Patent Publication Number: US-6663380-B2

Title: Method and apparatus for advanced staged combustion utilizing forced internal recirculation

Description:
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DE-FC07-95ID13333 awarded by the U.S. Department of Energy. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for staged combustion of a fossil fuel which has the capability of reducing the formation of nitrogen oxides (NO x ) while simultaneously providing substantially complete combustion at low excess oxidant, that is, an overall stoichiometric ratio that does not exceed 1.25, although higher or lower ratios may be employed, if desired. More particularly, this invention relates to a burner for boilers and other process heating equipment such as fluid heaters, furnaces, radiant tubes, and kilns which are fueled by gaseous or liquid fuels, in which the fuel and/or oxidant are introduced in stages, and combustion products are substantially recirculated within the combustion zone. 
     2. Description of Prior Art 
     Conventional combustion of fossil fuels produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen, forming various oxides of nitrogen as byproducts of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NO x . Concern over the role of NO x  and other combustion byproducts, such as sulfur oxides, carbon monoxide, total hydrocarbons and carbon dioxide, in “acid rain” and other environmental problems has generated considerable interest in reducing the formation of these environmentally harmful byproducts of combustion. 
     Known methods of combustion for reducing NO x  emissions from combustion processes include flue gas recirculation and staged combustion. U.S. Pat. No. 4,004,875 teaches a low NO x  burner for combustion of liquid and gaseous fuels in which the combustion area is divided into at least two stages and combustion products are recirculated, cooled and reintroduced into the primary combustion zone, resulting in a reduction of NO x  emissions. The secondary combustion air is introduced into a secondary combustion zone downstream of the primary combustion zone in an amount sufficient to complete combustion therein. Fuel and primary combustion air are introduced into a primary combustion zone formed by a burner tile which provides a high temperature environment for the fuel and air mixture to promote combustion. Except for the opening into the secondary combustion zone, the tile is completely surrounded by a steel enclosure forming an annular space around the tile. Thus, as fuel and air are injected into the primary combustion zone, part of the partially combusted fuel and air is recirculated around the outside of the tile in the annular space between the tile and steel enclosure and back into the upstream end of the primary combustion zone. U.S. Pat. No. 5,350,293 teaches a combustion process and apparatus employing air staging in which at least two combustion zones, a primary combustion zone and a secondary combustion zone disposed downstream of the primary combustion zone, are formed. In addition, at least a portion of the products of combustion generated in the primary combustion zone are recirculated from a downstream region of the primary combustion zone to an upstream region thereof. U.S. Pat. No. 5,573,391 and U.S. Pat. No. 5,636,977 teach a multi-stage combustion process and apparatus in which internal recirculation of combustion products is carried out only in the secondary combustion zone and a portion of the primary, fuel-lean air/fuel mixture is introduced into the secondary combustion zone. See also U.S. Pat. No. 4,629,413 which teaches a low NO x  burner utilizing staged combustion in which a mixture of primary combustion air and fuel is introduced into a primary combustion chamber and secondary combustion air is introduced into the combustion chamber in a manner such that the mixing of the secondary combustion air with the flame generated by the mixture of fuel and primary combustion air is delayed; U.S. Pat. No. 5,044,932 which also teaches a process and apparatus for reducing the NO x  content of the flue gas effluent from a furnace in which cooled flue gases are internally recirculated from the downstream end of the combustion chamber into the upstream end of the combustion chamber where it undergoes reaction with the flame generated by the fuel and air introduced into the upstream end of the combustion chamber; U.S. Pat. No. 4,575,332 which teaches staged combustion in a swirl combustor with forced annular recycle of flue gases to the upstream end of the primary combustion zone; and U.S. Pat. No. 4,395,223 which teaches staged combustion with excess air introduced into the primary combustion zone with additional fuel being introduced into the secondary combustion zone. 
     It is also known that, in addition to limiting the oxygen available in a combustion process for formation of NO x  emissions, NO x  emissions may also be controlled by maintaining the temperature in the combustion zone below the temperature required for formation of significant amounts of NO x , about 2600° F. Cooling of the products of combustion is suggested by U.S. Pat. No. 4,004,875 discussed hereinabove. However, by recirculating the cooled partial combustion products from the downstream of the primary combustion zone to the upstream end of the primary combustion zone, any heat removed from the primary combustion zone as a result of cooling is reintroduced into the secondary combustion zone, resulting in no net heat removal from the combustion process. Consequently, the temperatures in the primary and secondary combustion zones are not maintained below the level required for significant NO x  formation. 
     In spite of numerous advances toward reducing NO x  emissions from boilers and other process heating equipment, such as fluid heaters, furnaces, radiant tubes and kilns, which are fueled by gaseous or liquid carbonaceous or hydrocarbon fuels, a substantial amount of NO x  continues to be produced by such heating equipment. As a result, there continues to be a need for further reducing the amount of NO x  produced by such heating equipment, particularly if future standards restricting NO x  emissions are to be met. 
     It is also apparent that the number of boilers and process heating apparatuses producing NO x  emissions is very large and that replacement of these apparatuses with more up-to-date equipment as a means for reducing NO x  emissions is not practical. As a result, proposed solutions for reducing NO x  emissions must be able to address issues relating to the age and physical condition of the boilers and process heating equipment, such as the lack of gas-tight seals between various portions thereof which may result in the undesirable intake of air from around the heating equipment. For boilers and process heating equipment which lack a gas-tight seal between the combustion zone and convective pass, of which tangent-tube boilers are a typical example, combustion products from the primary combustion zone can leak into the convective pass. In addition, proposed solutions should be suitable for retrofitting to the boilers and process heating equipment, regardless of the physical condition of the tube walls. 
     As previously stated, combustion processes and apparatuses in which oxidant—typically air, oxygen, or oxygen-enriched air—is introduced into the processes and apparatuses in two or more stages are well known as means for reducing NO x  emissions. However, in oxidant staging systems applied to boilers and process heating equipment lacking gas-tight seals between the combustion zone and the convective pass as previously discussed, the combustion products from the primary combustion zone that leak into the convective pass may contain high levels of CO, which can result in unacceptably high CO in the stack. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is one object of this invention to provide a method and apparatus for combustion of fossil fuels which is capable of reducing NO x  emissions compared to conventional methods and apparatuses. 
     It is another object of this invention to provide an apparatus for combustion of fossil fuels which is suitable for retrofitting to conventional boilers and process heating equipment. 
     These and other objects of this invention are addressed by a combustion apparatus comprising at least one combustion chamber wall forming at least one burner opening and enclosing a combustion chamber, and at least one multi-stage fuel/multi-stage oxidant burner having a fuel and oxidant inlet end and a fuel and oxidant outlet end attached to the at least one combustion chamber wall. At least a portion of the fuel and oxidant outlet end of the burner extends through the at least one burner opening into the combustion chamber. 
     The at least one multi-stage fuel/multi-stage oxidant burner comprises a first-stage plenum chamber wall enclosing a first-stage plenum chamber and forming a first-stage inlet for a first-stage fuel and a first-stage oxidant and a first-stage outlet for the first-stage fuel and the first-stage oxidant. The first-stage outlet is in fluid communication with the at least one burner opening. An end wall is disposed within the first-stage outlet having a periphery in contact with the first-stage plenum chamber wall and forming at least one first-stage nozzle opening and at least one second-stage nozzle opening. The at least one first-stage nozzle opening provides a fluid communication between the combustion chamber and the first-stage plenum chamber. A second-stage plenum chamber wall encloses a second-stage plenum and forms a second-stage inlet for a second-stage fuel and a second-stage oxidant and a second-stage outlet for the second-stage fuel and the second-stage oxidant. At least a portion of the second-stage plenum chamber is disposed within the first-stage plenum chamber and extends through the end wall, thereby providing fluid communication between the second-stage outlet and the combustion chamber. 
     In the method for combustion of a fossil fuel in accordance with one embodiment of this invention, a first-stage fuel and a first-stage oxidant are introduced into a combustion chamber and ignited, forming a primary combustion zone comprising first-stage combustion products. At least about 5% of the total heat output produced by combustion of the first-stage fuel and first-stage oxidant is removed from the primary combustion zone, forming cooled first-stage combustion products. A portion of the cooled first-stage combustion products is recirculated from a downstream region of the primary combustion zone to an upstream region of the primary combustion zone. A second-stage fuel is introduced into the combustion chamber downstream of the primary combustion zone, forming a secondary combustion zone and igniting the second-stage fuel, forming second-stage combustion products. At least about 5% of the heat from the secondary combustion zone is also removed. In accordance with one preferred embodiment of this invention, the first stage of oxidant comprises more than a stoichiometric requirement for complete combustion of the first-stage fuel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: 
     FIG. 1 is a partial cross-sectional lateral view of a combustion apparatus in accordance with one embodiment of this invention in which second-stage fuel and oxidant are premixed external to the burner and introduced into the combustion chamber at a point downstream of the primary combustion zone and a primary combustion zone recirculation means; 
     FIG. 2 is a partial cross-sectional lateral view of a combustion apparatus in accordance with another embodiment of this invention in which second-stage fuel and oxidant are not premixed, but rather mixed in the region where one or more second-stage fuel jets introduce second-stage fuel into a chamber containing flowing second-stage oxidant, resulting in partial mixing of the second-stage fuel and oxidant prior to entering the combustion chamber; 
     FIG. 3 is a partial cross-sectional lateral view of a combustion apparatus in accordance with yet another embodiment of this invention in which a secondary recirculation means is provided for recirculating secondary combustion products from the downstream region of the secondary combustion zone to an upstream region of the secondary combustion zone, which means can also be employed in combination with the embodiment shown in FIG. 1; and 
     FIG. 4 is a partial cross-sectional lateral view of a combustion apparatus in accordance with yet a further embodiment of this invention whereby fuel is introduced in two stages and oxidant is introduced in three stages into a combustion chamber. 
    
    
     DETAILED DESCRIPTION OF PRESENTLY PREFERRED EMBODIMENTS 
     As used herein, the term “stoichiometric ratio” is defined as the oxidant-to-fuel ratio divided by the oxidant-to-fuel ratio at stoichiometric conditions. 
     The apparatus of this invention is a burner for boilers and other process heating equipment, such as fluid heaters, furnaces, radiant tubes, and kilns, which are fueled by gaseous and/or liquid fossil fuels, which is designed to reduce the formation of NO x  emission from said boilers and process heating equipment simultaneous with complete combustion of the fuel at low excess oxidant, that is overall stoichiometric ratios preferably not exceeding about 1.25, although higher or lower ratios can be used, if desired. This design results in lower levels of NO x  in the flue gases than comparable burner designs that employ only oxidant staging, that is where fuel is introduced into the burner only in a single stage. Additionally, when compared to oxidant staging designs, this invention is advantageous for retrofit to boilers and process heating equipment which lack a gas-tight seal between the combustion zone and the convective pass. That is, implementation of this invention maintains levels of CO in the stack which are acceptable in spite of the existence of leaks between the combustion zone and the convective pass. 
     The combustion apparatus of this invention comprises a combustion chamber having a primary combustion zone and a secondary combustion zone and at least one burner comprising primary and secondary stages into which the fuel and oxidant are introduced, either premixed, partially premixed or nozzle-mixed. A plurality of first-stage nozzles are provided in an array, preferably a circular array, around the central axis of the burner. Forced internal recirculation is employed to recirculate products of combustion to the region of flame ignition in the primary combustion zone or in both the primary and secondary combustion zones. To provide recirculation of the products of combustion, a fixed component of the burner, referred to herein as a recirculation sleeve, which provides recirculation of the combustion products generated in the primary combustion zone induced by the kinetic energy of the primary combustion zone oxidant-fuel jets is provided. In addition, the recirculation sleeve also promotes stabilization of the flame and provides heat transfer by radiation from the primary combustion zone to the cooled walls of the boiler or other process heating equipment, thereby reducing the flame temperature and, consequently, reducing NO x  formation. As described hereinbelow, the burner apparatus may contain a plurality of recirculation sleeves. 
     FIG. 1 shows an exemplary combustion apparatus in accordance with one embodiment of this invention. As shown, the combustion apparatus  10  comprises at least one combustion chamber wall  12  forming at least one burner opening  34  and enclosing combustion chamber  35 . Attached to combustion chamber wall  12  is at least one multi-stage fuel/multi-stage oxidant burner  36  having a fuel/oxidant inlet end  37  and a fuel/oxidant outlet end  38 , whereby at least a portion of fuel/oxidant outlet end  38  extends through said at least one burner opening  34  into combustion chamber  35 . 
     Multi-stage fuel/multi-stage oxidant burner  36  comprises first-stage plenum chamber wall  13  enclosing first-stage plenum chamber  14  and forming a first-stage fuel/first-stage oxidant inlet  39  and a first-stage fuel/first-stage oxidant outlet  40 , which first-stage fuel/first-stage oxidant outlet is in fluid communication with burner opening  34 . Disposed within first-stage fuel/first-stage oxidant outlet  40  is end wall  11  having a periphery in contact with first-stage plenum chamber wall  13  and forming at least one first-stage nozzle opening  15 , which provides fluid communication between combustion chamber  35  and first-stage plenum chamber  14 , and a second-stage nozzle opening  41 . Second-stage plenum chamber wall  17  encloses second-stage plenum chamber  18  and forms second-stage fuel/second-stage oxidant inlet  42  and second-stage fuel/second-stage oxidant outlet  43 . At least a portion of second-stage plenum chamber  18  is disposed within first-stage plenum chamber  14  and extends through second-stage nozzle opening  41  of end wall  11  into combustion chamber  35 , thereby providing a fluid communication between second-stage fuel/second-stage oxidant outlet  43  and combustion chamber  35 . Disposed around at least a portion of the outer surface of second-stage plenum chamber wall  17  is second-stage cooling jacket wall  19  forming a second-stage cooling jacket  20  around and in contact with at least a portion of an outer surface of second-stage plenum chamber wall  17 , thereby providing cooling of second-stage plenum chamber wall  17  as well as the contents of second-stage plenum chamber  18 . The coolant circulated in second-stage cooling jacket  20  may be any fluid that provides the desired cooling. In accordance with one embodiment of this invention, the cooling fluid is selected from the group consisting of water, oil, air, nitrogen, carbon dioxide and flue gases. 
     Combustion chamber  35  comprises primary combustion zone  16  and secondary combustion zone  24  disposed downstream of primary combustion zone  16 . The at least one multi-stage fuel/multi-stage oxidant burner  36  comprises recirculation means for recirculating products of combustion from a downstream region of primary combustion zone  16  to an upstream region of primary combustion zone  16  as denoted by arrows  21 . In accordance with one preferred embodiment of this invention, said recirculation means comprises a recirculation wall  22 , preferably in the shape of a hollow cylinder, disposed in combustion chamber  35  and surrounding at least a portion of second-stage plenum chamber  18 . Recirculation wall or sleeve  22  is disposed in primary combustion zone  16  parallel to and at an axial distance from the axes of first-stage nozzle openings  15 , thereby forming recirculation annulus  23  between recirculation wall or sleeve  22  and second-stage cooling jacket wall  19  disposed around second-stage plenum chamber wall  17 . Without wishing to be bound by any particular theory or explanation of the mechanics of the recirculation of combustion products within primary combustion zone  16 , it is believed that as a result of negative pressure generated in the upstream region of primary combustion zone  16  by the kinetic energy provided by the introduction of the first-stage fuel/first-stage oxidant mixture through first-stage nozzle openings  15 , a portion of the partial products of combustion entering the downstream region of primary combustion zone  16  are recirculated through recirculation annulus  23 , as indicated by arrows  21 , and reintroduced through the upstream region of primary combustion zone  16  into primary combustion zone  16 . 
     Connected to second-stage fuel/second-stage oxidant outlet of second-stage plenum chamber  18  are a plurality of second-stage nozzles  25  through which second-stage plenum chamber  18  is in fluid communication with secondary combustion zone  24 . Second-stage nozzles are preferably disposed in an array surrounding the central axis of second-stage plenum chamber  18 , which is perpendicular to end wall  11 . A mixture of second-stage fuel and second-stage oxidant flows continuously into second-stage plenum chamber  18  and thereafter through second-stage nozzles  25  into secondary combustion zone  24  in which combustion of the second-stage fuel/second-stage oxidant mixture occurs. 
     A combustion apparatus in accordance with another embodiment of this invention is shown in FIG.  2 . As shown, the combustion apparatus  10  comprises at least one combustion chamber wall  12  forming at least one burner opening  34  and enclosing combustion chamber  35 . Attached to combustion chamber wall  12  is at least one multi-stage fuel/multi-stage oxidant burner  36  having a fuel/oxidant inlet end  37  and a fuel/oxidant outlet end  38 , whereby at least a portion of fuel/oxidant outlet end  38  extends through said at least one burner opening  34  into combustion chamber  35 . 
     Multi-stage fuel/multi-stage oxidant burner  36  further comprises second-stage oxidant plenum chamber wall  26 , which extends through end wall  11  perpendicular to the plane of end wall  11  and encloses second-stage oxidant plenum chamber  27 . Second-stage oxidant plenum chamber  27  comprises second-stage oxidant inlet end  44  and second-stage oxidant outlet end  45 . Multi-stage fuel/multi-stage oxidant burner  36  in accordance with the embodiment shown in FIG. 2 further comprises second-stage fuel plenum chamber wall  28  enclosing second-stage fuel plenum chamber  29 , a portion of which is disposed within second-stage oxidant plenum chamber  27 . Second-stage fuel plenum chamber comprises second-stage fuel inlet end  46  and second-stage fuel outlet end  47 . During combustion, second-stage oxidant enters second-stage oxidant plenum chamber  27  at second-stage oxidant inlet end  44  and second-stage fuel enters second-stage fuel plenum chamber  29  at second-stage fuel inlet end  46 . In accordance with the embodiment shown in FIG. 2, second-stage nozzles  25  are connected to second-stage oxidant outlet end  45  of second-stage oxidant plenum chamber  27  and second-stage fuel nozzles  30  are connected to second-stage fuel outlet end  47  of second-stage fuel plenum chamber  29 . Second-stage fuel nozzles  30  are disposed upstream of second-stage nozzles  25  as a result of which second-stage fuel nozzles  30  are in direct fluid communication with second-stage oxidant plenum chamber  27 . Both second-stage nozzles  25  and second-stage fuel nozzles  30  are disposed in an array about the central axis of multi-stage fuel/multi-stage oxidant burner  36 , which is perpendicular to end wall  11  of combustion chamber  35 . In operation, second-stage fuel flows through second-stage fuel nozzles  30  into second-stage oxidant plenum chamber  27  where it mixes with second-stage oxidant prior to flowing through second-stage nozzles  25  and into secondary combustion zone  24  in which combustion of the second-stage fuel/oxidant mixture occurs. 
     As in the embodiment shown in FIG. 2, multi-stage fuel/multi-stage oxidant burner  36  comprises recirculation means for recirculating products of combustion from a downstream region of primary combustion zone  16  to an upstream region of primary combustion zone  16  as denoted by arrows  21 . In accordance with one preferred embodiment of this invention, said recirculation means comprises a recirculation wall  22 , preferably in the shape of a hollow cylinder, disposed in combustion chamber  35  and surrounding at least a portion of second-stage oxidant plenum chamber  27 . Recirculation wall or sleeve  22  is disposed in primary combustion zone  16  parallel to and at an axial distance from the axes of first-stage nozzle openings  15 , thereby forming recirculation annulus  23  between recirculation wall or sleeve  22  and second-stage oxidant plenum chamber wall  26 . 
     Yet another embodiment of the combustion apparatus of this invention is shown in FIG. 3 in which multi-stage fuel/multi-stage oxidant burner  36 , substantially as shown in FIG. 2, further comprises secondary recirculation wall or sleeve  31 , preferably in the form of a hollow cylinder, disposed downstream of second-stage nozzles  25  and substantially parallel to the central axis of multi-stage fuel/multi-stage oxidant burner  36 . Although not wishing to be bound by any single theory or explanation, it is believed that, as a result of negative pressure generated at the upstream end of secondary combustion zone  24  by the kinetic energy provided by the introduction of the second-stage fuel/second-stage oxidant mixture through second-stage nozzles  25 , a portion of the secondary combustion products entering the downstream end of secondary combustion zone  24  are recirculated through secondary annulus  32 , as indicated by arrows  33 , and reintroduced through the upstream region of secondary combustion zone  24  into secondary combustion zone  24 . It will be apparent to those skilled in the art that the improvements shown in FIG. 3 may be applied to the embodiment of the combustion apparatus of this invention shown in FIG.  1 . 
     FIG. 4 shows yet a further embodiment of this invention which is particularly suitable for use in applications such as steel manufacturing in which relatively low Btu fuel gases such as blast furnace gas and coke oven gas are employed as first stage and second stage fuels. It should be noted that reference to the use of this invention in steel manufacturing applications is in no way intended to limit the scope of applications in which the invention can be employed. 
     As shown in FIG. 4, the combustion apparatus in accordance with one embodiment of this invention comprises a second stage fuel plenum chamber wall  51  enclosing a second stage fuel plenum chamber  52  in first-stage plenum chamber  14 . End wall  11  forms at least one second stage fuel nozzle opening  54  providing fluid communication between second stage fuel plenum chamber  52  and combustion chamber  35 . Fuel gases injected through nozzle openings  34  and  54  are premixed with oxidant prior to injection into combustion chamber  35 , thereby, in addition to providing first stage and second stage fuel, also providing first stage and second stage oxidant. Third-stage plenum chamber wall  67  encloses third-stage plenum chamber  66 , at least a portion of which is disposed within second-stage fuel plenum chamber  52 , and comprises third-stage oxidant inlet end  68  and third-stage oxidant outlet end  69  through which tertiary oxidant is introduced into combustion chamber  35 . 
     In accordance with the embodiment shown in FIG. 4, combustion chamber  35  comprises primary combustion zone  16  and secondary combustion zone  50 . As shown, secondary combustion zone  50  is formed within recirculation annulus  23  formed between recirculation wall  22  and third-stage plenum chamber wall  67  whereby products of combustion are recirculated from a downstream end of recirculation wall  22  to an upstream end of recirculation wall  22  as indicated by arrows  21 . Combustion chamber  35  further comprises tertiary combustion zone  65  disposed downstream of third-stage outlet end  69  of third-stage plenum chamber  66 . 
     In accordance with one embodiment of this invention, end wall  11  forms a second stage array of second-stage fuel nozzle openings  54  disposed around a central axis  80  of third-stage plenum chamber  66 , which central axis is disposed substantially perpendicular to end wall  11 , and a first-stage array of first stage fuel nozzle openings  34  concentrically disposed around said second-stage array of second-stage fuel nozzle openings  54 , thereby forming an annular ring  70  between the first-stage array and the second stage array. In accordance with one embodiment of this invention, recirculation wall  22  is disposed within an area  71  bounded by a longitudinal axis  72  of each of second-stage fuel nozzle openings  54  shown in phantom in FIG.  4 . In this embodiment, both first-stage fuel and second-stage fuel are introduced into combustion chamber  35  along the outside of recirculation wall  22 . In accordance with another embodiment of this invention, recirculation wall  22  is disposed within an annular space formed between a boundary formed by the longitudinal axis  72  of each of the second-stage fuel nozzle openings  54  and the longitudinal axis  73  of each of the first-stage fuel nozzle openings  34 . In this embodiment, second-stage fuel is introduced into recirculation annulus  23  and first-stage fuel is introduced into combustion chamber  35  along the outside of recirculation wall  22 . 
     In accordance with one embodiment of the combustion method of this invention, a first-stage fuel and a first-stage oxidant are introduced into a combustion chamber and ignited, forming a primary combustion zone comprising first-stage combustion products. At least about 5% of a total heat output produced by the combustion of the first-stage fuel and first-stage oxidant is removed from the primary combustion zone, forming cooled first-stage combustion products. A portion of the cooled first-stage combustion products are recirculated from a downstream region of the primary combustion zone to an upstream region of the primary combustion zone. A second-stage fuel is introduced into the combustion chamber downstream of the primary combustion zone, forming a secondary combustion zone and igniting the second-stage fuel, forming second-stage combustion products. At least about 5% of the heat generated by the combustion of the second-stage fuel is removed from the secondary combustion zone. In accordance with one embodiment of the method of this invention, the first-stage fuel comprises more than a stoichiometric requirement for complete combustion of the first-stage fuel. In accordance with one embodiment of this invention, a second-stage oxidant is introduced into the secondary combustion zone, preferably in an amount comprising less than the stoichiometric requirement for complete combustion of the second-stage fuel. In accordance with one particularly preferred embodiment of this invention, the amount of second-stage fuel is less than the amount of first-stage fuel. The preferred ratio of second-stage fuel to first-stage fuel is in the range of about 0.05 to 0.33 (based on heat content). The preferred stoichiometric air- to-fuel ratio in the first stage is in the range of about 1.20 to 1.40 and the stoichiometric air-to-fuel ratio in the second stage is preferably in the range of about 0.00 to 0.70. These ratios are interrelated such that the overall stoichiometric air-to-fuel ratio is in the preferred range of about 1.05 to 1.25, although higher or lower ratios may be employed if desired. In accordance with one embodiment of this invention, at least one of the first-stage oxidant and the second-stage oxidant is preheated. 
     Fuels that may be used in the method of this invention include, but are not limited to, natural gas, propane, hydrogen, producer gas, refinery gas, synthesis gas, coke oven gas, blast furnace gas, and hydrocarbon liquids. In accordance with one particularly preferred embodiment of this invention, the fuel is natural gas. Suitable oxidants for use in the method of this invention are air, oxygen-enriched air, and oxygen. In accordance with one preferred embodiment of this invention, recirculated flue gases are mixed with either the first-stage oxidant or the second-stage oxidant. 
     Laboratory test data, shown in Table 1, show that application of fuel staging in accordance with the method and apparatus of this invention results in lower NO x  and CO per unit volume of flue gas than with a similar burner employing only air staging. In addition, cooling of the primary zone further decreases emissions. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Primary 
                 Firing 
                 Overall 
                   
                   
               
               
                 Test 
                 Staging 
                 zone 
                 rate, 
                 Stoichiometric 
                 CO, 
                 NO x   
               
               
                 No. 
                 type 
                 cooling 
                 MM Btu/h 
                 Ratio 
                 vppm* 
                 vppm* 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 1 
                 Air only 
                 No 
                 4.0 
                 1.09 
                 24.9 
                 12.7 
               
               
                 2 
                 Fuel/air 
                 No 
                 4.0 
                 1.08 
                 2.4 
                 8.2 
               
               
                 3 
                 Fuel/air 
                 Yes 
                 4.0 
                 1.08 
                 0.1 
                 6.7 
               
               
                   
               
               
                 *Corrected to 3% O 2    
               
            
           
         
       
     
     While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.