Patent Publication Number: US-8113824-B2

Title: Large diameter mid-zone air separation cone for expanding IRZ

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of fuel burners and in particular to a new and useful air separation cone for expanding the internal recirculation zone near the exit of one or more air zones surrounding a fuel delivery nozzle. 
     Low-NOx fossil fuel burners operate on the principle of controlled separation and mixing of fuel and oxidizer for minimizing the oxidation of fuel-bound nitrogen and nitrogen in the air to NOx (i.e., NO+NO2). Use of overfire air in conjunction with fuel-rich combustion is referred to as external (or air) staging. Internal staging involves the creation of fuel-rich and fuel-lean combustion zones within the burner flame. With proper design, fuel-air mixing and swirl patterns can be optimized to create a reverse flow region or “internal recirculation zone” (IRZ) near the burner exit for recycling heat and combustion products including NOx from fuel-lean regions into fuel-rich zones to sustain ignition, maintain flame stability, and convert NOx to N2. Both internal and external staging are often necessary for maximum NOx reduction. Flames with large, high temperature, sub-stoichiometric (oxygen-deficient) IRZ&#39;s generally produce very low NOx levels since such conditions are conducive for NOx destruction. Low-NOx burner designs produce the IRZ by imparting swirl on the air and/or fuel streams as well as flow deflecting devices such as flame holders and air separation cones. 
       FIG. 1  shows a low-NOx pulverized coal fired burner  900  having a conventional air separation cone. Primary air and pulverized coal  902  are blown into an inlet and pass through a burner elbow  904 . The pulverized coal concentrates along the outer radius at the elbow exit. The pulverized coal enters the inlet end of a fuel nozzle or tubular burner nozzle  906 , and encounters a deflector  908  which redirects the coal stream into a conical diffuser  912 , which disperses the majority of the pulverized coal particles entrained in the primary air to a location near the inside surface of the tubular burner nozzle  906 , leaving the central portion of the nozzle  906  relatively free of pulverized coal particles. 
     Secondary air  910 , or the majority of combustion air, is delivered to inner and outer secondary air zones  914  and  916  from the burner windbox. Swirl can be imparted into the zones  914  and  916  via adjustable angle spin vanes  922  in the inner air zone  914  and both fixed spin vanes  920  and adjustable angle spin vanes  922  in the outer air zone  916 . The inner and outer secondary air zones  914  and  916  are formed by concentrically surrounding walls. The inner air zone  914  concentrically surrounds the tubular burner nozzle  906  and the outer air zone  916  concentrically surrounds the inner air zone  914 . 
     An air separation cone  924 , concentrically surrounding the end of the tubular burner nozzle  906 , helps channel the secondary air  910  leaving the inner and outer air zones  914  and  916 . A flame stabilizer  926  and a slide damper  928  control the secondary air  910 . The flame stabilizer  926  is mounted at the end of the tubular burner nozzle  906  while the air separation cone  924  is installed on a cylindrical sleeve that separates the inner and outer secondary air zones  914  and  916 . 
     The inner and outer zones  914  and  916  direct the secondary air radially outward by the combined action of the burner throat and the swirl imparted by the spin vanes  922 , generating internal recirculation zones (IRZ)  930 .  FIG. 1  shows the predicted reverse flow IRZ streamlines for a low-NOx pulverized coal fired burner  900  having a conventional air separation cone  924 . NOx is formed along the outer air-rich periphery of the flame as secondary air is introduced from the inner and outer air zones. The IRZ causes the NOx formed at the outer fringe of the flame to recirculate back along the fuel rich flame core, where hydrocarbon radicals react to reduce the NOx. 
     The size of the IRZ can be increased somewhat by imparting more swirl on the secondary air flow, and extending the flow deflection devices, or increasing their angle of attack. Generation of high swirling flows require fan power boosting due to higher pressure drop. High swirl combustion can also intensify the fuel/oxidizer mixing and generate high NOx emissions. Extension of flow deflecting devices (flame holder or air separation cone) into the furnace could expose those parts to high flame temperatures and cause damage. Increasing the angle of attack on the flow deflecting devices could restrict the air flow passages, raise the pressure drop, and diminish the swirl effects. Therefore, a device is needed for safely and effectively increasing the size of the IRZ, without damaging flow deflecting devices, causing increased NOx emissions, or raising pressure drop. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a device which safely and effectively increases the size of the IRZ, without damaging flow deflecting devices, causing increased NOx emissions, or raising pressure drop. 
     Accordingly, a large diameter mid-zone air separation cone is provided for increasing the IRZ and decreasing NOx. The air separation cone has a larger diameter than the conventional air separation cone. The mid-zone air separation cone has a short cylindrical leading edge that fits in the outer air zone of a burner. The mid-zone air separation cone is supported by standoffs inside the outer air zone. The mid-zone air separation cone splits the outer air zone secondary air flow into two equal or unequal streams depending on the position of the air separation cone with respect to the outer air zone, and deflects a portion of the secondary air flow radially outward. Since the radial position of the mid-zone air separation cone is farther from the burner centerline than the radial position of the conventional air separation cone, the size of the IRZ is expanded and NOx emissions are minimized. 
     The mid-zone air separation cone can be used with many types of burners. The mid-zone air separation cone can be used with burners fueled by pulverized coal, oil, or natural gas. The mid-zone air separation cone can be used with burners with primary air and coal in the center or a large central passage of secondary air surrounded by primary air and coal. The mid-zone air separation cone can essentially be used with any burner where there is at least one air zone surrounding a fuel delivery nozzle or annulus, where the air separation cone is of a large diameter and therefore the IRZ is enlarged. 
     Thus, some of the advantages of using the mid-zone air separation cone of the present invention are expansion of the IRZ, better flame stabilization and attachment, and lower NOx emissions. Also, there is no adverse effect on burner operation, such as damage to air separation cone or other components of the burner and pressure drop is not raised. The mid-zone air separation cone is a simple cost-effective solution that requires no additional conduits inside a burner and can be installed with relative ease inside the air zone of many burners. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a schematic drawing showing the predicted reverse flow IRZ streamlines for a low-NOx pulverized coal fired burner having the conventional air separation cone; 
         FIG. 2  is a schematic drawing of the mid-zone air separation cone of the present invention at the end of a burner; 
         FIG. 3  is a graph plotting reverse volumetric flow rate versus axial distance for both a conventional air separation cone and the mid-zone air separation cone of the present invention; 
         FIG. 4  is a schematic drawing of the low NOx DRB-XCL® pulverized coal burner incorporating the mid-zone air separation cone of the present invention; 
         FIG. 5  is a schematic drawing of the low NOx DRB-4® burner incorporating the mid-zone air separation cone of the present invention; and 
         FIG. 6  is a schematic drawing of the low NOx central air jet pulverized coal burner incorporating the mid-zone air separation cone of the present invention. 
         FIG. 7  is a schematic drawing of the low NOx XCL-S pulverized coal burner incorporating the mid-zone air separator cone of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements,  FIG. 2  shows the end of a burner  2  which is adjacent or near a furnace. The end of the burner  2  includes a large diameter mid-zone air separation cone  1  with a short cylindrical leading edge  3  that fits in the middle of an outer secondary air zone  4 . Additionally, as illustrated by  FIGS. 2 ,  4 ,  5 ,  6  and  7 , the leading edge  3  along its entire length is parallel to the horizontal axis A of the mid-zone separation cone. The device is supported by standoffs (not shown) inside the outer secondary air zone  4  and is not directly connected to any conduits in the burner. It essentially splits the outer air zone  4  secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone  1  is farther from the burner centerline than the radial position of the conventional air separation cone shown in  FIG. 1 , it expands the IRZ size and with that, the NOx emissions are minimized. 
     The diverging angle of the mid-zone air separation cone can be between 25 to 45° from the horizontal axis A (50 to 90° included angle). Although the embodiment in  FIG. 2  shows that mid-zone air separation cone fits at approximately the middle of the outer air zone annulus, the cone may also be fitted anywhere within the outer air zone annulus to divide the secondary air stream in any desired proportion. The length of the cone  1  can vary depending on the air zone gap and burner size. The mid-zone air separation cone  1  can also be used in burners designed for firing pulverized coal, fuel oil, and natural gas. 
       FIG. 3  shows the computer modeling predictions of reverse (recirculating) flow rates in the near-burner region of the flame at different axial distances up to 2.5 burner diameters (x/D=2.5). The plots clearly indicate a larger IRZ (more reverse flow) for the case with the mid-zone air separation cone relative to conventional air separation cone. It is noted that the calculations correspond to staged combustion of an eastern bituminous coal at 0.85 burner stoichiometry. 
       FIGS. 4 through 7  show four possible installations of the mid-zone air separation cone  1  in four different types burners. Although four different embodiments of the invention are shown, the invention is not limited to these embodiments. The mid-zone air separation cone of the present invention can also be installed in other burners not shown here, where there is at least one air zone surrounding a fuel delivery nozzle or annulus. 
       FIG. 4  shows installation of the mid-zone air separation cone  1  in a low NOx DRB-XCL® pulverized coal burner  10 , which is described in more detail as prior art ( FIG. 2 ) in U.S. Pat. No. 5,829,369, which is incorporated by reference. The burner  10  includes a conical diffuser  12  and deflector  34  situated within the central conduit of the burner  10  which is supplied with pulverized coal and air by way of a fuel and primary air (transport air) inlet  14 . A windbox  16  is defined between the inner and outer walls  18 ,  20  respectively. The windbox  16  contains the burner conduit which is concentrically surrounded by walls which contain an outer array of fixed spin vanes  22  and adjustable angle spin vanes  24  within an outer air zone  26 . An inner air zone  27  is provided concentrically within the outer air zone  26 . The burner  10  is provided with a flame stabilizer  30  and a slide damper  32  for controlling the amount of secondary air  28 . 
     A mid-zone air separation cone  1  of the present invention is provided for increasing the IRZ zone and decreasing NOx. The air separation cone  1  has a larger diameter than the air separation cone shown in  FIG. 1 . The mid-zone air separation cone  1  also has a short cylindrical leading edge  3  that fits in the middle of the outer air zone  26 . The mid-zone air separation cone  1  is supported by standoffs (not shown) inside the outer air zone  26 . The mid-zone air separation cone  1  splits the outer air zone  26  secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone  1  is farther from the burner centerline than the conventional air separation cone shown in  FIG. 1 , it expands the IRZ size and accordingly, NOx emissions are minimized. 
       FIG. 5  shows a burner generally depicted  40  in accordance with then present invention. Burner  40 , which is also referred to as the DRB-4Z® burner, comprises a series of zones created by concentrically surrounding walls in the burner conduit which deliver a fuel such as pulverized coal with a limited stream of transport air (primary air), and additional combustion air (secondary air)  28  provided from the burner windbox  16 . The central zone  42  of the burner  40  is a circular cross-section primary zone, or fuel nozzle, that delivers the primary air and pulverized coal by way of inlet  44  from a supply (not shown). Surrounding the central or primary zone  42  is an annular concentric wall  45  that forms the primary-secondary transition zone  46  which is constructed either to introduce secondary combustion air or to divert secondary air to the remaining outer air zones. The transition zone  46  acts as a buffer between the primary and secondary streams to provide improved control of near-burner mixing and flame stability. The transition zone  46  is configured to introduce air with or without swirl, or to enhance turbulence levels to improve combustion control. The remaining annular zones of burner  40  consist of the second inner air zone  48  and the outer air zone  50  formed by concentrically surrounding walls which deliver the majority of the combustion air. 
     The burner  40  includes a mid-zone air separation cone  1  having a short cylindrical leading edge  3  that fits in the middle of the outer air zone  50 . The mid-zone air separation cone  1  is supported by standoffs (not shown) inside the outer secondary air zone annulus. The mid-zone air separation cone  1  splits the outer air zone  50  secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone  1  is farther from the burner centerline than the conventional air separation cone shown in  FIG. 1 , it expands the IRZ size and accordingly, NOx emissions are minimized. 
     Structurally, the design of the burner  40  (DRB-4Z®) according to the present invention is based largely on that for the DRB-XCL® burner shown in  FIG. 4 . A detailed explanation of the differences between the two types of burners is provided in U.S. Pat. No. 5,829,369. 
       FIG. 6  shows a low NOx central air jet pulverized coal burner  60  in which pulverized coal and primary air (PA/PC)  61  enter at an inlet and pass through a burner elbow  62 . The pulverized coal mostly travels along the outer radius of the elbow  62  and concentrates into a stream along the outer radius at the elbow exit. The pulverized coal enters a coal pipe  63  and encounters a deflector  64  which redirects the coal stream into a conical member  65 , dispersing the coal. A core or central pipe  66  is attached to the downstream side of conical member  65 . The coal pipe  63  expands in section  63 A to form a larger diameter section  63 B. The dispersed coal travels into an annulus  71  formed between central pipe  66  and the coal pipe  63 A and then  63 B. The PA/PC  61  then exits the coal annulus  71  into the burner throat  68 , and then out into the furnace (not shown). The core or central pipe  66  and the annulus  71  form a fuel nozzle. 
     Secondary air  78  is supplied by forced draft fans or the like, preheated in air heaters, and supplied under pressure. Feeder duct  69  supplies core air to central zone  66 . Wedged shaped pieces  68 A and  69 B provide a more contoured flow path for the PAIPC  61  as it travels past the core air supply feeder duct  69 . The core air proceeds down central zone  66  until it exits. Some secondary air flows into transition zone  76  or outer air zone  77 . Secondary air can be throttled to one zone or the other, or to supply lesser quantities of air to both zones to cool the burner when the burner is out of service. The transition zone  76  is separated from the outer air zone  77 . The transition zone  76  is constructed to provide air for near-burner mixing and stability. Adjustable angle spin vanes  81  are situated in the transition zone  76  to provide swirl to transition air. Outer air proceeds through fixed spin vanes  80  and adjustable angle spin vanes  82  which impart swirl to the outer air. 
     A large diameter mid-zone air separation cone  1  with a short cylindrical leading edge  3  (not shown in  FIG. 6 ) fits in the middle of the outer air zone  77 . The cone  1  is supported by standoffs (not shown) inside the outer air zone  77  and is not directly connected to any conduits in the burner. The cone  1  splits the outer air zone  77  secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone  1  is farther from the burner centerline than the conventional air separation cone shown in  FIG. 1 , it expands the IRZ size and with that, the NOx emissions are minimized. 
     Performance of the mid-zone air separation cone was further tested with low NOx central air jet pulverized coal burner at 100 million Btu/hr while firing a pulverized eastern bituminous coal. At 17% overall excess air level, and 0.80 burner stoichiometry, NOx emissions were 0.276 lb/million Btu with the conventional air separation cone installed on the end of the cylindrical sleeve  5  separating the transition zone  76  from outer air zone  77 , and 0.238 lb/million Btu with the mid-zone air separation cone, shown in  FIG. 6 , while maintaining low CO and unburned carbon levels. 
       FIG. 7  show another low NOx burner embodiment according to the present invention. A fossil fuel, such as pulverized coal, and primary air enter burner  100  via burner inlet  102 , and pass through burner elbow  104 . Secondary air  106  is provided to outer air zone  108 , wherein swirl may be added via adjustable vanes  110 . 
     Mid-zone air separation cone  1  having a short cylindrical leading edge  3  is provided within outer air zone  108 . Air separation cone  1  is supported by standoffs (not shown) inside the outer air zone  108 . Air separation cone  1  splits the outer air zone  108  secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone  1  is farther from the burner centerline than the conventional air separation cone shown in  FIG. 1 , it expands the IRZ size and provided a means for minimizing NOx emissions. 
     While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.