Abstract:
An active system for regenerating a NO x  adsorber and a particulate filter, the system comprising a fuel source, a reformer for generating hydrogen and carbon monoxide in fluid communication with the fuel source, a first valve, a second valve, and a third valve in fluid communication with the reformer, an oxidation catalyst, a NO x  adsorber located downstream from the oxidation catalyst, a particulate filter located downstream from the NO x  adsorber; and wherein the first valve, the second valve, and the third valve control fluid flow from the reformer to the oxidation catalyst, the NO x  adsorber, and the particulate filter.

Description:
BACKGROUND 
     This disclosure relates generally to a method and system for regenerating and/or desulfating NO x  adsorbers and/or regenerating particulate filters. 
     In general, diesel engines generally emit less nitrogen oxides (NOx) than a gasoline engine under most conditions, but because diesel engines mostly or exclusively operate on a high air to fuel ratio, the chemistry of the exhaust gas does not favor NOx reduction, because of the excess of oxidizing species. Thus, the reduction of nitrogen oxides, e.g., nitric oxide (NO), nitrogen dioxide (NO 2 ), and nitrous oxide (N 2 O), in exhaust gas is a widely addressed problem as a result of environmental concerns and mandated government emissions regulations, particularly in the transportation industry. One proposed solution is the use of a three-way conversion catalyst, which can be employed to treat the exhaust gases. Such three-way conversion catalysts, contain precious metals such as platinum, palladium, and rhodium, and can promote the oxidation of unburned hydrocarbons and carbon monoxide, and the reduction of nitrogen oxides in exhaust gas provided that the engine is operated around a balanced stoichiometry for combustion (also referred to as “combustion stoichiometry”). The balanced combustion stoichiometry is typically at an air to fuel ratio between about 14.4 to about 14.7. 
     However, fuel economy and global carbon dioxide emission concerns have made engine operation under lean-burn conditions desirable in order to realize a benefit in fuel economy. Under such lean-burn conditions, the air-to-fuel ratio may be greater than the balanced combustion stoichiometry, i.e., greater than about 14.7 and may be between about 19 to about 35. When lean-burn conditions are employed, three-way conversion catalysts are generally efficient in oxidizing the unburned hydrocarbons and carbon monoxides, but are generally inefficient in the reduction of nitrogen oxides. 
     One approach for treating nitrogen oxides in exhaust gases is to incorporate a NO x  adsorber, also referred to as a “lean-NO x  trap,” in the exhaust lines. The NO x  adsorber promotes the catalytic oxidation of nitrogen oxides by catalytic metal components effective for such oxidation, such as precious metals. The formation of NO 2  is generally followed by the formation of a nitrate when the NO 2  is adsorbed onto the catalyst surface. The NO 2  is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form. The system can be periodically operated under fuel-rich combustion to regenerate the NO x  adsorber. During this period of fuel-rich combustion, the absence of oxygen and the presence of a reducing agent promote the release and subsequent reduction of the stored nitrogen oxides. However, this period of fuel-rich combustion may also result in a significant fuel penalty. 
     As previously mentioned, exhaust gas streams can further comprise particulate matter such as carbon-containing particles or soot. A particulate filter, commonly used with compression ignition engines, can be used to prevent the carbon particles or the soot from exiting a tailpipe. The particulate filter may be a stand-alone device separate and distinct from devices employing catalytic elements for removing undesirable NO x  gaseous components. Carbon particles can be trapped in the particulate filter and then periodically burned to regenerate the filter. 
     Regeneration of particulate filters can be accomplished by the use of auxiliary devices such as a burner or other heating element. For example, an air-fuel nozzle and an ignition device can be used and operated, when desired, to heat the exhaust gases and the particulate filter to a combustion temperature of the trapped particulate matter. In this manner, the trapped particulate matter can be burned from the filter surfaces to permit a continuous flow of the exhaust gases. Alternatively, an electric heater can be used to generate the heat to initiate the combustion of the trapped particulates. However, these approaches are limited by their energy efficiency, durability, and cost. 
     BRIEF SUMMARY 
     Disclosed herein is a system for regenerating and/or desulfating a NOx adsorber and/or a system for regenerating a particulate filter. The system comprises regeneration system comprising an exhaust conduit in fluid communication with an exhaust fluid from an engine, wherein the exhaust conduit comprises a first oxidation catalyst, a NOx adsorber, and a second oxidation catalyst coupled to a particulate filter; a fuel source in fluid communication with a reformer, wherein the reformer is adapted to generate a hydrogen and carbon monoxide containing fluid from a fuel supplied by the fuel source; a regeneration conduit in fluid communication with the exhaust conduit and the reformer; and valve means disposed in the regeneration conduit for selectively controlling and directing the hydrogen and carbon monoxide containing fluid from the reformer to the first oxidation catalyst, the coupled second oxidation catalyst and particulate filter, the NOx adsorber, or a combination thereof. 
     In accordance with another embodiment, a regeneration system comprises an exhaust conduit in fluid communication with an exhaust fluid from an engine, wherein the exhaust conduit comprises a first oxidation catalyst, a second oxidation catalyst coupled to a NOx adsorber, and a third oxidation catalyst coupled to a particulate filter; a fuel source in fluid communication with a reformer, wherein the reformer is adapted to generate a hydrogen and carbon monoxide containing fluid from a fuel supplied by the fuel source; a regeneration conduit in fluid communication with the exhaust conduit and the reformer; and valve means disposed in the regeneration conduit for selectively controlling and directing the hydrogen and carbon dioxide containing fluid from the reformer to the first oxidation catalyst, the second oxidation catalyst coupled to the NOx adsorber, and the third oxidation catalyst coupled to the particulate filter, or a combination thereof. 
     A process for regenerating and desulfating a NO x  adsorber and/or a regenerating a particulate filter comprises periodically supplying a fuel to a reformer; converting said fuel to a hydrogen and carbon monoxide containing fluid; selectively feeding the hydrogen and carbon monoxide containing fluid into an oxidation catalyst or into a NOx adsorber, or into the oxidation catalyst and the NOx adsorber catalyst; and generating an exotherm in the oxidation catalyst and heating an exhaust fluid passing therethrough to a temperature effective to regenerate a particulate filter disposed downstream from the oxidation catalyst or reducing nitrogen oxides adsorbed by the NOx adsorber or generating the exotherm in the oxidation catalyst and heating the exhaust fluid to the temperature effective to regenerate the particulate filter downstream from the oxidation catalyst and reducing the nitrogen oxides trapped by the NOx adsorber. 
     The above-described and other features will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the figures, which are exemplary embodiments and wherein like elements are numbered alike: 
     FIG. 1 is a schematic view of a regeneration and/or desulfating system for light duty diesel architecture; 
     FIG. 2 is a schematic view of a regeneration and/or desulfating system for heavy duty diesel architecture; 
     FIG. 3 a schematic view of a system for regenerating a NOx adsorber in accordance with a third embodiment; 
     FIG. 4 is a schematic view of a system for regenerating a NOx adsorber in accordance with a fourth embodiment; 
     FIG. 5 is a schematic view of a system for regenerating a NOx adsorber in accordance with a fifth embodiment; 
     FIG. 6 is a schematic view of a system for regenerating a NOx adsorber in accordance with a sixth embodiment; and 
     FIG. 7 is a schematic view of a system for regenerating a NOx adsorber in accordance with a seventh embodiment. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a system, generally designated  10  for regenerating a particulate filter and/or for regenerating and/or desulfating a NOx adsorber is shown. System  10  depicts a preferred architecture for light-duty diesel architecture applications, e.g., passenger cars. The system  10  generally comprises a fuel source  12  in fluid communication with an inlet  14  of a reformer  16 . An outlet  18  of the reformer  16  is fluidly connected to a reformer conduit  20 . The general flow of reformat from the reformer  16  is indicated by an arrow labeled reformat flow direction. Valves  22 ,  24 , and  26  are dispose in the reformer conduit  20  to selectively provide fluid communication from the reformer  16  to an exhaust conduit  28 . Disposed in serial fluid communication within the exhaust conduit  28  are an oxidation catalyst  30 , an NOx adsorber  32 , and an oxidation catalyst  34  coupled to a particulate filter  36 . Valve  22  provides controlled fluid communication from the reformer  16  to the oxidation catalyst  30 . Valve  24  provides controlled fluid communication from the reformer  16  to the NOx adsorber  32 . Valve  26  provides controlled fluid communication from the reformer  16  to the coupled oxidation catalyst  34  and particulate filter  36 . For light-duty applications, the general directional flow of exhaust fluid from an engine (as shown by an arrow labeled exhaust flow direction) is through the oxidation catalyst  30 , the NOx adsorber  32 , and then the coupled oxidation catalyst  34  and particulate filter  36 . The fluid passing through the exhaust conduit  28  is then discharged into the external environment. 
     FIG. 2 illustrates system  50 , which provides architecture for heavy-duty diesel architecture applications, e.g., over the highway tractors, trucks, and the like. In system  50 , the oxidation catalyst  30 , the NO x  adsorber  32 , and the coupled oxidation catalyst  34  and particulate filter  36  are disposed in serial fluid communication within the exhaust conduit  28 . Thus, the general directional flow of exhaust fluid (as shown by an arrow labeled exhaust flow direction) from the engine to the external environment is through the coupled oxidation catalyst  34  and particulate filter  36 , then through the oxidation catalyst  30 , and then through the NO x  adsorber  32 . 
     Fuel source  12  preferably includes hydrocarbon fuels, including, but not limited to, liquid fuels, such as gasoline, diesel, ethanol, methanol, kerosene, and others; gaseous fuels, such as natural gas, propane, butane, and others; and alternative fuels, such as hydrogen, biofuels, dimethyl ether, and others; and mixtures of at least one of the foregoing fuels. The selection of fuel source  12  is based upon application, expense, availability, and environmental issues relating to fuel source  12 . 
     Reformer  16  generates a reformat gas from the fuel source  12 . The reformat includes hydrogen, carbon monoxide, and other byproducts that may include carbon dioxide. Reformer  16  may be configured for partial oxidation, steam reforming, or dry reforming. Preferably, reformer  16  is configured for partial oxidation. 
     Partial oxidation reformers are based on sub-stoichiometric combustion to achieve the temperatures necessary to reform the hydrocarbon fuel. Decomposition of fuel to primarily hydrogen and carbon monoxide occurs through thermal reactions at temperatures of about 700° C. to about 1,000° C. Catalysts can be used with partial oxidation systems (catalytic partial oxidation) to promote conversion of various sulfur-free fuels, such as ethanol, into a synthetic gas. The use of the catalyst can result in acceleration of the reforming reactions and can provide this effect at lower reaction temperatures than those that would otherwise be required in the absence of a catalyst. An example of the partial oxidation reforming reaction is as follows: 
      CH 4 +½O 2 →CO+2H 2 +heat 
     In contrast, steam configured reformers react fuel and steam (H 2 O) in heated tubes filled with catalysts to convert hydrocarbons in the fuel into primarily hydrogen and carbon monoxide. An example of the steam reforming reaction is as follows: 
     
       
         CH 4 +H 2 O→CO+4H 2   
       
     
     Dry reforming systems form hydrogen and carbon monoxide in the absence of water, for example, by using carbon dioxide. An example of the dry reforming reaction is depicted in the following reaction: 
     
       
         CH 4 +CO 2 →2CO+2H 2   
       
     
     Reformer  16  preferably comprises a catalyst and a substrate. The catalyst can be washcoated, imbibed, impregnated, physisorbed, chemisorbed, precipitated, or otherwise applied to the substrate. Possible catalyst materials include metals, such as platinum, palladium, rhodium, iridium, osmium, ruthenium, and the like, and oxides, mixtures, and alloys comprising at least one of the foregoing metals. 
     The reformer substrate is preferably capable of operating at temperatures up to about 1,200° C.; capable of withstanding reducing and oxidizing environments containing, for example, hydrocarbons, hydrogen, carbon monoxide, water, oxygen, sulfur and sulfur-containing compounds, combustion radicals, such as hydrogen and hydroxyl ions, and the like, and carbon particulate matter; and has sufficient surface area and structural integrity to support the desired catalyst. 
     Materials that can be used as the reformer substrate include alumina, zirconia, cordierite, silicon carbide, metals (e.g., stainless steel, aluminum, and the like), as well as oxides, alloys, cermets, and mixtures comprising at least one of the foregoing materials, with alumina, zirconia, and mixtures comprising alumina and/or zirconia preferred. 
     Although the reformer substrate can have any size or geometry, the size and geometry are preferably chosen to optimize the surface area in the given catalytic converter design parameters. The reformer substrate can have an open cell foam structure, or an extruded honeycomb cell geometry, with the cells being any multi-sided or rounded shape, with substantially square, hexagonal, octagonal or similar geometries preferred due to increased surface area and structural integrity. The substrate is formed into a cell structure with a plurality of cells arranged in a honeycomb pattern using a foam process, and the like. 
     The oxidation catalyst,  30  or  34 , preferably comprises a catalytic metal including, but not limited to, platinum, palladium, ruthenium, rhodium, osmium, iridium, gold, silver, aluminum, gallium, indium, tin, titanium, and other metals, as well as oxides, alloys, salts, and mixtures comprising at least one of the foregoing metals. Moreover, the catalyst utilized for the oxidation catalyst  30  or  34  may also be employed as the catalyst in the NO x  adsorber  32  and particulate filter  36 . 
     The NO x  adsorber  32  generally comprises a porous support, a catalytic metal component, and one or more NO x  trapping materials. Suitable NOx trapping materials include alkali metals, alkaline earth metals, and the like, and combinations comprising at least one of the foregoing. The catalytic metal component and NO x  trapping materials can be washcoated, imbibed, impregnated, physisorbed, chemisorbed, precipitated, or otherwise applied onto and/or within the porous support. 
     The porous support can comprise any material designed for use in a spark ignition or diesel engine environment. Preferably, the porous support is selected to be capable of operating at temperatures up to about 1,200° C.; capable of withstanding exposure to hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide, sulfur and/or sulfur oxides; and has sufficient surface area and structural integrity to support the desired catalyst. Some possible materials include zirconium toughened alumina, cordierite, silicon carbide, metallic foils, alumina sponges, porous glasses, and the like, and mixtures comprising at least one of the foregoing materials, with zirconium toughened alumina preferred. 
     Although the porous support can have any size or geometry, the size and geometry are preferably chosen to optimize surface area in the given catalytic converter design parameters. Generally, the porous support has a honeycomb geometry, with the combs being any multi-sided or rounded shape, with substantially square, triangular, hexagonal, or similar geometries preferred due to ease of manufacturing and increased surface area. 
     The porous support further comprises one or more other support materials suitable for use at the high operation temperatures associated with an internal combustion engine (e.g., up to about 1,200° C.). Such materials include, but are not limited to, aluminates (e.g., hexaaluminates), alumina, and the like, as well as combinations comprising at least one of the foregoing, with gamma-alumina, theta-alumina, delta-alumina, and combinations thereof being preferred. 
     The catalytic metal component comprises precious metals, such as, platinum, rhodium, palladium, ruthenium, iridium and osmium, as well as alloys and combinations comprising at least one of the foregoing metals. Where the catalytic metal component is a combination of rhodium with one or more other metals, the other metals, e.g., palladium, platinum, and the like, are typically present in an amount less than the rhodium. More particularly, with a a platinum/rhodium combination, the catalytic metal component can comprise up to about 95 wt % rhodium and up to about 30 wt % platinum; with about 70 wt % to about 85 wt % rhodium and about 2.5 wt % to about 20 wt % platinum preferred; and about 70 wt % to about 80 wt % rhodium and about 5 wt % to about 10 wt % platinum especially preferred, weight percent based on the total weight of the alloy. 
     In addition to the catalytic metal component, the porous support may be further loaded with one or more NO x  trapping materials, such as alkali metals, alkaline earth metal, and mixtures comprising at least one of the foregoing metals. Suitable trapping materials include barium, lithium, potassium, magnesium, sodium, cesium, strontium, and combinations comprising at least one of the foregoing, with a mixture of barium and potassium being preferred. 
     The particulate filter  36  generally comprises a shell, an insulation material, and a filter element. The insulation material substantially covers the filter element, and the shell substantially covers the insulation material. 
     Possible materials for the shell include ferrous materials, such as ferritic stainless steels. Ferritic stainless steels include stainless steels such as the 400-Series, for example, SS-409, SS-439, and SS-441, and alloys, and combinations comprising at least one of the foregoing stainless steels, with grade SS-409 generally preferred. 
     The insulation material comprises materials such as fiberglass, intumescent materials, non-intumescent materials, ceramic mats, and/or mica based materials, including combinations comprising at least one of the foregoing insulation materials, and the like. 
     The filter element can comprise one or more monoliths, substrates, supports, and the like, comprising a ceramic, metallic, cermet, and carbides, silicides, nitrides, such as silicon carbide, silicon nitride, and the like, or composite material, and the like, and combinations comprising at least one of the foregoing materials. Such materials preferably possess a sufficient porosity to permit passage of reformat through the monolith walls, and yet filter out a substantial portion, if not all of the particulate matter present in the exhaust. 
     Preferably, the filter element includes a catalyst material such as precious metals such as platinum, palladium, rhodium, nickel, iron, cobalt, molybdenum, tungsten, vanadium, niobium, tantalum, their oxides and sulfides, and combinations comprising at least one of the foregoing precious metals and the like. Further, the filter element can optionally include a washcoat material such as aluminum oxide, silicon oxide, zirconium oxide, titanium oxide, cerium oxide, combinations comprising at least one of the following washcoat materials, and the like. 
     In operation of system  10 ,  50 , the reformer  16  converts the fuel from the fuel source  12  to produce a reformate including, among other products, hydrogen and carbon monoxide gases. In a preferred embodiment, the reformer is a partial oxidation reformer. The reformate as it exits the reformer  16  is preferably at a temperature of about 1,000° C. or less. Depending on the particular system architecture, the system may include an optional heat exchanger to reduce the reformate temperature to a temperature effective to cause regeneration and or desulfurization of the particulate component, e.g., NOx adsorber  32 , particulate filter  36 , and the like. The heated reformate can then be used to regenerate and desulfate NO x  adsorber  32  and/or regenerate the particulate filter  36 . For example, in system  10 , when valve  24  is open (valves  22 ,  26  closed), hydrogen and carbon monoxide from the reformer  16  can be fed directly to the exhaust fluid stream entering the NO x  adsorber  32 . The heated reformate allows the NOx adsorber to be regenerated using hydrogen and carbon monoxide. Preferably, the reformate is at a temperature of about 200° C. to about 600° C. as it enters the NOx adsorber  32 , with about a temperature of about 300° C. to about 500° C. even more preferred, with a temperature at about 400° C. most preferred. Valve  24  can be programmed to provide intermittent flow, i.e., a pulse, of hydrogen and carbon monoxide into NO x  adsorber  32 . Valve  24  allows hydrogen and carbon monoxide to flow to NO x  adsorber  32  as needed for regeneration, which can also reduce the amount of fuel consumed during regeneration, when compared to other regeneration processes that use direct injection of fuel as the reducing agent. (In system  50 , valve  26  is opened and valves  22 ,  24  are closed to provide a similar regeneration of the NOx adsorber contained therein.) Additionally, the hydrogen and carbon monoxide can be used for desulfurization purposes. For example, in system  10  when valve  22  is opened hydrogen and carbon monoxide from reformer  16  is fed to oxidation catalyst  30 . Reaction of the hydrogen with the oxidation catalyst creates an exotherm, which heats the exhaust fluid flowing into the NOx adsorber  32 . The exotherm is preferably sufficient to heat the exhaust fluid to a temperature effective to remove sulfur from NO x  adsorber  32 . Preferably, the exhaust fluid as it enters the NOx adsorber  32  is at a temperature of about 60° C. to about 1,000° C., and with about 260° C._to about 460° C. more preferred. Likewise, in system  50 , valve  24  is opened instead of, or in combination with valve  26  to regenerate the oxidation catalyst. 
     The hydrogen gas generated in the reformer  16  can also be used to regenerate the particulate filter  36 . As previously mentioned, the particulate filter  36  is coupled to an oxidation catalyst  34 , which contains a catalyst material and creates an exotherm. The exotherm preferably raises the temperature to a temperature less than or equal to about 550° C., and with less than or equal to about 500° C. more preferred. The heat generated initiates combustion of trapped particulates in the particulate filter  36 . 
     Other possible system architectures are schematically illustrated in FIGS. 3-6, wherein opening and closing selected valves can be used to periodically regenerate and/or desulfurize the various components of the system. FIG. 3 illustrates system  70 , wherein the NO x  adsorber  32 , the coupled oxidation catalyst  34  and particulate filter  36 , and the oxidation catalyst  30 , are serially disposed in the exhaust conduit  28 . In this embodiment, exhaust from an engine flows through the serially disposed components, wherein the reformer  16  can be used to selectively regenerate the NOx adsorber  32  and/or particulate filter  36  and/or desulfurized the NOx adsorber  32   
     FIGS. 4-7 schematically illustrate various system architectures  80 ,  90 ,  100 ,  110 , wherein additional valves  40  and  42  are employed to direct the exhaust flow into a selected one of a duplicate component to provide system redundancy. Reformate is selectively injected into the selected system component by opening the appropriate valve. In this manner, regeneration can occur for the selected system component. 
     The valves  22 ,  24 , and  26  are preferably in electrical communication with an on-board computer. The computer can be programmed such that the NOx adsorber  32  can be regenerated and/or desulfated, or the particulate filter  36  can be regenerated as need. For example, in system  10 , the computer can be programmed to open and/or close any of the valves  22 ,  24 , and/or  26  based upon operating conditions such as idle speed or load, exhaust temperature, pressure differential across the diesel particulate filter, or it can be a time based program. One skilled in the art will appreciate that embodiments of the above mentioned systems could be used for processes that include desulfurization and regeneration of a NO x  adsorber, regeneration of a particulate filter, and the like. 
     Advantageously, the system provides on-demand regeneration capabilities for the oxidation catalyst  30  or  34 , the NOx adsorber  32 , and/or the particulate filter  36 . Additional chemicals do not have to be carried on-board, since reformer  16  can readily produce hydrogen and carbon monoxide, as needed. Further, the use of hydrogen and carbon monoxide also increases NO x  performance at lower temperatures as well as particulate filter regeneration at lower exhaust temperatures. Another advantage may be an increase in fuel efficiency when compared to rich-combustion reduction of a NO x  adsorber. Yet another advantage may be reducing platinum and rhodium loading of the NO x  adsorber, which may lead to substantial cost savings. 
     While the disclosure has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.