JPH04346816A - Treatment of exhaust gas - Google Patents

Abstract

PURPOSE:To remove simultaneously carbon dioxide and sulfur oxides from a exhaust gas by using potassium carbonate as a absorbent of carbon dioxide and by separating potassium salt of sulfur oxides, which is fixed with calcium ion, to suppress loss of potassium. CONSTITUTION:Carbon dioxide and sulfur oxides in an exhaust gas is brought into contact with the absorbing solution 3 containing potassium carbonate in the absorption column 2 and is absorbed thereinto. Sulfur oxides absorbed into potassium carbonate further reacts with oxygen in the exhaust gas to form potassium sulfate. The carbon dioxide and sulfur oxides absorbed absorbing solution is sent to the desorbing column 5 and heated by the heating medium 6 to exhaust carbon dioxide desorbed through the desorbing duct 7. On the other hand, a part of the absorbing solution containing crystalline potassium sulfate is introduced into the solid-liquid separater 8 where solid potassium sulfate is separated and furthermore reacts with calcium carbonate or calcium hydroxide at a reactor 9 to form calcium sulfate. Solid calcium sulfate is separated from the slurry introduced into the solid-liquid separater 10.

Description

Description: FIELD OF THE INVENTION The present invention relates to an exhaust gas treatment method for simultaneously removing sulfur oxides and carbon dioxide from the exhaust gas of a boiler or the like. [0002] The present invention takes into consideration the problem of global warming caused by large amounts of carbon dioxide emissions and the problem of acid rain caused by sulfur oxides, which have been drawing attention rapidly in recent years. There is interest in developing methods to remove it. [0003] Carbon dioxide has traditionally been considered to be harmless, and therefore there are few technologies that dare to remove it from exhaust gas, and at present, the technology that removes it at the same time as sulfur oxides has not yet reached the stage where it can be used industrially. . [0004] Therefore, as a prior art, a technology for treating carbon dioxide and sulfur oxides separately will be briefly described here. Current techniques for removing carbon dioxide from exhaust gas include a wet chemical absorption method using an organic absorbent such as amine, and a dry adsorption method using an adsorbent. On the other hand, the mainstream technology for removing sulfur oxides from exhaust gas is a wet lime plaster method using calcium carbonate as an absorbent. [0005] In principle, it is possible to simultaneously treat carbon dioxide and sulfur oxides using a wet chemical absorption method using an organic absorbent such as an amine, but in this case, the following problems arise. has. ■ Organic absorbents generally decompose in the presence of sulfur oxides, resulting in large losses of the absorbent. ■ In general, wet absorption methods produce a large amount of wastewater, but when organic absorbents are used, COD (chemical oxygen demand) components resulting from the absorbent or the decomposition products of the absorbent are contained in the wastewater. worsen the condition. Furthermore, although it is theoretically possible to simultaneously absorb carbon dioxide and sulfur oxides with potassium carbonate in the same way as with organic absorbents such as amines, the absorbed sulfur oxides absorb potassium and potassium from the absorbent. It has the problem of forming salts, reducing its ability as an absorbent and increasing potassium loss. SUMMARY OF THE INVENTION In view of the above-mentioned state of the art, the present invention makes it possible to simultaneously remove carbon dioxide and sulfur oxides from exhaust gas, which can eliminate the problems of the prior art and methods conceivable from the prior art. This paper aims to provide a method for doing so. [Means for Solving the Problems] In the above-mentioned situation, the present inventors have discovered that a sulfur oxide is stable even in the coexistence of sulfur oxide,
In addition, we focused on potassium carbonate, which is a carbon dioxide absorbent that does not cause COD components in wastewater, and in order to further suppress the loss of potassium, we separated the potassium salt of sulfur oxide.
A method of immobilizing calcium ions was discovered, and as a result, the present invention solved the problems by the following means. That is, the present invention provides a method for treating exhaust gas containing sulfur oxides and carbon dioxide with an absorption liquid containing potassium carbonate, including: (1) an absorption step for removing sulfur oxides and carbon dioxide from the exhaust gas; ) Diffusion step of heating the absorption liquid extracted from the absorption step to diffuse carbon dioxide (3) A first separation step of separating solidified potassium sulfate from the absorption liquid extracted from the diffusion step (4) In the first separation step A second separation step of reacting the obtained potassium sulfate with calcium hydroxide and/or calcium carbonate and separating the produced calcium sulfate (5) A step of reusing the mother liquor obtained in the second separation step in the absorption step. This is a method for treating exhaust gas characterized by the following. [Operation] FIG. 1 shows an embodiment of the present invention, and the operation of the present invention will be explained with reference to FIG. Carbon dioxide and sulfur oxides in the exhaust gas led from the inlet duct 1 come into contact with an absorption liquid 3 containing potassium carbonate in an absorption tower 2, and are absorbed according to the following reaction formula. (Carbon dioxide) CO2 +K2 CO3 +H2 O → 2KH
CO3 (sulfur oxide) SO2 +2K2 CO3 +H2 O →K2 S
O3 +2KHCO3 K2 SO3 + 1/2O2 → K2SO4
[0011] Sulfur oxides produce potassium sulfite when absorbed by potassium carbonate, and further react with oxygen in the exhaust gas to produce potassium sulfate. If potassium sulfite is not oxidized sufficiently due to reasons such as low oxygen concentration in exhaust gas or high sulfur oxide concentration, add air or oxygen from outside the system to the absorption liquid to remove sulfur dioxide. It may also oxidize potassium. The absorbed exhaust gas is discharged from the outlet duct 4. The absorption liquid that has absorbed carbon dioxide and sulfur oxides is sent to a stripping tower 5 and heated by a heating medium 6 such as steam. Carbon dioxide is diffused from the heated absorption liquid according to the following reaction formula, and is discharged through the diffusion duct 7. However, since potassium sulfate has no partial pressure, it is not dissipated. 2KHCO3 → CO2 +K2 CO3 +
On the other hand, at this time, the concentration of potassium sulfate in the absorption liquid is maintained at a concentration higher than its solubility, and crystals of potassium sulfate are present in the absorption liquid. This is figure 2
As shown in Figure 2, this technique cleverly takes advantage of the fact that potassium sulfate has a lower solubility than potassium carbonate or potassium bicarbonate. A portion of the absorption liquid containing crystals of potassium sulfate is led to a solid-liquid separator 8, where solid potassium sulfate is separated, and the mother liquor is returned to the stripping tower 5. The separated potassium sulfate reacts with calcium ions provided by calcium carbonate or calcium hydroxide in the reactor 9 as follows to produce calcium sulfate. K2 SO4 +Ca2+ → CaSO4 +2
K+ Calcium sulfate crystallizes because of its low solubility. This slurry is again introduced into the solid-liquid separator 10 to separate solid calcium sulfate 11. The mother liquor is returned to the stripping tower 5. Note that the solid-liquid separator 8 and the solid-liquid separator 1
In this embodiment, the mother liquor from 0 is returned to the stripping tower 5, but even if it is returned to the absorption tower 2, there is no major change in principle or performance. [Example] Specific examples will be given below, and the specifications of the apparatus used therein will be shown. (a) Absorption tower 2 Inner diameter 0.13m x height 8.0m (effective contact area 6.0m) (b) Stripping tower 5
Inner diameter 0.13 m x height 8.0 m (pressurized steam heating) (c) Solid-liquid separator 8 Centrifugal separation method; throughput 20 liters/h (d) Solid-liquid separator 10 Centrifugal separation method; throughput 20 liters /h [0017] An example of the operating state of the embodiment is shown below.・Exhaust gas properties 1 Flow rate: 200m3 N/h (dry) Composition: CO
2: 14.5vol% (dry)
SO2: 800ppm
(dry)
O2: 4.5 vol% (dry)・Absorption tower 2
Circulation flow rate: 3.5m3/h
temperature
; 55℃
Pressure; atmospheric pressure
Potassium concentration: 2.3 kgmol/m3
- Stripping tower 5 supply flow rate; 3.
5m3/h Temperature: 100℃
Pressure; Atmospheric pressure [001
8] With the above apparatus and operating conditions, the removal rate of carbon dioxide from exhaust gas was 35% or more, and the removal rate of sulfur oxides was 99% or more. In addition, the amount of calcium component used was 0.7 kg/in the case of calcium carbonate.
In the case of calcium hydroxide, it was 0.5 kg/h. Furthermore, the carbon dioxide gas recovered as a by-product is 9
m3 N/h, and calcium sulfate was 1.3 kg/h. [Effects of the Invention] As specifically described in the examples above,
The present invention uses potassium carbonate, which is a carbon dioxide absorbent that is stable even when sulfur oxides coexist and does not produce COD components in wastewater. This method separates potassium salts and immobilizes them with calcium ions, making it possible to simultaneously remove carbon dioxide and sulfur oxides from exhaust gas, which had not been commercially practical in the past.

[Brief explanation of the drawing]

Claims (1)

[Claims] Claim 1: A method for treating exhaust gas containing sulfur oxides and carbon dioxide with an absorption liquid containing potassium carbonate, comprising: (1) an absorption step for removing sulfur oxides and carbon dioxide from the exhaust gas; (2) an absorption step. Diffusion step of heating the extracted absorption liquid to diffuse carbon dioxide (3) First separation step of separating solidified potassium sulfate from the absorption liquid extracted from the diffusion step (4) Sulfuric acid obtained in the first separation step A second separation step of reacting potassium with calcium hydroxide and/or calcium carbonate and separating the produced calcium sulfate (5) A step of reusing the mother liquor obtained in the second separation step in the absorption step. How to treat exhaust gas.