JP2006233938A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device not influenced by sulfur poisoning with a simple control by using a three-way conversion catalyst and a component separation mechanism separating oxygen and NOx in exhaust gas. <P>SOLUTION: This invented exhaust emission control device 1 is provided in an exhaust line of an internal combustion engine and is provided with a main exhaust gas flow passage 12 discharging exhaust gas discharged from a combustion chamber 2 of an internal combustion engine E to an outside, a bypass flow passage 14 branching off at an upstream part of the main exhaust gas flow passage 12 and merging in a downstream part of the main exhaust gas passage, a component separation means 16 installed in the branch part of the main exhaust flow passage and the bypass flow passage and separating predetermined component in exhaust gas, a reducing agent 20 installed in the bypass flow passage and reducing NOx, a reducing agent adding means 18 arranged in an upstream of the reducing catalyst and adding reducing agent reducing NOx, and a pressure difference producing means 22 installed in the bypass passage and introducing exhaust gas into the component separating means. The component separation means 16 is provided with a zeolite film separating NOx and oxygen in exhaust gas. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine such as a lean gasoline engine or a diesel engine.

  Conventionally, various filters and catalysts have been used to reduce nitrogen oxides (hereinafter referred to as NOx) and particulates (PM) in the exhaust gas of an internal combustion engine. When NOx is reduced by the catalyst, it is common to perform rich combustion to increase hydrocarbons in the exhaust gas and supply this to the catalyst as a reducing agent. However, since rich combustion also causes deterioration of fuel consumption, a technique for promoting the reduction of NOx by other methods has been desired.

  For this reason, a technique for reducing oxygen in the exhaust gas and improving the NOx purification performance of the storage catalyst has been proposed (see Patent Document 1). In this technology, an oxygen enrichment system is provided in the exhaust pipe of an engine, and the exhaust gas is separated into an oxygen enriched gas having a relatively high oxygen concentration and an oxygen depleted gas having a relatively low oxygen concentration by an oxygen enriched membrane. The oxygen-depleted gas is supplied to the catalyst during the NOx reduction treatment to promote the reduction of NOx, and a known oxygen-enriched film that enriches the oxygen in the exhaust gas is used. I can do it. Known oxygen-enriched membranes include non-porous polymer membranes made of materials such as silicone rubber, poly, polysulfone, and polyamide.

  However, oxygen-enriched films made of these materials have a problem in heat resistance because the exhaust gas temperature is high, and stable oxygen-enrichment characteristics cannot be maintained over a long period of time.

In addition, the above-described exhaust gas purification apparatus according to the related art has a problem that a switching mechanism is required depending on the operating state of the internal combustion engine in order to use the NOx storage catalyst as the catalyst, and the influence of sulfur poisoning is large.
JP 2003-27926 A

  The present invention has been made to solve the above-described problems. By utilizing a component separation mechanism that separates oxygen and NOx in exhaust gas and a three-way catalyst, the sulfur coverage can be achieved with simple control. It is an object of the present invention to provide an exhaust purification device that is not affected by poison.

An exhaust purification device of the present invention is an exhaust purification device provided in an exhaust path of an internal combustion engine, and a main exhaust passage that exhausts exhaust gas discharged from a combustion chamber of the internal combustion engine to the outside;
A detour channel that branches at the upstream portion of the main exhaust flow channel and joins at the downstream portion of the main exhaust flow channel, and a predetermined component in the exhaust gas that is interposed between the branch portion of the main exhaust flow channel and the detour flow channel. A component separating means for separating; a reduction catalyst for reducing NOx interposed in the bypass channel; a reducing agent adding means for adding a reducing agent arranged upstream of the reduction catalyst to reduce NOx; And a differential pressure generating means for introducing the exhaust gas into the component separation means, and the component separation means comprises a zeolite membrane for separating NOx and oxygen in the exhaust gas.

  Since the exhaust gas purification apparatus of the present invention has component separation means for separating NOx and oxygen in the exhaust gas at the branch portion between the main exhaust flow path and the bypass flow path, the NOx enriched gas is provided in the bypass flow path. Can be introduced, and a reducing agent can be added to efficiently reduce NOx by the reduction catalyst. Here, since the zeolite membrane is used as the separation membrane for separating NOx in the exhaust gas, the separation characteristics can be stably maintained even in high-temperature exhaust gas with high heat resistance.

  In the exhaust emission control device of the present invention, the component separation means is preferably a rectangular wave cross-sectional structure in which a zeolite membrane is integrally formed on the substrate surface.

  Since the component separation means is a rectangular wave cross-section structure in which a zeolite membrane is integrally formed on the substrate surface, the NOx separation area can be expanded, and thus a compact and high separation efficiency exhaust purification device is provided. It is possible.

  Further, the exhaust gas inflow end face of the rectangular wave cross-section structure is formed so that the sealing portions and the opening portions are alternately adjacent to each other, and the exhaust gas preferably flows into the opening portion.

  By providing a sealing portion and an opening at the exhaust gas inflow end surface of the rectangular wave cross-section structure, it is possible to prevent the exhaust gas from freely flowing into the bypass channel and to enrich NOx in the bypass channel. it can. Further, since the exhaust gas can freely flow to the main exhaust passage, the exhaust gas enriched with oxygen, carbon dioxide, sulfur oxide, etc. by separating NOx is sent to the outside through the main exhaust passage. Can be discharged. Sulfur oxide is a factor that affects sulfur poisoning of the reduction catalyst, but it is discharged to the outside through the main exhaust passage without flowing into the detour passage interposing the reduction catalyst. The reduction catalyst does not suffer from sulfur poisoning and can maintain good reduction characteristics. The exhaust gas rendered harmless through the reduction catalyst is discharged to the outside after joining the main exhaust passage, so that NOx is separated to enrich oxygen, carbon dioxide, sulfur oxide, etc. Exhaust gas can be diluted.

  Here, the zeolite membrane has pores having a pore diameter of 0.26 to 0.5 nm, and the base material of the rectangular cross-section structure is a porous body having through holes having a diameter of 0.1 to 50 μm. Is desirable.

  By setting the pore diameter of the zeolite membrane within the above range, NOx and oxygen in the exhaust gas can be efficiently separated. Moreover, while making a base material the porous body which has a through-hole with a diameter of 0.1-50 micrometers, while ensuring the intensity | strength of a base material, permeation | transmission of NOx is not inhibited.

  Such a porous body is desirably a ceramic, and the ceramic is preferably cordierite. Cordierite is preferable because it has good moldability and can easily obtain desired air permeability (through-hole diameter, through-hole density, etc.) and strength by blending raw materials and firing methods.

  In the exhaust emission control device of the present invention, the reduction catalyst can be a three-way catalyst. As described above, only the reducing agent and NOx are introduced into the reduction catalyst, and oxygen and sulfur oxides are not introduced. Therefore, it is not necessary to use an occlusion reduction catalyst that requires complicated operation, and a three-way catalyst that is inexpensive and easy to handle may be used.

  In the exhaust emission control device of the present invention, the component separation means can carry a NOx adsorbing substance. The separation performance of NOx can be further improved by supporting a substance having high adsorptivity to NOx, such as Ba, on the component separation means.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows a system configuration of an exhaust emission control device in a diesel engine which is a preferred embodiment of the present invention.

The exhaust purification apparatus 1 of this aspect includes a main exhaust flow path 12 that discharges exhaust gas G ₀ discharged from the combustion chamber 2 of the engine E to the outside via a silencer (muffler) 24, and a main exhaust flow path 12. And a bypass flow path 14 for bypassing. The bypass flow path 14 is branched by the main exhaust flow path 16 and the branch portion 14 a and communicates with the main exhaust flow path 16 at the merge section 14 b upstream of the silencer 24.

A component separation means 16 for separating a predetermined component in the exhaust gas G ₀ is interposed in the branch portion 14 a of the main exhaust flow path 12. Further, the detour channel 14 is provided with a reducing agent addition means 18 for adding a reducing agent for reducing NOx, a reduction catalyst 20 for reducing NOx, and the exhaust gas G ₀ via the component separation means 16. The differential pressure generating means 22 to be introduced to are arranged in order from upstream to downstream. In the figure, 4 is an injector for injecting fuel, 6 is an intake valve, and 8 is an exhaust valve.

In the exhaust purification apparatus 1 having the configuration described above, the present invention is the main exhaust passage 12 and the bypass passage 14 component separating means 16 interposed in the branch portion 14a with the, NOx and oxygen in the exhaust gas G ₀ It is characterized by having a zeolite membrane that separates.

Since the zeolite membrane selects a molecular sieve having pores of a size that allows NOx to pass through but does not allow oxygen to pass through, the exhaust gas G ₀ passes through the component separation means 16 and has a high NOx concentration. The exhaust gas g ₁ having a low concentration and the exhaust gas g ₂ having a high oxygen concentration and a low NOx concentration are separated.

The exhaust gas g ₁ having a high NOx concentration and a low oxygen concentration is introduced into the bypass channel 14, and a reducing agent (for example, fuel) is added by the reducing agent addition means 18, and then flows into the reduction catalyst 20 to reduce NOx. The harmless exhaust gas g ₃ is obtained.

Further, the exhaust gas g ₂ having a high oxygen concentration and a low NOx concentration flows through the main exhaust passage 12 and is discharged to the outside. However, the exhaust gas g ₂ joins the exhaust gas g ₃ that has been rendered harmless by the joining portion 14b and is exhausted. ₁ and discharged to the outside. Therefore, the NOx concentration of the exhaust gas G ₁ can be kept extremely low.

  As schematically shown in FIG. 2, the component separating means 16 is a rectangular wave cross-sectional structure K in which a zeolite membrane 24 is integrally formed on the surface of a base material 26. The rectangular wave cross-sectional structure K includes a base material (hereinafter also referred to as a partition wall) 26 and a large number of gaps (hereinafter referred to as slits) 30 defined by connecting portions 28 that connect adjacent partition walls 26. .

Here, the exhaust gas G that opens to the exhaust gas inflow end surface (for example, X ₀ ) of the main exhaust flow channel 12 of the slit 30 that opens to the surface (the upper surface Y _{1 in} FIG. 2) that faces the bypass flow channel 14. ₀ inflow portions 30a are sealed to prevent the flow of exhaust gas G _0.

That is, the inflow end surface of the exhaust gas of the rectangular wave sectional structure K X _0, since the sealing portion 30a and the opening 30b are formed so as to be adjacent to each other alternately, the end face X ₀ a rectangular wave sectional structure K Is located at the upstream side of the exhaust gas G ₀ and the X ₀ -X ₁ direction of the slit 30 is parallel to the axis of the main exhaust flow path 16, the exhaust gas G ₀ is sealed. The inside of the slit 30 that is not present can be freely distributed.

However, since free inflow of the exhaust gas G ₀ into the bypass channel 14 is blocked by the sealing portion 30a and the connecting portion 28, the exhaust gas G ₀ cannot freely flow. However, the zeolite membrane 24 formed on the surface of the base material 26 is permeable to NOx in the exhaust gas G ₀ , and the base material 26 is formed of a porous body having fine through holes. By reducing the pressure at the branching portion 14a side of the bypass channel 14 with the differential pressure generating means 22, NOx that permeates the zeolite membrane 24 can be enriched in the bypass channel 14 as shown in FIG. 2 (b). it can. FIG. 2B is an enlarged view of a partial cross section of the rectangular wave cross section of FIG. 2A. By passing the exhaust gas G ₀ through the rectangular wave cross section structure K, NOx (NO) is reduced. FIG. ₂ schematically shows a state in which the exhaust gas g ₁ enriched and the exhaust gas g ₂ enriched mainly in oxygen are separated.

  The slit density at the end faces X and X ′ of the slit 30 of the rectangular wave cross-section structure K as described above is preferably 5 to 15 pieces / cm. If the slit density is less than 5 pieces / cm, the NOx separation area is insufficient, and if it exceeds 15 pieces / cm, the pressure loss of the exhaust gas increases, which is not preferable.

  The thickness of the partition wall 26 separating the slits 30 is desirably 0.05 to 1.0 mm. If the thickness of the partition wall 26 is less than 0.05 mm, the strength is insufficient, and if it exceeds 1.0 mm, the pump 22 having a large driving pressure is required, which is not appropriate. More preferably, it is 0.1-0.5 mm.

  As shown in FIG. 2B, a separation membrane 24 that selectively permeates and separates NOx in the exhaust gas G0 is integrally formed with the base material 26 on the surface of the partition wall 26 of the rectangular wave sectional structure K. Is formed. The separation membrane 24 is formed on the upstream side of the partition wall 26 with respect to the flow of the exhaust gas G0, and the NOx that has permeated through the separation membrane 24 passes through the fine through-holes of the partition wall 26 made of a porous base material to generate a rectangular wave. It is enriched on the downstream side of the cross-section structure K (the bypass channel 14 side).

  The separation membrane 24 uses a molecular sieve membrane made of a zeolitic material in order to separate NOx from the high temperature exhaust gas G0. Zeolite is a crystalline inorganic oxide having a pore size of molecular size, and gas components having different molecular sizes can be selected by selecting the pore size.

FIG. 3 schematically shows how the NOx (NO) in the exhaust gas is separated by the molecular sieve membrane 24 of zeolite. In the exhaust gas, many gas components such as oxygen O ₂ , nitrogen N ₂ , carbon dioxide CO ₂ , NO (NOx) or water vapor H 2 O are mixed. However, these gas components have different sizes at the molecular level, and as is well known, their molecular diameters are in the order of water vapor <NO (NOx) <carbon dioxide <oxygen <nitrogen. The molecular diameter of NO (NOx) is about 0.32 nm, and that of oxygen is 0.35 nm. Therefore, by making the pore diameter of the molecular sieve film 24 larger than the molecular diameter of NOx and smaller than the molecular diameter of oxygen, it is possible to permeate NOx and separate it from other gas components such as oxygen molecules. In consideration of the transmission efficiency of NOx and the like, specifically, the pores of the molecular sieve membrane 24 are desirably 0.33 to 0.5 nm.

  The thickness of the zeolite membrane 24 is preferably 0.1 to 100 μm. If the film thickness is less than 0.1 μm, defects such as insufficient strength and pinholes may occur, and if it exceeds 100 μm, the pressure difference for permeating gas components becomes large, which is not appropriate. More preferably, it is 1-50 micrometers.

  As described above, the zeolite membrane 24 integrally formed on the surface of the porous substrate 26 can separate NOx in the exhaust gas and enrich it in the bypass channel 14. Therefore, the base material 26 is desirably a porous body that does not inhibit the permeability of the zeolite membrane 24.

  The material of such a porous body is not particularly limited, and ceramics such as metals and metal oxides, carbon, and organic polymers can be used. However, from the viewpoint of strength and rigidity, ceramics such as metals and metal oxides, metal nitrides, and metal carbides are preferable. Furthermore, considering that the difference in coefficient of thermal expansion from the zeolite membrane is small or that the affinity is high, the sintered metal made of stainless steel as the metal, alumina, zirconia, silica, mullite, cordierite as the metal oxide , Titania, zeolite or zeolite analogues are preferred. Among these, cordierite, which is a porous ceramic, has good formability, and can easily obtain desired air permeability (pore diameter, hole density, etc.) and strength by blending raw materials and firing methods. Is preferred. The average through-hole diameter of such a porous body is preferably 100 μm or less, and more preferably 0.1 to 50 μm.

In the present embodiment, the differential pressure generating means 22 is disposed on the downstream side of the reduction catalyst 20, and the exhaust gas G0 is reduced as the exhaust gas g ₁ through the rectangular cross-section structure K by reducing the pressure of the bypass passage 14. Introduced into the catalyst 20. The differential pressure generating means is not particularly limited, and a normal vacuum pump or blower can be used. As shown in FIG. 4, the NOx amount V separated by the zeolite membrane 24 increases in proportion to the pressure P of the pump that is the differential pressure generating means. Therefore, in the exhaust purification device 1 having the rectangular cross-sectional structure K of FIG. 2 as the component separation means 16, the amount of NOx introduced into the reduction catalyst 20 can be changed by changing the exhaust pressure of the pump 22. That is, the reduction amount of NOx can be controlled by controlling the pump pressure.

In the exhaust gas g ₁ from which NOx has been separated by the component separation means 16, unburned hydrocarbons, carbon monoxide and the like that function as a reducing agent mixed in the exhaust gas G ₀ are depleted together with oxygen. Therefore, it is desirable to add a reducing agent such as fuel (gasoline, light oil) of the internal combustion engine to the exhaust gas g ₁ in order to promote the reduction action of the reduction catalyst 20. The reducing agent adding means 18 for adding the reducing agent is not particularly limited, and a well-known reducing agent adding device that is usually used may be used.

In the present embodiment, the reduction catalyst 20 can suitably use a three-way catalyst used in a normal gasoline engine. This is because the exhaust gas g ₁ in which oxygen capable of reacting with the added reducing agent is depleted is introduced into the reduction catalyst, so that even the so-called lean combustion can be easily reduced with the three-way catalyst. is there.

Moreover, sulfur oxides, is a factor that affects the sulfur poisoning reduction catalyst, the sulfur oxides are also separated from the NOx by component separating means 16, are mixed in an oxygen-enriched gas g ₂ It is discharged outside. Therefore, the reduction catalyst 20 can maintain good NOx reduction characteristics without being subjected to sulfur poisoning.

  The exhaust emission control device of the present invention is not limited to the above embodiment, and can be changed without departing from the gist of the present invention.

For example, Ba having high NOx adsorptivity is supported on the surface of the base material 26 made of cordierite or the like of the component separating means 16 as shown in FIG. Ba stores NOx as Ba + 2NO ₂ + O ₂ → Ba (NO ₃ ) ₂ when the temperature of the exhaust gas is low. When the exhaust gas temperature is high or during rich operation, the NOx concentration can be increased by decomposition as Ba (NO ₃ ) ₂ → Ba + 2NO ₂ + O ₂ . Therefore, if the pump is driven in a state where the concentration of NOx is high so as to pass through the zeolite membrane 24, NOx can be enriched with high efficiency without always driving the pump. Energy saving can be achieved.

Moreover, the rectangular wave cross-section structure K is good also as a cylindrical shape as shown in FIG. By adopting a cylindrical shape, the inflow area of the exhaust gas G ₀ can be increased as compared with the rectangular wave cross-sectional structure K of FIG. In addition, the black coating part in a figure is the sealing part 30a or the connection part 28. FIG.

  The exhaust purification device of the present invention is suitable as an exhaust purification device for an internal combustion engine such as a lean gasoline engine or a diesel engine.

It is explanatory drawing which shows schematic structure of Embodiment-. It is explanatory drawing which shows the outline | summary of a rectangular wave cross-section. (A) is a perspective view which shows the whole shape of a rectangular wave cross-section structure. (B) is an explanatory view showing a part of a longitudinal section and explaining how NOx is separated. It is explanatory drawing explaining separation of NOx (NO) by a zeolite membrane. It is the graph which showed notionally the relation between pump pressure and NOx permeation amount. It is a conceptual diagram explaining a mode that Ba is supported on the surface of a substrate and occludes NOx. It is a perspective view which shows the other aspect of a rectangular wave cross-section structure.

Explanation of symbols

12: Main exhaust passage 14: Detour passage 16: Component separation means 18: Reducing agent addition means 20: Reduction catalyst 22: Differential pressure generation means (pump) 24: Zeolite membrane (separation membrane) 26: Base material (partition wall) 28: Connection part 30: Slit 30a: Sealing part 30b: Opening part K: Rectangular wave cross-section structure

Claims (10)

An exhaust purification device provided in an exhaust path of an internal combustion engine,
A main exhaust passage for exhausting the exhaust gas discharged from the combustion chamber of the internal combustion engine to the outside;
A detour channel that branches at an upstream portion of the main exhaust channel and merges at a downstream portion of the main exhaust channel;
Component separation means for separating a predetermined component in the exhaust gas interposed in a branch portion between the main exhaust passage and the bypass passage;
A reduction catalyst interposed in the bypass channel to reduce nitrogen oxides;
Reducing agent addition means for adding a reducing agent disposed upstream of the reduction catalyst to reduce nitrogen oxides;
Differential pressure generating means interposed in the bypass flow path for introducing the exhaust gas into the component separation means,
The exhaust gas purification apparatus, wherein the component separation means includes a zeolite membrane that separates nitrogen oxides and oxygen in the exhaust gas.   The exhaust emission control device according to claim 1, wherein the component separation means is a rectangular wave cross-sectional structure in which the zeolite membrane is integrally formed on a substrate surface.   The exhaust gas inflow end surface of the front rectangular wave cross-sectional structure is formed so that sealing portions and openings are alternately adjacent to each other, and the exhaust gas flows into the openings. Exhaust purification device.   The exhaust purification apparatus according to any one of claims 1 to 4, wherein the zeolite membrane has pores having a pore diameter of 0.26 to 0.5 nm.   The exhaust emission control device according to any one of claims 2 to 4, wherein the base material is a porous body having a through hole having a diameter of 0.1 to 50 µm.   The exhaust purification device according to claim 5, wherein the porous body is ceramic.   The exhaust emission control device according to claim 6, wherein the ceramic is cordierite.   The exhaust purification device according to claim 1, wherein the reduction catalyst is a three-way catalyst.   The exhaust emission control device according to claim 1, wherein the component separation means carries a nitrogen oxide adsorbing substance.   The exhaust emission control device according to claim 9, wherein the nitrogen oxide adsorbing substance is Ba.

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