JP2009228618A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent thermal damage of filter base material and improve fuel economy by reducing a fuel use quantity during a forcible regeneration process. <P>SOLUTION: A downstream side filter 5 having a high PM filtering function is disposed at a downstream of an upstream side filter 4 having low PM filtering function. Coarse and large PM particles including large quantity of soluble polymeric hydrocarbon (SOF) are collected with priority in the upstream side filter 4. Since SOF has lower ignition point than that of carbon, exhaust gas of which temperature is raised by reaction heat in the upstream side filter 4 flows into the downstream side filter 5. Consequently, PM burns even in the downstream side filter 5 at an early stage. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an exhaust gas purifying apparatus capable of efficiently collecting particulates (hereinafter referred to as PM) contained in diesel exhaust gas and the like and suppressing thermal damage of a filter during forced regeneration.

  As for gasoline engines, toxic components in exhaust gas are steadily decreasing due to strict regulations on exhaust gas and technological advances that can cope with it. On the other hand, for diesel engines, harmful components are emitted as PM (carbon fine particles, sulfur fine particles such as sulfate, soluble high molecular weight hydrocarbon fine particles (SOF), etc.), and therefore, more exhaust gas than gasoline engines. Purification is difficult.

  Therefore, a ceramic plug-type honeycomb body (diesel particulate filter (hereinafter referred to as DPF)) has been 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 wall that partitions the inflow side cell and the outflow side cell, and collects PM by filtering the exhaust gas through the pores of the cell partition wall.

  However, in DPF, exhaust pressure loss increases due to PM accumulation, so it is necessary to periodically remove and regenerate PM accumulated by some means. Therefore, conventionally, when exhaust pressure loss rises, DPF is regenerated by burning high-temperature exhaust gas and burning PM. For example, PM deposited by placing an oxidation catalyst upstream of the DPF, supplying hydrocarbons such as fuel into the exhaust gas, raising the exhaust gas temperature by the reaction heat in the oxidation catalyst, and supplying the hot exhaust gas to the DPF There are known methods for oxidizing the.

Also, for example, in Japanese Translation of PCT International Publication No. 2002-531762, PM deposited by placing an oxidation catalyst upstream of DPF, oxidizing NO into NO ₂ in the oxidation catalyst, and supplying NO ₂ with high oxidation activity to DPF. A method for oxidizing the is described.

  However, when the accumulated PM is oxidatively combusted and the DPF is forcibly regenerated, if the amount of PM deposited is large, acceleration combustion occurs and sometimes thermal runaway occurs, causing thermal damage to the center or downstream end of the DPF. May occur.

  Therefore, in recent years, for example, as described in Japanese Patent Publication No. 07-106290, a coating layer is formed on the surface of the cell partition wall of DPF from alumina or the like, and a catalytic metal such as platinum (Pt) is supported on the coating layer. Filter catalysts have been developed. According to this filter catalyst, the collected PM is oxidized and burned by the catalytic reaction of the catalytic metal, so that the filter catalyst can be regenerated continuously by burning simultaneously with the collection or continuously with the collection. . Since the catalytic reaction occurs at a relatively low temperature and can be burned while the amount collected is small, there is an advantage that the thermal stress acting on the filter catalyst is small and breakage is prevented.

  Japanese Patent Application Laid-Open No. 09-094434 describes a filter catalyst in which a coating layer supporting a catalyst metal is formed not only in the cell partition walls but also in the pores of the cell partition walls. By supporting the catalyst metal in the pores, the contact probability between the PM and the catalyst metal is increased, and the PM collected in the pores can be oxidized and burned.

  However, in the filter catalyst, PM is deposited in a low temperature range before the catalytic metal is activated. Even in a high temperature range, if a large amount of PM suddenly flows into the filter catalyst depending on the operating conditions, oxidation combustion by the catalyst metal may not catch up and PM may accumulate. When PM deposition progresses, PM that does not come into contact with the catalytic metal is present, and such PM is not oxidized and burned, and the deposition state is maintained, so that pressure loss increases.

  Therefore, the filter catalyst also needs to be subjected to a forced regeneration process for burning the deposited PM. However, when the amount of PM deposited is large, the base material may be thermally damaged due to thermal runaway.

In order to burn PM deposited on DPF, it is necessary to raise the temperature to about 600 ℃ or higher. In the method of heating the exhaust gas flowing into the DPF using the oxidation reaction by the oxidation catalyst arranged upstream, it is necessary to supply the fuel continuously in the exhaust gas for a long time in order to maintain a high temperature of about 600 ° C or higher. There is a problem that fuel consumption deteriorates.
Japanese Unexamined Patent Publication No. 09-094434 Special Table 2002-531762

  The present invention has been made in view of the above circumstances, and it is an object to be solved to prevent thermal damage to the filter base material and to reduce the amount of fuel used during forced regeneration processing to improve fuel efficiency.

A feature of the exhaust gas purification apparatus of the present invention that solves the above problems includes an upstream filter disposed in an exhaust gas passage through which exhaust gas containing PM flows, and a downstream filter disposed on the exhaust gas downstream side of the upstream filter. ,
The PM filtering function in the upstream filter is lower than the PM filtering function in the downstream filter.

  According to the exhaust gas purification apparatus of the present invention, the PM filtering function in the upstream filter is lower than the PM filtering function in the downstream filter. Accordingly, coarse PM particles are preferentially collected by the upstream filter, and PM particles that have passed through the upstream filter are collected by the downstream filter. Here, coarse PM particles are aggregates in which fine carbon particles are bonded by sticky SOF and are preferentially collected by the upstream filter because of their large particle size and stickiness. The Therefore, the PM collected by the upstream filter has a higher SOF content than the PM collected by the downstream filter.

  The spontaneous ignition temperature of carbon particles is about 550 ° C, but SOF spontaneously ignites at about 300 ° C. Therefore, when high-temperature exhaust gas is circulated through the exhaust gas purifying apparatus of the present invention in which PM is deposited as described above, SOF is first burned in the upstream filter, and carbon particles deposited on the upstream filter are burned by the reaction heat. Then, the exhaust gas whose temperature has been raised by the reaction heat in the upstream filter flows into the downstream filter. Therefore, PM combustion also occurs early in the downstream filter, and the upstream filter and the downstream filter can be efficiently and forcibly regenerated.

  That is, according to the exhaust gas purifying apparatus of the present invention, the time required for forced regeneration can be shortened, and the time for adding HC to the exhaust gas can be shortened, thereby improving fuel efficiency.

  Moreover, since two filters having different filtration functions are used, the total length (total length) of the filter can be shortened as compared with the case of collecting with one filter. Therefore, the apparatus can be miniaturized, and PM combustion propagation is stably maintained and the temperature inside the filter is made uniform, so that thermal runaway hardly occurs and thermal damage can be prevented.

  Furthermore, since the temperature inside the filter is made uniform, it becomes possible to increase the PM deposition limit amount per unit capacity of the filter. Therefore, the frequency of forced regeneration can be reduced, and fuel consumption is improved.

  In the exhaust gas purification apparatus of the present invention, an upstream filter and a downstream filter are arranged in series in this order in the exhaust gas flow direction in the exhaust gas flow path. The shape of the upstream filter and the downstream filter is not particularly limited as long as PM can be collected, and various filters such as a honeycomb shape and a foam shape can be used. Moreover, the material can also use the material which has heat resistance, such as ceramics and a metal.

  In the present invention, the PM filtering function in the upstream filter is configured to be lower than the PM filtering function in the downstream filter, and as a result, the PM collected in the upstream filter is collected in the downstream filter. SOF content is higher than PM.

  In order to form a difference in the filtration function between the upstream filter and the downstream filter as described above, for example, there is a method of making a difference in the porosity of the filter or a method of making a difference in the pore diameter of the filter. These methods are optimal when using a honeycomb-shaped ceramic filter generally used as a DPF.

  That is, a general honeycomb-shaped DPF includes an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, an inflow side cell, and an outflow side cell. And a porous cell partition wall having a plurality of pores. When such a DPF is used for the upstream filter and the downstream filter, the porosity of the cell partition wall in the upstream filter is preferably 50% or more. Moreover, the average pore diameter of the cell partition wall in the upstream filter may be 100 μm or more, and it is also preferable that both the porosity is 50% or more and the average pore diameter is 100 μm or more.

  By using a DPF having such a cell partition with pores in the upstream filter, the upstream filter preferentially collects coarse particle size PM that contains a large amount of SOF and aggregates it. A small amount of fine PM tends to slip through the upstream filter. By setting the porosity or the pore diameter of the cell partition of the downstream filter so that the fine PM that has passed through the upstream filter is collected by the downstream filter, the fine PM is removed by the downstream filter. It can be reliably collected.

  Therefore, it is preferable that the cell partition in the downstream filter has a porosity of 30 to 50% or an average pore diameter of 5 to 20 μm. When the porosity is 50% or more or the average pore diameter is 20 μm or more, it becomes difficult to collect fine PM, and the PM emission amount increases. Further, if the porosity is less than 30% or the average pore diameter is less than 5 μm, the increase in exhaust pressure loss is remarkably impractical.

  The capacity ratio of the upstream filter and the downstream filter is preferably in the range of X / Y = 1/2 to 1/1, where X is the upstream filter capacity and Y is the downstream filter capacity. If the capacity ratio is smaller than this range, the amount of PM accumulated in the downstream filter becomes too large and damage is likely to occur during forced regeneration. If the capacity ratio is larger than this range, the exhaust pressure loss in the downstream filter tends to increase and is forced. It is necessary to increase the frequency of the regeneration process, resulting in a reduction in fuel consumption.

  It is also preferable to use a metal filter as the upstream filter. A metal filter has a high thermal conductivity and a smaller heat capacity than a ceramic filter, so that it can be ignited early during forced regeneration and heat propagation is fast. Therefore, PM combustion in the downstream filter is also promoted, and the time required for forced regeneration can be further shortened. As such a metal filter, a filter in which corrugated metal thin plates and metal nonwoven fabrics are alternately laminated is known.

  For the upstream filter and the downstream filter, it is preferable to use a filter catalyst in which a catalyst coat layer is formed in the surface and pores of the cell partition walls. By using the filter catalyst, the accumulated PM can be oxidized and burned during use, and the time until forced regeneration can be extended, so that the fuel efficiency is further improved. If at least the upstream filter is a filter catalyst, the ignition temperature at the forced regeneration is lowered, so that the time required for the forced regeneration can be further shortened and the fuel consumption is improved.

The catalyst coating layer of the filter catalyst includes an oxidation catalyst in which noble metals such as Pt, Rh, and Pd are supported on a porous oxide such as alumina, zirconia, ceria, and titania, and an alkali metal, alkaline earth metal, etc. A NO _x storage reduction catalyst carrying a NO _x storage material can be used.

  During forced regeneration, the exhaust gas temperature flowing into the upstream filter is raised. As a method therefor, there is a method of heating the exhaust gas or the upstream filter with a heater or the like, but when the upstream filter is a filter catalyst, hydrocarbons such as light oil are supplied into the exhaust gas upstream of the upstream filter. It is desirable to provide HC supply means. Since the supplied HC is oxidized from a low temperature region by the catalytic reaction in the upstream filter catalyst, the exhaust gas can be quickly heated by the reaction heat, and the PM deposited in the downstream filter can be combusted.

  It is also preferable to dispose an oxidation catalyst upstream of the upstream filter and add HC such as fuel to the exhaust gas upstream of the oxidation catalyst. In this case, since the temperature of the exhaust gas is raised by the reaction heat in the oxidation catalyst, PM deposited on the upstream filter and the downstream filter can be burned without using the upstream filter as a filter catalyst.

  As the oxidation catalyst, a straight flow structure honeycomb base material in which a noble metal is supported on a porous oxide such as alumina is preferably used. As the noble metal to be supported, Pt or Pd having high HC oxidation activity is preferably used.

  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. A catalytic converter 2 is connected to an exhaust pipe 11 extending from the exhaust manifold 10 of the diesel engine 1. In the catalytic converter 2, an oxidation catalyst 3, an upstream filter catalyst 4, and a downstream filter catalyst 5 are housed in series in this order from the exhaust gas upstream side to the downstream side. An injector 6 for adding light oil to the exhaust gas is disposed upstream of the catalytic converter 2.

  The upstream filter catalyst 4 uses a honeycomb substrate having a cell partition wall porosity of 70% and an average pore diameter of 70 μm. The downstream filter catalyst 5 uses a honeycomb substrate having a cell partition wall porosity of 45% and an average pore diameter of 15 μm. Therefore, according to the exhaust gas purifying apparatus of the present embodiment, as shown in FIG. 2, coarse PM particles of about 100 μm or more aggregated containing a large amount of SOF are preferentially collected on the upstream filter catalyst 4, and the SOF Fine PM particles having a small content and passing through the upstream filter catalyst 4 are collected by the downstream filter catalyst 5.

  Hereafter, the manufacturing method of each catalyst is demonstrated and it replaces with the detailed description of a structure.

<Oxidation catalyst 3>
A honeycomb substrate made of cordierite and having a straight flow structure (400 cells / in ² , diameter 130 mm, 1 liter) was prepared. On the other hand, Pt / Al ₂ O ₃ powder in which Pt was previously supported on γ-Al ₂ O ₃ powder was prepared, mixed with alumina sol and ion-exchanged water, and milled to prepare a slurry. This slurry was wash coated onto a honeycomb substrate, dried and fired to form a catalyst coating layer of 150 g per liter of the honeycomb substrate. 2 g of Pt is supported per liter of honeycomb substrate.

<Upstream side filter catalyst 4>
An inflow side cell formed from cordierite and clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, and an inflow side cell and an outflow side cell A honeycomb substrate having a wall flow structure (200 cells / in ² , diameter 130 mm, 1 liter, cell partition porosity 70%) having porous cell partition walls having pores was prepared.

On the other hand, Pt / Al ₂ O ₃ powder in which Pt was previously supported on γ-Al ₂ O ₃ powder was prepared, mixed with alumina sol and ion-exchanged water, and milled to prepare a slurry. This slurry was filled into the inflow side cell of the honeycomb base material, sucked from the outflow side cell, washed, dried and fired to form a catalyst coating layer of 50 g per 1 L of the honeycomb base material. 2 g of Pt is supported per liter of honeycomb substrate.

<Downstream filter catalyst 5>
A honeycomb substrate similar to the upstream filter catalyst 4 is prepared except that the cell partition wall porosity is 45% and the average pore diameter is 15 μm. A catalyst coat layer of 50 g per liter of material was formed. 2 g of Pt is supported per liter of honeycomb substrate.

(Example 2)
As shown in FIG. 3, it is the same as Example 1 except that the oxidation catalyst 3 was not used.

(Comparative Example 1)
FIG. 4 shows the exhaust gas purifying apparatus of Comparative Example 1. In this exhaust gas purifying apparatus, only the filter catalyst 7 is disposed on the downstream side of the oxidation catalyst 3 as in the first embodiment. The filter catalyst 7 is the same as the upstream filter catalyst 4 of Example 1 except that the cell partition wall porosity is 50% and the volume is 2 liters, and 50 g per liter of honeycomb substrate. The catalyst coat layer is formed, and 2 g of Pt is supported per 1 L of the honeycomb substrate.

(Comparative Example 2)
As shown in FIG. 5, it is the same as Comparative Example 1 except that only the filter catalyst 7 is used without using the oxidation catalyst 3.

(Comparative Example 3)
As shown in FIG. 6, a downstream filter catalyst 5 similar to that of the first embodiment is disposed on the downstream side of the oxidation catalyst 3, and an upstream filter catalyst 4 is disposed on the downstream side of the downstream filter catalyst 5. No. 3 exhaust gas purification device.

<Test Example 1>
For each of the exhaust gas purification devices described above, 10 g of PM in total was deposited while the diesel engine 1 was in steady operation. Meanwhile, the exhaust gas pressure on the upstream side and the downstream side of the catalytic converter 2 was measured to detect the exhaust pressure loss.

  In Example 1 and Example 2, it was confirmed that 5 g of PM was deposited on the upstream filter catalyst 4 and the downstream filter catalyst 5 respectively. In each of Examples and Comparative Examples 1 and 2, no abnormal increase in exhaust pressure loss during operation was observed. However, in Comparative Example 3, an abnormal increase in exhaust pressure loss was observed before 10 g of PM was accumulated, and most of the PM was deposited on the downstream filter catalyst 5 disposed on the upstream side, and the upstream disposed on the downstream side. Only a small amount of PM was deposited on the side filter catalyst 4.

  After depositing PM as described above, the diesel engine 1 was operated for 5 minutes under the condition that the exhaust gas temperature flowing into the catalytic converter 2 was 300 ° C. Immediately thereafter, a forced regeneration treatment was performed in which the gas oil was added to the exhaust gas from the injector 6 at an addition amount of 2 g / min and the operation was continued for another 5 minutes.

  After this test, the weight of each catalyst was measured and the PM removal rate was calculated. Separately from this test, the test was performed in the same manner except that the operation time while adding light oil was 10 minutes, and the PM removal rate was calculated in the same manner. The results are shown in Table 1. In Table 1, the exhaust pressure loss results are indicated by ○ (no abnormality) and × (abnormal).

  From Table 1, of course, the PM removal rate is higher as the light oil addition time is longer. And Example 1 and Example 2 have higher PM removal rate than Comparative Example 1 and Comparative Example 2, and PM removal rate equivalent to 10 minutes of Comparative Example 1 and Comparative Example 2 even when the light oil addition time is 5 minutes. It can be seen that it is expressed. This is clearly the effect of using two filter bases with different porosity. Moreover, it can be seen from comparison between Example 2 and Comparative Example 2 that this effect is exhibited regardless of the presence or absence of an oxidation catalyst.

  Further, in Comparative Example 3, although the PM removal rate is higher than that of Comparative Example 2, the exhaust pressure loss is high, so that it is not practical as an exhaust gas purification device.

<Test Example 2>
For the exhaust gas purification apparatuses of Examples 1 and 2 and Comparative Examples 1 to 3, PM was deposited so that the total deposition amount was 10 g, 12 g, 14 g, 16 g, 18 g, and 20 g. In Example 1 and Example 2, it was confirmed that half amount of PM was deposited on each of the upstream filter catalyst 4 and the downstream filter catalyst 5.

  Thereafter, the diesel engine 1 was operated for 5 minutes under the condition that the exhaust gas temperature flowing into the catalytic converter 2 was 300 ° C. Immediately thereafter, a forced regeneration treatment was performed in which the gas oil was added to the exhaust gas from the injector 6 at an addition amount of 2 g / min and the operation was continued for another 5 minutes.

  After this test, each filter catalyst was taken out and visually inspected for damage and cracks. The results are shown in Table 2.

  From Table 2, damages and cracks occur at a PM deposition amount of 12 g in each comparative example, whereas damages and cracks do not occur even at a PM deposition amount of 16 g in each example. That is, according to the exhaust gas purifying apparatus of each embodiment, the PM deposition limit value increases compared to the conventional case even with the same catalyst capacity as the conventional one, and the time until forced regeneration can be extended. Further, if the forced regeneration process is performed when the PM accumulation amount becomes the same as that in the comparative example, the forced regeneration time can be shortened in the embodiment. Therefore, the amount of light oil used during forced regeneration can be reduced, and fuel efficiency is improved.

  Furthermore, if the forced regeneration is performed for the same amount of time as in the comparative example at the same PM accumulation amount as in the comparative example, the capacity of the filter catalyst can be reduced, that is, the length can be shortened. Therefore, the apparatus can be miniaturized.

It is sectional drawing which shows the structure of the exhaust gas purification apparatus which concerns on one Example of this invention. It is explanatory drawing which shows typically the effect | action of the exhaust gas purification apparatus which concerns on one Example of this invention. It is sectional drawing which shows the structure of the exhaust gas purification apparatus which concerns on the 2nd Example of this invention. It is sectional drawing which shows the structure of the exhaust gas purification apparatus which concerns on the comparative example 1. It is sectional drawing which shows the structure of the exhaust gas purification apparatus which concerns on the comparative example 2. It is sectional drawing which shows the structure of the exhaust gas purification apparatus which concerns on the comparative example 3.

Explanation of symbols

2: catalytic converter 3: oxidation catalyst 4: upstream filter catalyst 5: downstream filter catalyst 6: injector (HC supply means)

Claims (6)

An upstream filter disposed in an exhaust gas flow path through which exhaust gas containing particulates flows, and a downstream filter disposed on the exhaust gas downstream side of the upstream filter,
An exhaust gas purifying apparatus, wherein a particulate filtering function in the upstream filter is lower than a particulate filtering function in the downstream filter.   The upstream filter partitions an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell clogged on the exhaust gas upstream side adjacent to the inflow side cell, and the inflow side cell and the outflow side cell. The exhaust gas purifying apparatus according to claim 1, further comprising a porous cell partition wall having a large number of pores, wherein the porosity of the cell partition wall is 50% or more.   The upstream filter partitions an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell clogged on the exhaust gas upstream side adjacent to the inflow side cell, and the inflow side cell and the outflow side cell. The exhaust gas purifying apparatus according to claim 1 or 2, further comprising a porous cell partition wall having a large number of pores, wherein an average pore diameter of the cell partition wall is 100 µm or more.   The exhaust gas purification device according to any one of claims 1 to 3, wherein at least the upstream filter is a filter catalyst having a catalyst coat layer formed thereon.   The exhaust gas purification apparatus according to claim 4, further comprising HC supply means for supplying hydrocarbons into the exhaust gas flowing into the upstream filter on the upstream side of the exhaust gas of the upstream filter.   The exhaust gas purification according to any one of claims 1 to 4, further comprising an oxidation catalyst on the upstream side of the exhaust gas of the upstream filter, and HC supply means for supplying hydrocarbons into the exhaust gas flowing into the oxidation catalyst. apparatus.

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