JPH0412165B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a wet desulfurization method using a magnesium compound. For more details,
The present invention relates to a wet desulfurization method for removing sulfur oxides by bringing combustion exhaust gas containing sulfur oxides into contact with an aqueous solution or dispersion containing magnesium sulfite. [Background of the Invention] Petroleum-based fuel or coal Since sulfur oxides ( _SO is actually used. Typical examples include a method in which sulfur oxides are recovered as gypsum using milk of lime, and a method in which sulfur oxides are absorbed into a sodium hydroxide solution, neutralized, and then discharged. Wet desulfurization methods using magnesium compounds have also been studied, and various methods have already been proposed. Most of these methods utilize magnesium sulfite (MgSO ₃ ) obtained by reacting an aqueous magnesium hydroxide solution with sulfur dioxide as a sulfur dioxide absorbing compound. This reaction can be expressed by the following formula. MgSO ₃ + SO ₂ + H ₂ O→Mg(HSO ₃ ) ₂ () Mg(HSO ₃ ) ₂ + Mg(OH) ₂ →2MgSO ₃ +2H ₂ O () In other words, sulfur dioxide is absorbed into magnesium sulfite and Mg(HSO ₃ ) ₂ (magnesium bisulfite), a circulation reaction is used in which Mg(HSO ₃ ) ₂ is returned to magnesium sulfite by contacting newly supplied magnesium hydroxide. Note that since the combustion exhaust gas also contains oxygen, a part of the magnesium sulfite is oxidized by the oxygen to become magnesium sulfate, which coexists in the circulating reaction liquid. During this circulation system, a portion of each of magnesium sulfite and magnesium sulfate (approximately the same amount in magnesium terms as newly supplied magnesium hydroxide) is extracted from the circulation system and subjected to an oxidation reaction. . This oxidation reaction converts magnesium sulfite into harmless magnesium sulfate, which is then discharged as wastewater. Alternatively, a method has been proposed in which sulfur oxides are recovered as gypsum using lime or the like from extracted magnesium sulfite and magnesium sulfate. As mentioned above, sulfur oxides (most of which exist as sulfur dioxide) are easily absorbed by magnesium sulfite, so in order to increase absorption efficiency, it is important to have as much magnesium sulfite as possible in the circulating fluid. desirable. However, it is known that magnesium sulfite tends to form a sparingly soluble hexahydrate (MgSO ₃ .6H ₂ O) and easily precipitate from the circulating fluid. The precipitated MgSO ₃ .6H ₂ O adheres to the walls of devices that make up the circulation system, such as circulating fluid reservoirs, pipelines, spray equipment, etc., and becomes scale, which tends to impede normal operation. Therefore, in the wet desulfurization method using a magnesium compound, there is a problem in that if the absorption efficiency of sulfur oxides in the combustion exhaust gas is to be increased, it is likely to become difficult to continue normal operation. For the above reasons, conventional methods operate while maintaining a low concentration of magnesium sulfite in the circulating fluid, which makes it difficult to obtain sufficient absorption performance, complicates the absorption tower, and There is also a problem when the amount of drainage increases. On the other hand, by taking a part of the circulating fluid outside the circulation system, oxidizing it to change magnesium sulfite to highly soluble magnesium sulfate, and returning a part of the oxidized liquid to the circulation system, the circulating fluid can be recycled. A method has been proposed in which the amount of waste water is reduced while maintaining a constant concentration of magnesium sulfite in the liquid, but this requires an external oxidation device.
Considering economic efficiency, this method is not necessarily a satisfactory method either. [Structure of the Invention] An object of the present invention is to provide a wet method for removing sulfur oxides in combustion exhaust gas using a magnesium compound. The main object of the present invention is to provide a method for absorbing and removing sulfur oxides in combustion exhaust gas, which reduces the generation of scale and improves the absorption efficiency of sulfur oxides. In particular, the present invention makes it possible to absorb and remove sulfur oxides with high absorption efficiency while suppressing scale formation by adjusting the ratio of magnesium sulfite and magnesium sulfate in the circulating fluid for sulfur oxide absorption. It is an object of the present invention to provide an improved method for wet removal of sulfur oxides in combustion exhaust gas using a magnesium compound, which makes it possible to reduce the amount of sulfur oxides in combustion exhaust gas. The present invention allows sulfur oxides to be absorbed into the aqueous solution or aqueous dispersion by bringing combustion exhaust gas containing sulfur oxide into contact with an aqueous solution or aqueous dispersion containing magnesium sulfite released into the gas from a spray device. Afterwards, this aqueous solution or aqueous dispersion is collected into a liquid reservoir, and then magnesium hydroxide or oxide is added to this collected liquid, and then this collected liquid is led to the spray device to absorb the above-mentioned sulfur oxides. This is an improved method for the circulating wet desulfurization method, which is used repeatedly for operations. A part of the recovered liquid containing magnesium hydroxide or oxide is pumped through a nozzle installed in the liquid reservoir. By ejecting it toward the bottom of the reservoir, it introduces oxygen present on the surface into the recovered liquid in the reservoir, and also suppresses the precipitation and growth of magnesium sulfite within the reservoir. This method consists of a wet desulfurization method using a magnesium compound. By spraying and adding the recovered liquid containing magnesium hydroxide or oxide to the bottom of the reservoir, not only forced stirring but also suppression of crystal growth of magnesium sulfite in the reservoir is achieved, and further recovery is possible as necessary. An advantage is that the pH of the liquid can be adjusted at the same time. It is advantageous to eject the recovered liquid in the form of a spray using a plurality of nozzles. This jetting can be done in any direction in the liquid reservoir, but
In order to efficiently introduce oxygen in the exhaust gas present on the surface of the recovered liquid in the liquid reservoir into the recovered liquid, it is advantageous to carry out the process toward the surface of the recovered liquid.
In addition, if it is also considered to suppress the accumulation of scale on the bottom of the device in the liquid reservoir, it is advantageous to carry out the treatment toward the bottom of the recovered liquid in the liquid reservoir. Next, the wet desulfurization method of the present invention will be explained in more detail with reference to an example of a wet desulfurization apparatus shown in the accompanying drawings. FIG. 1 is a schematic diagram showing a system of a wet desulfurization apparatus that can be used to carry out the wet desulfurization method of the present invention. Except for the forced stirring system placed in the liquid reservoir, this equipment is a conventional wet desulfurization system that removes sulfur oxides by bringing the flue gas containing sulfur oxides into contact with an aqueous solution or dispersion containing magnesium sulfite. Since there is no essential difference from the desulfurization equipment used in the method, the description will be given to the extent necessary for explaining the wet desulfurization method of the present invention. The wet desulfurization apparatus shown in FIG. 1 consists of a cooling dust removal section 11 and a desulfurization section 12. Combustion exhaust gas sent from a boiler etc. is introduced from the top of the cooling dust removal section 11, where it is subjected to cooling dust removal treatment and preliminary desulfurization treatment, and then the remainder of the submerged acid gas is desulfurized in the desulfurization section 12. It is discharged from the top. A spray device 13 is disposed above the desulfurization section 12. An aqueous solution or an aqueous dispersion containing magnesium sulfite is sprayed from this spray device 13 and comes into contact with the combustion exhaust gas that has been subjected to cooling dust removal treatment and preliminary desulfurization treatment. As a result, most of the sulfur oxides in the combustion exhaust gas are absorbed into the aqueous solution or aqueous dispersion. The above aqueous solution or aqueous dispersion is sent to the desulfurization section 12.
The liquid is collected in a liquid reservoir 14 provided at the bottom of the tank, and is temporarily stored there. Next, the recovered liquid in the liquid reservoir 14 is transferred to the spray device 13 by the circulation pump 15.
and 21 to be used repeatedly for absorbing sulfur oxides. Incidentally, magnesium hydroxide (or oxide) is added to the recycled recovered liquid during the circulation, thereby reinforcing the sulfur oxide absorption capacity. Further, a part of the recovered liquid is taken out to the outside, subjected to necessary oxidation treatment, etc., and then discharged by means such as discharge. In addition, the concentration of the magnesium compound in the recovered liquid is detected by a concentration detection device 17 attached to the circulation system, and if necessary, industrial water is replenished using the industrial water installed above the spray device 13 of the desulfurization section 12. The recovered liquid is replenished via the spray 18. In the wet desulfurization apparatus shown in FIG. 1, the recovered liquid to which magnesium hydroxide (or oxide) has been added is sent to a spray device 16 (hereinafter referred to as an underwater spray device) having a spout in the recovered liquid in the liquid reservoir. It plays the role of forcibly stirring the recovered liquid in the liquid reservoir 14 and suppressing the crystal growth of magnesium sulfite. The concentration, ratio, etc. of each component of the recovered liquid in the liquid reservoir 14 are as follows:
It is detected by a pH detection device 19 attached to the circulation system, and the amount of magnesium hydroxide is automatically adjusted based on this value. [Effect of the invention] According to the present invention, the recovered liquid to which magnesium hydroxide (or oxide) has been added is forcibly stirred using a method such as spouting it into the recovered liquid in a liquid reservoir. As a result, the recovered liquid in the liquid reservoir is vigorously stirred, and as a result, oxygen in the combustion exhaust gas present on the surface of the recovered liquid in the liquid reservoir is taken into the liquid, so that some of the magnesium sulfite in the liquid is oxidized. It is rapidly converted to magnesium sulfate. Therefore, the ratio of magnesium sulfite to magnesium sulfate in the collected liquid in the liquid reservoir can be easily adjusted by adjusting the above-mentioned stirring conditions, such as the ejection amount and ejection speed. By utilizing this operation, it is possible to increase the concentration of magnesium sulfite in the collected liquid in the reservoir within a range that can suppress the generation of scale, and by using such collected liquid as a circulating absorption liquid, It becomes possible to absorb and remove sulfur oxides with high absorption efficiency. In addition, the stirring action of the liquid reservoir suppresses the crystal growth of magnesium sulfite, making it possible to operate at high concentrations, which also makes it possible to absorb and remove sulfur oxides with high absorption efficiency. Furthermore, there is an advantage that the amount of absorbed wastewater can be reduced. Furthermore, by using a method of spouting the recovered liquid to which magnesium hydroxide (or oxide) has been added into the recovered liquid in the liquid reservoir, it is possible to adjust the pH of the recovered liquid in the liquid reservoir if desired. There is also the advantage of being Next, examples of the present invention and comparative examples will be shown. Example 1 The following experiment was conducted using a wet desulfurization apparatus (combustion exhaust gas processing capacity: 1000 Nm ³ /hour) having the configuration shown in FIG. Combustion exhaust gas: Exhaust gas from a furnace using petocoke as fuel, temperature 180-210℃, sulfur dioxide concentration
650ppm, oxygen content 14-17% by volume. Absorption liquid spray (in exhaust gas): Holo-cone type spray nozzles were installed in the cooling dust removal section 11 and the desulfurization section 12, respectively, facing downward, and the absorption liquid (circulated recovery liquid) was sprayed from each spray. Liquid/gas ratio: 3.6 to 4.2 (/Nm ³ ) Adjustment of PH and density of absorption liquid (recovered liquid): Adjusted by automatically adjusting the amount of drainage, industrial water supply amount, magnesium hydroxide supply amount, etc.
maintained constant. PH: 5.5 to 5.7 Density: 1075 to 1200 Kg/m ³ Underwater spray A Holo-cone type spray 16 is installed facing downward in the recovered liquid in the liquid reservoir, and serves as a spray plate for spraying the recovered liquid supplemented with magnesium hydroxide. , the replenishment recovery liquid was sprayed. Flow rate: 1m ³ /hour. Measurement: (1) Sulfur oxide concentration in exhaust gas and treated gas is
It was measured by turbidimetry specified in JIS-K-0103. (2) Magnesium sulfite (MgSO ₃ ) and magnesium hyposulfite (Mg
(HSO ₃ ) ₂ ) and magnesium sulfate (MgSO ₄ ) concentrations were calculated from KMnO ₄ titration and quantitative analysis of sulfur (S), respectively. Comparative Example 1 A flue gas desulfurization operation was carried out in the same manner as in Example 1, except that the underwater spray device was not used, while various adjustments were made, and the effect of the underwater spray device was investigated. However, the concentration of magnesium sulfite in the absorption liquid was adjusted to be at the same level as in Example 1. Example 2 A flue gas desulfurization operation accompanied by an underwater spray operation was carried out in the same manner as in Example 1, except that the concentration of magnesium sulfite in the absorption liquid was doubled, while making the same various adjustments. . [Measurement Results] Table 1 shows the measurement results in Examples 1 and 2 and Comparative Example 1.

[Table] From the results of Example 1 and Comparative Example 1, the desulfurization equipment was operated so that the desulfurization rate when using underwater spray was at the same level as the desulfurization rate when underwater spray was not used. In this case, the former is
It can be seen that it is possible to operate under conditions of a high concentration (salt concentration: magnesium sulfite) of 1.4 times (9.12/6.43). Therefore, when operating under these conditions, by installing an underwater sprayer in the liquid reservoir and using it for stirring, the amount of water discharged will be approximately 25% lower than when no underwater sprayer is installed.
It will be reduced to 70%. In Example 2, the operation was carried out while maintaining the concentration of magnesium sulfite at about twice that in Example 1. No scale trouble occurred during this operation, and a high desulfurization rate was achieved. In addition, since it was possible to operate under the conditions of a high absorbent salt concentration of about three times (19.4/6.43) compared to Comparative Example 1, the amount of waste water was reduced to about 33%.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a system of a wet desulfurization apparatus that can be used to carry out the wet desulfurization method of the present invention. 11: Cooling dust removal section, 12: Desulfurization section, 13: Spray device, 14: Liquid reservoir, 15: Circulation pump, 1
6: Underwater spray device (forced stirring means), 17:
Concentration detection device, 18: Industrial water replenishment spray, 1
9: PH detection device.

Claims (1)

[Claims] 1. By bringing combustion exhaust gas containing sulfur oxide into contact with an aqueous solution or aqueous dispersion containing magnesium sulfite released into the gas from a spray device, sulfur oxides are added to the aqueous solution or aqueous dispersion. After absorption, this aqueous solution or aqueous dispersion is collected in a liquid reservoir, and then magnesium hydroxide or oxide is added to this collected liquid, and then this collected liquid is led to the spray device to perform the above-mentioned sulfur oxidation. In a circulating wet desulfurization method that involves repeated use for absorbing substances, a portion of the recovered liquid to which magnesium hydroxide or oxide has been added is pumped into the liquid reservoir through a nozzle installed in the liquid reservoir. By ejecting it toward the bottom of the liquid reservoir, oxygen existing on the surface of the recovered liquid in the liquid reservoir is introduced, and precipitation and growth of magnesium sulfite in the liquid reservoir is suppressed. Wet desulfurization method using magnesium compounds. 2. Claim 1, wherein the spouting operation of the recovered liquid to which magnesium hydroxide or oxide is added in the liquid reservoir is carried out in a spray form using a plurality of nozzles.
Wet desulfurization method described in section.