Abstract:
A method of operating a burner includes providing supplying a combustible mixture containing a ratio of fuel and air that is incapable of maintaining a stable flame to a combustion chamber. The combustible mixture is ignited by an igniter, and presence of a flame is sensed. The igniter is maintained active to sustain combustion of the combustible mixture within the combustion chamber so that a space exterior to the combustion chamber is heated to a temperature at or above an auto-ignition temperature of the combustible mixture. The temperature of the space exterior is monitored and the combustible mixture is provided at a second flow rate, which is higher than the first flow rate, to extinguish the flame in the combustion chamber such that combustion occurs in the space exterior to the combustion chamber. When combustion occurs in the space exterior to the combustion chamber, the igniter is deactivated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of U.S. Provisional Patent Application No. 61/701,212, filed Sep. 14, 2013, which is incorporated herein in its entirety by this reference. 
     
    
     BACKGROUND 
       [0002]    The use of high velocity gas burners is well known. In such burners, fuel gas and oxidant are mixed with one another and ignited in the interior of the burner. The resultant hot combustion gases then flow at high velocity through an outlet and into the furnace chamber for direct heating or into a radiant tube for indirect heating. The combustion of the fuel gas with an oxidant within the burner results in a greatly elevated temperature environment in the burner. To increase system efficiency, the oxidant can be pre-heated to result in higher temperatures. The preheating of the oxidant may be achieved by using a recuperative or regenerative system that uses the residual heat in the exhaust gas. This high temperature combustion environment provides two challenges. First, the burner internals and combustion chamber are exposed to the very high temperature environment. Second, when combustion is carried out at extremely high temperatures, thermal nitrogen oxides (NOx) formation is promoted. As combustion temperatures increase, the levels of NOx production also increase. In order to deal with higher combustion temperatures, burners may be constructed from high temperature grade materials, for example, the combustion chambers can be made of ceramic materials, which can withstand the high temperature environment. However, the difficulties associated with high NOx emissions still remain unaddressed. 
       SUMMARY 
       [0003]    A method and apparatus for a burner adapted to heat a furnace, radiant tube, or other environment of use is described herein. In particular, a burner for providing a fuel gas in combination with an oxidant to effect controlled combustion (or oxidation) of the fuel gas in a manner to reduce NOx emissions is described. Combustion of the fuel gas is shifted from within the burner combustor to a location outside the burner once the temperature within the furnace/radiant tube has reached a sufficient level to complete combustion of the fuel gas. 
         [0004]    The burner can provide oxidant and fuel at a ratio and/or velocity that does not permit the burner to maintain a stable flame. Accordingly, the burner can be provided with a stabilization device that is capable of maintaining a flame in the burner combustor notwithstanding the instability created by the oxidant and fuel ratio and/or velocity. The stabilization device can be turned on or off as desired. 
         [0005]    More particularly, the fuel gas may be delivered through a fuel tube for discharge, such as axial and/or radial discharge, into a burner combustor for mixing with oxidant at a ratio and/or velocity that is not capable of maintaining a stable flame. During a start-up stage, the stabilization device is activated, and the fuel gas/oxidant mixture is ignited to combust within the burner combustor. The stabilization device maintains the flame in the burner combustor. During this period, the flame inside the burner combustor can be monitored with a flame sensor, such as a flame rod or UV scanner. 
         [0006]    Once the temperature in the furnace/radiant tube reaches a pre-defined level at or above the auto-ignition temperature, the stabilization device can be turned off. When this occurs, flame will be destabilized and extinguished in the burner combustor such that all combustion will take place in the furnace chamber/radiant tube, and the flame sensor will detect a loss of flame inside the combustor. Due to the elevated temperature above auto ignition level in the furnace/radiant tube, this movement of the flame to the furnace/radiant tube space leads to combustion in the furnace/radiant tube in the absence of a flame in the burner. While the temperature levels within the furnace/radiant tube are sufficient to cause combustion of the fuel gas, these temperature levels nonetheless are low enough to avoid substantial NOx generation. Moreover, the high exit velocity of the air and fuel provides substantial blending and recirculation of the furnace/radiant tube atmosphere with the air/fuel mix, resulting in reduced temperature spikes formed in the core of the flame jet in the furnace/radiant tube, which are normally experienced during the standard operating mode of typical burners. After the flame ceases to exist in the burner combustor, the flow rate of the mixture of fuel gas and oxidant can be maintained, decreased, or increased, according to the needs of the furnace operator. 
         [0007]    Examples of suitable stabilization devices include a hot surface igniter, a continuous spark igniter, a plasma igniter, an arc igniter, a backflow fluid flow, a pilot flame, an electric field generator, a magnetic field generator, and an electromagnetic field generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a diagrammatic views illustrating a burner and control system for delivery of fuel gas and combustion air adapted to heat a furnace, radiant tube, or other chamber, in accordance with the disclosure; 
           [0009]      FIG. 2  is a fragmentary sectional view of a fuel gas discharge nozzle mounted within a combustor for the burner of  FIG. 1 ; 
           [0010]      FIG. 3  is a fragmentary sectional view of the fuel gas discharge nozzle of  FIG. 2  taken along line  3 - 3  in  FIG. 4 ; and 
           [0011]      FIG. 4  is a sectional view taken generally along line  4 - 4  of FIG,  2  showing the orientation of an air flow control disk surrounding the fuel gas discharge nozzle of  FIG. 2 ; 
           [0012]      FIG. 5  is a diagrammatic view illustrating a first embodiment of a stabilization device for the burner of  FIG. 1 ; 
           [0013]      FIG. 6  is a diagrammatic view illustrating a second embodiment of a stabilization device for the burner of  FIG. 1 ; 
           [0014]      FIG. 7  is a diagrammatic view illustrating a third embodiment of a stabilization device for the burner of  FIG. 1 ; 
           [0015]      FIG. 8  is a diagrammatic view illustrating a fourth embodiment of a stabilization device for the burner of  FIG. 1 ; 
           [0016]      FIG. 9  is a diagrammatic view illustrating a fifth embodiment of a stabilization device for the burner of  FIG. 1 ; 
           [0017]      FIG. 10  is a diagrammatic view illustrating a sixth embodiment of a stabilization device for the burner of  FIG. 1 ; and 
           [0018]      FIG. 11  is a diagrammatic view illustrating a seventh, eighth, and ninth embodiment of a stabilization device for the burner of  FIG. 1 . 
           [0019]      FIG. 12  is a flowchart for a method of operating a burner in accordance with the disclosure. 
       
    
    
       [0020]    Before the embodiments of the burner and method are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and/or the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for purposes of description only and should not be regarded as limiting. The use herein of “including,” “comprising,” and variations thereof is meant to encompass the items listed thereafter and equivalents, as well as additional items and equivalents thereof. 
       DETAILED DESCRIPTION 
       [0021]    Reference will now be made to the drawings wherein like elements are designated by like reference numbers in the various views.  FIGS. 1-4  illustrate a burner  10  including a generally hollow tubular cover tube  12  having an open end  14  that projects into a furnace/radiant tube  16  or other environment to be heated. By way of example only, the burner  10  may project into an enclosed radiant heating tube, or the like, used for indirect heating of a furnace while avoiding substantial introduction of combustion products into the furnace. As another example, the burner  10  may project into a furnace for direct heating of a furnace with substantial introduction of combustion products into the furnace. In the illustrated embodiment, the cover tube  12  is disposed in surrounding relation to a hollow heat recuperator  18  of ceramic or the like having a convoluted surface extending outwardly from a housing  20 . The recuperator  18  can surround fuel tube  22 , which provides fuel to a nozzle assembly  24  disposed within a burner combustion chamber  26  (also referred to as a combustor) located adjacent to the open end  14  of the burner. An annular air passageway  28  can be disposed between the inner walls of the heat recuperator  18  and outer wall of the fuel tube  22 . 
         [0022]    As shown, an air supply  30  provides combustion air for delivery from a blower or other supply source (not shown) to the annular air passageway  28  for transmittal to the nozzle assembly  24 . An oxidant control valve  32  is used to control the flow of oxidant. In this regard, the oxidant control valve  32  may be operatively connected to a controller  34  such as a PLC, computer, or the like which opens or closes the oxidant control valve  32  in accordance with pre-established commands based on conditions in the furnace/radiant tube and/or the burner. Likewise, a fuel supply  40  provides natural gas or other gaseous fuel for delivery to the fuel tubes  21  and  22  for transmittal to the nozzle assembly  24 . A fuel control valve  42  is used to control the flow of fuel gas. In this regard, the fuel control valve  42  may be operatively connected to the controller  34 , which adjusts fuel feed in accordance with pre-established commands based on conditions in the furnace, radiant tube, and/or the burner. It will be appreciated that the fuel gas, air and oxidant can pass into the nozzle assembly in any suitable manner. 
         [0023]    A sensor  46 , such as a flame sensor, or the like, may be present to continuously monitor the presence of a flame within the burner combustion chamber  26 , and to communicate such data to the controller  34 . As will be described further herein, the controller  34  may utilize the data from the sensor  46  in combination with temperature data from the furnace/radiant tube to control the burner. It will be appreciated that the sensor  46  can be any suitable sensor and can be disposed in any suitable location. In one embodiment, the sensor  46  is embodied as an ultra-violet radiation or flame detector  69  that is disposed to sense for the presence of a flame directly within the combustor chamber  26 , as shown in  FIG. 5 . 
         [0024]    Referring now back to  FIGS. 2-4 , the nozzle assembly  24  can be in the form of a sleeve that is secured about the distal end of the fuel tube  22 . In this regard, the illustrated nozzle assembly  24  includes a forward nipple portion  50  and a radial disk portion  52  disposed rearward (i.e. upstream) of the nipple portion  50 . In this arrangement, the radial disk portion  52  can have a generally concave forward face projecting towards the outlet of the burner. 
         [0025]    As best seen through joint reference to  FIGS. 2 and 4 , stand-offs  54  can be located at positions around the circumference of the radial disk portion  52  to provide centered spacing relative to the surrounding body. This can result in an annular gap  56  ( FIG. 5 ) extending substantially around the perimeter of the radial disk portion  52 . The radial disk portion  52  also includes a pattern of interior air passages  58 . During operation, oxidant and/or air delivered from the supply  30  may flow through the annular gap  56  and the interior air passages  58  towards the burner outlet as shown by the arrows in  FIG. 2 . 
         [0026]    As shown in  FIG. 3 , the forward nipple portion  50  can include an axial gas passage opening  64  and an arrangement of radial gas passage openings  66  aligned with corresponding openings in the fuel tube  22  for outward conveyance of the fuel gas. During operation, fuel gas can be passed outwardly from the axial gas passage opening  64  and the radial gas passage openings  66  and can mix with the oxidant. 
         [0027]    The burner  10  may be operated in a flame mode with ignition within the burner combustor or in a flameless mode during which the oxidant and fuel gas combusts only downstream of the combustor outlet in an area  16  ( FIG. 1 ), which is external to the combustor, The flameless mode may also be referred to as a volume combustion mode, i.e., when combustion is occurring in the volume of the furnace chamber or radiant tube in the absence of a flame in the combustion chamber  26  of the burner. The flame mode provides the initial start-up of the furnace/radiant tube  16  using combustion of fuel gas in the burner combustion chamber  26  to heat up the furnace/radiant tube. The flame mode can be followed by the flameless mode during which the fuel gas and oxidant are ejected from the burner  10  and is allowed to undergo combustion downstream of the combustor outlet. This dual mode operation results in substantially reduced NOx emissions. 
         [0028]    Referring again to  FIGS. 1-4 , by way of example only, and not limitation, upon initiation of the flame mode, both the air control valve  32  and the fuel control valve  42  are set to an open condition that provides a flow of oxidant and fuel, which need not be capable of maintaining a stable flame within the burner. Unlike previously proposed burners, in which a combustible mixture capable of maintaining a stable flame after initial ignition was required within the burner, in the present embodiments, a mixture and/or flow rate of fuel, air and, optionally, the oxidant, may be insufficient to maintain a stable flame within the burner during operation in the flame mode. As used herein, oxidant is meant to describe any substance that contains oxygen, such as air, and/or other additives intended to make the combustion of fuel more efficient and/or to lower emissions. 
         [0029]    When combusting in the open condition, air and/or another oxidant will pass along the annular air passageway  28  to the nozzle assembly  24  and fuel gas will pass along the fuel tube  22  to the nozzle assembly  24 . At the nozzle assembly  24 , a portion of the oxidant can flow through the annular gap  56  surrounding the radial disk portion  52 , while the remainder of the oxidant can pass through the interior air passages  58 . Concurrently, the fuel gas can be expelled from the nozzle assembly  24  to mix with the oxidant in the burner combustion chamber  26 . As these materials mix, a flame stabilization device  90 , as shown in  FIG. 5 , for stabilizing a flame in the burner combustor can be activated to initiate a flame, and remain active to perpetuate the flame as required. 
         [0030]    The stabilization device or a suitable igniter, such as a spark rod, hot surface igniter, direct spark igniter, plasma igniter, electrical arc igniter, field igniter, pilot light igniter, and the like, can be activated by the controller  34  to ignite the fuel/air mixture in the burner combustion chamber  26  based on, or in response to, a signal provided by an ultra-violet detector, flame rod, or other type of flame sensor  69  disposed to sense the presence of a flame within the burner combustion chamber  26 , as shown in  FIG. 5 . This on-demand ignition, which can be activated continuously, can provide stable combustion occurring in the burner while the flame mode operation is active. This flame can be maintained continuously or intermittently, as required, by the stabilization device until the auto-ignition temperature in the furnace or in an area outside of the radiant tube is achieved. Throughout the flame mode, thermocouples or other devices can continuously monitor the interior temperature of the furnace/radiant tube  16  and a flame sensor  69  can monitor the presence or absence of flame inside the burner combustor  26  to provide such data to the controller  34  by means of any suitable link. 
         [0031]    Once the temperature within the furnace/radiant tube reaches a pre-established level (normally about 1550 degrees Fahrenheit or greater) the controller  34  can communicate with the stabilization device to deactivate the stabilization device. The deactivation of the stabilization device causes the flame in the burner combustor  26  to be extinguished when operation transitions to the flameless mode of operation of the burner  10 . The absence of the flame in the burner can be detected using the flame sensor (e.g., a flame rod or UV sensor), which can be used as an indication that the flameless mode has been reached. 
         [0032]    During the flameless mode, the fuel gas and oxidant can be passed out of the burner  10  without undergoing combustion. Upon entering the auto-ignition temperature furnace/radiant tube environment, the fuel gas and oxidant are raised to a temperature sufficient to activate combustion without requiring continuous or intermittent ignition. Alternatively, a sustained combustion within the furnace may not require a combustible mixture to be provided at all through the burner, Thus, the location of the onset of combustion is moved from the burner combustor  26  downstream to the furnace chamber/radiant tube  16 . Due to the relatively disperse combustion zone outside of the burner  10  and the entrainment of the flue gas within the fuel/oxidant mixture, there is not a substantial localized temperature spike. NOx production is thereby substantially reduced. As will be appreciated, once the flameless combustion mode has been initiated, the flows of fuel gas and oxidant may thereafter be cycled on and off, or otherwise maintained, decreased, or increased, to adjust the temperature within the furnace/radiant tube as desired anywhere above an auto-ignition level. 
         [0033]    The stabilization device can be any suitable device that is capable of maintaining a flame in the combustion chamber when the flow rate and/or flow mixture of oxidant and fuel gas would otherwise destabilize and either extinguish, blow out or not otherwise maintain a flame within the burner combustion chamber without the stabilization device. In one embodiment, shown in  FIG. 5 , the stabilization device can be a hot surface igniter  90 . The hot surface igniter is a device that uses electrical power in the form of heat provided when an electric current passes through an electrical resistive element  92 . The resistive element  92  is disposed at the end of a rod  94  that extends into the burner chamber  26  so that the resistive element  92  is adjacent the fuel flow orifices of the nozzle assembly  24 . The rod  94  may be hollow to accommodate electrical conduits  96  that interconnect the resistive element  92  with appropriate connections to the controller  34 . In this way, the controller  34  can control operation of the hot surface igniter  90 . 
         [0034]    During operation, the resistive element  92  is activated and heated to a temperature that is sufficient to ignite the oxidant/fuel mixture in the combustion chamber  26 . The controller is connected to the hot surface igniter to turn it on and off. In operation, the hot surface igniter is turned on to reach a temperature sufficient to ignite the oxidant/fuel mixture, and is left in the on condition to maintain a flame in the burner combustor. Once the furnace/radiant tube has reached the desired temperature, the controller can turn off the hot surface igniter to destabilize the flame in the burner combustor and initiate the flameless mode in the burner combustor, It will be appreciated that the hot surface igniter can have any suitable shape and size. In addition, the hot surface igniter can be disposed in any suitable position. In one embodiment, more than one such igniter may be used in the same burner chamber. 
         [0035]    In another embodiment, shown in  FIG. 6 , the stabilization device can be a direct spark igniter  98 . The direct spark igniter  98  is a device that uses electrical power at a high voltage or that includes a voltage multiplier coil  100  associated with a spark-producing tip  102  that provide electrical arcing that serves to ignite a combustible mixture. The tip  102  is disposed at the end of a rod  104  that extends into the burner chamber  26  so that the tip  102  is generally adjacent the fuel flow orifices of the nozzle assembly  24 . The rod  104  may be hollow to accommodate electrical conduits  105  that interconnect the tip  102  with appropriate connections to the controller  34 . In this way, the controller  34  can control operation of the direct spark igniter  98 . 
         [0036]    During operation, the tip  102  is activated to produce an arc that is sufficient to ignite the oxidant/fuel mixture in the combustion chamber  26 . The controller is connected to the direct spark igniter to turn it on and off. In operation, the direct spark igniter is turned on to produce a spark sufficient to ignite the oxidant/fuel mixture, and is left in the on condition to maintain a flame in the burner combustor. Once the furnace/radiant tube has reached the desired temperature, the controller can turn off the direct spark igniter to destabilize the flame in the burner combustor and initiate the flameless mode in the burner combustor. It will be appreciated that the direct spark igniter can have any suitable shape, size or configuration. For example, the tip  102  need only be disposed within the burner chamber, while the coil  100  may be located remotely from the tips at an external location relative to the burner  10 . In addition, the tips can be disposed in any suitable position, or at multiple positions within the burner chamber. In one embodiment, more than one such igniter may be used in the same burner chamber. 
         [0037]    In yet another embodiment, shown in  FIG. 7 , the stabilization device can be a plasma igniter  106 . The plasma igniter  106  is a device that uses an electrical discharge to produce an arc in a gas disposed between two electrodes  108 . The electrodes  108  are disposed at the end of a rod  110  that extends into the burner chamber  26  so that the electrodes  108  are adjacent the fuel flow orifices of the nozzle assembly  24 . The rod  110  may be hollow to accommodate electrical conduits  112  that interconnect the electrodes  108  with appropriate connections to the controller  34 . In this way, the controller  34  can control operation of the direct spark igniter  98 . 
         [0038]    During operation, the electrodes  108  are activated to produce an arc that is sufficient to ignite the oxidant/fuel mixture in the combustion chamber  26 . The controller is connected to the plasma igniter to turn it on and off. In operation, the plasma igniter is turned on to produce an electrical arc sufficient to ignite the oxidant/fuel mixture, and is left in the on condition to maintain a flame in the burner combustor. Once the furnace/radiant tube has reached the desired temperature, the controller can turn off the plasma igniter to destabilize the flame in the burner combustor and initiate the flameless mode in the burner combustor. It will be appreciated that the plasma igniter can have any suitable shape, size or configuration. For example, the electrodes  108  need only be disposed within the burner chamber and controlled remotely by the controller  34  through an induction coil, capacitor, or other electrical device that is disposed within or outside of the burner  10 . In addition, the electrodes can be disposed in any suitable position, or at multiple positions within the burner chamber. In one embodiment, more than one such igniter may be used in the same burner chamber. 
         [0039]    In another embodiment, shown in  FIG. 8 , the stabilization device can be an arc igniter  114 . The arc igniter  114  is a device that uses electrical power to provide electrical arcing that serves to ignite a combustible mixture. Tips  116 , between which the arc is created, are disposed at the end of a rod  118  that extends into the burner chamber  26  so that the tips  116  are adjacent the fuel flow orifices of the nozzle assembly  24 . The rod  118  may be hollow to accommodate electrical conduits  120  that interconnect the tips  116  with appropriate connections to the controller  34 . In this way, the controller  34  can control operation of the arc igniter  114 . 
         [0040]    During operation, the tips  116  are activated to produce an arc that is sufficient to ignite the oxidant/fuel mixture in the combustion chamber  26 . The controller is connected to the arc igniter to turn it on and off. In operation, the arc igniter is turned on to produce an electrical ark or spark sufficient to ignite the oxidant/fuel mixture, and is left in the on condition to maintain a flame in the burner combustor. Once the furnace/radiant tube has reached the desired temperature, the controller can turn off the arc igniter to destabilize the flame in the burner combustor and initiate the flameless mode in the burner combustor. It will be appreciated that the are igniter can have any suitable shape, size or configuration. For example, the tips  116  need only be disposed within the burner chamber. In addition, the tips can be disposed in any suitable position, or at multiple positions within the burner chamber. In one embodiment, more than one such igniter may be used in the same burner chamber. 
         [0041]    In another alternative embodiment, a directional secondary airflow may be provided to the burner chamber  26  to provide a counter-flow of air and/or oxidant in a direction generally toward the nozzle assembly  24  and away from the outlet opening  14  of the combustion chamber, as shown in  FIG. 9 . In this figure, an igniter  90  is used, which can be any appropriate igniter type operating to ignite a self-sustaining flame within the combustion chamber  26  or, alternatively, maintain a continuous flame within the chamber  26  of an otherwise non-flame-sustaining mixture. In one embodiment, the secondary air flow  200  is provided through one or more openings  202  formed in the sidewall of the hollow heat recuperator  18  in a region overlapping with the combustion chamber  26 . The air entering the combustion chamber  26  through each opening  202  is provided, in the illustrated embodiment, by a respective conduit  204  having a valve  206  associated therewith that is responsive to commands from the controller  34  and operable to selectively fluidly block the conduit  204 . In this way, air and/or an oxidant can selectively be provided to the combustion chamber  26 . The conduits  204  are associated with an air source  208  which can be at the same pressure as the air supply  30  or at a different pressure, for example, higher pressure, such that a stream of counter-direction air can be formed within the combustion chamber  26  when the valve(s)  206  is/are open. 
         [0042]    While the secondary air flow  200  is provided to the combustion chamber, a flame region  210  may be formed in an area where air, oxidant and fuel provided by the nozzle assembly  24  meets the counter-flowing air from the conduits  204 . In the illustrated embodiment, the region  210  overlaps with the igniter  90  such that the resulting flame can be sustained more efficiently within the combustion chamber  26 . In operation, the controller  34  can open the valve(s)  206  to provide a counter-flow of oxidant to the oxidant and fuel passing the nozzle assembly. An ignition device, such as a spark, can ignite the oxidant/fuel mixture to create a flame in the combustion chamber. The counter-flow of oxidant can stabilize the flame in the combustion chamber. Once the furnace/radiant tube has reached the desired temperature, the controller can close the valve(s) supplying oxidant to the backward oxidant pathways, which will destabilize the flame in the burner combustor to initiate the flameless mode in the burner combustor. It will be appreciated that the flow pathways can have any suitable shape and size. In addition, the flow pathways can be disposed in any suitable position. 
         [0043]    In another embodiment, shown in  FIG. 10 , the stabilization device can be a pilot flame igniter  122 . The pilot flame igniter  122  is a device that maintains a relatively small flame lit by providing a predetermined and metered flow of fuel or a fuel/air mixture continuously. The relatively small flame, which is commonly referred to as a pilot flame, serves to ignite a larger fuel flow during operation. The pilot flame  124  is disposed at the end of a fuel conduit  126  that extends into the burner chamber  26  so that the pilot flame  124  is adjacent the fuel flow orifices of the nozzle assembly  24 . Flow of pilot fuel in the conduit  126  may be controlled by a valve  128 , and also ignition of the pilot flame  124  periodically may be accomplished by an igniter  129 . 
         [0044]    During operation, the pilot flame  124  is continuously kept lit to ignite the oxidant/fuel mixture in the combustion chamber  26 . In operation, the pilot flame is turned on to produce a flame sufficient to ignite the oxidant/fuel mixture, and is left in the on condition to maintain a flame in the burner combustor. Once the furnace/radiant tube has reached the desired temperature, the controller can turn off the pilot flame or, alternatively, leave it on but otherwise increase the flow of fuel, air and oxidant to push the flame outside of the combustion chamber and initiate the flameless mode in the burner combustor. In other words, the increased velocity of the fuel and air may prevent the dwell of the flame within the combustion chamber, In such condition, the fuel for the pilot flame, which represents a very small portion of the fuel provided by the nozzle assembly  24 , may be carried with the remaining fluids and combust outside of the combustion chamber  26 . It will be appreciated that the pilot flame igniter can have any suitable shape, size or configuration. For example, the pilot flame  124  need only be disposed within the burner chamber. In addition, the pilot flame can be disposed in any suitable position, or at multiple positions within the burner chamber. In one embodiment, more than one pilot flame may be used in the same burner chamber. 
         [0045]    In another embodiment, shown in  FIG. 11 , the stabilization device can be an electric, magnetic or electromagnetic field generator igniter  130  that produces an induction heating effect on a heater element, which can reach a temperature sufficient for ignition of a combustible mixture. The induction igniter  130  can be a device that uses a process for heating an electrically conductive material such as a metal by electromagnetic induction, where so-called eddy currents in alternating directions are generated within the material, whose electrical resistance causes heating of the material. Heat may also be generated by magnetic hysteresis losses in materials that have significant relative permeability. A heated tip  132  of the induction igniter is disposed at the end of a rod  134  that extends into the burner chamber  26  so that the tip  132  is adjacent the fuel flow orifices of the nozzle assembly  24 . The rod  134  may be hollow to accommodate electrical conduits  136  that interconnect the tip  132  with appropriate connections to the controller  34 . In this way, the controller  34  can control operation of the induction igniter  130 . 
         [0046]    During operation, the tip  132  is heated to a temperature sufficient to initiate combustion of the oxidant/fuel mixture in the combustion chamber  26 . The controller is connected to the induction igniter to turn it on and off. In operation, the induction igniter is turned on to produce in the heated element a temperature sufficient to ignite the oxidant/fuel mixture, and is left in the on condition to maintain a flame in the burner combustor. Once the furnace/radiant tube has reached the desired temperature, the controller can turn off the induction igniter to destabilize the flame in the burner combustor and initiate the flameless mode in the burner combustor. It will be appreciated that the induction igniter can have any suitable shape, size or configuration. For example, the tip  132  need only be disposed within the burner chamber. In addition, the tips can be disposed in any suitable position, or at multiple positions within the burner chamber. In one embodiment, more than one such igniter may be used in the same burner chamber. 
         [0047]    A flowchart for a method of operating a burner in accordance with the disclosure is shown in  FIG. 12 . When the burner is turned on, an igniter is activated at  301 , and a combustible mixture is provided to an internal burner chamber at  302 . In one embodiment, the combustible mixture forms within the burner chamber as streams of fuel, air and/or an oxidant are provided to the chamber and mix. Unlike past burner designs, the combustible mixture need not be capable of self-sustaining a flame within the burner chamber in the absence of a sustained ignition source, which operates to stabilize the flame within the combustion chamber. The presence of a flame is sensed at  304 , and a determination of presence of a flame within the burner chamber is made at  306 . At a positive flame determination, i.e., when a flame is detected in the combustion chamber, a temperature external to the burner is sensed at  308 . The external temperature is compared to an auto-ignition temperature threshold at  310  and, when the auto-ignition temperature is reached or exceeded, the flow rates of fuel, air and/or oxidant may be altered to transition the flame outside of the burner chamber, or extinguish the flame altogether. 
         [0048]    In one embodiment, the burner is configured to maintain flame within the burner chamber by continuously monitoring for presence of a flame while the external temperature is below the auto-ignition threshold, and to maintain an igniter in an active state as a form of flame stabilizer for an otherwise unstable flame. In the event no flame is detected at the determination  306 , the system is shut-down and restarted by discontinuing the flow of combustible mixture at  314 , ensuring the igniter is active at  301 , and providing the combustible mixture at  302 . The igniter is maintained in an active state continuously while the temperature is below the predetermined value for as long as a stable flame is desired. When the external temperature has been exceeded, the igniter is turned off at  316  to extinguish the flame or to transition the flame to an area external to the burner. Following the flame transition or extinction, the flame sensor is interrogated to ensure no flame is present or remains within the burner chamber at  317 . When no flame is present, the process ends. However, when a flame is still present in the burner chamber after deactivation of the igniter, the flow rate of the combustible mixture may be incrementally increased or decreased at  318  to destabilize the flame present and to push the flame outside of the burner until the flame is no longer present. Alternatively, the system is shut-down and restarted, for example, as previously described, by restarting the igniter and restarting the combustible mixture supply. 
         [0049]    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, 
         [0050]    The use of the terms “a” and “an” and “the” and “at least one” 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 use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), 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. 
         [0051]    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.