JPS6018449B2 - Method for removing sulfur oxides from exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a wet flue gas desulfurization method using a crude basic magnesium compound.

A wet flue gas desulfurization method in which sulfur oxides in combustion exhaust gas are absorbed and removed using a basic compound such as magnesium or calcium as an absorbent is well known.

The disadvantage of this method is that the sulfite produced is poorly soluble in water, so if a practical amount of absorbent is used, it is necessary to operate with a slurry-like absorbent, which can lead to clogging of pipes and valves, tank walls, pipe walls, etc.
Problems such as scale formation on fillers, nozzles, etc., and abrasion of pumps, piping, and nozzles were caused. A method to improve this is to use magnesium compounds such as magnesium hydroxide and magnesium oxide as absorbents.
A method is known in which a part of the magnesium sulfite produced is oxidized to sulfate, and a part of it is discharged from the system while maintaining the sulfite concentration below the saturation concentration (Tokuko Kokō 5).
1-16192).

This method makes good use of the fact that magnesium sulfate has a solubility of about 1 lower than that of magnesium sulfite, and this method makes it possible to use the absorbent in the form of a solution. However, in this method, since the solubility of calcium sulfite is about 1/100 of that of magnesium sulfite, unmoated salts resulting from the basic calcium compound present as an impurity in the basic magnesium compound still remain as crystals. Because of the precipitation, a large amount of waste water had to be drained in order to operate the absorption liquid as a complete solution. Inexpensive crude basic magnesium compounds supplied industrially usually contain significant amounts of basic calcium compounds, for example crude magnesium hydroxide usually contains M
For g0 equivalent Mg content 60 to 65% by weight, Ca○ equivalent C
a minute to 0.5 to 2. % by weight, so the influence of insoluble salts based on calcium cannot be ignored.

(See Reference Example 1) Furthermore, magnesium compounds purified by removing calcium are extremely expensive and cannot be used industrially. The present inventors should improve the drawbacks of the flue gas desulfurization method using the above basic magnesium compound.
As a result of repeated studies, we found that by adjusting the pH of the absorption liquid to a range of approximately 3.0 to 4.5, we were able to suppress the precipitation of insoluble salts caused by calcium-based impurities, thereby preventing scale formation and reducing the amount of water discharged. succeeded in reducing the amount by half.

That is, the present invention allows sulfur oxides in exhaust gas to be absorbed into an absorbent liquid, circulates and uses the absorbent liquid while substantially completely oxidizing the absorbed sulfur oxides, and at least part of the absorbent liquid is transferred to the system. In the method of discharging outside and replenishing new absorbent, catalyst and make-up water, MgO equivalent M is used as absorbent.
For a g content of 65% by weight, a Ca content of 0.5 to 2.
The present invention relates to a method for removing sulfur oxides from combustion exhaust gas, which comprises adjusting the pH of the absorption solution to about 3.0 to 4.5 using an aqueous solution of a basic magnesium compound containing % by weight of sulfur oxides.

In the present invention, the crude basic magnesium compound refers to industrially produced and commercially available basic magnesium compounds such as magnesium hydroxide, magnesium oxide, magnesium carbonate, etc., and includes basic calcium compounds such as calcium hydroxide. , calcium oxide, calcium carbonate, etc. as impurities. Crude magnesium hydroxide usually contains about 60 to 65% by weight in terms of magnesium Mg0 and about 0% calcium in terms of Ca0.
．． 5-2. Contains % by weight of weir. Most of the sulfur oxides present in exhaust gas are sulfur dioxide, which is
In an aqueous solution with an H of about 4.5 or less, it cannot exist as sulfite ion, and most of it exists in the form of dissolved sulfur dioxide and bisulfite ion.

The solubility of magnesium bisulfite and calcium salts is very high, so in this method there is no need to take into account the precipitation of precipitates due to the formation of sulfites or bisulfites.
The absorption efficiency of sulfur dioxide in exhaust gas depends especially on the pH of the absorption liquid.
When it is low, it is greatly affected by the partial pressure of dissolved sulfur dioxide and hydrogen sulfite ions, so it is necessary to oxidize it to restore absorption capacity.

Suitable oxidation catalysts for such oxidation treatments are iron, cobalt, nickel, manganese, vanadium, and the like. These salts are dissolved in an absorption liquid, and the dissolved sulfur dioxide is brought into contact with air by an appropriate method such as a water jet method to oxidize it to sulfate ions. This lowers the partial pressure of sulfur dioxide in the absorption liquid and restores the absorption capacity, but on the other hand, the sulfate ions produced form calcium ions and poorly soluble salts, and when this exceeds a certain range, they precipitate as insoluble crystals. Therefore, a part of the oxidized absorption liquid is taken out of the system as wastewater. The solubility of calcium compounds in the sulfur dioxide absorption liquid is approximately twice as high in the pH range of 3 to 4.5 as in the case of pH 6 or higher, and even if we focus only on this point, the amount of discharged liquid can be reduced to approximately 1/2. I can do it. To describe the advantages of the method of the present invention in more detail, when the method described in Japanese Patent Publication No. 51-16192 is carried out using a crude basic magnesium compound, crystals based on the basic magnesium compound present are precipitated.

In order to prevent this, the concentration of sulfite ions may be reduced as shown in Reference Example 2. However, if the sulfite ion concentration is excessively lowered by oxidizing the sulfite ions, the pH buffering ability of the absorption liquid will be reduced, making stable pH control difficult. In the method of the present invention, the pH is adjusted to 3.0 to 4.5 and hydrogen sulfite ions with excellent solubility are produced, so there is no such difficulty. Furthermore, the combustion exhaust gas contains carbon dioxide gas, and at a pH of 6.5 or higher, this gas is absorbed by the absorption liquid to form carbonate, which is more soluble than sulfite.

This relationship is shown in the reference example. As is clear from Reference Example 33, when absorbing sulfur oxides using a crude basic magnesium compound, in order to prevent the formation of slurry in the absorption liquid and to reduce scale formation, at pH 7, a magnesium content of 65% by weight in terms of Mg○ is required. It is necessary to use magnesium hydroxide of good quality, in which the calcium content in the magnesium hydroxide is about 1% by weight or less, preferably about 0.5% by weight or less, calculated as Ca◯. Furthermore, it is necessary to strictly control the pH, concentration, and oxidation rate of sulfite ions of the absorption liquid within a narrow range. This management is extremely difficult in actual treatment of combustion exhaust gas, where the amount of exhaust gas fluctuates and the oxygen concentration in the exhaust gas changes. However, when the pH is set to 4.5 or lower as in the present invention, no carbonate ions can exist in the absorption liquid, and the formation of calcium carbonate can be ignored. Furthermore, in order to oxidize sulfite ions and hydrogen sulfite ions, a method is generally used in which metal ions such as iron, nickel, cobalt, manganese, and vanadium are oxidized with oxygen as a catalyst; Since it is quickly oxidized by oxygen and precipitated as a hydroxide or oxide, and the concentration of the oxidation catalyst decreases, it is necessary to replenish a large amount of catalyst continuously or intermittently. (See Reference Example 5). When operating at a pH of 4.5 or less as in the present invention, the decrease in catalyst activity is extremely small and economical. A specific example of an apparatus for carrying out the present invention is shown in FIG.

The exhaust gas containing sulfur oxides discharged from the combustion furnace 1 enters the absorption device 3 through the inlet flue 2 and comes into contact with the absorption liquid.

Here, sulfur oxides in the gas are removed by the absorption liquid. The droplets entrained in the gas are separated by the mist separator 4 and then discharged through the outlet flue 18. On the other hand, the absorption liquid in the absorption liquid tank 19 is sprayed onto the exhaust gas from the inlet flue by the absorption circulation pump 5 and the absorption liquid water supply pipe 6 (this spray is called the primary spray), and the sulfur oxides are absorbed. After that, return it to the absorption liquid tank. This sulfur oxide absorption liquid is injected from a nozzle onto the water surface of the absorption liquid tank by an absorption liquid circulation pump 8 through an absorption liquid water supply pipe 9 (this operation is called oxidation spray). When this liquid enters below the liquid surface, it attracts surrounding gas, resulting in the formation of microbubbles in the liquid.
The oxygen in the bubbles oxidizes sulfite ions, hydrogen sulfite ions, etc. in the absorption liquid. When seeking even higher sulfur oxide removal performance, the absorption liquid circulation pump 5 is used to spray the absorption liquid sent from the absorption liquid water pipe 7 (this is called secondary spray) into the gas to be treated. absorbs remaining sulfur oxides. Since the pH of the oxidized absorption liquid has decreased, it is sent from the absorbent storage tank 10 to the absorption liquid tank via the absorbent liquid sending pipe 11 and the absorbent liquid sending pipe 12, where the pH is adjusted. The absorption liquid whose sulfur oxide absorption performance has been restored through oxidation and pH adjustment accumulates sulfate during recycling and use, so a portion of it is discharged out of the system through the absorption liquid discharge pipe 16 to maintain a constant concentration. do.
The absorbent and catalyst reduced by the discharge are transferred to the liquid tank 19 through the absorbent liquid sending pipe 12 and the catalyst liquid sending pipe 15.
supply to. Water will be supplied through makeup water pipes. Reference example 1 2% by weight of calcium hydroxide as CaO
Sulfur dioxide gas is blown into the crude magnesium hydroxide (magnesium content 65% by weight calculated as Mg○) solution while maintaining the temperature at 60:00.

The sulfite ion (including hydrogen sulfite ion) concentration and calcium ion concentration are measured when the pH reaches 8.2, 6.8, and 6.3. The measurement results are shown in Table 1. Table 1 As is clear from Table 1, at pH 6.8, CaHO. 0
It is judged that calcium sulfite precipitation occurs when the amount exceeds 0.006mo.

Since the MgH at that time is approximately 0.28ho/so,
The molar ratio of Mg+10:CaH is 1:0.0023,
The weight ratio of Mg0:Ca○ is 65:0.22. Therefore, it can be seen that when crude magnesium hydroxide is used as an absorbent, impurities therein, that is, basic lucium compounds, are an important factor in slurry formation. That is, in the system of this example (S03--Toka S03-, pH 6.8), if the Ca content in terms of Ca○ in magnesium hydroxide (Mg content 65% by weight) is more than 0.22% by weight. There is a possibility of precipitation of crystals. Reference Example 2 A magnesium sulfite solution obtained by bubbling sulfur dioxide gas into a crude magnesium hydroxide (Mg content 65 parts by weight in terms of Mg0, calcium content in terms of Ca○ 22% by weight) solution is partially oxidized to produce sulfate ions and sulfite ions. and an absorption liquid containing hydrogen sulfite ions (Mg++ concentration 0.50-0.
55ho so/so, pH 5.9-6.0, temperature 60q○)
shall be.

Table 2 shows the results of measuring the concentration of C corresponding to sulfite ions in this solution. Table 2 From the above results, it can be seen that as the sulfate ion concentration increases, the solubility of calcium salts improves.

However, in the oxidation absorption method at pH 6.0, when operating under the conditions of an inlet S02 concentration of 1000 ppm and L/G (So/N) of 10, the lower limit concentration of sulfite ions that can be pH controlled is usually 0.1 mo. It is impossible to obtain a stable desulfurization rate at a concentration lower than this. S0
3--Toka S03-concentration 0.15ho〆/evening, pH6.
0. Under the conditions of temperature 6000, the acceptable purity of magnesium hydroxide is that the precipitation of calcium sulfite does not precede the precipitation of magnesium sulfite, based on the results in Table 2, the purity of magnesium hydroxide (Mg content 65% by weight in terms of Mg○) is Ca0 to
It is necessary that the converted Ca content is 1.0% by weight or less (
Calculation is performed in the same manner as in Reference Example 1). Reference Example 3 The magnesium sulfite aqueous solution obtained in Reference Example 2 was completely oxidized to 0. Shoe socks/magnesium sulfate solution.

This solution was kept at 60 oo and gas containing CO2 at various concentrations was blown into it to measure the relationship between pH and Ca+0 concentration. The results are shown in Table-3. Table 3 It is clear from Table 3 that when using a gas containing 15% CO2 (heavy oil boiler exhaust gas contains about 10 to 15% CO2) and the pH of the absorption liquid is 7.05, the concentration of Ca++ is 0. When the temperature is larger than 0055, calcium carbonate crystals are precipitated.

This shows that the calcium content in magnesium hydroxide with an Mg content of 65% by weight in terms of Mg○ must be 0.8% by weight or less in terms of Ca○. Reference Example 4 A magnesium sulfite solution was completely oxidized in the same manner as in Reference Example 2, and the resulting magnesium sulfate solution (0.25 to 2
．． As a result of determining the solubility of calcium sulfate in 5mo so/so), at a liquid temperature of 40 to 60°C and a pH of 5 to 2.5, C
aHO. 012-0.014mo evening/soon.

From the above measured values, the amount of calcium compound allowed in crude magnesium hydroxide (Mg content 65% by weight in terms of Mg0) is as shown in Table 4 when converted to Ca0. Table -
4 Reference example 5 Magnesium sulfite and magnesium hydrogen sulfite 0.17
Add 1 pm of divalent iron ion to 2.0 liters of absorbent solution containing ~0.1 chlorine (liquid temperature: about 60 °C), and blow air through a glass furnace plate at a rate of 1.0 min/min.

Table 5 shows the relationship between blowing time and FeH concentration. Table -
As shown in Table 5, the higher the pH, the faster the Fe++ reduction rate, which indicates that the amount of catalyst replenishment to the absorption liquid must be increased.

Example 1 C heavy oil (S content 2.5%) combustion boiler exhaust gas flow rate 40
00 to 5000 N per hour to the gas absorption device.

On the other hand, an absorbing liquid having the following composition is injected from the primary spray at a flow rate of 50 to 100 per hour, and brought into contact with exhaust gas in the scrubber (gas flow rate of 14 to 17 skins/second). Next, the absorption liquid was sprayed onto the liquid surface using 10 nozzles with a diameter of 6 ribs (at a speed of 20
/hour) gas-liquid contact with the exhaust gas at a gas velocity of 5 to 7 /second (oxidizing spray). Through this operation, the S02 component is oxidized with oxygen in the exhaust gas drawn in at the same time. Then 1% by weight crude magnesium hydroxide slurry was added to adjust the pH.
Adjust. The absorption liquid whose absorption capacity has been restored through the above operations is again guided to the gas absorption device by a pump and used for circulation, and a portion of it is discharged so that the magnesium concentration of the absorption liquid is kept constant. Boiler exhaust gas inlet composition and temperature 170-190 OS
02 1300-150 other pages
m02 3~4%CQ
13-15%QO
9-10% absorption liquid composition and processing conditions Absorbent Magnesium hydroxide Mg○ equivalent Mg69% by weight Ca female calculation Ca0, amount of flies % MgH 0.45-0.58ho〆/SoHS〇3- 0.0001-0 .00
08h.

So/so FeH ten FeH ten 10
~15ppm temperature 5
7-5 scales ○ pH (top of tower) 4, 3.75,
3.5 3 Change the primary spray nozzle to adjust the liquid-gas ratio (L
The S02 concentration at the entrance and exit of the absorption tower was measured while changing the L/G ratio), and the relationship between L/G and the S02 removal rate was determined.
The results are shown in Table-6. Table 6 Continuous operation was performed for two weeks under the conditions of pH 3.75 and L/G 20, but no crystals were deposited and no scale was generated.

It has also been found that HS03- can be maintained at 0.0001 mo or less and oxidation can be sufficiently achieved.
Example 2 The oxidation step is carried out outside the absorption tower (absorption liquid flow rate: 20 w/w)
The relationship between L/G and desulfurization rate was measured in the same manner as in Example 1, except that the oxidation nozzles: diameter 6 side, 10 nozzles).

The results are shown in Table-7. Table 7 After two weeks of continuous operation at pH 3.75 and L/G 20, no crystals or scale were observed.

Note that the HS03 concentration is o. oool~o. 000 shoes o
I was able to hold the saw. Example 3 Next spray 75〆/hour (L/GI5〆/N〆),
Other than setting the oxidation absorption spray to 20/hour and further varying the secondary spray in the range of 25 to 50/hour,
The relationship between L/G and desulfurization rate was measured in exactly the same manner as in Example 1.

The test results are shown in Table-8. Table-8 Example 4 The desulfurization rate was measured by changing L/G in the same manner as in Example 1 except that the magnesium sulfate (Mg++) concentration was increased to lmo.

The test results are shown in Table-9. Table 9: When the magnesium sulfate concentration is set to lmo/so, pH control becomes possible down to 4.9 degrees (control becomes unstable when the MgH concentration is above 0.5 degrees/so).

Although continuous operation was carried out for two weeks at pH 4.5, no occurrence of calcium sulfate crystals or scale was observed.

In addition, the Ca10 concentration in this absorption liquid is 0.007 to 0.0
0 shoes o〆/sleeves. Example 5 Fuel oil C (S content: 2.5% by weight) combustion exhaust gas was cooled by water spray, and an empty tower was placed in a Terraret L-type packed tower (cross-sectional area: 1, filling height: 2) filled with filler. Speed 1.5~
Exhaust gas is introduced at a rate of 2.0 m/sec, and sulfur oxides are absorbed while the magnesium sulfate-sulfuric acid mixture is flowing down.

The absorption liquid is injected onto the liquid surface of the absorption liquid at a flow rate of 20/hour using an oxidation nozzle (diameter 6 side, 1 solid) outside the absorption tower, and is oxidized by the entrained and absorbed air in the atmosphere. Add 1% by weight crude magnesium hydroxide slurry to the absorption liquid, adjust the pH to a predetermined value, and spray again from the top of the absorption tower using the upper pump. Boiler exhaust gas inlet composition and temperature 170-19pi OS0
2 1300-150 pm
Absorbing liquid absorbent: Magnesium hydroxide with the same composition as in Example 1 was used as Mze+ 0.5 to 0.8ho/SoSQ--+HS03 0.0001 to 0.0
00 legs o〆/kuFe++++Fe++++
10-15ppm temperature
56-5 p○pH
4, 3.75 The relationship between the L/G of the absorption tower and the desulfurization rate in operations of 3.5 or higher was measured, and the results are shown in Table 10.

Table-lo Comparative Example 1 Using a spray tower and a bottom pressure drop venturi scrubber, the injection from the oxidizing nozzle is stopped and sulfur oxides are absorbed by the sulfite method.

The pH of the absorption liquid is adjusted to 7.0, 6.5, and 6.0. Primary spray L/G is 8-10, secondary spray L/G
is 2 to 3. Other than the above, treatment was carried out in substantially the same manner as in Example 1, and L/G and desulfurization rate were measured. The results are shown in Table 11. In all of Table 11 (A), (B), and (C), the liquid concentration was measured and the discharge amount was adjusted so that magnesium sulfite crystals were not formed in the absorption liquid.

Although no magnesium sulfite crystals or scale were generated during the absorption process, magnesium sulfite crystals were deposited on parts that are cooled by the outside air, such as the exhaust pipe.

The concentration of sulfite ions is 20-25oo, (A) 0.1moso/so, (B) 0.15moso/so, (C) 0.25
Mo〆/sleeve, and depending on the difference, crystals of magnesium sulfite are deposited in the pipe. In addition, as a result of continuous operation for one week under the condition of pH 6.5, calcium sulfite crystals precipitated and scale appeared on the nozzle and pipe walls. In this case, scaling occurred after one week of continuous operation. Comparative Example 2 The oxidation rate of sulfite ions in the absorption liquid was changed by changing the spray pressure of the oxidation spray nozzle.

Adjust the pH of the absorption solution to 6.5. The desulfurization rate was measured in the same manner as in Example 1 except that the primary spray of the absorption liquid was L/G 8 to 10 and the secondary spray L/G was 2 to 3. The MgH concentration of the absorption liquid was adjusted to 0.4 by adjusting the amount of absorption liquid discharged.
~0.5ho so/kuko adjustment. Table 12 shows the relationship between the sulfite + hydrogen sulfite ion concentration of the absorption liquid and the desulfurization rate.
Table 12 When the S03--Toka S03- is below 0.1 mo, the desulfurization rate decreases, and further below 0.07 mo, the pH decreases.
Control became difficult.

S03--Toka S03--0.2~0.3mo so/so and 0.10~0.19ho evening/1 each under the conditions
After continuous operation for a week, scale formation was observed on the 0.2-0.3mo sleeves, and 0.10-0.19ho
That/sleeve was not recognized.

In addition, under conditions of pH 7 or higher, the sulfite ion concentration is 0.10 h.
At temperatures above o/c, calcium sulfite scale formation was severe, and 3 out of 21 nozzles were clogged with scale within one week.

Furthermore, at pH 7.5 or higher, the nozzle will become clogged with calcium carbonate scale in a few days, regardless of the concentration of sulfite ions.
As the gas temperature at the outlet of the absorber rose, the rotation was stopped.
Magnesium concentration in absorption liquid 0.15-0.2mo/
When the operation was carried out at pH 6.5, sulfite ions were 0.1 ㎇〆/ Below that, no scale formation was observed after one week of continuous operation, but sulfite ions were 0.1 ㎇
Above that level, scale formation was observed.

From the above results, when using a crude basic magnesium compound as an absorbent for sulfur oxides in combustion exhaust gas, the pH of the absorbent should be adjusted to about 4 to prevent crystal precipitation and scale formation due to the calcium compounds present. It is understood that carrying out the oxidation absorption method under conditions of .5 or less is extremely effective.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing the flow of gas and absorption liquid when carrying out the exhaust gas treatment method of the present invention. In the figure, each number indicates the following device. 1... combustion furnace,
2... Inlet flue, 3... Gas absorption device, 4...
Mist separator, 5...Absorption liquid circulation pump, 6...Absorption liquid circulation water pipe, 7...Absorption liquid circulation water pipe, 8...Absorption liquid circulation pump, 9...Absorption liquid Circulating water pipe, 10...absorbent storage tank, 11
...Absorbent liquid feeding pump, 12...Absorbent liquid feeding pipe, 13...Catalyst liquid storage tank, 14...Catalyst liquid water feeding pipe, 15...Catalyst liquid feeding pipe, 16... Absorption liquid discharge pipe, 17...Makeup water supply pipe, 18...External flue, 19...Liquid tank. Figure 1

Claims (1)

[Claims] 1. Absorbing sulfur oxides in exhaust gas into an absorption liquid, circulating and using the absorption liquid while substantially completely oxidizing the absorbed sulfur oxides, and discharging at least a portion of the absorption liquid to the outside of the system; In the method of replenishing new absorbent, catalyst, and make-up water, an aqueous solution of a basic magnesium compound containing 65% by weight of Mg content in terms of MgO and 0.5 to 2.0% by weight of Ca content in terms of CaO as an absorption liquid. A method for removing sulfur oxides from combustion exhaust gas, the method comprising adjusting the pH of the absorption liquid to 3.0 to 4.5.