JPH0592124A - Nitrogen oxide removing method - Google Patents

Abstract

PURPOSE:To provide a method to sufficiently remove nitrogen oxides from a combustion waste gas containing the nitrogen oxides and excessive oxygen. CONSTITUTION:A catalyst prepared by carrying 0.1-1.5wt.% of (as calculated in terms of element) silver or a silver oxide in 100wt.% of porous inorganic oxide is set in the middle of a waste gas leading pipe and in the upper stream side of the catalyst, gaseous alkane and/or alkene is added in 200vol.% or less of the amount of the nitrogen oxide in the waste gas and at 400-600 deg.C the waste gas is brought into contact with the catalyst to react the nitrogen oxide with the added alkane and/or alkene as well as the unburnt alkanes and/or alkenes in the waste gas and thus the nitrogen oxide is removed.

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 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 (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 needed to burn the components.

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 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 the amount of injected ammonia is measured while measuring the concentration of nitrogen oxides in the exhaust gas so that unreacted ammonia is not discharged. There are problems that it must be controlled 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, 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 described above can remove nitrogen oxides with high efficiency from simulated exhaust gas that does not contain water, actual exhaust gas contains about 10% of water. It was found that the nitrogen oxide removal rate was significantly reduced.

Therefore, an object of the present invention is to theoretically react with unburned components such as nitrogen oxides and carbon monoxide and hydrocarbons, such as combustion exhaust gas from a fixed combustion device and a gasoline engine that burns in 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 an amount of oxygen or more.

[0010]

As a result of earnest research in view of the above problems, the present inventors have added to the exhaust gas an amount of gaseous alkane and / or alkene in proportion to the amount of nitrogen oxides contained in the exhaust gas. However, if the exhaust gas containing the alkane and / or alkene is brought into contact with a specific amount of silver supported on a porous inorganic oxide or a catalyst composed of silver oxide, the exhaust gas containing about 10% of water has a nitrogen content. The present inventors have completed the present invention by discovering that oxides can be effectively removed.

That is, 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 with respect to the coexisting unburned components is 100% by weight of a porous inorganic oxide. 0.1 to 5 silver or silver oxide
A catalyst supporting weight% (elemental conversion value) is installed in the middle of the exhaust gas conduit, and a gaseous alkane and / or alkene of 200% by volume or less of the amount of nitrogen oxides in the exhaust gas is provided on the upstream side of the catalyst. The catalyst is characterized by adding and contacting the exhaust gas with the catalyst at 400 to 600 ° C., thereby reacting the nitrogen oxide with the alkane and / or alkene in the exhaust gas to remove the nitrogen oxide. In a preferred embodiment, 1 ml of exhaust gas (value converted to standard conditions)
Is contacted with 1 g of the catalyst for 0.03 seconds or more.

The present invention will be described in detail below. The catalyst used in the present invention comprises silver or silver oxide supported on a porous inorganic oxide. First, as the porous inorganic oxide, porous alumina, titania, zirconia, and composite oxides thereof can be used, but preferably γ
-Using alumina. If γ-alumina is used, the reaction between the added alkane and / or alkene 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. When the specific surface area of the porous inorganic oxide is less than 30 m ² / g, the contact area between the exhaust gas and the inorganic oxide (and silver or silver oxide supported on the exhaust gas) becomes small, so that good nitrogen oxide It cannot be removed.

In the present invention, the catalyst is silver or silver oxide supported on the above-mentioned inorganic oxide such as γ-alumina. The amount of silver or silver oxide supported varies somewhat depending on the type of gaseous hydrocarbon added to the exhaust gas, but is 0.1 to 5% by weight (as silver element) of the weight of the inorganic oxide. If the amount is less than the lower limit of the above range, the effect of supporting silver is not remarkable, and if the amount of silver exceeding the upper limit is supported, the NOx removal performance deteriorates.

As a method for supporting silver on an inorganic oxide such as γ-alumina, a known dipping method or the like can be used. At that time, it is preferable that the porous inorganic oxide is immersed in an aqueous solution of silver nitrate, dried at about 70 ° C., and then gradually heated at 100 to 500 ° C. to be baked. This firing is preferably carried out in a nitrogen atmosphere or in a flow of hydrogen gas, which prevents silver sintering and deterioration of nitrogen oxide purification characteristics.

The silver supported on the inorganic oxide is in the state of metal or oxide in the temperature range of exhaust gas and can be easily converted into each other.

The inorganic oxide such as γ-alumina carrying a silver catalyst is placed in the exhaust gas conduit in the form of pellets, powder, honeycomb, foam or the like.

Since the reaction between the alkane and / or alkene and the nitrogen oxide occurs at the site where the above catalyst is installed, it is important to add the alkane and / or alkene to the exhaust gas upstream of the site where this catalyst is installed. Be on the side.

The alkane and / or alkene used in the present invention is gaseous in the standard state. Of the alkanes, propane and butane are particularly preferable. Among the alkenes, ethylene, propylene, butylene and the like are particularly preferable.

The amount of alkane and / or alkene added to the exhaust gas is 200% by volume or less of the volume of nitrogen oxides in the exhaust gas. Alkanes and / or alkenes are present in the exhaust gas as unburned components. Therefore, the use of the catalyst described above has the effect of reducing NOx without adding alkanes and / or alkenes. When an amount of alkane and / or alkene exceeding 200% by volume is added, the amount of alkane and / or alkene becomes excessive as compared with nitrogen oxide, and unreacted alkane and / or alkene remains in the exhaust gas to cause secondary pollution. Will be born.

In the present invention, the time during which the exhaust gas is in contact with the above-mentioned catalyst is adjusted so that the reaction between the alkane and / or alkene and the nitrogen oxides proceeds efficiently. Alkane and /
Alternatively, the contact time between the exhaust gas containing an alkene and the catalyst is 0.
03g · sec / ml or more. The unit of contact time (g · sec / ml) here is 1 ml of exhaust gas containing alkane and / or alkene (however, volume converted to standard state)
Represents the time (seconds) of contact with 1 g of the catalyst. That is, the contact time of 0.03 g · sec / ml means that when 1 g of the catalyst is used, 1 ml of exhaust gas is in contact with 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 alkane and / or alkene and the nitrogen oxide does not sufficiently occur, and the removal rate of the nitrogen oxide decreases. .. In addition, unreacted alkane and / or alkene will also remain 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 the alkane and / or alkene and the nitrogen oxide occurs is kept at 400 to 600 ° C. If the temperature of the exhaust gas is less than 400 ° C., the reaction between the alkane and / or alkene and the nitrogen oxide does not proceed, and the nitrogen oxide cannot be removed satisfactorily. On the other hand, if the temperature exceeds 600 ° C., combustion of the alkane and / or alkene itself starts, and the nitrogen oxide cannot be reduced and removed by adding the alkane and / or alkene. When alkyne is contained in addition to alkane and alkene, NOx is reduced and removed at an exhaust gas temperature of 250 to 600 ° C.

[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 (each having a diameter of 1.5 mm and a length of about 6 mm) was added with an aqueous silver nitrate solution (concentration 0.02 g / ml).
And dried at 70 ° C., and then under a nitrogen stream containing 5% by volume of hydrogen, 150 ° C., 200 ° C., 300 ° C., 400 ° C.
And calcination at 500 ° C for 2 hours each, and then 10% oxygen
The mixture was calcined at 500 ° C. for 2 hours under the nitrogen stream contained therein, and 2% by weight (elemental conversion value) of silver was supported on the pelletized γ-alumina.

Next, each gas having the composition shown in Table 1 (a total of 100% by volume of dry components consisting of nitric oxide, carbon dioxide, oxygen, ethylene, and nitrogen, to which 10% by volume of water was added) was added. A flow rate of 2 liters (standard state) was applied (contact time: 0.3 g · sec / ml), the exhaust gas temperature in the reaction tube was maintained in the range of 300 to 550 ° C., and ethylene was reacted with 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, and the conversion rate of nitrogen oxides to nitrogen was determined. I asked. The results are shown in Figure 1.

[0027] Water content 10% by volume based on the amount of gas consisting of the above components

Example 2 Using the same catalyst as in Example 1, a nitrogen oxide removal test was conducted in the same manner as in Example 1 except that ethylene was changed to propylene in the gases shown in Table 1. The test results are shown in FIG.

Example 3 Using the same catalyst as in Example 1, a nitrogen oxide removal test was conducted in the same manner as in Example 1 except that propane was used instead of ethylene in the gases shown in Table 1. Figure 1 shows the test results
Shown in.

Comparative Examples 1 to 3 Gases shown in Table 1 (Comparative Example 1) and gases shown in Table 1 were prepared using the same pelletized γ-alumina as used in Example 1 (without supporting silver). Using a gas in which ethylene was replaced by propylene (Comparative Example 2) and a gas in which ethylene was replaced by propane (Comparative Example 3) shown in Table 1 (Comparative Example 3), a nitrogen oxide removal test was conducted in the same manner as in Example 1. went. The results are shown in Figure 2.

Comparative Examples 4 to 6 Also, the same pelletized γ-alumina used in Example 1 was used, and silver was added to the pelletized γ-alumina in an amount of 6% by weight in the same manner as in Example 1. A supported catalyst was prepared. Regarding this catalyst, the gas shown in Table 1 (Comparative Example 4), the gas shown in Table 1 in which ethylene was replaced by propylene (Comparative Example 5), and the gas shown in Table 1 in which ethylene was replaced by propane A nitrogen oxide removal test was conducted in the same manner as in Example 1 using gas (Comparative Example 6). Results are shown in FIG.

Examples 4 to 6 Using the same pelletized γ-alumina as in Example 1, a catalyst carrying 0.5% by weight of silver was prepared in the same manner as in Example 1.

Using this catalyst, a nitrogen oxide removal test (Example 4) was conducted in the same manner as in Example 1.

The catalyst, the gas shown in Table 1 in which ethylene was replaced by propylene (Example 5), and the gas shown in Table 1 in which ethylene was replaced by propane (Example 6) were used. A nitrogen oxide removal test was conducted in the same manner as in Example 1. The results are shown in Fig. 4.

Examples 7 to 9 Using the same pelletized γ-alumina as in Example 1, a catalyst carrying 4% by weight of silver was prepared in the same manner as in Example 1.

Using this catalyst, a nitrogen oxide removal test (Example 7) was conducted in the same manner as in Example 1.

The catalyst, the gas shown in Table 1 in which ethylene was replaced by propylene (Example 8), and the gas shown in Table 1 in which ethylene was replaced by propane (Example 9) A nitrogen oxide removal test was conducted in the same manner as in Example 1. Results are shown in FIG.

As can be seen from the above, in each example, good removal of nitrogen oxides was observed at the exhaust gas temperature of 400 to 550 ° C. On the other hand, when a catalyst that does not support silver or silver oxide is used (Comparative Examples 1 to 3), good removal of nitrogen oxide is not observed at an exhaust gas temperature of 550 ° C. or lower. Further, when the amount of silver or silver oxide supported was as high as 6% by weight (Comparative Examples 4 to 6), the removal rate of nitrogen oxides was smaller than that in Comparative Example 1 described above.

[0039]

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. 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. Further, the removal temperature of nitrogen oxides (exhaust gas temperature) is 550 ° C. or lower, which is lower than the conventional removal methods.

The method for removing nitrogen oxides 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 NOx conversion rate in Examples 1 to 3.

FIG. 2 is a graph showing the relationship between exhaust gas temperature and NOx conversion rate in Comparative Examples 1 to 3.

FIG. 3 is a graph showing the relationship between exhaust gas temperature and NOx conversion rate in Comparative Examples 4 to 6.

FIG. 4 is a graph showing the relationship between exhaust gas temperature and NOx conversion rate in Examples 4 to 6.

FIG. 5 is a graph showing the relationship between exhaust gas temperature and NOx conversion rate in Examples 7 to 9.

Front page continuation (72) Inventor Kiyohide Yoshida 810 Kumagaya, Kumagaya, Saitama Prefecture

Claims (3)

[Claims] 1. 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 silver or silver is added to 100% by weight of a porous inorganic oxide. A catalyst supporting 0.1 to 5% by weight (elemental conversion value) of an oxide is installed in the middle of an exhaust gas pipe, and a gas of 200% by volume or less of the amount of nitrogen oxides in the exhaust gas is provided upstream of the catalyst. -Like alkane and / or alkene is added, and the exhaust gas is brought into contact with the catalyst at 400 to 600 ° C., thereby reacting the nitrogen oxide with the alkane and / or alkene in the exhaust gas to remove the nitrogen oxide. A method characterized by: 2. The method according to claim 1, wherein the time during which 1 ml of exhaust gas (value converted into a standard state) contacts with 1 g of the catalyst is 0.03 seconds or more. 3. The method according to claim 1 or 2, wherein
The method, wherein the porous inorganic oxide is alumina.