JP2009002225A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that NOx reduction rate is deteriorated when addition of reducing agent is started when a NOx storage reduction catalyst is regenerated with shutting off flow of exhaust gas. <P>SOLUTION: An exhaust emission control device comprises a pair of NOx storage reduction catalysts 4, 4 installed in the middle of an exhaust pipe 3 in parallel, exhaust change over valves 6, 7 distributing exhaust gas 2 to either of NOx storage reduction catalysts 4, 4, a reducing agent adding means (fuel reforming device 9) individually adding reducing agent (reformed gas 10) to each inlet side of the NOx storage reduction catalysts 4, 4, and a valve open close control means (control device 13) inhibiting a rapid increase of desorbing NOx by variably controlling open close speed of the exhaust change over valves 6, 7 according to operation states. <P>COPYRIGHT: (C)2009,JPO&INPIT

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.

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, when using the NOx storage reduction catalyst as an exhaust purification device for a diesel engine, fuel is added to the exhaust gas upstream of the NOx storage reduction catalyst, and this added fuel is used as a reducing agent on the NOx storage reduction catalyst. the O ₂ concentration in the exhaust gas by reacting with O ₂ is required to be reduced.

  However, a large amount of fuel needs to be added to keep the atmosphere around the NOx storage and reduction catalyst in an oxygen-deficient state by adding fuel to the exhaust gas that continuously flows away, resulting in a significant deterioration in fuel consumption. It will end up.

  Therefore, it has been studied to regenerate the NOx occlusion reduction catalyst one by one by arranging a pair of NOx occlusion reduction catalysts in parallel and alternately distributing the exhaust gas by the exhaust gas switching valve (see, for example, Patent Document 1). .

That is, in this way, by adding fuel to the inlet side of the NOx occlusion reduction catalyst that shuts off the flow of exhaust gas, the O ₂ concentration in the atmosphere is effectively reduced, and the NOx occlusion reduction catalyst is made efficient. It is possible to reproduce well.
JP 2002-97940 A

However, in the type in which the flow of exhaust gas is blocked and the fuel is added as in Patent Document 1 above, the O ₂ concentration in the atmosphere decreases immediately after the start of the fuel addition, and the HC rich (reductant rich) ), The amount of NOx stored is desorbed all at once and the amount of NOx released increases rapidly. As a result, the NOx storage reduction catalyst cannot be completely reduced and purified, and part of the desorbed NOx leaks, resulting in a reduction in NOx. There was a problem that it deteriorated temporarily.

  The present invention has been made in view of the above circumstances, and solves the problem that the NOx reduction rate deteriorates at the start of addition of a reducing agent when the exhaust gas flow is shut off to regenerate the NOx storage reduction catalyst. The purpose is that.

  The present invention provides a pair of NOx occlusion reduction catalysts installed in parallel in the middle of an exhaust pipe, an exhaust gas switching valve that distributes exhaust gas to any one of the NOx occlusion reduction catalysts, and each of the NOx occlusion reduction catalysts. A reducing agent adding means for individually adding a reducing agent to the inlet side of the exhaust gas, and a valve opening / closing control means for variably controlling the opening / closing speed of the exhaust gas switching valve according to the operating state to suppress a sudden increase in desorption NOx. The present invention relates to an exhaust emission control device characterized by the above.

  Thus, in this way, the flow of the exhaust gas flowing through the exhaust pipe is distributed to one NOx storage reduction catalyst by the exhaust switching valve, and the reducing agent is added to the inlet side of the other NOx storage reduction catalyst for regeneration. When starting, it is possible to variably control the opening / closing speed of the exhaust gas switching valve by the valve opening / closing control means, and to narrow down the flow rate of the exhaust gas over an optimal operation time according to the operating situation.

That is, immediately after the start of the addition of the reducing agent, the exhaust gas flow rate is gently throttled so that the O ₂ concentration in the atmosphere does not decrease and the reducing agent becomes suddenly rich, and the stored NOx is gradually reduced from the NOx storage reduction catalyst. Since it can be desorbed and efficiently reduced and purified, a situation in which a part of the desorbed NOx leaks without being completely reduced and purified on the surface of the NOx storage and reduction catalyst is prevented.

  Further, the present invention provides a NOx occlusion reduction catalyst installed in the middle of an exhaust pipe, a bypass passage that bypasses the NOx occlusion reduction catalyst and flows exhaust gas, and any of the bypass passage and the NOx occlusion reduction catalyst On the other hand, an exhaust switching valve for distributing exhaust gas, a reducing agent adding means for adding a reducing agent to the inlet side of the NOx storage reduction catalyst, and desorption by variably controlling the opening / closing speed of the exhaust switching valve according to the operating conditions The present invention also relates to an exhaust emission control device comprising valve opening / closing control means for suppressing a rapid increase in NOx.

  Thus, in this way, when the exhaust gas flow flowing through the exhaust pipe is distributed to the bypass flow path by the exhaust switching valve, and the reductant is added to the inlet side of the NOx storage reduction catalyst, the regeneration is started. The opening / closing speed of the exhaust gas switching valve can be variably controlled by the valve opening / closing control means, and the flow rate of the exhaust gas can be narrowed down over an optimum operation time according to the operating situation.

That is, immediately after the start of the addition of the reducing agent, the exhaust gas flow rate is gently throttled so that the O ₂ concentration in the atmosphere does not decrease and the reducing agent becomes suddenly rich, and the stored NOx is gradually reduced from the NOx storage reduction catalyst. Since it can be desorbed and efficiently reduced and purified, a situation in which a part of the desorbed NOx leaks without being completely reduced and purified on the surface of the NOx storage and reduction catalyst is prevented.

  Furthermore, in the present invention, it is preferable that the reducing agent adding means is composed of a fuel reforming device that decomposes and supplies fuel into reformed gas having high reactivity. It is possible to regenerate the NOx storage reduction catalyst from a lower temperature condition than when adding NO as it is.

  In addition, in this case, since the reactivity of the reformed gas is high, the rapid increase of desorbed NOx appears more conspicuously. Therefore, the opening / closing speed of the exhaust gas switching valve can be variably controlled according to the operating conditions. It can be said that the effectiveness of suppressing the rapid increase of desorbed NOx is high.

  Further, when concretely implementing the present invention, for example, the valve opening / closing control means is configured to read and control the opening / closing speed of the exhaust gas switching valve from the two-dimensional map of the catalyst bed temperature of the NOx storage reduction catalyst and the engine speed. What is necessary is just to comprise.

  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 first and second aspects of the present invention, when regenerating the NOx occlusion reduction catalyst by shutting off the flow of exhaust gas, the oxygen in the atmosphere is immediately after the start of the addition of the reducing agent. _{(2) Since the} exhaust gas flow rate is slowly throttled so that the concentration does not drop and the reductant becomes rich suddenly, the stored NOx can be desorbed from the NOx storage reduction catalyst step by step, so that it can be efficiently reduced and purified. In addition, it is possible to prevent a situation in which a part of the desorbed NOx leaks without being reduced and purified on the surface of the NOx occlusion reduction catalyst, and solves the problem that the NOx reduction rate deteriorates at the start of the addition of the reducing agent. be able to.

  (II) According to the invention described in claim 3 of the present invention, the NOx occlusion reduction catalyst can be regenerated from a lower temperature condition than when the fuel is directly added as a reducing agent. The rapid increase of desorbed NOx that is likely to occur due to high reactivity can be effectively suppressed.

  (III) According to the invention described in claim 4 of the present invention, the NOx occlusion that can be grasped by existing sensors by adopting a two-dimensional map of the catalyst bed temperature of the NOx occlusion reduction catalyst and the engine speed. Since the opening / closing speed of the exhaust gas switching valve can be variably controlled using the catalyst bed temperature of the reduction catalyst and the engine speed as an index, the present invention can be implemented without causing a significant increase in cost.

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

  FIG. 1 shows an example of an embodiment for carrying out the present invention. In the exhaust purification apparatus of this embodiment, a pair of NOx occlusions are provided in the middle of an exhaust pipe 3 through which exhaust gas 2 discharged from a diesel engine 1 flows. A reduction catalyst 4 is provided in parallel, and the exhaust pipe 3 is branched into two branch flow paths 3A and 3B so that the exhaust gas 2 can be appropriately distributed to the NOx storage reduction catalysts 4 and 4 respectively. And connected to each of the NOx occlusion reduction catalysts 4 and 4 and joined again at the downstream side thereof to be guided to the particulate filter 5. Exhaust gas switching valves 6 and 7 are respectively provided so that the exhaust gas 2 flowing through the exhaust pipe 3 can be switched alternately to the branch flow paths 3A and 3B.

  Further, reducing agent addition nozzles 8 and 8 for adding a reducing agent into the exhaust gas 2 are arranged on the inlet side of the NOx storage reduction catalysts 4 and 4, respectively. , 8, the reformed gas 10 from the fuel reformer 9 constituting the reducing agent addition means is individually supplied as a reducing agent.

Here, the fuel reformer 9 does not supply fuel (light oil) as a reducing agent as it is, but reforms the fuel with a high reactivity such as H ₂ or CO using a reforming catalyst or a plasma device. The gas 10 is decomposed and supplied. When the reformed gas 10 is supplied to the reducing agent addition nozzles 8 and 8, a part of the exhaust gas 2 is used as a carrier gas and sent out. It is possible to configure so that only the reformed gas 10 is sent out as it is.

  Further, NOx sensors 11 and 12 for detecting the NOx concentration in the exhaust gas 2 are provided in the exhaust pipes 3 before and after the branch flow paths 3A and 3B, respectively. These detection signals 11a and 12a are inputted to a control device 13 constituting an engine control computer (ECU: Electronic Control Unit).

  On the other hand, in the control device 13, based on the detection signals 11a and 12a from the NOx sensors 11 and 12, how much the NOx value decreases by passing through the NOx occlusion reduction catalyst 4 on the side where the exhaust gas 2 is distributed. The amount of NOx occluded in the NOx occlusion reduction catalyst 4 is estimated by accumulating the decrease.

  When the control device 13 determines that the estimated value of the stored amount of NOx exceeds a predetermined threshold value, the control device 13 confirms various conditions and then directs the exhaust switch valves 6 and 7 and the fuel reformer 9 to each other. Control signals 6a, 7a, 9a are output, the flow of the exhaust gas 2 to the NOx storage reduction catalyst 4 to be regenerated is interrupted, the exhaust gas 2 is distributed to the NOx storage reduction catalyst 4 on the opposite side, and the exhaust gas 2 The regeneration control for adding the reformed gas 10 to the inlet side of the NOx occlusion reduction catalyst 4 to be regenerated, which is the one that cuts off the flow, is executed.

  At this time, the control device 13 does not simply control the opening / closing operation of the exhaust gas switching valves 6, 7 at a uniform opening / closing speed, but variably controls the opening / closing speed of the exhaust gas switching valves 6, 7 according to the operating condition. More specifically, as shown in FIG. 2, as shown in FIG. 2, the exhaust gas switching valve 6 is determined from a two-dimensional map of the catalyst bed temperature of the NOx storage reduction catalyst 4 and the engine speed. , 7 is read out, and the opening / closing operation of the exhaust gas switching valves 6, 7 is executed during the set time.

  Here, the catalyst bed temperature of each NOx storage reduction catalyst 4, 4 is detected by the temperature sensor 14 using the temperature of the exhaust gas 2 on the outlet side of each NOx storage reduction catalyst 4, 4 as a substitute value, and is detected as a detection signal 14 a. The engine speed is detected by a rotation sensor 15 attached to the diesel engine 1 and is input to the control device 13 as a detection signal 15a. .

  Note that when the NOx occlusion reduction catalyst 4 is regenerated by shutting off the flow of the exhaust gas 2, the amount of NOx released during the regeneration is greatly affected by the difference in the opening and closing speeds of the exhaust switching valves 6 and 7. Is the knowledge that the present inventors have independently found after intensive research. In fact, according to the test results by the present inventors, the set time required for the opening / closing operation of the exhaust switching valves 6 and 7 is about 200 msec in general. In this case, a rapid increase in the amount of NOx released (NOx concentration) as shown in the graph of FIG. 3 was confirmed, but when the set time was increased to 2000 msec, the amount of NOx released as shown in the graph of FIG. It has been confirmed that the rapid increase in (NOx concentration) is remarkably suppressed, and the NOx reduction rate at that time has also deteriorated to about 85% in the case of FIG. In this case, it is confirmed that the improvement is about 92%.

In addition, when the open / close speed of the exhaust gas switching valves 6 and 7 is variably controlled according to the operating conditions, the NOx occlusion reduction employs a two-dimensional map of the catalyst bed temperature of the NOx occlusion reduction catalyst 4 and the engine speed. the reaction at the time of reproduction in the catalyst 4 is greatly influenced by the catalyst bed temperature (catalytic activity) and the amount of O ₂ in the atmosphere (also increase or decrease the amount of O ₂ according to the exhaust gas flow to increase or decrease the engine rotational speed), these conditions This is because the optimum opening / closing speed of the exhaust gas switching valves 6 and 7 for suppressing the rapid increase in the amount of NOx released due to the change of the engine is different, but the two-dimensional map of the catalyst bed temperature and the engine speed of the NOx storage reduction catalyst 4 If this is adopted, it is possible to optimize the opening and closing speed of the exhaust gas switching valves 6 and 7.

  However, since the estimated value of the storage amount of NOx is already grasped in the control device 13, this is used as a parameter for further optimization, and the NOx value is compared with the set time read from the two-dimensional map. It is also possible to add further correction to the set time by appropriately multiplying a correction coefficient according to the estimated value of the occlusion amount.

  Thus, in the control device 13, based on the detection signals 11a and 12a from the NOx sensors 11 and 12, for example, the estimated value of the stored amount of NOx in the NOx storage and reduction catalyst 4 on the branch flow path 3A side is a predetermined threshold value. If it is determined that the value exceeds the control range, the control signals 6a, 7a and 9a are output to the exhaust gas changeover valves 6 and 7 and the fuel reformer 9 after confirming the various conditions, the exhaust gas changeover valve 6 is closed and the exhaust gas changeover valve is 7 opens, the flow of the exhaust gas 2 flowing through the exhaust pipe 3 is distributed to the NOx storage reduction catalyst 4 on the branch flow path 3A side, and the fuel reforming is performed on the inlet side of the NOx storage reduction catalyst 4 on the branch flow path 3B side. The reformed gas 10 from the apparatus 9 is added through the reducing agent addition nozzle 8 and regeneration is started.

  At this time, a set time required for opening / closing the exhaust gas switching valves 6 and 7 is read from the two-dimensional map (see FIG. 2) of the catalyst bed temperature of the NOx storage reduction catalyst 4 and the engine speed in the control device 13. As a result of the opening and closing operation of the exhaust gas switching valves 6 and 7 being executed in the set time, the opening and closing speed of the exhaust gas switching valves 6 and 7 is variably controlled, and the flow rate of the exhaust gas 2 on the branch flow path 3B side is optimal according to the operating situation It will be narrowed down over the operation time.

That is, immediately after the addition of the reformed gas 10 is started, the flow rate of the exhaust gas 2 is slowly throttled so that the O ₂ concentration in the atmosphere does not decrease and the reducing agent becomes suddenly rich, and the NOx occlusion reduction catalyst 4 As a result, it is possible to desorb the NOx stepwise and efficiently reduce and purify it. Therefore, there is a situation in which a part of the desorbed NOx leaks without being completely reduced and purified on the surface of the NOx occlusion reduction catalyst 4. To be prevented.

Therefore, according to the above embodiment, when the NOx storage reduction catalyst 4 is regenerated by blocking the flow of the exhaust gas 2, the O ₂ concentration in the atmosphere decreases immediately after the addition of the reformed gas 10 is started. NOx occlusion reduction is possible because the exhaust gas 2 is slowly throttled so that it does not suddenly become rich in reducing agent, and the NOx occlusion reduction catalyst 4 is gradually desorbed from the NOx occlusion reduction catalyst 4 for efficient reduction and purification. A problem that a part of the desorbed NOx leaks without being reduced and purified on the surface of the catalyst 4 can be prevented in advance, and the NOx reduction rate deteriorates at the start of the addition of the reformed gas 10 as a reducing agent. Can be solved.

  In particular, in the present embodiment, the reformed gas 10 having high reactivity is used as the reducing agent, so that the NOx storage reduction catalyst 4 can be regenerated from a lower temperature condition than when the fuel is added as it is. In addition, it is possible to effectively suppress the rapid increase of desorbed NOx that is likely to occur due to the high reactivity of the reformed gas 10.

  Furthermore, a two-dimensional map of the catalyst bed temperature of NOx occlusion reduction catalyst 4 and engine speed is adopted, and the catalyst bed temperature of NOx occlusion reduction catalyst 4 and engine speed that can be grasped by existing sensors are used as indices. Since the opening / closing speed of the exhaust gas switching valves 6 and 7 can be variably controlled, it can be carried out without significantly increasing the cost of existing equipment.

  FIG. 5 shows another embodiment of the present invention. In this embodiment, one pair of NOx occlusion reduction catalysts 4 and 4 is provided in parallel in FIG. The NOx storage reduction catalyst 4 is provided only in the branch flow path 3A, the NOx storage reduction catalyst 4 in the other branch flow path 3B is eliminated, and the exhaust gas 2 flows through the branch flow path 3B by bypassing the NOx storage reduction catalyst 4. This is used as a bypass flow path.

Thus, even in this case, the flow of the exhaust gas 2 flowing through the exhaust pipe 3 is distributed to the branch flow path 3B side forming the bypass flow path by the exhaust switching valves 6 and 7, and the NOx occlusion reduction catalyst 4 enters. When the reformed gas 10 is added to the side and the regeneration is started, the control device 13 variably controls the opening / closing speed of the exhaust gas switching valves 6 and 7 so that the flow rate of the exhaust gas 2 is set to the optimum operating time according to the operating conditions. If the exhaust gas 2 is narrowed down, the flow rate of the exhaust gas 2 is slowly reduced so that the O ₂ concentration in the atmosphere decreases immediately after the start of the addition of the reformed gas 10 and does not suddenly become rich in the reducing agent. The NOx occluded from the NOx 4 can be desorbed stepwise and efficiently reduced and purified. Therefore, the NOx occluded reduction catalyst 4 cannot be completely reduced and purified, and a part of the desorbed NOx leaks. Is prevented It will be.

  The exhaust emission control device of the present invention is not limited to the above-described embodiment. The exhaust gas switching valve may be configured as a three-way valve at a branch point of the exhaust pipe, and the particulate filter is NOx occlusion reduction. It may be arranged upstream of the catalyst, and the reducing agent addition means may not necessarily employ a fuel reformer, and may be a system in which a reducing agent such as fuel is added as it is. Of course, 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 diagram which shows an example of the two-dimensional map used with the control apparatus of FIG. It is a graph which shows the NOx discharge | release amount in the general opening-and-closing speed of an exhaust gas switching valve. It is a graph which shows NOx discharge | release amount at the time of variably controlling opening and closing speed and optimizing. It is the schematic which shows another form example of this invention.

Explanation of symbols

2 Exhaust gas 3 Exhaust pipe 3A Branch channel 3B Branch channel (bypass channel)
4 NOx occlusion reduction catalyst 6 Exhaust gas switching valve 7 Exhaust gas switching valve 9 Fuel reformer (reducing agent addition means)
10 Reformed gas (reducing agent)
13 Control device (valve opening / closing control means)
14 Temperature sensor 15 Rotation sensor

Claims (4)

  A pair of NOx occlusion reduction catalysts installed in parallel in the middle of the exhaust pipe, an exhaust switching valve for distributing exhaust gas to one of the NOx occlusion reduction catalysts, and an inlet side of each NOx occlusion reduction catalyst A reducing agent adding means for individually adding a reducing agent, and a valve opening / closing control means for variably controlling the opening / closing speed of the exhaust gas switching valve according to an operating state to suppress a sudden increase in desorption NOx. Exhaust purification device.   NOx occlusion reduction catalyst installed in the middle of the exhaust pipe, a bypass passage for flowing exhaust gas bypassing the NOx occlusion reduction catalyst, and exhaust gas is supplied to either the bypass passage or the NOx occlusion reduction catalyst An exhaust switching valve that distributes, a reducing agent addition means that adds a reducing agent to the inlet side of the NOx storage reduction catalyst, and an open / close speed of the exhaust switching valve are variably controlled according to operating conditions to suppress a sudden increase in desorbed NOx. An exhaust purification device comprising a valve opening / closing control means.   The exhaust emission control device according to claim 1 or 2, wherein the reducing agent addition means comprises a fuel reforming device that decomposes and supplies fuel into reformed gas having high reactivity.   The valve opening / closing control means is configured to read and control the opening / closing speed of the exhaust gas switching valve from a two-dimensional map of the catalyst bed temperature of the NOx storage reduction catalyst and the engine speed. The exhaust emission control device according to 3.

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