Patent Application
20100077739
The present disclosure is directed to an exhaust system and, more particularly, to an exhaust system that implements selective catalytic reduction (SCR) in two stages.
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 gaseous compounds including, among other things, the oxides of nitrogen (NOX). Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amount of 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 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 elemental nitrogen N2. 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 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 the ammonia gas required for the reduction of NOX and to evenly distribute through the substrate. This spacing may further increase packaging difficulties of the exhaust system.
An exemplary SCR-equipped exhaust system for use with a combustion engine is disclosed in U.S. Pat. No. 7,272,924 (the '924 patent) issued to Itoh et. al on Sep. 25, 2007. This exhaust system includes an upstream catalytic converter having a vanadium titania substrate, and a downstream catalytic converter having a copper zeolite substrate. The exhaust system also includes a feed pump that draws an ammonia-generating compound from a tank and directs the compound through an electromagnetically-controlled flow control valve into an exhaust flow upstream of the two catalytic converters. A majority of the compound is absorbed onto the vanadium titania substrate, while unused ammonia flowing out of the upstream catalytic converter is absorbed onto the copper zeolite substrate. As exhaust passes through these substrates, NOX contained therein is reduced.
The exhaust system of the present disclosure solves one or more of the problems set forth above and/or other problems of the prior art.
One aspect of the present disclosure is directed to an exhaust system. The exhaust system may include a passageway, a flow control device situated to feed reductant into the passageway, and a particulate collection device located in fluid communication with the passageway downstream of the flow control device. The particulate collection device may be catalyzed to promote NOX reduction in a presence of the reductant. The exhaust system may also include a reduction device located in fluid communication with the passageway downstream of the flow control device. The reduction device may be catalyzed to promote NOX reduction in a presence of the reductant, and the particulate collection device and the reduction device may receive reductant from only the flow control device.
A second aspect of the present disclosure is directed to another exhaust system. This exhaust system may include a first substrate catalyzed to promote NOX reduction, and a second substrate catalyzed to promote NOX reduction. The exhaust system may also include a supply of gaseous ammonia, and a flow control device situated to feed the gaseous ammonia to the first substrate and the second substrate.
Multiple separate sub-systems may be included within power system 10 to promote power production. For example, power system 10 may include an air induction system 16 and an exhaust system 18. Air induction system 16 may be configured to direct air or an air/fuel mixture into power system 10 for subsequent combustion. Exhaust system 18 may treat and discharge byproducts of the combustion process to the atmosphere.
Air induction system 16 may include components that cooperate to condition and introduce compressed air into cylinders 14. For example, air induction system 16 may include an air cooler 20 located downstream of one or more compressors 22. Compressors 22 may be connected to pressurize inlet air directed through cooler 20. It is contemplated that air induction system 16 may include different or additional components than described above such as, for example, a throttle valve, variable valve actuators associated with each cylinder 14, filtering components, compressor bypass components, and other known components, if desired. It is further contemplated that compressor 22 and/or cooler 20 may be omitted, if a naturally aspirated engine is desired.
Exhaust system 18 may include components that condition and direct exhaust from cylinders 14 to the atmosphere. For example, exhaust system 18 may include an exhaust passageway 24, one or more turbines 26 driven by exhaust flowing through passageway 24, a particulate collection device 28 located downstream of turbine 26, and a reduction device 30 fluidly connected downstream of particulate collection device 28. It is contemplated that exhaust system 18 may include different or additional components than described above such as, for example, bypass components, an exhaust compression or restriction brake, an attenuation device, additional exhaust treatment devices, an exhaust gas recirculation (EGR) circuit, and other known components, if desired.
Turbine 26 may be located to receive exhaust leaving power system 10, and may be connected to one or more compressors 22 of air induction system 16 by way of a common shaft 32 to form a turbocharger. As the hot exhaust gases exiting power system 10 move through turbine 26 and expand against vanes (not shown) thereof, turbine 26 may rotate and drive the connected compressor 22 to pressurize inlet air.
Particulate collection device 28 may include a filtration substrate 34 located downstream of turbine 26 and utilized to remove particulate matter or soot from the exhaust flow of power system 10. It is contemplated that filtration substrate 34 may be fabricated from an electrically conductive or non-conductive coarse mesh metal or porous ceramic honeycomb medium. In one example, particulate collection device 28 may have an effective filtration volume of about 1.3-2.6 times a displacement of power system 10. As exhaust flows through filtration substrate 34, particulates may be blocked by thereby and left behind within particulate collection device 28 (i.e., particulates may collect within particulate collection device 28.
At least a portion of filtration substrate 34 may be catalyzed to promote reduction of an exhaust constituent. In one example, the constituent may be an oxide of nitrogen (NOX), and filtration substrate 34 may be coated or impregnated with a selective catalytic reduction (SCR) type catalyst, for example, vanadia on titania or a zeolite with an active base metal formulation. This coating, in conjunction with the effective volume of particulate collection device 28 and the use of a gaseous or liquid reductant added to the exhaust, may help reduce approximately 40-90% of the NOX produced by power system 10 to innocuous substances while simultaneously capturing particulate matter from the exhaust.
To help promote NOX reduction within filtration substrate 34, a gaseous or liquid reductant may be sprayed or otherwise advanced from a supply 36 into the exhaust upstream of filtration substrate 34 by an injection control device 38, for example a control valve or an injector. In one example, supply 36 may include pressurized a urea solution, ammonia gas, liquefied anhydrous ammonia, ammonium carbonate, or an ammine salt. Injection control device 38 introduces one or more of these reductants into the exhaust stream, ultimately resulting in the presence of ammonia gas in the exhaust of passageway 24. The ammonia gas in the exhaust may be absorbed onto a surface of filtration substrate 34 that is coated with the SCR catalyst, where it may react with NOX (NO and NO2) from the exhaust to form water (H2O) and elemental nitrogen (N2). In one example, about 40-90% of the injected reductant may be consumed within particulate collection device 28 by the reduction process.
Injection control device 38 may be located a distance upstream of filtration substrate 34 to allow the injected reductant sufficient time to mix with exhaust from power source 10 before entering filtration substrate 34. That is, an even distribution of reductant within the exhaust passing through filtration substrate 34 may enhance NOX reduction therein. In one example, this distance may be based on a flow rate of exhaust exiting power system 10 and/or on a cross-sectional area of passageway 24. In most applications, this distance may be less for ammonia gas than it would be if a liquid reductant, for example urea or a urea/water solution, were utilized, as the ammonia gas is not required to first break down before it is distributed.
The reduction process performed by filtration substrate 34 may be most effective when a concentration of NO to NO2 supplied to filtration substrate 34 is about 1:1. To help provide the correct concentration of NO to NO2, an oxidation catalyst 40 may, in some embodiments, be located upstream of filtration substrate 34. Oxidation catalyst 40 may be, for example, a diesel oxidation catalyst (DOC). As a DOC, oxidation catalyst 40 may include a porous ceramic honeycomb structure or a metal mesh substrate coated with a material, for example a precious metal, that catalyzes a chemical reaction to alter the composition of the exhaust. For example, oxidation catalyst 40 may include platinum that facilitates a conversion of NO to NO2.
Reduction device 30 may receive exhaust from particulate collection device 28 and further reduce constituents of the exhaust. In one example, reduction device 30 may embody a selective catalytic reduction (SCR) device having a catalyzed substrate 42. Similar to filtration substrate 34, substrate 42 may be coated or impregnated with an SCR type catalyst that causes NOX in the exhaust to react with ammonia and form water and elemental nitrogen. It is contemplated that the catalyst material of reduction device 30 may be different than the catalyst material of particulate collection device 28 to accommodate downstream conditions that are different from upstream conditions such as exhaust temperatures, if desired. For example, substrate 42 may be coated with a catalyst having a lower activation temperature than that applied to upstream-located filtration substrate 34. In one embodiment, the catalyst coating of substrate 42 may be copper zeolite.
Reduction device 30 may be configured to reduce a different amount of NOX, as compared to particulate collection device 28. Specifically, reduction device 30 may have an effective volume of about 1.1-2.2 times a displacement of power system 10 (i.e., the effective volume of reduction device 30 may be less than the effective volume of particulate collection device 28). This volume, combined with a location of reduction device 30 and a type of catalyst coating applied to substrate 42, may result in a reductant consumption of about 10-40% of the reductant advanced into the exhaust by injection control device 38, and a NOX reduction of about 10-55% of the NOX produced by power system 10. Thus, reduction device 30 may reduce a lesser amount of NOX than particulate collection device 28.
Substrates 34 and 42 may receive all reductant from injection control device 38. That is, injection control device 38 may introduce ammonia into exhaust passage 24 upstream of particulate collection device 28, and particulate collection device 28 may consume a majority of the ammonia gas. Residual ammonia gas passing through filtration substrate 34 may be consumed by substrate 42. No other source of reductant may be available within exhaust system 18. Ammonia in excess of that expected to be consumed in particulate collection device 28 can be provided to help insure that at least some ammonia is in the exhaust gas entering reduction device 30.
During operation of power system 10, it may be possible for too much ammonia gas to be advanced into the exhaust (i.e., ammonia gas in excess of that required for appropriate NOX reduction). In this situation, known as “ammonia slip”, some amount of ammonia may pass through substrates 34 and 42 to the atmosphere, if not otherwise accounted for. To help minimize the magnitude of ammonia slip, an oxidation catalyst (AMOX) 44 may be located downstream of reduction device 30. Oxidation catalyst 44 may include a substrate coated with a catalyst that oxidizes residual NH3 in the exhaust. It is contemplated that oxidation catalyst 44 may be omitted, if desired.
The exhaust system of the present disclosure may be applicable to any power system having constituent-reducing capabilities, where a reduction efficiency and packaging are important issues. The disclosed exhaust system may increase an efficiency of constituent reduction while improving packaging by increasing an effective volume of reduction devices, utilizing multiple small reduction devices, and by using a reductant that requires minimal space and time for reductant breakdown and even distribution. Operation of power system 10 will now be described.
Referring to
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 equivalent.
