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Alumina-based lean NOx trap system and method of use

a technology of nox traps and nox, which is applied in the direction of separation processes, machines/engines, mechanical equipment, etc., can solve the problems of loss of both catalyst activity, low efficiency, and low durability of conventional nox traps, and achieve efficient desulphurization and extended use

Inactive Publication Date: 2006-08-03
FORD GLOBAL TECH LLC
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0009] The present invention meets those needs by providing a lean NOx trap (LNT) system for use in lean burn engines such as diesel engines which includes at least one LNT which utilizes alumina as a NOx storage (absorbent) material. The alumina-based L

Problems solved by technology

However, lean burn engines such as diesel engines, which offer 10 to 30% better fuel economy than conventional gasoline engines, require special treatment since a TWC cannot effectively reduce the level of NOx under the oxidizing atmosphere present in the exhaust gas of such engines which are operated under fuel-lean conditions (λ>1, where λ represents the air-to-fuel ratio divided by the stoichiometric air-to-fuel ratio).
However, the LNT has not been widely used in diesel engine-powered vehicles because the exhaust gas temperature is usually somewhat lower than the operating temperature window normally provided by the trap.
In addition, a conventional LNT is not very durable, suffering loss of both catalyst activity (e.g., through thermally activated coarsening of precious metal particles) and NOx storage capacity (e.g., through aqueous-based leaching).
In addition, fuel rich operation is required for the de-SOx treatment, which takes away some of the efficiency provided by diesel engine operation.
However, none of these approaches has provided a satisfactory solution to the removal of NOx in lean exhaust gas due to low efficiency, inability to regenerate and / or undergo efficient desulphurization, and the high system cost.

Method used

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Examples

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example 1

[0067] An alumina-based lean NOx trap (LNT) was prepared in accordance with the present invention using only Pt as the catalyst and was exposed to synthetic exhaust gas at temperatures ranging from about 1000 to 450° C. The composition of synthetic exhaust gas used in the LNT laboratory tests is listed in Table 1 below.

TABLE 1Gases used in LNT Lab TestGasLeanRichCO500 ppm4%H2167 ppm1.33%C3H6300 ppm C15000 ppm C1NOx500 ppm500 ppmO210%1%CO25%5%H2O5%5%λ = 2.0λ = 0.90

FIG. 6 illustrates the resulting NOx conversion efficiency under varying conditions in a lean / rich cycle of 24 sec. / 6 sec. The conditions for the alumina-based LNT included operation under lean conditions at 700° C. for 20 minutes, and an additional 10 minutes under rich conditions at 350° C. As can be seen, the fresh alumina LNT exhibits good NOx conversion efficiency at low temperatures, but the 700° C. lean treatment causes the NOx activity to deteriorate. The rich treatment at 350° C. does not restore the activity.

example 2

[0068] An alumina-based LNT was prepared in accordance with the present invention using both Pt and Rh as catalysts. The trap was exposed to exhaust gas at temperatures ranging from about 100 to 450° C. FIG. 7 illustrates the NOx conversion efficiency under varying conditions in a lean / rich cycle of 24 s / 6 s. The conditions for the alumina-based LNT included operation under lean conditions at 700° C. for 20 minutes and an additional 10 minutes under rich conditions at 350° C.

[0069] As can be seen, the fresh alumina-based LNT has good NOx conversion efficiency at low temperatures, and the 700° C. lean treatment causes the NOx activity to deteriorate, but the rich treatment at 350° C. largely restores the activity. It can be concluded that the combination of Pt and Rh catalysts is preferred for use in the alumina-based LNT of the present invention.

[0070]FIG. 8 illustrates the NOx storage capacities of the alumina-based LNTs of the present invention, with either pure alumina or alumi...

example 3

[0075] An alumina-based LNT was prepared in accordance with the present invention using both Pt and Rh as catalysts and was exposed to 30 ppm SO2 at 200° C. to 450° C. and λ=2.0 for 3200 seconds. A desulphurization (de-SOx) treatment was then performed at 650° C. under slightly rich conditions. FIG. 11 illustrates the de-SOx process of the alumina LNT at 650° C. and λ=0.987. The sulfur loading was 1.14 g per liter and the desulphurization started at t=5500 seconds. More than 95% by weight of the sulfur was released in about 60 seconds as SO2. Similar efficient desulphurization properties were shown when the alumina-based LNT was modified by a very small amount (less than 2.0 wt %) of calcium oxide through solution ion exchange as shown in FIG. 12.

[0076]FIG. 13 compares the de-SOx performance of the alumina-based LNT of the present invention with a conventional LNT comprised of Ba oxide supported on alumina as the NOx storage material with similar sulfur loading (1.14 g / liter). The ...

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Abstract

An alumina-based lean NOx trap system for use in a lean burn engine such as a diesel engine is provided which includes at least one alumina-based lean NOx trap comprising a catalyst, an alumina NOx absorbent material, and optionally, from 0 to about 4 wt % of an alkaline earth metal oxide. The system preferably includes at least a first alumina-based lean NOx trap, and a second lean NOx trap which is positioned downstream from the first lean NOx trap in an engine exhaust. The second lean NOx trap may comprise an alumina-based trap or a conventional lean NOx trap. The lean NOx trap system converts at least a portion of NOx contained in the exhaust gas to N2 at a temperature between about 150° C. to about 500° C. The alumina-based lean NOx trap system also undergoes efficient desulphurization and maintains its activity with extended use.

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application No. 60 / 649,331, entitled SYSTEM FOR TREATING EMISSIONS FROM DIESEL EXHAUST GAS, filed Feb. 2, 2005. The entire contents of said application are hereby incorporated by reference.BACKGROUND OF THE INVENTION [0002] The present invention relates to a lean NOx trap system for treating emissions from lean burn engines such as diesel engine exhaust gas, and more particularly, to a system including at least one lean NOx trap which utilizes alumina as a NOx storage material. The alumina-based lean NOx trap achieves high NOx conversion efficiency at low temperatures encountered in the exhaust gas of diesel engine-powered vehicles, maintains its activity with extended use, and undergoes efficient desulphurization. [0003] Government regulations, created in response to environmental and health concerns, have necessitated the treatment of exhaust gas from vehicle engines in order to ...

Claims

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Application Information

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IPC IPC(8): F01N3/00F01N3/10
CPCB01D53/9422B01D2253/104B01D2255/1021B01D2255/1025B01D2255/204B01D2255/91F01N3/0821F01N3/0842F01N3/0878F01N3/0885F01N2370/02Y02T10/24F01N13/009F01N13/0097F01N13/0093Y02T10/12
Inventor XU, LIFENGGRAHAM, GEORGEHOARD, JOHNMCCABE, ROBERT
Owner FORD GLOBAL TECH LLC