JPH0576757A - Catalyst for removal of nitrogen oxide and method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To effectively remove NOx from exhaust gas contg. NOx and excess oxygen. CONSTITUTION:A catalyst made of a porous inorg. oxide or obtd. by supporting a metal component on the oxide is set in an exhaust gas duct and exhaust gas is brought into contact with the catalyst at 250-550 deg.C. At this time, 1ml (expressed in terms of a standard state) of the exhaust gas is allowed to keep contact with 1g of the catalyst for >=0.03sec. Residual acetylene and NOx in the exhaust gas are brought into a reaction to remove the NOx.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Excessive amounts of combustion exhaust gas emitted from internal combustion engines such as automobile engines, combustion equipment installed in factories, household fan heaters, etc. Nitrogen oxides such as nitric oxide and nitrogen dioxide are contained together with oxygen. Here, "containing excess oxygen" means containing more oxygen than the theoretical oxygen amount necessary to burn unburned components such as carbon monoxide, hydrogen, and hydrocarbons contained in the exhaust gas. To do. Moreover, the nitrogen oxide in the following refers to nitric oxide and / or nitrogen dioxide.

This nitrogen oxide is considered to be one of the causes of acid rain and has 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 a combustion exhaust gas containing excess oxygen, a selective catalytic reduction method using ammonia is used particularly for a large-scale fixed combustion device (large combustor in a factory etc.). It 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, so that the nitrogen oxides in the exhaust gas are prevented so that unreacted ammonia is not discharged. There are problems that the ammonia injection amount must be controlled while measuring the concentration, and that the apparatus is generally 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, which 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 with a low residual oxygen concentration burned in the vicinity of the stoichiometric 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, although these methods can remove nitrogen oxides with high efficiency from 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 as high as about 400-600 ° C.

Therefore, an object of the present invention is to remove nitrogen oxides, carbon monoxide, hydrogen hydrocarbons, etc., such as combustion exhaust gas from a fixed combustion apparatus and a gasoline engine, a diesel engine, etc. that burn under an excess oxygen condition. From the combustion exhaust gas that contains more than the theoretical reaction amount of oxygen for combustion,
It is an object of the present invention to provide a nitrogen oxide removal catalyst and a removal method capable of efficiently removing nitrogen oxides.

[0010]

As a result of earnest research in view of the above problems, the present inventor has found that a porous inorganic oxide or a catalyst having a metal component supported on the porous inorganic oxide is used at a specific temperature and If the exhaust gas is contacted for the contact time, the water content will be 10%
It was discovered that even with exhaust gas containing a small amount, nitrogen oxides can be effectively removed by residual acetylene in the exhaust gas and / or acetylene added to the exhaust gas to match the amount of nitrogen oxides contained in the exhaust gas. Then, the present invention has been completed.

That is, the catalyst of the present invention for removing nitrogen oxides from combustion exhaust gas containing nitrogen oxides and oxygen in a larger amount than the theoretical reaction amount for coexisting unburned components is a porous inorganic oxide or a metal component thereof. Carrying 250,
At 550 ° C., residual acetylene in the exhaust gas and / or acetylene externally added to the exhaust gas is used as a reducing agent to accelerate the reaction of reducing nitrogen oxides in the exhaust gas.

Further, the 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 is a porous inorganic oxide or a metal component thereof. The catalyst supporting the catalyst is installed in the middle of the exhaust gas conduit, and the exhaust gas is brought into contact with the catalyst at 250 to 550 ° C., in which case 1 ml of the exhaust gas (value converted into the standard state) comes into contact with 1 g of the catalyst. Set the time to 0.
By setting the time to be at least 03 seconds, the residual acetylene in the exhaust gas is reacted with the nitrogen oxides to remove the nitrogen oxides.

In a preferred embodiment of the method of the present invention, acetylene of 200% by volume or less of the amount of nitrogen oxides in the exhaust gas is added upstream of the site where the catalyst is installed, and the residual acetylene in the exhaust gas and the added acetylene are added. To reduce and remove the nitrogen oxide.

The present invention will be described in detail below. In the present invention, the catalyst shown below is used, and by contacting the exhaust gas with this catalyst, acetylene in the exhaust gas and / or acetylene added to the exhaust gas on the upstream side from the site where the catalyst is installed are used as reducing agents for nitrogen in the exhaust gas. The oxide is reduced and removed.

The catalyst of the present invention comprises a porous inorganic oxide or a metal component supported on it. As the porous inorganic oxide, porous alumina, titania, zirconia, and their composite oxides can be used, but γ-alumina or alumina-based composite oxide is preferably used. By using γ-alumina or an alumina-based composite oxide, the reaction between the added acetylene and / or residual acetylene in the exhaust gas and the nitrogen oxide in the exhaust gas occurs efficiently.

In the present invention, it is preferable to use a catalyst obtained by further supporting a metal component on the above-mentioned inorganic oxide such as γ-alumina. Examples of the metal component include silver and cobalt, and it is particularly preferable to use silver.

The supported amount of silver is preferably 5% by weight (elemental conversion value) or less of the weight of the inorganic oxide. When the above-mentioned amount of silver is supported on the inorganic oxide, the removal rate of nitrogen oxides is further improved. When silver is loaded in an amount exceeding the above upper limit, acetylene itself is easily burned, and the nitrogen oxide removal rate is rather lowered. The silver supported on the inorganic oxide is in a metal or oxide state, although it varies depending on the exhaust gas temperature.

Next, the method of the present invention will be described. First, the catalyst described above is installed in the middle of the exhaust gas conduit. γ-
The inorganic oxide such as alumina, or the catalyst obtained by supporting a metal component such as silver on it can be used in the form of pellet, powder, honeycomb, foam, plate or the like.

In the method of the present invention, nitrogen oxides are removed using residual acetylene in exhaust gas as a reducing agent. Generally, the unburned unsaturated hydrocarbons in the exhaust gas discharged from the diesel engine are about 40 ppm, but the amount of (residual) acetylene in the exhaust gas can be increased by adjusting the engine and the engine surroundings. However, when acetylene in an amount sufficient to reduce the nitrogen oxides in the exhaust gas does not remain in the exhaust gas, acetylene is added from the outside to the upstream side of the above-mentioned catalyst installation site.

The amount of acetylene added to the exhaust gas is preferably 200% by volume or less of the volume of nitrogen oxide in the exhaust gas. When acetylene in an amount exceeding 200% by volume of the amount of nitrogen oxides is added, acetylene becomes excessive as compared with nitrogen oxides, and unreacted acetylene remains in the exhaust gas to cause secondary pollution.

In the present invention, the time during which the exhaust gas containing acetylene is in contact with the above-mentioned catalyst is adjusted so that the reaction between acetylene and nitrogen oxides proceeds efficiently. The contact time between the exhaust gas containing acetylene and the catalyst is 0.03 g · sec / ml or more. Unit of contact time here (g · sec / ml)
Represents the time (seconds) in which 1 ml of the exhaust gas containing acetylene (however, the volume converted to the standard state) is in contact with 1 g of the catalyst. That is, the contact time is 0.03 g · sec / ml
Means, for example, when 1 g of catalyst is used, 1 ml of exhaust gas contacts this catalyst for 0.03 seconds.

When the contact time with the catalyst is less than 0.03 g · sec / ml, the reaction between the added acetylene and the nitrogen oxides does not sufficiently occur and the removal rate of the nitrogen oxides decreases. In addition, unreacted acetylene also remains in the exhaust gas. The contact time is preferably 0.1 g · sec / ml or more.

Further, in the present invention, the temperature of the exhaust gas at the catalyst installation site where the reaction between acetylene and nitrogen oxide occurs is maintained at 250 to 550 ° C. If the temperature of the exhaust gas is less than 250 ° C., the reaction between acetylene and nitrogen oxides does not proceed, and good removal of nitrogen oxides cannot be performed. On the other hand, if the temperature exceeds 550 ° C., the combustion of acetylene itself starts, and the nitrogen oxide cannot be reduced and removed by the addition of acetylene.

[0024]

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, length: about 6 mm) was filled in a reaction tube, and a gas having the composition shown in Table 1 (nitrogen monoxide, carbon dioxide, oxygen, acetylene, and nitrogen) was used. 100% by volume of the dry component consisting of 10% by volume of water was further added) at a flow rate of 2 liters per minute (standard condition) (contact time 0.3 g · sec / ml), Keep the exhaust gas temperature in the range of 300-550 ° C,
Acetylene was reacted with nitrogen oxide.

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. The results are shown in Table 2.

Table 1 Component Concentration Nitric oxide 500 ppm Carbon dioxide 10% by volume Oxygen 10% by volume Acetylene 500 ppm Nitrogen balance Moisture 10% by volume with respect to the amount of gas composed of the above components

Comparative Examples 1 to 3 For comparison, in the gas components shown in Table 1, ethylene, propylene, or propane (each having a concentration of 500 ppm on a dry matter basis) was used in place of acetylene. A nitrogen oxide removal test was conducted in the same manner as in Example 1. The results are shown in Table 2.

Table 2 Nitrogen oxide removal rate (%) Example No. Additive 300 ° C 350 ° C 400 ° C 450 ° C 500 ° C 550 ° C Example 1 Acetylene 2 8 32 45 45 33 Comparative Example 1 Ethylene 0 1 3 3 5 2 Comparative Example 2 Propylene 0 1 4 10 22 28 Comparative Example 3 Propane 0 0 1 4 17 33

Example 2 Of the gas components shown in Table 1, the concentration of acetylene was 100.
Removal of nitrogen oxides in exhaust gas in the same manner as in Example 1 except that the same gas as in Table 1 was used except that the content was changed within the range of ppm to 800 ppm, and the exhaust gas temperature in the reaction tube was maintained at 475 ° C. I asked for the rate. The results are shown in Table 3.

Table 3 Acetylene Concentration Nitrogen Oxide Removal Rate (ppm) (%) 100 15 200 200 26 300 30 400 400 38 500 47 47 600 520 700 56 56 800 82

As can be seen from Table 3, the higher the acetylene concentration, the higher the nitrogen oxide removal rate.

Example 3 Using the gases shown in Table 1, the contact time of the exhaust gas containing acetylene was changed within the range of 0.15 to 0.60 g · sec / ml, and the temperature was within the range of 400 to 550 ° C. The nitrogen oxide removal rate was determined in the same manner as in Example 1. The results are shown in Table 4.

Table 4 Contact time Nitrogen oxide removal rate (%) (g · sec / ml) 400 ° C 450 ° C 500 ° C 550 ° C 0.15 18 34 40 40 0.30 32 45 45 33 0.45 41 57 54 44 0.60 53 61 54 44

Example 4 Of the gas components shown in Table 1, the concentrations of nitric oxide (NO) and acetylene were changed in the range of 200 ppm to 1000 ppm (where the molar ratio of nitric oxide (NO) and acetylene was 1: 1). The nitrogen oxide removal rate was determined in the same manner as in Example 1 except that the reaction temperature (exhaust gas temperature in the reaction tube) was 475 ° C. The results are shown in Table 5.

Table 5 NO Concentration Acetylene Concentration Nitrogen Oxide (ppm) (ppm) Removal Rate (%) 200 200 33 33 500 500 47 47 1000 1000 55

As can be seen from Table 5, the NO concentration is 200
In the range of up to 1000 ppm, the higher the NO concentration, the higher the nitrogen oxide removal rate.

Examples 5 and 6 and Comparative Examples 4 to 7 The γ-alumina pellets used in Example 1 were immersed in an aqueous silver nitrate solution (silver nitrate concentration 0.02 g / ml), dried at 70 ° C., and then 5% by volume. Under a nitrogen stream containing hydrogen at 150 ℃, 2
After calcination at 00 ° C., 300 ° C., 400 ° C., and 500 ° C. for 2 hours each, under a nitrogen stream containing 10% oxygen, 500
The pellet was fired at 2 ° C. for 2 hours, and 1% by weight of silver (elemental conversion value, the same applies hereinafter) was supported on the pellet (Example 5). Also,
The pellets of γ-alumina impregnated with an aqueous solution of silver nitrate were dried in the same manner as in Example 5 and dried in air at 100 ° C. and 200 ° C.
2 hours each at ℃, 300 ℃, and 400 ℃,
1% by weight of silver was supported (Example 6).

The removal rate of nitrogen oxides was measured in the same manner as in Example 1 except that the silver-supported γ-alumina obtained above was used.

For comparison, the nitrogen oxide purification rate was determined in the same manner as in Example 5 except that the exhaust gas temperature in the reaction tube was set to 600 ° C. (Comparative Example 4).

Further, silver-supported γ-alumina was used, and ethylene (Comparative Example 5), propylene (Comparative Example 6) and propane (Comparative Example 7) were used in place of acetylene,
Others were the same as in Example 5, and the removal rate of nitrogen oxides was measured. The results are shown in Table 6.

Table 6 Nitrogen oxide removal rate (%) Example No. 250 ° C 300 ° C 350 ° C 400 ° C 450 ° C 500 ° C 550 ° C 600 ° C Example 5 23 34 47 52 43 35 30-Example 6 11 15 29 40 39 32 17 − Comparative example 4 − − − − − − − 10 Comparative example 5 0 0 0 2 5 10 18 0 Comparative example 6 0 2 5 13 21 29 30 8 Comparative example 7 0 0 1 4 10 23 32 24

[0042]

As described above in detail, according to the method of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed. In addition, according to the method of the present invention, good removal of nitrogen oxides can be performed even when the exhaust gas contains about 10% of water. Furthermore, the nitrogen oxide removal temperature (exhaust gas temperature) can be set to 250 to 550 ° C., which is a lower temperature than the conventional removal methods.

INDUSTRIAL APPLICABILITY The catalyst for removing nitrogen oxides and the removing method according to the present invention can be widely used for removing nitrogen oxides contained in exhaust gas from various combustors, automobiles and the like.

Claims (7)

[Claims] 1. A catalyst 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, which comprises a porous inorganic oxide or a metal component thereof. Supported, 250-550 ℃
In the residual acetylene in the exhaust gas and / or acetylene added from the outside in the exhaust gas as a reducing agent,
A catalyst for removing nitrogen oxides, which promotes a reaction for reducing nitrogen oxides in the exhaust gas. 2. The nitrogen oxide removing catalyst according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide. 3. The catalyst according to claim 1 or 2, wherein
The catalyst is 5% by weight with respect to the porous inorganic oxide.
A nitrogen oxide-removing catalyst, which comprises silver or a silver oxide having an element conversion value or less. 4. A method for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than the theoretical reaction amount for coexisting unburned components, wherein a porous inorganic oxide or a metal component thereof is supported. The catalyst formed as described above is installed in the middle of the exhaust gas pipe, and the exhaust gas is brought into contact with the catalyst at 250 to 550 ° C., at which time, 1 ml of the exhaust gas (value converted into the standard state) is brought into contact with the catalyst 1 g for 0 A method for removing nitrogen oxides, wherein the residual acetylene in the exhaust gas and the nitrogen oxides are reacted to remove the nitrogen oxides by setting the time to 0.03 seconds or more. 5. The method according to claim 4, wherein 200% by volume or less of the amount of nitrogen oxides of acetylene is added upstream of the site where the catalyst is installed, and the residual acetylene in the exhaust gas and the added acetylene A method comprising reducing and removing the nitrogen oxide. 6. The method according to claim 4 or 5, wherein
The method, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide. 7. The method according to claim 4, wherein the catalyst supports silver or silver oxide in an amount of 5% by weight (elemental conversion value) or less with respect to the porous inorganic oxide. A method for removing nitrogen oxides, which comprises: