Abstract:
A radiant tube burner system comprises a novel flue gas recirculating assembly for reducing nitrous oxides emissions. In the burner system, the combustion air conduit running to the radiant tube burner intersects the exhaust conduit at a location between the radiant tube and the eventual exhaust outlet. A venturi is formed at this intersection. The venturi is configured to suction a portion of the flue gas from the exhaust conduit into the combustion air flow. The assembly includes a may be interposed along the exhaust conduit mounted directly between the radiant tube and a downstream section of the exhaust conduit.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention pertains generally to radiant tube burners and more particularly to methods and apparatus for recirculating flue gas so as to reduce nitrous oxides (NO x ) emissions.  
       BACKGROUND OF THE INVENTION  
       [0002]     Radiant tube burner systems are well know as generally disclosed in various U.S. patents assigned to the present assignee, Eclipse Combustion, Inc., including U.S. Pat. No.: 4,673,350 to Collier; U.S. Pat. No. 4,705,022 to Collier; U.S. Pat. No. 5,241,949 to Collier; and U.S. Pat. No. 6,024,083 to Smirnov. The entire disclosures of all of these patents are hereby incorporated by reference. As generally disclosed in these patents, a radiant tube burner system generally comprises a burner having a combustion air inlet, a fuel inlet, and a burner head that extends into to a radiant tube (often a U-shaped or W shaped tube). The radiant tube is mounted through a furnace wall with inlet and outlet openings on the outside of the furnace. The advantage of using a radiant tube is that the internal environment of a furnace may be kept as a clean environment substantially free of products of combustion, known as flue gas. During operation, the burner convey combustion oxidant (e.g. combustion air generated by a blower) and gaseous fuel to the burner head for combustion and into the radiant tube, where heat is radiated. The products of combustion or flue gases are then conveyed to an exhaust stack for exhausting outside the factory into the ambient environment.  
         [0003]     As with any industrial burner, the products of combustion or flue gases as they are known contain nitrous oxides (NO x ) emissions, which are undesirable and regulated by regulatory agencies. As a result there is a desire (and a need in some instances where emissions are too high) to reduce nitrous oxides (NO x ) emissions.  
         [0004]     A well known method for reducing nitrous oxides (NO x ) emissions in industrial burners is to recirculate a portion of the flue gas to reduce interaction between oxygen and gaseous fuel and thereby lower the temperature, which inherently lowers the nitrous oxides (NO x ) emissions. Proposals for recirculating a portion of the flue gas in radiant tube burners are disclosed in U.S. Pat. No. 6,190,159 to Moore et al. and U.S. Pat. No. 4,800,866, and the entire disclosures of these patents is hereby incorporated by reference. These proposals appear to have several drawbacks relating to complexity and cost, and might be difficult to implement and control.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     The general objective of the present invention is to provide an improved way to recirculate a portion of flue gas in radiant tube burner systems to reduce nitrous oxides (NO x ) emissions.  
         [0006]     The present invention is directed toward a novel flue gas entrainment mechanism for a radiant tube burner system. The radiant tube burner system generally comprises a burner that is adapted to convey combustion air and fuel from the combustion air inlet and the fuel inlet to the burner head for combustion. A radiant tube receives the burner head. An exhaust conduit is connected to radiant tube and is adapted to convey flue gas toward an exhaust outlet. A blower may be provided to generate a pressurized source of combustion air. A combustion air conduit connects the blower to the combustion air inlet of the burner. In accordance with the present invention, the combustion air conduit intersects the exhaust conduit between the radiant tube and the exhaust outlet and a venturi is formed at the intersection between the combustion air conduit and the exhaust conduit. The venturi is arranged to suction a portion of the flue gas from the exhaust conduit into the combustion air conduit.  
         [0007]     In accordance with one aspect of the present invention, an assembly for accomplishing the radiant tube burner system is provided. This assembly includes a tubular conduit section that can be interposed along the exhaust conduit which carries the flue gas toward the exhaust outlet. The tubular conduit section includes at least four openings including a first flue gas port, a second flue gas port, a first oxidant port and a second oxidant port. The first and second flue gas ports connected by a flue gas passageway for communicating flue gas through the exhaust conduit toward the exhaust outlet. The tubular conduit section further includes a throat portion integral therewith that extends transverse relative to the flue gas passageway. The throat portion connects the flue gas passageway to the second oxidant port. The assembly also includes an oxidant inlet pipe extending into the first oxidant port of the tubular conduit section. The oxidant inlet pipe terminating in a nozzle that is directed toward the second oxidant port. With this arrangement and when oxidant flows through the oxidant inlet pipe and flue gas flows through the flue gas passageway, a portion of the flue gas is suctioned into the second oxidant port for recirculation for reducing nitrous oxides (NO x ) emissions.  
         [0008]     The invention can be used on new burner systems or used to retrofit old existing burner system units.  
         [0009]     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a perspective view, shown in partly schematic form, of a radiant tube burner system according to an embodiment of the present invention.  
         [0011]      FIG. 2  is a top view of a portion the radiant tube burner system shown in  FIG. 1 , illustrating the novel tubular conduit section interposed on the exhaust conduit in accordance with an embodiment of the present invention.  
         [0012]      FIG. 3  is a frontal view of a portion the radiant tube burner system shown in  FIGS. 1 and 2 .  
         [0013]      FIG. 4  is a top view of the gas entrainment assembly including the tubular conduit section and an oxidant inlet pipe used in the embodiment shown in the previous Figures.  
         [0014]      FIG. 5  is a cross section of  FIG. 4  taken about line  5 - 5 .  
         [0015]      FIG. 6  is a cross section of  FIG. 3  taken about lines  6 - 6 .  
         [0016]      FIG. 7  is an isometric view of the tubular conduit section shown in  FIG. 4 .  
         [0017]      FIG. 8  is a cross section similar to  FIG. 5 , but showing a different embodiment that has the addition of a baffle plate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring to  FIG. 1 , the present invention has been illustrated as embodied in a radiant tube burner system  10 . The burner system  10  includes a radiant tube  12  (which may be U shaped) that is mounted to a furnace wall  14  to include U-shaped portion  16  (or other appropriately shaped portion) on the inside of the furnace, and inlet and outlet openings on the outside of the furnace which typically terminate in and are surrounded by mounting flanges  18 ,  20 .  
         [0019]     As is often conventional with radiant tube burners, a burner  22  is mounted the inlet side flange  18  on the outside of the furnace. The burner  22  has a combustion air inlet  24 , a gaseous fuel inlet  26 , a burner head  28  and an igniter  30 . The burner head  28  extends into the inlet leg of the radiant tube  12  and includes conduits for air (or another form of oxidant) and fuel, and a burner nozzle  32  (also referred to as mixing plate or combustion section) that is adapted to mix fuel and oxidant for combustion in the inlet leg of the radiant tube  12 . Accordingly, the igniter  30  typically extends with the burner head (with an electrical terminal on the outside of the burner and furnace) and is positioned in close proximity to burner nozzle for igniting fuel and air. Further structural details of an appropriate burner can be had to any of the previously referenced Eclipse patents incorporated by reference in the background section above.  
         [0020]     To communicate fuel and air to the burner  22 , the system  10  includes a combustion air inlet conduit  34  in fluid communication with the combustion air inlet  24  and a fuel conduit  36  connected to the fuel inlet  26 . One or more valves  38  may be interposed along the fuel conduit for setting a fuel flow rate and/or for providing a fuel shut-off function. The entrance of the air inlet conduit  34  is connected to the output of a blower  40  or other suitable combustion oxidant source (e.g. such as pressurized oxygen or oxygen enriched air). In this embodiment of the present invention, and according to one feature of the preferred embodiment of the present invention, a single blower  40  may be used to generate a pressurized combustion air flow to two or more burners  22  arranged in parallel circuit as shown schematically in  FIG. 1 . However, one blower  40  may also be dedicated to only one of the burners  22  if desired.  
         [0021]     The return leg of the radiant tube  12  terminates in the outlet mounting flange  20 . An exhaust conduit  42 , which may comprise one or multiple sections, is mounted to the outlet mounting flange  20  and connects the radiant tube  12  to an exhaust outlet  44 . The exhaust outlet  44  serves to exhaust flue gases produced by combustion generated in the radiant tube  12  to the external ambient atmosphere (e.g. typically via an exhaust stack on the outside of the factory).  
         [0022]     In accordance with the present invention, a flue gas recirculation assembly  46  is provided that includes an oxidant inlet pipe  48  of the combustion air conduit  34  and a first conduit section  50  of the exhaust conduit. The oxidant inlet pipe  48  intersects the first conduit section  50  of the exhaust conduit  42 , providing a novel configuration for recirculating a portion of the flue gas using the venturi effect. The first conduit section  50  includes an flue gas entrance port  52  and a flue gas exit port  54 , connected by a flue gas passageway  55  for communicating flue gas through the exhaust conduit  42  toward the exhaust outlet  44 . The entrance and exit ports may be coaxially surrounded by mounting flanges  51  that facilitate interposition of the first conduit section  50  directly on the exhaust conduit  42 . As shown herein, one flange  51  is mounted directly to the return leg mounting flange  20  of the radiant tube  12 , while the other flange  51  is mounted to a downstream section  53  of the exhaust conduit  42 .  
         [0023]     The conduit section  50  also includes an oxidant entrance port  56  and an oxidant exit port  58  for communicating combustion air oxidant transversely through the exhaust conduit  42 . The tubular conduit section  50  also integrally includes a throat portion  60  that extends transverse relative to the flue gas passageway  55 . The throat portion  60  connects the flue gas passageway  55  to the oxidant outlet port  58  and integrally provides the oxidant outlet port  58 . As shown best in  FIG. 5 , the throat portion  60  starts at a larger diameter and flow area and converges to the oxidant exit port  58  which is at a reduced diameter and flow area. The throat portion  60  extends toward a threaded end  62  that surrounds the exit port  58 . The threaded end  62  provides for mounting the of the tubular conduit section  50  to a downstream section of the combustion air conduit  34  to deliver combustion air to the combustion air inlet  24  of the burner  22 .  
         [0024]     The oxidant inlet pipe  48  is slidably inserted through the entrance port  56  of the exhaust conduit section  50  and extends toward the oxidant exit port  58 . When mounted, the oxidant inlet pipe  48  defines the flow area of the oxidant entrance port  56 , and provided an internal passageway  65  extending transverse relative to the exhaust gas passage. The inlet pipe  48  includes a threaded end  64  that is adapted to be connect to an upstream section of the combustion air conduit  34 . The inlet pipe  48  extends linearly from the threaded end  64  toward an integrally formed nozzle  86  which is disposed internally with respect to the exhaust conduit section  50  with a trajectory directed toward the combustion air exit port  58 . In the disclosed embodiment, the position of the inlet pipe  48  can be axially adjusted and can be linearly translated relative to the exhaust conduit section  50 . To provide for linear adjustment, clamping means is provided by a bolt  68  that releasably clamps the inlet pipe  48  in set position. The bolt is screwed into a threaded hole  70  formed into a boss structure  72  integrally extending from the exhaust conduit section  50 . The boss structure  72  defines the opening that provides for the oxidant entrance port  56  and receipt of inlet pipe  48 .  
         [0025]     In addition, an annular seal  74  is provided between the inlet pipe  48  and the exhaust conduit section  50  to prevent leakage of flue gas. The seal  74  is retained by a sleeve shaped nut  76  that is coaxial about the inlet pipe  48  and threaded into a threaded sleeve  78  extending from the boss structure  72 .  
         [0026]     As shown in  FIG. 5 , outer surface of the inlet pipe  48  is spaced radially inward of the inner surface of the throat portion  60  such that a flow passage  80  is defined therebetween for introducing a portion of the flue gas into the flow of the combustion air oxidant. With this arrangement, a venturi generally indicated at  82  is formed such that during operation combustion air being jetted through the nozzle  86  (with a trajectory extending with the throat portion toward the exit port  58 ) draws a portion of the flue gas through the throat portion  60  to the combustion air the exit port  58 .  
         [0027]     By adjusting the axial position of the inlet pipe  48  the nature and characteristic of the venturi  82  will change, thereby allowing for adjustment as to how much flue gas will be recirculated at selected operating parameters. Ordinarily, once the desired axial position of the inlet pipe  48  is set, no further adjustments may be necessary.  
         [0028]     There are several advantages of the flue gas recirculating assembly  46 . First, flue gas recirculation is effected by intersecting the combustion air and exhaust conduits  34 ,  42 , and as a result minimal components may be needed. Additional plumbing and parallel conduit arrangements are not necessary to achieve flue gas recirculation.  
         [0029]     An additional feature which may be provided is shown in the additional embodiment of  FIG. 7 . As shown therein, an annular restrictor plate  84  that includes a mounting sleeve portion  86  is arranged coaxial about the inlet pipe  48 . The restrictor plate  84  axially slides onto the inlet pipe  48  and is secured thereto via a clamping screw  88 . The clamping screw  88  is mounted into a threaded hole formed in the mounting sleeve portion  86 . With the addition of the restrictor plate  84 , a narrower restriction  90  is formed in the venturi arrangement  82  that serves to reduce and limit the amount of flue gas being recirculated. The position of the restrictor plate  84  relative to the throat portion  60  and the inlet pipe  48  can be selected to provide a desired environmental performance characteristic for the radiant tube burner system  10 .  
         [0030]     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.  
         [0031]     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.  
         [0032]     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.