Patent Publication Number: US-2012042637-A1

Title: Tall vertical scr

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 &#39;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 &#39;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&#39;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&#39;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&#39;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.