JPH04200722A - Exhaust gas purification method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is directed to the treatment of nitrogen oxides and organic compounds such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans contained in exhaust gas discharged from incinerators, etc. The present invention relates to a method for purifying exhaust gas by removing chlorine compounds and ammonia.

[Conventional technology]

Exhaust gas generated from incinerators and other facilities used to process industrial and municipal waste contains nitrogen oxides (NOx) as well as trace amounts of highly toxic polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCs).
Contains organic chlorine compounds such as DPs). Removal of such harmful nitrogen oxides and organic chlorine compounds from exhaust gas is extremely important for pollution prevention.

As a method for removing nitrogen oxides from exhaust gas, for example,
Japanese Patent Publication No. 54-29419 discloses that by adding ammonia as a reducing agent to the exhaust gas and bringing the ammonia-added exhaust gas into contact with a nitrogen oxide reduction catalyst, nitrogen oxidation in the exhaust gas is carried out. A method for reducing and removing substances is disclosed.

As a method for removing organic chlorine compounds from exhaust gas, for example, Japanese Patent Application Laid-Open No. 63-290314 discloses a method for removing organic chlorine compounds from exhaust gas.
An oxidation catalyst such as platinum supported on a ceramic carrier and 30
A method is disclosed in which organic chlorine compounds in exhaust gas are oxidized and removed by contacting at a temperature of 0 to 900°C.

Furthermore, JP-A No. 62-65721 discloses that ammonia is added to exhaust gas, and the exhaust gas to which ammonia has been added is brought into contact with a catalyst for reducing nitrogen oxides to reduce nitrogen oxides in the exhaust gas. Discloses a method in which the exhaust gas from which nitrogen oxides have been removed is brought into contact with an oxidation catalyst to oxidize and remove carbon monoxide, hydrocarbons, and oxygen-containing organic compounds contained in the exhaust gas. has been done.

[Problem to be solved by the invention]

According to the method disclosed in Japanese Patent Publication No. 54-29419 mentioned above, only nitrogen oxides can be removed from exhaust gas, and according to the method disclosed in Japanese Patent Publication No. 63-290314, only nitrogen oxides can be removed from exhaust gas. , it is possible to remove only organic chlorine compounds from exhaust gas. However, none of the above methods can remove both nitrogen oxides and organic chlorine compounds from exhaust gas. Furthermore, although ammonia (NH*) is contained in the exhaust gas discharged from incinerators and the like, the above-mentioned method cannot remove such ammonia together with nitrogen oxides and organic chlorine compounds.

Furthermore, according to the method disclosed in JP-A No. 62-65721, nitrogen oxides, carbon oxides, hydrocarbons, and oxygen-containing organic compounds can be removed from exhaust gas; It is not possible to remove existing organic chlorine compounds.

Therefore, an object of the present invention is to provide a method for purifying exhaust gas by efficiently removing harmful nitrogen oxides, organic chlorine compounds, and ammonia contained in exhaust gas discharged from an incinerator, etc. Our goal is to provide the following.

[Means to solve the problem]

The present inventors have conducted extensive research in order to solve the above-mentioned problems. As a result, the exhaust gas is brought into contact with a catalyst for reducing nitrogen oxides in the presence of ammonia to reduce nitrogen oxides in the exhaust gas, and then brought into contact with a catalyst for oxidizing organic chlorine compounds to reduce organic chlorine compounds in the exhaust gas. It has been found that nitrogen oxides, organic chlorine compounds, and ammonia can be removed from exhaust gas with high efficiency by oxidation.

This invention was made based on the above findings, and the method of this invention is capable of treating nitrogen oxides and organic chlorine compounds such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans discharged from incinerators, etc. In addition, ammonia as a reducing agent is continuously or intermittently added to the ammonia-containing exhaust gas, and the exhaust gas to which ammonia has been added is brought into contact with a nitrogen oxide reduction catalyst at a temperature of 200 to 500°C. reducing and removing the nitrogen oxides contained in the exhaust gas,
Next, the exhaust gas from which nitrogen oxides have been removed is
By contacting with a catalyst for oxidizing organic chlorine compounds at a temperature of 00°C, the organic chlorine compounds contained in the exhaust gas are oxidized and removed, and the ammonia contained in the exhaust gas is decomposed. It is characterized by its removal.

[Effect]

In this invention, first, ammonia is added as a reducing agent to exhaust gas, and the exhaust gas to which such ammonia has been added is brought into contact with a catalyst for reducing nitrogen oxides, thereby reducing nitrogen oxide contained in the exhaust gas. The exhaust gas from which nitrogen oxides have been removed is brought into contact with a catalyst for oxidizing organic chlorine compounds to oxidize and remove ammonia contained in the exhaust gas.

Therefore, in order to increase the removal efficiency of nitrogen oxides, NHs
Even if ammonia is added to the exhaust gas so that /N0x (molar ratio) is 1 or more, a problem will occur because most of the unreacted ammonia in the exhaust gas will be removed in the reactor for removing organic chlorine compounds. Never.

TIT Sll A as a catalyst for oxidizing organic chlorine compounds
Pt, Pd on the surface of a substrate made of an oxide of at least one metal selected from the group consisting of Pt and Zr.
.

NOx, SO by using a seta catalyst supported with at least one metal selected from the group consisting of Ru, Mn, Cu, Cr and Fe or its oxide.
x, HCI, CO, halogen gas, etc., toxic organic chlorine compounds such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans can be efficiently (decomposed and removed) over a long period of time.

Further, a nitrogen oxide reduction catalyst and an organic chlorine compound oxidation catalyst can be stacked and provided in one reactor,
This makes it possible to make the device more compact.

Next, the present invention will be explained with reference to the drawings. 1st
The figure is a process diagram showing one embodiment of the method of this invention.

As shown in FIG. 1, in the method of the present invention, first, in the mixing chamber 1, ammonia (NH*
) are added continuously or intermittently and mixed. Then, the exhaust gas to which ammonia has been added in this way is
At a temperature of 00 to 500°C, the nitrogen oxides (NOx) in the exhaust gas are introduced into the denitrification reactor 2 equipped with a nitrogen oxide reduction catalyst. contact with a catalyst for product reduction and are reduced and selectively removed.

In this way, the exhaust gas from which nitrogen oxides have been removed in the denitrification reactor 2 is guided at a temperature of 200 to 500°C to the organochlorine compound removal reactor 3 provided with an organochlorine compound oxidation catalyst ( As a result, the organic chlorine compounds in the exhaust gas are brought into contact with the organic chlorine compound oxidation catalyst in the organic chlorine compound removal reactor 3, and are oxidized and removed.

The ammonia remaining in the exhaust gas after nitrogen oxides are selectively removed by the denitrification reactor 2 described above is
Unless the amount is excessive (approximately 200 ppm or more), it is decomposed into nitrogen and water and removed in the reactor 3 for removing organic chlorine compounds.

The temperature of the exhaust gas needs to be within the range of 200 to 500°C. When the temperature of the exhaust gas is less than 200'C, the reduction reaction of nitrogen oxides in the denitrification reactor 2 and
The oxidation reaction of the organic chlorine compound in the organic chlorine compound removal reactor 3 becomes insufficient (1), and nitrogen oxides and organic chlorine compounds cannot be efficiently removed from the exhaust gas. On the other hand, if the temperature of the exhaust gas exceeds 500° C., a problem arises in which ammonia remaining in the exhaust gas is not decomposed into nitrogen and water in the organic chlorine compound removal reactor 3, but instead becomes nitrogen oxides.

In order to increase the removal efficiency of nitrogen oxides in the denitrification reactor 2, it is desirable to add ammonia to the exhaust gas so that NHs/N0x (molar ratio) is 1 or more. Even if ammonia is added to the exhaust gas at such a ratio, unreacted ammonia in the exhaust gas will, as mentioned above,
Since most of the organic chlorine compounds are removed in the reactor 8, no problem occurs.

The denitrification reactor 2 and the organic chlorine compound removal reactor S are not provided separately, but are integrated into one reactor, and a nitrogen oxide reduction catalyst and an organic chlorine compound oxidation catalyst are contained in this one reactor. It may be provided by laminating them. In this way, the device can be made more compact.

Ammonia may be added to the exhaust gas continuously or intermittently. Even if ammonia is added intermittently to the exhaust gas, the nitrogen oxides contained in the exhaust gas are reduced by the ammonia adhering to the nitrogen oxide reduction catalyst in the denitrification reactor 2.

In order to control the amount of ammonia added to the exhaust gas, as shown in Fig. 2, a nitrogen oxide concentration measuring device 4 is installed on the outlet side of the reactor 3 for removing organic chlorine compounds to measure the nitrogen oxide concentration. It is desirable to control the amount of ammonia added to the exhaust gas in the mixing chamber 1 based on the concentration of nitrogen oxides in the exhaust gas measured by the device 4.

FIG. 3 is a process diagram for automatically controlling the amount of ammonia added. As shown in FIG. 3, a first nitrogen oxide concentration measuring device 4 is provided on the outlet side of the reactor 3 for removing organic chlorine compounds, and a second nitrogen oxide concentration measuring device 4 is provided on the inlet side of the mixing chamber 1.
A nitrogen oxide concentration measuring device 5 is provided. 6 is a computing unit. The concentration of nitrogen oxides in the exhaust gas before treatment measured by the second nitrogen oxide concentration measuring device 5 and the first nitrogen oxide concentration measuring device 4 on the outlet side of the reactor 3 for removing organic chlorine compounds. The measured concentration of nitrogen oxides in the treated exhaust gas is sent to the computing unit 6, and the computing unit 6 computes the difference between the two.

In this manner, an appropriate amount of ammonia is added to the exhaust gas in the mixing chamber 1 based on the calculated concentration difference of nitrogen oxides.

As the catalyst for reducing nitrogen oxides, commercially available catalysts such as
A catalyst or the like in which vanadium, tungsten, or the like is supported on the surface of a substrate made of an oxide containing titanium can be used.

Examples of catalysts for oxidizing organic chlorine compounds include Ti, Si,
At least one selected from the group consisting of Al and 2
Pt, P on the surface of a substrate made of a metal oxide.
It is preferable to use a catalyst on which at least one metal selected from the group consisting of d+ Ru, Mn+ Cu, Or and Fe or an oxide thereof is supported.

Particularly preferred catalysts are TiO, -5iO□ or Ti
e.

- Binary composite oxide of Zr0z, 3A1201 ” 2
An oxide in which TiO2 is coated on the surface of an oxide containing SiO, or Ti0t-310*-AlJs
, TiOx-510*-ZrOz is used as a base, and on the surface of this base, the above-mentioned Pt, P
d, Ru, Mn, Cu, Or and Fe, or an oxide thereof, is supported on the catalyst.

Such catalysts can be used for NOx, SOx, HCI, C
In an environment where O and halogen gases exist, organic chlorine compounds such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans can be efficiently decomposed and removed over a long period of time.

In particular, if the substrate has a structure in which Ties is coated on the surface of 3AlJs/2SI02, that is, an oxide containing mullite, fine irregularities will be formed on the surface of the substrate due to Ti (L) coated on the surface of mullite. Because it is formed, it is possible to secure a sufficient surface area to support catalyst components such as PT and obtain high activity, and it also has excellent acid resistance due to the TiO coating on the surface of mullite. will be granted.

The shape of the catalyst for reducing nitrogen oxides and the catalyst for oxidizing organic chlorine compounds can be any integrally formed shape, such as columnar, cylindrical, plate, ribbon, honeycomb, or pellet shape. . In particular, a catalyst consisting of a honeycomb structure A having a lattice-like cross section as shown in the cross-sectional view in FIG. 5 or a honeycomb structure B having a corrugated cross-section as shown in the cross-sectional view in FIG. 6 is particularly preferable. . If the catalyst is configured with such a honeycomb structure, the dust present in the exhaust gas will not adhere to the catalyst (therefore, there will be no increase in pressure loss or deterioration of performance due to the adhesion of dust, and it will be stable). It is possible to carry out operations based on the following conditions.

FIG. 4 is a process diagram showing an example of a method outside the scope of the present invention for comparison. In the example shown in FIG. 4, the exhaust gas is first led to an organic chlorine compound removal reactor 3 provided with an organic chlorine compound oxidation catalyst, and the organic chlorine compounds in the exhaust gas are oxidized and removed. Next, ammonia is added and mixed in the mixing chamber 1 into the exhaust gas from which the organic chlorine compounds have been removed. The exhaust gas to which ammonia has been added in this way is led to the denitrification reactor 2 equipped with a catalyst for reducing nitrogen oxides, and the nitrogen oxides in the exhaust gas are reduced.
removed.

In the case of the method described above, the following problems arise.

■ Since ammonia is added to the exhaust gas after organic chlorine compounds have been removed, it is necessary to sufficiently disperse and mix ammonia in the exhaust gas, so there is sufficient space between the mixing chamber 1 and the denitrification reactor 2. It is necessary to increase the distance, and the reactor 3 for removing organic chlorine compounds and the reactor 2 for denitration cannot be compactly combined into one reactor, resulting in an increase in the size of the apparatus.

(2) Ammonia contained in the exhaust gas is oxidized in the organic chlorine compound removal reactor 3 to become nitrogen oxides. As a result, the amount of nitrogen oxides in the exhaust gas on the outlet side of the reactor 8 for removing organic chlorine compounds increases. In order to remove nitrogen oxides increased in this way, mixing chamber 1
In this case, it is necessary to add a large amount of ammonia to the exhaust gas.

■ In order to increase the removal efficiency of nitrogen oxides, especially for the reason stated in (■) above, the amount of ammonia added to the exhaust gas is adjusted so that NHt/N0x (molar ratio) is 1 or more.
have to do a lot. However, when such a large amount of ammonia is added to the exhaust gas, unreacted ammonia remains in the exhaust gas after treatment. Such unreacted ammonia reacts with SOx in the flue gas in the denitrification reactor 2 or on its outlet side, becomes ammonium sulfate compounds such as acidic ammonium sulfate, and is transferred to equipment downstream from the denitrification reactor 2. adhere to. This results in corrosion of the above equipment.

■ In order to prevent the above-mentioned problem caused by unreacted ammonia in the exhaust gas, the amount of ammonia added to the exhaust gas should be adjusted so that the NHm/N0x (molar ratio) is less than 1, for example 0.7 to
0.9, and it is necessary to ensure that no unreacted ammonia remains in the exhaust gas. However, when the amount of ammonia added to the exhaust gas is reduced as described above, the removal rate of nitrogen oxides from the exhaust gas decreases to about 70%.

Next, the present invention will be explained in more detail using Examples and comparing with Comparative Examples.

[Example 1] As a nitrogen oxide reduction catalyst, vanadium was supported on a base made of titanium oxide, as shown in Fig. 5, consisting of a honeycomb structure with a lattice-like cross section, opening: 6 mm, wall thickness: A 1 mm catalyst was used. As a catalyst for oxidizing organic chlorine compounds, Pt(
A honeycomb structure with a lattice cross section shown in FIG. 5, carrying 1.5 g/catalyst ii), opening +3.2 m.
m, wall thickness: 0.51 catalyst was used.

The space velocity <SV> of the denitrification reactor provided with the nitrogen oxide reduction catalyst is 5000 hr-', and the organochlorine compound removal reactor provided with the organochlorine compound oxidation catalyst The space velocity (SV) of is 2000
h is "-'.

By using the above-mentioned catalyst and according to the process diagram shown in FIG. 1, nitrogen oxides (NOx) and polycarbonate are removed from exhaust gas containing NOx, PCD [lS and NH, as shown below] by the method of the present invention. Dibenzodioxin chloride (P
CDDs) and ammonia (NH*) were removed.

NOX content: 130-180ppm PCDDs
Content: 2000-4000ng/Nm"NHI content: 30-50ppm Table 1 shows exhaust gas temperature, amount of ammonia added as a reducing agent (NHm/N0x (mole ratio)), NOX removal rate, PCDDs removal rate, and Shows the degree of residual NHs in the treated exhaust gas.

Table 1 [Comparative Example 1] According to the process diagram shown in Figure 4 showing the method for comparison, organic chlorine compounds were removed from exhaust gas in a reactor for removing organic chlorine compounds, and then the organic chlorine compounds were removed. Using the same catalyst as in Example 1, the same amount of NOx and PCDDs as in Example 1 was obtained by a method consisting of adding ammonia to the exhaust gas and removing nitrogen oxides from the exhaust gas in a denitrification reactor. Nitrogen oxides (NOx), polychlorinated dibenzodioxins (PCDDs) and ammonia (N
HI) was removed.

Table 2 shows exhaust gas temperature, amount of ammonia added as a reducing agent (NHI/N0x (molar ratio)), NOx removal rate, P
The CDDs removal rate and the residual NH and concentration in the treated exhaust gas are shown.

Table 2 As is clear from a comparison between Table 1 showing the results of Example I and Table 2 showing the results of Comparative Example 1, according to the method of the present invention, nitrogen oxides are removed from exhaust gas. (NOx) and polychlorinated dibenzodioxins (PCDDs) could be removed with high efficiency, and the amount of residual ammonia in the exhaust gas after treatment was 1.0 ppm or less, which was an extremely small amount.

[Example 2] As a nitrogen oxide reduction catalyst, a honeycomb structure with a lattice-like cross section as shown in FIG. 5 was used, in which vanadium was supported on a substrate made of titanium oxide, opening: 6 mm, wall thickness: 1 mm. catalyst was used. As a catalyst for oxidizing an organic chlorine compound, a substrate having a Tie content of 60 wt%, the surface of which is a mullite-containing oxide coated with Ti0a,
A catalyst consisting of a honeycomb structure having a corrugated cross section shown in FIG. 6 and supporting Pt (2.0 g/catalyst If) was used. The dimensions of each part of the catalyst are as follows.

Width pitch (a): 3.7mm Length pitch (b): 7.5mm Thickness of wavy wall (c)
: 0.4mm side wall thickness (mountain: 0.4mm)
Note that the space velocity (SV) of the denitrification reactor equipped with the nitrogen oxide reduction catalyst and the space velocity (SV) of the organic chlorine compound removal reactor equipped with the organic chlorine compound oxidation catalyst ) are all 5000 hours.

Using the catalyst described above and following the process diagram shown in FIG. 1, the same amount of NOx,
Nitrogen oxides (NOx), polychlorinated dibenzodioxins (PCDDs), and ammonia (NHx) were removed from exhaust gas containing PCDDs and NH. The amount of ammonia added as a reducing agent to exhaust gas is
NHs/N0x (molar ratio) was set to 0.9.

Table 3 shows the exhaust gas temperature, NOx removal rate, PCDDs removal rate, and residual NH and concentration in the treated exhaust gas.

Table 3 [Comparative Example 2] According to the process diagram shown in Figure 4 showing the method for comparison, organic chlorine compounds were removed from exhaust gas in a reactor for removing organic chlorine compounds, and then the organic chlorine compounds were removed. Using the same catalyst as in Example 2, the same amount of NOx and PCDDs as in Example 1 was obtained by a method consisting of adding ammonia to the exhaust gas and removing nitrogen oxides from the exhaust gas in a denitrification reactor. Nitrogen oxides (NOx), polychlorinated dibenzodioxins (PCDDs) and ammonia (NHx) are extracted from the exhaust gas containing
, ) was removed.

Table 4 shows the exhaust gas temperature, NOx removal rate, PCDDs removal rate, and residual NH1 concentration in the treated exhaust gas.

Table 4 As is clear from a comparison between Table 3 showing the results of Example 2 and Table 4 showing the results of Comparative Example 2, according to the method of the present invention, nitrogen oxides are removed from exhaust gas. (NO and polychlorinated dibenzodioxins (PCDDs) could be removed with high efficiency, and the amount of residual ammonia in the exhaust gas after treatment was 1.0 ppm or less, which was an extremely small amount.

〔Effect of the invention〕

As described above, according to the present invention, nitrogen oxides, organic chlorine compounds such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans, and ammonia contained in the exhaust gas discharged from the incinerator are eliminated. Many industrially useful effects are brought about, such as being able to purify the exhaust gas by removing it from the exhaust gas with high efficiency, and making the device more compact.

[Brief explanation of the drawing]

FIG. 1 is a process diagram showing one embodiment of the method of this invention, FIGS. 2 and 3 are process diagrams showing other embodiments of the method of this invention, and FIG. 4 is a process diagram of this method for comparison. 5 and 6, which are process charts showing an example of a method outside the scope of the invention, are sectional views showing an example of the shape of a catalyst used in the method of the invention. In the drawings, 1... ~mixing chamber, 2...- reactor for denitrification, 3... reactor for removing organic chlorine compounds, 4.5...
- Nitrogen oxide concentration measuring device, 6-.- Arithmetic unit, A, B-... Catalyst.

Claims (1)

[Claims] 1. In exhaust gas containing nitrogen oxides, organic chlorine compounds such as polychlorinated dibenzodioxins and polychlorinated dibenzofurans, and ammonia, which are discharged from incinerators, etc., as a reducing agent. By adding ammonia continuously or intermittently and bringing the ammonia-added exhaust gas into contact with a catalyst for reducing nitrogen oxides at a temperature of 200 to 500°C, the nitrogen oxides contained in the exhaust gas can be reduced. The nitrogen oxides are reduced and removed, and the exhaust gas from which the nitrogen oxides have been removed is brought into contact with a catalyst for oxidizing organic chlorine compounds at a temperature of 200 to 500°C, thereby reducing the nitrogen oxides contained in the exhaust gas. A method for purifying exhaust gas, comprising: oxidizing and removing the organic chlorine compounds contained in the exhaust gas; and decomposing and removing ammonia contained in the exhaust gas. 2. Measure the concentration of nitrogen oxides in the exhaust gas after removing the organic chlorine compounds and ammonia contained in the exhaust gas by contacting with the organic chlorine compound oxidation catalyst. The method according to claim 1, wherein the amount of ammonia added as the reducing agent to the exhaust gas is controlled based on the concentration of nitrogen oxides. 3. As the catalyst for oxidizing the organic chlorine compound, Ti, Si
, Pt, Pt, Pt, Pt,
3. The method according to claim 1, wherein a catalyst on which at least one metal selected from the group consisting of Ru, Mn, Cu, Cr, and Fe or an oxide thereof is supported is used. 4. The substrate of the catalyst for oxidizing an organic chlorine compound is 3A
4. The method according to claim 3, wherein the oxide comprises an oxide containing 1_2O_3.2SiO_2 and TiO_2 coated on the surface of the oxide. 5. The substrate of the catalyst for oxidizing an organic chlorine compound is Ti.
O_2-SiO_2, TiO_2-ZrO_2, TiO
_2-SiO_2-Al_2O_3 and TiO_2-
4. The method according to claim 3, wherein the composite oxide is one of SiO_2-ZrO_2.