JP2516145B2 - Nitrogen oxide removal method - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Excessive combustion exhaust gas discharged 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 (generally called NOx) such as nitric oxide and nitrogen dioxide together with oxygen. Here, nitrogen oxides generally refer to nitrogen monoxide and / or nitrogen dioxide, and “containing excess oxygen” refers to carbon monoxide, hydrogen,
It means containing oxygen in excess of 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 method, ammonia used as a reducing agent for nitrogen oxides is expensive, and the amount of injected ammonia is measured while measuring the nitrogen oxide concentration in the exhaust gas so that unreacted ammonia is not discharged. Must be controlled, and the size of the device must be 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 No. 1-130735, and the 59th Annual Meeting of the Chemical Society of Japan
(1990) 2A526, 60th Autumn Meeting (1990) 3L420, 3L42
2, 3L423, etc.).

However, although these methods can remove nitrogen oxides with high efficiency from a simulated exhaust gas containing no water, the actual exhaust gas contains about 10% of water, so It was found that the oxide removal rate was significantly reduced. Also, with these methods,
There is also the disadvantage that the optimum temperature for the reduction reaction of nitrogen oxides is relatively high at around 400-600 ° C.

Further, a method of adding methanol to an exhaust gas and contacting the methanol-containing exhaust gas with alumina at about 300 ° C. to reduce nitrogen oxides (“Catalyst” vol.33,
No. 2, p. 59, 1991) has also been proposed, but this method is not practical because the nitrogen oxide reducing property is low.

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 excess oxygen conditions. 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.

[0011]

As a result of intensive research in view of the above problems, the present inventors have found that an oxygen-containing aliphatic organic compound having two or more carbon atoms and one or more oxygen atoms is subjected to nitrogen oxidation contained in exhaust gas. Exhaust gas containing about 10% water content if added in an amount commensurate with the amount of the substance and brought into contact with a purifying material in which silver or a silver oxide is supported on a porous inorganic oxide at a predetermined temperature. However, the inventors have found that nitrogen oxides can be effectively removed, and completed the present invention.

Namely, the nitrogen oxides, the method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing a greater than theoretical reaction amount of oxygen for combustion components end coexisting a carbon atom
Two or more, and one or more oxygen-containing organic compounds selected from the group consisting of alcohols, ketones, aldehydes, carboxylic acids, ethers and esters having at least one oxygen atom are added to the exhaust gas, and the oxygen-containing organic compounds are added. The exhaust gas containing the compound was added to a porous inorganic oxide containing silver or silver oxide in an amount of 0.
1 to 15% by weight (elemental conversion value) supported, and used as a purifying material obtained by coating the surface of a honeycomb-shaped or foam-shaped molded body with 2
The nitrogen oxide is removed by reacting the nitrogen oxide with the aliphatic oxygen-containing organic compound by bringing them into contact with each other at 00 to 600 ° C.

Hereinafter, the present invention will be described in detail. In the present invention, one or more kinds of aliphatic oxygen-containing organic compounds having two or more carbon atoms and one or more oxygen atoms are added to the exhaust gas, and the exhaust gas containing these additive substances is converted into a porous inorganic oxide. Contacting with a purifying material carrying silver or silver oxide on
The substances added to the exhaust gas are reacted with the nitrogen oxides in the exhaust gas to remove the nitrogen oxides. The aliphatic oxygen-containing organic compound will be described in detail later.

First, as a purifying material which becomes a reaction site between the aliphatic oxygen-containing organic compound added to the exhaust gas and the nitrogen oxide in the exhaust gas, porous inorganic oxide is supported with silver or silver oxide. Use one. As the porous inorganic oxide, porous alumina, titania, zirconia, composite oxides thereof, and the like can be used. Preferably, γ-alumina or an alumina-based composite oxide is used.

In the present invention, the purifying material in which silver or silver oxide is supported on the above-mentioned inorganic oxide such as γ-alumina and alumina-based composite oxide is used.
0.1 to 15% by weight of the inorganic oxide (element conversion value)
It is good to do. When the supported amount of silver or silver oxide is less than the lower limit of the above range, the effect of removing nitrogen oxides by supporting silver is not remarkable, and even when the amount of silver exceeding the upper limit is supported, nitrogen oxidation is carried out. There is no improvement in the removal rate, and the upper limit is 15
Weight% More preferably, the amount of silver or silver oxide supported is 0.5 to 10% by weight of the weight of the inorganic oxide. In addition, silver or silver oxide is supported on the inorganic oxide by immersing the above-mentioned porous inorganic oxide in an aqueous solution of silver nitrate or the like,
It can be carried out by drying at about 70 ° C. and then gradually raising the temperature at 70 to 550 ° C. and baking. In the case where the above calcination is performed under a flow of a reducing gas such as hydrogen, it is preferable that the calcination is further performed under a gas flow containing an oxidizing gas such as oxygen and then used. In the purification material obtained by firing, silver exists in the form of metal or oxide depending on the temperature.

The purifying material (inorganic oxide such as γ-alumina carrying silver or silver oxide) is installed in the exhaust gas conduit in the form of pellets, powder, honeycomb, foam or plate. To do. When the above-mentioned purification material is in the form of pellets or powder, it is preferable to put it in a casing and install it in the middle of the exhaust gas conduit. Further, (a) a cordierite or mullite excellent in heat resistance, or another ceramic material, or (b) a honeycomb-like or foam-like filter (support) made of a metal, on the surface of which is a powdery purification material (silver or Powdered inorganic oxide supporting silver oxide) is coated by a known method (for example, wash coating method), or the inorganic oxide is coated on the molded body , and then silver or silver oxide is supported and used. May be.

Since the reaction between the aliphatic oxygen-containing organic compound and the nitrogen oxide occurs at the site where the above-mentioned purification material is installed, the addition of the aliphatic oxygen-containing organic compound to the exhaust gas is the installation of this purification material. It becomes the upstream side of the part.

The aliphatic oxygen-containing organic compound added to the exhaust gas in the present invention has two or more carbon atoms and one or more oxygen atoms, and examples thereof include alcohols, ketones, aldehydes, carboxylic acids, ethers and esters. Etc. 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 oxides does not proceed well. It is particularly preferable to use acetone or the like as the ketone. Further, as the carboxylic acid, those having 2 or more carbon atoms (including the carbon of the carboxyl group) in the carboxylic acid are used. Acetic acid is particularly preferable. Acetaldehyde is preferred as the aldehyde.

In the present invention, alcohols, ketones,
It is preferable to maintain the temperature of the exhaust gas at the installation site of the purification material where the reaction of aldehyde, carboxylic acid, ether, ester and nitrogen oxide occurs, as follows. First, when the substance to be added is alcohol or ketone, the temperature of the exhaust gas at the site where the purification material is installed is maintained at 200 to 550 ° C. If the temperature of the exhaust gas is less than 200 ° C., the reaction characteristics between the alcohol or ketone and the nitrogen oxides are deteriorated, and the nitrogen oxides cannot be removed well. On the other hand, when the temperature exceeds 550 ° C., the added alcohol or ketone itself starts to burn, and the nitrogen oxide cannot be reduced and removed efficiently by adding these substances. Preferably, the exhaust gas temperature is set to 2
50 to 500 ° C.

When the carboxylic acid is added, it is preferable to keep the temperature of the exhaust gas at 300 to 600 ° C. If the exhaust gas temperature is lower than 300 ° C. or higher than 600 ° C., the reaction characteristics between the carboxylic acid and the nitrogen oxides are deteriorated, and the nitrogen oxides cannot be removed satisfactorily. More preferably, the temperature of the exhaust gas is 350 to 550 ° C.

[0021]

The present invention will be described in more detail by the following specific examples. Example 1 10 g of commercially available pelletized γ-alumina (diameter 1.5 mm,
Approximately 6 mm in length is a silver nitrate aqueous solution (silver nitrate concentration 0.035 g
/ Ml) and dried at 70 ° C., then under a nitrogen stream containing 5% by volume of hydrogen, 100 ° C., 125 ° C., 150 ° C., and 2
After firing at 00 ° C. for 2 hours each and then at 300 ° C. for 30 minutes, purging hydrogen with nitrogen gas, then under a nitrogen stream containing water, carbon dioxide gas, and oxygen at 10% each, 300 ° C., 400 ° C., and Firing was performed at 500 ° C. for 2 hours and at 550 ° C. for 5 hours to obtain a pellet-shaped purifying material carrying 2% by weight of silver (elemental conversion value) with respect to γ-alumina.

A gas having the composition shown in Table 1 (a total of 100% by volume of dry components consisting of nitric oxide, carbon dioxide, oxygen, ethanol and nitrogen) was filled with this purifying material in a reaction tube.
Further, 10% by volume of water was added) at a flow rate of 2 liters per minute (standard state) to make the temperature of the exhaust gas in the reaction tube 200 to 600 ° C. and to react ethanol and nitrogen oxides.

The concentration of nitrogen oxides (total amount of nitric oxide and nitrogen dioxide) in the gas after passing through the reaction tube was measured by a chemiluminescence type nitrogen oxide analyzer to determine the removal rate of nitrogen oxides. Table 2 shows the results.

Table 1 Concentration of components (volume ratio) Nitric oxide 500 ppm Carbon dioxide 10% Oxygen 10% Ethanol 500 ppm Nitrogen balance Moisture 10% with respect to the amount of gas composed of the above components

Comparative Example 1 The same purifying material as in Example 1 was used, except that the gas components shown in Table 1 were mixed with methanol instead of ethanol. A removal test was performed. The amount of methanol was 500 ppm, which is the amount of gas excluding water. The test results are shown in Table 2.

Comparative Example 2 Further , the same pellet-shaped γ-alumina as used in Example 1 (without supporting silver or silver oxide) was used, and the same procedure as in Example 1 was carried out except that nitrogen oxide was used. A removal test was performed. The test results are shown in Table 2.

Examples 2 to 6 Using the same purification material as in Example 1, the gas components shown in Table 1 were mixed with acetone (Example 2) instead of ethanol, and acetic acid (Example 3). Prepared, mixed with acetaldehyde (Example 4), mixed with 2-propanol (isopropyl alcohol) (Example 5), and 1-propanol (n-propyl alcohol)
The nitrogen oxide removal test was carried out in the same manner as in Example 1 except that the mixture (Example 6) was used. Here, in Examples 2 to 4, the amount of acetone, the amount of acetic acid,
Alternatively, the amount of acetaldehyde was 500 ppm of the amount of gas excluding water, and the concentrations of 2-propanol (Example 5) and 1-propanol (Example 6) were 333 ppm, respectively. The test results are shown in Table 2.

Example 7 10 g of commercially available pelletized γ-alumina (diameter 1.5 mm,
Approximately 6 mm in length is a silver nitrate aqueous solution (silver nitrate concentration 0.035 g
/ Ml), dried at 70 ° C., and baked at 100 ° C., 125 ° C., 150 ° C., and 200 ° C. for 2 hours each in a nitrogen stream containing 50% by volume of hydrogen, and then at 300 ° C. for 30 minutes. After purging hydrogen with nitrogen gas, it was fired at 300 ° C., 400 ° C., and 500 ° C. for 2 hours each at 550 ° C. for 5 hours in a nitrogen stream containing water, carbon dioxide gas, and oxygen at 10% each, and γ −
A pellet-like purification material carrying 2% by weight of silver (elemental conversion value) on alumina was obtained.

Using this purification material, the removal rate of nitrogen oxides was determined in the same manner as in Example 1. Table 2 shows the results.

Table 2 Nitrogen oxide removal rate (%) Example No. 200 ° C 250 ° C 300 ° C 350 ° C 400 ° C Example 1 ⁽¹⁾ 40 76 92 92 87 Example 2 ⁽²⁾ 45 89 98.9 99.3 99.2 Example 3 ⁽³⁾ -6 31 38 52 Example 4 ⁽⁴⁾ 36.9 67.2 76.9 76.8 72.7 Example 5 ⁽⁵⁾ 55 94 98 98.5 97.0 Example 6 ⁽⁶⁾ 58 79 82.5 75.3 65.6 Example 7 ⁽¹⁾ 48.6 86.7 95.2 90.4 84.2 Comparative example 1 ⁽⁷⁾ -15 12 12 6.5 Comparative example 2 ⁽¹⁾ -9 27 11 6 Table 2 (continued) Nitrogen oxide removal rate (%) Example No. 450 ℃ 500 ℃ 550 ℃ 600 ℃ Example 1 ⁽¹⁾ 78 63 45 − Example 2 ⁽²⁾ 97.6 85 60 − Example 3 ⁽³⁾ 58 51 41 35 Example 4 ⁽⁴⁾ 64.4 48.5 21.8 − Example 5 ⁽⁵⁾ 86.6 88.3 46.2 − Implementation Example 6 ⁽⁶⁾ 56.7 44.9 30.0-Example 7 ⁽¹⁾ 75.6 60.7 40.5-Comparative Example 1 ⁽⁷⁾ 3.1 1.6 1.3-Comparative Example 2 ⁽¹⁾ 6.6 10 28-Table 2 Note (1): Ethanol was added . (2): Acetone was added. (3): Acetic acid was added. (4): Acetaldehyde was added. (5): 2-Propanol was added. (6): 1-Propanol was added. (7): Methanol was added.

As can be seen from Table 2, in Examples 1, 2 and 4, effective removal of nitrogen oxides was observed in the exhaust gas temperature range of 200 to 550 ° C. Further, in Example 3, effective removal of nitrogen oxides was observed at 300 to 600 ° C. Particularly, in Example 1, when the exhaust gas temperature in the vicinity of the purification material was 250 to 400 ° C, the nitrogen oxide removal rate exceeded 75%. Further, Example 5
And, in Example 6, particularly high removal of nitrogen oxide was observed at 250 to 500 ° C. Further, as can be seen from Example 7, in the purification material having the catalyst that is fired in the nitrogen atmosphere having a high hydrogen concentration, the removal rate of nitrogen oxides is particularly high on the low temperature side. On the other hand, in the comparative example, the nitrogen oxide removal rate was remarkably smaller than that in the example over the measurement temperature.

[0032]

As described in detail above, according to the method of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed. Further, according to the method of the present invention, nitrogen oxides can be efficiently removed even when the exhaust gas contains about 10% of water. Furthermore, the nitrogen oxide removal temperature (exhaust gas temperature) is also 200 to 600 ° C.
And, the temperature can be lower than that of the conventional removal methods.

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-4-354536 (JP, A)

Claims (3)

(57) [Claims] 1. A 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, wherein at least two carbon atoms are contained.
In addition, one or more kinds of aliphatic oxygen-containing organic compounds selected from the group consisting of alcohols, ketones, aldehydes, carboxylic acids, ethers and esters having one or more oxygen atoms are added to the exhaust gas, and the aliphatic oxygen-containing compounds are added. Exhaust gas with organic compounds added to porous inorganic oxides with silver or silver oxides
0.1 to 15% by weight (elemental conversion value) is supported, and the nitrogen oxide and the fat are brought into contact with a purifying material formed by coating the surface of a honeycomb-shaped or foam-shaped molded body at 200 to 600 ° C. A method for removing nitrogen oxides, which comprises removing the nitrogen oxides by reacting with a group oxygen-containing organic compound. 2. The method according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide. 3. The method according to claim 1 or 2, wherein
The purifying material carries silver or silver oxide on a porous inorganic oxide.
A method for removing nitrogen oxides, which is characterized in that the method is carried out and the temperature is raised stepwise at 70 to 550 ° C. and baked .