JP2003254117A - Exhaust emission controlling method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To express high methane purifying activity over a long time by easily recovering the oxidizing activity of a SOX-poisoned methane oxidation catalyst under a lean atmosphere of excessive oxygen. <P>SOLUTION: In this exhaust emission controlling method for supplying the exhaust gas from a combustion engine including lean-burn combusted in the lean atmosphere of excessive air, to the catalyst capable of oxidizing at least methane, the exhaust gas enervated by combustion in the atmosphere where an air-fuel ratio is alternately changed between a lean atmosphere of excessive air and a rich atmosphere of excessive fuel, is supplied to the catalyst. The SOX-poisoned active species is easily reduced and decomposed on the catalyst exposed to the reducing atmosphere, and the active species is regenerated. Whereby the methane oxidizing activity can be recovered. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently oxidizing and purifying mainly methane in exhaust gas containing at least methane discharged from a combustion engine including lean combustion in which an air-fuel ratio is burned in a lean atmosphere with excess air. . INDUSTRIAL APPLICABILITY The present invention is suitable for purifying exhaust gas from an automobile, and particularly suitable for purifying exhaust gas containing a large amount of methane such as exhaust gas from a gas engine.

[0002]

BACKGROUND OF THE INVENTION Among hydrocarbons (hereinafter referred to as HC), olefinic HCs are relatively easily oxidized, but saturated HCs are less likely to be oxidized than olefinic HCs, and methane is particularly difficult to be oxidized and purified. Has been. Therefore, the development of a catalyst that can purify methane has been advanced, and it has been found that palladium (Pd) is effective as a catalyst metal. For example, JP-A No. 11-137998 discloses that Pd is added to an alumina carrier.
And a catalyst exhibiting methane purification ability, which carries at least one selected from Ru, Ir, and Cu. Further, JP-A-7-053976 discloses a methane oxidation catalyst using a coprecipitate of Pd and Co as a catalytic metal.

As described above, Pd has a high oxidative activity for methane, and it was considered that high activity can be maintained for a long period of time by supporting it on a carrier having excellent heat resistance such as alumina. However, it has been revealed that the methane oxidation activity of the methane oxidation catalyst in which Pd is supported on alumina or the like gradually decreases during use in exhaust gas in a lean atmosphere with excess oxygen.

In the methane oxidation catalyst, the active site that causes the oxidation reaction of methane in a lean atmosphere is that the precious metal is M
When it is said that it is a MO species. Then, it was clarified that the sulfur oxide (SO _x ) in the exhaust gas had a great influence as one of the causes of the activity of the methane oxidation catalyst gradually decreasing during use. That is, when the noble metal that is the active component of the methane oxidation catalyst is M, when SO _x coexists in a lean atmosphere with excess oxygen, MO-SO ₃ is produced, which greatly inhibits the methane oxidation reaction. . This phenomenon is called SO _x poisoning of the catalyst.

In order to avoid this problem, Japanese Patent Laid-Open No. 2000-
Japanese Patent No. 254500 discloses a methane oxidation catalyst in which Pd is supported on a carrier such as TiO ₂ or ZrO ₂ . By using such an acidic carrier, the proximity of SO _x is suppressed, so SO _x poisoning of the catalyst can be suppressed. The coexistence of an alkali metal or an alkaline earth metal, it is also an effective method these reacting SO _x preferentially.

[0006] However, although the above-mentioned means is effective in reducing the amount of SO _x attached (reacted) to the methane oxidation catalyst, the activity of the catalyst once poisoned with SO _x once changed to MO-SO _3. It is difficult to recover, and the methane purification rate after endurance will actually drop by 30% or more from before endurance.
MO-SO ₃ is about 600 ℃ in a lean atmosphere with excess oxygen.
Since it will not decompose unless it is heated above, it is difficult to decompose MO-SO _{3 in} exhaust gas in a lean atmosphere with a relatively high temperature and excess oxygen, such as exhaust gas from a gas engine. Therefore, when used in an exhaust gas atmosphere from a gas engine, a decrease in the methane purification rate with time cannot be avoided.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is possible to easily recover the oxidation activity of a SO _x poisoned methane oxidation catalyst in a lean atmosphere with excess oxygen. The purpose is to allow high methane purification activity to be exhibited for a long period of time.

[0008]

The feature of the exhaust gas purification method of the present invention for solving the above-mentioned problems is that at least methane is oxidized from exhaust gas from a combustion engine including lean combustion in which the air-fuel ratio is burned in a lean atmosphere with excess air. A possible exhaust gas purification method for supplying to a catalyst, in which exhaust gas produced by combustion in an atmosphere in which an air-fuel ratio is alternately changed between a lean atmosphere with excess air and a rich atmosphere with excess fuel is supplied to the catalyst. .

In the above exhaust gas purification method, the exhaust gas burned in a rich atmosphere is preferably supplied to the catalyst at a temperature of 400 ° C. or higher. The total amount of reducing components in the exhaust gas burned in the rich atmosphere and supplied to the catalyst is preferably equal to or more than the total amount of the sulfur components in the exhaust gas burned in the lean atmosphere and supplied to the catalyst.

Another feature of the exhaust gas purifying method of the present invention that solves the above-mentioned problems is that a catalyst capable of oxidizing at least methane from exhaust gas from a combustion engine including lean combustion in which the air-fuel ratio is burned in a lean atmosphere with excess air. A method of purifying exhaust gas, which comprises supplying a reducing gas to a catalyst.

In the above exhaust gas purification method, it is desirable that the reducing gas be supplied to the catalyst at a temperature of 400 ° C. or higher. The total amount of reducing components in the reducing gas supplied to the catalyst is preferably equal to or more than the total amount of sulfur components in the exhaust gas burned in the lean atmosphere and supplied to the catalyst.

In the exhaust gas purification method of the present invention, the catalyst preferably contains Pd.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have earnestly studied a method for decomposing a catalyst that has become MO-SO _3, and as a result, by making the atmosphere a reducing atmosphere, MO-SO ₃ can be easily reduced even in a low temperature range. The inventors have found that they decompose, and have completed the present invention.

That is, in the exhaust gas purification method of the present invention, the exhaust gas produced by burning in an atmosphere in which the air-fuel ratio is alternately changed between a lean atmosphere with excess air and a rich atmosphere with excess fuel is converted into a catalyst capable of oxidizing at least methane. We are supplying. Further, the exhaust gas purifying method of another invention includes a step of supplying a reducing gas to the catalyst in the exhaust gas purifying method of supplying exhaust gas from a combustion engine including lean combustion to at least a catalyst capable of oxidizing methane.

When lean combustion is performed in a lean atmosphere with an air-fuel ratio of excess air, the exhaust gas is also in a lean atmosphere of excess oxygen, so at least methane is oxidized and purified on the catalyst. However, when the exhaust gas contains SO _x , the precious metal M of the catalyst becomes MO-SO ₃ , and the active sites disappear and deteriorate.

Therefore, the exhaust gas burned in the rich atmosphere of excess fuel is supplied to the catalyst or the reducing gas is supplied to the catalyst. As a result, the catalyst is exposed to a reducing atmosphere, and MO-SO ₃ is easily reductively decomposed even in the low temperature range of 400 to 500 ° C, and the active species, MO species, is regenerated. As a result, the methane oxidation activity is restored, and a high methane purification rate is exhibited even after long-term use.

When the exhaust gas burned in a rich atmosphere with excess fuel is supplied to the catalyst or the reducing gas is supplied to the catalyst, the temperature of the gas supplied after being mixed with the exhaust gas is 400 ° C. or higher. Is desirable. This is because the reductive decomposition of MO-SO ₃ becomes difficult at less than 400 ° C. The upper limit of temperature is not particularly limited, but 800 ° C or lower is desirable. If the temperature is higher than 800 ° C, the activity of the precious metal supported on the catalyst may decrease due to grain growth.

When the exhaust gas burned in a rich atmosphere with excess fuel is supplied to the catalyst or the reducing gas is supplied to the catalyst, the total amount of reducing components in the supplied gas is lean in the lean atmosphere. It is desirable that the amount is equal to or more than the total amount of the sulfur component in the exhaust gas that is burned and supplied to the catalyst. If the total amount of reducing components is less than the equivalent amount to the total amount of sulfur components, MO-SO _{3 that} has not been reductively decomposed remains, and the deterioration of the catalyst gradually progresses. The optimum value of the total amount of reducing components to be introduced depends on the type of catalyst and oxygen and nitrogen oxides that are oxide components other than sulfur components, depending on the coexisting amount and storage amount of rich gas and reducing gas, It is preferably present in excess of the amount consumed by the oxidizing component. If the total amount of reducing components is more than the equivalent amount with respect to the total oxidizing components, the amount of HC and CO discharged during this recovery process will increase, so the total amount of reducing components should be equivalent to the total amount of oxidizing components. It is preferably 1 to 10 times.

In order to make the total amount of the reducing component equal to or more than the total amount of the sulfur component, a method of adjusting the concentration of the reducing component in the exhaust gas or the reducing gas burned in an atmosphere rich in fuel, and the supply of this gas There is a method of adjusting the time and a method of adjusting both of them.

As a catalyst capable of oxidizing at least methane, an oxide carrier composed of at least one of Al ₂ O ₃ , ZrO ₂ , TiO ₂ , SiO ₂ , CeO ₂ , MgO, etc. or a composite oxide composed of a plurality of these. A catalyst in which at least one kind of noble metal selected from Pt, Pd, Rh, Ir and the like is supported on the carrier can be used. The optimum loading amount of the noble metal is 0.1 to 10% by weight in the catalyst. Above all, a catalyst supporting Pd, which has a particularly high methane oxidation activity, is particularly preferable.

The exhaust gas burned in a rich atmosphere with excess fuel can be supplied to the catalyst by controlling the ratio of fuel to air (air-fuel ratio) supplied to the engine. To supply the reducing gas to the catalyst, CO, HC, H ₂
A reducing gas mainly containing a reducing component such as the above may be added to the exhaust gas to make the exhaust gas atmosphere a rich atmosphere. Fuel can also be used as the reducing gas.

As for the timing of carrying out this recovery process, it is desirable to estimate the amount of MO-SO ₃ produced on the catalyst and perform the recovery process when the amount reaches a predetermined value. The amount of MO-SO ₃ produced can be estimated by calculating the amount of sulfur component in the exhaust gas, the air-fuel ratio, the exhaust gas temperature, the exhaust gas circulation time, etc. as factors.

[0023]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

Example 1 Alumina powder was mixed with an alumina binder and water to prepare a slurry, which was wash-coated on a 35 cc test piece honeycomb substrate, and the slurry was mixed at 500 ° C. for 1 hour.
The coating layer was formed by firing for a period of time. The coat layer was formed in an amount of 120 g per 1 L of the honeycomb substrate.

A predetermined amount of an aqueous solution of palladium nitrate was absorbed into this coating layer, and the layer was pulled up and baked in the atmosphere at 300 ° C. for 3 hours to carry 5 g / L of Pd.

[0026]

[Table 1]

[0027]

[Table 2]

Arrangement of the obtained methane oxidation catalyst in an evaluation device
Then, using the model gas shown in Table 1, as shown in Table 2.
Alternately flow lean gas for 300 seconds and rich gas for 300 seconds.
While changing the concentration of methane, CO and S after passing through the catalyst
It was measured continuously. Gas flow rate is 30ml / min, reaction empty
Inter-speed is 51,400h^-1And the catalyst bed temperature is 450 ° C.
It SO in lean gas₂ Return in rich gas to total amount
The equivalent ratio of the total amount of the original components is 175. Also, Richga
SO during lean of excess reducing components in_x And NO _x of
The equivalent ratio to the sum is 8.5. Of methane after passing through the catalyst
The change in concentration is shown in FIG. In addition, the meta
CO, SO and_x FIG. 2 shows the change in the concentration of.

Then, the methane purification rate and the CO emission amount during the flow of the rich model gas were calculated by comparison with the concentration in the gas containing the catalyst, and the results are shown in Table 3.

(Comparative Example 1) The same methane oxidation catalyst as in Example 1 was placed in the evaluation apparatus, and the model gas shown in Table 1 was used. The change in methane concentration was measured continuously. The total gas flow rate is 30 L / min, the reaction space velocity is 51,400 h ^-1 , and the catalyst bed temperature is 450 ° C. The change in methane concentration after passing through the catalyst is shown in FIG. Then, the methane purification rate was calculated by comparison with the concentration in the gas containing the catalyst, and the results are shown in Table 3.

(Comparative Example 2) The same methane oxidation catalyst as in Example 1 was placed in the evaluation apparatus, and the model gas shown in Table 1 was used while flowing only the lean gas excluding SO ₂ as shown in Table 2. The change in methane concentration after passing through the catalyst was continuously measured. The total gas flow rate is 30 L / min, and the reaction space velocity is
It is 51,400 h ^-1 , and the catalyst bed temperature is 450 ° C. The change in methane concentration after passing through the catalyst is shown in FIG. Then, the methane purification rate was calculated by comparison with the concentration in the gas containing the catalyst, and the results are shown in Table 3.

<Evaluation>

[0033]

[Table 3]

The deterioration rate of the catalyst in the case of the method of Example 1 and the method of Comparative Example 1 was calculated from the difference in the methane purification rate with respect to the method of Comparative Example 2, and the results are shown in Table 3.

In Comparative Example 2, since no sulfur component was present in the model gas, SO _x poisoning of the methane oxidation catalyst did not occur. Therefore, the methane purification rate shows a high activity of 62%,
The methane concentration after passing through the catalyst remains low.

Further, in Comparative Example 1, since only lean gas containing SO ₂ is used, reductive decomposition of PdO-SO ₃ cannot occur. Therefore, the methane purification rate is as low as 8%, and it can be seen from FIG. 1 that the methane concentration after passing through the catalyst sharply increases with the lapse of usage time. It is thought that this is because PdO-SO ₃ was generated and the active sites of the catalyst were decreased because it was not decomposed.

However, according to the exhaust gas purification method of Example 1, the methane purification rate was 52%, which was 6 times or more that of Comparative Example 1, and the deterioration rate of the catalyst was low. Further, as shown in FIG. 1, the methane concentration after passing through the catalyst was high during the flow of rich gas, but the methane concentration after passing through the catalyst during the flow of lean gas was almost the same as in Comparative Example 1, and the methane purification capacity was greatly improved compared to Comparative Example 1. You can see that

Further, it can be seen from FIG. 2 and Table 3 that the CO emission rate was as low as 0.07% even when the rich gas was flowing, and that CO was consumed by the reaction with PdO-SO ₃ . Further, from FIG. 2, when the rich gas flows, the methane concentration once rises, then sharply falls, and then rises again. From this, it can be seen that the reaction between methane and PdO-SO ₃ is occurring.

In the exhaust gas from the actual gas engine,
The SO ₂ concentration is lower than that of the lean model gas in Table 1, and when converted to the amount of SO ₂ flowing, the lean model gas in Table 1
Circulating for 00 seconds corresponds to circulating the exhaust gas from the actual gas engine for 7 hours. In other words, by flowing the rich model gas of Table 1 for 300 seconds every time the exhaust gas from the actual gas engine is circulated for 7 hours, it is possible to recover the methane oxidation activity of the SO _x poisoned catalyst.

From the above, it is assumed that the actual purification of exhaust gas from the gas engine is performed in the same manner as in the first embodiment.
Based on the graph, the relationship between the actual equipment equivalent time and the methane purification rate was calculated for the exhaust gas purification methods of Example 1 and Comparative Example 1, and a graph is shown in FIG.

According to FIG. 3, the methane purification rate is stable at 50 to 60% according to the method of Example 1, and methane can be purified at a purification rate as high as about 6 times that of Comparative Example 1 even after 100 hours of use. it is obvious.

[0042]

[Effects of the Invention] That is, according to the exhaust gas purification method of the present invention, it is possible to easily recover the activity of a deteriorated catalyst even in a lean atmosphere with excess oxygen in a low temperature range. Therefore, high methane purification activity is exhibited for a long period of time.

[Brief description of drawings]

FIG. 1 is a graph showing a change with time of methane concentration after passing through a catalyst.

FIG. 2 is a graph showing a change in concentration of methane, CO, and SO _x after passing through a catalyst during a rich gas flow in an exhaust gas purification method of an example.

FIG. 3 is a graph showing the relationship between the actual machine equivalent time and the methane purification rate.

Front page continuation (51) Int.Cl. ⁷ Identification symbol FI theme code (reference) F01N 3/24 B01D 53/36 ZABG F02D 41/04 305 K (72) Inventor Masahiro Taki Nagakute-cho, Aichi-gun, Aichi Prefecture F-Term 1 at 41 Yokomichi, F Central in Toyota Central Research Institute Co., Ltd. (reference) 3G091 AA19 AB02 BA11 BA39 CB01 GA06 GB05W GB06W GB07W GB17W 3G301 HA18 JA21 MA01 4D048 AA13 AA18 AB01 AB07 BA03X BA31X BA41X BB02 BC01 BD03 DA01 DA03 DA06 DA03 DA03 DA06 DA03 DA06 DA06

Claims (7)

[Claims] 1. A method for purifying exhaust gas, comprising: supplying exhaust gas from a combustion engine including lean combustion, which is burned in a lean atmosphere having an air-fuel ratio of excess air, to a catalyst capable of oxidizing at least methane; An exhaust gas purification method, characterized in that exhaust gas produced by combustion in an atmosphere that is alternately changed to a lean atmosphere and a rich atmosphere with excess fuel is supplied to the catalyst. 2. The exhaust gas burned in the rich atmosphere is supplied to the catalyst at a temperature of 400 ° C. or higher.
The exhaust gas purification method described in. 3. The total amount of reducing components in the exhaust gas burned in the rich atmosphere and supplied to the catalyst is equal to or more than the total amount of sulfur components in the exhaust gas burned in the lean atmosphere and supplied to the catalyst. The exhaust gas purification method according to claim 1 or claim 2. 4. A method for purifying exhaust gas, which comprises supplying exhaust gas from a combustion engine including lean combustion, which is burned in a lean atmosphere with an air-fuel ratio of excess air, to a catalyst capable of oxidizing at least methane, wherein the reducing gas is used as the catalyst. An exhaust gas purification method comprising the step of supplying the exhaust gas to the exhaust gas. 5. The exhaust gas purification method according to claim 4, wherein the reducing gas is supplied to the catalyst at a temperature of 400 ° C. or higher. 6. The total amount of reducing components in the reducing gas supplied to the catalyst is equal to or more than the total amount of sulfur components in the exhaust gas burned in the lean atmosphere and supplied to the catalyst. 4. The exhaust gas purification method according to claim 4 or claim 5. 7. The exhaust gas purification method according to claim 1, wherein the catalyst contains Pd.