Abstract:
Methods and systems for producing a pressurized ammonia-containing gas stream from aqueous urea. The method comprising pumping an aqueous urea-containing solution to a fluid-tight enclosure at a rate to match the external demand for ammonia-gas, wherein a fluid heat transfer medium is applied to the exterior of the fluid-tight enclosure to transfer heat to the aqueous urea-containing solution sufficient to hydrolyze the solution to an ammonia gaseous product. The systems comprise means for carrying out the methods.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    Ammonia is a highly toxic gas that is usually stored under high pressure as a liquid. It presents substantial hazards in transportation, storage and handling. Ammonia is used in many industrial processes and as a reducing agent in selective catalytic reduction of nitrous oxide in many combustion processes. The Urea to Ammonia technology offers a safe means of producing ammonia on demand from urea, which is a safe non-toxic substance, to be utilized in processes, particularly selective catalytic reduction (SCR), in which a non-pure ammonia source can be utilized. 
         [0003]    2. Brief Description of Related Technology 
         [0004]    This invention relates to processes and systems for the removal of nitrogen oxides from combustion gas streams and for other industrial uses based on the hydrolysis of urea to ammonia. 
         [0005]    One method for providing a pressurized gas stream for removing nitrogen oxides from a combustion gas stream by SNCR (Selective Non-Catalytic Reduction) or SCR (Selective Catalytic Reduction) processes includes feeding an aqueous solution of urea or mixtures of urea-containing biuret and/or ammonium carbamate into a reactor and hydrolyzing the urea therein at temperatures of at least 110° C. and under pressure, Cooper, et. al. U.S. Pat. No. 6,077,91, the disclosure of which is expressly incorporated herein by reference. The reaction rate is controlled by the input of heat to the reactor to produce a gaseous product stream containing ammonia and feeding it into a combustion gas stream at a controlled rate which is approximately the amount necessary to the demand for external use in removing nitrogen oxides. The rate of hydrolysis of urea must be balanced with the demand for ammonia to achieve thorough removal of nitrogen oxides while avoiding ammonia slip. In the prior art, this balance has been based on manipulation of the reactor pressure and heat input to the hydrolysis reactor in response to demand. 
         [0006]    In the prior methods, typically there is a demand signal emitted by a detection system within the effluent combustion gas stream which is linked to the heat source and/or pressure controller at the hydrolysis reactor intended to maintain the ammonia-gas generation rate at a level equal to that required to scrub the nitrous oxides from the combustion gas. The urea solution level within the reactor is kept essentially constant by a fluid level detector governing the pumping rate of urea solution to the hydrolysis reactor. The solution level of the urea solution in the reactor is independent of the ammonia-gas requirement in the combustion gas stream. Since water is both a reactant in urea hydrolysis as well as being lost as steam in the ammonia going to the combustion gas stream, the effect of the feed rate of urea solution to the reactor being independent of ammonia demand while the urea solution level in the hydrolysis reactor remains essentially constant is that the concentration of water in the urea solution in the reactor varies. Variation in the water concentration causes fluctuation in the ammonia generation rate. 
         [0007]    According to the present invention, the volume of aqueous urea-containing solution in the hydrolyzer varies. More particularly, the urea concentration in the aqueous solution in the hydrolyzer and the temperature are kept more nearly constant by the matching of the aqueous urea-containing solution feed rate to the hydrolyzer with the demand for ammonia-gas for scrubbing NO X , flue gas conditioning or other industrial purposes. 
         [0008]    This invention is particularly useful for scrubbing NO X  from combustion gas streams, in flue gas conditioning and other industrial uses which require relatively low rates of ammonia-containing product gas on the order of about 0.1 to about 100 pounds of ammonia per hour. 
       SUMMARY OF THE INVENTION 
       [0009]    A method adapted to provide a pressurized ammonia-containing gas stream from aqueous urea, the method comprising feeding an aqueous urea-containing solution to a fluid-tight enclosure at a rate to match the external demand rate for ammonia-gas, wherein a fluid heat transfer medium is present on the exterior of said fluid-tight enclosure to transfer heat to the aqueous urea-containing solution within the enclosure to hydrolyze the solution and convert the urea-containing solution to an ammonia gaseous product comprising ammonia, carbon dioxide and water vapor, the pressure within said fluid-tight enclosure being maintained by back pressure control to ensure that the ratio of urea and water is maintained independent of the ammonia generation rate, and the urea is converted. 
         [0010]    One preferred method is adapted to provide a pressurized ammonia-containing gas stream from aqueous urea, the method comprising feeding an aqueous urea-containing solution to the tube side of a shell and tube heat exchanger at a rate to match the external demand rate for ammonia-gas, wherein a fluid heat transfer medium is present on the shell side to transfer heat to the aqueous urea-containing solution within the tubes sufficient to hydrolyze the solution and convert the urea-containing solution to an ammonia gaseous product comprising ammonia, carbon dioxide and water vapor, the pressure on the tube side being maintained by back pressure control to ensure that the ratio of urea and water is maintained independent of the ammonia generation rate, and the urea is converted. 
         [0011]    Alternatively, a method adapted to provide a pressurized ammonia-containing gas stream from aqueous urea, the method comprising feeding an aqueous urea-containing solution to a coil within a shell at a rate to match the external demand rate for ammonia-gas, wherein a fluid heat transfer medium is present within said shell on the coil exterior to transfer heat to the aqueous urea-containing solution within the coil sufficient to hydrolyze the solution and convert the urea-containing solution to an ammonia gaseous product comprising ammonia, carbon dioxide and water vapor, the pressure in the coil interior being maintained by back pressure control to ensure that the ratio of urea and water is maintained independent of the ammonia generation rate, and the urea is converted. 
         [0012]    Superheated inert gas at essentially constant temperature can be introduced into the evolved ammonia-containing product gas. 
         [0013]    A preferred method of the invention for producing a pressurized gaseous ammonia-containing product, useful for the catalytic reduction of nitrous oxides or flue gas conditioning, from an aqueous urea-containing solution, the method comprising feeding an aqueous urea-containing solution to a shell and tube heat exchanger in response to an external demand signal at a rate balanced to meet the external demand for gaseous ammonia, where the aqueous urea-containing solution flows through the tubes at essentially constant temperature and pressure, and is heated by hot fluid medium present on the shell side of the heat exchanger, admixing a hot carrier gas or steam with the evolved ammonia-containing product gas exiting the reactor to prevent condensation of the ammonia-containing product gas downstream and provide improved mixing at the point of use, said aqueous urea solution in said tubes being maintained under pressure utilizing a back pressure control valve to convert the urea to a gaseous ammonia-containing product. 
         [0014]    Another preferred method of the invention for producing a pressurized gaseous ammonia-containing product, useful for the catalytic reduction of nitrous oxides or in flue gas conditioning, from a solution of aqueous urea-containing solution, the method comprising feeding an aqueous urea-containing solution to a shell and tube heat exchanger in response to an external demand signal at a rate balanced to meet the external demand for gaseous ammonia, where the aqueous urea solution flows through the tubes at essentially constant temperature and pressure, and is heated by hot fluid medium present on the shell side of the heat exchanger, said heat exchanger containing a second tube bundle, pumping air or carrier gas through said second tube bundle to generate hot air or carrier gas, admixing said hot air or carrier gas or steam with the evolved ammonia-containing product gas exiting the first tubes to prevent condensation of the ammonia-containing product gas downstream and provide improved mixing at the point of use, said aqueous urea solution in said first tubes being maintained under pressure utilizing a back pressure control valve to convert the urea to a gaseous ammonia-containing product. 
         [0015]    Yet another preferred method of the invention for producing a pressurized gaseous ammonia-containing product, useful for the catalytic reduction of nitrous oxides or in flue gas conditioning, from an aqueous urea-containing solution, the method comprising feeding an aqueous urea-containing solution to a coil within a shell in response to an external demand signal at a rate balanced to meet the external demand for ammonia, where the aqueous urea-containing solution flows through said coil at essentially constant temperature and pressure, and is heated by hot fluid medium present in the shell in heat transfer contact with the external surface of said coil, admixing a hot air carrier, gas or steam with the evolved ammonia-containing product gas exiting the coil to prevent condensation of the ammonia-containing product gas downstream and provide improved mixing at the point of use, said aqueous urea solution being maintained under pressure utilizing a back pressure control valve to convert the urea to a gaseous ammonia-containing product. 
         [0016]    The invention includes systems for the production of gaseous ammonia-containing product to meet an external demand comprising a source of aqueous urea-containing solution, a fluid-tight enclosure, means for providing fluid heat transfer media in contact with the exterior surfaces of said fluid-tight enclosure, means for continuously introducing aqueous urea-containing solution from said source into the enclosure to provide for the evolution of gaseous ammonia, means for controlling said means for continuously introducing aqueous urea-containing solution to balance the rate of introduction of said urea-containing aqueous solution into said fluid-tight enclosure with said external demand for ammonia and means for maintaining essentially constant pressure within said fluid-tight enclosure. 
         [0017]    In another aspect, the invention further includes systems for the production of gaseous ammonia-containing product to meet an external demand comprising a source of aqueous urea-containing solution, a shell and tube heat exchanger having a tube bundle within the outer shell, means for providing fluid heat transfer media within said shell and in contact with the exterior surfaces of said tube bundle, means for continuously introducing aqueous urea-containing solution from said source into the tube bundle to provide for the evolution of gaseous ammonia, means for controlling said means for continuously introducing aqueous urea-containing solution to balance the rate of introduction of said urea-containing aqueous solution into said tube bundle with said external demand for ammonia and means for maintaining essentially constant pressure within said tube bundle. 
         [0018]    The invention also comprises a system for the production of gaseous ammonia-containing product to meet an external demand comprising a source of aqueous urea-containing solution, a shell having a coil within the shell, means for providing fluid heat transfer media within said shell and in contact with the exterior surfaces of said coil, means for continuously introducing aqueous urea-containing solution from said source into the coil to provide for the evolution of gaseous ammonia, means for controlling said means for continuously introducing aqueous urea-containing solution to balance the rate of introduction of said urea-containing aqueous solution into said coil with said external demand for ammonia and means for maintaining essentially constant pressure within said coil. 
         [0019]    The systems as just described can further comprise means for providing superheat to the gas as it is generated within said fluid-tight enclosure, tube bundle or coil, means within said fluid-tight enclosure, shell or coil for heating said heat transfer media, means within said shell or coil for blowing air in closed heat transfer relationship with said heat transfer media to continuously generate hot air or steam, and means for diluting the ammonia-containing product gas with said generated hot air or steam. 
         [0020]    The heat exchanger tubes or coil volume on both the tube or coil side and the shell side is sized to ensure that all urea solution will fully convert to the gaseous product at up to the maximum urea feed rate without the necessity of controlling the liquid level within the tubes or coil. 
         [0021]    The fluid-tight enclosure such as a tube bundle or coil may be enclosed in a shell as indicated in the drawings. The fluid-tight enclosure may also be incorporated in a duct or conduit through which hot combustion gases continuously pass when the system is in operation. For example, the fluid-tight enclosure can be incorporated in an external duct carrying the hot combustion gas stream from a gas turbine used to power an electric generator. 
         [0022]    The evolved product ammonia-containing gas mixture is preferably superheated within the upper portion of the interior of the heat exchange tubes or coil by a hot gas or fluid medium. This reduces the formation of water condensation in the downstream piping reducing possibility of forming condensed material pluggage and corrosion. 
         [0023]    The present invention has the advantage over other urea to ammonia processes in that it is a simpler, more responsive to demand change, less expensive and more compact process that is particularly well suited to smaller installations. 
         [0024]    Many variations are effective. For example, the fluid heat transfer medium can be heated by a heating coil within the shell or by a separate external heater. The preferred fluid heat transfer medium on the shell side is hot oil, steam or hot air where the tube bundle or coil is incorporated in an exhaust duct, the fluid heat transfer medium is a hot combustion gas stream which is continuously passed over the tube bundle or coil. 
         [0025]    The empty hydrolysis system can be preheated prior to aqueous urea-containing solution introduction to enable faster start up time and/or the aqueous urea-containing solution can be preheated prior to introduction into the hydrolysis tube bundle or coil to enable faster response rates or reduction in size of the tube bundle or coil. 
         [0026]    Hot air can be withdrawn from the carrier gas tube bundle or coil before it is mixed with the product ammonia-containing gas and recycled to a heater to reduce the energy requirement. 
         [0027]    Cold air can be introduced to the shell side of the hydrolysis tube bundle or exterior of the coil to remove heat, reduce the pressure and enable a fast shutdown. 
         [0028]    A surge tank can be connected to the interior of the tube bundle or coil, and sized to maintain a pressurized inventory of hot liquid urea solution or provide a product gas inventory in the gas space of the surge tank to enable more rapid changes in the product gas production rate for response to changes in ammonia demand. 
         [0029]    An aqueous urea-containing solution reservoir can also be provided at the bottom of the tube bundle which contains an inventory of urea solution and a separate heater coil to increase the capability of the system to quickly increase the production rate of ammonia-containing product gas. 
         [0030]    Quick fill valve and blow back valve provide a means to quickly establish required liquid level in the tube/coil reactor to meet fast demand changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    Turning to the drawings. 
           [0032]      FIG. 1  is a schematic drawing of a preferred embodiment of the method and system of the invention. 
           [0033]      FIG. 2  is a schematic drawing of slightly modified embodiment of shell and tube components used in the present invention. 
           [0034]      FIG. 3  is a schematic drawing of another embodiment of the shell and tube components used in this invention. 
           [0035]      FIG. 4  is another embodiment of the shell and tube components wherein steam is used as the fluid heat transfer medium. 
           [0036]      FIG. 5  is still another embodiment of the shell and tube components wherein hot air is used as the fluid heat transfer medium. 
       
    
    
       [0037]    Turning to the drawings in more detail, the aqueous urea solution storage/quench tank  1  and feed pump  2  supply the aqueous urea solution for hydrolysis. The feed rate of urea solution is governed by urea feed rate controller  3  which is electrically connected to a gas analyzer positioned downstream at the point of ammonia demand. Commonly, the gas analyzer is located in the effluent combustion gas stream of a fossil fueled power plant using ammonia to scrub NO X  and serves to balance the ammonia gas stream with the demand presented by the NO X  in the combustion gas stream to maintain adequate scrubbing without slip. 
         [0038]    The urea feed line  6  also serves as a blow back line and feeds the aqueous urea-containing solution into the hydrolysis heat exchanger tube bundle assembly  7 . Blow back valve  5  regulates reactor liquid blow back via line  16 . The urea feed line  6  can be provided with a quick fill valve  4 . 
         [0039]    The blow back valve  16  and quick fill valve  4  serve to quickly change the reacting fluid inventory in the reactor to shorten the time required to adjust the ammonia generation rate. 
         [0040]    The reactor  7  is equipped with a back pressure control valve  8 . 
         [0041]    The ammonia-gas generated in heat exchanger assembly  7  is discharged to a combustion gas stream to be scrubbed of NO X , flue gas conditioning, or for other chemical uses, via discharge line  9 . The ammonia in line  9  may be diluted by air from dilution air heat exchange coil  19 . The diluted gas stream is discharged via line  24 . Dilution air is forced through coil  19  by dilution air blower  18  which is normally provided with an inlet air filter  17 . 
         [0042]    The heat exchanger assembly  7  has a pressure transmitter  10  and a fluid level transmitter  11 . 
         [0043]    In this preferred embodiment, heat to the heat exchanger assembly  7  is provided by heat transfer oil  22  in the shell surrounding the tube bundle of heat exchanger assembly  7  which serves as a hot oil reservoir  14 . The heat transfer oil  22  is maintained by hot oil heater  13  which is equipped with hot oil temperature sensor  12 . 
         [0044]    Pressurized air may be provided to the heat exchanger tube bundle or coil for purging upon shut down or to enable pre-pressurization of tube or coil reactor for more rapid initial ammonia generation via line  15 . A dilution air temperature transmitter  20  is provided in line  21 . 
         [0045]      FIG. 2  depicts in more detail the hot oil hydrolyzer system with dilution air heater. The aqueous urea solution is supplied via line  101  which also serves as a blow back line. The tubes  103  communicate with aqueous urea inlet plenum  102  and ammonia-containing gas outlet plenum  104 . 
         [0046]    The hot oil reservoir  107  contains hot heat transfer oil  108  heated by hot oil heater  109 . The dilution air heat exchange coil  115  is also heated by heat transfer oil  108 . The dilution air is blown through coil  115  by dilution air blower  114  which is equipped with an inlet filter  113 . The temperature of the dilution air at the outlet of coil  115  has a dilution air temperature transmitter  117 . 
         [0047]    The liquid level at the outlet plenum  104  of the heat exchanger assembly is indicated by reactor level transmitter  110  and the pressure by reactor pressure transmitter  111 . 
         [0048]    Air or other inert gas supply to the inlet  102  plenum and tubes  103  to pressurize, or at shutdown, to purge is supplied via air or gas line  112 . 
         [0049]    The back pressure in the inlet plenum  102  and tubes  103  is controlled by valve  105 . The ammonia-gas generated in tubes  103  is discharged at line  106  and this gas is diluted with air at dilution product gas discharge line  116 . 
         [0050]    The embodiment of  FIG. 3  is similar to that of  FIG. 2  except that the dilution of the ammonia-containing product gas is eliminated along with the requirement for a dilution air heat exchange coil. 
         [0051]    In the alternate embodiment of  FIG. 4 , the hot oil reservoir  107  and hot heat transfer oil  108  are replaced by steam vessel shell  118  fitted with steam inlet nozzle  119  and condensate discharge nozzle  120 . In this embodiment, steam around tubes  103  provides the heat required for the hydrolysis reaction of the aqueous urea solution within tubes  103 . 
         [0052]    Turning to another alternate embodiment,  FIG. 5  employs hot air or other hot gas as the heat transfer medium in place of hot oil or steam. The hot air shell or duct  121  has a hot air inlet nozzle  122  and a hot air outlet nozzle  123 . The hot air at this point can be used to dilute the ammonia-containing gas coming from outlet plenum  104 . To enhance hot air circulation in the shell or duct  121  around tubes  103 , a series of baffles  124  can be provided. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0053]    The storage tank  1  located near the heat exchanger is adapted to serve as surge tank and possible a day tank containing sufficient aqueous urea-containing solution for one day. A preheater is not required since the feed line  6  submersed in the hot medium  22  supplying urea solution to the inlet plenum  102  can act as a preheater. 
         [0054]    The vertical tube bundle  103  is always partially filled and the reacting liquid temperature is few degrees lower than the heat media  22  due to heat required to sustain reaction and water evaporation. The upper portion  104  of tube bundle  103  which will be filled with product gas is available to heat the product gas up to heat media temperature as a small superheat. 
         [0055]    It is to be understood that the tube bundle  103  can be replaced by a coil submerged in fluid heat transfer media. The hydrolysis of urea takes place within the tube or coil. 
         [0056]    The aqueous urea-containing solution may also contain biuret or ammonium carbamate. 
         [0057]    The generated ammonia-gas is essentially free of urea, biuret and ammonium carbamate. 
         [0058]    The adequacy of heat is provided by heat stored in the hot oil, steam or hot air/gas. 
         [0059]    It is necessary to establish pressure in the hydrolysis tubes or coil. This is provided by a means of pre-pressurizing the tubes or coil prior to initial start and maintaining minimum pressure during the idled mode. 
         [0060]    In addition to above, to quickly establish the volume of urea-containing solution in the tubes or coil required to sustain the reaction rate (ammonia generation rate), a liquid level transmitter  11  and quick fill valve  4  are used which by passes the urea feed flow controller  3 . For major load (ammonia demand) change the quick fill valve  4  and blowback valve  5  will quickly control the reacting liquid level in the tube bundle  103 . In essence the capacity is controlled by varying the liquid volume in the tube bundle  103  while keeping the temperature and pressure constant. It is possible to operate in this way because the volume and ammonia design capacity of the method is fairly low (on the order of about 0.1 to 100 pounds per hour). 
         [0061]    A dilution air heater coil  19  can be added to generate hot dilution air from the same heat source (hot oil, hot air, steam or combustion exhaust gas) used to support the hydrolysis reaction. 
         [0062]    Prior art urea to ammonia hydrolysis technologies pump the urea solution into a hydrolysis reactor vessel where steam or electric heater coils submerged in the aqueous urea-containing reaction medium are controlled to heat the fluid to maintain a set pressure range or set temperature range or both predetermined temperature and pressure ranges within the vessel. Changes within these ranges occur in response to the demand for ammonia. The urea flow rate to the reactor vessel is controlled to maintain a relatively constant liquid level within the reactor regardless of ammonia demand utilizing level measuring instrumentation. The processes retain a liquid phase reaction medium in the reactor for further conversion to gaseous ammonia. Since the heat is provided within the liquid side of the reactor in the prior art method, the gases exiting the reactor are essentially saturated at all production rates which increases the risk of the formation of unwanted condensate downstream, requiring heat tracing. 
         [0063]    According to the present invention the aqueous urea-containing solution feed rate into the hydrolysis tube/coils is controlled to produce the required ammonia-gas for the process. The heat input into the process occurs at the essentially constant temperature of the hot oil, steam or hot air with heat input being a function of the level of solution in the tube bundle or coil. As the ammonia production rate increases in response to increased demand the fluid level in the tubes or coil increases until there is a balance between the heat transfer to react the amount of urea to meet the ammonia production demand requirement and to evaporate the water in the feed solution. Urea solution level control is obtained through the balance of the heat input with the feed solution. As the fluid level increases there is more heat transfer across the hydrolysis tube bundle or coil. The tube bundle or coil volume, heat transfer area and heat input are sized to ensure that all urea solution will be fully reacted to a gaseous product up to the maximum urea feed rate, within the bundle or coil volume. 
         [0064]    The tube bundle or the coil containing the urea solution is essentially submerged in hot oil or essentially surrounded by hot air, steam or combustion exhaust gases. There is no heating element within the tube bundle or coil containing the aqueous urea solution. A constant reaction temperature enables setting a corresponding constant reaction pressure for a given aqueous urea feed solution concentration independent of the ammonia generation (urea solution feed) rate thus allowing reaction water balance to be independent of the ammonia generation (urea solution feed) rate. The essentially constant temperature of hydrolysis is at least 110° C., and normally from about 130° C. to about 200° C., and the essentially constant pressure is from about 25 psig to about 170 psig. The aqueous urea-containing feed solution concentration is from about 1% to 76% by weight urea, and more typically from about 25% to 50% urea by weight. 
         [0065]    The gases exiting the tube bundle or coil will be heated above the saturation temperature and will contain some superheat, which reduces the potential for the ammonia to recombine with the carbon dioxide to make an ammonium carbamate solid and plug the line. Additionally combining a hot carrier medium with the product gas at the exit of the tube side or coil interior further reduces the gas dew point and in many cases can eliminate the need to heat trace the product gas line to the process. 
         [0066]    Another embodiment of the invention utilizes a heat transfer fluid which is heated by an electric heater to constant temperature with urea tube bundle or coil submersed within the thus heated heat transfer fluid. In cases where either steam or hot air are not available a separate heat exchanger is also submerged in the same hot fluid medium as the urea to ammonia hydrolyzer tube bundle or coil to heat air provided by an external source. This hot air steam is then mixed with the ammonia product gas exiting the hydrolysis tube bundle or coil to lower the gas dewpoint temperature.