WO2025075713A1 - Burner and method using a flexible fuel including hydrogen - Google Patents

Abstract

A flexible fuel burner includes provisions for combustion of fuels including various amounts of hydrogen while preventing flashback.

Description

BURNER AND METHOD USING A FLEXIBLE FUEL INCLUDING HYDROGEN

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of co-pending U.S. Provisional Patent Application No. 63/587,363, filed October 2, 2023 entitled “BURNER AND METHOD USING A FLEXIBLE FUEL INCLUDING HYDROGEN.”

The foregoing application is incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The inventions disclosed herein were made with U.S. Government support pursuant to Department of Energy, Office of Science Contract No. DE- SC0022909. Accordingly, the Government may hold certain rights in inventions disclosed and claimed herein.

SUMMARY

According to an embodiment, a flexible fuel burner includes a combustion air conduit configured to convey combustion air to a distal position in a furnace. A fuel and flue gas conduit is configured to convey flue gas and main fuel to the distal position in the furnace. A distal flame holder is configured to hold a combustion reaction of the main fuel and combustion air at a position distal to outlets of the combustion air conduit and the fuel and flue gas conduit in the furnace. According to another embodiment, a flexible fuel burner includes a mixing tube configured to convey at least combustion air to a distal position within a furnace. The flexible fuel burner includes at least one of a low hydrogen fuel nozzle disposed at a proximal position within the mixing tube and/or a high hydrogen fuel nozzle disposed at the distal position at or near an outlet of the mixing tube. A flame holder is disposed distal to the outlet of the mixing tube to receive the combustion air and the low hydrogen or high hydrogen fuel from the outlet of the mixing tube, the flame holder being configured to hold a main combustion reaction. A distal pilot burner is disposed at the distal position, the distal pilot burner being configured to combust pilot combustion air and the low hydrogen or the high hydrogen fuel to maintain a pilot flame at the distal position. According to an embodiment, the distal pilot burner is a pre-mixed burner that inserts fuel and entrained combustion air at or near the proximal position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side sectional diagram of a flexible fuel burner system, according to an embodiment.

FIG. 2A is a rendered view of a flexible fuel burner system, according to an embodiment.

FIG. 2B is another rendered view of a flexible fuel burner system, according to an embodiment.

FIG. 3A is a view of a distal flame holder, according to an embodiment.

FIG. 3B is a view of the distal flame holder of FIG. 3A, according to an embodiment.

FIG. 3C is a view of the distal flame holder of FIG. 3A, according to an embodiment.

FIG. 3D is a view of the distal flame holder of FIG. 3A, according to an embodiment. FIG. 4 is a side-sectional view of a flexible fuel burner system, according to an embodiment.

FIG. 5A is a rendered view of the flexible fuel burner system of FIG. 4, according to an embodiment.

FIG. 5B is a rendered view of the flexible fuel burner system of FIG. 4, according to an embodiment.

FIG. 6 is a flow chart outlining a method of operating a furnace, according to an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description and drawings are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein.

Various structures configured to convey fluids such as fuel, air, flue gas, etc., are described herein, and referred to using terms such as conduit, tube, mixing tube, etc. While the precise function of such a structure may vary according to the particular embodiment in which it is used. For example, a conduit that receives two or more gas phases at an inlet will typically induce mixing of the two or more gas phases as they move to an outlet to form a homogeneous mixture; in which case, the conduit may operate as and/or be referred to as a mixing tube.

FIG. 1 is a side sectional diagram of a flexible fuel burner system 100, according to an embodiment. FIG. 2A is a rendered view of a flexible fuel burner system 200, according to an embodiment. FIG. 2B is another rendered view of the flexible fuel burner system 200, according to an embodiment. Referring to FIGS. 1, 2A and 2B, the flexible fuel burner 100 includes a combustion air conduit 102 configured to convey combustion air 103 to a distal position 104 in a furnace 106. A fuel and flue gas conduit 108 is configured to convey flue gas 107 and main fuel 109 to the distal position 104 in the furnace 106. A distal flame holder 110 is configured to hold a combustion reaction of the main fuel 109 and combustion air 103 at the distal position 104 in the furnace 106. The distal position 104 is preferably selected to ensure enough length of the fuel and flue gas conduit 108 to cause the conveyed main fuel 109 and flue gas 107 to fully mix.

As used here, the distal position 104 refers to a region within the furnace volume 114 extending distally from the outlets of the combustion air conduit 102 and the fuel and flue gas conduit 108 — and, in other embodiments, other conduits and mixing tubes — to at least as far as the pilot burner and the flame holder 110.

The combustion air conduit 102 and the fuel and flue gas conduit 108 may be configured to cooperate to maintain the main fuel 109 above a fuel-rich combustion limit in the fuel and flue gas conduit 108, and maintain the combustion air 103 below a fuel-lean combustion limit in the combustion air conduit 102 until the main fuel 109, flue gas 107, and combustion air 103 mix coincident with the distal flame holder 110.

This approach substantially prevents the combustion reaction at the distal position 104 from flashing back through either the combustion air conduit 102 or the fuel and flue gas conduit 108. Flashback through the combustion air conduit does not happen because an amount of fuel in the air conduit 102 is too low to burn, the gas therein being below the fuel-lean combustion limit. Flashback through the fuel and flue gas conduit 108 is prevented because an amount of oxygen in the fuel and flue gas conduit 108 is too low to burn the main fuel 109, the mixture being above the fuel-rich combustion limit. This arrangement is particularly advantageous in embodiments in which the main fuel includes a portion of hydrogen, which has a high flame propagation speed and a correspondingly high risk of flashback. Meanwhile, the length of the fuel and flue gas conduit 108 permits good mixing of the main fuel and flue gas, which reduces oxides of nitrogen (NOx) production in the combustion reaction.

The flexible fuel burner 100 may further include a pilot burner 112 disposed to output a pilot flame at the distal position 104. The pilot burner 112 may be configured to operate to ensure the combustion reaction at the distal flame holder 110. The pilot burner 112 may be configured to cause a combustion reaction of gases including the main fuel 109, flue gas 107, and combustion air 103 to initiate at the distal position 104 in the furnace 106.

The pilot burner 112 may be configured to start combustion to reduce oxygen in a furnace volume 114 to below 5% molar concentration prior to output of main fuel 109 from a main fuel nozzle 116. Molar concentration is typically approximated by concentration by volume (assuming ideal gas behavior). This approach ensures that the main fuel 109 conveyed from the main fuel nozzle 116 to the distal position 104 by the fuel and flue gas conduit 108 is above a fuel-rich combustion limit, by virtue of the low oxygen concentration in the furnace volume 114, and particularly in the flue gas 107 that is conveyed within the fuel and flue gas conduit 108. The main fuel nozzle 116 and the fuel and flue gas conduit 108 may be configured to cause flow of the main fuel 109 into the fuel and flue gas conduit 108 to entrain flue gas 107.

The pilot burner 112 may be configured to cause combustion of oxygen in air present in the furnace volume 114 prior to commencing flow of main fuel 109 from the main fuel nozzle 116 to be consumed prior to flowing the main fuel 109 through the fuel and flue gas conduit 108. The pilot burner 112 may be configured to burn a fuel including a hydrocarbon, at least as a component. The pilot burner 112 may be configured to burn main fuel 109 conveyed by the fuel and flue gas conduit. The pilot burner 112 may include a pilot burner fuel nozzle 120 disposed near a proximal position 118. The pilot burner fuel nozzle 120 may entrain pilot combustion air 123 received via a pilot burner air register 122. Optionally, the pilot burner 112 may be controlled by an electronic burner controller 406 (described below) via a pilot burner fuel valve. The combustion air conduit 102 may include a combustion air outlet 205 at the distal position 104. The pilot burner 112 may be disposed within a perimeter defined by the combustion air outlet 205. The combustion air conduit 102 may include a flue gas inlet 202 at a position along the combustion air conduit 102 between a proximal position 118 and the distal position 104.

The fuel and flue gas conduit 108 may operate as a mixing tube configured to mix the main fuel 109 and flue gas 107 to produce mixed fuel and flue gas when the main fuel and flue gas arrive at the distal position 104.

According to an embodiment, the flexible fuel burner 100 further includes a combustion air preheater 124 configured to preheat combustion air 103a prior to introducing the preheated combustion air into the combustion air conduit 102. To preheat the combustion air 103a, the combustion air preheater 124 may use waste heat, heated gas or other fluid, electrical energy, etc. For example, heat may be harvested from flue gas via a heat exchanger to preheat the combustion air 103a.

Combustion air 103 and/or preheated combustion air 103a may be introduced into the combustion air conduit 102 at or near the proximal position 118 and/or at a position between the proximal position and the distal position 104.

FIG. 3A is a view of a distal flame holder 110, according to an embodiment. FIG. 3B is a view of the distal flame holder 110 of FIG. 3A, according to an embodiment. FIG. 3C is a view of the distal flame holder 110 of FIG. 3A, according to an embodiment. FIG. 3D is a view of the distal flame holder 110 of FIG. 3A, according to an embodiment.

Referring to FIGS. 3A, 3B, 3C and 3D, the distal flame holder 110 may include a perforated tile, a V-gutter, a venturi, a half-venturi, a swirler, and/or an apparatus to produce a low pressure region coincident with a flame front. The distal flame holder 110 may include a refractory material. The distal flame holder 110 may include silicon carbide. In the embodiment shown in FIGS. 3A-3D, the flame holder 110 includes a V-gutter flame holder element 302. A pilot burner 112 is positioned so as to support a pilot flame that is distal to the V-gutter flame holder element 302.

Additionally or alternatively, the inventors contemplate the use of various high temperature metals, such as superalloys (e.g., Hastelloy, Inconel, etc.), for example.

The inventors contemplate alternative distal flame holder 110 types, some of which include structures positioned between the distal position 104 and a proximal position 118. The distal flame holder 110 may include at least one selected from the group consisting of a jet stabilization apparatus, a diffuser, a choke, a half venturi, a bluff body, a solid bluff body, and a swirler.

The burner 100 may support a combustion reaction that outputs below 5 parts per million oxides of nitrogen (NOx).

The fuel and flue gas conduit 108 may include a plurality of fuel and flue gas conduits, the plural number of fuel and flue gas conduits 108 being selected to deliver volume of fuel corresponding to a burner capacity at a fuel pressure received by the plural number of main fuel nozzles 116.

The fuel and flue gas conduit 108 may have a cross sectional flow area sufficiently small to cause the fuel to emerge from a distal position end of the flue gas conduit and ignite only after travelling a distance sufficient to prevent ignition of fuel inside the distal end of the flue gas conduit.

According to an embodiment, the flexible fuel burner 100, 200 includes a combustion air conduit 102 configured to convey combustion air 103 to a distal position 104. A fuel and flue gas conduit 108 is configured to convey flue gas 107 and main fuel 109 to the distal position 104. A distal pilot burner 112 is configured to support a pilot flame disposed to ignite the main fuel, combustion air, and flue gas mixture at the distal position 104.

The combustion air conduit 102 and the fuel and flue gas conduit 108 may be configured to cooperate to maintain the main fuel 109 above a fuel-rich combustion limit and maintain the combustion air 103 below a fuel-lean combustion limit until the main fuel 109, flue gas 107, and combustion air 103 mix proximal to or coincident with the pilot flame.

In operation of the furnace 106, according to an embodiment, a fuel 109 containing hydrogen and flue gas 107 are introduced into a proximal end of a first conduit, i.e., the fuel and flue gas conduit 108. Combustion air 103 is introduced into a proximal end of a second conduit, i.e., the combustion air conduit 102. The main fuel 109 and flue gas 107 mix along the length of the first conduit 108 and the mixture is emitted from the distal end of the first conduit. Meanwhile, the combustion air 103 is emitted from the distal end of the second conduit 102, which is approximately adjacent to the distal end of the first conduit. A pilot flame supported by the pilot burner 112 ignites the main fuel 109, flue gas 107, and combustion gas 103 at a position between the distal ends of the first and second conduits and the flame holder 110 producing a combustion reaction that is held or stabilized by the flame holder.

According to an embodiment, a formulation of the mixture of fuel and flue gas within the first conduit 108 is controlled to be above a fuel-rich combustion limit of the fuel 109. This is done by controlling an oxygen concentration in the flue gas 107 which thereby controls an oxygen concentration in the first conduit 108. During normal operation of the furnace 106, the oxygen concentration in the flue gas 107 can be controlled using various known methods, including controlling the volume of combustion air 103 that is admitted, controlling the size of the pilot flame, regulating the volume of gases that exit the furnace 106, etc. However, prior to startup of the furnace 106, oxygen concentration in the interior of the furnace 106 is typically about equal to ambient levels outside the furnace. According to an embodiment, prior to startup of the furnace 106, the oxygen level may be reduced by operating the pilot burner 112 for a period of time while monitoring the oxygen level. Once the oxygen level has dropped to a desired value, a valve controlling a flow of main fuel 109 to a fuel nozzle 116 positioned near the proximal end of the first conduit 108 may be opened, emitting a stream of main fuel 109 from the nozzle into the first conduit. Flue gas 107 is drawn into the first conduit 108 and entrained by the stream of main fuel 109 from the nozzle 116.

According to an embodiment, the flue gas is controlled to have an oxygen concentration below 5%. According to another embodiment, the flue gas is controlled to have an oxygen concentration below 3%.

The length of the first conduit permits a substantially complete mixture of the main fuel and flue gas before they are emitted from the distal end of the first conduit, where the mixture is combusted with combustion air. The inventors have found that this configuration results in a production of nitrogen oxides in the furnace at a level below 5 parts per million. According to an embodiment, providing a supply of combustion air that is external to the first conduit can also reduce or prevent the occurrence of flashback into the first conduit even when fuels containing hydrogen are used.

FIG. 4 is a side-sectional view of a flexible fuel burner system 400, according to an embodiment. FIG. 5A is a rendered view 500 of the flexible fuel burner system 400 of FIG. 4, according to an embodiment. FIG. 5B is a rendered view 501 of the flexible fuel burner system 400 of FIG. 4, according to an embodiment.

According to an embodiment, the flexible fuel burner 400 includes a main conduit 403 configured to convey at least combustion air 103 to a distal position 104. The flexible fuel burner 400 includes at least one of a low hydrogen fuel nozzle 402 disposed at a proximal position 118 within the main conduit 403 and/or a high hydrogen fuel nozzle 404 disposed at the distal position 104 at or near an outlet of the main conduit 403. A flame holder 110 is disposed at the distal position 104 to receive the combustion air 103 and the low hydrogen or high hydrogen fuel from the outlet of the main conduit 403, the flame holder 110 being configured to hold a main combustion reaction. A distal pilot burner 112 disposed at the distal position 104, the distal pilot burner 112 being configured to combust pilot combustion air and the low hydrogen or the high hydrogen fuel 401 , 407, to maintain a pilot flame at or near the distal position 104. According to an embodiment, the distal pilot burner 112 is a pre-mixed burner that inserts fuel and entrained combustion air 103 at or near the proximal position 118.

The at least one of the low hydrogen fuel nozzle 402 or the high hydrogen fuel nozzle 404 may include both a low hydrogen fuel nozzle 402 and a high hydrogen fuel nozzle 404.

A control system 406 operatively coupled to at least one fuel valve 408a, 408b may be configured to control fuel flow to the high hydrogen fuel nozzle 404 and the low hydrogen fuel nozzle 402. The control system 406 may be configured to execute a computer program stored on a non-transitory computer readable medium, the control system 406 and at least one fuel valve 408a, 408b may be configured to cooperate to cause the high hydrogen fuel nozzle 404 to output high hydrogen fuel 407 only during a start-up period when an average oxygen molar concentration in the furnace volume 114 is greater than 9% to 11 %. The startup period may include at least a subperiod characterized by operation of the pilot burner until an oxygen concentration in the furnace volume 114 of 5% or less is achieved. The startup period may include at least a subperiod characterized by operation of the pilot burner until an oxygen concentration in the furnace volume 114 of about 3% is achieved. Selection of a target oxygen concentration may be fixed, or may optionally be adjustable to optimize for fast start-up, maximum stability, to accommodate user processes, such as for visual inspection, and/or to provide compatibility with burner controller characteristics, for example.

The burner system 400 may further include a control system 406 operatively coupled to at least one fuel valve 408a, 408b configured to control fuel flow to the high hydrogen fuel nozzle 404 and the low hydrogen fuel nozzle 402. According to an embodiment, the control system 406 may be configured to execute a computer program stored on a non-transitory computer readable medium. The control system 406 and the fuel valve 408b may be configured to cooperate to cause the low hydrogen fuel nozzle 402 to output only low hydrogen fuel 401 , such as a petroleum gas. The term "low hydrogen fuel" may be defined as a fuel having a hydrogen concentration of no more than about 55% by volume. In another embodiment, the term "low hydrogen fuel" may be defined as a fuel having a hydrogen concentration of no more than about 60% by volume, no more than 40% by volume, or another concentration determined to provide stable combustion under conditions at the installed location, depending on operating protocols, depending on burner and nozzle dimensions or number for a given installation, and/or depending on regulations and available fuel supply.

At least one secondary fuel nozzle 411 may be disposed at a proximal position 118 and configured to output a secondary fuel 414 into a corresponding at least one secondary conduit 409. The secondary conduit 409 may be arranged to function as a mixing tube and carry a mixed gas including the secondary fuel 414 from the proximal position 118 to the distal position 104.

The high hydrogen fuel nozzle 404 may include a startup burner configured to reduce an oxygen concentration in a furnace 406 defining a combustion space prior to commencing operation of the at least one secondary high hydrogen fuel nozzle 404. The secondary conduit 409 may be configured to convey a mixture of the secondary fuel 414 and flue gas 107 received from a flue gas source.

The flue gas source may include at least a startup combustion reaction in the furnace, e.g., supported by one or more of the high hydrogen fuel nozzles 404 or one or more low hydrogen fuel nozzles 402. The flue gas source may include combustion reactions in the furnace during normal operation. The flue gas source may include an external flue gas recirculation (EFGR) conduit.

According to an embodiment, the at least one secondary conduit 409 may be characterized by a cross-sectional area less than half a cross-sectional area of the main conduit 403. According to another embodiment, the at least one secondary conduit 409 may be characterized by a cross-sectional area less than one-third a cross-sectional area of the main conduit 403.

The at least one secondary conduit 409 may be characterized by a cross- sectional area selected to minimize both a NOx output by the main combustion reaction and an instability related to flashback from at least a portion of the distance from the distal position 104 to the proximal position 118 over at least one fuel range. The at least one secondary conduit 409 may be sized to maintain a gas velocity greater than a flame speed of the secondary fuel.

The distal pilot burner 112 may be configured to combust pilot combustion air and the same low hydrogen or the high hydrogen fuel 404, 407 output by the high hydrogen and/or low hydrogen fuel nozzle(s) 402, 404.

The distal flame holder 110 may include at least one of a solid refractory body, a solid ceramic body, and/or a perforated or porous ceramic such as a reticulated ceramic. Additionally, the distal flame holder 110 may be configured to support a combustion reaction upstream, downstream, and/or within the distal flame holder 110. In other embodiments, the distal flame holder may provide flame stabilization at a selected distance from an aerodynamic feature, such that the distal flame holder includes a ridge, choke, horn, swirler, or other feature that produces the flame stabilization effect.

The distal flame holder 110 may include an outward flare of the distal end 416 of the main conduit 403.

According to an embodiment, at least one of the low hydrogen fuel nozzle 402 or the high hydrogen fuel nozzle 404 may consist of the low hydrogen fuel nozzle 402. The flexible fuel burner 400 may be limited to fuels having no more than 50-70% hydrogen. According to another embodiment, the at least one of the low hydrogen fuel nozzle or the high hydrogen fuel nozzle may consist of the high hydrogen fuel nozzle 404. The flexible fuel burner 400 may be limited to fuels having at least 50-70% hydrogen.

According to a further embodiment, the at least one of the low hydrogen fuel nozzle 402 or the high hydrogen fuel nozzle 404 consists of at least one variable-position fuel nozzle configured to have at least a low hydrogen operating position at the proximal position 118 and a high hydrogen operating position at the distal position 104.

According to an embodiment, during operation of the furnace 406, a low- hydrogen fuel 401 is emitted from a first nozzle, e.g., the low hydrogen fuel nozzle 402 positioned near a proximal end 414 of the main conduit 403. Combustion air 103 is drawn into the main conduit 403 via a register 405 near a proximal end 414 of the main conduit and entrained by the stream of fuel emitted by the first nozzle 404. The fuel 401 and combustion air 103 mix along the length of the main conduit 403. The mixture is emitted from a distal end 416 of the main conduit 403 and ignited at the distal position 104. A combustion reaction of the ignited mixture is supported by a flame holder 110.

According to an embodiment, the mixture of fuel 401 and combustion air 103 is ignited by a pilot flame that is supported by a pilot burner 112.

According to an embodiment, a high-hydrogen fuel 407 is emitted from a second nozzle, e.g., the high hydrogen fuel nozzle 404, positioned in the main conduit 403 near the distal end 416 of the main conduit.

According to an embodiment, a percentage of oxygen in the furnace 406 may be reduced, prior to operation of the furnace, by burning fuel 407 emitted from the second nozzle 409 prior to emitting fuel 401 from the first nozzle 402.

FIG. 6 is a flow chart outlining a method 600 of operating a furnace similar to the furnace described above with reference to FIGS. 1-3D, according to an embodiment.

In step 602, oxygen is reduced in the furnace by operating a pilot flame within the furnace volume. This process may be performed prior to initiating normal operation of the furnace if an oxygen level is above a selected threshold, such as, for example, 5% or 3%.

For normal operation, a fuel containing hydrogen is introduced into a proximal end of a first conduit in step 604, flue gas is introduced into the proximal end of the first conduit in step 606, and combustion air is introduced into a proximal end of a second conduit in step 608. In step 610, a mixture of the fuel and flue gas is emitted from a distal end of the first conduit while, in step 612, the combustion air is emitted from a distal end of the second conduit, which is positioned approximately adjacent to the distal end of the first conduit. In step 614, the mixture of the fuel and flue gas and the combustion air are ignited by a pilot flame and the resulting combustion reaction is supported by a flame holder at a distal position within the furnace 406. While various aspects and embodiments have been disclosed herein, other aspects and embodiments are contemplated. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

CLAIMS What is claimed is:

1 . A flexible fuel burner, comprising: a combustion air conduit having an inlet and an outlet and being configured to convey combustion air to a distal position in a furnace; a fuel and flue gas conduit having an inlet and an outlet and being configured to convey flue gas and main fuel to the distal position in the furnace; and a distal flame holder configured to hold a combustion reaction of the fuel and combustion air at the distal position in the furnace.

2. The flexible fuel burner of claim 1 , wherein the combustion air conduit and the fuel and flue gas conduit are configured to cooperate to maintain a mixture of fuel and flue gas above a fuel-rich combustion limit in the fuel and flue gas conduit, and maintain gas, including the combustion air, below a fuel-lean combustion limit in the combustion air conduit until the fuel, flue gas, and combustion air mix coincident with the distal flame holder.

3. The flexible fuel burner of claim 1 , further comprising: a pilot burner disposed to output a pilot flame at the distal position.

4. The flexible fuel burner of claim 3, wherein the pilot burner is configured to cause a combustion reaction of gases including the fuel and flue gas, and the combustion air to initiate at the distal position in the furnace.

5. The flexible fuel burner of claim 3, wherein the pilot burner is configured to start combustion to reduce oxygen in a furnace volume to below 5% molar concentration prior to output of main fuel from a main fuel nozzle.

6. The flexible fuel burner of claim 5, wherein the main fuel nozzle and the fuel and flue gas conduit are configured to cause flow of the main fuel into the fuel and flue gas conduit to entrain flue gas.

7. The flexible fuel burner of claim 3, wherein the pilot burner is configured to burn a fuel including a hydrocarbon.

8. The flexible fuel burner of claim 3, wherein the pilot burner is configured to burn main fuel conveyed by the fuel and flue gas conduit.

9. The flexible fuel burner of claim 1 , wherein the combustion air conduit includes a flue gas inlet at a position along the combustion air conduit between a proximal position and the distal position.

10. The flexible fuel burner of claim 1 , wherein the fuel and flue gas conduit comprises a mixing tube configured to mix the fuel and flue gas to produce mixed fuel and flue gas when the fuel and flue gas arrive at the distal position.

11 . The flexible fuel burner of claim 1 , wherein the distal flame holder includes a perforated flame holder.

12. The flexible fuel burner of claim 1 , wherein the distal flame holder includes a V-gutter.

13. The flexible fuel burner of claim 1 , wherein the distal flame holder includes a refractory material.

14. The flexible fuel burner of claim 11 , wherein the distal flame holder includes silicon carbide.

15. The flexible fuel burner of claim 1 , wherein the burner supports a combustion reaction that outputs below 5 parts per million (ppm) oxides of nitrogen.

16. The flexible fuel burner of claim 1 , wherein the fuel and flue gas conduit includes a plurality of fuel and flue gas conduits.

17. The flexible fuel burner of claim 1 , wherein the fuel and flue gas conduit includes a plurality of fuel and flue gas conduits, the plural number of fuel and flue gas conduits being selected to deliver volume of fuel corresponding to a burner capacity at a fuel pressure received by the plural number of main fuel nozzles.

18. The flexible fuel burner of claim 1 , wherein the fuel and flue gas conduit has a cross sectional flow area sufficiently small to cause the fuel to emerge from a distal position end of the flue gas conduit and ignite only after travelling a distance sufficient to prevent ignition of fuel inside the distal end of the flue gas conduit.

19. The flexible fuel burner of claim 1 , further comprising: a combustion air preheater configured to preheat combustion air prior to introducing the preheated combustion air into the combustion air conduit.

20. A flexible fuel burner, comprising: a combustion air conduit configured to convey combustion air to a distal position; a fuel and flue gas conduit configured to convey flue gas and main fuel to the distal position; and a distal pilot burner configured to support a pilot flame disposed to ignite the main fuel, combustion air, and flue gas mixture at the distal position.

21 . The flexible fuel burner of claim 20, wherein the combustion air conduit and the fuel and flue gas conduit are configured to cooperate to maintain the main fuel above a fuel-rich combustion limit and maintain the combustion air below a fuel-lean combustion limit until the main fuel, flue gas, and combustion air mix coincident with the pilot flame.

22. A flexible fuel burner, comprising: a main conduit configured to convey at least combustion air to a distal position; at least one of: a low hydrogen fuel nozzle disposed at a proximal position within the main conduit; and a high hydrogen fuel nozzle disposed at the distal position at or near an outlet of the main conduit; and a flame holder disposed at the distal position to receive the combustion air and low hydrogen content or high hydrogen content fuel from the outlet of the main conduit, the flame holder being configured to hold a main combustion reaction; and a distal pilot burner disposed at the distal position, the distal pilot burner being configured to combust pilot combustion air and fuel to maintain a pilot flame at the distal position.

23. The flexible fuel burner of claim 22, wherein the distal pilot fuel includes natural gas.

24. The flexible fuel burner of claim 22, wherein the distal pilot fuel consists of the same fuel as is provided to the flame holder.

25. The flexible fuel burner of claim 22, wherein the distal pilot fuel consists of the low hydrogen content or the high hydrogen content fuel.

26. The flexible fuel burner of claim 22, wherein the at least one of the low hydrogen fuel nozzle or the high hydrogen fuel nozzle includes both a low hydrogen fuel nozzle and a high hydrogen fuel nozzle.

27. The flexible fuel burner of claim 26, further comprising: a control system operatively coupled to at least one fuel valve configured to control fuel flow to the high hydrogen fuel nozzle and the low hydrogen fuel nozzle; wherein the control system is configured to execute a computer program stored on a non-transitory computer readable medium, the control system and at least one fuel valve being configured to cooperate to cause: the high hydrogen fuel nozzle to output high hydrogen content fuel only during a start-up period when an average oxygen molar concentration in the furnace volume greater than 9% to 11 %.

28. The flexible fuel burner of claim 26, further comprising: a control system operatively coupled to at least one fuel valve configured to control fuel flow to the high hydrogen fuel nozzle and the low hydrogen fuel nozzle; wherein the control system is configured to execute a computer program stored on a non-transitory computer readable medium, the control system and at least one fuel valve being configured to cooperate to cause: the low hydrogen fuel nozzle to output only low hydrogen fuel having a hydrogen concentration substantially 0-55% by volume.

29. The flexible fuel burner of claim 22, further comprising: at least one secondary fuel nozzle disposed at a proximal position and configured to output a secondary fuel into a corresponding at least one secondary conduit; and the secondary conduit arranged to carry a mixed gas including the secondary fuel from the proximal position to the distal position; wherein the high hydrogen fuel nozzle comprises a startup burner configured to reduce an oxygen concentration in a furnace defining a combustion space prior to commencing operation of the at least one secondary high hydrogen fuel nozzle.

30. The flexible fuel burner of claim 29, wherein the secondary conduit is configured to convey a mixture of the secondary fuel and flue gas received from a flue gas source.

31 . The flexible fuel burner of claim 30, wherein the flue gas source comprises at least a startup combustion reaction in the furnace.

32. The flexible fuel burner of claim 30, wherein the flue gas source comprises combustion reactions in the furnace during normal operation.

33. The flexible fuel burner of claim 30, wherein the flue gas source comprises an external flue gas recirculation conduit.

34. The flexible fuel burner of claim 29, wherein the at least one secondary conduit is characterized by a cross-sectional area less than half a cross-sectional area of the main conduit.

35. The flexible fuel burner of claim 29, wherein the at least one secondary conduit is characterized by a cross-sectional area selected to minimize both a NOx output by the main combustion reaction and an instability related to flashback from at least a portion of the distance from the distal position to the proximal position over at least one fuel range.

36. The flexible fuel burner of claim 22, wherein the at least one secondary conduit is sized to maintain a gaseous velocity greater than a flame speed of the secondary fuel.

37. The flexible fuel burner of claim 22, wherein the at least one secondary conduit is sized to cause a residence time of secondary fuel within the secondary conduit shorter than a chemical ignition delay time of the fuel and oxidant in the flue gases traversing the mixing volume.

38. The flexible fuel burner of claim 22, wherein the distal pilot burner is configured to combust pilot combustion air and the same low hydrogen or the high hydrogen fuel output by the high hydrogen and/or low hydrogen fuel nozzle(s).

39. The flexible fuel burner of claim 22, wherein the distal flame holder includes at least one of the group consisting of a perforated tile, a V-gutter, a bluff body, a venturi, a half-venturi, a swirler, and an apparatus to produce a low pressure region coincident with a flame front.

40. The flexible fuel burner of claim 22, wherein the at least one of the low hydrogen fuel nozzle or the high hydrogen fuel nozzle consists of at least one variable-position fuel nozzle configured to have at least a low hydrogen operating position at the proximal position and a high hydrogen operating position at the distal position.

41 . A method of operating a furnace, comprising: introducing fuel containing hydrogen into a proximal end of a first conduit; introducing flue gas into the proximal end of the first conduit; introducing combustion air into a proximal end of a second conduit; emitting a mixture of the fuel and flue gas from a distal end of the first conduit; emitting the combustion air from a distal end of the second conduit, the distal ends of the first and second conduits being approximately adjacent to each other at a distal position within the furnace; and supporting a combustion reaction of the fuel, flue gas, and combustion air with a flame holder located at the distal position.

42. The method of claim 41 , comprising igniting the fuel, flue gas, and combustion gas at the distal position.

43. The method of claim 42, wherein igniting the fuel, flue gas, and combustion gas comprises supporting a flame at a pilot burner at the distal position.

44. The method of claim 41 , wherein introducing flue gas into the proximal end of the first conduit comprises maintaining a formulation of the mixture of fuel and flue gas above a fuel-rich combustion limit of the fuel by controlling an oxygen concentration in the first conduit.

45. The method of claim 44, wherein controlling an oxygen concentration in the first conduit comprises controlling an oxygen concentration in the flue gas.

46. The method of claim 45, wherein controlling an oxygen concentration in the flue gas comprises reducing oxygen in the furnace by operating a pilot burner for a period prior to introducing fuel into the proximal end of the first conduit.

47. The method of claim 45, wherein controlling an oxygen concentration in the flue gas comprises controlling an oxygen concentration in the flue gas to be below 5%.

48. The method of claim 45, wherein controlling an oxygen concentration in the flue gas comprises controlling an oxygen concentration in the flue gas to be below 3%.

49. The method of claim 41 , wherein introducing fuel into a proximal end of a first conduit comprises emitting a stream of fuel from a nozzle positioned near the proximal end of the first conduit.

50. The method of claim 49, wherein introducing flue gas into the proximal end of the first conduit comprises entraining flue gas with the stream of fuel.

51 . The method of claim 41 , comprising maintaining a production of nitrogen oxides in the furnace at a level below 5 parts per million by mixing the fuel and flue gas along a length of the first conduit.

52. The method of claim 41 , comprising, prior to introducing combustion air into the proximal end of the second conduit, preheating some or all of the combustion air.

53. A method for operating a burner, comprising: flowing both a gaseous fuel including hydrogen and a flue gas having an oxygen content less than 7% into a flue gas-and-fuel mixing tube; discharging mixed fuel and flue gas from the flue gas-and-fuel mixing tube at a distal position within a furnace; flowing combustion air to the distal position; supporting a pilot flame at the distal position to ignite the discharged mixture of fuel and flue gas and the combustion air at the distal location; and supporting a combustion reaction of the fuel, flue gas, and combustion air with a bluff body flame holder arranged at the distal position.

54. The method of claim 53, wherein supporting a combustion reaction of the fuel, flue gas, and combustion air with a bluff body flame holder comprises supporting the combustion reaction with a bluff body flame holder including a V- gutter-type flame holder element.

55. The method of claim 54, wherein supporting a pilot flame at the distal position comprises supporting the pilot flame at a location that is distal to at least a portion of the bluff body flame holder.

56. A method of operating a furnace, comprising: emitting a low-hydrogen fuel from a first nozzle positioned near a proximal end of a main conduit; entraining, with the fuel from the nozzle, combustion air via the proximal end of the main conduit; mixing the fuel and combustion air along a length of the main conduit; emitting the mixture of fuel and combustion air from a distal end of the main conduit; introducing a secondary fuel into a proximal end of a secondary conduit; introducing flue gas into the proximal end of the secondary conduit; mixing the secondary fuel and the flue gas along a length of the secondary conduit; emitting the mixture of secondary fuel and flue gas from a distal end of the secondary conduit; and igniting the mixture of fuel and combustion air and the mixture of secondary fuel and flue gas at a distal position within the furnace.

57. The method of claim 56, wherein: introducing a secondary fuel into a proximal end of a secondary conduit comprises introducing the secondary fuel into proximal ends of each of a plurality of secondary conduits; introducing flue gas into the proximal end of the secondary conduit comprises introducing flue gas into the proximal ends of each of the plurality of secondary conduits; emitting the mixture of secondary fuel and flue gas from a distal end of the secondary conduit comprises emitting the mixtures of secondary fuel and flue gas from the distal ends of each of the plurality of secondary conduits; and igniting the mixture of fuel and combustion air and the mixture of secondary fuel and flue gas at a distal position within the furnace comprises igniting the mixture of fuel and combustion air and the mixtures of secondary fuel and flue gas from each of the plurality of secondary conduits at the distal position within the furnace.

58. The method of claim 56, further comprising: supporting a combustion reaction of the ignited mixtures with a flame holder located at the distal position.

59. The method of claim 56, wherein igniting the mixture of fuel and combustion air and the mixture of secondary fuel and flue gas comprises supporting a pilot flame at the distal position.

60. The method of claim 56, comprising emitting a high-hydrogen fuel from a second nozzle positioned in the main conduit near the distal end of the main conduit.

61 . The method of claim 60, comprising reducing a percentage of oxygen in the furnace by burning fuel emitted from the second nozzle prior to emitting fuel from the first nozzle.