US20120042637A1

US20120042637A1 - Tall vertical scr

Info

Publication number: US20120042637A1
Application number: US12/858,454
Authority: US
Inventors: Stephan D. Roozenboom; Andrew J. Kieser
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2010-08-18
Filing date: 2010-08-18
Publication date: 2012-02-23

Abstract

An exhaust aftertreatment system including an exhaust conduit transmitting exhaust from an engine, a reductant introduction system introducing a reductant into the exhaust, and a selective catalytic reduction catalyst (SCR) receiving the exhaust and reductant. A SCR length divided by a SCR width is greater than 4. A SCR cell density is less than 180 cells per square inch of cross-sectional area of the SCR. The SCR is vertically mounted adjacent a corner of a cab of a machine.

Description

TECHNICAL FIELD

The present disclosure relates to engine exhaust aftertreatment systems and more particularly to the size, orientation, and locations of components in exhaust aftertreatment systems.

BACKGROUND

A selective catalytic reduction (SCR) system may be included in an exhaust treatment or aftertreatment system for a power system to remove or reduce nitrous oxide (NOx or NO) emissions coming from the exhaust of an engine. SCR systems use reductants, such as urea, that are introduced into the exhaust stream.
U.S. Pat. No. 6,182,443 (the '443 patent) discloses an aftertreatment system including an SCR system. The SCR includes a monolithic structure with a catalyst applied. The monolithic structure has channels or cells through which the exhaust passes and interacts with the applied catalyst. According to the '443 patent, the “[c]ell density should be maximized consistent with pressure drop limitations and is preferably in the range of 200-800 cells per square inch of cross-sectional area of the structure.”

SUMMARY

The present disclosure provides an exhaust aftertreatment system including an exhaust conduit transmitting exhaust from an engine, a reductant introduction system introducing a reductant into the exhaust, and a selective catalytic reduction catalyst (SCR) receiving the exhaust and reductant. In one aspect a SCR length divided by a SCR width is greater than 4. In another aspect a SCR cell density is less than 180 cells per square inch of cross-sectional area of the SCR. In yet another aspect the SCR is vertically mounted adjacent a corner of a cab of a machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a machine including a power system with an engine and an aftertreatment system.

FIG. 2 is a side view of a SCR known in the prior art.

FIG. 3 is a side view of a SCR from FIG. 1.

FIG. 4 is a side view of an alternative SCR from FIG. 1.

FIG. 5 is a cross-sectional view of the alternative SCR from FIG. 3.

FIG. 6 is another cross-sectional view of the alternative SCR from FIG. 3.

DETAILED DESCRIPTION

FIG. 1 shows a machine 1 including a cab 2 where an operator 3 sits and a power system 10. The machine 1 might be a tractor (as illustrated), on-highway truck, car, vehicle, off-highway truck, earth moving equipment, material handler, logging machine, compactor, construction equipment, generator, pump, aerospace application, locomotive application, marine application, or any other device or application requiring a power system 10.
The power system 10 includes an engine 12 and an aftertreatment system 14 to treat an exhaust stream 16 produced by the engine 12. The engine 12 may include other features not shown, such as controllers, fuel systems, air systems, cooling systems, peripheries, drivetrain components, turbochargers, exhaust gas recirculation systems, etc. The engine 12 may be any type of engine (internal combustion, gas, diesel, gaseous fuel, natural gas, propane, etc.), may be of any size, with any number of cylinders, and in any configuration (“V,” in-line, radial, etc.).
The aftertreatment system 14 includes an engine exhaust conduit 18 delivering the exhaust stream 16. The aftertreatment system 14 includes an exhaust conduit 18 and a Selective Catalytic Reduction (SCR) system 20. The SCR system 20 includes an SCR 22, and a reductant supply system 24.
In some embodiments, the aftertreatment system 14 may also include a diesel oxidation catalyst (DOC) 26, a diesel particulate filter (DPF) 28, and a clean-up catalyst 30. The DOC 26, DPF 28, SCR 22, and clean-up catalyst 30 involve the appropriate catalyst or other material disposed on a substrate. The substrate may consist of cordierite, silicon carbide, other ceramic, or metal structure. The substrates may form a honeycomb structure with a plurality of through going channels or cells for the exhaust stream 16 to pass through. The DOC 26, DPF 28, SCR 22, and clean-up catalyst 30 substrates may be housed in canisters 31. The DOC 26 and DPF 28 may be in the same canister 31, as shown, or separate. Likewise, the SCR catalyst 22 and clean-up catalyst 30 may also be in the same canister 31, as shown, or separate.
The aftertreatment system 14 is configured to remove, collect, or convert undesired constituents from the exhaust stream 16. The DOC 26 oxidizes Carbon Monoxide (CO) and unburnt hydrocarbons (HC) into Carbon Dioxide (CO2). The DPF 28 collects particulate matter or soot. The SCR catalyst 22 is configured to reduce an amount of NOx in the exhaust stream 16 in the presence of a reductant.
The clean-up catalyst 30 may embody an ammonia oxidation catalyst (AMOX). The clean-up catalyst 30 is configured to capture, store, oxidize, reduce, and/or convert reductant that may slip past or breakthrough the SCR catalyst 22. The clean-up catalyst 30 may also be configured to capture, store, oxidize, reduce, and/or convert other constituents present.
In the illustrated embodiment, the exhaust stream 16 exits the engine 12, passes through the DOC 46, DPF 48, then passes through the SCR system 20, and then passes through the clean-up catalyst 30 via the exhaust conduit 18. The SCR system 20 is downstream of the DPF 28 and the DOC 26 is upstream of the DPF 28. The clean-up catalyst 30 is downstream of the SCR system 20. In other embodiments, these devices may be arranged in a variety of orders and may be combined together. In one embodiment, the SCR catalyst 22 may be combined with the DPF 48 with the catalyst material deposited on the DPF 48. Other exhaust treatment devices may also be located upstream, downstream, or within the SCR system 20.
The reductant supply system 24 is configured to introduce the reductant in to the exhaust upstream of the SCR 22. The reductant supply system 24 may include a reductant source 32, reductant line 34, and an injector 36. The reductant supply system 24 may also include a pump and one or more valves to achieve and control the delivery of the reductant. Reductant may also be provided to the SCR 22 from the engine 12 or in a variety of other ways.
The reductant supply system 24 may also include a thermal management system to thaw frozen reductant, prevent reductant from freezing, or preventing reductant from overheating. Components of the reductant supply system 24 may also be insulated to prevent overheating of the reductant. The reductant supply system 24 may also include an air assist system for introducing compressed air. The air assist system may also be used to purge the reductant line 34 and other reductant supply system 24 components of reductant when not in use.
The injector 36 injects reductant in a mixing section 37 of the exhaust conduit 18 where the reductant may be converted and mix with the exhaust stream 16. A mixer may also be included in the mixing section 37 to help the conversion and mixing. While other reductants are possible, urea is the most common reductant. The urea reductant converts, decomposes, or hydrolyzes into ammonia (NH3) and is then adsorbed or otherwise stored in the SCR catalyst 22. The NH3 is then consumed in the SCR Catalyst 22 through a reduction of NOx into Nitrogen gas (N2).
A heat source may also be included to remove the soot from or regenerate the DPF 28, thermally manage the SCR catalyst 22, DOC 26, or clean-up catalyst 30, to remove sulfur from the DOC 26, DPF 28, or SCR catalyst 22, or to remove deposits of reductant that may have formed. The heat source may embody a burner, hydrocarbon dosing system to create an exothermic reaction on the DOC 46, electric heating element, microwave device, or other heat source. The heat source may also embody operating the engine 12 under conditions to generate elevated exhaust stream 16 temperatures. The heat source may also embody a backpressure valve or another restriction in the exhaust conduit 18 to cause elevated exhaust stream 16 temperatures.
The aftertreatment system 14 may also include a control system with NOX sensors. The control system may use the NOX sensor or engine maps to control the introduction of reductant from the reductant supply system 24 to achieve the level of NOX reduction required while controlling ammonia slip. The control system may also include soot sensors associated with the DPF 28 to control regeneration of the DPF 28.

INDUSTRIAL APPLICABILITY

Emission regulations have only recently necessitated the need for SCR systems 20. Prior art SCR systems utilize horizontally mounted, short and wide SCRs 38 with high cell densities, as shown in FIG. 2. The short and wide dimensions limit backpressure losses while the high cell densities provide high NOX conversion efficiencies by exposing the exhaust to a greater surface area of catalyst material. The horizontal mounting is utilized for structural reasons. Ceramic substrates are often used which may be heavy, especially when cell densities are high. The horizontal mounting allows the heavy substrate to be supported. The horizontal mounting is also conducive to receive the reductant, which is often injected in a horizontal section of the exhaust pipe.
However, many existing machines 1 were not designed to accommodate a short and wide SCR 38. The design changes required to accommodate such a short and wide SCR 38 may impact an operator's 3 visibility. Such design changes may include larger hoods or engine compartments. Such design changes are also expensive.
The disclosed SCR 22 is suited to be located in certain mounting locations of the machine 1. The mounting location may be selected for a number of different reasons. For example, the mounting location may be a location where the impact on operator 3 visibility is reduced or a location where the machine 1 was already designed to have a muffler located. Because an SCR often provides the level of sound dampening required, the SCR 22 may replace the muffler and therefore only limited design changes to the machine 1 would be required
An example of one such mounting location is shown in FIG. 1. FIG. 1 shows a tractor with the SCR 22 mounted along or adjacent a corner of the cab 2. While adjacent, the SCR 22 may still be spaced apart from the corner of the cab, which a common location for a muffler. The corner of the cab 2 provides the operator 3 with a greater degree of visibility than other solutions and is a location where a muffler is commonly located.
Many other mounting locations for mounting of the SCR 22 are also possible. For example, with a bulldozer or track-type tractor side visibility is important and the SCR 22 may be mounted more toward the front center of the cab 2 over the engine compartment. Other machines, such as motor graders, compactors, excavators, and wheel loaders often have rear-mounted engines so the SCR 22 may be vertically mounted behind the cab 2. Yet other machines, such as large mining trucks and wheel tractor-scrapers have side-mounted engines so the SCR 22 may be vertically mounted to the side of the cab 2. In another example, the mounting location for an on-highway truck may be the back corner of the cab 2, despite a front engine mounted design. The mounting location does not necessary require a vertical orientation, for many automotive applications the mounting location is a horizontal mounting along the length and underneath the vehicle.
Many of the mounting locations described above require the SCR not to be too wide. A wide SCR 38 could limit visibility in vertical mounting situations outside the cab 2. A wide SCR 38 could also be a clearance issue in horizontal mounted situations underneath the machine 1.
However, while the mounting locations described above often do not facilitate a wide SCR 38, they do often allow the SCR 22 to be long. In vertical mounting locations, the SCR 22 may also need to be light because the vertical mounting provides limited support. Meanwhile the SCR 22 must still achieve the level of NOX conversion needed without creating too much backpressure.
FIG. 3 illustrates an SCR 22 configured to meet the needs listed above. The illustrated SCR 22 has a SCR length 40 and a SCR width 42. The SCR width 42 may represent a diameter if the SCR 22 is circular. The SCR length 40 and SCR width 42 establish an aspect ratio of SCR length 40 divided by SCR width 42 of greater than 4. In other embodiments the aspect ratio of SCR length 40 divided by SCR width 42 may be greater than 3.5. In yet other embodiments the aspect ratio of SCR length 40 divided by SCR width 42 may be greater than 5, between 4 and 8, or between 5 and 8. By way of comparison, prior art wide SCRs 38, as shown in FIG. 2, may have lower aspect ratios of typically between 1 and 2.
In contrast to the prior art's teachings of higher cell densities, the long SCR length 40 enables lower cell density and larger cells or channels. Because of the long SCR length 40, high cell densities are not needed to create the surface area for exhaust stream 16 contact needed for high NOX conversion efficiencies. The long SCR length 40 creates the high amount of surface area for exhaust stream 16 contact for high NOX conversion efficiencies. The larger cells prevent excessive amounts of backpressure created from small cells which block exhaust stream 16 flow. The larger cells enable a narrower SCR width 42 while still limiting backpressure.
The SCR 22 cell density may be less than 180 cells per square inch of cross-sectional area. In other embodiments, The SCR 22 cell density may be between 50 and 180. By way of comparison, prior art wide SCRs 38, as shown in FIG. 2, may have cell densities between 200 and 800 cells per square inch of cross-sectional area.
The SCR cell density may be a function of SCR length 40 and the power system's 10 characteristics. The longer the SCR length 40, the less the cell density may need to be to achieve the desired SCR efficiency. The SCR length 40 may be between 2 and 8 feet. When the SCR length 40 is between 4 and 5 feet the SCR cell density may be between 100 and 150 cells per square inch of cross-sectional area. When the SCR length 40 is between 5 and 6 feet the SCR cell density may be between 60 and 120 cells per square inch of cross-sectional area. When the SCR length 40 is between 3 and 4 feet the SCR cell density may be between 130 and 180 cells per square inch of cross-sectional area.
The SCR 22 substrate may also be metallic, which is often lighter than ceramic. The lightweight achieved by the low cells per square inch of cross-sectional area and lighter metallic material helps enable vertical mounting because less weight needs to be supported. The long SCR length 40 also helps provide greater surface area between the canister 31 and the SCR 22 to help achieve the vertical mounting. Metallic substrates may also be able to be formed in longer structures with through going cells than ceramic can be extruded into.
A support 44 may also be needed to achieve the vertical mounting. The support 44 may be located underneath the SCR 22 to help support the weight of the SCR 22. The support 44 may be configured to allow the exhaust stream 16 to pass and not block the SCR 22. As seen in FIG. 3, the support 44 may embody tabs or a thin ring welded or otherwise secured to the inside wall of the canister 31. The support 44 may also be thick ring with openings, as seen in FIG. 5. The support 44 could also be thin cross-members extending from one side of the canister 31 to another side.
The reductant mixing section 37 may need to be sufficiently long and may need to be horizontal. Spraying a liquid reductant vertically upward may be problematic due to gravity. In the current configuration the injector 36 is mounted horizontal and most of the reductant is gaseous before turning vertical to pass through the SCR 22. The reductant mixing section 37 length allows a majority of the urea reductant to convert into gaseous ammonia before turning vertical. If the reductant were injected in the gaseous form these limitations on the reductant mixing section 37 could be removed or decreased.
FIGS. 4-6 illustrate a split SCR 50 as an alternative embodiment for the SCR 22 while still achieving the same aspect ratios described above. Unlike the SCR 22 in FIG. 3, the split SCR 50 includes multiple SCR 22 bodies with the exhaust stream 16 being split. The split SCR 50 includes an interior SCR 52, exterior SCR 54, exterior passage 56, and an interior passage 58. The split SCR 50 configuration allows for individual split SCRs 52 and 54 to have shorter lengths and larger cell densities, like prior art wide SCRs 38.
The interior SCR 52 has a cross-section area that is smaller than the cross-sectional area of the canister 31. The space between the interior SCR 52 and the canister forms a lower portion 60 of the exterior passage 56. An upper portion 62 of the exterior passage 56 widens in a transition zone 64 between the interior and exterior SCRs 52 and 54 to meet with the exterior SCR 54. The exterior SCR 54 has cross-section with a through-going opening that forms an upper portion 66 of the interior passage 58. A lower portion 68 of the interior passage 58 mates between the interior SCR 52 and the upper portion of the interior passage 58 in the transition zone 64. A dividing wall 70 isolates the flow of exhaust in the interior passage 58 from the exterior passage 56 in the transition zone 64. The support 44 for the SCR 52 may extend across yet still allow the exhaust stream 16 to pass through or around to enter the lower portion 60 of the exterior passage 56.
A portion of the exhaust stream 16 passes through the interior SCR 52 and then through the interior passage 58. The other portion of the exhaust stream 16 passes through the exterior passage 56 and then through the exterior SCR 54. The exhaust stream 16 then exits the split SCR 50 and may pass through the clean-up catalyst 30.
Others configurations of the split SCR 50 are possible. For example, the order of interior and exterior SCRs and passages 52, 54, 56, 58 may be reversed. The clean-up catalyst 30 may also be split in a similar manner as the split SCR 50.
Although the embodiments of this disclosure as described herein may be incorporated without departing from the scope of the following claims, it will be apparent to those skilled in the art that various modifications and variations can be made. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.

Claims

What is claimed is:

1. A exhaust aftertreatment system comprising:

an exhaust conduit transmitting exhaust from an engine;

a reductant introduction system introducing a reductant into the exhaust; and

a selective catalytic reduction catalyst (SCR) receiving the exhaust and reductant, wherein a SCR length divided by a SCR width is greater than 4.

2. The exhaust aftertreatment system of claim 1 wherein the SCR includes a plurality of cells with a cell density of less than 180 cells per square inch of cross-sectional area of the SCR.

3. The exhaust aftertreatment system of claim 1 wherein the SCR includes a plurality of cells with a cell density of between 130 and 180 cells per square inch of cross-sectional area of the SCR.

4. The exhaust aftertreatment system of claim 3 wherein the SCR length is between 3 and 4 feet.

5. The exhaust aftertreatment system of claim 1 wherein the SCR is vertically mounted on a machine.

6. The exhaust aftertreatment system of claim 5 wherein the reductant introduction system introduces the reductant into a horizontal section of the exhaust conduit.

7. The exhaust aftertreatment system of claim 5 wherein the SCR is mounted adjacent a corner of a cab.

8. The exhaust aftertreatment system of claim 5 wherein the SCR includes a metallic substrate.

9. The exhaust aftertreatment system of claim 5 further including a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) upstream of the SCR and a clean-up catalyst downstream of the SCR.

10. A exhaust aftertreatment system comprising:

an exhaust conduit transmitting exhaust from an engine;

a reductant introduction system introducing a reductant into the exhaust; and

a selective catalytic reduction catalyst (SCR) receiving the exhaust and reductant, wherein the SCR includes a plurality of cells with a cell density of less than 180 cells per square inch of cross-sectional area of the SCR.

11. The exhaust aftertreatment system of claim 1 wherein a SCR length divided by a SCR width is greater than 4.

12. The exhaust aftertreatment system of claim 11 wherein the SCR includes a plurality of cells with a cell density of between 130 and 180 cells per square inch of cross-sectional area of the SCR.

13. The exhaust aftertreatment system of claim 11 wherein the SCR length is between 3 and 4 feet.

14. The exhaust aftertreatment system of claim 11 wherein the SCR is vertically mounted on a machine.

15. The exhaust aftertreatment system of claim 14 wherein the reductant introduction system introduces the reductant into a horizontal section of the exhaust conduit.

16. The exhaust aftertreatment system of claim 14 wherein the SCR is mounted adjacent a corner of a cab.

17. The exhaust aftertreatment system of claim 14 wherein the SCR includes a metallic substrate.

18. The exhaust aftertreatment system of claim 14 further including a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) upstream of the SCR and a clean-up catalyst downstream of the SCR.

19. A machine comprising:

an exhaust conduit transmitting exhaust from an engine;

a reductant introduction system introducing a reductant into the exhaust; and

a selective catalytic reduction catalyst (SCR) receiving the exhaust and reductant, wherein the SCR is vertically mounted adjacent a corner of a cab of the machine.

20. The exhaust aftertreatment system of claim 19 wherein a SCR length divided by a SCR width is greater than 4 and the SCR includes a plurality of cells with a cell density of less than 180 cells per square inch of cross-sectional area of the SCR.