JP7280169B2 - Exhaust purification device - Google Patents

Description

The present invention relates to an exhaust purification device.

2. Description of the Related Art Conventionally, as an exhaust purification device for purifying exhaust gas discharged from an engine, for example, a device described in Patent Document 1 is known. This exhaust purification device includes, in order from the upstream side of an exhaust passage through which exhaust gas flows, a first reduction catalyst that purifies NOx in the exhaust gas, a first reduction catalyst that purifies CO and HC in the exhaust gas, and oxidizes NO of the NOx. a filter that collects particulate matter in the exhaust gas; and a second reduction catalyst that purifies NOx in the exhaust gas.

JP 2018-159334 A

In the exhaust purification device described above, the first reduction catalyst is arranged upstream of the oxidation catalyst, the filter, and the second reduction catalyst. With this arrangement, the high-temperature exhaust gas immediately after being discharged from the engine flows to the first reduction catalyst, and the first reduction catalyst can be activated early, so that NOx in the exhaust gas is reduced. It is suitable for improving the purification efficiency of However, in the above-described exhaust purification device, it cannot be said that the catalyst is configured after sufficiently considering the component ratio in the exhaust gas flowing into the catalyst. Performance may not be fully demonstrated. Therefore, there is room for further improvement in terms of improving the purification efficiency of NOx.

SUMMARY OF THE INVENTION An object of the present invention is to provide an exhaust purification device capable of improving the NOx purification efficiency.

An exhaust purification device according to one aspect of the present invention is an exhaust purification device for purifying exhaust gas discharged from an engine, and is arranged in an exhaust passage through which the exhaust gas flows, and is a first exhaust gas purification device that reduces NOx contained in the exhaust gas. a reduction catalyst disposed downstream of the first reduction catalyst in the exhaust passage and oxidizing at least NO of NOx; and a particle contained in the exhaust gas disposed downstream of the oxidation catalyst in the exhaust passage. a second reduction catalyst disposed downstream of the filter in the exhaust passage to reduce NOx contained in the exhaust gas; and a first reduction catalyst disposed upstream of the first reduction catalyst in the exhaust passage. a first reducing agent injection unit configured to inject a reducing agent into the exhaust gas; a second reducing agent injection unit, a sensor for detecting the temperature of the oxidation catalyst, and controlling the first reducing agent injection unit and the second reducing agent injection unit based on the temperature of the oxidation catalyst detected by the sensor. and a controller, wherein the first reduction catalyst contains copper (Cu), and the controller controls, when the temperature of the oxidation catalyst detected by the sensor is lower than the activation temperature of the oxidation catalyst, the first The second injection scheduled to be injected from the second reducing agent injection unit at the first injection amount scheduled to be injected from the first reducing agent injection unit without injecting the reducing agent from the second reducing agent injection unit. The added amount of the reducing agent is injected from the first reducing agent injection part.

In this exhaust purification device, since the first reduction catalyst is arranged upstream of the oxidation catalyst, the filter, and the second reduction catalyst, the high temperature exhaust gas immediately after being discharged from the engine is subjected to the first reduction can be flowed to the catalyst. As a result, the first reduction catalyst can be activated early, and the NOx purification efficiency can be improved. Furthermore, the first reduction catalyst is a Cu-based reduction catalyst containing Cu. Since the exhaust gas discharged from the engine contains a large amount of NO, the exhaust gas with a high NO ratio flows into the first reduction catalyst. In such an environment where a large amount of NO exists (or only NO exists), it is preferable to use a Cu-based reduction catalyst rather than using other reduction catalysts from the viewpoint of sufficiently exhibiting the NOx purification performance. preferred. Therefore, by using a Cu-based reduction catalyst as the first reduction catalyst, the reduction reaction of NOx can be sufficiently advanced in the first reduction catalyst disposed immediately after the engine, and NOx can be efficiently purified. be able to.

After passing through the first reduction catalyst, the exhaust gas flows into the second reduction catalyst after passing through the oxidation catalyst and the filter. Here, when starting the engine, it takes time until the temperature of the exhaust gas on the downstream side rises sufficiently. Therefore, in the period until the temperature of the exhaust gas on the downstream side rises sufficiently, the temperature in the vicinity of the second reduction catalyst on the downstream side reaches the temperature necessary for generating NH ₃ from the reducing agent. Therefore, even if the reducing agent is injected from the second reducing agent injection part, it is impossible to generate NH ₃ necessary for the NOx reduction reaction in the second reducing catalyst from this reducing agent. However, during such a period, the temperature of the downstream oxidation catalyst does not reach the activation temperature, so most of the _NH3 in the exhaust gas that has passed through the first reduction catalyst is not oxidized in the oxidation catalyst, It is supplied to the second reduction catalyst.

Therefore, in the exhaust purification device described above, when the temperature of the oxidation catalyst is equal to or higher than the activation temperature, the reducing agent is not injected from the second reducing agent injection unit, and is planned to be injected from the first reducing agent injection unit. The reducing agent is injected from the first reducing agent injection part in an amount obtained by adding the second injection amount to be injected from the second reducing agent injection part to the first injection amount of . In this way, by injecting the surplus reducing agent that cannot be processed in the first reduction catalyst from the first reducing agent injection unit, NH ₃ generated from the reducing agent is directed downstream of the first reduction catalyst. It can be actively leaked. Then, when the temperature of the oxidation catalyst is lower than the activation temperature, most of the leaked _NH3 is supplied to the second reduction catalyst without being oxidized in the oxidation catalyst, so the reduction reaction of NOx in the second reduction catalyst can proceed. In other words, the NOx reduction reaction can be efficiently progressed until the temperature of the exhaust gas on the downstream side rises sufficiently. Therefore, according to the exhaust purification device described above, the NOx purification efficiency can be improved.

When the temperature of the oxidation catalyst detected by the sensor is equal to or higher than the activation temperature of the oxidation catalyst, the control unit causes the first reducing agent injection unit to inject a first injection amount of the reducing agent to perform the second reduction. A second injection amount of the reducing agent may be injected from the agent injection unit. In this case, when NH ₃ leaks to the oxidation catalyst, the leaked NH ₃ is oxidized to N ₂ O, which is difficult to purify and is highly harmful in the oxidation catalyst. Therefore, the leakage of NH ₃ to the oxidation catalyst is suppressed by injecting a predetermined amount (first injection amount) of reducing agent instead of excess reducing agent from the first reducing agent injection unit. As a result, the situation in which N ₂ O is generated in the oxidation catalyst can be suppressed. Further, when the temperature of the oxidation catalyst is equal to or higher than the activation temperature of the oxidation catalyst, _NH3 can be generated from the reducing agent in the vicinity of the second reducing catalyst, so the predetermined amount (second 2 injection amount) of the reducing agent allows the NOx reduction reaction to proceed in the second reduction catalyst.

The upstream portion of the second reduction catalyst may contain Fe and the downstream portion of the second reduction catalyst may contain Cu. Since the NO in the exhaust gas that has passed through the first reduction catalyst is oxidized to _NO2 by the oxidation catalyst, a state with an increased _NO2 ratio (for example, NO and NO ₂ proportions) flow in. In such an environment where NO and NO ₂ coexist, it is preferable to use an Fe-based reduction catalyst containing Fe rather than using other reduction catalysts from the viewpoint of sufficiently exhibiting NOx purification performance. is. Therefore, by using the Fe-based reduction catalyst for the upstream portion of the second reduction catalyst, the NOx reduction reaction can be sufficiently advanced in the upstream portion, and NOx can be efficiently purified. can. By purifying NO, NO 2, etc. in the upstream portion of the second reduction catalyst, NO, NO _{2, etc.} in the exhaust gas flowing into the downstream portion (Cu-based reduction catalyst) of the second reduction catalyst decreases. By reducing the amount of NO ₂ flowing into the Cu-based reduction catalyst in this way, it is possible to prevent the Cu-based reduction catalyst from generating a large amount of N ₂ O, which is difficult to purify and is highly harmful. The Cu-based reduction catalyst allows the NOx reduction reaction to proceed sufficiently even in an environment where only NO is present due to the reduction of NO ₂ , so that the exhaust gas that has passed through the Fe-based reduction catalyst NOx can be purified efficiently. Therefore, according to the configuration described above, it is possible to further improve the purification efficiency of NOx in the exhaust gas.

The exhaust purification device described above may further include a first ammonia slip catalyst disposed between the first reduction catalyst and the oxidation catalyst in the exhaust passage. When the reducing agent is injected into the exhaust gas upstream of the first reduction catalyst, _NH3 is produced from the reducing agent. Since the first reduction catalyst containing Cu (Cu-based reduction catalyst) has the property of adsorbing a large amount of _NH3 , most of the generated _NH3 is Adsorbs to the reduction catalyst of 1. The NH ₃ adsorbed on the first reduction catalyst leaks from the first reduction catalyst due to the rapid temperature rise caused by the exhaust gas immediately after being discharged from the engine. In the configuration described above, since the first ammonia slip catalyst is arranged between the first reduction catalyst and the oxidation catalyst, the leaked _NH3 is converted into NOx such as NO and _NO2 by the first ammonia slip catalyst. After being converted, it flows into the oxidation catalyst. As a result, it is possible to prevent the leaked NH ₃ from being oxidized to N ₂ O, which is difficult to purify and is highly harmful, in the oxidation catalyst. As a result, the NOx purification efficiency can be further improved.

The exhaust purification device described above may further include a second ammonia slip catalyst arranged downstream of the second reduction catalyst in the exhaust passage. In this case, _NH3 in the exhaust gas is oxidized to NOx such as NO and _NO2 by the second ammonia slip catalyst, and then discharged to the outside of the exhaust purification device, so that _NH3 , which is a harmful substance, is released into the atmosphere. It is possible to suppress the situation that is released inside.

ADVANTAGE OF THE INVENTION According to this invention, purification efficiency of NOx can be improved.

FIG. 1 is a schematic configuration diagram showing an exhaust purification device according to one embodiment. FIG. 2 is a schematic configuration diagram showing the first reduction catalyst and first ammonia slip catalyst in FIG. FIG. 3 is a schematic diagram showing the second reduction catalyst of FIG. 1. FIG. FIG. 4 is a schematic configuration diagram showing an exhaust purification device according to a comparative example. FIG. 5 is a graph showing the temperature of the exhaust gas and the temperature of each catalyst in chronological order. FIG. 6 is a graph showing in chronological order the temperature of the exhaust gas flowing into the inlet of the oxidation catalyst and the concentration of each component in the exhaust gas flowing out of the outlet of the oxidation catalyst.

An embodiment of the present invention will be described in detail below with reference to the drawings. In the drawings, the same or equivalent elements are denoted by the same reference numerals, and overlapping descriptions are omitted as appropriate.

FIG. 1 is a schematic configuration diagram showing an exhaust purification device 1 according to this embodiment. The exhaust purification device 1 is mounted on a vehicle such as a truck, for example, and purifies exhaust gas G emitted from an engine 2, which is an internal combustion engine. The engine 2 is, for example, a diesel engine. The vehicle on which the exhaust purification device 1 is mounted may be a large-sized vehicle such as a tractor or a bus, a medium-sized vehicle, a small-sized vehicle, or a light vehicle, in addition to a truck. In the following description, "upstream side" means the upstream side in the flow direction of the exhaust gas G, and "downstream side" means the downstream side in the flow direction of the exhaust gas G.

The exhaust purification device 1 is mounted in an exhaust passage 5 through which the exhaust gas G flows. The exhaust purification device 1 includes, in order from the upstream side to the downstream side of the exhaust passage 5, SCR 11 (first reduction catalyst), ASC 12 (first ammonia slip catalyst), DOC 13 (oxidation catalyst), and DPF 14 ( filter), SCR 15 (second reduction catalyst), and ASC 16 (second ammonia slip catalyst). Further, the exhaust purification device 1 includes a urea water injection part 21 (first reducing agent injection part) for injecting urea water into the exhaust gas G flowing upstream of the SCR 11 and a urea water injection part into the exhaust gas G flowing upstream of the SCR 15 . A urea water injection unit 22 (second reducing agent injection unit) that injects water, a control unit 23 that controls the urea water injection unit 21 and the urea water injection unit 22, and the temperature of the exhaust gas G flowing upstream of the SCR 11. and a sensor 32 for detecting the temperature of the exhaust gas G flowing through the SCR 15 .

The SCR (Selective Catalytic Reduction) 11 is a selective reduction catalyst that selectively reduces and purifies NOx (nitride oxide) in the exhaust gas G. are placed in Directly below the engine 2 is a position on the downstream side of the outflow port of the engine 2 and close to the outflow port. The position immediately below the engine 2 may be a position within a range where the cooling of the exhaust gas G flowing out from the engine 2 is substantially negligible. The SCR 11 converts NOx in the exhaust gas G into N ₂ (nitrogen) and H ₂ O ( _water ) into harmless substances.

ASC (Ammonia Slip Catalyst) 12 is a catalyst that reduces NH ₃ leaked from SCR 11 . When NH ₃ leaking from SCR 11 flows into DOC 13, NH ₃ is oxidized to N ₂ O in DOC 13. Since N ₂ O is difficult to purify and highly harmful, it is desirable to suppress the generation of N ₂ O as much as possible. Therefore, by arranging the ASC 12 between the SCR 11 and the DOC 13, the situation in which a large amount of N ₂ O is generated in the DOC 13 is suppressed.

A DOC (Diesel Oxidation Catalyst) 13 is an oxidation catalyst that oxidizes NO (nitrogen monoxide), HC (hydrocarbon), CO (carbon monoxide), etc. in the exhaust gas G. The DOC 13 is composed of, for example, a carrier such as zeolite on which a metal or metal oxide is carried. A DPF (Diesel Particulate Filter) 14 is a filter that collects and removes particulate matter (PM: Particulate Matter) in the exhaust gas G that has passed through the DOC 13 . The DPF 14 is made of, for example, ceramics or metal porous bodies. A light oil supply unit that supplies light oil to the DOC 13 may be provided upstream of the DOC 13 .

The SCR 15 is a selective reduction catalyst that selectively reduces NOx in the exhaust gas G to purify it. The SCR 15 converts NOx in the exhaust gas G into harmless substances such as N ₂ and H ₂ O through a reduction reaction using NH ₃ produced from the urea water injected from the urea water injection part 22 as a reducing agent. do. ASC16 is an oxidation catalyst that oxidizes _NH3 leaked from SCR15. As the ASC 16, for example, a carrier such as zeolite on which a noble metal such as Pt is supported may be used.

The urea water injection part 21 is arranged between the engine 2 and the SCR 11 in the exhaust passage 5 . The urea water injection part 21 is configured to be able to inject urea water into the exhaust gas G flowing between the engine 2 and the SCR 11 . The urea water injection part 22 is arranged between the DPF 14 and the SCR 15 in the exhaust passage 5 . The urea water injection part 22 is configured to be able to inject urea water into the exhaust gas G flowing between the DPF 14 and the SCR 15 . The urea water injection unit 21 and the urea water injection unit 22 inject urea water under the control of the control unit 23 . When the exhaust gas G is at a predetermined temperature (for example, 180° C.) or higher, when the urea water is injected into the exhaust gas G, NH ₃ is produced by thermal decomposition reaction and hydrolysis reaction.

The sensor 31 is arranged, for example, between the engine 2 and the SCR 11 in the exhaust passage 5 and upstream of the urea water injection section 21 . The sensor 31 detects the temperature of the exhaust gas G flowing between the engine 2 and the SCR 11 and outputs the detected temperature to the controller 23 . The sensor 32 is arranged at the DOC 13, for example. The sensor 32 detects the temperature of the DOC 13 and outputs the detected temperature to the controller 23 . "Detecting the temperature of the DOC 13" includes both the direct detection of the DOC 13 temperature and the indirect detection of the DOC 13 temperature. The sensor 32 may indirectly detect the temperature of the DOC 13 by, for example, detecting the temperature of the DOC 13 or the temperature of the exhaust gas G flowing upstream of the DOC 13 and estimating the temperature of the DOC 13 from the detected temperature. The sensor 32 may be arranged in the exhaust passage 5 on the upstream side of the DOC 13 when detecting the temperature of the exhaust gas G flowing upstream of the DOC 13 . For example, sensor 32 may be positioned between ASC 12 and DOC 13 in exhaust passage 5 .

The control unit 23 is, for example, a computer configured including a CPU, a RAM, a ROM, and an input/output interface, etc. According to a control program stored in the ROM, the control unit 23 takes in various sensor information and the like from the input interface circuit, and outputs it to the output interface. Outputs the command signals necessary for various controls from the circuit. The control unit 23 is electrically connected to the urea water injection unit 21 , the urea water injection unit 22 , the sensor 31 and the sensor 32 . The control unit 23 may be, for example, a vehicle ECU (Electronic Control Unit).

The control unit 23 controls the urea water injection unit 21 and the urea water injection unit 22 based on the temperatures detected by the sensors 31 and 32 . When the temperature detected by the sensor 31 is equal to or higher than a predetermined first temperature (for example, 200° C.), the control section 23 causes the urea water injection section 21 to inject the urea water. On the other hand, when the temperature detected by sensor 31 is lower than the first temperature, control unit 23 does not cause urea water injection unit 21 to inject urea water. The first temperature is the SCR 11 activation temperature, which is the temperature required to generate NH ₃ from urea water.

When the temperature detected by the sensor 32 is equal to or higher than a predetermined second temperature (for example, 200° C.), the control section 23 causes the urea water injection section 22 to inject the urea water. On the other hand, when the temperature detected by sensor 32 is equal to or higher than the second temperature, control unit 23 does not cause urea water injection unit 22 to inject urea water. The second temperature is the activation temperature of DOC13. Although the first temperature and the second temperature are the same in this embodiment, the first temperature and the second temperature may be different.

When the temperature detected by the sensor 31 is equal to or higher than the first temperature and the temperature detected by the sensor 32 is equal to or higher than the second temperature, the controller 23 injects the urea water at the first injection amount. The urea water injection part 21 is caused to inject, and the urea water injection part 22 is caused to inject the second injection amount of the urea water. The first injection amount is a scheduled injection amount to be injected from the urea water injection part 21, and is an amount of NH ₃ necessary for the progress of the NOx reduction reaction in the SCR 11 (that is, the NOx reduction reaction in the SCR 11). This is the amount of urea water injected that can generate the amount of NH ₃ that can be consumed at the time of . The second injection amount is a scheduled injection amount to be injected from the urea water injection part 22, and is an amount of NH ₃ necessary for the progress of the NOx reduction reaction in the SCR 15 (that is, the NOx reduction reaction in the SCR 15). This is the amount of urea water injected that can generate the amount of NH ₃ that can be consumed at the time of . That is, when the temperature detected by the sensor 31 is equal to or higher than the first temperature and the temperature detected by the sensor 32 is equal to or higher than the second temperature, the control unit 23 reduces the planned injection amount. The urea water is injected as it is from the urea water injection part 21 and the urea water injection part 22 respectively.

When the temperature detected by the sensor 31 is equal to or higher than the first temperature, the temperature of the exhaust gas G flowing upstream of the SCR 11 is equal to or higher than the temperature required to generate _NH3 from urea water. Under this environment, when the first injection amount of urea water is injected from the urea water injection part 21 on the upstream side of the SCR 11, _NH3 is generated from the injected urea water by thermal decomposition reaction and hydrolysis reaction. . When the produced NH ₃ flows into the SCR 11, the NOx reduction reaction proceeds in the SCR 11. Also, when the temperature detected by the sensor 32 is equal to or higher than the second temperature, the temperature of the DOC 13 is equal to or higher than the activation temperature. If the temperature of the DOC 13 is higher than the activation temperature, the temperature of the exhaust gas G on the downstream side is _sufficiently increased. above temperature. Under this environment, when the second injection amount of urea water is injected from the urea water injection part 22, NH ₃ is produced from the injected urea water by thermal decomposition reaction and hydrolysis reaction. When the generated NH ₃ flows into the SCR 15, NOx reduction reaction proceeds in the SCR 15.

When the temperature detected by the sensor 31 is equal to or higher than the first temperature and the temperature detected by the sensor 32 is lower than the second temperature, the controller 23 controls the first injection amount and the second injection amount. The urea water injection part 21 is caused to inject the total amount of urea water and the injection amount, and the urea water injection part 22 is not caused to inject the urea water. That is, when the temperature detected by the sensor 31 is equal to or higher than the first temperature and the temperature detected by the sensor 32 is lower than the second temperature, the control unit 23 causes the urea water injection unit 21 to inject The urea water injection part 21 injects the urea water in an amount obtained by adding the planned injection amount to be injected from the urea water injection part 22 to the planned injection amount.

When the temperature detected by the sensor 31 is equal to or higher than the first temperature, the temperature of the exhaust gas G flowing upstream of the SCR 11 is equal to or higher than the temperature required to generate _NH3 from urea water. Under this environment, when the total amount of urea water of the first injection amount and the second injection amount is injected from the urea water injection unit 21, the total amount of urea water is generated by thermal decomposition reaction and hydrolysis reaction. A large amount of NH ₃ is produced depending on . When this NH ₃ flows into the SCR 11, the NOx reduction reaction proceeds in the SCR 11. During the NOx reduction reaction in the SCR 11, an amount of NH ₃ corresponding to the first injection amount is consumed, and an amount of NH ₃ corresponding to the second injection amount cannot be consumed by the SCR 11, and is downstream of the SCR 11. leak to. NH ₃ leaked from SCR 11 reaches SCR 15 via ASC 12 , DOC 13 and DPF 14 . Then, in SCR 15, the NOx reduction reaction using the leaked NH ₃ proceeds.

Next, the catalyst configuration of the SCR 11 and ASC 12 will be described with reference to FIG. FIG. 2 is a schematic diagram showing the catalyst configuration of SCR11 and ASC12. As shown in FIG. 2, SCR 11 is an SCR containing Cu (copper). An SCR containing Cu is hereinafter referred to as a "Cu-based SCR". Cu-based SCRs include, for example, those in which copper zeolite is supported on a carrier such as ceramic. A Cu-based SCR can allow the NOx reduction reaction to proceed sufficiently in an environment where a large amount of NO is present. An environment in which a large amount of NO exists means an environment in which the proportion of NO in NOx is higher than the proportion of NO ₂ (nitrogen dioxide) in NOx.

Cu-based SCR has the property of adsorbing a large amount of _NH3 . Therefore, most of the NH ₃ produced from the urea water on the upstream side of the SCR 11 is adsorbed on the SCR 11 when passing through the SCR 11 . The NH ₃ adsorbed on the SCR 11 leaks from the SCR 11 due to the rapid temperature rise caused by the exhaust gas G immediately after being discharged from the engine 2 . In the example shown in FIG. 2, the entire SCR 11 is a Cu-based SCR, but only part of the SCR 11 may be a Cu-based SCR.

ASC 12 is a catalyst that has a function as an SCR in addition to a function as an oxidation catalyst. The ASC 12 has an oxidation section 12a functioning as an oxidation catalyst and a reduction section 12b functioning as an SCR. The oxidation section 12 a is arranged upstream of the ASC 12 . The oxidized portion 12a contains a noble metal such as Pt (platinum), Pd (palladium), or Ru (ruthenium). As the oxidized portion 12a, for example, a carrier such as zeolite carrying Pt can be used.

The oxidation unit 12a oxidizes _NH3 leaked from the SCR 11 into NOx such as NO and _NO2 , and oxidizes NO in the exhaust gas G that has passed through the SCR 11 into _NO2 . _If the oxidizing power of the oxidizing part _12a to _{NH3 is} too high, the oxidizing part 12a will oxidize _NH3 to _N2O . It is set to the extent that it is possible to oxidize _NH3 to NO, _NO2, etc. without

The reducing section 12b is arranged downstream of the ASC 12 . The reducing part 12b is, for example, an SCR containing metal such as Fe (iron) or Cu (copper). In the present embodiment, the case where the reducing part 12b is an SCR containing Fe is exemplified. An SCR containing Fe is hereinafter referred to as an "Fe-based SCR". Examples of Fe-based SCRs include those in which iron zeolite is carried on a carrier such as ceramic. The reducing section 12b selectively reduces and purifies NOx in the exhaust gas G that has passed through the oxidizing section 12a. Specifically, the reduction unit 12b reduces NOx that has not been reduced in the SCR 11, and reduces NOx that is produced by the oxidation of NH ₃ in the oxidation unit 12a.

The NH ₃ in the exhaust gas G that has flowed into the ASC 12 is oxidized into NOx such as NO and NO ₂ in the oxidation section 12a, and is consumed in the reduction section _12b for the reduction reaction of NOx. Leaks are suppressed. Furthermore, the NOx in the exhaust gas G flowing out from the ASC 12 is purified by the NOx reduction reaction in the reducing section 12b. The oxidized portion 12a and the reduced portion 12b are formed respectively in the upstream and downstream portions of the ASC 12 by, for example, two-layer coating or zone coating.

Next, the catalyst configuration of the SCR 15 will be described with reference to FIG. FIG. 3 is a schematic diagram showing the catalyst configuration of the SCR 15. As shown in FIG. As shown in FIG. 3, the upstream portion 15a of the SCR 15 is a Fe-based SCR, and the downstream portion 15b of the SCR 15 is a Cu-based SCR. The Fe-based SCR can sufficiently promote the reduction reaction of NOx in an environment where NO and NO ₂ coexist in the exhaust gas G. An environment in which NO and _NO2 coexist is an environment in which both NO and _NO2 exist to some extent, for example, the ratio of NO in NOx (the ratio of NO) and the ratio of _NO2 in NOx. (ratio of NO ₂ ) is about the same. On the other hand, the Cu-based SCR, as described above, allows the NOx reduction reaction to proceed sufficiently in an environment in which a large amount of NO exists (or only NO exists). NOx in the exhaust gas G that has flowed into the SCR 15 is purified in the Fe-based SCR and then further purified in the Cu-based SCR.

In the example shown in FIG. 3, the proportion of Fe-based SCR in the SCR 15 (the proportion of Fe-based SCR) and the proportion of the Cu-based SCR in the SCR 15 (percentage of Cu-based SCR) are both about 1/2. However, it is not limited to this example, and the ratio of Fe-based SCR and the ratio of Cu-based SCR can be changed as appropriate. The proportion of Fe-based SCR may be determined, for example, according to the magnitude of the NO ₂ proportion and the NO proportion in the DOC 13 .

Next, the operation and effect of the exhaust gas purification device 1 according to this embodiment will be described together with the problems of the comparative example.

FIG. 4 is a schematic configuration diagram showing an exhaust purification device 100 according to a comparative example. As shown in FIG. 4, the exhaust purification device 100 includes, in order from the upstream side of the exhaust passage 5, a DOC 113 that purifies CO and HC in the exhaust gas G and oxidizes NO out of NOx, and a particulate It has a DPF 114 that collects substances, an SCR 115 that purifies NOx in the exhaust gas G, and an ASC 116 that oxidizes NH ₃ leaked from the SCR 115 . In the exhaust purification device 100, the sensor 132 arranged on the upstream side of the SCR 115 detects the temperature of the exhaust gas G, and when the temperature of the exhaust gas G reaches or exceeds a predetermined temperature, the control unit 123 causes the urea water injection unit 122 to operate. Inject urea water from On the other hand, when the temperature of the exhaust gas G is lower than the predetermined temperature, even if the urea water is injected from the urea water injection part 122, _NH3 cannot be generated from the urea water, and NOx reduction reaction occurs in the SCR 115. is not promoted.

In the exhaust purification device 100, as shown in FIG. 4, the SCR 115 is arranged downstream of the DOC 113 and the DPF 114, so the distance from the engine 2 to the SCR 115 is long. Therefore, when starting the engine 2, it takes time for the temperature of the exhaust gas G on the upstream side of the SCR 115 to reach a predetermined temperature. Therefore, the reduction reaction of NOx is not promoted in the SCR 115 until the temperature of the exhaust gas G reaches the predetermined temperature. Therefore, it is difficult to improve the NOx purification efficiency in the exhaust gas purification device 100 .

On the other hand, in the exhaust purification device 1 according to the present embodiment, as shown in FIG. 1, the SCR 11 is arranged upstream of the DOC 13 and the DPF 14. can flow to SCR 11. As a result, the SCR 11 can be activated early, and the NOx purification efficiency can be improved. Furthermore, the SCR 11 is a Cu-based SCR. Since the exhaust gas G discharged from the engine 2 contains a large amount of NO, the exhaust gas G with a high NO ratio flows into the SCR 11 . In such an environment where a large amount of NO exists (or only NO exists), it is preferable to use a Cu-based SCR rather than using other SCRs from the viewpoint of sufficiently exhibiting the NOx purification performance. . Therefore, by using a Cu-based SCR as the SCR 11, the reduction reaction of NOx can be sufficiently advanced in the SCR 11 arranged immediately after the engine 2, and NOx can be purified efficiently.

Exhaust gas G that has passed through SCR 11 flows into SCR 15 after passing through DOC 13 and DPF 14 . Here, when starting the engine 2, it takes time for the temperature of the exhaust gas G on the downstream side to rise sufficiently. During the period until the temperature of the exhaust gas G on the downstream side rises sufficiently, the temperatures of the catalysts such as the DOC 13 and the SCR 15 on the downstream side do not rise sufficiently. FIG. 5 is a graph showing in chronological order the temperature of the exhaust gas G and the temperature of each catalyst through which the exhaust gas G passes. In FIG. 5, the horizontal axis indicates the operating time of the vehicle, and the vertical axis indicates the temperature of the exhaust gas G and the temperature of each catalyst through which the exhaust gas G passes. In FIG. 5, "gas temperature" indicates the temperature of the exhaust gas G immediately after being discharged from the engine 2, "SCR IN" indicates the temperature at the inlet of the SCR 11, and "DOC IN" indicates the temperature at the inlet of the DOC 13. "DPF OUT" indicates the temperature at the outlet of DPF 14, and "SCR2 IN" indicates the temperature at the inlet of SCR 15.

As shown in FIG. 5, the "gas temperature" and "SCR IN" immediately below the engine 2 sharply rise after the start of operation, reaching around 200°C 50 seconds after the start of operation. has reached On the other hand, "DOC IN", "DPF OUT", and "SCR2 IN" on the downstream side away from the engine 2 gradually increase as the operating time elapses. "DOC IN" does not reach around 200°C until 300 seconds have passed since the start of operation, and "SCR2 IN" reaches around 200°C until 600 seconds after the start of operation. not reach. In this way, the temperature of the SCR 15 does not reach the temperature (approximately 200° C.) required for generating NH ₃ from the urea water during the period from the start of operation until 600 seconds have passed. Even if urea water is injected from the water injection part 22, _NH3 cannot be produced from this urea water. However, the temperature of the DOC 13 has not risen sufficiently and has not reached the activation temperature of the DOC 13 (around 200° C.) during the period from the start of operation until 300 seconds have passed. That is, since DOC13 is not activated, the oxidation reaction in DOC13 does not proceed sufficiently. Therefore, during the period until the temperature of the catalyst on the downstream side sufficiently rises, most of the NH ₃ in the exhaust gas G that has passed through the SCR 11 flows out of the DOC 13 without being oxidized in the DOC 13. Become.

FIG. 6 is a graph showing the temperature of the exhaust gas G flowing into the inlet of the DOC 13 and the concentration of each component in the exhaust gas G flowing out of the outlet of the DOC 13 in time series. In FIG. 6 , the horizontal axis indicates the operating time of the vehicle, the left vertical axis indicates the concentration of each component in the exhaust gas G flowing out from the outlet of the DOC 13, and the right vertical axis indicates the exhaust gas flowing into the inlet of the DOC 13. The temperature of gas G is shown. In FIG. 6 , “gas temperature” indicates the temperature of the exhaust gas G flowing into the inlet of the DOC 13 (that is, the temperature of the inlet of the DOC 13), and “NH ₃ ” indicates NH in the exhaust gas G flowing out of the outlet of the DOC 13. ₃ , “NO” indicates the concentration of NO in the exhaust gas G flowing out from the outlet of the DOC 13, and “NO ₂ ” indicates the concentration of NO ₂ in the exhaust gas G flowing out from the outlet of the DOC 13. , and “N ₂ O” indicates the concentration of N ₂ O in the exhaust gas G flowing out from the outlet of the DOC 13 .

As shown in FIG. 6, in the period until the "gas temperature" reaches around 200° C., the concentration of NH ₃ is extremely high, and the concentrations of NO, NO ₂ and N ₂ O are low. It's becoming Then, when the "gas temperature" exceeds around 200°C, the concentration of _NH3 drops extremely and the concentrations of NO, _NO2 , and _N2O increase. This is because most of the NH ₃ in the exhaust gas G that has flowed into the DOC 13 is oxidized into NOx such as NO, NO ₂ and N ₂ O by the DOC 13 until the temperature of the DOC 13 reaches around 200°C. It means that it is flowing out from the DOC 13 without being processed. On the other hand, when the temperature of the DOC 13 exceeds around 200° C., the NH ₃ in the exhaust gas G is oxidized to NOx by the DOC 13, so the concentration of NH ₃ decreases and the concentration of NOx increases. In this way, during the period until the temperature of the DOC 13 reaches around 200° C., which is the activation temperature, most of the NH ₃ in the exhaust gas G flows out to the downstream side of the DOC 13 as it is without being oxidized in the DOC 13. I understand. Then, the NH ₃ that has flowed out downstream of the DOC 13 is supplied to the SCR 15 via the DPF 14 . Therefore, if NH ₃ is actively leaked to the SCR 15 using this period, it is considered that the NOx reduction reaction using the leaked NH ₃ can proceed in the SCR 15 .

Therefore, in the exhaust purification device 1 according to the present embodiment, when the temperature of the DOC 13 is equal to or higher than the activation temperature, the urea water injection unit 22 does not inject the urea water, and the urea water injection unit 21 does not inject the urea water. The urea water injection unit 21 injects the urea water in an amount obtained by adding the second injection amount to be injected from the urea water injection unit 22 to the injection amount of 1. By injecting excess urea water that cannot be processed in the SCR 11 from the urea water injection unit 21 in this way, NH ₃ produced from the urea water can be actively leaked downstream of the SCR 11 . When the temperature of the DOC 13 is lower than the activation temperature, most of the leaked NH ₃ is supplied to the SCR 15 without being oxidized in the DOC 13, so that the reduction reaction of NOx can proceed in the SCR 15. That is, the reduction reaction of NOx can be efficiently advanced during the period until the temperature of the exhaust gas G on the downstream side rises sufficiently. Therefore, according to the exhaust emission control device 1 according to the present embodiment, the NOx purification efficiency can be improved.

In the exhaust purification device 1 according to the present embodiment, when the temperature of the DOC 13 detected by the sensor 32 is equal to or higher than the activation temperature of the DOC 13, the control unit 23 causes the urea water injection unit 21 to is injected, and the second injection amount of the urea water is injected from the urea water injection part 22 . When the temperature of the DOC 13 is equal to or higher than the activation temperature of the DOC 13, if NH ₃ leaks into the DOC 13, the leaked NH ₃ is oxidized into N ₂ O, which is difficult to purify and highly harmful. Therefore, the leakage of NH ₃ to the DOC 13 is suppressed by injecting a predetermined amount (first injection amount) of urea water instead of excess urea water from the urea water injection unit 21 . Thereby, the situation in which N ₂ O is generated in the DOC 13 can be suppressed. Moreover, when the temperature of the DOC 13 is equal to or higher than the activation temperature of the DOC 13, NH ₃ can be generated from the urea water near the SCR 15. Therefore, by injecting a predetermined amount (second injection amount) of urea water from the urea water injection unit 22 , the NOx reduction reaction can be advanced in the SCR 15 .

In the exhaust purification device 1 according to this embodiment, the upstream portion 15a of the SCR 15 is a Fe-based SCR, and the downstream portion 15b of the SCR 15 is a Cu-based SCR. Since the NO in the exhaust gas G that has passed through the ASC 12 is oxidized to NO ₂ by the DOC 13, the upstream side portion 15a of the SCR 15 is in a state where the proportion of NO ₂ is increased (for example, the proportions of NO and NO ₂ are the same). Exhaust gas G in a state of about In such an environment where NO and NO ₂ coexist, it is preferable to use the Fe-based SCR as described above. Therefore, by using the Fe-based SCR for the upstream portion 15a of the SCR 15, the reduction reaction of NOx can be sufficiently advanced in the upstream portion 15a, and NOx can be purified efficiently.

On the other hand, the Fe-based SCR cannot sufficiently progress the NOx reduction reaction in an environment in which only NO exists (or the proportion of NO is high). Therefore, if the exhaust gas G in which the ratio of NO is relatively high with respect to the ratio of NO ₂ flows into the upstream portion 15a, NO is reduced during the NOx reduction reaction in the Fe-based SCR. It is assumed that NO will remain in the exhaust gas G because the exhaust gas G cannot be exhausted. On the other hand, the Cu-based SCR, as described above, allows the NOx reduction reaction to proceed sufficiently in an environment in which only NO exists (or the proportion of NO is high). Therefore, by making the downstream portion 15b of the SCR 15 a Cu-based SCR, it is possible to efficiently purify NOx in the exhaust gas G that has passed through the Fe-based SCR. Further, NO, NO ₂ , etc., are purified in the upstream portion 15a of the SCR 15, thereby reducing NO, NO _2, etc. in the exhaust gas G flowing into the downstream portion 15b (Cu-based SCR) of the SCR 15. By reducing the amount of NO ₂ flowing into the Cu-based SCR in this way, it is possible to prevent the Cu-based SCR from generating a large amount of N ₂ O, which is difficult to purify and is highly harmful. Therefore, according to the exhaust emission control device 1 according to the present embodiment, the purification efficiency of NOx in the exhaust gas G can be further improved.

The exhaust purification device 1 according to this embodiment includes an ASC 12 arranged between the SCR 11 and the DOC 13 in the exhaust passage 5 . As described above, the SCR 11, which is a Cu-based SCR, has the property of adsorbing a large amount of NH ₃ , so most of the NH ₃ produced from the urea water is adsorbed on the SCR 11, and the NH ₃ adsorbed on the SCR 11 is SCR 11 leaks due to a rapid temperature rise caused by the exhaust gas G immediately after being discharged from the SCR 11 . When ASC12 is placed between SCR11 and DOC13, the leaked _NH3 flows into DOC13 after being reduced by ASC12. As a result, in the DOC 13, it is possible to prevent the leaked NH ₃ from being oxidized to N ₂ O, which is difficult to purify and is highly harmful. As a result, the NOx purification efficiency can be further improved.

The exhaust purification device 1 according to this embodiment includes an ASC 16 arranged downstream of the SCR 15 in the exhaust passage 5 . As a result, the NH ₃ in the exhaust gas G is oxidized into NOx such as NO and NO ₂ by the ASC 16, and then discharged outside the exhaust purification device 1, so that NH ₃ , which is a harmful substance, is released into the atmosphere. You can suppress the situation that will be done.

The invention is not limited to the embodiments described above. The configuration of the exhaust purification device is not limited to the above-described embodiment, and can be changed as appropriate without departing from the gist of the claims. For example, the exhaust purification device may not include ASC12 and ASC16, or may include only one of ASC12 and ASC16. ASC 12 may have a configuration similar to ASC 16 . That is, the ASC 12 may have only the oxidized portion 12a. Conversely, ASC 16, like ASC 12, may have an oxidation portion 12a and a reduction portion 12b. The arrangement of the urea water injection part 21, the urea water injection part 22, the sensor 31, and the sensor 32 can be changed as appropriate. For example, the sensor 31 may be arranged on the downstream side of the urea water injection section 21 , and the sensor 32 may be arranged on the downstream side of the urea water injection section 22 . The exhaust purification device may not include the sensor 31 . When the temperature detected by the sensor 32 on the downstream side is equal to or higher than the second temperature, the temperature detected by the sensor 31 on the upstream side is considered to be equal to or higher than the first temperature. The urea water injection part 21 and the urea water injection part 22 may be controlled based only on the temperature detected by the sensor 32 . That is, when the temperature detected by the sensor 32 is equal to or higher than the second temperature, the control unit 23 causes the urea water injection unit 21 and the urea water injection unit 22 to inject the first injection amount of urea water and the second injection amount, respectively. 2 injection amount of the urea water may be injected.

DESCRIPTION OF SYMBOLS 1... Exhaust purification device, 2... Engine, 5... Exhaust passage, 11... SCR (first reduction catalyst), 12... ASC (first ammonia slip catalyst), 13... DOC (oxidation catalyst), 14... DPF ( filter), 15... SCR (second reduction catalyst), 15a... upstream part, 15b... downstream part, 16... ASC (second ammonia slip catalyst), 21... urea water injection part (first reducing agent Injection part), 22... Aqueous urea injection part (second reducing agent injection part), 23... Control part, 32... Sensor, G... Exhaust gas.

Claims (5)

An exhaust purification device for purifying exhaust gas emitted from an engine,
a first reduction catalyst disposed in an exhaust passage through which the exhaust gas flows to reduce NOx contained in the exhaust gas;
an oxidation catalyst disposed downstream of the first reduction catalyst in the exhaust passage and oxidizing at least NO of the NOx;
a filter arranged downstream of the oxidation catalyst in the exhaust passage and collecting particulate matter contained in the exhaust gas;
a second reduction catalyst arranged downstream of the filter in the exhaust passage and reducing NOx contained in the exhaust gas;
a first reducing agent injecting part disposed upstream of the first reduction catalyst in the exhaust passage and configured to inject a reducing agent into the exhaust gas;
a second reducing agent injection unit disposed between the filter and the second reduction catalyst in the exhaust passage and configured to inject a reducing agent into the exhaust gas;
a sensor that detects the temperature of the oxidation catalyst;
a control unit that controls the first reducing agent injection unit and the second reducing agent injection unit based on the temperature of the oxidation catalyst detected by the sensor;
with
The first reduction catalyst contains copper (Cu),
When the temperature of the oxidation catalyst detected by the sensor is lower than the activation temperature of the oxidation catalyst, the control unit does not inject the reducing agent from the second reducing agent injecting unit. The amount of the reducing agent obtained by adding the second injection amount to be injected from the second reducing agent injection unit to the first injection amount to be injected from one reducing agent injection unit is added to the first reducing agent injection unit. Exhaust purification device that injects from the agent injection part. When the temperature of the oxidation catalyst detected by the sensor is equal to or higher than the activation temperature of the oxidation catalyst, the control unit supplies the first injection amount of the reducing agent from the first reducing agent injection unit. 2. The exhaust emission control system according to claim 1, wherein the second reducing agent injecting portion injects the reducing agent in the second injection amount. The upstream portion of the second reduction catalyst contains iron (Fe),
3. The exhaust purification device according to claim 1, wherein the downstream portion of said second reduction catalyst contains copper (Cu). 4. The exhaust purification device according to claim 1, further comprising a first ammonia slip catalyst disposed between said first reduction catalyst and said oxidation catalyst in said exhaust passage. The exhaust purification device according to any one of claims 1 to 4, further comprising a second ammonia slip catalyst disposed downstream of said second reduction catalyst in said exhaust passage.

Images