Abstract:
Disclosed are methods and apparatus enabling the efficient utilization of urea for purposes such as selective catalytic reduction (SCR) of NO x , which enable feeding urea to a chamber designed to efficiently and completely gasify the urea to enable ammonia feed. Preferably, aqueous urea is fed to a gasification chamber, which is also fed with heated gases. An injector means, capable of distributing the aqueous urea as fine droplets, is positioned centrally of a gas distribution plate in the chamber. An arrangement of spaced holes in the gas distribution plate provides higher gas velocity in the vicinity of the injector means than near the walls of the chamber. Uniform gas distribution without equipment fouling is achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to and is a continuation in part of U.S. Provisional Patent Application No. 61/056,121, filed May 27, 2008, the entire disclosure of which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The invention relates generally to the efficient utilization of urea for purposes such as selective catalytic reduction (SCR) of NO x , and more particularly to feeding urea to a chamber designed to efficiently and completely gasify (by pyrolysis and/or hydrolyzation) the urea to feed an SCR unit. 
       BACKGROUND OF THE INVENTION  
       [0003]    There are a number of processes for which ammonia is useful in a heated gas stream. In the case of air pollution control, examples are flue gas conditioning wherein a small amount of ammonia is injected and SCR system which depend on ammonia in relatively large amounts. Wherever ammonia is required in a hot gas stream, it would be desirable to avoid the danger and expense of dealing with ammonia per se. 
         [0004]    SCR has been proven to be highly effective at NO x  reduction, and SCR units can generally be scaled to the size required. However, SCR units typically require the use of ammonia as a reducing reagent, and it is a common problem that ammonia is difficult and dangerous to store, especially in populated areas. Thus, the use of urea and ammonia generators such as described in U.S. Pat. No. 7,090,810 to Sun, et al., and U.S. Pat. No. 6,077,491 to Cooper, et al., are often effective, but the ability to fully gasify the urea on an as-needed basis can cause problems if not done correctly. 
         [0005]    When urea for such a gasification chamber or other like chemical for other commercial units is introduced, effective operation without fouling of equipment requires uniform distribution and rapid pyrolysis and/or hydrolysis. A proper velocity distribution of hot air before and after introduction of urea is critical for the operation of such a gasification chamber. While the concept of a perforated plate has been suggested to provide uniform flow prior to urea injection to provide a desirable gas pattern for urea distribution, in practice these devices can cause improper reagent back flow or recirculation which can result in solid urea encrusting on the plate, chamber walls or near the nozzle, causing fouling and related problems. It would be desirable to avoid fouling, especially on the nozzle. 
         [0006]    There is a present need for a process, apparatus and system for efficient utilization of urea for purposes such as selective catalytic reduction (SCR) of NO x , and more particularly for gasification apparatus, methods and systems that enable feeding urea to a chamber designed to efficiently and completely gasify (by pyrolysis and/or hydrolyzation) the urea to feed a SCR unit. 
         [0007]    There is a particular need for such a system which converts urea to ammonia, yet maintains the ability to fully control ammonia generation without equipment fouling or excessive reagent usage or loss of pollution control effectiveness. 
       SUMMARY OF THE INVENTION  
       [0008]    The present invention provides processes for introducing ammonia into a heated gas stream without actually storing or handling ammonia in bulk form. 
         [0009]    The present invention provides a process, apparatus and system for gasifying urea for reducing the concentration of nitrogen oxides in combustion gases. 
         [0010]    In one aspect, a process is provided comprising: feeding urea to a gasification chamber, feeding heated gases into the gasification chamber upstream of the point for introducing the urea by injector means capable of distributing the urea as fine particles or droplets, providing a gas distribution plate in the chamber in proximity to the injector means, providing an arrangement and spacing of the holes in the gas distribution plate to provide higher gas velocity in the vicinity of the injector means than near the walls of the chamber, and adjusting the feed rates of the urea and the heated gases effectively to gasify the urea prior to exit from the chamber. Preferably, the urea is employed as an aqueous solution 
         [0011]    In another aspect, an apparatus is provided comprising: a gasification chamber having top bottom and side walls, injector means for feeding urea to the gasification chamber and capable of distributing the urea within the chamber as fine particles or droplets, duct means for feeding heated gases into the gasification chamber upstream of the injector means, a gas distribution plate in the chamber in proximity to the injector means said plate having an arrangement and spacing of the holes effective to provide higher gas velocity in the vicinity of the injector means than near the side walls of the chamber, and gas exit means for directing the heated gases containing gasified urea from the chamber. 
         [0012]    Preferably, the method and apparatus are employed in combination with a catalyst for selective catalytic NO x  reduction. 
         [0013]    Systems employing the process and apparatus as disclosed are also provided. 
         [0014]    Other and preferred aspects of the invention are described below. 
     
    
     
       DESCRIPTION OF THE DRAWINGS  
         [0015]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. As shown throughout the drawings, like reference numerals designate like or corresponding parts. 
           [0016]      FIG. 1  is a schematic side elevation of a preferred embodiment of the process and system of the invention. 
           [0017]      FIG. 2  is a schematic top plan view of a system as shown in  FIG. 1 . 
           [0018]      FIG. 3  is a top plan view of a preferred distribution plate design for use in a system as shown in  FIG. 1   
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    In describing the present invention, reference is made to the drawings, wherein there is seen a preferred embodiment shown schematically in  FIG. 1 . The drawings and the process they represent will be described briefly below, without undue recitation of various components described in U.S. Pat. No. 7,090,810, which is incorporated herein in its entirety. 
         [0020]    The term “urea” is meant to include the reagents that are equivalent to urea in the sense that they form ammonia and HNCO when heated, whether or not they contain large amounts of the pure chemical urea in the form introduced; however, the reagents that are equivalent to urea typically contain measurable quantities of urea in their commercial forms and thus comprise urea. Among the NO x -reducing reagents that can be gasified are those that comprise a member selected from the group consisting of: ammelide; ammeline; ammonium carbonate; ammonium bicarbonate; ammonium carbamate; ammonium cyanate; ammonium salts of inorganic acids, including sulfuric acid and phosphoric acid; ammonium salts of organic acids, including formic and acetic acid; biuret; triuret, cyanuric acid; isocyanic acid; urea formaldehyde; melamine; tricyanourea and mixtures of any number of these. Yet other NO x -reducing reagents are available that do not form HNCO, but decompose to a mixture of gases including hydrocarbons. Among this group are various amines and their salts (especially their carbonates), including guanidine, guanidine carbonate, methyl amine carbonate, ethyl amine carbonate, dimethyl amine carbonate, hexamethylamine; hexamethylamine carbonate; and byproduct wastes containing urea from a chemical process. Amines with higher alkyls can be employed to the extent that the hydrocarbon components released do not interfere with the NO x -reduction reaction. 
         [0021]    The term “urea” is thus meant to encompass urea in all of its commercial and equivalent forms. Typically, commercial forms of urea will consist essentially of urea, containing 95% or more urea by weight. This relatively pure form of urea is preferred and has several advantages in the process of the invention. It is preferably supplied to the process as an aqueous solution at a concentration of from about 5 to about 70%, with about 30 to about 60% being most typical. Urea can be used also as a finely divided solid or as a melt. When certain of these reagents are gasified, the reactant gas will also contain HNCO which can react with water to convert to ammonia and carbon dioxide. It is an advantage of the invention that this can be easily achieved without prehydrolysis of the NO x -reducing reagent which has the attendant risk of plugging nozzles and other equipment. By the term “gasification” we mean that substantially all of the urea is converted into a gas, leaving no significant dissolved or free solids or liquid to contact the SCR catalyst. 
         [0022]    With reference to  FIG. 1 , there is shown a gasification chamber  10 , having a gas inlet  12 , a gas outlet  14 , a gas distribution plate  16  and an injector  18  for introducing aqueous urea solution, fed through line  20 , as a spray  19  of fine particles (in the case of solid urea) or droplets (in the case of liquid urea). A preferred arrangement of the gas inlet  12  and the outlet  14  in relation to the chamber  10  can be seen by taking  FIG. 1  and  FIG. 2  together. The chamber  10  is shown to include a top wall  102 , a bottom wall  104  and a side wall  106 . 
         [0023]    The detail of one embodiment of a distribution plate is shown in  FIG. 3 . In the figure, there are illustrated a central hole  160 , and eight circular arrays of smaller holes,  162  and  164 . The central hole  160  is of a size sufficient to permit an injector to introduce urea through it and meet the heated gases in the chamber  10  at a velocity that prevents gas recirculation. The injector is preferably juxtaposed with the plate. The injector opening can be above, below or at the elevation of the plate depending on the nozzle design and flow rates. Preferably, the nozzle will be spaced from the plate to allow flow through the hole  160  and between the injector  18  and the plate  16 . One preferred set of dimensions is shown in the drawing. For holes of this exemplary type and dimension, the flow rate for urea solution (for a 35% solution) can be between about 0.1 and about 10 liters per minute and the gas flow rate can be between about 50 and about 1000 cubic feet per minute. 
         [0024]    The urea injector  18  introduces finely dispersed particles or droplets. The spray pattern  19  is preferably designed to be conical or otherwise as will provide uniform distribution. Any suitable injector or nozzle can be employed, e.g., air assisted, airless and mechanical atomizers can be utilized. Droplet or particle sizes less than 500 microns but typically less than 100, and preferably below 50, microns are desirable to rapidly evaporate any water and decompose the urea. Also in consideration of vessel size, small and slow droplets generated from, e.g., ultrasonic nozzles, can be more desirable than large and fast droplets. If desired, steam can be utilized as an atomizing fluid. Urea feed line  20  can provide a central channel for the urea and a surrounding annular channel for the atomizing fluid which can protect the urea from decomposition in the line  20  prior to exiting the injector  18 . 
         [0025]    The heated gases entering chamber  10  via inlet  12  will gasify the urea by pyrolyzation and/or hydrolysis, and the gases containing gasified urea exit from the chamber  10  via outlet  14 . The gases are preferably introduced into the chamber  10  at a temperature of at least 400° F., preferably greater than 500° F., and more preferably from about 600° F. to about 1300° F., e.g., from about 700° F. to about 1200° F.. The temperature of the gases and the residence time prior to exit from the chamber  10  will be effective to achieve full gasification. The entry temperature should be high enough also to maintain an exit temperature of at least about 350° F. and preferably at least 450° F.. The presence of moisture from the entering gases or a urea solution will facilitate hydrolysis, which is desired but not essential. The invention will provide improved urea decomposition chamber design through gas velocity shaping using a perforated plate design with varying sizes of openings effective to prevent back flow of urea or byproducts toward the nozzle and solids encrustation of the nozzle. 
         [0026]    As a precaution to the possibility of solids or liquids exiting the chamber harming the SCR unit downstream, an element  15  can be employed. Element  15  can be a screen, series of baffles or vanes, filter or the like, which designed to trap solids or liquids, from whatever source. It can optionally contain a catalyst to hydrolyze HNCO or urea or byproduct to ammonia. 
         [0027]    A proper velocity distribution of hot air before and after introduction of urea is critical for the operation of the decomposition chamber  10  and is achieved by the invention. The invention provides urea injection into a desirable hot gas flow pattern to achieve urea distribution for effective gasification without causing nozzle fouling and related problems. Effective gas velocity shaping is achieved by using the specially designed perforated plate  16  and proper positioning of the injector  18  outlet, to create a gas velocity profile that nearly matches the gas with the urea particle or droplet velocity near the injector and provides a reduced gas velocity near the wall  106 . The design of the holes in plate  16  and the flow parameters of the urea and hot gases can be achieved by computational fluid dynamics or cold flow modeling, or trial and error with greater difficulty. 
         [0028]    It is an advantage that the invention provides gas and liquid velocity shaping, which avoid a flow recirculation zone near the injector  18 . The recirculation near the injector, as has occurred in the past, is undesirable because it can cause droplets to deposit on the injector body. Once deposited, the reagent solidifies and accumulates over time. This solid mass tends to grow towards the injector spray and eventually interferes with the spray pattern causing large particles or droplets that can impinge on the chamber walls. Impingement on the walls generates solid deposits on the wall. By nearly matching gas velocity to spray velocity, this recirculation zone and its adverse effects are avoided. 
         [0029]    It is another advantage of the invention that velocity shaping reduces the magnitude of gas flow rate through the chamber  10 . A uniformly high gas velocity could reduce near-injector recirculation; however, this would require a higher volume of hot gas, increasing heating and gas blower requirements. The invention preferably provides a near zero gas velocity near the wall and high gas velocity at the injector at the center, with a net reduction in the quantity of gas flow required. It is thus an advantage of the invention that effective flow rates can be achieved without increasing the height of the chamber. 
         [0030]    It is yet another advantage of the invention that velocity shaping stabilizes the flow pattern within the chamber  10 . If gas were fed near the injector only, the recirculation zone near injector would disappear but the downstream flow pattern would become unstable. Instead of a stable high velocity core in the center, it would move closer to a wall, increasing the likelihood of urea impingement. 
         [0031]    It is a further advantage of the invention that velocity shaping allows a large cross sectional area for injection. An alternative method to stabilize the flow would be to shape the chamber as an upside-down funnel to force the core to be at the center. However, this would substantially reduce the area for chemical injection and thus increase the likelihood of droplet impingement on a wall. The invention avoids the problem while providing a large cross-sectional area for injection. 
         [0032]    Preferably, the method and apparatus are employed in combination with a catalyst for selective catalytic NO x  reduction, for selective noncatalytic NO x  reduction and for other purposes such as flue gas conditioning, and the like. 
         [0033]    Systems employing the process and apparatus combine the disclosed features and incorporate details as necessary for a wide variety of industrial applications. 
         [0034]    The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence which is effective to meet the objectives there intended, unless the context specifically indicates the contrary.