JPH02229527A - Method for simultaneously removing sulfur oxide and nitrogen oxide - Google Patents

Abstract

PURPOSE:To simultaneously remove SOx and NOx in exhaust gas by previously blowing gaseous chlorine into the exhaust gas and bringing this exhaust gas into contact with an aq. NaOH soln. contg. ammonium ions and bromine ions. CONSTITUTION:Gaseous chlorine is previously blown into exhaust gas to be wet-treated. This exhaust gas is brought into contact with an aq. NaOH soln. contg. ammonium ions and bromide ions as an absorbing soln. Since both the gaseous chlorine and bromide ions are present, reactive nitrosyl bromide is formed and reacts with ammonia, SOx and NOx can be simultaneously removed and a high rate of removal of NOx is attained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to the treatment of sulfur oxides (SO
The present invention relates to a method for effectively removing x) and nitrogen oxides (NOx) simultaneously by a wet method, thereby contributing to improving the atmospheric environment.

[Prior Art and Problems] Conventionally, as this type of simultaneous removal technology of SOx and NoX in exhaust gas, wet NH3 absorption method, complex salt absorption method,
Sulfuric acid and nitric acid methods are known. Further, as dry methods, an activated carbon method, an electron beam irradiation method, etc. have been proposed. However, as is well known, simultaneous removal of SOx and NOx is extremely difficult in both wet and dry methods, and this technology has not yet been established or put to practical use.

In the NH3 absorption method, which is known as a wet method, SOX is absorbed by NH3 water, but for NOx, ozone is also added in advance, NO is oxidized to NO2, and then this is absorbed with NH3 water. This is the way to do it. The problem with this method is that even after ozone oxidation, the NOx absorption efficiency is not very high and the NOx removal rate is low. Complex salt absorption method is E
Although DTA is used and the sulfuric acid/nitric acid method is produced using nitrosyl sulfuric acid as an intermediate, these methods also have a low NOx removal rate.

Regarding dry methods, all of the methods listed above are still in the development stage and have not been put into practical use.

[Means for solving the problem] One of the methods for simultaneously removing SOx and NOx from exhaust gas according to the present invention relates to wet treatment, in which chlorine gas (C/2) is added in advance to the exhaust gas to be treated. The method is characterized in that the exhaust gas is then brought into contact with an absorption liquid consisting of an aqueous solution of caustic soda containing ammonium ions (NH4'') and bromide ions (Br).

According to the method of the present invention, not only is SOx removal and NOx removal almost completely achieved, but in particular, SO2 in SOx is oxidized and absorbed by S03 to form sulfates.

Therefore, it is not necessary to oxidize the sulfite in the solution after SOx absorption to sulfate as in the case of absorption using a normal alkaline aqueous solution.

Regarding the removal of NOx, the mechanism is not necessarily clear, as will be described later, but there is a strong possibility that nitrogen and water are ultimately reduced, and the reaction presents a unique reaction situation of reduction in a low temperature range.

Another method according to the present invention is characterized in that bromine gas (Br2) is added in advance to the exhaust gas to be treated, and then the exhaust gas is brought into contact with an absorption liquid made of a caustic soda aqueous solution containing NH4+. .

The difference between this second method and the first method described above is that the additive gas injected into the exhaust gas has changed from CI2 to Br2, and the absorption liquid has changed from an NH4+-containing caustic soda aqueous solution containing Br- to a Br--containing aqueous solution containing Br-. It has become something that does not exist. The components in these two methods are relatively similar;
The reaction mechanism will also be similar.

The absorption of SOx, that is, the absorption of S02 and S03 in the first method is as follows.

C/2 +2NaOH- Na C / O+Na C / +H2 0・-
・−−(1>SO2 +NaC/O+2NaOH −
N a 2 S O 4 + N a C / +
H 2 0 − − (2)? O3 +2NaO
H − Na2 SO4 +H2 0・・・・・・・・・
・・・・・・・・・(3) Equation (1) is the absorption of CI2 into caustic soda, Equation (2) is the oxidation absorption of S02 by NaC/0, and Equation (3) is the absorption of CI2 into caustic soda. It is absorption into. Furthermore, according to this invention, B in the absorption liquid
``Because - and NH4+ are included, in addition to the above reactions, the following reactions occur, accelerating the oxidation and absorption of SO■,
More efficient SOx removal is achieved.

C/2 +2Br-→Br2+2C/-=(4) Although not clear, it is an important factor in NOx absorption.

Next, looking at NOx absorption, almost no oxidative absorption occurs only by the NaC/0 and caustic soda absorbent produced by the formula (l), and NOx removal is not achieved. That is, NOx removal is not achieved when Br-NH4+ is not present in the absorption liquid.

This reaction requires the presence of both Br- and NH4'', and if either one is missing, NOx will hardly be removed.

C/2 +28r- → Br2+2C/-・・・・・・・・・(4) NO+1/
2Br2 → NOBr (6) SO2
+Br2 +4NaOH-e-Na2 SO4
+2NaBr+2H2 0・・・・・・(5) The effect of NH4+ in SOx absorption method is NOBr+
NH3 +H2 0→ NH4 NO2 +HB r... (7) As shown in these reaction equations, Br2 produced in equation (4) produces highly reactive nitrosyl bromide (NOB r) death,
This NOBr reacts with NH to generate NH4NO2. Here, when a caustic soda aqueous solution containing NH4+ is used, NOx is significantly reduced compared to when it does not contain NH4+.
High removal rate. Even though they are the same basic substances, NH40H and NaOH have distinct differences in their effects. The reason for this is not clear, but in the case of NH40H, NOB r has a partial pressure of NH3 in the gas-liquid boundary layer.
This is thought to be a factor that increases the reactivity with and the rate of absorption. In this sense, in the above formula (7), the reactant is expressed as NH3 instead of NH4+.

NH4NO2 is a substance that is very easily decomposed as shown in the following formula (8).

NH4No2→N2 + 2 H2 0・・・・・・(
8) Also in the results of the examples described later, the amounts of NO2- and NO3- on the drainage side of the absorption liquid were qualitatively very small.

In the second method, Br2 is not generated by Br- and C/2, but Br2 is added directly to the exhaust gas, and SO x and NOx are absorbed without going through C/2. It's a method.

Removal of SOx is performed using equation (5), and removal of NOx is performed using equation (8) (
7) This is considered to be based on equation (8).

Here, important reaction conditions when carrying out the first and second methods include the amount of C/2 or Br2 injected into the exhaust gas in advance, the Br-fl and NH4 dish in the absorption liquid, and is the pH value adjusted by caustic soda. These conditions are shown below.

1) C/2 amount; C/2/(S 02 +1/
2 No) Molar ratio 1 or more 2) Brz amount; B r2 / (S 02 +1/2
No) Molar ratio 1 or more 3) NH4+ amount; NH4''/No molar ratio 1 or more 4) Absorbent inlet pH; 3 to 9 (adjusted with caustic soda)> In the process of implementing this invention, C/2 and Br2 are not consumed. C/2 and Br2 change their form within the system and are captured in the absorption liquid.Therefore, this C/2 and Br2 can be recycled and used by diaphragm electrolysis.

NH4" is used as a reducing agent for NOx, so it is necessary to replenish the amount consumed.NH4 sources include NH,
, NH40H can of course be used, but industrially, salts such as ammonium sulfate and ammonium chloride can be used economically and without any problem in terms of reaction.

As the type of absorption tower in the present invention, various kinds of absorption towers, such as a packed tower such as a Schichling, a spray tower, and a tray tower, which are commonly known, are used. The type of absorption tower to be used is determined in relation to the operating conditions of these absorption towers, ie, liquid-gas ratio, gas superficial velocity, etc.

[Effects of the Invention] According to the method of the present invention, simultaneous removal of SOx and NOx, which has been difficult with conventional wet methods, can be achieved economically while maintaining a particularly high NOx removal rate.

[Example] Example 1 A SOx and NOx removal test was conducted using the test apparatus shown in FIG. Here, as SOx and NOx, s02 and NO, which account for most of SOx and NOx in the exhaust gas, were used, and SO3 and NO2 were not mixed.

The reaction column (1) was made of a Vilex glass tube with a diameter of 30 mm and a height of 510 mm, and was filled with spherical glass beads having a diameter of 2 lal to a height of 331 mm. Absorption tower (2
) is of exactly the same type and size as the reaction column (1). Both the reaction tower (1) and the absorption tower (2) have hot water jackets (3) and (4), and are maintained at a predetermined temperature (70°C). The reaction tower (1) is a device for adding C/2 or Br2 into the reaction gas in advance.
The reaction solution of 1) does not directly absorb S02 and No. On the other hand, the absorption tower (2) is a device that absorbs S02 and NO. The removal rate of S02 and the removal rate of No were determined by measuring the respective concentrations before and after the absorption tower (2).

Table 1 shows common standard reaction conditions in this test device. In the same table, case A is Ce in the test exhaust gas.
Case B is the condition when Br2 is added (first method), and case B is the condition when Br2 is added (second method). Looking at the operating conditions of the absorption tower, in both Case A and Case B, the B concentration was 50 mg//, but in Case B, Br- was not added. Br- and NH4
'The sources are KBr and (NH4)2S, respectively.
O4 was used.

The results for Case A are shown in Table 2. As is clear from the table, a high S02 removal rate and a high No removal rate can be obtained. However, when the S02 concentration is 500 ppm, C/2 is consumed by S02, resulting in a C/2 shortage, and the No removal rate is reduced. The results for Case B are shown in Table 3. In this case, the S02 concentration is 500 pp
A high No removal rate can be obtained even with m.

Comparative Example 1 Using the apparatus of Example 1, under the conditions shown in Table 1, ? The experimental exhaust gas bypassed the reaction tower (1), that is, was introduced directly into the absorption tower (2) without adding C/2 and Br2.

The absorption liquid in the absorption tower (2) was adjusted to p119 with caustic soda, and NH4 (10+sg//) and Br- (
Tests were conducted in each case with and without 50 mg//). The results are shown in Table 4. Effective S02 and No removal is not achieved in systems without the addition of C/2 and Br2.

Comparative Example 2 Using the apparatus of Example 1, No. 2 in Table 2 was used. Based on the gas composition of No. 2, a test for removing SO■ and NO was conducted without adding NH, + and Br- to the absorption liquid. As a result, the S02 removal rate was 99.5% and the NO removal rate was 4.3%.
I got it.

That is, it can be seen that when NH4+ and B'- are not included in the absorption branch, effective NO removal is not achieved.

Comparative Example 3 Using the apparatus of Example 1, No. 3 in Table 3 was used. From the gas composition of 2, without adding NH4+ to the absorption liquid, S02
and No. removal tests were conducted. As a result, an S02 removal rate of 99.5% and a No removal rate of 3.2% were obtained. In other words, in a system where NH4+ is not included in the absorption liquid, the effective N
It can be seen that O removal is not achieved.

Table 1 (Margin below) Table 2 Table 3 4.

[Brief explanation of the drawing]

The drawing is a flow sheet showing an embodiment of this invention. It is. Below Up

Claims (2)

[Claims] (1) A method for simultaneous removal of sulfur oxides and nitrogen oxides, characterized by adding chlorine gas in advance to various combustion exhaust gases, and then bringing the exhaust gases into contact with a caustic soda aqueous solution containing ammonium ions and bromine ions. . (2) A method for simultaneously removing sulfur oxides and nitrogen oxides, which comprises adding bromine gas to various combustion exhaust gases in advance, and then bringing the exhaust gases into contact with a caustic soda aqueous solution containing ammonium ions.