[0002] As a result of the interest in recent years to reduce the emission of polluants from burners used in large furnaces and boilers, 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.

[0003] 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. In recent years, a wide variety of mobile and stationary sources of NOX emissions have come under increased scrutiny and regulation.

[0004] One 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, it represents a less desirable alternative to improvements in burner design.

[0005] Burners used in large industrial furnaces may use either liquid or gaseous fuel. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and mix combustion air with the fuel at the zone of combustion.

[0006] 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.

[0007] Raw gas burners inject fuel directly into the air stream, such that 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.. In addition, many raw gas burners produce luminous flames.

[0008] 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.

[0009] 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.

[0010] In gas fired industrial furnaces, NO, is formed by the oxidation of nitrogen drawn into the burner with the combustion air stream. The formation of NOx 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 NOX emissions.

[0011] One technique for reducing NOx 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 NOx 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 NOx. Since NOX formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature 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 as well.

[0012] 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.

[0013] 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.

[0014] Another technique for achieving lower flame temperatures involves 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.

[0015] U. S. Patent 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 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 02 in the combustion air, which lowers flame temperature and thereby reduces NOX emissions.

[0016] Analysis of burners of the type described in U. S. Patent 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)] where G = Flue-gas drawn into venturi, (lb) F = Fuel combusted in burner, (lb), and A = Air drawn into burner, (lb).

[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. Further closing of the primary air dampers will produce lower pressures in the primary air chamber and thus enable increased FGR ratios. However, when the ratio of FGR is increased, the flame becomes more susceptible to entrainment into the FGR duct, which raises combustion temperature, which, in turn raises NOx and may cause damage to metal parts.

[0018] Commercial experience and modeling have shown when flue gas recirculation rates are raised, there is a tendency of the flame to be drawn into the FGR duct. Often, it is this phenomenon that constrains the amount of flue gas recirculation. When the flame enters directly into the flue gas recirculation duct, the temperature of the burner venturi tends to rise, which raises flame speed and causes the recirculated flue gas to be less effective in reducing NOX. From an operability perspective, the flue gas recirculation rate needs to be lowered to keep the flame out of the FGR duct to preserve the life of the metallic FGR duct. [0019] Therefore, what is needed is a burner for the combustion of fuel wherein the amount of FGR can be increased without the problems associated with flame entrainment into the FGR duct, yielding further reductions in NOxemissions.

[0020] The present invention relates to a burner for use in furnaces such as in steam cracking. The burner is located within a first flame opening in a furnace and includes a primary air chamber, a burner tube including a downstream end, an upstream end in fluid communication with the primary air chamber and a burner tip mounted on the downstream end of the burner tube and directed to the first flame opening in the furnace so that combustion of the fuel takes place downstream of the burner tip, at least one flue gas recirculation duct having a first end at a second opening in the furnace and a second end opening into the primary air chamber, and a wall extending into the furnace between the first flame opening and the first end of the flue gas recirculation duct to substantially lengthen a flow path therebetween and thereby providing a substantial barrier to flow [0021] In addition to the use of a wall as described above, the present invention effectively moves the entrance of the FGR duct opening further away from the flame to avoid or at least minimize flame entrainment. Therefore, the amount of flue gas recirculation can be increased to reduce overall flame temperature and therefore reduce NOx production.

[0022] 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: FIG. 1 illustrates an elevation partly in section of an embodiment of the burner in accordance with the present invention; FIG. 2 is an elevation partly in section taken along line 2--2 of FIG.

1; FIG. 3A is a plan view taken along line 3--3 of FIG. 1; FIG. 3B illustrates an alternate embodiment of the present invention employing a curved wall as opposed to the straight wall in FIG. 3A; FIG. 4 illustrates an elevation partly in section of another embodiment of the burner with a separation wall in accordance with the present invention; FIG. 5 is an elevation partly in section taken along line 5--5 of FIG.

4; FIG. 6 is a plan view taken along line 6--6 of FIG. 4; FIG. 7 is a perspective view of a separation wall in accordance with the instant invention; FIG. 8 is an elevation view of an embodiment of the present invention employing external FGR; FIG. 9 is a plan view of an embodiment of the present invention employing external FGR; FIG. 10 illustrates an elevation partly in section of an embodiment of a flat-flame burner of the present invention; FIG. 11 is an elevation partly in section of the embodiment of a flat- flame burner of FIG. 10 taken along line 11-11 of FIG. 10; FIG. 12 illustrates an elevation partly in section of another embodiment of a flat-flame burner of the present invention; and FIG. 13 is an elevation partly in section of the embodiment of a flat- flame burner of FIG. 12 taken along line 13--13 of FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0023] 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.

[0024] Referring 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 within gas spud 24 is at the top end of a gas fuel riser 65 and 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 opening 80 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 the burner tip 20.

[0025] A plurality of air ports 30 (FIGS. 2 and 3A and 3B) 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. Patent No. 4,629, 413.

[0026] 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 flue gas recirculation (FGR) duct 76 into a primary air chamber 26 by the inspirating effect of the fuel passing through the venturi portion 19. The duct 76 is shown as a metallic FGR duct.

[0027] As shown in FIG. 1, an aspect of the instant invention angles the FGR duct 76 outwardly at 84 such that the opening 82 of the duct 76 is physically further spaced away from the base of the burner tip 20. The angled FGR duct inlet 84 thus avoids or at least reduces the potential for the burner flame to be entrained into the FGR duct 76. This embodiment enables higher flue gas recirculation (FGR) rates to be induced into the burner 10. Such higher FGR rates, in turn, reduce overall flame temperature and NOX production.

[0028] With reference to FIG. 3A and FIG. 3B, a flame opening 23 is circular and has a radius R, and the distance that the. distance (d) that the duct opening 82 is laterally spaced from the flame opening 23 is defined by d > 0.5R for avoiding entrainment of the flame into the duct opening 82.

[0029] The angle outward at 84 also permits the continued use of the relatively small burner box 17. It should be noted that such FGR burners may be in the order of 6 feet in height by 3 feet in width.

[0030] Referring to FIGS. 2 and 5, 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. This is accomplished through steam injection tubes 15, which may or may not be present. Steam can be injected in the primary air or the secondary air chamber. Preferably, steam may be injected upstream of the venturi portion 19.

[0031] An optional embodiment of the invention serves to further increase the effective distance between the opening 82 of the FGR duct 76 and the base of the burner flame. In this embodiment, a physical wall 95 is installed between the burner tip 20 and the opening 82 to the FGR duct 76. The wall 95 also avoids or at least reduces the potential for the burner flame to be entrained into the FGR duct 76, and therefore enables higher flue gas recirculation (FGR) rates to be induced into the burner 10.

Such higher FGR rates, in turn, reduce overall flame temperature and NOx production. According to the teachings of the present invention, wall 95 may be straight as shown in FIG. 3A, curved as shown in FIG. 3B or other shapes as would be obvious to one of skill in the art.

0032] Flue gas containing, for example, 0 to 15% Os is drawn from near the furnace floor through the duct 76 with 5 to 15% Os preferred, 2 to 10% Oz more preferred and 2 to 5% Os 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, and as a result, reducing NOx emissions.

[0033] 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.

[0034] Advantageously, a mixture of from 20% to 80% flue gas and from 20% to 80% ambient air should be drawn through passageway 76. It is particularly preferred that a mixture of 50% flue gas and 50% ambient air be employed. The desired proportions of flue gas and ambient air may be achieved by proper placement and/or design of the duct 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.

[0035] 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.

[0036] Referring now to FIGS. 4-7, another embodiment of the present invention is discussed. In this embodiment, 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 at the top end of a gas fuel riser 65 and 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 the burner tip 20.

[0037] A plurality of air ports 30 (FIGS. 5 and 6) 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. Patent No. 4,629, 413.

[0038] 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.

[0039] With reference to FIGS. 4-7, a wall 60 encircles the burner tip 20 mounted on the downstream end 18 of the burner tube 12 to provide a barrier between a base of a flame downstream of the burner tip 20 and both the second opening 76 in the furnace and the at least one air port 30.

[0040] In one embodiment of the present invention, the wall 60 is perforation-free to prevent flow of flue gas and air therethrough. In another embodiment of the instant invention, the wall 60 has a plurality of wall openings 61 spaced therearound. In the embodiment shown, the openings 61 are rectangular in shape, and are located at the base of the wall 60 and spaced around the wall. The advantage achieved by the use of the openings 61 lies in their alignment, which is optimized to reduce the amount of oxygen from the staged air ports 30 that reaches the flame, reducing the level of interaction of oxygen with the flame. As may be appreciated by one skilled in the art, when aligned in this manner, each wall opening is aligned so as to maximize the effective flow path from the air ports to the flame. However, flue gas is permitted to advantageously enter the flame, enabling a reduction in NOX emission levels.

[0041] In accordance with a preferred embodiment of the present invention, each one of the air ports 30 is positioned between adjacent wall openings 61 to minimize the amount of oxygen flowing from the air ports 30 through the wall openings 61 to the base of the flame.

[0042] Sight and lighting port 50 provides access to the interior of burner 10 for a lighting element (not shown).

[0043] Referring to FIGS. 8 and 9, wherein like numbers refer to like elements, another embodiment of the present invention is illustrated. In this embodiment, the teachings above with respect to the separation wall and angled duct of the present invention may be applied in connection with a furnace having one or more burners and 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 may be located within the furnace, all of which may feed furnace exhaust 300 into external FGR duct 376. As may be appreciated, wall 60, encircling the burner tip 20 mounted on the downstream end 18 of the burner tube 12 provides a barrier between a base of a flame at the burner tip 20 and the at least one air port 30.

[0044] Benefits similar to those described above through the use of the flue gas recirculation system of the present invention can be achieved in flat-flame burners, as will now be described by reference to FIGS. 10 through 13. FIGS. 10 and 11 illustrate a flat-flame burner embodiment similar to the novel burner arrangement illustrated in FIG. 1 while FIGS. 12 and 13 illustrate a flat-flame burner embodiment similar to the novel burner arrangement illustrated in FIG. 4.

[0045] A burner 110 includes a freestanding burner tube 112 located in a well in a furnace floor 114. Burner tube 11, 2 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.

[0046] 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 15% 02 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.

[0047] In operation, fuel orifice 111, which may be, located within gas spud 124 discharges fuel into burner tube 112, 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 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. 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 120 [0048] Optionally, one or more steam injection tubes 115 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 119 for inducing the flow of fuel, steam and flue gas, air and mixtures thereof into the burner tube 112.

[0049] Referring now to FIGS. 10-11, as with a previous embodiment, the FGR duct 176 may be angled outwardly at 184 such that the opening 182 of the duct 176 is physically further spaced away from the base of the burner tip 120. The angled FGR duct inlet 184 thus avoids or at least reduces the potential for the burner flame to be entrained into the FGR duct 176. This enables higher flue gas recirculation (FGR) rates to be induced into the burner 110. Such higher FGR rates, in turn, reduce overall flame temperature and NO,, production.

[0050] The angle outward at 184 also permits the continued use of the relatively small burner box 117. It should be noted that such FGR burners may be on the order of 6 feet in height by 3 feet in width.

[0051] Still referring now to FIGS. 10-11, the benefits of the present invention in connection with a flat-flame burner embodiment may be further enhanced by increasing the effective distance between the opening 182 of the FGR duct 176 and the base of the burner flame. In this embodiment, a physical wall 195 as described above is installed between the burner tip 120 and the opening 182 to the FGR duct 176. The wall also avoids or at least reduces the potential for the burner flame to be entrained into the FGR duct 176, and therefore enables higher flue gas recirculation (FGR) rates to be induced into the burner 110. Such higher FGR rates, in turn, reduce overall flame temperature and NOx production.

[0052] In the context of a flat-flame burner embodiment, as represented by FIGS. 12 and 13 (similar to the FIG. 4 embodiment), a wall 160 peripheral surrounds the burner tip 120 mounted on the upstream end 116 of the burner tube 112 to provide a barrier between a base of a flame at the burner tip 120 and both of the second opening 176 in the furnace and the at least one air port (not shown). The wall 160 reduces the amount of oxygen flowing into the base of said flame.

EXAMPLES Example 1 [0053] To demonstrate the benefits of the present invention, a pre- mix burner employing flue gas recirculation, of the type described in U. S.

Patent No. 5,092, 761, without a wall encircling the burner tip to provide a barrier between the base of the flame and both the flue gas recirculation duct and the secondary air ports, of the present invention, was operated at a firing rate of 5.8 million BTU/hr. , using a fuel gas comprised of 30% H2/70% natural gas. The burner yielded NOx emissions of 49 ppm.

Example 2 [0054] A wall encircling the burner tip to provide a barrier between the base of the flame and both the flue gas recirculation duct opening and the secondary air ports of the present invention, was installed in the pre- mix burner of Example 1. The burner was operated at a firing rate of 6.135 million BTU/hr. , with a fuel gas comprised of 30% H2/70% natural gas. The NOx emissions were observed to be 46.11 ppm.

[0055] Computational fluid dynamics modeling and, as indicated above, actual tests on a commercial unit have shown that the existing design, without a wall peripherally surrounding the burner tip, possesses a high concentration oxygen zone in the furnace above the FGR duct (s). It is believed that a part of this oxygen flows into the base of the flame and may be responsible for higher NOx production as a result of the large amount of oxygen interacting with the flame base. While it is believed that such co- current flow causes good mixing and high combustion rate, higher temperatures and higher levels of NOx emissions likely result from this effect.

In an effort to solve this problem, it has been discovered that the use of a wall, in accordance with the present invention, between the flame and the oxygen recirculation zone can serve to greatly reduce the interaction between the two.

[0056] 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.

[0057] It will also be understood that the flue gas recirculation system and methodologies described herein also has utility in 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.

[0058] 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.