JPH09225258A - Desulferizing method of exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To desulferize exhaust gas by a double alkali method with simple and small-sized equipment by supplying an excess quantity of treating liquid to a double decomposition process, and also adding Ca(OH)2 in the double decomposition process such that pH in a desulferizing process becomes constant. SOLUTION: A desulferizing process treating liquid is fed from a desulferizing tower 1 to an oxidation tank 3 with a pump 2 and piping L2, and aerated with air to oxidize and produce MgSO4 and a small quantity of H2 SO4 aqueous solution. The oxidation process treating liquid is fed to a double decomposition tank 4 with piping L3 in more excess than a theoretical flow rate. A water slurry of Ca(OH)2 is added to the double decomposition tank 4 from a Ca(OH)2 supply tank 5 with piping L4 under being controlled such that pH of the treating liquid in the desulferizing tower 1 becomes 5.9. Then, the MgSO4 and H2 SO4 are reacted with Mg(OH)2 , and solid particles of gypsum dihydrate and Mg(OH)2 are produced. Thus, absorption efficiency of sulfur oxide with the treating liquid becomes high.

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 limestone gypsum method is known as a typical method for wet desulfurization of various exhaust gases. This method
Since calcium carbonate or calcium hydroxide is directly added to the desulfurization tower as an absorbent, calcium ions are dissolved in the treatment liquid. Therefore, when this reacts with sulfur oxides etc. in the desulfurization tower, it is difficult to maintain smooth operation because scales composed of precipitates of gypsum dihydrate, calcium dihydrosulfite, calcium carbonate, etc. occur in the desulfurization tower and piping. However, it took a lot of time to remove it. In addition, calcium hydroxide is originally a desulfurizing agent that can absorb two molecules of sulfurous acid gas, but calcium sulfite, which is a desulfurizing agent that absorbs one molecule of sulfurous acid gas, is significantly larger than magnesium sulfite, which is a similar desulfurizing agent. Since the solubility is low, the absorption efficiency of the sulfur oxide in the treatment liquid is low, which leads to an increase in the size of equipment such as a desulfurization tower and a circulation pump, which is a problem in terms of economy.

On the other hand, the absorption of sulfur oxides in the desulfurization tower is carried out by using a basic desulfurization agent such as basic sodium, ammonia or basic magnesium, and the desulfurization agent is carried out outside the desulfurization tower by using a quick lime for metathesis. The double alkali method for regenerating is also known. The double alkali method is less likely to cause scale generation. Particularly, the method using basic magnesium as a desulfurizing agent also has a high absorption efficiency of sulfur oxides, has a high solubility of the generated magnesium sulfite, and has little scale generation in the absorption tower. It is one of the methods. However, in this method using a basic desulfurizing agent, two types of crystals, gypsum dihydrate and magnesium hydroxide, precipitate in the metathesis process, and their separation is not easy, so there is a problem that the device becomes complicated. .

The Kawasaki magnesium gypsum method is known as an eclectic method of the lime gypsum method and the double alkali method (Practical pollution control technology list (1), published by Kagaku Kogyosha,
p. 14). In this method, a mixed slurry of magnesium hydroxide and calcium hydroxide is used as a desulfurizing agent to absorb sulfur oxides in a desulfurization process, and then the treatment liquid is treated with sulfuric acid to remove p
While adjusting H to 2.0-4.0, it is oxidized with air or the like to generate magnesium sulfate and gypsum dihydrate, and separated into gypsum dihydrate and an aqueous magnesium sulfate solution by a sedimentation separation step and a centrifuge. The separated magnesium sulfate aqueous solution is circulated and supplied to the desulfurizing agent regeneration step in which a mixed slurry of magnesium hydroxide and calcium hydroxide is added,
Here, a metathesis reaction with a part of the calcium hydroxide in the mixed slurry produces magnesium hydroxide and gypsum dihydrate, and a mixed liquid containing this and the balance calcium hydroxide is circulated and supplied to the absorption step as a desulfurizing agent. It However, this method is the same as the lime gypsum method in that it introduces calcium hydroxide or calcium carbonate and gypsum dihydrate into the desulfurization tower,
The problem that scale adheres easily to the desulfurization tower and to the circulation pump and piping has not been solved.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust gas desulfurization method which has a high absorption efficiency of sulfur oxides in a treatment liquid and can be carried out by a simple and small-sized facility.

Another object of the present invention is to provide an exhaust gas desulfurization method capable of preventing the scale from adhering to or blocking the inside of the desulfurization tower or the circulation pump / pipe so that smooth operation can be maintained.

[0007]

Means for Solving the Problems As a result of earnest studies on simplification of the process in the double alkali method using a magnesium-based desulfurizing agent, the present inventor has found that dihydrate gypsum and magnesium hydroxide conventionally obtained by a metathesis process. To separate the
Only the magnesium hydroxide slurry was returned to the desulfurization tower, but if calcium hydroxide is not included in the treatment liquid returned to the desulfurization tower, desulfurization is performed as a mixed slurry without separating gypsum dihydrate and magnesium hydroxide. No scale is generated in the desulfurization tower even when returned to the tower, and the pH of the treatment liquid in the desulfurization process
If a basic calcium compound is added in the metathesis process based on the standard, stable operation is possible even if the load changes and the process liquid is returned directly to the desulfurization process without providing a large-capacity buffer tank after the metathesis process. Therefore, the present invention has been completed.

That is, the present invention provides a desulfurization process 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, and a desulfurization treatment liquid. And a step of oxidizing the magnesium salt in the treatment liquid into magnesium sulfate by contacting with a gas containing oxygen, and a basic calcium compound is added to the treatment liquid, and magnesium sulfate in the treatment liquid is mixed with magnesium hydroxide and dihydrate. In a desulfurization method that has a metathesis step of decomposing into gypsum and returns the mixed slurry obtained in the metathesis step to the desulfurization step, and in the desulfurization method of extracting dihydrate gypsum from the treatment liquid to the outside of the system, an oxidation step treatment The metathesis process is performed so that the flow rate of the liquid is larger than the theoretical flow rate calculated from the absorption amount of sulfur oxides, and the pH of the treatment liquid in the desulfurization process maintains a constant value. The basic calcium compound is a method for desulfurizing exhaust gas, characterized in that the addition.

[0009]

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

In the method for desulfurizing exhaust gas of the present invention, the exhaust gas containing sulfur oxide is brought into contact with a treatment liquid consisting of an aqueous solution containing a magnesium-based desulfurizing agent as a main component in the desulfurization step, so that the sulfur oxide becomes the treatment liquid. Be absorbed.

The treatment liquid returned from the metathesis process to the desulfurization process is a mixed slurry containing magnesium hydroxide, gypsum dihydrate and a trace amount of calcium sulfite, and a small amount of magnesium sulfate dissolved therein. Since magnesium is consumed as a desulfurizing agent, the magnesium hydroxide fine particles have disappeared in the desulfurization process.

An apparatus suitable for the desulfurization process is a tower having a structure for efficiently contacting these gas and liquid,
An example is a type in which an aqueous solution containing a desulfurizing agent is sprayed with a nozzle and a gas is caused to flow therethrough in a countercurrent or a cocurrent. Since the treatment liquid contains gypsum dihydrate coarse particles, the nozzle is required to be such that clogging does not occur. In addition, in order to improve the efficiency of gas-liquid contact, a packing, a shelf, etc. may be installed inside.

The treatment liquid exiting the desulfurization process (hereinafter abbreviated as "desulfurization process treatment liquid") was a mixture of magnesium sulfite, magnesium hydrogen sulfite, and magnesium sulfate produced by the reaction of a magnesium-based desulfurizing agent aqueous solution with sulfur oxides. An aqueous solution of the composition, containing gypsum dihydrate as a suspension.

In the desulfurization step, the temperature of the treatment liquid is usually 50.
It is set to -60 ° C. Further, in the desulfurization step, since the solubility of magnesium sulfite in water is low, in order to prevent its precipitation, blowing in air or the like to oxidize it to give magnesium sulfate having high solubility in water, and reduce the concentration of magnesium sulfite to the solubility or less. It is generally controlled. As a method of controlling the magnesium sulfite concentration in the desulfurization step to be less than or equal to its solubility without blowing air or the like, a portion of the treatment liquid that exits the oxidation step described below is circulated to the desulfurization step, and magnesium sulfite in the desulfurization step is used. A method of diluting the concentration is also known. Regarding the pH of the treatment liquid in the desulfurization step,
Although it is set to 5.0 to 6.2, the relationship with the basic calcium compound added in the metathesis step will be described later.

In the method of the present invention, the gypsum dihydrate contained in the treatment liquid is taken out of the system by separating gypsum dihydrate from the treatment liquid for the desulfurization step or the treatment liquid for the oxidation step described later. Since these treatment liquids contain almost no solid components other than gypsum dihydrate, they can be easily separated.

Separation and removal of gypsum dihydrate from the desulfurization process liquid may be carried out by separately taking the desulfurization process liquid from the desulfurization tower and returning the residual liquid to the desulfurization tower or leading to the next oxidation process. You may implement it with respect to the desulfurization process liquid in the middle. A wet classifier such as a wet cyclone, a centrifugal settler or a Dorsickner can be used for separating and removing gypsum dihydrate, and a wet cyclone is particularly preferable. The separated gypsum dihydrate is taken out of the system and can be widely used for cement, gypsum board and the like.

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. Usually, the concentration of magnesium sulfate in the treatment liquid is 3 to 10% by weight and the pH is 1 to 3, but it may be neutralized for the corrosion resistance of the device. In the oxidation step, a tank reactor is usually used, and the treatment liquid may be mixed by stirring,
You don't have to.

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.

The treatment liquid that has exited the oxidation process (hereinafter abbreviated as "oxidation process treatment liquid") is then introduced to the metathesis process. In the method of the present invention, the flow rate of this treatment liquid supplied to the metathesis process is set to the minimum amount of treatment required for converting all of the absorption amount of sulfur oxide of the treatment liquid in the absorption process into dihydrate gypsum. It should be larger than the theoretical flow rate calculated as the liquid flow rate. That is, the oxidation step treatment liquid containing magnesium sulfate in an excess amount over the amount of regenerated magnesium sulfate is supplied to the metathesis step. The excess ratio expressed as the ratio of the supply flow rate of this processing liquid to the theoretical flow rate is 1.07 to 1
The range of 0.0 is preferable, and the range of 1.2 to 5.0 is more preferable. If the excess ratio is less than 1.07, unreacted calcium hydroxide in the metathesis process may flow out to the desulfurization process when the load changes, and in that case, the scale in the desulfurization tower internal column May occur. on the other hand,
When the excess ratio exceeds 10, the concentration of calcium sulfate in the metathesis tank becomes thin, so that the growth of dihydrate gypsum slows down and the flow rate of the circulating treatment liquid increases by that much. Is required, which is not preferable.

When the amount of sulfur oxides such as sulfurous acid gas fluctuates, the flow rate of the treatment liquid can be adjusted by interlocking with the amount of sulfur oxides while keeping the above-mentioned excess rate constant. preferable. Note that here, the adjustment of the flow rate of the oxidation treatment liquid is described, but if the desulfurization treatment liquid is sent to the metathesis process as the oxidation treatment liquid at a constant ratio, the flow rate of the desulfurization treatment liquid is targeted. Even if the excess rate is adjusted as described above, the same result can be obtained.

In the desulfurization method of the present invention, a basic calcium compound is added and stirred and mixed in the metathesis process to the oxidizing process solution containing magnesium sulfate and sulfuric acid as the main components supplied as described above. . Here, sulfuric acid reacts with a basic calcium compound to form gypsum dihydrate, and magnesium sulfate and basic calcium react to form gypsum dihydrate and magnesium hydroxide.

In the metathesis process, a tank reactor is usually used, and the higher the reaction temperature, the more preferable. However, from the viewpoint of operation and operation, the temperature may be the same as that of the desulfurization process. The residence time is preferably 4 to 5 hours or longer, and the gypsum dihydrate produced thereby grows into coarse particles having an average particle diameter (major axis) of 70 μm or more, usually up to 200 μm. On the other hand, magnesium hydroxide is fine particles having a size of 1 μm or less, usually about 0.3 to 1 μm, and these particles agglomerate between the particles to make apparently 1
The size is about 0 to 20 μm.

The basic calcium compound used in the metathesis step is preferably calcium hydroxide, calcium oxide, calcium carbonate or a mixture thereof, and the form of supplying to the reaction tank may be powder, but an aqueous slurry of these is used. Most preferred in terms of sex.

In the method of the present invention, the addition of the basic calcium compound in the metathesis step is carried out while controlling the pH of the treatment liquid in the desulfurization step to a predetermined constant value, for example, 5.9.

The addition of the basic calcium compound in the metathesis step in the conventional double alkali method has been carried out based on the pH of the metathesis tank so that the metathesis reaction of magnesium sulfate is completed. However, the target reaction rate of magnesium sulfate excessively supplied in the metathesis process, in other words, the amount of sulfur oxide absorbed by the treatment liquid in the desulfurization process is adjusted to react in an amount that is adjusted to adjust the pH in the metathesis tank. Cannot be implemented based on This is because the solubility of magnesium hydroxide slightly changes depending on the concentration of dissolved magnesium sulfate, the liquid temperature, and the like.

In the exhaust gas desulfurization facility, the load may change depending on the operating condition of the boiler, and at night, 1 / day
The load may decrease to about 3. The residence time of the treatment liquid in the desulfurization process, the oxidation process, and the metathesis process is usually 2 each.
It's quite long, like ~ 3 hours, 2-3 hours, 5-7 hours. Therefore, as in the conventional case, the treatment in each of these steps is handled independently, and if there is no large capacity buffer tank (storage tank) before returning the mixed slurry to the desulfurization step, there will be a case of load fluctuation. , Regardless of the amount of magnesium hydroxide consumed in the desulfurization process at that time, the amount of mixed slurry corresponding to the amount of magnesium hydroxide consumed 10 to 14 hours ago is sent from the metathesis tank, so desulfurization is performed. PH of process
Is difficult to control. However, if the amount of basic calcium compound added to the metathesis tank is adjusted and controlled as described above, not only the flow rate of the mixed slurry returned to the desulfurization process but also the concentration of magnesium hydroxide changes, so a large buffer tank is used. It is possible to keep the pH of the treatment liquid in the desulfurization step constant without disposing. In addition, since a larger amount of magnesium sulfate than the amount to be subjected to the metathesis reaction is dissolved in the treatment liquid supplied to the metathesis step, such a control temporarily adds a little excess amount of the basic calcium compound in the metathesis tank. However, the pH does not rise too much.

In the method of the present invention, 100% of the magnesium sulfate in the treatment liquid does not react in the metathesis process. Therefore, even if the treatment liquid in any of the systems is blown out of the system, magnesium sulfate is entrained. Magnesium loss,
The concentration of magnesium sulfate in the treatment liquid in the system gradually decreases. Therefore, it is necessary to detect the salt concentration of the treatment liquid and supply magnesium hydroxide into the system. In the method of the present invention, it is the oxidation step treatment liquid that has the lowest solid content and the highest purity of magnesium sulfate as a dissolved component. Therefore, it is appropriate to measure the density of the oxidizing process liquid and replenish the magnesium hydroxide slurry based on the measured value.

Various modifications can be made to the desulfurization method of the present invention. For example, a part of the oxidation step treatment liquid may be returned to the desulfurization step (desulfurization tower). When the oxidation step treatment liquid is added to the desulfurization tower, the proportion of magnesium sulfate in the treatment liquid in the desulfurization tower increases, and the proportion of sulfite decreases.
Precipitation of magnesium sulfite can be suppressed. Further, the desulfurization method of the present invention can be applied to exhaust gas containing hydrogen chloride and the like in addition to sulfur oxide.

[0029]

EXAMPLES Hereinafter, the exhaust gas desulfurization method of the present invention will be described according to examples with reference to the drawings, but the present invention is not limited thereto. Example 1 This example is an example of desulfurization treatment of exhaust gas from a heavy oil-fired boiler, and FIG. 1 shows an outline of this process.

A treatment liquid in which the magnesium-based desulfurizing agent is dissolved and containing gypsum dihydrate coarse particles as a suspension is flowed from above the desulfurization tower 1 at a rate of 500 t / hr in a shower shape, and the sulfur oxide introduced from below is removed. The exhaust gas G1 contained therein was brought into gas-liquid contact, the sulfur oxides were absorbed and fixed in the treatment liquid as magnesium sulfite, magnesium hydrogen sulfite, etc., and the exhaust gas G2 from which the sulfur oxides were removed was discharged from the upper part to the outside of the column.

Since the exhaust gas supplied to the desulfurization tower was at a high temperature, industrial water was sprayed through a nozzle to cool it. The exhaust gas flow rate at maximum load is 10 ⁵ wetNm ³ / hr and the SO ₂ concentration is 200.
It was 0 ppm.

The desulfurization liquid which has absorbed the sulfur oxide and has flowed down to the bottom of the desulfurization tower 1 is sent down to the upper part of the tower through the pump P1 and the pipe L1 together with the treatment liquid newly supplied from the magnesium hydroxide slurry supply tank 7. By repeating this, the desulfurization tower 1 was continuously circulated. At the bottom of the tower,
Air was blown in to prevent the precipitation of magnesium sulfite. In addition, 65 t /
It was led to the gypsum separator 2 by hr, the gypsum dihydrate suspended in the treatment liquid was separated and discharged out of the system at 1.4 t / hr, and the residual liquid was returned to the desulfurization tower. The concentration of gypsum dihydrate in the treatment liquid was about 2%. The salt concentration of the treatment liquid in the desulfurization tower is 7.5% by weight when the total sulfur content is expressed in terms of magnesium sulfate,
Among them, the total concentration of magnesium sulfite and magnesium hydrogen sulfite was 1.5% by weight in terms of magnesium sulfate. The exhaust gas G2 had a SO ₂ concentration of 100 ppm and a desulfurization rate of 95%.

The treatment liquid for the desulfurization step was supplied from the desulfurization tower to the oxidation tank 3 through the pump P2 and the pipe L2, and aerating the air to oxidize it to obtain an aqueous solution containing 7.5% by weight of magnesium sulfate and a small amount of sulfuric acid. This oxidation step treatment liquid was supplied to the metathesis tank 4 at 36 t / hr with the excess rate from the theoretical flow rate set to 3.0 through the pipe L3. A water slurry of 30% by weight of calcium hydroxide was added to the metathesis tank 4 from the calcium hydroxide supply tank 5 through a pipe L4 while controlling the pH of the treatment liquid in the desulfurization tower to be 5.9, and added with a stirrer. The reaction of magnesium sulfate and sulfuric acid with calcium hydroxide was carried out with stirring and mixing, to produce solid particles of gypsum dihydrate and magnesium hydroxide. The reaction temperature was 50 ° C.

The water slurry containing the obtained dihydrate gypsum and solid particles of magnesium hydroxide, in which unreacted magnesium sulfate was dissolved at a concentration of 5.0 wt% on average over time, was desulfurized through a pipe L7. It was circulated and supplied to the tower 1.

The exhaust gas supplied to this desulfurization tower had an SO ₂ concentration of 2000 ppm at night, but the exhaust gas flow rate decreased to 4 × 10 ⁴ wetNm ³ / hr. However, the production of gypsum dihydrate in the metathesis tank was successful, and the pH of the treatment liquid in the desulfurization tower could be maintained at 5.9.

[0036]

By the exhaust gas desulfurization method of the present invention, the desulfurization process of the double alkali method using a magnesium-based desulfurizing agent can be carried out with simple and small equipment without separating two kinds of solid particles. . Gypsum dihydrate circulates in the system along with the treatment liquid, but this can be handled as an inert SS, and scale is not attached to the desulfurization tower or pipes, stable operation is maintained, and efficient exhaust gas desulfurization is performed. Was able to be carried out. In addition, although the flow rate of the treatment liquid supplied to the metathesis tank increased slightly, stable operation could be continued against the load change of sulfurous acid gas without providing a large capacity buffer tank.

[Brief description of drawings]

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

[Explanation of symbols]

 1 desulfurization tower 2 gypsum separator 3 oxidation tank 4 metathesis tank 5 calcium hydroxide supply tank 6 magnesium hydroxide slurry replenishment tank 7 dehydrator G1 exhaust gas before desulfurization G2 exhaust gas after desulfurization P pump L pipe

Claims (1)

[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 the desulfurization treatment liquid containing oxygen. Oxidation step of contacting with gas to convert magnesium salt in treatment solution to magnesium sulfate, and oxidation step Add basic calcium compound to treatment solution to decompose magnesium sulfate in treatment solution into magnesium hydroxide and gypsum dihydrate In the desulfurization method, which has a metathesis step to allow the mixed slurry obtained in the metathesis step to be returned to the desulfurization step and to take out gypsum dihydrate from the treatment solution, the flow rate of the oxidation step treatment solution supplied to the metathesis step Increase the theoretical flow rate calculated from the absorption of sulfur oxides, and make basic calcium in the metathesis process so that the pH of the treatment liquid in the desulfurization process remains constant. A method for desulfurizing exhaust gas, which comprises adding a compound.