Patent Application
20140170576
Flare stacks are used to burn off vented volatile organic compounds. For example, in an oil refinery, a flare stack may be used to maintain a substantially atmospheric pressure at a node of the system. In an oil field, a flare stack may be used to burn off natural gas that is produced as a byproduct of crude oil production. In a landfill, a flare stack may be used to burn off methane released by decomposition processes. Because volatile organics are considered pollutants, it is generally considered more preferred to burn the volatile organics than to vent the volatile organics directly to the atmosphere.
In flare stack applications, it can be important to control the height of a flame envelope created by the burner. In some applications, it may be required or desired that the flame not exceed the height of the flare stack itself. By keeping the flame inside the flare stack, safety may be improved. Moreover, aesthetics may be improved sufficiently to avoid complaints about a visible flame.
Enclosed flare stacks or ground flares can be used for burning off unusable waste field gas in a variety of oil and gas production applications, for example. Waste gases may be released during over-pressuring of plant equipment. The waste gases may be transported to a corresponding ground flare. Some ground flares are enclosed. By “enclosed” it is meant that a flame envelope is substantially blocked from view by persons outside a controlled access area.
Flame length may determine a required height, girth, or other dimensions of the ground flare structure. A problem may arise when the flame becomes visible (e.g., is too high). One parameter that can cause undesirable flame lengthening is insufficient air in combustion regions of a ground flare. Poor mixing of fuel and air may similarly cause flame lengthening.
Excessively high flame length may substantially halt operational permitting, and/or may be expressed as greater capital cost, increased operating expenses, and/or other remediation expenses.
For the foregoing reasons, it is desirable to reduce flame length and/or improve the overall combustion efficiency in ground flares.
According to an embodiment, a ground flare structure is configured for the application of electrical effects to a flame. The application of electrical effects can include application of a charge or voltage to the flame, and/or application of an electric field to a flame. The ground flare structure can include a vertical stack, a burner to support the flame, air inlets to allow air flow necessary for combustion, piping that transports fuel or waste gas to burner, and a power source connected to one or more electrodes.
According to an embodiment, a power source can generate a time-varying voltage waveform that can be applied to the flame through one or more electrodes. This time-varying voltage waveform can introduce alternating positive and negative charges to the flame, creating continuous expansion and contraction of the flame in an oscillating effect. This oscillating effect on the flame can enhance the mixing of air and fuel, improving combustion efficiency and reducing flame length.
According to another embodiment, a power source can generate one or more DC voltages that can be applied to the flame through one or more electrodes. The DC voltages can be used to control the flame shape.
According to an embodiment, a system for volatile compound venting with a flare stack can include a flare stack combustor configured to at least intermittently receive volatile compound flow and support a flame at least partially fueled by the volatile compound flow. An electrical energy application system can be configured to apply electrical energy to at least a portion of the flare stack combustor supporting the flame, and to cause the flame to be substantially contained within the flare stack.
By reducing flame length within the vertical stack, material requirements for building ground flare structures can be significantly reduced as less material would be required to support a shorter flame. The technique for reducing flame length disclosed herein can assist compliance with regulation standards about flame length and can also be applicable for elevated flares and retrofit applications.
In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, which are not to scale or to proportion, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings and claims, are not meant to be limiting. Other embodiments may be used and/or and other changes may be made without departing from the spirit or scope of the present disclosure.
The system 100 for volatile compound venting with a flare stack 102 includes a flare stack combustor 104. The flare stack 102 is configured to at least intermittently receive volatile compound flow and support a flame 106 at least partially fueled by the volatile compound flow.
The system 100 for volatile compound venting with a flare stack 102 includes an electrical energy application system 108 configured to apply electrical energy to at least a portion of the flare stack combustor 104 supporting the flame 106, and to cause the flame 106 to be substantially contained within the flare stack 102.
According to an embodiment, the electrical energy application system 108 can include a controller 110, a voltage source 112 operatively coupled to and responsive to the controller 110 and one or more electrodes 114 operatively coupled to the voltage source 112 and the flare stack combustor 104.
The voltage source 112 can be disposed outside a grounded vertical stack 116. The electrical energy application system 108 and/or the voltage source 112 can further include at least one electrical isolator and/or insulator 118. The at least one electrical isolator and/or insulator 118 can be configured to maintain electrical insulation and/or isolation between the voltage provided by the voltage source 112 and ground.
According to an embodiment, the one or more electrodes 114 can be configured to affect a rate of combustion in the flare stack combustor 104.
The one or more electrodes 114 can be configured to affect an ionic wind in the flare stack combustor 104.
The one or more electrodes 114 can be configured to flatten the flame 106 to substantially prevent flame height from exceeding the height of a visual barrier 120. The visual barrier 120 can include a top edge of the flare stack.
The flare stack combustor 104 can be configured to receive ignition fuel from an igniter fuel source 122 and to receive a volatile gas fuel from a volatile gas fuel source 124, to maintain a pilot flame 106 or initiate ignition with the ignition fuel, and to maintain ignition of the volatile gas fuel using the pilot flame 106 or ignition. An igniter controller 126 can be configured to cause the flare stack combustor 104 to establish and/or maintain ignition.
The electrical energy application system 108 can include a controller 110, which can be referred to as an ECC controller 110. The ECC controller 110 can be configured to control the application of an electrical voltage, an electrical charge, an electrical field, and/or a combination thereof to the flare stack combustor 104.
The electrical energy application system controller 110 may be configured to cause the electrical energy application system 108 to apply a spark discharge to the flare stack combustor 104 when fuel is present without ignition or a pilot flame 106.
According to various embodiments, the igniter controller 126 can be operatively coupled to the electrical energy application system controller 110. For example, portions of the igniter controller 126 and the ECC controller 110 can include hardware and/or software that is shared. The ECC controller 110 can include igniter control as part of its function. The igniter controller 126 can include electrical energy application system 108 control as part of its function. One or more electrodes 114 can cooperate to form a spark (or arc) discharge ignition source for the igniter fuel 122 and/or the volatile compound flow.
The charge source 202 can be configured to apply charge to one or more fuel streams 204 that support the flame 106.
The charge source 202 can include a serrated, ion-ejecting electrode. The serrated, ion ejecting electrode can be disposed to convey ejected ions to the flame 106.
The charge source 202 can include an ionizer configured to convey ions to the flame 106.
A current-limiting resistor 205 can be included. The current-limiting resistor 205 can be operatively coupled between the voltage source 112 and the charge source 202. The current-limiting resistor 205 can be configured to reduce or eliminate the formation of electrical arcs to or from the charge source 202.
The one or more electrodes 114 can include a charge source 202 configured to supply a charge to the flame 106 and at least one field electrode 206 configured to flatten the flame 106.
According to an embodiment, the at least one field electrode 206 can include a distally-disposed repulsion electrode 208 configured to receive a voltage having the same polarity as the charge applied to the flame 106. The distally-disposed repulsion electrode 208 can be configured to exert a downward Coulombic pressure on the flame 106 to cause the most distal tip of the flame 106 to be below a top edge 120 of the of the flare stack.
The distally-disposed repulsion electrode 208 can be disposed at or below the top edge 120 of the flare stack. Additionally and/or alternatively, the distally-disposed repulsion electrode 208 can be disposed at or above a nominally designated flame tip.
According to an embodiment, the at least one field electrode 206 can include a proximally-disposed attraction electrode 210 configured to receive a voltage having the opposite polarity as the charge applied to the flame 106.
The proximally-disposed attraction electrode 210 can be configured to exert a downward Coulombic attraction force on the flame 106 to cause a higher amount of combustion at or near a flame holder 212 than a flare stack combustor 104 not including the proximally-disposed attraction electrode 210.
According to an embodiment, a current limiting resistor 214 can be included and operatively coupled between the voltage source 112 and the attraction electrode 210. The current-limiting resistor 214 can be configured to reduce or eliminate the formation of electrical arcs to or from the attraction electrode 210.
The flare stack combustor 104 can also include at least one fuel nozzle 216. The fuel nozzle(s) 216 can optionally be maintained at a voltage relative to ground and/or relative to a voltage placed on a nearby electrode 114, such as the ion-ejecting electrode 202 and/or the proximally disposed electrode 210. The fuel nozzle can cooperate with a nearby electrode 114 to at least intermittently form an electric field therebetween. According to an embodiment, the flame holder 212 can include or consist essentially of a flame holding conductive surface, which can be referred to as an electrode 114.
The system can include a sensor 302 operatively coupled to the electrical energy application system controller 110. The sensor 302 can be configured to sense a flame parameter.
The sensor 302 can be configured to sense flame height. The sensor 302 can be configured to sense a parameter proportional to flame behavior. Additionally, the sensor 302 can include an infrared sensor and/or pyrometer.
One or more fuel flow sensors can be included. The one or more fuel flow sensors can be operatively coupled to the electrical energy application system controller 110 and can be configured to detect a fuel flow rate to the flare stack combustor 104.
The electrical energy application system controller 110 can be configured to cause the electrical energy application system 108 to apply at least one of one or more voltages, one or more duty cycles, one or more charge densities, and/or one or more electric fields having a magnitude proportional to the fuel flow rate. Additionally or alternatively, the electrical energy application system controller 110 can be configured to dynamically modulate the electrical energy application system 108 responsive to dynamic changes in the fuel flow rate. The electrical energy application system controller 110 can include a proportional controller, an integral controller, a differential controller, and/or a combination thereof.
The electrical energy application system 108 can be configured to apply one or more DC voltages to the flame 106.
The electrical energy application system 108 can be configured to apply one or more time-varying voltages to the flame 106. The electrical energy application system 108 can be configured to apply one or more alternating current (AC) voltages to the flame 106. The electrical energy application system 108 can be configured to apply one or more voltage waveforms to the flame 106. Additionally, the electrical energy application system 108 can be configured to apply one or more of a sinusoidal voltage waveform, a square voltage waveform, a sawtooth voltage waveform, a triangular voltage waveform, a truncated sawtooth or triangular voltage waveform, a logarithmic voltage waveform, or an exponential voltage waveform to the flame 106.
According to an embodiment, the one or more time-varying voltages can be selected to increase flame mixing to cause substantially complete consumption of fuel within the flare stack 102.
The dimensions or scale, geometric relationships and forms of the vertical stack 116 and the fuel nozzle(s) 216 can vary according to the application.
Two or more air inlets 408 can be located at the bottom regions of the vertical stack 116 for allowing air 410 flow to support the combustion. A fuel stream 204 to be disposed by burning is fed to fuel nozzle(s) 216 through piping 414. The fuel stream 204 can include waste gases originated from over-pressuring of plant equipment, or other hydrocarbon-based fuels. For example, the fuel stream 204 can include a refinery mixture of 25% hydrogen, 50% methane, and 25% propane, where there is enough carbon content for the technique described herein to work accordingly.
The flame 106 enclosed in the vertical stack 116 can include a plurality of charged and uncharged species. During combustion, charged species such as ions 420 are produced within the flame 106. Such ions 420 can include HCO+, C3H3+, H3O+, among others, along with their corresponding but dissociated electrons. Uncharged or neutral species can include uncharged combustion products, unburned fuel stream 204 and air 410.
One or more electrodes 114 can be configured to apply voltage, charge, and/or electric field to the flame 106. One or more electrodes 114 can be isolated from the ground flare structure 400 and can be connected to a voltage source 112. The voltage source 112 can be configured to produce a plurality of voltage waveforms for driving one or more electrodes 114. In an embodiment, a controller can be connected to the voltage source 112 to determine voltage waveforms for driving one or more electrodes 114 according to received combustion feedback or sensed combustion values from a plurality of sensors.
In another embodiment, fuel nozzle(s) 216 can be configured to apply voltage, charge, and/or electric field to the flame 106. Yet in another embodiment, a charged pilot flame can be configured for the application of voltage, charge, and/or electric field to flame the 106.
As shown in
A time-varying voltage waveform 502 generated from voltage source 112 and applied through one or more electrodes 114 can first introduce a positive charge at high voltage but low amperage into the flame 106 to remove electrons and enhance the concentration of cations. Exit of electrons from the flame 106 can occur very rapidly as electrons are considerably less massive than ions 420. As a result, the higher concentration of positive ions 420 in the flame 106 can disperse as charges of same polarity mutually repel. While charge imbalance affects primarily ions 420, collisions between ions 420 and uncharged or neutral species can occur, producing a net dispersive bulk flow away from flame 106 and toward a region of lower electrical potential, in this case, the vertical stack 116 which is grounded. At this time, the flame 106 can expand toward the vertical stack 116.
Ions 420 within the flame 106 can capture electrons when reaching grounded the vertical stack 116 or any oppositely charged structure. In order to avoid this condition, the time-varying voltage waveform 502 generated from the voltage source 112 and applied through electrodes 114 can introduce a negative charge to the flame 106 to bring back electrons and reduce concentration of cations. With a higher concentration of electrons and lower concentration of cations, the flame 106 can repel from the vertical stack 116 and can contract to its original shape.
The time-varying voltage waveform 502 can continue reversing the polarity of one or more electrodes 114, producing continuous expansion/contraction of flame 106. This can be referred to as oscillation 504 of the flame 106.
In addition, oscillation 504 of the flame 106 can provide higher entrainment and mixing with the neutral species without the need for additional excess air 410. The excess air 410 requirements can be reduced since the bulk momentum of air 410 used for mixing can be assisted by increased turbulence originated from continuous oscillation 504 of the flame 106.
The reduced flame length can be maintained as long as the voltage source 112 operates at ON mode and continues driving one or more electrodes 114 for the application of time-varying voltage waveform to the flame 106. When the voltage source 112 is deactivated or at OFF mode, the flame 106 can immediately return to its normal combustion state as described in
Different levels of flame length reduction can be achieved according to application requirements. The flame length reduction can depend on the voltage amplitude and frequency of the time-varying voltage waveform 402 applied by one or more electrodes 114, as well as fuel type and/or the overall ground flare structure 400 configuration.
The period P can include a duration tL corresponding to low voltage VL and another duration tH corresponding high voltage VH, where tL plus tH can equal P. Frequency of the time-varying voltage waveform 502 can be the inverse of period P. According to an embodiment, flame length reduction can be controlled by modulating the frequency of time-varying voltage waveform 502, which can vary between 10 Hz and 2 kHz, with 200 Hz being preferred.
As described in
The technique herein described for the reduction of flame length can also be applicable to elevated flares or open flare arrays that can require improved mixing of air and fuel, and higher combustion efficiency.
While various aspects and embodiments have been disclosed herein, other aspects and embodiments may be 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.
The present application claims priority benefit from U.S. Provisional Patent Application No. 61/736,524, entitled “CONTAINED FLAME FLARE STACK”, filed Dec. 12, 2012; which, to the extent not inconsistent with the disclosure herein, is incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61736524 | Dec 2012 | US |
