JP2014088816A - Exhaust gas cleaning system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning system which enhances the conversion ratio from urea to ammonia even when heating a hydrolysis catalyst from an outer circumferential part of an exhaust pipe.SOLUTION: The exhaust gas cleaning system which is provided with at least a selective reduction catalyst and supplies urea N into an exhaust pipe 3 as reductant of the selective reduction catalyst includes a hydrolysis catalyst 9 which is arranged between a supply portion of the urea N in the exhaust pipe 3 and the selective reduction catalyst and facilitates hydrolysis of the urea N, and a heating means 10c which heats the hydrolysis catalyst 9 from the outer circumference part of the exhaust pipe 3 and, therein, the urea N is ejected in a cross-sectional doughnut shape to the hydrolysis catalyst 9 and the hydrolysis catalyst 9 is preferably in the cross-sectional doughnut shape.

Description

  The present invention relates to an exhaust gas purification system that purifies exhaust gas discharged from an internal combustion engine of a vehicle.

The vehicle exhaust gas purification system is provided with a selective reduction catalyst (SCR [Selective Catalytic Reduction]) to purify NOx, which is one of the environmental pollutants contained in the exhaust gas discharged from the engine. . The SCR reduces and purifies NOx by chemically reacting ammonia (NH ₃ ) and NOx. The SCR has a urea SCR system that uses urea to obtain ammonia. The urea SCR system injects urea into exhaust gas, hydrolyzes urea, and converts it into ammonia. Patent Document 1 discloses that a hydrolysis catalyst is provided on the upstream side of the SCR catalyst, urea is supplied to the hydrolysis catalyst, and hydrolysis is promoted by the hydrolysis catalyst to convert urea into ammonia. .

Special Table 2002-543424

  In the hydrolysis from urea to ammonia, the temperature of the exhaust gas needs to be somewhat high (about 180 ° C. is said to be necessary). Therefore, when the temperature of the exhaust gas is low, such as immediately after starting the engine, NOx cannot be purified by SCR because it cannot be converted from urea to ammonia. Therefore, some exhaust gas purification systems are provided with a heating device on the outer peripheral portion of the exhaust pipe, and when the temperature of the exhaust gas is low, the hydrolysis catalyst is heated from the outer peripheral portion of the exhaust pipe. However, in the case of the cylindrical hydrolysis catalyst disclosed in Patent Document 1, when the low-temperature exhaust gas passes through the cylindrical hydrolysis catalyst, the hydrolysis catalyst is cooled. Therefore, when heated from the outer periphery of the exhaust pipe, the heat transfer distance from the outer periphery of the exhaust pipe becomes longer as the hydrolysis catalyst is closer to the center of the cylindrical shape. It ’s hard to go up. As a result, even if urea is injected over the entire cylindrical hydrolysis catalyst, the closer to the center, the more it cannot be converted to ammonia, and the conversion rate from urea to ammonia decreases.

  Then, this invention makes it a subject to provide the exhaust-gas purification system which improves the conversion rate from urea to ammonia, even when a hydrolysis catalyst is heated from the outer peripheral part of an exhaust pipe.

  An exhaust gas purification system according to the present invention is an exhaust gas purification system that purifies exhaust gas discharged from an internal combustion engine of a vehicle, and supplies urea into the exhaust pipe as a selective reduction catalyst and a reducing agent of the selective reduction catalyst. A urea supply means, a hydrolysis catalyst disposed between the urea supply portion of the urea supply means in the exhaust pipe and the selective reduction catalyst, for promoting hydrolysis of urea supplied by the urea supply means, and a hydrolysis catalyst And heating means for heating from the outer periphery of the exhaust pipe, and the urea supply means jets urea so as to form a donut cross-section with respect to the hydrolysis catalyst.

  This exhaust gas purification system includes at least a selective reduction catalyst for purifying exhaust gas, and includes urea supply means for supplying urea as a reducing agent of the selective reduction catalyst. Furthermore, the exhaust gas purification system includes a hydrolysis catalyst provided between the urea supply portion of the urea supply unit in the exhaust pipe and the selective reduction catalyst, and includes a heating unit that heats the hydrolysis catalyst from the outer periphery of the exhaust pipe. ing. Since the hydrolysis catalyst is heated from the outer periphery of the exhaust pipe by the heating means when the temperature of the exhaust gas is low, heat is more easily transferred to the outer periphery side of the hydrolysis catalyst disposed in the exhaust pipe (heating efficiency is higher). ) The temperature rises. Therefore, in the urea supply means, urea is sprayed so as to form a cross-sectional donut shape with respect to the hydrolysis catalyst. Thus, urea is injected in the shape of a donut in cross section, so that urea is not injected around the central portion of the exhaust pipe but is injected to the outer peripheral side, and urea concentrates on the outer peripheral side of the hydrolysis catalyst. Therefore, on the outer peripheral side of the hydrolysis catalyst whose temperature is rising, hydrolysis of urea sprayed intensively is promoted, and urea is converted into ammonia. In this way, the exhaust gas purification system injects urea so that the cross section of the hydrolysis catalyst has a donut shape, so that the conversion rate from urea to ammonia can be achieved even when the hydrolysis catalyst is heated from the outer periphery of the exhaust pipe. Will improve. As a result, even when the temperature of the exhaust gas is low, ammonia can be obtained efficiently, and the selective reduction catalyst can purify NOx through a chemical reaction between ammonia and NOx, thereby improving the purification rate.

  In the exhaust gas purification system of the present invention, it is preferable that the hydrolysis catalyst has a cross-sectional donut shape.

  In the urea supply means, urea is sprayed in a donut cross-section so that urea concentrates on the outer peripheral side of the hydrolysis catalyst having high heating efficiency by the heating means. Therefore, urea is not sprayed around the center of the exhaust pipe, and no hydrolysis catalyst is required around the center. Therefore, in the exhaust gas purification system, the hydrolysis catalyst has a cross-sectional donut shape (a shape in which the catalyst is disposed on the outer peripheral side) in the exhaust pipe. Therefore, urea is intensively jetted from the urea supply means to the hydrolysis catalyst having a donut cross section. Since the periphery of the central portion of the exhaust pipe where the hydrolysis catalyst is disposed is a space, the exhaust gas can pass through without resistance. Therefore, the exhaust gas discharge resistance is reduced. Thus, the exhaust gas purification system can reduce the pressure loss in the system by making the hydrolysis catalyst into a cross-sectional donut shape, and the fuel efficiency is improved.

  In the exhaust gas purification system of the present invention, the heating means may be a chemical heat storage device. Thus, the exhaust gas purification system can reduce the energy loss for heating the hydrolysis catalyst by applying the chemical heat storage device as the heating means, and the fuel efficiency is improved.

  According to the present invention, the urea conversion rate from urea to ammonia is improved even when the hydrolysis catalyst is heated from the outer peripheral portion of the exhaust pipe by injecting urea so that the hydrolysis catalyst has a donut shape in cross section.

1 is a schematic configuration diagram of an exhaust gas purification system according to the present invention. It is a sectional side view around a hydrolysis catalyst.

  Hereinafter, an embodiment of an exhaust gas purification system according to the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected about the element which is the same or it corresponds in each figure, and the overlapping description is abbreviate | omitted.

  In the present embodiment, the exhaust gas purification system according to the present invention is applied to an exhaust gas purification system provided in an exhaust system of a vehicle engine. The exhaust gas purification system according to the present embodiment is a system that purifies harmful substances (environmental pollutants) contained in exhaust gas discharged from an engine (particularly a diesel engine). The exhaust gas purification system according to the present embodiment includes a catalyst DOC [Diesel Oxidation Catalyst], an SCR, an ASC [Ammonia Slip Catalyst], and a filter DPF [Diesel Particulate Filter]. The exhaust gas purification system according to the present embodiment is also a urea SCR system in which the SCR is a urea water supply device that supplies urea (particularly urea water) as a reducing agent. Furthermore, the exhaust gas purification system according to the present embodiment includes a hydrolysis catalyst that promotes hydrolysis of urea water, and also includes a chemical heat storage device for warming up the hydrolysis catalyst.

  With reference to FIG.1 and FIG.2, the exhaust gas purification system 1 which concerns on this Embodiment is demonstrated. FIG. 1 is a schematic configuration diagram of an exhaust gas purification system according to the present invention. FIG. 2 is a side sectional view around the hydrolysis catalyst.

  The exhaust gas purification system 1 includes a diesel oxidation catalyst (DOC) 4, a diesel exhaust particulate removal filter (DPF) 5, a selective reduction catalyst from the upstream side to the downstream side of the exhaust pipe 3 connected to the exhaust side of the engine 2. (SCR) 6, ammonia slip prevention catalyst (ASC) 7, and urea water supply device 8 for SCR 6. In the present embodiment, the engine 2 corresponds to the internal combustion engine described in the claims, the SCR 6 corresponds to the selective reduction catalyst described in the claims, and the urea water supply device 8 is described in the claims. This corresponds to the urea supply means.

  DOC4 is a catalyst that oxidizes HC, CO, etc. contained in the exhaust gas. The DPF 5 is a filter that collects and removes PM contained in the exhaust gas. The SCR 6 is a catalyst that reduces and purifies NOx by chemically reacting ammonia hydrolyzed from urea water as a reducing agent and NOx contained in the exhaust gas. The ASC 7 is a catalyst that oxidizes ammonia that has passed through the SCR 6 and has flowed downstream.

  The urea water supply device 8 is a device for supplying urea water (reducing agent) to the upstream side of the SCR 6 in the exhaust pipe 3. Specifically, the urea water supply device 8 includes a pump (not shown), a urea water tank 8a, a supply pipe 8b, an injector 8c, and the like. The urea water tank 8a is a tank that stores urea water. When the pump is operated, urea water is sent from the urea water tank 8a to the supply pipe 8b. The supply pipe 8b is a pipe line that connects the urea water tank 8a and the injector 8c and moves urea water from the urea water tank 8a to the injector 8c. The injector 8c is disposed between the DPF 5 and the SCR 6 in the exhaust pipe 3, and injects (sprays) urea water into the exhaust pipe 3 (in the exhaust gas). The injection control of the injector 8c and the ON / OFF control of the pump are performed by an ECU [Electronic Control Unit] (not shown) that controls the engine 2. In addition, the spray shape of the urea water of the injector 8c will be described later in detail.

  In order to promote hydrolysis of urea water supplied by the urea water supply device 8, the exhaust gas purification system 1 is provided between the position where the injector 8 c is disposed in the exhaust pipe 3 (the supply location of urea water) and the SCR 6. It has a hydrolysis catalyst 9. For the hydrolysis of urea water, the temperature of the exhaust gas needs to be high to some extent. However, immediately after the engine 2 is started, the temperature of the exhaust gas discharged from the engine 2 is relatively low. Therefore, the exhaust gas purification system 1 has a chemical heat storage device 10 for warming up the hydrolysis catalyst 9 so that it can be hydrolyzed even when the temperature of the exhaust gas is low. Furthermore, the exhaust gas purification system 1 also has a dispersion plate 11 for dispersing ammonia converted from urea water. In the present embodiment, the hydrolysis catalyst 9 corresponds to the hydrolysis catalyst described in the claims, and the chemical heat storage device 10 corresponds to the heating means described in the claims.

  The hydrolysis catalyst 9 is a catalyst for promoting hydrolysis of urea water. A conventionally well-known catalyst is applied as a catalyst for hydrolysis. The hydrolysis catalyst 9 is provided between the injector 8 c and the SCR 6 in the exhaust pipe 3. As shown in FIG. 2, the hydrolysis catalyst 9 has a cross-sectional donut shape in which the periphery of the central portion of the exhaust pipe 3 is a space. This cross-sectional donut shape has a diameter that fits inside the exhaust pipe 3, and the shape is a shape corresponding to the inner shape of the exhaust pipe 3. For example, if the inner shape of the exhaust pipe 3 is circular, the shape is circular. The cross-sectional donut shape. The cross section of the cross-sectional donut shape is a surface obtained by cutting the hydrolysis catalyst 9 perpendicularly to the direction in which the exhaust gas flows. The hydrolysis catalyst 9 is formed in a structure such as a honeycomb structure or a fin structure, and the catalyst is supported or embedded in these structures. For example, in the case of a honeycomb structure, a corrugated plate attached to one surface of a flat plate is wound several times around a pipe whose outer diameter is smaller than the inner diameter of the exhaust pipe 3 to form a honeycomb structure having a cross-sectional donut shape. Further, the hydrolysis catalyst 9 has a predetermined width (length in the exhaust gas flow direction) so as to sufficiently function as a catalyst for promoting hydrolysis when urea water passes.

  The reason why the hydrolysis catalyst 9 has a donut shape in cross section is that the hydrolysis catalyst 9 is heated from the outer periphery (outside) of the exhaust pipe 3 by the chemical heat storage device 10 (reactor 10c). This is because the closer to the part, the shorter the heat transfer distance, the better the heat transfer, and the higher the heating efficiency. When the catalyst for hydrolysis is disposed on the outer peripheral side of the exhaust pipe 3 due to the cross-sectional donut shape, the catalyst is quickly warmed up and the hydrolysis of urea water can be promoted. On the other hand, since the heat transfer distance from the outer periphery of the exhaust pipe 3 becomes longer around the center of the exhaust pipe 3, the heating efficiency is low and it is difficult to warm up even if heated. Therefore, when the low-temperature exhaust gas passes, the center portion is cooled. For this reason, even if there is a catalyst for hydrolysis in the center of the exhaust pipe 3, the temperature does not rise and the hydrolysis of urea water cannot be promoted.

  When urea water enters the hydrolysis catalyst 9 whose temperature is high, hydrolysis is promoted and the conversion rate to ammonia is increased. The hydrolysis catalyst 9 has a donut shape in cross section, and the hydrolysis catalyst 9 is warmed to the outer peripheral side of the exhaust pipe 3 by heating by the chemical heat storage device 10, so that urea water is sprayed around the center of the exhaust pipe 3. However, hydrolysis is not promoted. Therefore, as shown in FIG. 2, the sprayed urea water spreads radially to the outer peripheral side in the exhaust pipe 3 so as to have a cross-sectional donut shape (concentrates on the hydrolysis catalyst 9 having a cross-sectional donut shape) The spray shape by the injector 8c is set. For example, the injection port at the tip of the injector 8c has a donut shape, and the distance between the injector 8c and the hydrolysis catalyst 9 is adjusted in consideration of the injection pressure of urea water. The cross-section of the doughnut-shaped cross section is a surface obtained by cutting the sprayed urea water perpendicular to the direction in which the exhaust gas flows.

  The chemical heat storage device 10 is a device for heating and warming the hydrolysis catalyst 9 without energy. That is, the chemical heat storage device 10 normally stores the heat (exhaust heat) of the exhaust gas, and uses the heat to warm up the hydrolysis catalyst 9 when necessary. The chemical heat storage device 10 includes an adsorber (heat storage device) 10a, a connecting pipe 10b, a reactor 10c, and the like.

  The adsorber 10a contains activated carbon as an adsorbent that physically adsorbs ammonia. In the adsorber 10a, in the heat storage state, ammonia is stored in a state of being physically adsorbed with the activated carbon.

  The connecting pipe 10b is a pipe line that connects the adsorber 10a and the reactor 10c and moves ammonia between the adsorber 10a and the reactor 10c. An opening / closing valve (not shown) is provided in the connecting pipe 10b, and when the opening / closing valve is opened, ammonia can move between the adsorber 10a and the reactor 10c. The on / off control of the on / off valve is performed by an ECU or the like that controls the engine 2.

The reactor 10c is provided on the outer peripheral surface of the exhaust pipe 3 where the hydrolysis catalyst 9 is disposed. The reactor 10c has a solid or powdery reaction material 10d that chemically reacts with ammonia. Examples of the reaction material include MgCl ₂ , CaCl ₂ , NiCl ₂ , ZnCl ₂ , SrCl ₂ , bromides such as MgBr ₂ and CaBr _2, and iodides such as MgI ₂ and CaI ₂ . As illustrated in FIG. 2, the reaction material 10 d has a cross-sectional donut shape that is disposed at a location where the hydrolysis catalyst 9 is disposed on the outer peripheral surface of the exhaust pipe 3 and surrounds the exhaust pipe 3. The width of the reaction material 10d (the length in the flow direction of the exhaust gas) is preferably wider than the width of the hydrolysis catalyst 9. Furthermore, the reactor 10c is provided with a heat insulating material 10e so as to cover the reaction material 10d, and stores them in a case 10f.

  The dispersion plate 11 is a dispersion plate for dispersing ammonia converted from urea water and introducing ammonia into the SCR 6. The dispersion plate 11 is disposed between the hydrolysis catalyst 9 and the SCR 6 in the exhaust pipe 3. The dispersion plate 11 has a disk shape having a predetermined thickness and is disposed in the exhaust pipe 3. The structure for dispersing ammonia in the dispersion plate 11 is a conventionally known structure such as a mixer or a swirler.

  Operations of the urea water supply device 8, the hydrolysis catalyst 9, and the chemical heat storage device 10 in the exhaust gas purification system 1 configured as described above will be described. While the vehicle is stopped (when the engine 2 is stopped), the on-off valve provided in the connection pipe 10b of the chemical heat storage device 10 is closed. Therefore, even if ammonia is separated from the activated carbon in the adsorber 10a, the ammonia is not supplied to the reactor 10c through the connection pipe 10b.

When the temperature of the exhaust gas discharged from the engine 2 is lower than a predetermined temperature after the engine 2 is started (for example, immediately after the engine 2 is started), the on-off valve disposed in the connection pipe 10b is opened by control by the ECU. Ammonia is supplied to the reactor 10c through the connecting pipe 10b. At this time, the pressure of the adsorber 10a is higher than the pressure of the reactor 10c, and ammonia moves to the reactor 10c side. In the reactor 10c, the supplied ammonia and the reaction material 10d (for example, MgCl ₂ ) chemically react and chemisorb (coordinate bond) to generate heat. And the hydrolysis catalyst 9 is heated from the outer peripheral part of the exhaust pipe 3 by the heat | fever generate | occur | produced from the reactor 10c, the temperature of the hydrolysis catalyst 9 rises, and it becomes the temperature which can hydrolyze urea water. At this time, the heat from the reactor 10c is efficiently used in the hydrolysis catalyst 9 having a cross-sectional donut shape because the heat transfer distance from the outer periphery of the exhaust pipe 3 is short.

  At this time, when the urea water in the urea water tank 8a is sprayed from the injector 8c through the supply pipe 8b under the control of the ECU, the sprayed urea water N is formed in a cross-sectional donut shape as shown in FIG. Become. Therefore, the urea water N concentrates on the outer peripheral side in the exhaust pipe 3 and efficiently enters the hydrolysis catalyst 9 having a cross-sectional donut shape. In the hydrolysis catalyst 9 whose temperature is rising, the hydrolysis of the urea water N is promoted, and the urea water N is efficiently converted to ammonia A. Ammonia A from the hydrolysis catalyst 9 is dispersed in the exhaust pipe 3 by the dispersion plate 11 and enters the SCR 6. In the SCR 6, ammonia A and NOx contained in the exhaust gas chemically react to efficiently reduce and purify NOx.

  When the temperature of the exhaust gas discharged from the engine 2 becomes higher than a predetermined temperature, the exhaust heat of the exhaust gas is given to the reactor 10c, whereby the ammonia and the reactant 10d are separated, and high temperature ammonia is generated. The separated ammonia returns from the reactor 10c to the adsorber 10a through the connection pipe 10b. At this time, the pressure of the reactor 10c is higher than the pressure of the adsorber 10a, and ammonia moves to the adsorber 10a side. In the adsorber 10a, activated carbon physically adsorbs and stores ammonia. Incidentally, when the temperature of the exhaust gas is high, the hydrolysis catalyst 9 can promote hydrolysis without warming up the hydrolysis catalyst 9, and the urea water N sprayed from the injector 8c is efficiently converted into ammonia A. Can be converted.

  According to this exhaust gas purification system 1, by spraying urea water from the injector 8c to the hydrolysis catalyst 9 so as to have a cross-sectional donut shape, the hydrolysis catalyst 9 is generated from the outer periphery of the exhaust pipe 3 by the chemical heat storage device 10. Even when is heated, the hydrolysis catalyst 9 on the outer peripheral side of the exhaust pipe 3 is quickly warmed up, and the conversion rate from urea water to ammonia is improved. As a result, even when the temperature of the exhaust gas is low, ammonia can be obtained efficiently. In the SCR 6, ammonia and NOx can be chemically reacted to purify NOx, and the purification rate is improved.

  Furthermore, according to the exhaust gas purification system 1, since the hydrolysis resistance 9 is made to have a cross-sectional donut shape, the exhaust gas discharge resistance can be reduced, so that the pressure loss in the system can be reduced and the fuel efficiency is improved. Moreover, according to the exhaust gas purification system 1, since the chemical heat storage device 10 is applied as the heating means for the hydrolysis catalyst 9, energy loss for heating the hydrolysis catalyst 9 can be reduced, and fuel efficiency is improved.

  As mentioned above, although embodiment which concerns on this invention was described, this invention is implemented in various forms, without being limited to the said embodiment.

  For example, in the present embodiment, the present invention is applied to an exhaust gas purification system including DOC and ASC in addition to SCR as a catalyst and DPF as a filter, but can be applied to exhaust gas purification systems having various other configurations. Further, although the present embodiment has a configuration having a dispersion plate, a configuration without a dispersion plate may be used.

  In the present embodiment, the chemical heat storage device is applied as the heating means, but other heating means such as a heater may be used.

  In the present embodiment, the hydrolysis catalyst has a donut shape in cross section, but may have a circular shape in cross section (the entire hydrolysis catalyst has a cylindrical shape). In the case where the hydrolysis catalyst has a cylindrical shape, the structure is simple and the production is easy. However, the injector sprays urea water in a donut shape.

  DESCRIPTION OF SYMBOLS 1 ... Exhaust gas purification system, 2 ... Engine, 3 ... Exhaust pipe, 4 ... Diesel oxidation catalyst (DOC), 5 ... Diesel exhaust particulate removal filter (DPF), 6 ... Selective reduction catalyst (SCR), 7 ... Ammonia slip prevention Catalyst (ASC), 8 ... Urea water supply device, 8a ... Urea water tank, 8b ... Supply pipe, 8c ... Injector, 9 ... Hydrolysis catalyst, 10 ... Chemical heat storage device, 10a ... Adsorber, 10b ... Connection pipe, 10c ... reactor, 10d ... reaction material, 10e ... heat insulating material, 10f ... case, 11 ... dispersion plate.

Claims (3)

An exhaust gas purification system for purifying exhaust gas discharged from an internal combustion engine of a vehicle,
A selective reduction catalyst;
Urea supply means for supplying urea into the exhaust pipe as a reducing agent of the selective reduction catalyst;
A hydrolysis catalyst that is disposed between the urea supply portion of the urea supply means in the exhaust pipe and the selective reduction catalyst, and promotes hydrolysis of urea supplied by the urea supply means;
Heating means for heating the hydrolysis catalyst from the outer periphery of the exhaust pipe;
With
The exhaust gas purification system according to claim 1, wherein the urea supply means injects urea so as to form a donut shape in cross section with respect to the hydrolysis catalyst.   The exhaust gas purification system according to claim 1, wherein the hydrolysis catalyst has a cross-sectional donut shape.   The exhaust gas purification system according to claim 1 or 2, wherein the heating means is a chemical heat storage device.

Images