Abstract:
One aspect of the invention relates to a device for storing ammonia for use in SCR on board a vehicle. The device comprises an adsorption bed with a high capacity for storing ammonia. The device can be designed to hold a long-lasting charge of ammonia comparable to a urea tank, but will not release substantial amounts of ammonia into the environment even if the device is accidentally ruptured. In one embodiment, the devices are charged at stationary locations. In another embodiment, the devices are charged by vehicle-mounted ammonia synthesis plants. The device facilitate the use of small ammonia synthesis plants that operate at low pressures and give low conversions. Preferably, the devices are operated through temperature swing adsorption.

Description:
PRIORITY  
       [0001]     This application is a continuation-in-part of U.S. Provisional Application No. 60/467,871, filed May 5, 2003. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of pollution control devices for internal combustion engines.  
       BACKGROUND OF THE INVENTION  
       [0003]     NO x  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 NO x  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 NO x  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.  
         [0004]     Several solutions have been posed 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, homogenizing fuel-air mixtures, and inducing sparkless ignition 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 NO x  catalysts, NO x  adsorber-catalysts, and selective catalytic reduction (SCR).  
         [0005]     Lean-burn NOx catalysts promote the reduction of NO x  under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proved 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 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.  
         [0006]     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 BaCO 3  and the catalyst can be a precious metal, such as Ru. A drawback of this system is that the precious metal catalyst and the adsorbant may be poisoned by sulfur.  
         [0007]     SCR involves using ammonia as the reductant. The NOx can be temporarily stored in an adsorbant or ammonia can be fed continuousy into the exhaust. SCR can achieve NOx reductions in excess of 90%. SCR is widely considered to be the one proven technology for NOx control and has been selected for implementation by European heavy-duty vehicle manufacturers.  
         [0008]     In connection with SCR, the provision of ammonia is a concern. Compressed or liquid ammonia on vehicles is considered an unacceptable safety and enviromental hazard. Alternatives include urea, which can be hydrolyzed as needed to form ammonia, and ammonia salts, such as carbamate, which can be decomposed to give ammonia. The European heavy-duty vehicle manufacturers in particular have chosen to create a distribution system for a 32.5% solution of urea in water (AdBlue). While this distribution system will be difficult and expensive to create and maintain, no better alternatives have been identified.  
         [0009]     U.S. Pat. Appl. No. 2003/0136115 suggests an emission control systems in which ammonia is generated by a reaction between NO with H 2 . During a special rich mode of engine operation, the ammonia is generated in a first catalytic converter and stored in a second, downstream catalytic converter. During a normal lean mode of operation, NO is reduced by the ammonia in the second catalytic converter. When sensors indicate the stored ammonia is exhausted, the engine is returned to rich operation for a period to regenerate the ammonia.  
         [0010]     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.  
       SUMMARY OF THE INVENTION  
       [0011]     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. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, 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.  
         [0012]     One aspect of the invention relates to a device for storing ammonia for use in SCR on board a vehicle. The device comprises an adsorption bed with a high capacity for storing ammonia. Preferably, the device can store at least about 10% ammonia by weight and preferably the device is adapted to release the ammonia by heating. An ammonia storage device according to the invention can be designed to hold a long-lasting charge of ammonia comparable to a urea tank, but will not release a substantial amount of ammonia into the environment even if the device is accidentally ruptured.  
         [0013]     Another aspect of the invention relates to systems and methods of supplying ammonia to vehicles for SCR. The ammonia is adsorbed into an adsorption bed of a storage device. The device is mounted on a vehicle and used to treat the vehicle exhaust. After the supply of ammonia is depleted, the device can be replaced by another with a fresh charge.  
         [0014]     A further aspect of the invention also relates to a vehicle provided with an ammonia synthesis reactor. Ammonia precursors undergo partial conversion as they pass through the reactor. Ammonia is adsorbed into an ammonia storage device and unconverted reagents are recycled through the reactor for further conversion. After the ammonia storage device is charged, it is used to supply an SCR reactor. The invention allows for efficient use of a low pressure ammonia synthesis reactor in which complete conversion of reagents cannot be expected. Preferably, the vehicle is provided with at least two ammonia storage devices whereby one can be supplying ammonia while the other is being charged.  
         [0015]     A still further aspect of the invention relates to a vehicle provided with two devices that store ammonia by adsorption. The vehicle is adapted to supply ammonia from the first device to an SCR reactor and, at a point generally corresponding to depletion of ammonia from the first device, to switch to supplying ammonia from the second device. One of the devices can be charging while the other is being used. Alternatively, the depleted devices can be replaced or recharged during a vehicle stop.  
         [0016]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is an illustration of an ammonia storage device comprising a porous-walled monolith;  
         [0018]      FIG. 2  is an illustration of an ammonia storage device comprising a cohesive mass of adsorbant pellets;  
         [0019]      FIG. 3  is an illustration of an ammonia storage device comprising a stack of coated screens;  
         [0020]      FIG. 4  is a cross-sectional view of the ammonia storage device of  FIG. 3 ;  
         [0021]      FIG. 5  is an illustration of an ammonia storage device comprising an annular monolith adsorbant bed and a central channel for heat exchange;  
         [0022]      FIG. 6  is a cross-sectional view of the ammonia storage device of  FIG. 5 ;  
         [0023]      FIG. 7  is another cross-sectional view of the ammonia storage device of  FIG. 5 ;  
         [0024]      FIG. 8  is an illustration of an ammonia storage device comprising a rolled, coated screen adsorbant bed surrounding a central channel for heat exchange;  
         [0025]      FIG. 9  is a cross-sectional view of the ammonia storage device of  FIG. 8 ;  
         [0026]      FIG. 10  is an illustration of an ammonia storage device comprising coated screens radially arrayed around a central channel;  
         [0027]      FIG. 11  is a cross-sectional view of the ammonia storage device of  FIG. 10 ;  
         [0028]      FIG. 12  is a schematic illustration of an ammonia synthesis plant. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     One aspect of the present invention relates to the idea of generating ammonia in small synthesis plants and storing the ammonia by adsorption. A synthesis plant can form ammonia from H 2  and N 2  or from H 2  and NO. H 2 , N 2 , and NO can be generated from just air, fuel, and water. The synthesis plant can be stationary or vehicle-mounted. Storing the ammonia in adsorption beds serves the dual functions of extracting ammonia from a dilute stream, which is the typical product of a small scale ammonia synthesis plant, and of providing a safe system for storing substantial quantities of ammonia on vehicles.  
         [0030]     An ammonia storage device according to the present invention is adapted for use in a vehicle exhaust system. Vehicle exhaust systems create restriction on weight, dimensions, and durability. For example, an adsorption bed for a vehicle exhaust systems must be reasonably resistant to degradation under the vibrations encountered during vehicle operation. A vehicle is typically powered by an internal combustion engine burning a fuel such as diesel, gasoline, natural gas, or propane and produces an exhaust.  
         [0031]     The mass of an ammonia storage device according to the present invention can be substantial in terms of the device sizes typically found in a vehicle exhaust system. To limit the total mass, the adsorbant bed preferably comprises a high loading of adsorbant per unit bed mass. Preferably, an adsorbant bed according to the present invention comprises at least about 40% adsorbant by weight, more preferably at least about 60%, still more preferably at least about 80%, and most preferably at least about 90%. The weight of an adsorbant bed includes any inert substrate and any binders, but does not include any housing.  
         [0032]     Adsorbant beds according to the invention generally carry more adsorbant per unit volume than prior art beds. In one embodiment, an adsorbant bed according to the invention is at least about 20% adsorbant by volume, in another embodiment, at least about 35% adsorbant by volume, in a further embodiment, at least about 50% adsorbant by volume, and in a still further embodiment, at least about 65% adsorbant by volume.  
         [0033]     Temperature swing adsorption is the preferred method of operating an ammonia storage device according to the present invention. In contemplation of temperature swing adsorption, ammonia storage devices according to the invention may be provided with mechanisms for heating and/or cooling. For example, an adsorption 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 adsorption bed. 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 catalytic reformer, or an adsorber.  
         [0034]     In one embodiment of the invention, the ammonia storage device has a small number of heat-exchange passages, for example less than five, and preferably just one. A single channel can pass through the center of the adsorption bed. A central channel is typically rather large, having for example a cross-sectional area of at least about 1 square inch. The channels can be provided with heat exchanger fins. Advantages of heat exchange through a single central passage include simplicity, low pressure drop, and easy coupling and decoupling from a vehicle.  
         [0035]     An ammonia storage device can also include a provision for electrical heating. Where the adsorption bed includes a metal substrate, the metal substrate can be used as an electrical resistance heater. An adsorption bed can also be permeated by wires for electrical resistance heating.  
         [0036]      FIG. 1  illustrates an ammonia storage device  30  with a design for improving the utilization of an adsorbant, especially a molecular sieve adsorbant that has very small pores. The ammonia storage device  30  comprises a monolith  31  within a housing  32 . The monolith  31  is preferably a self-supporting structure without an inert substrate. The monolith can be cast or extruded. Casting may be accomplished by pressing a coarse mixture of adsorbant pellets and binder into a mold and then curing the mixture. Alternately, the pellets can be pored into the mold and sintered to form a cohesive mass. Extrusion can be carried out in a similar fashion with heat applied at the point of extrusion to cure the binder or sinter the pellets. The pellets themselves are typically a mixture of adsorbant and binder. The walls  33  of the monolith  31  have a macro-porous structure, whereby the diffusion path length from the macro-pores to the innermost parts of the walls  33  is substantially less than the diffusion path length from the channels to the centers of the walls. Because the monolith  31  lacks an inert substrate, it comprises a large fraction of adsorbant by weight. Preferably, the walls of the monolith, exclusive of the channel volume and exclusive of any pores having an effective diameter less than 1 μm (an effective diameter being defined with reference to mercury porosimetry) have a void volume fraction of at least about 0.1, more preferably at least about 0.2, still more preferably at least about 0.3.  
         [0037]      FIG. 2  illustrates an ammonia storage device  35  comprising a cohesive mass of pellets  36  in a housing  37 . Loose pellets in a packed bed have a tendency to erode when mounted on a vehicle. The ammonia storage device  35  mitigates this problem by forming the pellets into a cohesive mass. The pellets can be formed into a cohesive mass by, for example, sintering the pellets together or mixing them with a binder. The individual pellets are preferably themselves made up of smaller pellets. Smaller pellets can themselves be formed onto larger pellets by a binder or a sintering process. The intersticies between the larger pellets correspond to the channels of the monolith  31  and the voids in the pellets (intersticies between the smaller pellets, where appropriate) correspond to the voids in the walls of the monolith  31 . The comments regarding preferred composition and void sizes for the monolith  31  apply to the cohesive mass  36 . The ammonia storage device  35  is provided in a pancake design. A pancake design gives a large cross-sectional area in the direction of flow and thereby reduces the pressure drop for a given bed volume.  
         [0038]     The packed bed designs of the present invention can provide very high adsorbant densities. Density can be increased by using a mixture of pellet sizes, for example, a mixture of 1/16 inch and 1/8 inch pellets.  
         [0039]      FIGS. 3 and 4  illustrates a device  40  in the form of a stack  41  of coated metal screens  42  in a housing  43 . An adsorbant forms a coating over the screens  42 . Exhaust flows between the screens  42 . The spacing between the screens is controlled by spacers  44 . The openings in the screens  42  provide additional surface area for the adsorbant. Optional electrical leads  45  are connected to the screens along either side of the adsorbent bed. By connecting a power source to the electrical leads  45 , the device  40  can be heated.  
         [0040]     FIGS.  5  to  7  illustrate a device  50  comprising an annular monolith  51  enclosed in a housing  52  and surrounding a central channel  53 . The central channel  53  is in fluid isolation from the monolith  51 , but can be used to heat or cool the monolith. For example, the monolith can be heated by passing hot exhaust through the central channel  51  and cooled by driving ambient air through the central channel  51 . The monolith itself can have any suitable structure. In one embodiment, the monolith is made up of metal foil coated with an adsorbant. The structure can be made by spiraling together two rolled sheets of metal, one flat and one articulated, about the central channel. A metal foil substrate can be used for electrical resistance heating. The adsorbent bed occupying the annular region can alternatively be, for example, a cohesive mass of pellets or layered coated screening.  
         [0041]      FIGS. 8 and 9  illustrate an ammonia storage device  60  that has a housing  63  and a central channel  64 . The adsorbent bed  61  comprises a metal screen coated with adsorbant and rolled into a hollow cylinder to form roughly annular passages. The widths of the passages are controlled by spacers  62 . The housing  63  is different from the housing  53  of the ammonia storage device  50  in that the central passage vents out the ends rather than the sides. These housings can be interchanged.  
         [0042]     The housing  52  and  63  and their associated beds and central channels can have any appropriate dimensions for a particular application. The length, central channel diameter, and bed outer diameter are selected in view of the required volume, bed thermal conductivity, requirements for temperature uniformity, requirements for heat exchange, and limitations on pressure drops through the bed and central channel. Mathematical calculations and/or computer simulations can be used to identify appropriate designs for particular applications. The frontal area of the bed and channel is typically from about 4 square inches to about 120 square inches, more typically from about 7 square inches to about 50 square inches. The inner channel diameter is typically from about 1 to about 3 inches. The difference between the inner and the outer channel diameter is typically from about 1 to about 3 inches. The length to outer diameter ratio is typically from about 12:1 to about 3:1.  
         [0043]      FIGS. 10 and 11  illustrate an ammonia storage device  70  using the housing  52  and the central channel  53 . The ammonia storage device  70  comprises an adsorbant bed  71  made of metal screens  72  coated with adsorbant or catalyst, attached edgewise, and arrayed radially about the central channel  53 . Attaching the screens  72  edgewise to the central channel  53  may facilitate heat transfer between the adsorbant bed  71  and the central channel  53 . Optionally, the central channel  53  includes heat-exchanger fins extending from the edges of the channel towards its interior. The screens  72  curve as they extend away from the central channel  53 . The curvature limits or eliminates the tendency for the spacing between screens  72  to increase with distance from the central channel  53 . The curvature also makes the ammonia storage device  70  more compact and may further facilitate heat exchange with a fluid in the central channel  53 . The spacing between screens is controlled with spacers  73 .  
         [0044]     In one embodiment, the adsorption bed has a large capacity for adsorbing NH 3  at 25° C. and one atmosphere pressure. In this and similar contexts, one atmosphere pressure means, in substance, one atmosphere of pure ammonia. Pressures are absolute pressure unless otherwise specified. Preferably at 25° C. and one atmosphere pressure the adsorption bed can take up at least about 5% ammonia by weight, more preferably at least about 10% ammonia by weight, still more preferably at least about 20% ammonia by weight. The weight of adsorbant bed includes the weight of any binders or inert substrates but does not include the weight of any housing or couplings.  
         [0045]     The weight of the storage device can be significant. To minimize total weight, the adsorbant preferably accounts for at least about 40% of the ammonia storage device weight, more preferably at least about 60%, and still more preferably at least about 80%.  
         [0046]     An ammonia storage device can be charged at a stationary location and mounted on a vehicle or can be charged onboard the vehicle. Where the ammonia storage devices are charged at stationary locations, preferably the one or more ammonia storage devices provided on the vehicle can collectively adsorb at least about 3 kg of ammonia at 1 atmosphere and 25° C., more preferably at least about 6 kg, still more preferably at least about 12 kg. Where the ammonia storage devices are charged onboard, preferably the one or more ammonia storage device on the vehicle can collectively adsorb at least about 0.6 kg of ammonia at 1 atmosphere ammonia and 25° C., more preferably at least about 1.2 kg, still more preferably at least about 2.4 kg.  
         [0047]     For safety, the adsorbant is preferably adapted for temperature swing adsorption. An adsorbant that has a capacity for adsorbing NH 3  that changes relatively slowly with pressure but rapidly with temperature is preferred. The heat (energy) of adsorption is a critical factor in determining the temperature increase that will induce desorption. Solid adsorbants generally have a plurality of types of binding sites with a range of heats of adsorption, but an average or approximate value can be determined by analyzing changes in partial pressure with temperature. A larger heat of adsorption means a more rapid increase in partial pressure of adsorbants with temperature. Preferably, the heat of adsorption for NH 3  on the adsorbant is at least about 50 kJ/mol, more preferably at least about 70 kJ/mol, still more preferably at least about 90 kJ/mol.  
         [0048]     Any suitable adsorbant material can be used. Examples of adsorbants are 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 phoshates of titanium and zirconium.  
         [0049]     Molecular seives 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. While the selection of an adsorbant depends on such factors as the desired adsorption temperature and desorption method, preferred zeolites for ammonia storage generally include faujasites and rare earth zeolites. Faujasites include X and Y-type zeolites. 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.  
         [0050]     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. 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%. A preferred composition for small adsorbant pellets that can be used to form monoliths, larger pellets, or a porous coatings over an inert substrate such as screening, is molecular sieve crystals with about 8% or less portland cement as a binder. This composition can provide structural integrity and high utilization of the molecular sieve&#39;s adsorption capacity. Where the molecular sieve is H—Y or NH 4 —Y zeolite, this mixture can adsorb about 23% NH 3  by weight at 25° C. and one atmosphere ammonia partial pressure. At 350° C. and one atmosphere ammonia partial pressure, the adsorption capacity is reduce to about 5% by weight. H—Y and NH 4 —Y zeolites have relatively flat isotherm (small effect of pressure on adsorption capacity), which is advantageous in temperature swing adsorption processes.  
         [0051]     According to one aspect of the invention, the ammonia storage devices are charged at stationary plants and interchanged during fuel stops. For these applications, preferably the ammonia storage device is adapted for mounting on a vehicle. Preferably the device can be mounted and dismounted by hand. Hand-operated mounting means can include, for example, clamps, clips, snap-fitting members, sliding connections, interlocking members, and screw connections. A mounting means that involved a small tool mounted on the vehicle or on the ammonia storage device would still be considered a hand-operated mounting means.  
         [0052]      FIG. 12  is a schematic illustration of an ammonia synthesis plant  110  that can be used to charge an ammonia storage device according to the present invention. The ammonia synthesis plant  110  can be mounted on a vehicle or at a stationary location, such as a fuel station. The ammonia storage device  110  comprises a nitrogen source  111  and a hydrogen source  112 . Under the control of valves  120  and  122 , N 2  and H 2  from these sources are taken up by a compressor  113  and supplied under pressure to a recirculating loop that includes an ammonia synthesis reactor  114  and one of the ammonia storage devices  116  and  117 . Recirculation is driven by circulator  118 . The circulator  118  can be a simple fan. Alternatively, it can be a compressor.  
         [0053]     An optional heat exchanger  115  is provided to cool the recirculating gas as it leaves the ammonia synthesis reactor  114 . Cooling can alternatively be provided as the gas leaves the compressor, in the ammonia synthesis reactor  114 , in the ammonia storage devices  116  and  117 , or elsewhere in the recirculating loop. N 2  and H 2  are partially converted to NH 3  in the ammonia synthesis reactor  114 . The ammonia storage device  116  or  117  adsorbs the ammonia produced. Unreacted N 2  and H 2  are returned to the ammonia synthesis reactor  114 . A portion of the recirculation gas is released through valve  126  to limit the accumulation of non-reacting impurities.  
         [0054]     Valves  122 - 125  allow one or the other of the ammonia storage devices  116  and  117  to be selectively taken out of the recirculating loop. In  FIG. 12 , valves  122  and  124  are open while valves  123  and  125  are closed, whereby the ammonia storage device  116  is in the recirculating loop and the ammonia storage device  117  is not. On a vehicle, the ammonia storage device  117  might be used to supply ammonia to an SCR reactor while the ammonia storage device  116  is charging. In a stationary system, the ammonia storage device  117  might be swapped with an ammonia storage device requiring a charge. Optional couplings  130 - 133  can be used to removably mount the ammonia storage devices  116  and  117  to the ammonia synthesis plant  110 .  
         [0055]     The nitrogen source is typically a system for obtaining pure nitrogen from air. One simple system is a membrane separator. Other examples include pressure and temperature swing adsorption systems. Typically, such a membrane will also admit argon. The argon concentrates in the recirculating loop and is removed by the purge through the valve  126 . A typical purge rate is one part in ten or one part in 20.  
         [0056]     The hydrogen source can be a reformer, which can be vehicle mounted. A reformer can convert fuel, such as diesel, gasoline, propane, methane, or natural gas into synthesis gas (syn gas). A reformer can be a catalytic reformer or a plasma reformer. A reformer can use oxygen and/or steam. Relatively pure hydrogen can be extracted from syn gas by any suitable method, for example, temperature or pressure swing adsorption. Hydrogen can also be obtained by electrolysis of water.  
         [0057]     The ammonia synthesis reactor  114  comprises a catalyst for the reaction of N 2  and H 2  to for NH 3 . The catalyst is provided as a coating on a substrate. Any suitable substrate can be used, including any of the structures described above for ammonia storage devices. A typical structure is a ceramic monolith. Additional options, particularly for stationary applications, are packed and fluidized bed reactors. Examples of potentially suitable catalysts include Group VIII metal compounds, such as a Group VIII metal with a Group VIB metal, Fe optionally with oxides of Al, Mg, Ca, and/or K, Fe 2 O 3 , Ni with Mo, and Ru with an alkali metal and Ba compound, and molybdenum oxycarbonitride.  
         [0058]     Preferably, the ammonia synthesis reactor  114  is designed for operation at a relatively low pressure (for an ammonia synthesis reactor), for example, a pressure of about 100 atm or less, more preferably about 50 atm or less. At these pressures, maximum conversion may be in the 5-30% range. Adsorption in ammonia storage devices and recirculation of reagents allows the reagents to be efficiently used in spite of low conversions.  
         [0059]     The exemplary ammonia synthesis plant  110  includes two ammonia storage devices  116  and  117 . At any given time, one can be charging and the other can be supplying ammonia or undergoing exchange. Optionally, more than two ammonia storage devices can be provided with one or more charging and one or more discharging, waiting, or undergoing exchange.  
         [0060]     Desorption from an ammonia storage device to supply ammonia can be carried out in any suitable manner, however, a temperature change is preferred. Desportion can also be controlled in any suitable manner. For example, a heating device can be selectively actuated to maintain a target pressure, e.g., 15 psig, of ammonia while a valve is used to control the flow rate of ammonia to a SCR reactor. A state of discharge can be detected through a fall off in concentration or a fall off in pressure. Alternatively, a state of discharge can be estimated from data relating to usage. For example, knowing the pressure in the ammonia storage device and the position of a discharge valve as a function of time can provide the information from which the degree of discharge is estimated. Likewise, during charging, a state of complete charge can be determined either from sensors or estimates.  
         [0061]     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.