JP2006037768A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device for achieving a high NOx reduction rate from a relatively low temperature region. <P>SOLUTION: The exhaust emission control device comprises: a pair of NOx storage reduction catalysts 5 disposed in parallel in the middle of an exhaust pipe 4; an exhaust selector valve 8 (first exhaust selector means) distributing exhaust gas 3 into one of the NOx storage reduction catalysts 5; a bypass pipe 13 disposed upstream the valve 8 to guide part of the exhaust gas 3 to an inlet side of each NOx storage reduction catalyst 5; a reforming catalyst 14 disposed in the middle of the bypass pipe 13 and decomposing diesel oil 20 (fuel) into H<SB>2</SB>and CO; and fuel adding means 21 adding the diesel oil 20 as reducer to the inlet side of the reforming catalyst 14; and an open/close valve 25 (second exhaust switching means) distributing the exhaust gas 3, which has been guided by the bypass pipe 13 through the reforming catalyst, into one of the NOx storage reduction catalysts 5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust emission control device.

Conventionally, exhaust purification is carried out with an exhaust purification catalyst installed in the middle of the exhaust pipe. As this type of exhaust purification catalyst, NOx in exhaust gas is oxidized when the exhaust air-fuel ratio is lean. Thus, a NOx occlusion reduction catalyst having the property of temporarily storing in the form of nitrate and decomposing and releasing NOx through the intervention of unburned HC, CO, etc. when the O ₂ concentration in the exhaust gas decreases is reduced and purified. Are known.

  As this type of NOx occlusion reduction catalyst, platinum / barium / alumina catalyst, platinum / potassium / alumina catalyst, and the like have already been known.

In the NOx occlusion reduction catalyst, when the occlusion amount of NOx increases and reaches the saturation amount, no more NOx can be occluded, and therefore, O _{2 of} the exhaust gas flowing into the NOx occlusion reduction catalyst periodically. It is necessary to decompose and release NOx by reducing the concentration.

For example, when used in a gasoline engine, the operating air-fuel ratio of the engine is reduced (the engine is operated at a rich air-fuel ratio), thereby reducing the O ₂ concentration in the exhaust gas and unburned HC in the exhaust gas. It is possible to promote the decomposition and release of NOx by increasing reducing components such as CO and CO. However, when the NOx storage reduction catalyst is used as an exhaust purification device of a diesel engine, it is difficult to operate the engine at a rich air-fuel ratio. is there.

For this reason, by adding fuel (HC) to the exhaust gas upstream of the NOx storage reduction catalyst, the added fuel is used as a reducing agent to react with O ₂ on the NOx storage reduction catalyst. ₂ It is necessary to reduce the concentration (see, for example, Patent Document 1).
JP 2000-356127 A

However, in the method of adding fuel upstream of the NOx storage reduction catalyst in this way, a part of HC generated by evaporation of the added fuel reacts with O ₂ in the exhaust gas on the surface of the NOx storage reduction catalyst. (Combustion), and NOx decomposition and release is started after the O ₂ concentration in the atmosphere around the NOx storage reduction catalyst becomes almost zero, so that HC is O ₂ on the surface of the NOx storage reduction catalyst. NOx is efficiently removed from the NOx occlusion reduction catalyst under operating conditions where the combustion temperature (about 220-250 ° C) required for reaction (combustion) with NOx cannot be obtained (for example, slow driving in cities with heavy traffic). There was a problem that the NOx occlusion reduction catalyst could not be decomposed and released, and the regeneration of the NOx occlusion reduction catalyst did not proceed efficiently, so that the recovery rate of the NOx occlusion site in the catalyst volume was reduced and the occlusion capacity was lowered.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an exhaust emission control device capable of obtaining a high NOx reduction rate from a relatively low temperature range.

The present invention provides a pair of NOx occlusion reduction catalysts installed in parallel in the middle of an exhaust pipe, first exhaust gas switching means for distributing exhaust gas to any one of the NOx occlusion reduction catalysts, and the first exhaust gas A bypass pipe that extracts a part of the exhaust gas upstream from the switching means and leads to the inlet side of each NOx storage reduction catalyst, and a reforming catalyst that is provided in the middle of the bypass pipe and decomposes the fuel into H ₂ and CO And a fuel addition means for adding fuel as a reducing agent to the inlet side of the reforming catalyst, and a second for distributing exhaust gas guided through the reforming catalyst by the bypass pipe to any one of the NOx storage reduction catalysts. And an exhaust gas switching device.

Thus, the flow of the exhaust gas from the engine is distributed to one NOx storage reduction catalyst by the first exhaust switching means, and the exhaust gas guided through the reforming catalyst by the bypass pipe is sent to the other by the second exhaust switching means. When the fuel is added to the reforming catalyst as a reducing agent by the fuel addition means, the fuel reacts with the O ₂ coexisting with the fuel in the reforming catalyst to raise the ambient temperature. After O ₂ is consumed, HC in the fuel is decomposed into H ₂ and CO and introduced into the other NOx occlusion reduction catalyst, so that the concentration of O ₂ in the atmosphere becomes almost zero from the introduction stage and NOx is reduced. The decomposition and release are immediately started, and NOx is efficiently reduced to N ₂ from a temperature lower than the combustion temperature of the conventional HC by H ₂ and CO having high reactivity on the surface of the NO x storage reduction catalyst as it is.

  At this time, the addition of fuel by the fuel addition means is only performed to the exhaust gas having a relatively small flow rate guided by the bypass pipe, so that the excess air ratio in the exhaust gas is effectively reduced even when the addition amount is smaller than the conventional amount. Therefore, the NOx storage reduction catalyst can be efficiently regenerated by adding the minimum amount of fuel.

  If the other NOx occlusion reduction catalyst is regenerated while the NOx occlusion reduction catalyst is performing the NOx occlusion reduction in this way, one of the NOx occlusion reduction catalysts is always kept in a usable state. Thus, it is possible to alternately regenerate the pair of NOx storage reduction catalysts one by one while reducing NOx.

Furthermore, in the present invention, it is preferable that a bypass pipe is provided so as to bypass the turbine of the turbocharger. In this way, the fuel added as a reducing agent is converted into H ₂ and CO by the reforming catalyst. Even if the temperature of the exhaust gas rises due to the heat generated during the decomposition, the heated exhaust gas bypasses the turbine and is introduced into one NOx storage reduction catalyst, thus preventing abnormal turbine rotation in advance. In this case, the exhaust gas having a relatively high temperature that has just been exhausted from the engine before working in the turbine is extracted into the bypass pipe. Thus, the heat of the exhaust gas is effectively utilized to promote the fuel decomposition reaction at the reforming catalyst to the maximum extent.

  According to the exhaust emission control device of the present invention described above, various excellent effects as described below can be obtained.

(I) According to the invention described in claim 1 of the present invention, the fuel added as the reducing agent is decomposed into H ₂ and CO by the reforming catalyst, and these relatively reactive H ₂ and CO Since each NOx storage reduction catalyst can be efficiently regenerated alternately from the low temperature range with the minimum amount of fuel added, the exhaust temperature is low at low loads, such as slow driving in cities with heavy traffic. Even under operating conditions in which the driving state is likely to continue, NOx contained in the exhaust gas discharged outside the vehicle can be reduced more effectively than before, and an exhaust purification device using a NOx storage reduction catalyst is practically used. Can greatly improve the performance.

(II) According to the invention described in claim 2 of the present invention, the exhaust gas is heated by heat generated when the fuel added as the reducing agent is decomposed into H ₂ and CO by the reforming catalyst. However, since the heated exhaust gas can be bypassed and introduced into one NOx occlusion reduction catalyst, abnormal rotation of the turbine can be avoided and torque fluctuation can be reliably prevented, Moreover, by effectively utilizing the heat of the high-temperature exhaust gas just exhausted from the engine, the decomposition reaction of the fuel with the reforming catalyst can be accelerated to the maximum, so that H ₂ and CO are efficiently removed from the fuel. Can be generated.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 and FIG. 2 show an example of an embodiment for carrying out the present invention. As shown in FIG. 1, in the exhaust purification apparatus of this embodiment, the exhaust gas discharged from the diesel engine 1 through the exhaust manifold 2 is shown. A pair of NOx occlusion reduction catalysts 5 having a flow-through type honeycomb structure are held in parallel by a casing 6 in the middle of an exhaust pipe 4 through which 3 flows, and the first half of the internal space of the casing 6 Is divided into two by a partition plate 7 so that the exhaust gas 3 can flow individually to each NOx storage reduction catalyst 5.

  An exhaust switching valve 8 (first exhaust switching means) that distributes the flow of the exhaust gas 3 to any one of the NOx storage reduction catalysts 5 is rotatably provided at the inlet portion of the casing 6. The exhaust gas switching valve 8 is driven by an actuator 9 disposed outside the casing 6 via a lever and a link mechanism.

  Further, in the example shown here, a particulate filter 10 that integrally carries an oxidation catalyst is disposed at the subsequent stage of each NOx storage reduction catalyst 5, and the exhaust gas 3 that has passed through each NOx storage reduction catalyst 5. And the particulates in the exhaust gas 3 are collected.

Further, a part of the exhaust gas 3 from the outlet of the exhaust manifold 2 upstream of the turbine 12 of the turbocharger 11 with respect to the inlet side of each NOx storage reduction catalyst 5 to which the flow of the exhaust gas 3 is distributed by the exhaust gas switching valve 8. The bypass pipe 13 that extracts and guides the gas is branched and connected to the reforming catalyst 14 that decomposes the light oil into H ₂ and CO at an appropriate position upstream from the branch position of the bypass pipe 13. Is equipped.

  Here, as this type of reforming catalyst 14, for example, a catalyst in which Pd, Pt, Rh or the like is supported as an active metal using an oxide such as alumina or silica or a composite oxide such as zeolite as a support may be used. .

  Further, an injection nozzle 15 is provided on the inlet side of the reforming catalyst 14 in the bypass pipe 13, and a light oil supply pipe 17 connects between the injection nozzle 15 and a light oil tank 16 provided at a required place. Light oil 20 (fuel as a reducing agent) in the light oil tank 16 is supplied to the reforming catalyst 14 through the injection nozzle 15 by driving a supply pump 18 provided in the middle of the light oil supply pipe 17 and opening the light oil injection valve 19. The fuel addition means 21 is constituted by the injection nozzle 15, the light oil tank 16, the light oil supply pipe 17, the supply pump 18, and the light oil injection valve 19.

  And in the control apparatus 24 which comprises an engine control computer (ECU: Electronic Control Unit), the rotation speed signal 22a from the rotation sensor 22 which detects the engine rotation speed of the diesel engine 1, and the accelerator sensor 23 (the depression angle of the accelerator pedal) The appropriate addition amount and injection timing of the light oil 20 are determined based on the current operation state determined from the load signal 23a from the sensor to be detected), and the addition of the light oil 20 at the appropriate addition amount and injection timing is executed. As described above, the drive command signal 18a to the supply pump 18 and the valve opening command signal 19a to the light oil injection valve 19 are output from the control device 24.

  In addition, in each branched flow path of the bypass pipe 13, an open / close valve 25 (second valve) is provided so that the exhaust gas 3 guided through the reforming catalyst 14 can be distributed to any one of the NOx storage reduction catalysts 5. Exhaust gas switching means) are provided, and these open / close valves 25 are interlocked with an exhaust gas switching valve 8 that is switched by a switching command signal 9a directed to the actuator 9 from the control device 24. The opening / closing command signal 25a controls the opening / closing of the exhaust gas 3 from 1 so that the exhaust gas 3 having passed through the reforming catalyst 14 can be guided to the NOx occlusion / reduction catalyst 5 on the side where the exhaust gas 3 is blocked.

  In FIG. 1, reference numeral 26 denotes a flow rate adjusting valve provided on the upstream side of the bypass pipe 13, 27 denotes an intake pipe, and 28 denotes an intercooler.

Thus, the flow of the exhaust gas 3 from the diesel engine 1 is distributed to one NOx occlusion reduction catalyst 5 by the exhaust switching valve 8, and the exhaust gas 3 guided through the reforming catalyst 14 by the bypass pipe 13 is opened by the opening / closing valve 25. distributed to the other of the NOx storage reduction catalyst 5, when added to the inlet side of the reforming catalyst 14 light oil 20 as a reducing agent by the fuel addition means 21, wherein the light oil 20 coexists with gas oil 20 at the reforming catalyst 14 O ₂ Since the HC in the light oil 20 is decomposed into H ₂ and CO and introduced into the other NOx occlusion reduction catalyst 5 after the ambient temperature is raised by reaction with O ₂ and O ₂ is consumed, O ₂ concentration is substantially zero and is an exploded release of NOx is started immediately, than the combustion temperature of conventional HC by highly reactive H ₂ and CO as it is on the surface of the NOx storage reduction catalyst 5 NOx from the stomach temperature is reduced processed efficiently N _2.

  At this time, the addition of the light oil 20 by the fuel addition means 21 is only performed on the exhaust gas 3 having a relatively small flow rate guided by the bypass pipe 13, and therefore, the excess air ratio in the exhaust gas 3 even with a smaller addition amount than in the past. Is effectively reduced, and the NOx occlusion reduction catalyst 5 can be efficiently regenerated by adding the minimum necessary amount of light oil 20.

  If the other NOx occlusion reduction catalyst 5 is regenerated while the NOx occlusion reduction catalyst 5 is performing NOx occlusion in this way, one of the NOx occlusion reduction catalysts 5 can always be used. While continuously reducing NOx as a state, the pair of NOx storage reduction catalysts 5 can be alternately regenerated one by one.

Further, particularly in this embodiment, since the bypass pipe 13 is provided so as to bypass the turbine 12 of the turbocharger 11, the light oil 20 added as a reducing agent is decomposed into H ₂ and CO by the reforming catalyst 14. Even if the temperature of the exhaust gas 3 rises due to the heat generated during the operation, the raised exhaust gas 3 bypasses the turbine 12 and is introduced into one NOx storage reduction catalyst 5, so that the turbine 12 rotates abnormally. In this case, the exhaust gas 3 having a relatively high temperature that has just been exhausted from the diesel engine 1 before working in the turbine 12 is prevented. Is extracted to the bypass pipe 13, the heat of the exhaust gas 3 is effectively utilized, and the decomposition reaction of the light oil 20 in the reforming catalyst 14 is promoted to the maximum extent.

Therefore, according to the above embodiment, the light oil 20 added as a reducing agent is decomposed into H ₂ and CO by the reforming catalyst 14, and the minimum necessary from a relatively low temperature range by these highly reactive H ₂ and CO. Since the NOx occlusion reduction catalyst 5 can be efficiently regenerated with the addition of the limited amount of light oil 20, an operation state in which the exhaust temperature is low with a low load, such as slow driving in a city with a lot of traffic, is achieved. Even under operating conditions that are easily continued, NOx contained in the exhaust gas 3 exhausted outside the vehicle can be reduced more effectively than before, and the practicality of the exhaust emission control device using the NOx storage reduction catalyst 5 can be reduced. Can be greatly improved.

In fact, according to the results of experiments conducted by the present inventor, as shown in the graph of FIG. 2, the HC produced from the light oil 20 is directly produced on the NOx occlusion reduction catalyst 5 and produced from the light oil 20. Comparison of case B, in which HC was decomposed into H ₂ and CO and reacted on the NOx storage reduction catalyst 5, shows that case B can obtain a higher NOx reduction rate from a lower temperature range than case A. Was confirmed. The vertical axis in the graph of FIG. 2 indicates the NOx reduction rate, and the horizontal axis indicates the exhaust temperature.

Further, even if the exhaust gas 3 is heated by heat generated when the light oil 20 added as a reducing agent is decomposed into H ₂ and CO by the reforming catalyst 14, the heated exhaust gas 3 is converted into the turbine 12. Can be bypassed and introduced into one NOx storage and reduction catalyst 5, abnormal rotation of the turbine 12 can be avoided in advance, and torque fluctuation can be reliably prevented, and the engine has just been discharged from the engine. Since the decomposition reaction of the light oil 20 in the reforming catalyst 14 can be promoted to the maximum by effectively using the heat of the high-temperature exhaust gas 3, it is possible to efficiently generate H ₂ and CO from the light oil 20. it can.

  The exhaust emission control device of the present invention is not limited to the above-described embodiment. For fuel added as a reducing agent, in addition to using light oil that is a general diesel engine fuel, kerosene or the like. Of course, different types of fuels may be used, and various modifications can be made without departing from the scope of the present invention.

It is the schematic which shows an example of the form which implements this invention. It is a graph which shows the relationship between NOx reduction rate and catalyst temperature with a comparative example.

Explanation of symbols

1 Diesel engine (engine)
3 Exhaust gas 4 Exhaust pipe 5 NOx storage reduction catalyst 8 Exhaust gas switching valve (first exhaust gas switching means)
11 Turbocharger 12 Turbine 13 Bypass pipe 14 Reforming catalyst 20 Light oil (fuel)
21 Fuel addition means 24 Control device 25 Open / close valve (second exhaust gas switching means)

Claims (2)

A pair of NOx occlusion reduction catalysts installed in parallel in the middle of the exhaust pipe, a first exhaust gas switching means for distributing the exhaust gas to any one of the NOx occlusion reduction catalysts, and an upstream side of the first exhaust gas switching means A bypass pipe that extracts a part of the exhaust gas on the side and leads to the inlet side of each NOx storage reduction catalyst, a reforming catalyst that is provided in the middle of the bypass pipe and decomposes fuel into H ₂ and CO, and the modified A fuel addition means for adding fuel as a reducing agent to the inlet side of the catalyst, and a second exhaust gas switching means for distributing the exhaust gas guided through the reforming catalyst by the bypass pipe to any one of the NOx storage reduction catalysts And an exhaust emission control device.   The exhaust emission control device according to claim 1, wherein a bypass pipe is provided so as to bypass the turbine of the turbocharger.

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