JPH06126186A - Catalyst for removing nitrogen oxygen and removing method for the same - Google Patents

Abstract

PURPOSE:To provide a catalyst for removing nitrogen oxide which is capable of efficiently removing the same from combustion exhaust gas containing oxygen not less than theoretical reaction amount for nitrogen oxide and unburnt substance such as carbon monoxide and hydrocarbon. CONSTITUTION:A silver catalyst is obtained by making silver of silver oxide inorganic oxide at 0.2-15wt.% expressed in terms of silver element. A catalyst for removing nitrogen oxide is produced by mixing a zeolite catalyst with the silver catalyst. Nitrogen oxide contained in exhaust gas is removed by adding gaseous hydrocarbon into exhaust gas from the outside and bringing the exhaust gas into contact with the mixed catalyst at 200-600 deg.C.

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 discharged 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 is 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 this method, ammonia used as a reducing agent for nitrogen oxides is expensive, and ammonia is toxic, so that unreacted ammonia is discharged so that nitrogen oxides in exhaust gas are not discharged. There are problems that the amount of ammonia injection 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, 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, 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 as high as 400 to 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 device 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 unburned components,
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 the amount of gaseous hydrocarbons commensurate with the amount of nitrogen oxides contained in the exhaust gas is added to the exhaust gas, and the zeolite catalyst is added. By contacting with a mixed catalyst consisting of and a catalyst in which a specific amount of silver component is supported on a porous inorganic oxide,
Even in exhaust gas containing about 10% of water, 200 to 600
The present invention has been completed by discovering that nitrogen oxides can be effectively removed at ℃.

That is, the catalyst of the present invention 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 is silver or silver as a porous inorganic oxide. 0.2 in terms of oxides
Is a mixed catalyst composed of a silver catalyst and a zeolite catalyst supported by ˜15% by weight, and a gaseous hydrocarbon is externally added to the exhaust gas, and the exhaust gas is contacted with the mixed catalyst at 200 to 600 ° C., Therefore, the nitrogen oxide in the exhaust gas is removed.

The method of the present invention 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 is silver or silver as a porous inorganic oxide. 0.2 to 1 in terms of oxides converted to elemental silver
A mixed catalyst composed of a silver catalyst and 5% by weight supported thereon and a zeolite catalyst was installed in the middle of an exhaust gas pipe, and a gaseous hydrocarbon was added to the exhaust gas from the outside to give a mixture of 200 to 600
The exhaust gas is brought into contact with the catalyst at 0 ° C., hydrocarbons are added to the upstream side of the site where the catalyst is installed, and the residual hydrocarbons in the exhaust gas are reacted with the nitrogen oxides to remove the nitrogen oxides. It is characterized by

The present invention will be described in detail below. In the present invention, the catalyst shown below is used, and exhaust gas is brought into contact with this catalyst to reduce and remove nitrogen oxides in the exhaust gas by using the hydrocarbon added to the exhaust gas on the upstream side of the catalyst installation site as a reducing agent. .

The catalyst of the present invention is a mixed catalyst comprising a silver catalyst in which a silver component is supported on a porous inorganic oxide and a zeolite catalyst. First, as the porous inorganic oxide, porous alumina, titania, zirconia, and composite oxides thereof can be used, but preferably γ
-Alumina or an alumina-based composite oxide is used. By using γ-alumina or an alumina-based composite oxide,
The reaction between the added hydrocarbon and the nitrogen oxide in the exhaust gas occurs efficiently, and the nitrogen oxide purification characteristics are improved.

The inorganic oxide such as γ-alumina can be used in the form of pellets, powder, honeycomb, foam, plate or the like. In consideration of mixing with the zeolite catalyst, a shape such as a pellet shape or a powder shape that facilitates mixing is preferable.

The specific surface area of the porous inorganic oxide is 30 m ²
/ G or more is preferable. Specific surface area of 30m ² / g
If it is less than the above range, the dispersion of the silver component (silver active species) in the inorganic oxide becomes poor, and the nitrogen oxide cannot be removed well.

The amount of the silver component supported on the above-mentioned inorganic oxide such as γ-alumina is 0.2 to the amount of the inorganic oxide.
15% by weight (silver element conversion value). If the silver component is less than 0.2% by weight, the removal rate of nitrogen oxides on the low temperature side is lowered. In particular, this tendency becomes remarkable when the contact time of the exhaust gas becomes short. Further, when silver is loaded in an amount of more than 15% by weight, the nitrogen oxide removal rate is rather lowered. Preferably, the amount of silver supported is 0.5 to 10% by weight based on the inorganic oxide.

As a method for supporting the silver component on the inorganic oxide such as γ-alumina, a known dipping method or the like can be used. At that time, the porous inorganic oxide is dipped in a solution having a silver component such as an aqueous solution of silver nitrate and dried at about 70 ° C.
It is preferable to raise the temperature stepwise at 00 to 550 ° C. and to bake.

Next, as the zeolite catalyst, mordenite, zeolite-A, zeolite-L, faujasite, or the like can be used. It can also be composed of a mixture of two or more of these components. Zeolites are crystalline aluminosilicates and are represented by the following general composition formula. _{^{(M 1, M 2 1/2)}} m (Al m Si n O 2 (m + n)) · xH
₂ O, (n ≧ m) where M ¹ is Li ⁺ , Na ⁺ , K ⁺ , Pb ^+, etc., and M ² is Ca ²⁺ , Mg ²⁺ , Ba ²⁺ , Sr ^2+, etc. . The M ¹ and M ² ions of zeolite can be used by reversibly exchanging some or all of them with other cations. Zeolite catalysts have the function of converting hydrocarbons to oxygen-containing organic compounds such as alcohols through hydration.

The zeolite catalyst is in the form of pellets, powder,
It can be used in a honeycomb shape, a foam shape, a plate shape, or the like. In consideration of mixing with a silver catalyst, pellets, powders, and other shapes that are easy to mix are preferable.

The mixed catalyst is a mixture of a silver catalyst and a zeolite catalyst. With only the silver catalyst, the reduction reaction of nitrogen oxides by alkane, alkene or alkyne does not sufficiently occur at an exhaust temperature of 400 ° C. or lower in an oxygen atmosphere, and the removal rate of nitrogen oxides decreases. However, since silver-based catalysts generally have high reactivity of reducing nitrogen oxides by oxygen-containing organic substances, it is possible to use a zeolite catalyst in combination with 200-6
It is possible to effectively reduce nitrogen oxides in a wide range of 00 ° C.

Mixing a silver catalyst and a zeolite catalyst means, when the shapes of both catalysts are pellets or powders, mixing them artificially or mechanically. The shape of both catalysts or one of them is honeycomb. When the exhaust gas passes through the mixed catalyst, it has a structure in which the exhaust gas comes into macro contact with both catalysts when the exhaust gas passes through the mixed catalyst.

The mixing ratio of the zeolite catalyst is 1 in the mixed catalyst.
It is 0 to 60% by weight. If it is less than 10% by weight, the effect of the zeolite catalyst is not sufficient, and if it exceeds 60% by weight,
The proportion of the silver catalyst becomes too small, and the ability to remove nitrogen oxides decreases.

Next, the nitrogen oxide removing method of the present invention will be described. First, the above-mentioned mixed catalyst is installed in the middle of the exhaust gas conduit.

The exhaust gas contains acetylene, methane, ethane, propylene and the like as residual hydrocarbons, but when the residual hydrocarbons are not contained in an amount sufficient to reduce NOx in the exhaust gas, Hydrocarbons are introduced into the exhaust gas from the outside. The introduction position of the hydrocarbon is upstream of the position where the catalyst is installed.

As the hydrocarbon introduced from the outside, a gaseous alkane, alkene or alkyne in a standard state is used. Of the alkanes, propane and butane are particularly preferable. Among alkenes, especially ethylene, propylene,
Butylene is preferred. Among the alkynes, acetylene and methylacetylene are particularly preferable. Besides, hydrocarbons which are liquid in the standard state can also be used. Specific examples of the liquid hydrocarbon in the standard state include light oil, cetane, heptane, and kerosene. These liquid hydrocarbons can be introduced into the exhaust gas by a method such as spraying.

The amount of hydrocarbons introduced from the outside has a ratio of:
It is preferable that (weight of added liquid hydrocarbon / weight of NOx in exhaust gas) be 0.2 to 5.

The contact time of the exhaust gas containing hydrocarbon with the above catalyst is 0.006 g · sec / ml or more. Here, the contact time represents the time (second) in which 1 ml of the exhaust gas containing hydrocarbon (however, the volume converted to the standard state) is in contact with 1 g of the catalyst. For example, contact time is 0.1g
• sec / ml means that 1 g of catalyst is used and 1 ml of exhaust gas is contacted with this catalyst for 0.1 second.

Further, in the present invention, the temperature of the exhaust gas at the catalyst installation site, which is the site where hydrocarbons and nitrogen oxides react, is maintained at 200 to 600 ° C. Exhaust gas temperature is 200
If the temperature is lower than ° C, the reaction between hydrocarbons and nitrogen oxides does not proceed, and it is impossible to remove nitrogen oxides satisfactorily. On the other hand, if the temperature exceeds 600 ° C., the combustion of the hydrocarbon itself starts, and the nitrogen oxide cannot be reduced and removed by the hydrocarbon. The exhaust gas temperature is preferably 250 to 600 ° C.

[0030]

The present invention will be described in more detail by the following specific examples. Example 1 Commercially available pelletized porous γ-alumina (diameter 1.5 mm,
A length of about 6 mm and a specific surface area of 200 m ² / g) of 10 g was dipped in an aqueous solution of silver nitrate, dried at 70 ° C., and then at a temperature of 150 ° C., 200 ° C., and 300 ° C. under a nitrogen stream containing 5% by volume of hydrogen. And calcination at 400 ° C and 500 ° C for 2 hours at 550 ° C in a nitrogen stream containing 10% oxygen.
Calcination was performed for 5 hours, and 2% by weight (element conversion value) of silver was supported on the pelletized γ-alumina. This and 10 g of pelletized H-type mordenite were alternately stacked to form a mixed catalyst.

Next, about 20 g of the obtained mixed catalyst was placed in a reaction tube, and a gas having a composition shown in Table 1 (a total of 100% by volume of dry components consisting of nitric oxide, carbon dioxide, oxygen, propylene, and nitrogen). (With 10% by volume of water added) at a flow rate of 2 liters per minute (standard condition) (contact time 0.6 g · sec / ml), and the exhaust gas temperature in the reaction tube is in the range of 250 to 550 ° C. Then, propylene was reacted with nitrogen oxide.

The concentration of nitrogen oxides (total amount of nitric oxide and nitrogen dioxide) of the gas after passing through the reaction tube was measured by a chemiluminescence type nitrogen oxide analyzer to obtain the conversion rate of nitrogen oxides to nitrogen. It was The results are shown in Fig. 1.

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

Comparative Example 1 Of the mixed catalysts described in Example 1, only a silver catalyst in which 2% by weight of silver was supported on γ-alumina pellets was prepared. Using this catalyst and the gases shown in Table 1, a nitrogen oxide removal test was conducted in the same manner as in Example 1. The test results are shown in FIG.

Comparative Example 2 Of the mixed catalysts described in Example 1, only the pelletized H-type mordenite, which is a zeolite catalyst, was used, and the nitrogen oxide removal test was conducted in the same manner as in Example 1. The results are shown in Fig. 1.

As can be seen from the above, in Example 1, good removal of nitrogen oxides was observed at an exhaust gas temperature of 250 to 550 ° C. On the other hand, when only the catalyst supporting silver or silver oxide is used (Comparative Example 1), good removal of nitrogen oxide is not observed at the exhaust gas temperature of 400 ° C. or lower. Also,
Almost no removal of nitrogen oxides was obtained when only the zeolite catalyst, mordenite, was used (Comparative Example 2).

[0037]

As described in detail above, by using the catalyst of the present invention, nitrogen oxides in exhaust gas containing excess oxygen can be efficiently removed. Further, in the method of the present invention,
Even if the exhaust gas contains about 10% of water, the nitrogen oxides can be removed efficiently. Furthermore, the nitrogen oxide removal temperature (exhaust gas temperature) is also 200 to 600 ° C.
And, it can be performed at a lower temperature than the conventional removal methods.

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

[Brief description of drawings]

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tatsuo Miyadera 16-3 Onogawa, Tsukuba, Ibaraki Prefecture

Claims (4)

[Claims] 1. A catalyst for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen in an amount larger than a theoretical reaction amount for coexisting unburned components, wherein silver or silver oxide is added to a porous inorganic oxide. It is a mixed catalyst composed of a silver catalyst and a zeolite catalyst which are supported by 0.2 to 15% by weight in terms of silver element, and a hydrocarbon gas of 200 to 600 is added from the outside into the exhaust gas. A catalyst for removing nitrogen oxides, which comprises contacting exhaust gas with the mixed catalyst at 0 ° C. to remove nitrogen oxides in the exhaust gas. 2. The nitrogen oxide removing catalyst according to claim 1, wherein the mixing ratio of the zeolite catalyst in the mixed catalyst is 10 to 60% by weight in the mixed catalyst. 3. The nitrogen oxide removing catalyst according to claim 1 or 2, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide. 4. A method for removing nitrogen oxides from a combustion exhaust gas containing nitrogen oxides and oxygen larger than the theoretical reaction amount for coexisting unburned components, wherein silver or silver oxide is added to the porous inorganic oxide. A mixed catalyst composed of a silver catalyst and 0.2 to 15% by weight calculated as silver element and a zeolite catalyst is installed in the middle of an exhaust gas conduit, and a gaseous hydrocarbon is added to the exhaust gas from the outside. The exhaust gas is brought into contact with the catalyst at 200 to 600 ° C., hydrocarbons are added to the upstream side of the site where the catalyst is installed, and the residual hydrocarbons in the exhaust gas are reacted with the nitrogen oxides to carry out the nitrogen oxidation. A method for removing nitrogen oxides, which comprises removing a substance.