WO2009015001A2

WO2009015001A2 - Methods and apparatus for operating an emission abatement assembly

Info

Publication number: WO2009015001A2
Application number: PCT/US2008/070417
Authority: WO
Inventors: Prasad Tumati; Navin Khadiya; Wilbur H. Crawley
Original assignee: Emcon Technologies Llc
Priority date: 2007-07-20
Filing date: 2008-07-18
Publication date: 2009-01-29
Also published as: WO2009015001A3; US20080028754A1

Abstract

A diesel exhaust system includes a supply of diesel fuel. The system further includes a first reduction path having a diesel fuel-fired burner operated to partially oxidize diesel fuel supplied thereto from the supply of diesel fuel and to introduce at least one of CO and H2 into an exhaust stream. The first reduction path further includes an oxidation catalyst and a first emissions reduction component that is configured to be regenerated by CO and H2 in the exhaust system. An associated method is disclosed.

Description

METHODS AND APPARATUS FOR OPERATING AN EMISSION ABATEMENT ASSEMBLY

FIELD OF THE DISCLOSURE. The present disclosure relates to exhaust emission reduction systems for diesel engine exhaust streams, and, more particularly, to emission reduction systems having a nitrogen oxides (NOx) reduction component.

BACKGROUND Diesel engine combustion exhaust includes various emissions, such as carbon dioxide, carbon monoxide, unburned hydrocarbons, NOx, and particulate matter (PM). Increasingly, environmental regulations call for emissions controls to aggressively lower diesel exhaust emission levels for NOx and PM. These standards include, for example, EURO 4 (2005) and EURO 5 (2008) and U.S. Year 2004 and U.S. Phased 2007-2010 Emissions Limit Standards. Regulations are increasingly limiting the amount of NOx that can be emitted during a specific drive cycle, such as the FTP (Federal Test Procedure) in the United States or the MVEG (Mobile Vehicle Emission Group) in Europe.

One of the ways known in the art to remove NOχ from diesel engine exhaust gas is by catalyst reduction. The catalyst reduction method essentially includes passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. For example, known emission reduction systems include systems for supplying fuel oil as hydrocarbon (HC) reductant or ammonia provided in the form of urea, either of which are injected into the exhaust gas upstream of the NOx catalyst.

Another way to remove NOχ from diesel exhaust gas is by use of a NOχ absorber catalyst. In this case, NOχ is trapped in the absorber catalyst as the exhaust gas stream passes therethrough. Over time, the absorber catalyst may become saturated with NOχ. To alleviate such a condition, the absorber catalyst is periodically regenerated to remove the NOχ from the absorber catalyst by converting the trapped to NOχ to nitrogen.

Diesel particulate filters (DPF) for the removal of PM from a diesel engine exhaust stream have been proven effective to remove carbon soot. A widely used DPF is the wall flow filter which filters the diesel exhaust by capturing the particulate material on the porous walls of the filter body.

SUMMARY OF THE DISCLOSURE

According to one aspect of the disclosure, an exhaust emission reduction system for reducing exhaust stream emissions produced by a diesel engine includes a supply of diesel fuel, NOx absorber, a diesel fuel fired burner, an oxidation catalyst, and a diesel particulate filter (DPF). The NOx absorber absorbs substances attributable to diesel fuel sulfur content and oxides of nitrogen. The burner operates to produce CO and H₂ to the exhaust stream, thereby regenerating and increasing the capacity of the NOx absorber.

In a specific exemplary implementation, the exhaust emission reduction system includes a bypass path through which most of the exhaust stream may be redirected during periodic firing of the burner and regeneration of the NOx absorber. Additionally, the DPF is disposed in the exhaust stream between the burner and NOx absorber to remove particulate matter (PM) produced by the burner and diesel engine. In another specific exemplary implementation, the system includes two parallel reduction paths each including a burner, an oxidation catalyst, a DPF, and a NOx absorber. The exhaust stream may be selectively directed to one of the reduction paths while the other path is regenerating.

In certain implementations, the emission reduction system may be configured to include an aggregate box for receiving the exhaust stream and high- temperature valves for selectively directing portions of the exhaust stream from the aggregate box through the reduction path and the bypass path. The reduction path includes the diesel fuel fired burner, the oxidation catalyst, the DPF, and the NOx absorber. The bypass path may simply include an exhaust pipe for advancing the exhaust stream. The exhaust streams passing through the reduction path and the bypass path may be re-joined and thereafter directed through a muffler and exhaust pipe. A controller controls the burner and the valves. The system may be operated in a reducing mode in which substantially the entire exhaust stream is directed through the reduction path components and the burner is not operating or is minimally operating. Additionally, in the reducing mode, the DPF removes PM and the NOx absorber traps NOx emissions from the exhaust stream. In order to regenerate the NOx absorber, the controller periodically operates the system in a regenerating mode by controlling the valves to redirect a large portion of the exhaust stream through the bypass path and operating the burner at a highly enrichened fuel- to-air mixture, thereby producing excessive CO and H₂ for regenerating the NOx absorber. The oxidation catalyst further oxidizes unburned fuel supplied to the burner to produce more CO and H₂. Additionally, the controller may also periodically operate the burner at a normal mixture setting and a high temperature, which, along with the oxidation catalyst, regenerates the DPF by providing a temperature suitable for incineration of PM trapped in the DPF.

In the case of an emission reduction system that includes two parallel reduction paths while one reduction path is being regenerated, the other path is available for reducing emissions in the engine exhaust stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an assembly view of an exhaust emission reduction system for use with a diesel engine of a vehicle,

FIG. 2 is a plan view of a first exemplary embodiment of the exhaust emission reduction system of FIG. 1; and

FIG. 3 is a plan view of a second exemplary embodiment of the exhaust emission reduction system of FIG. 1;

FIG. 4 is a plan view of a third exemplary embodiment of the exhaust emission reduction system of FIG. 1; and FIG. 5 is a plan view of a fourth exemplary embodiment of the exhaust emission reduction system of FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS

A vehicle 10, shown in FIG. 1, is powered by a diesel engine (not shown) and includes an exhaust emission reduction system 20 for removing nitrogen oxides

(NOx) and particulate matter (PM) from a diesel exhaust stream as shown in FIG. 2.

The emission reduction system 20 generally includes an aggregate box junction 24, a reduction path 26, a bypass path 28, an exit junction 30, a muffler 32, and an exhaust pipe 34. The aggregate box junction 24 receives an exhaust stream 22 and is coupled to the reduction path 26 and the bypass path 28, which are parallel paths for transmitting the exhaust stream 22. A two-way high temperature valve 38 controls a first stream portion 42 of the exhaust stream 22 flowing into the reduction path 26. A two-way high temperature valve 40 controls a second stream portion 44 of the exhaust stream 22 flowing into the bypass path 28. The reduction path 26 and the bypass path 28 are also joined at an exit junction 30 which is coupled to the muffler 32. The exhaust pipe 34 is coupled to the muffler 32 and vents a processed exhaust stream 36 to the atmosphere.

The reduction path 26 includes components for reducing emissions contained in the first stream portion 42. The reduction path 26 includes a diesel fuel fired burner 46, a diesel particulate filter (DPF) 48, and a NOx absorber 50, illustratively in that order relative to the flow of the first stream portion 42. Additionally, the reduction path 26 or the exit junction 30 may include a diesel oxidation catalyst (DOC) (not shown).

The NOx absorber 50 includes at least one catalyst absorber (not shown) that converts engine exhaust NOx into nitrogen. Such catalysts may include, for example, potassium or barium-based catalyst supported by a ceramic or metallic substrate. To recharge the NOx absorber 50 when it is nearing capacity, CO and H₂ are added to the first stream portion 42 and are carried into the NOx absorber 50 to desorb and regenerate the absorber. The CO and H₂ are produced by combustion of diesel fuel in the burner 46. Additional NOx absorbers may also be included in the reduction path 26 in parallel or in series with the NOx absorber 50. Exemplary NOx absorbers may be, for example, NOx absorbers manufactured by Engelhard Corporation of Iselin, New Jersey, and Johnson Matthey of London, England.

The DPF 48 includes a filter structure (not shown) for trapping and combusting diesel exhaust PM, such as carbon soot. The filter structure may be, for example, a porous ceramic forming a plurality of end-plugged honeycomb structures that are efficient at removing carbon soot from the exhaust of diesel engines. The filter structure may also include a catalyst that provides ignition and incineration of carbon soot at a lower temperature range. The DPF 48 may be, for example, a filter manufactured by Corning Incorporated of Corning, New York. The DPF 48 may also be embodied as any of the filters described in U.S. Patent No. 6,464,744.

The diesel fuel fired burner 46 receives a supply of diesel fuel at a supply line 52 and is capable of increasing the temperature of the first stream portion 42 of the exhaust of a diesel engine. The products of combustion of diesel fuel in the burner 46 include CO, H₂, and soot. CO and H₂ act as reducing compounds for removal of nitrogen oxides from the absorber 50, exhausting the nitrogen as N₂ thereby regenerating the absorber to trap additional NOx. If the burner 52 is operated at an enrichened setting, i.e., the fuel-to-air ratio is increased beyond that used for peak heat production and/or efficient combustion, the burner produces significant quantities of CO and H₂ which are then carried by the first stream portion 42 into the NOx absorber 50. Any soot produced by the burner 46 is trapped by the DPF 48 before the exhaust stream portion 42 reaches the NOx absorber 50.

The exhaust emission reduction system 20 includes a control device (not shown) for controlling actuation of the valves 38 and 40 and the burner 46. During operation of emission reduction system 20 in a reduction mode, the valve 38 is positioned in an open position thereby allowing the stream portion 42 of the exhaust stream 22 to flow into the reduction path 26, and the valve 40 is positioned in the closed position thereby reducing the second stream portion 44 of the exhaust stream 22 to substantially no flow. Additionally in the reduction mode, the burner 46 is off or in a reduced operating setting, the DPF 48 traps PM contained in the exhaust stream portion 42, and the NOx absorber 50 traps NOx. The reduction mode of emission reduction system 20 may provide approximately 60 to 90 seconds of emission reduction before regeneration is necessary, but may provide 50 to 100 seconds, or more or less, depending on the capacity of the NOx absorber 50 and the volume of the emissions of exhaust stream 22.

The emission reduction system 20 is operated in a regeneration mode to regenerate the NOx absorber 50. To do so, the controller switches the valve 38 to a closed position, substantially reducing the first stream portion 42 of the exhaust stream 22 flowing through the reduction path 26. The controller also switches the valve 40 to an open position, substantially increasing the second stream portion 44 of the exhaust stream 22 flowing through the bypass 28 and therefore around the burner 46, the DPF 48, and the NOx absorber 50. In an exemplary implementation, approximately 70% of the exhaust stream 22 flows through the bypass path 28 during the regeneration mode. The controller also operates the burner 46 at a very rich fuel-to-air mixture, thus producing significant quantities of CO and H₂ in the exhaust stream portion 42 that is provided to the NOx absorber 50 for regenerating, i.e., restoring the capacity of, the absorber catalyst. The DPF 48 traps any soot generated by operating the burner 46 at a rich mixture. The regeneration mode may continue for approximately 20 seconds but may last from 10 to 30 seconds, or more or less depending on the quantities of CO and H₂, the temperature of the exhaust stream 22, and the characteristics of NOx absorber 50.

The exhaust emission reduction system 20 will repeatedly cycle between the reduction and regeneration modes during operation of the diesel engine. Additionally, the DPF filter 46 may require periodic regeneration, for example, every two to four hours of operation, in order to more fully combust and remove soot trapped by the DPF 48.

The burner 46 may also be operated by the controller to raise the temperature of the first exhaust stream portion 42 entering the DPF 48 to a range of 600° to 650° C, but less than a temperature causing damage to the filter structure within the DPF 48, for example, less than 1000° C, perhaps less than 900° C. If a catalytic treated DPF is used, regeneration of the DPF 48 may only require elevating the temperature of the first stream portion 42 to between 300° to 350° C. Elevation of the temperature of the first stream portion 42 to more than 500° C. 600° C. provides desulfation (i.e., SOχ removal), and therefore regeneration, of the NOx absorber 50. Desulfation frees absorbed substances, primarily sulfur, from absorber storage sites and therefore restores capacity of the NOx absorber 50.

Referring to FIG. 3, a second exemplary embodiment of an exhaust emission reduction system 100 generally includes an aggregate box junction 102, a first reduction path 104, a second reduction path 106, an exit junction 108, a DOC 110, a muffler 112, and an exhaust pipe 114. The first reduction path 104 and the second reduction path 106 are parallel paths for the exhaust stream 22 and receive a first exhaust stream portion 116, controlled by a valve 118, and a second exhaust stream 120, controlled by a valve 122. Each reduction path 104 and 106 may include the same emission reduction components as the reduction path 26 of the emission reduction system 20, namely a diesel fuel fired burner 124 and 126, a DPF 128 and 130, and a NOx absorber 132 and 134. Each of these components of the first reduction path 104 and the second reduction path 106 may operate as described in regard to the reduction path 26 of the emission reduction system 20.

Because the emission reduction system 100 includes two parallel reduction paths, one reduction path (104 or 106) can receive and reduce emissions of the exhaust stream 22 while the other path is being regenerated. For example, the valve 118 may be positioned in its open position and the valve 122 positioned in its closed position. In such a case, the first stream portion 116 includes substantially all of the exhaust stream 22 and the burner 128, the DPF 128, the NOx absorber 132, and the DOC 110 create a processed exhaust stream 140 which has reduced levels of NOx, PM, and HC. While the first reduction path 104 is in the reduction mode, the second path 106 may be regenerating, as described above in regard to the exhaust emission reduction system 20.

The diesel oxidation catalyst (DOC) 110 receives the first and second exhaust stream portions 42 and 44 and reduces unburned HC and CO present in the exhaust stream. The DOC 110 catalyzes the oxidation of unburned HC and CO. Such a device is available from EMCON Technologies, LLC of Columbus, Indiana (formerly the exhaust unit of ArvinMeritor, Inc.). The muffler 112 and the exhaust pipe 114 provide engine exhaust noise reduction and directing of the processed exhaust stream 140 into the atmosphere.

As discussed above, the reduction path 26 may include a DOC. One illustrative embodiment of doing so is shown as system 200 in FIG. 4. This illustrative embodiment contains many of the emission reduction components, such as the DPF 48 and the NOx absorber 50, described in regard to FIG. 2. Furthermore, the regeneration scheme described in regard to FIG. 2 used to regenerate both the NOx absorber 50 and the DPF 48 can be implemented with the system 200 as well. However, it should be appreciated that other regeneration schemes can also be used. The system 200 illustratively includes a diesel fuel-fired burner 206 and a

DOC 204 disposed downstream of the burner 206 along the reduction path 26. During regeneration of either the NOx absorber 50 or the DPF 48, the burner 206 can be activated and supplied diesel fuel through a fuel line 208. Through burning the supplied diesel fuel, the burner 206 provides both heat for regenerating the DPF 48 and H₂ and CO for regenerating the NOx absorber 50. In one illustrative embodiment, the burner 206 is configured to partially oxidize fuel supplied to it. The partially-oxidized fuel will flow downstream to the DOC 204.

The DOC 204 is configured to catalyze an oxidation reaction between a gaseous component containing oxygen and hydrocarbons. It should be appreciated that the term "hydrocarbons" can refer to various molecules containing hydrogen, carbon, and oxygen, such as diesel fuel and partially-oxidized diesel fuel. Specifically, when hydrocarbons are advanced into contact with the DOC 204 in the presence of a gaseous component containing oxygen, the DOC 204 catalyzes an oxidation reaction converting the hydrocarbons and a portion of the oxygen into, amongst other things, H₂ and CO. The valves 38, 40 can be controlled to allow enough of the exhaust flow 22 to enter the reduction path 26, such that an adequate amount of oxygen is available for the reaction catalyzed by the DOC 204 to take place. Thus, in this illustrative embodiment, the DOC 204 may catalyze the reaction of partially-oxidized diesel fuel with oxygen present in the first stream portion 42. This can provide an increased amount of H₂ and CO provided to the NOx absorber 50 for regeneration than would be provided by the burner 206 alone. During periods of regenerating the DPF 48, the burner 206 can increase the temperature of the first stream portion 42 in order to burn trapped soot in the DPF 48. The DOC 204 can be used to further increase the temperature of the first stream portion 42. As previously described, the DOC 204 catalyzes the reaction between the partially-oxidized diesel fuel and oxygen present in the first stream portion 42. This reaction both produces the byproducts previously discussed and generates heat. This generated heat can be used to further increase the temperature of the first stream portion 42 to reach temperature levels sufficient for regenerating the DPF 48. Furthermore, at temperature levels sufficient for regenerating the DPF 48, the air-to- fuel ratio supplied to the burner 206 can be controlled such that the burner 206 and DOC 204 produce CO and H₂ for desulfating the NOx absorber 50.

It should be appreciated that other configurations may be implemented with the DOC 204 other than that shown in FIG. 4. For example, the burner 206 may be operated to fully oxidize fuel supplied thereto for supplying both heat and the byproducts of H₂ and CO. A fuel injector (not shown) may be placed directly upstream of the DOC 204 to inject fuel into the DOC 204. This will allow the DOC 204 to catalyze a reaction between the injected fuel and oxygen present in the exhaust gases, as previously described. Thus, the DOC 204 can provide an increased amount of H₂ and CO for regeneration of the NOx absorber 50 and heat for raising the exhaust gas temperatures to temperatures sufficient for regenerating the DPF 50. It should be further appreciated that other reforming catalysts besides the DOC 204 may be used in order to generate the H₂ and CO, as well as heat. FIG. 5 shows an illustrative embodiment of an exhaust emission reduction system 210, similar to the configuration shown in FIG. 3. As similarly described in regard to FIG. 3, the system 210 includes a first reduction path 224 and a second reduction path 226. Each reduction path 224, 226 includes a diesel fuel-fired burner 216, 218, respectively, with each burner 216, 218 being supplied diesel fuel through a fuel line 220, 222, respectively. Each reduction path 224, 226 further includes a DOC 212, 214, respectively, with each DOC 212, 214 being positioned downstream of a burner 216, 218, respectively. The regeneration scheme described in regard to the system 100 can be used with the system 210 to the extent that one of the reduction paths 224, 226 is performing reduction, while the other is undergoing regeneration in an alternating manner. In this illustrative embodiment, the burner/DOC combination in each reduction path 224, 226 can be operated in the manner described in regard to the illustrative embodiment shown in FIG. 4. This allows each reduction path 224, 226 to have its respective DPF 132, 134 and NOx absorber 128, 130 regenerated in the manner described in regard to FIG. 4. There are a plurality of advantages of the present disclosure arising from the various features of the apparatus and methods described herein. It will be noted that alternative embodiments of the apparatus and methods of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of an apparatus and method that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the present disclosure.

Claims

1. A diesel exhaust system comprising: a supply of diesel fuel, and a first reduction path having:

(i) a diesel fuel-fired burner operated to partially oxidize diesel fuel supplied thereto from the supply of diesel fuel and to introduce at least one of CO and H₂ into an exhaust stream,

(ii) an oxidation catalyst, and (iii) a first emissions reduction component, the first emissions reduction component being configured to be regenerated by CO and H₂ in the exhaust stream.

2. The system of claim 1, wherein: the oxidation catalyst is positioned downstream of the diesel fuel-fired burner and configured to catalyze a reaction between oxygen present in the exhaust stream and the partially-oxidized diesel fuel to introduce at least one of CO and H₂ into the exhaust stream, and the first emissions reduction component is positioned downstream of the oxidation catalyst.

3. The system of claim 1, wherein the first emission reduction component includes a catalytic NOx absorber.

4. The system of claim 3, wherein the burner is periodically operated at a fuel- to-air mixture providing increased production of at least one of CO or H₂.

5. The system of claim 3, further comprising a particulate filter disposed in the exhaust stream between the oxidation catalyst and the NOχ absorber.

6. The system of claim 5, wherein: the burner is operable to heat the exhaust stream to a first temperature, the oxidation catalyst is configured to further heat the exhaust stream to a second temperature greater than the first temperature, and the second temperature is sufficient for at least one of incinerating a substantial portion of the particulates trapped by the filter or removing SOχ from the

NOx absorber.

7. The system of claim 1, further comprising a bypass path for periodically redirecting at least a substantial portion of the exhaust stream from the first reduction path.

8. The system of claim 7, further comprising at least one valve capable of selectively directing a substantial portion of the exhaust stream from the first reduction path to the bypass path.

9. The system of claim 1, further comprising a second reduction path, having: a second diesel fuel-fired burner operated to burn at least a first portion of diesel fuel supplied thereto from the supply of diesel fuel and to introduce at least one of CO and H₂ into an exhaust stream, a second oxidation catalyst, and a second emissions reduction component positioned downstream of the second diesel fuel-fired burner, the second emissions reduction component being configured to be regenerated by CO and H₂ in the exhaust stream, and at least one valve operable to selectively direct portions of the exhaust stream between the first and the second reduction paths.

10. The system of claim 9, wherein: the oxidation catalyst is disposed downstream of the diesel fuel-fired burner and upstream of the emissions reduction component and configured to catalyze a reaction between oxygen present in the exhaust stream and the partially-oxidized diesel fuel to introduce at least one of CO and H₂ into the exhaust stream, and the second oxidation catalyst is disposed downstream of the second diesel fuel-fired burner and upstream of the second emissions reduction component and configured to catalyze a reaction between oxygen present in the exhaust stream and the partially-oxidized diesel fuel to introduce at least one of CO and H₂ into the exhaust stream.

11. The system of claim 9, wherein the first and the second emissions reduction components include NOx absorbers.

12. A method of operating a diesel exhaust system, the comprising the steps of: advancing an exhaust stream along a reduction path having an oxidation catalyst, operating a diesel fuel-fired burner to oxidize diesel fuel supplied thereto and to introduce at least one of CO or H₂ into the exhaust stream, and directing the exhaust stream to a first NOχ absorber positioned along the reduction path to regenerate the N0χ absorber.

13. The method of claim 12, wherein the operating step comprises processing an enriched fuel-to-air mixture of diesel fuel with the diesel fuel-fired burner.

14. The method of claim 12, further comprising the step of filtering particulate matter from the exhaust stream with a particulate matter filter.

15. The method of claim 14, further comprising the step of operating the burner to increase the temperature of the exhaust stream to a first temperature.

16. The method of claim 12, further comprising: the step of operating the burner to increase the temperature of the exhaust stream to a first temperature sufficient to remove SOχ from the N0χ absorber.

17. The method of claim 12, wherein the directing step comprises directing a substantial portion of the exhaust stream to bypass the first N0χ absorber.

18. The method of claim 12, further comprising the steps of wherein the directing step comprises redirecting a substantial portion of the exhaust stream from the first N0χ absorber to a second N0χ absorber.