JP2000282852A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device which can restrain deterioration of catalyst due to poisoning of NOx and due to heating, which is small-sized, and which can remove both PM and NOx simultaneously. SOLUTION: This exhaust emission control device, incorporated in an exhaust system of a combustion engine, includes a wall-flow type filter 1. The wall surface 3 of the filter 1 is coated thereover an NOx reduction catalyst layer 5 formed of NOx reduction catalyst on the engine side, and the NOx reduction catalyst layer 5 is coated thereover with an oxidizing catalyst layer 7 formed of an oxidizing catalyst, on the engine side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purification device which is provided in an exhaust system of a combustion engine and has a wall-flow type filter.

[0002]

2. Description of the Related Art In a conventional exhaust gas purification apparatus, a diesel engine is provided with a DPF (Diesel Engine) to remove particulates (particulate matter, hereinafter abbreviated as PM) and nitrogen oxides (NOx) in exhaust gas. (Particulate filter) and a NOx reduction catalyst are used in combination. 8 and 9 show the most commonly used wall-flow type filter (DPF) 20 at present.
A is shown. The inlet and the outlet of the channel Ch of porous cordierite or silicon carbide honeycomb are alternately plugged with Be so that PM is trapped when the exhaust gas flows through the wall surface W. ing.

[0003] Here, the flow area required for PM collection and the volume and weight for ensuring the strength are increasing the weight of the vehicle, which is a problem. Further, when PM adheres to the NOx reduction catalyst, the NOx reduction efficiency is reduced, so that it is necessary to arrange the DPF upstream of the NOx reduction catalyst. Such a normal arrangement of the DPF and the NOx reduction catalyst in series has a problem that it takes up space and increases the weight. Therefore, NOx
A method has been proposed in which DPF and the NOx reduction catalyst are integrated while avoiding PM attachment to the reduction catalyst. However, in such a method, as is evident from FIG. 7 showing the temperature in the filter, the temperature Ti in the filter is reduced by a dotted line (in FIG. 7) during the regeneration processing of the filter for burning and removing the trapped PM. As shown in (1), the temperature rises to a very high temperature close to 1000 ° C. (a so-called “peak” occurs). Therefore, there is a disadvantage that the catalyst is deteriorated. In FIG. 7, the solid line indicates the temperature To of the filter front surface (engine side surface) To.

[0004] Regarding the DPF and the NOx reduction catalyst, other conventional techniques include, for example, Japanese Patent Application Laid-Open No. 5-249292.
No. 3 discloses a technique for reducing power consumption for effectively operating a DPF and a lean NOx reduction catalyst.
Japanese Patent Application Laid-Open No. 6-148663 discloses a technique for reducing NOx by supplying a reducing agent in accordance with the operating state of an engine. A technique of injecting a system liquid and burning PM with reaction heat is disclosed. Further, Japanese Patent Application Laid-Open No. 9-88569 discloses that an upstream DPF and a downstream NPF are used.
A technology is disclosed in which a reflux outlet for EGR gas is provided between the Ox reduction catalyst and the amount of recirculation is adjusted to reduce NOx.
In the publication, a technology of a carrier structure of the trap is disclosed. However, none of these prior arts solves the above-mentioned various drawbacks and problems.

[0005]

SUMMARY OF THE INVENTION The present invention has been proposed in view of the above-mentioned problems of the prior art, and is small in size and light in weight with little poisoning of the NOx reduction catalyst and deterioration due to heat.
It is an object of the present invention to provide an exhaust gas purification device capable of simultaneously removing M and NOx.

[0006]

An exhaust gas purifying apparatus according to the present invention is provided in an exhaust system of a combustion engine. The exhaust gas purifying apparatus has a wall-flow type filter. And a layer comprising an oxidation catalyst is coated on the engine side of the layer.

According to the present invention, the PM in the exhaust gas is filtered by the oxidation catalyst layer applied and coated on the side wall of the engine of the DPF filter and does not adhere to the NOx reduction catalyst. NOx emissions are reduced at the same time.

According to another aspect of the present invention, there is provided an exhaust gas purifying apparatus which is provided in an exhaust system of a combustion engine and has a wall-flow type filter. The exhaust outlet side of the wall of the filter is covered with a layer made of a NOx reduction catalyst.

According to the present invention, since the PM in the exhaust gas is filtered by the oxidation catalyst layer and the filter wall surface coated and coated on the engine side wall surface of the DPF filter, NO
The function of the x reduction catalyst layer is maintained, and PM and NOx emissions are simultaneously reduced.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, an exhaust gas purifying apparatus 10 according to the present invention, which is applied to a diesel engine, will be described as an example.
An embodiment will be described. The exhaust gas purification device 10 is composed of a filter 1 and related devices and members not explicitly shown. FIG. 1 shows a local cross section A of the filter 1 shown in FIGS. 2 and 3. In FIG. 1, the upper part of the drawing is a gas inlet Ic leading to the engine, and the lower part of the drawing is a gas outlet Oc leading to the muffler on the exhaust outlet side. As in the case of the conventional wall-flow type filter, the outlet Ou of half of all the channels Ch formed by extrusion with porous cordierite or silicon carbide is sealed with a ceramic plugging 2o to form a plurality of gases. An inlet Ic is formed, and the inlet Ii of the channel Ch other than the gas inlet Ic is sealed with a ceramic plug 2i to form a plurality of gas outlets O.
c is formed.

[0011] Then, as shown in FIG.
Two types of catalyst layers are applied and coated on the gas entrance Ic side of the wall surface 3 of h. That is, the upper surface on the engine side of the wall 3 is N
An 0x reduction catalyst (for example, a zeolite-based catalyst) layer 5 is applied and coated, and an oxidation catalyst (for example, a noble metal-based catalyst) layer 7 is applied and coated on the upper surface on the engine side. FIG. 1 is an enlarged view of the local section A of the cross section, and each of the catalyst layers 5 and 7 is applied and coated over the entire area of the gas passage Ic of the wall surface 3. FIG.
Reference numeral 8 in the middle schematically shows a PM layer to which the exhaust gas Gi adheres when flowing through the wall surface 3.

The operation of the exhaust gas purifying apparatus 10 including the filter 1 having the above configuration will be described. In FIGS. 1 and 2, the inlet exhaust gas Gi flows into the gas inlet Ic of the filter 1,
The gas passes through the wall surface 3 from the upper to the lower stream of the gas inlet Ic, flows into the gas outlet Oc, becomes a purified outlet exhaust gas Go, and is discharged from the outlet Ou.

When the inlet exhaust gas Gi passes through the filter 1, the inlet exhaust gas Gi flows from the gas inlet Ic through the oxidation catalyst layer 7 applied to the upper surface of the wall 3, and then the NOx reduction catalyst layer. 5 and then the wall 3 to the gas outlet Oc. And the exhaust gas Gi is
PM is attached to the oxidation catalyst layer 7 and deposited as the PM layer 8. Thereby, PM does not adhere to the NOx reduction catalyst layer 5, and performance deterioration such as poisoning by PM is suppressed.

Further, the PM combustion temperature in the filter is set to 300 to 400 by coating and coating the oxidation catalyst layer 7 on the wall surface 3.
° C, which is significantly lower than 600 ° C or higher without a coating film. As a result, the temperature at the time of regeneration of the filter 1 by the PM combustion, that is, the temperature to which the NOx reduction catalyst is exposed becomes 600 ° C. or less, and is greatly reduced from the temperature Ti without an oxide film of 900 to 1000 ° C. Since the maximum operating temperature of the zeolite-based catalyst is about 600 ° C., this is an extremely effective measure. In other words, as shown in FIG. 4, by forming the coating layer of the oxidation catalyst, the temperature Ti in the filter and the front surface of the filter (the surface on the engine side) are formed.
As is clear from the comparison between the temperature To and the temperature characteristic in the conventional technique shown in FIG. 7, the temperature To is surely lowered. In addition, as for the temperature Ti in the filter (temperature characteristics are shown by dotted lines in FIGS. 4 and 7), when the coating layer of the oxidation catalyst shown in FIG. 4 is formed, a temperature peak occurs. In other words, the temperature rise or temperature change becomes gradual.

The PM layer 8 deposited as a result of the operation at the time of the normal operation is heated to a high temperature by any known method such as a burner or a heater, burned off, and the filter 1 is regenerated. Further, the NOx reduction catalyst layer 5 is protected and protected by heat by the coating of the oxidation catalyst layer 7 when the PM layer is burned during regeneration.

Further, in order to improve the reduction efficiency of the NOx reduction catalyst, a method of increasing the concentration of HC in exhaust gas by adding light oil has been adopted. However, the effect of gas oil is inferior to low molecular weight hydrocarbons having about 2 to 3 carbon molecules due to its long carbon chain. FIG. 5 shows this state. In the NOx reduction efficiency with respect to the reducing agent / NOx ratio, the low molecular weight hydrocarbon Hb is superior to the light oil K. In the present invention, since the oxidation catalyst layer 7 applied to the filter 1 decomposes light oil into low molecular weight hydrocarbons, the reduction efficiency is improved as compared with the case of light oil.

In the present invention, the SOF that is normally discharged to the atmosphere as it is is decomposed by the oxidation catalyst layer 7 and
Since it acts as a reducing agent for the Ox reduction catalyst, the amount of light oil added to the exhaust gas is reduced.

FIG. 6 shows another invention, in which the position of application and coating of the catalyst layer is changed from that of the above-mentioned embodiment, and the adhesion of PM to the NOx catalyst and the adverse effect are further reduced. The different parts will be mainly described with reference to FIG. 2 of the embodiment. In FIG. 6, the upper part of the drawing is a gas entrance Ic leading to the engine,
The lower part of the drawing shows the gas outlet Oc leading to the muffler.
In the same manner as in the previous embodiment, similarly to the conventional wall flow type filter, the outlet portion Au of half of all the channels Ch formed by extrusion with porous cordierite or silicon carbide or the like is sealed with ceramic material 2o. To form a plurality of gas inlet paths Ic, and the inlet portion Ii of the channel Ch other than the gas inlet path Ic is sealed with a ceramic plug 2i to form a plurality of gas outlet paths Oc.

An oxidation catalyst layer 7B is applied and coated on the upper surface of the wall 3B of each channel Ch, and N
The 0x reduction catalyst layer 5B is applied and coated. FIG. 6 is an enlarged view of the cross-sectional local portion A of FIG. 2 for reference used for description, but the catalyst layer is applied and coated over the entire gas inlet Ic of the wall surface 3B. Reference numeral 8B in FIG. 6 schematically shows the PM layer 8B to which the inlet exhaust gas Gi adheres when flowing through the wall surface 3B.

The operation of the exhaust gas purifying apparatus 10A including the filter 1A having the above configuration will be described. In FIG. 6, the exhaust gas Gi flows into the gas inlet Ic of the filter 1A, and flows into the wall surface 3B from above to downstream of the gas inlet Ic. The exhaust gas Gi, which has been purified by flowing through the wall surface 3B,
c, and is discharged from the filter 1A as the purified outlet exhaust gas Go.

When the inlet exhaust gas Gi passes through the filter 1A, the exhaust gas Gi passes from the gas inlet Ic through the oxidation catalyst layer 7B applied to the upper surface of the wall 3B, and then passes through the wall 3B. The gas passes through the NOx catalyst layer 5B to reach the gas outlet Oc. Here, the inlet exhaust gas Gi
Is to deposit PM as the PM layer 8 by attaching PM to the oxidation catalyst layer 7B. Since PM is further filtered on the wall 3B,
PM does not adhere to the NOx reduction catalyst layer 5, and performance deterioration such as poisoning by PM is suppressed. Therefore, the PM poisoning of the NOx reduction catalyst layer 5B is further reduced as compared with the tenth embodiment, and the heat effect due to the PM combustion heat during the regeneration of the filter 1A is reduced.

[0022]

The effects of the present invention are listed below.

(1) According to the exhaust gas purifying apparatus of the present invention,
By attaching PM to the oxidation catalyst layer coated on the wall,
PM and NOx can be removed at the same time by a small and lightweight device with little poisoning of the NOx reduction catalyst and deterioration due to heat.

(2) By coating the oxidation catalyst on the filter wall surface, the PM combustion temperature can be reduced, and the filter temperature at the time of the regeneration treatment can be lowered to suppress the thermal deterioration of the NOx reduction catalyst.

(3) The reduction efficiency of the NOx reduction catalyst as compared with light oil is improved by decomposing light oil added as a NOx reducing agent into exhaust gas into low molecular weight hydrocarbons having a large effect as a reducing agent. Can be. .

(4) In addition to the increase in HC concentration due to light oil added as a NOx reducing agent to the exhaust gas, the oxidation catalyst layer decomposes the SOF component in PM into low-molecular-weight hydrocarbons, thereby further supporting the increase in HC concentration. Thus, the amount of light oil added as a NOx reducing agent can be reduced.

[Brief description of the drawings]

FIG. 1 is a side sectional view showing a local wall portion of a DPF used in an exhaust gas purification apparatus of the present invention.

FIG. 2 is a side sectional view of the DPF of FIG. 1;

FIG. 3 is a front view of FIG. 1;

FIG. 4 is a diagram showing a temperature decrease during regeneration of a filter due to coating of the filter with an oxidation catalyst.

FIG. 5 is a graph showing the contribution of light oil and low molecular weight hydrocarbons to NOx reduction efficiency.

FIG. 6 is a side sectional view showing a local wall portion of a DPF used in an exhaust gas purification apparatus of another invention.

FIG. 7 is a diagram showing a temperature rise during regeneration processing of a filter that is not coated with an oxidation catalyst.

FIG. 8 is a side sectional view showing a local wall portion of a conventional DPF.

FIG. 9 is a front view of FIG. 8;

[Explanation of symbols]

 Gi ... Inlet exhaust gas Go ... Outlet exhaust gas Ic ... Gas inlet Ii ... Inlet Oc ... Gas outlet Ou ... Outlet 1,1A ... Filter 2i ... Inlet-side plugging 2o ... Outlet Side sealing 3 ・ ・ ・ Wall surface 5 ・ ・ ・ NOx reduction catalyst layer 7 ・ ・ ・ Oxidation catalyst layer 8 ・ ・ ・ PM layer 10, 10A ・ ・ Exhaust gas purification device

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 B01D 53/36 101A F-term (Reference) 3G090 AA02 CA04 EA01 3G091 AA18 AB02 AB04 AB05 AB13 BA07 BA11 BA14 BA15 BA19 BA33 CA02 CA03 FB10 FC02 GA06 GA18 GA24 GB01X GB05W GB09X GB13X GB17X 4D048 AA06 AA18 AB01 AB02 BA11Y BA30Y BA31Y BA32Y BA33Y BA34Y CA07 CC32 CD05 CD08

Claims (2)

[Claims] 1. An exhaust gas purification apparatus which is provided in a combustion engine exhaust system and has a wall flow type filter,
An exhaust emission control device, wherein an engine side of a wall surface of the filter is coated with a NOx reduction catalyst layer made of a NOx reduction catalyst, and an engine side of the NOx reduction catalyst layer is coated with an oxidation catalyst layer made of an oxidation catalyst. 2. An exhaust gas purifying apparatus interposed in an exhaust system of a combustion engine and having a wall flow type filter.
An exhaust emission control device, wherein an oxidation catalyst layer made of an oxidation catalyst is coated on an engine side of a wall surface of the filter, and a NOx reduction catalyst layer made of a NOx reduction catalyst is coated on an exhaust outlet side of a wall surface of the filter.