Abstract:
A combined flow mixing, tempering and noise suppressing apparatus for a selective catalytic reduction system, and method of use thereof is provided. The suppressing apparatus is positioned downstream of a gas input duct of the selective catalytic reduction system in order to provide improved flow uniformity and reduced noise at the entrance to the SCR system with minimal pressure loss. The combined flow mixing, tempering and noise suppressing apparatus includes lobe depressions that extend toward a centerline axis of a gas input duct of the selective catalytic reduction system and alternating lobe protrusions leading away from the centerline axis. The combined flow mixing, tempering and noise suppressing apparatus can include a means for introducing a secondary stream, such as an air manifold or an air bustle in fluid communication with the selective catalytic reduction system.

Description:
1. Field of the Invention 
     This invention relates generally to a combined flow mixing, tempering and noise suppressing apparatus for a selective catalytic reduction (SCR) system and method of use thereof, and more particularly to a lobe mixer positioned downstream of a gas input duct of the SCR system in order to provide improved flow uniformity and reduced noise at the entrance to the SCR system with minimal pressure loss. 
     2. Description of the Related Art 
     Flue gas waste streams produced from combustion, such as that produced by burning gas, coal, oil, or other carbon-based fuel sources in combustion turbine engines or for other similar uses, typically contain toxic elements, including carbon monoxide (CO) and nitrogen oxide (NO x ). NO x  refers to the cumulative emissions of nitric oxide (NO), nitrogen dioxide (NO 2 ) and trace quantities of other chemicals during combustion which are environmentally hazardous substances. Combustion of fossil fuels generates some level of NO x  due to high temperatures and availability of oxygen and nitrogen from both the fuel and air. Releasing such combustion waste streams without removing these toxic elements is bad for the environment. Additionally, many places have regulations or laws prohibiting such release. The extent to which CO and NO x  may be released into the environment varies widely under these various laws. California, in particular, has a very high standard regarding such emissions. Therefore, producers of such combustion waste streams frequently must process such waste streams to reduce the amount of toxic elements before releasing the waste streams into the environment. NO x  emissions may be controlled using low NO x  combustion technology and post-combustion techniques, such as selective catalytic reduction. SCR systems catalytically reduce flue gas NO x  to nitrogen and water using ammonia (NH 3 ) in a chemical reaction. 
     SCR systems treat the NO x  before the gas is released into the atmosphere. SCR systems rely on a catalyst to treat flue gas as the gas passes through the SCR system. Because the catalyst is an integral part of the SCR chemical reaction, SCR systems attempt to provide maximum exposure of the catalyst to the flue gas in order to ensure that all the flue gas comes sufficiently into contact with the catalyst for treatment. The catalysts used in SCR systems are carefully engineered and expensive. Thus, it is beneficial to be able to control the stoichiometry of the exhaust gas/ammonia/catalyst reaction. 
     Ammonia injection grids with zone controls have been installed to distribute a prescribed amount of ammonia for NO x  reducing SCR systems. To increase the mixing efficiency and reduce the required mixing distance, many SCR installations are equipped with static mixers. Static mixers typically utilize elaborate designs, are difficult to fabricate, have higher construction and installation costs, and cause significantly higher pressure drop. Static mixers are typically installed between the ammonia injection grid and the SCR catalyst bed; however, deflectors or baffles attached to the injection nozzles or turbulence enhancers installed between the injection tubes have also been utilized. 
     In the SCR exhaust stack, there is generally a need for noise suppression. Typically, a muffler, also called a silencer, is included in the SCR exhaust stack for noise suppression downstream of the ammonia injection grid and the catalyst bed to provide the required noise suppression. The muffler adds length to the SCR exhaust stack, increases exhaust stack system costs and increases the pressure drop in the stack. 
     It is therefore desirable to provide a combined flow mixing, tempering and noise suppressing apparatus for an SCR system. 
     It is also desirable to provide a method of using the apparatus for improved uniformity and noise reduction of a gas waste stream entering the SCR system with minimal pressure loss. 
     It is further desirable to provide a lobe mixer positioned downstream of a gas input duct of the SCR system in order to provide improved flow uniformity and reduce noise at the entrance to the SCR system. 
     It is still further desirable to provide a combined flow mixing, tempering and noise suppressing apparatus positioned downstream of a gas input duct and upstream of a catalyst bed in a SCR system to provide improved exhaust flow uniformity, noise suppression and to eliminate the need for a muffler in the exhaust stack in order to provide superior operational efficiency. 
     It is yet further desirable to provide a combined flow mixing, tempering and noise suppressing apparatus that can include a secondary stream, such as a tempering air, steam or an ammonia-steam mixture, introduced in the lobe depressions of the apparatus. 
     SUMMARY OF THE INVENTION 
     In general, in a first aspect, the invention relates to an SCR system having a waste stream gas input duct, an exhaust stack located downstream from the gas input duct and a combined flow mixing, tempering and noise suppressing apparatus positioned downstream from the waste stream gas input duct and upstream from the exhaust stack. 
     The ammonia injection grid assembly and an SCR catalyst assembly of the SCR system can include one or more of ammonia injection grid assemblies and one or more SCR catalyst assemblies, ordered such that each ammonia injection grid assembly is immediately followed by an SCR catalyst assembly. The SCR system can also include a distribution grid located intermediate of the gas input duct and the ammonia injection grid assembly and the SCR catalyst assembly. In addition, at least one air intake duct may be located intermediate of the gas input duct and the ammonia injection grid assembly and the SCR catalyst assembly and/or may be located upstream from the distribution grid, if present. Additionally, the SCR system can include at least one catalyst bed located downstream from the gas input duct or a plurality of catalyst beds, ordered such that each catalyst bed is immediately followed by an ammonia injection grid assembly. 
     The combined flow conditioner and noise suppression device of the SCR system can be constructed from a casting of metal, a formed or pressed sheet metal or welded sections of plate and/or can be coated with a refractory material. The combined flow mixing, tempering and noise suppressing apparatus can also include at least one lobe depression that extends toward a centerline axis of the gas input duct and at least one alternating lobe protrusion leading away from the centerline axis. Further, the combined flow mixing, tempering and noise suppressing apparatus can be a lobe mixer having six lobe depressions and six lobe protrusions in a sinusoidal profile. 
     The combined flow mixing, tempering and noise suppressing apparatus can be positioned internally within the SCR system or positioned externally at an entrance to the SCR system. The combined flow mixing, tempering and noise suppressing apparatus can also include a toroidal air manifold having a plurality of air manifold pipes. The manifold pipes are in fluid communication with the SCR system, such as via circumferentially spaced ports of a circular plate. The manifold pipes can be positioned substantially parallel with respect to the flow of the waste stream, or can be positioned at an angle with respect to the flow of the waste stream. Alternatively, the combined flow mixing, tempering and noise suppressing apparatus can include a hollow annular air bustle forming a chamber to which a secondary stream under pressure is supplied. The chamber of the air bustle is in fluid communication with the SCR system, such as via circumferentially spaced ports of a circular plate. 
     In general, in a second aspect, the invention relates to a SCR system having a gas input duct, a catalyst bed located downstream from the gas input duct, an ammonia injection grid assembly and an SCR catalyst assembly located downstream from the catalyst bed, and an exhaust stack located downstream from the catalyst bed, the ammonia injection grid assembly and the SCR catalyst assembly. The SCR system includes a combined flow mixing, tempering and noise suppressing apparatus located downstream from the gas input duct and upstream from the catalyst bed. 
     The combined flow conditioner and noise suppression device of the SCR system can be constructed from a casting of metal, a formed or pressed sheet metal or welded sections of plate and/or can be coated with a refractory material. The combined flow mixing, tempering and noise suppressing apparatus can also have at least one lobe depression that extends toward a centerline axis of the gas input duct and at least one alternating lobe protrusion leading away from the centerline axis. Further, the combined flow mixing, tempering and noise suppressing apparatus can be a lobe mixer having six lobe depressions and six lobe protrusions in a sinusoidal profile. 
     The combined flow mixing, tempering and noise suppressing apparatus can be positioned internally within the SCR system or positioned externally at an entrance to the SCR system. The combined flow mixing, tempering and noise suppressing apparatus can also include a toroidal air manifold having a plurality of air manifold pipes. The manifold pipes are in fluid communication with the SCR system, such as via circumferentially spaced ports of a circular plate. The manifold pipes can be positioned substantially parallel with respect to the flow of the waste stream, or can be positioned at an angle with respect to the flow of the waste stream. Alternatively, the combined flow mixing, tempering and noise suppressing apparatus can include a hollow annular air bustle forming a chamber to which a secondary stream under pressure is supplied. The chamber of the air bustle is in fluid communication with the SCR system, such as via circumferentially spaced ports of a circular plate. 
     In general, in a third aspect, the invention relates to a combined flow mixing, tempering and noise suppressing apparatus for a SCR system. The SCR system has a gas input duct, a catalyst bed located downstream from the gas input duct, an ammonia injection grid assembly and an SCR catalyst assembly located downstream from the catalyst bed, and an exhaust stack located downstream from the ammonia injection grid assembly and the SCR catalyst assembly. The combined flow mixing, tempering and noise suppressing apparatus includes a lobe mixer positioned downstream from the gas input duct and upstream from the catalyst bed in the SCR system. The lobe mixer has lobe depressions that extend toward a centerline axis of the gas input duct and alternating lobe protrusions leading away from the centerline axis. 
     The combined flow conditioner and noise suppression device of the SCR system can be constructed from a casting of metal, a formed or pressed sheet metal or welded sections of plate and/or can be coated with a refractory material. Further, the combined flow mixing, tempering and noise suppressing apparatus can be a lobe mixer having six lobe depressions and six lobe protrusions in a sinusoidal profile. The combined flow mixing, tempering and noise suppressing apparatus can also include a toroidal air manifold having a plurality of air manifold pipes. The manifold pipes are in fluid communication with the SCR system. The manifold pipes can be positioned substantially parallel with respect to the flow of the waste stream, or can be positioned at an angle with respect to the flow of the waste stream. Alternatively, the combined flow mixing, tempering and noise suppressing apparatus can include a hollow annular air bustle forming a chamber to which a secondary stream under pressure is supplied. The chamber of the air bustle is in fluid communication with the SCR system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevation view of an SCR system having an example of an internal lobe mixer positioned downstream of a gas input duct, within the SCR system and upstream of a catalyst bed in accordance with an illustrative embodiment of the combined flow mixing, tempering and noise suppressing apparatus for the SCR system disclosed herein; 
         FIG. 2  is a plan view of the SCR system shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view along line  3 - 3  of the SCR system shown in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view along line  4 - 4  of the SCR system shown in  FIG. 2 ; 
         FIG. 5  is a perspective view of the internal lobe mixer shown in  FIGS. 1 through 4 ; 
         FIG. 6  is a partial cutaway view of the internal lobe mixer shown in  FIGS. 1 through 5  positioned within the SCR system; 
         FIG. 7  is an elevation view of an SCR system having an example of an external lobe mixer positioned downstream of a gas input duct in accordance with an illustrative embodiment of the combined flow mixing, tempering and noise suppressing apparatus for the SCR system disclosed herein; 
         FIG. 8  is a plan view of the SCR system shown in  FIG. 7 ; 
         FIG. 9  is a cross-sectional view along line  9 - 9  of the SCR system shown in  FIG. 7 ; 
         FIG. 10  is a cross-sectional view along line  10 - 10  of the SCR system shown in  FIG. 8 ; 
         FIG. 11  is a rear perspective view of the external lobe mixer shown in  FIGS. 7 through 10 ; 
         FIG. 12  is an elevation view of an SCR system having an example of an external lobe mixer having a straight air manifold and positioned downstream of a gas input duct in accordance with an illustrative embodiment of the combined flow mixing, tempering and noise suppressing apparatus for the SCR system disclosed herein; 
         FIG. 13  is a plan view of the SCR system shown in  FIG. 12 ; 
         FIG. 14  is a cross-sectional view along line  14 - 14  of the SCR system shown in  FIG. 12 ; 
         FIG. 15  is a cross-sectional view along line  15 - 15  of the SCR system shown in  FIG. 13 ; 
         FIG. 16  is a rear perspective view of the external lobe mixer having the straight air manifold shown in  FIGS. 12 through 15 ; 
         FIG. 17  is an elevation view of an SCR system having an example of an external lobe mixer having an angled air manifold in accordance with an illustrative embodiment of the combined flow mixing, tempering and noise suppressing apparatus for the SCR system disclosed herein; 
         FIG. 18  is a plan view of the SCR system shown in  FIG. 17 ; 
         FIG. 19  is a cross-sectional view along line  19 - 19  of the SCR system shown in  FIG. 17 ; 
         FIG. 20  is a cross-sectional view along line  20 - 20  of the SCR system shown in  FIG. 18 ; 
         FIG. 21  is a rear perspective view of the external lobe mixer having the angled air manifold shown in  FIG. 17  through; 
         FIG. 22  is an elevation view of an SCR system having an example of an external lobe mixer having an air bustle in accordance with an illustrative embodiment of the combined flow mixing, tempering and noise suppressing apparatus for the SCR system disclosed herein; 
         FIG. 23  is a plan view of the SCR system shown in  FIG. 22 ; 
         FIG. 24  is a cross-sectional view along line  24 - 24  of the SCR system shown in  FIG. 22 ; 
         FIG. 25  is a cross-sectional view along line  25 - 25  of the SCR system shown in  FIG. 23 ; and 
         FIG. 26  is a rear perspective view of the external lobe mixer having the air bustle shown in  FIGS. 22 through 25 . 
     
    
    
     Other advantages and features will be apparent from the following description and from the claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatuses and methods discussed herein are merely illustrative of specific manners in which to make and use this invention and are not to be interpreted as limiting in scope. 
     While the apparatuses and methods have been described with a certain degree of particularity, it is to be noted that many modifications may be made in the details of the construction and the arrangement of the apparatuses and components without departing from the spirit and scope of this disclosure. It is understood that the apparatuses and methods are not limited to the embodiments set forth herein for purposes of exemplification. 
     Referring to the figures of the drawings, wherein like numerals of reference designate like elements throughout the several views, and initially referring to  FIGS. 1 and 2 , the SCR system  10  includes a gas input duct  12 , wherein a flue gas waste stream (arrow  14 ) from combustion enters the SCR system  10 . Downstream from the gas input duct  12  may be a distribution grid  16 . The distribution grid  16  axially distributes the waste stream  14  so as to limit the pressure drop on the successive elements. Downstream from the gas input duct  12  and the distribution grid  16 , if it is present, is a catalyst bed  20 . The catalyst bed  20  removes carbon monoxide from the waste stream  14 . 
     The combined flow mixing, tempering and noise suppressing apparatus  18  is positioned downstream from the gas input duct  12 , and aides in limiting the pressure drop by conditioning the flow of the waste stream  14 . The combined flow conditioner and noise suppressing apparatus  18  is positioned upstream of the catalyst bed  20  in the SCR system  10 , thereby improving the SCR system&#39;s  10  efficiency and reducing sound pollution. The combined flow conditioner and noise suppressing apparatus  18  can be a lobe mixer, as exemplified in the figures. 
     Downstream from the catalyst bed  20  is at least one ammonia injection grid assembly  22 , followed by at least one SCR catalyst assembly  24 . The ammonia injection grid assembly  22  and the SCR catalyst assembly  24  work in conjunction to remove NO x  present in the waste stream  14 . Downstream from the ammonia injection grid assembly  22  and the SCR catalyst assembly  24  could be additional ammonia injection grid assemblies and/or SCR catalyst assemblies to remove additional NO x  remaining in the waste stream  14 . 
     Downstream from the SCR catalyst assembly  24  is an exhaust stack  28  whereby the treated flue gas waste stream  14  exits the SCR system  10 . The combined flow mixing, tempering and noise suppressing apparatus  18  provides noise suppression and eliminates the need for a muffler or silencer in the SCR exhaust stack  28 . This allows the SCR system  10  to be used in locations where sound pollution is a concern. Further, by eliminating the need for a muffler in the SCR exhaust stack  28 , the SCR exhaust stack  28  requires less space, is less expensive and has less pressure loss for a more effective exhaust system. 
     The combined flow mixing, tempering and noise suppressing apparatus  18  decreases shear and increases mixing as the waste stream  14  enters the gas input duct  12  of the SCR system  10 . (The combined flow mixing, tempering and noise suppressing apparatus  18  partially preconditions the waste stream  14  to a predetermined profile for introduction into the SCR system. The suppressing apparatus rapidly mixes the waste stream  14  with gases already present in the gas input duct  12 , or gases introduced using a secondary stream, in order to reduce the density gradient between the waste stream  14  and the SCR gases, which reduces the generation of noise.) Further, the combined flow mixing, tempering and noise suppressing apparatus  18  provides a well-mixed, evenly tempered waste stream  14  in the SCR system  10  so as to prolong the life of the catalysts of the catalyst bed  20 . 
     The combined flow mixing, tempering and noise suppressing apparatus  18  can be constructed of any suitable materials, such as a casting of metal, formed (pressed) sheet metal, or welded sections of plate. In addition, the combined flow mixing, tempering and noise suppressing apparatus  18  may be coated with a refractory material. The combined flow mixing, tempering and noise suppressing apparatus  18  includes at least one lobe portion  32  that extends toward the centerline axis of the gas input duct  12  with at least one alternating lobe portion  34  leading away from the centerline axis. As exemplified throughout the figures, the combined flow mixing, tempering and noise suppressing apparatus  18  is a lobe mixer having six (6) lobes in a sinusoidal profile, but a person having ordinary skill in the art will readily appreciate that other profiles are possible. 
     As illustrated in  FIGS. 1 through 6 , the combined flow mixing, tempering and noise suppressing apparatus  18  is positioned downstream of the gas input duct  12 , upstream of the catalyst bed  20  and within the distribution grid  16 . In this example, the combined flow mixing, tempering and noise suppressing apparatus  18  does not include a secondary stream, but the SCR system  10  may have at least one air intake duct  30  located downstream from both the gas input duct  12  and the combined flow mixing, tempering and noise suppressing apparatus  18  and upstream of the catalyst bed  20 . This allows a secondary stream of tempering air, steam or an ammonia-steam mixture to be added to the combustion waste stream  14 . Connected to the air intake duct  30  may be a fan (not shown), and the air flow from the fan may be controlled by a damper (not shown), which may be located adjacent the air intake duct  30 . The fan allows tempering air to be forcibly added to the combustion waste stream  14 , and the damper allows a user to control the amount of air allowed to pass through the air intake duct  30 . 
     Referring now to  FIGS. 7 through 11 , the combined flow mixing, tempering and noise suppressing apparatus  18  is positioned at the entrance of the SCR system  10 , in particular, downstream of the gas input duct  12  and upstream of the distribution grid  16  and the catalyst bed  20 . As exemplified in this example, the forwardly disposed terminal ends of the lobe portion  32  and the lobe portion  34  of the combined flow mixing, tempering and noise suppressing apparatus  18  abuts and is attached to a rearwardly disposed face of a circular plate  36 . Similar to above, the combined flow mixing, tempering and noise suppressing apparatus  18  in this example does not include a secondary stream, but the SCR system  10  includes the air intake duct  30  located downstream from the combined flow mixing, tempering and noise suppressing apparatus  18 , within the distribution grid  16  and upstream of the catalyst bed  20 . 
     Referring now to the two examples of the combined flow mixing, tempering and noise suppressing apparatus  18  illustrated in  FIGS. 12 through 16  and in  FIGS. 17 through 21 , the SCR system  10  includes the combined flow mixing, tempering and noise suppressing apparatus  18  positioned intermediate of the gas input duct  12  and the distribution grid  16 . The forwardly disposed terminal ends of the lobe portion  32  and the lobe portion  34  of the combined flow mixing, tempering and noise suppressing apparatus  18  abuts and is attached to the rearwardly disposed face of the plate  36 . 
     In these examples, the combined flow mixing, tempering and noise suppressing apparatus  18  includes a means of introducing the secondary stream into the SCR system for mixing with the waste stream  14 , such as by means of a toroidal air manifold  38  having a plurality of air manifold pipes  40  circumferentially spaced and in fluid communication with circumferentially spaced ports  42  in the plate  36 . As can be seen in  FIGS. 14 and 16 , the manifold pipes  40  may be positioned substantially parallel with respect to the plane of the flow of the waste stream  14 , and are positioned within the lobe portions  32 . Alternatively as can be seen in  FIGS. 19 and 21 , the manifold pipes may be positioned at an angle with respect to the flow of the waste stream  14  so as to cause an angular impingement of the secondary stream with the flow of the waste stream  14 . The secondary stream is introduced to the air manifold  38  which blows downstream in the direction of the distribution grid  16 . The secondary stream penetrates the ports  42  in the plate  36  where it is added to the waste stream  14 , where the suppressing apparatus  18  effectively mixes and conditions the waste stream  14  and the SCR gases to reduce the generation of noise in the SCR system  10 . 
     Turning now to  FIGS. 22 through 26 , the combined flow mixing, tempering and noise suppressing apparatus  18  may include a hollow annular air bustle  44  forming a chamber  46  to which the secondary stream under pressure is supplied from a pipe  48 . In order to admit the secondary stream from the air bustle  44  into the SCR system  10 , ports  42  are provided in the plate  36  suitably arranged, such as circumferentially spaced, so as to afford fluid communication between the air bustle  44  and the SCR system  10 . A forwardly disposed wall of the air bustle  44  abuts and is attached to the rearwardly disposed face of the circular plate  36 . The secondary stream is introduced into the chamber  46  of the air bustle  44  and blows downstream to penetrate the ports  42  in the plate  36 . The combined flow mixing, tempering and noise suppressing apparatus  18  mixes the secondary stream with the waste stream  14  to improve the flow uniformity and to reduce the noise at the gas input duct  12  of the SCR system  10  with minimal pressure loss. 
     Whereas, the apparatuses and methods have been described in relation to the drawings and claims, it should be understood that other and further modifications, apart from those shown or suggested herein, may be made within the scope of this invention.