Patent Application
20060042235
The present invention relates to the field of pollution control devices for internal combustion engines.
NOx emissions from vehicles with internal combustion engines are an environmental problem recognized worldwide. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. In conventional gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles and vehicles with lean-burn gasoline engines, however, the exhaust is too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles and lean-burn gasoline engines. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and homogenizing fuel-air mixtures can reduce NOx emissions. These techniques alone, however, will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOx catalysts, NOx adsorber-catalysts, and selective catalytic reduction (SCR).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. Lean-burn NOx catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean burn NOx 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 NOx conversion efficiency with lean-burn catalysts is unacceptably low.
NOx adsorber-catalysts alternately adsorb NOx and catalytically reduce it. The adsorber can be taken offline during regeneration and a reducing atmosphere provided. The adsorbant is generally an alkaline earth oxide adsorbant, such as BaCO3 and the catalyst can be a precious metal, such as Ru.
SCR involves using ammonia as the reductant. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuously into the exhaust. SCR can achieve NOx reductions in excess of 90%, however, there is concern over the lack of infrastructure for distributing ammonia or a suitable precursor. SCR also raises concerns relating to the possible release of ammonia into the environment.
U.S. Pat. No. 6,560,958 describes a NOx adsorber-catalyst system in which hydrogen-rich synthesis gas (syn gas), including H2 and CO, is used as a reductant to regenerate the adsorber. In some embodiments, two adsorbers are provided in tandem, whereby one adsorber can be regenerated while the exhaust is diverted to the other adsorber.
U.S. Pat. No. 6,735,940 discloses a dual leg system wherein 80% of the exhaust is diverted to one leg while 20% of the exhaust is diverted to the other leg. The leg with the lower flow can be regenerated using less reducing agent than if the leg were receiving a full share of the exhaust. Less reducing agent is required because there is less lean exhaust with oxygen to be consumed by the reducing agent during regeneration.
U.S. Pat. No. 6,732,507 describes a hybrid exhaust treatment system using a NOx adsorber-catalyst and an SCR reactor in series. The SCR reactor captures ammonia produced by the NOx adsorber-catalyst during regeneration and uses the captured ammonia to increase the extent of NOx conversion.
There continues to be a long felt need for reliable, affordable, and effective systems for removing NOx from the exhaust of diesel and lean-burn gasoline engines.
The following presents a simplified summary in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. The primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the invention relates to a NOx trapping device for a vehicle exhaust system having a plurality of NOx adsorbers. The device has a mobile framework configured to hold the NOx adsorbers and move them in sequence through a plurality of positions. A manifold channels exhaust to one or more of the NOx adsorber positions. The mobile framework is adapted to move the NOx adsorbers in and out of position to receive the exhaust. The adsorbers can be regenerated while they are not receiving the exhaust.
The invention facilitates the use of multiple NOx adsorbers in an exhaust system, particularly the use of more than two adsorbers. Two NOx adsorbers are useful in that one can be regenerated offline while the other is treating the exhaust. Having more than two is further useful in that increasing the number of adsorbers often reduces the total amount of adsorbant and precious metal catalyst required to achieve a given emission reduction. The invention avoids complex exhaust valving, which is normally associated with multiple adsorber systems.
In one embodiment of the invention, the treated exhaust flow is combined with the exhaust from an adsorber undergoing regeneration. This allows ammonia produced during regeneration to further clean the exhaust in an SCR reactor placed downstream of the adsorbers. It also allows oxygen present in the exhaust to be used to oxidize excess ammonia and unconsumed reductants from the regeneration process.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
In a preferred embodiment, there are from three to about six of the NOx adsorbers 12. Two adsorbers are often provided so that one can be regenerating while the other is treating the exhaust. When only two adsorbers are used, each adsorber must provide sufficient adsorbant and catalyst to achieve a target degree of exhaust clean up. Inefficiencies arise because the adsorption cycle and the regeneration cycle are not the same length. In a common system, the regeneration cycle is much shorter, whereby one of the adsorbers is usually idle.
Where the regeneration cycle is short, efficiency can be improved by using three adsorbers instead of two. If 100 units of adsorber/catalyst are required to treat the exhaust, a two-adsorber system requires that each of the adsorber provide the full 100 units and the two-unit system requires 200 units in all. In a three-adsorber system, however, two adsorbers can be treating the exhaust at any given time and each unit need only provide 50 units of capacity. The third adsorber undergoes regeneration. The three-adsorber system requires only 150 units total, a significant improvement over the two-adsorber system.
If regeneration takes one ninth the time an adsorber can effectively adsorb NOx, reductions in the total amount of adsorbant and catalyst required can be realized by increasing the number of adsorber up to ten as shown in the following table:
As shown from this table, the majority of the benefit is realized by about six or fewer adsorbers. In this example, a manifold supplies the exhaust to all but one adsorber location. The other adsorber location is supplied with a reductant.
The foregoing system is easily controlled. A control system determines when to rotate the NOx adsorbers. In one example, a sensor downstream of the NOx adsorbers is used to determine when to rotate the array. The sensor can be placed in the combined adsorber exhaust, or immediately downstream of the NOx adsorber that has been receiving exhaust longest. When the NOx concentration exceeds a target level, the array is rotated. In another example, the amount of NOx adsorption is estimated from vehicle torque and speed conditions. A model is used to estimate the amount of NOx adsorbed in each adsorber and the array is rotated when the NOx adsorber longest online has reached a certain adsorption level.
It is also possible to rotate the array periodically. The adsorbers can be regenerated more frequently than is necessary. The main disadvantage to overly frequent regeneration is that excess reductant may be used. This consequence can be mitigated by a feedback control system for the reductant flow.
Although less common, it is also possible that the regeneration process might take longer than adsorption. An example of this could occur if temperature swing adsorption (TSA) were used. In TSA, the NOx adsorbers 12 are heated during regeneration to desorb NOx. The desorbed NOx is then treated in a separate location. An advantage of TSA is that the catalyst used to reduce NOx is never exposed to the main exhaust flow. This is useful, for example, with catalysts that are sensitive to oxygen and with catalysts that might be deactivated by high temperature conditions, which may be periodically used to remove sulfur from the NOx adsorbers 12. The time taken to heat and cool the NOx adsorbers 12 may exceed the time it takes to saturate the NOx adsorbers 12 with NOx. In such an example, a majority of slots in the rotary NOx trap 11 would be devoted to heating and cooling the NOx adsorbers 12 and capturing desorbed NOx.
The rotary NOx trap 11 has an indexing mechanism. An indexing mechanism is a mechanism that moves the NOx adsorbers 12 in a pre-defined order through a sequence of positions. In the case of the rotary NOx trap 11, this involves simply rotating the radially arrayed adsorbers 12. Any suitable mechanism can be used to move the adsorbers. A typical mechanism would be a spindle driven by an electric motor.
At any given time, one or more of the NOx adsorbers 12 are in position to couple with the manifold 13. The coupling between the manifold 13 and the NOx adsorbers 12 can be of any suitable type. Typically, the coupling involves abutting seals such as Teflon or brass rings. Optionally, the entire radial array is enclosed in a housing to capture any exhaust leaking from the seals. The captured exhaust can be channel to join with the treated exhaust, in manifold 15 for example.
The exhaust is typically generated by burning a fossil fuel such as diesel, gasoline, natural gas, or propane in an internal combustion engine. The exhaust comprises NOx. NOx includes NO, NO2, N2O, and N2O2. Generally the exhaust is lean, meaning that it contains oxygen. Typically, lean exhaust contains 3-5% oxygen.
The NOx adsorbers 12 can comprise any suitable adsorbant material. Examples of adsorbant materials include molecular sieves, such as zeolites, alumina, silica, and activated carbon. Further examples are oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Be or alkali metals such as K or Ce. Still further examples include metal phosphates, such as phosphates of titanium and zirconium.
Molecular sieves are materials having a crystalline structure that defines internal cavities and interconnecting pores of regular size. Zeolites are the most common example. Zeolites have crystalline structures generally based on atoms tetrahedrally bonded to each other with oxygen bridges. The atoms are most commonly aluminum and silicon (giving aluminosilicates), but P, Ga, Ge, B, Be, and other atoms can also make up the tetrahedral framework. The properties of a zeolite may be modified by ion exchange, for example with a rare earth metal or chromium. Preferred zeolites generally include rare earth zeolites and Thomsonite. Rare earth zeolites are zeolites that have been extensively (i.e., at least about 50%) or fully ion exchanged with a rare earth metal, such as lanthanum.
The adsorbant is typically combined with a binder and either formed into a self-supporting structure or applied as a coating over an inert substrate. A binder can be, for example, a clay, a silicate, or a cement. Portland cement can be used to bind molecular sieve crystals. Generally, the adsorbant is most effective when a minimum of binder is used. Preferably, the adsorbant bed contains from about 3 to about 20% binder, more preferably from about 3 to about 12%, most preferably from about 3 to about 8%.
The NOx adsorbers 12 are adapted for use in vehicle exhaust systems. Vehicle exhaust systems create restrictions on weight, dimensions, and durability. For example, an adsorption bed for a vehicle exhaust system must be reasonably resistant to degradation under the vibrations encountered during vehicle operation.
The adsorbant in the NOx adsorbers 12 is formed into adsorbant beds. Beds that have an adsorbant function tend to be large in comparison to beds that have only a catalytic function. To limit the total mass, the NOx adsorbers 12 preferably comprises a high loading of adsorbant per unit mass. Preferably, an adsorbant bed comprises at least about 40% adsorbant by weight. The weight of an adsorbant bed includes any inert substrate and any binders, but does not include any housing. Preferably an adsorbant bed comprises at least about 20% adsorbant by volume.
The NOx adsorbers 12 are optionally provided with mechanisms for heating and/or cooling. For example, a bed can be permeated with heat-exchange passages in fluid isolation from the passages provided for adsorbed and desorbed gases. A hot or cold fluid is circulated through the heat-exchange passages to heat or cool the adsorber. A cooling fluid could be, for example, engine coolant or ambient air. A heating fluid could be, for example, hot exhaust or a fluid that draws heat from hot exhaust or a heat-producing device such as an ammonia synthesis reactor, a fuel reformer, or an adsorber. Another option is electrical resistance heating. Where a bed includes a metal substrate, the metal substrate can be used as an electrical resistance heater. A NOx adsorber 12 can also be permeated by wires for electrical resistance heating.
A liquid coolant could be circulated through the NOx adsorbers 12 from a reservoir contained in a hub for the rotary NOx trap 11. A gaseous heating or cooling agent could be channeled through central passages formed through the centers of the NOx adsorbers 12. The central passages could couple with the heating or cooling agent in the same manner that the NOx adsorbers 12 couple with the manifolds as they rotate into position.
An adsorbant bed for a NOx adsorber 12 can have any suitable structure. Examples of suitable structures may include monoliths, packed beds, and layered screening. A packed bed is preferably formed into a cohesive mass by sintering the particles or adhering them with a binder. Preferably, any thick walls, large particles, or thick coatings have a macro-porous structure facilitating access to micro-pores where adsorption occurs. A macro-porous structure can be developed by forming the walls, particles, or coatings from small particles of adsorbant sintered together or held together with a binder.
Preferably the NOx adsorbers 12 have a large capacity for adsorbing a NOx species at a typical adsorption temperature and exhaust partial pressures. Preferably, the adsorbant can adsorb at least about 3% of a NOx species by weight adsorbant at a typical adsorption temperature and 1 torr partial pressure of the NOx species, more preferably at least about 5% by weight adsorbant, and still more preferably at least about 7% by weight adsorbant. The weight of adsorbant does not include the weight of any binders or inert substrates.
The system 10 generally includes a catalyst effective for reducing NOx in a reducing environment. Usually, the catalyst is provided in the NOx adsorbers 12, however, the catalyst can optionally be provided in a separate bed downstream of the NOx adsorbers 12. In one embodiment, a reducing catalyst bed is configured to receive the output from the one or more adsorber locations receiving the reductant flow. This configuration considerably reduces the required amount of catalyst.
The catalyst can be, for example, one or more precious metals, such as Au, Ag, and Cu, group VIII metals, such as Pt, Pd, Ru, Ni, and Co, Cr, Mo, or K. A typical catalyst includes Pt and Rh, although it may be desirable to reduce or eliminate the Rh to favor the production of NH3 over N2. Effective operating temperatures are generally in the range from about 200 to about 450° C. Lower temperatures may also be desirable in terms of favoring the production of NH3 over N2. A typical adsorption bed comprises a precious metal catalyst distributed through a zeolite adsorbant.
A reductant flow can include any suitable reductant. Examples of suitable reductants include synthesis gas (syn gas), hydrocarbons, and oxygenated hydrocarbons. Syn gas includes H2 and CO. Syn gas can be provided by a fuel reformer. A fuel reformer can be a catalytic reformer, a steam reformer, an autothermal reformer, or a plasma reformer. A reformer is typically supplied with an oxidant source, which is typically air or water, but may be lean exhaust.
In a typical regeneration process, reductant is supplied to the NOx adsorbers 12. The reductant causes NOx to desorb. The reductant also reacts with the NOx over the catalyst to reduce NOx to N2. While this is the main process, some of the desorbing NOx may escape the NOx adsorbers 12 during regeneration. Also, particularly in the later part of a regeneration cycle, some ammonia may be produced. An SCR reactor downstream of the regenerating NOx adsorbers 12 can capture ammonia and use it to reduce NOx.
In the exemplary system 10, the treated exhaust joins with exhaust from the regeneration process. The combined flows are supplied to an optional SCR reactor 17. In this configuration, ammonia produced during the regeneration process can be used to reduce NOx in the main exhaust flow. This can improve the overall extent of NOx removal and/or improve the efficiency with which reductant is used. Ammonia production can be favored during regeneration by regenerating the NOx adsorbers before they become saturated and using higher concentrations of reductant.
An SCR reactor is a reactor having an effective amount of catalyst for the reaction of NOx with NH3 to reduce NOx to N2 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, Rd, 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. Reaction can be obtained at relatively low temperatures, for example, temperatures in the range from about 230 to about 450° C.
The exemplary system 10 also has an optional oxidation catalyst 18 treating the combined exhaust flow. Unused reductant may escape the NOx adsorbers 12 during regeneration. Because reduction is generally carried out in a reducing atmosphere, there is usually no oxygen in the regeneration process exhaust. To oxidize unused reductant, oxygen must be supplied. In the exemplary system 10, that oxygen is supplied by the main exhaust flow. Thus, the oxidation catalyst 18 oxidizes unused reductant using oxygen from the main exhaust flow. The oxidation catalyst 18 can also oxidize any escaping NH3 into comparatively less harmful NOx. The oxidation catalyst 18 can be used with or without the optional SCR reactor 17.
The invention has been shown and described with respect to certain aspects, examples, and embodiments. While a particular feature of the invention may have been disclosed with respect to only one of several aspects, examples, or embodiments, the feature may be combined with one or more other features of the other aspects, examples, or embodiments as may be advantageous for any given or particular application.
