JP2010265753A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device capable of maintaining high NOx purifying performance by suppressing an excessive temperature rise of an NOx reduction catalyst such as an NOx occlusion reduction catalyst even though an operating state with a high exhaust temperature continues. <P>SOLUTION: An exhaust emission control device equipped with an NOx occlusion reduction catalyst 5 (NOx reduction catalyst) at some midpoint in an exhaust pipe 4, includes a cooler 14 capable of cooling exhaust gas 3 through heat exchange with a refrigerant, a bypass flow path 13 for introducing the exhaust gas 3 to the NOx occlusion reduction catalyst 5 by detouring the exhaust gas 3 from some midpoint of the exhaust pipe 4 through the cooler 14, a temperature sensor 20 for detecting exhaust temperature at an inlet of the NOx occlusion reduction catalyst 5, and a flow path switching means 19 for switching the flow of exhaust gas 3 to the bypass flow path 13 when a detected temperature with the temperature sensor 20 is a prescribed value or higher. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device.

  Particulate matter (particulate matter) discharged from a diesel engine is mainly composed of soot made of carbonaceous matter and SOF content (Soluble Organic Fraction) made of high-boiling hydrocarbon components. The composition contains a small amount of sulfate (mist-like sulfuric acid component). As a measure to reduce this type of particulates, a particulate filter is installed in the middle of the exhaust pipe through which the exhaust gas flows. It has been done conventionally.

  This type of particulate filter has a porous honeycomb structure made of ceramics such as cordierite, and the inlets of the flow paths partitioned in a lattice pattern are alternately sealed, and the inlets are not sealed. The outlet of the passage is sealed, and only the exhaust gas that has permeated through the porous thin wall defining each flow passage is discharged downstream, while the particulates in the exhaust gas are porous. It is collected on the inner surface of the thin wall.

  However, when purifying exhaust gas from a diesel engine, it is not sufficient to remove particulates in the exhaust gas as described above, and it is also necessary to remove NOx (nitrogen oxide) contained in the exhaust gas. In addition, it has been proposed to simultaneously reduce particulates and NOx by using a particulate filter and a NOx occlusion reduction catalyst in combination (for example, see Patent Document 1).

Here, the NOx occlusion reduction catalyst means that when the exhaust air-fuel ratio is lean, NOx in the exhaust gas is oxidized and temporarily stored in the form of nitrate, and unburned when the O ₂ concentration in the exhaust gas is reduced. It has the property of decomposing and releasing NOx through the intervention of hydrocarbons and CO to reduce and purify it. Generally, alkaline metals such as Li, Na and K, and alkaline earth metals such as Ba are treated with NOx. As a storage material, a material in which a porous material such as silica is supported together with a noble metal material such as Pt is known.

In such a NOx occlusion reduction catalyst, when the occlusion amount of NOx increases and reaches a saturation amount, no more NOx can be occluded, and therefore, O _{2 of} exhaust gas flowing into the NOx occlusion reduction catalyst periodically. For example, when the NOx storage reduction catalyst is applied to a diesel engine, it is difficult to operate the engine at a rich air-fuel ratio. Fuel (hydrocarbon) is directly added as a reducing agent to the upstream exhaust pipe via an injector or the like.

  On the other hand, the particulates (mainly apportioned) collected by the particulate filter are oxidized and purified (combustion removed) by active oxygen generated when NOx is occluded by the NOx occlusion reduction catalyst or occluded NOx is reduced. Will be.

JP 2004-176636 A

  However, this type of NOx occlusion reduction catalyst has an active temperature range in which the necessary catalytic activity can be obtained, and as shown in the graph of FIG. 4, the NOx occlusion amount peaks around 300 ° C., and the temperature is higher than this. However, there is a problem that high NOx purification performance cannot be maintained even if the amount of NOx occluded decreases. For example, a high rotation / high load operation state with a high exhaust temperature continues for a long time. In such a case, if the NOx occlusion reduction catalyst continues to be exposed to high temperature exhaust gas and becomes too warm due to overheating, the catalytic activity of the NOx occlusion reduction catalyst is reduced. There was a problem that NOx purification performance could not be obtained.

  The present invention has been made in view of the above circumstances, and even if an operation state with a high exhaust temperature is continued, excessive NO temperature reduction of a NOx reduction catalyst such as a NOx storage reduction catalyst is suppressed and high NOx purification performance is maintained. An object of the present invention is to provide an exhaust emission control device.

  The present invention is an exhaust emission control device provided with a NOx reduction catalyst in the middle of an exhaust pipe, a cooler capable of cooling the exhaust gas by heat exchange with a refrigerant, and an intermediate part of the exhaust pipe so as to pass through the cooler. A bypass flow path that bypasses the exhaust gas from the exhaust gas to the NOx reduction catalyst, a temperature sensor that detects the exhaust temperature at the inlet of the NOx reduction catalyst, and a flow of exhaust gas when the temperature detected by the temperature sensor exceeds a predetermined value And a flow path switching means for switching to a bypass flow path.

  Thus, if the exhaust gas temperature at the inlet of the NOx reduction catalyst rises and the temperature detected by the temperature sensor becomes equal to or higher than the predetermined value in the case where the operation state with a high exhaust gas temperature continues, The flow is switched from the middle of the exhaust pipe to the bypass flow path by the flow path switching means, and the exhaust gas is cooled by the cooler in the middle of the bypass flow path before being guided to the NOx reduction catalyst. The excessive temperature rise of the catalyst is suppressed to prevent a decrease in catalyst activity, and the high NOx purification performance of the NOx reduction catalyst is maintained.

  Further, in the present invention, when the exhaust air-fuel ratio is lean, NOx in the exhaust gas is oxidized and temporarily stored in the form of nitrate, and when the oxygen concentration in the exhaust gas decreases, the NOx is reduced by the intervening reducing agent. NOx occlusion reduction catalyst with the property of decomposing and releasing to reduce and purify may be used as a NOx reduction catalyst, or selective reduction type catalyst with the property of selectively reacting NOx with a reducing agent even in the presence of oxygen reduces NOx. It may be a catalyst.

  According to the exhaust gas purification apparatus of the present invention described above, the exhaust temperature at the inlet of the NOx reduction catalyst is monitored by the temperature sensor, and when the detected temperature exceeds a predetermined value, the flow of the exhaust gas is switched to the bypass flow path for cooling. As a result, the exhaust temperature is lowered by the vessel, thereby preventing a situation in which the temperature of the NOx storage reduction catalyst exceeds the activation temperature range and overheating is avoided. However, it is possible to achieve an excellent effect that the high NOx purification performance can be maintained by suppressing the excessive temperature rise of the NOx reduction catalyst such as the NOx storage reduction catalyst.

It is the schematic which shows an example of the form which implements this invention. It is a graph which shows the difference regarding the catalyst inlet gas temperature of a prior art example and this form example. It is a graph which shows the difference regarding the NOx purification rate of a prior art example and this form example. It is a graph which shows the temperature change of the NOx occlusion amount of a NOx occlusion reduction catalyst.

  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, the exhaust pipe 4 through which the exhaust gas 3 discharged from the diesel engine 1 through the exhaust manifold 2 flows is shown. In addition, when the exhaust air-fuel ratio is lean, NOx in the exhaust gas 3 is oxidized and temporarily stored in the form of nitrate, and when the oxygen concentration in the exhaust gas 3 is reduced, the NOx is decomposed and released by the intervening reducing agent. The NOx occlusion reduction catalyst 5 (NOx reduction catalyst) having the property of reducing and purifying is equipped.

  An injection nozzle 6 is installed upstream of the NOx occlusion reduction catalyst 5 and a fuel supply line 9 having a fuel injection valve 7 is connected between the injection nozzle 6 and a fuel tank 8 provided at a required location. In addition, fuel 11 such as light oil (reducing agent) in the fuel tank 8 is added to the upstream side of the NOx occlusion reduction catalyst 5 through the fuel injection valve 7 by driving a supply pump 10 installed in the middle of the fuel supply line 9. These fuel injection valves 7, fuel tank 8, fuel supply line 9, and supply pump 10 constitute a fuel addition device 12.

  Further, the exhaust pipe 4 is provided with a bypass flow path 13 that bypasses the exhaust gas 3 from the upstream side and guides the exhaust gas 3 to the position immediately before the injection nozzle 6. Is equipped with a cooler 14 capable of cooling the water by heat exchange with cooling water.

  Here, the cooler 14 employs a structure of a multi-tube heat exchanger such as an EGR cooler 15 for cooling the exhaust gas 3 recirculated from the exhaust side to the intake side so as to achieve water cooling with cooling water. It is also possible to adopt a radiator structure such as a radiator or an intercooler 16 (not shown) so that air cooling with cooling water or air cooling with running wind can be achieved.

  Furthermore, an opening / closing valve 17 capable of opening / closing the flow path on the exhaust pipe 4 side, and an opening / closing valve 18 capable of opening / closing the flow path on the bypass flow path 13 side are provided at a location where the exhaust pipe 4 and the bypass flow path 13 branch. These open / close valves 17 and 18 constitute flow path switching means 19 for switching the flow of the exhaust gas 3 flowing through the exhaust pipe 4 to the bypass flow path 13 side.

  Further, a temperature sensor 20 for detecting the exhaust temperature at the inlet is disposed at the inlet of the NOx storage reduction catalyst 5, and a detection signal 20 a of the temperature sensor 20 is output to the control device 21. The control device 21 outputs opening / closing signals 17a, 18a to the opening / closing valves 17, 18 when the temperature detected by the temperature sensor 20 exceeds a predetermined value (for example, about 470 ° C.). The flow of the exhaust gas 3 can be switched to the bypass flow path 13 side by closing one open / close valve 17 and opening the other open / close valve 18.

In the example shown here, a particulate filter 22 that collects and removes particulates from the exhaust gas 3 is provided downstream of the NOx occlusion reduction catalyst 5, and the NOx occlusion reduction catalyst. 5 is equipped with an oxidation catalyst 23 that promotes an oxidation reaction of NO in the exhaust gas 3 to NO ₂ .

  Thus, if the exhaust emission control device is configured in this way, the exhaust temperature at the inlet of the NOx storage reduction catalyst 5 rises and the temperature detected by the temperature sensor 20 increases when the operation state with a high exhaust temperature continues. When the value exceeds a predetermined value, the control device 21 receiving the detection signal 20a from the temperature sensor 20 outputs the opening / closing signals 17a, 18a to the opening / closing valves 17, 18 to close one opening / closing valve 17 and the other. The on-off valve 18 is opened, the flow of the exhaust gas 3 is switched to the bypass flow path 13 side, and the exhaust gas 3 flowing through the bypass flow path 13 is cooled by the cooler 14 on the way before being introduced to the NOx storage reduction catalyst 5. Therefore, the excessive temperature rise of the NOx occlusion reduction catalyst 5 is suppressed to prevent the catalyst activity from dropping, and the high NOx purification performance of the NOx occlusion reduction catalyst 5 is maintained.

  Therefore, according to the above embodiment, the exhaust temperature at the inlet of the NOx storage reduction catalyst 5 is monitored by the temperature sensor 20, and the flow of the exhaust gas 3 is switched to the bypass flow path 13 when the detected temperature exceeds a predetermined value. Therefore, it is possible to avoid a situation in which the exhaust temperature is lowered by the cooler 14 and the temperature of the NOx occlusion reduction catalyst 5 exceeds the activation temperature range and excessively rises in temperature. Even if the state continues, the excessive temperature rise of the NOx storage reduction catalyst 5 can be suppressed and high NOx purification performance can be maintained.

  In fact, according to the verification experiment by the present inventor, as shown in the graph of FIG. 2, when the conventional example A that does not include the bypass flow path 13 and the cooler 14 is compared with the present example B, the conventional example A Even in the operating conditions in which the catalyst inlet gas temperature of the NOx storage reduction catalyst 5 reaches about 470 ° C., the catalyst inlet gas temperature of the NOx storage reduction catalyst 5 is reliably set to 400 ° C. in this embodiment B. In addition, as shown in the graph of FIG. 3, the NOx storage reduction catalyst 5 is more active in the present embodiment B than in the conventional example A even when the NOx purification rate is compared as shown in the graph of FIG. 3. It could be used at a high temperature and a good NOx purification rate could be obtained.

  In the present embodiment described above, the NOx occlusion reduction catalyst 5 is exemplified as the NOx reduction catalyst. However, instead of this, NOx selectively reacts with the reducing agent even in the presence of oxygen. Alternatively, a selective reduction type catalyst having the property to be used may be employed as the NOx reduction catalyst.

  That is, even in the case of this type of selective reduction type catalyst, there is an active temperature range in which the necessary catalytic activity is obtained, and the catalytic activity of the selective reduction type catalyst declines when the temperature rises beyond the active temperature range. Therefore, as in the embodiment shown in FIG. 1, the bypass passage 13 and the cooler 14 are provided to manage the catalyst inlet gas temperature. Thus, it is possible to maintain the high NOx purification performance by suppressing the excessive temperature rise of the selective catalytic reduction catalyst.

  Note that the exhaust purification apparatus of the present invention is not limited to the above-described embodiment, and it is not possible to obtain a good NOx purification performance when the temperature rises beyond the activation temperature range. The NOx reduction catalyst other than the NOx occlusion reduction catalyst or the selective reduction catalyst may be used as long as the NOx reduction catalyst has the above problem, and the flow path switching means is not necessarily limited to a pair of on-off valves. Of course, for example, the flow path switching means may be constituted by a single three-way valve, and various changes may be made without departing from the scope of the present invention.

3 Exhaust gas 4 Exhaust pipe 5 NOx storage reduction catalyst (NOx reduction catalyst)
DESCRIPTION OF SYMBOLS 13 Bypass flow path 14 Cooler 17 Open / close valve 18 Open / close valve 19 Flow path switching means 20 Temperature sensor

Claims (3)

  An exhaust purification device having a NOx reduction catalyst in the middle of an exhaust pipe, a cooler capable of cooling the exhaust gas by heat exchange with a refrigerant, and exhaust gas from the middle of the exhaust pipe so as to pass through the cooler A bypass flow path that bypasses and leads to the NOx reduction catalyst, a temperature sensor that detects the exhaust temperature at the inlet of the NOx reduction catalyst, and a flow path of the exhaust gas when the temperature detected by the temperature sensor exceeds a predetermined value An exhaust gas purification apparatus comprising a flow path switching means for switching between the two.   When the exhaust air-fuel ratio is lean, NOx in the exhaust gas is oxidized and temporarily stored in the form of nitrate, and when the oxygen concentration in the exhaust gas decreases, the NOx is decomposed and released by the intervention of a reducing agent for reduction and purification. The exhaust purification apparatus according to claim 1, wherein the NOx occlusion reduction catalyst having properties is a NOx reduction catalyst.   2. The exhaust emission control device according to claim 1, wherein a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen is used as a NOx reduction catalyst.

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