US 20080028754 A1
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.
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
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
4. The system of
5. The system of
6. The system of
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 SOX from the NOx absorber.
7. The system of
8. The system of
9. The system of
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
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
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 NOX absorber positioned along the reduction path to regenerate the NOX absorber.
13. The method of
14. The method of
15. The method of
16. The method of
the step of operating the burner to increase the temperature of the exhaust stream to a first temperature sufficient to remove SOX from the NOX absorber.
17. The method of
18. The method of
This application is a continuation-in-part application of pending U.S. Non-Provisional patent application Ser. No. 10/745,363 filed on Dec. 23, 2003, which is incorporated by reference herein in its entirety.
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.
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 NOx 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 NOX from diesel exhaust gas is by use of a NOX absorber catalyst. In this case, NOX is trapped in the absorber catalyst as the exhaust gas stream passes therethrough. Over time, the absorber catalyst may become saturated with NOX. To alleviate such a condition, the absorber catalyst is periodically regenerated to remove the NOX from the absorber catalyst by converting the trapped to NOX 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.
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 rejoined 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.
A vehicle 10, shown in
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, N.J., 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, N.Y. The DPF 48 may also be embodied as any of the filters described in U.S. Pat. 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°0 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., SOX 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.
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, Ind. (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
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
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.