US20070056268A1

US20070056268A1 - LNT-SCR packaging

Info

Publication number: US20070056268A1
Application number: US11/582,039
Authority: US
Inventors: James McCarthy
Original assignee: Eaton Corp
Current assignee: Eaton Corp
Priority date: 2005-09-10
Filing date: 2006-10-17
Publication date: 2007-03-15

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_xwhen the exhaust from the fuel reformer is lean and to simultaneously reduce stored NO_xand 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

This application is a continuation-in-part of U.S. application Ser. No. 11/223,589, filed Sep. 10, 2005.

FIELD OF THE INVENTION

The present invention relates to pollution control devices for diesel engines.

BACKGROUND

NO_xand 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_xand 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_xemissions.
In gasoline-powered vehicles that use stoichiometric fuel-air mixtures, NO_xemissions 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.
One set of approaches for controlling NO_xemissions 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_xemissions, but these techniques alone are not sufficient. Another set of approaches involves removing NO_xfrom the vehicle exhaust. These approaches include the use of lean-burn NO_xcatalysts, selective catalytic reduction (SCR), and lean NO_xtraps (LNTs).
Lean-burn NO_xcatalysts promote the reduction of NO_xunder oxygen-rich conditions. Reduction of NO_xin an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NO_xcatalyst that has the required activity, durability, and operating temperature range. Lean-burn NO_xcatalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NO_xcatalysts 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_xconversion efficiencies for lean-burn NO_xcatalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NO_xby ammonia. The reaction takes place even in an oxidizing environment. The NO_xcan be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NO_xreduction, 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.
To clarify the state of a sometimes ambiguous nomenclature, in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NO_xcatalyst” 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_xcatalysis. Commonly, when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NO_xcatalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.
LNTs are devices that adsorb NO_xunder lean exhaust conditions and reduce and release the adsorbed NO_xunder rich exhaust conditions. A LNT generally includes a NO_xadsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO₃and 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_xadsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NO_xis 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_xand thereby regenerate (denitrate) the LNT.
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.
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.
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.
U.S. Pat. No. 7,082,753 (hereinafter “the '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.
The operation of a fuel reformer can be modeled in terms of the following three reactions:
0.684CH_1.85+O₂→0.684CO₂+0.632H₂O (1)
0.316CH_1.85+0.316H ₂0→0.316CO+0.608H₂ (2)
0.316CO+0.316H₂O→0.316CO₂+0.316H₂ (3)
wherein CH_1.85represents 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 H₂are effective for regeneration.
The inline reformer of the '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.
Depending on such factors as the exhaust oxygen concentration, the fuel injection rate, and the exhaust temperature, the inline reformer of the '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 '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.
During denitrations, much of the adsorbed NO_xis reduced to N₂, although a portion of the adsorbed NO_xis released without having been reduced and another portion of the adsorbed NO_xis deeply reduced to ammonia. The NO_xrelease occurs primarily at the beginning of the regeneration. The ammonia production has generally been observed towards the end of the regeneration.
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_xin order to facilitate ammonia production. The ammonia is utilized to reduce NO_xslipping past the LNT and thereby improves conversion efficiency over a stand-alone LNT.
U.S. Pat. Pub. No. 2004/0076565 (hereinafter “the '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_xrelease at the beginning of the regeneration, it is proposed to eliminate oxygen storage capacity from the LNT.
In addition to accumulating NO_x, LNTs accumulate SO_x. SO_xis the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SO_xproduced by combustion is significant. SO_xadsorbs more strongly than NO_xand 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.
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_xemissions from diesel engines to an extent that meets U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.

SUMMARY

One of the inventor'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_xwhen the exhaust from the fuel reformer is lean and to simultaneously reduce stored NO_xand 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.
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.
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_xand 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_xand a portion of the SO_xfrom the exhaust. The exhaust from these LNTs is passed through one or more SCR catalysts that reduce a second portion of NO_xin 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_xis 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.
The primary purpose of this summary has been to present certain of the inventor's concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventor's concepts or every combination of the inventor's concepts that can be considered “invention”. Other concepts of the inventor will 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

FIG. 1 is a schematic illustration of an exemplary power generation system.
FIG. 2 is a plot of temperatures and reductant concentration in a comparison power generation over the course of LNT desulfation.
FIG. 3 is a schematic of the power generation system that produced the data plotted in FIG. (2).
FIG. 4 is a schematic illustration of another exemplary power generation system.
FIG. 5 is a schematic illustration of yet another exemplary power generation system.
FIG. 6 is a schematic illustration of a further exemplary power generation system.

DETAILED DESCRIPTION

FIG. 1 is a schematic of an exemplary power generation system 100 embodying one of the inventor'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.
During lean operation (a lean phase), the LNTs 106 adsorb a first portion of the NO_xfrom 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_xfrom the lean exhaust.
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_xadsorbed in the LNTs 106. Some of this NO_xis reduced to NH₃, most of which is captured by the ammonia-SCR catalysts 107 and used to reduce NO_xduring a subsequent lean phase.
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.
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.
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.
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.
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 (H₂) 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 106i 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.
The inventor'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.
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.
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.
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.
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'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.
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.
FIG. 4 is a schematic of an exemplary power generation system 400 illustrating a second embodiment of the inventor'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.
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 H₂S in the rich exhaust.
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 NO₂. Oxidizing untreated NO to NO₂improves the overall performance of the exhaust treatment system.
H₂S may slip from the LNTs 106 during desulfation. H₂S has an offensive odor even in very small concentrations. By oxidizing this H₂S to SO₂, the unpleasant odor can be avoided.
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.
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_xslip 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.
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_xslip during regenerations.
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.
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_xconcentrations are high, facilitating continuous regeneration. A potential advantage of placing the DPF downstream from the fuel reformer 105 is that oxidation of NO to NO₂in 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.
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 NH₃. The preferred location for the DPF depends on the type of DPF and other particulars of the various system components.
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.
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.
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
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 NO₂. N0 ₂can 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.
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_xconsists of NO and NO₂.
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.
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.
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.
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, K₂O, 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.
The LNTs 106 can comprise any suitable NO_x-adsorbing material. Examples of NO_xadsorbing 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.
The LNTs 106 also comprise a catalyst for the reduction of NO_xin 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.
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_xadsorbent bed for a vehicle exhaust system must be reasonably resistant to degradation under the vibrations encountered during vehicle operation.
The ammonia-SCR catalysts 107 are catalysts functional to catalyze reactions between NO_xand NH₃to reduce NO_xto N₂in 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.
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 H₂S 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 H₂S, when required, may be facilitated by one or more additional components such as NiO, Fe₂O₃, MnO₂, CoO, and CrO₂.
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.

Claims

1. A power generation system, comprising:

a diesel engine operative to produce an exhaust containing NO_x

a fuel reformer configured to receive the exhaust and operative to produce reformate when the fuel reformer is sufficiently warm, the exhaust is rich, and the exhaust contains diesel fuel;

two or more separate LNT bricks configured in a parallel valveless;

arrangement wherein each simultaneously receives a separate portion of the exhaust leaving the fuel reformer, the LNTs each being adapted and configured to simultaneously store NO_xwhen the exhaust from the fuel reformer is lean and to simultaneously reduce stored NO_xand regenerate when the exhaust from the fuel reformer is rich and contains reformate.

2. The power generation system of claim 1, wherein the two or more separate LNT bricks comprise at least three separate LNT bricks.

3. The power generation system of claim 1, wherein each LNT brick is a monolith from about 7 cm to about 15 cm in length.

4. The power generation system of claim 1, wherein:

an equivalent diameter to equivalent length ratio of the two or more separate LNT bricks is at least about two;

the equivalent diameter is obtained by dividing the total frontal area of the two or more separate LNT bricks by pi, taking the square root, and multiplying by two; and

the equivalent length is obtained by dividing the total volume of the two or more separate LNT bricks by the total frontal area of the two or more separate LNT bricks.

5. The power generation system of claim 4, wherein the equivalent diameter to equivalent length ratio of the two or more separate LNT bricks is at least about three.

6. The power generation system of claim 4, wherein the equivalent diameter to equivalent length ratio of the two or more separate LNT bricks is at least about four.

7. A method of operating a power generation system, comprising:

operating a diesel engine to produce an exhaust containing NO_xand SO_x;

channeling the exhaust through a plurality of LNTs that adsorb and store a first portion of NO_xand a portion of the SO_xfrom the exhaust;

passing the exhaust from the plurality of LNTs through one or more SCR catalysts that reduce a second portion of NO_xin the exhaust by reaction with ammonia under lean conditions;

generating a first control signal to denitrate a first one or more of the LNTs;

in response to the control signal, supplying rich exhaust to the first one or more of the LNTs, whereby adsorbed NO_xin the first one or more LNTs is reduced producing ammonia-containing exhaust;

passing the ammonia containing exhaust through one or more of the SCR catalysts, whereby the one or more ammonia-SCR catalysts adsorb and store ammonia;

generating a second control signal to desulfate one or more of the LNTs; and

in response to the second control signal, desulfating a second one or more LNTs by heating the second one or more LNTs and making the exhaust supplying the second one or more LNTs rich such that over the course of the desulfation, the temperatures in the second one or more LNTs increase in the direction of the exhaust flow;

wherein the LNTs each comprise a separate brick and each LNT simultaneously receives a separate portion of the exhaust.

8. The method of claim 7, wherein the exhaust is divided among the plurality of LNTs by static structures that do not move in response to either control signal.

9. The method of claim 7, wherein the temperatures of the second one or more LNTs increase in the direction of flow during desulfation due to reactions involving residual oxygen carried by the exhaust during desulfation.

10. The method of claim 7, wherein the temperatures of the second one or more LNTs increases in the direction of flow during desulfation by reactions between reductants and oxygen stored in the LNTs.

11. The method of claim 7, wherein supplying rich exhaust to the first one or more of the LNTs comprises injecting hydrocarbons into the exhaust and passing the exhaust through a fuel reformer.

12. The method of claim 11, further comprising heating the fuel reformer in response to the first control signal in preparation for supplying rich exhaust to the first one or more of the LNTs and wherein the fuel reformer comprises an effective amount of a steam reforming catalyst.

13. The method of claim 7, wherein the first one or more LNTs and the second one or more LNTs each comprise all the LNTs.

14. The method of claim 13, wherein a single fuel reformer is configured to supply rich exhaust to all the LNTs.

15. The method of claim 14, wherein the fuel reformer is configured to receive all the exhaust from the diesel engine and the fuel reformer produces reformate by steam reforming reactions.

16. The method of claim 7, wherein there are three or more LNTs each comprising a monolith brick from about 7 cm to about 15 cm in length.

17. The method of claim 7, wherein the plurality of LNTs collectively have an equivalent diameter to equivalent length ratio of at least about three;

18. The method of claim 7, wherein the equivalent diameter to equivalent length ratio of the two or more separate LNT bricks is at least about four.