COMBUSTION SYSTEM WITH A CORONA

ELECTRODE

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority benefit from U.S. Provisional Patent Application No. 61/666,757, entitled "COMBUSTION SYSTEM WITH A SHARP ELECTRODE", filed June 29, 2012; and from U.S. Provisional Patent Application No. 61/694,207, entitled "COMBUSTION SYSTEM WITH A SERRATED

ELECTRODE", filed August 28, 2012; both of which applications, to the extent not inconsistent with the disclosure herein, are incorporated herein by reference in their entireties.

BACKGROUND

Combustion systems may benefit from applying one or more electric fields, charge, or electrical potential to a combustion reaction.

SUMMARY Little or no benefit has been heretofore reported regarding the application of the electric field(s) to a combustion reaction as a function of electrode shape. The inventors have determined that the shape of an electrode used to apply an electrical field to a combustion reaction can affect the shape and intensity of the electric field, as well as its effect on the combustion reaction. Moreover, the inventors have determined that corona electrodes can eject charges that are incorporated into the combustion reaction, without the necessity of maintaining physical contact between the combustion reaction and the corona electrode. What is needed are electrode shapes that provide desired electric field strength interacting with a combustion reaction.

In researching and developing electrodes for use in applying an electrical charge to a combustion reaction, the inventors have determined that some electrode shapes and materials, in some combustion systems, are subject to thermal ablation that can reduce the effectiveness or shorten the life of the electrodes. This is particularly the case where a sharp or thin (i.e., corona) electrode is used in a burner system that achieves very high temperatures, or in systems in which the electrode is in direct contact with a flame or other kind of combustion reaction.

According to various embodiments, structures are provided that address these concerns, and that provide additional benefits, as well.

According to an embodiment, combustion system benefits from the use of one or more electrodes configured to generate high electric field strength proximate to the surface of the electrode(s). An electrode configured to generate a high electric field strength proximate to its surface sufficient to eject charges is referred to as a corona electrode. Such an electrode can be alternatively referred to as a sharp electrode, an ionizing electrode, an ion-ejecting electrode, and ion-injecting electrode, or, in some contexts, simply an ionizer.

According to an embodiment, an electrode system for a combustion apparatus is provided that includes at least one corona electrode configured for mounting proximate to a combustion reaction. A power supply is operatively coupled to the corona electrode(s) and to the combustion reaction zone (e.g., flame). The power supply and the corona electrode(s) can be configured to apply an electric field to a region adjacent to the combustion reaction. The corona electrode can be characterized as producing an electric field having a maximum magnitude adjacent to the corona electrode at least double an average electric field magnitude in the region adjacent to the combustion reaction. The electric field and/or charges produced by the corona electrode(s) are configured to cause ions to be injected into the combustion reaction, thus providing increased mixing of fuel and oxidizer in the combustion reaction.

According to another embodiment, a combustion system includes a serrated or sawtooth corona electrode. The combustion system includes a fuel burner structure configured to support a combustion reaction. The combustion system includes a serrated corona electrode configured to form an electrical relationship with the combustion reaction. The serrated electrode includes a plurality of sharp projections configured to at least intermittently eject ions into a dielectric gap between the plurality of projections and the combustion reaction. Each of the plurality of projections is configured to at least intermittently eject ions into the dielectric gap responsive to receiving an ion ejection voltage from an electrical coupling.

According to an embodiment, a system is provided for applying a charge or voltage to a combustion reaction. The system includes a power supply configured to output a voltage of 1000 volts or more. The system includes one or more electrodes operatively coupled to the power supply and configured to eject ions into a region proximate to the combustion reaction. The system includes at least one counter electrode configured to at least intermittently receive or supply current to the combustion reaction responsive to the ions ejected by the one or more electrodes.

According to further embodiments, methods for applying an electric field to a combustion reaction are provided, which include supporting at least one corona electrode proximate to but not contacting a combustion reaction. The corona electrode(s) can be characterized as including one or more small radius tips or edges. A voltage is applied to the corona electrode(s) to cause ion ejection in a high electrical field strength volume peripheral to the small radius tip(s) or edge(s) of the electrode. A response in the combustion reaction is caused responsive to the application of the electric field strength and ion ejection.

According to another embodiment, a system is provided that includes a corona electrode positioned adjacent to a burner assembly and configured to form an electrical relationship with a combustion reaction supported by the burner assembly. A radiation shield is provided, positioned between at least a portion of the electrode and the combustion reaction, configured to attenuate or block radiant heat emanating from the combustion reaction that would otherwise impinge on the electrode.

According to another embodiment, a corona electrode is provided, configured for use in a combustion system

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a combustion system including a corona electrode configured to apply an electric field to a combustion reaction, according to an embodiment.

FIG. 2 is a diagram showing illustrative electric field strength and voltage between a corona electrode and a dull electrode, according to an embodiment.

FIG. 3 is a diagram of a combustion system including a corona electrode and a dull electrode, according to an embodiment.

FIG. 4 is a view of a corona electrode assembly including a corona electrode configured as a pointed cylinder, according to an embodiment.

FIG. 5 is a view of a corona electrode assembly including a corona electrode configured as a blade, according to an embodiment.

FIG. 6 is a view of a corona electrode assembly including a serrated electrode, according to an embodiment.

FIG. 7 is a view of a corona electrode assembly including a serrated electrode, according to another embodiment.

FIG. 8 is a flow chart showing a method for applying an electric field or voltage to a combustion reaction using a corona electrode, according to an embodiment. FIG. 9 is a view of a corona electrode assembly including a radiation shield configured to protect the electrode from radiant heat, according to an embodiment.

FIG. 10 is a view of a corona electrode assembly including a radiation shield configured to protect the electrode from radiant heat, according to another embodiment.

FIG. 11 is a view of a corona electrode assembly including a self- sharpening electrode, according to another 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.

For brevity, elements disclosed with respect to a system illustrated in one figure may not be disclosed or described in detail with respect to systems of other figures. Nevertheless, those of skill in the art will recognize the

combinability of many of the features disclosed herein.

When a voltage is applied to an electrode, an electric field is formed around the electrode. The relative strength of the electric field at any given location adjacent to the electrode is inversely related to the radius of the curvature of the electric field at that location. Thus, a corona electrode, i.e., an electrode with a very small radius at the point, will generate a large electric field strength near its curvature relative to a field strength adjacent to other portions of the electrode.

FIG. 1 is a diagram of a combustion system 101 including a corona electrode 102 configured for mounting proximate to a combustion reaction 104 such as, e.g., a flame, supported by a burner 1 12 in a combustion volume 103, according to an embodiment. The combustion volume 103 can be defined by furnace walls, boiler walls, a rotary kiln, etc. Such combustion volumes 103 are generally separated from work areas accessible to operating engineers and other persons. A power supply 106 is operatively coupled to the corona electrode 102 and to the burner 1 12. The power supply 106 and the corona electrode 102 are configured to cooperate to apply an electric field to a region 108 in the

combustion volume 103 adjacent to the combustion reaction 104, with the magnitude of the electric field near the corona electrode 102 being at least twice the average magnitude of the electric field in the region 108 adjacent to the combustion reaction 104.

In other words, a corona electrode 102 can be characterized as an ionizing electrode because a small physical radius of at least a portion of the electrode causes high curvature in the electric field, and hence high electric field strength E in near proximity to a sharp surface. The high electric field strength is associated with insertion of ions 1 1 1 from the corona electrode 102 into the dielectric layer, or region, 108 upon application of high voltage to the corona electrode 102. The inserted ions 1 1 1 can be referred to as a corona discharge.

According to other embodiments, the combustion system 101 may include a plurality of corona electrodes 102 configured for mounting proximate to the combustion reaction 104 and operatively coupled to the power supply 106, or a plurality of power supplies 106.

FIG. 2 is a graph 201 showing an illustrative variation of electric field strength E and an illustrative voltage V in the dielectric gap or region 108 between a corona electrode 102 and the combustion reaction 104. The position on the abscissa X is indicative of the distance between the corona electrode 102 and the combustion reaction 104 (increasing from left to right). The curves shown in FIG. 2 occur responsive to an electric potential being applied between the corona electrode 102 and the combustion reaction 104. The solid curve labeled E in the graph in FIG. 2 depicts the electric field strength between the corona electrode 102 and the combustion reaction 104. The maximum electric field strength occurs immediately adjacent to corona electrode 102. The dashed curve labeled V depicts the variation in voltage between the corona electrode 1 02 and the combustion reaction 1 04 under the same applied electric potential, where V _M AX is an electrical potential applied to the corona electrode 1 02 and VFLAME is an electrical potential (or a calculated potential corresponding to a charge density) of the combustion reaction 1 04.

Generally, corona electrodes 1 02 are characterized as having a small radius feature (which may include a point or a line) that tends to concentrate an applied voltage into a relatively small volume peripheral to the small radius feature. Some example electrode shapes used by the inventors include needle, blade, saw-tooth, and thin wire. By contrast, dull electrodes 1 1 0 are defined as having a large effective surface with little or no concentration of applied voltage, such that the electric field magnitude adjacent to them is low, relative to the field strength at the tip or edge of a corona electrode 1 02 energized at the same potential. For the purposes of this disclosure, the highest field magnitude adjacent to a dull electrode 1 1 0 is less than twice the average electric field magnitude between the dull electrode 1 1 0 and the cooperating corona electrode or electrodes 1 02.

Typically, in known systems that employ corona discharge, a corona electrode 102 is used in concert with a dull electrode 1 1 0 (also referred to as a counter electrode) that carries different electrical potential, such as ground potential or an opposing potential. The counter electrode 1 1 0 is dull so as to attract the ions 1 1 1 generated by the corona electrode 102 without generating ions 1 1 1 of its own. As shown in FIG. 1 , the burner 1 1 2 is coupled to ground and functions as a dull electrode 1 1 0.

However, according to other embodiments, the combustion reaction 104 itself can be considered to act as the dull electrode 1 10 because its fluid nature responds to charge concentrations in a surrounding dielectric (e.g. air) region by assuming a shape that distributes the charge concentration over a larger area of the combustion reaction 104, the fluid response acting to substantially prevent ion ejection from the combustion reaction 1 04 to the dielectric. According to other embodiments, a counter electrode 1 10 (not shown) can be provided to draw the emitted ions away from the corona electrode 102. The combination of the corona electrode 102 and the counter electrode 1 10 can create an ionic wind that causes ejected ions 1 1 1 to stream toward and combine with the combustion reaction 104.

Generally, a dull electrode 1 10 includes only large radius features that do not significantly concentrate an applied voltage into a small volume peripheral to the electrode. Dull electrodes 1 10 generally are not considered charge-ejecting or ionization-inducing bodies, whereas corona electrodes 102 are regarded as ionizing or charge ejecting bodies that launch charged particles into the surrounding volume when exposed to relatively high voltage. Many known devices take advantage of this property. Examples include electrostatic precipitators, which typically use corona electrodes 102 that deposit charges onto airborne particles that are then trapped by electrical attraction to a ground or counter electrode 1 10, which is typically a dull electrode 1 10.

In the combustion system of FIG 1., the electric field in the vicinity of the corona electrode 102, the cumulative particle charge emitted by the corona electrode 102, and/or the kinetic energy of the charged particles emitted by the corona electrode 102 affect the motion of particles in the combustion reaction 104. One such effect may be to cause increased mixing of the fuel and oxidizer components of the combustion reaction 104. Increased mixing of the fuel and oxidizer in the combustion reaction 104 produces several effects on combustion reaction 104, which can occur singly or in combination. The increased mixing can increase a reaction rate of the combustion reaction 104, and/or can increase fuel and air contact area within the combustion reaction 104. Increased mixing of fuel and air can cause a decrease of the combustion reaction temperature, a decrease of an evolution of oxides of nitrogen (NOx) and/or carbon monoxide (CO) in the combustion reaction 104, an increase of stability of the combustion reaction 104, and/or a decrease of a chance of combustion reaction blow-out. Increased fuel-air mixing can cause an increase in emissivity of the combustion reaction 104, or a decrease in the size (such as volume) of the combustion reaction 104 for a given fuel flow rate.

The voltage applied to the corona electrode(s) 102 by the power supply 106 can be a substantially constant DC voltage, a time-varying voltage, or a DC voltage with a superimposed time-varying voltage. A time-varying voltage can have a periodic voltage waveform with a frequency in the range from 50 to 10,000 Hertz, for example. According to some embodiments, the time-varying voltage can have a periodic voltage waveform with a frequency in the range from 200 to 800 Hertz. N.B. The waveform of the time-varying voltage can be any of a number of shapes, including a square waveform, sine waveform, triangular waveform, truncated triangular waveform, sawtooth waveform, logarithmic waveform, or exponential waveform, for example. Additionally or alternatively, the waveform shape can include a combination of square waveform, sine waveform, triangular waveform, truncated triangular waveform, sawtooth waveform, logarithmic waveform, or exponential waveform. The amplitude of the time-varying voltage can be in the range ±1 ,000 volts to ±1 15,000 volts, for example. According to some embodiments, the time-varying voltage can have an amplitude in the range ±8,000 volts to ±40,000 volts. The magnitude of the electric field in region 108 can be in the range from 0.3 kV/m (kilovolts per meter) to 1 ,000 kV/m, for example. According to some embodiments, the electric field strength in the region 108 can be between 80 kV/m and 400 kV/m.

Electrically conductive surfaces energized to high positive or negative voltages may exhibit the phenomenon known as corona discharge. Corona discharge typically occurs due to ionization of an adjacent dielectric medium.

The conditions under which corona discharge occurs can be calculated with a mathematical equation known as Peek's Law.

For example, one form of Peek's Law gives e _Vj the minimum voltage necessary for corona discharge to occur (the "corona inception voltage" in kilovolts) between two wires according to the formula: Where m _v is an irregularity factor depending on the condition of the wires, typically ranging between 0.9 and 1 .0, and

g _v is the visual critical potential gradient, a function of air density δ (which varies with air temperature and pressure) and the radius r of the wires, and

S is the distance between the wires.

According to Peek's Law, the smaller the radius of the wires, the less voltage is needed to initiate corona discharge. In general, corona discharge is more likely to occur from sharply angled or pointed electrodes such as corona electrode 102 than from dull electrodes such as dull electrode 1 10 (shown in FIG. 1 ), because the electric field gradient has its greatest magnitude close to the corona electrode 102. In the Peek's Law equation, a corona electrode 102 has a small effective value of r, and hence a lower corona inception voltage than a dull electrode 1 10 having a larger effective r.

Applying Peek's Law, the corona inception voltage for an apparatus such as that shown in FIG. 1 can be determined, and the output of the power supply 106 (shown in FIG. 1 ) can be adjusted so that the electric field strength near the corona electrode 102 is at least equal to the corona inception voltage. Such a condition causes corona discharge. According to an embodiment, the electrode is considered sufficiently sharp if the maximum electric field strength near the corona electrode 102 is at least double the average electric field strength E between the corona electrode 102 and a dull electrode 1 10 or between the corona electrode 102 and the combustion reaction 104, indicated by the horizontal line labeled E in FIG. 2.

Since the combustion reaction 104 generates many charged particles that are capable of supporting the flow of electric current, the surface of a flame 104 can be treated as a substantially equipotential conductive surface that

cooperates with the at least one corona electrode 102 to produce the electric field. In other words, the combustion reaction 104 can be considered to be the counter electrode 1 10. In an embodiment, at least one second corona electrode 102 is included, configured for mounting proximate to the combustion reaction 104 and

cooperating with the first corona electrode(s) 102 to produce the electric field.

The burner 1 12 can act as a second electrode 1 10 and can be in electrical continuity with a conductive surface of the combustion reaction 104. The burner 1 12 can be configured to define a counter voltage to cooperate with the corona electrode(s) 102 to produce the electric field.

According to an embodiment, the burner 1 12 also functions as the dull electrode 1 10, which is operatively coupled to the power supply 106. Likewise, where the combustion reaction 104 functions as the dull electrode 1 10, the countercharge can be applied via the burner 1 12, from which the charge is carried by the fuel stream into the combustion reaction 104. According to another embodiment, the countercharge is applied to the conduit that carries the fuel to the burner 1 12.

The electric potential of the burner 1 12 or fuel conduit can be held substantially at ground voltage. Alternatively, the burner 1 12 can be galvanically isolated from ground and from power supplies 106 other than the corona electrode 102, such that the burner 1 12 is floating.

FIG. 3 is a diagram showing a combustion system 301 including a corona electrode 102 and a dull electrode 1 10 proximate to the combustion reaction 104 supported by a burner 1 12, according to an embodiment. A power supply 106 is operatively coupled between the dull electrode 1 10 and the corona electrode 102 to provide a voltage difference to produce the electric field. Because the dull electrode 1 10 lacks the small-radius features found on corona electrode 102, it does not significantly concentrate an applied voltage into a small volume adjacent to the electrode and thus does not tend to eject charge into or induce ionization of the surrounding dielectric medium.

The dull electrode 1 10 is configured so that the electric field adjacent to it is about equal to or less than the average electric field magnitude in the region between electrodes 102 and 1 10, according to an embodiment. According to an embodiment, the dull electrode 1 10 can be configured in the shape of a toroid or torus, as shown. The dull electrode 1 10 is operatively coupled to the power supply 106. The dull electrode 1 10 can be held

substantially at ground potential, or can be configured to be driven to an instantaneous voltage substantially the same as the instantaneous voltage applied to the corona electrode 102. The dull electrode 1 10 can be configured to be galvanically isolated from ground and from other electrical potentials.

FIG. 4 is a view of a corona electrode assembly 401 including a corona electrode 102 configured as a pointed cylinder, according to an embodiment. The corona electrode 102 includes a cylindrical taper 402 to a tip 404 having a radius of 0.1 inch or less. The corona electrode 102 and/or the assembly 401 also includes an electrical coupling 406, which may include an electrical lug for attachment of a wire or other conductor. The corona electrode 102 and/or assembly 401 can include electrical insulation 408 to substantially prevent current flow between the corona electrode 102 and a surface or apparatus it is mounted to. An electrically-isolated mounting bracket 410 can include a flange configured to mount the corona electrode 102 to a mounting surface 1 14 (shown in FIG. 1 ), which can include a burner body, a boiler, a furnace wall or other structure.

FIG. 5 is a diagram of a corona electrode assembly 501 including a corona electrode 102 configured as a conductive blade, according to an embodiment. The corona electrode 102 includes a taper 502 to an edge 504 having a radius of 0.1 inch or less, for example. An electrical coupling 406 is configured as a tapped hole in the corona electrode 102 for receiving an electrical connection (not shown). An electrically-isolated mounting bracket 410 can include a clamp configured to compress electrical insulation 408 against the electrode 102 body. The mounting bracket 410 can include a mounting point 506 for mounting the assembly 501 to a burner body, a boiler, a furnace wall or other structure (not shown).

The corona electrode 102 can be configured to operate in a relatively high temperature environment, in or adjacent to a combustion reaction 104. The corona electrode 102 can be constructed from a conductive material capable of withstanding a relatively high temperature corresponding to a combustion volume. For example, the corona electrode 102 can be made from iron, steel, platinum, palladium, tungsten, a high-temperature alloy, compressed carbon, silicon carbide, or a conductive ceramic. In an embodiment, the corona electrode 102 is made of stainless steel. Optionally, the corona electrode 102 is actively cooled, for example by circulating water or another cooling fluid through coolant passages (not shown) in the body of the corona electrode 102.

FIG. 6 is a diagram of a combustion system 600 including a corona electrode {e.g., 102, shown in FIG. 1 ) structured as a serrated corona electrode 606 (also referred to as "serrated electrode"), according to another embodiment. The system 600 includes a fuel burner structure 1 12 configured to support a combustion reaction 104. The serrated electrode 606 is configured to form an electrical relationship with the combustion reaction 104. The serrated electrode 606 includes a plurality of projections 608a, 608b, each configured to generate an increased electric field strength at their respective tips, substantially as described above with reference to the corona electrodes 102 of FIGS. 1 -5, and thus at least intermittently eject ions 1 1 1 into a dielectric gap 108 between the plurality of projections 608a, 608b and the combustion reaction 104. The plurality of projections 608a, 608b are configured to at least intermittently eject the ions 1 1 1 responsive to receiving an ion ejection voltage from an electrical coupling 406.

According to an embodiment, the dielectric gap 1 12 between the plurality of projections 608a, 608b and the combustion reaction 104 includes air.

Additionally or alternatively, the dielectric gap 1 12 can include flue gas.

The system 600 includes the electrical coupling 406 to the serrated electrode 106. The electrical coupling 406 includes a current channel operatively coupled to the power supply 106 (not shown in FIG. 6), according to an embodiment.

The electrical relationship between the serrated electrode 106 and the combustion reaction 104 includes an addition of charge to the combustion reaction 104. Additionally or alternatively, the electrical relationship between the serrated electrode 106 and the combustion reaction 104 can include the application of a voltage to the combustion reaction 104.

According to an embodiment, the system 600 includes a fuel source 616 configured to provide a fuel stream 618 to support the combustion reaction 104.

The system 600 includes a dull electrode 1 10. The dull electrode 1 10 is configured to be maintained in at least an intermittent capacitive relationship to the ejected ions 1 1 1 . Additionally or alternatively, the dull electrode 1 10 can be configured to be maintained in at least an intermittent capacitive relationship to the plurality of projections 608a, 608b, to the serrated electrode 106, and/or to the electrical coupling 406.

The system 600 includes an electrode-mounting surface 1 14. The electrode-mounting surface 1 14 is configured to mechanically couple the serrated electrode 606 to the other elements of the burner system 600.

According to an embodiment, the mounting surface 1 14 is electrically insulated from the serrated electrode 606, as described with reference to FIG. 4.

According to other embodiments, the electrode-mounting surface 1 14 forms a portion of the electrical coupling 406. Additionally or alternatively, the electrical coupling 406 can form a portion of the electrode-mounting surface 1 14.

According to another embodiment, the electrode-mounting surface 1 14 and the electrical coupling 406 are substantially congruent. Additionally or alternatively, the electrode-mounting surface 1 14 and the electrical coupling 406 can be in electrical continuity with one another.

According to an embodiment, the electrode-mounting surface 1 14 includes a clamp configured to hold the serrated electrode 606 in a substantially constant position relative to the fuel burner structure 1 12. Additionally or alternatively, the clamp can be configured to hold the serrated electrode 606 in one or more positions relative to the fuel burner structure 1 12.

According to another embodiment, the electrode-mounting surface 1 14 can be configured to move the serrated electrode 606 to a time-varying plurality of positions relative to the fuel burner structure 1 12. The time-varying plurality of positions corresponds to one or more serrated electrode 606 loading actuation movements. The time-varying plurality of positions corresponds to vibration, translation along one or more axes, rotation about one or more axes, and/or yaw relative to one or more axes. Additionally or alternatively, the time-varying plurality of positions correspond to a heat-cycling movement of the serrated electrode 606 relative to the fuel burner structure 1 12 and the combustion reaction 104.

In an embodiment of the system 600, the serrated electrode 606 includes a sawblade originally configured to fit a powered saw body or a hand saw body. For example, the serrated electrode 606 can be at least derived from a sawblade configured to fit a powered saw body or a hand saw body.

According to an embodiment, the serrated electrode 606 includes an electrode body 624 operatively coupled to the plurality of projections 608a, 608b. According to another embodiment, the serrated electrode 606 includes the electrode body 624 operatively coupled to a plurality of corona electrode portions including the plurality of projections 608a, 608b. Additionally or alternatively, the electrode body 624 can include the plurality of corona electrode portions including the plurality of projections 608a, 608b.

In an embodiment, the combustion reaction 104 includes a flame.

FIG. 7 is a diagram of a system 700 for applying charge or voltage to a combustion reaction 104, according to an embodiment. The system 700 includes a power supply 106 configured to output a voltage of 1000 volts or more. The system 700 also includes one or more serrated electrodes 606 operatively coupled to the power supply 106. The one or more serrated electrodes 606 are configured to eject ions 1 1 1 into a region 108 proximate to a combustion reaction 104. The system 700 includes a counter electrode 1 10 configured to at least intermittently receive current responsive to the ions 1 1 1 ejected (or emitted) by the serrated electrode 606. The counter electrode 1 10 is configured to at least intermittently supply current to the combustion reaction 104 responsive to the ions 1 1 1 ejected by the serrated electrode 606. The region 108 proximate to the combustion reaction 104 can be a dielectric gap. The region 108 can, for example, include air or flue gas.

According to an embodiment, the system 700 includes a fuel burner structure 1 12 configured to support the combustion reaction 104.

According to an embodiment, the receipt or supply of ionic current by the counter electrode 1 10 can be selected to anchor the combustion reaction 104 proximate to the counter electrode 1 10. The counter electrode 1 10 can also be electrically coupled to ground.

According to an embodiment, the system 700 includes a conductive fuel nozzle tip 706 electrically coupled to ground. For example, the counter electrode 1 10 can include a toric structure held circumferential to the fuel stream 618 output by the fuel source 616 (shown in FIG. 6).

FIG. 8 is a flowchart showing a method 801 for applying an electric field or voltage to a combustion reaction, according to an embodiment. In step 802 a corona electrode is supported proximate to a combustion reaction. The corona electrode may includes a small radius tip or edge, or, alternatively, includes a plurality of tips, as in a serrated electrode. In step 806, a voltage is applied to the corona electrode to cause ion ejection in a voltage concentration volume peripheral to the small radius tip(s) or edge. In step 808, responsive to the application of the voltage and ion ejection, a response is caused in the

combustion reaction. According to an embodiment, step 802 includes supporting the corona electrode proximate to but not contacting the combustion reaction.

In embodiments that employ one or more serrated electrodes, each serrated electrode includes an electrode body and a plurality of projections coupled to or intrinsic to the electrode body. Each of the plurality of projections is shaped to cause corona ejection of ions responsive to the applied voltage.

According to an embodiment, at least a portion of the ejected ions travel across the dielectric gap to the combustion reaction to charge the combustion reaction. The dielectric gap can include air and/or can include flue gas.

According to an embodiment, the method 801 includes step 804, which includes supporting a second electrode proximate to or contacting the combustion reaction. Supporting a second electrode may include supporting a second corona electrode proximate to but not contacting the combustion reaction. Alternatively, supporting the second electrode proximate to or contacting the combustion reaction may include supporting at least one dull electrode to cooperate with the at least one corona electrode to produce an electric field.

In step 806, according to an embodiment, applying the voltage to the corona electrode results in an electric field magnitude adjacent to a dull electrode that is no larger than twice the average electric field magnitude between the electrodes, while an electric field magnitude adjacent to the corona electrode is at least twice the average electric field magnitude between the electrodes.

Supporting the second electrode proximate to or contacting the

combustion reaction can include supporting a toroid or torus. The method 801 can optionally include driving the second electrode to an instantaneous voltage substantially the same as the instantaneous voltage applied to the corona electrode. Alternatively, the second electrode can be held substantially at voltage ground. Alternatively, the second electrode can be isolated from ground and from voltages other than a voltage received from the corona electrode.

According to another embodiment, the method includes moving the serrated electrode to a time-varying plurality of positions relative to the fuel burner structure.

According to an embodiment, the corona electrode includes a cylindrical taper to a tip having a radius of 0.1 inch or less. This radius is preferably less than 0.004" for most applications. According to another embodiment, the corona electrode includes a conductive blade having a taper to an edge having a radius of 0.1 inch or less. Alternatively, the tip or edge can be larger than 0.1 -inch radius, especially under conditions of higher voltage or appropriate counter electrode/combustion reaction geometry is maintained to conform with Peek's Law.

Applying a voltage to the corona electrode in step 806 can include operating a power supply to apply a high voltage to the corona electrode(s). Applying the voltage to the at least one corona electrode can include applying an electric field to a region adjacent to the combustion reaction, the electric field having a maximum magnitude in the voltage concentration volume peripheral to the small radius tip or edge at least double an average electric field magnitude in the region adjacent to the combustion reaction.

Applying a voltage to the at least one corona electrode in step 806 can include applying a substantially constant voltage to the at least one corona electrode. Alternatively, applying a voltage to the at least one corona electrode in step 806 can include applying a time-varying voltage to the at least one corona electrode.

Applying the time-varying voltage can include applying a periodic voltage waveform having a 50 to 10,000 Hertz frequency. For example, applying the time-varying voltage can include applying a periodic voltage waveform having a 200 to 800 Hertz frequency. Applying the time-varying voltage can include applying a square waveform, sine waveform, triangular waveform, truncated triangular waveform, sawtooth waveform, logarithmic waveform, or exponential waveform. Applying the time-varying voltage can include applying a waveform having ±1000 volt to ±1 15,000 volt amplitude. For example, applying the time- varying voltage can include applying a waveform having ±8000 volt to ±40,000 volt amplitude. Applying the voltage to the at least one corona electrode in step 806 can include applying an average electric field magnitude in the region adjacent to the combustion reaction between 0.3 kV/m to 1000 kV/m. For example, applying the voltage to the at least one corona electrode can include applying an average electric field magnitude in the region adjacent to the combustion reaction between 80 kV/m to 400 kV/m. Applying the voltage to the at least one corona electrode can include applying an average electric field magnitude sufficient to meet a corona inception voltage according to Peek's law.

Referring to step 808, causing a response in the combustion reaction includes causing a visible response in the flame, according to an embodiment. Additionally or alternatively, causing a response in the combustion reaction can include causing increased mixing of fuel and oxidizer in the combustion reaction. Causing the increased mixing of fuel and oxidizer can increase a rate of combustion. Additionally or alternatively, causing the increased mixing of fuel and oxidizer can increase fuel and air contact in the combustion reaction.

Additionally or alternatively, causing the increased mixing of fuel and oxidizer can decrease a combustion reaction temperature. Additionally or alternatively, causing the increased mixing of fuel and oxidizer can decrease an evolution of oxides of nitrogen (NOx) by the combustion reaction. Additionally or

alternatively, causing the increased mixing of fuel and oxidizer may decrease an evolution of carbon monoxide (CO) by the combustion reaction. Causing the increased mixing of fuel and oxidizer may increase flame stability and/or decrease a chance of flame blow-out. Additionally or alternatively, causing the increased mixing of fuel and oxidizer can increase combustion reaction emissivity. Additionally or alternatively, causing the increased mixing of fuel and oxidizer can decrease combustion reaction size for a given fuel flow rate.

According to an embodiment, the method 801 includes causing a conductive surface of the combustion reaction to form a substantially

equipotential surface that cooperates with the corona electrode(s) to produce an electric field between the corona electrode(s) and the combustion reaction (not shown). Causing the conductive surface of the combustion reaction to form a substantially equipotential surface includes applying a voltage condition to a burner in electrical continuity with the combustion reaction. Applying a voltage condition to the burner includes operating a power supply that also applies the voltage to the at least one corona electrode. Additionally or alternatively, applying a voltage condition to the burner can include holding the burner substantially at voltage ground. Applying a voltage condition to the burner can include isolating the burner from ground and from voltage sources other than the corona electrode such that the burner is electrically floating.

One problem that the inventors have identified is the potential for thermal ablation of electrodes employed in applying a charge to a combustion reaction. Depending on the particular circumstances and configuration, the tip of an electrode can be heated by the combustion reaction to a point that it undergoes gradual sublimation, as molecules of the material of the electrode are gasified and dispersed. Particularly in cases where the electrode has a relatively sharp tip, the mass of the electrode at the point may not be sufficient to conduct heat away from the tip well enough to prevent overheating at the tip. As a result, the tip ablates and becomes more rounded, reducing the efficiency of the electrode.

In contrast to many other electrode designs, a corona electrode need not make contact with a combustion reaction, but can be positioned some distance from the reaction, thus reducing the heat to which it is subjected. On the other hand, because a corona electrode has a relatively sharp tip or edge, it is more susceptible to overheating.

Turning now to FIG. 9, a system 900 is shown, according to an

embodiment. In most respects, the system 900 is substantially similar to the system 100 of FIG. 1 , having, for example, a burner 1 12 configured to support a combustion reaction 104, and a corona electrode 102 configured to eject ions 1 1 1 toward the reaction 104. For brevity, other elements of the system that are previously described, or well known in the art are not shown.

System 900 includes a radiation shield 902 supported by a bracket 904 in a position directly between the electrode 102 and the combustion reaction 104. Heat radiated by the combustion reaction 104 is intercepted or reduced by the radiation shield 902 such that the temperature proximate to sharp (ion ejecting) features of the corona electrode 102 is reduced. Since a large majority of the heat energy applied to the corona electrode 102 is in the form of thermal radiation, and thermal radiation is transmitted along a line-of-sight, the radiation shield 902 prevents radiant heat from the combustion reaction 104 from impinging on at least the tip of the corona electrode 102. It was found by the inventors that high temperature may be associated with a reduction in ion ejection rate by a corona electrode 102 in proximity to a combustion reaction 104. The radiation shield 902 at least partially ameliorates this effect.

The bracket 904 can be electrically conductive, semi-conductive, or insulating. In an embodiment, bracket 904 is formed at least partly from an electrical insulator such as alumina. The radiation shield 902 is maintained at a floating electrical potential different from the electrical potential of the corona electrode 102. In another embodiment the radiation shield 902 may be made of a non-conductive material such as a ceramic. For example, alumina is a suitable non-conductive material choice in some embodiments.

The radiation shield 902 can be electrically conductive, semi-conductive, or insulating.

In an embodiment, the radiation shield 902 floats or is driven to an electrical potential between the electrical potential of the corona electrode 102 and the electrical potential of the combustion reaction 104. In another

embodiment, the radiation shield 902 is driven to the same potential as the corona electrode 102. In such a case, one or more counter electrodes 1 10 can be disposed to cause the corona electrode 102 to emit ions 1 1 1 . For example, the counter electrode(s) 1 10 can be disposed beside the radiation shield 902 to cause ions 1 1 1 emitted by the corona electrode 102 to pass around the radiation shield 902. In another example, the counter electrode(s) 1 10 can be disposed to cause the corona electrode 102 to emit ions 1 1 1 in a direction different than a line-of-sight between the combustion reaction 104 and the corona electrode 102.

In an embodiment, the radiation shield 902 is configured to not prevent ejected ions 1 1 1 from traveling in the direction of the combustion reaction 104. For example, as described above, the radiation shield 902 can be disposed in a direction that is different than an ion streaming direction. In another embodiment, the radiation shield 902 is formed from a screen or includes holes that allow ejected ions 1 1 1 to travel from the corona electrode 102 and through the radiation shield 902 to the combustion reaction 104.

In the example of FIG. 9, the radiation shield 902 is sized and positioned to protect primarily the tip and front portion of the corona electrode 102 from radiant heat, which constitute the most vulnerable portions of the corona electrode 102. The radiation shield 902 can be made larger to protect more of the corona electrode 102, but will tend to block portions of the ions 1 1 1 if made too large. Accordingly, the shield is preferably no larger than necessary to prevent direct transmission of radiant heat to the corona electrode 102. According to various embodiments, the size, shape, optical transparency (e.g., a portion of the radiation shield 902 that is perforated by holes formed therethrough), and position of the radiation shield 902 are selected to protect more or less of the corona electrode 102, according to the heat-tolerance of the corona electrode 102.

The bracket 904 can be mounted directly to the corona electrode 102, as shown in FIG. 9, or it can be coupled separately, according to the needs of a particular system.

FIG. 10 is a diagram of a system 1000, according to an embodiment. The System 1000 is substantially similar to the system 600 of FIG. 6, having, for example, a burner 1 12 configured to support a combustion reaction 104, and a serrated electrode 606 configured to eject ions 1 1 1 toward the reaction 104. The system 1000 also includes a radiation shield 902 supported by a bracket 904 in a position directly between the serrated electrode 606 and the combustion reaction 104. The radiation shield 902 of the system 1000 functions substantially identically to the radiation shield 902 of the system 900, but is shaped to protect the serrated electrode 606 from the radiant heat of the combustion reaction 104. As with the radiation shield 902 of the system 900, the radiation shield 902 is coupled by the bracket 904 directly to the electrode 606. However, this is merely illustrative, and can be configured in a manner convenient to the particular system configuration.

FIG. 11 is a diagrammatic view of a system 1 100, according to an embodiment. The system 1 100 includes a burner 1 12 configured to support a combustion reaction 104. The system 1 100 also includes a corona electrode 1 102. The corona electrode 1 102 includes a core 1 106, a support body 1 108, and a connector 1 109 configured to receive an electrical connection to a power supply (not shown).

A forward end of a corona electrode core 1 106 extends slightly beyond a forward end of a support body 1 108. Ablation of the core 1 106 and the support body 1 108 tends to result in and sustain a generally sharp shape on the forward end of the corona electrode 1 102. A radius can be substantially equal to a radius taken in a plane that lies perpendicular to a longitudinal axis of the corona electrode 1 102. The core 1 106 has a radius selected to be appropriate for the tip radius of a corona electrode 102 (shown in FIG. 1 ), as previously described.

The core 1 106 is formulated to have a greater resistance to heat and ablation than the support body 1 108, and therefore tends to ablate more slowly than the support body 1 108. Additionally, the support body 1 108 protects the core 1 106 from ablation except at the tip, as ablation of the support body 1 108 exposes the forward end of the core 1 106. As the corona electrode 1 102 ablates, the support body 1 108 protects areas of core 1 106 covered thereby, and the greater ablation resistance of the core 1 106 causes the core 1 106 to ablate more slowly than the support body 1 108. The geometry and material properties of the core 1 106 and the support body 1 108 cause the corona electrode 1 102 to "self-sharpen" such that the tip radius does not increase, but remains consistent.

The core 1 106 can be made of a relatively hard, non-reactive, and/or high melting point material and the support body 1 108 can be made of a relatively soft, reactive, and/or lower melting point material. In one example, the core 1 106 is carbon steel and the support body 1 108 is made of soft iron. In this example, a self-sharpening characteristic of the corona electrode 102 is provided primarily by a difference hardness between the core 1 106 and support body 1 108 materials. In another example, the core 1 106 is made of platinum and the support body 1 108 is made of tungsten. In this example, the self-sharpening characteristic of the corona electrode 102 is provided primarily by a difference in reactivity between the core 1 106 and support body 1 108 materials. Other combinations of core 1 106 and support body 1 108 materials are contemplated and fall within the scope of the claims.

In the example of FIG. 11 , the system 1 100 includes an electrode advancement mechanism 1 104 configured to advance the electrode 1 102 toward the combustion reaction 104 as the electrode becomes shorter due to ablation. The electrode advancement mechanism 1 104 includes a stepper motor 1 1 10 controlled by an advancement circuit and coupled to advancement rollers 1 1 12. The stepper motor 1 1 10 is controllable to extend the electrode 1 102 by small and precise increments. An electrode advance controller 1 1 14 contains a non- transitory computer-readable medium carrying computer executable instructions to (optionally) sense an electrode 1 102 position and advance or retract the position of the electrode 1 102.

In operation, as the electrode 1 102 shortens due to ablation, a sensor (not shown) configured to detect the forward end of the electrode provides a signal to the electrode advancement mechanism 1 104, which advances the electrode toward the combustion reaction 104, thereby maintaining the position of the forward end of the electrode, relative to the combustion reaction 104. Thus, the length of the portion of the core 1 106 that extends from the support body 1 108 remains substantially constant as the electrode 1 102 ablates. In an embodiment, the sensor can include a current or voltage sensor operatively coupled to the corona electrode 1 102. In another embodiment, no sensor is used. The electrode 1 102 is fed forward at a predetermined rate or is repositioned

manually. In a manual embodiment, the electrode advance controller includes a human interface configured to receive a control input from an operating engineer. In another embodiment, the electrode 1 102 is held in a fixed position, and the nominal position of the core material 1 106 is allowed to recede between scheduled service or replacement.

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.