JPH0691941B2 - Wet removal method of nitrogen oxides in various combustion exhaust gas - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to nitrogen oxides (NO) in combustion exhaust gas discharged from various boilers, various heating furnaces, and dust-burning furnaces.
x), especially nitric oxide (NO) is effectively removed, which contributes to the improvement of the environment.

Conventional technology and its problems Conventionally, as a NOx removal technology for this kind of exhaust gas, dry denitration methods such as catalytic reduction denitration method and non-catalytic reduction denitration method have been widely adopted. As is well known, the catalytic reduction method is a method in which a titanium / vanadium catalyst is used at a reaction temperature of 300 to 400 ° C. and ammonia is injected as a reducing agent. With this method, a denitrification rate of 90% or more was obtained, and there are many actual results. However, this system has problems that the equipment cost including the catalyst cost is high, and that a sophisticated control system is required. Therefore, it is often used for treating large-volume exhaust gas,
There are few examples of medium- and small-volume exhaust gas treatment.

On the other hand, the non-catalytic reduction denitration method is a method of injecting ammonia into a combustion furnace or a flue at about 1000 ° C., and it has a lot of results especially in a dust-burning furnace. However, the denitrification rate is 50 with this method.
%, Which is as low as about 10% or less and its use is limited.

In contrast to the above-mentioned dry denitration method, the wet denitration method has a low reaction temperature, relatively low equipment cost, and easy operation. It can be a very preferable method as a denitration method for treating exhaust gas.

In view of the above situation, the present inventors have previously proposed a method of treating exhaust gas with an oxidizing agent-containing aqueous solution containing bromine ions (Japanese Patent Application No.
-24146), a method of treating exhaust gas with an oxidizing agent-containing liquid containing ammonium ions or an oxidizing agent-containing liquid containing ammonium ions and bromine ions (Japanese Patent Application No. 63-14).
7606) was proposed.

The present invention is an extension of the invention of the above-mentioned patent application, and as a result of pursuing a more effective NOx removal method, chlorine is added to the exhaust gas to be treated in advance, and then ammonium ions and bromine are added. The inventors have found that bringing the above chlorine-containing exhaust gas into contact with an absorbent containing an aqueous caustic soda solution containing ions has a remarkable effect on denitration, and have completed the invention.

In the method for removing NOx from exhaust gas according to the present invention, when wet processing the exhaust gas, chlorine is previously added to the exhaust gas to be treated, and then ammonium ion (NH ₄ ⁺ ) and bromine ion (Br 4 ^- ) Containing an aqueous solution of caustic soda (NaOH) containing the chlorine-containing exhaust gas.

The method according to the invention is particularly suitable for NOx, especially nitric oxide (N
O) has a remarkable effect on absorption and removal. In contrast, NOx
The absorption of nitrogen dioxide (NO ₂ ), which accounts for a part of NO2, is not so remarkable and is small compared to the absorption of NO. NOx in various flue gas is composed of NO and NO ₂ , but the proportion of NO is usually very high, and 95% or more is considered to be NO. Therefore, the method of the present invention is an excellent method as a method for absorbing and removing NOx in various combustion exhaust gas from this viewpoint as well.

The amount of chlorine added to the exhaust gas is Cl ₂ / NO molar ratio,
It is preferably 1 to 3. As a method of adding chlorine to the exhaust gas, a method of directly injecting and diffusing a cylinder-filled chlorine gas can be adopted.

Further, an exhaust gas cleaning stage is provided on the upstream side of the NOx absorption stage, and as the cleaning liquid, a solution in which sodium hypochlorite (NaClO) is dissolved and whose pH is adjusted to a strong acidity of, for example, 3 or less with a mineral acid is used. A method of bringing the exhaust gas into contact with the cleaning liquid can also be adopted. By this operation, the generated chlorine gas is added to the exhaust gas. This reaction is as follows:

NaClO + 2H ⁺ → Na ⁺ + 1 / 2Cl ₂ + H ₂ O NaClO is commercially available as a large amount at low cost as an industrial chemical, so it can be used as it is. Also,
It is also possible to easily supply chlorine gas locally by electrolysis of saline solution. Furthermore, since a large amount of salt is usually contained in the smoke washing wastewater of the dust-burning furnace, an aqueous solution containing NaClO can be obtained by electrolyzing the wastewater without using a diaphragm. The effect of using this effluent is mainly on the economic side, and it is not on the reaction side such as the chlorine generation mechanism and the improvement of the NOx removal rate in the downstream stage.

The absorption liquid of NOx is basically an aqueous solution of NaOH, and the pH of the aqueous solution at the inlet of the absorption tower is preferably 3-9. This pH is
Not only does it affect the rate of absorption and removal of NOx, but the absorption liquid on the alkaline side also absorbs and removes excess of the chlorine added on the upstream side to the reaction, and it remains in the treated gas after purification. It plays a role in solving chlorine problems.

The presence of NH ₄ ⁺ and Br ^{− in the} NOx absorbing aqueous solution improves the NOx absorption efficiency, which is shown in the previously proposed patent applications (Japanese Patent Application Nos. 63-24146 and 63-147606). The case where the liquid contains these ions and NaClO at the same time is described. In this absorption, as shown in the following reaction formula,
It is considered that NO is oxidized by NaClO and becomes NO ₂ , which is absorbed, but Br ⁻ acts as a catalyst for this oxidation reaction, and NH ₄ ⁺ acts as a neutralizing agent for NO ₂ absorbed in the liquid. Was given.

NaClO + NO → NO ₂ ＋ NaCl ₂ NO ₂ ＋ 2NH ₄ OH → NH ₄ NO ₃ ＋ NH ₄ NO ₂ ＋ H ₂ O The inventors of the present invention conducted extensive studies to improve the denitration rate, and as a result, the absorption mechanism. In addition to obtaining new findings, we were able to achieve a significant improvement in the denitration rate.

As described above, the feature of the present invention is that chlorine is added to the exhaust gas to be treated in advance, and then NH ₄ ⁺ is added.
Br ^- aqueous NaOH solution and absorbent solution containing, in terms of contacting the flue gas in the absorption liquid. Here, as the NH ₄ ⁺ sources, NH ₃ (gas), although of course it can be used is NH ₄ OH (aq), the industrial ammonium salts such as ammonium sulfate _{((NH 4) 2 SO 4} ) , Ammonium chloride (NH ₄ Cl), etc. can be used economically and without any problems in terms of reaction. In addition to HBr as a Br ⁻ supply source, practically salts such as potassium bromide and ammonium bromide can be used. In this method, chlorine added in advance in the exhaust gas does not act directly on NOx, but acts to gasify Br ⁻ in the absorption liquid in the downstream stage, as shown in the following reaction formula.

1 / 2Cl ₂ + Br ⁻ → 1 / 2Br ₂ + Cl ⁻ And this Br ₂ reacts with NOx, especially NO.

NO + 1 / 2Br ₂ → NOBr This NOBr is very reactive and easily absorbed by aqueous solution.

NOBr + H ₂ O → HNO ₂ + HBr In this reaction, bromine returns to the aqueous solution as Br ⁻ ,
Bromine acts as a catalyst in this series of reactions.

NH ₄ ⁺ in the absorbing solution is considered to act as a neutralizing agent for HNO ₂ produced in the above reaction, but experimentally, it not only acts as a neutralizing agent, but also the absorption rate of NOBr into the absorbing solution. It was recognized as working to enhance.

HNO ₂ + NH ₄ ⁺ → NH ₄ NO ₂ + H ^{+ As described} above, the absorption of NOx according to the present invention is not due to the oxidation of NO to NO ₂ and chemical absorption as described above, but as an intermediate. This is probably due to the formation of highly reactive NOBr. The above reaction is not confirmed by a method such as analysis of an intermediate, but it is estimated by the following experimental phenomenon.

1) When chlorine was not added, the NO removal rate was extremely low.

2) A high denitration rate was obtained in an experiment in which bromine gas was directly added to the exhaust gas in a system containing no chlorine.

3) As a result of the treatment, NO was removed, but NO ₂ was hardly removed.

4) NO removal rate is not affected by oxygen concentration in exhaust gas,
Even when the oxygen concentration was 0%, NO was effectively removed.

5) A slight NO removal rate was observed when the Br ⁻ concentration was 0 mg / l, but the NO removal rate was almost 0% when NH ₄ ⁺ 0 mg / l.

6) the absorbing solution in the absorption tower outlet NO ₂ ^- and NO ₃ ^- is detected (NO ₂ ^- concentration> NO ₃ ^- concentration).

As described above, in any case, the NOx wet absorption method of the present invention can achieve a high denitrification rate that has not been heretofore recognized. Also, as an important effect of the present invention, conventional Na
In the one-stage absorption tower system using ClO, the pH of the absorbing solution had to be lowered in order to maintain a high denitration rate, but when the pH was lowered, chlorine leakage occurred at the outlet side of the absorption tower. If chlorine leakage is suppressed, the achievable denitration rate will be limited. On the other hand, according to the two-stage system such as the method of the present invention, the pH of the inlet of the absorption liquid can be on the alkaline side, so that even if excess chlorine is introduced into the absorption tower, the excess is completely removed. It is absorbed and removed without causing any problems.

Further, when this denitration system is applied to the exhaust gas treatment of a dust-burning furnace, since NaClO naturally exists in this absorbing solution, mercury in the exhaust gas is also removed at the same time.

Next, as important reaction conditions when carrying out the present invention, chlorine addition amount in the exhaust gas, Br ^- content and NH ₄ ⁺ content in the absorption liquid, and further the pH of the absorption liquid (adjusted by NaOH)
Is shown below.

1) Chlorine amount: Cl ₂ / NO molar ratio = 1 or 2) Br ^- content: Br ^- / NO molar ratio = 1 or 3) NH ₄ ⁺ content: NH ₄ ⁺ / NO molar ratio = 1 or 4 ) PH of absorption liquid inlet: 3 to 9 As a device type of the washing tower and the absorption tower used for carrying out the method of the present invention, conventionally known Raschig ring packed tower, spray tower, plate tower, etc. are appropriately used. It can be used. The operating conditions of these absorption towers, that is, the liquid gas ratio, the gas superficial velocity, etc., should be determined in relation to the desired denitration rate.

EFFECTS OF THE INVENTION According to the wet NOx removal method of the present invention, chlorine is added to exhaust gas in advance before the exhaust gas is treated with an absorption liquid, and thus it is almost impossible to remove by the conventional one-stage wet absorption method. NOx can be removed very effectively. Since NOx is almost NO in various combustion exhaust gases, the method of the present invention is very effective in this sense.
It can be said that this is a removal method. Further, according to the method of the present invention, no harmful substances such as chlorine are left in the treated gas after purification. Furthermore, when the method of the present invention is applied to the treatment of exhaust gas in a dust-burning furnace, mercury in exhaust gas can be simultaneously removed.

Examples Next, the present invention will be described with reference to Examples and Comparative Examples.

Example 1 As shown in FIG. 1, a washing tower (1) and an absorption tower (3)
An NO removal experiment was carried out using an experimental apparatus equipped with. In the figure, the washing tower (1) has a diameter of 30 mm and a height of 510 mm, is covered with a hot water jacket (2), and is filled with spherical glass beads having an average diameter of 2 mm up to a height of 331 mm. . The absorption tower (3) also has exactly the same type and size as the washing tower (1), and is equipped with a hot water jacket (4). The internal temperature of the washing tower (1) and the absorption tower (3) is controlled by the hot water jackets (2) and (4) to a predetermined temperature (usually 70
℃) is maintained. In addition, this figure also shows an analysis system for analyzing each gas composition of the prepared exhaust gas for treatment and the treated gas, and the liquid composition.

Table 1 shows the common standard conditions adopted in this experiment. The experimental results are shown in FIGS. 2 and 3. In these experiments, chlorine was added by injecting an aqueous NaClO solution into the washing tower (1) and adjusting the pH to 1-2 with hydrochloric acid.

FIG. 2 shows the effect of Br ⁻ addition with NH ₄ ⁺ in the solution kept constant at 10 mg / l. As is clear from the figure, the NO removal rate rises up to Br ^- 25 mg / l and becomes constant at higher concentrations. In addition, Br ^- 25 mg / l was used in this experiment.
Corresponds to an r ⁻ / NO molar ratio of 1.

FIG. 3 shows the effect of NH ₄ ⁺ concentration with Br ⁻ kept constant at 50 mg / l. As is clear from the figure, the NH ₄ ⁺ concentration is 0 m.
Although g / l hardly removes NO, a sharp increase in NO removal rate is observed as the NH ₄ ⁺ concentration increases. The NH ₄ ⁺ concentration at which the NO removal rate is maximum is 10 mg / l, and this value corresponds to an NH ₄ ⁺ / NO molar ratio of 1.5 in this experiment. The chlorine at the outlet of the washing tower was 0 ppm throughout the experiments of the examples.

Comparative Example 1 In the experimental apparatus shown in FIG. 1 shown in Example 1, Br ⁻ and NH ₄ ⁺ were added to the aqueous solution supplied to the washing tower (1) so that the concentration of each was 50 mg / l, and the pH of the solution was adjusted to 9. After adjusting, the prepared exhaust gas was brought into contact with the same solution, and the NO removal rate was measured. At this time, the operation in the absorption tower was not performed. This operation corresponds to a case where the two-stage operation of the washing tower and the absorption tower of Example 1 is so-called, so that it belongs to the prior art. Measured
The NO removal rate was 30%.

Comparative Example 2 In the experiment of Example 1, an absorbing solution containing NH ₄ ⁺ 10 mg / l and Br ^- 50 mg / l was used, and the prepared exhaust gas was directly introduced into the absorption tower (3) without passing through the washing tower (1). This experiment corresponds to the case where chlorine gas was not previously added to the exhaust gas. NO at this time
The removal rate was 0%.

Example 2 In the experiment of Example 1, an absorption liquid containing NH ₄ ⁺ 10 mg / l and Br ^- 50 mg / l was used, and the prepared exhaust gas was directly introduced into the absorption tower (3) without passing through the washing tower (1). However, in this experiment, a predetermined amount of chlorine gas was previously injected into the exhaust gas. The experimental results are shown in Table 2. As is clear from the table, a high NO removal rate was obtained when the Cl ₂ concentration at the inlet of the absorption tower was about 100 ppm or more.

Example 3 Using the experimental apparatus of FIG. 1 shown in Example 1, NO and
The removal rate of NO ₂ was calculated. Table 3 shows the reaction conditions, the obtained NO removal rate and NO ₂ removal rate. In these experiments, both NO and NO ₂ used were bomb gas diluted with nitrogen and further diluted with air immediately before being introduced into the experimental apparatus. As shown in the table, NO was removed with high efficiency, but NO ₂ was hardly removed.

Example 4 In order to see the form of NO in the exhaust gas absorbed in the absorption liquid,
By increasing the NO concentration in the prepared exhaust gas, the first concentration shown in Example 1
Experiments were conducted using the equipment shown in the figure, and NO ₂ and N
O ₃ ⁻ was analyzed. The conditions of the prepared exhaust gas and the results of the absorption liquid analysis are shown in Tables 4 and 5, respectively. NO removed
The amount and the equivalent NO amount in the liquid do not match well, but in the liquid
NO ₂ ⁻ and NO ₃ ⁻ were detected, and NO ₂ ⁻ > NO ₃ ⁻ .

Example 5 In order to investigate whether NO in the exhaust gas is an oxidation reaction absorption or an anoxic reaction absorption, the first example shown in Example 1 was used when the atmosphere gas was nitrogen and when it was air. An experiment was conducted using the apparatus shown in the figure. The results are shown in Table 6. No effect of oxygen in the atmosphere gas was observed, and both high NO removal rates were obtained.

Example 6 FIG. 4 shows a flow of the NOx absorber (exhaust gas treatment amount 60,000 Nm ² / hour) for treating exhaust gas from a refuse burning furnace. In this apparatus, the exhaust gas was introduced into the washing tower (11), then passed through the absorption tower (13), and then passed through the dehumidification tower (14). The washing tower (11) is a co-current Venturi tower type, and the absorption tower (13) and the dehumidifying tower (14) are both countercurrent packed tower types. In the cleaning tower (11), NaClO was injected into the circulating liquid of the cleaning tower in order to add chlorine to the exhaust gas in advance. Although the circulating fluid of this washing tower becomes strongly acidic due to the hydrochloric acid in the exhaust gas, pH3 was maintained by adding NaOH separately.

The NO concentration in the exhaust gas was about 50 ppm, but the amount of NaClO added was adjusted to a theoretical molar ratio of this NO amount (Cl ₂ / NO molar ratio) of 2 or more. NH ₄ ⁺ and Br ⁻ were added to the absorption tower circulation liquid. At this time, ammonium sulfate was used as the NH ₄ ⁺ supply source, and potassium bromide was used as the Br ⁻ source, and the addition amounts thereof were each at a molar ratio of NO amount of 1 or more. In the dehumidifying tower (14), the exhaust gas was dehumidified by the cooling water coming from the water tank (15) through the cooling tower (16), and then the same gas was discharged to the outside of the system.

The NO removal rate obtained by the above treatment was 80%.

[Brief description of drawings]

1 and 4 are flow sheets showing an embodiment of the present invention, FIG. 2 is a graph showing the relationship between Br ⁻ concentration and NO removal rate,
FIG. 3 is a graph showing the relationship between the NH ₄ ⁺ concentration and the NO removal rate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Hirotsuna 1-6-14 Edobori, Nishi-ku, Osaka-shi, Osaka Prefecture Hitachi Shipbuilding Co., Ltd. (72) Toshiharu Kobayashi 1-6-14 Edobori, Nishi-ku, Osaka-shi, Osaka Within Hitachi Shipbuilding Co., Ltd. (72) Zensuke Inoue, 10-14-43 Hanyuno, Habikino-shi, Osaka (72) Inventor, Nao Ito 10-6-16, Uegahara, Nishinomiya-shi, Hyogo (72) Inventor, Minoru Sawaji, Nara, Nara 3-316 A-102, Nankiji-cho, Tochigi, Japan (56) Reference Japanese Patent Laid-Open No. 1-315320 (JP, A) JP 58-34174 (JP, B2)

Claims (4)

[Claims] 1. When absorbing and removing nitrogen oxides from various combustion exhaust gases with an absorption liquid, chlorine is added to the exhaust gas in advance, and then the absorption liquid is formed of an aqueous caustic soda solution containing ammonium ions and bromine ions, A wet removal method of nitrogen oxides in various combustion exhaust gases, which comprises contacting the chlorine-containing exhaust gases. 2. In order to add chlorine to the exhaust gas, an exhaust gas cleaning stage is provided on the upstream side of the nitrogen oxide absorption stage, and a solution prepared by adjusting the aqueous solution of sodium hypochlorite to a strong acid is used as the cleaning solution. The method according to claim 1, wherein exhaust gas is brought into contact with the cleaning liquid. 3. The method according to claim 2, wherein an aqueous solution of sodium hypochlorite obtained by electrolyzing a saline solution or a smoke washing waste water without using a diaphragm is used. 4. The method according to claim 1, wherein the pH of the absorbing solution is adjusted to 3-9.