JP2000117057A - Method for improving durability of catalyst for oxidizing unburned hydrocarbon in exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the durability of the catalyst for oxidizing unburned hydrocarbon by previously removing a sulfur component from fuel before burning the fuel forming the waste gas in the case that exhaust gas is treated with a catalyst oxidizing the unburned hydrocarbon in exhaust gas with excessive oxygen and constituted by supporting platinum and palladium on alumina. SOLUTION: The fuel gas such as city gas is supplied to a dilute combustion gas engine C from an introducing pipe A together with the air from a combustion air introducing pipe B, and the gas formed after combustion is discharged from a conduit D. And the trace hydrocarbon in the waste gas from the conduit D is oxidized at an oxidizing catalyst layer and discharged from a discharge part F and sent to a heat recovering stage. In such a case, when the sulfur component is to be removed from the fuel supplied to the dilute combustion gas engine C, a desulfurization device X is disposed in the fuel introducing pipe A. Then the sulfur-containing compd. such as odorant incorporated in the fuel such as city gas is removed at the desulfurization device X. The activated carbon, the zeolite, etc., effective for removing the sulfur- containing compd. are exemplified for example as the desulfurization agent used at the desulfurization device X.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a catalyst for oxidizing unburned hydrocarbons in an oxygen-excess exhaust gas comprising platinum and palladium supported on alumina, or a catalyst for oxidizing unburned hydrocarbons in an oxygen-excess exhaust gas prepared by supporting palladium on alumina. The present invention relates to a method for improving the durability of a combustion hydrocarbon oxidation catalyst used in exhaust gas treatment.

[0002]

2. Description of the Related Art Exhaust gas emitted from automobiles, aircraft, thermal power plants, various factories, etc. contains unburned (unburned) hydrocarbons (HC) in addition to NOx and SOx, odorous substances, dust, and the like. ing. Various measures have been taken for exhaust gas containing these, and further research and development are underway. This point is due to cogeneration systems and air conditioners (GHP
The same applies to the exhaust gas discharged from the above.

Conventionally, in gas engines, gas turbines, boilers, heating furnaces, and the like, city gas and other fuel gases including methane, ethane, propane, butane, and the like have been used as fuel gas. To increase the air ratio, that is, the ratio of air to fuel gas, on the fuel gas lean side, that is, the amount of air to fuel gas is 5.0 times, in particular, more than the theoretical air amount required for complete combustion of fuel gas. A so-called lean burn system of 1.1 to 3.0 times has been applied.

[0004] These points also apply to a lean burn gas engine in a so-called cogeneration system in which electric power, mechanical energy, and heat energy are produced from a single drive source (energy source) and energy can be used with high efficiency. The same is true. However, in the case of such a lean burn system, a large amount of oxygen and water vapor coexist in the exhaust gas together with a small amount of lower hydrocarbons (particularly methane), nitrogen oxides (NOx), carbon monoxide and the like. Become.

[0005] As a method of oxidizing and removing hydrocarbons in a combustion exhaust gas containing a trace amount of a lower hydrocarbon (for example, about 5000 ppm or less), three components (H
A treatment method using a so-called three-way catalyst for purifying C, CO, NOx) with the same catalyst has been developed. However, the treatment method using a three-way catalyst can be effectively applied only to exhaust gas containing almost no oxygen, and is effective under conditions where oxygen is excessive and the hydrocarbon component in the exhaust gas is particularly methane. Does not act on Pt or Pd is used as an oxidation catalyst.
There are Pt / Al ₂ O ₃ , Pd / Al ₂ O _3, etc., which are used alone, and these are effective as an organic solvent or a CO oxidation catalyst. It has no effect on the oxidative removal of methane.

[0006] Therefore, for example, methane is particularly contained in the unburned hydrocarbon component containing oxygen in excess, which is discharged from a lean-burn gas engine, and depending on the operating conditions, it is usually 300 to 600 ° C. In order to effectively oxidize and remove hydrocarbons (especially methane) in the exhaust gas discharged to a certain degree, it is necessary to use an oxidation catalyst which can be effectively applied even if oxygen is excessively contained, or an effective treatment method. Development is required.

[0007]

SUMMARY OF THE INVENTION The inventors of the present invention have developed a method for oxidizing low-concentration hydrocarbons in an oxygen-excess exhaust gas by supporting platinum and palladium together on a honeycomb substrate via an alumina carrier. A catalyst and a method for oxidizing and removing hydrocarbons in exhaust gas containing excess oxygen and low-concentration hydrocarbons using this catalyst have been previously developed (JP-A-8-33239)
No. 2).

The above-mentioned oxidation catalyst is a catalyst having excellent durability, and can exhibit a catalytic action for a long period of time. However, when exhaust gas discharged from a lean burn gas engine or the like is treated using the oxidation catalyst, it has been observed that durability is reduced depending on the type of exhaust gas. When the cause was further pursued, it was found that durability was lowered particularly when SOx was contained as a component in the exhaust gas.

[0009] Based on the above facts, the inventors have further pursued, by previously removing the SOx component from the components in the exhaust gas or by removing the sulfur component from the fuel before burning the fuel for generating the exhaust gas. It has been found that the catalyst maintains excellent durability and can maintain its catalyst activity stably for a long period of time. In this regard, it was found that a similar result was obtained with a catalyst in which palladium was supported on alumina.

That is, the present invention relates to a catalyst for oxidizing unburned hydrocarbons in an oxygen-excess exhaust gas obtained by supporting platinum and palladium on alumina, or an unburned hydrocarbon in an oxygen-excess exhaust gas obtained by supporting palladium on alumina. An object of the present invention is to provide a method for further improving the durability of an oxidation catalyst used in exhaust gas treatment.

[0011]

According to the present invention, there is provided (1) an exhaust gas produced by treating an exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas comprising platinum and palladium supported on alumina. Provided is a method for improving the durability of an unburned hydrocarbon oxidation catalyst, which comprises removing a sulfur component from a fuel before burning the fuel.

Further, the present invention provides (2) removing SOx components from exhaust gas in advance when treating exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas comprising platinum and palladium supported on alumina. A method for improving the durability of an unburned hydrocarbon oxidation catalyst is provided.

Further, the present invention provides (3) a method of treating an exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas comprising palladium supported on alumina before burning the fuel for producing the exhaust gas. The present invention provides (4) a method for improving the durability of an unburned hydrocarbon oxidation catalyst, which comprises removing a sulfur component from a catalyst in advance. An object of the present invention is to provide a method for improving the durability of an unburned hydrocarbon oxidation catalyst, which comprises removing an SOx component from the exhaust gas before treating the exhaust gas with the burned hydrocarbon oxidation catalyst.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The catalyst used in the present invention is a catalyst for oxidizing unburned hydrocarbons in an oxygen-excess exhaust gas, and is a catalyst in which platinum and palladium are supported on alumina or a catalyst in which palladium is supported on alumina. It is a catalyst.
As described above, the catalyst of palladium alone is not considered to be effective for the oxidative removal of methane. However, according to the present invention, the sulfur component is previously removed from the fuel or SOx is previously removed from the exhaust gas. Can be used.

When the present oxidation catalyst is a catalyst in which platinum and palladium are supported on alumina, the supported amounts of platinum and palladium on alumina are in the range of 0.025 to 20.0 wt% with respect to alumina, respectively. More preferably, it is in the range of 0.8 to 9.0 wt%. It is still effective when the supported amount of platinum or palladium is less than 0.025 wt%, but the catalytic effect is reduced accordingly. Similarly, an effective catalytic effect can be obtained when each of the supported amounts exceeds about 20.0 wt%. However, if platinum and palladium are each supported up to about 20 wt%, the desired catalytic effects can be obtained. 20.0 upper limit in terms of cost, etc.
About wt% is sufficient. Of course, the above range 0.025 to
It may be around 20.0 wt%.

When the present oxidation catalyst is a catalyst in which palladium is supported on alumina, the amount of palladium supported on alumina is 0.025 to 20.0 wt.
%, More preferably 0.8 to 9.0 wt%.
Range. 0.025wt% palladium loading
Is still effective, but the catalytic effect is reduced accordingly. On the other hand, when each of the supported amounts exceeds about 20.0 wt%, an effective catalytic effect can be obtained similarly. However, if palladium is supported up to about 20 wt%, the expected catalytic effect can be obtained. From the viewpoint of cost and the like, an upper limit of about 20.0 wt% is sufficient. Of course, the above range may be around 0.025 to 20.0 wt%.

In the present invention, when the oxidation catalyst is used as a honeycomb body, the amount of platinum and palladium supported on the honeycomb body is 0.1 to 3 for the same reason as described above.
It is in the range of 0.0 g / L (g / L = gram / liter), preferably in the range of 0.7 to 20.0 g / L, and more preferably in the range of 1.0 to 15.0 g / L. In the case of palladium alone, it is carried according to this.

The method for producing the present oxidation catalyst is not particularly limited as long as it is a method capable of uniformly supporting platinum and palladium on alumina, or a method capable of uniformly supporting palladium on alumina. An impregnation method or an equilibrium adsorption method is applied. As a raw material, nitrates, chlorides, acetates, complex salts (dinitrodiammine platinum, trichlorotriammine platinum, etc.) are used for platinum, and nitrates, chlorides, acetates, complex salts (dichlorotetraammine palladium, etc.) are used for palladium. ) Etc. are used.

As one example of the impregnation method, platinum and palladium are used as an aqueous solution of these compounds. Alumina such as powder is poured into the aqueous solution and stirred, and the platinum compound and the palladium compound are added to the alumina. Impregnated. Next, it is dried and fired by a conventional method. In the case of a catalyst in which palladium alone is supported on alumina, palladium is used as an aqueous solution of these compounds with respect to alumina, and powdered alumina or the like is charged and impregnated. Next, it is dried and fired by a conventional method.

The catalyst may be used in any suitable form such as powder, granule, granule (including spherical), pellet (cylindrical or annular), tablet (tablet), or honeycomb (monolith). Can be used. In the present invention, since it is necessary to allow exhaust gas to pass therethrough, in the case of powder, the powder is sized or granulated in a predetermined particle size range so as not to escape from the catalyst layer filled with the powder, or pressurized. It is desirable to use it after molding or extrusion. Among them, in the case of extrusion molding, it is cut into a predetermined length and pelletized before use. In particular, when treating exhaust gas from a lean burn gas engine, it is preferably used as a honeycomb.

In the case of a honeycomb shape, for example, after alumina is wash-coated on a honeycomb-shaped substrate, platinum and palladium are supported on the alumina-supported honeycomb substrate as an aqueous solution of these compounds. . Next, it is dried and fired by a conventional method. In the case of a catalyst supporting only palladium, a substrate having a honeycomb structure after supporting alumina is impregnated with palladium as an aqueous solution of these compounds, dried and calcined by a conventional method.

As the honeycomb substrate, a ceramic or metal substrate can be used. Preferable examples of ceramics include cordierite,
Preferred examples of the metal include stainless steel and iron-aluminum-chromium alloys. FIG. 1 shows one or two examples of a honeycomb substrate.

FIGS. 2 and 3 are schematic diagrams showing an example of a system flow when the present invention is applied to exhaust gas from a lean burn gas engine. 2 and 3, fuel gas such as city gas is passed through a lean combustion gas engine C from an introduction pipe A together with air from a combustion air introduction pipe B, and gas generated after combustion is passed through a conduit D. From about 300 to 500 ° C. E denotes an oxidation catalyst layer in which the honeycomb-supported platinum-palladium / alumina catalyst or the honeycomb-supported palladium / alumina catalyst according to the present invention is filled and disposed. The trace HC in the exhaust gas from the conduit D is oxidized here, discharged from the outlet F, and sent to a heat recovery step or the like as necessary.

FIG. 2 shows an example in which a sulfur component is removed from the fuel supplied to the lean burn gas engine in the present invention. A desulfurization device X is disposed in a fuel introduction pipe A.
Fuels such as city gas contain sulfur-containing compounds such as odorants, which are removed by the desulfurizer X. Desulfurizer X
Uses a desulfurizing agent for removing sulfur-containing compounds. As the desulfurizing agent, any desulfurizing agent effective for removing sulfur-containing compounds can be used, and examples thereof include activated carbon and zeolite. FIG. 3 shows an example of a case where a sulfur component is removed from exhaust gas from a lean burn gas engine according to the present invention. SOx contained in the exhaust gas is removed by the desulfurization device Y. In the desulfurization device Y, a desulfurizing agent for removing SOx is used. As this desulfurizing agent, any desulfurizing agent that is effective for removing SOx such as SO ₂ may be used.
O) and calcia (CaO).

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below, but it goes without saying that the present invention is not limited to these embodiments. A test catalyst was prepared as described below, and a performance test on the test catalyst was performed.

<Preparation of Test Catalyst A> A cylindrical cordierite honeycomb having a diameter of about 20 mm and a length of about 8 mm was wash-coated with alumina as an aqueous suspension,
A platinum-palladium / alumina catalyst supported on cordierite honeycomb was prepared by supporting platinum and palladium on this. The amount of platinum and palladium supported on the carrier was 10 g / l (10 liters per liter of honeycomb).
g). This catalyst is referred to as Test Catalyst A.

<< Preparation of Test Catalyst B >> A cylindrical cordierite honeycomb having a diameter of about 20 mm and a length of about 8 mm was wash-coated with alumina as an aqueous suspension.
A palladium / alumina catalyst supported on a cordierite honeycomb was prepared by supporting palladium on this. The amount of palladium supported on the carrier was adjusted to 10 g / l. This catalyst is referred to as Test Catalyst B.

<< Performance Test >> Using each of the above test catalysts,
Each test catalyst was filled in a reaction tube having a reaction tube diameter of 20 mm, and the durability as a catalyst was evaluated under the following conditions. The test conditions were SV = 80,000 h ⁻¹ and temperature = 385 ° C.
The gases to be treated include those not containing SO ₂ (1) and SO ₂
And the following three types (2) and (3) when the amount is changed. (1) CH ₄ = 2000 ppm, CO = 820 ppm,
NO = 80 ppm, H ₂ O = 10%, O ₂ = 10.5%,
N ₂ = balance. (2) CH ₄ = 2000 ppm, CO = 820 ppm,
NO = 80 ppm, H ₂ O = 10%, O ₂ = 10.5%,
SO ₂ = 0.05 ppm, N ₂ = balance. (3) CH ₄ = 2000 ppm, CO = 820 ppm,
NO = 80 ppm, H ₂ O = 10%, O ₂ = 10.5%,
SO ₂ = 1 ppm, N ₂ = balance.

FIG. 4 shows the results when the test catalyst A was used. Example 1 uses the composition (1) of the gas to be treated,
The case where the composition (2) of the gas to be treated is used is Comparative Example 1, and the case where the composition (3) of the gas to be treated is used is Comparative Example 2. FIG. 5 shows the results when the test catalyst B was used. Example 2 uses the composition (1) of the gas to be treated,
The case where the composition (2) of the gas to be treated is used is Comparative Example 3, and the case where the composition (3) of the gas to be treated is used is Comparative Example 4.

The oxidation activity [conversion (%)] was determined by the following equation.

[Equation 1]

First, as shown in FIG. 4, when a catalyst supporting platinum and palladium is used, the methane oxidation activity is SO ₂
The gas to be treated containing no is higher than the gas to be treated containing SO ₂ from the beginning of the test, and the conversion at the start of the test is as high as 90%. Thereafter, the conversion rate gradually decreases, but the conversion rate of about 50% is maintained even after the elapse of 180 hours (h). On the other hand, with regard to the gas to be treated containing SO ₂ , the conversion is about 50% at the beginning of the test, and gradually decreases thereafter, and drops to about 17% after 150 hours.

Next, as shown in FIG. 5, even when a catalyst supporting palladium alone was used, the methane oxidation activity was still lower than that of SO.
_The gas to be treated containing no ₂ is higher than the gas to be treated containing SO ₂ from the beginning of the test. Thereafter, the conversion rate gradually decreases, but the conversion rate of about 30% is maintained even after the elapse of 180 hours (h). On the other hand, for the gas to be treated containing SO ₂ , it is around 39% at the beginning of the test, but drops rapidly thereafter, and the function as a catalyst becomes zero after 30 hours.

As described above, according to the present invention, an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas comprising platinum and palladium supported on alumina or an oxygen-excess exhaust gas formed by supporting palladium on alumina is used. When the unburned hydrocarbon oxidation catalyst is used for exhaust gas treatment, its durability is improved by removing SOx from the exhaust gas to be treated in advance, or removing sulfur components in advance before burning the fuel to eliminate SOx generation. Can be remarkably improved. In particular,
It has been described that a palladium-only catalyst does not effectively work for oxidative removal of unburned hydrocarbons in an oxygen-excess exhaust gas. However, according to the present invention, it can be used effectively.

[0034]

According to the present invention, the catalyst for oxidizing unburned hydrocarbons in an oxygen-excess exhaust gas obtained by supporting platinum and palladium on alumina, or the catalyst in an oxygen-excess exhaust gas obtained by supporting palladium on alumina. When using a combustion hydrocarbon oxidation catalyst for exhaust gas treatment, SOx
By not including, the durability can be remarkably improved.

Further, according to the present invention, the present invention can be effectively applied particularly to exhaust gas from a lean combustion gas engine in a cogeneration system or an air conditioner (such as a GHP) using city gas as a driving source. Furthermore, according to the present invention, since excellent durability can be maintained for the catalyst used, the frequency of replacement can be significantly reduced, and the cost of the exhaust gas treatment system can be reduced. Further, since the oxidation is accompanied by heat generation, heat recovery is possible.

[Brief description of the drawings]

FIG. 1 is a diagram (a schematic cross-sectional view) showing one or two examples of a honeycomb substrate used in the present invention.

FIG. 2 is a schematic diagram showing an example of a system flow when the present invention is applied to exhaust gas from a lean burn gas engine.

FIG. 3 is a schematic diagram showing another example of a system flow when the present invention is applied to exhaust gas from a lean burn gas engine.

FIG. 4 is a diagram showing the results of Example 1.

FIG. 5 is a diagram showing the results of Example 2.

[Explanation of symbols]

 A fuel gas introduction pipe B air introduction pipe C lean burn gas engine D exhaust gas pipe E unburned hydrocarbon oxidation catalyst layer F exhaust gas discharge pipe

Claims (9)

[Claims] When treating an exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas comprising platinum and palladium supported on alumina, a sulfur component is removed from the fuel before burning the fuel for producing the exhaust gas. A method for improving the durability of an unburned hydrocarbon oxidation catalyst, which comprises removing the catalyst in advance. 2. A method for treating an exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas comprising platinum and palladium supported on alumina, wherein SOx components are previously removed from the exhaust gas. A method for improving the durability of a combustion hydrocarbon oxidation catalyst. 3. A method for treating an exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas obtained by supporting palladium on alumina, wherein a sulfur component is previously removed from the fuel before burning the fuel for producing the exhaust gas. A method for improving the durability of an unburned hydrocarbon oxidation catalyst, comprising: 4. A method for treating an exhaust gas with an unburned hydrocarbon oxidation catalyst in an oxygen-excess exhaust gas obtained by supporting palladium on alumina, wherein SOx components are previously removed from the exhaust gas. A method for improving the durability of an unburned hydrocarbon oxidation catalyst. 5. The method for improving durability of an unburned hydrocarbon oxidation catalyst according to claim 1, wherein the form of the oxidation catalyst is a honeycomb shape. 6. The method for improving the durability of an unburned hydrocarbon oxidation catalyst according to claim 5, wherein the honeycomb-shaped substrate is a ceramic or a metal. 7. The method for improving durability of an unburned hydrocarbon oxidation catalyst according to claim 6, wherein the ceramic is cordierite and the metal is stainless steel. 8. The method for improving durability of an unburned hydrocarbon oxidation catalyst according to claim 1, wherein the main component of the unburned hydrocarbon in the oxygen-excess exhaust gas is methane. 9. An exhaust gas containing excess oxygen and containing unburned hydrocarbons is exhaust gas from a lean burn gas engine.
9. The method for improving durability of an unburned hydrocarbon oxidation catalyst according to any one of 8.