US20100139258A1

US20100139258A1 - Exhaust mixer with backward flow

Info

Publication number: US20100139258A1
Application number: US12/327,971
Authority: US
Inventors: Mark David Hackett; Jinhui Sun
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2008-12-04
Filing date: 2008-12-04
Publication date: 2010-06-10

Abstract

An exhaust gas mixer configured to turn exhaust gas including a reluctant into a backward flow is provided. The mixer is further configured to turn the backward flow into a forward flow and direct the exhaust gas into an SCR.

Description

TECHNICAL FIELD

The present disclosure relates to exhaust mixing systems, and more particularly to mixing systems for selective catalytic reduction systems.

BACKGROUND

Selective Catalytic Reduction (SCR) systems may be included in an aftertreatment system for a power system to remove or reduce nitrous oxide (NOx or NO) emissions coming from an engine. The SCR systems include the introduction of a reductant to the exhaust stream. Mixers are added to help mix the reductant in the exhaust stream. Thorough mixing may help the performance of the SCR system by improving the reactions and reducing slip or release of the reductant through the SCR system.
U.S. Patent Publication No. 2006/0191254 (the '254 publn) shows a system for mixing exhaust gas. The '254 publn discloses vanes added downstream of the introduction of ammonia in the exhaust stream and before the SCR.

SUMMARY

In one aspect, the present disclosure provides an exhaust gas mixer including a structure configured to receive an entering flow of exhaust gas and turn it into a backward flow at an angle greater than 90 degrees to the entering flow. In another aspect, the mixer is further configured to turn the backward flow into a forward flow an angle greater than 90 degrees to the backward flow. A reductant may be sprayed into the exhaust gas before entering the mixer and the mixer may direct the exhaust gas to an SCR.
In yet another aspect, the present disclosure provides a mixer including a housing and internal baffle. The internal baffle may include a front wall located in the path of the exhaust stream to turn the exhaust stream into the backward direction.
In still another aspect, the present disclosure provides a method of mixing exhaust gas components. The method includes receiving the exhaust stream and turning it in a backward direction that is at an angle greater than 90 degrees to the entering direction.
Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is a cross-sectional view of an SCR system and a mixer included in the aftertreatment system shown in FIG. 1;

FIG. 3 is a frontal cross-sectional view of a mixer included in the SCR system shown in FIG. 2; and

FIG. 4 is an alternative embodiment of a mixer included in the SCR system.

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 13 produced by the engine 12. The engine 12 may include other features not shown, such as fuel systems, air systems, cooling systems, peripheries, drivetrain components, turbochargers, 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 pre-SCR components 16, an SCR system 18, post-SCR components 20, and an exhaust pipe 22. The exhaust stream 13 exits the engine 12, passes through the pre-SCR components 16, then passes through the SCR system 18, and then passes through the post-SCR components 20 via the exhaust pipe 22. The pre-SCR and post-SCR components 16 and 20 may include devices such as regeneration devices, heat sources, oxidation catalysts, diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs), additional SCR systems, lean NOx traps (LNTs), mufflers, or other devices needed to treat the exhaust stream 13 before and after the SCR system 18 and before exiting the power system 10.
The SCR system 18 includes a reductant system 24, mixer 26, and SCR 28. The reductant system 24 introduces or supplies a reductant 30 into the exhaust stream 13. The mixer 26 mixes the reductant 30 with the exhaust stream 13 and introduces the mixture to the SCR 28. The reductant 30 may be urea, ammonia, diesel fuel, or other hydrocarbon used by the SCR 28 to reduce or otherwise remove NOx or NO emissions from the exhaust stream 13.
FIG. 2 illustrates a cross-sectional view of the SCR system 18. The reductant system 24 is shown to include a reductant source 32, pump 34, valve 36, and injector 38. The reductant source 32 may be a tank, vessel, absorbing material, or other device capable of storing and releasing the reductant 30. If the reductant 30 used is the same as the fuel used to power the engine 12, then the reductant 30 may be the engine's 12 fuel tank.
The pump 34 is an extraction device capable of pulling the reductant 30 from the reductant source 32. The valve 36 may be included to help regulate or control the delivery of the reductant 30. The injector 38 is a device capable of creating a reductant spray 40 or otherwise introducing the reductant 30 in the exhaust stream 13.
The reductant system 24 may also include a preliminary mixer or diffuser 42 as needed to aid in mixing of the reductant 30 with the exhaust stream 13. The diffuser 42 may be any structure to disrupt the flow of the exhaust stream 13 and facilitate dispersion of the reductant 30 into the exhaust stream 13. The diffuser 42 may include orifices, deflectors, swirlers, baffles, or other structures that disrupt flow of the exhaust stream 13.
FIG. 2 also illustrates the mixer 26. The exhaust pipe 22 is connected to the mixer entrance pipe 44, extending into an interior 46 of the mixer 26. A structure 47 of the mixer 26 includes a housing 48 and baffle 50. Defining the interior 46 is the housing 48. The baffle 50 is located in the interior 46 of the mixer 26.
The housing 48 may include a back wall 52 and outer wall 54. The mixer entrance pipe 44 enters through a pipe opening 55 in the back wall 52 of the housing 48. The outer wall 54 extends forward from the periphery of the back wall 52 to meet the SCR 28.
The internal baffle 50 may include a front wall 56, side wall 58, and openings 59. The front wall 56 is directly in the path of or in front of the exhaust stream 13 as it enters the mixer 26 through the mixer entrance pipe 44. The side wall 58 extends rearward from the periphery of the front wall 56. The openings 59 may be formed as cutouts in the side wall 58 and may be located in a portion of side wall 58 closer to the back wall 52 of the housing 48 than to the front wall 56 of the baffle 50. The openings 59 may also be a singular opening 59. The openings 49 may also be formed by the side wall 58 stopping short of the back wall 52.
Support structures (not shown) may also be added to support the baffle 50. The support structures may extend from the end of the side wall 58, the mixer entrance pipe 44, or the housing to support the baffle 50.
The SCR 28 includes an SCR entrance 60, SCR body 62, SCR exit 64, and SCR housing 66. The SCR entrance 60 is in fluid communication with the mixer 26 and the SCR body 62. The SCR housing 66 contains the SCR body 62 and may be coupled to, proximate, or be an extension from the mixer housing outer wall 54. In alternative embodiments, the SCR 28 may be separated or further downstream from the mixer 26 and not proximate each other. The mixer housing 48, SCR housing 66, and other aftertreatment system 14 components may be double walled or include insulation as needed to reduce skin temperatures.
The SCR body 62 includes a catalyst facilitating the reaction, reduction, or removal of NOx emissions from the exhaust stream 13 as it passes through the SCR 28 in a SCR flow direction 68. The SCR body 62 may be a honeycomb or other structure made from or coated with an appropriate material. The material may be an oxide, such as vanadium oxide or tungsten oxide, coated on an appropriate substrate, such as titanium dioxide.
The mixer 26 provides a torturous mixer flow path 70 to the exhaust stream 13 passing through it. The mixer flow path 70 may include multiple flow passages and flow directions, as described below. The exhaust stream 13 enters the mixer 26 in an entering flow direction 72 through the entrance pipe 44 within a entering flow passage 74. Exiting the entering flow passage 74, the exhaust stream 13 is directed into the front wall 56 of the baffle 50, causing the exhaust stream 13 to turn in the first turn flow direction 76 within a first turn passage 78. The first turn flow direction 76 redirects the exhaust stream 13 backward relative to the entering flow direction 72. The first turn flow direction 76 may be substantially a 180 degree backward turn.
Exiting the first turn passage 78, the exhaust stream 13 follows a backward flow direction 80 in a backward flow passage 82 defined by the mixer entrance pipe 44 and the side wall 58 of the baffle 50. The backward flow direction 80 may be substantially parallel, but in the reverse direction as the entering flow direction 72. In alternative embodiments, the backward flow direction 80 may diverge away from or toward the entering flow direction 72. In the shown embodiment, the first turn flow direction 76 is substantially a 180 degree forward turn. As a result, the backward flow direction 80 travels in a reverse, 180 degree, direction relative to the forward flow direction 90.
In alternative embodiments, the first turn flow direction 76 may be more or less than 180 degrees. The first turn flow direction 76 may be any turn greater than 90 degrees to provide the forward flow direction 90 that is opposed to the entering flow direction 72. As a result, the backward flow direction 80 travels at an angle greater than 90 degrees to the entering flow direction 72. The backward flow direction 80 may also vary along the backward flow passage's 82 length.
Exiting the backward flow passage 82, the exhaust stream 13 is directed into the back wall 52 of the housing 48 and through the openings 59, causing the exhaust stream 13 to move in the second turn flow direction 84 within a second turn flow passage 86. The second turn flow direction 84 redirects the exhaust stream 13 forward relative to the backward flow direction 80 and in the same general direction as the entering flow direction 72.
Exiting the second turn flow passage 86, the exhaust stream 13 follows a forward flow direction 90 in a forward flow passage 92 defined by the side wall 58 of the baffle 50 and the housing 48. A shown, the forward flow direction 90 may be substantially parallel, but in the reverse direction as the backward flow direction 80. In alternative embodiments, the forward flow direction 90 may diverge away from or toward the backward flow direction 80. In the illustrated embodiment, the forward flow direction 90 is in substantially the same direction as the entering flow direction 72. In alternative embodiments, the forward flow direction 90 may be different from the entering flow direction 72. In the shown embodiment, the second turn flow direction 84 is substantially a 180 degree forward turn. As a result, the forward flow direction 90 travels in a reverse, 180 degree, direction relative to the backward flow direction 80.
In alternative embodiments, the second turn flow direction 84 may be more or less than 180 degrees. The second turn flow direction 84 may be any turn greater than 90 degrees to provide the forward flow direction 90 that is opposed to the backward flow direction 80. As a result, the forward flow direction 90 travels at an angle greater than 90 degrees to the backward flow direction 80. The forward flow direction 90 may also vary along the forward flow passage's 92 length.
The mixer 26 accordingly creates overlapping flows that may be substantially parallel or may be at angles to one another. For example, the entering flow direction 72 may overlap to at least some extent the backward flow direction 80. Similarly, the forward flow direction 90 may overlap to at least some extent the backward flow direction 80.
Exiting the forward flow passage 92, the exhaust stream 13 follows an exit flow direction 94 in a exit flow passage 96 defined by the front wall 56 of the baffle 50, SCR entrance 60, and the outer wall 54 of the housing 48. The exit flow passage 96 opens and delivers the exhaust stream 13 to the SCR 28. The exhaust stream 13 then passes through the SCR 28 in the SCR flow direction 68. In the illustrated embodiment, the SCR flow direction 68 is in substantially the same direction as the entering flow direction 72.
In alternative embodiments, additional structures may be added to make the SCR flow direction 68 different from the entering flow direction 72, as needed for the application. In yet other embodiments, no forward flow direction 90 or forward flow passage 92 may be included. The backward flow passage 82 or second turn flow passage 86 may open into and deliver the exhaust stream 13 to the SCR 28.
FIG. 3 shows the mixer 26 to have a circular cross-section and a cylindrical shape. In alternative embodiments the mixer 26 may have a square, triangular, oval, oblong, rectangular, or other shape. In alternative embodiments the mixer 26 may have a conical, box, or other three-dimensional shape. The shape and size of the mixer 26 may vary depending on size constraints and flow considerations of a given application. The mixer 26 may symmetrically extend around or encompass the entrance pipe 44, as shown. In alternative embodiments the mixer 26 may extend around only a portion of the entrance pipe 44, be non-symmetrical, or be skewed to one side or another.
FIG. 4 shows an alternative embodiment of the baffle 50 that may include a deflector 98 and may also include a rear wall 100. The deflector 98 may extend at an angle from the periphery of the front wall 56 and extend to the side wall 58. The deflector 98 may also extend further forward and eliminate the front wall 56 or extend further backward and eliminate the side wall 58. The rear wall 100 is an extension from the side wall 58, providing the openings 59 at a distance from the back wall 52 of the housing 48.

INDUSTRIAL APPLICABILITY

The overlapping forward, backward, and entering flow directions 90, 80, and 72 provide a back and forth tortuous flow path 70 the exhaust stream 13 must follow. The forward, backward, and entering flow directions 90, 80, and 72 may follow substantially opposite directions compared to one another. This flow path 70 may cause mixing of the reductant 30 into the exhaust stream 13. The first and second turn flow directions 76 and 84 may make the flow path 70 tortuous to cause the mixing of the reductant 30 with the exhaust stream 13. As a result, the mixer 26 may provide a substantially homogenized dispersion of reductant 30 in the exhaust stream 13 being introduced into the SCR 28.
The mixer 26 provides this tortuous flow path 70 over a given mixer length 102. The mixer length 102 may be defined or determined by the packaging and size constraints of the application. Because of the overlapping flow path 70, the mixer length 102 may be substantially less than the length of the flow path 70. The length of the flow path 70 is the straight line length of the flow path 70 through the mixer 26. In some embodiments, the flow path 70 length may be more than twice as long as the mixer length 102. It is understood that the ratio of flow path 70 length to mixer length 102 will vary widely depending on the specific design implemented.
The elongated flow path 70 may provide a longer flow path than would otherwise be possible in a given application. The longer flow path 70 may provide increased mixing time and travel distance for a given mixer length 102. The increased mixing time and travel distance may provide for the creation of a homogenized dispersion of reductant 30 in the exhaust stream 13.
While the above description is directed to the mixing of the reductant 30 used for the SCR 28 into the exhaust stream 13, it is understood that other applications of the mixer 26 exist. The mixer 26 may be used to mix any exhaust gas components or any liquid 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. A mixer for mixing exhaust gas components comprising:

a structure configured to receive an entering flow of exhaust gas and turn the entering exhaust flow into a backward flow within the structure wherein the backward flow is at an angle greater than 90 degrees to the entering flow.

2. The mixer of claim 1 wherein the structure is further configured to direct the backward flow into a selective catalytic reduction (SCR) device.

3. The mixer of claim 1 wherein the structure is further configured to turn the backward flow into a forward flow within the structure, wherein the forward flow is at an angle greater than 90 degrees to the backward flow.

4. The mixer of claim 3 wherein the structure is further configured to direct the forward flow into a selective catalytic reduction (SCR) device.

5. The mixer of claim 4 wherein the entering flow includes a reductant.

6. The mixer of claim 3 wherein the structure further includes:

an internal baffle configured to create the backward and forward flows.

7. The mixer of claim 6 wherein the baffle further includes:

a front wall configured to direct the entering flow into the backward flow;

a side wall extending from the periphery of the front wall; and

an opening in the side wall.

8. The mixer of claim 1 wherein at least a portion of the backward flow overlaps at least a portion of the entering flow.

9. The mixer of claim 2 wherein at least a portion of the forward flow overlaps at least a portion of the entering flow and overlaps at least a portion of the backward flow.

10. A mixer for mixing exhaust gas components comprising:

a housing configured to receive an exhaust stream in a entering direction; and

a baffle inside the housing including a front wall located in the path of the exhaust stream entering direction and configured to turn the exhaust stream into a backward direction.

11. The mixer of claim 10 wherein the baffle further includes:

a side wall extending from the periphery of the front wall; and

an opening in the side wall.

12. The mixer of claim 11 wherein baffle is configured to pass the exhaust stream through the opening and turn the exhaust stream into a forward direction.

13. The mixer of claim 11 wherein the backward direction is at an angle greater than 90 degrees to the entering direction and the forward direction is at an angle greater than 90 degrees to the backward direction.

14. A method of mixing exhaust gas components comprising:

receiving an exhaust stream traveling in a entering direction; and

turning the exhaust stream traveling in the entering direction to travel in a backward direction, wherein the backward direction is at an angle greater than 90 degrees to the entering direction.

15. The method of claim 14 wherein the backward direction is substantially parallel to the entering direction.

16. The method of claim 14 further including:

turning the exhaust stream traveling in the backward direction to travel in a forward direction wherein the forward direction is at an angle greater than 90 degrees to the backward direction.

17. The method of claim 16 wherein at least a portion of the forward flow overlaps at least a portion of the entering flow and overlaps at least a portion of the backward flow.

18. The method of claim 16 further including:

directing the exhaust stream traveling into a selective catalytic reduction (SCR) device.

19. The method of claim 16 wherein the forward direction is substantially parallel to the backward direction and the entering direction.

20. The method of claim 19 wherein the exhaust stream flows through the SCR in a direction that is substantially the same as the entering direction.