JP2010013969A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of preventing an ammonia slip operation as well as improving a purification rate of NOx by regulating a concentration ratio of NO and NO<SB>2</SB>in an exhaust gas inflowing into an SCR catalyst in an exhaust emission control device provided in an exhaust passage of an internal combustion engine. <P>SOLUTION: There is provided the exhaust emission control device provided in the exhaust passage of an internal combustion engine. The exhaust emission control device includes an oxidative catalyst containing an oxygen occlusion material, a reduction agent adding means arranged on a more downstream side than the oxidative catalyst, and the SCR catalyst arranged on a more downstream side than the reduction agent adding means. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification device for an internal combustion engine. In particular, the present invention relates to an exhaust gas purification device that purifies NOx in exhaust gas discharged from an internal combustion engine using an oxidation catalyst and an SCR catalyst.

In recent years, from the viewpoint of measures against environmental pollution, for internal combustion engines such as diesel engines (hereinafter also referred to as engines), in addition to low fuel consumption, nitrogen oxides contained in exhaust gas discharged (hereinafter referred to as NOx) There is a strong demand for reducing the above. However, in order to meet both of these requirements, the response by the engine itself is already close to the limit, and an exhaust gas aftertreatment device (exhaust gas purification device) is required. In particular, since the fuel consumption and NOx are in a trade-off relationship, the fuel consumption can be improved if there is an exhaust gas purification device that can significantly reduce NOx.
Therefore, as one means for reducing this NOx, urea water is added as a reducing agent in the exhaust gas, and NOx is converted into nitrogen (N ₂ ) and water (NH ₃ ) by ammonia (NH ₃ ) generated by hydrolysis of urea. _2. Description of the Related Art An exhaust gas purification device that performs NOx reduction using a NOx catalyst that selectively reduces to H ₂ O), that is, a so-called urea SCR (Selective Catalytic Reduction) catalyst, is known.

  As an exhaust gas purification device as described above, for example, an exhaust gas purification device including an oxidation catalyst, an SCR catalyst, and a device that injects urea water as a reducing agent, which is disclosed in Patent Literature 1 and Non-Patent Literature 1, is known. It has been.

In this exhaust gas purification device, in order to efficiently purify NOx by the SCR catalyst, the NO component in the exhaust gas is oxidized to NO ₂ by the oxidation catalyst, and then urea water is injected into the exhaust gas, and then the SCR The catalyst is configured to reduce NOx to N ₂ and purify it.
Here, since the reaction with the SCR catalyst follows 2NH ₃ + NO + NO ₂ → 2N ₂ + 3H ₂ O, it is known that the purification ratio is highest when the molar ratio of NO and NO ₂ is 1: 1.
In addition, since the NOx concentration in the exhaust gas normally discharged from the engine is higher than the concentration of NO ₂ , in the oxidation catalyst, a part of NO in the exhaust gas is obtained by the reaction of 2NO + O ₂ → 2NO _2. It is varied to NO _2. Thus, the NOx purification rate is improved by allowing the NO and NO ₂ concentration ratios to flow into the SCR catalyst in a uniform state.
JP 2004-1000069 A Hirata Kiminobu et al., “Urea Selective Reduction System for Large Car Diesel”, Automotive Technology, VOL, 60, No. 9, 2006, PP28-33

However, it is difficult to stably adjust the ratio between the optimal NO concentration and the NO ₂ concentration described above by simply oxidizing NO with the oxidation catalyst disclosed in Patent Document 1 and Non-Patent Document 1. Met. That is, although the ratio of the NO concentration to the NO ₂ concentration in the exhaust gas varies depending on the temperature of the exhaust gas, the engine operating condition, etc., the SCR catalyst located downstream of the oxidation catalyst exhaust gas is not considered. It was supposed to flow into. For this reason, the SCR catalyst cannot be efficiently reduced, and the NOx purification rate cannot always be increased.

  Further, when NOx purification is not sufficiently performed, a part of urea added as a reducing agent is not used and may be released into the atmosphere as ammonia (ammonia slip phenomenon), which is avoided. Therefore, there is a problem that an oxidation catalyst for removing ammonia is further required on the exhaust gas downstream side of the SCR catalyst.

The present invention improves the NOx purification rate by adjusting the ratio of the NO concentration and the NO ₂ concentration in the exhaust gas flowing into the SCR catalyst in the exhaust gas purification device provided in the exhaust passage of the internal combustion engine. Another object is to provide an exhaust gas purifying device capable of preventing ammonia slip.

  In order to achieve the above object, an exhaust gas purifying apparatus according to a first aspect of the present invention is provided in an exhaust passage of an internal combustion engine, and is provided with an oxidation catalyst including an oxygen storage material, and downstream of the oxidation catalyst. A reducing agent adding means, and an SCR catalyst arranged on the downstream side of the reducing agent adding means.

According to the present invention, when the oxygen concentration in the exhaust gas is high, the oxygen storage material takes in oxygen, and conversely, when the oxygen concentration is low, the oxygen storage material releases oxygen which is taken up in the oxidation catalyst. The oxidation reaction from NO to NO ₂ can be stably performed, and the ratio of the NO concentration and the NO ₂ concentration in the exhaust gas after passing through the oxidation catalyst can be stably adjusted to an ideal ratio as described above. it can. Therefore, even if the NO concentration is high due to the operating conditions of the engine so far and the oxidation reaction to NO ₂ has not been sufficiently performed due to the rate of oxygen supply, the oxidation reaction can be efficiently performed and the reduction reaction in the SCR catalyst. closer positively to optimum molar ratio of NO and NO _2, it is possible to improve the purification rate of NOx. In addition, discharge of the reducing agent into the atmosphere (ammonia slip phenomenon) can be avoided.

Note that NO and NO ₂ are often converted to each other by chemical reaction or the like and exhibit relatively similar characteristics, and it is more convenient to handle them together, so NOx here is referred to as NO and NO ₂ Refers to two types of gas.

  In the exhaust gas purification apparatus according to the second invention, in the first invention, the oxygen storage material is at least one selected from the group consisting of zinc oxide, cerium oxide, and zirconium oxide. Formed from material.

  According to the present invention, zinc oxide, cerium oxide and zirconium oxide are materials having particularly excellent oxygen storage and release characteristics, and are therefore suitable for use as an oxygen storage material. Since the amount of catalytic metal used can be suppressed and the oxidation catalyst can be miniaturized, it is possible to maintain a high NOx purification rate while miniaturizing the exhaust gas purification apparatus as a whole.

  The exhaust gas purifying apparatus according to a third aspect of the present invention is the exhaust gas purifying apparatus according to the first or second aspect, wherein the oxidation catalyst has a honeycomb-shaped carrier substrate, and the catalyst layer containing the oxygen storage material and the catalyst metal is provided. It is formed on the surface of the carrier substrate.

According to this invention, since the catalyst layer containing the oxygen storage material is formed on the honeycomb-shaped carrier base material, the surface area with which the catalyst layer can come into contact with the exhaust gas can be increased. Can be occluded and released. Accordingly, it is possible to make the molar ratio of NO and NO ₂ optimal for the reduction reaction in the SCR catalyst even closer, and the NOx purification rate can be further improved.

  In the exhaust gas purifying apparatus according to the fourth invention, in the third invention, the honeycomb-shaped carrier substrate is formed of a wall flow type.

  Here, the wall-flow type carrier base material is a material that has pores or the like in the carrier base material and allows exhaust gas to pass through in the thickness direction of the carrier base material, such as a porous ceramic or foam metal Can be used.

  According to the present invention, since the support base material is a wall flow type, the exhaust gas passes through the support base material, so that the surface area where the catalyst layer can come into contact with the exhaust gas is further increased compared to the wall through type. Since it can be increased, oxygen can be stored and released more effectively, and the NOx purification rate can be further improved.

  Further, since the carrier base material is a wall flow type, a filter that collects PM (Particulate Matter) in the exhaust gas passing through the carrier base material by adjusting the pore diameter of the base material. It is also possible to have a function as Therefore, it is possible to provide an exhaust gas purification device that can purify not only NOx but also PM.

According to the present invention, in the exhaust gas purifying device provided in the exhaust passage of the internal combustion engine, the reducing agent is removed from the atmosphere by adjusting the ratio of the NO concentration and the NO ₂ concentration in the exhaust gas flowing into the SCR catalyst. It is possible to provide an exhaust gas purification device capable of improving the NOx purification rate without causing the exhaust gas to be released.

(Embodiment 1)
FIG. 1 is a perspective view schematically showing an oxidation catalyst used in an exhaust gas purification apparatus according to Embodiment 1 of the present invention. The oxidation catalyst 10 is formed by forming a catalyst layer containing an oxygen storage catalyst on each cell wall of a honeycomb-shaped carrier 12 having a large number of cells 11 penetrating in the exhaust gas flow direction.

  FIG. 2 is an enlarged cross-sectional view schematically showing the oxidation catalyst used in the exhaust gas purification apparatus according to Embodiment 1 of the present invention, cut in a direction perpendicular to the axis. As shown in the figure, the catalyst 10 includes an inner layer 14 formed on the surface of the cell wall 13 and an outer layer 15 overlaid on the inner layer 14.

  The carrier substrate of the catalyst constituting the cell 11 is ceramic, specifically cordierite, which is formed by a known extrusion molding and firing method.

The inner layer 14 is made of alumina containing at least one selected from the group consisting of zinc oxide, cerium oxide, and zirconium oxide, which are oxygen storage materials, and is formed by a known washcoat method. . The wash coat method is a method of forming a carrier layer (porous ceramic layer) on a molded body by immersing the molded body in a slurry of ceramic powder (alumina or the like) for forming a porous ceramic layer and drying it. .
Alternatively, it can be formed by a known sol-gel method as another supporting method.

  The outer layer 15 is made of alumina containing a catalyst metal such as platinum or palladium, and is formed by the same method as described above.

  FIG. 3 is a schematic cross-sectional view of the oxidation catalyst used in the exhaust gas purifying apparatus according to Embodiment 1 of the present invention cut along the axis, illustrating the flow of exhaust gas. In the figure, the inner layer 14 and the outer layer 15 are omitted in order to explain the flow of exhaust gas.

The catalyst shown in the figure is a wall-through type catalyst. Thus, the exhaust gas flowing into the oxidation catalyst 10, by going through while contacting in the cell 11 catalyst layer formed on the surface of the cell walls 13 (not shown), oxide NO is the NO ₂ Will be. Since the catalyst 10 has a honeycomb shape, the surface area of the carrier substrate can be increased, and NO can be efficiently oxidized to NO ₂ . Therefore, if the catalyst device 10 is applied to an exhaust gas purification device, NOx can be effectively purified.

  FIG. 4 is a cross-sectional view schematically showing a sectional view of the oxidation catalyst used in the exhaust gas purifying apparatus according to Embodiment 1 of the present invention cut along the axis, illustrating the flow of exhaust gas. . Also in this figure, the inner layer 14 and the outer layer 15 are omitted in order to explain the flow of exhaust gas.

  The catalyst shown in the figure is a wall flow type catalyst. The cell wall 13 which is a carrier substrate has pores so that exhaust gas can pass in the thickness direction of the cell wall 13, and a known porous ceramic or foam metal can be used.

  Further, as shown in the figure, the oxidation catalyst 10 is provided with a sealing portion 16, and the exhaust gas that has flowed in cannot pass through the catalyst 10 straightly as in the wall-through type. It is configured such that it must pass through the cell wall 13 which is a carrier substrate.

Since the cell wall 13 has many pores, and the inner layer 14 and the outer layer 15 (not shown) are formed on the surface of the pores, the entire surface area of the catalyst should be larger than the wall-through type. And can more efficiently oxidize NO to NO ₂ .

  Further, by adjusting the pore diameter, it is possible to provide a function as a filter for collecting PM (Particulate Matter) in the exhaust gas passing through the carrier base material. Therefore, if the oxidation catalyst 10 is applied to an exhaust gas purification device to be described later, an exhaust gas purification device that can purify not only NOx but also PM can be provided.

  If the collected PM is left as it is, clogging will occur in the oxidation catalyst 10, so that the collected PM can be adjusted by adjusting the engine operating conditions at an appropriate timing and raising the temperature of the exhaust gas. It can be burned to eliminate clogging. As another method, a heater may be incorporated in the oxidation catalyst 10, and PM may be heated and burned with this heater at a predetermined timing.

(Embodiment 2)
FIG. 5 is a perspective view schematically showing an oxidation catalyst used in the exhaust gas purification apparatus according to Embodiment 2 of the present invention. FIG. 6 is a perspective view schematically showing a state in which the flat plate and the corrugated plate are wound using the winding shaft, (a) is a state in which the winding shaft, the flat plate and the corrugated plate are coupled, and (b) is a winding shaft. It is a figure which shows the state which winds a flat plate and a corrugated sheet around.

  Here, the oxidation catalyst 20 is a metallic honeycomb carrier, and as shown in FIG. 6, a flat plate 22 mainly made of heat-resistant stainless steel and a corrugated plate 23 obtained by corrugating the flat plate 22. It is laminated by winding a flat plate 22 and a corrugated plate 23 around a cylindrical winding shaft 21 made of heat-resistant stainless steel.

  As shown in FIG. 6 (a), after slitting the winding shaft 21 and inserting and fixing the flat plate 22 and the corrugated plate 23 into the slit, or after sandwiching the flat plate 22 and the corrugated plate 23 with the winding shaft 21 being split. The winding shaft 21, the flat plate 22, and the corrugated plate 23 are coupled by a fixing method. Next, as shown in FIG. 6B, by rotating the winding shaft 21, the flat plate 22 and the corrugated plate 23 are wound in an alternating spiral shape, and are laminated to form a honeycomb-like structure. Further, an outer cylinder 24 is provided outside the structure to reinforce the strength.

  The winding shaft 21 may be left in the oxidation catalyst 20 as it is, or may be taken out after winding. In the case of taking out, it is possible to prevent the exhaust gas from leaking from the portion by inserting an insert as a cap into the taken out gap and sealing it.

  Further, as the material of the flat plate 22 and the corrugated plate 23 used as the carrier base material, heat resistant materials such as high heat resistant stainless steel and Ni-base superalloy capable of withstanding high temperature use of about 1000 degrees are suitable, and more preferably, In order to cope with thermal stress caused by rapid heating and cooling, a material having a thermal expansion coefficient equivalent to that of the outer cylinder 24 is preferable. Specifically, 20Cr-5Al stainless steel generally used for a catalyst support for automobiles can be used.

  Further, as described above, since the support base material of the oxidation catalyst 20 is made of metal, it is advantageous in that it has a high heat-resistant temperature and an extremely small difference in thermal expansion from the piping parts and is excellent in durability. .

  Moreover, although it is a wall flow type in the same figure, it can also be made into a wall through type by sealing similarly to FIG. 4, and PM can also be collected. In this case, the flat plate 22 may be made of a metal nonwoven fabric made of stainless steel.

  FIG. 7 is an enlarged cross-sectional view schematically showing an oxidation catalyst used in the exhaust gas purifying apparatus according to Embodiment 2 of the present invention cut in a direction perpendicular to the axis. As shown in the figure, the oxidation catalyst 20 includes cell walls 31 and 32 constituted by a flat plate 22 and a corrugated plate 23, an inner layer 33 formed on the surface of the cell walls 31 and 32, and the inner layer. And an outer layer 34 overlying 33.

The inner layer 33 contains alumina and contains at least one material selected from the group consisting of zinc oxide, cerium oxide, and zirconium oxide, which are oxygen storage materials, and is formed by a known washcoat method. Is done. The wash coat method is a method of forming a carrier layer (porous ceramic layer) on a molded body by immersing the molded body in a slurry of ceramic powder (alumina or the like) for forming a porous ceramic layer and drying it. .
Alternatively, it can be formed by a known sol-gel method as another supporting method.

  The outer layer 34 is made of alumina containing a catalyst metal such as platinum or palladium, and is formed by the same method as described above.

Therefore, if the catalyst device 20 is applied to the exhaust gas purification device, the catalyst device 20 can stably perform the oxidation reaction of NO to NO ₂ , so that NOx can be effectively purified.

(Embodiment 3)
FIG. 8 is a diagram for schematically illustrating an exhaust gas purifying apparatus according to Embodiment 3 of the present invention. The exhaust gas purification device 40 is a device using a urea selective reduction catalyst (SCR catalyst) that reduces and purifies NOx in exhaust gas to nitrogen gas or the like using urea. Moreover, in the same figure, the engine 41 is a diesel engine, for example.

As shown in the figure, the exhaust gas purification device 40 includes an oxidation catalyst 43 that oxidizes NO in the exhaust gas exhausted from the engine 41 to NO ₂ , and NOx in the exhaust gas that is disposed downstream of the NOx. _The SCR catalyst 47 is reduced to ₂ and H ₂ O.
Here, the oxidation catalyst 43 is the oxidation catalyst of the first embodiment or the second embodiment, and includes the oxygen storage material described above.

  A urea water addition device 45 for adding urea as a reducing agent is provided at the inlet of the SCR catalyst 47, and controls the spray amount of urea water necessary for purification by NOx reduction with the SCR catalyst. Control is performed by an apparatus (ECU) 46. The urea water is stored in the urea water tank 44.

  Further, this diesel engine is provided with an exhaust gas recirculation device (EGR) (not shown), and a control device (ECU) 42 is disposed for controlling fuel injection. Therefore, the control device (ECU) 46 that controls the spray amount of urea water can be shared with the control device (ECU) 42.

According to this, since the oxidation catalyst 43 containing the oxygen storage material is used, the NOx concentration in the exhaust gas changes every moment depending on the operating conditions of the engine 41, but when the oxygen concentration in the exhaust gas is high The ratio of the NO concentration and the NO ₂ concentration in the exhaust gas after passing through the oxidation catalyst 41 is described above by releasing the oxygen taken in by the oxygen storage material when the oxygen storage material takes in oxygen and conversely when the oxygen concentration is low. It can be stably adjusted to the ideal ratio.
Therefore, even if the NO concentration is high due to the operating conditions of the engine so far and the oxidation reaction to NO ₂ has not been sufficiently performed due to the rate of oxygen supply, the oxidation reaction can be efficiently performed and the reduction reaction in the SCR catalyst. closer positively to optimum molar ratio of NO and NO _2, it is possible to improve the purification rate of NOx. In addition, discharge of the reducing agent into the atmosphere (ammonia slip phenomenon) can be avoided.

  The structure of the embodiment of the present invention disclosed above is merely an example, and the scope of the present invention is not limited to the scope of these descriptions. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

  The exhaust gas purification apparatus of the present invention can purify NOx in exhaust gas generated from an internal combustion engine such as a vehicle or a ship.

It is a perspective view which shows typically the oxidation catalyst used for the exhaust-gas purification apparatus in Embodiment 1 of this invention. It is an expanded sectional view showing typically the oxidation catalyst used for the exhaust gas purification device in Embodiment 1 of the present invention in the direction perpendicular to the axis. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross-sectional view of an oxidation catalyst used in an exhaust gas purification apparatus according to Embodiment 1 of the present invention cut along an axis, illustrating the flow of exhaust gas. It is the modification of the sectional view shown typically which cut along the axis the oxidation catalyst used for the exhaust gas purification device in Embodiment 1 of the present invention, and explains the flow of exhaust gas. It is a perspective view which shows typically the oxidation catalyst used for the exhaust-gas purification apparatus in Embodiment 2 of this invention. It is a perspective view which shows typically the state which winds a flat plate and a corrugated sheet using a winding axis, (a) is a state which combined a winding axis, a flat plate, and a corrugated sheet, (b) is a flat plate around a winding axis. It is a figure which shows the state which winds a corrugated sheet. It is an expanded sectional view showing typically the oxidation catalyst used for the exhaust gas purification device in Embodiment 2 of the invention cut in the direction perpendicular to the axis. It is a figure for demonstrating typically the exhaust-gas purification apparatus in Embodiment 3 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Oxidation catalyst 11 Cell 12 Honeycomb carrier 13 Cell wall 14 Inner layer 15 Outer layer 16 Sealing material 20 Oxidation catalyst 21 Winding shaft 22 Flat plate 23 Corrugated plate 24 Outer cylinder 25 Honeycomb shaped carrier 26 Cell 31 Corrugated plate 32 Flat plate 33 Inner layer 34 Outer layer 40 Exhaust gas purification device 41 Engine 42 Control device (ECU)
43 Oxidation catalyst 44 Urea tank 45 Urea spraying device 46 Control unit (ECU)
47 SCR catalyst

Claims (4)

An exhaust gas purifying device provided in an exhaust passage of an internal combustion engine,
An exhaust gas comprising: an oxidation catalyst containing an oxygen storage material; a reducing agent adding means disposed downstream from the oxidizing catalyst; and an SCR catalyst disposed downstream from the reducing agent adding means. Purification equipment.   The exhaust gas purification device according to claim 1, wherein the oxygen storage material is made of at least one material selected from the group consisting of zinc oxide, cerium oxide, and zirconium oxide.   3. The exhaust according to claim 1, wherein the oxidation catalyst has a honeycomb-shaped carrier substrate, and a catalyst layer containing the oxygen storage material and a catalyst metal is formed on a surface of the carrier substrate. Gas purification device.   The exhaust gas purification device according to claim 3, wherein the honeycomb-shaped carrier substrate is a wall flow type.

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