JP2005201155A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress increase in exhaust pressure loss and improve PM trapping efficiency. <P>SOLUTION: The exhaust emission control device has: straight cells 10 whose upstream side and downstream side are not filled up; downstream outflow cells 11 which are adjacent to the straight cells 10 and whose upstream side is filled up; cell partition walls 14 partitioning between the straight cells 10 and the downstream outflow cell 11. Each plug part 12 of the downstream outflow cell 11 is formed downstream of the upstream end face, opened to the upstream end face, and provided with bag cell 13 whose bottom part is the plug part 12. After the exhaust gas flowing into the bag cell 13 is collided with the plug part 12 and is bounced back, the exhaust gas flows into the straight cells 10, so that a gas flow direction becomes at random and a gas flow becomes disturbed flow. Therefore, aggregation of the PMs is suppressed, aggregated coarse particles are decomposed to make the particle diameter small, so that the PMs are efficiently trapped in the cell partition walls 14. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification device that purifies exhaust gas containing particulates such as exhaust gas from a diesel engine, and more particularly to an exhaust gas purification device that achieves both a high purification rate and a low pressure loss.

  As for gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology that can cope with it. However, with regard to diesel engines, regulations are also advanced due to the unique situation that harmful components are emitted as particulates (particulate matter: carbon fine particles, sulfur fine particles such as sulfate, SOF fine particles, etc., hereinafter referred to as PM). Even behind the gasoline engine.

  As exhaust gas purification devices for diesel engines that have been developed so far, a trap type exhaust gas purification device (wall flow) and an open type exhaust gas purification device (straight flow) are known. Among these, as a trap-type exhaust gas purification device, a ceramic-made plug-type honeycomb body (diesel PM filter (hereinafter referred to as DPF)) is known. This DPF is formed by alternately sealing both ends of the openings of the cells of the ceramic honeycomb structure, for example, in a checkered pattern, and is adjacent to the inflow side cells and the inflow side cells clogged on the exhaust gas downstream side. It consists of an outflow side cell clogged on the exhaust gas upstream side and a cell partition that partitions the inflow side cell and the outflow side cell. PM is discharged by filtering exhaust gas through the pores of the cell partition and collecting PM. It suppresses. However, DPF has a defect that pressure loss increases due to PM accumulation.

  On the other hand, an open-type exhaust gas purification device forms a catalyst layer on the cell partition wall surface of a straight-flow honeycomb body, oxidizes and purifies HC and CO by precious metal supported on the catalyst layer, and oxidizes and purifies PM. To do. In this open type exhaust gas purification device, although an increase in pressure loss is suppressed, there is a problem that a large amount of PM passes through the cell and the PM purification rate is low.

  As described above, the PM collection efficiency and the exhaust pressure loss are in the relationship of the exhaust reaction, and various proposals have been made to satisfy both. For example, Japanese Patent Laid-Open No. 2002-256842 discloses an exhaust gas purification filter having a half-wall flow structure in which at least a part of a plugging portion of a DPF inflow side cell is constituted by a partial plug having an opening communicating with the outside of the cell. Has been described. According to this exhaust gas purification filter, the exhaust gas flowing into the inflow side cell having a partial plug passes through the cell partition while the amount of PM deposited on the cell partition is small, and when the amount of PM deposited on the cell partition increases, the partial plug Therefore, both high PM collection efficiency and suppression of pressure loss increase can be achieved.

  However, this technique has a problem that the pressure loss is high when the amount of accumulated PM increases because the ventilation resistance of the opening of the partial plug is set to be higher than the ventilation resistance of the cell partition wall.

  JP 2003-035126 also discloses an exhaust gas purification system in which a filter having a semi-wall flow structure in which only the upstream end face is alternately packed in a checkered pattern on the downstream side of the oxidation catalyst and the downstream end face is completely opened. An apparatus is disclosed. According to this exhaust gas purifying apparatus, the PM amount can be reduced by purifying PM with the oxidation catalyst during low load operation, and the PM amount deposited on the filter can be reduced. Further, during high load operation, PM accumulated on the filter can be burned, and the filter can be regenerated.

  Furthermore, Japanese Patent Application Laid-Open No. 2003-214141 describes a DPF having a half-wall flow structure in which at least a part of a plugged portion of an inflow side cell is formed from a partially plugged oxidation catalyst having a through hole. According to this technique, PM deposited by the oxidation catalyst can be purified, and thus an increase in pressure loss can be suppressed.

  In recent years, for example, as described in JP-A No. 09-173866, a coating layer is formed from alumina or the like on the surface and pores of the cell partition of DPF, and platinum (Pt) or the like is formed on the coating layer. A continuous regeneration type DPF (filter catalyst) carrying a catalytic metal has been developed. According to this continuous regeneration type DPF, the collected PM is oxidized and burned by the catalytic reaction of the catalytic metal, so that the DPF can be regenerated by burning the PM at the same time as or after the collection. . Since the catalytic reaction occurs at a relatively low temperature and PM can be burned while the amount of trapped is small, there is an advantage that the thermal stress acting on the DPF is small and damage is prevented.

  However, in the conventional DPF and filter catalyst, a plugging portion is formed on the upstream end face of the outflow side cell, and the exhaust gas is easily guided to the inflow side cell. For this reason, the exhaust gas is almost laminar and flows into the inflow side cell, so that PM tends to aggregate and the pores of the cell partition walls are blocked, and the pressure loss tends to increase.

In addition, in a DPF with a half-wall flow structure, the flow rate of exhaust gas flowing into a straight cell that does not have a plugging part or has a partial plug is large, and a differential pressure is generated between the gas in the adjacent downstream outflow cell. Therefore, a gas flow is generated from the straight cell to the downstream outflow cell through the cell partition, and at that time, PM is collected in the cell partition. However, when the PM in the exhaust gas flowing into the straight cell in a nearly laminar flow aggregates, it may be difficult to enter the cell partition wall, and coarse PM may be discharged from the straight cell as it is.
JP2003-035126 JP2003-214141 JP 09-173866

  The present invention has been made in view of the above-described circumstances, and it is an object to be solved to suppress an increase in exhaust pressure loss and further improve PM collection efficiency.

  A feature of the exhaust gas purification apparatus of the present invention that solves the above problems is a straight cell that is not clogged on the exhaust gas upstream side and the exhaust gas downstream side, and a downstream that is adjacent to the straight cell and clogged on the exhaust gas upstream side and not clogged on the exhaust gas downstream side. A honeycomb structure having a half-wall flow structure having an outflow cell and a porous cell partition wall having a large number of pores dividing the straight cell and the downstream outflow cell, and the plugging portion of the downstream outflow cell is a honeycomb structure The honeycomb structure is formed downstream of the upstream end face of the body, and the honeycomb structure has a bag cell having an opening at the upstream end face and having a plugging portion as a bottom.

  The plugging portion is preferably formed 5 to 15 mm downstream from the upstream end face of the honeycomb structure.

Further, it is desirable that a catalyst layer in which a catalyst metal is supported on an oxide carrier is formed on the surface of the cell partition wall and the inner surface of the pores, and the catalyst metal preferably includes a noble metal and a NO _x storage material. .

  The porosity of the cell partition walls of the honeycomb structure is preferably 60 to 80%, and the average pore diameter of the pores of the cell partition walls of the honeycomb structure is preferably 10 to 50 μm. Further, it is preferable that an oxidation catalyst is further disposed on the exhaust gas upstream side of the honeycomb structure.

According to the exhaust gas purifying apparatus of the present invention, the exhaust gas is stirred by the bag cell and flows into the straight cell as a turbulent flow. Therefore, PM aggregation is unlikely to occur and PM with a small particle diameter can be efficiently collected by the cell partition wall, so that the PM collection efficiency is greatly improved. Moreover, the rise in pressure loss is suppressed by the straight cell. If a catalyst layer is formed on the cell partition walls, PM collected by the catalyst layer can be continuously oxidized and burned, so that overheating can be prevented and heat shock of the honeycomb structure can be prevented. The collection ability can be regenerated. Further, if a NO _x storage material is supported on the catalyst layer together with a noble metal, NO _x can also be purified efficiently.

  As shown in FIG. 2, the exhaust gas purifying apparatus of the present invention has the bag cell 13 on the upstream end face. Therefore, after the exhaust gas flowing into the bag cell 13 collides with the plugging portion 12 and bounces back, the straight cell 10 Flow into. Therefore, the gas flow direction becomes random, and the exhaust gas in the straight cell 10 becomes almost turbulent. Therefore, the aggregation of PM is suppressed and the aggregated coarse particles are decomposed to reduce the particle size.

  The pressure in the straight cell 10 becomes higher than the pressure in the downstream outflow cell 11 having the plugging portion 12 due to the exhaust gas having a large flow velocity. Accordingly, the exhaust gas flowing in the straight cell 10 passes through the cell partition wall 14 and flows into the downstream outflow cell 11, and PM having a fine particle size in the exhaust gas is collected in the cell partition wall 14. In addition, the pressure of the exhaust gas flowing into the downstream outflow cell 11 is maintained lower than the pressure in the straight cell 10 due to the resistance when passing through the cell partition wall 14, so that the gas flow from the straight cell 10 toward the downstream outflow cell 11 is reduced. There is no interruption, and the PM having a small particle diameter continues to be collected in the cell partition wall 14.

  Therefore, according to the exhaust gas purifying apparatus of the present invention, PM can be collected efficiently, PM collection efficiency is greatly improved, and since a straight cell is provided, an increase in pressure loss is suppressed.

  The honeycomb structure includes a straight cell that is not clogged on the exhaust gas upstream side and the exhaust gas downstream side, a downstream outflow cell that is adjacent to the straight cell and that is clogged on the exhaust gas upstream side and that is not clogged on the exhaust gas downstream side, and the straight cell and the downstream outflow cell. A porous cell partition wall having a large number of pores, and the plugging portion of the downstream outflow cell is formed downstream of the upstream end surface of the honeycomb structure, and the honeycomb structure is formed on the upstream end surface. It has a half-wall flow structure provided with a bag cell that is open and has a plugging portion at the bottom.

  This honeycomb structure can be manufactured from heat-resistant ceramics such as cordierite and silicon carbide. For example, a clay-like slurry containing cordierite powder as a main component is prepared, and the slurry is formed by extrusion or the like and fired. Instead of cordierite powder, powders of alumina, magnesia and silica can be blended so as to have a cordierite composition. Then, the honeycomb structure is manufactured by sealing the cell opening on one end face on the upstream side with a similar clay-like slurry or the like at a position downstream from the end face.

  In order to form pores in the cell partition walls of the honeycomb structure, a combustible powder such as carbon powder, wood powder, starch, or resin powder is mixed in the slurry, and the combustible powder disappears upon firing. Thus, pores can be formed, and by adjusting the particle size and addition amount of the combustible powder, the distribution of the surface vacancies and the diameters of the internal pores and the opening area can be controlled.

  The pore distribution in the cell partition walls of the honeycomb structure is preferably in the range of a porosity of 60 to 80% or an average pore diameter of 10 to 50 μm. If the porosity or average pore diameter is out of this range, the collection efficiency of PM having a small particle diameter may be reduced, or the pressure loss may be increased.

  The plugging portion of the bag cell is preferably formed 5 to 15 mm downstream from the upstream end face of the honeycomb structure. When the depth of the plugging portion is less than 5 mm, the stirring action is not expressed and the PM collection efficiency cannot be improved. On the other hand, if it exceeds 15 mm, the bag cell becomes too deep and it becomes difficult to obtain an agitating action, and PM easily accumulates in the plugging portion, resulting in a large amount of heat generated during combustion and a heat shock.

It is preferable to form a catalyst layer in which a catalyst metal is supported on an oxide carrier on the surface of the cell partition wall and the pore inner surface. By forming this catalyst layer, it is possible to purify HC, CO, etc. in the exhaust gas, and to continuously oxidize and burn PM trapped in the cell partition walls, preventing overheating and heat shock of the honeycomb structure. Can be prevented, and the PM collection ability can be regenerated. Further, if a NO _x storage material is supported on the catalyst layer together with a noble metal, NO _x can also be purified efficiently.

  As the oxide carrier, an oxide such as alumina, ceria, zirconia, titania, or a composite oxide composed of a plurality of these can be used. As the catalyst metal, it is preferable to use one or more selected from platinum group noble metals such as Pt, Rh, Pd, Ir and Ru. The amount of catalyst metal supported is preferably 0.1 to 5 g per liter of the honeycomb structure volume. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased.

The catalyst layer preferably contains a NO _x storage material selected from alkali metals, alkaline earth metals, and rare earth elements. When the NO _x storage material is included in the catalyst layer, NO ₂ generated by oxidation with the catalyst metal can be stored in the NO _x storage material, so that the NO _x purification activity is further improved. The supported amount of the NO _x storage material is preferably in the range of 0.05 to 0.45 mol per liter of the volume of the honeycomb structure. If the supported amount is less than this, the activity is too low to be practical, and if the supported amount is more than this range, the catalyst metal is covered and the activity is lowered.

  In order to form a catalyst layer on the honeycomb structure, oxide powder or composite oxide powder is made into a slurry together with a binder component such as alumina sol and water, and the slurry is attached to the cell partition walls and fired. What is necessary is just to carry. A slurry can also be prepared from a catalyst powder in which a catalyst metal is previously supported on an oxide powder or a composite oxide powder. A normal dipping method can be used to attach the slurry to the cell partition walls. However, the slurry is forcibly filled into the pores of the cell partition walls by air blowing or suction, and the excess slurry contained in the pores is filled. It is desirable to remove things.

The formation amount of the catalyst layer is preferably 30 to 200 g per liter of the honeycomb structure volume. If the catalyst layer is less than 30 g / L, the durability of the catalyst metal or the NO _x storage material is inevitably lowered, and if it exceeds 200 g / L, the pressure loss becomes too high and is not practical.

It is desirable that an oxidation catalyst is further arranged on the exhaust gas upstream side of the honeycomb structure. By arranging the oxidation catalyst, the SOF in the PM is decomposed and the PM is further atomized. Therefore, coupled with the turbulent flow by the bag cell, PM aggregation can be further suppressed, and the PM collection efficiency is further improved. In addition, the purification performance of HC, CO, etc. in the exhaust gas is improved, and PM captured by NO ₂ generated by oxidation of NO is oxidized, so the combustion characteristics of PM are also improved.

  As this oxidation catalyst, a pellet catalyst or a honeycomb catalyst can be used. For example, a honeycomb structure having a straight flow structure in which a coat layer is formed from an oxide carrier and a noble metal is supported on the coat layer can be used. As the oxide carrier, an oxide such as alumina, ceria, zirconia, titania, or a composite oxide composed of a plurality of these can be used. As the noble metal, one or plural kinds selected from platinum group noble metals such as Pt, Rh, Pd, Ir, and Ru can be used, and Pt, Pd, and the like excellent in oxidation activity are preferable. The amount of the precious metal supported is preferably 0.1 to 5 g per liter of the honeycomb structure volume. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased.

  The oxidation catalyst is disposed on the exhaust gas upstream side of the honeycomb structure of the present invention. Although it can be arranged at a distance from the honeycomb structure such as on the upstream side of the turbocharger, it is desirable to arrange it adjacent to the honeycomb structure in order to suppress the temperature reduction of the exhaust gas. The composition ratio of the oxidation catalyst and the honeycomb structure is preferably in the range of oxidation catalyst: honeycomb structure = 3: 1 to 1: 1-3 by volume ratio.

  The outlet opening of the oxidation catalyst cell is preferably opposed to the inlet opening of the bag cell of the honeycomb structure. As a result, the effect of turbulence is further increased, and the PM collection efficiency is further improved.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

(Example 1)
FIG. 1 shows an exhaust gas purification apparatus of this embodiment. In this exhaust gas purifying apparatus, a honeycomb structure 1 having a catalyst layer (not shown) is disposed in a catalytic converter 2.

  Hereinafter, a method for manufacturing the honeycomb structure 1 will be described, and a detailed description of the configuration will be given.

A cordierite semi-wall flow honeycomb structure was prepared. The honeycomb structure 1 has a diameter of 130 mm, a length of 150 mm, a volume of about 2 liters, a cell partition wall 14 of 300 / inch ₂ (46.5 cells / cm ₂ ) and a thickness of 0.3 mm. . The cell partition 14 has a porosity of 65% and an average pore diameter of 25 μm. This honeycomb structure 1 includes straight cells 10 that are not clogged at both ends, and outflow side cells that are plugged with a clogged portion 12 at a position 5 mm downstream from the upstream end surface and are not clogged at the downstream end surface. 11 and the straight cells 10 and the downstream outflow cells 11 are alternately arranged. The straight cell 10 and the downstream outflow cell 11 are partitioned by a cell partition wall 14.

  Next, a mixed slurry in which powders of alumina, titania, zirconia, and ceria are dispersed in water is prepared, and the surface of the cell partition 14 of the honeycomb structure 1 and the surface of the pores inside the cell partition 14 are formed by a wash coat method. A coat layer was formed at 150 g / L. Thereafter, 2 g / L of Pt was supported and calcined by a water absorption method, and calcined at 500 ° C. to form a catalyst layer.

  The honeycomb structure 1 having the catalyst layer was disposed in the catalytic converter 2 to obtain an exhaust gas purification apparatus of this example.

(Example 2)
A honeycomb structure similar to that of Example 1 was used except that the plugging portion 12 was positioned 10 mm downstream from the upstream end face, and a catalyst layer was similarly formed.

  The honeycomb structure 1 having the catalyst layer was disposed in the catalytic converter 2 to obtain an exhaust gas purification apparatus of this example.

(Example 3)
A honeycomb structure similar to that of Example 1 was used except that the plugging portion 12 was positioned 15 mm downstream from the upstream end face, and a catalyst layer was similarly formed.

  The honeycomb structure 1 having the catalyst layer was disposed in the catalytic converter 2 to obtain an exhaust gas purification apparatus of this example.

(Comparative Example 1)
A catalyst layer was formed in the same manner using a honeycomb structure similar to that of Example 1 except that the plugging portion 12 was formed on the upstream end face.

  The honeycomb structure having the catalyst layer was disposed in the catalytic converter 2 to obtain an exhaust gas purification apparatus of this comparative example.

(Comparative Example 2)
A honeycomb structure similar to that of Example 1 was used except that the plugging portion 12 was positioned 20 mm downstream from the upstream end face, and a catalyst layer was similarly formed.

  The honeycomb structure having the catalyst layer was disposed in the catalytic converter 2 to obtain an exhaust gas purification apparatus of this comparative example.

<Test>
The exhaust gas purifying apparatuses of Examples and Comparative Examples were mounted on an exhaust system of a diesel engine with a displacement of 2 L, the diesel engine was driven at a rotational speed of 2900 rpm and an exhaust gas temperature of 300 ° C. for 1 hour, and the PM collection rate during that time was measured. The PM collection rate is indicated by an index with the PM collection rate of the exhaust gas purifying apparatus of Comparative Example 1 as 100. Further, for each honeycomb structure after the measurement, the inside of the bag cell 13 was visually observed to determine the degree of PM deposition. These results are shown in Table 1.

  From Table 1, in the comparative example 1 in which the depth of the plugging portion 12 is zero, the PM collection rate is low, and in the comparative example 2 in which the depth is 20 mm, the amount of PM deposited on the plugging portion 12 is large. However, according to the exhaust gas purification apparatus of each example, it is clear that the PM collection rate is high and the PM accumulation amount is small, and the plugging portion 12 needs to be formed on the downstream side from the upstream end surface, and the upstream side It is clear that it is particularly desirable to form 5-15 mm downstream from the side end face.

Example 4
FIG. 3 shows the exhaust gas purifying apparatus of this embodiment. In this exhaust gas purifying apparatus, an oxidation catalyst 3 and a honeycomb structure 1 on which a catalyst layer similar to that in Example 1 is formed are arranged in this order from the upstream side to the downstream side in the catalytic converter 2.

The oxidation catalyst 3 has a straight flow structure, a diameter of 130 mm, a length of 150 mm, a volume of about 2 liters, a cordierite honeycomb structure 30 with 400 / inch ² cells, and cell partition walls of the honeycomb structure 30 31 and a catalyst layer (not shown) formed on the surface. The catalyst layer is formed from alumina powder supporting Pt, and 150 g per liter of the honeycomb structure 30 is formed. The amount of Pt supported is 2 g per liter of honeycomb structure.

According to the exhaust gas purification apparatus of this embodiment, HC and CO in the exhaust gas are oxidized and purified by the oxidation catalyst 3, and SOF in PM is also oxidized and decomposed. As a result, the PM is atomized and flows into the straight cell 10 of the honeycomb structure 1, so that PM agglomeration is further suppressed coupled with the turbulent flow due to the stirring action in the bag cell 13, so that the PM collection efficiency is improved. Further improve. Moreover, since NO is oxidized to NO ₂ in the oxidation catalyst 3 and NO ₂ having high oxidation activity flows into the honeycomb structure 1, PM deposited in the bag cell 13 can be oxidized and burned, and the cell partition 14 is collected. Since the PM can be oxidized and burned, the PM trapping ability can be regenerated while suppressing the temperature rise of the honeycomb structure 1.

  The exhaust gas purification apparatus of the present invention can be used for an internal combustion engine such as a diesel engine from which exhaust gas containing PM is discharged.

It is sectional drawing which shows the structure of the exhaust gas purification apparatus of one Example of this invention. It is explanatory drawing which shows the effect | action of the exhaust gas purification apparatus of one Example of this invention. It is sectional drawing which shows the structure of the exhaust gas purification apparatus of the 4th Example of this invention.

Explanation of symbols

1: Honeycomb structure 2: Catalytic converter 3: Oxidation catalyst
10: Straight cell 11: Downstream outflow cell 12: Plugging part
13: Bag cell 14: Cell bulkhead

Claims (7)

A straight cell that is not clogged on the exhaust gas upstream side and the exhaust gas downstream side, a downstream outflow cell that is adjacent to the straight cell and that is clogged on the exhaust gas upstream side and that is not clogged on the exhaust gas downstream side, and the straight cell and the downstream outflow cell are partitioned And a honeycomb structure having a half wall flow structure having a porous cell partition wall having a large number of pores,
The plugging portion of the downstream outflow cell is formed on the downstream side of the upstream end surface of the honeycomb structure, and the honeycomb structure has a bag cell having an opening in the upstream end surface and having the plugging portion as a bottom. A featured exhaust gas purification device.   The exhaust gas purification device according to claim 1, wherein the plugging portion is formed 5 to 15 mm downstream from the upstream end face of the honeycomb structure.   The exhaust gas purifying apparatus according to claim 1 or 2, wherein a catalyst layer is formed by supporting a catalyst metal on an oxide carrier on a surface of the cell partition wall and a surface in the pores. The catalytic metal is an exhaust gas purifying apparatus according to claim 1 comprising a noble metal and _the NO _x storage material.   The exhaust gas purification apparatus according to any one of claims 1 to 4, wherein a porosity of the cell partition walls of the honeycomb structure is 60 to 80%.   The exhaust gas purification apparatus according to any one of claims 1 to 5, wherein an average pore diameter of pores of the cell partition walls of the honeycomb structure is 10 to 50 µm.   The exhaust gas purification apparatus according to any one of claims 1 to 6, wherein an oxidation catalyst is further disposed on the exhaust gas upstream side of the honeycomb structure.

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