Exhaust gas treatment system

Abstract

Methods and systems related to an exhaust gas treatment system including, in order: (i) a first means for injecting a nitrogenous reductant; (ii) a first selective catalytic reduction (SCR) catalyst; (iii) an ammonia slip catalyst (ASC); and (iv) a second selective catalytic reduction (SCR) catalyst,
wherein the ASC comprises an SCR catalyst and a supported palladium (Pd) component.

Claims

1. An exhaust gas treatment system comprising, in order: (i) a first means for injecting a nitrogenous reductant; (ii) a first selective catalytic reduction (SCR) catalyst; (iii) an ammonia slip catalyst (ASC); and (iv) a second selective catalytic reduction (SCR) catalyst, wherein the ASC comprises an SCR catalyst and a supported Palladium (Pd) component, and wherein the exhaust gas treatment system does not include a means for injecting a nitrogenous reductant downstream of the first selective catalytic reduction catalyst (SCR).

2. The exhaust gas treatment system according to claim 1, wherein the ASC has a layered structure, wherein an upper layer comprises the SCR catalyst and a lower layer comprises the supported Palladium (Pd) component.

3. The exhaust gas treatment system according to claim 1, wherein the system further comprises a first sensor for determining a NOx level in the exhaust gases, wherein the first means for injecting a nitrogenous reductant is configured, based on a measurement by the first sensor, to dose the nitrogenous reductant at an ANR of greater than 1, preferably from 1.1 to 1.5.

4. The exhaust gas treatment system according to claim 3, wherein the first sensor is a NOx measurement sensor arranged downstream of the first means for injecting a nitrogenous reductant, preferably downstream of the first SCR catalyst, or wherein the first sensor is one or more of a temperature sensor, an engine speed sensor and an engine load sensor.

5. The exhaust gas treatment system according to claim 1, further comprising upstream of the first means for injecting a nitrogenous reductant, in order: a second means for injecting a nitrogenous reductant; a third selective catalytic reduction (SCR) catalyst; a catalysed soot filter (CSF).

6. The exhaust gas treatment system according to claim 5, further comprising a diesel oxidation catalyst upstream of the second means for injecting a nitrogenous reductant or between the third SCR catalyst and the CSF.

7. The exhaust gas treatment system according to claim 5, wherein the exhaust gas treatment system further comprises a second sensor for determining a NOx level in the exhaust gases and wherein the second means for injecting a nitrogenous reductant is configured, based on a measurement by the second sensor, to dose the nitrogenous reductant at an ANR of less than 1, preferably from 0.5 to 0.9.

8. The exhaust gas treatment system according to claim 1, wherein the first selective catalytic reduction (SCR) catalyst and the ammonia slip catalyst (ASC) share a common substrate, or wherein the ammonia slip catalyst (ASC) and the second selective catalytic reduction (SCR) catalyst share a common substrate.

9. The exhaust gas treatment system according to claim 1, wherein the first and/or second SCR catalyst comprises a Cu or Fe-doped zeolite component.

10. The exhaust gas treatment system according to claim 1, wherein the ASC is substantially Pt-free.

11. The exhaust gas treatment system according to claim 1, wherein the Palladium component is supported on a particulate metal oxide or a zeolite, preferably alumina.

12. The exhaust gas treatment system according to claim 1, wherein the system further comprises in order a CSF, or a DOC and CSF, downstream of the second SCR, or wherein the system does not comprise any catalytic components downstream of the second SCR.

13. A diesel combustion and exhaust gas treatment system, the system comprising a diesel engine and the exhaust gas treatment system according to claim 1 arranged to treat an exhaust gas produced by the diesel engine.

14. A method for the treatment of an exhaust gas from a diesel engine, the method comprising passing the exhaust gas through the exhaust gas treatment system according to claim 1.

Description

FIGURES

(1) The present invention will now be described further with reference to the following non limiting Figures, in which:

(2) FIG. 1 shows the fundamental components of the exhaust system described herein.

(3) FIG. 2 shows a preferred embodiment of the exhaust system described herein.

(4) FIG. 3 shows a preferred embodiment of the exhaust system described herein.

(5) FIG. 4 shows a preferred embodiment of the exhaust system described herein.

(6) FIG. 5 shows a preferred embodiment of the exhaust system described herein.

(7) FIG. 6 shows a preferred embodiment of the exhaust system described herein.

(8) FIG. 7 shows a preferred embodiment of the exhaust system described herein.

(9) FIG. 8 shows a preferred embodiment of the exhaust system described herein.

(10) FIG. 9 shows a preferred embodiment of the exhaust system described herein.

(11) FIGS. 10A and 10B show comparative NOx conversion and N.sub.2O make for Pd and Pt ASC components in a conventional SCR/ASC configuration.

(12) FIGS. 11A and 11B show comparative NH.sub.3 conversion and slip for Pt and Pd-containing parts in a conventional SCR/ASC configuration.

(13) FIGS. 12 and 13 compare performance of a conventional SCR/SCR/ASC arrangement, compared to the SCR/ASC/SCR configuration described herein. FIGS. 12A and 12B look at NH.sub.3 conversion and slip, respectively. FIGS. 13A and 13B look at NOx conversion and N.sub.2O make, respectively.

(14) FIGS. 14A and 14B show the data obtained when testing the exhaust configuration on an engine.

(15) As shown in FIG. 1, there is provided and exhaust gas treatment system 1 comprising, in order from the engine 5 to the emissions outlet 10: a first means for injecting a nitrogenous reductant 15, a first selective catalytic reduction (SCR) catalyst 20, an ammonia slip catalyst (ASC) 25; and a second selective catalytic reduction (SCR) catalyst 30. The system further comprises a NOx sensor 31.

(16) The ASC 25 comprises an SCR catalyst layer 35 overlying a supported Palladium (Pd) containing layer 40.

(17) In use, the first means for injecting a nitrogenous reductant 15 is run aggressively with an ANR ratio above 1. This permits thorough SCR reactions to take place on the first selective catalytic reduction (SCR) catalyst 20. The dosing of the nitrogenous reductant by the first means for injecting a nitrogenous reductant 15 is controlled based on measurements taken by the NOx sensor 31.

(18) The ammonia slip catalyst (ASC) 25 serves to moderate the ammonia that passes to the second selective catalytic reduction (SCR) catalyst 30. This ensures that there is no ammonia slip and minimal NOx release. The position of the ammonia slip catalyst (ASC) 25 in the exhaust gas treatment system 1 means that it is running under optimal conditions for the Palladium component, such that it is highly selective for the NOx.

(19) As shown in FIG. 2, the exhaust gas treatment system 1 of FIG. 1 has now been provided with an upstream second means for injecting a nitrogenous reductant 45, a third selective catalytic reduction (SCR) catalyst 50 and a catalysed soot filter (CSF) 55. The system further comprises a further NOx sensor 51.

(20) In use the second means for injecting a nitrogenous reductant 45 is run with an ANR of less than 1. This ensures that there is no (or minimal) ammonia slip onto the CSF 55. Any ammonia passing through the system is handled by the downstream SCR catalyst 20. The ANR of the second means for injecting a nitrogenous reductant 45 may be controlled based on measurements taken by the further NOx sensor 51.

(21) On a cold start the third selective catalytic reduction (SCR) catalyst 50 handles the bulk of the SCR reactions required. However, as the exhaust gas temperature increases, the performance of the overly hot upstream third selective catalytic reduction (SCR) catalyst 50 will decline. It is therefore possible to save on ammonia by shifting the balance of the ammonia dosing from the second means for injecting nitrogenous reductant 45 which is upstream and hot, to the first means for injecting nitrogenous reductant 15 which is cooler and downstream. This means that the performance of the system can be enhanced with reduced usage of the nitrogenous reductant.

(22) FIGS. 3-9 show the same components as the above FIGS. 1 and 2 with the same reference numerals. Additional components are indicated with their usual acronyms as discussed herein.

EXAMPLES

(23) The present invention will now be described further in relation to the following non-limiting examples.

(24) Exhaust gas systems were prepared and tested as discussed below.

(25) Reactor Testing

(26) The testing reactor was run on the following settings: High Temperature (HT) aging: 650 C./264 h/10% H.sub.2O/Air SV.sub.acrossASC: 210,000 h.sup.1 Feedgas: 350 ppm NO, 500 ppm NH.sub.3, 10% O.sub.2, 6.5% H.sub.2O, 7% CO.sub.2, N.sub.2 balance ASCs tested: 2 g/ft.sup.3 Pt or Pd, or 5 g/ft.sup.3 Pd, PGM.Math.Al.sub.2O.sub.3-based bottom layer (0.35 g/in.sup.3 WCL excl. PGM), Cu.Math.CHA-based top layer (3.33 wt % Cu, 2.4 g/in.sup.3 WCL) SCR tested: Cu.Math.CHA-based (3.33 wt % Cu, 2.4 g/in.sup.3 WCL) Temperature: 500 C./30 min preconditioning in base gases, followed by ramp to 200 C., start NO/NH.sub.3 flow and measure emissions during ramp to 600 C. at 5 C./min

(27) FIGS. 10A and 10B show results for NOx conversion and N.sub.2O make for comparison of Pt and Pd ASC components. The parts had been subjected to high temperature aging at 650 C. for 264 hours in 10% H.sub.2O/Air. These are tested in a feedgas comprising 350 ppm NO, 500 ppm NH.sub.3, 10% O.sub.2, 6.5% H.sub.2O, 7% CO.sub.2, N.sub.2 balance, using the reactor rig as described above.

(28) FIG. 10A shows improved NOx conversion at high temperature for the two Pd-containing ASCs. The chart labels correspond in order (top to bottom) to the three lines at 550 C. FIG. 10B shows reduced N.sub.2O make for the Pd-containing ASCs. At 550 C., the lowest line is the Pd (2 g/ft.sup.3) ASC. The highest line at 250 C. is the Pt ASC.

(29) FIGS. 11A and 11B show results for NH.sub.3 conversion and NH.sub.3 slip for comparison of Pt and Pd ASC components. As can be seen, the Pd ASC shows slower NH.sub.3 light-off but approaches similar conversion with a Pt ASC at a high temperature regime. In FIG. 11A the order of the lines at 550 C is the opposite to the order in which they are listed on the figure. In FIG. 11B the order of the lines at 550 C. matches the order in which they are listed in FIG. 8A.

(30) FIG. 12A show improved NH.sub.3 conversion at higher temperatures for the SCR/ASC/SCR configuration disclosed herein. FIG. 12B show reduced NH.sub.3 slip at higher temperatures for the SCR/ASC/SCR configuration disclosed herein.

(31) FIG. 13A show improved NOx conversion at higher temperatures for the SCR/ASC/SCR configuration disclosed herein. FIG. 13B show reduced N.sub.2O production at temperatures below about 500 C for the SCR/ASC/SCR configuration disclosed herein (and comparable performance above).

(32) As shown in FIGS. 10-13, Pd ASC shows higher NOx conversion at high temperature regime than Pt ASC, with NH.sub.3 conversion approaching that of Pt ASC. Significantly lower N.sub.2O make is observed on Pd ASC compared to Pt ASC at low temperature regime. The disclosed ASC configuration with Pd ASC shows higher NOx conversion and slightly higher N.sub.2O make at high temperature regime compared to conventional configuration with Pt ASC.

(33) FIGS. 14A and 14B shows engine performance data for the exhaust system configured as described herein. As shown the NOx conversion is increased with a lower NOx output, despite the upstream SCR permitting greater NOx output due to slight variability in ammonia dosing. FIG. 14A shows NH.sub.3 slip is mitigated (i.e., essentially zero).

(34) Engine Testing Example

(35) DOC+inj (ANR 0.9)+SCR1+CSF+inj (ANR 1.2)+SCR2 (with and without Pd ASC)+SCR3 DOC (Ba promoter, PGM.Math.SiO.sub.2/Al.sub.2O.sub.3-based, 2.6 g/in.sup.3 WCL) SCR (4.25 wt % Cu, Cu.Math.CHA-based, 2.4 g/in.sup.3 WCL, without ASC) CSF (PGM.Math.Al.sub.2O.sub.3-based, 0.2 g/in.sup.3 WCL) SCR2 (3.33 wt % Cu, Cu.Math.CHA-based, 2.4 g/in.sup.3 WCL), with or without 5 L Pd ASC (0:1/2) SCR3 same as SCR2 (without ASC)

(36) The above configuration was tested and demonstrated a reduced NOx output (mg/mi) during a US06 cycle ran on a 6.6 L light duty diesel engine, together with an increased NOx conversion.

(37) The foregoing detailed description has been provided by way of explanation and illustration and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.