JPH11166410A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformly disperse reducing agents in exhaust gas by providing swirl generator for providing a swirl to an exhaust gas flow in an exhaust pipe upstream from a supply pipe in a device having an NOx catalyst in the exhaust pipe and the supply pipe for introducing reducing agents in the upstream thereof. SOLUTION: Exhaust gas from a diesel engine is passed through an exhaust pipe, cleaned by an NOx catalyst and then discharged into the atmosphere from a muffler. At this time, a pump 4 in a reducing agent supply pipe 10 is actuated, and the proper amount of hydrocarbon reducing agents in a reducing agent tank is injected and added from the supply pipe 10 to the downstream side of a swirl generator 9 provided in the exhaust pipe in the upstream position of the NOx catalyst at a proper time. This swirl generator 9 is constructed in such a manner that a plurality of arcwise laminas 22 are extended radially from the vicinity of the center through-hole 21 of a disk 20 in the radial direction of the disk 20. Thus, the reducing agents injected from the tip part of the supply pipe 10 are sprayed in mists by exhaust gas passed through the center through-hole 21, and then uniformly dispersed in the exhaust gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus using a NOx catalyst for reducing nitrogen oxides (NOx) contained in exhaust gas from a diesel engine, and more particularly, to an exhaust gas stream to be treated. The introduction method and introduction device of the hydrocarbon (HC) -based reducing agent, which is previously replenished and added from outside the system, is improved so that the added reducing agent is NO
x by uniformly distributing and dispersing the exhaust gas stream in a relatively short distance before reaching the x catalyst, i.e. by preventing uneven distribution of the reducing agent.
The present invention relates to an exhaust gas purifying apparatus having improved overall NOx reduction efficiency in a catalyst.

[0002]

2. Description of the Related Art Exhaust gas from a diesel engine is subjected to a purification treatment by a catalyst and a silencing treatment by a muffler device before being discharged into the atmosphere from an exhaust manifold through an exhaust pipe.
As a catalyst for reducing NOx contained together with excess air (oxygen) in diesel engine exhaust gas, copper-
Various catalysts have been proposed and known, such as zeolite-based (eg, Cu-ZSM5) catalysts, noble metal-refractory metal oxide-based (eg, Pt, Pd, Ir, Rh, etc./alumina, titania, zirconia, etc.) catalysts. It is. These catalysts are used in the form of pellets, or in the form of washcoats (dip-coating) on the surface of a carrier, such as a flow-through honeycomb (monolith), generally made of ceramics or metal, and are used in vehicles. This is a compact form so that the exhaust gas can be treated in a relatively short distance to the muffler and in a limited space, but a form in which the flow pressure drop of the exhaust gas is reduced as much as possible.

[0003]

SUMMARY OF THE INVENTION The inventor of the present invention has found that when purifying NOx contained in diesel engine exhaust gas using a NOx catalyst, a reducing agent added to the exhaust gas before the entrance of the catalyst, for example, a diesel engine fuel. NOx by modifying the introduction method of hydrocarbons (HC) such as light oil
We have studied extensively how the NOx reduction efficiency of the catalyst is affected. As a result, the flow of the exhaust gas in the exhaust pipe is generally laminar, so that when the reducing agent is introduced at one location in the cross section of the exhaust pipe, the reducing agent vapor reaches the NOx catalyst without being uniformly dispersed and distributed in the exhaust gas flow. and, NO _x catalyst enters only preferentially to a portion of the flow-through holes limited ones of a number of throughflow holes of honeycomb if it is supported on the honeycomb,
Insufficient amount of reducing agent entering the remaining through-holes occurs,
In the end, we obtained experimental results showing that the utilization of the catalytic function becomes unbalanced in the cross section of the honeycomb carrier. Therefore, the results of this experiment show that the function of the catalyst is evenly and sufficiently distributed over the entire cross section of the catalyst by making the distribution of the reducing agent uniform in the exhaust gas stream before the entrance of the catalyst and eliminating the uneven distribution of the reducing agent in the cross section of the catalyst. It was convinced that it could be used and that the NOx reduction efficiency could be improved. Therefore, based on the above experimental results, the present inventor has studied and studied means for uniformly dispersing and reducing the reducing agent in the exhaust gas at a short limited distance from the reducing agent introduction position to the NOx catalyst inlet, and as a result, The present invention has been completed.

[0004]

Thus, the present invention provides a NOx catalyst (7) provided in an exhaust pipe (2) for discharging exhaust gas from a diesel engine (1) to the outside and its NOx catalyst.
In the exhaust gas purifying apparatus, which faces the tip of the supply pipe (10) for introducing the hydrocarbon-based reducing agent (5) into the exhaust pipe portion (A) on the upstream side of the x catalyst, the exhaust pipe is located upstream of the supply pipe. There is provided an exhaust gas purifying apparatus characterized in that a vortex generator (9) for providing a vortex to an exhaust gas flow is provided in an exhaust pipe portion (A ') (see FIG. 1).

By providing such a vortex generator, a swirling flow is excited in an exhaust gas flow flowing almost laminarly on the upstream side, and a reduction flow supplied from a supply pipe by a throttle flow through a through hole is provided. The agent spouts radially, causing the exhaust gas flow to become disturbed. The hydrocarbon reducing agent injected from the tip of the reducing agent supply pipe downstream of the vortex generator is dispersed in the vortex or turbulent flow of the exhaust gas in a gaseous state,
The catalyst reaches the inlet of the catalyst arranged downstream in a state of being uniformly dispersed throughout the cross section of the exhaust pipe. Therefore, even when the catalyst is wash-coated on a ceramic or metal honeycomb structure base (monolith), the reducing agent flows into each through-hole of the honeycomb equally (or in the same amount) to reduce NOx throughout the catalyst. The function can be used effectively. If the reducing agent is injected into the laminar exhaust gas stream from the end of the supply pipe, NO
Since the distance to the x catalyst is short, the dispersion of the reducing agent does not proceed sufficiently and reaches the NOx catalyst inlet, and the honeycomb through-holes in which the reducing agent is distributed relatively more and the honeycomb through-holes in which the reducing agent is relatively less distributed. (Which in most cases would result in excess or deficiency of the reducing agent), and therefore, the use of the catalytic function is considered to be relatively incomplete.

The eddy current generator used in the exhaust gas purifying apparatus of the present invention comprises a disk fixed by a flange in an exhaust pipe so as to cross the exhaust pipe, and the disk is formed by, for example, cutting and folding. It is preferable to have a plurality of gas flow holes formed by processing or the like and inclined rising wings (or leaf-shaped pieces) corresponding to the gas flow holes. The number, size, shape, inclination angle, etc. of these standing wings (or leaf-shaped pieces) give a swirling flow moment to the exhaust gas flowing in the exhaust pipe, and generate sufficient turbulent flow to disperse the injection reducing agent well. It is designed to generate and not add excessive resistance to the exhaust gas flow.

FIG. 3 is a plan view showing an example of a preferred vortex generator. The illustrated vortex generator (9) has a circular plate (2).
0), having a central through hole (21), and extending a plurality of arc-shaped leaf pieces (22) radially extending from the vicinity of the central through hole in the radial direction of the disc by, for example, cutting and folding operations. It is erected on one side at an inclination angle of, for example, about 45 ° by press forming including the above. Such a vortex generator can be installed across the exhaust pipe by means of a flange, as shown in the partially enlarged view of FIG. The arc-shaped leaf piece (22) stands obliquely on the downstream surface, and imparts a swirling flow moment to the exhaust gas flow by its inclination angle. The distal end of the reducing agent supply pipe (10) is disposed immediately downstream of the central through-hole (21), where the injected reducing agent is atomized by the exhaust gas passing through the central through-hole (21), and Quickly taken into the surrounding swirling flow,
A uniform dispersion is achieved before reaching the inlet of the NOx catalyst (7).

The vortex generator (9) shown in FIG. 3 has arc-shaped leaf pieces (22).
Numbers, arrangements (positions, symmetrical or asymmetrical), etc. can be easily modified, such as triangular, rectangular,
The shape may be an elongated semi-elliptical shape, a trapezoid, or the like. The design should provide a moderate swirling flow moment to the exhaust gas flow but not excessive flow resistance. The upright surface of the leaf piece (22) is around 30 to 60 °, preferably around 40 to 50 °.

FIG. 1 shows an outline of the case where the exhaust gas purifying apparatus of the present invention is used for a diesel engine. Exhaust gas from a diesel engine (1) passes through a manifold and passes through an exhaust pipe (2). It is purified by a vortex generator (9) and a NOx catalyst (7), and finally discharged through a muffler (8). Although not shown, the engine (1) is provided with a rotation sensor and a load sensor,
A temperature sensor is attached to the exhaust pipe (2), and an output signal from the sensor is input to a computer control unit (ECU), and a control signal resulting therefrom is sent to the reducing agent supply pipe (1).
Activate pump (4) in 0) and reducer tank (6)
And an appropriate amount of the reducing agent (5) is injected into the exhaust pipe (2) downstream of the vortex generator (9) at an appropriate timing. The reducing agent is usually diesel fuel gas oil,
Other suitable hydrocarbons may be used. Ammonia can also be used as a reducing agent.

[0010] The reducing agent (eg, light oil) injected into the exhaust pipe (2) through the reducing agent supply pipe (10) is formed into a vortex flow which is disturbed by the action of a vortex generator (9) arranged upstream. The gaseous gas is uniformly dispersed in the exhaust gas, uniformly distributed over the entire cross section of the inlet of the NOx catalyst (7), and passes through a large number of honeycomb through-holes to purify NOx. Therefore, the function of the catalyst is utilized not locally but uniformly, so that NOx purification is efficiently achieved. The purified exhaust gas flowing out of the NOx catalyst (7) is then muffled (8) for silencing
After that, it is released into the atmosphere.

Further, conventionally, light oil (reducing agent) and compressed air (for example, supplied by an air compressor) are supplied by using a two-fluid nozzle device at the position of the supply pipe (10) without using the vortex generator in the device of FIG. Injection of compressed air).

However, according to this method, the light oil as the reducing agent is directly exposed to the high temperature of the exhaust gas at the tip of the two-fluid nozzle device, and is partially carbonized, and the carbide is removed at the tip of the two-fluid nozzle device. In some cases, a problem may occur in which the nozzle pores are deposited on the portion and block the nozzle pores.

However, in the exhaust gas purifying apparatus of the present invention as shown in FIG. 1, when a reducing agent (hydrocarbon) and air are simultaneously injected by using a two-fluid nozzle device, the exhaust pipe (2) has a temperature of 30 to 60 °. By attaching the two-fluid nozzle device (11) to the inner wall of the branch pipe (12) attached to join at an angle of, for example, by screwing (see FIG. 4), the problem of clogging of the nozzle due to the conventional carbon deposition is solved. This is significantly reduced, and the NOx reduction efficiency is improved in combination with the effect of the eddy current generator of the present invention. In this case, the tip of the nozzle of the two-fluid nozzle device (11) is disposed in the portion (14) of the exhaust pipe (2) inside the branch pipe (12) which is withdrawn from the exhaust gas main stream area (13), so that the hot exhaust gas main stream is formed. Avoiding direct contact further reduces the problem of carbide deposition.

Accordingly, the present invention provides a NOx catalyst (7) provided in an exhaust pipe (2) for discharging exhaust gas from a diesel engine (1) to the outside and an exhaust pipe portion (A) upstream of the NOx catalyst. In the exhaust gas purification device facing the end of the supply pipe (10) for introducing the hydrocarbon-based reducing agent (5) into the exhaust pipe section (A ') upstream of the supply pipe position, A swirling flow generating device (9) is provided for partially imparting a swirling flow moment, and a reducing agent is radially injected by a throttle flow, and the supply pipe (10) is provided.
Is a two-fluid nozzle device for injecting the reducing agent and the compressed air together, and is mounted in a branch pipe (12) attached to the exhaust pipe (2) at an angle of 30 to 60 °. An exhaust gas purifying apparatus characterized by the above feature is provided.

[0015]

Example 1 Preparation of Catalyst Silver nitrate and zeolite ZSM-5 were dispersed in water, and silica sol was further added as a binder to form a slurry. This was coated on a honeycomb, and then dried and fired. This was impregnated with a predetermined amount of an aqueous solution of copper nitrate, dried, and fired at 500 ° C. for 1.5 hours.

Tests The supported NOx catalysts thus obtained were mounted on an exhaust gas purifying device (I) as shown in FIG. 1 and a device (II: comparison) in which the vortex generator was omitted therefrom.

The exhaust gas of a diesel engine is controlled by changing the load to 250 ° C. to 550 ° C. in these devices, and is passed at a space velocity of 20,000 hr ⁻¹ , and light oil is used as a reducing agent in a weight ratio of light oil / NO = 2 Inject with NOx
The reduction rate was measured. The results are shown graphically in FIG.

[0018]

Embodiment 2 Reducing agent supply pipe (10) in the apparatus of FIG.
Instead of the apparatus I (invention) equipped with a two-fluid nozzle (11) in the manner of FIG. 4, a two-fluid nozzle was inserted and mounted in the exhaust pipe instead of the supply pipe without providing the branch pipe (12). The experiment of Example 1 was repeated using apparatus II (comparative). In the continuous test for a total of one week, the number of occurrences of carbide deposition trouble at the nozzle tip was 0 times / week for the apparatus I and 7 times / week for the apparatus II. FIG. 6 is a graph showing the result of the NOx reduction rate.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an exhaust gas purifying apparatus of the present invention.

FIG. 2 is a partially enlarged view of FIG. 1 showing attachment of a vortex generator.

FIG. 3 is a plan view of an example of a vortex generator.

FIG. 4 is a cross-sectional view of an example of the apparatus of the present invention employing a reducing agent addition method using a two-fluid nozzle.

FIG. 5 is a graph showing a measurement result of a NOx reduction rate in Example 1.

FIG. 6 is a graph showing a measurement result of a NOx reduction rate in Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Diesel engine 2 Exhaust pipe 5 Reducing agent (hydrocarbon type) 7 NOx catalyst 8 Muffler 9 Eddy current generator 10 Reducing agent supply pipe

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI F01N 3/24 B01D 53/34 129E ZAB

Claims (4)

[Claims] A NOx catalyst (7) provided in an exhaust pipe (2) for discharging exhaust gas from a diesel engine to the outside.
And an exhaust gas purifying apparatus in which an end of a supply pipe (10) for introducing a hydrocarbon-based reducing agent (5) into an exhaust pipe portion (A) on the upstream side of the NOx catalyst faces upstream of the supply pipe. A swirl generator (9) for providing a swirl to the exhaust gas flow in the exhaust pipe portion (A ') on the side of the exhaust gas purifier. 2. A vortex generator (9) having a central through hole (21).
And a plurality of arc-shaped leaf pieces (22) extending radially in a radial direction of the disc from a position outside the periphery of the center through hole and having a tilted upright standing on one surface. The exhaust gas purifying apparatus according to claim 1, wherein the arc-shaped leaf piece standing surface is provided across the exhaust pipe so as to be disposed on the downstream side. 3. A reducing agent supply pipe (10) for introducing a hydrocarbon-based reducing agent is a two-fluid nozzle device (11) for injecting a reducing agent and compressed air, and an exhaust pipe (2) having 30 to 60 nozzles.
3. The exhaust gas purifying apparatus according to claim 1, wherein the exhaust gas purifying apparatus is mounted in a branch pipe (12) provided so as to merge at an angle of °. 4. The two-fluid nozzle device (11) is characterized in that the tip of the nozzle is disposed in a portion (14) inside a branch pipe (12) which is withdrawn from a main exhaust gas flow area (13) in an exhaust pipe (2). The gas purifying apparatus according to claim 3, wherein