US20100154393A1

US20100154393A1 - Combination partial flow particulate filter and catalyst

Info

Publication number: US20100154393A1
Application number: US12/318,009
Authority: US
Inventors: Ronald Graham Silver; Thomas Edward Paulson
Original assignee: Caterpillar Inc
Current assignee: Caterpillar Inc
Priority date: 2008-12-19
Filing date: 2008-12-19
Publication date: 2010-06-24

Abstract

The present disclosure is directed to a filtering element. The filtering element may comprise at least one channel defined by at least one wall. The at least one wall extends longitudinally from a first end of the channel to a second end of the channel, the at least one channel having an unobstructed first end and an unobstructed second end. The filtering element further includes at least one partial barrier configured to reduce a path of a flow of exhaust through the at least one channel, the at least one partial barrier being positioned within the at least one channel so as to redirect a portion of the exhaust flow within the channel. The at least one channel includes a selective catalytic reduction catalyst on at least a portion of one of a surface of the at least one partial barrier or a surface of the at least one wall.

Description

TECHNICAL FIELD

The present disclosure relates generally to particulate filters and, more particularly, to a combination partial flow particulate filter and catalyst.

BACKGROUND

Internal combustion engines exhaust a complex mixture of pollutants as a byproduct of the combustion process. In diesel engines, these pollutants may include particulate matter and oxides of nitrogen (NOx). Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent over the years. As part of these emission standards, regulations prescribe the amount of particulate matter and NOx that can be emitted from an engine depending on the type, size, and/or class of the engine. One method used by engine manufacturers to comply with these regulations is to remove the pollutants from the exhaust flow of the engine. Different techniques are typically used to remove particulate matter and NOx from the exhaust flow. One common method used to remove particulate matter contained in the exhaust is to capture and oxidize particulate matter using diesel particulate filters (“DPF”). One common method used to remove NOx contained in the exhaust is to use selective catalytic reduction (“SCR”) to convert the NOx into nitrogen gas and water.
In an engine, DPFs are typically located along the path of the exhaust flow and they operate by forcing the exhaust flow through a filtering element of the DPF. There are many types of filtering elements that have been used in DPFs. Ceramic wall-flow monoliths are a common type of filtering element used in DPFs. A ceramic wall-flow filtering element has parallel channels alternately plugged at each end in order to force the exhaust gases through the porous ceramic walls. That is, channels that are unobstructed at the inlet end (called “inlet channels”) are plugged at the outlet end, and channels that are plugged at the inlet end (called “outlet channels”) are unobstructed at the outlet end. Therefore, exhaust gases that enter the inlet channels at the inlet end are forced to percolate through the walls of the filter element to the outlet channels in order to exit the filter element. Thus, the walls of the filtering element act as a filter. These filter elements are commonly made of ceramic materials, such as cordierite (a synthetic ceramic composition having the formula 2MgO-2Al₂O₃-5SiO₂), that are characterized by good porosity, high temperature resistance, and good mechanical strength. The ceramic walls of the filtering element block some or all of the particulate matter in the exhaust while allowing the exhaust gases to flow through. Over time, the particulate matter may clog the filtering element, impeding the flow of gas through it, resulting in increased pressure drop across the filter (engine back pressure) and reduced engine efficiency. Filter regeneration is one way to remove the particulate build up within the filtering element.
SCR is a process where gaseous or liquid reductant (most commonly urea) is added to the exhaust gas stream of an engine and is absorbed onto a catalyst. The catalyst may be disposed on a substrate within an exhaust system of the engine. In the presence of the catalyst, the reductant reacts with NOx in the exhaust gas to form H₂O and N₂. In order to decrease the number of system components, the catalyst of the SCR system may be included in a ceramic wall flow monolith of a DPF.
One system for selective catalytic reduction and particulate filtration is described in U.S. Patent Publication 2007/0137184 to Patchett et al. (“Patchett”). Specifically, Patchett describes a soot filter coated with a selective catalytic reduction catalyst. The soot filter of Patchet is described as a wall flow monolith. The system may inject an ammonia precursor into the exhaust stream and then the exhaust may flow into the soot filter. The wall of the soot filter may trap particulate matter and the NOx and ammonia precursor may react in the presence of the catalyst to become nitrogen gas and water.
While the system of Patchett may be effective at removing particulate and reducing NOx, due to the capped channels characteristic of a wall flow monolith, the soot filter may increase the backpressure of the system to unacceptable levels. The present disclosure is directed to solving one or more of the problems set forth above and/or other problems in existing technology.

SUMMARY

In one aspect, the present disclosure is directed to a filtering element. The filtering element includes at least one channel being defined by at least one wall extending longitudinally from a first end of the channel to a second end of the channel, the at least one channel having an unobstructed first end and an unobstructed second end. The filtering element further includes at least one partial barrier configured to reduce a path of a flow of exhaust through the at least one channel, the at least one partial barrier being positioned within the at least one channel so as to redirect a portion of the exhaust flow around the partial barrier within the channel; and wherein the at least one channel includes a selective catalytic reduction catalyst on at least a portion of one of a surface of the at least one partial barrier or a surface of the at least one wall.
In another aspect, the present disclosure is directed to a method of filtering a fluid. The method includes directing a flow of fluid extending between a first unobstructed end and a second unobstructed end; and redirecting a portion of the flow of fluid around the partial barrier within the channel. The method further includes selectively reducing a non-particulate pollutant level through contact between the flow of fluid and at least one of a wall of the channel and the partial barrier. The method still further includes trapping particulate matter within the at least one channel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary disclosed power system;

FIG. 2 is a perspective view of an exemplary disclosed diesel particulate filter that may be used with the power system of FIG. 1;

FIG. 3 is a cross-section of the exemplary diesel particulate filter of FIG. 2;

FIG. 4 is an illustration of a magnified view of the filtering element of the diesel particulate filter of FIG. 3; and

FIG. 5 is a flow diagram illustrating an exemplary disclosed method of operating the diesel particulate filter of FIG. 3.

DETAILED DESCRIPTION

FIG. 1 illustrates a power system 10 that may be configured to produce a power output for a machine (not shown). The machine may be a fixed or mobile machine that performs some type of operation associated with an industry, such as mining, construction, farming, transportation, or any other industry known in the art. The composition of an exhaust gas output of power system 10 may be subject to government or industry regulation. For this reason, power system 10 may include components configured to reduce or eliminate harmful elements of the exhaust gas while still maintaining the performance, life, and efficiency of power system 10.
Power system 10 may include, among other systems, an engine 12, an induction system 14, and an exhaust system 16. Engine 12 may include an engine such as, for example, a diesel engine. Fuel may be combusted in engine 12 to produce mechanical power. The byproducts of the combustion gas may be exhausted from engine 12 through the exhaust system 16.
Induction system 14 may be configured to introduce compressed air into combustion chambers (not shown) of engine 12. These components of induction system 14 may include any components known in the art such as a compressor 20 of a turbocharger 18, valves, air coolers, air cleaners, control system, etc.
Exhaust system 16 may be configured to extract energy from exhaust gas exiting engine 12 and treat the exhaust gas prior to allowing the exhaust gas to pass to the atmosphere. Exhaust system 16 may include a turbine 22 of turbocharger 18. The turbine 22 is driven by the expansion of hot exhaust gases exiting engine 12.
Exhaust gas may contain solid particulate matter and various chemicals in liquid or gaseous form. The solid particulate matter may include combustible organic constituents (such as elemental carbon) and incombustible inorganic constituents (such as ash). Some of the exhaust gas constituents may be subject to regulation by various agencies, such as government or environmental agencies, and hence may need to be removed/reduced before the exhaust gas is released to the atmosphere. Aftertreatment system 23 may include components configured to separate these regulated constituents from the exhaust gas. The components of aftertreatment system 23 may include, among others, a diesel oxidation catalyst (DOC) 24, a reductant injection system 30, a diesel particulate filter (DPF) 32 combined with a selective reduction catalyst 54, and an exhaust outlet 34 configured to direct exhaust to the atmosphere.
DOC 24 may include a porous ceramic honeycomb structure or metal mesh substrate coated with a material, for example a precious metal, that reacts with constituents of the exhaust gas to alter the composition of the constituents. For example, DOC 24 may include platinum or vanadium to facilitate the conversion of NOx, specifically the conversion of NO to NO₂. DOC 24 may be configured to achieve an NO to NO₂ratio of about 1:1.
Reductant injection system 30 may supply reductant configured to react with NOx in the exhaust gas and a catalyst to reduce the level of NOx in the exhaust gas. Reductant injection system may include a reductant supply 28, and a reductant injector 26 connected to the reductant supply 28. The reductant may be drawn from reductant supply 28, and sprayed by reductant injector 26 directly onto selective reduction catalyst 54 (shown in FIG. 3) within DPF 32. Alternatively, reductant may be drawn from reductant supply 28, and may be sprayed by reductant injector 26 into the exhaust gas and may be carried by the exhaust gas into contact with the selective catalytic reduction catalyst 54. Reductant supply 28 may be fluidly connected to reductant injector 26. The reductant contained in reductant supply 28 may be gaseous, liquid, or solid, and may be any reductant known in the art, such as, for example, urea, ammonia, or a hydrocarbon reductant.
Diesel particulate filter 32 of aftertreatment system 23 may be disposed downstream of turbine 22 to remove particulates from the exhaust gas. Diesel particulate filter 32 may be a partial flow particulate filter. By way of example, a wall-flow filter may have at least one channel, and either a first end or a second end of the channel may be capped, requiring that all of the exhaust gas passes through a wall of the filter. In contrast, a partial flow filter may have at least one channel, and an entrance end and exit end of the channel may be unobstructed, e.g. not capped, so that all of the exhaust gas is not required to pass through a wall of the filter to exit the filter. Diesel particulate filter 32 may be combined with selective reduction catalyst 54 to reduce the level of NOx within the exhaust gas as well as particulate levels in the exhaust gas. FIG. 2 illustrates a perspective view of an exemplary diesel particulate filter 32.
Diesel particulate filter 32 may include a housing 36 enclosing a filtering element 38. Although a cylindrical housing is depicted in FIG. 2, housing 36 may be of any appropriate shape and size. Housing 36 may be made of any material, such as steel, that may reliably withstand the temperatures and constituents of the exhaust gas. In some embodiments, heaters may also be provided with diesel particulate filter 32 to heat filtering element 38 to a regeneration temperature. Filtering element 38 may be positioned within housing 36 such that the exhaust gas flowing through diesel particulate filter 32 flows through filtering element 38.
FIG. 3 is a cross-sectional view of diesel particulate filter 32 showing filtering element 38. The exhaust gas may enter filtering element 38 via a filter inlet 42 and exit filtering element 38 via a filter outlet 44. Filtering element 38 may include components configured to direct exhaust gas through diesel particulate filter 32 and to physically trap or chemically reduce the level of constituents in the exhaust gas. In this manner, filtering element 38 may include at least one channel 46, at least one opening 52, at least one partial barrier 50 and a catalyst 54.
Channel 46 may extend longitudinally from an unobstructed first end 62 to an unobstructed second end 64, and may have at least one sidewall 48. Channel 46 may be defined by a single sidewall 48 (such as in a channel having a circular cross-section) or a plurality of sidewalls (such as in a channel having a rectangular or triangular cross-section). Sidewall 48 may include at least one opening 52. Opening 52 may allow the exhaust gas to pass from a first channel 46 to an adjacent channel 46. It is contemplated that opening 52 may be a structural feature, e.g. not characteristic of the material of sidewall 48 or, alternatively, opening 52 may be a characteristic of the material of channel 46, such as, for example, a pore. It is further contemplated, that there may be a plurality of openings 52, and openings 52 may be a combination of pores, characteristic in a material, and structural features. In addition, the number of openings 52 in sidewall 48 may vary from channel to channel. Filtering element 38 may be made of a porous material such as cordierite or silicon carbide, or of a metallic mesh or a metallic foam. The porous structure of filtering element 38 may allow exhaust gas to pass through while preventing some particulate matter in the exhaust gas from passing through it. Due to this filtering of particulate matter, the amount of particulate matter in the exhaust gas exiting filtering element 38 may be less than the amount of particulate matter in the exhaust gas entering filtering element 38. Filtering element 38 may be constructed of any non-porous material, such as, for example, a metal that may reliably withstand the temperatures and constituents of the exhaust gas.
Partial barrier 50 of channel 46 may be configured to reduce a path of the flow of exhaust gas between first and second unobstructed ends 62 and 64 of channel 46. Partial barrier 50 may direct some of exhaust gas around partial barrier 50 and may redirect a portion of exhaust gas through opening 52. Partial barrier 50 may also be configured to create turbulence within channel 46, and to trap particulate matter within channel 46. By re-directing exhaust gas within channel 46, for example around partial barrier 50, partial barrier 50 may increase the amount of time that the exhaust gas remains within filtering element 38 before exiting via filter outlet 44. An increase in the amount of time exhaust gas remains with diesel particulate filter 32 may correspond to an increased reduction of NOx or other constituents of exhaust gas. Partial barrier 50 is depicted in FIG. 3 as being perpendicular to sidewall 48; however, it is contemplated that partial barrier 50 may be at any angle relative to sidewall 48, may face filter inlet 42 or filter outlet 44, and may be of any length that will not completely obstruct the pathway between unobstructed ends 62 and 64.
As discussed above, diesel particulate filter 32 may be combined with a selective catalytic reduction catalyst to reduce the level of NOx in the exhaust gas. A catalyst 54 may be disposed on a portion of a surface of sidewall 48 and/or a surface of partial barrier 50. Catalyst 54 may be applied to a portion of a surface of sidewall 48 and/or a surface of partial barrier 50 in any known manner, such as, for example, it may be painted on as a wash coat or painted on in a different manner. Alternatively, the components to be coated may be dipped in the catalyst while in liquid form and the catalyst may solidify on the surfaces of components. The method for applying catalyst 54 may be chosen based on the construction of diesel particulate filter 32, e.g. whether the components of diesel particulate filter 32 include non-porous material or a porous material. For simplicity, FIG. 3 depicts catalyst 54 on a surface of channel 46 and on a surface of partial barrier 50; however, it is contemplated that catalyst 54 may be disposed on any surface of diesel particulate filter 32 and filtering element 38 that may be contacted by the exhaust gas as it passes through diesel particulate filter 32. Catalyst 54 may comprise any of a variety of materials. For example, catalyst 54 may include a support material and a metal promoter dispersed within the catalyst support material. The support material may include, for example, at least one of alumina, zeolite, aluminophosphates, hexaluminates, aluminosilicates, zirconates, titanosilicates, and titanates. Combinations of these materials may be used, and the support material may be chosen based on the type of fuel used, the reductant used, the air to fuel-vapor ratio desired, and/or for conformity with environmental standards. One of ordinary skill in the art will recognize that numerous other catalyst compositions, including catalyst compositions usable with a hydrocarbon reductant, may be used without departing from the scope of this disclosure.
FIG. 4 is an illustration of a magnified view of an exemplary portion of a sidewall 48 made from a porous material and partial barrier 50, also made from a porous material, of filtering element 38. When the exhaust gas percolates through the porous sidewall 48 and partial barrier 50 of channels 46 such as, for example, via a path 68, some of particulate matter 66 is filtered by side wall 48 and partial barrier 50. As particulate matter 66 builds up, the accumulated particulate matter 66 may also filter the exhaust gas leading to more particulate matter accumulation.
A portion of particulate matter 66 present in the exhaust gas may be filtered while percolating through side walls 48 and partial barrier 50 of filtering element 38. The remaining portion of particulate matter 66 may pass through second unobstructed end 64 of channel 46 and out of diesel particulate filter 32. A ratio of the amount of particulate matter 66 filtered by filtering element 38 to the total amount of particulate matter 66 present in the exhaust gas may be a measure of filtration efficiency of diesel particulate filter 32. The portion of particulate matter 66 escaping with the exhaust gas passing through second unobstructed end 64 may contribute to a reduction in filtration efficiency of diesel particulate filter 32.
As particulate matter 66 accumulates in filtering element 38, the resistance to exhaust flow through diesel particulate filter 32 may increase, however, the ability of the exhaust gas to exit filtering element 38 through second unobstructed end 64, may prevent catastrophic failure of exhaust system 16 and engine 12. Specifically, even though the continued accumulation of particulate matter 66 may eventually clog the pores of side wall 48 and partial barrier 50 thereby preventing further percolation of the exhaust gas, the exhaust gas may still pass through unobstructed second end 64.

INDUSTRIAL APPLICABILITY

The filtering element of the present disclosure may be applicable to any power system, especially one with low levels of particulate but that requires a particulate filter, and where performance of the system is enhanced when the change in backpressure on the engine is minimized. The filtering element may avoid detrimental backpressure on the engine relative to a wall flow filter, while still reducing the particulate pollutant and NOx level of the exhaust gas. Further, by combining the particulate filter with the selective catalytic reduction catalyst, the particulate matter level and NOx level may be simultaneously reduced while eliminating excess system components. The operation of power system 10 will now be explained.
Referring to FIG. 1, Atmospheric air may be drawn into air induction system 14 and directed through compressor 20 of turbocharger 18 where it may be pressurized to a predetermined level before entering the combustion chamber of engine 12. Fuel may be mixed with the pressurized air before or after entering the combustion chamber of engine 12. The fuel and air mixture may be ignited by engine 12 to produce mechanical work and an exhaust gas flow containing gaseous compounds. The exhaust gas may be a fluid that may also contain particulate matter and NOx. The exhaust gas may be directed from engine 12 to turbine 22 of turbocharger 18 where the expansion of hot exhaust gases may cause turbine 22 to rotate, thereby rotating connected compressor 20 to compress the inlet air. After exiting turbine 22 the exhaust may flow through DOC 24. A portion of the NO that may be in the exhaust gas that passes through DOC 24 may react with the catalyst in DOC 24 and may be chemically converted to NO₂.
FIG. 5 is a flow diagram illustrating an exemplary disclosed method for operating a diesel particulate filter 32. The exhaust gas may flow toward the diesel particulate filter 32 and injector 26 may inject reductant stored in reductant supply 28 into the exhaust stream. The exhaust gas may enter diesel particulate filter 32 via filter inlet 42 of housing 36 and may pass into the first unobstructed end 62 of channel 46 of filtering element 38 (Step 70). As the exhaust gas moves along a length of channel 46, it may encounter partial barrier 50. Partial barrier 50 may redirect the exhaust gas into sidewall 48, into another partial barrier 50, or through an opening 52 (Step 72). Alternatively, when channels 46 that make up filtering element 38 are comprised of a porous material, the exhaust gas may additionally pass through the porous material forming sidewalls 48 (Step 74), and in this manner, particulate matter within the exhaust gas may be trapped within the porous material (Step 76). Particulate matter within the exhaust gas may accumulate along sidewall 48 and may be trapped behind partial barrier 50 (Step 76). In this manner, the level of particulate matter within the exhaust gas may be reduced. Exhaust gas may pass through opening 52 in sidewall 48 and into an adjacent channel 46.
As the exhaust gas passes through diesel particulate filter 32, reductant that has been sprayed into the exhaust stream may be absorbed by catalyst 54 and may chemically react with catalyst 54 and NOx within the exhaust stream to form H₂O and N₂. Partial barrier 50 may create turbulence within channel 46 which also may increase mixing between the reductant and the NOx and may increase the amount of contact between the exhaust gas, reductant, and catalyst 54 (Step 78). By way of example, exhaust gas that may contact partial barrier 50 may be directed into multiple different directions, such as, for example into contact with sidewall 48 or through opening 52. In this manner portions of the exhaust gas that might not otherwise come into contact with catalyst 54 may contact catalyst 54. As a result, the level of NOx, within the exhaust gas may be reduced (Step 80). The exhaust gas may then leave diesel particulate filter 32 (Step 82) via filter outlet 44 and may enter the atmosphere via exhaust outlet 34. Particulate matter may be retained within diesel particulate filter 32 and may accumulate on sidewall 48, partial barrier 50, and within opening 52. Accumulation of particulate matter may reduce the path of flow of the exhaust gas and may cause the backpressure on engine 12 may increase. However, partial barrier 50 may only reduce a portion of the path of flow of exhaust gas in contrast to a wall-flow filter. In this manner, the backpressure on engine 12 may remain within an expected range.
The use of a diesel particulate filter as disclosed that partially blocks the exhaust flow serves to control the detrimental backpressure on the engine. By controlling detrimental engine backpressure, the efficiency of the engine may be maintained and engine damage from excessive backpressure may be avoided. Further, by using partial barriers to redirect exhaust flow and to create turbulence within the diesel particulate filter, the exhaust gas may more uniformly contact the selective catalytic reduction catalyst coating the inner surfaces of the filter.
It will be apparent to those skilled in the art that various modifications and variations can be made to the system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.

Claims

1. A filtering element, comprising:

at least one channel being defined by at least one wall extending longitudinally from a first end of the channel to a second end of the channel, the at least one channel having an unobstructed first end and an unobstructed second end;

at least one partial barrier configured to reduce a path of a flow of exhaust through the at least one channel, the at least one partial barrier being positioned within the at least one channel so as to redirect a portion of the exhaust flow around the partial barrier within the channel; and

wherein the at least one channel includes a selective catalytic reduction catalyst on at least a portion of one of a surface of the at least one partial barrier or a surface of the at least one wall.

2. The filtering element of claim 1, wherein the at least one wall is a non-porous material.

3. The filtering element of claim 2, wherein the non-porous material is metal.

4. The filtering element of claim 1, wherein the at least one channel includes at least two channels and wherein the at least two channels are in fluid communication with each other through at least one opening in the at least one wall.

5. The filtering element of claim 4, wherein the at least one opening includes a plurality of openings.

6. The filtering element of claim 1, wherein the partial barrier is perpendicular with the at least one wall.

7. A method of filtering an exhaust gas, comprising:

directing a flow of exhaust gas through at least one channel extending between a first unobstructed end and a second unobstructed end of a diesel particulate filter;

redirecting a portion of the flow of exhaust gas around at least one partial barrier within the channel;

selectively reducing a non-particulate pollutant level in the exhaust gas through contact between the flow of exhaust gas and a catalyst within the diesel particulate filter; and

trapping particulate matter of the exhaust gas within the at least one channel.

8. The method of claim 7, including creating turbulence within the flow of exhaust gas by the redirecting of a portion of the flow.

9. The method of claim 7, wherein the redirecting of a portion of the flow of exhaust gas further includes redirecting the flow of exhaust gas with at least one non-porous wall of the at least one channel.

10. The method of claim 7,.wherein trapping particulate matter includes redirecting a portion of the flow of exhaust gas through at least one porous wall of the at least one channel.

11. The method of claim 7, wherein the at least one channel includes at least two channels and the method further includes directing the flow of exhaust gas from a first of the at least two channels into a second of the at least two channels through at least one wall between the channels.

12. The method of claim 7, wherein selectively reducing a non-particulate pollutant level includes facilitating reaction of the non-particulate pollutant level with a catalyst disposed on at least one of a wall of the channel and the partial barrier.

13. The method of claim 12, wherein the facilitating reaction includes injecting a reductant into the exhaust flow.

14. A filter, comprising:

a housing having an inlet and an outlet; and

a filtering element, the filtering element including:

at least one channel defined by at least one wall extending longitudinally between first and second ends, the first and second ends being unobstructed;

at least one partial barrier extending from the at least one wall and configured to redirect a path of a flow of exhaust through the at least one channel; and

a selective catalytic reduction catalyst on at least a portion of one of a surface of the at least one partial barrier or a surface of the at least one wall.

15. The filter of claim 14, wherein the at least one wall is a non-porous material.

16. The filter of claim 15, wherein the non-porous material is metal.

17. The filter of claim 14, wherein the at least one channel includes at least two channels, the at least two channels being in fluid communication with one another through at least one opening in the at least one wall.

18. The filter of claim 17, wherein the at least one wall includes a plurality of openings, the openings configured to allow a flow of fluid from on of the least two channels into a second of the at least two channels.

19. The filter of claim 14, wherein the at least one partial barrier includes a plurality of partial barriers.

20. The filter of claim 14, wherein the at least one partial barrier is configured to collect particulate matter from the flow of exhaust as the flow of exhaust gas passes through the at least one channel.