Burner assembly for gaseous fuels

Abstract

A burner assembly and method for combusting gaseous fuels includes a combustion air inlet, a mixing tube assembly spaced from a heater or furnace wall to define an annular gap, and primary, secondary, and tertiary fuel circuits. Combustion air is discharged into the mixing tube assembly and acts as a primary fluid that entrains flue gas and an upstream‑injected tertiary fuel stream from a heater or furnace volume through the annular gap before combustion. The tertiary fuel is injected into the heater or furnace volume in a direction having an upstream component relative to a longitudinal burner flow axis, such that the tertiary fuel mixes with flue gas outside the burner body before entrainment. The entrained flue gas and tertiary fuel are mixed with combustion air and a secondary fuel stream to form a diluted combustible mixture that is ignited downstream to produce a stable, low‑NOx flame without post‑combustion treatment.

Description

Docket No.: P3037PC01 BURNER ASSEMBLY FOR GASEOUS FUELS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Patent Application No. 63 / 761,710, filed February 21, 2025, and incorporates by reference the provisional application in its entirety.BACKGROUND OF THE INVENTION1. Field of the Invention.

[0002] The present disclosure relates generally to burner assemblies for gaseous fuels. More particularly, the disclosure relates to burner assemblies for use in boilers, industrial furnaces, dircct-fircd heaters, flares, thermal oxidizers, and other enclosed combustion devices, and to burner assemblies configured to achieve ultra-low nitrogen oxide (NOx) emissions while maintaining stable combustion and efficient heat release.2. Description of the Related Art.

[0003] Burners for gaseous fuels are widely used in industrial heating applications to convert chemical energy into thermal energy through controlled combustion. Conventional burner designs typically introduce fuel and combustion air into a combustion chamber where ignition occurs, producing a flame that transfers heat to a process or working fluid. While such burners can provide reliable heat release, they often generate nitrogen oxides (NOx) as an undesirable by-product of combustion.

[0004] Nitrogen oxides are regulated pollutants associated with environmental and human health concerns, including the formation of ground -level ozone and smog. The formation of NOx in gaseous-fuel burners is strongly influenced by peak flame temperature, oxygen concentration, residence time, and local mixing conditions. As emissions regulations have become increasingly stringent, particularly in non-attainment regions, there has been a continuing need to reduce NOx emissions from combustion equipment.

[0005] Various approaches have been employed to reduce NOx emissions from burners. Some burner designs attempt to lower NOx formation through air staging, fuel staging, lean premixed combustion, or operation at high excess air levels. While these techniques can reduce NOx emissions relative to conventional burners, they often involve trade-offs such as reduced flame stability, increased sensitivity to fuel composition, limited turndown capability, or reduced thermal efficiency. In addition, fixed-geometry burners may perform well only over a narrow operating range and may not be adaptable to varying heat release requirements or alternative fuel compositions.Docket No.: P3037PC01

[0006] In applications requiring extremely low NOx emissions, post-combustion treatment systems, such as selective catalytic reduction (SCR), have been widely deployed. These systems reduce NOx emissions downstream of the combustion process by chemically converting NOx in the flue gas. Although effective, SCR systems add significant cost, complexity, and maintenance requirements to combustion equipment, and often require handling and storage of chemical reagents. As a result, combustion-only solutions capable of achieving ultra-low NOx emissions remain highly desirable.

[0007] Some advanced burner designs utilize internal flue gas recirculation, staged fuel injection, or partial premixing of fuel and combustion air to moderate flame temperature and suppress NOx formation. However, existing designs may still struggle to achieve ultra-low NOx emissions at low excess oxygen levels, particularly while maintaining a compact flame, stable operation across a wide turndown range, and complete combustion with minimal carbon monoxide formation.

[0008] Accordingly, there remains a need for burner assemblies that can achieve extremely low NOx emissions using combustion-based techniques, without reliance on post-combustion treatment, while maintaining stable flames, efficient heat transfer, fuel flexibility, and robust operation over a wide range of firing conditions.SUMMARY OF THE INVENTION

[0009] In view of the foregoing considerations, it would be desirable to provide a burner assembly that addresses the limitations of existing low-NOx and ultra-low-NOx combustion technologies by reducing nitrogen oxide formation through controlled combustion dynamics rather than post-combustion treatment.

[0010] The present disclosure provides burner assemblies and methods for combusting gaseous fuels in heaters, furnaces, boilers, flares, thermal oxidizers, and other enclosed combustion devices. The burner assemblies are configured to reduce nitrogen oxide formation while maintaining stable combustion over a wide operating range using combustion-based techniques.

[0011] In general, a burner assembly includes a combustion air inlet and a mixing tube assembly defining an internal passage. The mixing tube assembly is spaced from a heater or furnace wall to define an annular gap that places the internal passage in fluid communication with a heater or furnace volume. The burner assembly further includes a secondary fuel circuit having one or more forward-facing secondary fuel ports configured to inject a secondary fuel stream into the mixing tube assembly, and a tertiary fuel circuit having one or moreDocket No.: P3037PC01 rearward-facing tertiary fuel ports configured to inject a tertiary fuel stream into the heater or furnace volume in a direction having an upstream component relative to a longitudinal burner flow axis.

[0012] During operation, combustion air is discharged into the mixing tube assembly and acts as a primary fluid providing a principal motive force. Momentum of the combustion air entrains flue gas and the tertiary fuel stream from the heater or furnace volume through the annular gap and into the mixing tube assembly prior to combustion. Within the mixing tube assembly, the entrained flue gas and tertiary fuel stream are mixed with combustion air and the secondary fuel stream to form a diluted combustible mixture that is ignited downstream of the mixing tube assembly to produce a stable flame.

[0013] The burner assembly may include a swirler hub assembly disposed downstream of the mixing tube assembly to impart swirl to the diluted combustible mixture, and a flame holder configured to stabilize combustion at low flow rates. The burner assembly may further include a primary fuel circuit configured to provide a primary anchoring flame for ignition.

[0014] The burner assembly may include a controller operatively coupled to one or more fuel circuits and a combustion air delivery device to regulate fuel and air flows in response to operating conditions. The burner assembly may be configured to operate with natural gas, hydrogen-containing fuels, biogas, or combinations thereof, and may permit injection of steam or inert gases into one or more fuel streams or the combustion air stream.

[0015] Methods of operating burner assemblies are also provided. The methods include supplying combustion air into a mixing tube assembly, injecting a tertiary fuel stream into a heater or furnace volume in an upstream direction, entraining flue gas and the tertiary fuel stream into the mixing tube assembly through an annular gap using momentum of the combustion air, injecting a secondary fuel stream into the mixing tube assembly, and igniting a diluted combustible mixture downstream of the mixing tube assembly.

[0016] In one aspect, a burner assembly for combusting gaseous fuels has a combustion air inlet configured to receive a combustion air stream. A mixing tube assembly defines an internal passage and is positioned to discharge into a heater or furnace volume. The mixing tube assembly is spaced from a heater or furnace wall to define an annular gap therebetween. At least one combustion air conduit fluidly connects the combustion air inlet to the internal passage of the mixing tube assembly, and the at least one combustion air conduit is oriented to discharge the combustion air stream into the internal passage with momentum sufficient to draw gases through the annular gap. A secondary fuel circuit has one or more forward-facingDocket No.: P3037PC01 secondary fuel ports configured to inject a secondary fuel stream into the mixing tube assembly upstream of an outlet of the mixing tube assembly. A tertiary fuel circuit has one or more rearward-facing tertiary fuel ports disposed downstream of the outlet of the mixing tube assembly and is configured to inject a tertiary fuel stream into the heater or furnace volume in a direction having an upstream component relative to a longitudinal burner flow axis. The combustion air stream provides a principal motive force that entrains flue gas and the tertiary fuel stream from the heater or furnace volume through the annular gap and into the mixing tube assembly prior to combustion.

[0017] In an embodiment, the annular gap is defined between an outer surface of the mixing tube assembly and an interior face of the heater or furnace wall.

[0018] In an embodiment, the at least one combustion air conduit comprises a plurality of combustion air pipes arranged circumferentially about the mixing tube assembly.

[0019] In an embodiment, the plurality of combustion air pipes is oriented at an acute angle relative to the longitudinal burner flow axis to promote stratification of entrained flue gas toward an outer wall of the mixing tube assembly.

[0020] In an embodiment, the one or more forward-facing secondary fuel ports inject the secondary fuel stream upstream of outlet ports of the plurality of combustion air pipes.

[0021] In an embodiment, the one or more forward-facing secondary fuel ports are oriented at an angle of about 0° to about 45° downstream relative to the longitudinal burner flow axis.

[0022] In an embodiment, the one or more rearward-facing tertiary fuel ports are oriented at an angle of about 15° to about 45° upstream relative to the longitudinal burner flow axis.

[0023] In an embodiment, the one or more rearward-facing tertiary fuel ports are oriented at about 35° upstream relative to the longitudinal burner flow axis.

[0024] In an embodiment, a swirler hub assembly is disposed downstream of the mixing tube assembly, and the swirler hub assembly has swirl vanes configured to impart swirl to a mixture flowing from the internal passage.

[0025] In an embodiment, the swirl vanes are oriented at an angle of about 15° to about 75° relative to the longitudinal burner flow axis.

[0026] In an embodiment, the swirl vanes are oriented at about 45° relative to the longitudinal burner flow axis.Docket No.: P3037PC01

[0027] In an embodiment, the swirler hub assembly comprises an expanding cone defining a diverging flow passage downstream of the swirl vanes.

[0028] In an embodiment, the expanding cone includes a first divergence angle greater than a second divergence angle to promote controlled expansion and internal recirculation.

[0029] In an embodiment, a primary fuel circuit has at least one forward-facing primary fuel port configured to provide a primary anchoring flame.

[0030] In an embodiment, an ignitor is positioned to ignite fuel discharged from the at least one forward-facing primary fuel port.

[0031] In an embodiment, a flame holder is disposed within an expanding cone and configured to stabilize the primary anchoring flame at low flow rates.

[0032] In an embodiment, the tertiary fuel circuit comprises one or more tertiary fuel poker tubes extending through the mixing tube assembly and terminating in the one or more rearward-facing tertiary fuel ports.

[0033] In an embodiment, air-cooled support pipes support the tertiary fuel poker tubes, the support pipes being cooled by combustion air flowing through the burner assembly.

[0034] In an embodiment, a controller is operatively coupled to at least one of the secondary fuel circuit, the tertiary fuel circuit, or a combustion air delivery device to regulate at least one of fuel flow or combustion air flow in response to operating conditions.

[0035] In an embodiment, at least one sensor is operatively coupled to the controller, the sensor being configured to detect at least one of flame presence, flame temperature, infrared radiation, ultraviolet radiation, or spectral characteristics of a flame.

[0036] In an embodiment, at least one of steam, nitrogen, or another inert gas is injected into at least one of the combustion air stream, the secondary fuel circuit, or the tertiary fuel circuit.

[0037] In an embodiment, different fuel sources are supplied to the primary, secondary, and tertiary fuel circuits.

[0038] In an embodiment, the combustion air stream comprises a primary fluid and the flue gas comprises a secondary fluid entrained through the annular gap by momentum of the primary fluid.

[0039] In an embodiment, the momentum of the primary fluid entrains the flue gas as the combustion air stream passes through a point of maximum hydraulic resistance.

[0040] In an embodiment, the tertiary fuel stream comprises a secondary fluid entrained with the flue gas through the annular gap prior to combustion.Docket No.: P3037PC01

[0041] In another aspect, a method of operating a burner assembly to combust gaseous fuels includes the steps of: supplying combustion air into a mixing tube assembly; injecting a tertiary fuel stream into a heater or furnace volume in a direction having an upstream component relative to a longitudinal burner flow axis; using momentum of the combustion air as a primary fluid to entrain flue gas and the tertiary fuel stream from the heater or furnace volume through an annular gap and into the mixing tube assembly; injecting a secondary fuel stream into the mixing tube assembly in a downstream direction relative to the longitudinal burner flow axis; and igniting a diluted combustible mixture downstream of the mixing tube assembly.

[0042] In an embodiment, the tertiary fuel stream is injected from rearward-facing fuel ports oriented at an angle of about 15° to about 45° upstream relative to the longitudinal burner flow axis.

[0043] In an embodiment, the combustion air is supplied through a plurality of conduits arranged circumferentially about the mixing tube assembly.

[0044] In an embodiment, the tertiary fuel stream is entrained together with flue gas prior to ignition.

[0045] In an embodiment, the method also includes imparting swirl to the diluted combustible mixture downstream of the mixing tube assembly.

[0046] In an embodiment, the method also includes stabilizing a flame with a bluff-body flame holder at low flow rates.

[0047] In an embodiment, the method also includes controlling at least one of fuel flow or combustion air flow with a controller responsive to a sensor signal.

[0048] In an embodiment, the sensor signal is indicative of flame presence or flame characteristics.

[0049] In an embodiment, the method also includes injecting at least one of steam, nitrogen, or another inert gas into the combustion air or a fuel stream.

[0050] In an embodiment, the gaseous fuel comprises natural gas, a hydrogen-containing fuel, biogas, or combinations thereof.

[0051] In an embodiment, the method also includes transitioning flame stabilization between a flame holder and a swirler hub assembly based on operating conditions.

[0052] In an embodiment, the combustion air stream comprises a primary fluid and the flue gas comprises a secondary fluid entrained through the annular gap by momentum of the primary fluid.Docket No.: P3037PC01

[0053] In an embodiment, the tertiary fuel stream is entrained together with the flue gas prior to ignition.BRIEF DESCRIPTION OF DRAWINGS

[0054] The above and other objects and advantages of this invention may be more clearly seen when viewed in conjunction with the accompanying drawing wherein:

[0055] Figure 1 is an isometric view of an example of a burner assembly in accordance with an illustrative embodiment of the invention disclosed herein.

[0056] Figure 2 is another isometric view of the burner assembly shown in Figure 1.

[0057] Figure 3 is a side view of the burner assembly shown in Figure 1.

[0058] Figure 4 is a top view of the burner assembly shown in Figure 1.

[0059] Figure 5 is a cross-sectional, side elevation view of the burner assembly shown in Figure 4 along lines 5-5.

[0060] Figure 6 is an isometric view of an example of a secondary or tertiary fuel injection assembly in accordance with an illustrative embodiment of the invention disclosed herein.

[0061] Figure 7 is another isometric view of the fuel injection assembly shown in Figure 6.

[0062] Figure 8 is a side view of the fuel injection assembly shown in Figure 6.

[0063] Figure 9 is a rear view of the fuel injection assembly shown in Figure 6.

[0064] Figure 10 is a cross-sectional view of the fuel injection assembly shown in Figure 6 along lines 10-10.

[0065] Figure 11 is a cross-sectional view of the fuel injection assembly shown in Figure 6 along lines 11-11.

[0066] Figure 12 is a front view of the fuel injection assembly shown in Figure 6.

[0067] Figure 13 is a detailed view of the fuel injection assembly shown in Figure 11 at area 13.

[0068] Figure 14 is a detailed view of the fuel injection assembly shown in Figure 11 at area 14.

[0069] Figure 15 is an isometric view of an example of a mixing tube assembly in accordance with an illustrative embodiment of the invention disclosed herein.

[0070] Figure 16 is another isometric view of the fuel injection assembly shown in Figure 15.

[0071] Figure 17 is a side view of the mixing tube assembly shown in Figure 15Docket No.: P3037PC01

[0072] Figure 18 is a rear view of the mixing tube assembly shown in Figure 15.

[0073] Figure 19 is a cross-sectional view of the mixing tube assembly shown in Figure 17 along lines 19-19.

[0074] Figure 20 is a cross-sectional view of the mixing tube assembly shown in Figure 18 along lines 20-20.

[0075] Figure 21 is an isometric view of an example of a swirler hub assembly in accordance with an illustrative embodiment of the invention disclosed herein.

[0076] Figure 22 is a side view of the swirler hub assembly shown in Figure 21

[0077] Figure 23 is a rear view of the swirler hub assembly shown in Figure 21.

[0078] Figure 24 is a front view of the swirler hub assembly shown in Figure 21.

[0079] Figure 25 is a cross-sectional view of the swirler hub assembly shown in Figure 24 along lines 25-25.

[0080] Figure 26 is a side view of an example of a flame holder and a primary fuel gas port in accordance with an illustrative embodiment of the invention disclosed herein.

[0081] Figure 27 is a cross-sectional view of the flame holder and the primary fuel gas port shown in Figure 26 along lines 27-27.

[0082] Figure 28 is a front view of the burner assembly shown in Figures 1 through 5.

[0083] Figure 29 is a side view of an example of a burner assembly illustrating fluid flows in accordance with an illustrative embodiment of the invention disclosed herein.

[0084] Figure 30 are representative flame images at different firing rates: (1) Light-off before the addition of the flame visibility port in the swirler; (2) Light-off with the flame visibility port in the swirler (current installation); (3) Increasing rate to 60%; (4) Flame at 70%; (5) Flame at 80%; and (6) Flame at 115%.

[0085] Figure 31 shows a representative axial velocity field illustrating internal recirculation downstream of the burner assembly. Positive values flow away from the burner while negative values flow toward the burner.DETAILED DESCRIPTION

[0086] While this invention is susceptible to embodiments in many different forms, there are shown in the drawings and will be described hereinafter in detail some specific embodiments of the invention. It should be understood, however, that the present disclosure is to be considered an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments described.Docket No.: P3037PC01

[0087] A burner assembly is disclosed for gaseous fuels in boilers, industrial furnaces, direct-fired heaters, flares, thermal oxidizers, and other enclosed combustion devices to produce a heating source for energy, chemical, and manufacturing processes. The inventive burner assembly is a next-generation ultra-low NOXburner assembly that employs advanced combustion technologies: staged combustion to optimize flame temperature and minimize nitrogen oxide (NOx) formation; internal flue gas recirculation (IFGR) to lower peak flame temperature; and flame stabilization to ensure even heat distribution and minimize NOXproduction.

[0088] The burner assembly is designed to operate on various fuel gases, including natural gas, hydrogen blends, or other combustible fuels. The burner assembly produces extremely low NOXemissions and is configured for complete combustion of fuel gases, preventing the emission of partial combustion products, such as carbon monoxide.

[0089] The burner assembly includes a mixing tube assembly spaced from a heater or furnace wall to define an annular gap, thereby placing an internal mixing region of the mixing tube assembly in fluid communication with a heater or furnace volume. Combustion air is discharged into the internal mixing region and acts as a primary fluid providing a principal motive force. A tertiary fuel stream is injected into the heater or furnace volume through rearward-facing fuel ports in a direction having an upstream component relative to a longitudinal burner flow axis. As a result, the tertiary fuel stream mixes with flue gas outside the burner assembly. Momentum of the combustion air entrains both flue gas and the tertiary fuel stream through the annular gap and into the internal mixing region prior to combustion. Within the mixing tube assembly, the entrained flue gas and tertiary fuel stream are mixed with combustion air and a secondary fuel stream to form a diluted combustible mixture, which is ignited downstream of the mixing tube assembly.

[0090] By using combustion air as a primary fluid to entrain flue gas and an upstream-injected tertiary fuel stream through the annular gap prior to combustion, peak flame temperature is moderated and the effective mixing length is increased. This configuration promotes dilution of the combustion zone and stabilizes the flame over a wide operating range, thereby reducing nitrogen oxide formation while maintaining stable combustion without reliance on external recirculation systems or post-combustion treatment.

[0091] The burner assembly has a primary fuel circuit having forward-facing primary fuel ports, which can be configured to flow about 1% to about 20%, preferably about 5%, of the gaseous fuel flow, a secondary fuel circuit having forward-facing secondary fuel ports,Docket No.: P3037PC01 which can be configured to flow about 0% to about 100% of the fuel flow, and a tertiary fuel circuit having rearward-facing tertiary fuel ports, which can be configured to flow about 0% to about 100% of the fuel flow required to satisfy the heat requirement of the burner assembly. The types of fuel gases and / or the fuel flow rates can be independently varied through each of the fuel circuits in the burner assembly. In addition, steam, nitrogen, or another inert gas or mixtures thereof can be injected into one or more of the fuel gas circuits.

[0092] Referring now to the figures of the drawings, wherein like numerals of reference designate like elements throughout the several views, and initially to Figures 1 through 5 and 28, a burner assembly 100 is shown configured for installation in a fired heater or furnace wall. The burner assembly 100 integrates a blower inlet assembly 110, a transition spool assembly 130, a secondary or tertiary fuel injection assembly 150, a mixing tube assembly 170, and a swirler hub assembly 1 0 arranged along a longitudinal burner flow axis A-A. The burner assembly 100 includes mounting flanges, sealing elements, and interfaces for combustion air, fuel supplies, ignition, and instrumentation. During operation, combustion air flows through the burner assembly 100 from upstream to downstream along the burner flow axis A-A, entrains flue gas (and tertiary fuel gas) from a heater or furnace volume 108, mixes with staged fuel, and exits the burner assembly 100 to form a stable, low-NOx flame within the heater or furnace volume 108.

[0093] The blower inlet assembly 110 of the burner assembly 100 is configured to receive combustion air from a forced-draft blower or ducted air supply (not shown). The combustion air is generally atmospheric air, but can include steam, nitrogen, or other inert gases or mixtures thereof. The blower inlet assembly 110 includes a generally cylindrical or transitional housing body 112 having an upstream flanged or threaded interface 114 configured for receipt of an ignitor assembly 210 and a primary fuel gas pipe 212. The housing body 112 also has a downstream flanged or threaded interface 116 configured for attachment to the transition spool assembly 130. The blower inlet assembly 110 may include one or more combustion air pressure taps 118.

[0094] The blower inlet assembly 110 is configured to align the combustion air flow with the longitudinal burner flow axis A-A and to uniformly distribute combustion air to downstream components of the burner assembly 100 while accommodating thermal expansion and vibration associated with burner operation. A blower inlet 120 can be disposed at any angle relative to a longitudinal burner flow axis A-A, including parallel or perpendicular. Depending on the orientation of the blower inlet 120 relative to the burner flow axis A-A, the inlet assemblyDocket No.: P3037PC01 110 may include a damper or turning vane 122 to control the flow of the combustion air within the inlet assembly 110. If the blower inlet 120 is orthogonal to the burner flow axis A- A, the turning vane 122 may be used depending on the air-side pressure drop required at the point of maximum hydraulic resistance to fluid flow within the burner assembly 100.[00951 The transition spool assembly 130 is disposed downstream of the blower inlet assembly 110 and fluidly connects the blower inlet assembly 110 to the secondary or tertiary fuel injection assembly 150. The transition spool assembly 130 provides a controlled geometric transition between differing cross-sectional areas or shapes. The transition spool assembly 130 includes a contoured body 132 configured to minimize pressure loss and flow separation while maintaining alignment of the combustion air flow with the longitudinal burner flow axis A-A. The contoured body 132 terminates at upstream and downstream flanged or threaded interfaces 134 and 136, which facilitate attachment to the inlet assembly 110 and the fuel injection assembly 150.[0096 J Phe burner assembly 100 also includes the secondary or tertiary fuel injection assembly 150, which is disposed intermediate the transition spool assembly 130 and the mixing tube assembly 170 and is fluidly connected to both. The fuel injection assembly 150 includes an upstream flanged or threaded interface 152 configured to be attached to the transition spool assembly 130 and a downstream flanged or threaded interface 154 configured to be attached to an exterior face 104 of a heater or furnace wall 102. The fuel injection assembly 150 also includes one or more gas and / or poker manifold assemblies 156, each having a manifold body 158 with a manifold nozzle 160 for receiving fuel from an external fuel supply. From the manifold body 158, a plurality of elongated gas or poker tubes 162 extend into the mixing tube assembly 170 or the heater or furnace volume 108. The gas or poker tubes 162 are arranged circumferentially and oriented to inject fuel into a mixing region within (e.g., secondary fuel circuit) or upstream of (e.g., tertiary fuel circuit) the mixing tube assembly 170. In some embodiments, separate gas or poker manifold assemblies 156 are provided for the secondary and tertiary fuel circuits, enabling independent control of fuel flow across different fuel types and / or injection stages. In some embodiments, the gas or poker tubes 162 are arranged circumferentially and substantially parallel to the burner flow axis A-A. In other embodiments, the gas or poker tubes 162 can be arranged circumferentially and angled so as to be nonparallel to the burner flow axis A-A. The fuel injection assembly 150 may further include one or more access features, such as a sight port 157, and provisions for mounting instrumentation, including pressure taps (e.g., a secondary pressure tap 159 and / or a tertiary pressure tap 161),Docket No.: P3037PC01 and flame-monitoring components. In some embodiments, an ultraviolet or infrared flame scanner (not shown) is provided for optical detection of flame presence within the heater or furnace volume 108.

[0097] The burner assembly 100 also includes the mixing tube assembly 170 disposed downstream of and fluidly connected to the fuel injection assembly 150. The mixing tube assembly 170 has a mixing tube body 172 that defines an internal passage 174 configured to receive combustion air from one or more combustion air pipes 176 and to entrain flue gas (and tertiary fuel gas if utilized) from the heater or furnace volume 108 into the burner assembly 100. An annular gap 178 is formed between the mixing tube assembly 170 and an interior face 106 of the heater or furnace wall 102, allowing flue gas (and tertiary fuel gas if utilized) to be drawn into the mixing tube assembly 170 by the momentum of the combustion air stream. In this configuration, the combustion air stream provides the principal motive force for drawing the flue gas and tertiary fuel gas into the mixing tube assembly 170.

[0098] As shown in more detail in Figures 15 through 20, the combustion air pipes 176 of the mixing tube assembly 170 extend into fluid communication with the internal passage 174 of the mixing tube body 172. Although one combustion air pipe 176 may be used, a plurality of combustion air pipes 176 increases mixing between the combustion air and recirculated flue gas (and tertiary fuel gas if utilized) from the heater or furnace volume 108. The combustion air pipes 176 include combustion inlet ports 180 that are fluidly connected to outlet ports 168 of the secondary or tertiary fuel injection assembly 150. The burner assembly 100 increases the interfacial area between the combustion air streams and the recirculated and entrained gases from the heater or furnace volume 108 compared with a single round orifice or a lobed mixer configuration. The combustion air pipes 176 are arranged circumferentially and oriented at an acute angle relative to the longitudinal burner flow axis A-A to promote stratification and mixing of combustion air with entrained flue gas and fuel.

[0099] As shown in more detail in Figures 6 through 14, the secondary fuel circuit includes a secondary fuel manifold body 156A having elongated secondary fuel gas tubes 162A that extend into fluid communication with the mixing tube assembly 170. The secondary fuel gas tubes 162A are fluidly connected to principally forward-facing secondary fuel gas ports 164, which are configured to inject a secondary fuel gas stream upstream of outlet ports 182 of the combustion air pipes 176. The forward-facing secondary fuel gas ports 164 are configured to inject the secondary fuel gas stream in a principal direction downstream with respect to the longitudinal burner flow axis A-A, where 0° corresponds to directly downstream. In illustrativeDocket No.: P3037PC01 embodiments, the secondary fuel gas ports 164 are oriented at an angle S of about 0° to about 45° (and any range or value therebetween), measured downstream relative to the burner flow axis A-A. This orientation directs the secondary fuel gas stream partially toward the burner periphery while maintaining a downstream injection component, thereby enhancing mixing with the combustion air emerging from the combustion air pipes 176. The stream of secondary fuel gas mixes with the flue gas (and tertiary fuel gas, if utilized) from the heater or furnace volume 108, upstream of and before mixing with the combustion air flowing from the combustion air pipes 176. Forward-facing secondary injection promotes staged combustion downstream of the mixing tube assembly 170, enabling additional flame-stabilization or heat-release control modes depending on the firing strategy and desired emissions profile.

[0100] The tertiary fuel circuit includes a tertiary fuel manifold body 156B, which has one or more elongated tertiary fuel poker tubes 162B that pass through the mixing tube assembly 170 and extend into fluid communication with the heater or furnace volume 108. The tertiary fuel poker tubes 162B are fluidly connected to rearward-facing tertiary fuel gas ports 166. The tertiary fuel gas ports 166 are oriented to inject a tertiary fuel gas stream in a principal upstream direction with respect to the longitudinal burner flow axis A-A, where 0° corresponds to directly upstream. In illustrative embodiments, the tertiary fuel gas ports 166 are oriented at an angle T of about 15° to about 45° (and any range or value therebetween), and more particularly about 35°, measured upstream relative to the burner flow axis. At these angles, the tertiary fuel gas stream is directed upstream and radially outward, away from the downstream combustion-air flow and into the heater or furnace volume 108. This upstream injection promotes premixing of the tertiary fuel with flue gas outside the mixing tube assembly 170 prior to entrainment through the annular gap 178. The orientation facilitates a recirculating, dilute-gas function, increasing the effective mixing length, suppressing peak flame temperature, and supporting ultra-low-NOx combustion performance. If the tertiary fuel gas ports 166 were to be aligned on the axis or inclined in a principal direction downstream along the burner flow axis A-A, the tertiary fuel gas ports 166 would take on a different “staged gas” flame function as opposed to the tertiary recirculating dilute gas function of the inventive burner assembly 100.

[0101] The mixing tube assembly 170 also includes a plurality of air-cooled support pipes 184 that support the elongated tertiary poker tubes 162B, which pass through the mixing tube body 172 from the upstream fuel injection assembly 150 and are in fluid communication downstream with the heater or furnace volume 108. The support pipes 184 are actively cooledDocket No.: P3037PC01 by combustion air drawn from the blower inlet assembly 110 through the fuel injection assembly 150. Further, the relatively cool mixture of secondary fuel, combustion air, and flue gas / tcrtiary fuel acts to shield the support pipes 184 within the mixing tube body 172 from heat transfer by radiation and / or from flue gas that has not been sufficiently cooled by premixing with other, lower-temperature fluids.

[0102] The burner assembly 100 also includes the swirler hub assembly 190 positioned downstream of the mixing tube assembly 170 and upstream of the main combustion zone of the heater or furnace volume 108. The swirler hub assembly 190 is configured to impart a controlled swirl motion to the flow of combustion air, secondary fuel, and entrained flue gas / tertiary fuel mixture. As shown in more detail in Figures 21 through 25, the swirler hub assembly 190 includes a central hub 192 and a plurality of angularly disposed or canted swirl vanes 194 disposed radially about the central hub 192. The swirl vanes 194 arc oriented at an angle V relative to the longitudinal burner flow axis A-A (e.g., about 15° to about 75° (and any range or value therebetween), and more particularly about 45°) (Figures 22 and 25) to induce rotational flow and internal recirculation downstream of the swirler hub assembly 190. The swirl vanes 194 can also have a canted angle to help shape the formed flame. The swirler hub assembly 190 further includes a central passage 196 configured to accommodate the primary fuel gas pipe 212 and / or the ignitor assembly 210, which extends through the swirler hub assembly 190. In some embodiments, the ignitor assembly 210 and primary fuel gas pipe 212 are disposed in separate passages along the longitudinal burner flow axis A-A and through the swirler hub assembly 190. The central passage 196 is configured to accommodate the primary fuel gas pipe 212, and an adjacent secondary passage 198 is configured to accommodate the ignitor assembly 210.

[0103] The primary fuel gas pipe 212 passes through the swirler hub assembly 190. The primary fuel gas pipe 212 is fluidly connected to a principally forward-facing primary fuel gas port 214. The forward-facing primary fuel gas port 214 is configured to inject the primary fuel gas stream in a principal direction downstream with respect to the longitudinal burner flow axis A-A (e.g., inject the primary fuel gas stream at an angle P of about 0° to about 45° in a downstream-and-radially outward direction in relation to the burner flow axis (and any range or value therebetween)) (Figure 27). The swirler hub assembly 190 includes an expanding cone 200 coupled to the angularly disposed swirl vanes 194. The expanding cone 200 defines a diverging flow passage 202 downstream of the swirl vanes 194. The expanding cone 200 includes an upstream inlet 204 positioned adjacent to the trailing edges of the swirl vanes 194Docket No.: P3037PC01 and a downstream outlet 206 having a substantially larger diameter than the upstream inlet 204. The expanding cone 200, as illustrated in Figure 22, has a primary divergence angle C’ (e.g., about 39° to about 47°, and more particularly about 42° to about 44° (and any range or value therebetween)) greater than a secondary divergence angle C” (e.g., about 15° to about 26°, and more particularly about 18° to about 23° (and any range or value therebetween)), configured to promote controlled expansion of the swirling mixture, enhance internal recirculation downstream of the swirler hub assembly 190, and provide a stable zone for anchoring a primary or partially premixed flame.

[0104] The primary fuel gas is injected from the forward-facing primary fuel gas port 214 and ignited by the ignitor assembly 210 (e.g., by a high-energy spark ignitor, hot surface ignitor, and / or a pilot flame burner) to provide a primary flame that expands inside the expanding cone 200 before exiting the swirler hub assembly 190. The expanding cone 200 geometry can be modified to adjust the flame length and diameter. This allows the flame to be modified for various heater configurations without altering the burner assembly 100. Furthermore, the installed expanding cone 200 includes a flame holder 216 that enables stable operation across a wider turndown range.

[0105] The swirler hub assembly 190 is configured to stabilize the primary flame by combining wake stabilization and swirl stabilization. The primary fuel circuit produces the nozzle-mix primary flame, which becomes more premixed as the flow rate of the combustion air, secondary fuel, and flue gas / tertiary fuel mixture increases, due to the co-annular swirler hub assembly 190. At low flow rates, the primary flame is initially anchored by a variable or slotted bluff body flame holder 216 (Figure 26) concentrically disposed inside the expanding cone 200 of the swirler hub assembly 190. As the flow rate increases, more of the primary flame becomes swirl-stabilized, and more of the combustion air, secondary fuel, and flue gas / tertiary fuel mixture are mixed with the primary fuel gas prior to combustion in the main combustion zone of the heater or furnace volume 108. The primary fuel stream can be provided throughout the operating range of the burner assembly 100 or shut off.

[0106] The burner assembly 100 can further include a controller operatively coupled to the primary, secondary, and tertiary fuel circuits. The burner assembly 100 can have one or more sensors operatively coupled to the controller. For example, the sensor can include a heat sensor configured to detect infrared or ultraviolet radiation or a temperature of the nozzle mix primary flame. The controller can be configured to control the burner assembly 100 responsive to input from the sensor. In addition, a fuel control valve (not shown) can be operativelyDocket No.: P3037PC01 coupled to the controller and configured to selectively control a flow of the primary fuel, the secondary fuel, and / or the tertiary fuel from a primary fuel source, a secondary fuel source, and / or a tertiary fuel source, respectively, to the burner assembly 100. Additionally or alternatively, the blower can include a variable-frequency drive and a fan, and the blower or damper can be operatively coupled to the controller and configured to control the flow of the combustion oxidant supplied to the burner assembly 100. The controller can also be operatively coupled to the primary fuel circuit to transition the primary flame between the flame holder 216 at low flow rates and the swirler hub assembly 190 at high flow rates.

[0107] The sensor can further include a combustion sensor operatively coupled to the control circuit, configured to detect a temperature, video image, and / or spectral characteristic of the primary flame held by the flame holder 216. The controller can be configured to control the fuel control valve responsive to input from the combustion sensor. The controller can be programmed with air and fuel curves to control the fuel control valve and / or the blower or damper to control a preheat flame type of the heater assembly 100 and / or to change the fuel flows responsive to a heat demand change as a percentage of the requested duty received as data via the data interface. Once the primary flame is ignited (e.g., by the ignitor assembly 210 or dedicated pilot burner) and proven, the control circuit receives a signal from a thermocouple 163 (e.g., the process oil outlet temperature, a flue gas control temperature for a flare, or a steam temperature) requesting more or less fuel flow when compared to a temperature required to maintain or optimize combustion.

[0108] Referring now to Figure 29, the burner assembly 100 previously described herein is illustrated schematically to depict representative flow paths of primary fuel gas 300, combustion air 302, secondary fuel gas 304, tertiary fuel gas 306, and flue gas 308 drawn from the heater or furnace volume 108 during operation of the burner assembly 100. As illustrated, the stream of the tertiary fuel gas 306 premixes with the flue gas 308 in the heater or furnace volume 108, and the force of the combustion air 302 being used as the primary fluid draws the tertiary fuel gas 306 and the flue gas 308 into the mixing tube body 172 through the annular gap 178 between the mixing tube assembly 170 and the interior face of the heater or furnace wall 102, where the flue gas 308 and tertiary fuel gas 306 are entrained in the heater or furnace volume 108.

[0109] Using the tertiary fuel circuit increases the effective mixing length compared to axial injection while still maintaining a small ultimate flame volume. The tertiary fuel gas 306 does not manifest as a flame until it is circulated through the mixing tube assembly 170 andDocket No.: P3037PC01 exits into the main combustion zone 310. In the example below, using the tertiary fuel circuit reduces NOx emissions by an additional approximately 33% compared to using only the primary and secondary fuel circuits.

[0110] The dynamic pressure of the combustion air 302 flowing from the combustion air pipes 176 through the point of maximum hydraulic resistance 312 uses the combustion air 302 to provide the principal motive force as a primary fluid to draw in the flue gas 308 and tertiary fuel gas 306 through the annular gap 178 in the mixing tube assembly 170 from the heater or furnace volume 108 and premixes the flows of combustion air 302, the secondary fuel gas 304, the tertiary fuel gas 306, and the flue gas 308 in the axially disposed mixing tube body 172 of the mixing tube assembly 170. The combustion air pipes 176 may be disposed at an acute angle pointing toward the longitudinal burner flow axis A- A of the burner assembly 100 and configured to provide stratification to the flow of combustion air 302, the secondary fuel gas 304, the tertiary fuel gas 306, and the flue gas 308. The mixture of the combustion air 302, the secondary fuel gas 304, and the mixture of the tertiary fuel gas 306 and flue gas 308 (which may initially be air) is then more thoroughly mixed in the axially disposed mixing tube body 172 over the length of the traverse. The flue gas 308 may stratify toward the outer wall of the mixing tube body 172 due to the disposition of the combustion air pipes 176, preventing preignition inside the mixing tube assembly 170 before the main combustion zone 310.

[0111] A portion 314 of the mixture of combustion air 302, secondary fuel gas 304, tertiary fuel gas 306 / flue gas 308 flows from the mixing tube body 172 to the swirler hub assembly 190, where the primary fuel gas is injected from the primary fuel gas pipe 212 and ignited to produce a primary anchoring flame. Another portion 316 of the mixture of combustion air 302, secondary fuel gas 304, tertiary fuel gas 306 / flue gas 308 bypasses the swirler hub assembly 190 and flows around the primary fuel circuit. No additional fuel gas is injected into this bypassed portion 316 of the mixture, which is ignited downstream by the primary flame of the primary fuel circuit. This internal recirculation increases the effective mass flow through the flame region, moderates peak flame temperature, and suppresses nitrogen oxide formation while maintaining stable combustion.

[0112] Example 1 - Fired Heater Operation Using a Burner Assembly

[0113] In one example, a burner assembly according to the present disclosure was installed in a fired heater operating under typical hot-oil heater conditions. The burner assembly included a primary fuel circuit, a secondary fuel circuit, and a tertiary fuel circuit, withDocket No.: P3037PC01 combustion air supplied by a forced-draft blower and controlled to maintain a target excess oxygen level in the flue gas. The burner assembly also included a diffuser.

[0114] The burner assembly was operated across a range of firing duties, including part-load and full-load operation. Representative flame images at different firing rates are shown in Figure 30, which illustrates stable flame anchoring and substantially consistent flame shape and length across an operating range from light-off through greater than 100% of the design firing rate. The flame stability illustrated in Figure 30 demonstrates that staged fuel injection and internal recirculation enable wide turndown operation while maintaining stable combustion. The burner assembly exhibited stable operation over a fuel turndown ratio of at least 20:1.

[0115] Measured emissions data collected during operation of the fired heater are summarized below in Table 1. At near-dcsign firing conditions, with only the primary and secondary fuel circuits activated, and with approximately 3% excess oxygen (dry) in the flue gas, nitrogen oxide (NOx) emissions were approximately 3 parts per million (ppm) while carbon monoxide (CO) emissions were approximately 0 ppm. At intermediate firing rates, activation of the tertiary fuel circuit further reduced NOx emissions to approximately 2 ppm while maintaining CO emissions at approximately 0 ppm.

[0116] At lower firing rates and lower firebox temperatures, elevated CO emissions were observed; however, as the firing rate increased and the firebox temperature exceeded the oxidation temperature of CO, CO emissions decreased to near zero while NOx emissions decreased with increasing firing rate, as reflected in Table 1.Docket No.: P3037PC01

[0117] The burner assembly was operated with lower excess air relative to conventional premixed burners, thereby improving fired-heater efficiency while maintaining ultra-low NOx emissions. The burner assembly maintained complete combustion at excess oxygen levels of approximately 2%-3.8% without producing measurable CO at normal operating temperatures, while maintaining NOx emissions at 2 ppm.

[0118] Computational modeling of the fired heater configuration was performed to evaluate flow fields, flame characteristics, and emissions behavior. A representative axial velocity field illustrating internal recirculation and entrainment of flue gas downstream of the burner assembly is shown in Figure 31. As illustrated in Figure 31, the momentum of the combustion air stream induces a reverse-flow region that entrains flue gas toward the flame root. This internal recirculation increases effective mass flow through the combustion zone and suppresses peak flame temperature, thereby reducing nitrogen oxide formation. This reverseflow region also ensures a well-anchored and stable flame.

[0119] In this example, the reduction in NOx emissions was attributed to suppression of peak flame temperature and dilution of the combustion zone through entrainment of flue gas driven by the combustion air stream, combined with staged fuel injection through the primary, secondary, and tertiary fuel circuits. This example demonstrates that burner assemblies according to the present disclosure can achieve ultra- low NOx emissions via combustion-based techniques without post-combustion treatment, while maintaining stable operation and high thermal efficiency in fired heater applications.

[0120] As used herein, the term “fluidly connected” means connected by a fluid transfer conduit or any other method that permits fluid transfer, with or without intervening elements, such as, without limitation, containers, filters, devices, pumps, valves, etc. A non-limiting example, two tanks or vessels may be “fluidly connected” if they are connected to each other through a pipe or tube, even if a pump, manifold, valve, or other device is placed in-line between the vessels. Two elements are considered to be “fluidly connected” even though there is no pipe or tubing making the connection if the first element leaks or otherwise drains, overflows, siphons, or transfers into the second element, though there may be no actual physical connection between the two elements in the form of a pipe or tube. As used herein, the term “in fluid communication with” means that a fluid-carrying or fluid-transporting member (e.g., vessel, tank, pump, pipe, tubing, disc, valve, channel, port, etc.) is coupled to another fluidcarrying or fluid-transporting member so as to permit the fluid to flow, leak, or otherwise migrate from one member to the other. In reference to a process or circuit, the termDocket No.: P3037PC01 “downstream” means a later in the direction of general process and / or fluid flow, and “upstream” means earlier in the direction of general process and / or flow.

[0121] The description of the invention is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. In the description, relative terms such as “front,” “rear,” “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly” etc.) should be construed to refer to the orientation as then described or as shown in the drawings under discussion. These relative terms are for convenience of description and do not require that the machine be constructed or the method to be operated in a particular orientation. Terms such as “connected,” “connecting,” “attached,” “attaching,” “join,” and “joining” are used interchangeably and refer to one structure or surface being secured to another structure or surface or integrally fabricated in one piece.[0122 J For purposes of the disclosure, the term “at least” followed by a number is used herein to denote the start of a range beginning with that number (which may be a ranger having an upper limit or no upper limit, depending on the variable being defined). For example, “at least 1” means 1 or more than 1. The term “at most” followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending upon the variable being defined). For example, “at most 4” means 4 or less than 4, and “at most 40%” means 40% or less than 40%. Terms of approximation (e.g., “about”, “substantially”, “approximately”, etc.) should be interpreted according to their ordinary and customary meanings as used in the associated art unless indicated otherwise. Absent a specific definition and absent ordinary and customary usage in the associated art, such terms should be interpreted to be ± 10% of the base value.

[0123] When, in this document, a range is given as “(a first number) to (a second number)” or “(a first number) - (a second number)”, this means a range whose lower limit is the first number and whose upper limit is the second number. For example, 25 to 100 should be interpreted to mean a range whose lower limit is 25 and whose upper limit is 100. Additionally, it should be noted that where a range is given, every possible subrange or interval within that range is also specifically intended unless the context indicates to the contrary. For example, if the specification indicates a range of 25 to 100 such range is also intended to include subranges such as 26 -100, 27-100, etc., 25-99, 25-98, etc., as well as any other possible combination of lower and upper values within the stated range, e.g., 33-47, 60-97, 41-45, 28-Docket No.: P3037PC01 96, etc. Note that integer range values have been used in this paragraph for purposes of illustration only and decimal and fractional values (e.g., 46.7 - 91.3) should also be understood to be intended as possible subrange endpoints unless specifically excluded.

[0124] Although an overview of the disclosed subject matter has been described with reference to specific example embodiments, various modifications and changes may be made to these embodiments without departing from the broader scope of embodiments of the present invention. For example, various embodiments or features thereof may be mixed and matched or made optional by a person of ordinary skill in the art. Such embodiments of the present subject matter may be referred to herein, individually or collectively, by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or present concept if more than one is, in fact, disclosed.

[0125] The embodiments illustrated herein arc believed to be described in sufficient detail to enable those skilled in the art to practice the teachings disclosed. Other embodiments may be used and derived therefrom, such that structural and logical substitutions and changes may be made without departing from the scope of this disclosure. The Detailed Description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled.

Claims

Docket No.: P3037PC01 WHAT IS CLAIMED:

1. A burner assembly (100) for combusting gaseous fuels, comprising:a burner body (100) configured for mounting to a heater or furnace wall (102) and defining a longitudinal burner flow axis (A- A);a mixing tube assembly (170) extending through the burner body (100) along the longitudinal burner flow axis (A- A) and defining an internal mixing region (174), the mixing tube assembly (170) being spaced from the heater or furnace wall (102) to define an annular gap (178) therebetween, the annular gap (178) placing the internal mixing region (174) in fluid communication with a heater or furnace volume (108);a combustion air delivery system (110, 176) configured to discharge combustion air (302) into the internal mixing region (174);a secondary fuel circuit (156A, 162 A, 164) comprising one or more forward-facing secondary fuel ports (164) configured to inject a secondary fuel stream (304) into the internal mixing region (174) in a downstream direction relative to the longitudinal burner flow axis (A-A); anda tertiary fuel circuit (156B, 162B, 166) comprising one or more rearward-facing tertiary fuel ports (166) disposed downstream of the mixing tube assembly (170) and configured to inject a tertiary fuel stream (306) into the heater or furnace volume (108) in a direction having an upstream component relative to the longitudinal burner flow axis (A-A);wherein the combustion air (302) discharged into the internal mixing region (174) acts as a primary fluid that entrains flue gas (308) and the tertiary fuel stream (306) from the heater or furnace volume (108) through the annular gap (178) and into the internal mixing region (174) prior to combustion.

2. The burner assembly (100) according to claim 1, wherein the combustion air delivery system comprises a plurality of combustion air conduits (176) arranged circumferentially about the mixing tube assembly (170).

3. The burner assembly (100) according to claim 2, wherein the combustion air conduits (176) are oriented at an acute angle relative to the longitudinal burner flow axis (A-A) to promote stratification of entrained flue gas (308) toward an outer wall of the mixing tube assembly (170).

4. The burner assembly (100) according to any one of claims 1 to 3, wherein the one or more forward-facing secondary fuel ports (164) are oriented at an angle of 0° to 45° downstream relative to the longitudinal burner flow axis (A-A).Docket No.: P3037PC01 5. The burner assembly (100) according to any one of claims 1 to 4, wherein the one or more rearward-facing tertiary fuel ports (166) are oriented at an angle of 15° to 45° upstream relative to the longitudinal burner flow axis (A-A).

6. The burner assembly (100) according to any one of claims 1 to 5, further comprising a swirler hub assembly (190) disposed downstream of the mixing tube assembly (170) and comprising swirl vanes (1 4) configured to impart swirl to a mixture flowing from the internal mixing region (174).

7. The burner assembly (100) according to claim 6, wherein the swirler hub assembly (190) comprises an expanding cone (200) defining a diverging flow passage (202) downstream of the swirl vanes (194).

8. The burner assembly (100) according to any one of claims 1 to 7, further comprising a primary fuel circuit (212, 214) having at least one forward-facing primary fuel port (214) configured to supply a primary fuel stream (300) for ignition.

9. The burner assembly (100) according to claim 8, further comprising a flame holder (216) disposed downstream of the at least one forward-facing primary fuel port (214) and configured to stabilize a flame at low flow rates.

10. The burner assembly (100) according to any one of claims 1 to 9, wherein the tertiary fuel stream (306) does not manifest as a flame until after being entrained through the annular gap (178) and mixed in the internal mixing region (174).

11. The burner assembly (100) according to any one of claims 1 to 10, further comprising a controller configured to regulate at least one of combustion air flow or fuel flow in response to operating conditions.

12. The burner assembly (100) according to any one of claims 1 to 11, wherein the gaseous fuel comprises natural gas, a hydrogen-containing fuel, biogas, or a combination thereof.

13. A method of operating a burner assembly (100) for combusting gaseous fuels, the method comprising:supplying combustion air (302) into an internal mixing region (174) of a mixing tube assembly (170);injecting a tertiary fuel stream (306) into a heater or furnace volume (108) in a direction having an upstream component relative to a longitudinal burner flow axis (A-A) using one or more rearward-facing tertiary fuel ports (166):Docket No.: P3037PC01 using momentum of the combustion air (302) as a primary fluid to entrain flue gas (308) and the tertiary fuel stream (306) from the heater or furnace volume (108) through an annular gap (178) and into the internal mixing region (174);injecting a secondary fuel stream (304) into the internal mixing region (174) in a downstream direction relative to the longitudinal burner flow axis (A-A) using one or more forward-facing secondary fuel ports (164); andigniting a mixture formed in the internal mixing region (174) downstream of the mixing tube assembly (170).

14. The method according to claim 13, wherein the tertiary fuel stream (306) is injected from the one or more rearward-facing tertiary fuel ports (166) oriented at an angle of 15° to 45° upstream relative to the longitudinal burner flow axis (A-A).

15. The method according to claim 13 or 14, wherein the combustion air (302) is supplied through a plurality of conduits (176) arranged circumferentially about the mixing tube assembly (170).

16. The method according to any one of claims 13 to 15, wherein the tertiary fuel stream (306) is entrained together with flue gas (308) into the internal mixing region (174) prior to ignition.

17. The method according to any one of claims 13 to 16, further comprising imparting swirl to the mixture downstream of the internal mixing region (174) using swirl vanes (194) of a swirler hub assembly (190) to stabilize combustion.

18. The method according to any one of claims 13 to 17, wherein at least one of steam or an inert gas is injected into the combustion air (302) or a fuel stream (300, 304, 306).

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