Abstract:
An apparatus and process for removing acidic gases and NOx from flue gases produced by utility and industrial plants. The process and apparatus convert NOx, and particularly nitric oxide, to nitrogen dioxide, which is then reacted to form a valuable byproduct. The process generally entails contacting a flue gas with a scrubbing medium to absorb acidic gases from flue gas and produce an intermediate flue gas. The intermediate flue gas is then cooled to cause nitric oxide present therein to be oxidized to form nitrogen dioxide, which is then absorbed from the flue gases to produce a nitrogen dioxide-containing solution and a scrubbed flue gas. The nitrogen dioxide in the nitrogen dioxide-containing solution is then reacted with ammonium hydroxide to form ammonium nitrate as a valuable byproduct.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     This invention generally relates to gas-liquid contactors used in the removal of acidic gases, such as from utility and industrial flue gases. More particularly, this invention is directed to a wet flue gas desulfurization (FGD) process and apparatus that remove nitrogen oxides (NOx) from a flue gas following the removal of acidic gases such as sulfur dioxide. 
     2. Description of the Related Art 
     Acidic gases, including sulfur dioxide (SO 2 ), hydrogen chloride (HCl) and hydrogen fluoride (HF), are known to be hazardous to the environment, and as a result their emission into the atmosphere is closely regulated by clean air statutes. For the removal of acidic gases from flue gases produced by utility and industrial plants, gas-liquid contactors and absorbers, or scrubbers, are widely employed. Scrubbers generally employ a liquid media that is brought into intimate contact with a flue gas to remove acidic gases by absorption. The process by which acidic gases are removed from flue gases in this manner is generally referred to as wet flue gas desulfurization (wet FGD). 
     The cleansing action produced by scrubbers is generally derived from the passage of a flue gas through a tower cocurrently or countercurrently to a descending liquid medium. Calcium-based slurries, sodium-based solutions and ammonia-based solutions are typical alkaline scrubbing media used in flue gas scrubbing operations. The cleansed gases are allowed to exit the tower, typically passing through a mist eliminator to atmosphere. The liquid medium and its absorbed gases are collected in a tank, typically at the bottom of the tower, where the absorbed gases are reacted to form byproducts that are useful or at least not harmful to the environment. While scrubbers utilizing calcium-based slurries generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer, as taught by U.S. Pat. Nos. 4,690,807 and 5,362,458, each of which are assigned to the assignee of the present invention. In these processes, the scrubbing solution is accumulated in a tank where the absorbed sulfur dioxide reacts with ammonia (NH 3 ) to form ammonium sulfite ((NH 4 ) 2 SO 3 ) and ammonium bisulfite (NH 4 HSO 3 ), which are oxidized in the presence of sufficient oxygen to form ammonium sulfate ((NH 4 ) 2 SO 4 ) and ammonium bisulfate (NH 4 HSO 4 ), the latter of which reacts with ammonia to form additional ammonium sulfate. A portion of the ammonium sulfate solution and/or ammonium sulfate crystals that form in the solution can then be drawn off to yield the desired byproduct of this reaction. 
     Nitrogen oxides (NOx), which include nitric oxide (NO) and nitrogen dioxide (NO 2 ), are also commonly found in flue gases produced by utility and industrial plants. The presence of NOx in the effluent of a scrubber is environmentally undesirable because of links to smog and ozone deterioration. Because nitric oxide readily oxidizes at room temperature (about 20° C. to about 25° C.) to form nitrogen dioxide, nitric oxide present in flue gases tends to form nitrogen dioxide which, depending on atmospheric conditions, can produce an unsightly yellow-brown plume. The elimination of NOx from flue gases has typically focused on the avoidance of forming nitrogen dioxide. For example, nitrogen gas (N 2 ) is produced in processes involving NOx dissociation by selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR). As the primary component of air, the release of nitrogen gas into the atmosphere does not pose an environmental concern. However, an ongoing demand of desulfurization processes is the reduction of emissions. From an economics standpoint, it is also desirable for a desulfurization process to produce valuable byproducts. Electron beam and low NOx burners have also been proposed as methods for removing Nox. For example, irradiation with an electron beam has been used to convert NOx to ammonium nitrate (NH 4 NO 3 ) in the presence of ammonia, as disclosed in U.S. Pat. Nos. 5,834,722 and 6,179,968. However, disadvantages of ion beam treatments include their relatively high cost and low efficiency. 
     SUMMARY OF INVENTION 
     The present invention provides an apparatus and process for removing acidic gases and NOx from flue gases produced by fuel combustion operations of the type carried out in utility and industrial plants. The process and apparatus of this invention intentionally convert NOx, and particularly nitric oxide, to nitrogen dioxide, which is then reacted to form a valuable byproduct. As such, the process and apparatus of this invention differs from prior processes in which the object was to convert NOx to nitrogen gas, which is then released into the atmosphere. 
     The process of this invention generally entails contacting a flue gas with a scrubbing medium, preferably an ammonium sulfate-containing scrubbing solution, to absorb acidic gases from the flue gas and produce an acidic gas-containing solution and an intermediate flue gas. According to the invention, after contact with the scrubbing medium, the intermediate flue gas is cooled to cause nitric oxide to be oxidized to form nitrogen dioxide, which is then absorbed from the intermediate flue gas to produce a NO 2 -containing solution and a more fully scrubbed flue gas. The nitrogen dioxide in the NO 2 -containing solution is then reacted with an aqueous ammonia solution to form ammonium nitrate (NH 4 NO 3 ) as the valuable byproduct of the process. A flue gas scrubbing apparatus suitable for use with this invention comprises means for contacting the flue gas with the scrubbing medium to remove acidic gases and produce the intermediate flue gas, means for reducing the temperature of the intermediate flue gas to convert nitric oxide present in the intermediate flue gas to nitrogen dioxide, means for absorbing nitrogen dioxide from the intermediate flue gas to produce the NO 2 -containing solution and a scrubbed flue gas, and means for reacting the nitrogen dioxide in the NO 2 -containing solution with aqueous ammonia solution to produce ammonium nitrate. 
     According to the above, NOx present in a flue gas is intentionally converted to nitrogen dioxide, which is the species typically avoided in prior art processes because of environmental concerns and the visible plume produced by nitrogen dioxide. However, the present invention uses a combination of oxidation of nitric oxide to nitrogen dioxide, followed by a controlled reaction with an aqueous ammonia solution, the result of which does not reduce NOx to nitrogen gas but instead captures NOx to produce a valuable byproduct. 
     Other objects and advantages of this invention will be better appreciated from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will now be described, by way of example, with reference to FIG. 1, which is a schematic representation of a flue gas scrubbing apparatus configured in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 schematically illustrates a flue gas scrubbing apparatus  10  that has been modified in accordance with the teachings of this invention. The scrubber  10  is generally of the type that scrubs flue gases produced by the burning of fossil fuels or another process that results in the flue gas containing acidic gases, such as sulfur dioxide, hydrogen chloride and/or hydrogen fluoride, as well as particulate matter and, of interest to the present invention, nitrogen oxides (NOx). 
     The conventional components of the scrubber  10  include a contact region  16  in which an alkaline contact medium, referred to as a scrubbing slurry or solution  14 , is brought into contact with a flue gas that enters the scrubber  10  through an inlet duct  12 . The solution  14  is shown as being delivered with a pump  22  through a pipe  46  to the contact region  16 , where the solution  14  is dispersed with spray nozzles  20  or another suitable delivery device. After being scrubbed by the solution  14 , the flue gas flows up through a demister  17 , and is eventually released to atmosphere through a chimney  24  or other suitable structure. As with many existing wet flue gas desulfurization facilities, the scrubber  10  is equipped for in situ forced oxidation of the solution  14  that has collected in a tank  18  below the contact region  16 . In FIG. 1, an oxygen-containing gas (e.g., air) is represented as being introduced into the tank  18  with a sparger  26  connected to a suitable source  28 . In this manner, the reaction product of contacting the acidic gases of the flue gas with the solution  14  is oxidized, which in the present invention preferably yields a useful fertilizer byproduct. A particular example is ammonia-based scrubbing processes taught by commonly-assigned U.S. Pat. Nos. 4,690,807 and 5,362,458. In these processes, acidic gases present in a flue gas are absorbed by an ammonium sulfate solution  14 , which collects in the tank  18  where aqueous ammonia (ammonium hydroxide, NH 4 OH) or another source of ammonia is introduced, such as with the sparger  26 . The absorbed sulfur dioxide reacts with the ammonia to form ammonium sulfite (NH 4 ) 2 SO 3  and ammonium bisulfite (NH 4 HSO 3 ), which are then oxidized in the presence of sufficient oxygen (introduced by the sparger  26 ) to form precipitates of ammonium sulfate and ammonium bisulfate (NH 4 HSO 4 ). Ammonium bisulfate undergoes a second reaction with ammonia to form additional ammonium sulfate precipitate. A portion of the ammonium sulfate solution  14  is then removed through a pipe  50  and dewatered with a suitable dewatering device  30  to precipitate ammonium sulfate, which can then be sold as a valuable fertilizer. If hydrogen chloride and hydrogen fluoride were present in the flue gas, as is typically the case with flue gas produced by the combustion of coal, these acidic gases are also captured to form ammonium chloride and ammonium fluoride, which can be removed in the same manner. Further details regarding the desulfurization of flue gases can be obtained in the prior art, including the above-noted U.S. Pat. Nos. 4,690,807 and 5,362,458, and therefore will not be discussed in any further detail here. 
     In addition to the above, the scrubber  10  of this invention removes NOx from the scrubbed flue gas emerging from the scrubbing process performed in the contact region  16 . The flue gas that has passed through the contact region  16  will be referred to here as an “intermediate” flue gas, since it is not the final “scrubbed” flue gas that will be released to atmosphere through the chimney  24 . Further processing of the intermediate flue gas occurs in additional zones provided between the demister  17  and the chimney  24  of the scrubber  10 . A first of these zones is a heat exchanger  32  or other device capable of reducing the temperature of the intermediate flue gas to something below the auto-oxidation temperature of nitric oxide (NO), which readily oxidizes to form NO 2  at or near room temperature. In conventional scrubbers of the type represented in FIG. 1, flue gases exit the contact region  16  at a temperature of typically at least 125° F. (about 50° C.), and often higher, such that the nitrogen dioxide content of the flue gas is relatively low, i.e., the NOx content of flue gases is primarily NO. In a conventional FGD process, oxidation of nitric oxide occurs after the scrubbed flue gases are released to atmosphere. A preferred heat exchanger  32  is capable of reducing the flue gas temperature so that the majority of the nitric oxide content of the intermediate flue gas is converted to nitrogen dioxide, e.g., the flue gas is reduced from a temperature of about 125 E F or higher to something near or below room temperature. Various types of heat exchangers known to those skilled in the art are believed to be suitable for use in the scrubber  10  shown in FIG. 1, and therefore the construction of the heat exchanger  32  will not be discussed here in any detail. 
     After conversion of nitric oxide to nitrogen dioxide within the heat exchanger  32 , the intermediate flue gas passes through an absorption zone  34  where nitrogen dioxide is absorbed so that the scrubbed flue gas exiting the chimney  24  is essentially free of nitrogen dioxide. Nitrogen dioxide is more readily absorbed in water and ammonia solutions than is nitric oxide. In a preferred embodiment, the flue gas is contacted with both water (or a water-containing solution) and an ammonia-containing solution, which are introduced into the absorption zone  34  as represented in FIG.  1 . Absorption of nitrogen dioxide with water forms nitric acid (HNO 3 ), while absorption of nitrogen dioxide with a ammonia-containing (e.g., ammonium sulfate or hydroxide) solution produces a blend of desirable fertilizers, as discussed below. Water is shown as being delivered from a source  38 , while the ammonia-containing solution is represented as being recycled through a pipe  40  from a solution collection tank  44  below the absorption zone  34 . The ammonia-containing solution, designated with reference number  36  in FIG. 1, is collected from the absorption zone  34 , and therefore is an aqueous ammonia solution containing absorbed nitrogen dioxide. The water and the ammonia-containing solution  36  can be brought into intimate contact with the flue gas within the absorption zone  34  in any suitable manner, such as with spray nozzles or packed column. 
     The ammonia-containing solution  36  preferably has a pH of less than 7 and contains ammonia slightly above the stoichiometric amount. A suitable and convenient source of the ammonia-containing solution  36  for use in the absorption zone  34  is the ammonium sulfate solution  14  used in the contact region  16  to remove the acidic gases from the flue gas, such that the ammonium hydroxide present in the solution  14  is the source of ammonia in the solution  36 . A secondary benefit of using the ammonium sulfate solution  14  is that the absorption zone  34  provides a second opportunity for removing sulfur dioxide and other acidic gases from the flue gas. For the purpose of delivering the solution  14  to the absorption zone  34 , the solution collection tank  44  is shown as being coupled by a pipe  48  to the pipe  46  connected to the pump  22 . The solution collection tank  44  is also shown as being connected by a pipe  42  through which the ammonia-containing solution  36  can be returned to the tank  18 . The final reaction of the process is the conversion of the absorbed nitrogen dioxide in the ammonia-containing solution  36  into a valuable byproduct, ammonium nitrate (NH 4 NO 3 ), which occurs according to the following reaction: 
     
       
         NO 2 +NH 4 OH→NH 4 NO 3 +H  
       
     
     The ammonium nitrate formed or otherwise collected in the tank  18  can be withdrawn and dewatered in the same manner as the ammonium sulfate produced by the absorption of sulfur dioxide. 
     In view of the above, the present invention can be seen as providing a method and apparatus for capturing nitrogen oxides from a flue gas. Benefits of this invention include the substantial elimination of undesirable NOx emissions from the scrubber  10 , producing a useful nitrogen-containing byproduct instead of releasing nitrogen gas to atmosphere, and providing a second opportunity for additional removal of sulfur dioxide, leading to a cleaner scrubbed flue gas released to the atmosphere. 
     While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the invention is to be limited only by the following claims.