Abstract:
In a method of storing and releasing gaseous ammonia from solid storage materials a first solid storage material ( 14 ) capable of releasing ammonia by desorption in a first container ( 10 ) and a second solid storage material ( 24 ) capable of ad- or absorbing ammonia reversibly and having a higher affinity for ammonia than the first storage material ( 14 ) in a second container ( 20 ) smaller than said first container ( 10 ) are in fluid communication. The pressure in at least the first container ( 10 ) is kept below the equilibrium pressure between ammonia and the storage material contained therein by means of a pump ( 28 ). When the pressure in the first container ( 10 ) is below a pressure threshold where the first storage material ( 14 ) does not release an amount of ammonia required by an ammonia consuming device connected with the containers via the vacuum pump ( 28 ), the second storage material ( 24 ) is heated such that the ammonia pressure of the second material ( 24 ) is higher than the ammonia pressure of the first material ( 14 ). The ammonia released by the second material ( 24 ) is continuously pumped off so as to deliver sufficient ammonia to said ammonia consuming unit. A device for carrying out the method is also described.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an improved method and device for storing and releasing gaseous ammonia from solid storage materials using a vacuum pump. 
       BACKGROUND OF THE INVENTION 
       [0002]    Ammonia is a widely used chemical with many applications. One specific application is as reductant for selective catalytic reduction (SCR) of NO x  in exhaust gas from combustion processes. 
         [0003]    For most applications, and in particular in automotive applications, the storage of ammonia in the form of a pressurized liquid in a vessel is too hazardous. Urea is a safe, but an indirect and impractical method for mobile transport of ammonia since it requires to be transformed into ammonia by a process involving thermolysis and hydrolysis ((NH 2 ) 2 CO+H 2 O→2 NH 3 +CO 2 ). 
         [0004]    A storage method involving ad- or absorption in a solid can circumvent the safety hazard of anhydrous liquid ammonia and the decomposition of a starting material. 
         [0005]    Metal ammine salts are ammonia absorbing and desorbing materials, which can be used as solid storage media for ammonia (see, e.g. WO 2006/012903 A2), which in turn, as mentioned above, may be used as the reductant in selective catalytic reduction to reduce NO x  emissions. 
         [0006]    Usually, ammonia is released by thermal desorption, e.g. from metal ammine salts, by external heating of a storage container, see e.g. WO 1999/01205 A1. The heating elements may also be placed inside the storage container, see e.g. U.S. Pat. No. 5,161,389 and WO 2006/012903 A2. 
         [0007]    In WO 2007/000170 A1 the release of ammonia from the storage material is facilitated by lowering the ammonia pressure in the gas phase. 
         [0008]    WO 2008/077652 A2 and DE 10 2007 022 858 A1 disclose systems that use at least two storage materials having different vapor pressures in at least one operation mode. 
         [0009]    The present invention seeks to improve the method of desorption of ammonia from an ammonia storage material using a vacuum pump. 
       SUMMARY OF THE INVENTION 
       [0010]    The invention relates to a method of storing and releasing gaseous ammonia from solid storage materials wherein a first solid storage material capable of releasing ammonia by desorption in a first container and a second solid storage material capable of ad- or absorbing ammonia reversibly and having a higher affinity for ammonia than the first storage material in a second container, which is smaller than the first container, are in fluid communication. The pressure in at least the first container is kept below the equilibrium pressure between ammonia and the storage material contained therein by means of a vacuum pump. When the pressure in the first container is below a pressure level where the first storage material does not release an amount of ammonia required by an ammonia consuming device connected with the containers via the vacuum pump, the second storage material is heated such that the ammonia pressure of the second material is higher than the ammonia pressure of the first material, and the ammonia released by the second material is continuously pumped off so as to deliver sufficient ammonia to the ammonia consuming unit. 
         [0011]    The invention is further directed to a device for storing and delivering ammonia to an ammonia consuming unit, the device comprising:
       a first container containing a first solid storage material capable of releasing ammonia by desorption;   a second container which is in fluid communication with said first container and smaller than that first container containing a second solid storage material capable of ad- or absorbing ammonia reversibly and having a higher affinity for ammonia than the first storage material;   heating means for the second container;   optional heating means for the first container;   a vacuum pump in fluid communication with both containers which can lower the pressure thereof below the equilibrium pressure between ammonia and at least the first storage material;   a controller that activates the heating of the second storage material; and   a connection between the vacuum pump and the ammonia consuming unit.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  schematically shows a first container with a first solid ammonia storage material, a second container with a second solid ammonia storage material, means for heating and a vacuum pump in a first embodiment of the device according to the invention. Optionally, a one-way valve is located in front of the first container. 
           [0020]      FIG. 2  schematically shows a first container with a first solid ammonia storage material, a second container with a second solid ammonia storage material and means for heating in a second embodiment of the device according to the invention. 
           [0021]      FIG. 3  schematically shows a first container with a first solid ammonia storage material, a second container with a second solid ammonia storage material and means for heating in a third embodiment of a device according to the invention. 
           [0022]      FIG. 4  schematically shows a first container with a first solid ammonia storage material, a second container with a second solid ammonia storage material and means for heating in a fourth embodiment of the device according to the invention. 
           [0023]      FIG. 5  schematically shows a first container with a first solid ammonia storage material, a second container with a second solid ammonia storage material and means for heating in a fifth embodiment of the device according to the invention. 
           [0024]      FIG. 6  schematically shows the device according to the invention connected to an ammonia consuming unit. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    Embodiments of the invention will now be described. 
         [0026]    When solid storage and delivery systems are used for ammonia dosing for SCR in automotive applications, the temperature range for operation may be from −40° C. to 80° C. Particularly the low temperature regime can limit the performance of a vacuum operated system both from a practical and a fundamental point of view: The equilibrium pressure of the storage materials decreases exponentially with temperature. For example a typical storage material like Sr(NH 3 ) 8 Cl 2  has an equilibrium pressure of about 0.5 bar at room temperature and 0.1 bar at 0° C. To facilitate vacuum-assisted desorption, a pump is needed that can operate effectively at pressures well below the equilibrium pressure, i.e. 0.05 bar at 0° C. and even lower at lower temperatures. 
         [0027]    The cost and power requirement of the pump is directly linked to the ability to perform at the lowest pressure. From the fundamental point of view the desorption process is an activated process. The kinetics is in the simplest approximation governed by an Arrhenius expression Exp(−Ea/kT), where Ea is the desorption energy, k is Boltzmann&#39;s constant and T is the temperature. This means that the desorption process is slowed down exponentially when the temperature is lowered. This will limit the desorption ability at low temperatures. 
         [0028]    On long time scales a single storage material can be heated to a temperature where both kinetics is sufficiently fast and the pressure is sufficiently high for the pump to deliver useful amounts of ammonia. However, for practical system sizes (a passenger car may need at least 8-10 kg storage material to operate for e.g. 30,000 km), with realistic power limitations for heating, e.g. 200-400 W, the heating time to reach an appropriate pressure level suitable for operation of the pump will be unacceptably long. 
         [0029]    By the present invention vacuum-operated systems are improved to assure acceptable start up times at very low temperatures. 
         [0030]    In one embodiment at least one first or main container is filled with a first storage material having ammonia ad- and/or absorbed therein. A second or further container, also referred to as “booster”, is filled with a second, ammonia ad- and/or absorbing and desorbing material. The second material has a higher affinity for ammonia than the first material. Both containers are in fluid communication with each other, and at least the booster can be heated. Usually, the booster will be significantly smaller that the main container and have a significantly faster thermal response upon heating. A vacuum pump is connected. Above a certain temperature level sufficient amounts of ammonia can be withdrawn directly from the main container, and the booster is passive. At lower temperatures where the pump cannot withdraw enough ammonia from the main container to deliver the amounts of ammonia required by an ammonia consuming unit, the booster and optionally the main container are heated. A controller activates the heating of the second and optionally the first storage material. Upon heating, stored ammonia is activated in the booster and the pump withdraws ammonia from the booster without significantly raising the pressure in the main tank. The immediate withdrawal of ammonia activated in the booster is advantageous, since it otherwise would allow for a significant increase in the pressure level in the main tank and thereby re-adsorb or -absorb in depleted areas of the main tank. Later, when the pressure level of the main container has increased to a level where the pump can withdraw the required ammonia flow from the main container alone, heating of the booster is stopped. As the temperature of the booster decreases it reaches a point where the equilibrium pressure of the booster is below that of the main container. At that point re-saturation of the booster begins—it may even occur when the pump is running. Re-saturation will continue until the booster is fully saturated or the booster is heated to an equilibrium pressure above the system pressure. If the vacuum pump under certain low-temperature conditions cannot pump to a pressure below the equilibrium pressure of the first material, an optional one-way valve can be inserted to prevent partial re-absorption in the main container. 
         [0031]    At low temperatures the system pressure will be below ambient pressure. In any practical system there will be a non-vanishing leak rate of air into the system. If the system in an extreme case is filled with air up to one bar total pressure, the ammonia transport from the main container to the booster changes from a mass flow to a diffusion of ammonia in air. The diffusion transport is proportional to the area and inversely proportional to diffusion distance. If the main container and the booster are connected by a tube of some (small) diameter and (significant) length the diffusion transport will be extremely slow, essentially preventing any re-saturation. 
         [0032]    Accordingly, the diffusion area, A, is preferably maximized and the diffusion length, l, minimized. This gives a system, where re-saturation can be assured even in the case of slow leaking of air into the system. Preferably (A)/(l)≧0.1 cm. 
         [0033]    Heating of the containers may be achieved by various means: Both containers can be heated internally or externally with contact heating or non-contact heating. For example a electric resistance heating element can be placed inside the container thereby delivering heat to the storage material. Alternatively, waste heat in the form of hot coolant or hot gas can pass through a heating element placed inside the storage material. If electric heating or waste heat is not available, burning of some fuel may supply the heat. If non-contact heating is desired this can be accomplished by e.g. infrared radiation onto the container, induction heating or microwave heating of either the surface of the container or material inside the container. In the case of induction heating of the material inside the container, the material could be mixed with a ferromagnetic material to make the energy transfer more efficient. 
         [0034]    All combinations of heating of the first and second containers may be used. However, for SCR in automotive applications where waste heat is only available some time after engine start, heating of the second container has to be electric (electric resistance, electric induction, microwave, infrared) or by burning fuel. 
         [0035]    Two specific heating combinations are particularly mentioned: 1. Internal electric resistance heating of both containers and 2. electrical resistance heating of the second container combined with heating of the first container using waste heat from the coolant either internally or externally. 
         [0036]    In one embodiment the booster container is integrated onto or into the main container. In this way the diffusion area, A, can be maximized and the diffusion length, l, minimized. This gives a system, where re-saturation can be assured even in the case of slow leaking of air into the system. 
         [0037]    Another benefit of this embodiment is that the heat supplied to the booster will later migrate to the main unit instead of being dissipated to the surroundings. 
         [0038]    Even further, movement of ammonia from the first material to the second material consists of an endothermic desorption from the first material and an exothermic absorption in the second material. This means that the second material will heat up and the first material will cool down. This will slow down the process, however, since the first material is surrounding the second material, the heat generated in the second material efficiently migrates to the first material thereby keeping the re-saturation speed at a maximum. Since the second material has a higher ammonia affinity than the first material, the total process is exothermic. 
         [0039]    In one embodiment the heating element and the geometry of a booster integrated in the main container is chosen such that the single heating element heats both materials. Preferably, the second material should be heated first. 
         [0040]    In another embodiment, the booster is heated using an external heat source. The heat source could for example be (but is not limited to) an electric resistance heater, or waste heat from the engine or exhaust system. In one variation the heat source is radiant, such as infrared heating, microwave heating or induction heating. This has the advantage that an intimate thermal contact between the container and the heating element is unnecessary. 
         [0041]    The vacuum pump is connected with an ammonia consuming unit, usually via a valve. A buffer may be placed between the pump and the ammonia consuming unit. 
         [0042]    The ammonia consuming unit may, e.g., be a fuel cell working with ammonia or a catalyst splitting ammonia into nitrogen and hydrogen or preferably a catalyst for selective catalytic reduction of NO x  in exhaust gases from combustion processes, such as exhaust gases from a combustion engine in a motor vehicle, using ammonia as a reductant. 
         [0043]    The first solid storage material capable of releasing ammonia by desorption is a material in which ammonia is adsorbed or absorbed. Materials that may contain adsorbed ammonia are e.g. acidic carbon or certain zeolites. Materials in which ammonia is absorbed are, e.g., metal ammine salts. These materials can absorb ammonia reversibly, i.e. after the desorption of the ammonia contained therein, they can re-absorb ammonia. The second solid storage material is one which reversibly desorbs and ad- or absorbs ammonia, e.g., acidic carbon or a metal ammine salt. 
         [0044]    Preferred metal ammine salts in the present invention have the general formula: M a (NH 3 ) n X z , wherein M is one or more cations selected from alkali metals such as Li, Na, K or Cs, alkaline earth metals such as Mg, Ca, Ba or Sr, and/or transition metals such as V, Cr, Mn, Fe, Co, Ni, Cu, or Zn or combinations thereof such as NaAl, KAl, K 2 Zn, CsCu, or K 2 Fe, X is one or more anions selected from fluoride, chloride, bromide, iodide, nitrate, thiocyanate, sulphate, molybdate, and phosphate ions, a is the number of cations per salt molecule, z is the number of anions per salt molecule, and n is the coordination number of 2 to 12. 
         [0045]    More preferably, the first storage material is CaCl 2 , SrCl 2 , BaCl 2  or mixtures thereof and the second storage material is MgCl 2 , FeCl 2 , NiCl 2  or mixtures thereof or mixtures of one or more of the latter with the first storage material. 
         [0046]    It is particularly preferred that the first storage material is SrCl 2  and the second storage material is MgCl 2 . 
         [0047]    The achievements of the invention are among others
       a minimum time to dosing sufficient ammonia even at very low temperatures,   a minimum power requirement even at very low temperatures, and   an efficient re-saturation of the booster unit during any driving pattern, when the system is used in automotive applications, even at very low temperatures.       
 
         [0051]    Turning now to  FIG. 1 , first or main container  10  filled with first material  14  is connected in fluid contact  16 , optionally via one-way valve  17 , with smaller container (booster)  20  containing second material  24 . Means for heating  22  is shown for the booster  20  and optional means for heating  12  is shown for the main container  10 . Two possible, alternative connection lines shown as dotted lines  18   a,    18   b  lead to vacuum pump  28 . The process requiring NH 3  from the system (e.g SCR) is not shown, but is located downstream of the pump. 
         [0052]      FIG. 2  shows booster  120  having perforated walls which is integrated into the main container  110 . Two different means  112 ,  122  for heating first material  114  and second material  124  are shown. The heating means  112  for the first material  114  is optional. Line  118  leads to a vacuum pump (not shown). 
         [0053]      FIG. 3  shows an example of a geometry where a single heating element  222  can heat both second material  224  contained in booster  220  having perforated walls and first material  214  in main container  210 . Line  218  leads to the vacuum pump (not shown). 
         [0054]      FIG. 4  shows an example of external heating  322  of booster  320  having perforated walls and containing second material  324  which is integrated in main container  310  containing first material  314 . Line  318  leads to a vacuum pump (not shown). 
         [0055]      FIG. 5  shows an example of external radiant heating  422  of booster  420  having perforated walls and containing second material  424  which is integrated in main container  410  containing first material  414 . Line  418  leads to a vacuum pump (not shown). 
         [0056]      FIG. 6  shows an example of a device  500  for storing and controlled release of gaseous ammonia from a solid storage medium. The device  500  comprises
       a first container  510  containing a first solid storage material  514  capable of releasing ammonia by desorption;   a second container (booster)  520  which is built into the first container  510  in fluid communication with the first container  510  through a perforated wall  521  and smaller than that first container  510  containing a second solid storage material  524  capable of ad- or absorbing ammonia reversibly and having a higher affinity for ammonia than the first storage material;   heating means  522  for the second container  520 ;   a vacuum pump  528  in fluid communication with both containers through line  518  and perforated wall  521 , which can lower the pressure thereof below the equilibrium pressure between ammonia and at least the first storage material  521 ;   a connection  530  between the vacuum pump  528  and the ammonia consuming unit  540 , optionally comprising a valve  536 , and   an active or passive controller  538  that activates the heating of the second storage material  524 , usually in response to a demand signal;   optional pressure sensing  534 , and   optional buffer volume  532 .       
 
       EXAMPLES 
     Example 1 
       [0065]    In an experiment similar to  FIG. 1  with a booster containing 100 g of MgCl 2  and a main container filled with 5 kg of saturated Sr(NH 3 ) 8 Cl 2 , resaturation of the booster was investigated. At room temperature the system pressure was 0.4 bar (absolute). Re-saturation rates (of ammonia absorbing in the depleted MgCl 2 ) between 2-4 g/hour were measured. At 5° C. the re-saturation rate drops to about 1 g/hour. 
       Example 2 
       [0066]    A system similar to  FIG. 2  was investigated comprising saturated MgCl 2  (i.e. Mg(NH 3 ) 6 Cl 2 ) in the booster and a main container filled with 5 kg of saturated Sr(NH 3 ) 8 Cl 2 . The heating element of the booster has a power of 240 W. Ammonia was pumped from the system with a vacuum pump. Before the experiment was started, the system was in equilibrium at a certain temperature. When the experiment was initiated the pump and booster heating element was switched on and the ammonia flow from the pump was measured. The time-to-dose is the time measured until the ammonia flow exceeds 0.2 g/min via the pump. At system temperatures above 10° C. the time-to-dose was essentially zero, since the main container can immediately deliver more than 0.2 g/min. At lower temperature the time-to-dose gradually increased until 4 minutes at −25° C. When the pump is not modified, the time-to-dose from the booster-enhanced system is mainly determined by a combination of the mass of material in the booster and the power of the heating element. A low mass of material and high power of the heater gives a rapid activation of the storage mass in the booster, and the pressure level in the booster enables the pump to withdraw sufficient ammonia flow after a short heating time. 
         [0067]    All cited patents, patent applications and journal articles referred to in this specification are hereby incorporated by reference.