JP2016050523A - Exhaust emission control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control system capable of enhancing a reduction in the amount of NOx.SOLUTION: The exhaust emission control system includes an exhaust passage 10 in which exhaust gas flows, a burner 20 of which a combustion space 15 for fuel is formed on the upstream side of the internal of the exhaust passage 10, a NOx adsorbent 50 located on the downstream side of the combustion space 15 for adsorbing nitrogen oxide contained in the exhaust gas, a selective reduction type catalyst 51 located on the further downstream side than the NOx adsorbent 50, an addition valve 53 located on the further upstream side than the selective reduction type catalyst 51, and a connection passage 54 located in the combustion space 15 for supplying urea water to the addition valve 53, and connected to the addition valve 53 for carrying the urea water to the addition valve 53, part of the connection passage 54 passing through the combustion space 15. A DPF 30 is provided upstream of the NOx adsorbent 50. With the DPF 30, a selective reduction type catalyst 29 is integrated which is located upstream of the selective reduction type catalyst 51.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an exhaust purification system equipped with a selective reduction catalyst.

  Conventionally, as disclosed in Patent Document 1, for example, as an exhaust gas purification system for reducing nitrogen oxides (hereinafter referred to as NOx) contained in exhaust gas, a urea SCR (Selective) using a urea water addition device and a selective reduction catalyst is used. There is a Catalytic Reduction) system. In the urea SCR system, the exhaust gas to which urea water is added by the urea water adding device flows into the selective catalytic reduction catalyst, and the urea water is converted into ammonia by hydrolysis. In the selective reduction catalyst, NOx is reduced by ammonia and converted into nitrogen and water.

JP 2013-11193 A

By the way, the urea water added to the exhaust gas is hydrolyzed and converted into ammonia by the heat of the exhaust gas and the heat of the selective catalytic reduction catalyst. For this reason, when the temperature of the exhaust gas or the temperature of the selective catalytic reduction catalyst is low, the urea water is not easily converted into ammonia, and urea contained in the urea water is crystallized, so the amount of urea water added is limited. I have to. Such restriction of urea water causes a reduction in the amount of NOx reduction.
An object of the present invention is to provide an exhaust purification system capable of increasing the amount of NOx reduction.

  An exhaust purification system that solves the above problems includes an exhaust passage through which exhaust gas flows and a burner disposed in the exhaust passage, and a combustion space for fuel in the burner is part of the exhaust passage. The burner, a NOx adsorbent that is located downstream of the combustion space in the exhaust passage and adsorbs nitrogen oxides contained in the exhaust gas, and a selection that is located downstream of the NOx adsorbent in the exhaust passage A reduction catalyst, an addition valve located between the selective reduction catalyst and the combustion space in the exhaust passage, and a connection passage through which urea water is connected to the addition valve and flows toward the addition valve; A part of the connection passage includes the connection passage passing through the combustion space.

  According to the above configuration, even when the catalyst temperature is lower than the activation temperature, urea water heated in the combustion space by driving the burner or hydrolyzed ammonia can be supplied to the exhaust gas. As a result, the amount of NOx reduction increases. Further, the NOx adsorbent located upstream of the selective reduction catalyst can adsorb NOx even if the catalyst temperature of the selective reduction catalyst is lower than the activation temperature. Therefore, it is possible to prevent NOx from being released into the atmosphere even during a cold start or a light load operation that is lower than the activation temperature at which the selective catalytic reduction catalyst is activated.

  In the exhaust purification system, a DPF (Diesel Particulate Filter) may be provided in the exhaust passage between the burner and the NOx adsorbent.

  According to the above configuration, the particulate matter generated by the combustion of fuel in the engine can be captured by the DPF.

  In the exhaust purification system, the selective reduction catalyst is a downstream selective reduction catalyst, the addition valve is a downstream addition valve, and the connection passage is a connection passage for the downstream addition valve. The DPF may be integrated with an upstream selective reduction catalyst located upstream of the downstream selective reduction catalyst in the exhaust passage. In this case, the upstream addition valve positioned further upstream than the upstream selective reduction catalyst in the exhaust passage, and the upstream flow of urea water connected to the upstream addition valve and flowing toward the upstream addition valve. A connection passage for a side addition valve, wherein a part of the connection passage on the upstream side includes a connection passage for the upstream addition valve that passes through the combustion space.

  According to the above configuration, NOx can be reduced on the upstream side of the NOx adsorbent and the downstream selective reduction catalyst, and the burden on the NOx adsorbent and the downstream selective reduction catalyst can be reduced. In the DPF, the upstream selective reduction catalyst is integrally provided on the upstream surface, and the NOx adsorbent is integrally provided on the downstream surface, thereby simplifying the configuration and reducing the number of parts. Reduction can be achieved.

  In the exhaust purification system, the exhaust passage includes a combustion space forming wall that forms the combustion space, and the combustion space forming wall partitions the space where the selective catalytic reduction catalyst is located and the combustion space, The selective reduction catalyst may be heated through the combustion space forming wall when the fuel is burned by the burner.

  According to the above configuration, when the urea water is heated by driving the burner, the selective catalytic reduction catalyst is heated through the combustion space forming wall. Thereby, the time required for the catalyst temperature of the selective catalytic reduction catalyst to reach the activation temperature can be shortened.

  The exhaust purification system may further include a control device that acquires a catalyst temperature of the selective catalytic reduction catalyst and drives the burner when the acquired catalytic temperature is lower than an activation temperature of the selective catalytic reduction catalyst. It may be.

  According to the said structure, the fuel consumption for heating urea water with a burner can be suppressed to the minimum.

  In the exhaust purification system, the burner is an upstream burner located upstream of the NOx adsorbent in the exhaust passage. In the exhaust passage, a downstream burner may be further provided between the NOx adsorbent and the selective catalytic reduction catalyst. In this case, the control device drives the downstream burner when the acquired catalyst temperature is lower than the activation temperature of the selective catalytic reduction catalyst.

  According to the above configuration, the selective reduction catalyst can be heated and supplied to the selective reduction catalyst by driving the downstream burner when the catalyst temperature of the selective reduction catalyst is lower than the activation temperature. The urea water is heated and converted to ammonia by hydrolysis. Therefore, the time required for the catalyst temperature of the downstream selective catalytic reduction catalyst to reach the activation temperature can be further shortened.

It is a figure which shows schematic structure of the exhaust gas purification system in 1st Embodiment. It is a figure which shows schematic structure of a control apparatus. It is a flowchart which shows an example of the process which adds urea water. It is a flowchart which shows an example of the process which calculates the NOx amount adsorbed by the NOx adsorbent. It is a graph which shows an example of the relationship between catalyst temperature and NOx reduction rate. It is a figure which shows schematic structure of the exhaust gas purification system in 2nd Embodiment.

Hereinafter, an embodiment of the exhaust purification system will be described with reference to FIGS.
(First embodiment)

  As shown in FIG. 1, the exhaust purification system 1 is supplied with exhaust gas from an engine 2, purifies and discharges the supplied exhaust gas, and is disposed outside the inner cylinder 3 and the inner cylinder 3. It has a double-pipe structure composed of the outer cylinder 4 that is formed. The outer cylinder 4 has a bottomed cylindrical structure, and the tip part which is the end opposite to the end close to the engine 2 out of the two ends of the inner cylinder 3 is the outer cylinder 4. It is inserted in and is separated from the bottom surface 5 of the outer cylinder 4. Thereby, the inner space of the inner cylinder 3 communicates with the outer space between the outer surface of the inner cylinder 3 and the inner surface of the outer cylinder 4, and the entire inner cylinder 3 and outer cylinder 4 constitute the exhaust passage 10. To do.

  Specifically, the exhaust passage 10 includes a first exhaust passage 11 configured from the inner surface of the inner cylinder 3, a second exhaust passage 12 configured from the outer surface of the inner cylinder 3 and the inner surface of the outer cylinder 4, and the outer cylinder. 4, and a third exhaust passage 13 communicating with the first exhaust passage 11 and the second exhaust passage 12. Further, the exhaust passage 10 has a fourth exhaust passage 14 for exhausting the exhaust gas that has passed through the second exhaust passage 12. In the exhaust passage 10, the exhaust gas from the engine 2 is supplied to the first exhaust passage 11, then turned back in the third exhaust passage 13 and supplied to the second exhaust passage 12, and finally, the fourth exhaust passage. 14 is discharged. Such an exhaust passage 10 is configured such that the exhaust gas flow path is folded back at the third exhaust passage, and the second exhaust passage 12 is positioned outside the first exhaust passage 11, so that the exhaust gas flow path is configured. Is made long in the extending direction of the inner cylinder 3 and the outer cylinder 4.

  A combustion space 15 in which fuel is burned by the burner 20 is formed in a portion of the first exhaust passage 11 that is located closest to the engine 2. The combustion space 15 is surrounded by a combustion space forming wall 16 that is a part of the inner cylinder 3. The burner 20 includes a fuel supply unit 21 that supplies fuel to the combustion space 15 and a spark plug 22 that ignites the fuel supplied to the combustion space 15. The burner 20 is a catalyst of the downstream selective catalytic reduction catalyst 51 that is positioned downstream of the upstream selective catalytic reduction catalyst 29 when the catalyst temperature of the upstream selective catalytic reduction catalyst 29 is lower than the activation temperature or in the exhaust flow direction. Driven when the temperature is lower than the activation temperature. The drive of the burner 20 heats the upstream selective reduction catalyst 29 and the downstream selective reduction catalyst 51 to such an extent that the urea water is hydrolyzed to ammonia.

  The fuel supply unit 21 connects the fuel tank 24 and the injection nozzle 23 provided on the combustion space forming wall 16 through a fuel passage 25 such as a pipe, and the fuel in the fuel tank 24 is injected by the fuel pump 26 into the injection nozzle 23. To pump. A fuel opening / closing valve 27 is provided in the fuel passage 25 between the injection nozzle 23 and the fuel pump 26. The fuel on-off valve 27 opens the fuel passage 25 and supplies fuel to the injection nozzle 23 only when the burner 20 is driven. The spark plug 22 is, for example, a spark plug or a glow plug, and is provided on the combustion space forming wall 16 to ignite the fuel injected from the injection nozzle 23. The fuel burns with oxygen remaining in the exhaust gas as an oxidant. The injection nozzle 23 may be an injector in which the function of the fuel on-off valve 27 is built.

In the first exhaust passage 11, a DPF (Diesel Particulate Filter) 30 that captures particulate matter (PM) contained in the exhaust gas is provided on the downstream side of the combustion space 15. The DPF 30 is a wall flow filter made of, for example, ceramic or stainless steel having excellent heat resistance, and captures particulate matter contained in the exhaust gas when the wall is filtered.
The particulate matter captured by the DPF 30 is incinerated when the exhaust gas heated by combustion of the fuel in the burner 20 flows in.

  The filter constituting the DPF 30 supports the upstream selective reduction catalyst 29 such as zeolite, alumina, zirconia, etc., so that the DPF 30 can have the function of the selective reduction system. Examples of zeolite include copper zeolite, iron zeolite, silver zeolite, zinc zeolite, and cobalt zeolite. When reducing NOx contained in the exhaust, the upstream side selective reduction catalyst 29 plays an auxiliary role of the downstream side selective reduction catalyst 51 provided in the second exhaust passage 12 described later. That is, the NOx reduction performance increases as the amount of the selective catalytic reduction catalyst increases. On the other hand, if the selective reduction type catalyst of the filter is increased, that is, if it is too thick, the exhaust gas hardly flows through the exhaust passage, and the pressure loss of the filter increases. Therefore, the upstream selective reduction catalyst 29 here plays an auxiliary role for the reduction of the downstream selective reduction catalyst 51. The upstream selective reduction catalyst 29 is heated to the activation temperature or higher by the exhaust gas, and is heated to the activation temperature or more by the burner 20 when the temperature is lower than the activation temperature during cold start or light load operation.

  As the reducing agent used in the upstream selective reduction catalyst 29 provided in the DPF 30, urea water is used. Therefore, a urea water supply unit 31 that supplies urea water to the upstream side of the DPF 30 is provided on the upstream side of the DPF 30 in the first exhaust passage 11, that is, in the combustion space 15. The urea water supply unit 31 includes an upstream side addition valve 32 that adds urea water serving as a reducing agent or hydrolyzed ammonia to the exhaust gas from the combustion space 15. The upstream side addition valve 32 is provided on the combustion space forming wall 16 and adds urea water or ammonia in the flow direction of the exhaust gas.

  The urea water supply unit 31 connects the urea water tank 33 and the upstream side addition valve 32 provided on the combustion space forming wall 16 through a connection passage 34 such as a pipe. The urea water is pumped to the upstream side addition valve 32. In the connection passage 34, an open / close valve 36 is provided between the upstream side addition valve 32 and the urea water pump 35. The on-off valve 36 opens the connection passage 34 to supply urea water to the upstream addition valve 32, and shuts off the connection passage 34 to stop the supply of urea water to the upstream addition valve 32.

  A portion of the connection passage 34 that supplies urea water to the upstream side addition valve 32 that is located downstream of the on-off valve 36 functions as an upstream heating unit 37 that heats the urea water added to the selective reduction catalyst of the DPF 30. The upstream heating unit 37 is located in the combustion space 15 in the first exhaust passage 11 and is a pipe made of copper, stainless steel or the like having high thermal conductivity so that the length of the flow path in the combustion space 15 is increased. Is a passage formed in a coil shape or a zigzag shape, and efficiently heats the urea water. The upstream heating unit 37 heats the urea water to such an extent that the aqueous urea is hydrolyzed into ammonia when the fuel in the combustion space 15 is burned by the burner 20. From the upstream side addition valve 32, when the burner 20 is driven, heated urea water or ammonia is added upstream from the DPF 30.

  Further, in the first exhaust passage 11, an upstream side NOx concentration sensor 42 that detects the NOx concentration of the exhaust gas flowing into the combustion space 15 and a temperature of the exhaust gas that flows into the upstream selective reduction catalyst 29 of the DPF 30 are selected. An upstream temperature sensor 43 that detects the catalyst temperature of the reduction catalyst is provided.

  The first exhaust passage 11 is provided with a NOx adsorbent 50 that adsorbs NOx contained in the exhaust gas on the downstream side of the DPF 30 in the exhaust flow direction. The NOx adsorbent 50 is a filter that physically adsorbs NOx, and the material forming the NOx adsorbent 50 is an oxide of rare earth, alkali metal, alkaline earth metal, zeolite, or the like. The NOx adsorbent 50 releases NOx when heated to a predetermined temperature. For example, the NOx adsorbent 50 releases NOx at a temperature of about 150 to 250 ° C., for example. The NOx that has passed through the NOx adsorbent 50 and the NOx released in the regeneration process are converted into nitrogen by the downstream selective reduction catalyst 51 provided in the downstream second exhaust passage 12. This NOx adsorbent 50 can physically adsorb NOx even at a temperature lower than the activation temperature at which the selective catalytic reduction catalyst is activated, and at the time of cold start lower than the temperature at which the selective catalytic reduction catalyst is activated. Even during light load operation, NOx can be prevented from being released into the atmosphere.

  The exhaust gas from which particulate matter has been removed by the DPF 30 is turned back in the third exhaust passage 13 and then supplied to the second exhaust passage 12. The second exhaust passage 12 is located outside the first exhaust passage 11, and the exhaust gas turned back in the third exhaust passage 13 flows into the second exhaust passage 12. The second exhaust passage 12 is formed by a space between the outer surface of the inner cylinder 3 and the inner surface of the outer cylinder 4, and in particular, the outer side of the combustion space 15 is an auxiliary upstream selective reduction type of the DPF 30 described above. A space for disposing the main downstream selective reduction catalyst 51 with respect to the catalyst 29 is provided.

  The downstream selective reduction catalyst 51 is a monolithic catalyst, and is configured, for example, by coating a cordierite honeycomb carrier with zeolite, vanadium, zirconia, or the like. Examples of zeolite include copper zeolite, iron zeolite, zinc zeolite, and cobalt zeolite. In general, the downstream selective reduction catalyst 51 is not activated when the temperature is lower than the activation temperature, and does not have sufficient NOx purification ability. The downstream selective catalytic reduction catalyst 51 is heated above the activation temperature by the exhaust gas, and is also heated above the activation temperature by the burner 20 when the temperature is lower than the activation temperature during cold start or light load operation. . When the burner 20 is driven, the downstream selective catalytic reduction catalyst 51 is heated through the combustion space forming wall 16 that is a part of the inner cylinder 3 or heated by the exhaust gas heated by the burner 20. .

  For example, urea water is also used as a reducing agent in the downstream selective reduction catalyst 51. Therefore, a urea water supply unit 52 that supplies a reducing agent to the downstream selective reduction catalyst 51 is provided on the upstream side of the downstream selective reduction catalyst 51 in the second exhaust passage 12. The urea water supply unit 52 includes a downstream side addition valve 53 that adds urea water or hydrolyzed ammonia to the exhaust gas from the third exhaust passage 13. The downstream addition valve 53 is provided in the outer cylinder 4 constituting the second exhaust passage 12, and adds urea water or ammonia in the flow direction of the exhaust gas.

  The urea water supply unit 52 shares some components with the urea water supply unit 31 described above, and the upstream side of the urea water pump 35 is common with the urea water supply unit 31 and is downstream of the urea water pump 35. The side is connected to the downstream addition valve 53 through a connection passage 54 such as a pipe. In the connection passage 54, an on-off valve 55 is provided between the downstream side addition valve 53 and the urea water pump 35. The on-off valve 55 opens the connection passage 54 and supplies urea water in the direction of the downstream side addition valve 53.

  A part of the connection passage 54 that supplies urea water to the downstream addition valve 53 functions as a downstream heating unit 56 that heats urea water added to the downstream selective reduction catalyst 51 downstream of the on-off valve 55. . The downstream heating unit 56 is located in the combustion space 15 in the first exhaust passage 11 in the same manner as the upstream heating unit 37 described above. The downstream heating unit 56 heats the urea water to such an extent that the aqueous urea is hydrolyzed into ammonia when the fuel is burned by the burner 20 in the combustion space 15. From the downstream side addition valve 53, when the burner 20 is driven, heated urea water or hydrolyzed ammonia is added upstream from the downstream side selective reduction catalyst 51.

  Further, in the second exhaust passage 12, a downstream side NOx concentration sensor 44 that detects the NOx concentration of the exhaust gas flowing into the downstream side selective reduction catalyst 51, on the upstream side of the downstream side selective reduction catalyst 51, and the downstream side A downstream temperature sensor 45 that detects the temperature of the exhaust gas flowing into the selective reduction catalyst 51 as the catalyst temperature of the downstream selective reduction catalyst 51 is provided. An exhaust NOx concentration sensor 46 that finally detects the NOx concentration contained in the exhaust gas is provided in the fourth exhaust passage 14 downstream of the downstream selective reduction catalyst 51 provided in the second exhaust passage 12. Is provided.

  As shown in FIG. 2, the exhaust gas purification system 1 configured as described above is controlled by a control device 47 configured by a microcomputer or the like. The control device 47 has a configuration such as a ROM, a RAM, and a CPU. The control device 47 performs calculations using input values input from various sensors 41, 42, 43, 44, 45, 46, etc. according to a control program stored in the ROM, and controls ignition of the spark plug 22 of the burner 20. In addition, on / off control of the on / off valves 36 and 55 of the urea water supply units 31 and 52, drive control of the fuel pump 26 and urea water pump 35, and the like are performed.

With reference to FIG. 3, an example of the process which adds urea water is demonstrated. This process is started and repeated at the time of cold start or light load operation.
In step S1, the control device 47 controls the intake air amount Ga from the intake air amount sensor 41 provided in the intake passage of the engine 2, the NOx concentrations Cx1, Cx2, Cx3 from the NOx concentration sensors 42, 44, 46, and the temperature sensor. Catalyst temperatures Tc1 and Tc2 from 43 and 45 are acquired.

  In step S <b> 2, the control device 47 acquires the acquired intake air amount Ga, the NOx concentration Cx1 acquired from the upstream NOx concentration sensor 42 of the first exhaust passage 11, and the catalyst acquired from the upstream temperature sensor 43 of the first exhaust passage 11. Based on the temperature Tc1, the NOx amount Gx1 of the first exhaust passage 11 is calculated. In step S3, the urea water addition amount Gu1 supplied in the direction of the upstream addition valve 32 supplied to the upstream selective reduction catalyst 29 is calculated based on the NOx amount Gx1.

  Further, the control device 47 is based on the intake air amount Ga, the NOx concentration Cx2 acquired from the downstream NOx concentration sensor 44 of the second exhaust passage 12, and the catalyst temperature Tc2 acquired from the downstream temperature sensor 45 of the second exhaust passage 12. Thus, the NOx amount Gx2 of the second exhaust passage 12 is calculated. In step S3, based on the NOx amount Gx2, the addition amount Gu2 of urea water supplied in the direction of the upstream addition valve 32 supplied to the downstream selective reduction catalyst 51 is calculated.

  In step S4, the control device 47 at least one of the catalyst temperature Tc1 acquired from the upstream temperature sensor 43 of the first exhaust passage 11 and the catalyst temperature Tc2 acquired from the downstream temperature sensor 45 of the second exhaust passage 12 in step S1. Is determined to be less than the activation temperature Tca. When at least one of the catalyst temperatures Tc1 and Tc2 is lower than the activation temperature Tca, the process proceeds to step S5 which is a process for heating the urea water. When the temperature is not lower than the activation temperature Tca, that is, when the catalyst temperatures Tc1 and Tc2 are equal to or higher than the activation temperature Tca, the process proceeds to step S9 where the urea water is not heated by the burner 20. By making such a determination, the control device 47 supplies urea water to the downstream selective reduction catalyst 51 and the upstream selective reduction catalyst 29 below the activation temperature, and the urea contained in the urea water is crystallized. To prevent it.

  When at least one of the catalyst temperatures Tc1 and Tc2 is lower than the activation temperature Tca, a fuel amount Gf for driving the burner 20 corresponding to the urea water addition amounts Gu1 and Gu2 is calculated in step S5. In step S6, the control device 47 opens the fuel on-off valve 27, drives the fuel pump 26, and injects fuel for the fuel amount Gf from the injection nozzle 23. Then, the spark plug 22 is driven to ignite the fuel.

  In step S7, the control device 47 determines whether or not the calculation of the fuel amount Gf is continuous. That is, the control device 47 determines whether or not the burner 20 continues to be driven by determining whether or not the calculation of the fuel amount Gf is continuous. The control device 47 proceeds to step S8 when the calculation of the fuel amount Gf is continuous, and returns to step S4 when it is not continuous. When not continuous, the control device 47 repeats the processing (calculation) from step S4.

  In step S8, the control device 47 opens the on-off valve 36, and urea water heated by the upstream heating unit 37 or ammonia generated by hydrolysis of the urea water is supplied from the upstream addition valve 32. It is added before the selective reduction catalyst 29 on the upstream side of the DPF 30. Further, the on-off valve 55 is opened, and the urea water heated by the downstream heating unit 56 or ammonia generated by hydrolysis of the urea water is sent from the downstream side addition valve 53 to the downstream side selective reduction catalyst 51. Add before this.

  Thus, by driving the burner 20, the urea water is heated so that hydrolysis is promoted in the downstream heating unit 56 and the upstream heating unit 37 of the combustion space 15. Further, the upstream selective reduction catalyst 29 of the DPF 30 is heated to the activation temperature or higher, and the downstream selective reduction catalyst 51 of the second exhaust passage 12 is also heated to the activation temperature or higher via the combustion space forming wall 16. As a result, the urea water is easily converted into ammonia, or the urea water is hydrolyzed to generate ammonia. Therefore, the exhaust gas temperature, the catalyst temperature Tc1 of the downstream selective reduction catalyst 51, and the upstream side selection are selected. Even when the catalyst temperature Tc2 of the reduction catalyst 29 is lower than the activation temperature Tca, it is possible to add heated urea water or ammonia. Thereby, it is possible to prevent NOx from being released into the atmosphere during a cold start or a light load operation.

  If the controller 47 has already determined that the catalyst temperatures Tc1, Tc2 are equal to or higher than the activation temperature Tca in step S4, the controller 47 does not drive the burner 20 in step S9, and adds urea water not heated by the burner 20 to the addition valves 32, From 53, the catalyst is added to the upstream selective reduction catalyst 29 and the downstream selective reduction catalyst 51. In such a case, the downstream selective reduction catalyst 51 and the upstream selective reduction catalyst 29 have already been sufficiently heated to the activation temperature or higher, and it is not necessary to drive the burner 20 and only add urea water. This is because hydrolysis proceeds and ammonia is supplied to the upstream selective reduction catalyst 29 and the downstream selective reduction catalyst 51.

  By the way, NOx is physically adsorbed by the NOx adsorbent 50 in the exhaust gas that has passed through the upstream selective reduction catalyst 29 and the DPF 30. The NOx adsorbent 50 adsorbs NOx even if the catalyst temperatures Tc1 and Tc2 are lower than the activation temperature Tca. Thereby, it is possible to prevent NOx from being released into the atmosphere even during a cold start or a light load operation that is lower than the activation temperature at which the selective catalytic reduction catalyst is activated. On the other hand, the NOx adsorbent 50 has a limit in the amount of NOx adsorbed. The control device 47 manages the NOx adsorption amount for the NOx adsorbent 50 located downstream of the DPF 30 in the exhaust flow direction, and when the adsorption amount reaches a threshold value close to the performance limit value, the regeneration process of the NOx adsorbent 50 is performed. To start.

  Specifically, as shown in FIG. 4, in step S11 and step S12, as in step S1 and step S2 described above, the control device 47 controls the intake air amount Ga from the intake air amount sensor 41, the NOx concentration sensor 42, NOx concentrations Cx1 and Cx2 from 44 and catalyst temperatures Tc1 and Tc2 from temperature sensors 43 and 45 are acquired. Based on the acquired intake air amount Ga, the NOx concentrations Cx1 and Cx2 acquired from the NOx concentration sensors 42 and 44, and the catalyst temperatures Tc1 and Tc2 acquired from the temperature sensors 43 and 45, the control device 47 controls the first exhaust passage 11. The NOx amount Gx1 and the NOx amount Gx2 of the second exhaust passage 12 are calculated.

  The NOx amount Gx1 in the first exhaust passage 11 is the NOx amount Gx1 in the combustion space 15, and is the NOx amount in the exhaust gas supplied from the engine 2 and before the purification process by the exhaust purification system 1. Further, the NOx amount Gx2 in the second exhaust passage 12 becomes the NOx amount after passing through the upstream selective reduction catalyst 29 and the NOx adsorbent 50.

  In step S13, the control device 47 performs a process of subtracting the NOx amount Gx2 after passing through the NOx adsorbent 50 from the NOx amount Gx1 before the purification process, and calculates the NOx amount Gx3 adsorbed on the NOx adsorbent 50. When calculating the NOx amount Gx3, when considering the NOx amount processed by the upstream selective reduction catalyst 29, the NOx amount and NOx amount processed by the upstream selective reduction catalyst 29 from the NOx amount Gx1. Gx2 is subtracted. The amount of NOx processed by the upstream selective reduction catalyst 29 can be calculated with reference to a table based on the catalyst temperature Tc1 from the upstream temperature sensor 43, for example.

  In step S14, the control device 47 calculates the current integrated value GxT by adding the current NOx amount Gx3 to the previous integrated value of NOx amount Gx3. In step S15, the control device 47 determines whether or not the current integrated value GxT exceeds a predetermined adsorption amount, and the processing from step S11 to step S14 until the current integrated value GxT exceeds the predetermined adsorption amount. repeat. Then, when the control device 47 determines that the current integrated value GxT exceeds a predetermined adsorption amount, the control device 47 proceeds to step S16. In step S <b> 16, the control device 47 drives the burner 20, heats the NOx adsorbent 50 via the DPF 30, and performs a regeneration process for releasing the adsorbed NOx from the NOx adsorbent 50. The released NOx is processed by the downstream selective reduction catalyst 51.

  FIG. 5 is a graph showing an example of the relationship between the catalyst temperature and the NOx reduction rate, and is a graph showing the results of an experiment conducted on the NOx reduction rate. In FIG. 5, the NOx reduction rate of the example is a value obtained by the exhaust purification system 1. The NOx reduction rate of the comparative example is a value obtained from the exhaust purification system 1 by the exhaust purification system in which the heating units 37 and 56 and the NOx adsorbent 50 are omitted. As shown in FIG. 5, in the comparative example, the NOx reduction rate is remarkably low in the range of 100 ° C. to 180 ° C., which is the temperature range of the catalyst temperature during cold start and light load operation. On the other hand, in Examples, it is recognized that NOx is reduced even when the catalyst temperature is in the range of 100 ° C to 180 ° C. Moreover, it was recognized that the NOx reduction rate is higher than that of the comparative example at each temperature of 200 ° C. or lower.

According to the exhaust purification system 1 of the first embodiment, the effects listed below can be obtained.
(1) In the exhaust purification system 1, the urea water is heated and easily converted to ammonia by driving the burner 20. Therefore, even if the catalyst temperatures Tc1 and Tc2 of the selective reduction catalysts 29 and 51 are lower than the activation temperature Tca, urea water heated by the heating portions 37 and 56 of the combustion space 15 or hydrolyzed ammonia with respect to the exhaust gas. Can be supplied. As a result, the amount of NOx reduction increases.

  (2) In the exhaust purification system 1, the NOx adsorbent 50 located upstream of the downstream selective reduction catalyst 51 adsorbs NOx even if the catalyst temperatures Tc1 and Tc2 are lower than the activation temperature Tca. Therefore, it is possible to prevent NOx from being released into the atmosphere even during a cold start or a light load operation that is lower than the activation temperature at which the selective catalytic reduction catalyst is activated.

  (3) Since the DPF 30 is disposed downstream of the burner 20, particulate matter accompanying the driving of the burner 20 can be captured by the DPF 30. As a result, an increase in the amount of particulate matter discharged due to the drive of the burner 20 is suppressed.

  (4) Since the upstream selective reduction catalyst 29 is integrally provided in the DPF 30, NOx can be reduced upstream of the NOx adsorbent 50 and the downstream selective reduction catalyst 51, The burden on the NOx adsorbent 50 and the downstream selective reduction catalyst 51 can be reduced.

  (5) When the burner 20 is driven to heat the urea water by the heating units 37 and 56, the upstream side selective reduction catalyst 29 is simultaneously heated. At the same time, the downstream selective reduction catalyst 51 is heated through the combustion space forming wall 16. As a result, the time required for the catalyst temperatures Tc1, Tc2 of the selective reduction catalysts 29, 51 to reach the activation temperature Tca can be shortened.

  (6) The burner 20 is driven only when the catalyst temperatures Tc1 and Tc2 of the selective reduction catalysts 29 and 51 are lower than the activation temperature Tca. Therefore, the fuel consumption for heating urea water with the burner 20 can be suppressed to the minimum.

(7) The ammonia supplied to the selective reduction catalysts 29 and 51 when the catalyst temperatures Tc1 and Tc2 are lower than the activation temperature Tca is held in the selective reduction catalysts 29 and 51 as they are, and the catalyst temperatures Tc1 and Tc2 are the activation temperatures. When Tca is reached, it can be immediately converted to nitrogen by the retained ammonia.
(Second Embodiment)

  A second embodiment of the exhaust purification system will be described with reference to FIG. The exhaust purification system of the second embodiment has the same main configuration as the exhaust purification system of the first embodiment. Therefore, in the second embodiment, portions different from the first embodiment will be described in detail. Parts similar to those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

  As shown in FIG. 6, in this exhaust purification system, a downstream burner 60 is provided outside the NOx adsorbent 50. The downstream burner 60 includes a fuel supply unit 61 that supplies fuel to the second exhaust passage 12 and an ignition plug 62 that ignites the fuel supplied to the second exhaust passage 12. Some of the plurality of components constituting the fuel supply unit 61 are common to the fuel supply unit 21 described above. That is, the components located upstream of the fuel pump 26 in the fuel supply unit 61 are common to the fuel supply unit 21, and the connection passage 63 such as a pipe located downstream of the fuel pump 26 is connected to the injection nozzle 64. It is connected. In the connection passage 63, a fuel opening / closing valve 65 is provided between the injection nozzle 64 and the fuel pump 26. The fuel on / off valve 65 opens the connection passage 63 to supply fuel to the injection nozzle 64, and shuts off the connection passage 63 to stop the supply of fuel to the injection nozzle 64. The spark plug 62 is provided in the second exhaust passage 12 of the inner cylinder 3 and ignites the fuel injected from the injection nozzle 64. The fuel burns with oxygen remaining in the exhaust gas as an oxidant.

  Further, as shown in FIG. 6, not only the DPF 30 and the upstream selective reduction catalyst 29 are integrated, but also the NOx adsorbent 50 is integrated to simplify the configuration and reduce the number of parts. it can. In this case, the upstream selective reduction catalyst 29 is supported on the upstream surface of the DPF 30, and the NOx adsorbent 50 is supported on the downstream surface. The DPF 30 is formed so that the upstream selective reduction catalyst 29 and the NOx adsorbent 50 do not overlap. Thereby, increase of the pressure loss of a filter can be prevented.

According to the exhaust purification system of the second embodiment, in addition to the effects equivalent to (1) to (7) described in the first embodiment, the effects listed below can be obtained.
(8) The downstream burner 60 is driven when the catalyst temperature of the downstream selective reduction catalyst 51 downstream of the second exhaust passage 12 is lower than the activation temperature, and can heat the downstream selective reduction catalyst 51. At the same time, the urea water added from the downstream addition valve 53 is heated and converted to ammonia by hydrolysis. Therefore, the time required for the catalyst temperature Tc2 of the downstream selective reduction catalyst 51 to reach the activation temperature Tca can be further shortened.

  (9) The downstream burner 60 heats the NOx adsorbent 50 through the inner cylinder 3 when the downstream selective reduction catalyst 51 is regenerated. Thereby, NOx adsorbed on the NOx adsorbent 50 is released downstream. The released NOx can be removed by the downstream selective reduction catalyst 51.

  (10) Since the upstream selective reduction catalyst 29 and the NOx adsorbent 50 are integrally provided in the DPF 30, it is possible to reduce the number of parts and realize downsizing. Even when the upstream selective reduction catalyst 29 and the NOx adsorbent 50 are integrally provided in the DPF 30, the upstream selective reduction catalyst 29 and the NOx adsorbent 50 are provided so as not to overlap. Can prevent the pressure loss of the filter.

In addition, you may change the said 1st and 2nd embodiment as follows.
The position of the NOx adsorbent 50 is not particularly limited as long as it is downstream of the DPF 30 in the exhaust flow direction. For example, the NOx adsorbent 50 can be arranged in the second exhaust passage 12 immediately before the downstream selective reduction catalyst 51.

  The exhaust passage 10 is not limited to a double pipe structure composed of the inner cylinder 3 and the outer cylinder 4. For example, the exhaust passage 10 is constituted by a single cylinder, one end of the exhaust passage 10 is connected to the engine 2, and the burner 20, DPF 30, NOx adsorbent 50 are sequentially arranged from the upstream side in the exhaust flow direction. The downstream selective reduction catalyst 51 may be arranged. At this time, the cylinder constituting the exhaust passage 10 may be linear, or at least a part thereof may be curved.

  Although the configuration in which the upstream selective reduction catalyst 29 is supported on the DPF 30 has been described, if the NOx adsorbent 50 and the downstream selective reduction catalyst 51 are configured to sufficiently remove NOx, the upstream side in the exhaust purification system The selective reduction catalyst 29 may be omitted.

  The NOx adsorbent 50 is not a material that physically adsorbs NOx, and may be one using a DPNR catalyst (Diesel Particulate-NOx Reduction). In this case, in addition to THC and CO, fine particles and NOx can be simultaneously reduced, and the DPF 30 can be omitted. Note that the NOx adsorbent 50 may be a combination of a structure that physically adsorbs NOx and a structure using a DPNR catalyst.

  -You may make it arrange | position an oxidation catalyst (DOC: Diesel Oxidation Catalyst) upstream of DPF30. An ammonia oxidation catalyst that oxidizes ammonia may be further provided downstream of the downstream selective reduction catalyst 51.

The burner 20 may be driven when the catalyst temperatures Tc1 and Tc2 reach the activation temperature Tca. Even in such a configuration, the fuel amount Gf is preferably controlled in accordance with the urea water addition amounts Gu1 and Gu2.
The fuel amount Gf may be constant regardless of the urea water addition amounts Gu1 and Gu2. Even with such a configuration, urea water is easily converted to ammonia.

The fuel amount Gf may be adjusted based on a detection value of a sensor that detects the temperature of the urea water in the urea water tank 33. According to such a configuration, the fuel consumption accompanying the drive of the burner 20 is further suppressed.
-The heating parts 37 and 56 should just be located in the combustion space 15, and the shape is not restricted to a coil shape or a spiral shape.

  -The urea water supply parts 31 and 52 may provide a heat insulating material so that a pipe may be covered between the heating parts 37 and 56 and the addition valves 32 and 53, and may prevent the temperature reduction of the heated urea water.

  The burners 20 and 60 may be provided with an air supply unit that supplies air upstream of the injection nozzles 23 and 64 so that an air-fuel mixture of fuel and air is generated. The burners 20 and 60 may be configured to supply a premixed gas in which fuel and air are mixed in advance.

  In the exhaust purification system 1 of FIG. 1, as shown in FIG. 6, a filter in which the DPF 30, the upstream side selective reduction catalyst 29, and the NOx adsorbent 50 are integrated may be used. Further, in the exhaust purification system 1 of FIG. 6, the DPF 30 in which the upstream selective reduction catalyst 29 is integrated and the NOx adsorbent 50 may be separated.

  DESCRIPTION OF SYMBOLS 1 ... Exhaust purification system, 2 ... Engine, 3 ... Inner cylinder, 4 ... Outer cylinder, 5 ... Bottom surface, 10 ... Exhaust passage, 11 ... First exhaust passage, 12 ... Second exhaust passage, 13 ... Third exhaust passage, DESCRIPTION OF SYMBOLS 14 ... 4th exhaust passage, 15 ... Combustion space, 16 ... Combustion space formation wall, 20 ... Burner, 21 ... Fuel supply part, 22 ... Spark plug, 23 ... Injection nozzle, 24 ... Fuel tank, 25 ... Fuel passage, 26 DESCRIPTION OF SYMBOLS ... Fuel pump, 27 ... Fuel on-off valve, 29 ... Upstream selective reduction type catalyst, 30 ... DPF, 31 ... Urea water supply part, 32 ... Upstream side addition valve, 33 ... Urea water tank, 34 ... Connection passage, 35 ... Urea water pump, 36 ... open / close valve, 37 ... upstream heating section, 41 ... intake air amount sensor, 42 ... upstream NOx concentration sensor, 43 ... upstream temperature sensor, 44 ... downstream NOx concentration sensor, 45 ... downstream Side temperature sensor, 46 ... discharge Ox concentration sensor, 47 ... control device, 50 ... NOx adsorbent, 51 ... downstream selective reduction catalyst, 52 ... urea water supply unit, 53 ... downstream addition valve, 54 ... connection passage, 55 ... open / close valve, 56 ... Downstream heating unit, 60 ... downstream burner, 61 ... fuel supply unit, 62 ... spark plug, 63 ... connection passage, 64 ... injection nozzle, 65 ... fuel on-off valve.

Claims (7)

An exhaust passage through which exhaust gas flows;
A burner disposed in the exhaust passage, wherein a combustion space of fuel in the burner is part of the exhaust passage;
A NOx adsorbent that is located downstream of the combustion space in the exhaust passage and adsorbs nitrogen oxides contained in the exhaust gas;
A selective catalytic reduction catalyst located downstream of the NOx adsorbent in the exhaust passage;
An addition valve positioned between the selective reduction catalyst and the combustion space in the exhaust passage;
A connection passage through which urea water flows to the addition valve and is connected to the addition valve, and a part of the connection passage passes through the combustion space;
An exhaust purification system comprising: The exhaust purification system according to claim 1, further comprising a DPF in the exhaust passage between the burner and the NOx adsorbent. The selective catalytic reduction catalyst is a downstream selective catalytic reduction catalyst,
The addition valve is a downstream addition valve;
The connection passage is a connection passage for the downstream addition valve;
The DPF is integrated with an upstream selective reduction catalyst located upstream of the downstream selective reduction catalyst in the exhaust passage,
And an upstream addition valve located upstream of the upstream selective reduction catalyst in the exhaust passage;
A connection passage for the upstream addition valve through which urea water is connected to the upstream addition valve and flows toward the upstream addition valve, wherein a part of the upstream connection passage passes through the combustion space; A connecting passage for the side addition valve;
An exhaust purification system according to claim 2. The exhaust according to claim 3, wherein the DPF is integrally provided with the upstream selective reduction catalyst on an upstream surface and the NOx adsorbent is integrally provided on a downstream surface. Purification system. The exhaust passage includes a combustion space forming wall that forms the combustion space, and the combustion space forming wall partitions the space where the selective catalytic reduction catalyst is located from the combustion space,
The exhaust purification system according to claim 1 or 2, wherein the selective reduction catalyst is heated through the combustion space forming wall when the fuel is burned by the burner. Furthermore, the control apparatus which acquires the catalyst temperature of the said selective reduction catalyst, and drives the said burner when the acquired catalyst temperature is less than the activation temperature of the said selective reduction catalyst is provided. The exhaust gas purification system according to any one of the above. The burner is
An upstream burner located upstream of the NOx adsorbent in the exhaust passage;
Furthermore, in the exhaust passage, a downstream burner is further provided between the NOx adsorbent and the selective reduction catalyst,
The exhaust purification system according to claim 6, wherein the control device drives the downstream burner when the acquired catalyst temperature is lower than an activation temperature of the selective catalytic reduction catalyst.

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