Abstract:
Method and apparatus for use in burners of furnaces such as those used in steam cracking. The apparatus includes a burner tube having a downstream end and an upstream end for receiving fuel and air, flue gas or mixtures thereof. A burner tip is mounted on the downstream end of the burner tube adjacent a first opening in the furnace, so that combustion of the fuel takes place downstream of the burner tip. At least one passageway has a first end at a second opening in the furnace and a second end in a primary air chamber adjacent the upstream end of the burner tube. The passageway also has structure for injecting steam into the passageway and a means for drawing flue gas from the furnace through the passageway.

Description:
RELATED APPLICATIONS  
       [0001]    This patent application claims priority from Provisional Application Serial No. 60/365,226, filed on Mar. 16, 2002, the contents of which are hereby incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to an improvement in a burner such as those employed in high temperature furnaces in the steam cracking of hydrocarbons. More particularly, it relates to the use of steam to provide a more homogeneous mixture of flue gas, steam and air entering a fuel-gas-recirculation (FGR) burner to achieve a reduction in NO x  emissions.  
         BACKGROUND OF THE INVENTION  
         [0003]    As a result of the interest in recent years to reduce the emission of pollutants from burners used in large industrial furnaces, burner design has undergone substantial change. In the past, improvements in burner design were aimed primarily at improving heat distribution. Increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants.  
           [0004]    Oxides of nitrogen (NO x ) are formed in air at high temperatures. Reduction of NO x  emissions is a desired goal to decrease air pollution and meet government regulations. In recent years, a wide variety of mobile and stationary sources of NO x  emissions have come under increased scrutiny and regulation.  
           [0005]    A strategy for achieving lower NO x  emission levels is to install a NO x  reduction catalyst to treat the furnace exhaust stream. This strategy, known as Selective Catalytic Reduction (SCR), is very costly and, although it can be effective in meeting more stringent regulations, represents a less desirable alternative to improvements in burner design.  
           [0006]    Burners used in large industrial furnaces may use either liquid fuel or gas. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and combustion air is mixed with the fuel at the zone of combustion.  
           [0007]    Gas fired burners can be classified as either premix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.  
           [0008]    Raw gas burners inject fuel directly into the air stream, and the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763, which patent is incorporated herein by reference. In addition, many raw gas burners produce luminous flames.  
           [0009]    Premix burners mix some or all of the fuel with some or all of the combustion air prior to combustion. Since premixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Premixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, premix burners are often compatible with various steam cracking furnace configurations.  
           [0010]    Floor-fired premix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Therefore, a premix burner is the burner of choice for such furnaces. Premix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.  
           [0011]    In gas fired industrial furnaces NO x  is formed by the oxidation of nitrogen drawn into the burner with the combustion air stream. The formation of NO x  is widely believed to occur primarily in regions of the flame where there exist both high temperatures and an abundance of oxygen. Since ethylene furnaces are amongst the highest temperature furnaces used in the hydrocarbon processing industry, the natural tendency of burners in these furnaces is to produce high levels of NO x  emissions.  
           [0012]    One technique for reducing NO x  that has become widely accepted in industry is known as staging. With staging, the primary flame zone is deficient in either air (fuel rich) or fuel (fuel lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NO x  formation than an air-fuel ratio closer to stoichiometry. Staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NO x . Since NO x  formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature dramatically reduce NO x  emissions. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase as well.  
           [0013]    In the context of premix burners, the term primary air refers to the air premixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air are more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.  
           [0014]    Thus, one set of techniques achieves lower flame temperatures by using staged-air or staged-fuel burners to lower flame temperatures by carrying out the initial combustion at far from stoichiometric conditions (either fuel-rich or air-rich) and adding the remaining air or fuel only after the flame has radiated some heat away to the fluid being heated in the furnace.  
           [0015]    Another set of techniques achieves lower flame temperatures by diluting the fuel-air mixture with inert material. Flue-gas (the products of the combustion reaction) or steam are commonly used diluents. Such burners are classified as FGR (flue-gas-recirculation) or steam-injected, respectively.  
           [0016]    U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NO x  emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through a pipe or pipes by the aspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. The flue gas mixes with combustion air in a primary air chamber prior to combustion to dilute the concentration of O 2  in the combustion air, which lowers flame temperature and thereby reduces NO x  emissions. The contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.  
           [0017]    Burners of the type disclosed in U.S. Pat. No. 5,092,761 have optionally employed steam injection for the primary purpose of providing a motive force for enhancing the flow of recirculated flue gas, fuel gas, air and steam into the burner tube located in the primary chamber at the base of the burner.  
           [0018]    Analysis of burners of the type described in U.S. Pat. No. 5,092,761 has indicated the flue-gas-recirculation (FGR) ratio is generally in the range 5-10% where FGR ratio is defined as:  
             FGR  ratio (%)=100 [G /( F+A )] 
           [0019]    where G=Flue-gas drawn into venturi, (Ib)  
           [0020]    F=Fuel combusted in burner, (Ib), and  
           [0021]    A=Air drawn into burner, (Ib).  
           [0022]    The ability of these burners to generate higher FGR ratios is limited by the inspirating capacity of the gas spud/venturi/FGR flow ducting combination. Further closing of the primary air dampers will produce lower pressures in the primary air chamber and thus enable increased FGR ratios.  
           [0023]    Despite these advances in the art, a need exists for a burner having a desirable heat distribution profile that meets increasingly stringent NO x  emission regulations.  
           [0024]    Therefore, what is needed is a burner for the combustion of fuel gas wherein the temperature of the fuel and air, flue-gas or mixtures thereof is advantageously reduced and which also enables higher flue gas recirculation ratios (FGR) to be utilized, yielding further reductions in NO x  emissions.  
         SUMMARY OF THE INVENTION  
         [0025]    The present invention is directed to a method and apparatus for use in burners of furnaces such as those used in steam cracking. In accordance with a broad aspect of the invention, there is provided an apparatus comprising a furnace having a first opening, and a burner located adjacent the first opening in said furnace. The burner has (i) a primary air chamber, and (ii) a burner tube including a downstream end, an upstream end for receiving fuel and air, flue gas or mixtures thereof from said primary air chamber, and a burner tip mounted on the downstream end of the burner tube adjacent the first opening in the furnace for combusting the fuel downstream of the burner tip. At least one passageway is provided with a first end at a second opening in the furnace and a second end in a primary air chamber adjacent the upstream end of the burner tube. The passageway is provided with means for injecting steam into the passageway. Means are provided for drawing flue gas from the furnace through the passageway and air from a source of air in response to an inspirating effect created by uncombusted fuel. The fuel and air flowing through the burner tube from its upstream end towards its downstream end creates the means for drawing flue gas and air.  
           [0026]    In accordance with another broad aspect of the present invention, a method is provided that includes the steps of combining fuel and air, flue gas or mixtures thereof at a predetermined location; passing the fuel and air, flue gas or mixtures thereof through a venturi; combusting the fuel at a combustion zone downstream of the venturi; drawing flue gas from the furnace through at least one passageway to a primary air chamber containing said predetermined location and injecting steam into said at least one passageway.  
           [0027]    The injection of steam into the stream of flue gas before the flue gas mixes with the air results in a more homogenous mixture of flue gas, steam, and air entering the burner. A more homogeneous mixture results in higher venturi capacity, higher flue gas entrainment capacity, lower peak flame temperature and lower NO x . This location also tends to reduce the temperature of the passageway, which extends its life.  
           [0028]    An object of the present invention is to provide a burner arrangement that permits the temperature of the fuel/air/flue-gas mixture in the venturi to be reduced, thus reducing NO x  emissions.  
           [0029]    These and other objects and features of the present invention will be apparent from the detailed description taken with reference to accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The invention is further explained in the description that follows with reference to the drawings illustrating, by way of non-limiting examples, various embodiments of the invention wherein:  
         [0031]    [0031]FIG. 1 illustrates an elevation partly in section of an embodiment of the burner of the present invention;  
         [0032]    [0032]FIG. 2 is an elevation partly in section taken along line  2 - 2  of FIG. 1;  
         [0033]    [0033]FIG. 3 is a plan view taken along line  3 - 3  of FIG. 1;  
         [0034]    [0034]FIG. 4 is a schematic illustration of another embodiment of the burner of the present invention;  
         [0035]    [0035]FIG. 5 is a plan view taken along line  5 - 5  of FIG. 4;  
         [0036]    [0036]FIG. 6 is an elevation view of an embodiment of the present invention employing external FGR;  
         [0037]    [0037]FIG. 7 is a plan view of an embodiment of the present invention employing external FGR;  
         [0038]    [0038]FIG. 8 illustrates an elevation partly in section of an embodiment of a flat-flame burner of the present invention; and  
         [0039]    [0039]FIG. 9 is an elevation partly in section of the embodiment of a flat-flame burner of FIG. 8 taken along line  9 - 9  of FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    Although the present invention is described in terms of a burner for use in connection with a furnace or an industrial furnace, it will be apparent to one of skill in the art that the teachings of the present invention also have applicability to other process components such as, for example, boilers. Thus, the term furnace herein shall be understood to mean furnaces, boilers and other applicable process components.  
         [0041]    Referring particularly to FIGS.  1 - 3 , a burner  10  includes a freestanding burner tube  12  located in a well in a furnace floor  14 . The burner tube  12  includes an upstream end  16 , a downstream end  18  and a venturi portion  19 . A burner tip  20  is located at the downstream end  18  and is surrounded by an annular tile  22 . A fuel orifice  11 , which may be located in gas spud  24 , is located at the upstream end  16  and introduces fuel into the burner tube  12 . Fresh or ambient air is introduced into a primary air chamber  26  through an adjustable damper  28  to mix with the fuel at the upstream end  16  of the burner tube  12  and pass upwardly through the venturi portion  19 . Combustion of the fuel and fresh air occurs downstream of burner tip  20 . Optionally, one or more steam injection tubes  15  may be provided so as to be positioned in the direction of flow so as to add to the motive force provided by venturi portion  19  for inducing the flow of fuel, steam and flue gas, air and mixtures thereof into the burner tube  12 .  
         [0042]    A plurality of air ports  30  (FIGS. 2 and 3) originate in a secondary air chamber  32  and pass through the furnace floor  14  into the furnace. Fresh or ambient air enters the secondary air chamber  32  through adjustable dampers  34  and passes through the staged air ports  30  into the furnace to provide secondary or staged combustion, as described in U.S. Pat. No. 4,629,413, which is hereby incorporated herein by reference.  
         [0043]    Unmixed low temperature fresh or ambient air, having entered the secondary air chamber  32  through the dampers  34  and having passed through the air ports  30  into the furnace, is also drawn through a passageway  76  into a primary air chamber  26  by the inspirating effect of the fuel passing through the venturi portion  19 . The passageway  76  is shown as a metallic FGR duct.  
         [0044]    U.S. Pat. No. 5,092,761 contemplates locating a steam injection point(s) at the base of the venturi for the purpose of reducing NO x . This is also known as deNO x  steam injection. In accordance with an aspect of the present invention, means for injecting steam in the form of deNO x  steam injection tube(s)  53  are located in the passageway  76  upstream of the air source  80 . This location results in a more homogenous combination of flue gas, steam, air or mixtures thereof and air entering the burner venturi  19 . A more homogeneous mixture can result in higher venturi capacity, higher flue gas entrainment capacity, lower flame temperature and lower NO x . This location also tends to reduce the temperature of the metallic FGR duct, which extends the life of the duct.  
         [0045]    Lighting port  50  provides access to the interior of burner  10  for lighting element (not shown).  
         [0046]    Flue gas containing, for example, about 0 to about 15% O 2  is drawn from near the furnace floor through the passageway  76  with about 5 to about 15% O 2  preferred, about 2 to about 10% O 2  more preferred and about 2 to about 5% O 2  particularly preferred, by the inspirating effect of fuel passing through venturi portion  19  of burner tube  12 . In this manner, the primary air and flue gas are mixed in primary air chamber  26 , which is prior to the zone of combustion. Therefore, the amount of inert material mixed with the fuel is raised, thereby reducing the flame temperature and, as a result, reducing NO x  emissions. This is in contrast to a liquid fuel burner, such as that of U.S. Pat. No. 2,813,578, in which the combustion air is mixed with the fuel at the zone of combustion, rather than prior to the zone of combustion.  
         [0047]    Closing or partially closing damper  28  restricts the amount of fresh air that can be drawn into the primary air chamber  26  and thereby provides the vacuum necessary to draw flue gas from the furnace floor.  
         [0048]    Advantageously, a mixture of about 50% flue gas and from about 50% ambient air should be drawn through the passageway  76 . The desired proportions of flue gas and ambient air may be achieved by proper placement and/or design of the passageway  76  in relation to the air ports  30 . That is, the geometry of the air ports, including but not limited to their distance from the burner tube, the number of air ports, and the size of the air ports, may be varied to obtain the desired percentages of flue gas and ambient air.  
         [0049]    [0049]FIG. 4 illustrates another embodiment of the invention for using steam injection to enhance the flue gas recirculation ratio of a burner  100 . With reference to FIG. 4, fuel exits a fuel orifice  111 , which may be located within gas spud  102 , at a high velocity at the entrance to a venturi portion  104  of a burner tube  106 , thus inspirating air from a primary air chamber  110  into the venturi portion  104 . Partially closing the primary air dampers  108  generates a sub-ambient pressure in the primary air chamber  110 . A flue gas recirculation (FGR) duct  112  connects the furnace  114  to the primary air chamber  110  of the burner  100 , thus permitting the flow of the flue gas into the primary air chamber  110  to be mixed with fuel from the fuel orifice  111 , which may be located within gas spud  102  and primary air from the dampers  108 . The flue gas recirculation duct  112  has a venturi section  116 . Steam for NO x  reduction is injected at the entrance of the venturi section  116  through an orifice, which may be located within spud  120  of steam injection tube  118 , for generating a high velocity steam jet at the entrance to venturi section  116 . The steam jet/venturi combination inspirates flue gas from the floor  122  of the furnace  114  into the primary air chamber  110  of the burner  100 . With this arrangement, the pressure in the primary air chamber  110  does not need to be reduced as far below ambient as does the burner of U.S. Pat. No. 5,092,761, the mixture of flue gas, air and steam is more homogeneous and a greater volume of flue gas can be recycled, providing higher FGR ratios and lower NO x  emissions, while still maintaining sufficient primary air flow to assure good burner stability.  
         [0050]    Optionally, one or more steam injection tubes  115  may be provided and positioned in the direction of flow so as to add to the motive force provided by venturi portion  104  for inducing the flow of fuel, steam and flue gas, air and mixtures thereof into the burner tube  106 .  
         [0051]    Referring to FIGS. 4 and 5, a plurality of staged air ports  130  originate in a secondary air chamber  132  and pass through the furnace floor  122  into the furnace  114 . Fresh or ambient air enters the secondary air chamber  132  through adjustable dampers  135  and passes through the staged air ports  130  into the furnace  114  to provide secondary or staged combustion.  
         [0052]    Referring to FIGS. 6 and 7, another embodiment of the present invention is illustrated. In this embodiment, the teachings above with respect to the steam injection techniques of the present invention may be applied in connection with a furnace having one or more burners utilizing an external FGR duct  376  in fluid communication with a furnace exhaust  300 . It will be understood by one of skill in the art that several burners  310  are located within the furnace, all of which feed furnace exhaust  300  and external FGR duct  376 . In this case, steam injection tube(s)  353  are located in the passageway  376  upstream of the primary air dampers  28 . The benefit of the present invention serves to increase the motive force available to draw flue gas through FGR duct  376 , eliminating or minimizing the need for an external fan to supply adequate levels of FGR.  
         [0053]    Benefits similar to those described above through the use of the steam injection techniques of the present invention can be achieved in flat-flame burners, as will now be described by reference to FIGS. 8 and 9.  
         [0054]    A burner  410  includes a freestanding burner tube  412  located in a well in a furnace floor  414 . Burner tube  412  includes an upstream end  416 , a downstream end  418  and a venturi portion  419 . Burner tip  420  is located at downstream end  418  and is surrounded by a peripheral tile  422 . A fuel orifice  411 , which may be located in gas spud  424 , is located at upstream end  416  and introduces fuel into burner tube  412 . Fresh or ambient air may be introduced into primary air chamber  426  to mix with the fuel at upstream end  416  of burner tube  412 . Combustion of the fuel and fresh air occurs downstream of the burner tip  420 . Fresh secondary air enters secondary chamber  432  through dampers  434 .  
         [0055]    In order to recirculate flue gas from the furnace to the primary air chamber, a flue gas recirculation passageway  476  is formed in furnace floor  414  and extends to primary air chamber  426 , so that flue gas is mixed with fresh air drawn into the primary air chamber from opening  480 , through dampers  428 . Flue gas containing, for example, 0 to about 15% O 2  is drawn through passageway  476  by the inspirating effect of fuel passing through venturi portion  419  of burner tube  412 . Primary air and flue gas are mixed in primary air chamber  426 , which is prior to the zone of combustion.  
         [0056]    Optionally, one or more steam injection tubes  484  may be provided so as to be positioned in the direction of flow so as to add to the motive force provided by venturi portion  419  for inducing the flow of fuel, steam and flue gas, air and mixtures thereof into the burner tube  412 .  
         [0057]    In operation, a fuel orifice  411 , which may be located within gas spud  424 , discharges fuel into burner tube  412 , where it mixes with primary air, recirculated flue-gas or mixtures thereof. The mixture of fuel and recirculated flue-gas, primary air or mixtures thereof then discharges from burner tip  420 . The mixture in the venturi portion  419  of burner tube  412  is maintained below the fuel-rich flammability limit; i.e. there is insufficient air in the venturi to support combustion. Secondary air is added to provide the remainder of the air required for combustion. The majority of the secondary air is added a finite distance away from the burner tip  420 .  
         [0058]    As with previous embodiments, means for injecting steam in the form of deNO x  steam injection tube(s)  453  are located in the passageway  476  upstream of the primary air dampers  428 . This location results in a more homogenous mixture of flue gas, steam and air entering the burner venturi  419 . A more homogeneous mixture results in higher venturi capacity, higher flue gas entrainment capacity, lower flame temperature and lower NO x . This location also tends to reduce the temperature of the metallic FGR duct, which extends the life of the duct.  
       EXAMPLES  
     Example 1  
       [0059]    This example explores the advantages of a burner of the type depicted in FIGS. 4 and 5, as modeled based on material balance calculations. The following burner condition was studied: fuel rate=255 lb./hr of methane fuel gas, with a fuel pressure upstream of the fuel orifice of 35-50 psig. The fuel orifice/gas spud is preferably of the type disclosed in Patent Application No. Awaited (Attorney Docket No. PM 2002B018), filed (concurrently herewith) by D. B. Spicer and G. Stephens for a Fuel Spud for High Temperature Burners, which application is hereby incorporated herein by reference.  
         [0060]    A total of 5,063 lb/hr of air (dry basis) is consumed in the burner  100 , permitting combustion of the fuel with a slight excess of air. A total of 914 lb/hr of air is drawn into the primary air chamber  110 . Steam is injected at a rate of 120 lb/hr of steam is injected in the steam injection tube  118 , and the steam pressure upstream of the spud  120  may be in the range 20-100 psig to generate a high velocity steam jet. A suitable typical pressure may be 40 psig.  
         [0061]    The action of the high velocity steam jet in the FGR venturi section  116  would inspirate approximately 800 lb/hr of flue gas into the FGR duct  112 , providing an FGR ratio of approximately 15%. The embodiments of the instant invention are designed to generate FGR ratios in the range 10-25%.  
         [0062]    In a typical ethylene furnace application, the burner  100  generates a mixture of fuel, air, flue gas and steam in the venturi section  104 . The oxygen concentration in the venturi section  104  is approximately 9% (dry volume basis) and the temperature in the venturi section  104  is approximately 700° F. The mixture in the venturi section  104  contains approximately 20% of the stoichiometric air requirement of the fuel.  
         [0063]    The mixture in the venturi section  104  exits through a series of ports or holes in the burner tip  124 . Initial combustion occurs downstream of a plurality of side ports  126 , where the combination of air. in the venturi mixture, plus the air passing between the burner tip  124  and an annular tile  128  provides sufficient air for combustion for the fuel exiting the side ports  126 . The majority of the fuel exits the burner tip  124  through a plurality of center ports  129 , generating a high velocity air-fuel-steam jet projecting into the furnace  114 . The mixture projecting into the furnace  114  is a fuel rich mixture of fuel (in this example methane) and air, diluted with flue gas and steam. Combustion occurs gradually as staged air from the staged air ports  130  mix with the air-fuel jet. FGR and steam also raise the total heat capacity, which lowers overall flame temperature, which, in turn, reduces NO x .  
       Example 2  
       [0064]    To further demonstrate the benefits of the present invention, a burner, of the type depicted in FIGS. 4 and 5 was tested. The fuel orifice/gas spud was the type disclosed in FIG. 5E of Patent Application No. Awaited (Attorney Docket No. PM 2002B018), filed (concurrently herewith) by D. B. Spicer and G. Stephens for a Fuel Spud for High Temperature Burners. The burner of this example also employed flue gas recirculation of the type described in U.S. Pat. No. 5,092,761 (as depicted in FIG. 5) and was operated at a firing rate of 6 million BTU/hr., using a fuel gas comprised of 30% H 2 /70% natural gas, without steam injection. A very stable flame was observed, with NO x  emissions measured at 67 ppm.  
       Example 3  
       [0065]    In this example, the burner of Example 2 was used. Once again, the burner employed flue gas recirculation of the type described in U.S. Pat. No. 5,092,761 and was operated at a firing rate of 6 million BTU/hr., using a fuel gas comprised of 30% H 2 /70% natural gas, with steam injected to the FGR duct (only) at a rate of 143 lb./hr. A very stable flame was observed, with NO x  emissions measured at 42 ppm.  
       Example 4  
       [0066]    Again, the burner of Example 2 was used, employing flue gas recirculation of the type described in U.S. Pat. No. 5,092,761. The burner was operated at a firing rate of 6 million BTU/hr., using a fuel gas comprised of 30% H 2 /70% natural gas, with steam injected in the region of the burner tube venturi (only) at a rate of 143 lb./hr. A very stable flame was observed, with NO x  emissions measured at 37 ppm.  
         [0067]    Although the burners of this invention have been described in connection with floor-fired hydrocarbon cracking furnaces, they may also be used in furnaces for carrying out other reactions or functions.  
         [0068]    Thus, it can be seen that, by use of this invention, NO x  emissions may be reduced in a burner. The flue gas recirculation system of the invention can also easily be retrofitted to existing burners.  
         [0069]    It will also be understood that the steam injection techniques described herein also has utility in traditional raw gas burners and raw gas burners having a pre-mix burner configuration wherein flue gas alone is mixed with fuel gas at the entrance to the burner tube. In fact, it has been found that the pre-mix, staged-air burners of the type described in detail herein can be operated with the primary air damper doors closed, with very satisfactory results.  
         [0070]    Although the invention has been described with reference to particular means, materials and embodiments, it is to be understood that the invention is not limited to the particulars disclosed and extends to all equivalents within the scope of the claims.