Abstract:
Solid metal ammine complexes are applied for safe and high-density storage of ammonia to be released for use as reducing agent in selective catalytic reduction of NO x  in exhaust gases. The compositional formula of the metal ammine complexes is M(NH 3 ) n X z , where M Z+  represents one or more metal ions capable of binding ammonia, X represents one or more anions, n is the coordination number (from 2 to 12), and z the valency of the metal ion (and thus the total number of compensating anion charges). Ammonia is released by generating a reduced gas-phase pressure of ammonia inside the container, which is below the equilibrium desorption pressure of ammonia at the operating temperature of the storage material thus enabling the unit to be operated is a safe manner with lower operating temperature and pressure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to the use of metal ammine complexes for storage of ammonia in solid form and a system utilizing the solid storage material for delivery of ammonia by controlled release of ammonia from the material using vacuum as a driving force. Upon release, ammonia may be used as the reducing agent in selective catalytic reduction (SCR) of NO x  in exhaust gases from combustion processes or as a fuel for a fuel cell. 
         [0003]    Other applications using ammonia in mobile or portable units or in special chemical synthesis routes where storage of liquid ammonia is too hazardous are also considered embodiments of the present invention. 
         [0004]    2. Description of the Related Art 
         [0005]    Current environmental regulations necessitate the use of catalysts in the treatment of exhaust gas from automotive vehicles, boilers and furnaces for control of NO x  emissions. Particularly, vehicles equipped with diesel or other lean burn (gasoline) engines offer the benefit of improved fuel economy, but catalytic reduction of NO x  using conventional car exhaust catalysts (three-way catalyst) is not feasible because of the high oxygen content in the exhaust gas. Instead, selective catalytic reduction (SCR) has proven useful for achieving the required low levels of NO x  in the exhaust gas both in stationary and mobile units. In such systems NO x  is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NO x . 
         [0006]    So far, the most efficient reductant has been ammonia, which is usually introduced into the exhaust gas by controlled injection either of gaseous ammonia (delivered from liquid ammonia stored under pressure), aqueous ammonia or indirectly as urea dissolved in water. In all cases, the amount of reductant being dosed has to be very precisely controlled. Injection of too high amount of reductant will cause a slip of ammonia in the exhaust gas whereas injection of a too small amount of reductant causes a less than optimal conversion of NO x . 
         [0007]    In many mobile units, the only available technology is to use an aqueous solution of urea as the reductant since in this way potential hazards or safety issues relating to the transport of liquid ammonia are eliminated. However, there are several disadvantages related to the use of aqueous urea as the reductant. First of all, the use of urea solutions requires that a relatively large storage volume is available in order to enable transport sufficient amounts of ammonia. In typical systems, about 30 wt % urea solution is preferred meaning that about 70 wt % of a container holding the urea solution is used only to transport water. During operation the urea solution is sprayed into the exhaust gas, the droplets evaporate and the urea decomposes more or less selectively to ammonia (one molecule of urea forms two molecules of NH 3  and one CO 2 ) which by mass is roughly 50 wt % of ammonia in the urea molecule. Similar concentrations of ammonia can be achieved using aqueous solutions of ammonia as reductant. 
         [0008]    Furthermore, for technologies using aqueous solutions a specially designed spray nozzle combined with a precision liquid pump is required to ensure that a) the aqueous urea is delivered to the exhaust system at a desired (and dynamically changing) flow rate and b) that the solution is efficiently dispersed as fine droplets in the gas phase before entering the catalyst. Furthermore, the aqueous solutions might freeze in extreme weather conditions, or the urea solution may simply form precipitates, which might block the dosing system, e.g. the nozzle. Therefore, all lines have to be heated. Furthermore, the decomposition of urea may not proceed as desired. There may be undesired side-reactions giving byproducts in the form of solid deposits of polymers (melamine) and these side reactions also make it difficult to dose a very specific amount of ammonia since the amount of free ammonia released from a given amount of urea can vary according to the decomposition conditions. 
         [0009]    Altogether, these difficulties may limit the possibilities of using SCR technology in pollution abatement, particularly in connection with mobile units. To circumvent these difficulties, the present invention devises an alternative method for transporting and dosing ammonia to exhaust gases prior to entering SCR catalyst systems. 
         [0010]    As disclosed in applicant&#39;s copending application No. WO 2006/012903 metal ammine salts can be used as a solid storage media for ammonia which in turn may be used as the reductant in selective catalytic reduction to reduce NO x  emissions from automotive vehicles, boilers and furnaces. Thus, the metal-ammine salt constitutes a solid storage medium for ammonia, which represent a safe and practical option for storage and transportation of ammonia. Usually, ammonia is released thermally at a gauge pressure (i.e. above atmospheric pressure) from the preferred metal ammine salt by external heating, see e.g. European Patent No. EP 0 932 440 B1. The metal ammine salt is held in a container from which the released ammonia is dosed through a controllable valve directly into the exhaust gas in the desired proportion. Between the container and the valve, there may be a small buffer volume to increase the controllability of the system. 
         [0011]    Useful metal ammine salts have the general formula M(NH 3 ) n X z , where M z+  is one or more metal ions capable of binding ammonia (For example M may be Li, Mg, Ca, Sr, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, etc.), n is the coordination number (2-12), and X is one or more anions, where representative examples of X are F, Cl, Br, I, SO 4 , MoO 4 , PO 4  etc. 
         [0012]    During release of ammonia, the original metal-ammine salt M(NH 3 ) n X z  is gradually transformed into M(NH 3 ) m X z  with m&lt;n. When all the desired ammonia has been released, the resulting M(NH 3 ) m X z  can usually be converted back into M(NH 3 ) m X z  by an absorption treatment with an ammonia-containing gas stream. 
         [0013]    Typical ammonia contents of the metal ammine complexes are in the range of 20-60 wt %, preferably above 30 wt %. As an example, a typical and inexpensive compound such as Mg(NH 3 ) 6 Cl 2  contains 51.7 wt % ammonia. Another example is Ca(NH 3 ) 8 Cl 2 , which contains 55 wt % NH 3 . Sr(NH 3 ) 8 Cl 2  has a similar ammonia storage density. Using a compaction method such as the one disclosed in applicant&#39;s copending application Nos. PCT/DK 2006/00059 and WO 2006/012903 it is demonstrated that it is possible to obtain an ammonia density within few percent from the theoretical maximum storage density of the solid, which gives above 90% of the volumetric capacity of liquid ammonia. This is a preferred unique combination of high density and safety in ammonia handling. 
         [0014]    The use of applicant&#39;s technology enables storage of ammonia at significantly higher densities (both on a volume and a weight basis) than both aqueous ammonia and aqueous urea solutions. For several metal ammine salts it is possible to release all ammonia and then transform the resulting material back into the original metal ammine salt in a large number of cycles. This obviously constitutes preferred embodiments. Additionally, the ammonia is directly delivered into the exhaust in the form of ammonia gas, which is an advantage in itself—both for the simplicity of the flow control system and for an efficient mixing of reducing agent, ammonia, with the exhaust gas—but it also eliminates potential difficulties related to blocking of the dosing system because of precipitation or impurities in a liquid-based system. 
         [0015]    For mobile units, it is particularly useful to hold the metal ammine in a container that can be easily separated from mobile unit and replaced by a new metal ammine container. In preferred embodiments, the metal ammine containers are recycled and recharged with ammonia in a separate recharging unit or recharging facility. 
         [0016]    Usually, ammonia is released from absorbed state by heating the material by electrical resistance in heating elements or by using the heat from e.g. an exhaust gas. In such a process, ammonia is made available and delivered to the desired location by generating a gauge pressure of ammonia by desorption in the storage container in order to control the flow of ammonia through a valve and further on into e.g. an exhaust gas line. When ammonia is thermally desorbed from the solid to generate an elevated ammonia supply pressure, the operating temperature of the desorption process is increased significantly due to the thermodynamic relation between vapour pressure and temperature. Consequently, the system is less safe due to the availability of desorbed ammonia at a pressure above the standard pressure of the surroundings. 
         [0017]    The present invention offers a simple solution for overcoming the drawbacks of the prior art and enables operation of a unit for absorbing/desorbing ammonia in a more safe manner at lower operating temperature and pressure as appears from the below. 
       SUMMARY OF THE INVENTION 
       [0018]    The present invention relates to a method of storing and delivering ammonia, said method comprising the steps of: 
         [0000]    (a) providing a container;
 
(b) placing solid material capable of storing/delivering ammonia by absorption/desorption in said container;
 
(c) effecting ammonia release by desorption using a vacuum pump to reach an ammonia gas phase pressure in said container, which is below the corresponding thermodynamic equilibrium ammonia pressure of the solid at the operating temperature of the container.
 
         [0019]    In a second aspect the invention relates to a system for removing NOx from an oxygen-containing exhaust gas of a combustion engine or combustion process, the system comprising: 
         [0000]    (a) a container with a solid material capable of storing/delivering ammonia by absorption/desorption inside said container;
 
(b) means for generating a reduced pressure of gas-phase ammonia in the container, which is below the corresponding equilibrium ammonia pressure of the solid at the operating temperature of the container, thereby effecting ammonia release by desorption
 
(c) optional means for heating in order to compensate for the reduction in temperature caused by the ammonia desorption process thereby maintaining a desired operating temperature
 
(d) means for introducing gaseous ammonia from the container into an exhaust gas,
 
(e) a catalyst for reducing NOx by reaction with the dosed ammonia, and
 
(f) means for controlling the amount of ammonia to give an optimal ratio between NOx and ammonia in order to obtain high NOx conversion while minimizing ammonia slip from the gas down-steam from catalyst.
 
         [0020]    In a third aspect the invention relates to a device for providing ammonia for a selective catalytic reduction of NOx in an oxygen-containing exhaust gas of a combustion engine or combustion process by using gaseous ammonia and a reduction catalyst, the device comprising: 
         [0000]    (a) a container for containing a solid ammonia storage material capable of storing/delivering ammonia by absorption/desorption;
 
(b) means for generating a reduced pressure of gas-phase ammonia in the container, which is below the corresponding equilibrium ammonia pressure of the solid at the operating temperature of the container, thereby effecting ammonia release by desorption,
 
(c) optional means for heating in order to compensate for the drop in temperature caused by the ammonia desorption process thereby maintaining a desired operating temperature
 
(d) means for introducing gaseous ammonia from the container into an exhaust line before a NOx reduction catalyst;
 
(e) means for controlling the amount of ammonia introduced into the exhaust line, depending on the operating conditions of the engine.
 
         [0021]    In a forth aspect, the invention relates to the use of a vacuum pump to generate a reduced ammonia gas-phase pressure in a container with a solid ammonia storage material thereby facilitating ammonia release by desorption and furthermore means for heating said storage container in order to compensate for the drop in temperature caused by the ammonia desorption process thereby maintaining the container at a desired operating temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention is disclosed more in detail with reference to the drawings in which 
           [0023]      FIG. 1  schematically shows an embodiment of a system according to the invention, 
           [0024]      FIG. 2  schematically shows another embodiment of a system according to the invention, 
           [0025]      FIG. 3  schematically shows a third embodiment of a system according to the invention, 
           [0026]      FIG. 4  schematically shows a forth embodiment of a system according to the invention, 
           [0027]      FIG. 5  schematically shows a fifth embodiment of a system according to the invention, 
           [0028]      FIG. 6  schematically shows a sixth embodiment of a system according to the invention, and 
           [0029]      FIG. 7  schematically shows a seventh embodiment of a system according to the invention, 
       
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
       [0030]    The present invention relates to a method of storing and delivering ammonia, said method comprising the steps of: 
         [0000]    (a) providing a container;
 
(b) placing solid material capable of storing/delivering ammonia by absorption/desorption in said container, said material containing absorbed ammonia;
 
(c) effecting ammonia release by desorption using a vacuum pump to reach an ammonia gas phase pressure in said container, which is below the corresponding thermodynamic equilibrium ammonia pressure of the solid at the operating temperature of the container.
 
         [0031]    Ammonia can e.g. be stored safely and efficiently as a solid material—more specifically as metal ammine complexes of the general formula M a (NH 3 ) n X z , wherein M is one or more cations selected from alkaline earth metals such as Mg, Ca or Sr, and/or one or more transition metal ions, such as Mn, Fe, Co, Ni, Cu, and/or Zn, X is one or more anions, 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. Such complexes have well-defined absorption and desorption characteristics. From a saturated or partly saturated metal ammine salt, there is a given equilibrium pressure of ammonia above the solid as a function of temperature. Ammonia can be released by heating the material to a temperature resulting in a desorption pressure of ammonia, which is higher than the (partial) pressure of ammonia in the gas phase. 
         [0032]    According to the present invention, storage and delivery is made even more safe and versatile by using a vacuum pump to facilitate the release of ammonia from the storage material. Thereby, the storage container is maintained at a lower pressure and a lower temperature and in case of an accident there will be almost no leak of ammonia since a hole in the container will result in gas from the surroundings flowing into the low-pressure container in stead of ammonia leaking out of a heated container with a gauge pressure of ammonia. After desorption controlled by the low-pressure state in the container, ammonia is dosed into the desired location/process/fluid/gas by controlling a flow of ammonia from the container—through an optional buffer volume—using an adjustable valve. 
         [0033]    Using vacuum as the driving force for ammonia desorption from the storage material reduces the operating temperature of the system since it is not necessary to operate the storage/desorption container at the temperature where the material would release all ammonia under an elevated ammonia pressure. 
         [0034]    In applicant&#39;s co-pending applications WO 2006/012903 and PCT/DK 2006/000059 is disclosed methods and devices for ammonia storage and delivery using compacted metal ammine salts. According to these applications the use of a solid storage material with a low equilibrium pressure of ammonia (preferably below 0.1 bar at room temperature) gives superior safety of the storage method. Using the method of compaction, the saturated material into e.g. dense rods gives a very high ammonia density compared to e.g. granulated material. The ammonia densities in such compacted structures give an ammonia density above 90% of the volumetric ammonia density of liquid ammonia. 
         [0035]    In accordance with the present invention, the method of delivery is made even more safe and versatile by using a vacuum pump to effect release of ammonia from the storage material into a (facultative) small buffer volume with a moderate pressure, e.g. 0.5-3 bar. Ammonia is then dosed into the desired process or fluid/gas by controlling a flow of ammonia from the buffer volume using an adjustable valve such as a reduction valve. 
         [0036]    Using Mg(NH 3 ) 6 Cl 2  as on example of a material where the present invention would be useful, the operating conditions are changed substantially. A normal thermally driven desorption of ammonia into a container (and optional buffer volume) with a pressure of e.g. 2 bar would require an operating temperature of the desorption unit of approximately 675 K in order to drive out al ammonia from the storage material. Reducing the pressure in the storage container to e.g. 0.01 bar, ammonia will forcibly be desorbed from the storage material much closer to room temperature. All ammonia is released using a reduced operating temperature of 500K, i.e. a reduction of almost 200K. If the pressure of 0.001 bar in the storage unit is used, the operating temperature is further reduced to 450K. This is summarized in table 1. 
         [0037]    Table 1: Operating temperature as a function of pressure level in the storage/delivery unit. The data shown in Table 1 are for Mg(NH 3 ) 6 Cl 2 . Similar trends are found for all other metal amine salts, e.g. SrCl 2 - or CaCl 2 -based storage material. For comparison, the operating temperature is shown for the case where the driving force for getting ammonia out is by thermal desorption; generating a pressure of 2 bar in the storage/delivery unit. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Approximate operating temperature 
               
               
                   
                 Pressure level in storage 
                 of storage container 
               
               
                   
                 container 
                 in order to release 
               
               
                   
                 (bar) 
                 ammonia 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0.5 
                 640 K 
               
               
                   
                 0.1 
                 560 K 
               
               
                   
                 0.01 
                 500 K 
               
               
                   
                 0.001 
                 450 K 
               
               
                   
                 2 
                 680 K 
               
               
                   
                   
               
             
          
         
       
     
         [0038]    The overall considerations of the present invention are as follows:
       The driving force for getting ammonia out of the storage material
           is not provided from thermal desorption using heat to provide a gauge pressure of ammonia above the level of the surroundings.   but is provided from generating vacuum (reduced pressure) in the storage container using a pump and thus pumping ammonia from the storage unit into a small buffer volume from which a controlled ammonia delivery is made through an adjustable valve.   
           Suitable kinetics of the vacuum-driven desorption process are obtained when the unit is maintained at a given temperature. For Mg(NH 3 ) 6 Cl 2 , the operating temperature is suitably in the range of 273-560K. For Ca(NH 3 ) 8 Cl 2  and Sr(NH 3 ) 8 Cl 2  the operating temperature is suitably in at a given level by means of heating since desorption of ammonia is endothermic. Thus, optional heating may compensate for a drop of temperature of the storage container during desorption of ammonia, which is controlled by the pump.   The present invention enhances the safety of the storage/delivery system since the storage container is not operated a pressure above the level of the surroundings (i.e. atmospheric pressure). During e.g. an accident, a crack in the storage container would not result in a sudden release of an amount of gaseous ammonia since the container is operated at an absolute pressure below 1 bar. Contrary, the container would take up air from the surroundings up to a pressure of roughly 1 bar and additionally the humidity from the surrounding air would passivate the surface of the metal ammine salt thus slowing down any potential release of ammonia from the material.   The start-up and termination of the ammonia release is made faster since the driving force (reduced pressure) can be obtained faster than an initial heating step of the thermal desorption process e.g. of WO 99/01205 or WO 2006/012903.       
 
         [0045]    The present invention is useful in any system requiring safe and controlled delivery/dosing of ammonia from a high-density solid storage material containing ammonia. The invention is useful both as a general method but also specifically for devices based on ammonia delivery from a solid ammonia storage medium, where the driving force for ammonia release is reduced pressure/vacuum. 
         [0046]    The invention is useful for different kinds of formulated solid forms of ammonia storage materials, e.g. solid rods/cylinders/blocks with low porosity and also for granulated storage material with a porosity generated from the loss in packing density due to the space between the individual granules. 
         [0047]    The present invention is particular useful in the controlled delivery of ammonia to an exhaust gas containing NOx for the purpose of removing NOx by the well-known selective catalytic reduction (SCR) process. 
         [0048]    The present invention is furthermore useful for both mobile and stationary SCR-applications but is considered particularly useful in the delivery of ammonia for automobiles, trucks and ships fuelled on diesel or using lean-burn gasoline or even for ammonia or hydrogen-fuelled engines. 
         [0049]    The present invention is also useful for delivery of ammonia to solid oxide fuel cells (SOFC), which can operate on a number of fuels, e.g. hydrogen, methane but also directly on gaseous ammonia. It is also useful for the delivery of ammonia to an integrated ammonia decomposition unit for the production of hydrogen, which subsequently may be used as fuel in a PEM Fuel Cell, an alkaline fuel cell or a molten carbonate fuel cell. 
         [0050]    In a preferred embodiment of the invention, a given mass of storage material capable of binding ammonia by absorption is present in a container with a small vacuum pump connected to it. When the vacuum pump is active it generates a reduced gas-phase pressure of ammonia inside the container and when the pressure has reached a level, which is below the thermodynamic equilibrium of ammonia desorption from the storage material, there will be a driving force for ammonia desorption into the gas phase solely controlled by a pressure gradient and not by an increase in desorption pressure as a result of heating. 
         [0051]    In another preferred embodiment, the ammonia storage material is fully saturated with ammonia before the material is placed inside the storage container. 
         [0052]    When ammonia is desorbed from the solid controlled by the pressure gradient, the enthalpy of desorption may cause the material to cool down as the desorption proceeds. In a situation of extremely efficient thermal insulation or very high desorption rates, this drop in temperature will result in a corresponding reduction in equilibrium vapour pressure of the desorption process and eventually the desorption pressure will be equal to the pressure level in the container controlled by the vacuum pump. At this point in time, desorption will terminate. Therefore, an embodiment of the invention involves means of maintaining the storage container at a constant temperature level thus compensating for the cooling effect of the desorption process. A preferred method of heat compensation is by well-known electric resistive heating or using heat generated in a chemical process. Another preferred method is to allow for thermal equilibration between the storage container and its surroundings. This is preferred if the operating temperature of the container is designed to be near room temperature. Yet another preferred method of heat compensation is to use waste heat from a combustion process such as the waste heat from the exhaust gas of an automotive unit such as a diesel or gasoline engine exhaust gas. Such an engine may e.g. be operating on gasoline, diesel or other hydro-carbon fuels such as ethanol or on synthetic fuel such as ammonia or hydrogen. 
         [0053]    In a preferred embodiment of the invention, the solid ammonia storage material is a metal ammine complex capable of binding ammonia by absorption into the crystal lattice of the inorganic salt. Such salts are of 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 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, 
         [0054]    Choice of preferred salts will depend of the desired operating temperature and degree of safety. One preferred choice of salt complex could be based on CaCl 2 , where the fully saturated compound is Ca(NH 3 ) 8 Cl 2 . The vapour pressure of ammonia at room temperature is approximately 0.5 bar and in such an embodiment, a vacuum pump could release ammonia at a temperature of the surroundings by reducing the pressure inside the container to below 0.5 bar. Operating temperature may be maintained simply by normal thermal equilibrium between the container and its surroundings or it may be maintained by electrical heating, exhaust gas heat or by circulating engine coolant around the container. Another compound with the same range of ammonia desorption pressure (0.8 bar) at room temperature is Sr(NH 3 ) 6 Cl 2 . 
         [0055]    Another preferred embodiment—with a higher degree of safety involved—is based on MgCl 2  (with Mg(NH 3 ) 6 Cl 2  in saturated form). The vapour pressure at room temperature is as low as 0.002 bar and establishment of a vacuum below 0.002 bar would have to be obtained in order to facilitate ammonia desorption at room temperature but suitable desorption kinetics may be obtained at slightly a higher temperature as already seen from table 1. There is a dramatic change in operating temperature when vacuum is applied as opposed to thermally activated desorption delivering ammonia at a gauge pressure. 
         [0056]    In a preferred embodiment of the method of the invention the ionic salt is MgCl 2 , CaCl 2 , SrCl 2  or mixtures thereof. 
         [0057]    A particularly preferred embodiment of the invention is related to applicants own co-pending invention (No. PCT/DK 2006/00059) according to which very high storage capacities are obtained when the saturated storage material is processed from a low-density powder to a high-density, shaped solid with almost no internal pore volume. It is particularly advantageous to use the high-density storage capacity of PCT/DK 2006/00059 in combination with the method of ammonia desorption of the present invention. 
         [0058]    Thus, it is preferred that the saturated ammonia containing material is compacted to a dense block, rod, cylinder, ring or edged unit such as cube with a density above 70% of the theoretical maximum skeleton density of the saturated solid material before placing in the container. 
         [0059]    Yet another preferred embodiment uses a control of the vacuum pump to control the desorption rate of ammonia from the solid. The desorption process can be rapidly terminated by shutting down the vacuum pump and similarly, fast start-up can be made by activating the vacuum pump. In such embodiments, there will be further components in the total system, i.e. fittings, control valves, an optional buffer volume, reduction valve etc. 
         [0060]    The present invention is particularly advantageous as an ammonia storage/delivery technology for selective catalytic reduction of NOx from e.g. a combustion process or more directly an engine. In such an embodiment, the system would comply of storage material, vacuum pump, optional means for heating (compensation of desorption heat) and a control valve for accurate dosing of ammonia to the NOx-containing exhaust. The system would also involve a catalyst for converting NOx and NH to nitrogen (N 2 ) and water. 
         [0061]    As on-board storage of ammonia as the reductant for DeNOx, the present is particularly advantageous for NOx-aftertreatment in exhaust of diesel and lean-burn gasoline vehicles or stationary engines such as power generators. These engines and/or combustion process may be fuelled by a range of fuels such as diesel, gasoline, natural gas or other fossil fuels or a bio-fuel (methane, ethane, methanol, ethanol or similar) and even ammonia and hydrogen. 
         [0062]    The present invention is particularly advantageous for DeNOx-technology on passenger automobiles, trucks, ships, trains, small/medium generators and even power plants. 
         [0063]    In a preferred embodiment the released ammonia is used in NOx emission reduction from stationary or mobile combustion engines fuelled by methanol, ethanol, hydrogen, methane, ethane or any other bio- or synthetic fuel such as ammonia and hydrogen 
         [0064]    The method is thus suitable for methods wherein the released ammonia is used in NOx emission reduction from stationary or mobile power plants fuelled by coal, natural gas, oil or any other fossil fuels. 
         [0065]    The present invention is generally applicable in methods and systems where safe storage of ammonia is made feasible by the controlled release technology controlled by the active vacuum pumping and the corresponding heat-compensation to maintain a suitable kinetics of the process. Ammonia made available in such an embodiment may have a range of other applications such as ammonia supply to remote and/or small chemical synthesis process involving ammonia. Such processes may also be related to power generation where ammonia is an indirect fuel as a source for hydrogen for a hydrogen-fuelled fuel cell or by direct ammonia feed to a fuel cell capable of running directly on ammonia (e.g. a solid oxide fuel cell). 
         [0066]    In a second aspect the invention relates to a system for removing NOx from an oxygen-containing exhaust gas of a combustion engine or combustion process, said the system comprising: 
         [0000]    (a) a container with a solid material capable of storing/delivering ammonia by absorption/desorption inside said container;
 
(b) means for generating a reduced pressure of gas-phase ammonia in the container, which is below the corresponding equilibrium ammonia pressure of the solid at the operating temperature of the container, thereby effecting ammonia release by desorption
 
(c) optional means for heating the contents of the container in order to compensate for the drop in temperature caused by the ammonia desorption process thereby maintaining a desired operating temperature
 
(d) means for introducing gaseous ammonia from the container into an exhaust gas,
 
(e) a catalyst for reducing NOx by reaction with the dosed ammonia, and
 
(f) means for controlling the amount of ammonia to give an optimal ratio between NOx and ammonia in order to obtain high NOx conversion while minimizing ammonia slip from the gas down-steam from catalyst.
 
         [0067]    In a third aspect the invention relates to a device for providing ammonia for a selective catalytic reduction of NOx in an oxygen-containing exhaust gas of a combustion engine or combustion process by using gaseous ammonia and a reduction catalyst, said the device comprising: 
         [0000]    (a) a container with a solid material capable of storing/delivering ammonia by absorption/desorption inside said container;
 
(b) means for generating a reduced pressure of gas-phase ammonia in the container, which is below the corresponding equilibrium ammonia pressure of the solid at the operating temperature of the container, thereby effecting ammonia release by desorption
 
(c) optional means for heating the contents of the container in order to compensate for the drop in temperature caused by the ammonia desorption process thereby maintaining a desired operating temperature
 
(d) means for introducing gaseous ammonia from the container into an exhaust gas,
 
(e) a catalyst for reducing NOx by reaction with the dosed ammonia, and
 
(f) means for controlling the amount of ammonia introduced into the exhaust line, depending on the operating conditions of the engine.
 
         [0068]    Such a device is very suitable for use for reduction of NO x  according to the method of the invention when applied to automobiles, trucks, trains, ships or any other motorized machine. 
         [0069]    Furthermore, such a device is very suitable for use for reduction of NO x  according to the method of the invention when applied to power plants generating electricity. 
         [0070]    In a fourth aspect the invention relates to the use of a pump to generate a reduced gas-phase pressure in a container with a solid ammonia storage material capable of storing/delivering ammonia by absorption/desorption for effecting ammonia release by desorption. 
         [0071]    In a preferred embodiment, container further comprises means for heating said storage container in order to compensate for the drop in temperature caused by the ammonia desorption process thereby maintaining the container at a desired operating temperature. 
         [0072]    In one embodiment of the use according to the invention, the released ammonia is used as fuel in an ammonia-fuelled fuel cell. 
         [0073]    In another embodiment of the use according to the invention, the released ammonia is fed to an ammonia decomposition reactor wherein ammonia is decomposed to nitrogen and hydrogen, and at least the hydrogen is used as fuel in a hydrogen-fuelled fuel cell. 
       DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0074]    The invention is now explained more in detail with reference to the drawings showing preferred embodiments of the invention. 
         [0075]    Reference is made to  FIG. 1  which schematically shows a system according to the invention comprising a storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt shown as a hatched unit, which system is operated at a reduced pressure using a vacuum pump ( 2 ). The pump draws ammonia from the storage material and delivers ammonia into a buffer volume ( 3 ) and ammonia is subsequently delivered to a consumer ( 5 ) such as a process or an ammonia consuming unit which has a need of a supply of ammonia through a dosing valve ( 4 ). Ammonia can alternatively be delivered directly from the pump ( 2 ) to the consumer ( 5 ) as indicated by the dotted line. 
         [0076]    In the embodiment shown in  FIG. 2  a storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt shown as a hatched unit, is operated at a reduced pressure using a vacuum pump ( 2 ). The pump delivers ammonia into a buffer volume ( 3 ) and ammonia is subsequently delivered to a consumer ( 5 ) through a dosing valve ( 4 ). Ammonia can also be delivered directly from the pump ( 2 ) to the consumer ( 5 ). The entire dosing system is monitored and/or controlled by a controller ( 6 ). 
         [0077]    In the embodiment shown in  FIG. 3  a storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt shown as a hatched unit, is operated at a reduced pressure using a vacuum pump ( 2 ). The pump delivers ammonia into a buffer volume ( 3 ) and ammonia is subsequently delivered to a consumer ( 5 ) through a dosing valve ( 4 ). Ammonia can also be delivered directly from the pump ( 2 ) to the consumer ( 5 ). The entire dosing system is monitored and/or controlled by a controller ( 6 ). The heat, which is necessary to maintain a given operating temperature, i.e. to compensate for the endothermic desorption of ammonia controlled by the vacuum pump ( 2 ), is supplied to the unit ( 1 ) through a heater ( 7 ). The heat can be provided by electrical heating or by heat produced in a separate process step or unit, e.g. an engine or a fuel cell. The heat compensating for ammonia desorption may also be provided by normal thermal equilibration between the container ( 1 ) and the surroundings. 
         [0078]      FIG. 4  shows an embodiment of a device for controlled delivery of ammonia to the exhaust gas of a combustion process. A storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt shown as a hatched unit, is operated at a reduced pressure using a vacuum pump ( 2 ). The pump delivers ammonia into a buffer volume ( 3 ) and ammonia is subsequently delivered, through a dosing valve ( 4 ), to a conduit carrying an exhaust gas ( 8 ) containing NO x  form a mobile or stationary combustion process or engine, wherein the gas mixture is passed through a SCR (Selective Catalytic Reduction) DeNOx catalyst ( 9 ). Ammonia can also be delivered directly from the pump ( 2 ) to the exhaust gas conduit ( 8 ). The entire dosing system is monitored and/or controlled by a controller ( 6 ). A heater ( 7 ) supplies heat which is necessary to maintain a given operating temperature, i.e. to compensate for the endothermic release of ammonia controlled by the vacuum pump ( 2 ), to the unit ( 1 ). The heat can be provided by electrical heating or in the form of waste heat from a separate process step, e.g. an engine or a combustion process. As indicated by the dotted arrow, it may also be provided from the exhaust gas ( 8 ) to the container ( 1 ). The heater ( 7 ) in  FIG. 4  may be redundant if the operating temperature of the container ( 1 ) is close to the temperature of the surroundings. 
         [0079]      FIG. 5  shows an embodiment of a device for controlled delivery of ammonia to an ammonia-fuelled fuel cell. A storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt shown as a hatched unit, is operated at a reduced pressure using a vacuum pump ( 2 ). The pump delivers ammonia into a buffer volume ( 3 ) and ammonia is subsequently delivered to an ammonia-fuelled fuel cell ( 10 ) through a dosing valve ( 4 ). Ammonia can also be delivered directly from the pump ( 2 ) to the fuel cell ( 10 ). The entire dosing system is monitored and/or controlled by a controller ( 6 ). A heater ( 7 ) supplies heat which is necessary to maintain a given operating temperature, i.e. to compensate for the endothermic desorption of ammonia controlled by the vacuum pump ( 2 ), to the unit ( 1 ). The heat can be provided by electrical heating or in the form of waste heat from a separate process step, e.g. the fuel cell ( 8 ) as indicated by the dotted line. The heat compensating for ammonia desorption may also be provided by normal thermal equilibration between the container ( 1 ) and the surroundings. 
         [0080]      FIG. 6  shows an embodiment of a device according to the invention for controlled delivery of ammonia to a hydrogen-fuelled fuel cell through a reactor decomposing ammonia into nitrogen and hydrogen. A storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt shown as a hatched unit ( 1 ) is operated at a reduced pressure using a vacuum pump ( 2 ). The pump delivers ammonia into a buffer volume ( 3 ) and ammonia is delivered to an ammonia decomposition reactor ( 11 ) through a dosing valve ( 4 ). In the ammonia decomposition reactor ( 11 ), ammonia is decomposed into hydrogen and nitrogen, which is delivered to the hydrogen fuelled fuel cell ( 12 ). The entire dosing system is monitored and/or controlled by a controller ( 6 ). The heat ( 7 ), which is necessary to maintain a given operating temperature, i.e. to compensate for the endothermic desorption of ammonia controlled by the vacuum pump ( 2 ), is supplied to the unit ( 1 ) through a heater. The heat can be provided by electrical heating or by waste heat from a separate process step, e.g. the fuel cell ( 12 ) or the ammonia decomposition reactor ( 11 ) as indicated by the dotted lines. The heat compensating for ammonia desorption may also be provided by normal thermal equilibration between the container ( 1 ) and the surroundings. 
         [0081]      FIG. 7  shows another embodiment of a device according to the invention for controlled delivery of ammonia to a hydrogen-fuelled fuel cell through a reactor decomposing ammonia into nitrogen and hydrogen. A storage unit ( 1 ) containing an ammonia storage material, preferably being a compacted metal ammine salt, is operated at a reduced pressure using a vacuum pump ( 2 ). The pump withdraws ammonia from the storage unit ( 1 ) through an ammonia decomposition reactor ( 11 ) and delivers hydrogen and nitrogen into a buffer volume ( 3 ), and hydrogen is subsequently delivered to the hydrogen-fuelled fuel cell ( 12 ) through a dosing valve ( 4 ). Hydrogen—generated in the reactor ( 11 )—can alternatively be delivered directly from the pump ( 2 ) to the fuel cell ( 12 ) as indicated by the dotted line. The entire dosing system is monitored and/or controlled by a controller ( 6 ). A heater ( 7 ) supplied heat which is necessary to maintain a given operating temperature, i.e. to compensate for the endothermic release of ammonia controlled by the vacuum pump ( 2 ), to the unit ( 1 ). The heat can be provided by electrical heating or in the form of waste heat from a separate process step, e.g. the fuel cell ( 12 ). The heat compensating for ammonia desorption may also be provided by normal thermal equilibration between the container ( 1 ) and the surroundings. The advantage of the embodiment shown in  FIG. 7  as compared to the embodiment shown in  FIG. 6  is that a low pressure in both the storage delivery unit ( 1 ) and the decomposition reactor ( 9 ) during operation is advantageous for the equilibrium-limited decomposition of ammonia. 
         [0082]    It will be obvious to the skilled in the art how to build systems and devices according to the present invention from units, components and/or assemblies known per se considering the specific system or device and the contemplated use.