Patent Application
20130047583
The present disclosure is directed to an aftertreatment system and, more particularly, to an aftertreatment system including a selective catalytic reduction catalyst.
Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art exhaust a complex mixture of air pollutants. These air pollutants are composed of particulates and gaseous compounds including, among other things, oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amounts of particulates and NOX emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to comply with the regulation of particulates and NOX, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR). SCR is a process where a reductant, most commonly urea ((NH2)2CO) or a water/urea solution, is selectively injected into the exhaust gas stream of an engine and absorbed onto a downstream substrate. The injected urea solution decomposes into ammonia (NH3), which reacts with NOX in the exhaust gas to form water (H2O) and diatomic nitrogen (N2). Engine manufacturers implementing the SCR process typically include an oxidation catalyst upstream of the SCR substrate to assist in altering the composition of the exhaust gas stream before it passes to the SCR substrate. Such oxidation catalysts typically include a porous substrate made from, coated with, or otherwise including a catalyzing material such as palladium, platinum, vanadium, and/or other precious metals. Such materials facilitate a conversion of NO to NO2, thereby increasing the ratio of NO2 to NO upstream of the SCR substrate. The elevated level of NO2 provided by the oxidation catalyst may assist in both improving NOx conversion over the SCR catalyst and oxidizing soot particles that collect in a particulate filter.
In some applications, the substrate used for SCR purposes may need to be very large to help ensure it has enough surface area or effective volume to absorb appropriate amounts of the ammonia required for sufficient catalytic reduction of NOX. These large substrates can be expensive and require significant amounts of space within the exhaust system. In addition, the substrate must be placed far enough downstream of the injection location for the urea solution to have time to decompose into ammonia and to evenly distribute within the exhaust flow for the efficient reduction of NOX. Due to the size of the SCR substrate and the required spacing between the substrate and the injector, packaging an exhaust system utilizing such components can be difficult, and packaging an exhaust system utilizing an oxidation catalyst upstream of the SCR substrate can be even more difficult. In addition, due to the precious metals used to manufacture oxidation catalysts, utilizing an oxidation catalyst can significantly increase the cost of the exhaust system.
An exemplary SCR-equipped system for use with a combustion engine is disclosed in JP Patent Publication No. 2008/274,851 (the '851 publication) of Makoto published on Nov. 13, 2008. This system includes an exhaust gas purification device having a gas accumulation canister, a separate dispersion canister, and a mixing pipe connected between the gas accumulation and gas dispersion canisters. A particulate filter and an oxidation catalyst are disposed in the gas accumulation canister, while an SCR catalyst and ammonia reduction catalyst are disposed within the gas dispersion canister. A urea injector is located in the mixing pipe, upstream of the SCR catalyst.
Although the exhaust system of the '851 patent may be configured to treat exhaust gases, such a system may be problematic in many aftertreatment applications. In particular, the multiple canisters and catalysts used in the '851 system may increase the cost, packaging complexity, and an overall size of the system.
The aftertreatment systems of the present disclosure solve one or more of the problems set forth above and/or other problems of the prior art.
In an exemplary embodiment of the present disclosure, an aftertreatment system for an internal combustion engine includes a treatment device having a combined particulate filter and SCR catalyst. The combined particulate filter and SCR catalyst treats uncatalyzed exhaust from the internal combustion engine including between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr.
In another exemplary embodiment of the present disclosure, an aftertreatment system for an internal combustion engine includes a treatment device having a first SCR catalyst disposed on a particulate filter substrate, and a second SCR catalyst downstream of the first SCR catalyst. The first SCR catalyst treats uncatalyzed exhaust from the internal combustion engine. The treatment device is characterized by a NOx conversion efficiency greater than approximately 95 percent.
In an additional exemplary embodiment of the present disclosure, an exhaust treatment method includes generating exhaust with an internal combustion engine, the exhaust including between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr. The method also includes directing the exhaust uncatalyzed from the internal combustion engine to a combined particulate filter and SCR catalyst. The method further includes catalytically reducing NOx in the exhaust with the combined particulate filter and SCR catalyst, the combined particulate filter and SCR catalyst forming a first treated exhaust.
An exemplary aftertreatment system 10 is shown in
The treatment device 28 may include one or more canisters 12 fabricated from a one or more corrosion-resistant materials. Such materials may include, for example, stainless steel or other like metals. In further exemplary embodiments, such materials may be treated, coated, and/or other wise provided with corrosion protection. In the embodiment shown in
As shown in
In further embodiments, the treatment device 28 may include an additional catalyst 32 disposed within the canister 12 downstream of the CDS catalyst 30. The additional catalyst 32 may be, for example, an SCR catalyst. In additional exemplary embodiments, the additional catalyst 32 may include an upstream region 32A that functions as an SCR catalyst, and a downstream region 32B that functions as a cleanup catalyst such as, for example, a diesel oxidation catalyst or an ammonia oxidation catalyst. In an alternative embodiment, the additional catalyst 32 may be a dedicated cleanup catalyst (e.g., catalyst 32 may not provide SCR functionality).
The CDS catalyst 30 may be configured to perform particulate trapping functions. In particular, CDS catalyst 30 may include filtration media configured to remove particulate matter from an exhaust flow. In one embodiment, the filtration media of the CDS catalyst 30 may embody a generally cylindrical deep-bed type of filtration media configured to accumulate particulate matter throughout a thickness thereof in a substantially homogenous manner. The filtration media may include a low density material having a flow entrance side and a flow exit side, and may be formed through a sintering process from metallic or ceramic particles. It is contemplated that the filtration media may alternatively embody a surface type of filtration media fabricated from metallic or ceramic foam, a wire mesh, or any other suitable material.
The CDS catalyst 30 may also be configured to perform SCR functions. Specifically, the filtration media of CDS catalyst 30 may be fabricated from or otherwise coated with a ceramic material such as titanium oxide, a base metal oxide such as vanadium and tungsten, zeolites, and/or a precious metal. With this composition, decomposed reductant entrained within an exhaust flow passing through the CDS catalyst 30 may be absorbed onto the surface of and/or within the filtration media, where the reductant may react with NOx (NO and NO2) in the exhaust gas to form water (H2O) and diatomic nitrogen (N2). It is contemplated that CDS catalyst 30 may perform both particulate trapping and SCR functions throughout the media of CDS catalyst 30 or, alternatively, in serial stages, as desired. If NO2 levels within the exhaust are sufficiently high, the exothermic reaction between the reductant and the NOx may assist in passively regenerating the CDS catalyst 30.
As described above, the additional catalyst 32 may comprise an upstream region 32A and a downstream region 32B. In particular, a single substrate brick of catalyst 32 may include an upstream region (32A) located proximate and/or adjacent the CDS catalyst 30. The upstream region 32A may be fabricated from or otherwise coated with a material that absorbs reductant onto its surface or otherwise internalizes reductant for reaction with NOx (NO and NO2) in the exhaust gas passing therethrough. Such a reaction may form water (H2O) and diatomic nitrogen (N2). Similarly, the substrate brick of catalyst 32 may include a downstream region (32B) located proximate and/or adjacent the outlet 16. The downstream region 32B may be disposed adjacent to and downstream of the upstream region 32A, and may be coated with or otherwise contain a different catalyst than the upstream region 32A. Such catalysts may include an oxidation catalyst configured to oxidize residual reductant in the exhaust.
In exemplary embodiments in which the downstream region 32B of the additional catalyst 32 comprises an oxidation catalyst, such an exemplary oxidation catalyst may be, for example, a diesel oxidation catalyst (DOC) or an ammonia oxidation (AMOx) catalyst. Such oxidation catalysts may comprise any suitable substrate coated with or otherwise containing a catalyzing material, for example a precious metal, that catalyzes a chemical reaction to alter a composition of exhaust passing through the oxidation catalyst. In one embodiment, such an oxidation catalyst may include palladium, platinum, vanadium, or a mixture thereof that facilitates oxidation of residual ammonia gas and/or entrained reductant. Such catalysts may also facilitate the oxidation of NO in the exhaust to NO2. In another embodiment, the additional catalyst 32 may alternatively or additionally perform particulate trapping functions (e.g., the additional catalyst 32 may include a particulate trap such as a continuously regenerating technology particulate filter or a catalyzed continuously regenerating technology particulate filter), hydrocarbon oxidation functions, carbon monoxide oxidation functions, and/or other functions known in the art.
As shown in
It is contemplated that access to the catalysts 30, 32 of the aftertreatment system 10 may be helpful in some situations. Thus, in exemplary embodiments, end portions 46, 48 of the canister 12 enclosing the gaps 34 at the inlet 14 and outlet 16, respectively, may be removably connected to a center portion of canister 12 that encloses the CDS catalyst 30 and the additional catalyst 32. For example, the end portions 46, 48 may be bolted or latched to the center portion, if desired. With this configuration, the end portions 46, 48 may be selectively removed for inspection and/or replacement of the various catalysts 30, 32.
One or more removable couplings 26 may be connected to the end portions 46, 48 to facilitate the selective removal of the treatment device 28 from the aftertreatment system 10. Such couplings 26 may comprise any removable air-tight coupling device known in the art. In an exemplary embodiment, such couplings 26 may comprise one or more clamps, brackets, pieces of flexible tubing, and/or other like devices configured to facilitate a removable connection between the canister 12 and other components of the aftertreatment system 10. In further exemplary embodiments, the couplings 26 may embody cobra-head type couplings that are capable of bending through an angle of about 90 degrees. Such couplings 26 may have an elliptical opening at the canister 12 and a circular opening at an opposite end of the coupling 26. In still further embodiments, other types of couplings may be utilized, if desired.
As shown in
To enhance incorporation of the reductant with the exhaust, a mixer 38 may be disposed within the mixing tube 24. In an exemplary embodiment, the mixer 38 may include vanes or blades inclined to generate a swirling motion of the exhaust as it flows through the mixing tube 24. In another exemplary embodiment, the mixer 38 may include a ring extending from internal walls of the mixing tube 24 radially inward a distance toward a longitudinal axis of the mixing tube 24. Such a ring may be configured to promote exhaust flow turbulence within the mixing tube 24, thereby assisting in incorporating the reductant into the exhaust. In either embodiment, the mixer 38 may be disposed upstream or downstream (shown in
The aftertreatment system 10 may also include one or more probes situated to monitor operating characteristics and/or other parameters of the aftertreatment system 10. For example, a first probe 40 may be situated within the gap 34 proximate the inlet 14 upstream of the CDS catalyst 30. In addition, a second probe 42 may be situated within the gap 34 proximate the outlet 16 downstream of the additional catalyst 32. In one embodiment, first probe 40 may be a temperature probe configured to generate a first signal indicative of a temperature of the exhaust entering CDS catalyst 30. The first signal may be utilized by a controller (not shown) to determine, among other things, an operating temperature and predicted efficiency of the CDS catalyst 30. The second probe 42 may be utilized to detect a constituent of the exhaust exiting catalyst 32, for example a concentration of NOx or an amount of residual reductant. The second probe 42 may generate a second signal indicative of this constituent, and the second signal may be utilized to determine, among other things, an actual effectiveness of the CDS catalyst 30 and/or the additional catalyst 32. It is contemplated that at least one of the probes 40, 42 may be configured to monitor other parameters of the aftertreatment system 10, and may be utilized for other purposes, if desired.
The aftertreatment system 10 of the present disclosure may be applicable to any engine configuration requiring the treatment of exhaust where component packaging is an important issue. While known aftertreatment systems utilize a DOC catalyst upstream of an SCR catalyst for converting NO to NO2, such DOC catalysts are typically disposed in large canisters, and due to the precious metals utilized in DOC catalysts, such catalysts are costly. The aftertreatment system 10 of the present disclosure, on the other hand, operates without using a DOC catalyst upstream of the treatment device 28. As a result, the disclosed system 10 takes up less space downstream of the engine 22, and is less expensive, less complicated, and easier to package on vehicles utilizing the engine 22 than known systems.
To compensate for the conversion of NO to NO2 provided by the upstream DOC catalyst of known aftertreatment systems, the engine 22 of the present disclosure may be calibrated to generate exhaust having increased NOx levels (thereby increasing the amount of NO2 in the exhaust) and decreased soot levels. For example, the timing of in-cylinder fuel injections may be advanced, the pressure of such injections may be increased, the flow of recirculated exhaust gas into the engine 22 may be reduced or eliminated, and/or the throughput of the compressor 20 and/or the turbocharger 18 may be increased in order to facilitate generating such exhaust. The elevated NOx levels resulting from such engine calibration may ensure passive regeneration of the CDS catalyst 30 due to the exothermic reduction reaction at the SCR catalyst contained therein, and may also increase the fuel efficiency of the engine 22. Such NOx levels may be, for example, between approximately 7 g NOx/kW-hr and approximately 10 g NOx/kW-hr. In further exemplary embodiments, such NOx levels may be greater than approximately 10 g NOx/kW-hr. Due to such calibration, the engine 22 may generate exhaust having a NO2 to NO ratio of approximately 1 to 2. In addition, such engine calibration may result in an improvement in fuel efficiency and a reduction in soot production by the engine 22. The exhaust flow through the aftertreatment system 10 will now be described.
Referring to
The uncatalyzed exhaust 44 may be directed from the mixing tube 24 into the canister 12 via the inlet 14. The exhaust 44 may flow from inlet 14 into the gap 34 upstream of CDS catalyst 30, and due to the change in cross-sectional area and/or volume between the mixing tube 24 and the end portion 46, the uncatalyzed exhaust 44 may expand upstream of the CDS catalyst 30. Such expansion may facilitate a substantially equal distribution of the exhaust 44 across a face of the CDS catalyst 30. By the time the exhaust 44 reaches the CDS catalyst 30, the bulk of the reductant may be decomposed, thereby facilitating NOx reduction within the CDS catalyst 30 and the additional catalyst 32.
The CDS catalyst 30 may treat the uncatalyzed exhaust 44 as the exhaust 44 passes through the CDS catalyst 30. For example, particulate matter may be removed from the exhaust 44, and NOx within the exhaust 44 may react with the reductant at the SCR catalyst. In particular, the exhaust 44 may be catalytically reduced by the SCR catalyst to form water and diatomic nitrogen. As a result, the CDS catalyst 30 may form a first treated exhaust from the uncatalyzed exhaust 44. Such a first treated exhaust may be, for example, exhaust that has undergone a catalytic reduction process in which NO has been formed from NO2. The first treated exhaust may exit the CDS catalyst 30 and enter the additional catalyst 32, where catalytic reduction of NOx contained in the first treated exhaust may occur and residual reductant carried by the first treated exhaust may be absorbed. For example, in embodiments in which the additional catalyst 32 comprises an SCR catalyst, the SCR catalyst may catalytically reduce the first treated exhaust. As a result, the SCR catalyst of the additional catalyst 32 may form a second treated exhaust from the first treated exhaust. Such a second treated exhaust may be, for example, treated exhaust that has undergone an additional catalytic reduction process in which NO has been formed from NO2.
In exemplary embodiments, the treatment device 28 may be characterized by a NOx conversion efficiency greater than approximately 95 percent. As used herein with regard to the SCR catalysts of the treatment device 28, the term “NOx conversion efficiency” means the percentage of NOx contained by the exhaust that is catalytically reduced to N2 upon passing through the SCR catalysts or the treatment device 28. Further, unless otherwise specified, the NOx conversion efficiency values discussed herein are associated with treatment devices at or near the beginning of their useful life. In exemplary embodiments, the CDS catalyst 30 may be characterized by a NOx conversion efficiency of approximately 90 percent or less. The CDS catalyst 30 may provide such a NOx conversion efficiency while producing advantageous levels of backpressure upstream of the treatment device 28 and while having a size suitable for packaging in the aftertreatment system 10 of the engine 22. In such exemplary embodiments, the SCR catalyst of the additional catalyst 32 may be characterized by a NOx conversion efficiency of at least approximately 50 percent. In further exemplary embodiments, the SCR catalyst of the additional catalyst 32 may be characterized by a NOx conversion efficiency of between approximately 50 percent and approximately 80 percent. Thus, the respective NOx conversion efficiencies of the SCR catalysts may combine to result in a NOx conversion efficiency of the treatment device 28 greater than approximately 95 percent. Such NOx conversion efficiency levels may be required to comply with exhaust emission standards.
In addition, exemplary treatment device embodiments including a CDS catalyst 30 having a NOx conversion efficiency of approximately 90 percent or less, and an additional catalyst 32 having an SCR catalyst with a NOx conversion efficiency of between approximately 50 percent and approximately 80 percent, may be capable of oxidizing a greater amount of soot per unit volume of exhaust than, for example, a treatment device including a single CDS catalyst having a NOx conversion efficiency of approximately 95 percent. It is understood that, for example, passive soot regeneration may improve inversely with NOx conversion to N2. This relationship is a result of NO2 being consumed by the passive soot oxidation reaction and the NOx reduction reactions taking place at the one or more SCR catalysts of the treatment device 28. For example, lower levels of NOx reduction at the CDS catalyst 30 may allow for higher levels of soot reduction on the substrate of the CDS catalyst 30. Such NOx reduction may be furthered and/or completed at the additional catalyst 32 in order to achieve a NOx conversion efficiency of the treatment device greater than approximately 95 percent.
In embodiments in which the additional catalyst 32 further comprises one of a DOC catalyst and an AMOx catalyst, the second treated exhaust may pass from the SCR catalyst of the additional catalyst 32 to the downstream oxidation catalyst of the additional catalyst 32. The oxidation catalyst may catalytically oxidize the second treated exhaust. In particular, as the second treated exhaust passes through the oxidation catalyst, residual reductant entrained within the second treated exhaust may be oxidized. After treatment within the additional catalyst 32, the exhaust may pass through the gap 34 proximate the end portion 48, and may be discharged to the atmosphere or other downstream exhaust system components via the outlet 16.
The aftertreatment system 10 may be simple, compact, and relatively inexpensive. For example, the aftertreatment system 10 may be simple and compact because it may utilize only a single canister having catalysts that provide multiple functions. In addition, the CDS catalyst 30 may provide both particulate trapping and NOx reduction functionality, while the additional catalyst 32 may provide NOx reduction and oxidation functionality. The simplicity of the aftertreatment system 10 may result in a lower cost solution to exhaust aftertreatment and may require less packaging space than known systems.
It will be apparent to those skilled in the art that various modifications and variations can be made to the aftertreatment system 10 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 aftertreatment 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 equivalent.
