JP2001190931A - Method for purifying waste gas containing methane - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel technique for purifying waste gas containing methane, in which nitrogen can be steadily purified over a long period of time and discharge of methane can be suppressed in a process of purifying waste gas containing methane. SOLUTION: This method for purifying waste gas containing methane is characterized by using a methane oxidation catalyst and a NOX occluding and reducing catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for removing nitrogen oxides (NOx) contained in methane-containing exhaust gas.

[0002]

2. Description of the Related Art Nitrogen oxides contained in exhaust gas such as combustion exhaust gas are harmful to the human body and also contribute to natural environmental problems such as acid rain. Therefore, the reduction is an urgent issue. At present, a so-called three-way catalyst is used for the treatment of nitrogen oxides in automobile exhaust gas, and the ammonia selective reduction method is widely used for the treatment of nitrogen oxides in exhaust gas discharged from power generation facilities. I have. However, the former is only applicable to exhaust gas with a stoichiometric air-fuel ratio in which no excess oxygen remains in the exhaust gas, while the latter is
There is a danger of toxic and odorous ammonia being emitted.

In consideration of such a situation, Japanese Patent Application Laid-Open No. 5-9895
No. 4 discloses a NOx storage reduction catalyst that absorbs nitrogen oxide on its catalyst in an oxidizing atmosphere in which excess oxygen exists, and releases and reduces the absorbed nitrogen oxide in a reducing atmosphere. I have. In the exhaust gas purifying catalyst and the exhaust gas purifying method described above, not only nitrogen oxides,
It is said that carbon monoxide and hydrocarbons can also be removed.
However, when an exhaust gas containing methane having low reactivity is treated using a nitrogen oxide removing catalyst, a considerable proportion of methane remains in the treated gas. Methane, unlike ammonia, is virtually non-toxic and poorly photoreactive, and is not considered to significantly degrade the local environment. However, from the viewpoint of global environmental protection in the future, it is desirable to reduce the emission as much as possible.

It is well known that a catalyst carrying a platinum group element has high activity in oxidizing hydrocarbons. For example, JP-A-51-106691 discloses an exhaust gas purification catalyst in which platinum and palladium are supported on an alumina carrier. However, among hydrocarbons,
It is also well-known that methane has high stability and is difficult to oxidize and remove, especially since flue gas contains catalytic activity inhibitors such as steam and sulfur oxides. It is inevitable that the catalytic activity decreases over time.

[0005] Lampert et al. Found that when methane oxidation was carried out using a palladium catalyst, only 0.1 p.
The presence of sulfur dioxide at only pm indicates that its catalytic activity is almost lost within a few hours, revealing that the presence of sulfur oxides has a significant effect on the catalytic activity of methane oxidation (Applied Cat. Lysis B: Environmental (Applied Catalysis B: Environment), vo
l. 14, 211-223 (1997)). As apparent from the above, it has been difficult to oxidize and remove methane with the conventional exhaust gas treatment technology. Therefore, for purification of exhaust gas such as combustion exhaust gas of natural gas, for example, establishment of a new technology capable of effectively suppressing methane emission is required.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a new method for purifying methane-containing exhaust gas, which can stably purify nitrogen oxides for a long period of time and can suppress emission of unburned components such as methane. The main object of the present invention is to provide a technology for purifying exhaust gas containing methane.

[0007]

Means for Solving the Problems As a result of intensive studies, the inventor has determined that a specific catalyst is used in combination, and more specifically, methane is oxidized and decomposed by a methane oxidation catalyst, and a NOx storage reduction type is used. It has been found that purifying methane-containing exhaust gas is possible by performing occlusion reduction of nitrogen oxides with a catalyst.

The present invention has been completed on the basis of such new findings, and provides the following method for purifying methane-containing exhaust gas. 1. A method for purifying methane-containing exhaust gas, comprising using a methane oxidation catalyst and a NOx storage reduction type catalyst. 2. 2. The method for purifying methane-containing exhaust gas according to 1 above, wherein the methane oxidation catalyst is arranged on the upstream side, and the NOx storage reduction catalyst is arranged on the downstream side. 3. 3. The method for purifying methane-containing exhaust gas according to the above 1 or 2, wherein the methane oxidation catalyst is a catalyst in which palladium is supported on tin oxide. 4. 3. The method for purifying methane-containing exhaust gas according to the above item 1 or 2, wherein the methane oxidation catalyst is a catalyst in which palladium and platinum are supported on tin oxide.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the method for purifying methane-containing exhaust gas of the present invention, a methane oxidation catalyst and a NOx storage reduction type catalyst are used in combination. The methane oxidation catalyst in the present invention refers to a catalyst that oxidizes hydrocarbon compounds such as methane; and unburned components such as carbon monoxide.

In the purification method of the present invention, if necessary, the methane-containing exhaust gas is exhausted in excess of air (l
ean) and excess fuel (rich). That is,
When the exhaust gas is in the lean state, hydrocarbons such as methane and unburned components such as carbon monoxide are oxidized and removed by the methane oxidation catalyst, and NOx is absorbed and removed by the NOx storage reduction catalyst. On the other hand, exhaust gas is rich
In this state, the methane oxidation catalyst does not function and NOx
The NOx stored in the storage reduction catalyst is released and reduced to nitrogen. In the reduction reaction of NOx, rich
The unburned components in the exhaust gas in the state are used as a reducing agent, and the unburned components are oxidized and decomposed. Therefore, in the present invention,
By controlling the rich period and the lean period of the exhaust gas flow, it is possible to simultaneously suppress the emission of nitrogen oxides and methane. Switching between the rich state and the lean state is achieved by adjusting the air-fuel ratio in the exhaust gas source (for example, a gas engine) at predetermined time intervals, and obtaining the composition of the exhaust gas, the amount of exhaust gas,
The control time may be provided in advance as a map based on the temperature or the like, and may be appropriately performed. Alternatively, a sensor for detecting the NOx concentration and / or the oxygen concentration may be provided, and the control may be performed in accordance with the measured value. Good.

The methane oxidation catalyst used in the present invention is:
A catalyst in which palladium is supported on tin oxide or a catalyst in which palladium and platinum are supported on tin oxide.

The amount of palladium carried on the methane oxidation catalyst is not particularly limited, but is usually about 1 to 25%, more preferably about 2 to 20%, based on the weight of tin oxide. If the supported amount of palladium is too small, the catalytic activity will not be sufficiently exhibited, whereas if it is too large, the particle size of palladium will be too large and palladium will not be effectively used as an active metal.

When palladium and platinum are used together in the methane oxidation catalyst used in the present invention, the amount of platinum carried is not particularly limited, but is usually about 5 to 50% by weight relative to palladium. Preferably 10-5
It is about 0%. If the amount of supported platinum is too small, the effect of the combined use of both metals is not sufficiently improved, whereas if it is too large, the function of palladium as an active metal may be impaired. . In the methane oxidation catalyst, when palladium and platinum are used in combination, the loading amount of these metals is not particularly limited, but the loading amount of palladium is usually 1 to 2 with respect to the weight of tin oxide.
About 5%, more preferably about 2 to 20%, and the amount of supported platinum is usually 5 to 100% by weight relative to palladium.
And more preferably about 10 to 50%.

The specific surface area of the oxidation catalyst used in the present invention is not particularly limited as long as the obtained catalyst can be used stably and the dispersion of the supported metal can be maintained, but it is usually 0.5 to 50 m ² / g as measured by the BET method. Degree, preferably 1-20 m ² / g
It is about.

The shape of the methane oxidation catalyst used in the present invention is not particularly limited. For example, it can be used by molding into a pellet by adding a binder as needed, or by wash-coating on a refractory honeycomb.

The methane oxidation catalyst used in the present invention is:
For example, it can be obtained by a method in which tin oxide is impregnated with a solution in which a palladium compound and, if necessary, a platinum compound are further dissolved, dried, and fired in an oxidizing atmosphere.

The tin oxide used is not particularly limited, and tin oxide produced by a known production method or commercially available tin oxide can be used. As a known production method, for example,
A method of firing tin hydroxide, tin oxalate, or the like under an oxidizing atmosphere such as air or oxygen to obtain tin oxide is given.

The palladium compound and the platinum compound used as required are not particularly limited as long as they dissociate the respective metal ions in the solution. For example, nitrates and ammine complexes of these metals can be mentioned. The solvent is preferably an aqueous solution, but may be a mixed solvent to which a water-soluble organic solvent such as acetone or ethanol is added.

Next, the tin oxide impregnated with the predetermined active metal is dried, and then fired in an oxidizing atmosphere such as air. The firing temperature is usually about 450 ° C to 700 ° C, more preferably about 500 ° C to 650 ° C. If the firing temperature is too high, the grain growth of the supported active metal proceeds,
Also, if it is too low, the active metal grains grow during use of the catalyst, so that it is not possible to obtain a catalyst that stably exhibits a high catalytic activity. The firing time is usually about 1 to 20 hours, preferably about 2 to 10 hours.

When the methane oxidation catalyst used in the present invention is wash-coated on a refractory honeycomb, even if the catalyst prepared by the above method is slurry-formed and wash-coated, the catalyst is previously tin oxide or a precursor thereof. A wash coat of tin hydroxide or the like may be wash-coated on the refractory honeycomb, and may be fired if necessary before carrying palladium or platinum according to the above method.

The NOx storage reduction catalyst used in the present invention is not particularly limited, and a known NOx storage reduction catalyst can be used. As such a NOx storage-reduction type catalyst, for example, an alkali metal (potassium,
NOx storage components and precious metals such as sodium, lithium, rubidium, etc.) and / or alkaline earth metals (barium, calcium, magnesium, strontium, etc.)
Examples thereof include a catalyst supporting a NOx reducing component such as (platinum and rhodium), a Ba-Fe-Pt-based catalyst, and a Ba-Pt-Rh-based catalyst. The porous carrier to be used can be appropriately selected from known porous carriers according to the active ingredient and the like. As the porous carrier for the catalyst, for example, alumina,
Titania, zeolite, zirconia, silica alumina,
Silica and the like can be exemplified.

Such a NOx storage type reduction catalyst can be produced according to a conventional method. For example, NOx storage component, NOx
A method of co-precipitating the reducing component and the carrier, a method of impregnating the carrier with the NOx storage component and the NOx reducing component, and the like can be given.

The shape of the NOx storage reduction catalyst used in the present invention is not particularly limited. For example, as in the case of the methane oxidation catalyst, it can be used as a honeycomb catalyst by adding a binder as necessary and molding into pellets, or by wash-coating on a refractory honeycomb. When wash-coating on a refractory honeycomb, even if the catalyst prepared as described above is slurry-coated and wash-coated, the carrier is previously wash-coated on the refractory honeycomb, and then the NOx storage component and the NOx reduction component. May be carried.

In the present invention, the methane oxidation catalyst and the NOx storage reduction catalyst may be supported on the same carrier (for example, a refractory honeycomb carrier). Alternatively, a methane oxidation catalyst and a NOx storage reduction catalyst may be mixed and used as a single catalyst layer.

Although the amount of the methane oxidation catalyst used in the present invention is not particularly limited, the space velocity per hour (GHS)
V), usually about 500000h ^-1 or less, more preferably 1
It is about 000-300000h ^-1 . The amount of the NOx storage reduction type catalyst can be appropriately set depending on the type of the catalyst used, etc., but in GHSV, it is usually about 60,000 h ^-1 or less, more preferably about 1,000 to 30,000 h ^-1. . If the usage of either / both the methane oxidation catalyst and the NOx storage reduction catalyst is too small, that is, if the GHSV value is too large, an effective purification rate may not be obtained. On the other hand, if the usage of these catalysts is too large, that is, if the GHSV value is too small, the purification rate is improved, but in addition to the disadvantage in cost, the pressure loss in the catalyst layer increases. Problems may occur.

The temperature of the methane oxidation catalyst used in the present invention is not particularly limited, but is usually about 350 to 650 ° C., preferably about 400 to 600 ° C. NO used in the present invention
The temperature of the x-storage-reduction catalyst can be appropriately set depending on the type of the catalyst to be used and the like.
Preferably it is about 350 ° C to 550 ° C. If the temperature of these catalysts is too low, the activity of the catalyst may be reduced and a desired conversion may not be obtained. On the other hand, if the temperature of the catalyst is too high, the durability of the catalyst may deteriorate. The temperature of the catalyst may or may not be the same for the methane oxidation catalyst and the NOx storage reduction catalyst.

The method for arranging the two catalysts is not particularly limited. For example, a method of separately disposing the catalyst layer of the methane oxidation catalyst and the catalyst layer of the NOx storage reduction type catalyst, filling the methane oxidation catalyst on the upstream side of a single catalyst container and filling the NOx storage reduction type catalyst on the downstream side Method, methane oxidation catalyst and NOx storage reduction type catalyst on the same carrier (for example, refractory honeycomb carrier, etc.)
A method in which a methane oxidation catalyst and a NOx occlusion reduction type catalyst are mixed and used as a single catalyst layer may be used. Among these, a method of separately disposing a catalyst layer of the methane oxidation catalyst and a catalyst layer of the NOx storage reduction type catalyst is preferable. In particular, a method in which a methane oxidation catalyst is arranged upstream of an exhaust gas source and a NOx storage reduction catalyst is arranged downstream is preferable. This is because the methane oxidation catalyst has higher activity at higher temperatures within a predetermined temperature range, and the NOx storage reduction catalyst has higher activity at lower temperatures within a predetermined temperature range.

The method of the present invention can be carried out, for example, by using an apparatus using a methane oxidation catalyst and a NOx storage reduction catalyst. In this apparatus, a methane oxidation catalyst and a NOx storage reduction catalyst that can be used in the present invention can be used. The arrangement method of the two catalysts in this apparatus is not particularly limited. For example, a reactor having a methane oxidation catalyst and a reactor having a Nox occlusion reduction type catalyst are separately installed, and each reactor is connected by a gas flow pipe.The methane oxidation catalyst is placed upstream of a single catalyst container. A method of arranging a catalyst layer filled with a NOx storage reduction catalyst on the downstream side, a catalyst layer in which a methane oxidation catalyst and a NOx storage reduction catalyst are supported on the same carrier (e.g., a refractory honeycomb carrier). And a method in which a methane oxidation catalyst and a NOx storage reduction type catalyst are mixed and used as a single catalyst layer. Among these, a method in which a reactor having a methane oxidation catalyst and a reactor having a Nox occlusion reduction type catalyst are separately installed, and the individual reactors are preferably connected by a gas flow pipe. In particular, a method is preferable in which a reactor having a methane oxidation catalyst is arranged upstream of the exhaust gas source, a reactor having a Nox storage reduction catalyst is installed downstream, and the individual reactors are connected by gas flow pipes.

[0029]

According to the present invention, a high conversion rate of methane is high even for an exhaust gas containing water vapor, although the combustion exhaust gas usually contains about 5% to 15% of water vapor. can get.

The exhaust gas usually contains, in addition to water vapor, sulfur oxides which are known to significantly reduce the catalytic activity. Since the catalyst used in the present invention has high resistance to the decrease in activity due to the sulfur component, the exhaust gas purifying performance is maintained at a high level according to the present invention.

In the method for purifying combustion exhaust gas of the present invention, an oxidation catalyst for oxidizing methane and a NOx storage reduction type catalyst are used in combination. According to the present invention, it is possible to simultaneously reduce nitrogen oxides and suppress methane emission.

[0032]

EXAMPLES Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

Example 1 First, a NOx storage reduction catalyst was produced as follows.

An aqueous dinitroammine platinum solution containing 10 g of platinum was impregnated with 1000 g of alumina powder at room temperature for 15 hours, and then evaporated to dryness. This is dried and then at 600 ° C for 5
After calcination for a period of time, a Pt-supported alumina powder was prepared. Next, Ba
The above Pt-supported alumina powder was dispersed in an aqueous barium acetate solution containing 275 g, and evaporated to dryness, then dried and calcined at 600 ° C. for 5 hours to prepare a Pt—Ba-supported alumina powder. To 500 g of the Pt-Ba-supported alumina powder, 150 ml of water and 350 g of alumina sol (alumina content: 10% by weight) were added and mixed by stirring to form a slurry. A 1.3 L cordierite honeycomb monolithic carrier was immersed in the slurry, pulled up, and then the excess slurry was blown off, dried at 80 ° C, and fired at 500 ° C. In the obtained catalyst, Pt was 1.0 g /
L and Ba were supported at 0.2 mol / L.

Next, a methane oxidation catalyst was produced as follows. Palladium nitrate aqueous solution containing 0.25 g as palladium and 0.083 g of dinitrodiammine platinum 69%
Mix with a solution heated and dissolved in 1 ml of nitric acid, add pure water and add 20 ml
Was prepared. The obtained solution was impregnated with 5 g of tin oxide (SL, manufactured by Nippon Chemical Industry Co., Ltd.) at 0 ° C. for 15 hours. This is dried and further calcined in air at 550 ° C for 2 hours to give 5% Pd-1% P
A t / SnO ₂ catalyst was obtained.

The obtained NOx storage reduction catalyst and methane oxidation catalyst were installed as follows. Pd-Pt / SnO ₂
1 ml of the catalyst was arranged on the upstream side, and 3 ml of the NOx storage reduction type catalyst was arranged on the downstream side, and charged in a reaction tube to form a catalyst layer. The temperature of the catalyst layer was maintained at 450 ° C, and a gas was passed at a flow rate of 1 liter per minute (0 ° C conversion) at a ratio of lean gas time of 55 seconds and rich gas time of 5 seconds to convert NOx after the start of the reaction. Rate and concentration of residual methane were measured. Each gas composition is as follows.

Lean gas composition: 100 ppm of nitric oxide, methane
1600ppm, carbon dioxide 6%, oxygen 10%, steam 9%, sulfur dioxide 0.3ppm and residual nitrogen rich gas composition: nitric oxide 100ppm, methane 1600ppm, carbon dioxide 6%, carbon monoxide 3000ppm, steam 9%, sulfur dioxide Table 1 shows the obtained results. The NOx conversion and the outlet methane concentration here are time-weighted average values when using lean gas and when using rich gas. The NOx conversion in the table was determined according to the following equation. NOx conversion rate (%) = 100 × (1−NOx-out / NOx-in) where “NOx-in” is the NOx concentration at the inlet of the methane oxidation catalyst, “NO
“x-out” indicates the NOx concentration at the outlet of the NOx storage reduction catalyst.

[0038]

[Table 1]

Comparative Example 1 Only 3 ml of the same NOx storage reduction catalyst as used in Example 1 was charged into a reaction tube to form a catalyst layer. Raise the temperature of the catalyst layer to 45.
Keeping the temperature at 0 ° C, the gas flows at a rate of 1 second per minute (converted to 0 ° C), lean gas time 55 seconds, rich gas time 5 seconds,
The NOx conversion and the concentration of residual methane after the start of the reaction were measured. Each gas composition is the same as the gas composition used in Example 1.

Table 2 shows the results.

[0041]

[Table 2]

Comparative Example 2 NK-124 alumina manufactured by Sumitomo Chemical Co., Ltd. was calcined in air at 800 ° C. for 2 hours. 5 g of this palladium was immersed in 20 ml of an aqueous palladium nitrate solution containing 0.25 g of palladium for 15 hours to carry impregnation of palladium. This was dried and further calcined at 550 ° C. for 2 hours in air to obtain a Pd / alumina catalyst.

3 ml of the same NOx storage reduction catalyst as used in Example 1 was placed on the upstream side, and 1 ml of Pd / alumina catalyst was placed on the downstream side, and they were charged into a reaction tube to form a catalyst layer. 1 liter per minute (converted to 0 ° C) while maintaining the temperature of the catalyst layer at 450 ° C
The gas was flowed at a ratio of lean gas time of 55 seconds and rich gas time of 5 seconds at a flow rate of, and the conversion of NOx and the concentration of residual methane after the start of the reaction were measured. Each gas composition is the same as the gas composition used in Example 1.

Table 3 shows the results.

[0045]

[Table 3]

In Example 1, nitrogen oxides could be removed while unreacted methane was oxidatively decomposed even when the hydrocarbon component in the exhaust gas was a hydrocarbon mainly composed of methane. . In addition, both methane oxidative decomposition and nitrogen oxide removal were efficiently performed for a long time.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) F01N 3/28 301 B01D 53/36 103B (72) Inventor Masataka Masuda 4-1-2 Hirano-cho, Chuo-ku, Osaka-shi, Osaka No. Osaka Gas Co., Ltd. (72) Inventor Ken Tabata 4-1-2, Hirano-cho, Chuo-ku, Osaka-shi, Osaka Osaka Gas Co., Ltd. (72) Inventor Hirano Takenori 1385, Izumi, Fuchi, Izumi-shi, Kagoshima -2 F term (reference) 3G091 AA19 AB02 AB06 BA14 BA15 BA19 BA39 CB02 DA01 DA02 DB10 EA30 EA33 EA34 FB10 FB12 GA01 GA06 GA19 GB01W GB01X GB02W GB02Y GB03W GB03Y GB05W GB06W GB07W GB10X GB16XHA08 AB03 AB03 BA09X BA15X BA21X BA30X BA31X BA41X BB01 BB02 CC32 CC47 CD08 DA08 EA04 4G069 AA03 BA01B BB04A BB04B BC13B BC22A BC22B BC72A BC72B BC75A BC75B CA02 CA03 CA07 CA08 CA13 CA15 DA06 EA19

Claims (4)

[Claims] 1. A method for purifying methane-containing exhaust gas, comprising using a methane oxidation catalyst and a NOx storage reduction catalyst. 2. The method for purifying methane-containing exhaust gas according to claim 1, wherein the methane oxidation catalyst is arranged on the upstream side, and the NOx storage reduction catalyst is arranged on the downstream side. 3. The method for purifying methane-containing exhaust gas according to claim 1, wherein the methane oxidation catalyst is a catalyst in which palladium is supported on tin oxide. 4. The method for purifying methane-containing exhaust gas according to claim 1, wherein the methane oxidation catalyst is a catalyst in which palladium and platinum are supported on tin oxide.