Abstract:
A power generation system comprises a diesel engine and a fuel reformer configured to receive the engine exhaust. Two or more separate LNT bricks are configured in a parallel valveless arrangement wherein each simultaneously receives a separate portion of the exhaust leaving the fuel reformer. The LNTs are each adapted and configured to simultaneously store NO x  when the exhaust from the fuel reformer is lean and to simultaneously reduce stored NO x  and regenerate when the exhaust from the fuel reformer contains reformate. This parallel multi-brick arrangement reduces the effective length to width ratio of the LNTs as a group without the packaging difficulties associated with a single LNT having an equivalently reduced length to width ratio. Axial temperature gradients that develop in the LNTs during desulfation are thereby mitigated.

Description:
PRIORITY  
       [0001]     This application is a continuation-in-part of U.S. application Ser. No. 11/223,589, filed Sep. 10, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to pollution control devices for diesel engines.  
       BACKGROUND  
       [0003]     NO x  and particulate matter (soot) emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NO x  and particulate matter emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. Diesel particulate filters (DPFs) have been proposed for controlling particulate matter emissions. A number of different solutions have been proposed for controlling NO x  emissions.  
         [0004]     In gasoline-powered vehicles that use stoichiometric fuel-air mixtures, NO x  emissions can be controlled using three-way catalysts. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.  
         [0005]     One set of approaches for controlling NO x  emissions from diesel-powered vehicles involves limiting the creation of pollutants. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful in reducing NO x  emissions, but these techniques alone are not sufficient. Another set of approaches involves removing NO x  from the vehicle exhaust. These approaches include the use of lean-burn NO x  catalysts, selective catalytic reduction (SCR), and lean NO x  traps (LNTs).  
         [0006]     Lean-burn NO x  catalysts promote the reduction of NO x  under oxygen-rich conditions. Reduction of NO x  in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NO x  catalyst that has the required activity, durability, and operating temperature range. Lean-burn NO x  catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NO x  catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NO x  conversion efficiencies for lean-burn NO x  catalysts are unacceptably low.  
         [0007]     SCR generally refers to selective catalytic reduction of NO x  by ammonia. The reaction takes place even in an oxidizing environment. The NO x  can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NO x  reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.  
         [0008]     To clarify the state of a sometimes ambiguous nomenclature, in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NO x  catalyst” are occasionally used interchangeably. Where the term “SCR” is used to refer just to ammonia-SCR, as it often is, SCR is a special case of lean NO x  catalysis. Commonly, when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NO x  catalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.  
         [0009]     LNTs are devices that adsorb NO x  under lean exhaust conditions and reduce and release the adsorbed NO x  under rich exhaust conditions. A LNT generally includes a NO x  adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO 3  and the catalyst is typically a combination of precious metals, such as Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NO x  adsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NO x  is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to remove accumulated NO x  and thereby regenerate (denitrate) the LNT.  
         [0010]     Creating a reducing environment for LNT regeneration involves eliminating most of the oxygen from the exhaust and providing a reducing agent. Except when the engine can be run stoichiometric or rich, a portion of the reductant reacts within the exhaust to consume oxygen. The amount of oxygen to be removed by reaction with reductant can be reduced in various ways. If the engine is equipped with an intake air throttle, the throttle can be used. However, at least in the case of a diesel engine, it is generally necessary to eliminate some of the oxygen in the exhaust by combustion or reforming reactions with reductant that is injected into the exhaust.  
         [0011]     The reactions between reductant and oxygen can take place in the LNT, but it is generally preferred that the reactions occur in a catalyst upstream from the LNT, whereby the heat of reaction does not cause large temperature increases within the LNT at every regeneration.  
         [0012]     Reductant can be injected into the exhaust by the engine fuel injectors or by separate injection devices. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. Alternatively, or in addition, reductant can be injected into the exhaust downstream of the engine.  
         [0013]     U.S. Pat. No. 7,082,753 (hereinafter “the &#39;753 patent”) describes an exhaust treatment system with a fuel reformer placed in the exhaust line upstream from a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate.  
         [0014]     The operation of a fuel reformer can be modeled in terms of the following three reactions:
 
0.684CH 1.85 +O 2 →0.684CO 2 +0.632H 2 O  (1)
 
0.316CH 1.85 +0.316H 2 0→0.316CO+0.608H 2   (2)
 
0.316CO+0.316H 2 O→0.316CO 2 +0.316H 2   (3)
 
 wherein CH 1.85  represents an exemplary reductant, such as diesel fuel, with a 1.85 ratio between carbon and hydrogen. Reaction (1) is exothermic complete combustion by which oxygen is consumed. Reaction (2) is endothermic steam reforming. Reaction (3) is the water gas shift reaction, which is comparatively thermal neutral and is not of great importance in the present disclosure, as both CO and H 2  are effective for regeneration. 
 
         [0015]     The inline reformer of the &#39;753 patent is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby Reaction (1) takes place. After warm up, the fuel injection rate is increased to provide a rich exhaust.  
         [0016]     Depending on such factors as the exhaust oxygen concentration, the fuel injection rate, and the exhaust temperature, the inline reformer of the &#39;753 patent tends to either heat or cool as reformate is produced. In theory, heating can be limited by increasing the fuel injection rate and thereby increasing the rate of endothermic reaction (2). In practice, due to differences in the locations at which reactions (1) and (2) occur and limitations on one more of heat transfer rates, reformer reaction rates, and the efficiency with which an LNT can use reformate, the reformer cannot always be cooled in this manner. As an alternative, the &#39;753 patent suggests pulsing the fuel injection to the reformer during LNT regenerations. The reformer cools between fuel pulses and thereby remains within an acceptable operating temperature range.  
         [0017]     During denitrations, much of the adsorbed NO x  is reduced to N 2 , although a portion of the adsorbed NO x  is released without having been reduced and another portion of the adsorbed NO x  is deeply reduced to ammonia. The NO x  release occurs primarily at the beginning of the regeneration. The ammonia production has generally been observed towards the end of the regeneration.  
         [0018]     U.S. Pat. No. 6,732,507 proposes a hybrid system in which a SCR catalyst is configured downstream from the LNT in order to utilize the ammonia released during denitration. The LNT is provided with more reductant over the course of regeneration than is required to remove the accumulated NO x  in order to facilitate ammonia production. The ammonia is utilized to reduce NO x  slipping past the LNT and thereby improves conversion efficiency over a stand-alone LNT.  
         [0019]     U.S. Pat. Pub. No. 2004/0076565 (hereinafter “the &#39;565 publication”) also describes hybrid systems combining LNT and SCR catalysts. In order to increase ammonia production, it is proposed to reduce the rhodium loading of the LNT. In order to reduce the NO x  release at the beginning of the regeneration, it is proposed to eliminate oxygen storage capacity from the LNT.  
         [0020]     In addition to accumulating NO x , LNTs accumulate SO x . SO x  is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SO x  produced by combustion is significant. SO x  adsorbs more strongly than NO x  and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. In the case of a lean-burn gasoline engine, the temperature of the exhaust can generally be elevated by engine measures. In the case of a diesel engine, however, it is generally necessary to provide additional heat. Typically, this heat can be provided through the same types of reactions as those used to remove excess oxygen from the exhaust. Once the LNT is sufficiently heated, the exhaust is made rich by measures like those used for LNT denitration. If an inline reformer is used to make the exhaust rich for LNT desulfation, it may be necessary to pulse the fuel injection over the course of desulfation to prevent the fuel reformer from overheating.  
         [0021]     In spite of advances, a long felt need exists for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NO x  emissions from diesel engines to an extent that meets U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.  
       SUMMARY  
       [0022]     One of the inventor&#39;s concepts relates to a power generation system, comprising a diesel engine and a fuel reformer configured to receive the exhaust from the diesel engine. Two or more separate LNT bricks are configured in a parallel valveless arrangement so that each simultaneously receives a separate portion of the exhaust leaving the fuel reformer. The LNTs are each adapted and configured to simultaneously store NO x  when the exhaust from the fuel reformer is lean and to simultaneously reduce stored NO x  and regenerate when the exhaust from the fuel reformer contains reformate. This parallel multi-brick arrangement reduces the effective length to width ratio of the LNTs as a group without the packaging difficulties that occur when equivalently reducing the length to width ratio with a single LNT brick.  
         [0023]     A small length to width ratio is particularly useful in this system for reducing axial temperature gradients within the LNTs during desulfation. When fuel injection is pulsed to limit the inline reformer temperature, it has been observed that significant axial temperature gradients develop within the downstream LNTs; their temperatures increase along the direction of flow. Desulfation rates are highly sensitive to temperature. Having the temperatures increasing along the direction of flow can substantially prolong desulfation and concomitant thermal degradation of the LNTs, particularly considering that sulfur deposits primarily at the fronts of the LNTs, where the LNTs are coolest. Reducing the length to width ratio ameliorates these gradients. Multiple LNT bricks in a parallel valveless arrangement are largely equivalent to a single LNT with a very small length to width ratio, but can be packaged more easily than the single brick.  
         [0024]     Another concept relates to a method of operating a power generation system. The method comprises operating a diesel engine to produce an exhaust containing NO x  and SO x . The exhaust is channeled through a plurality of LNTs, each comprising a separate brick and each receiving a separate portion of the exhaust flow. The LNTs adsorb and store a first portion of NO x  and a portion of the SO x  from the exhaust. The exhaust from these LNTs is passed through one or more SCR catalysts that reduce a second portion of NO x  in the exhaust by reactions with ammonia under lean conditions. The method further comprises generating a first control signal to denitrate one or more of the LNTs. In response to the control signal, a rich exhaust is supplied to the one or more of the LNTs, whereby adsorbed NO x  is reduced producing ammonia-containing exhaust. The ammonia containing exhaust is passed through one or more of the SCR catalysts, whereby the SCR catalysts adsorb and store ammonia. A second control signal to desulfate one or more of the LNTs is also eventually generated. In response to the second control signal, one or more LNTs are regenerated by heating them and making the exhaust supplying them rich. The manner of making the exhaust rich is such that the temperatures in the LNTs being desulfated increase in the direction of the exhaust flow. The provision of multiple LNTs each receiving a separate portion of the exhaust flow mitigates the temperature gradients that develop in the LNTs during desulfation.  
         [0025]     The primary purpose of this summary has been to present certain of the inventor&#39;s concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventor&#39;s concepts or every combination of the inventor&#39;s concepts that can be considered “invention”. Other concepts of the inventor will be conveyed to one of ordinary skill in the art by the following detailed description together with the drawings. The specifics disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventor claims as his invention being reserved for the claims that follow. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a schematic illustration of an exemplary power generation system.  
         [0027]      FIG. 2  is a plot of temperatures and reductant concentration in a comparison power generation over the course of LNT desulfation.  
         [0028]      FIG. 3  is a schematic of the power generation system that produced the data plotted in FIG. ( 2 ).  
         [0029]      FIG. 4  is a schematic illustration of another exemplary power generation system.  
         [0030]      FIG. 5  is a schematic illustration of yet another exemplary power generation system.  
         [0031]      FIG. 6  is a schematic illustration of a further exemplary power generation system. 
     
    
     DETAILED DESCRIPTION  
       [0032]      FIG. 1  is a schematic of an exemplary power generation system  100  embodying one of the inventor&#39;s concepts. The power generation system  100  comprises an engine  101  and an exhaust aftertreatment system  102 . The exhaust aftertreatment system  102  includes a controller  103 , a fuel injector  104 , a fuel reformer  105 , a plurality of lean NO x -traps (LNT)  106  (including at least two LNTs  106  more specifically identified as  106   a  and  106   b ), and a plurality of ammonia-SCR catalysts  107 . The controller  103  may be an engine control unit (ECU) that also controls the exhaust aftertreatment system  102  or may include several control units that collectively perform these functions.  
         [0033]     During lean operation (a lean phase), the LNTs  106  adsorb a first portion of the NO x  from the exhaust. The ammonia-SCR catalysts  107  may have ammonia stored from a previous regeneration of the LNTs  106  (a rich phase). If the ammonia-SCR catalysts  107  contain stored ammonia, they remove a second portion of the NO x  from the lean exhaust.  
         [0034]     From time to time, the LNTs  106  must be regenerated in a rich phase to remove accumulated NO x  (denitrated). Denitration may involve heating the reformer  105  to an operational temperature and then injecting fuel using the fuel injector  104  to make the exhaust rich. The fuel reformer  105  uses the injected fuel to consume most of the oxygen from the exhaust while producing reformate. The reformate thus produced reduces NO x  adsorbed in the LNTs  106 . Some of this NO x  is reduced to NH 3 , most of which is captured by the ammonia-SCR catalysts  107  and used to reduce NO x  during a subsequent lean phase.  
         [0035]     From time to time, the LNTs  106  must also be regenerated to remove accumulated sulfur compounds (desulfated). Desulfation involves heating the reformer  105  to an operational temperature, heating the LNTs  106  to a desulfating temperature, and providing the heated LNTs  106  with a rich atmosphere. Desulfating temperatures vary, but are typically in the range from about 500 to about 800° C., with optimal temperatures typically in the range of about 650 to about 750 ° C. Below a minimum temperature, desulfation is very slow. Above a maximum temperature, the LNTs  106  may be damaged.  
         [0036]     The primary means of heating the LNTs  106  is heat convection from the reformer  105 . To generate this heat, fuel can be supplied to the reformer  105  under lean conditions, whereby the fuel combusts in the reformer  105 . Once the reformer  105  is heated, the fuel injection rate can be controlled to maintain the temperature of the reformer  105  while the LNTs  106  are heating.  
         [0037]     The LNTs  106  can also be heated in part by combustion within them. Heating the LNTs  106  in part in this way reduces the peak temperatures at which the reformer  105  must be operated. One method of achieving combustion within the LNTs  106  is to design and operate the fuel reformer  105  in such a way that a portion of the fuel supplied to the fuel reformer  105  slips to the LNTs  106 . For example, the catalyst loading of the fuel reformer  105  or its mass transfer coefficient can be kept low to facilitate this mechanism. Another method of achieving combustion in the LNTs  106  is to use rapid cycling between rich and lean phases. Oxygen for the lean phases can mix with fuel or reformate from the rich phases to combust in the LNTs  106 . This mixing and combustion can be facilitated by a capacity of the LNTs  106  to adsorb reductants or oxygen.  
         [0038]     Even when the LNTs  106  are not specifically designed to adsorb either reductants or oxygen, it has become evident that when fuel is pulsed to the fuel reformer  105  in order to maintain its temperature over the course of a desulfation, reductant and oxygen mix and combust in the LNTs  106 . Data regarding this phenomenon are provided in  FIG. 2 .  
         [0039]     The data in  FIG. 2  were gathered for a power generation system  300  configured as illustrated in  FIG. 3 . In the system  300  of  FIG. 3 , two LNT bricks  106   a  and  106   b  are arranged in series. The LNTs  106  are provided in two separate bricks in the system  300  to give a target total LNT volume using conventionally sized LNT bricks. During desulfation, the fuel injection is pulsed to give the reformate concentration profile illustrated by line  201  (CO) and line  202  (H 2 ) in  FIG. 2 . Line  203  plots temperature readings obtained from a thermocouple in the LNT brick  106   a  2.5 cm from its entrance. Line  204  plots temperature readings obtained from a thermocouple in the LNT brick  106 i a 2.5 cm from its exit. Line  205  plots temperature readings obtained from a thermocouple in the LNT brick  106   b  2.5 cm from its exit. Both LNTs were about 24 cm long and 15 cm in diameter. The plots show that peak temperatures increase along the direction of flow, with peak temperatures near the exit of the two brick system being about 150° C. higher than peak temperatures near the front of the system.  
         [0040]     The inventor&#39;s concept is to replace a series arrangement of LNTs such as illustrated by  FIG. 3  with a parallel arrangement of LNTs such as illustrated by  FIG. 1 . By reducing the collective lengths of the LNTs  106 , the axial temperature gradients can be ameliorated. Temperatures still increase along the direction of flow when fuel injection is pulsed, but to a lesser degree. Axial conduction through the substrates of the LNT bricks smoothes the temperature profiles. The area available for this transport is increased and the distance over which heat must be transported is reduced when the LNTs  106  are arranged in parallel.  
         [0041]     For simplicity of representation,  FIG. 1  shows only two separate LNT bricks arranged in parallel. Preferably, however, more than two separate LNT bricks are used in order to achieve a very small overall effective length to width ratio for the LNT in comparison to the length to width ratios of the individual LNT bricks. Preferably, three or more LNTs bricks are used. More preferably, four or more separate LNT bricks are used.  
         [0042]     Preferably, the equivalent diameter to equivalent length ratio of the LNTs  106  collectively is at least about two, more preferably at least about three, and still more preferably at least about four. Equivalent diameter and equivalent length are calculated on the basis of a single cylindrical LNT brick having the same total frontal area and total volume as the LNTs  106  collectively. The equivalent diameter is obtained by dividing the total frontal area of the LNTs  106  by pi, taking the square root, and multiplying by two. The equivalent length is obtained by dividing the total volume of the LNTs  106  by the total frontal area of the LNTs  106 .  
         [0043]     Each of the LNTs  106  is preferably a separate monolith brick. A monolith is a structure providing an array of parallel passages. A brick is a cohesive unit, for example, an extruded structure or a structure formed by rolling one or more stacked sheets of metal into a cylinder. Monolith bricks generally have aspect ratios from about 0.5 to about 2.0, with a 1.0 aspect ratio being typical. These dimensions provide structural stability. Bricks with aspect ratios greater than 2.0 are less strong and are more difficult to manufacture and effectively can. Typical diameters and lengths of monolith bricks range from about 15 cm to about 36 cm. According to the present concept, shorter bricks are preferable, e.g., bricks from about 7 cm to about 15 cm in length.  
         [0044]     Each brick preferably provides a high degree of axial heat conduction per unit of surface area. Combustion that produces heat occurs at a rate proportional to the surface area whether the rate of combustion is kinetically or mass transfer rate controlled. For high porosity monoliths, increasing the wall thickness increases the degree of axial heat conduction. Metal conducts heat better than ceramic. A preferred LNT brick according to the inventor&#39;s concept is constructed with relatively thick metal walls. A thick metal wall is about 100 μm or thicker, preferably about 200 μm or thicker, more preferably about 400 μm or thicker.  
         [0045]     The benefit of arranging LNTs  106  in parallel can be realized whether or not the LNTs  106  are desulfated one at a time. In the power generation system  100 , the LNTs  106  are desulfated simultaneously using a single reductant source. One advantage of the power generation system  106  is that it can be constructed and operated without exhaust system valves. Exhaust valves are undesirable because they lack durability and reliability. Mobile dampers are within the scope of valves for the purpose of this description. The system  106  divides the flow among the various branches passively; the division of flow is independent of the control signals that trigger regeneration.  
         [0046]      FIG. 4  is a schematic of an exemplary power generation system  400  illustrating a second embodiment of the inventor&#39;s concept. The most significant difference between this embodiment and that exemplified by the power generation system  100  is that in the power generation system  400  each LNT  106  is provided with an independent mechanism for making the exhaust supplying it rich, in this case a separate inline reformer  105  for each of the exhaust branches  109 . This configuration allows one or more of the LNTs  106  to be regenerated independently of the others.  
         [0047]     A significant advantage of independently regenerating the LNTs  106  is that rich exhaust from LNTs  106  being regenerated can be combined with lean exhaust from LNTs  106  not being regenerated. Oxygen from the lean exhaust can be used to oxidized residual reductants, slipping NO, and H 2 S in the rich exhaust.  
         [0048]     NO tends to slip from the LNTs  106  being regenerated, particularly at the start of a regeneration. Some of this NO may be reduced in the SCR catalysts  107 . Some, however, is not so reduced either because of limitations on the catalyst efficiency or on the amount of available ammonia. NO is environmentally more harmful than NO 2 . Oxidizing untreated NO to NO 2  improves the overall performance of the exhaust treatment system.  
         [0049]     H 2 S may slip from the LNTs  106  during desulfation. H 2 S has an offensive odor even in very small concentrations. By oxidizing this H 2 S to SO 2 , the unpleasant odor can be avoided.  
         [0050]     Additional benefits are realized if the SCR catalysts  107  are arranged after the point in the exhaust line where the lean and rich flows are combined.  FIG. 5  is a schematic of an exemplary power generation system  500  in which the flow is combined while the SCR catalyst  107  still consists of multiple separate bricks in a parallel arrangement. This embodiment realizes the benefits of a combined flow and an arrangement of SCR catalysts  107  that fits compactly with the arrangement of LNTs  106  contemplated by the inventor.  
         [0051]     One benefit of combining the flows of separately regenerated LNTs  106  prior to supplying the combined flow to SCR catalysts  107  is that ammonia produced by the LNTs  106  during the regenerations is distributed to SCR catalysts  107  more evenly in time. This more even distribution in time increases the efficiency with which the ammonia is used. In the case of a single LNT  106  followed by a single SCR catalyst  107 , the ammonia concentration in the SCR catalyst  107  is highest immediately following regeneration. Immediately following regeneration, NO x  slip from the LNT  106  is generally at its lowest. As a result, much of the ammonia remains in the SCR catalyst  107  for an extended period prior to being used to reduce NO x . Over this period, a significant portion of the stored ammonia can be lost to decomposition. By staggering the regenerations and spreading out the times over which the LNT bricks  106  are regenerated and ammonia is produced, the average time that ammonia must be stored in the SCR catalysts  107  is significantly reduced, which results in increased ammonia utilization.  
         [0052]     Another benefit is that the environment of the SCR catalysts  107  can be maintained continuously lean. SCR catalysts function more effectively in the presence of oxygen. Maintaining a continuously lean environment in the SCR catalyst  107  can improve its performance and reduce NO x  slip during regenerations.  
         [0053]     In the exemplary power generation systems  100 ,  400 , and  500 , the exhaust is made rich using inline reformers  105 . The concepts, however, extend to methods of making the exhaust rich that do not include or entirely rely upon inline reformers. The engine  101  can be used remove excess oxygen from the exhaust: the engine  101  could be operated with a stoichiometric or rich fuel-air mixture, if the engine is of such a design that this is possible. Reformate or another reductant other than diesel fuel can be injected into the exhaust. Excess oxygen can be removed by combustion of reductant in a device other than a fuel reformer  105 , such as an oxidation or three-way catalyst. In addition, it should be noted that diesel fuel can be injected into the exhaust by an engine fuel injector rather than by an exhaust line fuel injector.  
         [0054]     At least one DPF will typically be included in a diesel exhaust aftertreatment system. The DPF can be placed at any suitable location. Examples of suitable locations are upstream from the fuel reformer  105 , between the fuel reformer  105  and the LNTs  106 , between the LNTs  106  and the SCR catalysts  107 , and downstream from the SCR catalysts  107 . A potential advantage of placing the DPF upstream from the LNTs  106  is that NO x  concentrations are high, facilitating continuous regeneration. A potential advantage of placing the DPF downstream from the fuel reformer  105  is that oxidation of NO to NO 2  in the fuel reformer  105  can facilitate DPF regeneration. Also, if placed downstream from the fuel reformer  105 , the fuel reformer  105  can be used to heat the DPF for intermittent regeneration.  
         [0055]     If the DPF is placed between the fuel reformer  105  and the LNTs  106 , the DPF can provide a thermal mass ameliorating temperature excursion in the LNTs  106  during denitrations. Repeated exposure to high temperatures can reduce the life of the LNTs  106 . Between the LNTs  106  and the SCR catalysts  107 , the DPF can have a similar effect: protecting the SCR catalysts  107  from desulfation temperatures; some SCR catalysts undergo degradation if exposed to desulfation temperatures. Downstream from the SCR catalysts  107  may be a preferred location if the DPF has a catalyst that could oxidize NH 3 . The preferred location for the DPF depends on the type of DPF and other particulars of the various system components.  
         [0056]      FIG. 6  provides a schematic illustration of an exemplary power generation system  600  comprising an exhaust treatment system  602  in which a DPF  108  is configured. Other components of the system  600  are the same as described for the system  500 . The DPF  108  is placed downstream from the LNTs  106  at a point where the exhaust flow is unified. This configuration allows a continuously lean environment to be maintained in the DPF  108 , provided the LNTs  106  are not all regenerated simultaneously. The environment in the SCR catalyst  107  would also be continuously lean. A lean environment allows the DPF  108  to be regenerated simultaneously with desulfation of one or more of the LNTs  106 . Heat from the desulfations helps achieve soot combustion. Consumption of oxygen in one or more of the LNTs  106  reduces the risk the DPF  108  will overheat at internal hot spots.  
         [0057]     A DPF can be a wall flow filter or a pass through filter and can use primarily either depth filtration or cake filtration. Cake filtration is the primary filter mechanism in a wall flow filter. In a wall flow filter, the soot-containing exhaust is forced to pass through a porous medium. Typical pore diameters are from about 0.1 to about 1.0 μm. Soot particles are most commonly from about 10 to about 50 nm in diameter. In a fresh wall flow filter, the initial removal is by depth filtration, with soot becoming trapped within the porous structure. Quickly, however, the soot forms a continuous layer on an outer surface of the porous structure. Subsequent filtration is through the filter cake and the filter cake itself determines the filtration efficiency. As a result, the filtration efficiency increases over time.  
         [0058]     In contrast to a wall flow filter, in a flow through filter the exhaust is channeled through macroscopic passages and the primary mechanism of soot trapping is depth filtration. The passages may have rough walls, baffles, and bends designed to increase the tendency of momentum to drive soot particles against or into the walls, but the flow is not forced though micro-pores. The resulting soot removal is considered depth filtration, although the soot is generally not distributed uniformly with the depth of any structure of the filter. A flow through filter can also be made from temperature resistant fibers, such as ceramic or metallic fibers, that span the device channels. A flow through filter can be larger than a wall flow filter having equivalent thermal mass  
         [0059]     Diesel particulate filters must be regenerated from time-to-time to remove accumulated soot. Two general approaches to DPF regeneration are continuous and intermittent regeneration. In continuous regeneration, a catalyst is provided upstream from the DPF to convert NO to NO 2 . N 0   2  can oxidize soot at typical diesel exhaust temperatures and thereby effectuate continuous regeneration. Intermittent regeneration involves heating the DPF to a temperature at which soot combustion is self-sustaining in a lean environment. Typically this is a temperature from about 400 to about 600° C., depending in part on what type of catalyst coating has been applied to the DPF to lower the soot ignition temperature.  
         [0060]     While the engine  9  is preferably a compression ignition diesel engine, the various concepts of the inventor are applicable to power generation systems with lean-burn gasoline engines or any other type of engine that produces an oxygen rich, NO x -containing exhaust. For purposes of the present disclosure, NO x  consists of NO and NO 2 .  
         [0061]     The power generation system can have any suitable type of transmission. A transmission can be a conventional transmission such as a counter-shaft type mechanical transmission, but is preferably a CVT. A CVT can provide a much larger selection of operating points than can a conventional transmission and generally also provides a broader range of torque multipliers. The range of available operating points can be used to control the exhaust conditions, such as the oxygen flow rate and the exhaust hydrocarbon content. A given power demand can be met by a range of torque multiplier-engine speed combinations. A point in this range that gives acceptable engine performance while best meeting a control objective, such as minimum oxygen flow rate, can be selected. In general, a CVT prevents or minimizes power interruptions during shifting.  
         [0062]     Examples of CVT systems include hydrostatic transmissions, rolling contact traction drives, overrunning clutch designs, electrics, multispeed gear boxes with slipping clutches, and V-belt traction drives. A CVT may involve power splitting and may also include a multi-step transmission.  
         [0063]     A preferred CVT provides a wide range of torque multiplication ratios, reduces the need for shifting in comparison to a conventional transmission, and subjects the CVT to only a fraction of the peak torque levels produced by the engine. These can be achieved using a step-down gear set to reduce the torque passing through the CVT. Torque from the CVT passes through a step-up gear set that restores the torque. The CVT is further protected by splitting the torque from the engine, and recombining the torque in a planetary gear set. The planetary gear set mixes or combines a direct torque element transmitted from the engine through a stepped automatic transmission with a torque element from a CVT, such as a band-type CVT. The combination provides an overall CVT in which only a portion of the torque passes through the band-type CVT.  
         [0064]     The fuel reformer  105  is a device that converts heavier fuels into lighter compounds without fully combusting the fuel. The fuel reformer  105  can be a catalytic reformer or a plasma reformer. Preferably, the fuel reformer  105  is a partial oxidation catalytic reformer comprising a steam reforming catalyst. Examples of reformer catalysts include precious metals, such as Pt, Pd, and Rh, and oxides of Al, Mg, and Ni, the latter group being typically combined with one or more of CaO, K 2 O, and a rare earth metal such as Ce to increase activity. The fuel reformer  105  is preferably small compared to an oxidation catalyst that is designed to perform its primary functions at temperatures below 450° C. The reformer  105  is generally operative at temperatures within the range of about 450to about 1100° C.  
         [0065]     The LNTs  106  can comprise any suitable NO x -adsorbing material. Examples of NO x  adsorbing materials include oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. Further examples of NO x -adsorbing materials include molecular sieves, such as zeolites, alumina, silica, and activated carbon. Still further examples include metal phosphates, such as phosphates of titanium and zirconium. Generally, the NO x -absorbing material is an alkaline earth oxide. The adsorbent is typically combined with a binder and either formed into a self-supporting structure or applied as a coating over an inert substrate.  
         [0066]     The LNTs  106  also comprise a catalyst for the reduction of NO x  in a reducing environment. The catalyst can be, for example, one or more transition metals, such as Au, Ag, and Cu, group VIII metals, such as Pt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical catalyst includes Pt and Rh. Precious metal catalysts also facilitate the adsorbent function of alkaline earth oxide adsorbers.  
         [0067]     Adsorbents and catalysts according to the present invention are generally adapted for use in vehicle exhaust systems. Vehicle exhaust systems create restriction on weight, dimensions, and durability. For example, a NO x  adsorbent bed for a vehicle exhaust system must be reasonably resistant to degradation under the vibrations encountered during vehicle operation.  
         [0068]     The ammonia-SCR catalysts  107  are catalysts functional to catalyze reactions between NO x  and NH 3  to reduce NO x  to N 2  in lean exhaust. Examples of SCR catalysts include oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Mo, W, and Ce, zeolites, such as ZSM-5 or ZSM-11, substituted with metal ions such as cations of Cu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCR catalysts  107  are designed to tolerate temperatures required to desulfate the LNTs  106 .  
         [0069]     Although not illustrated in any of the figures, a clean-up catalyst can be placed downstream from the other aftertreatment device. A clean-up catalyst is preferably functional to oxidize unburned hydrocarbons from the engine  101 , unused reductants, and any H 2 S released from the LNTs  106  and not oxidized by the ammonia-SCR catalyst  107 . Any suitable oxidation catalyst can be used. To allow the clean-up catalyst to function under rich conditions, the catalyst may include an oxygen-storing component, such as ceria. Removal of H 2 S, when required, may be facilitated by one or more additional components such as NiO, Fe 2 O 3 , MnO 2 , CoO, and CrO 2 .  
         [0070]     The invention as delineated by the following claims has been shown and/or described in terms of certain concepts, components, and features. While a particular component or feature may have been disclosed herein with respect to only one of several concepts or examples or in both broad and narrow terms, the components or features in their broad or narrow conceptions may be combined with one or more other components or features in their broad or narrow conceptions wherein such a combination would be recognized as logical by one of ordinary skill in the art. Also, this one specification may describe more than one invention and the following claims do not necessarily encompass every concept, aspect, embodiment, or example described herein.