Patent Publication Number: US-8113821-B2

Title: Premix lean burner

Description:
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. 
     SUMMARY OF THE INVENTION 
     A burner assembly and related methods are disclosed for lean, low NOx combustion. A burner assembly includes: a combustion air inlet; a gaseous fuel inlet manifold located downstream from the combustion air inlet; counter-swirl vanes located proximate the fuel inlet manifold; and a nozzle assembly that is located downstream from and spaced apart from the counter-swirl vanes and that is located downstream from and spaced apart from the gaseous fuel inlet manifold. The manifold has multiple ports for introducing gaseous fuel. The counter-swirl vanes include inner vanes oriented to impart a swirl in a first orientation and outer vanes oriented to impart a swirl in a second orientation that is opposite to that of the first orientation. Accordingly, mixing between the fuel and the combustion air is enhanced. 
     The spacing of the nozzle relative to the vanes forms a mixing zone between the vanes and the nozzle assembly. The nozzle assembly includes at least one flame anchor formed by a bluff surface located proximate a front of the nozzle assembly for anchoring the flame. The burner assembly preferably includes a fan coupled to the combustion air inlet for providing combustion air through the combustion air inlet to the nozzle assembly in excess of the stoichiometric amount such that the fuel-air mixture is fuel-lean. The burner assembly may also be supplied with combustion air from a manifold or other means. 
     At least a portion of the ports of the gaseous fuel manifold in the burner assembly are distributed in a plane that generally is perpendicular to a longitudinal axis of the burner assembly. Preferably, said burner assembly includes a converging housing cone generally located between the vanes and the front of the nozzle assembly wherein the nozzle assembly includes at least one converging nozzle cone that cooperates with the converging housing cone to direct flow of the fuel-air mixture. Said nozzle assembly includes at least one converging nozzle cone to direct flow of the fuel-air mixture, wherein the bluff surface of the nozzle assembly is preferably formed proximate the converging nozzle cone. 
     The burner assembly further includes a diverging cone extending forward from the nozzle assembly, whereby the diverging cone inhibits entrainment toward the front of the nozzle. The burner assembly preferably comprises a cooling air tube extending from the combustion air inlet, through the gaseous fuel manifold, and into a burner housing wherein the nozzle assembly optionally includes an oil nozzle, and the burner assembly optionally includes an oil supply tube capable of providing oil to said oil nozzle and an atomizing air tube capable of providing atomizing air to the oil nozzle. 
     In an alternate embodiment, a burner assembly for low NOx combustion comprises: a combustion air inlet; a gaseous fuel inlet; a housing; and a nozzle assembly. Said housing defines a mixing zone downstream of the combustion air inlet and downstream of the gaseous fuel inlet for enabling mixing of fuel and combustion air to form a lean fuel-air mixture. The nozzle assembly includes at least: a converging cone for directing the fuel-air mixture; at least one flame anchor formed by a bluff surface located proximate a front of the nozzle assembly for anchoring the flame; an optional oil nozzle located concentric with the converging cone; an optional oil supply tube for providing oil to the oil nozzle; and an air tube extending from the combustion air inlet capable of providing cooling air to the oil nozzle during operation of the burner assembly on oil and providing cooling air to the oil nozzle during operation of the burner assembly only on gaseous fuel. Preferably, the bluff surface is formed proximate a front of the burner assembly and said front of the burner is vaneless. 
     The burner assembly, in accordance with said alternate embodiment, further comprises a swirl vane assembly for mixing the combustion air with the gaseous fuel upstream of the nozzle assembly. Said swirl vane assembly includes 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. Preferably, the swirling motion imparted by the plurality of inner vanes is opposite in orientation to the swirling motion imparted by the plurality of outer vanes. 
     A method for generating low NOx, premixed combustion comprises: supplying and controlling flow of excess combustion air at a burner inlet; introducing gaseous fuel to the burner through a multi-port manifold located downstream the burner inlet; mixing the excess combustion air with the gaseous fuel by means of counter-swirl vanes located proximate the multi-port manifold; directing the air-fuel mixture flow through a nozzle assembly located generally within a converging housing cone; and providing a flame anchor formed by a bluff surface located proximate a front of the nozzle assembly for anchoring the flame. Preferably, the bluff surface is formed proximate the converging nozzle cone. 
     The combustion air is preferably supplied by a fan that is coupled to the burner inlet. Combustion air may also be supplied to the burner assembly from a manifold or other means. A portion of the fuel-air mixture is directed through a converging housing cone generally located between the vanes and the front of the nozzle assembly wherein said nozzle assembly includes at least one converging nozzle cone that cooperates with the converging housing cone to direct flow of the fuel-air mixture. 
     The burner assembly, according to the method described herein, further comprises a cooling air tube extending from the combustion air inlet, through the gaseous fuel manifold, and into the burner housing for providing cooling air to the nozzle assembly. Said nozzle assembly optionally includes an oil nozzle and the burner assembly optionally comprises an oil supply tube for providing oil to the oil nozzle and an atomizing air tube for providing atomizing air to the oil nozzle. Combustion according to the method described herein achieves NOx emissions levels below 20 ppm at 3 percent O 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation of an embodiment of a burner assembly and a combustion air fan 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 sectional perspective view of the nozzle assembly shown in  FIG. 1 . 
         FIG. 8  is an enlarged sectional view of the nozzle assembly portion of the burner assembly of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  and  FIG. 2  depict an embodiment of a premix lean burner assembly for low NOx combustion. As shown, a burner assembly  10  may include a combustion air inlet  12 , a gaseous fuel inlet manifold assembly  14 , a counter-swirl vane assembly  16  and a nozzle assembly  18 . 
     Combustion air 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  may include 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  may be concentric with the outer cylindrical shell  32  to form a plenum for distributing gas fuel. 
     Referring now to  FIGS. 2 and 3 , the cylindrical housing section  71  further comprises 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 . Furthermore, frusto-conical housing section  75  may further include 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 . In addition, as shown in  FIGS. 2 and 3 , 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 . As  FIG. 4  shows, the gaseous fuel inlet manifold assembly  14  further comprises 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 , the plurality of tubes  51  may be of varying 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  shows an embodiment of the preferred configuration of the counter-swirl vane assembly, which is indicated by reference numeral  16  and includes a plurality of inner vanes  120  and a plurality of outer vanes  122 , each of which is configured radially. Counter-swirl vane assembly  16  is housed in a central cylindrical shell  124 , an intermediate cylindrical shell  125 , and a peripheral cylindrical shell  126 . Cylindrical shells  124  and  125  may be used for attaching the inner vanes  120  and cylindrical shells  125  and  126  may be used for attaching the outer vanes  122 . In the exemplary embodiment shown in  FIG. 6 , the inner vanes  120  may be located between the central cylindrical shell  124  and the intermediate cylindrical shell  125  whilst the outer vanes  122  may be located between the intermediate cylindrical shell  125  and the peripheral cylindrical shell  126 . Preferably, the inner vanes  120  are disposed in an orientation opposite that of the outer vanes  122 . As shown in  FIG. 6 , the counter-swirl vane assembly  16  may further comprise a flange  128  with a plurality of bolt holes  129 , for example, for attaching to the gaseous fuel inlet manifold assembly  14 . 
       FIG. 7  depicts an embodiment of the nozzle assembly  18 . As shown, the nozzle assembly  18  includes a first cylindrical shell  201 , a second cylindrical shell  202 , and a converging nozzle cone  203 . The first cylindrical shell  201  preferably is concentric with the second cylindrical 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 . A cooling air tube  305  (as more fully described below) is located within shell  201 , and preferably includes a bluff structure having a third bluff surface  213 . Bluff surfaces  211 ,  212 , and  213  may be integral with cylindrical shells  201  and  202 , and cooling air tube  305 , respectively, or may each be attached to corresponding structure  201 ,  202 , and  305  as separate pieces, respectively. Furthermore, bluff surfaces  211 ,  212 , and  213  may generally be located toward the front, or downstream end, of the nozzle assembly  18 . Cylindrical shells  201  and  202 , and converging cone  203  may extend from a same upstream longitudinal location toward the front of the nozzle assembly  18 . As  FIG. 7  shows, cylindrical shells  201  and  202  may generally extend longitudinally downstream the same distance as each other to the downstream end of the burner assembly. Converging nozzle cone  203  may extend longitudinally a fraction of the distance that cylindrical shells  201  and  202  extend such that a space is formed between the second bluff surface  212  and the downstream end of the converging cone  203  where fuel-air mixture may flow. The present invention is not limited to the structure of the cones or bluff bodies particularly disclosed herein. 
     Referring again to  FIGS. 1 and 2 , the burner assembly  10  may include cooling air tube  305  having a combustion air opening  307  located proximate the combustion air inlet  12 . The cooling air tube  305  extends along the longitudinal axis of burner assembly  10  through gaseous fuel inlet manifold assembly  14 , counter-swirl vane assembly  16  and nozzle assembly  18 . Burner assembly  10  may also include an oil supply tube  314 , a primary air tube  315  and an atomizing air tube  410 , which preferably are concentric with the cooling air tube  305 . Oil supply tube  314 , primary air tube  315  and atomizing air tube  410  extend the length of the burner assembly  10  through the gaseous fuel inlet manifold assembly  14 , counter-swirl vane assembly  16 , and nozzle assembly  18 . Oil supply tube  314  and atomizing air tube  410  may further extend into and through the fan housing  81 . The oil supply tube  314  may extend outside of fan housing  81  where the oil supply tube  314  is capable of receiving a flow of oil from an external oil source (not shown) through an oil hose  321 . The primary air tube  315  may be used for centering an oil nozzle  220  and for providing cooling air to nozzle assembly  18 . The atomizing air tube  410  may extend out of the fan housing  81  where the compressed air tube may receive a flow of compressed air from an external compressed air source (not shown) through a compressed air pipe  413 . 
       FIG. 8  illustrates another embodiment of the nozzle assembly  18 . As shown in  FIG. 8 , nozzle assembly  18  includes oil nozzle  220  for receiving oil from the oil supply tube  314 . Said oil nozzle  220  may be in fluid communication with atomizing air tube  410 . Oil nozzle  220  may be configured to burn any liquid fuel, such as fuel oil, biodiesel, recycled motor oil, and the like. As shown in  FIG. 8 , said fuel may be directed to the oil nozzle  220  through oil supply tube  314 . The fuel may then be atomized, for example, with compressed air, which may flow to the oil nozzle  220  through atomizing air tube  410  before being ignited at the front of the nozzle assembly  18 . 
     As will be apparent from the discussion below, the description of the function and operation of the burner assembly  10  is provided simultaneously with a description of a method 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 inlet  12 . Upon passing through the combustion air 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 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 . 
     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 . 
     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 cooling air tube  305  and the first cylindrical shell  201 . Converging nozzle  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 fuel-air mixture exiting the nozzle assembly  18  may be ignited to form a flame which 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 cooling air tube  305 . 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. A fraction of the combustion air that entered cooling air tube  305  through cooling air tube inlet  307  flows the length of cooling air tube  305  and may assist in cooling the nozzle assembly  18  when the burner assembly  10  is in operation. 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. 
     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 . Reduced peak flame temperatures result in lower emissions of NOx. NOx emissions, for instance, of burner  10  may be reduced to levels below 20 ppm at 3 percent O 2 . Further, the premixing in burner  10  produces reduced levels of CO, VOCs, and the like. 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. 
     The present invention is not limited to the particular structures 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.