of sulfurous acid gas, the COD of the absorbing solution increases. In the above method (1), the concentrations of magnesium sulfite and magnesium magnesium sulfite in the absorbing solution are high, and the solubility of magnesium sulfite and magnesium magnesium sulfite in water is as low as 0.64 g / 100 g water. There is a possibility of precipitation. Therefore, it is necessary to dilute the absorbing solution in order to reduce the COD of the absorbing solution.

Also in the method (2), in order to suppress the formation of gypsum, it is necessary to treat the COD of the absorbing solution within a certain range determined by the magnesium sulfate concentration in the absorbing solution. COD may be reduced. There is a problem that the dilution increases the amount of absorbing liquid, and the ratio L / G, which is the ratio of the amount of exhaust gas to be treated (G) to the amount of absorbing liquid (L) in the desulfurization tower, increases. As the circulating fluid volume increases, the pump capacity and power need to be increased.

An object of the present invention is to provide a method of suppressing COD by suppressing an increase in L / G when treating an exhaust gas containing a high concentration of sulfurous acid gas.

[0010]

The above object is achieved by one or two aspects of the present invention described below.

1. A desulfurization step of contacting an exhaust gas containing sulfur oxides with an absorbent containing a magnesium-based desulfurizing agent in a desulfurization tower to absorb and remove sulfur oxides contained in the exhaust gas, and treating the treated liquid after the desulfurization step with a gas containing oxygen Including the oxidizing step for treatment, the pH of the desulfurizing step absorbent is in the range of 5.5 to 7.0, and the chemical oxygen demand does not exceed the upper limit determined by the temperature of the absorbent or the concentration of magnesium sulfate in the absorbent. In the desulfurization method of desulfurizing exhaust gas in a range, the desulfurization tower is made vertical, the inside of the desulfurization tower is divided into a plurality of sections by a tray through which the absorbent can stay and pass, and the absorbent is showered from the uppermost section. A desulfurization method, wherein the exhaust gas and the absorbing solution are brought into contact with each other in parallel, and a part of the treatment liquid for the oxidation step is supplied into one of the upper and lower compartments of the tray.

2. In addition to the desulfurization step of absorbing and removing the sulfur oxides, and the oxidation step after the desulfurization step, a metathesis step of reacting the treatment liquid after the oxidation step with a basic calcium compound, and magnesium hydroxide regenerated in the metathesis step The gypsum separation step further comprising the step of returning the slurry after the metathesis step to the desulfurization step or / and the oxidation step in a state containing gypsum, and removing the gypsum from the treatment liquid in the desulfurization step or / and the oxidation step. The desulfurization method described.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in more detail with reference to the drawings.

The invention 1 can be applied to a desulfurization process using an apparatus including the desulfurization tower 1, the oxidation tank 3, and the accompanying pumps and pipes shown in FIG.

FIG. 1 is a schematic view of a desulfurization tower used in the present invention.

As shown in FIG. 1, the desulfurization tower 1 is of a vertical type, and a pH measuring device (symbol pH) and a chemical oxygen demand measuring device (symbol COD) are installed at the bottom of the desulfurization tower. .
Here, the chemical oxygen demand (hereinafter abbreviated as COD) is defined as the oxygen demand measured by measuring the total amount of sulfite ion and acid sulfite ion concentration by iodometric titration according to 40 (sulfite ion) of JIS K0102. It is expressed in terms of oxygen equivalent concentration mg / l. The COD measuring device reads a change in the electrical conductivity of the target liquid at the time of titration by an iodine titration method and measures the COD of the liquid. The value of COD may be calculated by measuring the sulfite ion concentration or acidic sulfite ion concentration in the absorbing solution and calculating from the equilibrium data. The COD measurement is performed continuously or appropriately, and it is necessary to keep the COD in the absorbing solution within the range shown in FIG. 2 in order to prevent precipitation of magnesium sulfite. That is, as shown in FIG. 2, the optimal range of pH and COD that does not impair the desulfurization rate is C at pH 6.0 or lower.
If the OD is 3000 mg / 1 or less, and the pH is 6.0 or more, the COD value must be lower than the COD value determined by the temperature and pH.

In the desulfurization tower 1, the desulfurization tower absorption liquid containing magnesium hydroxide is circulated as a water slurry in the form of a shower and circulated, and comes into gas-liquid contact with the exhaust gas containing sulfur oxides in parallel with the sulfur oxides. And absorbed as acid magnesium sulfite.

Mg (OH) ₂ + SO ₂ = MgSO ₃ + H ₂ O (1) MgSO ₃ + SO ₂ + H ₂ O = Mg (HSO ₃ ) ₂ (2) Mg (HSO ₃ ) ₂ + Mg (OH) ₂ = ₂ MgSO ₃ + 2H ₂ O (3) The liquid after the desulfurization tower treatment is sent to the oxidation tank 3 by a pump and oxidized by a gas containing oxygen (oxygen-enriched air or air can be used, but usually air is used). Magnesium sulfite and acidic magnesium sulfite are converted to readily soluble magnesium sulfate and sulfuric acid. The generated sulfuric acid further reacts with magnesium hydroxide supplied for pH adjustment and is converted into magnesium sulfate.

MgSO ₃ + 1 / 2O ₂ = MgSO ₄ (4) Mg (HSO ₃ ) ₂ + O ₂ = MgSO ₄ + H ₂ SO ₄ (5) H ₂ SO ₄ + Mg (OH) ₂ = MgSO ₄ + 2H ₂ O (6 In the desulfurization tower 1, since magnesium sulfite and acidic magnesium sulfite are produced by the formulas (1), (2) and (3), the oxidation tank treatment liquid (COD = 0) is supplied from the oxidation tank 3 so that the concentration does not exceed the solubility. Back and dilute. Simultaneously with the dilution, a gas containing oxygen is blown from the bottom of the desulfurization tower 1 and a part of the magnesium sulfite and the magnesium acid sulfite generated by the above-mentioned reaction formulas (1), (2) and (3) react with the reaction in the oxidation tank 3. Similarly, it is oxidized in the desulfurization tower and converted into easily soluble magnesium sulfate. This operation is particularly necessary when the sulfur dioxide gas concentration in the exhaust gas is high.

The desulfurization tower absorption liquid is a mixed solution of magnesium hydroxide, magnesium sulfite which is a reaction product of magnesium hydroxide and sulfur dioxide contained in the exhaust gas, acidic magnesium sulfite, sulfuric acid, and magnesium sulfate. When air is blown to convert magnesium sulfite or acidic magnesium sulfite into magnesium sulfate, the pH of the treatment liquid is reduced due to by-produced sulfuric acid, and the sulfur dioxide gas absorption efficiency is reduced.

According to the present invention, the desulfurization tower absorption liquid and the exhaust gas come into contact with each other, and when the COD in the absorption liquid becomes the highest, the dilution is performed by comparing the exhaust gas amount (G) and the absorption liquid amount (L). The present invention is the most effective in reducing the ratio L / G, which has led to the present invention.

As shown in FIG. 1, the desulfurization tower 1 is divided into a plurality of sections (four sections 11, 12, 13, and 14 in FIG. 1) by means of a tray or a net in which the absorbent can stay and flow. )).

The absorbing liquid flows down in a shower form from the uppermost section 11 and the exhaust gas contacts the absorbing liquid in parallel. Let the amount of circulation of the absorbent be L, the COD concentration of the absorbent injected in section 11 be C, and the amount of sulfurous acid gas absorbed in section 12 (CO
D), the COD concentration of the absorbing solution discharged from the lowermost section 14 is C + S, where S1 is the amount of sulfurous acid gas finally absorbed in the absorption tower (COD converted value).
t / L. This is returned from the oxidation tank (COD = 0
Of the return liquid Lr (C
(OD = 0) is St / C. Actually, in addition to dilution with the processing liquid (COD = 0 liquid) returned from the oxidation tank,
Since oxidation by air blown into the bottom of the desulfurization tower reduces COD, Lr for setting the value of COD to C is St /
It may be smaller than C (Lr <St / C).

Under this condition, the maximum allowable COD of the absorbing solution is C + St / L, which does not exceed the upper limit determined by the magnesium concentration of the absorbing solution.

When the circulation amount of the absorbent is L1 (L1 <L), the COD in the section 12 is C + S1 / L1, and it is necessary to satisfy the following conditions. (The amount of sulfurous acid gas absorbed does not change if the required amount of magnesium hydroxide is contained irrespective of the amount of the absorbing solution.) C + S1 / L1 ≦ C + St / L That is, L1 ≧ (S1 / St) L Since the lower section absorbs sulfurous acid gas and the COD exceeds the allowable amount, it is necessary to dilute with the return liquid Lr from the oxidizing tank at this point and to make the COD equal to or less than the allowable amount in any of the lower sections. .

The COD in the section 14 is (L1C + St) /
(L1 + Lr). If the reduction of COD due to oxidation caused by blowing air into the absorbing liquid is not considered, (L1C + St) / (L1 + Lr) = C That is, Lr = St / C.

From the above considerations, the circulation amount of the desulfurization tower absorption liquid conceptually corresponds to the ratio of the amount of sulfurous acid gas absorbed in the uppermost section 12 divided by trays to the total amount of sulfurous acid gas absorbed in the desulfurization tower. It is understood that it is possible to reduce to a percentage. Under actual conditions, this is not always the case because there is a portion that does not contribute to COD due to natural oxidation of sulfurous acid gas or the like.

As described above, the section for supplying the return liquid from the oxidation tank is usually mixed with the liquid that has absorbed the sulfurous acid gas in the uppermost section. It is designed not to exceed the COD allowance.

Next, the second invention will be described.

An example of the apparatus to which the invention 2 is applied is a desulfurization tower 1, a gypsum separator 2, an oxidation tank 3, a double decomposition tank 4, a calcium hydroxide supply tank 5, a magnesium hydroxide slurry supply tank 6, and ancillary equipment shown in FIG. It is a configuration consisting of a pump and piping.

As shown in FIG. 1, the desulfurization tower 1 is provided with a pH measuring device for a desulfurizing column absorption liquid and a chemical oxygen demand measuring device as shown in FIG. The value of COD may be calculated by measuring the sulfite ion concentration or acidic sulfite ion concentration in the absorbing solution by the method described above, or by calculating from the equilibrium data. The measurement of COD needs to be performed continuously or appropriately and kept within a range not exceeding an upper limit determined according to the concentration of magnesium sulfate in the absorbing solution in order to prevent the precipitation of gypsum sulfite. When the concentration of magnesium sulfate in the absorbing solution fluctuates, CO determined according to the concentration of magnesium sulfate in the absorbing solution
Since the value of D also changes at the same time, it is preferable to control the magnesium sulfate concentration to be constant.

The mixture of magnesium hydroxide and gypsum regenerated in the double decomposition tank 4 is introduced into the desulfurization tower 1, and the desulfurization tower absorption liquid containing magnesium hydroxide is circulated in the form of a shower as a water slurry. Gas and liquid contact with exhaust gas containing sulfur oxides, and the sulfur oxides are absorbed and fixed as magnesium sulfite or acidic magnesium sulfite. The reaction between the magnesium hydroxide and the sulfur dioxide gas in the desulfurization tower 1 and the reaction in the oxidation tank 3 are represented by the above-mentioned formulas (1) to (6).

In the desulfurization tower 1, magnesium sulfite and acidic magnesium sulfite are formed by the formulas (1) to (3), so that the COD determined by the concentrations does not exceed the value determined by the magnesium sulfate concentration in the absorbing solution. The oxidation tank treatment liquid is returned from step 3 and diluted. At the same time as the dilution, a gas containing oxygen is blown from the bottom of the desulfurization tower 1 and a part of the magnesium sulfite and the acidic magnesium sulfite generated by the above-mentioned reaction formulas (1), (2) and (3) are reacted with the reaction in the oxidation tank 3. Similarly, it is oxidized in the desulfurization tower and converted into easily soluble magnesium sulfate. This operation is particularly necessary when the sulfur dioxide gas concentration in the exhaust gas is high.

The liquid treated in the oxidation tank is sent to a double decomposition tank 4 and reacts with calcium hydroxide supplied from a calcium hydroxide supply tank 5 to convert magnesium sulfate into magnesium hydroxide and gypsum. Will be returned to
This reaction is shown in equation (7).

MgSO ₄ + Ca (OH) ₂ + 2H ₂ O = Mg (OH) ₂ + CaSO ₄ .2H ₂ O (7) The magnesium hydroxide regenerated in the double decomposition tank 4 is not separated from the gypsum produced simultaneously. To be returned to the desulfurization tower 1, the desulfurization tower absorption liquid is magnesium hydroxide, dihydrate gypsum, magnesium sulfite which is a reaction product of magnesium hydroxide and sulfur dioxide gas contained in exhaust gas, magnesium acid sulfite and sulfuric acid, sulfuric acid, It becomes a mixed solution with magnesium. When air is blown to convert magnesium sulfite or acidic magnesium sulfite into magnesium sulfate, the pH of the treatment liquid is reduced due to by-produced sulfuric acid, and the sulfur dioxide gas absorption efficiency is reduced. Further, gypsum sulfite has a lower solubility than gypsum gypsum, and depending on the pH and COD conditions of the absorbing solution, calcium sulfite and sulfite ions in the absorbing solution may form and precipitate gypsum. Precipitation of sulfite gypsum inhibits the crystallinity of gypsum dihydrate, and
Significantly degrades quality. In order to maintain the filterability of gypsum and maintain the operation of the apparatus stably, it is necessary to prevent the precipitation of gypsum.

FIG. 3 shows the optimum ranges of pH and COD which prevent the precipitation of gypsum sulfite and do not impair the desulfurization rate. That is, in order to prevent the precipitation of gypsum, the pH of the desulfurization tower absorption solution is 5.5-7.0, and the COD varies depending on the concentration of magnesium sulfate in the absorption solution.
As shown in the figure, when the magnesium sulfate concentration is 5% by weight
When the amount of magnesium sulfate is 1% by weight, the content needs to be 600 mg / l or less. In addition,
FIG. 3 shows the COD when the gypsum is saturated. C determined by the concentration of magnesium sulfate shown in FIG.
Below OD, there is no precipitation of sulfite gypsum due to saturated gypsum.

The sulfite ion and the acid sulfite ion are both indicated as COD. The sulfite ion has an ability to absorb sulfur dioxide gas as shown in the formula (2), but the acid sulfite ion has no ability to absorb sulfur dioxide gas. . The sulfite ion and the acid sulfite ion are in equilibrium. When the pH is low, the acid sulfite ion is dominant, and when the pH is high, the sulfite ion is dominant. The cause of the low pH is H
SO ₃ ^- is increasing (including changing the sulfuric acid by oxidation). For this reason, an excessively low pH is not preferable for absorption of sulfurous acid gas.

As shown in FIG. 3, the pH and C
The allowable range of OD varies depending on the concentration of magnesium sulfate in the absorbing solution. COD is shown as an upper limit in the allowable range for magnesium sulfate concentration. The concentration of magnesium sulfate can be adjusted in the range of 1-10% by weight,
Generally, 3-8% by weight is employed. Since the set value of COD is determined by the concentration of magnesium sulfate, controlling the COD becomes easy by keeping the concentration of magnesium sulfate constant.

Factors that change the pH, magnesium sulfate concentration and COD of the desulfurization tower absorption liquid are the amount of sulfur oxide contained in the exhaust gas, the amount of magnesium hydroxide returned from the metathesis process, and the amount of magnesium hydroxide for replenishment. , The amount of air blown, the amount extracted to the oxidation step, and the like. Under the conditions where these amounts are constant, the pH, magnesium sulfate concentration and COD of the desulfurization column absorption liquid are in a pseudo equilibrium state.

Under such restrictions on the desulfurization tower absorption liquid,
When an exhaust gas containing a high concentration of sulfurous acid gas is treated, the concentration of sulfite ions and acidic sulfite ions in the desulfurization tower absorption liquid increases, the COD of the absorption liquid becomes large, and the production of gypsum sulfite is promoted. In order to reduce the COD, it is necessary to reduce the COD to a predetermined value or less by taking measures such as diluting the absorbing solution and oxidizing the generated sulfite ion / acid sulfite ion by supplying a gas containing oxygen. There is.

In general, a method is employed in which the treatment liquid for the oxidation step is partially returned and diluted while supplying a predetermined amount of gas containing oxygen.

According to the present invention, the ratio between the amount of exhaust gas (G) and the amount of absorbent (L) is determined by the fact that the desulfurization tower absorbent and the exhaust gas come into contact with each other and the COD of the absorbent reaches the highest level. Therefore, the present invention is the most effective for reducing L / G, which is the present invention.

The structure of the desulfurization tower 1 does not change between Invention 1 and Invention 2. Further, the considerations regarding the reaction in the desulfurization tower 1, the circulation amount of the absorbing solution, and the dilution amount are the same except that the reference COD value is different.

[0044]

The present invention will be described in more detail with reference to the following examples.

Example 1 In the process shown in FIG. 6, a desulfurization test was performed using the desulfurization tower shown in FIG.

As shown in FIG. 1, the desulfurization tower has three trays and is divided into four upper and lower sections.
An absorbing solution containing 3,000 mg / l magnesium hydroxide is sprayed at 1,400 m ³ / h and circulates down. The magnesium sulfate concentration in the absorbing solution was 8% by weight. An exhaust gas containing 175,000 Nm ³ / h containing 3,500 ppm of sulfurous acid gas was introduced from the upper part of the desulfurization tower, and was brought into contact with the absorbing liquid in a parallel flow. The COD of the absorption liquid at the bottom of the desulfurization tower is 3,0
Air from the bottom of the desulfurization tower to maintain
It was introduced at 0 m ³ / h. A part of the absorption solution in the desulfurization tower was drawn out to an oxidation tank, and air was blown into the solution to oxidize COD in the absorption solution to almost 0, thereby obtaining an oxidizing solution mainly composed of magnesium sulfate. 36.4 m ³ / h of the return liquid from the oxidation tank to the desulfurization tower was introduced into the section 12, and the absorption liquid was diluted to lower the COD value. The COD of the absorbing solution is increased by absorbing sulfurous acid gas in the exhaust gas.
g / l. The remaining oxidizing solution was discharged after removing SS.

L / G in this example is about 8.0. The sulfur dioxide gas in the processing gas discharged from the lower part of the desulfurization tower is 100p
pm, and the desulfurization efficiency was 97%.

Comparative Example 1 In the process shown in FIG. 6, a desulfurization test was carried out using the desulfurization tower shown in FIG. In this desulfurization tower, the place where the treatment liquid for the oxidation tank is introduced into the bottom of the tower is the same as in FIG.
Different from the tower.

The absorption liquid containing 3,000 mg / l of COD and containing magnesium hydroxide from the section 11 was 1,930 m ³ / h
And circulated down in the form of a spray. The concentration of magnesium sulfate in the absorbing solution was 8% by weight. 175,000N exhaust gas containing 3,500 ppm of sulfurous acid gas from the top of the desulfurization tower
m ³ / h was introduced and brought into contact with the absorbing solution in cocurrent.

In order to keep the COD of the absorbing solution at 3,000 mg / l, air was introduced from the bottom of the desulfurization tower at 2,400 m ³ / h, and the treatment liquid for the oxidation tank was introduced at 36.4 m ³ / h.

Under these conditions, the COD of the absorbing solution is 3,000 mg / l at the time of spraying, but increases by absorbing sulfurous acid gas in the exhaust gas.
It was 160 mg / l.

The sulfur dioxide in the processing gas discharged from the lower part of the desulfurization tower is 100 ppm, the desulfurization efficiency is 97%,
Same as Example, but L / G is about 11.0,
Approximately 38% more was required than in the example.

Example 2 In the process shown in FIG. 5, the desulfurization tower shown in FIG. 1 was used.
It was carried out. As shown in Fig. 1, the desulfurization tower has three trays
It is provided and divided into four upper and lower sections.
Absorbent solution containing D750mg / l magnesium hydroxide
840m ^Three / H and is circulating down. absorption
The magnesium sulfate concentration in the liquid was 6% by weight. Desulfurization
Exhaust gas 1 containing 3,400 ppm of sulfurous acid gas from the top of the tower
00,000Nm^Three/ H is introduced and comes into contact with the absorbing liquid in parallel
I let it. Drain a part of the absorption liquid in the desulfurization tower into the oxidation tank,
Air to oxidize COD in the absorbent to almost 0,
An oxidizing solution mainly composed of magnesium acid was obtained. From the oxidation tank
90m return liquid to desulfurization tower^Three / H into the compartment 12 and
The collected liquid was diluted to lower the COD value. COD of absorption liquid is exhaust gas
Increased by absorbing sulfurous acid gas in the
In the section 14 of, it was 830 mg / l. Of bottom liquid
Empty from the bottom of the desulfurization tower to keep COD at 750 mg / l or less.
I feel 1,259Nm^Three / H. Oxidation liquid residue
Was sent to the double decomposition tank. In the double decomposition tank, the pH is 9-11.
Slaked lime is supplied so that
As a result, a mixed slurry of magnesium hydroxide and gypsum
Obtained. Magnesium hydroxide in this mixed slurry has pH
Gypsum is supplied to the outflow tower and oxidation tank for adjustment, and gypsum is absorbed
It was recovered from the liquid via a gypsum separator.

L / G in this example is about 8.4. The sulfur dioxide gas in the processing gas discharged from the lower part of the desulfurization tower is 100p
pm, and the desulfurization efficiency was 97%.

Comparative Example 2 The process shown in FIG. 6 was performed using the desulfurization tower shown in FIG. An absorption liquid containing magnesium hydroxide at a COD of 750 mg / l was circulated from the section 11 in a spray form at 1,130 m ³ / h. The concentration of magnesium sulfate in the absorbing solution was 6% by weight. 100,000 Nm of exhaust gas containing 3,400 ppm of sulfurous acid gas from the top of the desulfurization tower
³ / h were introduced and brought into contact with the absorbing solution in cocurrent.

In order to maintain the COD of the absorbing solution at 750 mg / l, air was introduced from the bottom of the desulfurization tower at 1,250 Nm ³ / h, and the treatment liquid for the oxidation tank was introduced at 90 m ³ / h. Under these conditions, the COD of the absorbing solution is 750 at the time of spraying.
mg / l, but increased by absorbing sulfurous acid gas in the exhaust gas, and the COD in the compartment 14 was 850 mg / l.
Met.

The sulfur dioxide gas in the processing gas discharged from the lower part of the desulfurization tower is 100 ppm, the desulfurization efficiency is 97%,
Same as Example, but L / G is about 11.3,
It required an increase of about 35% compared to Example 2.

[0058]

According to the method of the present invention, a part of the treatment liquid in the oxidation tank is introduced into the section of the desulfurization tower partitioned by trays, nets, etc. to dilute and reduce the COD. The ratio L / G of the circulating amount of the absorbing solution and the gas to be processed is different from that of the conventional method.
Can be greatly reduced.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an example of a desulfurization tower of a desulfurization apparatus to which the method of the present invention is applied.

FIG. 2 is a diagram showing a relationship between temperature and pH in an absorbing solution and COD.

FIG. 3 is a graph showing the relationship between the amount of magnesium sulfate in the absorbing solution and COD.

FIG. 4 is a conceptual diagram showing an example of a conventional desulfurization tower.

FIG. 5 is a conceptual diagram showing the entire conventional desulfurization process.

FIG. 6 is a conceptual diagram showing an entire conventional discharge process.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Desulfurization tower 2 Gypsum separator 3 Oxidation tank 4 Double decomposition tank 5 Calcium hydroxide supply tank 6 Magnesium hydroxide supply tank 11-14 Desulfurization tower section G1 Exhaust gas before desulfurization G2 Exhaust gas after desulfurization pH pH measuring device COD Chemical oxygen demand Quantity measuring device

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4D002 AA02 BA02 BA05 CA01 CA02 DA05 DA06 DA12 EA02 EA03 EA07 EA11 EA12 EA13 EA14 FA03 GB01 GB06 GB09

Claims (2)

[Claims] 1. A desulfurization step of contacting an exhaust gas containing sulfur oxides with an absorbent containing a magnesium-based desulfurizing agent in a desulfurization tower to absorb and remove sulfur oxides contained in the exhaust gas. Including an oxidation step of treating with a gas containing oxygen, the pH of the desulfurization step absorbent is set in the range of 5.5 to 7.0, and the chemical oxygen demand is determined by the temperature of the absorbent or the concentration of magnesium sulfate in the absorbent. In the desulfurization method of performing desulfurization of exhaust gas within the range not exceeding the upper limit, the desulfurization tower is vertical, the interior of the desulfurization tower is divided into a plurality of sections with a tray that allows the liquid to stay and pass, and the top section is The absorption liquid is caused to flow down in a shower shape, the exhaust gas and the absorption liquid are brought into contact in parallel, and a part of the oxidation process treatment liquid is supplied into one of the upper and lower compartments of the tray. Desulfurization method. 2. A desulfurization step of absorbing and removing the sulfur oxide, and a metathesis step of reacting the treatment solution after the oxidation step with a basic calcium compound in addition to the oxidation step after the desulfurization step.
And the slurry after the metathesis step containing magnesium hydroxide regenerated in the metathesis step is returned to the desulfurization step or / and the oxidation step in a state containing the gypsum, and the dihydrate gypsum from the treatment liquid in the desulfurization step or / and the oxidation step The desulfurization method according to claim 1, further comprising a gypsum separation step of taking out gypsum.