JP6686516B2 - Exhaust gas treatment method and device using LNT - Google Patents

Description

  The present invention relates to an exhaust gas treatment method using LNT and an apparatus therefor.

  Urea SCR (Selective Catalytic Reduction) and LNT (Lean NOx Trap) systems are known as deNOx (denitration) systems.

The catalyst of LNT is a catalyst carrier such as alumina (Al ₂ O ₃ ), a noble metal catalyst such as Pt or Pd, an alkali metal such as Na, K or Cs or an alkaline earth metal such as Ca or Ba, Y or La. , Ce and the like are loaded with an occlusion material having a NOx occlusion function such as rare earths, and exert two functions of NOx occlusion and NOx emission / purification depending on the oxygen concentration in the exhaust gas.

The LNT system has a mechanism for repeating lean and rich to perform deNOx. That is, under conditions where the oxygen concentration in the exhaust gas is high (lean air-fuel ratio) such as in a normal operating state, NO in the exhaust gas is oxidized to NO ₂ by a noble metal catalyst such as Pt or Pd, and the storage material It is stored as nitrate (Ba (NO ₃ ) ₂ ) to purify NOx, and thereafter, at the timing when the storage amount in the storage material is saturated, rich reduction is performed under conditions of low oxygen concentration (rich air-fuel ratio), and fuel ( By reducing (HC) on the noble metal catalyst, CO, HC, and H ₂ are produced in the exhaust gas, and the released NOx is reduced and purified.

  In this way, the LNT system adsorbs or stores NOx when lean, and when rich, the adsorbed or stored NOx is released from Ce or Ba, and HC, CO and NOx in the exhaust gas are harmless gases due to the three-way catalytic function. It is said that

In the LNT system, the storage material adsorbs and stores SOx in the exhaust gas at the same time as NOx. Therefore, in order to release S accumulated in the storage material, the ambient temperature is set to a high temperature of about 650 ° C and the air-fuel ratio is rich. By making the atmosphere, Ba ₂ SO ₄ generated by occlusion is used as carbonate + SO ₂ for S purging (desulfurization).

  As a characteristic of the storage material, when the catalyst temperature is a high temperature of 400 ° C. or higher, the NOx storage amount per unit volume of the catalyst decreases, so that a problem is that the deNOx effect in the high temperature range (= high load range) decreases.

  As one of the countermeasures, the addition of a second LNT catalyst (2ndLNT) to the underfloor (U / F) of the vehicle is being considered. The effect is that the amount of NOx adsorbed increases with an increase in the catalyst capacity, and the catalyst temperature becomes lower than that of the upstream LNT catalyst (1st LNT), so that the deNOx efficiency can be maintained even in the high load range (expansion of the temperature window of the catalyst). It can be mentioned.

JP, 2009-174445, A JP, 2014-070520, A JP, 2012-087703, A

  However, when the LNT catalyst is arranged on the 1st LNT close to the engine and on the underfloor (U / F), the following two problems can be considered.

  (1) Insufficient reducing agent for rich reduction of 2ndLNT. Since the reducing agent at the rich time is consumed by the 1st LNT and the amount reaching the 2nd LNT is reduced, the reduction of NOx stored in the 2nd LNT may be insufficient.

  (2) Desulfurization of 2ndLNT becomes difficult. When the temperature of the 1st LNT is controlled to be equal to or higher than the desulfurization temperature and equal to or lower than the heat deterioration temperature of the catalyst (for example, 650 ° C to 750 ° C), the temperature of the 2nd LNT is equal to or lower than the desulfurization temperature or a borderline temperature (550 ° C to 650 ° C) It becomes difficult to desulfurize.

  Therefore, an object of the present invention is to solve the above problems, improve the deNOx effect in the high temperature range (= high load range), and further improve the rich reduction efficiency and desulfurization efficiency, and an exhaust gas treatment method using LNT, To provide the device.

  In order to achieve the above object, the present invention arranges 1stLNT and CSF close to an engine, arranges 2ndLNT at a position where exhaust gas temperature decreases by 100 ° C to 200 ° C, arranges an HC injector between them, and rich This is an exhaust gas treatment method using an LNT catalyst that controls the desulfurization temperature of the 2nd LNT catalyst by additionally injecting HC by the HC injector during reduction and by HC injection from the HC injector during S purge.

  Further, the present invention provides a front-stage processing unit in which a 1st LNT catalyst and CSF are arranged in the vicinity of an engine, a rear-stage processing unit in which a 2ndLNT is arranged on the vehicle underfloor, and between the CSF and the 2ndLNT. Is an exhaust gas treatment device using an LNT catalyst provided with an HC injector disposed in the.

The present invention exhibits the following excellent effects.
(1) The urea-free deNOx effect can be obtained in a wide temperature range such as an RDE test (exhaust gas test by actual running).
(2) Desulfurization can be reliably performed and catalyst performance can be maintained for a long period of time.
(3) It can be realized at a relatively low cost in terms of additional cost.

It is a schematic diagram showing an embodiment of the invention. It is a figure which shows the control flow in the exhaust gas processing method of this invention. In the control flow of FIG. 2, a map for calculating the NOx reduction amount is shown, (a) is a reduction efficiency map with respect to the catalyst temperature , and (b) is a diagram with a reduction efficiency map with respect to the air-fuel ratio . FIG. 3 is a diagram showing a post / HC injection timing, a temperature change of LNTs 1 and 2 and a change of excess air ratio at that time when S purge is performed in the control flow of FIG. 2. It is a figure which shows the catalyst temperature of a LNT catalyst, and the relationship of NOx purification rate.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows an exhaust gas aftertreatment device 10 using an LNT catalyst.

  The turbocharger 11 and the EGR pipe 12 are connected to the intake / exhaust system of the engine E, and the air taken in from the air cleaner 13 is compressed by the compressor 14 of the turbocharger 11 and is sent under pressure to the intake passage 15, It is supplied into the engine E from the E intake manifold 16. The intake passage 15 is provided with an intake valve 17 for adjusting the amount of air to the engine E.

  The exhaust gas discharged from the engine E is discharged from the exhaust manifold 18 to the turbine 19 of the turbocharger 11, drives the turbine 19, and is discharged to the exhaust system 20.

  The EGR pipe 12 is connected to the intake manifold 16 and the exhaust manifold 18, the EGR cooler 21 for cooling the exhaust gas from the exhaust manifold 18 to the intake manifold 16 is connected to the EGR pipe 12, and the EGR amount is adjusted. The EGR valve 22 is connected.

  The exhaust gas post-treatment device 10 is a vehicle in which the exhaust gas temperature decreases by about Δ100 ° C. to 200 ° C. in the downstream side of the turbine 19 and in the upstream side processing unit 10a provided near the engine E and the downstream side of the upstream side processing unit 10a. And a rear-side processing unit 10b disposed on the underfloor (U / F) of the.

  The pre-stage side processing unit 10a is configured by providing a vertical type pre-stage side canning container 23 with a 1st LNT 24 and a CSF (Catalyzed Soot Filter) 25 provided downstream thereof.

  A pre-stage side HC injector 26 is provided in the pre-stage side canning container 23 of the pre-stage side processing unit 10a. The pre-stage side HC injector 26 supplies HC to the 1st LNT 24 in the pre-stage side canning container 23 at the time of rich reduction or S purge, and is configured to supply HC by post injection on the engine E side. It may not be provided.

  The post-stage processing unit 10b is connected to the pre-stage processing unit 10a via the exhaust pipe 20a, and the 2ndLNT 28 is arranged in the post-stage canning container 27 connected to the exhaust pipe 20a.

  A second-stage HC injector 29 that injects HC to the upstream side of the 2ndLNT 28, which is the downstream side of the CSF 25 in this embodiment, is provided in the upstream-stage canning vessel 23.

  A first temperature sensor 30 for detecting the exhaust gas temperature is provided in the upstream side canning vessel 23 upstream of the 1stLNT 24, and a second temperature sensor 31 is provided in the upstream side canning vessel 23 between the 1stLNT 24 and the CSF 25. A third temperature sensor 32 is provided in the exhaust pipe 20a downstream of the side processing unit 10a.

  A first NOx sensor 33 is provided in the upstream-side canning container 23 on the downstream side of the CSF 25, and a second NOx sensor 34 is provided in the downstream-side canning container 27 on the downstream side of the 2ndLNT 28.

  The detection values of the first to third temperature sensors 30 to 32 and the first and second NOx sensors 33 and 34 are input to the ECU 40.

The engine E is controlled by the ECU 40 to control the fuel injection amount of the fuel injection valve IN according to the operating condition, and various general controls necessary for the operation are performed. The ECU 40 includes a NOx adsorption amount calculation unit 41 that calculates the NOx storage amount in the 1st LNT 24 and the 2nd LNT 28 from the detection values of the first and second NOx sensors 33 and 34, and a map of the reduction efficiency with respect to the catalyst temperature during the rich reduction (FIG. 3). Referring to (a)) and the map of reduction efficiency with respect to the air-fuel ratio (FIG. 3 (b)), post injection or HC injection by the front-stage HC injector 26 and HC injection by the rear-stage HC injector 29 are performed. The rich reduction control means 42 for performing rich reduction by performing post injection or HC injection by the front side HC injector and HC injection by the rear side HC injector at the time of S purging to perform the temperature and air-fuel ratio comparison of the 1st LNT 24 and the 2nd LNT 28. The S purge control means 43 for controlling is formed.

  Next, an exhaust gas treatment method using LNT will be described.

  Under conditions where the oxygen concentration in the exhaust gas is high (lean air-fuel ratio), such as in the normal operating state, NOx in the exhaust gas is stored in the 1st LNT 24 of the pretreatment unit 10a, and carbon monoxide and unburned fuel in the exhaust gas are CSF25. PM (particulate matter) is removed by the CSF 25 while being oxidized by the oxidation catalyst. After this, NOx that was not stored in the 1stLNT 24 of the upstream processing unit 10a is stored in the 2ndLNT 28 of the downstream processing unit 10b.

  With regard to deNOx with this lean air-fuel ratio, when the exhaust gas temperature discharged from the turbine 19 becomes a high temperature of 400 ° C. or higher, the NOx purification rate in the 1st LNT 24 becomes poor, but it becomes possible to increase the NOx purification rate in the 2nd LNT 28.

  This will be described with reference to FIG.

  FIG. 5 shows the relationship between the NOx purification rate and the exhaust gas temperature of LNT.

  In FIG. 5, the solid line a shows the purification rate characteristic of 1st LNT, and the dotted line b shows the purification rate characteristic of 2nd LNT.

  In the 1st LNT, the purification rate is high when the exhaust gas temperature is 250 to 400 ° C, but the NOx purification rate becomes worse at a high temperature (high load range) exceeding 400 ° C. However, since the 2ndLNT is arranged on the underfloor, even if the temperature of the exhaust gas exhausted from the engine E exceeds 400 ° C, the temperature of the exhaust gas flowing into the 2ndLNT decreases by 100 to 200 ° C, so the catalyst temperature of the 2ndLNT is Since the temperature is maintained at 250 to 400 ° C., NOx that could not be purified by 1st LNT can be purified with high efficiency.

  The NOx adsorption amount calculation means 41 during deNOx at this lean air-fuel ratio calculates the NOx storage amount at the 1st LNT 24 and the 2nd LNT 28 from the detection values of the first and second NOx sensors 33, 34, and the NOx storage amount is equal to or greater than the threshold value, or When the NOx purification rate becomes equal to or less than the threshold value, rich reduction is performed. The threshold values of the NOx storage amount and the NOx purification rate are set when the NOx storage amount in the 1stLNT24 is saturated or after the NOx storage amount in the 1stLNT24 is saturated and the NOx storage amount in the 2ndLNT28 approaches saturation. It is set arbitrarily until is less than or equal to the threshold.

Next, when performing rich reduction, the rich reduction control means 42 reduces the catalyst temperature at the 1st LNT 24 and the 2nd LNT 28 from the map of FIG. 3A based on the detection values of the first to third temperature sensors 30 to 32. The efficiency is obtained, and the NOx reduction amount is estimated from the air-fuel ratio of FIG. 3 (b) from the reduction efficiency, and based on this, the post injection amount or the HC (reducing agent) injection amount and the rear stage HC at the front side HC injector 26 is calculated. The HC injection amount in the injector 29 is determined, and the rich reduction is performed with the HC injection amount based on this. In this case, the rich reduction at the 2ndLNT 28 estimates the reduction efficiency in consideration of the reducing agent amount slipping the 1stLNT 24 and the CSF 25 to determine the HC amount, and the HC injector 29 at the subsequent stage intermittently determines the HC amount based on this. By performing the rich reduction by injecting, the reducing agent slip is prevented.

  Further, depending on the catalyst temperature in the 1stLNT24 and the 2ndLNT28, when the outlet exhaust gas temperature of the engine E is low (for example, 200 ° C to 400 ° C), rich reduction is performed only by the 1stLNT24, and at high temperature (for example, 350 ° C to 550 ° C). Then, the control may be performed so that the rich reduction is performed only by the 2ndLNT 28.

  The S purge control means 43 performs S purge assuming that the 1st LNT 24 and the 2nd LNT 28 are S-poisoned when the integrated value of the NOx storage amount reaches a set value or when the traveling distance exceeds 10,000 km. Then, as shown in FIG. 4, post-injection or HC injection is performed by the upstream-side HC injector 26 so that the exhaust gas temperature reaches a target temperature (for example, 650 ° C.), and then HC is intermittently discharged by the upstream-side HC injector 26. And the second-stage HC injector 29 also injects HC to control the temperature of the 1st LNT 24 and the 2nd LNT 28 to a desulfurization temperature or higher (for example, 650 ° C. or higher) and a thermal deterioration temperature of the catalyst or lower (for example, 750 ° C. or lower). Purge.

  At the time of desulfurization of this S purge, the feedback control of the temperature of the 2nd LNT catalyst is performed based on the detection value of the third temperature sensor 32 by the additional injection by the rear side HC injector 29.

Further, when the air-fuel ratio becomes small during S purge rich, oxygen becomes insufficient, and S is likely to be released in the form of H ₂ S instead of SO ₂ , so the HC injection amount is controlled so as to keep the excess air ratio λ at 1. .

  Next, the control flow of the present invention will be described with reference to FIG.

  In FIG. 2, when the control is started in step S10, the NOx storage amounts of 1stLNT and 2ndLNT are calculated from the NOx sensor value in step S11.

In step S11, when the NOx storage amount reaches the set value, the combustion is switched to rich combustion in the determination in step S12, and in step S13, the reduction efficiency map with respect to the catalyst temperature of FIG. 3 (a) and the empty space of FIG. 3 (b). After the NOx reduction amount is calculated by LNT based on the reduction efficiency map with respect to the fuel ratio, and after the post injection or the HC injection by the front side HC injector and the HC injection by the rear side HC injector, the rich reduction is performed. The control is ended (step S15).

Further, in step S11, the S poisoning amount of 1stLNT and 2ndLNT is estimated based on the integrated value of the NOx storage amount, and when the S poisoning amount reaches a certain amount, or when the traveling distance of the vehicle reaches a set distance. In step S12, it is determined as S purge, and in step S14, in order to raise the temperature of 1stLNT and 2ndLNT, post-injection or HC injection by the front-stage side HC injector and HC injection by the rear-stage side HC injector are performed. After increasing the temperatures of the 1st LNT and the 2nd LNT to a target temperature (for example, 650 ° C. to 750 ° C.) and performing the S purge while keeping the excess air ratio λ at 1, the control is ended (step S15).

10 Exhaust gas after-treatment device 10a Pre-stage side treatment section 10b Post-stage side treatment section 24 1stLNT
25 CSF
28 2nd LNT
29 Rear-stage HC injector E engine

Claims (4)

1stLNT and CSF are arranged close to the engine, 2ndLNT is arranged at a position downstream of CSF , a rear side HC injector is arranged between CSF and 2ndLNT, and HC is added at the rear side HC injector at the time of rich reduction. Injection, and at the time of S purging, the HC injection from the HC injector on the latter stage controls the desulfurization temperature of the 2nd LNT catalyst ,
At the time of S purging, post-injection or HC is injected by a pre-stage side HC injector provided upstream of 1stLNT to raise the 1stLNT to a desulfurization temperature,
During the S purge, the post injection or the front side HC injector intermittently injects HC, and the rear side HC injector continuously injects HC, and the excess air ratio is set to 1 during post injection or HC injection by the front side HC injector. An exhaust gas treatment method using an LNT catalyst, in which the HC injection amount is controlled so as to lower the air excess ratio to 1 at other times.   The exhaust gas treatment method according to claim 1, wherein at the time of rich reduction, a reducing agent is injected by a post-injection or an HC injector provided upstream of 1stLNT to perform rich reduction. The exhaust gas treatment method using the LNT catalyst according to claim 1 or 2, wherein the 2ndLNT is arranged at a position where the exhaust gas temperature decreases by 100 to 200 ° C. A front-side processing unit which 1stLNT catalyst and CSF is arranged close to the engine, provided in a vehicle underfloor, and the rear stage side processing unit which 2NdLNT is placed, arranged subsequent stage between the said CSF 2NdLNT An exhaust gas treatment device using an LNT catalyst equipped with an HC injector ,
During the rich reduction, additional injection of HC is performed by the latter-stage HC injector, and during S purge, the HC injection from the latter-stage HC injector controls the desulfurization temperature of the 2nd LNT catalyst,
At the time of S purging, post-injection or HC is injected by a pre-stage side HC injector provided upstream of 1stLNT to raise the 1stLNT to a desulfurization temperature,
During the S purge, the post injection or the front side HC injector intermittently injects HC, and the rear side HC injector continuously injects HC, and the excess air ratio is set to 1 during post injection or HC injection by the front side HC injector. An exhaust gas treatment device using an LNT catalyst that lowers the amount of HC and controls the HC injection amount to keep the excess air ratio at 1 at other times.

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