Abstract:
A method for remediating a NOx-containing lean diesel emission includes providing a LNT/SCR catalyst system including a SCR catalyst and a first and second LNT. The SCR catalyst is disposed downstream of the second LNT which is disposed downstream of the first LNT. The lean NOx-containing diesel emission is introduced to the first LNT with the NOx being absorbed on to the first LNT forming a substantially NOx-free lean diesel emission. An exotherm generating agent is introduced to the substantially NOx-free diesel emission between the first LNT and the second LNT to form a reactive lean diesel emission. The reactive lean diesel emission is introduced to the second LNT generating a quantity of heat effective for desorbing absorbed NOx. A reducing agent is introduced into the desorbed NOx between the second LNT and SCR catalyst. The desorbed NOx diesel emission is remediated in the SCR catalyst.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/277,285 filed Mar. 23, 2006, which issued as U.S. Pat. No. 7,827,782 on Nov. 9, 2010. 
    
    
     BACKGROUND 
     1. Technical Field 
     One aspect of the present invention relates to a method for remediating emissions. 
     2. Background Art 
     Environmental regulations regarding emissions from engines and electrical generation stations are in effect in many countries. Among the emissions of regulatory concern are oxides of nitrogen. The oxides of nitrogen include, but are not limited to, nitric oxide, NO, and nitrogen dioxide, NO 2 . These compounds are frequently referred to as NOx as prescribed by the United States Environmental Protection Agency. 
     NOx emissions in an emissions stream may occur under lean burn conditions of a diesel engine. In an exhaust system of the diesel engine of a heavy truck, controlling NOx emissions may be a significant challenge due to rapidly changing temperatures and chemical composition of the emissions. Such changes may arise during a rapid acceleration or deceleration of the heavy truck. In such scenario, an inlet emissions temperature of the emissions may change relatively quickly from as low as 130° C. to as high as 600° C. and an air-to-fuel (A/F) ratio of the emissions stream may change relatively quickly from as low as 15 to as high as 100. 
     Aftertreatment systems have been proposed to remediate NOx in the emissions from a diesel engine. These systems suffer from one or more disadvantages. For example, a lean NOx trap may have difficulty providing relatively good NOx storage capacity at relatively low temperatures of the emissions. By contrast, some lean NOx traps may provide effective NOx remediation techniques for engines with lean exhaust when the inlet emissions temperature is predominantly above 300° C. However, the lean NOx trap may reach capacity with regards to its ability to trap NOx. Restoring the trapping capacity of the lean NOx trap may involve purging. Purging the lean NOx trap may involve providing a fuel-rich emission. Providing the fuel-rich emissions throughout the entire diesel engine and exhaust system may waste expensive fuel. 
     Another challenge in remediating NOx in diesel systems is matching the concentration of supplied reducing agent to the concentration of NOx in the emissions during transient operations. During transient operations, such as acceleration and deceleration, the exhaust flow rates, temperature of the emissions, and NOx concentrations can change rapidly. Rapid changes in some cases may be controlled by a selective catalytic reduction (SCR) catalyst. The SCR can store NH 3 , particularly at relatively low temperatures of the emissions. However, at temperatures of 450° C. and above, the SCR effectiveness for controlling NOx emissions in the emissions stream may decrease because the amount of ammonia which the SCR catalyst can store drops to a relatively low level. 
     In light of the foregoing, what is needed is an effective emissions remediation method for NOx emissions suitable for lean air-to-fuel ratio conditions for a relatively wide range of temperature and NOx concentrations in the emissions. 
     What is further needed is an emissions remediation method for NOx emissions that avoids wasting expensive fuel. 
     SUMMARY 
     One aspect of the present invention is a method for remediating a NOx-containing lean diesel emission having a directional flow. The method includes providing a LNT/SCR catalyst system including a SCR catalyst. The catalyst system also includes a first LNT having a first NOx storage capacity and a second LNT having a second NOx storage capacity. The second LNT is disposed downstream of the first LNT relative to the direction of flow of the NOx-containing lean diesel emission. The second LNT has a down-stream exit and fluidly communicates with the first LNT. The SCR catalyst is disposed downstream of and fluidly communicating with second LNT. The SCR catalyst is capable of communicating with the first LNT. A second LNT has a portion of absorbed NOx. The method also includes introducing the lean NOx-containing diesel emission to the first LNT. A portion of the NOx from the NOx-containing lean diesel emission is absorbed on the first LNT to form a substantially NOx-free lean diesel emission exiting downstream from the first LNT. The method also includes introducing an exotherm generating agent (EGA) into the substantially NOx-free lean diesel emission between the first LNT and the second LNT to form a reactive lean diesel emission. An EGA introduction time period. The method also includes introducing the reactive lean diesel emission to the second LNT generating a quantity of heat effective for desorbing a portion of the absorbed NOx from the second LNT to form a lean desorbed NOx emission. The method further includes streaming the lean, desorbed NOx emission downstream from the exit of the second LNT and introducing a reducing agent into the lean, desorbed NOx diesel emission between the second LNT and the SCR catalyst for an RA introduction time period. The method also includes remediating the lean, desorbed NOx diesel emission in the SCR catalyst to obtain a remediated diesel emission. 
     In at least one embodiment, a method for remediating a NOx-containing lean diesel emission includes directing the NOx-containing lean diesel emission into a dual-LNT, reversing flow emission remediation system that includes absorbed NOx. The dual-LNT, reversing flow emission remediation system includes a first LNT, a second LNT disposed serially relative to the first LNT and a SCR catalyst disposed downstream of the second LNT relative to the direction of flow of the NOx-containing lean diesel emission. The remediation system further includes a switching valve disposed between the first LNT, the second LNT and the SCR catalyst. The method also includes maintaining an average lean air-to-fuel ratio throughout the dual-LNT, reversing-flow emission system during all steps of the method. The method absorbing substantially all of the NOx from the lean diesel emission in the emission remediation system to form a substantially NOx-free, lean diesel emission. The method further includes introducing an exotherm generating agent (EGA) into the substantially NOx-free lean diesel emission between the first and second LNTs to form a reactive lean diesel emission. The reactive lean diesel emission reacts to form a lean, desorbed NOx emission in the emission remediation system. A reducing agent (RA) is introduced into the lean, desorbed NOx emission between the SCR catalyst and at least one of the first LNT or the second LNT to form a lean, SCR catalyst-reactive emission. The SCR catalyst-reactive emission is remediated to form a lean, remediated diesel emission. 
     In another embodiment, a LNT/SCR catalyst controlled system for use in remediating a NOx-containing lean diesel emission having a direction of flow and an inlet temperature sensor includes a first LNT having a NOx storage component. The control system also includes a SCR catalyst downstream of the first LNT with respect to the flow of the NOx containing lean diesel emission. Upstream of the first LNT is an exotherm generating agent (EGA) introduction port that is capable of introducing an EGA adjacent to the inlet to the first LNT. Between the first LNT and the SCR catalyst is positioned a reducing agent (RA) introduction port. A signaling device capable of responding to a timed signal or at least one sensor is also included in the control system. The sensor can be at least one of the emission temperature, an inlet temperature, an LNT bed temperature, a rate of emission flow, an emission air-to-fuel ratio, or a NOx concentration sensor. The control system also includes a controller having an engine model and communicating with the EGA and the RA introduction ports. The controller combines one or more signals with the engine model such that either an effective quantity of EGA is introduced at the EGA introduction port in order to release NOx from the NOx storage component or an effective amount of reducing agent is introduced at the RA introduction port when controlling a transient NOx concentration increase. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows an emissions remediation system according to certain embodiments of the present invention; 
         FIGS. 2 and 2   a  show a lean NOx trap plus a layered SCR/LNT system according to certain embodiments of the present invention; 
         FIGS. 3 ,  3   a , and  3   b  show a zoned SCR/LNT system according to certain embodiments of the present invention. 
         FIGS. 4   a  and  4   b  show an emissions remediation system according to certain embodiments of the present invention; and, 
         FIGS. 5   a  and  5   b  show another emissions remediation system according to certain embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to compositions, embodiments, and methods of the present invention known to the inventors. However, it should be understood that disclosed embodiments are merely exemplary of the present invention which may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, rather merely as representative bases for teaching one skilled in the art to variously employ the present invention. 
     Except where expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the present invention. Practice within the numerical limits stated should be desired and independently embodied. 
     The description of a group or class of materials as suitable for a given purpose in connection with the present invention implies that mixtures of any two or more of the members of the group or class are suitable. Description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among constituents of the mixture once mixed. The first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation. Unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property. 
     At least one embodiment of the present invention is a method for remediating an emissions stream containing NOx emissions by using a first lean NOx trap and a layered SCR/LNT. 
     Referring to  FIG. 1 , which describes an overall exemplary environment, the emissions stream  46  ( FIG. 2 ) is produced by an engine  2  and flows out of the engine through an emissions inlet  4 . In some embodiments, the emissions inlet  4  is defined by a casing  6 . The emissions inlet  4  may be considered as positioned on the upstream side  8  of the remediation system  10 . A noble metal containing member  11  is positioned upstream of a SCR/LNT  14 . An exotherm generating agent may be introduced through a first introduction port  16 . A reducing agent may be introduced through a second introduction port  18 . The introduction ports may pass through, be attached to, or be supported by the casing  6 . In some embodiments, the casing  6  may continue beyond the SCR/LNT  14  to an emissions outlet  20  located downstream  22  of the SCR/LNT  14 . 
     Referring to  FIGS. 2 and 2   a , in one embodiment of the present invention the exotherm generating agent introduction port  16  and the reducing agent introduction port  18  are attached to the casing  6 . The exotherm generating agent introduction port  16  is positioned upstream of an LNT  12  portion which precedes the portion having a layered SCR/LNT  24 . This forms a zoned LNT plus a layered SCR/LNT system  21 . The reducing agent introduction port  18  is positioned between the first LNT  12  portion and the portion having the layered SCR/LNT  24 . A substrate  26  may be connected to the casing  6  by a compression-fit web  28 . The layered SCR/LNT  24  may be applied to the substrate  26 . The substrate  26  has a fifth opposing surface  30  adjacent to some portions of a fourth opposing surface  32  of an LNT layer  34 . The LNT layer  34  has a third opposing surface  36  adjacent in some portions to a second opposing surface  38  of an SCR layer  40 . The SCR layer  40  also has a first opposing surface  42  which is adjacent to the open space  44  defined, in part, by the SCR layer  40 . An emissions stream  46  containing NOx emissions is exposed to the SCR layer  40  as the emissions  46  flow through the space  44 . An inlet emissions temperature probe  48  is optionally present in the emissions  46  upstream of the layered SCR/LNT  24 . An optional bed temperature probe  50  is present in a portion of the layered SCR/LNT  24 . It should be understood that temperature may be also signaled by other methods. A non-limiting example of such is a model-based calculation based on engine activity. 
     In this embodiment of the present invention the method for remediation may include the introduction of the exotherm generating agent to the emissions  46  and exposure of the emissions  46  to the noble metal containing member  11  ( FIG. 1 ). The noble metal containing member  11  may include the first lean NOx trap  12 , a lightoff catalyst, a three-way catalyst, a bimetallic catalyst, a noble metal catalyst, or combinations thereof. The introduction of exotherm generating agent in such a manner may allow saving of expensive fuel since the remediation system  10  ( FIG. 1 ) remains substantially lean regarding A/F ratio. The method may also include introduction of the reducing agent to the emissions  46  that subsequently interacts with the SCR layer  40 . 
     In certain embodiments of the present invention, the reducing agent may be introduced into the emissions stream  46  between the LNT  12  and the SCR  24  when the emissions inlet temperature of the emission stream  46  upstream of the LNT  12  is in the range of 130° C. to 600° C. This introduction may overlap introducing the exotherm generating agent into the emissions stream  46  before the LNT  12  when the inlet temperature of the emissions  46  is the range of 300° C. to 600° C. The A/F ratio downstream of the SCR  24  remains greater than or equal to 15 before, during, and after the time period when the exotherm generating agent and reducing agent are introduced. These introductions may relatively improve the remediation of the emissions stream containing NOx emissions when exposing the emissions to the SCR/LNT. 
     In another embodiment of the present invention, a zoned SCR/LNT may include an LNT on a separate substrate upstream relative to an SCR. Referring to  FIGS. 3 ,  3   a , and  3   b , the LNT  91  is formed when an LNT layer  96  is washcoated on to a carbide-whiskered, first stainless steel metal foil substrate  94  which is confined by a first stainless steel case  98 . The SCR  93  is formed when a second stainless steel metal foil substrate  104  is washcoated with an SCR layer  102 . The second stainless steel metal foil is contained in a second stainless steel case  100 . The LNT  91  is welded to the upstream side of the SCR  93  at a weld  106 . The reducing agent introduction port  108  is positioned near the weld  106  upstream of the SCR  93  and downstream of the LNT  91 . The exotherm generating agent introduction port  110  is positioned upstream of the LNT  91 . It should be understood that a cavity may optionally exist between the downstream end of the LNT  91  and the upstream end of the SCR  93 . The reducing agent introduction port  108  may be positioned in such a cavity to allow effective blending of the emissions stream  46  and the reducing agent. 
     In certain embodiments, the remediation system  10  ( FIG. 1 ) may be viewed as a kit where the emissions  46  ( FIG. 2 ) are streaming into an emissions inlet  4  which is defined by the casing  6 . The casing may include a compression-fitting web  28  adjacent to the substrate  26 , and which in certain embodiments may include components of the exhaust train from the engine manifold to the muffler. The casing  6  assists in directing the emissions  46  to portions where catalytic and trapping action occur. The casing  6  also directs the emissions  46  to an emissions outlet  20  which is positioned downstream  22  of the SCR/LNT  14 . Non-limiting examples of such portions where catalytic and trapping actions occur may include a noble metal containing member  11  ( FIG. 1 ) such as the lightoff catalyst, the first lean NOx trap  12  ( FIG. 2 ); and/or the LNT portion  98  ( FIG. 3 ) of the SCR/LNT  92 . The lightoff catalyst may include a noble metal, a bimetallic or a three-way catalyst. 
     In certain embodiments of the present invention, the remediation occurs under essentially assured lean emissions conditions. The A/F ratio averaged over the remediation system  10  (FIG.  1 )could be greater than 15, 20, 25, 30, 35, or 40, and less than 100, 90, 80, 70, 60, or 50, with the range selected independently from these values. 
     In certain embodiments of the present invention, introduction of the reducing agent can expose the selective catalytic reduction catalyst, either as the SCR layer  40  ( FIG. 2 ) of the SCR/LNT  24 ; or the SCR  93  ( FIG. 3 ) portion of the zoned SCR/LNT  92 , to the reducing agent. Non-limiting examples of reducing agents may include ammonia, ammonium compounds, hydrazine, urea, or combinations thereof. 
     In certain embodiments of the present invention, the exotherm generating agent may be introduced to the emissions  46  ( FIG. 2 ). The introduction of the exotherm generating agent can expose the lean NOx trap either as a first LNT  12  ( FIG. 2 ) portion; or LNT  91  ( FIG. 3 ) portion of the zoned SCR/LNT  92 . Non-limiting examples of the exotherm generating agents may include fuel; emissions exhaust, such as exhaust gas recirculation (EGR); or combinations thereof. Non-limiting examples of fuel may include hydrocarbons, aliphatic compounds, cycloaliphatic compounds, aromatic compounds, alkanes, gasoline, alcohols, propane, biofuels, biodiesel, diesel fuel, propylene, petroleum distillates, liquefied petroleum gas, natural gas, or combinations thereof. 
     Operating costs may be relatively reduced when the exotherm generating agent is introduced only intermittently to the LNT such as at times when the LNT approaches a fraction of storage capacity used. This fraction may be greater than 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% of the storage capacity available. Exceeding the fraction of storage capacity may trigger a regeneration signal. The regeneration signal may cause the introduction of the exotherm generating agent. Non-limiting examples of means to trigger a regeneration signal for the introduction of the exotherm generating agent include a timer; a NOx sensor, such as a chemiluminescent detector or an electrochemical sensor; a signal from a model-based calculation of the engine output and the amount of NOx stored; or combinations thereof. 
     In certain embodiments of the present invention, non-limiting examples of the SCR layer  40  (FIG.  2 )of the SCR/LNT  24 ; or the SCR  93  ( FIG. 3 ) portion of the zoned SCR/LNT  92  may include one or more of the following compounds: a silica, an alumina, a hydrated alumina compound, an activated alumina compound, a mullite, a cordierite, a steatite, a clay, or combinations thereof, and one or more compounds chosen from a lanthanide metal compound, a transition metal compound, or combinations thereof. 
     In certain embodiments of the present invention, non-limiting examples of the lean NOx trap layer  34  (FIG.  2 ); the first LNT  12  portion ( FIG. 2 ); or the LNT  91 ( FIG. 3 ) portion of the zoned SCR/LNT  92  may include of one or more of the following compounds: an alkali metal compound, an alkali-earth metal compound, silica, alumina, hydrated alumina compounds, activated alumina compounds, mullite, cordierite, steatite, clay, or combinations thereof; and one or more of the following compounds: a catalytic component such as a noble metal compound; lanthanide metal compounds; transition metal compounds; or combinations thereof. 
     Non-limiting examples of the substrate  26  ( FIG. 2 ) used in certain embodiments include a particulate trap, a wire mesh, a whiskered non-corroding metal, a coated metal foil, a cordierite substrate, an aluminum titanate substrate, a mullite substrate, a kyanite substrate, an aluminosiloxane substrate, a magnesium aluminum silicate substrate, a refractory ceramic substrate, a ceramic substrate, a technical ceramic substrate, a honeycombed wall-flow monolith having alternating ends open and closed, a honeycombed ceramic monolith having both ends open, a ceramic foam, a zirconia substrate, a silica substrate, a silicon carbide substrate, or a zeolite. 
     The formulations of the SCR/LNT may vary with the emissions to which they could be exposed. As a non-limiting example, formulations can have metal compound additives to mitigate sulfur compounds. Sulfur compounds in emissions may poison or diminish the catalytic and/or trapping activity. Non-limiting examples of emissions to which the SCR/LNT may be exposed include those which may emanate from combustion processes such as found in a diesel truck; a gasoline powered vehicle; a portable hydrocarbon-powered device, such as a generator, a lawn mower, a snow mobile, a chain saw, as well as a one-, two-, or four-cycle engine; an electric power generation plant; a gas turbine; an airplane; a locomotive; a boat; a personal watercraft; and a ship. Emissions  46  (FIG.  2 ) can arise from a hybrid power system such as a vehicle intermittently using a combustion process with other power sources such as electricity, liquefied petroleum gas, natural gas, fuel cells, and solar power. 
     Referring to  FIGS. 4   a  and  4   b , a remediation system  114  is illustrated showing an embodiment of the positioning of various components of the system. The remediation system  114  of this illustration may take the following actions when the emissions inlet temperature is greater than 400° C. In this illustration, the casing  113  supports a first LNT  120 , a second LNT  124 , and an SCR  128 . Also, supported by the casing  113  are a first port  122  for introducing an exotherm generating agent and a second port  126  for introducing a reducing agent. An emissions stream  112  flows into the casing  113  and is directed to a channel  132  by a valve  116  on a pivot  118 . The first LNT  120  is exposed to the emissions stream  112 . Downstream of the first LNT  120 , an exotherm generating agent is introduced to the emissions stream  112  at the first port  122 . The exotherm generating agent may be introduced for an exotherm generating agent introduction period that may be independently selected from a range consisting of from 0.5, 1, 2, 3, 4, and 5 seconds to 7, 8, 9, 10, 12, 15, 20, 30, 40, 50, and 60 seconds. In certain embodiments, this introduction period may correspond to a second time segment associated with a remediation sequence, such as in Examples 9 and 10. The exotherm introduction period may be delayed after the switching of the valve  116  by an exotherm generating agent introduction delay period that may be independently selected from a range consisting of from 0, 1, 2, 3, 4, 5, and 6 seconds to 7, 8, 9, 10, 12, 15, 20, and 30 seconds. In certain embodiments, this introduction period may correspond to a first time segment associated with the remediation sequence, such as in Examples 9 and 10. The A/F ratio of the emissions stream  112  is greater than or equal to 15 during the exotherm generating agent introduction period. It should be understood that the A/F ratio values do not consider any particular relatively localized A/F ratio, but rather describe a vicinity average. The emissions stream  112  including the exotherm generating agent is exposed to the second LNT  124 . The exotherm generating agent generates heat at the second LNT  124 . If NOx has been trapped on the second LNT  124 , it should be purged by the heat. Downstream of the second LNT  124 , the reducing agent is introduced at the second port  126 . The emission  112  is exposed to the SCR  128 . At the SCR  128  the remainder of the NOx is remediated. The SCR  128  may be comprised of a zoned SCR. It should be understood that SCR  128  could be a layered SCR/LNT or a combination zoned and layered, but purging efficiency may be reduced if sufficient heating cannot be provided by convection even when augmented with other heating elements such as electrical heating. The emission  112  exits at the emissions outlet  130 . 
     In certain embodiments, after a first switching period, the range of which may be independently selected from at least 10, 20, 30, 40, or 50 seconds to at least 60, 70, 80, 90, 100, 150, 200, or 300 seconds, the valve  116  is pivoted on pivot  118  to a channel-directed position  134 , and the emissions cease entering the channel  132 , and commence entering the channel  136 . This switch starts the second switching period. The second LNT  124  is exposed to the emissions stream  112 . Downstream of the second LNT  124 , an exotherm generating agent is introduced to the emissions stream  112  at the first port  122  while maintaining the air to fuel ratio at greater than or equal to 15. The emissions stream  112  including the exotherm generating agent is exposed to the first LNT  120 . The exotherm generating agent generates heat at the first LNT  120 . If NOx has been trapped on the first LNT  120 , it may be released by the heat. Downstream of the first LNT  120 , the reducing agent is introduced at the second port  126 . The emission  112  is exposed to the SCR  128 . At the SCR  128  the remainder of the NOx may be remediated. 
     While the second LNT  124  is exposed to the emission  112 , the exotherm, which is caused by exposing the first LNT  120  to the exotherm generating agent, is purging the first LNT  120 . The purging continues during the second switching period. The second switching period, the range of which may be independently selected from at least 10, 20, 30, 40, or 50 seconds to at least 70, 80, 90, 100, 150, 200, or 300 seconds, ends when the valve  116  is switched back from the position  134  to the position shown in  FIG. 4   a.    
     The valve  116  may be any type of emission diverting device. Non-limiting examples include a four-way valve, an electronically-controlled valve, a baffle, or combinations thereof. 
     In certain embodiments when a four-way valve is used, a remediation sequence described below may be used. A non-limiting example of the remediation sequence may start with the first time segment having a duration independently selected from 1, 3, 5, or 7 seconds to 8, 10, 12, 15, 20, and 30 seconds. During the first time segment, the reducing agent introduction may precondition the SCR  128 . The end of the first time segment, the second time segment begins allowing the introduction of both the exothermic generating agent and reducing agent. The second time segment may have a duration independently selected from 1, 3, 5, or 7 seconds to 8, 10, 12, 15, 20, and 30 seconds. After the end of the second time segment, a third time segment begins and has a duration of independently selected from 1, 3, 5, or 7 seconds to 8, 10, 12, 15, 20, and 30 seconds during which only the reducing agent is introduced. While not wishing to be bound by any particular theory, this may help to reduce the bleeding of NOx from the downstream end of the NOx trap. After the end of the third time segment, a fourth time segment begins. During the fourth time segment, neither reducing agent nor exotherm generating agent are introduced. The fourth time segment may have a duration independently selected from 15, 20, 25, 30 or 34 seconds to 35, 40, 50, 60, 100, and 297 seconds. A total cycle time of the remediation actions described above for certain embodiments of the present invention may comprise the switch period. The length of time periods may vary depending upon the types of SCR and LNT used as well as the engine mathematical models based on the actual engine and emission conditions. 
     In certain embodiments of the present invention, during periods when the emission temperature is less than 400° C., introduction of the exotherm generating agent at the first port  122  may cease and the valve  116  will direct emissions into the channel  132  or channel  136  without switching. 
     Referring to  FIGS. 5   a  and  5   b , a remediation system  158  is illustrated. As a non-limiting illustration, the remediation system  158  may involve the following actions when the emissions inlet temperature is greater than 400° C. In this illustration, the casing  164  supports a first LNT  146 , a second LNT  150 , a first SCR  144 , and a second SCR  148 . Also, supported by the casing  164  are a first port  170  for introducing an exotherm generating agent, a second port  140  for introducing a reducing agent for a channel  166 , a third port  142  for introducing a reducing agent for a channel  168 , and a low emission temperature reducing agent port  138 . Further, as a non-limiting example, the reducing agent may optionally be introduced at the low emission temperature reducing agent port  138  with a continuous, semi-continuous, pulsed, or intermittent period when the emissions inlet temperature is less than 400° C. 
     As a non-limiting example, the remediation system  158  may involve the following actions when the emissions inlet temperature is greater than 400° C. In this example, an emissions stream  160  flows into the casing  164  and is directed to the channel  166  by a valve  152 . The first SCR  144  is exposed to the emissions stream  160 . Downstream of the first SCR  144 , the first LNT  146  is exposed to the emissions stream  160 . Downstream of the first LNT  146 , an exotherm generating agent is introduced to the emissions stream  160  at the first port  170 . The emissions stream  160 , which includes the exotherm generating agent, is exposed to the second LNT  150 . The exotherm generating agent generates heat at the second LNT  150 . If NOx has been trapped on the second LNT  150 , it is purged by the heat. Downstream of the second LNT  150 , the reducing agent is introduced at the third port  142 . The emissions stream  160  is exposed to the second SCR  148 . At the second SCR  148  the remainder of the NOx is remediated. The second SCR  148  may be comprised of a zoned SCR, a layered SCR/LNT, or a combination thereof. The emissions stream  160  exits at the emissions outlet  162 . 
     In certain embodiments of the present invention, after the first switching period, the range of which may be independently selected from at least 10, 20, 30, 40, or 50 seconds to at least 70, 80, 90, 100, 150, 200, or 300 seconds, the valve  152  is switched to the position  154 , and the emissions cease entering the channel  166 , and commence entering the channel  168 . This switch starts the second switching period. The second SCR  148  is exposed to the emissions stream  160 . Downstream of the second SCR  148 , the second LNT  150  is exposed to the emissions stream  160 . Downstream of the second LNT  146 , an exotherm generating agent is introduced to the emissions stream  160  at the first port  170 . The emissions stream  160  including the exotherm generating agent is exposed to the first LNT  146 . The exotherm generating agent generates heat at the first LNT  146 . If NOx has been trapped on the first LNT  146 , it is purged by the heat. Downstream of the first LNT  146 , the reducing agent is introduced at the second port  140 . The emissions stream  160  is exposed to the first SCR  144 . At the first SCR  144  the remainder of the NOx is remediated. The first SCR  144  may be comprised of a zoned SCR, a layered SCR/LNT, or a combination thereof. The emissions stream  160  exits at the emissions outlet  162 . 
     While the second LNT  150  is exposed to emissions stream  160 , the exotherm, which is caused by exposing the first LNT  146  to the exotherm generating agent, is purging the first LNT  146 . The purging continues during the second switching period. In certain embodiments, the second switching period, the range of which may be independently selected from at least 10, 20, 30, 40, or 50 seconds to at least 70, 80, 90, 100, 150, 200, or 300 seconds, ends when the valve  152  is switched back from the position  154  to the position shown in  FIG. 5   a.    
     In certain embodiments of the present invention, during periods when the emission temperature is less than 400° C., introduction of the exotherm generating agent at the first port  170  may cease and the valve  152  will direct emissions into the channel  166  without switching. 
     The reducing agent may then be introduced at the low emission temperature reducing agent port  138 . When the emission temperature is less than 400° C., the reducing agent may be introduced continuously, in a pulsed fashion, or when introduction is signaled by a sensor and/or a computer model. The continuous introduction should be understood to include introductions that are substantially continuous including high frequency introduction, introductions with modulations in flow, introductions with brief gaps, intended or not, and combinations thereof. 
     In certain embodiments of the present invention additional reducing agent may be introduced to augment remediation of NOx. As a non-limiting example, when the emissions inlet temperature is less than 400° C., reducing agent may also be introduced at port  142  when the emissions stream  160  is directed to the channel  166 . While not wishing to be bound by any particular theory, this introduction of reducing agent at the third port  142  may allow the SCR layer of the layered SCR/LNT  148  to reduce additional NOx. 
     In certain embodiments of the present invention the valve  116  ( FIG. 4   a ) and/or the valve  152  ( FIG. 5   a ) may help establish time periods during remediation. As a non-limiting example of a sequence of remediation actions, a remediation sequence may start with the first time segment having a duration independently selected from 1, 3, 5, or 7 seconds to 8, 10, 12, 15, 20, and 30 seconds. During the first time segment, the reducing agent introduction may precondition the SCR. The end of the first time segment, the second time segment begins allowing the introduction of both the exothermic generating agent and reducing agent. The second time segment may have a duration independently selected from 1, 3, 5, or 7 seconds to 8, 10, 12, 15, 20, and 30 seconds. After the end of the second time segment, a third time segment begins and has a duration of independently selected from 1, 3, 5, or 7 seconds to 8, 10, 12, 15, 20, and 30 seconds during which only the reducing agent is introduced. While not wishing to be bound by any particular theory, this may help to reduce the bleeding of NOx from the downstream end of the NOx trap. After the end of the third time segment, a fourth time segment begins. During the fourth time segment, neither reducing agent nor exotherm generating agent are introduced. The fourth time segment may have a duration independently selected from 15, 20, 25, 30 or 34 seconds to 35, 40, 50, 60, 100, and 297 seconds. A total cycle time of the four-way valve actions described above for certain embodiments of the present invention may comprise the switch period. The length of time periods may vary depending upon the types of SCR and LNT used as well as the engine mathematical models based on the actual engine and emission conditions. 
     EXAMPLE 1 
     This example of the SCR/LNT shows the mitigation of the emissions control problem for mobile sources such as the diesel-powered truck using the method and remediation system of certain embodiments of the present invention. 
     A relatively difficult control scenario arises when controls try to meter ammonia or urea to match the concentration of NOx during transient engine operations. Transient engine operations may cause the exhaust flow rate, the temperature of the emissions and the NOx concentration to change rapidly. In certain embodiments, to provide relatively high NOx conversion regardless of the concentration of NOx emissions contained in the emissions, the SCR/LNT will make use of its SCR capability, its LNT capability, or both. 
     Some aspects of this example depend on the exhaust temperature. At the relatively low inlet temperature of emissions, such as in the range of 150 to 400° C., the SCR properties of the SCR/LNT are predominantly being used for remediation by NOx conversion. Relatively high conversions of NOx can be achieved with SCR technology at temperatures as low as 180-200° C. when ammonia is introduced to the emissions. In addition, the SCR layer  40  ( FIG. 2 ) is able to store relatively large quantities of ammonia at these temperatures using its reducing agent storage component. Therefore, the ammonia does not have to be injected at relatively high concentrations into the exhaust to continually match the concentration of NOx from the occasional peak engine emission. If the NOx concentration decreases suddenly, such as during a deceleration transient operation, excess ammonia may be stored in the SCR layer  40 . In the converse, if the NOx concentration increases rapidly, such as during an acceleration transient operation, ammonia stored in the SCR layer  40  may supplement the introduced reducing agent to reduce the NOx. 
     At an intermediate inlet temperature of the emissions  46  ( FIG. 2 ), such as 400-550° C., the ability of the SCR layer  40  to store ammonia may decrease significantly. As a possible result, precisely matching the NOx concentration from the engine with the amount of ammonia injected may present a control problem. As a non-limiting example, during sudden acceleration transient operations, where emissions flow rates and NOx concentrations may increase rapidly, introducing insufficient amounts of NH 3  may lead to NOx breakthrough to the emissions outlet  20  ( FIG. 1 ). In the converse, during a sudden deceleration from a high-load condition, the emissions flow rate and NOx concentrations contained in the emissions  46  may decrease rapidly. If the catalyst bed temperature has been previously heated to these intermediate temperatures, the SCR layer  40  may be relatively unable to store effectively any excess NH 3 . As a consequence, over-introduction of NH 3  may lead to NH 3  slip emissions to the emissions outlet  20 . Further, over-introduction may waste the reducing agent, thereby requiring more frequent and more costly refillings. 
     Instead of relatively great reliance on the remediation function of the SCR in this intermediate inlet temperature range of emissions, the SCR/LNT may use its NOx storage component of the LNT to store the NOx. While not wishing to be bound by any particular theory, the NOx is probably stored as nitrates in the washcoat. 
     At relatively high inlet temperature of the emissions  46 , such as a range greater than 550° C. to 600° C., the SCR/LNT may still make use of its NOx storage capability to store NOx. The stored NOx may be purged by injecting enough exotherm generating agent into the emissions  46  to produce a net rich A/F ratio. 
     EXAMPLE 2 
     In this example, the LNT portion of the SCR/LNT, either the first LNT  12  ( FIG. 2 ), the LNT layer  34  portion of the layered SCR/LNT  24 , and/or the zoned LNT  91  ( FIG. 3 ), may approach its capacity to store NOx. In some cases, the LNT may need to be purged in order to restore the NOx storage capacity. In certain embodiments of the present invention, the LNT may be purged when the exotherm generating agent, such as diesel fuel, is introduced into the emissions  46  ( FIG. 2 ) upstream of the noble metal containing member such as the lightoff catalyst, the first LNT  12  ( FIG. 2 ), the LNT layer ( FIG. 2 ) portion of the layered SCR/LNT  24 , and/or the zoned LNT  91  ( FIG. 3 ), while maintaining the overall lean A/F ratio in the emissions stream  46  ( FIG. 2 ). While not wishing to be bound by any particular theory, the fuel may combust to increase the temperature of the emissions  46  which may purge the LNT of stored NOx. The oxidation of the hydrocarbons by the noble metal component may produce the exotherm which may cause the thermal release of NOx that may be stored on the LNT. This purge method may take advantage of the decreasing NOx storage capacity of the lean NOx trap with increasing temperature. 
     Expensive fuel may be conserved by this method because the fuel may be introduced only as needed in such quantities as needed to increase the emissions  46  ( FIG. 2 ) temperature. The method may avoid using relatively greater quantities of fuel to alter the emissions  46  from the lean A/F ratio to a rich A/F ratio. 
     A non-limiting example of how this purging method interacts with the integrated SCR/LNT is described. The combined remediation system incorporates several interacting remediation and control methods in order to accommodate the wide ranges of environmental properties in the emissions  46 . The emissions  46  are remediated by the introduction of relatively smaller amounts of the reducing agent during all temperature regimes. When the lean NOx trap approaches capacity or the effectiveness of the lean NOx trap decreases, the exotherm generating agent may be introduced to purge the NOx so that it can react with the reducing agent over the SCR portion. The exotherm generating agent, in this example, hydrocarbons, may be injected upstream of the first LNT  12  ( FIG. 2 ) while maintaining an overall lean mixture in the engine and portions of the exhaust system. Ammonia may be introduced into the emissions at approximately the same time as the hydrocarbon introduction. The second introduction port  18  ( FIG. 2 ) from where the ammonia may be introduced may be positioned after the first LNT  12  portion and upstream of the SCR/LNT  24 , providing reducing agent to reduce the NOx in the lean mixture when the emissions  46  is exposed to the SCR  40  portion of the SCR/LNT  24 . 
     The introduction of ammonia and/or fuel may only be triggered periodically in this example. Thus, the method may offer advantages in terms of control during transient driving conditions and conservation of expensive fuel. 
     In a non-limiting example, the introduction may occur during a period when the engine is at a steady-state condition or a semi-steady state condition. The amount of NO x  stored on the LNT can be calculated from lookup tables relating the engine-out NO x  to the speed and load conditions, and therefore the amount of NH 3  needed to reduce the NO x  can be determined. 
     EXAMPLE 3 
     In this example, the layered SCR/LNT is made. The LNT layer washcoat can be applied on the honeycombed substrate of mullite with channels open at both ends. The wash coat is dried in place. The LNT is then completed by impregnation of the washcoat with noble metal solutions to yield a product having 150 grams of noble metal, expressed as metal, per cubic foot of dried, washcoated LNT. The SCR washcoat layer is applied over at least portions of the LNT layer washcoat. As part of the washcoating process, the SCR washcoat layer is dried. 
     EXAMPLE 4 
     In this example, the zoned SCR/LNT  92  ( FIG. 3 ) is made. The LNT  96  layer washcoat is applied on the first stainless steel corrugated foil  94  having carbide whiskers and contained within a stainless steel case  98  with channels open at both ends. The wash coat is dried in place. The LNT  91  is then completed by impregnation of the washcoat with noble metal solutions to yield a product having less than 150 grams of noble metal, expressed as metal, per cubic foot of dried, washcoated LNT. The washcoat is then dried. 
     The SCR  102  layer washcoat is applied on the second stainless steel corrugated foil  104  having carbide whiskers and contained within the second stainless steel case  100  with channels open at both ends. As part of the washcoating process, the layer is dried. 
     The two separate stainless steel cases are welded together with the LNT  91  located upstream of the SCR  93 . The introduction port  108  of the reducing agents may be located near the weld  106  and between the LNT  91  and SCR  93 . The introduction port  110  of the exotherm generating agent may be located upstream of the LNT  91 . It should be understood that the LNT  91  and SCR  93  need not be adjacent each other. A gap between them may be present without changing the intent of the present invention. Without wishing to be bound by any particular theory, such a gap may facilitate mixing of the reducing agent with the emissions  46  ( FIG. 2 ). 
     EXAMPLE 5 
     This example is drawn to a laboratory reactor design for studying the SCR/LNT. Nitric oxide, NO 2 , CO 2 , O 2 , N 2 , and H 2 O are premixed before passing through the SCR/LNT sample placed in a vertical furnace. Hydrocarbons and/or NH 3  can be injected into the feed gas either periodically or continuously using electronically-controlled solenoids valves. Thermocouples are used to measure the inlet and bed temperatures of the SCR/LNT sample. After passing through the sample, the exhaust is diluted 10:1 with N 2  to reduce any water concentration and prevent water condensation in unheated sections of the tubing or the gas analyzers. The diluted exhaust is then analyzed for CO 2 , CO, hydrocarbons, O 2 , NOx, NO, and N 2 O, Separate NOx analyzers are used to measure NOx and NO so that the fractions of NO and NO 2  can be determined. The analyzers can be calibrated using span gases either as supplied or after the span gas has been diluted 10:1 with N 2 . A fraction of the diluted exhaust is heated to 600° C. over a platinum catalyst to oxidize NH 3  to NO, and this treated exhaust stream can be analyzed with a third NOx analyzer. The difference of this reading and the reading from the other NOx analyzers can provide a measure of the NH 3  concentration. 
     EXAMPLE 6 
     This example shows the feasibility of using hydrocarbon oxidation to cause the thermal release of NOx from the LNT. This example also demonstrates that, with the proper formulation, the LNT can be designed to provide maximum NOx storage capacity in the range of temperature independently selected from at least 350° C. and less than 600° C. In this example, the LNT is used to store NOx for 60 seconds. Propylene is injected as the fuel into the feed gas for a period of 15 seconds while maintaining an overall lean A/F ratio. This cycle is repeated continually as the temperature dropped from 550 to 200° C. over a period of approximately two hours. During the non-injection periods, the NOx at the reactor exit is at lower concentration than in the feed gas level. During the hydrocarbon fuel injection periods, NOx is released from the trap. The reducing agent, ammonia, is injected into the exhaust and then is used to reduce this released NOx over the SCR catalyst. 
     The average storage efficiency during the last 45 seconds of the 60 second non-introduction periods is a function of temperature for a barium-only formulation and for a formulation containing barium and an alkali metal. Results are shown in Table 1 for 5, 10, and 15-seconds fuel injection periods for a barium plus alkali metal trap provided by a catalyst supplier as well as a 5-second injection period for a barium-only trap. The barium-only formulation provides peak storage efficiency near 300° C., while the barium/alkali metal formulation provided peak storage efficiency closer to 400° C. Also, the barium-only formulation provides relatively high storage efficiency with only 5-second fuel injection periods, while the barium plus alkali metal formulation required up to 15-second fuel injection periods to provide relatively lower storage efficiencies. The barium plus alkali metal formulation has a NOx storage capacity of 46 mg/in 3  at its peak temperature of 400° C., while the barium-only formulation has a storage capacity near 29 mg/in 3  at its peak temperature of 300° C. It is also relatively easier to thermally purge the trap with lower capacity. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 AVERAGE NO x  STORAGE EFFICIENCY IN LAST 45-SEC OF 
               
               
                 60-SEC CYCLE IN PERCENTAGE AS A FUNCTION OF FUEL INJECTION 
               
               
                 TIME, LNT FORMULATION, AND BED TEMPERATURE 
               
             
          
           
               
                   
                   
                 BARIUM PLUS 
                 BARIUM PLUS 
                 BARIUM PLUS 
               
               
                   
                 BARIUM- 
                 ALKALI- 
                 ALKALI- 
                 ALKALI- 
               
               
                 AVERAGE BED 
                 ONLY LNT 
                 METAL LNT 
                 METAL LNT 
                 METAL LNT 
               
               
                 TEMPERATURE 
                 5-SEC FUEL 
                 5-SEC FUEL 
                 10-SEC FUEL 
                 15-SEC FUEL 
               
               
                 (° C.) 
                 PER 60-SEC 
                 PER 60-SEC 
                 PER 60-SEC 
                 PER 60-SEC 
               
               
                   
               
               
                 200 
                 40% 
                 10% 
                 14% 
                 14% 
               
               
                 250 
                 97% 
                 25% 
                 41% 
                 54% 
               
               
                 300 
                 100%  
                 50% 
                 73% 
                 79% 
               
               
                 350 
                 78% 
                 60% 
                 78% 
                 87% 
               
               
                 400 
                 56% 
                 63% 
                 85% 
                 92% 
               
               
                 450 
                 37% 
                 54% 
                 78% 
                 83% 
               
               
                 500 
                 30% 
                 40% 
                 58% 
                 66% 
               
               
                 550 
                 18% 
                 28% 
                 42% 
                 48% 
               
               
                 600 
                  5% 
                 14% 
                 30% 
                 36% 
               
               
                   
               
             
          
         
       
     
     EXAMPLE 7 
     This example shows the NOx conversion of a fully-formulated SCR catalyst from a second catalyst supplier. The conversion is measured as a function of temperature using either all NO, a 50/50 mix of NO and NO 2 , or all NO 2 . Ammonia is injected at approximately 95% of the NOx concentration to prevent NH 3  slip. The NOx conversion exceeds 90% at temperatures between 275° C. and 500° C. when using a 50/50 by mass mix of NO and NO 2 . The conversion obtained when the same mass of only NO 2  is introduced is lower than the conversion obtained with same mass of NO or the 50/50 by mass mix of NO and NO 2 . The conversion with the same concentration of NO 2  achieved a maximum of 86% NOx conversion. 
     EXAMPLE 8 
     This example shows and assesses the effect of positioning the introduction ports for reducing and exotherm generating agents when releasing NOx thermally from the LNT with hydrocarbon oxidation and then converting the released NOx over the SCR with ammonia in the zoned SCR/LNT configuration. The LNT is placed in front of a SCR catalyst. Propylene and ammonia are injected in front of the LNT. However, instead of reducing the NOx during the hydrocarbon injection periods the NOx actually increased. While not wishing to be limited by any one theory, the increase may be due to oxidation of the ammonia over the platinum in the LNT. Placing the ammonia introduction port downstream of the LNT prevents oxidation of the ammonia over the LNT. In a similar manner, placing the hydrocarbon introduction port upstream of the LNT may prevent hydrocarbons from deteriorating the effectiveness of the SCR. 
     EXAMPLE 9 
     This example of certain embodiments of the present invention illustrates a method of using zoned SCR/LNTs in a dual-LNT reversing-flow emission remediation system. A non-limiting example of the dual-LNT reversing-flow emission remediation system is illustrated in  FIGS. 4   a  and  4   b . When the NOx storage capacity of the first LNT  120  has been reduced sufficiently to warrant purging of the first LNT  120 , the valve  116  is positioned in a second switch position as illustrated in  FIG. 4   b . In this example, the valve  116  is an electronically-controlled four-way valve. The switching of the four-way valve initiates four time segments of a remediation sequence which comprise each switching period. The time segments are effective at temperatures above 400° C. During the second switch period, when the first LNT  120  is being purged, a first time segment begins when the reducing agent, in this example, urea, is introduced from the second port  126  upstream of the SCR  128  to precondition the SCR  128 . After 5-10 seconds, a second time segment of the remediation sequence begins. During the second time segment, the exotherm generating agent introduction begins while the reducing agent introduction continues. The exotherm generating agent is introduced into the emission at the first port  122 . The second time segment is 10 seconds in duration. As the first LNT  120  is exposed to the exotherm generating agent, an exotherm is generated on the front portion of the first LNT  120  and causes NOx to be thermally released from that portion of the first LNT  120 . After the second time segment, a third time segment of the remediation sequence begins. During the third time segment, the introduction of the exotherm generating agent ceases, while the reducing agent introduction continues. The third time segment duration is 5-15 seconds. While not wishing to be bound by any particular theory, the continued release of the reducing agent during the third time segment may help to reduce the bleeding of NOx from the downstream end of the NOx trap. After the third time segment, the remediation sequence begins the fourth time segment. During the fourth time segment, the reducing agent introduction ceases. The first LNT  120  is exposed to the emissions stream  112  comprising approximately NOx-free emissions coming from the second LNT  124 . Without wishing to be limited by any particular theory, this wait period may allow the exothermic heat generated in the front part of the first LNT  120  to begin to be transported downstream and purge the NOx stored in the rear part of the first LNT  120 . After the fourth time segment, the first LNT  120  will be purged and ready to store NOx again. While not wishing to be limited by any particular explanation, the desire that the LNT  120  be exposed to relatively limited amounts of NOx arises from the desire to prevent the adsorption of NOx on to the first LNT  120  during the third and fourth time segments, which may decrease the NOx storage efficiency when the NOx is re-introduced after the next valve switch. The fourth time segment is approximately 45-50 seconds in duration. Combined the duration of the first, second, third, and fourth time segments of the remediation sequence comprise the duration of the second switch period, which is 60 seconds. 
     The first switch period begins when the four-way valve switches to the first switch position illustrated in  FIG. 4   a . With the four way valve in this position, each of the four time segments are repeated again. The first switch period is 60 seconds and ends when the four-way valve switches back to the second switch position. 
     EXAMPLE 10 
     This example of certain embodiments of the present invention illustrates using layered SCR/LNTs in a dual-LNT reversing-flow emission remediation system. The switch periods and time segments of this Example may be similar to the switch periods and time segments of Example 9. A non-limiting example of the dual-LNT reversing-flow emission remediation system having the first and second SCRs being layered SCR/LNTs is illustrated in  FIGS. 5   a  and  5   b . When the NOx storage capacity of the first LNT  146  has been reduced sufficiently to warrant purging of the first LNT  146 , the valve  152  is positioned in a second switch position as illustrated in  FIG. 5   b . In this example, the valve  152  is an electronically-controlled four-way valve. The four-way valve yields four time segments which comprise each switching period. The time segments are effective at temperatures above 400° C. During the second switch period for purging the first LNT  146 , a first time segment begins when the reducing agent, in this example, urea, is introduced from the second port  140  upstream of the SCR  144  to precondition the SCR  144 . After 5-10 seconds, a second time segment of the valve begins. During the second time segment, the exotherm generating agent introduction begins at first port  170  while the reducing agent introduction continues. The second time segment is 10 seconds in duration. As the first LNT  146  and the LNT layer of the first SCR  144  are exposed to the exotherm generating agent, an exotherm is generated on the front portion of the first LNT  146  and the LNT portion of the first SCR  144  and causes NOx to be thermally released from that portion of the first LNT  146  and the first SCR  144 . After the second time segment, a third time segment of the valve  152  begins. During the third time segment, the introduction of the exotherm generating agent ceases, while the reducing agent introduction continues. The third time segment duration is 5-15 seconds. While not wishing to be bound by any particular theory, the continued release of the reducing agent during the third time segment may help to reduce the bleeding of NOx from the downstream end of the NOx trap. After the third time segment, the valve  152  begins the fourth time segment. During the fourth time segment, the reducing agent introduction ceases or is gradually diminished to match the concentration of NOx released from the first LNT  146  and/or the LNT layer of the SCR  144 . The first LNT  146  and the LNT layer of the SCR  144  is exposed to the emissions stream  160  comprising approximately NOx-free emissions coming from the second LNT  150 . Without wishing to be limited by any particular theory, these illustrated third and fourth time segments may allow the exothermic heat generated in the front part of the LNT  146  to be transported downstream and purge the NOx stored in the rear part of the first LNT  146 . After the fourth time segment, the first LNT  146  will be purged and ready to store NOx again. While not wishing to be limited by any particular explanation, the desire that the first LNT  146  be exposed to relatively limited amounts of NOx arises from the desire to prevent the adsorption of NOx on to the first LNT  146  or the LNT layer of the SCR  144  during the third and fourth time segments, which may decrease the NOx storage efficiency when the NOx is re-introduced during the second switch period. The fourth time segment is approximately 45-50 seconds in duration. Combined the duration of the first, second, third, and fourth time segments comprise the duration of the second switch period, which is 60 seconds. 
     The first switch period begins when the valve  152  switches to the first switch position illustrated in  FIG. 5   a . With the valve  152  in this position, each of the four time segments are repeated again. The first switch period is 60 seconds and ends when the valve  152  switches back to the second switch position. 
     It should be understood that while a layered SCR/LNT is used in this example for the first SCR  144  and the second SCR  148 , the first SCR  144  and the second SCR  148  could also be a zoned SCR or a combination zoned and layered SCR/LNT. 
     After approximately 60 seconds, approximately when the NOx storage capacity of the second LNT  150  has been reduced sufficiently to warrant purging of the second LNT  150 , the valve  152  switches back to a position illustrated in  FIG. 5   a . The emission flow  160  switches back to the channel  166 . In this positioning, the exhaust flows over the first SCR  144  and the first LNT  146  passing the first port  170  to expose the second LNT  150  and the LNT layer of the second SCR  148 , which are positioned in the channel  168 . The exotherm generating agent, in this non-limiting example, hydrocarbons, are introduced to the emissions from the first port  170 . The hydrocarbons are oxidized on the second LNT  150  positioned in the channel  168 , releasing NOx from the second LNT  150  and from the second SCR  148 . Ammonia, a non-limiting example of the reducing agent, can be introduced at the third port  142  positioned between the second LNT  150  and the second layered SCR/LNT  148  relatively simultaneously with the introduction of hydrocarbon. 
     After approximately 60 seconds, approximately when the NOx storage capacity of the first LNT  146  and the LNT layer of the first SCR  144  have been reduced sufficiently to warrant purging of the first LNT  146 , the valve  152  switches back to a position illustrated in  FIG. 5   b ; and the remediation and purging processes repeat. 
     EXAMPLE 11 
     This example of certain embodiments of the present invention illustrates options for methods of remediation when operating at temperatures below 400° C. The remediation system of Example 10 may cease to the introduction of hydrocarbons from the first port  170 . The valve  152  is switched to flow emissions into the channel  166  as shown in  FIG. 5   a . Ammonia or other reducing agent can be introduced continuously at the third port  142  positioned relatively upstream, in certain embodiments, of the second SCR  148  to reduce the NOx over the SCR layer. In addition, ammonia or other reducing agent may optionally be introduced continuously at the low emission temperature reducing agent port  138 . 
     While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention.