Abstract:
The present invention is a method for achieving low NO x  single-stage combustion. A high velocity burner is run normally, with combustion occurring within both the burner combustion chamber and furnace chamber, until the gases within the furnace exceed the auto-ignition temperature of the gaseous fuel. The gaseous fuel is then shut off until the temperature within the burner combustion chamber drops below the auto-ignition temperature of the gaseous fuel. The gaseous fuel is then turned back on and the gaseous fuel auto ignites within the furnace chamber. The combustion then occurs only within the furnace chamber and not in the burner combustion chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/675,975 filed Apr. 29, 2005, which application is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is generally directed to a method of combustion and specifically directed to a low nitrogen oxide method of combustion.  
       BACKGROUND OF THE INVENTION  
       [0003]     One of the by-products created by the combustion of hydrocarbon (HC) fuels in burners that use atmospheric air is nitrogen oxide (NO x ). A well-known problem in the industry for many years with the use of conventional burner designs utilizing preheated combustion air is higher flame temperatures which in turn contribute to an exponential increase in NO x  emissions. Efforts to save fuel and increase combustion efficiency via recuperative and/or regenerative combustion systems combined with increasingly strict governmental regulations to achieve acceptable NO x  emissions from furnaces has led to a much greater awareness and need to solve this problem in recent years. Ideally, the only by-products of stoichiometric hydrocarbon combustion should be water (H 2 O), carbon dioxide (CO 2 ), and nitrogen (N 2 ) with no carbon monoxide (CO), unburned hydrocarbons (HCs), or NO x  emissions.  
         [0004]     Techniques for controlling and inhibiting NO x  formation in furnace combustion processes are well known and may include, for example, provisions for staging fuel, staging combustion air, recirculating flue gas into the burner, recirculating flue gas into the burner flame, altering combustion patterns with different degrees of swirl, and injection of water or steam into the burner or flame. Factors that contribute to the formation of NO x  in burner-fired combustion chambers are the oxygen content of the flame or combustion chamber, the temperature of the combustion chamber, the temperature of the combustion air, the burner-firing rate, turbulence, and the residence time for complete combustion. However, these factors are difficult to predict because burners for different industrial processes must operate at various furnace chamber temperatures, have various oxygen concentrations in the work chambers, may or may not have preheated combustion air, and are required to operate at different heat inputs depending on changing heat load requirements.  
         [0005]     Previous efforts to solve the problem include the Staged Air, Low NO x  Burner with Internal Recuperative Flue Gas Recirculation, disclosed in U.S. Pat. No. 5,413,477. This design utilizes a combination of air staging and flue gas recirculation (FGR) for NO x  reduction. However, the added capital expense for piping and controlling the recirculated flue gases are substantial.  
         [0006]     What is needed is a method that can be used with existing burners to reduce NO x  emissions in high velocity burners that are capable of reduced NO x  emissions when fired on either ambient or preheated combustion air.  
       SUMMARY OF THE INVENTION  
       [0007]     An embodiment of the present invention is a method for achieving low NO x  single-stage combustion comprising providing a burner, the burner comprising a combustion chamber disposed within the burner, the combustion chamber being in fluid communication with a combustion air supply, a gaseous fuel supply, a furnace chamber, and a flame stabilizer nested within the burner, the flame stabilizer being in fluid communication with the combustion chamber, a combustion air supply, and a gaseous fuel supply. The method further comprises flowing the combustion air into the combustion chamber and flowing gaseous fuel into the combustion chamber. The method further comprises mixing the gaseous fuel with the combustion air within the combustion chamber, forming a fuel/air mixture. The method further comprises flowing the fuel/air mixture from the combustion chamber into the furnace chamber through a high velocity exit nozzle. The method further comprises igniting the fuel/air mixture within the combustion chamber to form a flame, the flame being stabilized by the flame stabilizer. The method further comprises combusting the fuel/air mixture, the combustion occurring in the combustion chamber, and in the furnace chamber, the combustion forming hot gases of combustion, hot gases of combustion produced in the combustion chamber flowing into the furnace chamber through the high velocity exit nozzle. The method further comprises continuing to combust the fuel/air mixture for a first preselected period of time until attaining a first preselected temperature of gases within the furnace sufficient to auto ignite the fuel. The method further comprises ceasing to provide gaseous fuel, causing combustion of the fuel/air mixture to cease. The method further comprises waiting for a second preselected period of time until the burner combustion chamber cools to below the auto-ignition temperature of the fuel, wherein the furnace gases remain sufficiently hot to auto ignite the fuel. The method further comprises restarting the provision of gaseous fuel, causing the auto-ignition of the fuel/air mixture solely within the furnace chamber, without igniting the fuel/air mixture in the burner combustion chamber, an exit velocity of the fuel/air mixture through the high velocity exit nozzle being sufficiently high to prevent flashback of flame into the burner combustion chamber.  
         [0008]     Another embodiment of the present invention is a method for achieving low NO x  single-stage combustion comprising providing a burner, wherein the burner is at least partially self-recuperative, the burner comprising a combustion chamber disposed within the burner, the combustion chamber being in fluid communication with a combustion air supply, a gaseous fuel supply, a furnace chamber, and a flame stabilizer nested within the burner, the flame stabilizer being in fluid communication with the combustion chamber, a combustion air supply, and a gaseous fuel supply, and an exhaust housing, the exhaust housing being in fluid communication with the furnace chamber and an exhaust air outlet passage. The method further comprises flowing the combustion air into the combustion chamber and flowing gaseous fuel into the combustion chamber. The method further comprises mixing the gaseous fuel with the combustion air within the combustion chamber, forming a fuel/air mixture. The method further comprises flowing the fuel/air mixture from the combustion chamber into the furnace chamber through a high velocity exit nozzle. The method further comprises igniting the fuel/air mixture within the combustion chamber to form a flame, the flame being stabilized by the flame stabilizer. The method further comprises combusting the fuel/air mixture, the combustion occurring in the combustion chamber, and in the furnace chamber, the combustion forming hot gases of combustion, hot gases of combustion produced in the combustion chamber flowing into the furnace chamber through the high velocity exit nozzle, the hot gases of combustion flowing back into the exhaust housing and out of the burner through the exhaust air outlet passage. The method further comprises continuing to combust the fuel/air mixture for a first preselected period of time until attaining a first preselected temperature of gases within the furnace sufficient to auto ignite the fuel. The method further comprises ceasing to provide gaseous fuel, causing combustion of the fuel/air mixture to cease. The method further comprises waiting for a second preselected period of time until the burner combustion chamber cools to below the auto-ignition temperature of the fuel, wherein the furnace gases remain sufficiently hot to auto ignite the fuel. The method further comprises restarting the provision of gaseous fuel, causing the auto-ignition of the fuel/air mixture solely within the furnace chamber, without igniting the fuel/air mixture in the burner combustion chamber, an exit velocity of the fuel/air mixture through the high velocity exit nozzle being sufficiently high to prevent flashback of flame into the burner combustion chamber.  
         [0009]     An advantage of the present invention is that it provides a method for use with a currently installed high velocity gaseous fuel burner capable of lowering NO x  emissions without altering the burner.  
         [0010]     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is an end view of a prior art exemplary high velocity burner used with the method of the present invention.  
         [0012]      FIG. 2  is a partial cross-sectional view of a prior art exemplary high velocity burner taken along line  2 - 2  of  FIG. 1 .  
         [0013]      FIG. 3  is a partial cross-sectional view of an exemplary prior art high velocity burner showing the normal prior art combustion mode, which is also the first combustion mode of the present invention taken along line  2 - 2  of  FIG. 1 .  
         [0014]      FIG. 4  is a cross-sectional view of an exemplary high velocity burner showing the lower NO x  second combustion mode of the present invention also taken along line  2 - 2  of  FIG. 1 .  
         [0015]      FIG. 5  is an end view of a prior art exemplary self-recuperative high velocity burner used with the method of the present invention.  
         [0016]      FIG. 6  is a partial cross-sectional view of a prior art exemplary self-recuperative high velocity burner taken along line  6 - 6  of  FIG. 5 .  
         [0017]      FIG. 7  is a partial cross-sectional view of an exemplary self-recuperative prior art high velocity burner showing the normal prior art combustion mode, which is also the first combustion mode of the present invention taken along line  6 - 6  of  FIG. 5 .  
         [0018]      FIG. 8  is a partial cross-sectional view of an exemplary self-recuperative high velocity burner showing the lower NO x  second combustion mode of the present invention also taken along line  6 - 6  of  FIG. 5 . 
     
    
       [0019]     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]      FIG. 1  depicts an end view of an exemplary prior art high velocity burner  100  designed for firing on gaseous fuel, while  FIG. 2  depicts a cross-sectional view of the prior art burner of  FIG. 1  taken along line  2 - 2 . The high-velocity burner  100  has an upstream end  105  and a downstream end  110 . Additionally, the burner  100  has a main burner body  115 , with a combustion air inlet passage  120  and a gaseous fuel inlet passage  125  located within the main burner body  115 . A flame igniter  130  and flame detector (not shown) are disposed in the main burner body  115 . The main burner body  115  also has a flame stabilizer  140  disposed within the body  115 , the flame stabilizer  140  having a structural element that functions as a means of mixing fuel and air, such as, for example, apertures  145 , and a flame stabilizer chamber  150  disposed within the flame stabilizer  140 . Attached to the main burner body  115  is burner tile  155 , having an upstream end  160  and a downstream end  165 . The burner tile  155  preferably comprises a material selected from the group consisting of ceramic and metal capable of withstanding high temperatures. A combustion chamber  170  is located within the burner tile  155 . The burner  100  extends through a furnace shell  185  and a furnace wall  190  to a furnace chamber  205 . A high velocity exit nozzle  180  fluidly connects the combustion chamber  170  and the furnace chamber  205 . The burner tile  155  extends through the furnace shell  185  and furnace wall  190 .  
         [0021]     The gaseous fuel inlet passage  125  is connected to a gaseous fuel supply (not shown). The gaseous fuel may be any gaseous fuel known in the art, such as, for example, natural gas, methane, coke oven gas(es), blast furnace gas(es), and combinations thereof. The gaseous fuel inlet passage  125  is in fluid communication with the combustion chamber  170 . The combustion air inlet passage  120  is connected to a combustion air supply (not shown). The combustion air inlet passage  120  is in fluid communication with the combustion chamber  170 .  
         [0022]     An embodiment of the method of the present invention has two combustion modes, a first higher NO x  combustion mode and a second reduced NO x  combustion mode. In the first combustion mode, combustion air is first provided to the burner  100  from the combustion air supply (not shown) and gaseous fuel is provided to the burner  100  from the gaseous fuel supply (not shown). The main combustion air flows into and through the combustion air inlet passage  120 . As shown by combustion airflow arrows  210 , the combustion air flows directly into the combustion chamber  170 . The combustion air is preferably provided at a static air inlet pressure in the range of about 8 ounces per square inch gauge (osig) to about 20 osig. The gaseous fuel flows directly into the combustion chamber  170  as shown by gaseous fuel flow arrows  215 .  
         [0023]     In the combustion chamber  170 , the gaseous fuel  215  mixes with the combustion air  210 , forming a fuel/air mixture. As shown by fuel/air mixture flow arrows  220 , the fuel/air mixture flows from the combustion chamber  170 , through the high velocity exit nozzle  180 , and into the furnace chamber  205 .  
         [0024]     In a preferred embodiment, the combustion air  210  and gaseous fuel  215  flows are provided at room temperature. Optionally, the combustion air may be preheated to a temperature up to about 1000° F. as known in the art. The burner can be fired providing an amount of air in excess of the amount required to combust the gaseous fuel, preferably an excess in the range of about 0% to about 30%. The burner may also be fired providing an amount of fuel in excess of the amount of air available to combust the fuel, preferably excess fuel in the range of about 0% to about 10%.  
         [0025]     Once both the combustion air  210  and the gaseous fuel  215  are flowing through the burner  100 , the flame igniter  130  is used to ignite a first combustion mode flame  225 , shown in  FIG. 3 , within the combustion chamber  170 . Upon ignition, the first combustion mode flame  225  is stabilized by flame stabilizer  140  and spreads through combustion chamber  170  and into the furnace chamber  205 . The first combustion mode is continued for a first preselected period of time until the temperature within the furnace chamber  205  rises above the auto-ignition temperature of the gaseous fuel such that the fuel/air mixture can auto ignite upon entry into the furnace chamber  205 .  
         [0026]     Once the temperature of the gases within the furnace exceeds the auto-ignition temperature of the gaseous fuel, the supply of gaseous fuel is shut off, causing the combustion of the fuel/air mixture to cease. For example, if natural gas, with an auto-ignition temperature of about 1400° F. is used, then the supply of natural gas may be shut off after the gases within the furnace chamber  205  have exceeded about 1400° F. Such inactivation of the flow of gaseous fuel  215  may be accomplished by any means known in the art, such as, for example, using a control panel (not shown) to activate a solenoid valve  175 , where the solenoid valve  175  controls the flow of gaseous fuel  215  through a gaseous fuel line  200  that connects the gaseous fuel supply to the gaseous fuel inlet passage  125 . Any functional valve known in the art may be used, such as for example a butterfly valve, etc. The first combustion mode may be run for any first preselected period of time sufficient to cause the hot gases within the furnace chamber  205  to exceed the auto-ignition temperature of the gaseous fuel  215 . The preselected time for the first combustion mode may exceed the time necessary for the hot gases within the furnace chamber  205  to exceed the auto-ignition temperature of the fuel.  
         [0027]     Optionally, once the temperature of the gases within the furnace exceeds the auto-ignition temperature of the gaseous fuel, the supply of combustion air  210  may be shut off in addition to the gaseous fuel  215  being shut off. Such inactivation of the flow of combustion air  210  may be accomplished by any means known in the art, such as, for example, using a control panel (not shown) to activate a butterfly valve  235  where the butterfly valve  235  controls the flow of gaseous fuel  215  through a combustion air line  230  that connects the combustion air supply to the combustion air inlet passage  120 . Any functional valve known in the art may be used, such as, for example, a solenoid valve, etc.  
         [0028]     During the first combustion mode, the temperature of portions of the combustion chamber  170  are heated to a temperature above the auto-ignition temperature of the gaseous fuel  215 , such that resuming a flow of the gaseous fuel immediately after terminating flow of the gaseous fuel would cause the gaseous fuel to reignite within the combustion chamber  170 . After a second preselected period of time, the second preselected period of time being sufficient to ensure that no auto-ignition of the air/fuel mixture  220  will occur within the combustion chamber  170 , the flow of gaseous fuel  215  is resumed. The second preselected period of time cannot be so long so that the temperature of the hot gases within the furnace chamber  205  fall below the auto-ignition temperature of the fuel  215 . The gases within the furnace chamber  205  must be maintained at or above a temperature sufficient to auto ignite the fuel  215 . Such activation of the flow of gaseous fuel  215  may be accomplished by any means known in the art, such as, for example, using the control panel to inactivate the solenoid valve  175 , where the solenoid valve  175  controls the flow of gaseous fuel  215  through a gaseous fuel line  200  that connects the gaseous fuel supply to the gaseous fuel inlet passage  125 .  
         [0029]     Optionally, if the supply of combustion air  210  was shut off as well, the combustion air  210  is activated as well as the gaseous fuel  215 . Such activation of the flow of combustion air  210  may be accomplished by any means known in the art, such as, for example, using the control panel to again activate the butterfly valve  235 , where the butterfly valve  235  controls the flow of combustion air  210  through the combustion air line  230  that connects the combustion air supply to the combustion air inlet passage  120 .  
         [0030]     Once the flow of gaseous fuel  215  is resumed, the gaseous fuel  215  flows into the combustion chamber  170  and mixes with the combustion air  210  without igniting. Optionally, if the flow of combustion air was shut off as well, the flow of combustion air  210  is resumed as well, the combustion air flows into the combustion chamber  170  and mixes with the gaseous fuel  215  without igniting. The fuel/air mixture  220  flows through the combustion chamber  170  and into the furnace chamber  205  through the high velocity exit nozzle  180 . Once in the furnace chamber  205 , the fuel/air  220  mixture auto ignites, initiating the second combustion mode, shown in  FIG. 4 . During the second combustion mode, combustion only occurs within the furnace chamber  205 . Because of the high velocity of the fuel/air mixture  220  flowing out of the high velocity exit nozzle  180 , the second combustion mode flame  250 , which is now substantially invisible to the naked eye, does not propagate back through the high velocity exit nozzle  180 . The amount of NO x  generated by the second combustion mode is lower than the amount of NO x  generated by the first combustion mode by about 50 percent or more. High velocity burners may be fired in any position or orientation, so such burners may be installed in the roof, walls, or floor of a furnace.  
         [0031]     The burner  100  of the present invention may be any high velocity burner known in the art as long as a mixture of combustion air and gaseous fuel exits the burner at a speed sufficient to prevent a flame  250  that exists solely within the furnace chamber  205  from re-entering the burner.  
         [0032]      FIG. 5  depicts an end view of an exemplary self-recuperative high velocity burner  300  designed for firing on gaseous fuel and designed for direct furnace heating, while  FIG. 6  depicts a cross-sectional view of the self-recuperative prior art high velocity burner of  FIG. 5  taken along line  6 - 6 . Self-recuperative burners require an effective built-in heat exchange to transfer heat which is otherwise lost as waste from the exhaust gas to the incoming combustion air. Referring now to  FIG. 5  and  FIG. 6 , the self-recuperative burner  300  comprises an exhaust housing  330  and a main burner assembly  322 . The exhaust housing  330  extends from a first end  310  to a second end  320 . Housing  330  extends through an aperture  304  in a furnace wall  306 . A support structure  308  extends between the first end  310  of housing  330  and the furnace wall  306  and support structure  308  firmly attaches and stabilizes burner  300  to furnace wall  306 . The main burner assembly  322  nests within the exhaust housing  330  and extends from the air inlet housing  307  to the second end  454  of the discharge nozzle  450 . Additionally, the self-recuperative burner  300  has a gaseous fuel inlet passage  340 , a combustion air inlet passage  355  and an exhaust air outlet passage  365 .  
         [0033]     The main burner assembly  322  comprises an air inlet housing  307 , a primary air passage  414 , a secondary air passage  440 , a flame stabilizer  460  for flame stabilization, a combustion chamber  475 , and a discharge nozzle  450 . The flame stabilizer  460  comprises, for example, flame stabilizer vanes  466 . The flame stabilizer vanes  466  begin at the end  411  of the primary air tube  410 . The flame stabilizer vanes  466  extend to the first end  452  of the discharge nozzle  450 . The combustion chamber  475  includes the flame stabilizer  460  and begins at the end  411  of the primary air tube  410  and extends to the second end  454  of the discharge nozzle  450 . The primary air tube  410  surrounds fuel tube  406 . Fuel tube  406  extends from end  402  to the fuel tube apertures  420 . Primary air passageway  414  is formed between fuel tube  406  and inner diameter  412  of primary air tube  410 . A secondary air passageway  440  has its inner diameter  418  on primary air tube  410  and an outer diameter  416  and extends from air inlet housing  307  to the flame stabilizer vanes  460 .  
         [0034]     Fuel is provided to fuel tube  406  by means of the gaseous fuel inlet passage  340  that is connected to a gaseous fuel line  335 , which is connected to a fuel supply (not shown) at end  402  of fuel tube  406 . Combustion air, which is split into primary combustion air and secondary combustion air in the main burner assembly  322 , is provided to the main burner assembly  322  by means of the combustion air inlet passage  355 , which is connected to a combustion air line  350  that is connected to a combustion air supply (not shown). Primary air passageway  414  supplies primary combustion air to mix with fuel from fuel tube  406 . The combustion air is preferably provided at a static air inlet pressure in the range of about 8 osig to about 20 osig. The flow of primary combustion air is shown by arrows  370 . The flow of secondary combustion air is shown by arrows  375 . The flow of gaseous fuel is shown by arrows  380 . Primary air tube  410  terminates at combustion chamber  475 . A secondary air tube  422  has an outside diameter  416  surrounding primary air tube  410  and extends to the flame stabilizer vanes  466 . Secondary air tube  422  includes a plurality of secondary air passageways  440  that extend along its length. These secondary air passageways  440  are formed by helical walls  442  that extend inward from tube  422 . The radial inward boundary of these passageways  440  is the outer diameter of primary air tube  410 . Secondary air  375  is introduced into the secondary air tube  422  at air inlet housing  307 .  
         [0035]     Fuel tube  406  terminates at the combustion chamber  475  just beyond primary air tube  410 . Fuel tube  406  includes a plurality of apertures  420  having at least a partial radial orientation to form a nozzle that distributes fuel at least partially radially outward from the fuel tube  406 . A flame igniter  424  is positioned proximate to fuel tube apertures  420 . A flame sensor  430 , also disposed proximate to fuel tube apertures  420 , extends beyond the termination of tube  410 .  
         [0036]     At the end  462  of flame stabilizer  460  is a conically-shaped high velocity exit nozzle  450  to discharge the flame. Nozzle  450  has a first end  452  that is received within tube  422  and a second end  454  with aperture  456  to direct the flame into furnace chamber  480 . The flow of the air and fuel mixture through combustion chamber  475  is shown by arrows  385 .  
         [0037]     Once both the combustion air  370 ,  375  and the gaseous fuel  380  are flowing into the combustion chamber  475 , the flame igniter  424  is used to ignite a first combustion mode flame  390 , shown in  FIG. 7 , within the combustion chamber  475 . Upon ignition, the first combustion mode flame  390  is stabilized by flame stabilizer  460  and exits through the nozzle  450  and into the furnace chamber  480 . Exhaust gases are drawn back into an exhaust passageway  470  within the exhaust gas housing  330  as shown by arrows  490  and out of the burner  300  through the exhaust air outlet passage  365 .  
         [0038]     Once the temperature of the gases within the furnace exceeds the auto-ignition temperature of the gaseous fuel, the supply of gaseous fuel  380  is shut off, causing the combustion of the fuel/air mixture to cease. For example, if natural gas, with an auto-ignition temperature of about 1400° F. is used, then the supply of natural gas may be shut off after the gases within the furnace chamber  480  have exceeded about 1400° F. Such inactivation of the flow of gaseous fuel may be accomplished by any means known in the art, such as, for example, using a control panel (not shown) to activate a solenoid valve  345 , where the solenoid valve  345  controls the flow of gaseous fuel  380  through a gaseous fuel line  335  that connects the gaseous fuel supply to the gaseous fuel inlet passage  340 . Any functional valve known in the art may be used, such as, for example, a butterfly valve, etc. The first combustion mode may be run for any first preselected period of time sufficient to cause the hot gases within the furnace chamber  480  to exceed the auto-ignition temperature of the gaseous fuel. The preselected time for the first combustion mode may exceed the time necessary for the hot gases within the furnace chamber  480  to exceed the auto-ignition temperature of the fuel.  
         [0039]     Optionally, once the temperature of the gases within the furnace chamber  480  exceeds the auto-ignition temperature of the gaseous fuel  380 , the supply of combustion air  370 ,  375  may be shut off in addition to the gaseous fuel being shut off. Such inactivation of the flow of combustion air  370 ,  375  may be accomplished by any means known in the art, such as, for example, using a control panel (not shown) to activate a butterfly valve  360  where the butterfly valve  360  controls the flow of combustion air through a combustion air line  350  that connects the combustion air supply to the combustion air inlet passage  355 . Any functional valve known in the art may be used, such as, for example, a solenoid valve, etc.  
         [0040]     During the first combustion mode, the temperature of portions of the combustion chamber  475  are heated to a temperature above the auto-ignition temperature of the gaseous fuel  380 , such that resuming a flow of the gaseous fuel immediately after terminating flow of the gaseous fuel would cause the gaseous fuel to reignite within the combustion chamber  475 . After a second preselected period of time, the second preselected period of time being sufficient to ensure that no auto-ignition of the air/fuel mixture  385  will occur within the combustion chamber  475 , the flow of gaseous fuel  380 , and optionally the combustion air  370 ,  375  as well, is resumed. The second preselected period of time cannot be so long so that the temperature of the hot gases within the furnace chamber  480  fall below the auto-ignition temperature of the fuel  380 . The gases within the furnace chamber  480  must be maintained at or above a temperature sufficient to auto ignite the fuel  380 . Such activation of the flow of gaseous fuel  380  may be accomplished by any means known in the art, such as, for example, using the control panel to inactivate the solenoid valve  345 , where the solenoid valve controls the flow of gaseous fuel  380  through a gaseous fuel line  335  that connects the gaseous fuel supply to the gaseous fuel inlet passage  340 .  
         [0041]     Optionally, if the supply of combustion air was shut off as well, the combustion air  370 ,  375  is activated as well as the gaseous fuel  380 . Such activation of the flow of combustion air  370 ,  375  may be accomplished by any means known in the art, such as, for example, using the control panel to again activate the butterfly valve  360 , where the butterfly valve  360  controls the flow of combustion air through the combustion air line  350  that connects the combustion air supply to the combustion air inlet passage  355 .  
         [0042]     Once the flow of gaseous fuel  380  is resumed, and, if it was shut off, the combustion air  370 ,  375 , as well, the gaseous fuel  380  mixes with the combustion air in combustion chamber  475  without igniting. The fuel/air mixture  385  flows through the burner  300  and into the furnace chamber  480  through the high velocity exit nozzle  450 . Once in the furnace chamber  480 , the fuel/air mixture  385  auto ignites, initiating the second combustion mode, shown in  FIG. 8 . During the second combustion mode, combustion only occurs within the furnace chamber  480 . Because of the high velocity of the combustion air  370 ,  375  and the fuel/air mixture  385  flowing out of the high velocity exit nozzle  450 , the second combustion mode flame  395 , which is now substantially invisible to the naked eye, does not propagate back through the high velocity exit nozzle  450 . The amount of NO x  generated by the second combustion mode is lower than the amount of NO x  generated by the first combustion mode by about 50 percent or more. High velocity burners may be fired in any position or orientation, so such burners may be installed in the roof, walls, or floor of a furnace.  
         [0043]     The self-recuperative burner  300  of the present invention may be any self-recuperative high velocity burner known in the art as long as a mixture of combustion air and gaseous fuel exits the burner at a speed sufficient to prevent a flame  395  that exists solely within the furnace chamber  480  from re-entering the burner.  
         [0044]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.