Abstract:
An improved process and apparatus for combustion flue gas conditioning in which ammonia is produced in situ from the hydrolysis of urea and injected into a stream of combustion flue gases, wherein key components of the process and apparatus are made to function independently of other components to prevent the shut-down of the entire apparatus in the case of a single component break-down.

Description:
RELATED APPLICATIONS  
       [0001]    The present application claims priority from pending provisional application 60/342,227 filed Dec. 21, 2001, entitled “An Improved Method for the Production of Ammonia for Use in the Removal of Nitrogen Oxides by the Hydrolysis of Urea,” the entirety of which is incorporated by reference herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to a process and apparatus for conditioning combustion flue gas produced by an industrial plant by injecting gaseous ammonia derived from the hydrolysis of urea into a flue containing flue gases.  
         BACKGROUND OF THE INVENTION  
         [0003]    Industrial plants such as incinerators and electric power generation plants produce undesired quantities of nitrogen oxide and fly ash as a byproduct of combustion of fossil fuels. In order to reduce the amount of nitrogen oxide and fly ash released from the combustion process by industrial plants, ammonia gas is mixed with the flue gases. The ammonia that is used in the process to condition the flue gases is either transported to the plant as anhydrous ammonia, in an aqueous solution or directly produced on-site, which is also known as in situ ammonia production.  
           [0004]    Ammonia transported to the plant in anhydrous aqueous form is the least desired because it is dangerous to handle and an environmental hazard. Aqueous ammonia is less dangerous, but still represents a hazard to plant workers and the local community. Accordingly, the on-site or in situ production of ammonia is most preferred because it avoids the hazards of ammonia transport and handling and can be made from solid urea, a non-toxic, safely transportable substance. The production of ammonia for use in the conditioning and reduction of nitrogen oxide and fly ash is described in U.S. Pat. 5,985,224, which is incorporated in its entirety by reference.  
           [0005]    The ammonia is used in the conditioning process to convert nitrogen oxide into nitrogen, a naturally occurring element in the atmosphere. Reactions between ammonia and nitrogen oxides, in the presence of oxygen, result in the formation of nitrogen and water in accordance with the following formula: 
           ΔNH 3 +2NO+2O 2 ⇄6H 2 O+3N 2   
           [0006]    Since aqueous ammonia solutions containing 20% or more ammonia by weight and anhydrous ammonia are classified as hazardous materials, they are strictly regulated with regard to transportation and handling. To eliminate the risk of transporting and handling ammonia, safer forms of on-site ammonia production are highly needed and in demand. One attempt to address this problem is the production of ammonia from urea at or near the point of use. Since urea is a non-toxic compound, it can be easily transported and handled with minimal risk of environmental harm or danger to personnel. In this process, urea is mixed with water and hydrolyzed to produce aqueous ammonia. The ammonia is then stripped from the aqueous solution in a gaseous form. The process of producing gaseous ammonia from an aqueous solution of ammonia by injecting steam into the solution is known as steam stripping.  
           [0007]    Urea is easily dissolved in water forming a urea solution. Urea is known to hydrolyze and produce ammonia and carbon dioxide in solution at temperature and pressure conditions in the range of about 180° C. to 250° C. at 15 to 50 bar in accordance with the following formula: 
           (NH 2 ) 2 CO+H 2 O→2NH 3 +CO 2 . 
           [0008]    The concentration and volume of urea solution needed to produce the necessary amount of ammonia gas for flue gas conditioning can be calculated from the estimated amount of NO production.  
           [0009]    Because the need is great and still unmet for satisfactory on-site ammonia production, more efficient and less costly means of production are always in demand and highly sought after. Ever increasing importance is placed on high ammonia production using the least amounts of energy, equipment and process steps as possible. Because of the extreme conditions associated with ammonia production, mechanical failures can occur, which result in very costly repair and lost production. Accordingly, there is a need for an improved process and apparatus for on-site gaseous ammonia production.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is directed to an improved process and apparatus that satisfies the need for more efficient and economical on-site production of ammonia from urea for the conditioning of combustion flue gases. The design of this invention is further enhanced in that each element of the apparatus can operate independently to facilitate maintenance of or prevent failure within the system. In addition, the present invention requires a reduced number of components as compared to a known apparatus in the field, making the present invention a more cost effective and efficient new apparatus embodying a safer and less energy demanding process.  
           [0011]    In one embodiment of the present invention, a hopper, having a conical shape at its bottom is used to direct solid urea to a narrowed outlet. The bottom of the hopper is equipped with a vibrating mechanism to dislodge solid urea as it is directed past the outlet. From the hopper, the solid urea is transferred to a dissolver where it is mixed with water to form a solution. The dissolver has a separate inlet for water. To improve efficient use of materials and to reduce energy use, heated water is obtained from condensation or steam vapors produced elsewhere in the industrial plant or added from an independent source. The dissolver is equipped with a stirring mechanism to aid in the dissolution of the urea in the water. By means of these improvements, urea concentrations can be obtained and controlled over a range from 10% to 70% by weight.  
           [0012]    Once the urea is dissolved in water in the dissolver, it is transferred to a solution storage tank for subsequent use. The storage tank provides dissolved urea on demand as needed by the process and ensures that ammonia production is not limited by the rate at which the urea dissolves in water. In another embodiment, the solution storage tank includes a heating element to maintain temperature as needed to prevent precipitation.  
           [0013]    The urea solution is then pumped from the solution storage tank to a pre-heater. The pre-heater increases the temperature of the urea solution before it enters the hydrolyzer. In one embodiment of the invention, the urea solution is both pre-heated and pressurized before entering the hydrolyzer.  
           [0014]    After pre-heating, the urea solution is transferred to at least one hydrolyzer through a urea solution inlet. Once in the hydrolyzer, additional heat is applied to the urea solution to raise its temperature above 300° F. in order to induce hydrolysis and the production of ammonia and carbon dioxide. The interior of the hydrolyzer is divided into a plurality of stages by baffles to enhance hydrolysis and stripping. In one embodiment, the apparatus includes a plurality of hydrolyzers. In another embodiment of the invention, a pressure-sensing device is mounted to the surface of at least one hydrolyzer to control the rate of ammonia production.  
           [0015]    After the urea is hydrolyzed, steam is introduced into the hydrolyzer through a steam inlet to strip the ammonia and carbon dioxide from the solution. In one embodiment of the invention, steam is introduced through a series of inlets at the bottom of the hydrolyzer. Ammonia and carbon dioxide are then released from the hydrolyzer through a gas outlet. In another embodiment, the gas outlet is located near the top of the hydrolyzer to optimize the separation of ammonia gas from the urea solution. As the gaseous ammonia and carbon dioxide are released from the hydrolyzer, they are directed to a flue containing combustion flue gases for conditioning.  
           [0016]    Any residual water remaining in the hydrolyzer after the stripping process is transferred to a holding tank through a residual water outlet in the hydrolyzer. Excess heat is transferred from the residual water as it leaves the hydrolyzer to the pre-heater. The residual water from the holding tank is recycled back to the dissolver for the continued production of urea solution. In one aspect of the invention, the holding tank operates at ambient air pressure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:  
         [0018]    [0018]FIG. 1 is an illustration of an improved process and apparatus for conditioning of combustion flue gases with ammonia from hydrolyzed urea according to one embodiment of the invention.  
         [0019]    [0019]FIG. 2 is a graphic description correlating urea concentration and dissolution temperature.  
         [0020]    [0020]FIG. 3 is a flow diagram of the process for conditioning of combustion flue gases with ammonia from hydrolyzed urea according to one embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 depicts an illustration of an improved process and apparatus  10  for conditioning of combustion flue gases with ammonia from hydrolyzed urea according to one embodiment of the invention. Referring to FIG. 1, urea is placed into an opening  12  in the top of a hopper  14 , having a main body  16  and a shaped bottom section  18 . The hopper  14  has a capacity to hold enough solid urea to produce enough ammonia to condition flue gases produced by the industrial plant (not shown) for at least one day of operation. In one embodiment, urea is supplied to the hopper  14  in the form of prills or granules. Urea is commercially available as either prills or granules as well as in other forms. Prills are spherical formations of urea typically having diameters between about 0.1 mm to about 1 mm. Granules are larger spherical formations of urea, typically 1 to 4 mm in diameter, and are harder and more resistant to moisture.  
         [0022]    In one embodiment of the invention, the hopper  14  has a bottom section  18  with a conical shape  19 . The conical shape  19  of the bottom section  18  of the hopper  14  helps to direct the solid urea to the hopper outlet  20  located at the bottom of the hopper  14 .  
         [0023]    Because of the hygroscopic nature of urea, the prills or granules tend to absorb water from the air, which has the effect of cementing the individual urea granules or prills into a single large mass and leads to clogging of the output from the hopper. In one aspect, the hopper  14  is equipped with a series of porous plates  22  that transverse the hopper&#39;s interior and through which dry air is injected.  
         [0024]    Air dried with chemical desiccants is passed through plates  22  near the bottom  18  of the hopper  14 . It is directed through the urea as the urea passes over the porous plates  22  to further prevent any sticking. The injection of the dried air drives out excess moisture present in the urea and prevents the influx of ambient air into the hopper  14 . Once the urea has been dried it is released from the hopper by a slide valve  26 .  
         [0025]    In one aspect of the invention, the bottom  18  of the hopper  14  is also equipped with a vibrating mechanism that dislodges any urea that may stick in hopper  14 . In one embodiment, the conically shaped bottom section  18  of the hopper  14  is flexibly connected to the main body  16  of the hopper  14 . The conically shaped bottom section  18  is vibrated periodically by a large attached vibrating electric motor. In another embodiment, the conically shaped bottom section  18  is equipped with strike plates  28  for the manual dislodging of the urea.  
         [0026]    Solid urea passes from the hopper  14  to a dissolver  32  through a urea inlet  34  where it is mixed with water to form a urea solution. The dissolver  32  has water inlet for water  36 . Water for making the urea solution may also be obtained from an external source. The dissolver  32  is also equipped with a stirring mechanism  38  that mixes the solution to speed the dissolution of the solid urea in the water.  
         [0027]    The urea solution made in the dissolver  32  is typically between about 10% to 70% urea by weight. In one embodiment, the composition is between about 35% to about 50% urea by weight. The dissolver  32  includes a heating element  40 . The temperature of the water in the dissolver  32  is between 80° F. to about 200° F., but it is preferably between about 125° F. to about 150° F.  
         [0028]    [0028]FIG. 2, illustrates the crystallization temperatures of urea solutions. Urea that has been dissolved in water at a concentration of about 70% by weight will remain dissolved if the water temperature is maintained at about 134° F. at ambient pressure. Exact concentrations and temperature may vary from those presented in FIG. 2 due to variations in pressure and impurities.  
         [0029]    Once the urea solution is made in the dissolver  32 , it is transferred to a solution storage tank  42  through an outlet  44 . The inclusion of the solution storage tank  42  in the apparatus of the invention is advantageous because it allows a surplus of urea solution to be available for ammonia production. Therefore, ammonia can be produced independent of urea solution production. In addition, a mechanical break-down in the dissolver  32  will not cause a complete shut-down of the system.  
         [0030]    In one aspect of the invention, the solution storage tank  42  includes a heating element  46  that is capable of maintaining the temperature of the urea solution at a temperature sufficient to prevent urea from precipitating. The temperature of the urea solution in the solution storage tank  42  is maintained between about 50° F. to about 90° F. In another embodiment of the invention, the solution in the storage tank can be maintained at above 100° F.  
         [0031]    The solution storage tank  42  allows urea solution to be available for hydrolysis regardless of any mechanical failures that may occur in association with the dissolver  32 . It is this stored supply of urea solution that can be used for the production of ammonia even if the hopper  14  or dissolver  32  become clogged or break down. By maintaining the temperature of the solution, the heating element prevents any dissolved urea from precipitating out of solution and therefore prevents any solid urea formation in the solution storage tank  42 .  
         [0032]    The urea solution in the solution storage tank  42  is pumped by a mechanical pump  48  to a pre-heater  50 . The pump pressurizes the urea solution and the pre-heater elevates the temperature of the urea solution prior to hydrolysis.  
         [0033]    After pre-heating, the urea solution is transferred to a feed line  52  to the hydrolyzer  54  through a urea solution inlet  56  in the hydrolyzer. The interior of the hydrolyzer  54  contains a plurality of baffles  58  that create a series of interior compartments that are in fluid contact with one another. The interior compartments created by the baffles  58  provide local environments for solution in the hydrolyzer  54 . The changing concentrations of the solution in each compartment or local environment allow more complete reaction and higher efficiency compared with a solution in a non-compartmentalized hydrolyzer. Because the efficiency of hydrolysis and stripping are dependent on the temperature and concentration of the solution, the baffles within the interior of the hydrolyzer  54  serve to optimize both processes. The spacing, size, and number of baffles  58  within the hydrolyzer  54  can be varied.  
         [0034]    Once the urea solution has been transferred to the hydrolyzer  54 , the urea solution is subject to sufficient heat to hydrolysis the urea and create a hydrolyzed solution comprising ammonia and carbon dioxide in water. In one embodiment, the urea solution is heated to about 195° C. The temperature, pressure and time for hydrolysis can be varied to optimize ammonia production rates. In another embodiment of the invention, the rate of production of ammonia and carbon dioxide from hydrolysis is controlled by a pressure sensing device that is flush mounted to the surface of hydrolyzer  54 . In yet another embodiment of the invention, more than one hydrolyzer is connected to a common urea solution inlet  56 . Each hydrolyzer can be used alone or simultaneously with at least one other. Multiple hydrolyzers reduce the chances of a complete shut-down of the apparatus because of mechanical failure, allowing one hydrolyzer to be serviced while the others remain in operation.  
         [0035]    After a sufficient time has elapsed for hydrolysis, steam is injected into the hydrolyzer  54  through at least one steam inlet  60  to strip the ammonia and carbon dioxide from the hydrolyzed solution. The steam inlet contains a pressure valve  62  to maintain the pressure of the contents of the hydrolyzer  54 . Steam for stripping may be obtained from steam produced by other industrial processes occurring at the plant or it can be made from water from an independent source.  
         [0036]    The ammonia and carbon dioxide are stripped from the hydrolyzed solution in gaseous form. Ammonia and carbon dioxide gas are then released from the hydrolyzer  54  through a gas outlet  64  and sent to an output line  66 . In one e aspect of the invention, the gas outlet  64  is located on the hydrolyzer to optimize the separation of ammonia gas from the hydrolyzed solution. The output line includes a pressure valve  68  necessary to maintain the pressure of the contents of the hydrolyzer  54 . Gaseous ammonia and carbon dioxide are then directed to a flue where they enter a stream of combustion flue gases. In one embodiment of the invention, multiple hydrolyzers are connected to the same output line  66 .  
         [0037]    In another embodiment of the invention, the output line  66  can feed ammonia and carbon dioxide gas to multiple gas ducts  68 . These ducts  68  allow the ammonia that is produced from the hydrolysis of the urea to be distributed to several different flues or different areas of a single flue. The flow of gas to each gas duct  68  can be independently controlled.  
         [0038]    After stripping, residual water remains in the hydrolyzer  54 . The residual water is transferred from the hydrolyzer  54  through a residual water outlet  70  in the hydrolyzer  54  to a holding tank  72 . The residual water is passed from the hydrolyzer  54  to the holding tank  72  through a pressure valve  74  that maintains the pressure of the contents of the hydrolyzer  54  during hydrolysis.  
         [0039]    The residual water leaving the hydrolyzer  54  is well above the temperature necessary for the dissolution of urea. Excessive heat from the residual water can be transferred from the residual water as it leaves the hydrolyzer  54  to the pre-heater  50 , reducing the energy requirement of the pre-heater  50  while allowing the water in the holding tank  72  to remain warm enough to dissolve urea at the proper concentrations when sent to the dissolver  32 . In one aspect of the invention, water is sent through a line  76  that contacts the pre-heater  50  and loops back to the holding tank  72 , allowing excess heat from the water to transfer to the pre-heater  50 .  
         [0040]    In one aspect of the invention, the holding tank  72  is open to ambient air pressure. The holding tank  72  can be covered to prevent excessive evaporation of the residual water in the holding tank  72  and can release steam through an open vent  78  if the pressure becomes too great. In another embodiment, the holding tank  72  includes a heating element  80  to maintain the temperature of the residual water at a temperature above about 80° F. Residual water in the holding tank is directed back to the dissolver  32  through a residual water inlet  82  in the dissolver  32  for the dissolution of urea.  
         [0041]    In U.S. Pat. No. 5,985,224 (Lagana), a separator was included in the apparatus for further separation of ammonia remaining in the residual water. The residual water in the separator was heated under pressure to further separate any ammonia remaining in the residual water from the residual water. Gases produced from the heating of the residual water in the separator were sent to the flue containing flue gases. The present invention eliminates the need for the separator because the stripping of the hydrolyzed solution eliminates essentially all of the ammonia. Therefore, the separator has been made unnecessary in the new apparatus.  
         [0042]    [0042]FIG. 3 illustrates one embodiment of the process  100  of conditioning combustion flue gases with ammonia from hydrolyzed urea. In this embodiment, solid urea is added to a hopper  110  and dried with air that has been dried over chemical desiccants  112 . Any solid urea that may clog the hopper is dislodged by mechanically vibrating or striking the hopper  114 . The dried urea is then fed through a roll-type feeder to a dissolver where it is mixed with water to form a urea solution  116 . Dissolution of the urea in water may be quickened with mechanical stirring. The urea solution is then stored in a solution storage tank until it is needed for the production of ammonia  118 . The temperature of the urea solution is maintained to prevent precipitation. The temperature of the solution depends on the concentration of the urea, but is typically between about 50° F. to about 90° F.  
         [0043]    When the urea solution is needed for ammonia production, it is pre-heated and pressurized  120 . The pre-heated and pressurized urea solution is then hydrolyzed to form a hydrolyzed solution including ammonia, carbon dioxide and residual water  122 . The ammonia and carbon dioxide in the hydrolyzed solution are then stripped from the solution by steam injected into the hydrolyzed solution  124 . Stripping causes the ammonia and carbon dioxide to be released from the solution in gaseous form. The gaseous ammonia and carbon dioxide are then injected into an industrial flue containing flue gases  126 .  
         [0044]    After the ammonia and carbon dioxide are stripped from the hydrolyzed solution, residual water is used to pre-heat additional urea solution prior to hydrolysis  130 . The temperature of the residual water in the holding tank can be maintained between about 80° F. to about 200° F. The residual water is recycled to produce more urea solution in the dissolver.  
         [0045]    These and other advantages of the present invention will be apparent to those skilled in the art from the foregoing specification. Accordingly, it will be recognized by those skilled in the art that changes or modifications may be made to the abovedescribed embodiments without departing from the broad inventive concepts of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein, but is intended to include all changes and modifications that are within the scope and spirit of the invention.