Abstract:
A burner ( 10 ) for reducing NO x  emissions where supply fuel ( 16 ) and supply air ( 20 ) are supplied to a combustion tunnel ( 52 ) at high and low velocities and secondary air ( 26 ) is supplied to a secondary combustion zone ( 60 ), wherein products of combustion ( 59 ) exiting into the secondary combustion zone ( 60 ) from the combustion tunnel ( 52 ) are drawn back into the combustion tunnel ( 52 ) and back into the secondary air conduit ( 54 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of earlier filed United States Provisional Patent Application Ser. No. 60/171,073, filed Dec. 16, 1999, entitled “Air and Fuel Staged Burner”. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to burners and, more particularly, to low NO x  emission burners having staged air and staged fuel capabilities.  
           [0004]    2. Brief Description of the Prior Art  
           [0005]    Low NO x  burners are known in the art. For example, U.S. Pat. Nos. 5,180,300 and 4,983,118 both disclose low NO x  regenerative burners. Likewise, U.S. Pat. No. 4,732,093 to Hansen et al. discloses a method and apparatus for burning fuel in an annular nozzle burner. However, there exists a need for a burner that further reduces NO x  generation.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides an air and fuel staged burner that reduces NO x  generation. One embodiment of a burner according the present invention generally includes a main burner body defining an internal cavity, an air connection fluidly connected to the internal cavity, and a combustion tunnel. A distribution tee may be fluidly connected to the internal cavity defined by the main burner body and a burner nozzle may be positioned in the interior cavity of the main burner body. The burner nozzle may define a primary air orifice, an annulus, and a fuel orifice. The air connection may be configured to receive supply air and divide the supply air into primary air and secondary air, where the ratio of primary air to secondary air is approximately in the range of 40/60 to 70/30 respectively, with a 50/50 ratio being preferred. The primary air preferably flows through the primary air orifice at a rate of approximately 300-400 feet/second (91-122 meters/second).  
           [0007]    The main burner body generally extends longitudinally about an imaginary burner centerline, and the primary air orifice is preferably oriented to form a convergent angle as measured from the imaginary burner centerline, such as an angle of approximately 30-60° as measured from the imaginary burner centerline. Alternatively, the primary air orifice may be oriented to produce a swirl pattern of primary air in the combustion tunnel, where the swirl is approximately less than or equal to 0.7 times an internal diameter of the combustion tunnel.  
           [0008]    The burner may also include a secondary air conduit fluidly connected to the distribution tee, the secondary air conduit having a secondary air jet fluidly connected to a secondary combustion zone. The main burner body generally extends longitudinally about an imaginary burner centerline and the secondary air jet is oriented substantially parallel to the imaginary burner centerline. Alternatively, the main burner body may extend longitudinally about the imaginary burner centerline with the secondary air jet oriented at an angle convergent with the imaginary burner centerline. The secondary air exits the secondary air jet at a velocity of approximately 150-400 feet/second (46-122 meters/second).  
           [0009]    A fuel connector is configured to receive a supply fuel and divide the supply fuel into a primary fuel and a secondary fuel. The split ratio of primary fuel to secondary fuel split ratio is approximately in the range of 20/80 to 40/60 respectively, with a split ratio of 22/78 being preferred. A primary fuel path and a secondary fuel path may also be included, with the primary fuel path fluidly connected to the annulus, the secondary fuel path fluidly connected to the fuel orifice, and the primary fuel path and the secondary fuel path fluidly connected to each other. The primary fuel may exit the annulus defined by the burner nozzle at a velocity approximately less than 100 feet/second (30 meters/second). The secondary fuel may exit the fuel orifice defined by the burner nozzle at a velocity approximately greater than 350 feet/second. The fuel orifice and the fuel annulus may lie in the same plane, substantially perpendicular to an imaginary burner centerline and the distribution tee may be positioned adjacent to the internal cavity of the main burner body and opposite the combustion tunnel ( 52 ).  
           [0010]    One method of decreasing NO x  emissions in a burner having a main burner body defining a combustion tunnel may include the steps of flowing supply air into the main burner body, dividing the supply air into primary air and secondary air, flowing the primary air into the combustion tunnel at a given velocity, flowing primary fuel into the combustion tunnel at a velocity lower than the velocity of the primary air, flowing secondary fuel into the combustion tunnel at a velocity higher than the velocity of the primary fuel, flowing secondary air into a secondary combustion zone by a secondary air jet at a velocity higher than the velocity of the primary fuel, and igniting the primary fuel, the secondary fuel, and primary air in the combustion tunnel to form products of combustion. Additional steps may include exhausting products of combustion into the secondary combustion zone and drawing products of combustion into the combustion tunnel and into the secondary air jet.  
           [0011]    The device and method according to the present invention helps to reduce burner NO x  emissions.  
           [0012]    These and other features and advantages of the present invention will be clarified in the description of the preferred embodiment taken together with the attached drawings in which like reference numerals represent like elements throughout. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a partial cross-sectional side view of one embodiment of the present invention;  
         [0014]    [0014]FIG. 2 is a full cross-sectional side view of the embodiment shown in FIG. 1 excluding the secondary air jets for clarity and rotating the location of the primary air connection by 90 degrees; and  
         [0015]    [0015]FIG. 3 is a front view of a burner nozzle shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    The preferred embodiment of a burner  10  according to the present invention is shown in FIGS.  1 - 3 . FIG. 2 shows the burner  10  having a main burner body  22  defining an air connection  12 , an internal cavity  13 , and a combustion tunnel  52 . A fuel connector  14  is provided through which supply fuel  16  enters the burner  10 , except in the event a gas pilot (not shown) is used through a port  18 . An electrode (not shown) is used to ignite the burner  10 ; however, a gaseous pilot could be used.  
         [0017]    As best shown in FIG. 2, supply air  20  enters the air connection  12 , passes into the internal cavity  13  defined by the main burner body  22 , and is divided into primary air  24  and secondary air  26 . A secondary air orifice  28  permits the secondary air  26  to enter a secondary air distribution tee  30  while the primary air  24  passes through at least one primary air orifice  32  defined by a burner nozzle  46 , with the number of primary air orifices  32  preferably in the range of four to eight orifices  32 . The primary air  24  is accelerated through the primary air orifice or orifices  32  to a range of approximately 300 feet/second-400 feet/second (91-122 meters/second), depending on the air preheat available, nominal burner  10  ratio, and rated input. The primary air  24  is preferably directed in a convergent manner toward an imaginary burner centerline C; however, the primary air orifice or orifices  32  may also be slightly offset to induce a swirl pattern on the primary air  24 . A convergence angle a of the primary air orifice or orifices  32  can be approximately 30°-60°, as measured from the imaginary burner centerline C. The swirl or offset can be as much as 0.7 times the primary port, or combustion tunnel, diameter D.  
         [0018]    The supply fuel  16  entering fuel connector  14  passes into a fuel sparger  34  which divides the supply fuel  16  via holes  36  into primary fuel  38  and secondary fuel  40 . The primary fuel  38  travels along one or more primary fuel paths  42 , preferably parallel to the secondary fuel  40  which travels through a secondary fuel path  44 . The primary fuel path  42  is preferably fluidly connected to an annulus  47  defined by the burner nozzle  46  positioned in the internal cavity  13  defined by the main burner body  22 . The secondary fuel path  44  is preferably fluidly connected to a fuel orifice  48 , also defined by the burner nozzle  46 . The primary fuel  38  exits the burner nozzle  46  through the annulus  47  into the combustion tunnel  52  at a low velocity, ideally less then 100 feet/second (30 meters/second), depending on rated input. The secondary fuel  40  passes down the secondary fuel path  44  and exits into the combustion tunnel  52  through fuel orifice  48 , preferably accelerated to a velocity approximately greater than 350 feet/second (107 meters/second), depending on rated input. As shown in FIG. 3, the fuel annulus  47  preferably has a first width W 1  and the fuel orifice  48  preferably has a second width W 2 , with the first width W 1  of the fuel annulus  47  being less than the second width W 2  of the fuel orifice  48 .  
         [0019]    Referring again to FIG. 2, the velocities of the primary and the secondary fuels  38 ,  40  exiting the annulus  47  and the fuel orifice  48  of the burner nozzle  46  will depend on the velocity of the primary air  24  exiting the primary air orifice or orifices  32 . The primary fuel  38  exiting the annulus  47  mixes in a highly turbulent region with the primary air  24  exiting the primary air orifice or orifices  32 , creating a highly reducing combustion region within the combustion tunnel  52 . The secondary fuel  40  exiting the fuel orifice  48  is accelerated to the point that there is only a partial mixing of the secondary fuel  40  with the primary air  24  and products of combustion  59  in a primary combustion zone  50  of the combustion tunnel  52 . Therefore, the profile of combustion exiting the combustion tunnel  52  is more oxidizing toward the perimeter of combustion tunnel  52  and more reducing along the imaginary burner centerline C.  
         [0020]    As best shown in FIG. 1, the secondary air  26  passes through the distribution tee  30  and into a secondary air conduit  54 . The secondary air conduit  54  communicates the secondary air  26  to a secondary air jet  56  spaced apart from a combustion tunnel exit  62  of the combustion tunnel  52  and in fluid communication with a secondary combustion zone  60 . Secondary air  26  exits the secondary air jet  56  at a velocity in the range of 150 feet/second to 400 feet/second (46-122 meters/second), depending on the air preheat, nominal design ratio of the burner  10 , and rated input.  
         [0021]    The burner  10  is capable of being operated with a single secondary air jet  56  or a plurality of secondary air jets  56 . The secondary air jets  56  may be oriented parallel or convergent to the imaginary burner centerline C, shown as angle β in FIG. 1. The secondary air  26  exits the secondary air jets  56  at a furnace wall  58  and creates a negative pressure region pulling the products of combustion  59  from the second combustion zone  60  back into the secondary air orifice  56 , highly vitiating the secondary air  26  before the secondary air  26  reaches the sub-stoichiometric ratio mixture exiting the combustion tunnel  52 . The resultant combustion expansion in the primary combustion zone  50  of combustion tunnel  52  also creates a suction at the furnace wall  58  in the vicinity of the combustion tunnel exit  62  which also induces the furnace products of combustion  59  back to the combustion tunnel exit  62 .  
         [0022]    The burner  10  configuration of the present invention provides vitiation in the primary and secondary combustion zones  50 ,  60  such that the stoichiometry to the burner  10  must be on the oxidizing side to initiate stable combustion in the secondary combustion zone  60  when below 1200° F. furnace temperature. At approximately 1200° F. (649° C.), the stoichiometry can be brought to approximately 10% excess air with the resulting main flame stability and the secondary combustion reactions completing without the generation of free combustibles. Minor traces of CO will be apparent with furnace temperature between 1200° F. and 1400° F. (649° C.-760° C.). The primary fuel  38  to secondary fuel  40  split ratio can be approximately 20/80 to 40/60, respectively, while the primary air  24  to secondary air  26  split ratio can be 40/60 to 70/30, respectively. The optimum primary fuel  38  to secondary fuel  40  split ratio is approximately 22/78, respectively, and the optimum primary air  24  to secondary air  26  split is approximately 50/50.  
         [0023]    The air and fuel staged burner  10  according to this first embodiment significantly improves NO x  emission capabilities, as illustrated in the following table:  
                                           TABLE 1                           COMPARISION OF PRESENT INVENTION WITH AN       AIR STAGED BURNER AT AN AIR TEMPERATURE OF       750° F. (399° C.) AND A FURNACE TEMPERATURE OF       1600° F. (871° C.)                AIR STAGED   FUEL &amp; AIR STAGED                        NO x  PPM @ 3%   44   22                  
 
         [0024]    The invention has been described with reference to the preferred embodiment. Obvious modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.