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  Hybrid system comprising hc-scr, nox-trapping, and nh3-scr for exhaust emission reductionUSPTO Application #: 20070012032Title: Hybrid system comprising hc-scr, nox-trapping, and nh3-scr for exhaust emission reduction Abstract: An exhaust aftertreatment system is provided with a first SCR catalyst, a NOx adsorber-catalyst, and an ammonia-SCR catalyst. The first catalyst is generally a hydrocarbon-SCR catalyst, but can be a carbon monoxide-SCR catalyst or a hydrogen-SCR catalyst. The first catalyst is functional to reduce NOx in lean exhaust using the corresponding reductant. The NOx adsorbant-catalyst is functional to adsorb NOx and to produce ammonia during regeneration. The ammonia SCR catalyst is configured to adsorb ammonia so produced and is functional to subsequently use that ammonia to reduce NOx in lean exhaust. The first SCR catalyst is useful to reduce the frequency with which the NOx adsorber-catalyst needs to be regenerated, and can thereby extends the life of that catalyst. In one embodiment, reductant for the first SCR catalyst is stored during regeneration of the NOx adsorber-catalyst and is used to convert additional NOx in a subsequent lean phase. (end of abstract)
Agent: Paul V. Keller, LLC - South Euclid, OH, US Inventor: Haoran Hu USPTO Applicaton #: 20070012032 - Class: 060286000 (USPTO) Related Patent Categories: Power Plants, Internal Combustion Engine With Treatment Or Handling Of Exhaust Gas, By Means Producing A Chemical Reaction Of A Component Of The Exhaust Gas, Condition Responsive Control Of Heater, Cooler, Igniter, Or Fuel Supply Of Reactor The Patent Description & Claims data below is from USPTO Patent Application 20070012032. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates to pollution control systems and methods for diesel engines. BACKGROUND [0002] NO.sub.x emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NO.sub.x emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have already put considerable effort toward meeting those regulations. [0003] In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NO.sub.x emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective. [0004] Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NO.sub.x catalysts, selective catalytic reduction (SCR), and lean NO.sub.x traps (LNTs). [0005] Lean-burn NOx catalysts promote the reduction of NO.sub.x under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NO.sub.x catalyst that has the required activity, durability, and operating temperature range. Currently, peak NOx conversion efficiencies for lean-burn catalysts are unacceptably low. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. [0006] Ammonia-SCR refers to selective catalytic reduction of NOx by ammonia. Often, this is referred to simply as SCR. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment. [0007] LNTs are NOx adsorbers combined with catalysts for NOx reduction. The adsorbant is typically an alkaline earth oxide adsorbant, such as BaCO.sub.3 and the catalyst is typically a precious metal, such as Pt or Ru. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. Accumulated NOx is removed by creating a rich environment within the LNT through the introduction of a reductant. In a rich environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed, preferably as N.sub.2. [0008] A LNT must periodically be regenerated to remove accumulated NOx. This type of regeneration may be referred to as denitration in order to distinguish desulfation, described below. The conditions for denitration can be created in several ways. One approach uses the engine to create a rich fuel-air mixture. For example, the engine can inject extra diesel fuel into the exhaust of one or more cylinders prior to expelling the exhaust. Reductant may also be injected into the exhaust downstream of the engine. In either case, a portion of the reductant is generally expended to consume excess oxygen in the exhaust. [0009] Reductant can consume excess oxygen by either combustion or reforming reactions. Typically, the reactions take place upstream of the LNT over an oxidation catalyst or in a reformer. The reductant can also be oxidized directly in the LNT, but this tends to result in faster thermal aging. [0010] U.S. Pat. Pub. No. 2003/0101713 describes an exhaust system with a fuel reformer placed inline with the exhaust and upstream of an LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate. [0011] In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily diesel fuel. Even with reduced sulfur fuels, the amount of SOx produced by diesel combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. The elevated temperatures required for desulfation can be produced by oxidizing reductant. [0012] A NOx adsorber-catalyst can produce ammonia during denitration. The ammonia can be captured by a downstream SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone NOx adsorber-catalyst with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes a system with an ammonia SCR catalyst configured downstream of a LNT for this purpose. U.S. Pat. Pub. No. 2004/0076565 describes such systems wherein both components are encased by a single shell and/or co-disbursed. WO 2004/090296 describes such a system wherein there is an inline reformer upstream of the NOx adsorber-catalyst and the SCR catalyst. [0013] U.S. Pat. No. 5,727,385 describes a system in which a hydrocarbon-SCR (HC--SCR) catalyst is configured upstream of an LNT. The two components together are said to provide higher NOx conversion than either of the components individually. [0014] U.S. Pat. No. 6,677,264 describes a combined LNT/HC--SCR catalyst. The catalyst comprises two layers on a support. The first layer is a NOx adsorber-catalyst and the second layer is an HC--SCR catalyst having a HC-storing function provided by a zeolite. The HC-storage function is intended to concentrate hydrocarbon reductants in the vicinity of the catalyst and thereby increase activity. [0015] U.S. Pat. No. 6,202,407 describes an HC--SCR catalyst that has a hydrocarbon-storing function. In one embodiment, a diesel fuel reductant supply is pulsed and the catalyst continues to show activity for extended periods between the pulses. [0016] In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and can practically be used to reduce NOx emissions across the spectrum of diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations. SUMMARY [0017] One concept relates to an exhaust aftertreatment system. The system comprises a first SCR catalyst, a NOx adsorber-catalyst, and an ammonia-SCR catalyst. The first catalyst is generally a hydrocarbon-SCR catalyst, but can be a carbon monoxide-SCR catalyst or a hydrogen-SCR catalyst. The first catalyst is functional to reduce NOx in lean exhaust using the corresponding reductant. The NOx adsorbant-catalyst is functional to adsorb NOx from lean exhaust and to produce ammonia during regeneration. The ammonia SCR catalyst is configured to adsorb ammonia so produced and is functional to subsequently use that ammonia to reduce NOx in lean exhaust. The first SCR-catalyst is useful to reduce the frequency with which the NOx adsorber-catalyst needs to be regenerated and can thereby extends the life of that catalyst. In one embodiment, reductant for the first SCR-catalyst is stored during regeneration of the NOx adsorber-catalyst and is used by the first SCR-catalyst to convert additional NOx during a subsequent lean phase. [0018] Another concept relates to a method of treating NOx-containing lean exhaust. The method includes a first phase in which: the exhaust is contacted with a first SCR catalyst to reduce a portion of the NOx by reactions with a first reductant; the exhaust is contacted with a NOx adsorber-catalyst to remove another portion of the NOx from the exhaust by adsorption; and the exhaust is contacted with an ammonia-SCR catalyst to reduce a further portion of the NOx by reactions with stored ammonia. In a second phase, the environment of the NOx-adsorber catalyst is made rich, whereby stored NOx is reduced. In the process, ammonia is generated and becomes stored in the SCR catalyst. The first reductant can be provided and stored in the second phase, or can be limited to that normally present in the exhaust. [0019] A further concept relates to a method of treating NOx-containing lean exhaust. The method includes a first phase comprising: contacting the exhaust with a first SCR catalyst to reduce a portion of the NOx by reactions with a stored reductant selected from the group consisting of hydrocarbons, carbon monoxide, and hydrogen; contacting the exhaust with a NOx adsorber-catalyst to remove another portion of the NOx from the exhaust by adsorption; and contacting the exhaust with an ammonia-SCR catalyst to reduce a further portion of the NOx by reactions with stored ammonia. The method also include a second phase comprising: making the exhaust rich; storing the first reductant in the first SCR catalyst; and reducing NOx stored in the NOx adsorber-catalyst and in the process producing ammonia that becomes stored by the SCR-catalyst. [0020] The primary purpose of this summary has been to present certain of the inventor's concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventor's concepts or every combination of the inventor's concepts that can be considered "invention". Other concepts of the inventor will become apparent to one of ordinary skill in the art from the following detailed description and annexed drawings. The concepts disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventor claim as his invention being reserved for the claims that follow. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading... 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