JPH07284664A - Waste gas purification material and method for purifying waste gas - Google Patents

Abstract

PURPOSE:To efficiently remove NOx from waste combustion gas contg. excess oxygen by using a purification material obtd. by carrying silver and/or a silver compd. or a mixture of them on a porous alumina-based multiple oxide consisting of alumina and silica, titania or zirconia. CONSTITUTION:When a waste gas purification material is made of a catalyst obtd. by carrying 0.2-15wt.% (expressed in terms of elements) silver and/or silver compd. or mixture of them on a porous inorg. oxide, a porous alumina- based multiple oxide consisting of alumina and at least one kind of oxide selected from among silica, titania and zirconia is used as the porous inorg. oxide to obtain the objective waste gas purification material for reduction and removal of NOx from waste combustion gas contg. NOx and a larger amt. of oxygen than the theoretical amt. of oxygen reacting with coexistent unburned components. This waste gas purification material is put in the middle of a waste gas duct, hydrocarbon and/or >=2C oxygen-contg. org. compd. is added to waste gas at the upper stream side of the purification material and then the waste gas is brought into contact with the purification material.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas discharged from internal combustion engines such as automobile engines, combustion equipment installed in factories, household fan heaters, etc. Nitrogen oxides (generally called NOx) such as nitric oxide and nitrogen dioxide are contained together with oxygen. Here, the nitrogen oxides generally refer to nitric oxide and / or nitrogen dioxide, and the phrase "containing excess oxygen" means carbon monoxide, hydrogen,
It is meant to contain more oxygen than the theoretical amount of oxygen required to burn unburned components such as hydrocarbons.

Such nitrogen oxides are considered to be one of the causes of acid rain and have become a serious environmental problem. Therefore, various methods for removing nitrogen oxides in exhaust gas discharged from various combustion devices have been studied.

As a method for removing nitrogen oxides from combustion exhaust gas containing excess oxygen, a selective catalytic reduction method using ammonia is used, particularly for large-scale fixed combustion devices (large combustors such as factories). It has been put to practical use.

However, in the above-mentioned method, ammonia used as a reducing agent for nitrogen oxides is expensive, and ammonia is toxic, so that the unreacted ammonia is not oxidized by nitrogen oxidation in the exhaust gas. There are problems that the ammonia injection amount must be controlled while measuring the substance concentration, and that the device becomes large.

[0006] As another method, there is a non-selective catalytic reduction method for reducing nitrogen oxides by using a gas such as hydrogen, carbon monoxide or hydrocarbon as a reducing agent, but this method is effective. In order to reduce and remove nitrogen oxides, it is necessary to add a reducing agent in an amount equal to or larger than a theoretical reaction amount with oxygen in exhaust gas, and there is a drawback that a large amount of reducing agent is consumed. Therefore, the non-selective catalytic reduction method is practically effective only for the exhaust gas having a low residual oxygen concentration that is burned in the vicinity of the theoretical air-fuel ratio, and is not versatile and impractical.

Therefore, there has been proposed a method for removing nitrogen oxides by adding a reducing agent in an amount equal to or less than a theoretical reaction amount with oxygen in exhaust gas by using zeolite or a catalyst supporting a transition metal thereon (for example, a special method). Kaisho 63-100919, 63-28372
No. 7, JP-A-1-130735, and 59th Annual Meeting of the Chemical Society of Japan
(1990) 2A526, 60th Autumn Meeting (1990) 3L420, 3L42
2, 3L423, "Catalyst" vol.33 No.2, page 59, 1991 etc.).

However, it has been found that these methods have a narrow temperature range for removing nitrogen oxides and that the exhaust gas containing water has a significantly low nitrogen oxide removal rate. Therefore, the present inventors have a silver catalyst, and
Even in an exhaust gas containing 100% of water, a purifying material that can effectively remove nitrogen oxides by the action of ethanol and a silver catalyst added to the exhaust gas was previously proposed (Japanese Patent Laid-Open No. 5-317647).
issue). However, the nitrogen oxide removal rate in the presence of SOx is not yet sufficient.

Therefore, an object of the present invention is to provide unburned components such as nitrogen oxides, carbon monoxide, and hydrocarbons such as combustion exhaust gas from a fixed combustion device and a gasoline engine, a diesel engine, etc. that burn under an excess oxygen condition. It is an object of the present invention to provide a method capable of efficiently removing nitrogen oxides from a combustion exhaust gas containing oxygen in an amount equal to or more than the theoretical reaction amount with respect to, particularly a combustion exhaust gas containing sulfur oxides.

[0010]

As a result of earnest research in view of the above problems, the present inventor has added a reducing agent to exhaust gas and purifying it by supporting silver and / or a silver compound on an alumina-based composite oxide. The inventors have found that nitrogen oxides can be effectively removed even in exhaust gas containing sulfur oxides by contacting the material at a predetermined temperature, and completed the present invention.

That is, 0.2 to 15% by weight of a porous inorganic oxide containing silver and / or a silver compound or a mixture thereof.
(Elementary conversion value) The exhaust gas purifying material of the present invention, which comprises a supported catalyst and reduces and removes nitrogen oxides from combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components, The porous inorganic oxide is an alumina-based composite oxide composed of alumina and at least one selected from the group consisting of silica, titania and zirconia.

Further, the exhaust gas purification method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components uses the above exhaust gas purification material, The exhaust gas purifying material is installed in the middle of an exhaust gas conduit, and the exhaust gas to which hydrocarbon and / or oxygen-containing organic compound having 2 or more carbon atoms or a fuel containing the same is added on the upstream side of the purifying material is 150 to 650 ° C. In step 1, the cleaning material is brought into contact with the purifying material to remove the nitrogen oxides.

The present invention will be described in detail below. [1] Purifying material In the present invention, an aliphatic oxygen-containing organic compound and / or a hydrocarbon having two or more carbon atoms is added to the exhaust gas, and the exhaust gas containing these added substances is converted into a porous inorganic oxide. A substance is added to exhaust gas by reacting it with a nitrogen oxide in the exhaust gas by contacting with a purifying material in which silver and / or a silver compound or a mixture thereof is supported on a certain alumina-based oxide to form the nitrogen oxide. Remove.

The first preferred form of the exhaust gas purifying material of the present invention is a purifying material obtained by coating a purifying material base with a catalyst comprising a powdery porous inorganic oxide carrying a catalytically active species, or a powdery material. This is a purifying material obtained by coating a purifying material base with the porous inorganic oxide of (1) and then carrying a catalytically active species. As a ceramic material forming the base of the purification material, γ-alumina and its composite oxide (γ-alumina-titania, γ-alumina
Alumina-silica, γ-alumina-zirconia, etc.), zirconia, titania-zirconia, and other heat-resistant materials having a large surface area are mentioned. When high heat resistance is required, cordierite, mullite, alumina and their composites are preferably used. Also, a known metal material can be used for the substrate of the exhaust gas purifying material.

The shape and size of the substrate of the exhaust gas purifying material is
Various changes can be made according to the purpose. Examples of the structure of the substrate include a honeycomb structure type, a foam type, a three-dimensional network structure type made of fibrous refractory, a granular form, a pellet form and the like.

A second preferred embodiment of the exhaust gas purifying material of the present invention is a pellet-like, granular-like or powder-like porous inorganic oxide carrying a catalytically active species, or a porous inorganic oxide carrying a catalytically active species. It is a purifying material obtained by filling a casing having a desired shape with a catalyst formed by molding an article into a pellet, granule or powder.

(1) Porous Inorganic Oxide As a purifying material which becomes a reaction site between the reducing agent added to the exhaust gas and the nitrogen oxide in the exhaust gas, silver and / or a silver compound, or porous inorganic oxide, or The thing which carries those mixtures is used. As the porous inorganic oxide, an alumina-based composite oxide containing at least one selected from the group consisting of silica, titania and zirconia is used. The content of alumina is preferably 50% by weight or more.
If the content of alumina is less than 50% by weight, the initial removal characteristics of the purification material will be significantly deteriorated. By using the alumina-based composite oxide, the reaction between the added reducing agent or the fuel containing the reducing agent and the nitrogen oxide in the exhaust gas occurs efficiently. In particular, by using an alumina-based composite oxide, S
Effective removal of nitrogen oxides can be performed even in the presence of O ₂ gas.

The specific surface area of the porous inorganic oxide is 10 m ²
/ G or more is preferable. Specific surface area of 10 m ² / g
If it is less than the above range, the contact area between the exhaust gas and the inorganic oxide (and the silver component carried on the exhaust gas) becomes small, and the nitrogen oxide cannot be removed well. The more preferable specific surface area of the porous inorganic oxide is 30 m ² / g or more.

(2) Silver Component Examples of the silver component supported on the above-mentioned alumina composite oxide as an active species include silver, silver oxide, silver halide and the like. The preferred silver component is silver and / or silver chloride. The loading amount of silver and / or silver compound or their mixture is 0.2 to 15% by weight based on the weight of the alumina composite oxide.
(Elemental conversion value) is recommended. When the supported amount of silver and / or a silver compound, or a mixture thereof is less than the lower limit of the above range, the effect of removing nitrogen oxides by supporting silver is not remarkable, and the amount of silver exceeding the upper limit is No improvement in the removal rate of nitrogen oxides was observed even when loaded, and the upper limit is set to 15% by weight. More preferably, the supported amount of silver and / or a silver compound, or a mixture thereof, is 0.5 to 10% by weight of the weight of the alumina composite oxide.

The silver component supported on the porous inorganic oxide is in the form of fine particles, and preferably has an average particle size of 10 to 1000 nm. Generally, the smaller the average particle size of the silver component particles is, the better the reaction characteristics are. However, if the average particle size is less than 10 nm, the oxidation reaction of the reducing agent hydrocarbon or the oxygen-containing organic compound proceeds too much. The removal rate is low. On the other hand, when the average particle size of the silver component particles exceeds 1000 nm, the reaction characteristics of the silver component deteriorate, and the nitrogen oxide removal rate decreases. The average particle diameter of the silver component particles is preferably 20 to 200 nm. Here, the average particle diameter is obtained by the arithmetic mean of the diameters of the particles.

(3) Supporting method The supporting of the silver and / or silver compound or a mixture thereof on the inorganic oxide is carried out by immersing the above-mentioned porous inorganic oxide in an aqueous solution of silver nitrate and drying at about 70 ° C. Then, it can be carried out by gradually raising the temperature at 70 to 550 ° C. and firing. When the above firing is carried out under a reducing gas flow such as hydrogen, it is desirable to use it after further firing under an air flow containing an oxidizing gas such as oxygen.

In the first preferred form of the purification material, the thickness of the catalyst provided on the purification material substrate is generally limited due to the difference in thermal expansion characteristics between the substrate material and the catalyst. . The thickness of the catalyst provided on the purification material substrate should be 300μ
It is preferably m or less. With such a thickness, it is possible to prevent the purification material from being damaged by thermal shock during use. The method of forming the catalyst on the surface of the purification material substrate is performed by a known wash coat method, powder method or the like.

The amount of the catalyst provided on the surface of the purification material substrate is preferably 20 to 300 g / liter of the purification material substrate. If the amount of catalyst is less than 20 g / liter, good NOx cannot be removed. On the other hand, the amount of catalyst is 300 g /
If it exceeds liter, the removal characteristic does not improve so much and the pressure loss increases. More preferably, the first catalyst provided on the surface of the purification material substrate is 50 to 250 g / of the purification material substrate.
Let it be liters.

If the purifying material having the above structure is used,
In a wide temperature range of up to 650 ° C., good nitrogen oxides can be removed even with exhaust gas containing about 10% of water.

Next, the method of the present invention will be described. [2] Exhaust Gas Purification Method Although the exhaust gas contains ethylene, propylene and the like as residual hydrocarbons to some extent, it is generally not sufficient to reduce NOx in the exhaust gas, so hydrocarbons and / or external A reducing agent composed of an oxygen-containing organic compound having 2 or more carbon atoms or a mixed fuel containing them is introduced into the exhaust gas. The introduction position of the reducing agent is upstream of the position where the purification material is installed.

(1) Hydrocarbon As the hydrocarbon introduced from the outside, a gaseous or liquid alkane, alkene and / or alkyne in a standard state can be used. The gaseous hydrocarbon in the standard state is preferably an alkane, alkene, or alkyne having 2 or more carbon atoms. As a liquid hydrocarbon in the standard state,
Specific examples thereof include hydrocarbons such as heptane, cetane, kerosene, light oil, gasoline and heavy oil. Among them, boiling point 5
Hydrocarbons at 0-350 ° C are particularly preferred.

(2) Oxygen-containing organic compound As the oxygen-containing organic compound introduced from the outside, for example,
Examples thereof include alcohols, ketones, aldehydes, carboxylic acids, ethers and esters. The alcohol is an aliphatic monohydric alcohol having 2 or more carbon atoms. Among these, it is particularly preferable to use a relatively low molecular weight alcohol such as ethanol or propanol (n-propyl alcohol, isopropyl alcohol). When methanol is used, the reaction with nitrogen oxide does not proceed well. As the ketone, it is particularly preferable to use acetone or the like.
Further, as the carboxylic acid, one having 2 or more carbon atoms (including carbon of the carboxyl group) in the carboxylic acid is used.
Acetic acid is particularly preferable. Acetaldehyde is preferred as the aldehyde. The amount of reducing agent introduced from the outside is
The weight ratio (weight of reducing agent to be added / weight of nitrogen oxide (NO) in exhaust gas) is preferably 0.1 to 5. If this weight ratio is less than 0.1, the nitrogen oxide removal rate does not increase. On the other hand, if the weight ratio exceeds 5, it leads to deterioration of fuel efficiency.

When a fuel containing a hydrocarbon or an oxygen-containing organic compound is added, it is preferable to use gasoline, light oil, kerosene or the like as the fuel. In this case, the amount of the reducing agent is set so that the weight ratio (weight of reducing agent to be added / weight of nitrogen oxide in exhaust gas) is 0.1 to 5 similarly to the above.

(3) Contact time In the present invention, the contact time between the purifying material and the exhaust gas is 0.006 g · second in order to efficiently proceed the reduction and removal of nitrogen oxides by the oxygen-containing organic compound or hydrocarbon. / Ml or more. When the contact time is less than 0.006 g · sec / ml, the reduction reaction of nitrogen oxides does not sufficiently occur and the removal rate of nitrogen oxides decreases. The preferred contact time is 0.007
g · sec / ml or more.

(4) Catalytic reaction temperature In the present invention, the temperature of the exhaust gas at the purification material installation site, which is the site where the reducing agent reacts with the nitrogen oxide, is 150 to 65.
Keep at 0 ° C. If the temperature of the exhaust gas is less than 150 ° C., the reaction between the reducing agent and the nitrogen oxide does not proceed, and the nitrogen oxide cannot be removed satisfactorily. On the other hand, when the temperature exceeds 650 ° C., the reducing agent itself is preferentially burned and the reduction rate of nitrogen oxides is reduced. The preferred exhaust gas temperature is 250-
It is 600 ° C.

[0031]

The present invention will be described in more detail by the following specific examples. Example 1 10 g of commercially available powdery silica-alumina (SiO ₂ content 5% by weight, specific surface area 350 m ² / g) was loaded with 5% by weight (elemental conversion value) of silver by using an aqueous solution of silver nitrate, and had a diameter of 0. It was molded into a pellet having a size of 0.5 to 1 mm and a length of 2 to 3 mm, dried and then fired stepwise in air to 600 ° C. to prepare a purification material.

About 1.5 g of the purification material was set in the reaction tube. Next, gases having the composition shown in Table 1 (nitrogen monoxide, oxygen,
Ethanol, sulfur dioxide, nitrogen and water) 1.8 per minute
Flow at a flow rate of liter (standard state) (contact time is about 0.05 g · sec / ml), change the temperature of exhaust gas in the reaction tube from 250 ° C to 600 ° C, and perform ethanol and nitrogen oxidation at each temperature. Reacted with things.

Table 1 Component Concentration Nitric oxide 800 ppm (dry basis) Oxygen 10% by volume (dry basis) Ethanol 3 times the mass of nitric oxide (dry basis) Sulfur dioxide 80 ppm (dry basis) Nitrogen Residual moisture 10 volume % (Relative to the total volume of the above ingredients)

Nitrogen oxides (NO +) of the gas after passing through the reaction tube
The concentration of NO ₂ ) was measured by a chemiluminescence type nitrogen oxide analyzer to determine the nitrogen oxide removal rate. The results are shown in Fig. 1.

Example 2 The same purifying material as in Example 1 was used at a temperature of 450 ° C. for 200 times in a gas having the composition shown in Table 1 excluding ethanol.
After exposure for a period of time, a nitrogen oxide removal test was conducted in the same manner as in Example 1. The test results are shown in FIG.

Comparative Example 1 Commercially available powdery γ-alumina (specific surface area 260 m ² / g)
5 g of silver was carried on 1.5 g by the same method as in Example 1 (element conversion value), the diameter was 0.5 to 1 mm, and the length was 2 to 3 m.
m was molded into pellets, dried and then fired stepwise in air to 600 ° C. to prepare a purification material.

After the purifying material was exposed to the SO _x -containing gas in the same manner as in Example 2, a nitrogen oxide removal test was conducted on the gas components shown in Table 1. The test results are shown in FIG.

Comparative Example ² 1.5 g of commercially available powdery silica-alumina (SiO ₂ content 60% by weight, specific surface area 490 m ^{2 /} g) was added to Example 1
In the same manner as above, 5% by weight of silver (elemental conversion value) was carried, and the pellet was formed into a pellet having a diameter of 0.5 to 1 mm and a length of 2 to 3 mm, dried and then fired stepwise in air to 600 ° C. A purification material was prepared.

In the same manner as in Example 1, a nitrogen oxide removal test was conducted on the gas components shown in Table 1. The test results are shown in FIG.

As can be seen from FIG. 1, in Examples 1 and 2, effective removal of nitrogen oxides was observed in the exhaust gas temperature range of 250 to 600 ° C. Particularly, in Example 2 in which the purifying material was exposed to the sulfur dioxide gas for a long time, the nitrogen oxide removal rate was not significantly decreased. On the other hand, in Comparative Examples 1 and 2, the removal rate of nitrogen oxides was remarkably smaller than that of the Examples over the measurement temperature.

[0041]

As described in detail above, according to the purifying material of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed. In particular, the nitrogen oxides can be efficiently removed even when the exhaust gas contains sulfur oxides.

The exhaust gas purifying material and the exhaust gas purifying method of the present invention can be widely used for removing nitrogen oxides contained in the exhaust gas of various combustors, automobiles and the like.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between nitrogen oxide removal rate and exhaust gas temperature in Examples 1 and 2 and Comparative Examples 1 and 2.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location B01J 27/25 ZAB A B01D 53/36 102 B

Claims (5)

[Claims] 1. A catalyst comprising a porous inorganic oxide carrying 0.2 to 15% by weight (elemental conversion value) of silver and / or a silver compound or a mixture thereof, and coexists with a nitrogen oxide. In an exhaust gas purification material for reducing and removing nitrogen oxides from a combustion exhaust gas containing oxygen with a larger amount than the theoretical reaction amount for unburned components, the porous inorganic oxide is at least one selected from the group consisting of silica, titania and zirconia. An exhaust gas purifying material, which is an alumina-based composite oxide composed of alumina. 2. The exhaust gas purifying material according to claim 1, wherein the silver compound is silver oxide and / or silver chloride. 3. The exhaust gas purifying material according to claim 1, wherein the purifying material is obtained by coating the surface of a ceramic or metal base with the catalyst. 4. The exhaust gas purifying material according to claim 1 or 2, wherein the porous inorganic oxide is in the form of pellets or granules. 5. An exhaust gas purification method for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components.
4 is used, the exhaust gas purifying material is installed in the middle of an exhaust gas conduit, and a hydrocarbon and / or an oxygen-containing organic compound having 2 or more carbon atoms is provided upstream of the purifying material, or Exhaust gas added with fuel containing it, 150
An exhaust gas purification method, which comprises contacting the purification material at ˜650 ° C. to remove the nitrogen oxides.