JP2020041528A - Exhaust gas emission control system - Google Patents

Abstract

To provide an exhaust gas emission control system capable of preventing breakage of a selective reduction type catalyst device while preventing an emission control performance of the selective reduction type catalyst device.SOLUTION: An exhaust gas emission control system comprises a reducer injection device and a selective reduction type catalyst device sequentially from an upstream side on an exhaust pipe constituting an exhaust passage of an internal combustion engine. The system includes a crystal passage prevention member provided between the reducer injection device and the selective reduction type catalyst device in a flow direction of exhaust gas passing through the exhaust pipe, and preventing passage of crystals obtained by crystallizing the reducer injected from the reducer injection device.SELECTED DRAWING: Figure 3

Description

  The present disclosure relates to an exhaust gas purification system.

  2. Description of the Related Art As an exhaust gas purification system for purifying NOx in exhaust gas of a diesel engine mounted on a vehicle such as a truck or a bus, a selective catalytic reduction (NOx reduction to nitrogen and water using urea water or the like as a reducing agent) ( An SCR (Selective Catalytic Reduction) system has been developed (for example, see Patent Document 1).

  The selective catalyst reduction system supplies the urea water stored in the urea water tank to an exhaust pipe upstream of the selective reduction catalyst device (SCR), hydrolyzes the urea water with the heat of the exhaust gas to produce ammonia, The NOx is reduced by the catalyst in the selective reduction catalyst device using ammonia. An appropriate amount of urea water is injected by, for example, a urea water injector provided in an exhaust passage.

JP-A-2000-303826

  However, the following inconveniences may occur due to the urea water injected into the exhaust pipe. That is, when the temperature of the exhaust gas is low, such as during low load operation of the internal combustion engine, when the injection amount of the urea water is abnormally large, or when the injection of the urea water is continuous despite the low flow rate of the exhaust gas. In addition, the hydrolysis of urea water becomes insufficient, and specifically, urea water does not sufficiently evaporate and crystallized products are produced. The crystals adhere to and accumulate on the outer peripheral portion of the selective catalytic reduction device in the flow of the exhaust gas, or collide directly with the front surface of the selective catalytic reduction device.

  When the crystals accumulate on the outer peripheral portion of the selective catalytic reduction device, the effective diameter of the selective catalytic reduction device and, consequently, the reduction region are reduced as compared with the case where the crystals are not deposited, and the purification performance of the selective catalytic reduction device is reduced. However, there is a problem that is reduced. Further, when a crystal collides with the front surface of the selective reduction catalyst device, the front surface is worn or the front surface is cracked, thereby causing a problem that the selective reduction catalyst device is damaged. there were.

  An object of the present disclosure is to provide an exhaust gas purification system capable of preventing a reduction in purification performance of a selective reduction catalyst device and preventing damage to the selective reduction catalyst device.

Exhaust gas purification system according to the present disclosure,
An exhaust gas purification system including, in order from an upstream side, a reducing agent injection device and a selective reduction catalyst device in an exhaust pipe constituting an exhaust passage of an internal combustion engine,
Passage of a crystallized product of the reducing agent injected from the reducing agent injection device, which is provided between the reducing agent injection device and the selective catalytic reduction device in the flow direction of the exhaust gas passing through the exhaust pipe. And a crystal passage prevention member for preventing the occurrence of a crystal.

  Advantageous Effects of Invention According to the present disclosure, it is possible to prevent a reduction in the purification performance of the selective reduction catalyst device and to prevent damage to the selective reduction catalyst device.

FIG. 1 is a diagram illustrating a configuration of a vehicle according to the present embodiment. FIG. 9 is a diagram illustrating a problem of a conventional configuration. FIG. 2 is a diagram showing a configuration around a selective catalytic reduction device in the present embodiment. It is a figure which shows the modification of a structure of the crystal passage prevention member in this Embodiment.

  Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration of a vehicle 1 according to the present embodiment. As shown in FIG. 1, an internal combustion engine 10 and an exhaust system 20 are mounted on a vehicle 1 such as a truck or a bus. The exhaust system 20 functions as an “exhaust gas purification system” of the present disclosure.

  First, the configuration of the internal combustion engine 10 will be described. The internal combustion engine 10 is, for example, a diesel engine. In the combustion chamber 11 of the internal combustion engine 10, the fuel injector 13 injects fuel into the combustion chamber 11. Note that the fuel injector 13 may inject fuel into the intake port of the combustion chamber 11. Fuel injection is controlled by an ECM (engine control module) not shown. The fuel in the combustion chamber 11 is compressed by the operation of the piston 19 and burns.

  The intake valve 15 and the exhaust valve 17 are configured to be openable and closable. When the intake valve 15 is opened, fresh air from the intake pipe 50 is sucked into the combustion chamber 11. When the exhaust valve 17 is opened, the exhaust gas generated by burning the fuel in the combustion chamber 11 is sent out to the exhaust system 20 (specifically, the exhaust pipe 21 forming the exhaust passage of the internal combustion engine 10).

  Next, the configuration of the exhaust system 20 will be described. The exhaust system 20 is provided, for example, at the lower part of the vehicle 1 and has a metal exhaust pipe 21 mainly. The exhaust pipe 21 guides exhaust gas generated by combustion of fuel in the internal combustion engine 10 into the atmosphere (outside the vehicle).

  Further, various post-processing devices are provided in the exhaust pipe 21 in order to purify (make harmless) the exhaust gas. In the present embodiment, a DOC (oxidation catalyst) 23A, a DPF 23B, and an SCR 23C (corresponding to the “selective reduction catalyst device” of the present disclosure) are provided as post-treatment devices.

The DOC 23A is formed by supporting rhodium, cerium oxide, platinum, aluminum oxide and the like on a metal carrier. The DOC 23A decomposes and removes hydrocarbons (HC) and carbon monoxide (CO) contained in the exhaust gas. The DOC 23A also has a function of oxidizing nitrogen monoxide (NO), which accounts for most of NOx contained in exhaust gas, to generate nitrogen dioxide (NO ₂ ). By utilizing this function, the NOx purification efficiency of the SCR 23C can be improved.

  In the exhaust pipe 21, on the upstream side of the DOC 23A (specifically, on the upstream side in the flow direction of the exhaust gas), fuel is temporarily supplied into the exhaust gas to convert hydrocarbons (HC) in the fuel into the DOC 23A. A fuel supply unit 22 (fuel supply injector) is provided for raising the temperature of the exhaust gas by utilizing the heat of the oxidation reaction.

  The DPF 23B is formed of a monolithic honeycomb type wall flow filter in which inlets and outlets of porous ceramic honeycomb channels (cells) are alternately sealed. The DPF 23B collects and removes particulate matter (PM) contained in the exhaust gas.

  In the exhaust pipe 21, a urea water injector for injecting (specifically, spraying) urea water as a reducing agent downstream of the DPF 23B and upstream of the SCR 23C in the flow direction of the exhaust gas. 24 (corresponding to the “reducing agent injection device” of the present disclosure).

  For example, a temperature sensor (not shown) for detecting the temperature of the exhaust gas is provided near the inlet of the SCR 23C in the exhaust pipe 21. This temperature sensor is used for controlling the injection of urea water in the urea water injector 24 and the like.

  The SCR 23C has, for example, a columnar shape and has a honeycomb carrier made of ceramic. A catalyst such as zeolite or vanadium is carried or coated on the honeycomb wall surface.

  The SCR 23C as described above is disposed downstream of the DPF 23B in the exhaust pipe 21. Urea water as a reducing agent is injected between the DPF 23B and the SCR 23C in the exhaust pipe 21 by the urea water injector 24 and supplied to the exhaust gas that has passed through the DOC 23A and the DPF 23B. As a result, the urea water is hydrolyzed to ammonia. Then, while the exhaust gas containing ammonia is passing through the SCR 23C, a nitrogen oxide (so-called NOx) reacts with nitrogen and water by the action of a catalyst in the SCR 23C (reduction reaction). Thereby, nitrogen oxides in the exhaust gas are purified.

  Here, the hydrolysis occurs when the temperature of the exhaust gas passing through the SCR 23C is equal to or higher than a predetermined temperature. Therefore, it is preferable that the urea water injector 24 supplies the urea water to the exhaust gas in the exhaust pipe 21 when the temperature of the exhaust gas flowing into the SCR 23C is equal to or higher than the predetermined temperature. Here, the injection of the urea water is controlled by a DCU (Dosing Control Unit) not shown. The predetermined temperature is appropriately determined by experiments and simulations at the stage of designing and developing the exhaust system 20 while taking into consideration the reaction temperature of ammonia and NOx.

  Note that an ammonia slip catalyst may be disposed in the exhaust pipe 21 immediately downstream of the SCR 23C. The ammonia slip catalyst is a second-stage oxidation catalyst, and has a configuration similar to that of the DOC 23A. The ammonia slip catalyst mainly oxidizes and removes the slipped ammonia so that the ammonia that is not used in the reduction reaction in the SCR 23C and is slipped is not released into the atmosphere. In addition, the ammonia slip catalyst may have a function similar to that of the SCR23C.

  Water, nitrogen, and carbon dioxide generated by processing the exhaust gas in each of the above-described post-processing devices are discharged into the atmosphere via a muffler (not shown) or the like.

  By the way, the following inconvenience may occur due to the urea water injected into the exhaust pipe 21. FIG. 2 is a diagram for explaining a problem of the conventional configuration related to the exhaust system 20 (particularly, around the SCR 23C). That is, when the temperature of the exhaust gas is low, such as during low load operation of the internal combustion engine 10, when the injection amount of the urea water is abnormally large, or when the injection of the urea water is continuous despite the low flow rate of the exhaust gas. The urea water is insufficiently hydrolyzed, and specifically, the urea water does not evaporate sufficiently to produce a crystallized product 30. The crystal 30 adheres to and accumulates on the outer peripheral portion of the SCR 23C or collides directly with the front surface of the SCR 23C in the flow of the exhaust gas.

  When a crystal is deposited on the outer peripheral portion of the SCR 23C, there is a problem that the effective diameter d1 of the SCR 23C and thus the reduction region are reduced as compared with a case where the crystal is not deposited, and the purification performance of the SCR 23C is reduced. Further, when a crystal collides with the front surface of the SCR 23C, there is a problem that the front surface is worn, or the front surface is cracked, so that the SCR 23C is damaged.

  Therefore, in the present embodiment, as shown in FIGS. 1 and 3, urea injected by the urea water injector 24 is provided between the urea water injector 24 and the SCR 23C in the flow direction of the exhaust gas passing through the exhaust pipe 21. A crystal-passage preventing member 25 is provided to prevent the passage of the crystal by capturing the crystallized water. The crystal passage preventing member 25 is sandwiched between two divided exhaust pipes, for example, and attached by welding or bolting.

  FIG. 3A is a diagram showing a configuration around the SCR 23C. The crystal passage preventing member 25 is formed horizontally (flat) with respect to the radial direction of the exhaust pipe 21, and prevents the passage of the crystal 30 over the entire radial direction of the exhaust pipe 21. FIG. 3B is a view of the crystal passage prevention member 25 in the exhaust pipe 21 in the flow direction of the exhaust gas. As shown in FIG. 3B, the crystal material passage preventing member 25 is formed of a wire mesh having a mesh size through which the crystal material 30 does not pass.

  As shown in FIG. 3A, even if the urea water injected from the urea water injector 24 does not sufficiently evaporate and crystallized crystal 30 is formed, the crystal 30 riding on the flow of the exhaust gas is prevented from passing through the crystal. It is captured by the member 25 and is prevented from passing downstream. Therefore, the crystal 30 is prevented from adhering and depositing on the outer peripheral portion of the SCR 23C and from directly colliding with the front surface of the SCR 23C. Even if the crystal 30 adheres and accumulates on the outer periphery of the crystal passage preventing member 25, the exhaust gas that has passed through the crystal passage preventing member 25 is directed outward from the center in the radial direction of the exhaust pipe 21. Since the gas flows while diffusing, the effective diameter d2 of the SCR 23C, and thus the reduction area, does not decrease. As described above, it is possible to prevent the purification performance of the SCR 23C from decreasing and to prevent the SCR 23C from being damaged.

  Note that, in the above-described embodiment, an example has been described in which the crystal passage prevention member 25 is formed horizontally with respect to the radial direction of the exhaust pipe 21, but the present disclosure is not limited to this. For example, as shown in FIG. 4A, the crystal passage preventing member 25 may be formed in a cone shape extending toward the center in the radial direction of the exhaust pipe 21 and extending downstream in the flow direction of the exhaust gas. In this case, the crystal 30 accumulates near the center of the crystal passage preventing member 25 in the radial direction. Further, as shown in FIG. 4B, the crystal passage prevention member 25 may be formed in a cone shape extending toward the center in the radial direction of the exhaust pipe 21 and extending upstream in the flow direction of the exhaust gas. In this case, the crystal 30 is deposited unevenly near the outside of the crystal passage preventing member 25 in the radial direction.

  As the shape of the crystal passage preventing member 25, the cone shape shown in FIG. 4A or FIG. 4B can be selectively adopted according to the temperature of the exhaust gas. For example, in a case where the temperature of the exhaust gas is high and the crystal 30 deposited on the crystal passage preventing member 25 is in an environment in which the crystal 30 is easily melted, the crystal 30 is biased toward the center of the crystal passage preventing member 25 in the radial direction. By adopting a cone shape (see FIG. 4A) on which the deposited crystal 30 is deposited, the deposited crystal 30 is suitably melted, and a large amount of exhaust gas is passed near the center of the SCR 23C having a high purification performance to reduce the purification performance. Can be suppressed. On the other hand, when the temperature of the exhaust gas is low and the crystal 30 deposited on the crystal passage preventing member 25 is in an environment in which it is difficult to melt, the crystal 30 By adopting a cone shape (see FIG. 4B) in which a large amount of exhaust gas is passed near the center of the SCR 23C having high purification performance, a decrease in purification performance can be suppressed.

  As described above, by forming the crystal passage preventing member 25 in a cone shape, the total area in which the crystal 30 can be captured by the crystal passage preventing member 25 increases, and the crystal 30 is 25 are deposited near the center or outside in the radial direction. Therefore, as compared with the case where the crystal passage preventing member 25 is formed horizontally with respect to the radial direction of the exhaust pipe 21 (that is, the case where the crystal 30 is uniformly deposited on the crystal passage preventing member 25). Therefore, the resistance of the gas flow due to the deposition of the crystal 30 on the crystal passage preventing member 25 can be reduced.

  Further, in the above-described embodiment, an example has been described in which the crystal passage preventing member 25 is configured to prevent the passage of the crystal 30 over the entire radial direction of the exhaust pipe 21. The passage of the crystal 30 may be prevented in a part of the radial direction. However, from the viewpoint of reliably preventing the purification performance of the SCR 23C from deteriorating and preventing the SCR 23C from being damaged, the crystal passage preventing member 25 allows the crystal 30 to pass through the entire exhaust pipe 21 in the radial direction. It is desirable to be configured to prevent

  In addition, each of the above-described embodiments is merely an example of a specific embodiment for carrying out the present disclosure, and the technical scope of the present disclosure should not be interpreted in a limited manner. That is, the present disclosure can be implemented in various forms without departing from the gist or the main features thereof.

  INDUSTRIAL APPLICABILITY The present disclosure is useful as an exhaust gas purification system capable of preventing a reduction in purification performance of a selective reduction catalyst device and preventing damage to the selective reduction catalyst device.

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Internal combustion engine 11 Combustion chamber 13 Fuel injection injector 15 Intake valve 17 Exhaust valve 19 Piston 20 Exhaust system 21 Exhaust pipe 22 Fuel supply unit 23A DOC
23B DPF
23C SCR
Reference Signs List 24 Urea water injector 25 Crystal material passage preventing member 30 Crystal material 50 Intake pipe

Claims (6)

An exhaust gas purification system including, in order from an upstream side, a reducing agent injection device and a selective reduction catalyst device in an exhaust pipe constituting an exhaust passage of an internal combustion engine,
Passage of a crystallized product of the reducing agent injected from the reducing agent injection device, which is provided between the reducing agent injection device and the selective catalytic reduction device in the flow direction of the exhaust gas passing through the exhaust pipe. Comprising a crystal passage prevention member for preventing
Exhaust gas purification system. The crystal passage preventing member prevents the passage of the crystal over the entire radial direction of the exhaust pipe.
The exhaust gas purification system according to claim 1. The crystal passage preventing member prevents passage of the crystal by trapping the crystal,
The exhaust gas purification system according to claim 1. The crystal passage prevention member is formed in a cone shape extending downstream in the flow direction as it goes toward the center in the radial direction of the exhaust pipe,
The exhaust gas purification system according to claim 1. The crystal passage preventing member is formed in a cone shape extending toward the center in the radial direction of the exhaust pipe and extending upstream in the flow direction.
The exhaust gas purification system according to claim 1. The crystal passing prevention member is a wire mesh having a mesh size through which the crystal does not pass,
The exhaust gas purification system according to claim 1.

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