Burner with high flow area tip

Abstract

An improved burner and a method for combusting fuel in burners used in furnaces, such as those used in steam cracking, are disclosed. The burner includes a burner tube having an upstream end and a downstream end and means for drawing flue gas from the furnace or air from a source of air or mixtures thereof in response to an inspirating effect of uncombusted fuel flowing through the burner tube from its upstream end towards its downstream end. A burner tip is mounted on the downstream end of the burner tube adjacent a first opening in the furnace, the burner tip having a plurality of main ports in an external surface thereof so that combustion of the fuel takes place downstream of the burner tip, the number and dimensions of said main ports in said external surface being such that the ratio of the total area of the main ports in said external surface is at least 1 square inch per million (MM) Btu/hr burner capacity. The inspirating effect of the fuel flowing though the burner tube can optionally be further assisted by steam injection upstream of the burner tube.

Description

RELATED APPLICATIONS

[0001] This patent application claims priority from Provisional Application Serial No. 60/365,227, filed on Mar. 16, 2002, the contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to an improved burner of the type employed in high temperature furnaces. More particularly, the invention relates to a burner having a tip with increased flow area so as to allow increased flue gas recirculation and thereby reduce NOx emissions.

BACKGROUND OF THE INVENTION

[0003] As a result of the interest in recent years to reduce the emission of pollutants from burners of the type used in large industrial furnaces, significant improvements have been made in burner design. In the past, burner design improvements were aimed primarily at improving heat distribution to provide more effective heat transfer. However, increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants.

[0004] Oxides of nitrogen (NOx) are formed in air at high temperatures. These compounds include, but are not limited to, nitrogen oxide and nitrogen dioxide. Reduction of NOx emissions is a desired goal to decrease air pollution and meet government regulations.

[0005] The rate at which NOx is formed is dependent upon the following variables: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. The rate of formation of NOx increases as flame temperature increases. However, the reaction takes time and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less NOx than the same mixture at a lower temperature, over a longer period of time.

[0006] A strategy for achieving lower NOx emission levels is to install a NOx 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.

[0007] 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.

[0008] Gas fired burners can be classified as either premix or raw gas burners, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.

[0009] 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. In addition, many raw gas burners produce luminous flames.

[0010] 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.

[0011] 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.

[0012] One technique for reducing NOx that has become widely accepted in industry is known as combustion staging. With combustion 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 NOx formation than an air-fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature can dramatically reduce NOx 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.

[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] U.S. Pat. No. 4,629,413 discloses a premix burner that employs combustion staging to reduce NOx emissions. The premix burner of U.S. Pat. No. 4,629,413 lowers NOx emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The entire contents of U.S. Pat. No. 4,629,413 are incorporated herein by reference.

[0015] U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NOx emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Air flow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The flue gas recirculating system may be retrofitted into existing burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.

[0016] Analysis of burners of the type disclosed in U.S. Pat. No. 5,092,761 has shown that the flue gas recirculation (FGR) ratio is generally in the range of 5 to 10%, where the FGR ratio is defined as: 1 FGR ⁢   ⁢ ratio ⁢   ⁢ ( % ) = 100 × [ ( l ⁢ b .   ⁢ of ⁢   ⁢ flue ⁢   ⁢ gas ⁢   ⁢ drawn ⁢   ⁢ into ⁢   ⁢ venturi ) ⁢   ( l ⁢ b .   ⁢ fuel ⁢   ⁢ combusted ⁢   ⁢ in ⁢   ⁢ burner + lb .   ⁢ air ⁢   ⁢ drawn ⁢   ⁢ into ⁢   ⁢ burner ) ]

[0017] The ability of existing burners of this type to generate higher FGR ratios is limited by the inspirating capacity of the fuel orifice/gas spud/venturi combination. Although further closing of the primary air dampers can further reduce the pressure in the primary air chamber and thereby enable increased FGR ratios; the resultant reduction of primary air flow is such that insufficient oxygen is present in the venturi for acceptable burner stability.

[0018] There is therefore a need to provide a burner that allows improved flue gas recirculation while minimizing any accompanying reduction in burner stability. In accordance with the invention, this result is achieved by increasing the flow area of the burner tip. It is to be appreciated that, in designing a burner tip, it is normal to target a specific velocity for the fuel/air mixture flowing through the tip. The velocity should be high enough to prevent flashback, but low enough to prevent the flame lifting off the tip.

SUMMARY OF THE INVENTION

[0019] In one aspect, the present invention is directed to a burner for the combustion of fuel in a furnace, said burner comprising: (a) a burner tube having a downstream end and an upstream end; (b) a fuel orifice located adjacent the upstream end of said burner tube, for introducing fuel into said burner tube; (c) a burner tip mounted on the downstream end of said burner tube and adjacent a first opening in the furnace, said burner tip having a plurality of main ports in an external surface thereof so that combustion of the fuel takes place at said external surface of said burner tip, the number and dimensions of said main ports in said external surface being such that the total area of the main ports in said external surface is at least 1 square inch per million (MM) Btu/hr burner capacity; and (d) means for drawing flue gas from said furnace and primary air from a source of air in response to an inspirating effect of uncombusted fuel flowing through said burner tube from its upstream end towards its downstream end. In one embodiment, a venturi includes a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3.

[0020] In another embodiment, ratio of the length to maximum internal cross-sectional dimension of said throat portion is from about 4 to about 10, more preferably from about 4.5 to about 8 and most preferably from about 6.5 to 7.5.

[0021] In a further aspect, the invention resides in a method for combusting fuel in a burner of a furnace, comprising the steps of: (a) combining fuel with air, flue gas or a mixture thereof at a predetermined location adjacent a fuel orifice; (b) passing the fuel and air, flue gas or mixture thereof through a burner tube; (c) discharging the fuel and air, flue gas or mixture thereof at a burner tip downstream of the predetermined location, the burner tip having a plurality of main ports in an external surface thereof and a plurality of further side burner openings in a peripheral surface thereof; and (d) combusting the fuel at the external surface of said burner tip, wherein the total area of the main ports in said external surface of the burner tip is at least 1 square inch per million (MM) Btu/hr burner capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is further explained in the description that follows with reference to the drawings wherein:

[0023] FIG. 1 is an elevation partly in section of a burner according to the present invention;

[0024] FIG. 2 is an elevation partly in section taken along line 2-2 in FIG. 1;

[0025] FIG. 3 is a perspective view of the burner tip of the FIG. 1 embodiment;

[0026] FIGS. 4A and 4B are plan views of the tip of the burner of the FIG. 1 embodiment and the tip of a conventional burner, respectively;

[0027] FIG. 5 is an elevation view of a burner according to another embodiment of the present invention employing an external flue gas recirculation duct;

[0028] FIG. 6 is an elevation partly in section of a flat-flame burner according to a further example of the present invention; and

[0029] FIG. 7 is an elevation partly in section taken along line 7-7 of FIG. 6.

[0030] FIGS. 8A and 8B are plan views of the tip of the burner of the FIG. 6 embodiment and the tip of a conventional burner, respectively;

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0031] 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.

[0032] Referring to FIGS. 1 to 3, the burner of this embodiment of the invention includes a freestanding burner tube 12 located in a well in a furnace floor 14. Burner tube 12 includes an upstream end 16, a downstream end 18 and a venturi 19. Burner tip 20 is located at downstream end 18 of tube 12 and is surrounded by an annular tile 22. A fuel orifice 11, which may be located within gas spud 24 is located at upstream end 16 of tube 12 and introduces fuel into burner tube 12. Fresh or ambient air is introduced into primary air chamber 26 through adjustable damper 28 to mix with the fuel at upstream end 16 of burner tube 12. Combustion of the fuel/air mixture occurs downstream of the burner tip 20.

[0033] Burner tip 20 has an upper end 66 which, when installed, faces the burner box and a lower end 68 adapted for mating with the downstream end 18 of burner tube 12. Mating of the lower end 68 of burner tip 20 to the burner tube 12 can be achieved by swaging or, more preferably, by welding or threaded engagement.

[0034] Referring specifically to FIG. 3, the upper end 66 of the burner tip 20 includes a plurality of main ports 64 in a centrally disposed end surface 69 and a plurality of side ports 68 in an annular side surface 60. In operation, the side ports 68 direct a portion of the fuel/air mixture across the face of the tile 22, whereas the main ports 64 direct the major portion of the mixture into the furnace.

[0035] Referring now to FIG. 2, a plurality of air ports 30 originate in secondary air chamber 32 and pass through furnace floor 14 into the furnace. Fresh air enters secondary air chamber 32 through adjustable dampers 34 (see FIG. 1) and passes through staged air ports 30 into the furnace to provide secondary or staged combustion.

[0036] In order to recirculate flue gas from the furnace to the primary air chamber, FGR duct 76 extends from opening 82, in the floor of the furnace into the primary air chamber 26. Alternatively, multiple FGR ducts may be used instead of a single FGR duct 76. Flue gas is drawn through duct 76 by the inspirating effect of fuel passing through venturi 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 NOx emissions. 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.

[0037] Unmixed low temperature ambient air, having entered secondary air chamber 32 through dampers 34 and having passed through air ports 30 into the furnace, is also drawn through duct 76 into the primary air chamber 26 by the inspirating effect of the fuel passing through venturi 19. The ambient air may be fresh air as discussed above. The mixing of the ambient air with the flue gas lowers the temperature of the hot flue gas flowing through duct 76 and thereby substantially increases the life of the duct and permits use of this type of burner to reduce NOx emission in high temperature cracking furnaces having flue gas temperature above 1900° F. in the radiant section of the furnace.

[0038] Advantageously, a mixture of from about 20% to about 80% flue gas and from about 20% to about 80% ambient air should be drawn through recirculation duct 76. It is particularly preferred that a mixture of about 50% flue gas and about 50% ambient air be employed. The desired proportions of flue gas and ambient air may be achieved by proper sizing, placement and/or design of duct 76, and air ports 30, as those skilled in the art will readily recognize. That is, the geometry and location of the air ports may be varied to obtain the desired percentages of flue gas and ambient air.

[0039] A lighting port 50 is provided in the primary chamber 26, both to allow inspection of the interior of the burner assembly, and to provide access for lighting of the burner with lighting element (not shown). As shown, a tube 84 provides access to the interior of secondary air chamber 32 for an optional pilot 86.

[0040] Referring now to FIGS. 4A and 4B, the upper end 66 of the burner tip 20 of FIG. 1 is shown in FIG. 4A, whereas FIG. 4B shows the upper end 66 of a conventional burner tip 20. It will be seen that the number and size of the main ports 64 in the centrally disposed end surface 69 of the burner tip of the invention are significantly larger than those of the conventional tip. In particular, the number and dimensions of the main ports 64 in the tip of the invention are such that the total area of the main ports 64 in the end surface 69 is at least 1 square inch, preferably at least 1.2 square inch, per million (MM) Btu/hr burner capacity. In contrast, in the conventional burner tip shown in FIG. 4B, the total area of the main ports 64 in the end surface 69 is less than 1 square inch per million (MM) Btu/hr burner capacity. In one practical embodiment of a burner tip according to the invention, wherein the design firing rate of the burner is 6.0 MMBtu/hr, the total area of the main ports 64 in the end surface 69 is 8.4 in2 whereas, in the conventional burner tip for use at the same design firing rate, the total area of these openings is only 5.8 in2.

[0041] Intuitively, it would be expected that raising the tip flow area would proportionally reduce tip velocity, but instead, it is found that the drop in velocity can be mitigated by the fact that raising tip flow area raises FGR.

[0042] Increasing the total area of the main ports 64 in the burner tip 20 increases the flow area of the burner tip 20, which in turn enables higher FGR, rates to be induced without increasing the velocity for the fuel/air mixture flowing through the tip. In this way, stable operation of the burner can be retained with higher FGR rates.

[0043] In a preferred embodiment of the invention, the venturi 19 of the burner shown in FIG. 1 includes a throat portion 19a that is of substantially constant internal cross-sectional dimensions along its length and a divergent cone portion 19b, wherein the ratio of the length to maximum internal cross-sectional dimension of the throat portion 19a is least 3, preferably from about 4 to about 10, more preferably from about 4.5 to about 8 and most preferably from about 6.5 to about 7.5. Increasing the ratio of the length to internal cross-sectional dimensions in the throat portion of the venturi allows the venturi to induce more flue gas recirculation thereby reducing flame temperature and NOx production. In addition, the increased flue gas recirculation mitigates the reduction of the tip velocity that results with the increased opening area in the tip. A longer venturi throat also promotes better flow development and hence improved mixing of the fuel/air stream prior to the mixture exiting the burner tip 20. Better mixing of the fuel/air stream also contributes to NOx reduction by producing a more evenly developed flame and hence reducing peak temperature regions.

[0044] In addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is through the use of steam injection. Steam can be injected in the primary air chamber through stream injection tube 15 or the secondary air chamber (not shown). Preferably, steam is injected upstream of the venturi 19.

[0045] Referring to FIG. 5, another embodiment of the present invention is illustrated in which the high flow area burner tip described above is employed with an external FGR duct 376 communicating with an exhaust 300 of the furnace. It will be understood by one of skill in the art that several burners may be located within the furnace, all of which may be connected to furnace exhaust 300 through the external FGR duct 376.

[0046] Benefits similar to those described above through the use of the burner tip of the present invention can also be achieved in flat-flame burners, as will now be described by reference to FIGS. 6 and 7.

[0047] A burner 110 includes a freestanding burner tube 112 located in a well in a furnace floor 114. Burner tube 112 includes an upstream end 116, a downstream end 118 and a venturi portion 119. Burner tip 120 is located at downstream end 118 and is surrounded by a peripheral tile 122. A fuel orifice 111, which may be located within gas spud 124 is located at upstream end 116 and introduces fuel into burner tube 112. Fresh or ambient air may be introduced into primary air chamber 126 to mix with the fuel at upstream end 116 of burner tube 112. Combustion of the fuel and fresh air occurs downstream of burner tip 120. Fresh secondary air enters secondary chamber 132 through dampers 134.

[0048] In order to recirculate flue gas from the furnace to the primary air chamber, a flue gas recirculation passageway 176 is formed in furnace floor 114 and extends to primary air chamber 126, so that flue gas is mixed with fresh air drawn into the primary air chamber from opening 180 through dampers 128. Flue gas containing, for example, 0 to about 15% O2 is drawn through passageway 176 by the inspirating effect of fuel passing through venturi portion 119 of burner tube 112. Primary air and flue gas are mixed in primary air chamber 126, which is prior to the zone of combustion.

[0049] A small gap exists between the burner tip 120 and the burner tile 122. By keeping this gap small, the bulk of the secondary staged air is forced to enter the furnace through staged air ports (not shown) located some distance from the primary combustion zone, which is located immediately on the furnace side of the burner tip 120.

[0050] In operation, fuel orifice 111, which may be located within a gas spud 124 discharges fuel into burner tube 112, where it mixes with primary air, recirculated flue-gas or a mixture thereof, before being discharged from burner tip 120. The mixture in the venturi portion 119 of burner tube 112 is maintained below the fuel-rich flammability limit; i.e. there is insufficient air in the venturi to support combustion. Staged, secondary air is added to provide the remainder of the air required for combustion. The majority of the staged air is added a finite distance away from the burner tip 120 through staged air ports (not shown). However, a portion of the staged, secondary air passes between the burner tip 120 and the peripheral tile 122 and is immediately available for combustion.

[0051] Referring now to FIGS. 8A and 8B, the upper end 166 of the burner tip 120 of FIG. 6 is shown in FIG. 8A, whereas FIG. 8B shows the upper end 166 of a conventional burner tip 120. It will be seen that the number and size of the main ports 164 in the centrally disposed end surface 169 of the burner tip of the invention are significantly larger than those of the conventional tip. In particular, the number and dimensions of the main ports 164 in the tip of the invention are such that the total area of the main ports 164 in the end surface 169 is at least 1 square inch, preferably at least 1.2 square inch, per million (MM) Btu/hr burner capacity. In contrast, in the conventional burner tip shown in FIG. 8B, the total area of the main ports 164 in the end surface 169 is less than 1 square inch per million (MM) Btu/hr burner capacity. In addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is through the use of steam injection. Steam can be injected in the primary air chamber through stream injection tube 15 or the secondary air chamber (not shown). Preferably, steam is injected upstream of the venturi 19.

[0052] In addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is through the use of steam injection. Steam can be injected into the primary air chamber through stream injection tube 184 or into the secondary air chamber (not shown). Preferably, steam is injected upstream of the venturi 119.

[0053] It will also be understood that the burner tip 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.

[0054] Although illustrative embodiments have been shown and described, a wide range of modification change and substitution is contemplated in the foregoing disclosure and in some instances, some features of the embodiment may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the embodiments disclosed herein.

Claims

1. A burner for the combustion of fuel in a furnace, said burner comprising:

(a) a burner tube having a downstream end and an upstream end;

(b) a fuel orifice located adjacent the upstream end of said burner tube, for introducing fuel into said burner tube;

(c) a burner tip mounted on the downstream end of said burner tube and adjacent a first opening in the furnace, said burner tip having a plurality of main ports in an external surface thereof so that combustion of the fuel takes place at said external surface of said burner tip, the number and dimensions of said main ports in said external surface being such that the total area of the main ports in said external surface is at least 1 square inch per million (MM) Btu/hr burner capacity; and

(d) means for drawing flue gas from said furnace or air from a source of air or mixtures thereof in response to an inspirating effect of uncombusted fuel flowing through said burner tube from its upstream end towards its downstream end.

2. The burner according to claim 1, wherein the burner is a pre-mix burner.

3. The burner according to claim 1, wherein the burner is a flat-flame burner.

4. The burner according to claim 1, wherein said means for drawing flue gas from said furnace is in fluid communication with an external FGR duct.

5. The burner of claim 1, wherein the total area of the main ports in said external surface is at least 1.2 square inch per million (MM) Btu/hr burner capacity.

6. The burner of claim 1, further comprising a venturi intermediate said upstream end and said downstream end of said burner tube.

7. The burner of claim 6, wherein said venturi includes a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3.

8. The burner of claim 6, wherein the ratio of the length to maximum internal cross-sectional dimension of said throat portion is from about 4 to about 10.

9. The burner of claim 6, wherein the ratio of the length to maximum internal cross-sectional dimension of said throat portion is from about 4.5 to about 8.

10. The burner of claim 6, wherein the ratio of the length to maximum internal cross-sectional dimension of said throat portion is from about 6.5 to about 7.5.

11. The burner according to claim 7, including one or more steam tubes terminating adjacent the upstream end of said burner tube for introducing steam into said burner tube.

12. The burner according to claim 1, including one or more steam tubes terminating adjacent the upstream end of said burner tube for introducing steam into said burner tube.

13. The burner according to claim 11, further comprising at least one air port for introducing staged secondary air.

14. The burner according to claim 1, further comprising at least one air port for introducing staged secondary air.

15. The burner according to claim 14 wherein said fuel orifice is located within a gas spud.

16. The burner according to claim 1 wherein said fuel orifice is located within a gas spud.

17. A method for combusting fuel in a burner of a furnace, comprising the steps of:

(a) combining fuel with air, flue gas or a mixture thereof at a predetermined location adjacent a fuel orifice;

(b) passing the fuel and air, flue gas or mixture thereof through a burner tube;

(c) discharging the fuel and air, flue gas or mixture thereof at a burner tip downstream of the predetermined location, the burner tip having a plurality of main ports in an external surface thereof and a plurality of further side ports in a peripheral surface thereof; and

(d) combusting the fuel downstream of said burner tip,

wherein the total area of the main ports in said external surface of the burner tip is at least 1 square inch per million (MM) Btu/hr burner capacity.

18. The method according to claim 17, wherein the burner is a pre-mix burner.

19. The method according to claim 17, wherein the burner is a flat-flame burner.

20. The method according to claim 17, wherein the burner further comprises an external FGR duct.

21. The method according to claim 17, further comprising the step of injecting steam into the burner tube to mix with the fuel and air, flue gas or mixtures thereof upstream of said zone of combustion.

22. The method according to claim 21 wherein the furnace is a steam-cracking furnace.

23. The method according to claim 17 wherein the furnace is a steam-cracking furnace.

24. The method of claim 17 wherein said fuel orifice is located within a gas spud.

25. The method of claim 17 wherein the total area of the main ports in said external surface is at least 1.2 square inch per million (MM) Btu/hr burner capacity.