POST THERMAL CONTROL DEVICE FOR USE WITH A NOx SLIP CATALYST

Abstract

A system and method for treating diesel exhaust in a diesel exhaust system, and specifically for improved NO.sub.x conversion efficiency during particulate filter regeneration, is disclosed. The exhaust gas treatment system includes a diesel oxidation catalyst (DOC); a diesel particulate filter (DPF) fluidly coupled to the DOC; a mixer fluidly coupled to the DOC and DPF, a thermal control device (TCD) positioned after the DPF, a NO.sub.x slip catalyst (NSC), at least one temperature sensor (TS), wherein the thermal control device reduces the temperature of the exhaust gas stream. The thermal control device used in conjunction with the NO.sub.x slip catalyst provides for improved overall exhaust system NO.sub.x conversion efficiency during active DPF regeneration.

Description

TECHNICAL FIELD

[0001] The present invention relates to a system and method for treating diesel exhaust in a diesel exhaust system. Particularly, the present invention provides a method for improving the reduction of nitrogen oxides (NO.sub.x) in an exhaust gas stream through installation of a thermal control device in conjunction with a NO.sub.x slip catalyst, which reduces exhaust gas temperature resulting in greater NO.sub.x conversion efficiency.

BACKGROUND

[0002] Diesel engines are efficient, durable and economical. Diesel exhaust, however, can harm both the environment and people. To reduce this harm, governments, such as the United States and the European Union, have proposed stricter diesel exhaust emission regulations. These environmental regulations require diesel engines to meet the same pollution emission standards as gasoline engines. Typically, to meet such regulations and standards, diesel engine systems require equipment additions and modifications.

[0003] For example, a lean burning engine provides improved fuel efficiency by operating with an amount of oxygen in excess of the amount necessary for complete combustion of the fuel. Such engines are said to run "lean" or on a "lean mixture." However, the increase in fuel efficiency is offset by the creation of undesirable pollution emissions in the form of nitrogen oxides (NO.sub.x). Nitrogen oxide emissions are regulated through regular emission testing requirements. One method used to reduce NO.sub.x emissions from lean burn internal combustion engines is known as selective catalytic reduction. When used to reduce NO.sub.x emissions from a diesel engine, selective catalytic reduction involves injecting atomized urea into the exhaust stream of the engine in relation to one or more selected engine

[0004] Another method for reducing NO.sub.x emissions is exhaust gas recirculation (EGR), which is a technique that re-circulates a portion of an engine's exhaust gas back to the engine cylinders. Engines employing EGR recycle part of the engine exhaust back to the engine air intake. The oxygen depleted exhaust gas blends into the fresh air entering the combustion chamber. Reducing the oxygen produces a lower temperature burn, reducing NO.sub.x emissions by as much as 50%. The recycled exhaust gas can then be cooled. This "cooled EGR", can create an even greater reduction in emissions by further lowering the combustion temperatures. When used with a DPF (diesel particle filter), emissions can be reduced up to 90%.

[0005] The DPF is made up of a diesel oxidation catalyst (DOC), which is a ceramic material that heats up in the DPF. The filter is used to collect particulate matter from the DPF. The DPF is cleaned of particulate matter at periodic intervals through a regeneration process. Regeneration is the process of removing the accumulated soot from the filter. This is done either passively (from the engine's exhaust heat in normal operation or by adding a catalyst to the filter) or actively by introducing very high heat (more than 600.degree. C. to burn off the particulate matter) into the exhaust system.

[0006] However, one potential disadvantage of the regeneration system is the generation of higher levels of NO.sub.x. In addition, the super heated exhaust may shorten the life of some engine components. Therefore, it would be advantageous to provide a system and method for improving the overall exhaust system NO.sub.x conversion efficiency during active DPF regeneration.

SUMMARY

[0007] A method for reducing NO.sub.x in an exhaust gas stream of a diesel-engine vehicle, is disclosed. Generally, the method comprises the steps of fluidly coupling components of an exhaust gas treatment system package to an engine exhaust gas system, injecting gaseous ammonia into the exhaust gas treatment system package, reducing an outgoing temperature of engine exhaust gas prior to exiting the system, and, reducing the level of NO.sub.x to an acceptable level. In a preferred embodiment, a thermal control device (TCD) in conjunction with a NO.sub.x slip catalyst (NSC) is incorporated into the system to reduce the temperature of the exhaust gas.

[0008] Further, an exhaust gas treatment system for use in reducing NO.sub.x in an exhaust gas stream of a diesel-engine vehicle, is disclosed. The system comprises a diesel oxidation catalyst (DOC); a mixer fluidly coupled to the DOC, a diesel particulate filter DPF fluidly coupled to the DOC and mixer, a thermal control device (TCD) positioned after the DPF, a NO.sub.x slip catalyst (NSC) positioned in conjunction with the TCD, and at least one temperature sensor (TS). In another embodiment, multiple temperature sensors may be used in a variety of locations among the components of the system.

[0009] In yet another embodiment, a method for reducing NO.sub.x in an exhaust gas stream in a diesel-engine vehicle after filter regeneration, is disclosed. The method comprises fluidly coupling components of an exhaust gas treatment system package to an engine exhaust gas system, superheating exhaust gas prior to entry into the exhaust gas treatment system package, injecting gaseous ammonia into the exhaust gas treatment system package, incorporating a NO.sub.x slip catalyst (NSC) into the exhaust gas treatment system, reducing the temperature of the exhaust gas prior to entry into the NSC, and reducing the level of NO.sub.x to an acceptable level.

[0010] These and other embodiments and their advantages can be more readily understood from a review of the following detailed description and the corresponding appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic of an EGR system as currently employed.

[0012] FIG. 2 is a schematic of an embodiment of an EGR system incorporating a thermal control device and NO.sub.x slip catalyst.

DETAILED DESCRIPTION

[0013] Referring to FIG. 1, there is illustrated a current design of an exhaust gas treatment system package 10. Exhaust gas is discharged from the diesel engine 100, through conduit such as exhaust piping to the exhaust gas treatment system 110. The exhaust gas treatment system 10 of the present application consists of, in order of exhaust gas flow, a pre-diesel oxidation catalyst (pre-DOC) 112, a main diesel oxidation catalyst (main DOC) 114, a mixing chamber 116, a diesel particulate filter (DPF) 118. The pre-DOC 112, main DOC 114, mixing chamber 116, and DPF 118 are exhaust gas treatment structures present in most diesel exhaust gas treatment systems. Such structures will be generally referenced herein and identified in the drawing figures but, as each of these additional exhaust treatment structures is commonly understood by those skilled in the art, a detailed discussion of the operation of each will not be presented.

[0014] In addition, in both the current system design and in the embodiment of the present system, the mixing chamber 116 is fluidly connected to a gaseous ammonia (NH.sub.3) injector 116a, through which NH.sub.3 is injected into the mixing chamber to mix with the exhaust stream. The gaseous ammonia may be supplied through a solid ammonia source. The NH.sub.3 reacts with the exhaust stream, further reducing the amount of NO.sub.x in the exhaust stream.

[0015] Referring to FIG. 2, there is illustrated an improved exhaust gas treatment system package 110 of the system package 10 described above. In the present embodiment, a thermal control device (TCD) 120 and a NO.sub.x slip catalyst (NSC) 122 are added to the components of the system 10 shown in FIG. 1. Incorporation of the TCD 120 in conjunction with the NSC 122 results in an exhaust gas treatment system 110 with enhanced overall exhaust system NO.sub.x conversion during active DPF regeneration. It should be understood, however, that while a specific embodiment and sequence of components is described, the sequence of components can be arranged in any desired fashion depending on vehicle specifications or other requirements.

[0016] During regeneration of the DPF 118, exhaust temperatures may be super heated to above 600.degree. C. in order to burn off the particulate matter in the filter. However, this super heating may lead to an increase in NO.sub.x output, during the burn off of the particulate matter. Therefore, the present system incorporates a thermal control device (TCD) 120 to assist in reducing the temperature of the exhaust stream through the system 110. The TCD 120 may be positioned after the DPF 118, but before the NSC 122 in the system 110; however, it should be understood that the TCD can be used anywhere in the system depending on design and specifications. In this manner, after the super heated exhaust stream passes through and cleans the DPF of particulate matter, the TCD 120 functions to reduce the exhaust temperature prior to entry into the NSC 122. By positioning the NSC 122 after the DPF 118 in the present system 110, any residual NO.sub.x generated by the DPF regeneration may be captured in the NSC.

[0017] Thermal control devices 122 useful in the present application may include a radiator-type device having a coolant flowing there through for absorbing the additional heat, a series of heat dissipating fins, having a high surface area for heat dissipation as the exhaust travels through or any of a wide variety of other known thermal control devices. While various thermal control devices may be useful in the present system, the ultimate goal is the reduction of the exhaust temperature to an acceptable level resulting in lower NO.sub.x generation to meet current emission standards.

[0018] In addition, the present system 110 includes at least one temperature sensor 124. The temperature sensors 124 may be positioned in a variety of locations among the other components of the system 110, including after the TCD 122 and before the NSC 120, after the NSC only, or after both the TCD and NSC. Alternatively, the temperature sensors 124 may be positioned before the pre-DOC 112, after the main DOC 114, or anywhere else within the components of the system 110 where it may be beneficial to monitor the temperature of the exhaust stream.

[0019] The temperature sensors 124, which may be resistance type temperature sensors, are useful for indicating the current temperature of the exhaust stream temperature during and after regeneration, as well as throughout the system 110 generally. In addition, and depending on what type of TCD 122 are desired to be used in the system 110, the temperature sensors 124 may send readings back to the electronic control module (not shown) of the system, which than activates or de-activates the TCD 122 depending on the temperature reading of the exhaust stream in the system.

[0020] Diesel particulate filters typically require periodic regeneration. The present method provides reducing NO.sub.x in an exhaust gas stream in a diesel-engine vehicle after particulate filter regeneration. The method includes fluidly coupling components of an exhaust gas treatment system package, including a DOC, mixer and DPF, to an engine exhaust gas system, applying superheated exhaust gas into the exhaust gas treatment system package for filter regeneration, injecting gaseous ammonia into the exhaust gas treatment system package for NO.sub.x reduction, incorporating a NO.sub.x slip catalyst (NSC) into the exhaust gas treatment system, reducing the temperature of the exhaust gas prior to entry into the NSC, and further reducing the level of NO.sub.x to an acceptable level.

Claims

1. A method for reducing NO.sub.x in an exhaust gas stream of a diesel-engine vehicle, the method comprising the steps of: fluidly coupling components of an exhaust gas treatment system package to an engine exhaust gas system; flowing a heated exhaust stream through the exhaust gas treatment system package; injecting gaseous ammonia into the exhaust gas treatment system package; reducing an outgoing temperature of engine exhaust gas after the exhaust passes through the system package; and, reducing the NO.sub.x to an acceptable level.

2. The method of claim 1, wherein the components of the exhaust gas treatment system package comprise a mixing chamber for reacting the gaseous ammonia with the exhaust gas stream to reduce NO.sub.x in the exhaust gas.

3. The method of claim 2, wherein the components of the exhaust gas treatment system package further comprise: a diesel oxidation catalyst (DOC); a diesel particulate filter (DPF); a thermal control device (TCD); a NO.sub.x slip catalyst (NSC); and, at least one temperature sensor (TS); wherein the DOC, DPF, TCD, TS and NSC are all fluidly coupled together and to the mixing chamber, and wherein the TCD operates in conjunction with the NSC to effectively reduce the amount of NO.sub.x in the exhaust stream.

4. The method of claim 3, wherein the thermal control device is positioned before the NSC.

5. The method of claim 3, wherein at least one of the temperature sensors is positioned after the thermal control device and before the NSC.

6. The method of claim 3, wherein at least one of the temperature sensors is positioned before the thermal control device.

7. The method of claim 1, wherein the step of reducing an outgoing temperature of engine exhaust gas prior to entry into a NO.sub.x slip catalyst (NSC) includes installation of a thermal control device (TCD) before the NSC.

8. The method of claim 7, wherein the step of reducing an outgoing temperature of engine exhaust gas prior to entry into a NO.sub.x slip catalyst (NSC) includes installation of a thermal control device (TCD) after the DPF.

9. An exhaust gas treatment system for use in reducing NO.sub.x in an exhaust gas stream of a diesel-engine vehicle, the system comprising: a diesel oxidation catalyst (DOC); a mixer fluidly coupled to the DOC; a diesel particulate filter (DPF) fluidly coupled to the mixer and DOC; a thermal control device (TCD) positioned after the DPF; a NO.sub.x slip catalyst (NSC) positioned downstream from the DPF; and, at least one temperature sensor (TS) positioned in conjunction with one of the TCD and NSC, wherein the TCD operates in conjunction with the at least one temperature sensor to reduce the temperature of the exhaust gas stream prior to entry into the NSC.

10. The exhaust gas treatment system of claim 9, wherein the temperature sensor is positioned after the NSC.

11. The exhaust gas treatment system of claim 9, wherein a first temperature sensor is positioned before the NSC, and a second temperature sensor is located after the NSC.

12. The exhaust gas treatment system of claim 9, wherein the thermal control device is an exhaust temperature reducing device.

13. The exhaust gas treatment system of claim 9, wherein the temperature sensor interacts with the thermal control device to reduce exhaust temperatures to a desired level.

14. A method for reducing NO.sub.x in an exhaust gas stream in a diesel-engine vehicle after particulate filter regeneration, the method comprising: fluidly coupling components of an exhaust gas treatment system package to an engine exhaust gas system; applying superheated exhaust gas into the exhaust gas treatment system package for filter regeneration; injecting gaseous ammonia into the exhaust gas treatment system package for NO.sub.x reduction; incorporating a NO.sub.x slip catalyst (NSC) into the exhaust gas treatment system; reducing the temperature of the exhaust gas prior to entry into the NSC; and, further reducing the level of NO.sub.x to an acceptable level.

15. The method of claim 14, wherein the components of the exhaust gas treatment system package further comprise: a diesel oxidation catalyst (DOC); a diesel particulate filter (DPF); a thermal control device (TCD); and, at least one temperature sensor (TS); wherein the DOC, DPF, TCD, TS and NSC are all fluidly coupled together and to the mixing chamber, and wherein the TCD operates to reduce the temperature of the superheated exhaust gas.

16. The method of claim 15, wherein the TCD operates in conjunction with the NSC to effectively reduce the amount of NO.sub.x in the exhaust stream.