JPS5923848B2 - Flue gas desulfurization method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a wet flue gas desulfurization method, and in particular, when performing wet flue gas desulfurization using a lime-gypsum method, it is possible to improve the desulfurization rate and prevent scale formation in the system, and it is easy to do so. This invention relates to a flue gas desulfurization method that can obtain high-quality by-product gypsum.

In the conventional wet flue gas desulfurization method using a basic calcium compound such as lime or calcium carbonate, two or more steps are required to produce high-grade calcium sulfate as a by-product and to obtain a high desulfurization rate. In order to obtain a high desulfurization rate, the pH of the slurry in the subsequent process relative to the gas flow is set to 7 or higher, and in order to oxidize the produced calcium sulfite at a high rate, sulfuric acid is added to the slurry after absorbing sulfur dioxide gas. Alternatively, highly concentrated sulfur dioxide gas is added to adjust the pH to, for example, 4 or less, and then high-pressure air is blown in a special device to promote oxidation under pressure to produce calcium sulfate as a by-product. It is being

Generally, when the pH of the absorption process is in the range of 5 to 8, calcium sulfite is not produced easily, and the calcium sulfite is needle-shaped fine crystals and has strong adhesive properties, and the slurry containing a large amount of calcium sulfite has a high viscosity. Scale easily adheres to the inside, and this gradually grows, eventually leading to a situation where the absorption device is completely closed.

Furthermore, the oxidation rate of calcium sulfite is very slow, so conventionally the oxidation process has been operated under pressure and complicated and special equipment has been used to improve gas-liquid contact.

This oxidation reaction is easier when the pH is 4 or less than when the pH is high, but there is a problem of material corrosion, which becomes a serious problem especially when oxidation is carried out under pressure.

The inventors of the present invention have conducted various studies to overcome these drawbacks of conventional methods. By making the absorbent slurry have a specific composition, the desulfurization rate can be increased, scale generation can be prevented, and high-quality products can be easily obtained. They discovered that it is possible to obtain gypsum as a by-product and completed the present invention.

That is, the present invention uses a basic calcium compound as a main ingredient,
When wet flue gas desulfurization is performed using this as an absorbent slurry containing manganous salt or ferric salt and alkali metal sulfate, the pH of the absorption tower and auxiliary absorption tower is 4.5 to 6. The present invention relates to a flue gas desulfurization method characterized by blowing into the slurry region of No. 5 a gas containing one or more equivalents of oxygen to calcium sulfite present in the system.

In the present invention, the basic calcium compound refers to calcium carbonate, calcium hydroxide, calcium oxide, dolomite, etc., and serves as the main ingredient of the absorbent slurry.

In addition, manganous salts refer to manganous chloride, manganese sulfate, manganous nitrate, etc., and ferric salts refer to ferric sulfate, ferric chloride, ferric nitrate, etc. acts as a pro-oxidation catalyst.

In addition, alkali metal sulfates refer to sodium sulfate, potassium sulfate, sodium hydrogen sulfate, potassium hydrogen sulfate, etc., which act as catalysts that promote absorption and gypsum formation of calcium sulfite (the effects of these catalysts will be discussed in this application). Please refer to the specification of Japanese Patent Application No. 49-10951 (Japanese Patent Publication No. 58-36619) which has already been filed by a person.
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In the present invention, an absorbent slurry having the above-described composition is used for desulfurizing flue gas, and by using the absorbent slurry having such a composition, the pH of the flue gas absorption process can be adjusted.
Oxygen or air is blown into the absorbent slurry liquid distillate having a particle size of 4.5 to 6.5 to oxidize the calcium sulfite in the absorbent slurry to form calcium sulfate.

In the present invention, the pH of the exhaust gas absorption step is set to 4.5 above.
The reason for setting the value to 6.5 is that when the pH is below 4, there is no problem of material corrosion as mentioned above, and the sulfur dioxide gas in the exhaust gas is hardly absorbed, and when the pH is above 7, the absorbent slurry This is because most of the solid matter becomes basic calcium, and there is almost no sulfite to be oxidized, so there is no point in blowing oxygen or air into it.

Further, in the present invention, the amount of oxygen or air blown into the exhaust gas absorption step is preferably such that the amount of oxygen is equal to or more than the equivalent amount of calcium sulfite present in the system.

This is because when the pH of the absorbent slurry changes within the range of 4.5 to 6.5, the solid composition of the absorbent slurry also changes within the range of many to about 60% (by weight). In order to oxidize sulfite in a slurry such as this, it is best to blow 2 to 3 times the amount of oxygen equivalent to the sulfite (or air in an amount of oxygen by weight) into the liquid distillation section. ) The efficiency of the blowing device varies depending on the efficiency of the blowing device (e.g., blowing from the end of a pipe, blowing from a perforated plate, or using a modified hole shape in the perforated plate, etc.). This is because, if an oxidation catalyst is used as in the method of the present invention, a large excess of oxygen is not required.

A specific flow for implementing the present invention will be described with reference to the accompanying drawings.

The flue gas from the flue gas source is introduced into the evacuation device through the flue 8, most of it passes through the flue 11 and enters the absorption tower 1 in the absorption process, and a part of the remaining flue gas goes through the flue 12 and is auxiliary. Absorption tower 2
A part of the exhaust gas is distributed and supplied to the oxidation tower 3 for the oxidation process through the flue 13, and in each unit, it comes into gas-liquid contact with the slurry to remove the sulfur dioxide gas in the exhaust gas, and then the flue gas is introduced into the flue 13. 9 and sent to absorption tower 4.

Here, gas-liquid contact is finally performed to remove the remaining sulfur dioxide gas that was not removed in the absorption tower 1, the auxiliary absorption tower 2, and the oxidation tower 3 in the oxidation step.

The purified gas is discharged from the flue 10 into the atmosphere through a chimney (not shown).

On the other hand, for example, an absorbent slurry containing calcium carbonate as an absorbent and manganous sulfate and sodium sulfate is sent from an absorbent replenishment tank 7 to an absorption tower 4 by a pump 14, and is circulated by a circulation pump 15 to mix exhaust gas and gas liquid. The sulfur dioxide gas in the exhaust gas is absorbed and removed through contact.

In this step, the solids in the absorbent slurry are mostly calcium carbonate, calcium sulfite produced by reaction with S02, and calcium sulfite which is oxidized by oxygen in the exhaust gas.

Next, a part of this circulating absorbent slurry is sent to the absorption tower 1 by the pump 15, and is circulated by the pump 16 and brought into gas-liquid contact with the exhaust gas introduced from the flue 11, thereby absorbing and removing 802 in the exhaust gas.

In the present invention, oxygen or air is blown into the absorption line 19 of the absorbent slurry liquid distillation section of the absorption tower 1 to oxidize calcium sulfite in the absorbent.

In this process, the solid composition in the absorbent slurry is approximately 60% by weight calcium sulfate, approximately 10% by weight calcium sulfite, and approximately 30% by weight calcium carbonate, and the pH of the absorbent slurry at this time is approximately 6. It is about .2.

Further, a part of the circulating absorbent slurry is sent to the auxiliary absorption tower 2 by the pump 16 and circulated by the pump 17, and comes into gas-liquid contact with the exhaust gas to absorb and remove SO2.

This auxiliary absorption tower 2 functions to consume the absorbent not consumed in the absorption towers 1 and 4.

Oxygen or air is also blown into the auxiliary absorption tower 2 from the absorbent slurry liquid distillation line 20 to oxidize the calcium sulfite in the absorbent slurry.

In this step, the composition of solids in the absorbent slurry is approximately 83% by weight calcium sulfate, approximately 10% by weight calcium sulfite, and approximately 11% by weight calcium carbonate, and the pH is approximately 6.

Note that this solid composition can also be used for cement in some cases, so a part of the slurry circulated by the pump 17 is taken out of the system and concentrated in the solid-liquid concentrator 5.
Next, solid-liquid separation is performed in a solid-liquid separator 6, and the solid is taken out as a product.

Furthermore, if calcium sulfate with a higher purity is required, the auxiliary absorption tower 2 is recycled.

A portion of the absorbent slurry is sent by pump 17 to oxidation tower 3 for the oxidation process.

In the oxidation tower 3, the absorbent slurry is also circulated by the pump 18, making gas-liquid contact with the exhaust gas, and removing S02 in the exhaust gas.
In addition to absorbing calcium sulfite, it also utilizes oxygen in exhaust gas to help oxidize calcium sulfite.

If oxidation is insufficient by this alone, oxygen or air is blown from the absorbent slurry liquid distillation section heliline 21 of the oxidation tower 3 to perform final oxidation.

Even in this case, most of the solid matter in the absorbent slurry has already turned into calcium sulfate at the inlet of the oxidation tower 3, so only a small amount of oxygen or air needs to be blown in, and the pH is 4.
It is about 6.

It should be noted that all air blowing operations are performed at normal pressure, and high pressure is not required, nor is a special gas-liquid contact device necessary; simply blowing air from the pipe is sufficient.

In this way, in the present invention, the oxidation of calcium sulfite produced in the absorbent slurry is carried out in several stages, and since the oxidation catalyst is present in the absorbent slurry, the pH is 4.5. Oxidation proceeds efficiently even in the high range of ~6.5.

In addition, at the outlet of the oxidation tower 3, calcium sulfate with a purity of 95% by weight or more and a thick plate-like material are obtained, and gypsum that can be used not only for cement but also for boards is obtained. can get.

The slurry sent out from the oxidation tower 3 is sent to the solid-liquid concentrator 5
For example, the slurry is supplied to a thickener, and the concentrated slurry is sent to a solid-liquid separator 6 for solid-liquid separation.

The mother liquor overflowing from the solid-liquid concentrator 5 is returned to the absorbent supply tank 7, where a new absorbent is supplied, and an absorbent slurry is adjusted and sent to the absorption tower 4 for circulation.

In this case, since manganous sulfate and manganese sulfate are dissolved in the mother liquor, there is no need to newly add them.

As described above, in the method of the present invention, an amount of oxygen equivalent to 1 to 10 times the amount of calcium sulfite produced in the absorption process is blown into the liquid distillation of the absorption equipment (absorption tower 1 and auxiliary absorption tower 2 in this case). Calcium sulfite can be oxidized at a high oxidation rate, and as a result, most of the solids in the absorbent slurry can be converted to calcium sulfate in the absorption process, and the remaining receiving portion can be converted to calcium carbonate. As the content of calcium sulfite is reduced, build-up and its growth within the absorber can be prevented.

Furthermore, since the amount of calcium sulfite in the loss slurry from the absorption process is small, it is not a scale that can simplify the next oxidation process (oxidation tower 3 in this case), and depending on the use of the by-product gypsum, The process can be omitted, and high-grade calcium sulfate with a plate-like thickness can be obtained even though the pH is as high as 4 to 6 at the exit of the oxidation process.

Next, examples of the present invention will be shown.

Example According to the flow shown in the attached drawing, air was blown into the absorption tower 1, the auxiliary absorption tower 2, and the oxidation tower 3, and as a result of the experiment under the following conditions, the total desulfurization rate was 95 cm, and the purity at the outlet of the oxidation tower 3 was 95% by weight. , thick plate-like calcium sulfate with an average particle size of 50 μm was obtained, and only a small amount of calcium sulfite was found in the solid matter of the slurry at each outlet of the absorption tower 1 and the auxiliary absorption tower 2. There was no occurrence of close-up.

The composition of each slurry at each outlet of absorption tower 1, auxiliary absorption tower 2 and oxidation tower 3 is shown in the above table.

Experimental conditions Inlet gas SO□ concentration 1300~isooppmO□
Concentration 5-6% by volume CO2 concentration 12-13 Volume gas temperature 140-150℃ Processing gas amount 1000 Nd/Hr absorbent slurry CaCO35 heavy slurry Na2804 2% by weight 4 Umbrella MnSO4300pI'' Air blowing amount Absorption tower 15N?729/ Hr auxiliary absorption tower 2 5 N7F+'/Hr Oxidation tower 3 5N/Hr Comparative Example 1 An experiment was conducted using the same flow and conditions as in the example except that air was blown into the oxidation tower 3 only.

Air blowing amount Oxidation tower 3 30 Ni/Hr The slurry compositions at each outlet of absorption tower 1, auxiliary absorption tower 2, and oxidation tower 3 in the Examples and Comparative Examples are shown in the table below.

As is clear from this table, according to the method of the present invention, the amount of calcium sulfite in absorption tower 1 and auxiliary absorption tower 2 is about X or less compared to the comparative example, and as a result, calcium sulfite is supplied to oxidation tower 3. Since the amount of air is reduced, the amount of air blown into the oxidizing device can be very small, and a very simple blowing device and normal pressure oxidizing device can be used, resulting in great industrial and economical effects.

Further, according to the observation after the experiment, in the case of the present invention, there was a small amount of fine, needle-like crystals and sticky calcium sulfite, and there was no buildup on the inner wall of the absorption tower.

Comparative Example 2 Slurry compositions of the auxiliary absorption tower 2 are shown under the same gas conditions as in the example but with different absorbent slurry compositions.

As described above, if wet flue gas desulfurization is performed based on the present invention, the oxidation process becomes very simple, and depending on the use of the by-product gypsum, the oxidation process (i.e., oxidation tower 3) or the oxidation process and auxiliary absorption Tower 2 can be omitted.

The by-product gypsum obtained by omitting this process is also of high quality,
It plays well even as a board.

In addition, since oxidation occurs at a high pH, there is no material corrosion, which is also a great economic advantage.

Furthermore, since there is little calcium sulfite in the slurry in the absorption process, there is no build-up on the inner wall of the absorption tower, making stable continuous operation possible over a long period of time.

[Brief explanation of the drawing]

The accompanying drawing is a flow sheet illustrating an embodiment of the method of the present invention.

Claims (1)

[Claims] 1 When performing wet flue gas desulfurization using a basic calcium compound as the main ingredient and containing manganous salts or ferric salts and alkali metal sulfates as an absorbent slurry, it consists of an absorption tower and an auxiliary absorption tower. In the absorption step, p)I of the absorption step is in a slurry region of 4.5 to 6.5,
A flue gas desulfurization method characterized by blowing in a gas containing more than one equivalent of oxygen than calcium sulfite present in the system.