JPH0842329A - Exhaust emission control device for engine - Google Patents

Abstract

PURPOSE:To improve the removal ratio of NOX by oxidizing NO to N02 high in reactivity before the NOX is reduced in an exhaust emission control device for lowering the concentration of the NOX contained in the exhaust gas of an engine. CONSTITUTION:A first catalyst 4 for oxidizing NO to NO2 and a second catalyst 5 for reducing NO2 to N2 located in the downstream part thereof are serially provided in the exhaust passage 2 of an engine 1. A means 7 for adding a reducing agent is provided in the upstream of the second catalyst 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine exhaust emission control device.

[0002]

2. Description of the Related Art EGR (exhaust gas recirculation) is well known as an effective means for reducing the concentration of NO _x contained in the exhaust gas of an engine, but a method of treating with a catalyst in the exhaust passage of the engine is also considered. It is (Japanese public Sho 51-725)
No. 2, JP-B-57-12003, JP-A-52
-53103, etc.).

[0003]

By the way, the reactivity of the catalyst is higher in NO ₂ than in NO as shown in FIG. 8, but most of NO _X is NO, so if the catalyst is not enlarged, There was a problem that a high removal rate could not be obtained.

The present invention aims to solve such problems.

[0005]

According to a first aspect of the present invention, in an exhaust emission control device for reducing the concentration of NO _x contained in the exhaust gas of an engine, the first aspect oxidizes NO into NO ₂ in an exhaust passage.
A catalyst and a second catalyst downstream of which NO ₂ is reduced to N ₂ are connected in series.

According to the second invention, means for adding the reducing agent is provided upstream of the second catalyst in the first invention.

[0007]

According to the first aspect of the invention, engine exhaust is discharged to the outside through the first catalyst and the second catalyst. In this case, NO in the exhaust gas is oxidized to NO ₂ by the first catalyst, so NO
As the concentration decreases, the highly reactive NO ₂ concentration rises, and the exhaust gas reduces NO ₂ to N ₂ in the second catalyst. Therefore, as compared with reducing exhaust gas rich in NO, N
O _X high removal rate is obtained.

Although the reducing agent (HC, etc.) in the exhaust gas is also oxidized by the first catalyst, according to the second invention, the reaction of the second catalyst is activated by the addition of the reducing agent, so that a higher NO The removal rate of _X can be secured.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, reference numeral 2 is an exhaust passage of a diesel engine 1, a tubular casing 3 is formed in the passage, and a first catalyst 4 for oxidizing NO to NO ₂ is provided on the upstream side inside the casing 3. , And a second catalyst 5 for reducing NO ₂ to N ₂ is placed in series on the downstream side thereof.

As the first catalyst 4, a catalyst in which an oxide such as alumina supports a noble metal such as platinum is used, and as the second catalyst 5, a zeolite catalyst in which a noble metal or a base metal is supported, an oxide catalyst or a metallosilicate catalyst is used. used.

A gap 6 is provided between the first catalyst 4 and the second catalyst 5 inside the casing 3. However, when the casing 3 does not have a sufficient size, as shown in FIG.
You may make it arrange | position the 1st catalyst 4 and the 2nd catalyst 5 continuously.

With this structure, the exhaust gas of the engine 1 is discharged to the outside through the first catalyst 4 and the second catalyst 5.
At this time, as shown in FIG. 2, NO _x in the exhaust gas is initially mostly occupied by NO, but when passing through the first catalyst 4, NO is oxidized and the ratio of NO _{2 in} the exhaust gas increases. Since this exhaust is rich in highly reactive NO ₂ , it is efficiently reduced to harmless N ₂ by the second catalyst 5.

Therefore, the first to oxidize NO to NO ₂
By using the catalyst 4 together with the second catalyst 5 for reducing NO ₂ to N ₂ , as compared with the conventional example in which NO-rich exhaust gas is passed through the reducing catalyst, as shown in FIG. To
A high NO _x removal rate can be obtained, as in the test results using an alumina-based catalyst as the second catalyst. Further, since the NO _x removal rate is improved, the catalyst can be downsized.

By the way, there is a possibility that the reducing agent (HC, etc.) in the exhaust gas is also oxidized by the first catalyst 4, and if it is left as it is, the reducing agent is insufficient in the second catalyst 5. FIG. 5 shows another embodiment. In the exhaust passage 2 of the engine 1, a device 7 for adding a reducing agent from the outside to the upstream side of the second catalyst 5 is added.

In this case, the pipe 7a is inserted into the space 6 in the casing 3 to drive the reducing agent from the tank 7b into the drive unit 7.
c passes from the downstream side of the first catalyst 4 to the second side through the pipe 7a.
It is designed to be added to the front surface of the catalyst 5. As the reducing agent, hydrocarbon such as light oil, hydroxyl group compound such as alcohol, urea, or the like is used.

According to this, by adding the reducing agent, the second catalyst 5
Since the reaction of No. 1 is activated, as shown in FIG. 6 (test results using a platinum / alumina catalyst as the first catalyst, an alumina catalyst as the second catalyst, and propylene as the reducing agent), A higher NO _x removal rate can be secured as compared with the example. FIG. 7 shows the relationship between the amount of reducing agent added and the NO _x removal rate (when the catalyst inlet temperature is constant).

The reducing agent may be added from the upstream side of the first catalyst 4 as long as the required amount for the second catalyst 5 can be secured. Incidentally, while the present invention has appreciated that it is also effective in the NO _X reduction in gasoline engines.

[0018]

According to the first aspect of the present invention, in the exhaust purification system for reducing the NO _X concentration contained in the exhaust gas of the engine, the first catalyst for oxidizing NO into NO ₂ in the exhaust passage and the NO downstream thereof. _Since the second catalyst that reduces ₂ to N ₂ is connected in series, NO in the exhaust gas is oxidized by the first catalyst and a large amount of highly reactive NO ₂ is produced, so that the second catalyst efficiently Can be reduced to harmless N ₂ . That is, there is an effect that a high removal rate of NO _X can be obtained.

According to the second aspect of the present invention, since the means for adding the reducing agent is provided on the upstream side of the second catalyst, the reaction of the second catalyst is activated by the addition of the reducing agent, so that a higher NO _{x is obtained.} The removal rate can be secured.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing changes in NO _{X in} a catalyst.

FIG. 3 is a characteristic diagram showing a NO _X removal rate in comparison with a conventional example.

FIG. 4 is a configuration diagram showing another embodiment.

FIG. 5 is a configuration diagram showing another embodiment.

FIG. 6 is a characteristic diagram showing the NO _x removal rate in comparison with the first embodiment.

FIG. 7 is a characteristic diagram showing the relationship between the amount of reducing agent added and the NO _x removal rate.

FIG. 8 is a characteristic diagram comparing the reactivity between NO and NO ₂ .

[Explanation of symbols]

 2 engine exhaust passage 3 casing 4 first catalyst 5 second catalyst 7 reducing agent addition device

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location F01N 3/08 ZAB B 3/28 J ZAB Q 301 C

Claims (2)

[Claims] 1. An exhaust emission control device for reducing the concentration of NO _x contained in exhaust gas of an engine, a first catalyst for oxidizing NO to NO ₂ in an exhaust passage, and a first catalyst for reducing NO ₂ to N ₂ downstream thereof. An engine exhaust gas purification device characterized in that two catalysts are interposed in series. 2. The exhaust emission control device according to claim 1, further comprising a means for adding a reducing agent upstream of the second catalyst.