JPH09276645A - Desulfurization of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the adhesion of scale to the inner surface of a column and piping to perform stable desulfurization by using a mixed slurry obtained by the double decomposition reaction of a soln. used in the digestion of an Mg ore containing CaO and MgSO4 as an Mg type desulfurizing agent. SOLUTION: A magnesium type ore such as light burnt magnesia containing magnesium oxide, calcium oxide and impurities is added to a digestion machine 1 and water is added to the magnesium type ore to heat and react the same to obtain a digestion slurry containing magnesium hydroxide and potassium hydroxide. An oxidizing process treatment soln. is supplied to a double decomposition tank 2 from piping L1 and the digestion slurry is supplied thereto from the digestion machine 1 through piping L2 so that the pH in a desulfurizing column 3 becomes constant. Thereupon, calcium hydroxide and magnesium sulfate react to form gypsum particles and magnesium hydroxide particles to become a mixed slurry having magnesium sulfate dissolved therein. This mixed slurry is used as a desulfurizing agent to be supplied to the desulfurizing tower 3 from piping L3 by a pump 3.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for desulfurizing exhaust gas containing sulfur oxides such as combustion exhaust gas of petroleum and coal.

[0002]

2. Description of the Related Art The magnesium sulfate discharge method is known as one of various wet desulfurization methods of exhaust gas, and has been widely used in recent years because of its high absorption efficiency of sulfur oxides. This method uses a magnesium hydroxide aqueous solution as a desulfurizing agent, absorbs sulfurous acid gas as magnesium sulfite in a desulfurization tower, oxidizes it in an oxidation tower to convert it to a magnesium sulfate aqueous solution, and then discharges it to the ocean for disposal. is there.

As the magnesium hydroxide for desulfurization, one obtained from seawater was used in the early stage, but now, the use of lightly burned magnesia (MgO) obtained by firing of magnesite (MgCO ₃ ) ore is shifted. are doing. Magnesite often contains calcium carbonate as an impurity because of its production process.However, if a substance containing a large amount of calcium is used, scale adhesion will occur in the desulfurization tower, circulation pumps and pipes, and therefore it will be used as a desulfurizing agent. As the material, there was used one with a small amount of calcium impurities.

[0004]

However, in order to obtain high-purity magnesite, it is usually necessary to sort the ore manually, so that it is more expensive than magnesite containing a large amount of calcium. It was also necessary to prevent problems from occurring in the absorption tower when using a mixture of low-purity magnesite.

The object of the present invention is to prevent the scale from adhering to or clogging the desulfurization tower and the circulation pump / pipe even when magnesite containing a large amount of calcium is used as a raw material for the treatment liquid, and stable operation is ensured. It is to provide a method of desulfurizing exhaust gas that can be maintained.

Another object of the present invention is to provide a method for desulfurizing exhaust gas which has a high absorption efficiency of sulfur oxides in the treatment liquid and can be carried out by a simple and small-sized facility.

[0007]

[Means for Solving the Problems] That is, the present invention is desulfurization in which an exhaust gas containing sulfur oxide is brought into contact with a treatment liquid containing a magnesium-based desulfurizing agent to absorb the sulfur oxide contained in the exhaust gas into the treatment liquid. Desulfurization method having a step and an oxidation step of bringing the treatment liquid that has undergone the desulfurization step into contact with a gas containing oxygen to convert the magnesium salt in the treatment liquid to magnesium sulfate, and taking the treatment liquid that has undergone the oxidation step out of the system At
A solution obtained by digesting a magnesium-based ore containing calcium oxide and a part of the treatment solution that has undergone the oxidation step are supplied to a metathesis tank, a metathesis reaction is performed to prepare a mixed slurry, and the mixed slurry is supplied as a treatment solution to a desulfurization step. The exhaust gas desulfurization method is characterized by

According to the method of the present invention, even if a digestive juice is produced by using an ore containing a large amount of calcium, the calcium contained in the digestive juice is fixed as gypsum particles in the metathesis tank. Therefore, in the mixed slurry, which is the treatment liquid sent to the desulfurization step, the calcium content has a low concentration close to the solubility of gypsum dihydrate. The gypsum dihydrate particles move in the apparatus together with the treatment liquid, but this can be handled as an inert SS, does not adhere to the desulfurization tower or piping as a scale, and enables stable desulfurization of exhaust gas.

[0009]

The exhaust gas desulfurization method of the present invention will be described below.

The treatment liquid used in the present invention is a liquid obtained by digesting magnesium ore containing calcium oxide. The magnesium-based ore that has been conventionally used in the magnesium sulfate discharge method has a calcium content of 1.0 as calcium oxide in order to prevent precipitation of dihydrate gypsum in the desulfurization tower.
In general, the content of about 2.5% by weight to 2.5% by weight is common, and the content of calcium of 2.0% by weight or less, which has no possibility of producing gypsum, has been used as a particularly preferable one. However, in the present invention, an ore containing 2.5% by weight or more of calcium as calcium oxide may be used, and it is also possible to use dolomite containing almost half the amount of calcium. Of course, no problem occurs even if a high-purity ore having a calcium content of 2% by weight or less is used.

Although the expression "a liquid obtained by digesting magnesium-based ore" is used here, the liquid may contain undigested magnesium oxide particles, and in some cases, only a small portion thereof may be contained. Also included are liquids that are reasonably described as magnesium oxide slurries that are only digested.

The digestion reaction of magnesium-based ore containing calcium oxide is carried out in the same manner as in the past. For example, magnesium hydroxide slurry containing calcium hydroxide (hereinafter abbreviated as "digestion liquid") can be obtained by adding 1 to 10 times the weight of ore hot water and reacting in a batch system for about 5 to 20 hours. .

In the present invention, this digestive juice is first supplied to the metathesis tank. A part of the treatment liquid (hereinafter, abbreviated as “oxidation treatment liquid”) that has been subjected to an oxidation process containing magnesium sulfate as a main component, which will be described later, is also supplied to the metathesis tank. In the double decomposition tank,
Calcium hydroxide and magnesium sulfate in both liquids react as follows to produce gypsum dihydrate particles and magnesium hydroxide particles.

The residence time of the reaction solution in the Ca (OH) ₂ + MgSO ₄ + 2H ₂ O → CaSO ₄ · 2H ₂ O + Mg (OH) ₂ metathesis tank is preferably 4 hours or longer. Generally grows into coarse particles having an average particle size (major axis) of 70 μm or more, usually up to 200 μm, and the calcium ion concentration in the liquid decreases to a level close to the solubility of dihydrate gypsum. On the other hand, the magnesium hydroxide produced in this reaction is 1
fine particles of less than μm, usually about 0.3 to 1 μm,
These particles agglomerate between the particles and have an apparent size of about 10 to 20 μm. The higher the reaction temperature, the more preferable, but it is not particularly limited. Incidentally, the metathesis tank in the present invention is different from the metathesis tank for the regeneration of the desulfurization agent used in the double alkali method, an apparatus for pretreating the calcium content contained in the digestive liquid supplied to the desulfurization tower. Therefore, the size of the device is remarkably small.

As for the amount of the oxidation treatment liquid supplied to the metathesis tank, the minimum amount necessary to convert all the calcium hydroxide in the digestive liquid to dihydrate gypsum is required, but an excessive amount is supplied. It is preferable. If the oxidizing step treatment liquid is insufficient, unreacted calcium hydroxide may flow out to the desulfurization step, and in that case, scale may occur in the desulfurization tower or the like for the first reason. is there. It is known that the solubility of gypsum dihydrate changes depending on the concentration of coexisting magnesium sulfate, and usually becomes the lowest when the concentration of magnesium sulfate is about 1 g / 100 ml. Therefore, if such a liquid having a low solubility of gypsum dihydrate is supplied into the desulfurization tower in which there is a treatment liquid in which magnesium sulfate is dissolved at a concentration of about 4 g / 100 ml and the solubility of gypsum dihydrate is larger. The second reason is that the situation in which gypsum dihydrate precipitates due to the change in solubility is less likely to occur. From this viewpoint, the dissolved amount of magnesium sulfate in the mixed slurry is 0.5 to 2.5.
It is preferable to supply the treatment liquid for the oxidation step to the metathesis tank so as to be g / 100 ml.

A magnesium hydroxide slurry (hereinafter abbreviated as “mixed slurry”) containing gypsum dihydrate particles thus obtained and containing magnesium sulfate dissolved therein is
It is sent to the desulfurization process as it is as a treatment liquid.

In the desulfurization step, the exhaust gas containing sulfur oxide is brought into contact with the treatment liquid consisting of an aqueous solution containing a magnesium-based desulfurizing agent as a main component, and the sulfur oxide is absorbed in the treatment liquid. Since the magnesium hydroxide in the treatment liquid is consumed as a desulfurizing agent, the magnesium hydroxide fine particles have disappeared in the desulfurization process, and the suspension of gypsum dihydrate particles is obtained.
The amount of gypsum dihydrate particles in the suspension depends on the amount of calcium contained in the magnesium-based ore used as a raw material of the digestive juice.

An apparatus suitable for the desulfurization step is a tower having a structure for efficiently contacting these gas and liquid,
Examples include a type in which the treatment liquid is sprayed by a nozzle and a gas is caused to flow in countercurrent or in parallel flow. Since the treatment liquid contains gypsum dihydrate particles, the nozzle needs to be such that it does not clog. In addition, in order to improve the efficiency of gas-liquid contact, a packing, a shelf, etc. may be installed inside.

In the desulfurization step, the temperature of the treatment liquid is usually 50 to 60, as in the case of the conventional magnesium sulfate discharge method.
C. and the pH of the treatment liquid are 5.0 to 6.2. Also,
Since the solubility of magnesium sulfite in water is low, in order to prevent its precipitation, it is common to blow air and oxidize it to obtain magnesium sulfate with high solubility in water, and control the concentration of magnesium sulfite to be below its solubility. Is.
As a method of controlling the magnesium sulfite concentration in the desulfurization step to be less than its solubility without blowing air or the like, a part of the oxidation step treatment liquid is circulated to the desulfurization step to dilute the magnesium sulfite concentration in the desulfurization step. Methods are also known.

The treatment liquid that has undergone the desulfurization process (hereinafter abbreviated as "treatment liquid for desulfurization process") is a mixture of magnesium sulfite, magnesium hydrogen sulfite, and magnesium sulfate produced by the reaction between the magnesium-based desulfurizing agent aqueous solution and the sulfur oxide. An aqueous solution of the composition, containing gypsum dihydrate as a suspension.

The desulfurization process liquid is then introduced into the oxidation process. In the oxidizing step, a gas containing oxygen is brought into contact with the treatment liquid, and magnesium sulfite or magnesium hydrogen sulfite in the treatment liquid is oxidized to generate magnesium sulfate and sulfuric acid. If the treatment liquid is discharged to the outside of the system with the sulfite as it is, the COD load becomes high, and this is the treatment for reducing the COD load. Moreover, since the pH of the treatment liquid after the oxidation treatment is 1 to 3, it is necessary to perform a neutralization treatment at the time of discharge to the outside of the system,
For the corrosion resistance of the apparatus, it is preferable to introduce a small amount of mixed slurry as a neutralizing agent from the metathesis tank to the oxidation step for neutralization. In the oxidation step, a tank reactor is usually used, and the treatment liquid is preferably mixed with stirring.

The gas component other than oxygen in the oxygen-containing gas supplied in the oxidation step may be of any type as long as it is inert to the treatment liquid for the desulfurization step. Air is usually used as the gas containing oxygen.

It is preferable that the treatment liquid for the oxidation step is then introduced into a separation step for separating and removing solid components in the solution. Most of the solid content in the liquid is gypsum dihydrate, but it also contains coal ash and undigested magnesium oxide from the carry-over from the electrostatic precipitator. A wet settler such as a filter, a wet cyclone, a centrifugal settler, or a Dorsickner can be used for separating and removing the solid components. The separation process can also be applied to the desulfurization process liquid before the oxidation process. However, in that case, when a mixed slurry containing gypsum dihydrate particles is used as a neutralizing agent in the oxidation step, it is necessary to treat the gypsum dihydrate particles again.

In this way, the aqueous solution containing magnesium sulfate as a main component from which the solid content has been removed is taken out of the system and is normally discharged to the ocean. In addition, a part of the oxidation treatment liquid is circulated to the metathesis tank and used to fix the calcium component in the digestive liquid as dihydrate gypsum particles.

In carrying out the desulfurization method of the present invention,
The supply of the oxidation treatment liquid to the metathesis tank can be carried out while controlling the pH of the solution in the metathesis tank to be 10.5 to 11. However, in order to carry out the dissolution magnesium sulfate in the mixed slurry at a high concentration such as 0.5 to 2.5 g / 100 ml, the desulfurization step is performed while supplying a certain excess amount of the oxidation step treatment liquid to the metathesis tank. It is convenient and preferable to carry out the process while controlling the supply amount of the digestive liquid to the metathesis tank so that the pH of the treatment liquid in step 1) maintains a constant value such as 5.9.

[0026]

EXAMPLES Hereinafter, the method for desulfurizing exhaust gas of the present invention will be described according to examples with reference to FIG. 1, but the present invention is not limited thereto. Example 1 A light-tightened magnesia 2.0t containing 80% by weight of magnesium oxide, 15% by weight of calcium oxide and 5.0% by weight of impurities such as SiO ₂ , Al ₂ O ₃ and Fe ₂ O ₃ was used as a digester 1.
5.4 t of water was added thereto, heated to 100 ° C. and reacted for 8 hours to obtain a digestive juice slurry containing magnesium hydroxide and calcium hydroxide in a total concentration of about 35% by weight.

To the multi-decomposition tank 2, the oxidizing process liquid is piped L
1 and 1.6 t / hr, and the digestive liquid slurry was supplied from the digester 1 through the pipe L2 so that the pH in the desulfurization tower 3 was maintained at 5.9. In the double decomposition tank, calcium hydroxide was reacted with magnesium sulfate while stirring and mixing with a stirrer to generate gypsum dihydrate particles and magnesium hydroxide particles. The capacity of the metathesis tank was 8 m ³ , the residence time of the reaction solution was about 5 hours, and the reaction temperature was 50 ° C. 5.7% by weight of gypsum dihydrate gypsum obtained in this way
A mixed slurry containing 14% by weight of magnesium hydroxide and 14% by weight of magnesium sulfate dissolved therein was supplied to the desulfurization tower at 1.6 t / hr by the pump 3 and the pipe L3.

In the desulfurization tower 3, 500 t / hr is applied from above.
Sulfur introduced from below by letting the treatment liquid flow in a shower shape.
Sulfur oxidation by making gas-liquid contact with the exhaust gas G1 containing oxides
The product is magnesium sulfite, magnesium hydrogen sulfite, etc.
It was absorbed and fixed in the treatment liquid and the sulfur oxides were removed.
Exhaust gas G2 was discharged from the top of the tower. Supplied to desulfurization tower
The generated exhaust gas is hot and sprayed with industrial water to cool it.
Was. Exhaust gas flow rate is 10 ^Five wetNm^Three / Hr, SO_Two 
The concentration was 1000 ppm.

The desulfurization liquid which has absorbed the sulfur oxides and has flowed down to the bottom of the desulfurization tower 3 is sent together with the mixed slurry supplied from the metathesis tank 2 to the top of the tower through a pump P4 and a pipe L4, and desulfurization is repeated by this repetition. The inside of the tower 3 was continuously circulated. Air was blown into the bottom of the column to prevent precipitation of magnesium sulfite.

The concentration of solid components in the treatment liquid was 0.7% by weight of gypsum dihydrate, 0.1% by weight of coal ash, and 0.13% by weight of insoluble residue of magnesium oxide. The salt concentration of the treatment liquid in the desulfurization tower was 4.0% by weight in terms of total sulfur content in terms of magnesium sulfate. Also, the SO of the exhaust gas G2
_{The 2} concentration was 20 ppm, and the desulfurization rate was 98%.

The desulfurization step treatment liquid is supplied from the desulfurization tower to the oxidation tank 4 at a rate of 13.6 t / hr through a pipe L5, aerating air to oxidize it to form an aqueous solution of magnesium sulfate and a small amount of sulfuric acid, and a neutralizing agent. The mixed slurry has a pH of 7.0.
Was added.

A part of the treatment liquid for the oxidation step thus obtained was supplied to the metathesis tank at a flow rate of 1.6 t / hr, and the residual liquid was introduced into the solid-liquid separator 5 and suspended in the treatment liquid. The solid content was separated at 0.15 t / hr and discharged to the outside of the system. The oxidation process treatment from which the solid content was removed was taken out of the system at a flow rate of 12 t / hr and discharged and discarded.

[0033]

According to the present invention, in the desulfurization of exhaust gas by the magnesium sulfate discharge method, the calcium content is 2% by weight.
Even when the magnesite containing the above, particularly, the magnesite containing 2.5% by weight or more is used as a raw material of the treatment liquid, the scale is prevented from adhering or clogging in the desulfurization tower, the circulation pump or the pipe,
Stable operation can be maintained. Further, the exhaust gas desulfurization method of the present invention can be carried out by simply changing the equipment by adding one small multi-decomposition tank to the apparatus of the conventional method, and the cost reduction effect is great.

[Brief description of drawings]

FIG. 1 is a schematic view showing one structural example of an apparatus used in an exhaust gas desulfurization method of the present invention.

[Explanation of symbols]

 1 digester 2 double decomposition tank 3 desulfurization tower 4 oxidation tank 5 solid-liquid separator G1 exhaust gas before desulfurization G2 exhaust gas after desulfurization P pump L piping

Claims (4)

[Claims] 1. A desulfurization step of bringing an exhaust gas containing sulfur oxide into contact with a treatment liquid containing a magnesium-based desulfurizing agent to absorb the sulfur oxide contained in the exhaust gas into the treatment liquid, and a treatment liquid that has undergone the desulfurization process. In the exhaust gas desulfurization method, which comprises contacting with a gas containing oxygen and converting the magnesium salt in the treatment liquid into magnesium sulfate, and removing the treatment liquid that has undergone the oxidation process to the outside of the system, a magnesium-based ore containing calcium oxide Exhaust gas characterized in that the digested liquid and a part of the treatment liquid that has undergone the oxidation step are supplied to a metathesis tank to prepare a mixed slurry by a metathesis reaction, and the mixed slurry is supplied as a treatment liquid to a desulfurization step. Desulfurization method. 2. The exhaust gas desulfurization method according to claim 1, further comprising a separation step of separating and removing solid components from the treatment liquid that has undergone the oxidation step. 3. The exhaust gas desulfurization method according to claim 1, wherein the mixed slurry is supplied to the oxidation step as a neutralizing agent. 4. The exhaust gas according to claim 1, 2 or 3, wherein the treatment liquid that has undergone the oxidation step is supplied to the metathesis tank so that the dissolved amount of magnesium sulfate in the mixed slurry becomes 0.5 to 2.5 g / 100 ml. Desulfurization method.