Patent Publication Number: US-9841189-B2

Title: Lean premix burner having center gas nozzle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This claims the benefit of U.S. Provisional Patent Application Ser. No. 61/758,892 filed on Jan. 31, 2013, the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to combustion equipment, and more particularly, it relates to apparatus and methods for lean premix low NOx combustion. 
     BACKGROUND OF THE INVENTION 
     Burners may be used in a wide range of well known applications, such as the drying and heating of materials. Stricter regulatory requirements have created a demand for burners that produce low levels of nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic compounds (VOCs). These emissions are a significant source of air pollution, and are thus undesirable. 
     Several well known techniques for reducing NOx emissions are not well suited for certain burner applications, where, for instance, a compact burner size is required. NOx reduction techniques, such as exhaust gas recirculation or water injection, may not be easy to implement in these applications and may produce undesirable secondary effects, such as reduced thermal and/or combustion efficiency. There is a need for improved burners producing low NOx. 
     U.S. Pat. No. 8,113,821, entitled “Premix Lean Burner,” which is owned by the owner of the present invention and marketed under the name Novastar™ burner, discloses a low NOx burner having oil firing capabilities. 
     SUMMARY OF THE INVENTION 
     The inventor has discovered that the Novastar burner described in the 821 patent when placed into service may be subject to process conditions that are sometimes detrimental to smooth operation. For example, when employed in an aggregate drying process that is part of asphalt manufacturing, the burner is sometimes subjected to pressure fluctuations or oscillations because of changes in the process. If the drum into which the burner fires has a momentary high pressure, a flashback may occur. In response to conditions of a flashback, the control system is designed to make an emergency stop on the burner. If the drum has low pressure, the location of the flame may change from the desired position of being attached to or very near the burner to a position that is spaced apart from the burner. Then the flame safety system might no longer see the flame and make an emergency stop. The emergency stop interrupts the process of a significant part of the entire manufacturing facility and is disruptive. 
     To partially cope with the process conditions and their effects on the flame safety system, the Novastar burner of the 821 patent has been mostly limited to an open-fired configuration, which is defined as including an open area around the annulus or housing of the burner to enable inflow of ambient air around the burner housing into the furnace, combustion chamber. Lean premix burners are prone flame envelope instabilities, and the Novastar burner of the 821 patent required a complicated draft system in an effort to control pressure and combustion-induced oscillations at the burner. 
     Moreover, industrial burners generally are designed and optimized for a particular firing rate range in BTU per hour. Lean premix burners generally have a turndown ratio (that is, the ratio of the highest to lowest firing rate) that is limited because of their goals of low NOx. But the inventor has found that Novastar 821 burners are often oversized, especially in aggregate drying applications. Operating the Novastar 821 burner at significantly less than its firing rate capacity exacerbates the above problems. 
     The present invention addresses the above problems, in general, by adding center gas capabilities to the Novastar prior art burner. In this regard, an improved burner assembly for low NOx combustion comprises a combustion air fan inlet; a gaseous fuel inlet; a housing that defines a mixing zone downstream of the combustion air fan inlet and downstream of the gaseous fuel inlet for enabling mixing of fuel and combustion air to form a lean fuel-air mixture; a main nozzle assembly for directing the fuel-air mixture; a gas pilot; and a nozzle-mix, center gas nozzle. The center gas nozzle includes a gas manifold and plural conduits that are oriented radially. The conduits include front-facing gas outlets, whereby gas from the gas manifold exits the conduit gas outlets and combustion air is supplied from the combustion air fan inlet. The gas pilot is configured to initiate combustion of fuel from the center gas nozzle and from the main nozzle. The center gas nozzle can be turned on or off independent of control of the fuel-air mixture. The turndown ratio of this configuration is greater than 10:1. 
     Preferably, the gas manifold is an annulus that extends to a front of the nozzle, and the conduits (which have individually outlet holes that are axially oriented) extend radially outwardly from the manifold. The center gas nozzles also may include radially-facing outlets near an outlet of the gas pilot. The gas pilot may be located at the center of the nozzle assembly such that an inner wall of the gas manifold forms an outer wall of the gas pilot. The main nozzle may include at least one converging cone, spaced radially apart from the housing, for directing the fuel-air mixture, and at least one flame anchor formed by a bluff surface located proximate a front of the nozzle assembly for anchoring the flame. 
     Preferably, the burner is a sealed-in burner that includes a swirl vane assembly for mixing the combustion air with the gaseous fuel upstream of the nozzle assembly. The burner may also include a plurality of inner vanes that impart a swirling motion in a first orientation and a plurality of outer vanes that impart a swirling motion in a second orientation wherein said first orientation may be the same as said second orientation. The swirling motion imparted by the inner vanes is opposite in orientation to the swirling motion imparted by the plurality of outer vanes. 
     The bluff surface preferably is formed proximate a front of the burner assembly and the downstream end is vaneless. Preferably the burner assembly has only gas fuel capacity and contains no oil firing capability. The burner turndown ratio can be between 10:1 and 30:1, more preferably between 14:1 and 28:1, even more preferably between 18:1 and 26:1. Also, the present invention encompasses a turndown ratio is greater than 20:1. 
     A corresponding method for operating a premix burner for low NOx combustion at high turndown ratios (that is, using the burner described herein) includes the steps of: (a) initiating pilot firing via the gas pilot. After the step of initiating pilot firing, (b) center firing by supplying gas to the main nozzle through a gas manifold and through radial conduits, and operating at a lowermost center firing rate indefinitely. And then (c) operating the burner on the main gaseous fuel, the burner having a capacity for operating during operating step (c) that is at least 10 times the lowermost center firing rate. 
     The present invention is not limited to structure that addresses all of the drawbacks of the prior art Novastar burner, as the description of the prior art Novastar burner is provided for context. Nor is the invention limited to the particular burner limited to the structure or steps described in this specification. The present invention should be given its scope according to the plain meaning of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation of an embodiment of a burner assembly, including a center fire nozzle assembly illustrating aspects of the present invention. 
         FIG. 2  is a sectional perspective view of the burner assembly and combustion air fan of  FIG. 1 . 
         FIG. 3  is an enlarged sectional perspective view of the burner assembly of  FIG. 1 . 
         FIG. 4  is an enlarged perspective view of the gaseous fuel inlet manifold assembly shown in  FIG. 3 . 
         FIG. 5  is an end view of the gaseous inlet manifold assembly of  FIG. 4 . 
         FIG. 6  is an enlarged perspective view of the counter-swirl vane assembly shown in  FIG. 3 . 
         FIG. 7  is an enlarged perspective view of the nozzle assembly shown in  FIG. 1 . 
         FIG. 8  is an another enlarged end view of nozzle assembly portion of  FIG. 1 ; 
         FIG. 9  is end view of the nozzle assembly of  FIG. 7 ; 
         FIG. 10  is a cross sectional view of the nozzle assembly of  FIG. 7   
         FIG. 11  is an enlarged perspective view of a rear portion of the nozzle assembly of  FIG. 7  with parts removed for clarity. 
         FIG. 12  is an enlarge view of a portion of  FIG. 10  identified by reference numeral  12 . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIGS. 1 and 2  depict an embodiment of a premix lean burner assembly for low NOx combustion having improved capabilities. As shown, a burner assembly  10  includes a combustion air fan inlet  12 , a gaseous fuel inlet manifold assembly  14 , a counter-swirl vane assembly  16 , and a center gas nozzle assembly  18 . Nozzle  18  is changed from the Novastar burner of the 821 patent. 
     Consistent with the burner of the 821 patent, combustion air fan inlet  12  may include a first flange  33  that may be integral with the gaseous fuel inlet manifold assembly  14  (as more fully described below) for attaching to a combustion air fan  80 . Combustion air fan  80  preferably is a conventional centrifugal fan having a tangential outlet. Combustion air fan  80  includes a fan housing  81 , a mating flange  82  and a fan wheel  83  having a plurality of blades  84  and a fan hub  86  for mounting the plurality of blades  84 .  FIG. 2  also shows a fan motor  90  and a fan driveshaft  88  for turning the fan hub  86 . The present invention is not limited to the structure of combustion air fan  80 , but rather encompasses employing any fan type or structure, and encompasses any source of air provided to the burner assembly such that no fan is required unless specifically stated in the claim. 
     The gaseous fuel inlet manifold assembly  14  has a second flange  35  for attaching to a burner housing. The burner housing preferably includes cylindrical housing section  71  and a frusto-conical housing section or converging housing cone  75 , as best shown in the embodiment depicted in  FIG. 2 . The housing portions of the manifold assembly may also be considered a portion of the burner assembly housing depending on the context, as will be clear to persons familiar with burner technology. 
       FIG. 3  shows an enlarged sectional perspective view of the burner assembly  10 . Gaseous fuel inlet manifold assembly  14  includes a gaseous fuel chamber  30 , which is partially defined by flanges  33  and  35 , both of which are attached to an inner cylindrical shell  31  and an outer cylindrical shell  32 . The inner cylindrical shell  31  is concentric with the outer cylindrical shell  32  to form a plenum for distributing gas fuel. 
     Referring to  FIGS. 2 and 3 , the cylindrical housing section  71  includes a first housing flange  72  for attaching to the gaseous fuel inlet manifold assembly  14  and a second housing flange  73  for attaching to the frusto-conical housing section  75 . Frusto-conical housing section  75  includes an upstream flange  76  for attaching to the cylindrical housing section  71  and a downstream flange  77  for attaching to a diverging cone  145  proximate the downstream end of the burner assembly  10 . As shown in  FIG. 2 , lifting eyes  98  and  99  may be attached to the combustion air fan  80  and the burner assembly  10  for easy lifting an relocation of the fully assembled unit shown in  FIG. 2 . 
       FIG. 4  depicts a perspective view of gaseous fuel inlet manifold assembly  14 . Gaseous fuel inlet manifold assembly  14  includes a main fuel inlet  55  and a plurality of tubes  51 , each tube having a multiplicity of perforations or ports  53 . The tubes  51  may be cylindrical in shape and may generally extend radially inward from the gaseous fuel chamber  30  (shown in  FIG. 3 ) toward the center of the circumference formed by the inner cylindrical shell  31 . As seen in  FIG. 4 , tubes  51  vary length and diameter. As an example, the gaseous fuel inlet manifold shown in  FIG. 4  is configured with alternating short tubes  51   a  and long tubes  51   b , with the short tubes  51   a  extending distally a fraction of the distance that the long tubes  51   b  extend. The exemplary long-short tube configuration just described may also be seen in  FIG. 5 , which shows an end view depiction of the gaseous fuel inlet manifold assembly  14 .  FIG. 6  illustrates counter-swirl vane assembly  16 . 
       FIGS. 7 and 8  depict an enlarged view of an embodiment of the nozzle assembly  18 . Nozzle assembly  18  includes outer shells, center fire gas capabilities, and a pilot burner. Nozzle  18  includes a first shell  201 , a second shell  202 , and a third shell  203 . The first shell  201  preferably is concentric with the second shell  202 , and preferably includes a bluff structure having a first bluff surface  211 . The second cylindrical shell  202  has a diameter greater than that of the first cylindrical shell  201  and preferably includes a bluff structure having a second bluff surface  212 . Outer shell  203  has a diameter greater than second shell  202  and preferably includes a bluff structure having a third bluff surface  213 . Outer shell  203  preferably is frusto-conical. 
     Bluff surfaces  211 ,  212 , and  213  may be integral with cylindrical shells  201 ,  202 , and  203 , respectively, or may each be attached to corresponding structure  201 ,  202 , and  203  as separate pieces. Furthermore, bluff surfaces  211 ,  212 , and  213  may generally be located toward the front, or downstream end, of the nozzle assembly  18 . Shells  201 ,  202 , and  203  may be cylindrical or conical or other shaped. As  FIG. 7  shows, shells  201 ,  202 , and  203  may generally extend longitudinally downstream the same distance as each other to the downstream end of the burner assembly such that the downstream end faces of the shells are parallel and lie in the same plane that is perpendicular to the long axis of the burner. The present invention is not limited to the structure of the shells or bluff bodies particularly disclosed herein. 
     As best shown in  FIGS. 7 and 8 , and overall in  FIGS. 1 through 3 , burner assembly  10  includes a center gas pilot assembly  20 . Gas pilot  20  is supplied with gas by a pilot gas tube  230 , which extends from the rear of the burner. Gas pilot  20  is located in a center air tube  305  that has an inlet  307  that receives pilot combustion air from fan  80 . 
     Nozzle  18  includes a gas manifold  42  that preferably is an annulus between inner manifold wall  41   a  and outer manifold wall  41   b . Manifold walls  41   a  and  41   b  are concentric with the burner longitudinal center line and with gas pilot assembly  20 . Manifold inner wall  41   a  can be coextensive with a portion of center air tube  305 . Inner manifold wall  41   a  thus forms a housing for gas pilot  20 . A gas diffuser plate  22  is located at the outlet end of inner manifold wall  41   a  to form a plenum  21  that stabilizes the pilot flame. Diffuser plate  22  is spaced apart from an end of inner manifold wall  41   a  to provide outlets or holes  24  for the pilot flame. Holes  24  are located about diffuser plate  22  to provide a ring of pilot flame outlets. 
     The fuel for the center fire capabilities of nozzle  18  is provided through manifold  42 , which is fed from a center fire gas supply tube  232 . Center fire gas supply tube  232  has valves and controls that are independent from the pilot gas supply to enable the pilot and center fire capabilities to be controlled independently from one another. Radial gas outlet conduits  44  extend from the forwardmost end of manifold  42 . Fuel exiting manifold  42  flows radially outwardly through radial conduits  44  and through holes  46  spaced along conduits  44 . The holes  46  are each (preferably axial oriented) and arranged along conduits  44 , which are radially oriented. Radial outlet conduits  44  preferably are radial tubes that extend outwardly from manifold  42  such that the gaseous fuel exiting conduits  44  extends across the entire radius of nozzle  18 , such as across the three shells  201 ,  202 , and  203 . An outermost tip of radial outlet conduits are attached to outermost bluff body  213 . The figures show three radial conduits  44 , and the present invention encompasses other configurations and quantities of outlets. 
     Manifold  42  also has a ring of outlet holes  48  located at or near the forwardmost end of manifold  42  such that holes  48  are located about the periphery diffuser plate  22 . As best shown in  FIG. 12 , holes  48  are formed at or near the front end of manifold  42  and have an outlet that is oriented radially outward at an oblique angle. 
     The description of the function and operation of the burner assembly  10  is provided below according to an aspect of the present invention. 
     Referring now to  FIG. 2 , power is supplied to fan motor  90  of combustion air fan  80  to provide combustion air to the burner assembly  10 . Motor  90  driving fan blades  84  via shaft  88  provides combustion air to burner assembly  10 . 
     As shown in  FIGS. 2-4 , mating flange  82  of the combustion air fan  80  is bolted to the first flange  33  of gaseous fuel inlet manifold assembly  14  such that combustion air discharged from combustion air fan  80  enters the burner assembly  10  through the combustion air fan inlet  12 . Upon passing through the combustion air fan inlet  12 , the combustion air discharge flows through the gaseous fuel inlet manifold assembly  14  around the plurality of tubes  51 . Gaseous fuel is introduced from an external source (not shown) into the main gaseous fuel inlet  55  of the gaseous fuel inlet manifold assembly  14  such that the gaseous fuel chamber  30  is filled with gaseous fuel and said gaseous fuel flows into the plurality of tubes  51   a  and  51   b  and exits through the multiplicity of ports  53  into the combustion air stream. Ports  53  preferably are generally directed in the downstream direction of the burner assembly  10 , and other configurations are contemplated. The combustion air stream flows around the plurality of tubes  51  of the gaseous fuel inlet manifold assembly  14  such that the combustion air entrains gaseous fuel flowing out of the multiplicity of port  53  in tubes  51   a  and  51   b  of the gaseous fuel inlet manifold assembly  14 . 
     As can be observed in  FIGS. 2, 3 and 6 , upon passing through gaseous fuel inlet manifold assembly  14 , the combustion air discharge and gaseous fuel flows through counter-swirl vane assembly  16  which is attached to the gaseous fuel inlet manifold assembly  14  by, for example, fastening with bolts (through holes  129 ) the flange  128  of the counter-swirl vane assembly to the second flange  35  of the gaseous fuel inlet manifold assembly  14 . The fuel-laden combustion air flow, downstream of the gaseous fuel inlet manifold assembly  14 , subsequently flows through the counter-swirl vane assembly  16 , where a first portion of the fuel-laden air flows through the plurality of inner vanes  120  and a second portion of the fuel-laden air flows through the plurality of outer vanes  122 . By passing through the plurality of inner vanes  120 , the first portion of fuel-laden air may be imparted a swirl motion in a first orientation, for example clockwise. By passing through the plurality of outer vanes  122 , the second portion of fuel-laden air may be imparted a swirl motion in a second orientation, for example, counter-clockwise, which is opposite to the first swirl orientation imparted by the plurality of inner vanes  120 . The simultaneous opposite swirling motions imparted by the plurality of inner vanes  120  and the plurality of outer vanes  122  results in enhanced mixing of the gaseous fuel and the combustion air to form a fuel-air mixture in the burner assembly  10 . Rings  124 ,  125 , and  126  provide structure to the vanes. 
     Referring now to  FIGS. 2 and 3 , once the fuel-air mixture exits the counter-swirl vane assembly  16 , it enters the cylindrical housing section  71  which may be attached to the counter-swirl vane assembly  16  by bolting, for example, the first housing flange  72  of the cylindrical housing section  71  to the flange  128  of the counter-swirl vane assembly  16 . After allowing the enhanced fuel-air mixing to develop in cylindrical housing section  71 , the air-fuel flow may be accelerated in the frusto-conical housing section  75  of the burner assembly  10 . Frusto-conical housing section  75  may be attached to the cylindrical housing section  71  of the burner assembly  10  by fastening with bolts, for example, the upstream flange  76  of the frusto-conical housing section  75  to the second housing flange  73  of the cylindrical housing section  71 . 
     The combustion air fan  80  may be controlled, for example, by a variable frequency drive (VFD), a damper mechanism or some other suitable mechanism which a person familiar with this technology would know how to select. The combustion air fan  80  may provide a flow of combustion air in excess of the stoichiometric amount required to burn the gaseous fuel supplied through the gaseous fuel inlet manifold assembly  14 . Precise control of the resulting air-to-fuel ratio (A/F) of the fuel-air mixture and the enhanced gaseous fuel mixing achieved with counter-swirl vane assembly  16  may help minimize peak flame temperatures produced by burner assembly  10 . The burner preferably operates at 40 percent excess air, more preferably at approximately 50 percent excess air, which provides an adiabatic flame temperate of a maximum of 2800 degrees F., which is generally considered a threshold for thermal NOx formation. 
     A first portion of the accelerated air-fuel mixture in frusto-conical housing section  75  may enter the nozzle assembly  18  and may flow into the first cylindrical shell  201 , the second cylindrical shell  202  and the converging nozzle cone  203 . A second portion of the air-fuel mixture in frusto-conical housing section  75  may flow around converging nozzle cone  203  through the annular volume formed between the converging nozzle cone  203  and the frusto-conical housing section  75 . Converging nozzle cone  203  aids in directing the first portion of flow toward the annular volume formed between the center air tube  305  and the first cylindrical shell  201 . Converging cone  203  also aids in directing said first portion of flow through the annular volume formed between the first cylindrical shell  201  and the second cylindrical shell  202 . 
     The pilot flame exits nozzle  18  from holes around diffuser plate  22 , as explained above. Main, premixed gas and air from fuel inlet manifold  14  and combustion air fan  80  and may be ignited by the pilot flame. Center fire flame from holes  46  and  48  may anchor and stabilize the flame from the premixed gas and air. The flame may be anchored to the nozzle assembly  18  by the first bluff body surface  211  of cylindrical shell  201 , the second bluff body surface  212  of second cylindrical shell  202 , and the third bluff body surface  213  of cone  203 . Furthermore, acceleration of the air-fuel mixture by the frusto-conical housing section  75  and the converging nozzle cone  203  may assist in preventing flashback of the flame into the burner assembly  10 . The flame formed at the front of the nozzle assembly  18  is allowed to develop with the aid of the diverging cone  145 , which may assist in anchoring and stabilizing said flame by, for example, inhibiting entrainment and blowoff. Furthermore, as shown in  FIG. 8 , nozzle assembly  18  optionally includes a plurality of spin vanes  225  located proximate the inlet portion of nozzle  18 , for stabilizing the burner flame. 
     Center fire air fuel may exit from holes  46  and from inner ring holes  48 , as supplied from center fuel manifold  42 . This center firing capability added to the prior art Novastar burner adds self-piloting functionality. Because burner  10  preferably is fitted with separate controls for center file fuel (through manifold  42 ) and main fuel (through main fuel assembly  14 ), center fire gas can be controlled or turned off to achieve high turndown ratios. 
     Sealed-in versions of the burner shown in  FIG. 1  have achieved approximately 30% reduction in CO ppm. The term “sealed-in” refers to enclosing an opening into which the burner system  10  is installed, such as by bolting flange  74  ( FIGS. 1 through 3 ) onto a wall of a furnace or combustion chamber or housing wall. In this regard, “sealed-in” is not “open-fired.” 
     The configuration of the burner described herein, including the ability to seal-in the burner, the center firing, and the self-piloting provides several advantages, including increasing overall efficiency and emissions reductions; improved ignition, reliability, and low-fire stability improved operating window and turndown, which improve the ability of burner  10  to be adapted to system requirements; ability to adjust and extend low-fire runtime, such as during preheating, and simplification of burner and draft control system scheme. Self-piloting refers to the common flame base for the flame from the center fire gas and the main flame gas. 
     Sealing in burner  10  enhances fuel efficiency by not wasting heat with higher excess air, which also improves heat transfer to the aggregate or other product. The center fire system improves burner ignition of main gas. 
     The center fuel firing enables the burner described herein to operate at a turndown ratio of 10:1 or greater. The inventors have demonstrated that turndown ratios of 30:1 can be achieved. Preferably, the turndown ratio is between 14:1 and 28:1, preferably between 18:1 and 26:1; preferably greater than 20:1. 
     The high turndown ratio with good combustion characteristics of the present burner enables an improved combustion operating window for better system adaptability and control over many individual plant variables that must work in unison, such as total system operation and operation of various plant components along. The predictability of operation of burner  10  also enables more reliable sizing and layout of the system and its components. There are also benefits to production rates (tph) and operating conditions, for example ambient conditions various, mix designs (such as aggregate particle size and their percentages in total mix), and moisture percentages of aggregate. 
     The burner of the present invention has advantages during the process of starting up the aggregate of other process in which the burner system  10  is installed. For example, the center fire nozzle has the ability to operate alone at “low-fire.” This low-fire capacity enables the burner to be adjusted to each individual plant to achieve indefinite run time for preheat of system components (such as a baghouse, ductwork, and the like) to achieve system temperature above dew point. Dew point for the process combustion gases typically are approximately 250-290 degrees F. High firing will provide gases above the dew point, but may reach unacceptable temperatures, such as a high stack temperature limit. In this way, the center firing capabilities of burner  10  can eliminate commonplace procedure of numerous cycles of burner starts and restarts due to reaching high stack temperature safety limit, in many circumstances. 
     Further, the improvements to burner  10  enable a simplified burner and draft control scheme. The self-piloting effect of the center fire burner improves ignition and low fire stability, and it enables eliminating complex control schemes. For example, in the prior art burner, an ignition and transition to low fire required careful control and adjustment of individual burner and draft throughout the transition. 
     The present invention is not limited to the particular structures and advantages disclosed herein, but rather encompasses variants as will be clear to persons familiar with burner technology and encompasses all structures recited and following from the language of the claims. For example, the present invention is not limited to a burner having, nor limited to the particular structure recited for, the counter-swirl vane assembly, fuel manifold, converging nozzle cone, and like components, unless the structure is stated in the claim. The embodiments described are illustrative, and the present invention is not limited to said embodiments.