JP4969865B2 - Exhaust gas purification device - Google Patents

Description

  The present invention relates to an exhaust gas purification device that collects particulate matter in exhaust gas.

  In recent years, regulations on the emission of particulate matter (particulate molecule, hereinafter referred to as “PM”) discharged from diesel engines together with exhaust gas have been strengthened. A technique for collecting the PM with a filter called a diesel particulate filter (hereinafter referred to as “DPF”) and reducing the amount of PM discharged to the outside has been developed. The exhaust gas purification apparatus using the DPF is installed in the engine exhaust pipe like other exhaust gas purification apparatuses, and purifies and discharges exhaust gas generated in the engine.

  This DPF for collecting PM is clogged with the collection of PM, and the exhaust resistance increases with the accumulation of the collected PM. Therefore, it is necessary to remove the collected PM from the DPF. .

  As one of the methods for removing PM, a method is known in which PM collected before the exhaust resistance increases due to clogging is combusted as appropriate, and a DPF regeneration process is performed. In this case, since a high exhaust temperature at which PM self-combusts cannot be obtained in normal diesel engine operation, an oxidation catalyst carrier carrying an oxidation catalyst is disposed upstream of the DPF, and the exhaust gas temperature is low. A method for burning PM is disclosed in, for example, Patent Document 1.

The oxidation catalyst carrier carries platinum or the like as an oxidation catalyst. This oxidation catalyst comprises hydrocarbons (hereinafter referred to as “HC”) and carbon monoxide (hereinafter referred to as “CO”) in exhaust gas passing through an oxidation catalyst carrier as CO ₂ (carbon dioxide) and H ₂ O (water). ) And NO (nitrogen monoxide) in the nitrogen oxide (hereinafter referred to as “NOx”) in the exhaust gas is efficiently changed to NO ₂ (nitrogen dioxide) by the reaction of (2NO + O ₂ → NO ₂ ). And plays a role of increasing the ratio of NO _{2 in} the exhaust gas.

When the exhaust gas that has passed through the oxidation catalyst carrier reaches the DPF (hereinafter also simply referred to as “filter”) on the downstream side, NO ₂ in the exhaust gas oxidizes PM deposited in the filter by its strong oxidizing power. PM (the main component is carbon) is oxidized to CO _{2 and} removed. This reaction occurs when the exhaust gas temperature is in the range of 250 to 450 ° C. In a diesel engine that is operating at a normal high speed, the exhaust gas temperature is within the above temperature range, and therefore PM is continuously burned.

  On the other hand, during low load operation such as low-speed traveling, downhill traveling, and idling, the exhaust gas temperature does not reach 250 ° C., so PM combustion due to the above reaction is not performed, and PM deposition progresses and exhaust resistance increases. It will be. In that case, in the fuel injection of the engine, the fuel injection control such as the delay operation (retard) operation of the main injection and the post injection (post injection) is performed, or the fuel is provided to the exhaust gas upstream from the oxidation catalyst carrier. The unburned hydrocarbon (HC) contained in the exhaust gas is burned in the oxidation catalyst carrier, the exhaust gas temperature is raised to 250 ° C. or higher by the reaction heat, and PM combustion is performed in the filter. .

JP 2002-276442 A

  When the operation of the diesel engine shifts from high speed operation to low speed or idling operation, the exhaust gas temperature falls below 250 ° C., PM combustion stops, and PM deposition proceeds. When a certain amount of PM deposition has progressed, processing such as post-injection is started, the exhaust gas temperature is raised, and PM combustion is resumed. Alternatively, when the operation shifts to high-speed operation, the exhaust gas temperature rises and PM combustion is resumed. When PM combustion resumes, the temperature inside the filter further rises due to the combustion heat of PM.

  When the exhaust gas temperature rises from a low temperature to a high temperature, if the exhaust gas temperature on the filter entrance side is observed, the exhaust gas temperature that passes through the central portion in the radial direction of the filter rises first, and the exhaust gas that passes through the outer periphery. There is a tendency for the temperature rise to be delayed. As a result, the radial center portion of the filter reaches the PM combustion temperature earlier than the outer peripheral portion. Therefore, PM in the vicinity of the central portion of the filter in the radial direction starts to burn, and as a result, the radial central portion and the outer peripheral portion The filter temperature difference is further increased. As a result, a temperature gradient is generated inside the filter, and thermal stress is generated. When the temperature gradient becomes extremely large, the thermal stress becomes excessive, and eventually the DPF is destroyed.

  The present invention relates to a filter that collects particulate matter in exhaust gas, and an exhaust gas purification device in which an oxidation catalyst carrier is disposed upstream of the filter. When the temperature of the exhaust gas rises in front of the filter, An object of the present invention is to provide an exhaust gas purifying device that makes the distribution uniform and allows PM combustion to start almost simultaneously in the radial direction of the filter.

That is, the gist of the present invention is as follows.
(1) In an exhaust gas purifying apparatus in which a filter 3 for collecting particulate matter in exhaust gas and an oxidation catalyst carrier 2 are arranged upstream thereof, the oxidation catalyst carrier 2 is at least 2 from the center in the radial direction to the outer periphery. It consists of various types of base materials, and the base material located in the center is 5/3 to 10/3 times thicker than the base material located in the outer periphery to increase the heat capacity. An exhaust gas purification apparatus comprising a flat foil made of foil and a corrugated foil wound around a metal foil, and an oxidation catalyst supported on the surface of the metal foil .

The exhaust gas purifying apparatus of the present invention has a filter that collects particulate matter in exhaust gas, and an oxidation catalyst carrier disposed upstream of the filter, and a base material that is located in the radial center of the oxidation catalyst carrier. heat capacity than the substrate located at the outer peripheral portion and the larger, when the exhaust gas temperature rise in the filter front to equalize the radial distribution of the exhaust gas temperature, the combustion of PM is initiated almost simultaneously in the radial direction of the filter. Thereby, the thermal stress which generate | occur | produces in a filter can be reduced and the breakage generation | occurrence | production of a filter can be prevented.

  In the exhaust gas purification apparatus of the present invention, as shown in FIG. 1, the diameter of the exhaust gas path is changed from the diameter of the exhaust pipe 6 to the diameter of the oxidation catalyst carrier 2 by the inlet cone 4 connected to the inlet exhaust pipe 6. After that, the oxidation catalyst carrier 2 is arranged, and further, a filter 3 having the same diameter as that of the oxidation catalyst carrier 2 is arranged on the downstream side, and the outlet cone 5 on the downstream side reduces the diameter of the exhaust gas path. The diameter is reduced to the diameter of the exhaust pipe 7 on the outlet side.

  The filter 3 is a DPF that collects PM, and a ceramic monolith honeycomb type wall flow type filter, a fiber type filter in which ceramic or metal is made into a fiber, or the like is used. The wall flow type filter is formed using an unglazed honeycomb monolith called cordierite, and PM is collected by this filter.

As the oxidizing catalyst carrier 2, a metal charge of body is used, a large number of cells provided exhaust gas passes through the oxidation catalyst such as platinum is supported on the wall surface of the cell. Main barrel carrier will form by winding after having superposing the corrugated foil were corrugating processed into flat foil and the metal foil made of a metal foil.

  The exhaust gas introduced into the exhaust gas purification device from the upstream exhaust pipe 6 reaches the front surface of the oxidation catalyst carrier 2 by expanding the exhaust gas passage in the inlet cone 4. The flow rate of the exhaust gas at the front surface of the oxidation catalyst carrier is not uniform in the radial direction of the oxidation catalyst carrier 2, the flow rate of the exhaust gas reaching the central portion in the radial direction is fast, and the exhaust gas becomes closer to the outer peripheral portion in the radial direction. It has a velocity distribution that the flow velocity of the is slow.

  When the oxidation catalyst carrier 2 is at a low temperature, the exhaust gas passing through the oxidation catalyst carrier 2 generates heat due to the catalytic action during the passage and the gas temperature rises, while the low temperature oxidation catalyst carrier 2 is deprived of heat by the temperature. The rise is prevented, or conversely the gas temperature falls. At this time, as the flow rate of the exhaust gas passing through the oxidation catalyst carrier 2 increases, the amount of heat taken by the oxidation catalyst carrier 2 decreases, and the gas temperature when passing through the oxidation catalyst carrier 2 and reaching the front surface of the filter 3 is reduced. Becomes higher. As described above, the exhaust gas flow velocity at the central portion in the radial direction of the oxidation catalyst carrier 2 and the filter 3 is faster than that at the outer peripheral portion. Therefore, the exhaust gas temperature at the front surface of the filter is higher at the central portion in the radial direction and lower at the outer peripheral portion. The result is as follows.

  In the exhaust gas purifying apparatus in which the particulate matter in the exhaust gas is collected and the oxidation catalyst carrier 2 is arranged on the upstream side, when the exhaust gas temperature rises from a low temperature to a high temperature, the exhaust gas on the filter inlet side When the temperature is observed, the exhaust gas temperature that passes through the central portion in the radial direction of the filter 3 rises in advance and the increase in the exhaust gas temperature that passes through the outer periphery tends to be delayed. It was found that the phenomenon occurred according to As a result of the existence of such a radial temperature distribution of the exhaust gas, the radial center portion of the filter 3 reaches the PM combustion temperature earlier than the outer peripheral portion. Starts combustion, and as a result, the difference in the filter temperature between the central portion in the radial direction and the outer peripheral portion becomes larger, resulting in a temperature gradient in the filter and thermal stress. When the temperature gradient becomes extremely large, the thermal stress becomes excessive, and eventually the DPF is destroyed.

  In the present invention, firstly, in order to eliminate the above-mentioned radial temperature non-uniformity of the exhaust gas at the front surface of the filter, the heat capacity of the oxidation catalyst carrier 2 is changed in the radial direction so that the heat capacity of the central part is more than the heat capacity of the outer peripheral part. Enlarge. The greater the heat capacity of the oxidation catalyst carrier 2, the more heat the exhaust gas has is taken away by the oxidation catalyst carrier 2, so the temperature rise of the exhaust gas is reduced. By making the heat capacity of the central part larger than that of the outer peripheral part, the temperature rise allowance of the exhaust gas passing through the central part can be made lower, reducing the radial temperature change of the exhaust gas when it reaches the front of the filter. The temperature can be made uniform.

  Here, the heat capacity of the oxidation catalyst carrier 2 means the heat capacity per aeration unit cross-sectional area of the oxidation catalyst carrier 2. Therefore, when increasing the heat capacity by increasing the metal foil thickness of the oxidation catalyst carrier made of a metal carrier, when increasing the heat capacity by increasing the axial length of the oxidation catalyst carrier, and by making the cell finer, the cell density To increase the heat capacity.

  Secondly, the present invention eliminates the above-mentioned radial temperature non-uniformity of the exhaust gas at the front surface of the filter by changing the ventilation resistance of the oxidation catalyst carrier 2 in the radial direction so that the ventilation resistance at the central portion is Make it larger than ventilation resistance. The greater the ventilation resistance of the oxidation catalyst carrier 2, the more difficult it is for the exhaust gas to pass through, so the exhaust gas flow rate can be slowed. In the conventional exhaust gas purifying device, the exhaust gas flow velocity when passing through the oxidation catalyst carrier 2 is fast at the radial center and slow at the outer peripheral portion, which is the cause of uneven temperature rise. In the present invention, as the ventilation resistance of the oxidation catalyst carrier 2, the ventilation resistance in the central portion is made larger than the ventilation resistance in the outer peripheral portion, thereby making it difficult for the cells in the central portion in the radial direction to pass through and the flow rate of the exhaust gas passing through the central portion. Accordingly, the exhaust gas flow velocity at the outer peripheral portion is increased accordingly, and as a result, the radial flow velocity distribution of the exhaust gas is made uniform. As a result, it is possible to reduce the radial temperature change of the exhaust gas when it reaches the front surface of the filter 3 and to make the temperature uniform.

  Next, specific means of the present invention will be described.

  When using a metal carrier as the oxidation catalyst carrier 2, the thickness of the metal foil constituting the metal carrier is changed in the radial direction of the oxidation catalyst carrier, and the thickness of the metal foil used in the central portion in the radial direction is used in the outer peripheral portion. By making the thickness thicker than this, it is possible to obtain an oxidation catalyst carrier having a larger heat capacity in the base material located in the central part than in the base material located in the outer peripheral part. Increasing the thickness of the metal foil also has the effect of reducing the exhaust gas passage cross-sectional area of the cell even at the same cell density, so that the ventilation resistance at the center is greater than the ventilation resistance at the outer periphery.

  The oxidation catalyst carrier 2 has a structure having a large number of cells through which exhaust gas passes. Here, if the cell density of the base material located in the central portion in the radial direction is denser than the cell density of the base material located in the outer peripheral portion, the higher the cell density, the larger the surface area per unit area of the carrier. As a result, exhaust gas ventilation resistance increases. Therefore, by making the cell density in the radial center part denser than the cell density in the outer peripheral part, the exhaust gas ventilation resistance in the radial center part becomes larger than the ventilation resistance in the outer peripheral part. The flow rate is reduced, and the exhaust gas flow rate at the outer periphery is accordingly increased. For this reason, it is possible to suppress the tendency that the exhaust gas flow velocity at the central portion is faster than the outer peripheral portion, and to make the flow velocity uniform. Further, the denser the cell density, the larger the heat capacity. In a metal carrier formed by winding a flat foil made of metal foil and a corrugated foil on the metal foil, by reducing one or both of the wave pitch and amplitude of the corrugated foil, the cell density is made dense. Is possible.

  By changing the axial length of the oxidation catalyst carrier 2 from the central portion in the radial direction to the outer peripheral portion, the axial length in the central portion is longer than the axial length in the outer peripheral portion. The heat capacity of the material is larger than that of the base material located at the outer peripheral portion, and at the same time, the ventilation resistance of the central portion can be made larger than the ventilation resistance of the outer peripheral portion as the ventilation resistance of the oxidation catalyst carrier. Both actions produce a synergistic effect, and the flow rate of the exhaust gas passing through the oxidation catalyst carrier can be made uniform in the radial direction, and at the same time, the temperature of the exhaust gas reaching the front surface of the filter can be made uniform in the radial direction.

In the present invention, the hand-stage to change the axial length to the outer portion of the foil thickness of the metal foil constituting the metal carrier means for varying the radial direction of the oxidation catalyst carrier, the oxidation catalyst carrier 2 from the radial center it a may be used alone, or tight means, than the cell density of the outer peripheral portion of the cell density of the radial center portion for changing the foil thickness of the metal foil constituting a metal carrier in the radial direction of the oxidation catalyst carrier For the oxidation catalyst carrier 2, a plurality of means for changing the axial length from the central part in the radial direction to the outer peripheral part may be used at the same time.

  When installing an exhaust gas purification device in the middle of an exhaust pipe having a diameter of 60 mm, a DPF in which a mat having a diameter of 190 mm and a length of 200 mm is wound outside as a filter 3 for collecting particulate matter in the exhaust gas has a thickness of 5 mm. And an oxidation catalyst carrier 2 composed of a metal carrier was disposed upstream of the filter. A cordierite monolith honeycomb wall flow type filter was used for the DPF. As the oxidation catalyst carrier 2, a flat foil made of stainless steel foil and a corrugated foil made of stainless steel foil are overlapped and wound into a honeycomb body, and platinum is used as a catalyst on the surface of the stainless steel foil. The supported one was used.

  As the shape of the oxidation catalyst carrier 2, the shape shown in FIGS. 2A to 2D was prepared. 2A and 2B, the outer diameter of the substrate 1 (the inner diameter of the substrate 2) is 100 mm, and the outer diameter of the substrate 2 is 200 mm. 2C and 2D, the outer diameter of the substrate 1 (the inner diameter of the substrate 2) is 60 mm, the outer diameter of the substrate 2 (the inner diameter of the substrate 3) is 140 mm, and the outer diameter of the substrate 3 is 200 mm. It is.

Table 1 shows the foil thickness, cell density, and axial length of the stainless steel foil used for each substrate. The cell density “cpsi” means cells / in ² .

  As a comparative example, an oxidation catalyst carrier having a single base material (base material 1) was used. Table 1 shows the specifications.

Regarding the temperature of the front surface of the DPF, the temperature at the central portion in the radial direction and a depth of 10 mm from the front surface was measured, and at the same time, the temperature at a depth of 20 mm from the outer peripheral portion and a depth of 10 mm from the front surface. The initial temperature of the oxidation catalyst carrier and DPF is 20 ° C., CO: 0.1 mol%, C ₃ H ₆ : 200 mol ppm, NO: 200 mol ppm, O ₂ : 16 mol%, CO ₂ : 4 mol%, H ₂ O: 4 mol%, The remaining portion of the N ₂ exhaust gas was supplied to the exhaust gas purification device at 0.076 Nm ³ / sec and the temperature rising rate was 4.3 ° C./sec, and the temperatures of the two locations of the DPF 100 seconds after the start of supply were measured. The temperature measurement results are shown in Table 1 as “central part” and “outer peripheral part”.

  In the comparative example, the same base material is used from the central part to the outer peripheral part of the oxidation catalyst carrier. As a result, the temperature of the central portion on the front surface of the DPF was 443 ° C., whereas the temperature of the outer peripheral portion was as low as 361 ° C.

Reference Example No. 1 and 2 roughen the cell density of the base material 2 located on the outer peripheral side, and at the same time, reduce the heat capacity of the base material on the outer peripheral part of the oxidation catalyst carrier to be smaller than that of the central part and simultaneously increase the amount of gas flowing to the outer peripheral part. It was. As a result, in all cases, the temperature of the outer peripheral portion increased as compared with the comparative example.

  Invention Example No. In Nos. 3 and 4, the heat capacity was increased by setting the foil thickness to 50 μm for the base material 1 at the center of the oxidation catalyst carrier. As a result, the temperature of the central portion of the DPF was lowered, and the temperature difference between the central portion and the outer peripheral portion could be reduced. No. In No. 4, since the heat capacity was reduced by reducing the foil thickness of the base material 2 at the outer peripheral portion to 20 μm, the temperature at the outer peripheral portion could be increased.

In Reference Examples 5 and 6, the length of the base material 2 on the outer peripheral portion of the oxidation catalyst carrier was shortened to reduce the heat capacity. As a result, the temperature of the outer periphery of the DPF could be increased.

Invention Example 7 and Reference Example 8 had a triple structure of base materials 1 to 3 as shown in FIG. No. For No. 7, the foil thickness for each substrate was changed to give a difference in heat capacity. No. For No. 8, the cell density for each substrate was changed to give a difference in heat capacity and gas passage. In any case, the temperature of the outer periphery of the DPF could be increased as compared with the comparative example.

It is sectional drawing which shows the exhaust-gas purification apparatus which has arrange | positioned the filter and the oxidation catalyst support | carrier. It is a figure which shows the oxidation catalyst support | carrier shape of each Example.

Explanation of symbols

2 Oxidation catalyst carrier 3 Filter (DPF)
4 Inlet cone 5 Outlet cone 6 Exhaust pipe (inlet)
7 Exhaust pipe (exit side)
8 exhaust gas

Claims (1)

In an exhaust gas purifying apparatus in which a particulate matter in an exhaust gas is collected and an oxidation catalyst carrier is disposed upstream of the filter, the oxidation catalyst carrier is composed of at least two kinds of base materials from the radial center to the outer periphery. The base material located in the center is 5/3 to 10/3 times thicker than the base material located in the outer periphery so that the heat capacity is large, and the oxidation catalyst carrier is a flat foil made of metal foil. An exhaust gas purifying apparatus comprising: a metal foil and a corrugated metal foil wound around a metal foil, and an oxidation catalyst supported on the surface of the metal foil.

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