US20120151902A1

US20120151902A1 - Biased reductant mixer

Info

Publication number: US20120151902A1
Application number: US12/970,352
Authority: US
Inventors: Yong Yi; Jinhui Sun
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2010-12-16
Filing date: 2010-12-16
Publication date: 2012-06-21

Abstract

An engine exhaust aftertreatment system including a mixer with a varying contact area that assists in the introduction and conversion of a reductant introduced by an injector.

Description

TECHNICAL FIELD

The present disclosure relates to engine exhaust aftertreatment systems and more particularly to exhaust aftertreatment systems employing mixer for the introduction of a reductant for NOx reduction technologies.

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 an exhaust stream of an engine. SCR systems use reductants, such as urea, that are introduced into the exhaust stream.
German Patent Publication DE 10 2007 052 262 A1 discloses a mixing mechanism with a lattice like asymmetric structure to mix the reducing agent in the exhaust gas stream The asymmetry is achieved by different angles of incidence or different sizes of guidance elements in the lattice.

SUMMARY

The present disclosure provides an engine exhaust aftertreatment system including an injector configured to introduce a reductant into an exhaust conduit and a mixer disposed in the exhaust conduit wherein a contact area of the mixer varies.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is a diagrammatic view of a dual leg aftertreatment system incorporating the mixer.

FIG. 3 is a front view of the mixer.

FIG. 4 is a side view of the mixer in FIG. 3.

FIG. 5 is a front view of another embodiment of the mixer.

FIG. 6 is a side view of the mixer in FIG. 5.

FIG. 7 is a front view of another embodiment of the mixer.

FIG. 8 is a side view of the mixer in FIG. 7.

FIG. 9 is a front view of another embodiment of the mixer.

FIG. 10 is a side view of the mixer in FIG. 9.

DETAILED DESCRIPTION

As seen in FIG. 1, a 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 engine 12 may be used to power any machine or other device, including on-highway trucks or vehicles, off-highway trucks or machines, earth moving equipment, generators, aerospace applications, locomotive applications, marine applications, pumps, stationary equipment, or other engine powered applications.
The aftertreatment system 14 includes an exhaust conduit 18 and a Selective Catalytic Reduction (SCR) system 20. The SCR system 20 includes an SCR catalyst 22, mixing conduit 24, mixer 26, and reductant supply system 28.
The SCR catalyst 22 includes a catalyst material disposed on a substrate. The substrate may consist of cordierite, silicon carbide, other ceramic, or metal. The substrate may include a plurality of through going channels and may form a honeycomb structure.
The reductant supply system 28 may include a reductant 30, reductant tank or source 32, pump 34, valve 36, reductant line 38, and injector 40. The reductant 30 is drawn from the reductant source 32 via the pump 34 and delivery to the injector 40 is controlled via the valve 36. The flow of reductant 30 may also be controlled by operation of the pump 34.
The mixing conduit 24 is the section of the exhaust conduit 18 where the reductant 30 is introduced. The mixing conduit 24 includes an inner wall 25 and outer wall 27.
The reductant supply system 28 may also include a thermal management system to thaw frozen reductant 30, prevent reductant 30 from freezing, or preventing reductant 30 from overheating. Components of the reductant supply system 28 may also be insulated to prevent overheating of the reductant 30.
The reductant 30 comes from a nozzle or injector tip 42 of the injector 40 to form a reductant spray 44 or is otherwise introduced into the exhaust stream 16 or SCR catalyst 22. The position of the injector tip 42 may be such to direct the reductant spray 44 directly down a centerline 71 of the mixing conduit 24 and the mixer 26 or it may be positioned to bias the spray 44 to one side of the mixing conduit 24.
The reductant supply system 28 may also include an air assist system for introducing compressed air to aid in the formation of small droplets in the reductant spray 44. The air assist system may also be used to purge the reductant lines 38 and other reductant supply system 28 components of reductant 30 when not in use.
The aftertreatment system 14 may also include a diesel oxidation catalyst (DOC) 46, a diesel particulate filter (DPF) 48, and a clean-up catalyst 50. The DOC 46 and DPF 48 may be in the same canister, as shown, or separate. The SCR catalyst 22 and clean-up catalyst 50 may also be in the same canister, as shown, or separate.
The aftertreatment system 14 is configured to remove, collect, or convert undesired constituents from the exhaust stream 16. The DOC 46 oxidizes Carbon Monoxide (CO) and unburnt hydrocarbons (HC) into Carbon Dioxide (CO2). The DPF 48 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 the reductant 30.
A heat source 52 may also be included to remove the soot from or regenerate the DPF 48, thermally manage the SCR catalyst 22, DOC 46, or clean-up catalyst 50, to remove sulfur from the SCR catalyst 22, or to remove deposits of reductant 30 that may have formed. The heat source 52 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 52 may also embody operating the engine 12 under conditions to generate elevated exhaust stream 16 temperatures. The heat source 52 may also embody a backpressure valve or another restriction in the exhaust to cause elevated exhaust stream 16 temperatures.
In the illustrated embodiment, the exhaust stream 16 exits the engine 12, passes by or through the heat source 52, passes through the DOC 46, DPF 48, then passes through the SCR system 20, and then passes through the clean-up catalyst 50 via the exhaust conduit 18.
Other exhaust treatment devices may also be located upstream, downstream, or within the SCR system 20. In the illustrated embodiment, the SCR system 20 is downstream of the DPF 48 and the DOC 46 is upstream of the DPF 48. The heat source 52 is upstream of the DOC 46. The clean-up catalyst 50 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.
While other reductants 30 are possible, urea is the most common source of reductant 30. Urea reductant 30 decomposes or hydrolyzes into ammonia (NH3) and is then adsorbed or otherwise stored in the SCR catalyst 22. The mixing conduit 24 may be long to aid in the mixing or even distribution of the reductant 30 into the exhaust stream 16 and provide dwell time for the urea reductant 30 to convert into NH3. The NH3 is consumed in the SCR Catalyst 22 through a reduction of NOx into Nitrogen gas (N2).
The clean-up catalyst 50 may embody an ammonia oxidation catalyst (AMOX). The clean-up catalyst 50 is configured to capture, store, oxidize, reduce, and/or convert NH3 that may slip past or breakthrough the SCR catalyst 22. The clean-up catalyst 50 may also be configured to capture, store, oxidize, reduce, and/or convert other constituents present.
Control and sensor systems may also be included to control the engine 12, heat source 52, reductant supply system 28, and other components in the power system 10 or its application.
FIG. 2 shows that in another embodiment, the aftertreatment system 14 may be a dual leg aftertreatment system 60. The dual leg aftertreatment system 60 includes first and second SCR legs 61 and 62, receiving the exhaust stream 16 and reductant 30 from the reductant supply system 28.
The exhaust stream 16 from the mixing conduit 24 is split or divided in a dividing section 63 of the exhaust conduit 18. The dividing section 63 may also include a splitting vane 64 to direct and divide the exhaust stream 16.
The dual leg aftertreatment system 60 may also include first and second entering legs 65 and 66, delivering the exhaust stream 16 to the reductant supply system 28. The exhaust stream 16 from the first and second entering legs 65 and 66 is split or divided in a combining section 67 of the exhaust conduit 18.
The first and second entering legs 65 and 66 are shown as including the DPF 48 and DOC 46 but may not include either or may include other components. In one embodiment, the first and second entering legs 65 and 66 do not include the DPF 48. The first and second entering legs 65 and 66 are also shown as being disposed at right angles relative to the first and second SCR legs 61 and 62, but may be disposed at various other angles or may be arranged linearly. The dual leg aftertreatment system 60 may also be contained in a box structure with interior walls dividing the flow of the exhaust.
The reductant spray 44 may be or become a biased spray 70 that travels at an angle relative to the centerline of the mixing conduit 24. This biased spray 70 may cause the spray 44 to predominately impinge an off center impingement region 72 of the mixer 26 more than a remainder 74 of the mixer 26. The biased spray 70 may be caused by the exhaust stream 16 interacting of the reductant spray 44 and pushing the reductant spray 44 to one side of the mixing conduit 24. The biased spray 70 may also result from a deflection off the inner wall 25 of the mixing conduit 24, as seen in FIG. 2. The biased spray 70 may also be caused by the alignment of the injector 40 relative to the mixing conduit 24.
The impingement region 72 and remainder 74 define portions of the mixer 26 corresponding to areas or portions or regions of the mixing conduit where the spray 44 preferentially does and does not travel respectively. The designation of the impingement region 72 and remainder 74 may be relative and may factor in many variables. The impingement region 72 and remainder 74 may represent a wide variety of different fractions of area inside the mixing conduit 24. The impingement region 72 and remainder 74 may each represent ½ of the area inside the mixing conduit 24. In other embodiments, the impingement region 72 may represent more or less than ½ of the area inside the mixing conduit 24. Additional regions may also be distinguished in the mixer 26.
The mixer 26 components may be constructed from steel or any other of a variety of materials. The mixer 26 may also be coated with materials to assist in the conversion or hydrolysis of the reductant 30 into NH3.
The mixer 26 has a frontal contact area 76 that varies or is biased. In one embodiment, the mixer 26 is biased with the impingement region 72 having a larger contact area 76 than the remainder 74. The contact area 76 is the total area at the front of the mixer 26 which the exhaust stream 16 and spray 44 contacts before passing into or through the mixer 26. The bias in distribution of the contact area 76 between the impingement region 72 and the remainder 74 may be accomplished in a variety of different ways, some of which are described below.
FIGS. 3-10 illustrate exemplary embodiments 100, 200, 300, and 400 of the mixer 26. FIGS. 3 and 4 show the mixer 100 may be a tilted cone with a front 102 extending across the mixing conduit 24 and a end 104 that is off center with the centerline 71 of the mixing conduit 24. The tilted cone mixer 100 includes a long surface 106 and a short surface 108 gradually transitioning back to the long surface 106 around the tilted cone. The long surface 106 is longer compared with the short surface 108. The long surface 106 is associated with the impingement region 72 and the short surface 108 is associated with the remainder 74. The tilted cone mixer 100 may also be defined by a large cone angle 110 and a small cone angle 112 gradually transitioning back to the large cone angle 110 around the tilted cone. The large cone angle 110 is larger compared with the small cone angle 112. The large cone angle 110 is associated with the impingement region 72 and the small cone angle 112 is associated with the remainder 74.
The tilted cone mixer 100 may have plurality of openings 114, including an end opening 116. The openings 114 may have a variety of sizes, may or may not be evenly distributed, and may have a variety of shapes including for example circular, oval, rectangular, square, hexagonal, or any other shape. The end opening 116 may be larger or smaller than the other openings 114 and also may have a variety of shapes. When included, the end opening 116 may be associated with the remainder 74.
The long surface 106 and the large cone angle 110 associated with the impingement region 72 is one way of creating a larger contact area 76 in the impingement region 72. The end opening 116 being associated with the remainder 74 is another way the tilted cone mixer 100 may create a larger contact area 76 in the impingement region 72 compared to the remainder 74.
FIGS. 5 and 6 show a symmetric cone mixer 200 can also be used to achieve a bias in contact area 76. The symmetric cone mixer 200 has a front 202 extending across the mixing conduit 24, a end 204 that is approximately aligned with the centerline 71 of the mixing conduit 24, a approximately constant frontal surface 206, and a approximately constant cone angle 208. The symmetric cone mixer 200 also includes varying sizes of openings, including large openings 210 and small openings 212 and any size in-between wherein the large openings 210 are larger compared to the small openings 212. The symmetric cone mixer 200 may also include a central opening 214. The larger contact area 76 in the impingement region 72 compared to the remainder 74 is achieved by locating the small openings 212 in the impingement region 72 and the large openings 210 in the remainder 74.
FIGS. 7 and 8 show planar disc mixer 300 can also be used to achieve a bias in contact area 76. The planar disc mixer 300 extends across the mixing conduit 24 and includes a front 302 and varying sizes of openings, including large openings 304 and small openings 306 and any size in-between wherein the large openings 304 are larger compared to the small openings 306. The larger contact area 76 in the impingement region 72 compared to the remainder 74 is achieved by locating small openings 306 in the impingement region 72 and large openings 304 in the remainder 74.
FIGS. 9 and 10 show a biased ring mixer 400 can also be used to achieve a bias in contact area 76. The biased ring mixer 400 includes a planar ring 402 disposed inside the mixing conduit 24. Spacers 404 may also be used for mounting and to define a gap 406 between the ring 402 and the inner wall 25. The gap 406 may serve as a drain to prevent reductant 30 from pooling along the outside of the ring 402. The ring 402 includes a wide width 408 and a narrow width 410 wherein the wide width 408 is wider compared to the narrow width 410. The width of the ring 402 may transition from the wide width 408 to the narrow width 410 gradually, abruptly, or in steps. The wide width 408 is associated with the impingement region 72 and the narrow width 410 is associated with the remainder 74. The larger contact area 76 in the impingement region 72 compared to the remainder 74 is achieved by the wide width 408 in the impingement region 72.
Another way of creating a larger contact area 76 in the impingement region 72 compared to the remainder 74 is to increase the spacing between any openings in the impingement region compared to the remainder 74. Any combination of the above techniques from the different embodiments 100, 200, 300, and 400 may also be used to create a larger contact area 76 in the impingement region 72 compared to the remainder 74.
The maximum contact area 76 for a portion of the mixer in the impingement region 72 may be between 10% and 40% greater than the minimum contact area 76 for a portion of the mixer in the remainder 74. In other embodiments, the maximum contact area 76 for a portion of the mixer in the impingement region 72 may be more than 30% greater than the minimum contact area 76 for a portion of the mixer in the remainder 74. In yet other embodiments, the maximum contact area 76 for a portion of the mixer in the impingement region 72 may be more than 10% greater than the minimum contact area 76 for a portion of the mixer in the remainder 74.

INDUSTRIAL APPLICABILITY

The mixer 26 assists in breaking up and atomizing the spray 44 of reductant 30, thereby promoting the conversion of the reductant 30 into NH3, and preventing the formation of deposits. The larger contact area 76 in the impingement region 72 provides more structure for the spray 44 to hit against and cause the spray 44 to break up. The larger contact area 76 in the impingement region 72 may also create more turbulence, further helping the conversion of reductant 30 into NH3.
Deposits may form when the reductant 30 is not quickly decomposed into NH3 and thick layers of reductant 30 collect. These layers may build as more and more reductant 30 is sprayed or collected, which may have a cooling effect that prevents decomposition into NH3. As a result, the reductant 30 sublimates into crystals or otherwise transforms into a solid composition to form the deposit. The deposit composition may consist of biuret (NH2CONHCONH2) or cyanuric acid ((NHCO)3) or another composition depending on temperatures and other conditions. These deposits may form in areas and on surfaces where the reductant spray 44 settles or lies stagnant.
These deposits may have negative impacts on the operation of the power system 10. The deposits may block flow of the exhaust stream 16, causing higher backpressure and reducing engine 12 and aftertreatment system 14 performance and efficiency. The deposits may also disrupt the flow and mixing of the reductant 30 into the exhaust stream 16, thereby reducing the decomposition into NH3 and reducing NOx reduction efficiency. The formation of the deposits also consumes reductant 30, making control of injection harder and potentially reducing NOx reduction efficiency. The deposits may also corrode and degrade components of the SCR system 20.
Limiting backpressure is also important and the smaller contact area 76 in the remainder 74 may help reduce backpressure. High backpressure can harm engine 12 performance. High backpressure may also lead to deposit formation and exhaust leaks.
The mixer 26 may also assist in straightening the flow of the exhaust stream 16, which is often naturally, repeatable, and/or predictably uneven and does not simply travel down the center of the exhaust conduit 18. Varying the contact area 76 of the mixer 26 may help to counteract and correct this uneven flow and cause the exhaust stream 16 to straighten and therefore may improve SCR system 20 efficiency. An uneven exhaust stream 16 flow may harm the efficiency of the SCR system 20 by sending more flow through some areas of the SCR catalyst 22 than other areas. A biased flow may be especially harmful in a dual leg aftertreatment system 60 by sending more exhaust flow through one SCR leg than another.
The mixer 26 may also be cheap. The mixer 26 may not employ a complex lattice structure with guide vanes of different sizes and therefore may provide cost savings. The designs described above all achieve a varied or biased contact area 76 by cutting and/or shaping a single piece of metal without the need for assembly and construction of multiple pieces as is needed to crate a lattice structure.
While the mixer 26 is described above to aid in the introduction of a reductant into an exhaust stream, it is also contemplated that the mixer 26 could be used to aid in the introduction of any of a variety of substances in any of variety of flows. 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

1. An engine exhaust aftertreatment system comprising:

an injector configured to introduce a reductant into an exhaust conduit of a engine; and

a mixer disposed in the exhaust conduit wherein a frontal contact area of the mixer varies.

2. The engine exhaust aftertreatment system of claim 1 wherein the frontal contact area of the mixer is greater in an impingement region where the reductant predominately strikes the mixer compared to a smaller contact area in a remainder of the mixer.

3. The engine exhaust aftertreatment system of claim 2 wherein the mixer is disposed downstream of the injector and upstream of a selective catalytic reduction catalyst.

4. The engine exhaust aftertreatment system of claim 2 wherein the mixer is a cone tilted off center from a centerline of the exhaust conduit with a longer surface associated with the impingement region compared to a shorter surface associated with the remainder of the mixer.

5. The engine exhaust aftertreatment system of claim 4 wherein the cone includes an end opening associated with the remainder of the mixer.

6. The engine exhaust aftertreatment system of claim 2 wherein the mixer includes openings and the openings in the impingement region are smaller than the openings in the remainder of the mixer.

7. The engine exhaust aftertreatment system of claim 2 wherein the mixer includes openings and the openings in the impingement region are spaced further apart than the openings in the remainder of the mixer.

8. The engine exhaust aftertreatment system of claim 2 wherein the mixer includes a ring with a wider width associated with the impingement region compared to a width associated with the remainder of the mixer.

9. The engine exhaust aftertreatment system of claim 2 wherein the mixer is constructed from only a single piece of metal.

10. The engine exhaust aftertreatment system of claim 2 wherein a maximum contact area for a portion of the mixer in the impingement region is between 10% and 40% greater than a minimum contact area for a portion of the mixer in the remainder of the mixer.

11. An engine exhaust aftertreatment system comprising:

a mixer disposed in the exhaust conduit wherein the mixer is a cone tilted off center from a centerline of the exhaust conduit.

12. The engine exhaust aftertreatment system of claim 11 wherein a longer surface of the tilted cone is associated with an impingement region where the reductant predominately strikes the mixer compared to a shorter surface associated with a remainder of the mixer.

13. The engine exhaust aftertreatment system of claim 12 wherein the tilted cone includes an end opening associated with the remainder of the mixer.

14. An engine exhaust aftertreatment system comprising:

a mixer disposed in the exhaust conduit, the mixer including openings that vary in size in different regions of the mixer.

15. The engine exhaust aftertreatment system of claim 14 wherein a frontal contact area of the mixer varies in the different regions.

16. The engine exhaust aftertreatment system of claim 15 wherein the frontal contact area of the mixer is greater in an impingement region where the reductant predominately strikes the mixer compared to a smaller contact area in a remainder of the mixer.

17. The engine exhaust aftertreatment system of claim 16 wherein the openings in the impingement region are smaller compared to the openings in a remainder of the mixer.

18. The engine exhaust aftertreatment system of claim 17 wherein a maximum contact area for a portion of the mixer in the impingement region is between 10% and 40% greater than a minimum contact area for a portion of the mixer in the remainder of the mixer.

19. The engine exhaust aftertreatment system of claim 14 wherein the mixer is a cone.

20. The engine exhaust aftertreatment system of claim 14 wherein the mixer is a planar disc.