Abstract:
According to embodiments, a co-fired or multiple fuel combustion system is configured to apply an electric field to a combustion region corresponding to a second fuel that normally suffers from poor combustion and/or high sooting. Application of an AC voltage to the combustion region was found to increase the extent of combustion and significantly reduce soot evolved from the second fuel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority benefit from U.S. Provisional Patent Application No. 61/616,223, entitled “MULTIPLE FUEL COMBUSTION SYSTEM AND METHOD”, filed Mar. 27, 2012; which, to the extent not inconsistent with the disclosure herein, is incorporated by reference. 
    
    
     SUMMARY 
     According to an embodiment, electro-dynamic and/or electrostatic fields may be applied to a co-fired combustion system to enhance combustion property(ies). In an example system, a bench-top scale model selectively introduced an AC field across a simulated tire-derived fuel (TDF) (a cut up bicycle inner-tube) held in a crucible over a propane pre-mixed flame. Without the electric field, the simulated TDF smoked profusely. With the electric field turned on, there was not any visible soot (although instrumentation detected a low level of soot). A cause and effect relationship was established by repeatedly turning on and turning off the electric fields. There was no observable hysteresis effect−switch on=no visible soot, switch off=visible soot. 
     According to an embodiment, a co-fired combustion apparatus may include a first fuel-introduction body defining a portion of a first combustion region. This may correspond to the premix nozzle and a flame region, for example. The first combustion region may be configured to combust a first fuel (e.g., propane) in a first combustion reaction. The apparatus may also include a second fuel-introduction body defining at least a portion of second combustion region. For example, the second fuel-introduction body may include the crucible described above. The second combustion region may be configured to combust a second fuel in a second combustion reaction. The first combustion reaction may be operable to sustain the second combustion reaction. For example, the simulated TDF was not readily ignited until heated by the propane flame. An electrode assembly associated with the second combustion region may be operable to be driven to or held at one or more first voltages. In the example above, the electrode assembly included the metallic crucible itself. A grounded 4-inch stack that was located approximately axial to the crucible may be envisioned as providing an image charge that varied to solve a field equation driven by the AC waveform. 
     Accordingly to another embodiment, a method of co-fired combustion may include maintaining the first combustion reaction by combusting the first fuel at the first combustion region. In other words, the propane combustion reaction C 3 H 8 +5 0 2 →3 C0 2 +4 H 2 0 may be a self-sustaining exothermic reaction. The first combustion region may have a portion thereof defined by the first fuel-introducing body. The method may further include maintaining a second combustion reaction by combusting a second fuel at a second combustion region having a portion defined by a second fuel-introducing body. The second combustion may be sustained by the first combustion reaction. According to embodiments, the method includes applying at least one first electrical potential (which may include a time-varying electrical potential) proximate the second combustion region. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a diagram of a co-fired combustion apparatus, according to an embodiment. 
         FIG. 2  is a diagram of a co-fired combustion apparatus, according to an embodiment. 
         FIG. 3  is a flow chart of a co-fired combustion method, according to an embodiment. 
         FIGS. 4-27  are thermographic images captured during a heat-exchange experiment wherein a voltage was applied to and removed over time from a crucible supporting a combustion, according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
       FIG. 1  is a diagram of a co-fired combustion apparatus  100 , according to an embodiment. The apparatus  100  may include a first fuel-introduction body  105  defining a portion of first combustion region  110 . The first combustion region  110  may be configured to combust a first fuel (not shown) in a first combustion reaction  115 . In an embodiment, the first fuel-introduction body  105  may be supported in a housing  120  by a first fuel-introduction-body support  125 . The first fuel may be provided by a first fuel supply  130 . The first fuel may be substantially liquid or gaseous. For example, the first fuel may include at least one of natural gas, propane, oil, or coal. In an embodiment, the first fuel-introduction body  105  may comprise a burner assembly that is configured to support a flame. 
     A second fuel-introduction body  135  may define a portion of a second combustion region  140 . The second combustion region  140  may be configured to combust a second fuel  145  in a second combustion reaction  150 . In an embodiment, the second fuel-introduction body  135  may comprise a crucible assembly, which may be operable to hold the second fuel  145 . Alternatively, the second fuel-introduction body  135  may include a grate, a screen, a fluidized bed support, or another apparatus configured to introduce, contain and/or hold the second fuel  145  proximate the second combustion region  140 . The second fuel-introduction body  135  may be supported in the housing  120  by a second fuel-introduction-body support  155 . In an embodiment, the second fuel  145  may be substantially solid under standard conditions. The second fuel  145  may melt, melt and vaporize, sublime, and/or be dried responsive to heating from the first combustion reaction  115 . In an embodiment, the second fuel  145  may include one or more of rubber, wood, glycerin, an industrial waste stream, a post-consumer waste stream, an industrial by-product, garbage, hazardous waste, human waste, animal waste, animal carcasses, forestry residue, batteries, tires, waste plant material, or landfill waste. In an embodiment, the second fuel  145  may be fluidized to form at least a portion of a fluidized bed. 
     In an embodiment, the first combustion reaction  115  may sustain the second combustion reaction  150 . For example, the first combustion reaction  115  may generate heat which initiates or supports the second combustion reaction  150 . Accordingly, in an embodiment, the first fuel-introduction body  105  may be positioned at a distance proximate to the second fuel-introduction body  135  so that the first combustion reaction  115  may support the second combustion reaction  150 . In an embodiment, a portion of the apparatus  100  may be enclosed within a flue, stack, or pipe configured to convey at least a portion of a combustion product stream generated by the first and/or second combustion reactions  115 ,  150 . 
     According to an embodiment, the first combustion region  110  may be substantially separated from the second combustion region  140 . According to another embodiment, the first combustion region  110  may extend to overlap or occupy the entirety of the second combustion region  140 . According to an embodiment, the first combustion reaction  115  may provide ignition for the second combustion reaction  150 . 
     An electrode assembly  160  associated with the second combustion region  140  may be operable to be driven to or held at one or more first voltages such as a constant (DC) voltage, a modulated voltage, an alternating polarity (AC) voltage, or a modulated voltage with a DC voltage offset. In an embodiment, the electrode assembly  160  may comprise at least a portion of one or more of the second fuel-introduction body  135 , the second fuel-introduction-body support  155 , the housing  120 , or an electrode (not shown) separate from the second fuel-introduction body  135 , the second fuel-introduction body support  155 , and the housing  120 . In an embodiment, any of the second fuel-introduction body  135 , the second fuel-introduction-body support  155 , the housing  120 , or a separate electrode assembly  160  may each be configured to be driven to or held at one or more voltage(s), which may or may not be the same voltage. For example, the housing  120  may be held at a ground voltage and the second fuel-introduction-body support  155  may be held at or driven to positive and/or negative voltages. In an embodiment, the housing  120  may rest on a grounding plate  180 , which may ground the housing  120 . 
     It was found that the smoke reduction was most pronounced when the first voltage included a high voltage greater than +1000 volts and/or less than −1000 volts. For example, in experiments, the voltage was an AC waveform with amplitude of +/−10 kilovolts. Other high voltages may be used according to preferences of the system designer and/or operating engineer. 
     The electrode assembly  160  may be configured to be driven to or held at a voltage produced by a voltage source including a power supply  165 . The power supply  165  may be operatively coupled to controller  170 , which is configured to drive or control the electrode assembly  160 . In some embodiments, the electrode assembly  160  may include one or more electrodes positioned proximate to the second combustion region  140 , which may or may not directly contact the second fuel-introduction body  135  or the second fuel  145 . Such electrodes may be positioned in any desirable arrangement or configuration. In an embodiment, a portion of the first fuel-introduction body  105 , a portion of the first fuel-introduction-body support  125 , or a portion of an electrode (not shown) proximate to the first combustion region  110  may be configured to be held at one or more second voltage(s). 
     The apparatus  100  may optionally include one or more sensor(s)  175  operable to sense one or more conditions of the apparatus  100 , components thereof, and/or the second fuel  145  combustion reaction  150 . For example, a sensor  145  may sense heat, voltage, fluid flow, fluid turbulence, humidity, particulate matter, or one or more compounds or species. In an embodiment, the sensor  175  may be used to sense the condition or state of a combustion product stream generated by the second combustion reaction  150 . A sensed state or condition of the combustion product stream generated by the second combustion reaction  150  may be used by a feedback controller  170  to modify or modulate the one or more voltages and/or waveforms that the electrode assembly  160  is held at or driven to. 
     For example, as further discussed herein, driving or holding the electrode assembly  160  at one or more voltages may affect the second combustion reaction  150 . Driving or holding the electrode assembly  160  at one or more voltages may modify the efficiency, rate, thermal output, or turbulence, of the second combustion reaction  150 . The sensor(s)  175  may be operable to detect such effects. 
     It was found that applying an electric field proximate to a combustion reaction may be used to improve the efficiency of the combustion reaction. The improvement in efficiency may include a reduction in undesirable combustion products such as unburned fuel, oxides of sulfur (SO X ), oxides of nitrogen (NO X ), hydrocarbons, and other species. Additionally, the improvement in efficiency may include an increase in thermal energy generated by the combustion reaction per the amount of fuel. In addition to being less harmful to the environment, supporting a cleaner combustion reaction may result in lower operating expense. Discharge of certain combustion pollutants may require the purchase of emission-permits for an amount of pollutant discharge. Reducing pollutant discharge in a given reaction may therefore allow a business to obtain fewer emission-permits and/or output more heat at a reduced cost. Additionally or alternatively, less fuel may be consumed to generate an equivalent amount of energy. 
     Increased efficiency of a combustion reaction may occur via one or more mechanisms. For example, applying an electric field proximate to a combustion reaction may increase the number of collisions between reactants, which may increase the reaction rate. In one example, applying an electric field proximate to a combustion reaction may increase the collision energy of reactants and therefore increase the rate of reaction. In another example, applying an electric field proximate to a combustion reaction may provide a self-catalysis effect for various desirable reactions and may reduce the reaction activation energy by urging reactants to come together in a correct reaction orientation. In a further example, applying an electric field proximate to a combustion reaction may increase the turbulence of a reaction and thereby increase the mixture or introduction rate of reactants (e.g., increased mixing of oxygen with fuel), which may promote a more efficient or complete combustion reaction (e.g., where reactants combust to produce a greater proportion of desired reaction products, fewer unreacted reactants and undesired products or by-products of the combustion reaction will be emitted). 
       FIG. 2  is a diagram of a co-fired combustion apparatus  200 , according to an embodiment. The apparatus  200  may include a first fuel-introduction body  105  defining a portion of first combustion region  110 . The first combustion region  110  may be configured to combust a first fuel from a first fuel supply  130  in a first combustion reaction  115 . In an embodiment, the first fuel-introduction body  105  may be supported in a housing  120  by a first fuel-introduction-body support  125 . 
     The apparatus  200  may also include a second fuel-introduction body  135  defining a portion of a second combustion region  140 . The second combustion region  140  may be configured to combust a second fuel (not shown) in a second combustion reaction (not shown). In an embodiment, the second fuel-introduction body  135  may comprise a crucible assembly, which may be configured to hold the second fuel. Alternatively, the second fuel-introduction body  135  may include a grate, a screen, a fluidized bed support, or another apparatus configured to introduce and/or contain or hold the second fuel proximate the second combustion region  140 . The apparatus may also include a stoker  210 , configured to introduce the second fuel to the fuel-introduction body  135 . 
     For example, in an embodiment, the second fuel may comprise timber waste products, and the stoker  210  may be configured to convey timber waste products into the fuel-introduction body  135  so that sufficient second fuel is present to sustain a relatively constant combustion fuel volume within the second fuel-introduction body  135 . For example, as the second fuel is consumed, additional second fuel may be introduced by the stoker  210  so that the second combustion reaction may continue. Optionally, the second fuel-introduction body  135  may include a containment body  1608  configured to prevent entrainment of unburned second fuel particles in flue gas exiting through the top of the body  120 . 
     In another embodiment, the second fuel may include black liquor, such as a residue from a sulfite pulp mill. The stoker  210  may be configured to convey liquid or semi-solid black liquor to the second combustion region  140 . 
     Optionally, the burner  200  may include a heat recovery system including one or more heat transfer surfaces such as water tube boiler tubes to convert heat output by the second (not shown) and/or first combustion reaction  115  to heated water or steam. According to an embodiment, the application of electrical energy to at least the second combustion reaction (not shown) may reduce tendency for combustion byproducts or entrained materials to be deposited on heat transfer surfaces. This may allow a longer operating duration between service shut-downs to clean heat transfer surfaces. 
     A first and second electrode assembly  160 A,  160 B associated with the second combustion region  140  may be operable to be driven to or held at one or more voltages using a substantially constant (DC) voltage, a modulated voltage, an alternating polarity (AC) voltage, or a modulated voltage with DC voltage offset. The first electrode  160 A assembly may be configured to be driven to or held at one or more first voltages. The second electrode  1608  assembly may be configured to be driven to or held at one or more second voltages. In an embodiment, the first and second one or more voltages may be the same. 
     The first and second electrode assemblies  160 A,  160 B may be electrically isolated from a portion of the housing  120  via respective insulators and/or air gaps  220 A,  220 B. In an embodiment, the first and second electrode assembly  160 A,  160 B may be held or driven to a first and second voltage respectively, and the housing  120  may be held at or driven to a third voltage. For example, the housing  120  may be held at ground potential via a grounding plate  180 . 
     The first and second electrode assembly  160 A,  160 B may each be configured to be driven to or held at a voltage produced by a voltage source including a power supply  165 . The power supply  165  may be operatively coupled to controller  170 , which may be configured to control the output voltage, current, and/or waveform(s) output by the power supply  165  to the first and/or second electrode assemblies  160 A,  160 B. 
     The apparatus  200  may optionally include a first and/or second sensor  170 A,  1708  operable to sense one or more conditions of the apparatus  200  or components thereof. For example, the first sensor  170 A may be associated with the first electrode assembly  160 A, and the second sensor  1708  may be associated with the second electrode assembly  160 B. 
       FIG. 3  is a flow chart showing a method  300  for operating a co-fired combustion system, according to an embodiment. The method  300  begins in block  310  where a first combustion is maintained at a first combustion region by combusting a first fuel. For example, referring to  FIGS. 1 and 2 , the first combustion  115  may be maintained at the first fuel-introduction body  105  in the first combustion region  110 . The first fuel may be a relatively free-burning fuel such as a hydrocarbon gas, a hydrocarbon liquid, or coal. The first fuel should be chosen to have a flame temperature that is sufficiently high to support and/or ignite combustion of the second fuel. 
     The method  300  continues in block  320 , where a second combustion reaction is sustained by heat and/or ignition from the first combustion reaction. The second combustion reaction may be maintained at a second combustion region by combusting the second fuel. For example referring to  FIGS. 1 and 2 , the second combustion reaction  150  may be sustained by the first combustion reaction  115 , at the second fuel-introduction body  135  in the second combustion region  140 . According to an embodiment, heat from the first combustion reaction may dry, volatilized, and/or raise a vapor pressure of the second fuel sufficiently to allow the second fuel to burn. Additionally or alternatively, the first combustion region may overlap with or contain the second combustion region. The first combustion reaction may provide ignition and/or maintain combustion of the second fuel. 
     The method  300  continues in block  330  where a first potential or sequence of potentials is applied to a first electrode operatively coupled to the second combustion region. For example, referring to  FIG. 1  a first potential or sequence of potentials may be applied to the electrode assembly  160  proximate to the second combustion region  140 . Referring to  FIG. 2 , a first potential may be applied to the first electrode assembly  160 A proximate to the second combustion region  140 . According to an embodiment, the first potential or sequence of potentials may include a substantially constant (DC) voltage, a modulated voltage, an alternating polarity (AC) voltage, or a modulated voltage with DC voltage offset. 
     The method  300  continues in block  340 , where a second electrical potential or sequence of potentials is applied to a second electrode operatively coupled to the second combustion region. For example, referring to  FIG. 1  a second potential may be applied to the housing  120  proximate to the second combustion region  140 . Referring to  FIG. 2 , a second potential may be applied to the second electrode assembly  160 B proximate to the second combustion region  140 . 
     The electrical potentials applied in steps  330  and  340  may be selected to cause an increase in reaction rate and/or an increase in the reaction extent reached by the second combustion reaction. According to an embodiment, the first electrical potential or sequence of potentials may include a time-varying high voltage. The high voltage may be greater than 1000 volts and/or less than −1000 volts. According to an embodiment, the high voltage may include a polarity-changing waveform with an amplitude of +/1 10,000 volts or greater. The waveform may be a periodic waveform having a frequency of between 50 and 300 Hertz, for example. In another example, the waveform may be a periodic waveform having a frequency of between 300 and 1000 Hertz. According to an embodiment, the second electrical potential may be a substantially constant (DC) ground potential. 
     The method is shown looping from step  340  back to step  310 . In a real embodiment, the steps  310 ,  320 ,  330 , and  340  are generally performed simultaneously and continuously while the second fuel is being burned (after start-up and before shut-down). 
     EXAMPLE 
     Referring to  FIG. 1 , a burner assembly  105  was disposed within a cylindrical housing  120 , defining a first combustion region  110 . The burner assembly  105  was operatively connected to a propane gas supply (first fuel supply  130 ), which was used to sustain a propane flame on the burner assembly  105  in a first combustion  115 . The housing  120  was approximately 3 inches in diameter and approximately 1 foot tall. The burner assembly  105  was substantially cylindrical having a diameter of approximately ¾ inch, and a height of approximately 1 inch. 
     A crucible  135  having a diameter of approximately ¾ inch was positioned within the housing  120  above the propane first combustion  115 . The crucible  135  held a mass of rubber pieces (second fuel  145 ), which were obtained by cutting pieces from a bicycle inner-tube. The propane first combustion  115  caused the rubber pieces to ignite, thus generating a second combustion  150 . The second combustion  150  of the rubber pieces generated a combustion product stream (not shown), which visually presented as black smoke. The housing  120  was used to contain and direct the combustion product stream, and rested on a grounding plate  180 , which held the housing  120  at a ground voltage. 
     A modulated voltage of 10 kV was then applied to the crucible  135  at a frequency of 300-1000 Hz. The smoke generated by the combustion of the rubber pieces changed from a black smoke to no visible smoke. This indicated that the combustion product stream included fewer particulates. The voltage was removed from the crucible  135  and the combustion product stream again presented as black smoke. The voltage was again applied to the crucible  135  and the combustion product stream again presented as a lighter or substantially no visible smoke. 
     In a first particulate-residue trial, a first volume of rubber pieces was burned in the crucible  135  and a first paper filter was positioned on the top end of the housing  120  to collect particulate matter in the combustion product stream. A voltage was not applied to the crucible  135 . 
     In a second particulate-residue trial, a second volume of rubber pieces (having substantially the same mass as the first volume of the first trial) was burned in the crucible  135  and a second paper filter was positioned on the top end of the housing  120  to collect particulate matter. A modulated voltage of 10 kV was then applied to the crucible  135  at a frequency of 300-1000 Hz. 
     The first and second filter papers were compared, and the first filter paper exhibited a substantial layer of black particulate matter. The second filter paper on exhibited a light discoloration of the paper, but did not have a layer of particulate matter. This result further indicated that the application of the voltage created a substantial reduction in particulate matter in the combustion product stream of the combusting rubber pieces. 
     In a first heat-exchange trial, a first volume of rubber pieces was burned in the crucible  135  and thermographic images of the combustion were recorded over time using a Fluke Ti20 Thermal Analyzer at a perspective substantially the same as the perspective of  FIG. 1 . A propane fuel volume of 0.4 actual cubic feet per hour (acfh) was supplied to the burner assembly  105  during the trial. A voltage was not applied to the crucible  135 . 
     In a second heat-exchange trial, a second volume of rubber pieces (having substantially the same mass as the first volume of the first trial) was burned in the crucible  135  and thermographic images of the combustion were recorded over time using a Fluke Ti20 Thermal Analyzer at a perspective substantially the same as the perspective of  FIG. 1 . A propane fuel volume of 0.2 actual cubic feet per hour (acfh) was supplied to the burner assembly  105  during the trial (i.e., half of the fuel compared to the first trial). A modulated voltage of 10 kV was then applied to the crucible  135  at a frequency of 300-1000 Hz. 
     The thermographic images of the first and second heat-exchange trial were compared over time. At 15 seconds, both burners registered approximately 130° F. At 45 seconds the first heat-exchange trial continued to register 130° F.; the second heat-exchange trial burner (with 50% fuel) registered approximately 186° F. These trials indicated that even with 50% fuel volume, application of a voltage to the crucible  135  generated a higher combustion temperature. 
     In a third heat-exchange trial, a volume of rubber pieces was burned in the crucible  135  and thermographic images of the combustion were recorded over time using a Fluke Ti20 Thermal Analyzer at a perspective substantially the same as the perspective of  FIG. 1 . Over time, a modulated voltage of 10 kv was then applied to the crucible  135  at a frequency of 300 Hz for a period of time; the voltage was removed for a period of time; a modulated voltage of 10 kv was then applied to the crucible  135  at a frequency of 1000 Hz for a period of time; and the voltage was removed for a period of time. The application and removal of these voltages was repeated six times. An image was captured at the end of each period. 
       FIGS. 4-27  depict the thermographic images captured during the heat-exchange trial from a time of 9:27:16 until 10:52:16 and show that application of a voltage to the crucible  135  generated a higher combustion temperature. 
     Schlieren photography was used to visualize the flow of the combustion product stream generated by the combustion of rubber pieces within the crucible  135 . When no voltage was applied to the crucible  135 , the flow of the combustion product stream appeared to be laminar flow; however, when a modulated voltage of 10 kV was then applied to the crucible  135  at a frequency of 300-1000 Hz, the combustion product stream appeared to have turbulent flow. In other words, the combustion product stream behaved according to a low Reynolds number, laminar flow regime when no voltage was applied, and exhibited a high amount of turbulence evocative of a high Reynolds number when a voltage was applied, even though mass flow rates were nearly identical. 
     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 claims.