COMBUSTION REACTION SUPPORTED BY A PERFORATED FLAME HOLDER

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit and is a Continuation-in- Part under 35 USC 120 from PCT Application No. PCT/US2014/016632, (docket number 2651 -188-04), entitled "FUEL COMBUSTION SYSTEM WITH A

PERFORATED REACTION HOLDER", filed February 14, 2014; and claims priority benefit to Provisional Patent Application No. 62/067,945, (docket number 2651 -238-02), entitled "APPLICATION OF AN ELECTRIC FIELD TO A

COMBUSTION REACTION SUPPORTED BY A PERFORATED FLAME

HOLDER", filed October 23, 2014; each of which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

SUMMARY

According to an embodiment, a combustion system includes a fuel and oxidant source configured to emit a fuel and oxidant mixture and a perforated flame holder aligned to receive the fuel and oxidant mixture and to hold a combustion reaction supported by the fuel and oxidant mixture. A voltage source and an electrode operatively coupled to the voltage source are configured to apply an electric field to the combustion reaction held by the perforated flame holder.

According to an embodiment, a method for operating a burner includes outputting a fuel and oxidant mixture, receiving the fuel and oxidant mixture at the perforated flame holder, and supporting a combustion reaction within the perforated flame holder. An electrical field is applied to the combustion reaction in the perforated flame holder. Combustion products can be output including about 3% oxygen and 5 ppm or less oxides of nitrogen (NOx).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a combustion system configured to hold a

combustion reaction with a perforated flame holder and apply an electric field to the combustion reaction, according to an embodiment.

FIG. 2 is a diagram of a combustion system including two electrodes configured to apply an electric field to a combustion reaction held by a perforated flame holder, according to an embodiment.

FIG. 3 is a diagram of a combustion system including a voltage source configured to apply a voltage having a first polarity to a first electrode and apply a second voltage different from the first voltage to a second electrode, the first and second electrodes being configured to apply an electric field to a combustion reaction held by a perforated flame holder, according to an embodiment.

FIG. 4 is a diagram showing a combustion system with an electrode disposed on or adjacent to the surface of the perforated flame holder and a charged particle source configured to apply charged particles to a fuel and/or oxidant, according to an embodiment.

FIG. 5 is a diagram of a combustion system including a plurality of first electrode portions extending into the perforated flame holder and a plurality of second electrode portions also extending into the perforated flame holder and not in contact with the plurality of first electrodes, the first and second electrodes being configured to apply a voltage to a combustion reaction held by the perforated flame holder, according to an embodiment.

FIG. 6 is a diagram of a combustion system including a plurality of first electrode portions extending into the perforated flame holder and a plurality of second electrode portions extending into the perforated flame holder and not in contact with the plural ity of first electrodes, with the plural ity of second electrodes being in continu ity with voltage ground , according to an embod iment.

FIG. 7 is a d iagram of a combustion system includ ing a prem ixed fuel and oxidant source configured to provide prem ixed fuel and oxidant to a perforated flame holder, and includ ing an electrode configured to apply an electric field to a combustion reaction held by the perforated flame holder accord ing to an embod iment.

FIG. 8 is a flow chart showing a method for operating a burner, accord ing to an embod iment.

FIG. 9 is a top view of a perforated flame holder including a pl ural ity of perforations, accord ing to one embod iment.

FIG. 10 is a side-sectional view of the perforated flame holder of FIG. 9, accord ing to one embodiment.

FIG. 11 is a top view of a conductive screen configured to be positioned on or near a perforated flame holder and configured to cooperate to apply an electric field to a combustion reaction held by a perforated flame holder, accord ing to one embodiment.

FIG. 12 is an enlarged side view of perforations of a perforated flame holder including electrodes positioned at the top and bottom of the perforations, accord ing to one embodiment.

FIG. 13 is an enlarged perspective view of one perforation of a perforated flame holder includ ing serpentine electrodes on the walls of the perforation configured to apply an electric field to a combustion reaction held by the perforated flame holder, accord ing to one embod iment

FIG. 14 is an enlarged perspective view of one perforation of a perforated flame holder includ ing straight electrodes on the walls of the perforation, accord ing to one embodiment.

FIG. 15 is an enlarged sectional view of one perforation of a perforated flame holder includ ing electrodes embedded in the walls of the perforated flame holder, accord ing to one embod iment. FIG. 16 is a diagram of a combustion system, according to one

embodiment.

FIG. 17A is a diagram of a combustion system, according to one embodiment.

FIG. 17B is a top view of the insulated electrode from FIG. 17A, according to one embodiment.

FIG. 17C is an enlarged cross-section of the insulated electrode of FIG. 17B, according to an embodiment.

FIG. 18A is a top view of an insulated electrode, according to one embodiment.

FIG. 18B is a cross-section of a portion of the insulated electrode of FIG. 18A, 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. Other embodiments may be used and/or other changes may be made without departing from the spirit or scope of the disclosure.

FIG. 1 is a diagram of a combustion system 100, according to an embodiment. A fuel and oxidant source 102 is configured to emit a fuel and oxidant mixture 104. A perforated flame holder 106 is aligned to receive the fuel and oxidant mixture 104 and configured to hold a combustion reaction supported by the fuel and oxidant mixture 104. A voltage source 108 and an electrode 1 10 operatively coupled to the voltage source 108 are configured to apply an electric field to the combustion reaction held by the perforated flame holder 106.

The perforated flame holder 106 is bounded by an extent. In an embodiment, the extent is bounded by an input surface 107 proximate to the fuel and oxidant source 102 and an output surface 109 distal from the fuel and oxidant source 102. The perforated flame holder 106 is configured to support the combustion reaction within its extent. In other words, the perforated flame holder 106 is configured to hold a majority of the combustion reaction between its input surface 107 and its output surface 109.

The perforated flame holder 106 is configured to receive heat from the combustion reaction and to output heat to the fuel and oxidant mixture 104. In receiving heat from the combustion reaction and outputting heat to the fuel and oxidant mixture, the perforated flame holder 106 stabilizes the combustion reaction. According to embodiments, the perforated flame holder 106 is configured to extend a stability limit of the fuel and oxidant mixture 104

supporting the combustion reaction. It is believed that this operates by ensuring sufficient heat transfer to the fuel and oxidant mixture 104 to maintain

combustion, even of a fuel and oxidant mixture that is too (fuel-) lean to support stable combustion in a conventional flame.

According to embodiments, the fuel and oxidant source 102 is configured to output the fuel and oxidant mixture 104 including a diluent that causes the fuel concentration to be relatively lean. The perforated flame holder 106 is then configured to stabilize the combustion reaction supported by the fuel and oxidant mixture 104 including the relatively lean fuel concentration.

The voltage source 108 is configured to output a voltage selected to cause the electrode 1 10 to apply an electric field to the combustion reaction (supported by the perforated flame holder 106) sufficient to broaden a stability and/or flammability limit of the fuel and oxidant mixture 104. Thus, the combustion reaction receives combined effects of stability and/or flammability limit

broadening from both the heat transfer effects of the perforated flame holder 106, and from the electric field effects from the voltage source 108 and electrode 1 10. The combined effects support cleaner combustion than can normally be supported by a conventional flame. In an embodiment, the combined effects support cleaner combustion than can normally be supported by either individual effect alone. "Cleaner combustion" refers to reduced output of undesirable reaction products such as oxides of nitrogen (NOx) and carbon monoxide (CO). PCT Patent Application No. PCT/US2014/073086, entitled "METHOD AND APPARATUS FOR EXTENDING FLAMMABILITY LIMITS IN COMBUSTION REACTION", filed December 31 , 2014, (docket no. 2651 -203- 04); describes flammability limit broadening responsive to applied electric field strength, and, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

In embodiments, the fuel and oxidant source 102 is configured to output the fuel and oxidant mixture 104 including a diluent that causes the fuel concentration to be lean. The voltage source 108 is configured to output a voltage selected to cause the electrode 1 10 to apply an electric field to the combustion reaction sufficient to stabilize the combustion reaction. According to embodiments, a feedback control mechanism can sense the presence of combustion, and adjust the voltage output by the voltage source (and/or electric field configuration provided by the electrode 1 10) to maintain combustion.

The perforated flame holder 106 can be configured to stabilize the combustion reaction supported by the lean fuel and oxidant mixture 104 and the voltage source 108 can be configured to output a voltage selected to cause the electrode 1 10 to apply an electric field to the combustion reaction sufficient to also stabilize the combustion reaction supported by the lean fuel and oxidant mixture 104. According to embodiments, the electric field and the geometry of the perforated flame holder 106 are selected, in combination, to stabilize a leaner combustion reaction than is stabilized by either the perforated flame holder 106 geometry alone or the electric field alone.

Various voltage, electrode, and electric field parameters can be used in the system 100. In one embodiment, the voltage source 108 is configured to output a DC voltage and the electrode 1 10 is configured to output a constant electric field to the combustion reaction. In another embodiment, the voltage source 108 is configured to output an AC voltage and the electrode 1 10 is configured to apply an alternating electric field to the combustion reaction.

Generally, the inventors have found that the high applied voltages are necessary and sufficient to affect the combustion reaction. In an embodiment, the voltage source 108 is configured to output a high voltage greater than 1000 volts to the electrode 1 10. In a preferred embodiment, the voltage source 108 is configured to output at least 10,000 volts to the electrode 1 10.

Various electrode configurations are contemplated by the inventors.

In the embodiment shown in FIG. 1 , the electrode 1 10 is disposed adjacent to the perforated flame holder 106. For example, the electrode 1 10 can include a metal screen disposed adjacent to the perforated flame holder 106. Additionally or alternatively, the electrode 1 10 can include a conductive material disposed on a surface of the perforated flame holder 106. Additionally or alternatively, the electrode 1 10 includes a conductor disposed within the volume bounded by the perforated flame holder 106. Approaches for coupling electrodes with a structure in a combustion environment are described more fully in PCT Patent Application No. PCT/US2014/031969, entitled "ELECTRICALLY

CONTROLLED COMBUSTION FLUID FLOW", filed March 27, 2014, (docket no. 2651 -174-04); which to the extent not inconsistent with the disclosure herein, is incorporated by reference in its entirety.

In embodiments where there is no explicit second electrode, the electrode 1 10 can form an electric field with any grounded surface nearby. In one embodiment, for example, the fuel and oxidant source 102 can be in continuity with a voltage ground 1 12, and the electric field can be formed between the electrode 1 10 and the fuel and oxidant mixture 104. In another embodiment, the perforated flame holder 106 becomes more conductive at an elevated

(combustion support) temperature, and the electrode 1 10 forms an electric field with one or more portions of the perforated flame holder 106 acting as a second electrode.

FIG. 2 is a diagram of an embodiment 200 wherein the electrode 1 10 includes two electrodes 1 10a and 1 10b. The two electrodes 1 10a and 1 10b can be configured to form an electric field between one another. The first electrode 1 10a can be disposed adjacent to the output surface 109 of the perforated flame holder 106 and the second electrode 1 10b can be disposed adjacent to the input surface 107 of the perforated flame holder 106. In another embodiment, the first electrode 1 10a is disposed on the output surface 109 of the perforated flame holder 106 and the second electrode 1 10b is disposed on the input surface 107 of the perforated flame holder 106.

With the first electrode 1 10a disposed on or adjacent to the output surface 109 of the perforated flame holder 106 and the second electrode 1 10b disposed on or adjacent to the input surface 107 of the perforated flame holder 106, the voltage source 108 can be configured to apply a voltage to one of the first electrode 1 10a or the second electrode 1 10b and the other of the first electrode 1 10a or the second electrode 1 10b can be operatively coupled to voltage ground 1 12.

FIG. 3 is a diagram showing an embodiment 300 including a perforated flame holder 106 bounded by an input surface 107 proximate to the fuel and oxidant source 102 and an output surface 109 distal from the fuel and oxidant source 102, a first electrode 1 10a disposed on or adjacent to the output surface 109 of the perforated flame holder 106, and a second electrode 1 10b disposed on or adjacent to the input surface 107 of the perforated flame holder 106. The voltage source 108 is configured to apply a voltage having a first polarity to the first electrode 1 10a and to apply a second voltage different from the first voltage to the second electrode 1 10b. It is preferable that the voltages respectively applied to the first and second electrodes 1 10a, 1 10b differ by at least 1000 volts. In an embodiment, the second voltage is opposite in polarity to the first voltage.

FIG. 4 is a diagram showing an embodiment 400 including a perforated flame holder 106 bounded by an input surface 107 proximate to the fuel and oxidant source 102 and an output surface 109 distal from the fuel and oxidant source 102. The electrode 1 10 can be disposed on or adjacent to the output surface 109 of the perforated flame holder 106, for example. The voltage source 108 is configured to apply a voltage having a first polarity to the electrode 1 10. A charged particle source 402 is configured to apply charged particles having a second polarity opposite to the first polarity to the fuel and oxidant mixture 104 before the fuel and oxidant mixture 104 is received by the input surface 107 of the perforated flame holder 106. In an embodiment, the charged particle source 402 includes a corona electrode operatively coupled to the voltage source 108, the corona electrode being configured to eject the charged particles into the fuel and oxidant mixture 104. In another embodiment, the charged particle source 402 includes a corona electrode operatively coupled to the voltage source 108, the corona electrode being configured to eject the charged particles into the fuel. In another embodiment, the charged particle source 402 includes a corona electrode operatively coupled to the voltage source 108, the corona electrode being configured to eject the charged particles into a gas including the oxidant. The gas including the oxidant can be combustion air, for example.

Apparatuses, methods, observed behavior, and hypotheses regarding charged particle sources for charging a fuel and oxidant mixture are described more fully in US Provisional Patent Application No. 62/030,960, entitled

"ASYMMETRICAL UNIPOLAR FLAME IONIZER USING A STEP-UP

TRANSFORMER" filed July 30, 2014, (docket no. 2651 -220-02); PCT Patent Application No. PCT/US2013/072392, entitled "PRECOMBUSTION

IONIZATION", filed November 27, 2013, (docket no. 2651 -065-04); U.S. Non- Provisional Patent Application No. 14/092,896, entitled "IONIZER FOR A

COMBUSTION SYSTEM, INCLUDING FOAM ELECTRODE STRUCTURE", filed November 27, 2013, (docket no. 2651 -100-03); PCT Patent Application No.

PCT/US2013/077882, entitled "COMBUSTION SYSTEM WITH A GRID

SWITCHING ELECTRODE", filed December 26, 2013, (docket no. 2651 -146-04); and U.S. Non-Provisional Patent Application No. 14/092,876, entitled

"MULTISTAGE IONIZER FOR A COMBUSTION SYSTEM", filed November 27, 2013, (docket no. 2651 -147-03); each of which, to the extent not inconsistent with the disclosure herein is incorporated by reference in its entirety.

FIG. 5 is a diagram of a system 500 wherein the electrode 1 10 includes a plurality of first electrodes 1 10a extending into the perforated flame holder 106 and a plurality of second electrodes 1 10b also extending into the perforated flame holder 106 and not in contact with the plurality of first electrodes 1 10a, according to an embodiment. The voltage source 108 is configured to apply a voltage having a first polarity to the first electrodes 1 10a and to apply a voltage having a second polarity to the plurality of second electrodes 1 10b. The first electrodes 1 10a and second electrodes 1 10b can be configured to apply electric fields having a direction component transverse to a direction of flow of the fuel and oxidant mixture 104.

Approaches for embedding electrodes in a cast dielectric flame holder may also be applicable to embedding electrodes in a perforated flame holder 106, and are described more fully in U.S. Non-Provisional Patent Application No. 14/101 ,328, entitled "BURNER HAVING A CAST DIELECTRIC ELECTRODE HOLDER", filed December 9, 2013, (docket no. 2651 -128-03); which to the extent not inconsistent with the disclosure herein, is incorporated by reference in its entirety.

FIG. 6 is a diagram of a system 600 wherein the electrode 1 10 comprises a plurality of first electrodes 1 10a extending into the perforated flame holder 106 and a plurality of second electrodes 1 10b extending into the perforated flame holder 106 and not in contact with the plurality of first electrodes 1 10a, according to an embodiment. The voltage source 108 is configured to apply a voltage to the first electrodes 1 10a. The plurality of second electrodes 1 10b are in continuity with a voltage ground 1 12. The first electrodes 1 10a and second electrodes 1 10b can be configured to apply electric fields having a direction component orthogonal to a direction of flow of the fuel and oxidant mixture 104. Optionally, the plurality of second electrodes can be formed from a conductive portion of the perforated flame holder 106.

Referring to FIG. 1 , the fuel and oxidant source 102 may include a fuel nozzle 1 14 configured to output a jet of fuel 1 16 and a combustion air source 1 18 configured to admit combustion air to a combustion volume 120 adjacent to the jet of fuel 1 16. The perforated flame holder 106 is disposed a distance D _D from the fuel nozzle 1 14 sufficient for the fuel 1 16 to entrain the combustion air to form the fuel and oxidant mixture 104.

FIG. 7 is a diagram of a combustion system 700 wherein the fuel and oxidant source 102 includes a mixing chamber wall 702 defining a mixing chamber 704, a fuel nozzle 1 14 configured to output a jet of fuel 1 16 into the mixing chamber 704, and an air nozzle 706 configured to output combustion air into the mixing chamber 704, according to an embodiment. The fuel and oxidant mixture 104 can thus be a premixed fuel and oxidant mixture 104.

The fuel and oxidant source 102 can also include a flame arrester 708 disposed between the mixing chamber 704 and the perforated flame holder 106.

In an embodiment, the fuel and oxidant mixture 104 includes insufficient fuel 1 16 to support stable combustion except in the perforated flame holder 106 under the influence of the electric field. Use of a premixed fuel and air source is described more fully in PCT Patent Application No. PCT/US2014/016632, entitled "FUEL COMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER", filed February 14, 2014, (docket no. 2651 -188-04); which to the extent not inconsistent with the disclosure herein, is incorporated by reference in its entirety.

FIG. 8 is a flow chart showing a method 800 for operating a burner, according to an embodiment. Beginning at step 802, a perforated flame holder is preheated to a starting temperature. Apparatuses, methods, observed behavior, and hypotheses regarding operation of perforated flame holder start up are described more fully in PCT Patent Application No. PCT/US2014/016628, entitled "PERFORATED FLAME HOLDER AND BURNER INCLUDING A

PERFORATED FLAME HOLDER", filed February 14, 2014, (docket no. 2651 - 172-04); PCT Patent Application No. PCT/US2014/016632, entitled "FUEL COMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER", filed February 14, 2014, (docket no. 2651 -188-04); PCT Patent Application No.

PCT/US2014/037743, entitled "COMBUSTION SYSTEM AND METHOD FOR ELECTRICALLY ASSISTED STARTUP", filed May 12, 2014, (docket no. 2651 - 179-04); each of which, to the extent not inconsistent with the disclosure herein is incorporated by reference.

At step 804, a fuel and oxidant mixture is output into a combustion chamber. Apparatuses, methods, observed behavior, and hypotheses regarding fuel and oxidant mixing are described more fully in U.S. Non-Provisional Patent Application No. 13/950,249, entitled "ELECTRICALLY STABILIZED BURNER", filed July 24, 2013 (docket no. 2651 -070-03); PCT Patent Application No.

PCT/US2013/043658, entitled "LOW NOx BURNER AND METHOD OF

OPERATING A LOW NOx BURNER", filed May 31 , 2013, (docket no. 2651 -074- 04); U.S. Non-Provisional Patent Application No. 14/061 ,432, entitled "LIFTED FLAME LOW NOx BURNER WITH FLAME POSITION CONTROL", filed October 23, 2013, (docket no. 2651 -126-03); PCT Patent Application No.

PCT/US2014/016626, entitled "SELECTABLE DILUTION LOW NOx BURNER", filed February 14, 2014, (docket no. 2651 -167-04); each of which, to the extent not inconsistent with the disclosure herein is incorporated by reference.

In step 806, the fuel and oxidant mixture is received at an input surface of a perforated flame holder. Proceeding to step 808, a combustion reaction is supported within the perforated flame holder. Perforated flame holder

embodiments, observed behavior, and hypotheses regarding stabilization effects on the combustion reaction by perforated flame holders are described more thoroughly in PCT Patent Application No. PCT/US2014/016632, entitled "FUEL COMBUSTION SYSTEM WITH A PERFORATED REACTION HOLDER", filed February 14, 2014, (docket no. 2651 -188-04); PCT Patent Application No.

PCT/US2014/057072, entitled "POROUS FLAME HOLDER FOR LOW NOx COMBUSTION", filed September 23, 2014, (docket no. 2651 -200-04); U.S.

Provisional Patent Application No. 62/021 ,549, entitled "BURNER SYSTEM INCLUDING A MOVEABLE PERFORATED FLAME HOLDER", filed July 7, 2014, (docket no. 2651 -218-02); each of which is incorporated by reference in its entirety. In step 810, which occurs simultaneously with and is superimposed on step 808, an electrical field is applied to the combustion reaction. Apparatuses which apply electric fields to combustion reactions, observed behaviors, and hypotheses regarding the effects of electric fields on combustion reactions are described more thoroughly in PCT Patent Application No. PCT/US2014/073086, entitled "METHOD AND APPARATUS FOR EXTENDING FLAMMABILITY LIMITS IN COMBUSTION REACTION", filed December 31 , 2014, (docket no. 2651 -203-04), U.S. Non-Provisional Patent Application No. 13/730,979, entitled "COOLED ELECTRODE AND BURNER SYSTEM INCLUDING A COOLED ELECTRODE", filed December 29, 2012, (docket no. 2651 -036-03); U.S. Non- Provisional Patent Application No. 13/962,917, entitled "SYSTEM AND

SACRIFICIAL ELECTRODE FOR APPLYING ELECTRICITY TO A COMBUSTION REACTION", filed August 8, 2013 (docket no. 2651 -039-03); PCT Patent Application No. PCT/US2012/024571 , entitled "SYSTEM AND METHOD FOR FLATTENING A FLAME", filed February 9, 2012, (docket no. 2651 -042-04), PCT Patent Application No. PCT/US2013/056913, entitled

"ELECTRODYNAMIC COMBUSTION SYSTEM WITH VARIABLE GAIN

ELECTRODES", filed August 27, 2013. (docket no. 2651 -043-04); U.S. Non- Provisional Patent Application No. 14/570,126, entitled "METHOD AND

APPARATUS FOR SHAPING A FLAME", filed December 15, 2014, (docket no. 2651 -045-03); PCT Patent Application No. PCT/US2013/048937, entitled

"COMBUSTION SYSTEM WITH A CORONA ELECRODE" filed July 1 , 2013, (docket no. 2651 -056-04); U.S. Non-Provisional Patent Application No.

14/092,91 1 , entitled "ELECTRODYNAMIC BURNER WITH A FLAME IONIZER", filed November 27, 2013, (docket no. 2651 -072-03); PCT Patent Application No. PCT/US2014/045707, entitled "COMBUSTOR HAVING A NONMETALLIC BODY WITH EXTERNAL ELECTRODES", filed July 8, 2014, (docket no. 2651 -095-04); PCT Patent Application No. PCT/US2014/056928, entitled "CONTROL OF COMBUSTION REACTION PHYSICAL EXTENT", filed September 23, 2014, (docket no. 2651 -103-04); U.S. Non-Provisional Patent Application No.

14/224,064, entitled "PREMIXED FLAME LOCATION CONTROL", filed March 24, 2014, (docket no. 2651 -143-03); and U.S. Provisional Patent Application No. 62/010,931 , entitled "FLAME POSITION CONTROL ELECTRODES", filed June 1 1 , 2014, (docket no. 2651 -212-02); each of which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.

Proceeding to step 812, heat is transferred from the perforated flame holder to a heat load. In step 814, combustion products including about 3% oxygen and 5 ppm or less oxides of nitrogen (NOx) are output from the perforated flame holder and/or a flue operatively coupled to the perforated flame holder. In the following description related to FIGS. 9-15, some terms and descriptions of embodiments may differ from those disclosed in the

preceding description, though similar concepts and structures are

described. Those of skill in the art will understand that many differing

terminologies can be used to describe similar or related concepts and

structures.

FIG. 9 is a top view of a perforated flame holder 106, according to one embodiment. The perforated flame holder 106 includes a plurality of perforations or openings 920 extending between the input surface 107 and the output surface 109 (see FIGS. 1 -7, 10 and 16) of the perforated flame holder 106. The perforations 920 are separated from each other by walls

922 of the perforated flame holder 106, which define the body of the

perforated flame holder.

FIG. 10 is a cross-section of the perforated flame holder 106 of

FIG. 9, according to one embodiment showing the perforations 920

extending between the input surface 107 and the output surface 109 of the perforated flame holder 106. The perforations 920 are separated from

each other by the walls 922 of the perforated flame holder 106.

As described previously in relation to FIG. 1 , the perforated flame holder 106 can be positioned to receive a fuel and oxidant mixture 104 at the input surface 107. The fuel and oxidant mixture 104 enters into the perforations 920. The perforated flame holder 106 supports a combustion reaction of the fuel and oxidant mixture 104 within the perforations 920. According to an embodiment, the perforated flame holder 106 is configured to hold a majority of the combustion reaction within the perforations 920 between the input surface 107 and the output surface 109.

The perforated flame holder 106 configured to receive heat from the combustion reaction and to output heat to the fuel and oxidant mixture 104. In receiving heat from the combustion reaction and outputting heat to the fuel and oxidant mixture, the perforated flame holder 106 stabilizes the combustion reaction. According to embodiments, the perforated flame holder 106 is configured to extend a stability limit of the fuel and oxidant mixture 104 supporting the combustion reaction. It is believed that this operates by ensuring sufficient heat transfer to the fuel and oxidant mixture 104 to maintain combustion, even of a fuel and oxidant mixture that is too (fuel-) lean to support stable combustion in a conventional flame.

FIG. 11 is a top view of a conductive screen 1 1 10, according to one embodiment. The conductive screen 1 1 10 is an electrode that can be placed on the input surface 107 or the output surface 109 (see FIGS. 1 -7,

10 and 16) of a perforated flame holder 106. Alternatively, a first screen 1 1 10 can be placed on the output surface 109 while a second screen

1 1 10 can be placed on the input surface 107 of the perforated flame

holder 106.

The conductive screen 1 1 10 includes a mesh of wires 1 124 having gaps between them through which the fuel mixture, air, or flue gas can pass. According to an embodiment, the conductive screen 1 1 10 receives a voltage V from the voltage source 108 (see FIGS. 1 -7), thereby

generating an electric field. The conductive screen 1 1 10 can be

positioned to apply the electric field to the combustion reaction within the perforations 920 of the perforated flame holder 106.

In one embodiment, the screen 1 1 10 can be positioned directly on the input or output surface 107, 109. Alternatively, the screen 1 1 10 can be supported near the input or output surface 107, 109 without being in direct physical contact with the perforated flame holder 106.

FIG. 12 is an enlarged side view of two perforations 920 of a

perforated flame holder 106, according to one embodiment. Electrodes

1210a are positioned on the walls 920 at the upper portions of the

perforations 920 and on the output surface 109 of the perforated flame holder 106. Electrodes 1210b are positioned on the walls 920 at the lower portions of the perforations 920 and on the input surface 107 of the

perforated flame holder 106. The electrodes 1210a, 1210b can be formed of a conductive paste, conductive ink or other conductive material applied to the top and bottom of the perforated flame holder 106. The conductive material can be rolled on by a roller that passes over the top and bottom surfaces of the perforated flame holder 106, by pad printing, or in any other suitable manner.

Though shown as physically separate in FIG. 12 due to the nature of the cross-sectional diagram, according to an embodiment, the electrodes 1210a are a single continuous electrode on the output surface 109 and upper portions of the perforations 920. Likewise, the electrodes 1210b are a single continuous electrode on the input surface 107 and lower portions of the perforations 920.

According to an embodiment, the electrodes 1210a, 1210b are coupled to the voltage source 108 (see FIGS. 1 -7). The voltage source 108 can apply a high voltage V+ to the electrode 1210a and a low voltage V- to the electrode 1210b, by which an electric field is generated within the perforations 920. In this manner the electric field can be applied to a combustion reaction of the fuel and oxidant mixture 104 (see FIGS. 1 -7) within the perforations 920, thereby stabilizing the combustion reaction.

According to an embodiment, the high voltage V+ can be greater than 1000 V while the low voltage V- is ground. Alternatively, the voltage on the electrode 1210a can be lower than the voltage on the electrode 1210b. The voltage source can also apply an alternating voltage between the electrodes 1210a and 1210b.

According to an embodiment, only one of the electrodes 1210a or

1210b may be present. Thus, the electrodes 1210a can be present while the electrodes 1210b are not present. Alternatively, the electrodes 1210b can be present without the electrodes 1210a being present.

FIG. 13 is an enlarged perspective view of a single perforation 920 of a perforated flame holder 106, according to one embodiment. The perforation 920 includes a first serpentine electrode 1310a and a second serpentine electrode 1310b each on a respective sidewall 922 of the same perforation 920 of the perforated flame holder 106.

According to an embodiment, the electrodes 1310a, 1310b are coupled to the voltage source 108 (see FIGS. 1 -7). The voltage source 108 can apply a high voltage V+ to the electrode 1310a and a low voltage V- to the electrode 1310b, by which an electric field is generated within the perforations 920. In this manner the electric field can be applied to a combustion reaction of the fuel and oxidant mixture 104 (see FIGS. 1 -7) within the perforations 920, thereby stabilizing the combustion reaction.

Though only a single perforation 920 is shown in FIG. 13, each perforation 920 of the perforated flame holder 106 can include a respective serpentine electrode 1310a and a respective serpentine electrode 1410b. Thus, an electric field can be applied to the combustion reaction within each perforation 920.

FIG. 14 is an enlarged perspective view of a single perforation 920 of a perforated flame holder 106, according to one embodiment. The perforation 920 includes a first straight electrode 1410a and a second straight electrode 1410b each on a respective sidewall 922 of the perforation 920 of the perforated flame holder 106.

According to an embodiment, the electrodes 1410a, 1410b are coupled to the voltage source 108 (see FIGS. 1 -7). The voltage source 108 can apply a high voltage V+ to the electrode 1410a and a low voltage V- to the electrode 1410b, by which an electric field is generated within the perforation 920. In this manner the electric field can be applied to a combustion reaction of the fuel and oxidant mixture 104 (see FIGS. 1 -7) within the perforation 920, thereby stabilizing the combustion reaction.

Though only a single perforation 920 is shown in FIG. 14, each perforation 920 of the perforated flame holder 106 can include a respective straight electrode 1410a and a respective straight electrode 1410b. Thus, an electric field can be applied to the combustion reaction within each perforation 920. FIG. 15 is an enlarged top view of a single perforation 920 of a perforated flame holder 106, according to one embodiment. Electrodes 1510a and 1510b are embedded in the walls 922 of the perforation 920 of the perforated flame holder 106. A high-voltage can be applied between the electrodes 1510a and 1510b as described previously to establish an electric field within the perforation 920 of the perforated flame holder 106. While a single perforation 920 is shown in FIG.15, in practice each perforation 920 of a perforated flame holder 106 can include an electrode 1510a and an electrode 1510b.

The electrodes 1510a, 1510b can be formed by scoring grooves into the walls 922 of the perforated flame holder 106 during production of the perforated flame holder 106. In one process for forming a perforated flame holder 106, the perforations 920 of the perforated flame holder 106 can be formed from a mass of material that has not yet hardened. To form the perforations, extruder bars are forced through the yet soft material of the perforated flame holder 106, leaving the perforations 920. The extruder bars can include sharp protrusions on opposite sides. Thus, when the extruders pass through the not-yet hardened material of the perforated flame holder 106, the sharp protrusions will score the walls 922 leaving grooves within the walls 922 of the perforation 920.

After the perforations 920 of the perforated flame holder 106 have been fully formed, a conductive material can be deposited on the walls 922 of the perforated flame holder 106. The conductive material will accumulate more thickly within the grooves than on the other portions of the walls 922. The perforated flame holder 106 can then be subjected to a timed wet etch that etches the conductive material. The duration of the wet etch can be selected to be long enough to remove the conductive material from the walls 922 of the perforated flame holder 106, but not long enough to fully remove the conductive material from the grooves where the conductive material is thicker, thereby leaving electrodes 1510a, 1510b as shown in FIG. 15. In an alternate embodiment, grooves are not formed in the walls 922. Instead, the electrodes 1510a, 1510b can be formed along the corners of the perforation 920 by depositing the conductive material into the perforation 920 and performing a timed wet etch as described above. Because the conductive material will accumulate more thickly at the corners of the perforation 920, the conductive material can be removed from the flat portions of the perforation 920 while some of the conductive material in the corners will remain. The remaining conductive material in the corners can be used as electrodes in a similar manner as the electrodes 1510a, 1510b.

FIG. 16 is a diagram of a combustion system 1600, according to an embodiment. The combustion system 1600 includes a fuel nozzle 1 14, a perforated flame holder 106, a voltage source 108, and electrodes 1610a, 1610b. The combustion system 1600 further includes an infrared camera 1626 disposed near the perforated flame holder 106 and a control circuit 1628 coupled to the infrared camera 1626 and to the voltage source 108.

According to an embodiment, the infrared camera 1626 captures infrared images of the combustion reaction within the perforations 920 of the perforated flame holder 106. The control circuit 1628 analyzes the images and determines whether the combustion reaction held by the perforated flame holder 106 is similar to 1626 a selected image pattern. If the image detected by the infrared camera 1626 does not correspond to the selected image pattern, the control circuit 1628 can modify the voltage between the electrodes 1610a, 1610b to change an electric field in the perforations 920 (see FIGS. 9-15) of the flame holder 106 sufficiently to cause the combustion reaction and perforated flame holder 106 to output infrared radiation corresponding to the selected image pattern.

For example, the control circuit 1628 can determine if the image pattern from the infrared camera 1626 corresponds to a desired ratio of temperature between the perforation walls within the perforated flame holder 106 (referred to as perforation core) and the ends of the perforation walls 922 (i.e., at the output surface 109). It was found that when a perforation core outputs less thermal radiation than the perforation walls at the output surface 109, then the combustion reaction could tend to be occurring in blue flames above the output surface 109, which is

disadvantageous with respect to NOx output.

According to an embodiment, when the infrared camera 1626 captures an image with "bright" wall ends, the control circuit 1628 causes the voltage source 108 to increase the voltage difference between the electrodes 1610a and 1610b. This increases reaction rate, and tends to pull the combustion reaction down into the perforation cores. Conversely, when the infrared camera 1626 captures an image with "dark" wall ends, the control circuit 1628 causes the voltage source 108 to decrease the voltage difference between the electrodes 1610a and 1610b. This decreases reaction rate, and tends to distribute the combustion reaction along the length of the perforation cores.

In another embodiment, the control circuit 1628 is operatively coupled to a fuel valve. When the wall ends are bright, the control valve is closed somewhat to reduce convective cooling of the perforated flame holder 106, which causes the combustion reaction to be more fully completed within the perforation cores. This approach can be used to modulate fuel during system start-up, for example, such that fuel flow rate is increased gradually such that a desired thermal image is maintained. The embodiment including the control circuit 1628 operatively coupled to a fuel valve can be useful even without an apparatus configured to apply a voltage to the combustion reaction carried by the perforated flame holder 106.

Although the above description relates to capturing an infrared image of the output surface 109 of the perforated flame holder 106, the techniques described can also be applied to respond to infrared images of the input surface 107 of the perforated flame holder 106. In one embodiment, the electrodes 1610a, 1610b can each include multiple individually addressable electrodes. The individually addressable electrodes can each be in or near a respective perforation 920 of the perforated flame holder 106. The voltage source, in conjunction with the control circuit 1628, can selectively apply a voltage to individual

addressable electrodes to generate an electric field in selected

perforations 920 or selected groups of perforations 920.

According to one embodiment, the infrared camera 1626 captures infrared images of the combustion reaction within the perforations 920 of the perforated flame holder 106. The control circuit analyzes the images and determines which perforations 920 of the perforated flame holder are below a selected threshold temperature or within a selected temperature range. The control circuit can then apply a high-voltage between selected addressable electrodes 1610a, 1610b in order to apply an electric field to those perforations 920 that are below the threshold temperature. The control circuit 1628 can also remove the high voltage from addressable electrodes in those perforations 920 that are higher than the threshold temperature or are already within the selected temperature range.

FIG. 17A is a diagram of a combustion system 1700 according to an embodiment. The combustion system 1700 includes a fuel nozzle 1 14, a perforated flame holder 106, and a voltage source 108 as described previously. The combustion system 1700 further includes insulated electrodes 1710a, 1710b positioned above and below the perforated flame holder 106, respectively. The insulated electrodes 1710a, 1710b are coupled to the voltage source 108.

The insulated electrodes each include a respective inner conductor 1732a, 1732b covered by a dielectric material 1730a, 1730b. The dielectric material 1730a, 1730b protects the inner conductors 1732a, 1732b from short-circuiting with each other, or with the perforated flame holder 106. At high temperatures, depending on the material from which the perforated flame holder 106 is made, it is possible that the perforated flame holder 106 can become conductive. This leads to the possibility of a short-circuit between un-insulated electrodes positioned near or in direct contact with the perforated flame holder 106. Likewise, in a situation in which only one electrode is present and a voltage is applied to the perforated flame holder 106, a short-circuit can occur between the perforated flame holder 106 and the un-insulated electrode. Furthermore, the combustion reaction itself, the gases that feed the combustion reaction, or products of the combustion reaction can be conductive, further increasing the risk of a short-circuit. Therefore, the electrodes 1710a, 1710b of the combustion system 1700 include the inner conductor 1732a, 1732b covered by the dielectric material 1730a, 1730b.

The dielectric material 1730a, 1730b is a material that maintains its dielectric properties at very high temperatures. In one embodiment, the dielectric material 1730a, 1730b is fused quartz. Alternatively, the dielectric material 1730a, 1730b can be any other suitable material that electrically insulates the inner conductors 1732a, 1732b at high

temperatures.

While two insulated electrodes 1730a, 1730b are shown in FIG. 17A, in some embodiments, only one insulated electrode is present.

Those skilled in the art will recognize that many configurations for a combustion system 1700 can be used. Such configurations can include more or fewer insulated electrodes than are shown in FIG. 17A.

FIG. 17B is a top view of the insulated electrode 1710a from FIG. 17A, according to one embodiment. The insulated electrode 1710a is in a substantially circular shape with two arms extending toward the voltage source 108. The inner conductor 1732a is coupled to the voltage source 108. The dielectric material 1730a completely covers the inner conductor 1732a in the circular portion of the insulated electrode 1710a.

In FIG. 17B the inner conductor 1732a protrudes from both arms of the insulated electrode 1710a. However, in one embodiment, the inner conductor 1732a can terminate within the dielectric material 1730a. FIG. 17C is an enlarged cross-section of the insulated electrode 1710a of FIG. 17B, according to an embodiment. The cross-section shown in FIG. 17C illustrates clearly that the inner conductor 1732a is completely covered by the dielectric material 1730a. Thus, the dielectric material 1730a electrically insulates the inner conductor 1732a of the insulated electrode 1710a.

FIG. 18A is a top view of an insulated electrode 1810, according to one embodiment. The insulated electrode 1810 is a screen including a mesh of conductive wires 1824. The conductive wires 1824 of the screen 1810 are covered in a dielectric material 1830. However, as is shown in the enlarged view of a portion of the screen 1810. The dielectric material 1830 electrically insulates the conductive screen 1810 in a similar fashion as described with respect to FIGS. 17A-17C above. In particular, the dielectric material maintains its dielectric properties at high temperatures, thereby preventing short-circuiting of the insulated electrode 1810 with other electrodes, the perforated flame holder 106, or conductive gases. In one embodiment, the dielectric material 1830 is fused quartz deposited on the wires 1824 by chemical vapor deposition (CVD) or by another suitable process. The insulated electrode 1810 can be placed above or below the perforated flame holder 106 and can receive a voltage from the voltage source 108 as described previously.

The individual wires 1824 are in electrical continuity with each other because they are in direct physical contact at the crossing points. Thus, all the wires carry the voltage applied to one of the wires 1824.

FIG. 18B is a cross-section of a portion of the insulated electrode

1810 of FIG. 18A. The cross-section of FIG. 18B shows that the wires 1824 are completely covered by the dielectric material 1830. Thus, the wires 1824 of the insulated electrode 1810 are electrically insulated from other nearby conductors. 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.