JPH06327941A - Removing method of nitrogen oxide - Google Patents

Abstract

PURPOSE:To efficiently remove nitrogen oxide by allowing the waste gas added with a hydrocarbon compd. or mixed hydrocarbons corresponding to the amount of the nitrogen oxide in the waste gas to contact at a specified temp. with the catalyst carrying a specified amount of silver or silver oxide and Pt, Pd, Ru, Rh, etc., on a porous inorg. oxide. CONSTITUTION:The catalyst carrying 100wt.% porous inorg. oxide 0.2-15wt.% silver or silver oxide (expressed in terms of silver element) and 1-(1/1000)wt.% at least one kind metal element selected from the group consisting of Pt, Pd, Ru and Ir (expressed in terms of metal element) is provided in the midst of the waste gas conduit. Less than 5 times mixed hydrocarbons consisting of either liquefied petroleum gas or city gas or liquefied natural gas, or the hydrocarbon such as methane, ethane per the weight of nitrogen oxide in the waste gas, is added at the upstream side of the catalyst, and the waste gas is allowed to contact with the catalyst at 200-650 deg.C. In this way, the nitrogen oxide is allowed to contact with the hydrocarbon, and the nitrogen oxide is removed efficiently.

Description

Detailed Description of the Invention

[0001]

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

[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, nitrogen oxides (NOx) refer to nitric oxide and / or nitrogen dioxide, and "containing excess oxygen" means that carbon monoxide, hydrogen, hydrocarbons, etc. contained in the exhaust gas are not burned. It is meant to contain more than the theoretical amount of oxygen required to burn the components.

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.

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 removal 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.

[0010]

As a result of earnest research in view of the above problems, the present inventor has found that exhaust gas to which an amount of a hydrocarbon compound or a mixed hydrocarbon corresponding to the amount of nitrogen oxides contained in the exhaust gas is added, Nitrogen oxidation can be achieved by contacting a porous inorganic oxide with a specific amount of (a) silver or silver oxide and (b) Pt, Pd, Ru, Rh and Ir at a specific temperature. The present invention has been completed by discovering that an object can be effectively removed.

That is, in a combustion exhaust gas from an engine which uses a mixed hydrocarbon composed of any one of liquefied petroleum gas, city gas and liquefied natural gas as a fuel, oxygen which is larger than the theoretical reaction amount with respect to an unburned component coexisting with nitrogen oxides. The method of the present invention for removing nitrogen oxides from a combustion exhaust gas containing a) includes: (a) 0.2 to 15% by weight of silver or silver oxide (100% by weight of a porous inorganic oxide). ) And (b) Pt, Pd,
1 to 1/1000 wt% of at least one metal element selected from the group consisting of Ru, Rh and Ir (metal element conversion value)
A supported catalyst is installed in the middle of an exhaust gas pipe, and a mixture of liquefied petroleum gas, city gas, or liquefied natural gas, which is 5 times or less the weight of nitrogen oxides in the exhaust gas, is provided upstream of the catalyst. Hydrocarbons or hydrocarbons such as methane and ethane are added, and exhaust gas is brought into contact with the catalyst at 200 to 650 ° C., thereby removing the nitrogen oxides by reacting the nitrogen oxides with the hydrocarbons. Characterize.

The present invention will be described in detail below. The catalyst used in the present invention is a porous inorganic oxide (a) silver or silver oxide, and (b) at least one metal element selected from the group consisting of Pt, Pd, Ru, Rh and Ir. Is carried.

First, as the porous inorganic oxide, porous alumina, zirconia, or a composite oxide thereof can be used, but γ-alumina or an alumina-based composite oxide is preferably used. When γ-alumina or an alumina-based composite oxide is used, 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 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 contact area between the exhaust gas and the inorganic oxide becomes small and the nitrogen oxide cannot be removed well. The more preferable specific surface area of the porous inorganic oxide is 80 m ² / g or more. Particularly, 100 m ² / g or more is preferable.

Inorganic oxides such as γ-alumina carry silver or silver oxide and Pt, Pd, Ru, Rh and Ir as will be described later, and these inorganic oxides are in the form of pellets or powders. It can be used in a honeycomb shape, a foam shape, a plate shape, or the like. It is also possible to coat a hamonica-like or foam-like substrate made of heat-resistant cordierite, mullite or the like with a porous inorganic oxide carrying active species. The preferred form of the catalyst of the present invention is to use a ceramic or metal honeycomb-shaped, foam-shaped, pellet-shaped, or granular three-dimensional structure coated with the catalyst. Further, another preferred embodiment of the present invention is to use a pellet-like or granular porous inorganic oxide carrying a catalytically active species.

In the present invention, (a) silver or silver oxide and (b) Pt, Pd, R are added to the above-mentioned inorganic oxides such as γ-alumina.
A catalyst carrying at least one metal element selected from the group consisting of u, Rh and Ir is used as a catalyst. The amount of silver or silver oxide supported varies somewhat depending on the type of gaseous hydrocarbon compound or mixed hydrocarbon added to the exhaust gas, the contact time with the exhaust gas, etc. 0.2 to 15% by weight (converted to silver element) If it is less than 0.2% by weight, the effect of supporting silver or silver oxide is not remarkable, and even if the amount of silver or silver oxide exceeding 15% by weight is supported, the NOx removal performance is not improved. Absent. The lower limit of the supported amount of silver or silver oxide is preferably 0.5% by weight, and the upper limit thereof is 12% by weight. The silver supported on the inorganic oxide is in the state of metal or oxide in the temperature range of the exhaust gas and can be easily converted into each other.

Of Pt, Pd, Ru, Rh and Ir, Pt, Pd, R
u and Rh are preferably used, but Pt is particularly preferably used. The total supported amount of Pt, Pd, Ru, Rh and Ir is 1 to 1/1000 wt% (elemental conversion value) with 100% by weight of the inorganic oxide. When the supported amount exceeds 1% by weight, the removal effect by the silver component is greatly reduced. In addition, the supported amount is 0.
It is preferably 5 to 1/500% by weight.

Inorganic oxides such as γ-alumina can be mixed with silver, Pt,
As a method for supporting d, Ru, Rh and Ir, a known impregnation method, coprecipitation method or the like can be used. At that time, the porous inorganic oxide is immersed in an aqueous solution of nitrate or hydrochloride,
After drying at about 0 ° C., it is preferable to raise the temperature stepwise at 100 to 600 ° C. and bake. The firing is preferably performed in an oxygen atmosphere, a nitrogen atmosphere or a hydrogen gas flow. When performing under a nitrogen atmosphere or under a flow of hydrogen gas, the oxidation treatment is performed last.

Next, the method of the present invention will be described. The present invention is a method for treating combustion exhaust gas from an engine that uses a mixed hydrocarbon composed of liquefied petroleum gas, city gas, or liquefied natural gas as a fuel. Such exhaust gas contains some residual hydrocarbons. If the residual hydrocarbons are not sufficient to reduce the nitrogen oxides in the exhaust gas, it is necessary to add organic compounds from the outside. At this time, the above-mentioned catalyst is installed in the middle of the exhaust gas conduit, and the mixed hydrocarbon or hydrocarbon compound that is the fuel of the engine is added to the exhaust gas on the upstream side of the installation site of the catalyst.

As the hydrocarbon compound, alkane, alkene, alkyne and the like are used. Gaseous hydrocarbons such as methane, ethane, propane and butane are preferably used. As the mixed hydrocarbon, liquefied petroleum gas, city gas, or liquefied natural gas is used. Also, other mixed hydrocarbons may be used. By using these mixed hydrocarbons, the nitrogen oxide removing performance in the high temperature region of 600 ° C. or higher becomes high. When a mixed hydrocarbon is used, if a saturated hydrocarbon having a small number of carbon atoms is the main component, the nitrogen oxide removal characteristics at low temperatures deteriorate, so it is preferable to add a hydrocarbon having a large number of carbon atoms before use. .

The amount of the hydrocarbon compound or the mixed hydrocarbon added to the exhaust gas is not more than 5 times the weight of nitrogen oxides in the exhaust gas. When it exceeds 5 times, the additive is often excessive, and unreacted hydrocarbon compound or mixed hydrocarbon is likely to remain in the exhaust gas. The preferable addition amount is set such that the total amount of hydrocarbons (residual hydrocarbons + added hydrocarbons) is 3 times or less with respect to the amount of nitrogen oxides, and the lower limit of the addition amount is 0.1 times excess. Is preferred. Since some unburned hydrocarbons such as alkanes, alkenes, and alkynes are present in the exhaust gas, even if only the above catalyst is used (without adding hydrocarbons), some NOx reduction effect can be obtained. Seen.

Further, in the present invention, the temperature of the exhaust gas at the catalyst installation site is maintained at 200 to 650 ° C. If the temperature of the exhaust gas is less than 200 ° C., the reaction between the additive and the nitrogen oxide does not proceed, and the nitrogen oxide cannot be removed satisfactorily. On the other hand, when the temperature is higher than 650 ° C., the added organic matter burns, and the reduction and removal characteristics of nitrogen oxides are significantly deteriorated.

Generally, when the contact time is shortened, the nitrogen oxide removing property is deteriorated. Therefore, when the contact time is short, it is preferable to support a large amount of the catalytic active species, and when the contact time is long, a small amount of the catalytic active species is supported. By doing so, the reaction between the hydrocarbon and the nitrogen oxide can proceed efficiently. In the catalyst of the present invention, the contact time for contacting exhaust gas with the above catalyst is preferably 0.006 seconds · g / 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. The preferable contact time is 0.007 seconds · g / ml or more.

[0024]

The present invention will be described in more detail by the following specific examples. Example 1 Commercially available pelletized porous γ-alumina (each having a diameter of 1.
5 mm, length about 6 mm, specific surface area 200 m ² / g) 10 g is immersed in an aqueous solution of chloroplatinic acid and an aqueous solution of silver nitrate, dried at 70 ° C., and then in air at 150 ° C., 200 ° C., 300 ° C., 400 ° C.
C., 500.degree. C., and 600.degree. C., respectively, for 2 hours to carry 5% by weight of silver (elemental conversion value) and 0.05% by weight of platinum (elemental conversion value) on pelletized .gamma.-alumina. It was

Next, 3.6 g of the obtained catalyst was placed in a reaction tube, and 2.2 liters / min of a gas having the composition shown in Table 1 (nitrogen monoxide, oxygen, liquefied petroleum gas, nitrogen, and water). Flow at the standard flow rate (at this time, the space velocity is 1
5,000 h ⁻¹ , the contact time was 0.1 sec · g / ml), the exhaust gas temperature in the reaction tube was kept in the range of 200 to 700 ° C., and the liquefied petroleum gas and the nitrogen oxide were reacted. It was

The concentration of nitrogen oxides 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 Fig. 1.

Table 1 Concentration of components Nitrogen monoxide 800 ppm Oxygen 10% by volume Liquefied petroleum gas 1700 ppm Nitrogen balance Moisture 10% by volume with respect to the amount of gas composed of the above components

Example 2 In the same manner as in Example 1, 5% by weight of silver was added to powdery γ-alumina (average particle size 40 μm, specific surface area 200 m ² / g).
(Element-converted value) and 1 g of catalyst supporting 0.02% by weight of platinum (element-converted value) were mixed with commercially available cordierite honeycomb molded bodies (diameter 30 mm, length 12.6 mm, 40).
0 cell / in 2) was coated by a wash coating method, dried and then calcined stepwise to 650 ° C. to prepare a catalyst-coated removal material. Using this removing material, a nitrogen oxide removal test was conducted under the same conditions as in Example 1 (space velocity 15,000 h ⁻¹ ). The test results are shown in FIG.

Example 3 In the same manner as in Example 1, except that propane was used instead of liquefied petroleum gas in the gas composition shown in Table 1 using the same removing material as in Example 2, nitrogen oxides of A removal test was performed. The test results are shown in FIG.

Comparative Example 1 Using the same γ-alumina pellets as in Example 1, a 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 Examples 1 to 3, good removal of nitrogen oxides was observed at an exhaust gas temperature of 200 to 700 ° C. On the other hand, in Comparative Example 1 in which the catalyst component was not supported, the nitrogen oxide removing performance was low in the entire temperature range.

[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. The removal temperature of nitrogen oxides (exhaust gas temperature) is about 300 ° C. or less, which is lower than 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.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between exhaust gas temperature and nitrogen oxide removal rate in Examples 1 to 3 and Comparative Example 1.

Claims (4)

[Claims] 1. A combustion exhaust gas from an engine that uses a mixed hydrocarbon of any one of liquefied petroleum gas, city gas, and liquefied natural gas as a fuel, and has a larger amount of oxygen than the theoretical reaction amount with respect to unburned components coexisting with nitrogen oxides. In a method for removing nitrogen oxides from combustion exhaust gas containing and, (a) silver or silver oxide is added in an amount of 0.2 to 1 in 100% by weight of a porous inorganic oxide.
5% by weight (silver element conversion value), (b) Pt, Pd, Ru, Rh and
A catalyst supporting at least one metal element selected from the group consisting of Ir in an amount of 1 to 1/1000 wt% (metal element conversion value) is installed in the middle of an exhaust gas conduit, and the exhaust gas is provided upstream of the catalyst. Add a mixed hydrocarbon consisting of liquefied petroleum gas, city gas, or liquefied natural gas, which is 5 times or less the weight of nitrogen oxides, or a hydrocarbon such as methane or ethane,
A method for removing nitrogen oxides, wherein exhaust gas is brought into contact with the catalyst at 200 to 650 ° C., and thereby the nitrogen oxides and the hydrocarbons are reacted to remove the nitrogen oxides. 2. The nitrogen oxide removing method according to claim 1, wherein the porous inorganic oxide is alumina or an alumina-based composite oxide. 3. The method for removing nitrogen oxides according to claim 1, wherein the catalyst is in the form of pellets or granules. 4. The removal method according to claim 1, wherein the catalyst is made of ceramic or metal having high fire resistance and has a three-dimensional structure having a honeycomb shape, a foam shape, a pellet shape, or a granular shape. A method for removing nitrogen oxides, which is characterized by being coated on.