JPH09262439A - Nitrogen oxide removing material and removing method of nitrogen oxide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitrogen oxide removing material capable of efficiently reducing and removing the nitrogen oxide from a combustion waste gas containing the nitrogen oxide and an oxygen more than theoretical reacting weight to an unburned combustibles content such as carbon monoxide, hydrogen and hydrocarbon and containing water content and to provide a removing method of the nitrogen oxide. SOLUTION: The material is abtd, by mixing the first catalyst constituted by depositing 0.2-15wt.% (in terms of metallic element) more than one kind element and/or compd. selected from a group consisting of silver and silver compd. on a porous inorg. oxide and zirconia or the second catalyst constituted by depositing <=5wt.% (in terms of metallic element) more than one kind element and/or compd. selected from the group consisting of silver and silver compd. on the zirconia.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide removing material capable of effectively reducing and removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and excess oxygen, and a method for removing the same using the same.

[0002]

2. Description of the Related Art Excessive combustion exhaust gas emitted from internal combustion engines such as automobile engines, combustion equipment installed in factories, household fan heaters, and the like is excessive. It contains nitrogen oxides such as nitric oxide and nitrogen dioxide together with oxygen. Here, "containing excess oxygen" means that the exhaust gas contains more oxygen than the theoretical amount of oxygen necessary to burn unburned components such as carbon monoxide, hydrogen, and hydrocarbons. I do. In the following, nitrogen oxide refers to nitric oxide and / or nitrogen dioxide.

[0003] This nitrogen oxide is one of the causes of acid rain and is a major environmental problem. Therefore, various methods for removing nitrogen oxides in exhaust gas discharged from various combustion equipments are being studied.

[0004] As a method for removing nitrogen oxides from a combustion exhaust gas containing excess oxygen, a selective catalytic reduction method using ammonia is used particularly for a large-scale fixed combustion device (a large-scale combustor in a factory or the like). Has been put to practical use.

However, in this method, ammonia used as a reducing agent for nitrogen oxides is expensive, and ammonia is toxic. Therefore, the nitrogen oxides in the exhaust gas must be removed so that unreacted ammonia is not discharged. There are problems that the amount of injected ammonia must be controlled while measuring the concentration, and that the apparatus generally becomes large.

As another method, there is a non-selective catalytic reduction method in which a nitrogen oxide is reduced by using a gas such as hydrogen, carbon monoxide, or a hydrocarbon as a reducing agent. In order to reduce and remove nitrogen oxides, it is necessary to add a reducing agent in an amount equal to or more than a theoretical reaction amount with oxygen in exhaust gas, and there is a disadvantage that a large amount of the reducing agent is consumed. For this reason, the non-selective catalytic reduction method is practically effective only for exhaust gas having a low residual oxygen concentration burned near the stoichiometric air-fuel ratio, and is not practical because of poor versatility.

Therefore, a catalyst for catalytic reduction of nitrogen oxides by a hydrocarbon comprising a metal oxide such as titania or alumina, a rare earth oxide and at least one of Ru, Rh, Pd, silver and Pt has been proposed ( JP-A-4-27431). However, according to the experimental results of the present inventors, this catalyst has a low nitrogen oxide removal rate at a high space velocity, and particularly in a low temperature region where the exhaust gas temperature is 400 ° C. or lower, the nitrogen oxide removal is low. Further, in an exhaust gas containing water, the removal rate of nitrogen oxides is significantly reduced.

Accordingly, it is an object of the present invention to provide a method for producing nitrogen oxides, carbon monoxide, hydrogen, hydrocarbons, and the like, such as combustion exhaust gas from a fixed combustion device and a gasoline engine, a diesel engine, or the like, which burns under oxygen excess conditions. By providing a nitrogen oxide removing material and a nitrogen oxide removing method capable of efficiently reducing and removing nitrogen oxides from flue gas containing moisture, which contains oxygen in excess of the theoretical reaction amount with respect to unburned components. is there.

[0009]

Means for Solving the Problems In view of the above-mentioned problems, as a result of intensive studies, the present inventor has found that a removing material obtained by mixing a catalyst carrying silver on alumina and zirconia or a catalyst carrying silver on zirconia is used. Discovered that nitrogen oxides can be effectively removed over a wide temperature range by adding hydrocarbons to the exhaust gas and bringing the exhaust gas into contact with the above-mentioned removal material, even in an exhaust gas containing about 10% of water. Thus, the present invention has been completed.

That is, the nitrogen oxide removing material of the present invention for reducing and removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in excess of the theoretical reaction amount for coexisting unburned components is a porous inorganic oxide. One or more elements and / or compounds 0.2 selected from the group consisting of silver and silver compounds
And at least one element selected from the group consisting of zirconia or silver and a silver compound on zirconia and / or a compound of 5% by weight or less (metal element). (Converted value) is mixed with a second catalyst carrying the same.

Further, the method for removing nitrogen oxides of the present invention for reducing and removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount of coexisting unburned components, Using a removing material, the nitrogen oxide removing material is installed in the middle of an exhaust gas conduit, and is selected from the group consisting of gaseous hydrocarbons, kerosene, light oil, and liquid hydrocarbons including gasoline on the upstream side of the removing material. An exhaust gas to which at least one of the above-mentioned additives has been added is brought into contact with the removing material at 200 to 600 ° C., whereby the nitrogen oxides are removed by a reaction with hydrocarbons in the exhaust gas.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. [1] Nitrogen oxide removing material The nitrogen oxide removing material of the present invention comprises a mixed catalyst obtained by mixing a first catalyst and a second catalyst described below.

[0013] In the present invention, the above-mentioned removal material is installed in an exhaust gas conduit, and the exhaust gas to which hydrocarbons are added upstream of the installation position of the removal material is brought into contact with the removal material to reduce nitrogen oxides in the exhaust gas. Remove.

A first preferred form of the nitrogen oxide removing material of the present invention is a removing material obtained by coating the above-mentioned mixed catalyst on a removing material substrate. Examples of the ceramic material forming the substrate of the removing material include heat-resistant porous materials having a large surface area such as alumina, zirconia, and titania-zirconia. When high heat resistance is required, it is preferable to use cordierite, mullite, alumina and a composite thereof. Further, a known metal material can be used for the substrate of the nitrogen oxide removing material.

The shape and size of the substrate of the nitrogen oxide removing material can be variously changed depending on the purpose. Examples of the structure include a honeycomb structure type, a foam type, a three-dimensional network structure type formed of a fibrous refractory, a granular shape, a pellet shape, and the like. The nitrogen oxide-removing material can be manufactured by coating the above-mentioned substrate with the mixed catalyst using a wash coat method, a sol-gel method, a powder method or the like, and then sintering.

In a second preferred embodiment of the nitrogen oxide removing material of the present invention, the mixed catalyst is formed into a honeycomb structure, a foam, a plate, a pellet or a granule, and then sintered to a desired shape. It is a removing material that is filled in the casing.

The following catalyst is formed on the removing material of the present invention. (1) First catalyst The first catalyst comprises a porous inorganic oxide carrying one or more elements and / or compounds selected from the group consisting of silver and silver compounds, and performs nitrogen oxidation in a wide temperature range. Acts on object removal. As the porous inorganic oxide, alumina alone, or titania, silica, zirconia, zinc oxide, tin oxide,
A composite or mixed oxide of either magnesium oxide or zeolite and alumina can be used. When an alumina-containing composite or mixed oxide is used, the alumina content is preferably 50% by weight or more. The use of alumina or a composite or mixed oxide of alumina improves the heat resistance and durability of the catalyst. The tin oxide referred to in the present invention includes tin oxide in various oxidation states, such as stannous oxide and stannic oxide.

The particle size of the porous inorganic oxide such as alumina used for the first catalyst is preferably 0.1 mm or less.
When the particle size exceeds 0.1 mm, the contact area between the catalyst and the exhaust gas is reduced, and the effect of the catalytically active species cannot be sufficiently exhibited.
The specific surface area of the porous inorganic oxide such as alumina used for the first catalyst is preferably 10 m ² / g or more. If the specific surface area is less than 10 m ² / g, the dispersion of the silver component is reduced, and good removal of nitrogen oxides cannot be performed. More preferred specific surface area of the porous inorganic oxide is 30 m ² / g or more.

The silver compound is at least one selected from the group consisting of silver oxide, silver halide, silver sulfate and silver phosphate, and preferably at least one of silver oxide, silver chloride and silver sulfate. And more preferably silver oxide and / or silver chloride. The supported amount of the silver component is 0.2 to 15% by weight (in terms of silver element) based on 100% by weight of the porous inorganic oxide. If the amount is less than 0.2% by weight, the removal rate of nitrogen oxides decreases. When the silver component is carried in an amount exceeding 15% by weight, hydrocarbons are easily generated, and the nitrogen oxide removal rate is rather lowered. The preferred amount of the silver component is 0.1.
5 to 12% by weight.

As a method for supporting silver on an inorganic oxide such as alumina, a known impregnation method, precipitation method, or the like can be used. When using the impregnation method, the porous inorganic oxide is immersed in an aqueous solution of silver nitrate, chloride, sulfate, carbonate, or the like, or an aqueous ammonia solution. Alternatively, the porous inorganic oxide is immersed in an aqueous solution of silver nitrate, dried, and then immersed again in an aqueous solution of ammonium chloride or ammonium sulfate. To prepare silver halide by the precipitation method, silver nitrate and ammonium halide are reacted to precipitate silver halide on the porous inorganic oxide. After drying at 50 to 150 ° C., particularly at about 70 ° C., it is preferable to raise the temperature stepwise at 100 to 600 ° C. for firing. The calcination is preferably performed in air, under a stream of nitrogen containing oxygen or under a stream of hydrogen gas. When the treatment is performed under a hydrogen gas stream, it is preferable to perform the oxidation treatment at 300 to 650 ° C. at last. In carrying silver on alumina, an alumina-based mixed or composite oxide, it is effective to use alumina hydrate such as boehmite as a starting material.

(2) Second Catalyst The second catalyst is composed of zirconia alone or one or more elements selected from the group consisting of silver and silver compounds and / or zirconia.
Alternatively, a compound is supported. As with the first catalyst, the particle size of zirconia is preferably 0.1 mm or less, and the specific surface area is preferably 10 m ² / g or more.

When a silver component is supported, the silver compound is at least one selected from the group consisting of silver oxide, silver halide, silver sulfate, silver phosphate, and the like, preferably silver oxide, silver chloride and silver chloride. It is at least one of silver sulfates, and more preferably silver oxide and / or silver chloride. The loading amount of the silver component is 5% by weight with respect to 100% by weight of zirconia.
The following (silver element conversion value) is used. If the silver component is supported in an amount exceeding 5% by weight, combustion of the hydrocarbon itself tends to occur, and the nitrogen oxide removal rate is rather lowered. The preferred amount of the silver component is 3% by weight or less.

The method of supporting the silver component on zirconia is the same as the method described in the above (1) First catalyst.

In the present invention, a mixed catalyst obtained by mixing the first catalyst and the second catalyst is used. The weight ratio of the first catalyst to the second catalyst (the ratio of the total weight of the porous inorganic oxide and the catalytically active species) is preferably 1: 1 to 100: 1. More preferably, the weight ratio of the first catalyst to the second catalyst is 10: 9.
８０80: 1.

In the case where the form of the removing material is the first preferred embodiment described above, the thickness of the mixed catalyst provided on the removing material base is generally determined by the difference in thermal expansion characteristics between the base material and this catalyst. Often restricted. It is preferable that the thickness of the catalyst provided on the removing material substrate is 300 μm or less. With such a thickness, it is possible to prevent the removal material from being damaged by a thermal shock or the like during use. The method of forming the mixed catalyst on the surface of the removing material substrate is performed by a known wash coat method or the like.

The amount of the mixed catalyst provided on the surface of the substrate for removing material is preferably 20 to 300 g / liter of the substrate for removing material. If the amount of the catalyst is less than 20 g / liter, good NOx removal cannot be performed. On the other hand, when the amount of the catalyst is 300
When the amount exceeds g / liter, the removal characteristics do not increase so much.
Pressure loss increases. More preferably, the mixed catalyst provided on the surface of the removing material substrate is 50 to 200 g of the removing material substrate.
/ Liter.

If the removing material having the above-described structure is used, 150
In a wide temperature range of up to 600 ° C., good removal of nitrogen oxides can be performed even with exhaust gas containing about 10% of water.

[2] Method for Removing Nitrogen Oxide Next, the method of the present invention will be described. First, a nitrogen oxide removing material comprising a mixed catalyst of the first catalyst and the second catalyst is provided in the middle of an exhaust gas conduit.

The exhaust gas contains ethylene, propylene and the like to some extent as residual hydrocarbons. However, the amount is generally not enough to reduce NOx in the exhaust gas. To be introduced. The introduction position of the reducing agent is on the upstream side of the position where the removing material is installed.

As the hydrocarbons introduced from the outside, gaseous or liquid alkanes, alkenes and / or alkynes can be used under standard conditions. In particular, in the case of an alkane or alkene, it preferably has 2 or more carbon atoms. Specific examples of hydrocarbons that are liquid in the standard state include gas oil, cetane,
Examples include hydrocarbons such as heptane, kerosene, gasoline and the like.
Among them, hydrocarbons having a boiling point of 50 to 350 ° C are particularly preferable.

The amount of hydrocarbons introduced from the outside is preferably such that the weight ratio (weight of added reducing agent / weight of nitrogen oxides in the exhaust gas) is 0.1 to 5. If the weight ratio is less than 0.1, the removal rate of nitrogen oxides does not increase. On the other hand, if it exceeds 5, fuel efficiency will be degraded.

In the present invention, the space velocity of the mixed catalyst of the first catalyst and the second catalyst is 300,000 h ^-1 or less in order to efficiently promote the reduction and removal of nitrogen oxides by hydrocarbons.
Preferably, it is 200,000h ^-1 or less.

Further, in the present invention, the temperature of the exhaust gas is maintained at 200 to 600 ° C. at the removal material installation site where the hydrocarbon reacts with the nitrogen oxide. Exhaust gas temperature is 20
When the temperature is lower than 0 ° C., the reaction between the reducing agent and the nitrogen oxide does not proceed, and good removal of the nitrogen oxide cannot be performed. On the other hand, if the temperature exceeds 600 ° C., combustion of the hydrocarbon itself starts, and reduction and removal of nitrogen oxides cannot be performed. The preferred exhaust gas temperature is from 250 to 550 ° C, more preferably from 3 to 5 ° C.
00-550 ° C.

[0034]

The present invention will be described in more detail with reference to the following specific examples. Example 1 A commercially available γ-alumina powder (specific surface area: 200 m ² / g) using an aqueous solution of silver nitrate was 3.0% by weight (in terms of a metal element).
After drying, the mixture was dried and calcined in air stepwise to 600 ° C. to prepare a first catalyst.

First catalyst and commercially available zirconia (manufactured by Sumitomo Osaka Cement Co., Ltd., specific surface area 20 m ² / g, second catalyst)
Are mixed in a weight ratio of 40: 1, a binder (alumina sol binder) is added and kneaded, dried and then fired in air in a stepwise manner up to 600 ° C., pulverized, and pellets having an average particle diameter of 3 mm. And a material for removing nitrogen oxides was prepared.

1.5 g of the above-mentioned nitrogen oxide removing material is placed in a reaction tube.
And a gas having the composition shown in Table 1 (nitrogen monoxide, oxygen, propylene, nitrogen, and moisture) was flowed at a flow rate of 1.8 liters per minute (standard condition) (the contact time of the removing material was 0.1 mm).
05g · sec / ml and apparent space velocity is about 3
It is 0000h- ¹ . ), The temperature of the exhaust gas in the reaction tube is 3
While maintaining the temperature in the range of 00 to 600 ° C, propylene was reacted with nitrogen oxide.

The concentration of nitrogen oxides in the gas after passing through the reaction tube was measured with a chemiluminescent nitrogen oxide analyzer to determine the nitrogen oxide removal rate. Table 3 shows the results.

Table 1 Component concentration Nitric oxide 800 ppm Oxygen 10% by volume Propylene 1714 ppm (3 times the mass of nitrogen oxide) Nitrogen Residual moisture 10% by volume (based on the total volume of the above components)

Examples 2 to 4 In the same manner as in Example 1, 0.5% by weight (Example 2), 1% by weight (Example 3) and 3% by weight of silver were added to commercially available zirconia.
A second catalyst was prepared by supporting the weight% (Example 4) (both in terms of metal element). The first catalyst of Example 1 and each of the second catalysts were mixed at a weight ratio of 40: 1, and the respective removal materials were prepared in the same manner as in Example 1.

The removing materials of Examples 2 to 4 were set in the reaction tubes. Under the same reaction conditions as in Example 1 (the contact time of the removing material is about 0.05 g · sec / ml, and the apparent space velocity is about 30,000 h ⁻¹ ), the gas having the composition shown in Table 1 was used. The evaluation was carried out. Table 3 shows the results.

Example 5 In the same manner as in Example 1, powdery γ was prepared using an aqueous solution of silver nitrate.
- Alumina Silica (silica content of 3% by weight, a specific surface area of 350 meters ² / g) carrying the silver to 3 wt% (in terms of metal element value) to produce a first catalyst. Next, zirconia (Sumitomo Osaka Cement Co., Ltd., specific surface area 20 m ² /
g) was loaded with 1% by weight (in terms of metal element) of silver to prepare a second catalyst.

The first catalyst and the second catalyst were mixed at a weight ratio of 10: 1. After slurrying 1 g of the mixed catalyst, a commercially available cordierite honeycomb-shaped formed body (30 m in diameter)
m, length 12.6 mm, 400 cells / inch ² ), dried and baked stepwise to 600 ° C to prepare a nitrogen oxide removing material (a removing material coated with a mixed catalyst).

A nitrogen oxide removing material was set in the reaction tube. The same reaction conditions as in Example 1 (flow rate 4.4 liter /
And the apparent space velocity of the removing material was about 30,000 h ^-1 ), and the evaluation was performed using a gas having a composition shown in Table 1.
Table 3 shows the results.

Example 6 The nitrogen oxide removing material of Example 5 was set in a reaction tube, and the same reaction conditions as in Example 5 (flow rate 4.4 liter / min, apparent space velocity of the removing material was about 30,000 h). ^-1 )
The evaluation was performed using a gas having a composition shown in Table 2. Table 3 shows the results.

Table 2 Component concentration Nitric oxide 800 ppm Oxygen 10% by volume Light oil 3 times the mass of nitrogen oxides Nitrogen Residual water 10% by volume (based on the total volume of the above components)

Example 7 In the same manner as in Example 1, 3% by weight (in terms of metal element) of silver was supported on powdered stannic oxide (specific surface area: 71 m ² / g) using an aqueous silver nitrate solution. One catalyst was made. Next, 1% by weight (in terms of metal element) of silver was supported on zirconia (manufactured by Sumitomo Osaka Cement Co., Ltd., specific surface area: 20 m ² / g) by the same method to prepare a second catalyst.

The first catalyst and the second catalyst were mixed at a weight ratio of 10: 1. After slurrying 1 g of the mixed catalyst, a commercially available cordierite honeycomb-shaped formed body (30 m in diameter)
m, length 12.6 mm, 400 cells / inch ² ), dried and baked stepwise to 600 ° C to prepare a nitrogen oxide removing material (a removing material coated with a mixed catalyst).

A nitrogen oxide removing material was set in the reaction tube. The same reaction conditions as in Example 1 (flow rate 4.4 liter /
And the apparent space velocity of the removing material was about 30,000 h ^-1 ), and the evaluation was performed using a gas having a composition shown in Table 1.
Table 3 shows the results.

Comparative Example 1 A binder (alumina sol binder) was added to the first catalyst of Example 1 and mixed, followed by firing to 600 ° C. to prepare a silver-based catalyst. This silver-based catalyst was formed into a pellet having an average particle diameter of 3 mm to prepare a silver-based nitrogen oxide removing material.

1.5 g of the silver-based nitrogen oxide-removing material was set in the exhaust gas conduit, and under the same reaction conditions as in Example 1 (apparent space velocity was about 30,000 h ^-1 ), the composition shown in Table 1 was obtained. The evaluation was performed using the following gases. Table 3 shows the results.

COMPARATIVE EXAMPLE 2 1.5 g of the silver-based removing material of Comparative Example 1 was set in an exhaust gas conduit, and reacted under the same reaction conditions as in Example 6 (apparent space velocity was about 3).
(Equivalent to 3,000 h ⁻¹ )), and evaluation was performed using a gas having a composition shown in Table 2. Table 3 shows the results.

Table 3 Exhaust gas temperature (° C.) Example No. 300 350 400 450 500 550 600 Example 1 5 28 68 92 92 60 18 Example 2 6 32 76 76 88 88 60 18 Example 3 6 33 78 90 90 58 58 16 Example 4 6 33 80 90 88 56 15 Example 5 6 22 68 85 78 52 15 Example 6 4 18 72 82 65 34 16 16 Example 7 5 22 69 86 78 50 50 Comparative Example 1 2 10 50 90 90 78 18 18 Comparative Example 2 17 45 70 54 26 16

As can be seen from Table 3, the first catalyst and the second catalyst according to the present invention were mixed in comparison with Comparative Examples 1 and 2 using only the silver catalyst as the first catalyst. In Examples 1 to 7 used, good removal of nitrogen oxides was observed in a wide exhaust gas temperature range.

[0054]

As described above in detail, the use of the nitrogen oxide removing material of the present invention makes it possible to efficiently remove nitrogen oxides in exhaust gas containing excess oxygen in a wide temperature range. INDUSTRIAL APPLICABILITY The nitrogen oxide removing material and the removing method of the present invention can be widely used for removing nitrogen oxides from exhaust gas from various combustors, automobiles and the like.

Claims (6)

[Claims] 1. A nitrogen oxide removing material for reducing and removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in excess of a theoretical reaction amount for coexisting unburned components. And a first catalyst supporting 0.2 to 15% by weight (in terms of metal element) of one or more elements and / or compounds selected from the group consisting of silver and silver compounds, and zirconia or zirconia from silver and silver compounds. A nitrogen oxide removing material obtained by mixing a second catalyst supporting at least one element and / or a compound selected from the group consisting of 5% by weight or less (in terms of metal element). 2. The nitrogen oxide removing material according to claim 1, wherein the silver compound is at least one selected from the group consisting of silver oxide, silver halide, silver sulfate and silver phosphate. Nitrogen oxide removing material. 3. The nitrogen oxide removing material according to claim 1, wherein the porous inorganic oxide of the first catalyst is alumina alone, or titania, silica, zirconia, zinc oxide, tin oxide, or oxide. A nitrogen oxide removing material, which is a composite or mixed oxide of either magnesium or zeolite and alumina. 4. The nitrogen oxide removing material according to claim 1, wherein the mixed catalyst obtained by mixing the first catalyst and the second catalyst is a surface of a ceramic or metal substrate. A material for removing nitrogen oxides, which is coated on a substrate. 5. The nitrogen oxide removing material according to claim 1, wherein the mixed catalyst obtained by mixing the first catalyst and the second catalyst is a honeycomb type, a foam type, a plate type,
A nitrogen oxide removing material, which is formed into a pellet or a granule. 6. A method for removing nitrogen oxides from a combustion exhaust gas containing the nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for the co-existing unburned components, using the nitrogen oxides removing material according to any one of claims 1 to 5. In the nitrogen oxide removal method of reducing and removing, the nitrogen oxide removal material is installed in the middle of an exhaust gas conduit, gaseous hydrocarbons upstream of the removal material, kerosene, light oil, liquid hydrocarbons including gasoline and Exhaust gas to which at least one selected from the group consisting of is added, is brought into contact with the removing material at 200 to 600 ° C., thereby removing the nitrogen oxides by reaction with hydrocarbons in the exhaust gas. Nitrogen oxide removal method.