JP2007002697A - Exhaust emission control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To protect a selective reduction type catalyst from high-temperature exhaust gas in forced regeneration of a particulate filter. <P>SOLUTION: The selective reduction type catalyst 5 is installed in the middle of an exhaust pipe 4, and urea water 11 is added as reducer to the upstream side of the selective reduction type catalyst 5 to reduction-control NOx in this exhaust emission control device. On an exhaust pipe 4 upstream of an addition position of urea water 11 (an opening position of an injection nozzle 6), the particulate filter 14 equipped at a front stage with an oxidation catalyst 13 is disposed. Between the particulate filter 14 and the addition position of urea water 11, a heat accumulating member 15 in permeable structure in which heat of exhaust gas 3 can be accumulated is disposed. A fuel adding means to add fuel (a fuel injection device 30) to exhaust gas 3 upstream of the oxidation catalyst 13 is provided. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust emission control device.

  Conventionally, a diesel engine is equipped with a selective reduction catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen in the middle of an exhaust pipe through which exhaust gas flows, and the selective reduction catalyst A required amount of a reducing agent is added to the upstream side of the catalyst so that the reducing agent undergoes a reduction reaction with NOx (nitrogen oxide) in the exhaust gas on the selective catalytic reduction catalyst, thereby reducing the NOx emission concentration. There is what I did.

On the other hand, in the field of industrial flue gas denitration treatment in plants and the like, the effectiveness of a method for reducing and purifying NOx using ammonia (NH ₃ ) as a reducing agent is already widely known. In recent years, it has been difficult to ensure the safety of traveling with ammonia itself, and in recent years, the use of non-toxic urea water as a reducing agent has been studied.

That is, if urea water is added to the exhaust gas upstream of the selective catalytic reduction catalyst, the urea water is thermally decomposed into ammonia and carbon dioxide gas in the exhaust gas, and NOx in the exhaust gas is converted onto the selective catalytic reduction catalyst. It is reduced and purified well by ammonia (see, for example, Patent Document 1).
JP 2002-161732 A

  On the other hand, when purifying exhaust gas from a diesel engine, it is not sufficient to remove NOx in the exhaust gas, and particulates contained in the exhaust gas are also collected through the particulate filter. However, when this type of particulate filter is employed, it is necessary to regenerate the particulate filter by appropriately burning and removing the particulate before the exhaust resistance increases due to clogging.

  For this reason, a flow-through type oxidation catalyst is attached to the preceding stage of the particulate filter, and fuel is added to the exhaust gas upstream from the oxidation catalyst when the amount of particulate deposition increases. It is considered to force playback.

  In other words, if fuel is added to the exhaust gas upstream of the oxidation catalyst, the added fuel (HC) undergoes an oxidation reaction while passing through the preceding oxidation catalyst. The catalyst bed temperature of the particulate filter immediately after that is raised, the particulates are burned out, and the particulate filter is regenerated.

  In general, as a specific means for performing the fuel addition as described above, the post-injection is executed at the timing of non-ignition later than the compression top dead center following the main injection of fuel performed near the compression top dead center. It is considered that the fuel is added to the exhaust gas, but in order to efficiently utilize the added fuel for forced regeneration and oxidize the added fuel before the temperature of the exhaust gas drops as much as possible, It is considered preferable to dispose the particulate filter and the oxidation catalyst in the preceding stage in the preceding stage of the selective catalytic reduction catalyst.

  However, if the particulate filter is forcibly regenerated before the selective catalytic reduction catalyst, heat is generated by the oxidation reaction of the added fuel at the preceding oxidation catalyst, and the particulates collected by the particulate filter burn. Due to heat generation, the temperature of the exhaust gas on the outlet side of the particulate filter rises to about 700 ° C. or more, and the selective reduction catalyst that originally has low heat resistance may be thermally deteriorated and the catalytic function may be greatly reduced. was there.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exhaust purification device that can protect a selective catalytic reduction catalyst from high-temperature exhaust gas during forced regeneration of a particulate filter. Yes.

  The present invention is an exhaust emission control device equipped with a selective reduction catalyst in the middle of an exhaust pipe and reducing and purifying NOx by adding urea water as a reducing agent upstream of the selective reduction catalyst, A particulate filter with an oxidation catalyst attached to the upstream stage is disposed in the exhaust pipe upstream from the urea water addition position, and the heat of the exhaust gas is transferred between the particulate filter and the urea water addition position. A heat storage material having a ventilation structure that can be stored is provided, and fuel addition means for adding fuel to the exhaust gas upstream of the oxidation catalyst is provided.

  Thus, in this way, particulates in the exhaust gas are collected by the particulate filter, and urea water is added to the exhaust gas in the subsequent stage and thermally decomposed into ammonia and carbon dioxide gas. Since NOx in the exhaust gas is satisfactorily reduced and purified by ammonia on the reduction catalyst, simultaneous reduction of particulates and NOx in the exhaust gas can be achieved.

  In addition, even during the forced regeneration of the particulate filter, even if the added fuel undergoes an oxidation reaction with the preceding stage oxidation catalyst and the collected particulate matter burns with the subsequent stage particulate filter, high temperature exhaust gas is generated. When the exhaust gas passes through the heat storage material, the heat is absorbed and the temperature is lowered, so that the thermal shock to the selective catalytic reduction catalyst is alleviated and the thermal degradation of the selective catalytic reduction catalyst is avoided.

  Furthermore, even if the temperature of the exhaust gas decreases due to a change in the operating state, the exhaust gas continues to be heated for a while due to the heat stored in the heat storage material, so that a sudden temperature decrease is avoided. The NOx reduction rate of the selective catalytic reduction catalyst in the operating region is improved.

  Further, in the present invention, a fuel injection device that injects fuel into each cylinder of the engine is adopted as a fuel addition means, and fuel injection into the cylinder is controlled to leave a large amount of unburned fuel in the exhaust gas. It is good to comprise so that fuel addition may be performed.

  According to the above-described exhaust gas purification apparatus of the present invention, even when high-temperature exhaust gas is generated upstream from the selective catalytic reduction catalyst, the heat of the exhaust gas is absorbed by the heat storage material to mitigate thermal shock to the selective catalytic reduction catalyst. As a result, the selective catalytic reduction catalyst can be reliably protected from the high-temperature exhaust gas during forced regeneration of the particulate filter, which enables the particulate filter to be placed in front of the selective catalytic reduction catalyst. This makes it possible to reduce particulates and NOx at the same time, and by effectively utilizing the heat stored in the heat storage material in the operating region where the temperature of the exhaust gas is low, the temperature of such exhaust gas can be reduced. Various excellent effects can be obtained, such as the NOx reduction rate of the selective catalytic reduction catalyst in the operating region where the operation is reduced can be improved as compared with the conventional technology.

  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, a selective catalytic reduction catalyst 5 having the property of selectively reacting NOx with ammonia even in the presence of oxygen is provided.

  An injection nozzle 6 is installed on the inlet side of the selective reduction catalyst 5, and urea water supply including a urea water injection valve 7 is provided between the injection nozzle 6 and a urea water tank 8 provided at a required place. The urea water 11 (reducing agent) in the urea water tank 8 is connected to the selective catalytic reduction catalyst 5 via the urea water injection valve 7 by driving a supply pump 10 connected in the middle of the urea water supply line 9. The urea water injection device 12 is configured by the urea water injection valve 7, the urea water tank 8, the urea water supply line 9, and the supply pump 10.

The exhaust pipe 4 upstream of the urea water 11 addition position (opening position of the injection nozzle 6) by the urea water addition device 12 is provided with an oxidation catalyst 13 in the preceding stage, and the oxidation catalyst is integrated with itself. A supported particulate filter 14 is provided, and a heat storage material 15 having a ventilation structure capable of storing heat of the exhaust gas 3 is provided between the particulate filter 14 and the urea water 11 addition position. In addition, immediately after the selective catalytic reduction catalyst 5, an NH ₃ slip catalyst 16 that oxidizes excess ammonia as a countermeasure against leaked ammonia is provided.

  Here, the heat storage material 15 having a ventilation structure capable of storing the heat of the exhaust gas 3 includes honeycomb structure ceramics, metal filters (thickness-sintered micron-order metal fibers, sintered metal powder, metal mesh) It is possible to employ a metal honeycomb or the like obtained by laminating and sintering a metal powder, a metal mesh sintered on a metal mesh, or the like.

  Further, the accelerator of the driver's seat (not shown) is provided with an accelerator sensor 17 (load sensor) that detects the accelerator opening as a load of the diesel engine 1, and the rotational speed is set at an appropriate position of the diesel engine 1. A rotation sensor 18 for detection is provided, and an accelerator opening signal 17a and a rotation speed signal 18a from the accelerator sensor 17 and the rotation sensor 18 constitute a control device 19 (fuel injection) that forms an engine control computer (ECU: Electronic Control Unit). Is input to the control device).

  On the other hand, in the control device 19, the fuel injection timing and the injection toward the fuel injection device 20 for injecting the fuel into each cylinder according to the current operation state determined from the accelerator opening signal 17a and the rotation speed signal 18a. A fuel injection signal 20a for commanding the amount is output.

  Here, the fuel injection device 20 is constituted by a plurality of injectors (not shown) provided for each cylinder, and the electromagnetic valves of these injectors are appropriately opened by a fuel injection signal 20 a from the control device 19. Thus, the fuel injection timing and the injection amount (valve opening time) are appropriately controlled.

  However, in the present embodiment, the control device 19 determines the fuel injection signal 20a in the normal mode based on the accelerator opening signal 17a and the rotational speed signal 18a, while the particulate filter 14 is forcibly regenerated. When it is necessary to perform this, the normal mode is switched to the regeneration mode, and the non-ignition timing (start timing) later than the compression top dead center following the main injection of fuel performed near the compression top dead center (crank angle 0 °). The fuel injection signal 20a is determined so as to perform post injection at a crank angle of 90 ° to 130 °.

  That is, when post-injection is performed at a non-ignition timing later than the compression top dead center following main injection, unburned fuel (mainly HC: hydrocarbon) is added to the exhaust gas 3 by this post-injection. The unburned fuel undergoes an oxidation reaction while passing through the oxidation catalyst 13 in the preceding stage of the particulate filter 14, and the particulate filter 14 immediately after the inflow of the exhaust gas 3 heated by the reaction heat. Thus, the catalyst bed temperature is raised and the particulates are burned and removed.

  Further, the control device 19 extracts the fuel injection amount determined from the rotational speed of the diesel engine 1 and the output value of the fuel injection signal 20a, and the diesel generation from the particulate generation map based on the rotational speed and the injection amount. The basic generation amount of particulates based on the current operation state of the engine 1 is estimated, and the basic generation amount is multiplied by a correction coefficient considering various conditions relating to the generation of particulates, and the current operation state The final generation amount is obtained by subtracting the particulate processing amount at, and the final generation amount is momentarily accumulated to estimate the particulate deposition amount. The deposition amount has reached a predetermined target value. When it is estimated, the fuel injection control is switched from the normal mode to the regeneration mode, and the exhaust gas 3 upstream of the particulate filter 14 is So that the fuel is added to.

  There are various ways of estimating the amount of particulate deposition, and it is of course possible to estimate the amount of particulate deposition using a method other than the estimation method exemplified here. It is also possible to estimate the accumulated amount of particulates based on the differential pressure before and after the curate filter, or to estimate the accumulated amount of particulates based on the operation time and travel distance.

  The control device 19 also estimates the amount of NOx generated based on the rotational speed of the diesel engine 1 and the amount of fuel injected, and the addition of a necessary amount of urea water 11 corresponding to the amount of NOx generated results in the urea water being added. More specifically, an instruction is given to the adding device 12, and more specifically, a drive command signal 10a to the supply pump 10 and a valve opening command signal 7a to the urea water injection valve 7 output from the control device 19 are provided. Thus, the addition of the urea water 11 by the urea water adding device 12 is executed.

  Thus, if the exhaust gas purification device is configured in this way, the particulate filter 14 collects the particulates in the exhaust gas 3 and, at the subsequent stage, the urea water 11 from the injection nozzle 6 of the urea water addition device 12. Is added to the exhaust gas 3 and thermally decomposed into ammonia and carbon dioxide, and the NOx in the exhaust gas 3 is favorably reduced and purified by ammonia on the selective catalytic reduction catalyst 5. Simultaneous reduction of particulates and NOx is achieved.

  Moreover, when it becomes necessary to perform forced regeneration of the particulate filter 14, the fuel injection control in the control device 19 is switched from the normal mode to the regeneration mode, and the fuel added on the diesel engine 1 side by the post injection is oxidized in the previous stage. Even if high temperature exhaust gas 3 is produced by oxidation reaction at the catalyst 13 and combustion of the particulates collected by the particulate filter 14 at the subsequent stage, the high temperature exhaust gas 3 passes through the heat storage material 15 immediately after that. In this case, heat is absorbed and the temperature is lowered, so that the thermal shock to the selective catalytic reduction catalyst 5 is alleviated and thermal degradation of the selective catalytic reduction catalyst 5 is avoided.

  For example, as shown in FIG. 2, when the temperature A of the particulate filter 14 at the time of forced regeneration is about 700 ° C., the temperature B of the selective catalytic reduction catalyst 5 without the heat storage material 15 is about 600 ° C. On the other hand, the temperature C of the selective catalytic reduction catalyst 5 when the heat storage material 15 is interposed between them can be suppressed to about 500 ° C. or less, and as indicated by X in FIG. When NO. 15 was used, the decrease in NOx reduction rate (thermal degradation) was very slight even after a mileage of 650,000 km, but as shown by Y in FIG. It was confirmed by a verification experiment that a decrease in NOx reduction rate (thermal degradation) appears remarkably in the absence of the NOx.

  Further, if the heat storage material 15 is provided as in the present embodiment, even if the temperature of the exhaust gas 3 decreases due to a change in the operating state, the exhaust gas 3 is heated for a while by the heat stored in the heat storage material 15. Since an abrupt temperature decrease is subsequently avoided, the NOx reduction rate of the selective catalytic reduction catalyst 5 in the operation region where the temperature of the exhaust gas 3 is lowered is improved.

  Therefore, according to the above embodiment, even if the high-temperature exhaust gas 3 is generated upstream from the selective catalytic reduction catalyst 5, the heat of the exhaust gas 3 is absorbed by the heat storage material 15 and the thermal shock to the selective catalytic reduction catalyst 5. Therefore, the selective catalytic reduction catalyst 5 can be surely protected from the high-temperature exhaust gas 3 during the forced regeneration of the particulate filter 14. By enabling the arrangement of the curative filter 14 to achieve simultaneous reduction of particulates and NOx, the heat stored in the heat storage material 15 can be effectively utilized in the operation region where the temperature of the exhaust gas 3 is lowered. Thus, the NOx reduction rate of the selective catalytic reduction catalyst 5 in the operation region where the temperature of the exhaust gas 3 becomes low can be improved as compared with the conventional art.

  The exhaust emission control device of the present invention is not limited to the above-described embodiment. In the above embodiment, the fuel injection device is used as the fuel addition means, and the fuel is performed near the compression top dead center. Fuel is added to the exhaust gas by performing post-injection at a timing of non-ignition later than the compression top dead center following the main injection of the engine, but the timing of main injection into the cylinder is delayed from normal The fuel may be added to the exhaust gas, and only the means for adding fuel by controlling the fuel injection into the cylinder and leaving a large amount of unburned fuel in the exhaust gas. In addition, an injector may be provided as a fuel addition means at an appropriate position of the exhaust pipe (or an exhaust manifold is acceptable), and fuel may be directly injected into the exhaust gas by this injector, It is of course that various changes and modifications may be made without departing from the scope and spirit of the present invention.

It is the schematic which shows an example of the form which implements this invention. It is the graph which compared the temperature of the selective reduction type catalyst at the time of forced regeneration with the presence or absence of a heat storage material. It is the graph which compared the relationship between a travel distance and a NOx reduction rate by the presence or absence of a heat storage material.

Explanation of symbols

DESCRIPTION OF SYMBOLS 3 Exhaust gas 4 Exhaust pipe 5 Selective reduction type catalyst 6 Injection nozzle 11 Urea water 12 Urea water addition apparatus 13 Oxidation catalyst 14 Particulate filter 15 Thermal storage material 20 Fuel injection apparatus (fuel addition means)

Claims (2)

  An exhaust gas purification apparatus equipped with a selective reduction catalyst in the middle of an exhaust pipe and adding urea water as a reducing agent upstream of the selective reduction catalyst to reduce and purify NOx, wherein urea water addition A ventilating structure in which an exhaust pipe upstream of the position is provided with a particulate filter with an oxidation catalyst attached to the preceding stage, and heat of the exhaust gas can be stored between the particulate filter and the urea water addition position An exhaust emission control device comprising a fuel addition means for arranging the heat storage material and adding fuel to the exhaust gas upstream of the oxidation catalyst.   Fuel injection device that injects fuel into each cylinder of the engine is adopted as fuel addition means, and fuel addition is executed by controlling fuel injection into the cylinder and leaving a large amount of unburned fuel in the exhaust gas The exhaust emission control device according to claim 1, which is configured.

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