Abstract:
An electrically enhanced combustor includes bilayer insulation. A thermal insulator protects an electrical insulator from high temperatures that could cause the electrical insulator to become at least somewhat electrically conductive.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a U.S. Continuation application which claims priority benefit under 35 U.S.C. §120 (pre-AIA) of co-pending International Patent Application No. PCT/US2014/058853, entitled “ELECTRICAL AND THERMAL INSULATION FOR A COMBUSTION SYSTEM,” filed Oct. 2, 2014 (docket number 2651-192-04); which application claims priority benefit from U.S. Provisional Patent Application No. 61/885,809, entitled “ELECTRICAL AND THERMAL INSULATION FOR A COMBUSTION SYSTEM,” filed Oct. 2, 2013 (docket number 2651-192-02), co-pending at the date of filing; each of which, to the extent not inconsistent with the disclosure herein, is incorporated herein by reference. 
     
    
     SUMMARY 
       [0002]    One embodiment is a combustor wall that includes a conductive wall defining an exterior surface, a thermal insulator defining an interior surface configured to lie adjacent to a combustion volume configured to be heated to an elevated temperature and to carry charged particles, and an electrical insulator disposed between the conductive wall and the thermal insulator. The thermal insulator is configured to thermally insulate the electrical insulator from the combustion volume. The electrical insulator is configured to electrically insulate the conductive wall from the thermal insulator and the combustion volume. 
         [0003]    In one embodiment, an electrically floating conductive foil is positioned between two layers of the thermal insulator material so as to redistribute any charge that finds its way past the first thermally insulating layer. 
         [0004]    According to an embodiment, a combustor includes a furnace wall defining a combustion chamber configured to enclose a combustion reaction, a power supply configured to output a high voltage, and a charger operatively coupled to the power supply and to the combustion chamber. The charger is configured to receive the high voltage from the power supply and to cause the combustion reaction to carry a majority charge. The furnace wall includes a conductive wall, a thermal insulator adjacent to the combustion chamber, and an electrical insulator disposed between the thermal insulator and the conductive wall. The thermal insulator is configured to thermally insulate the electrical insulator from the combustion volume. The electrical insulator is configured to electrically insulate the conductive wall from the thermal insulator and the combustion volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a sectional diagram of a combustor wall, according to an embodiment. 
           [0006]      FIG. 2  is a sectional diagram of a combustor wall, according to another embodiment. 
           [0007]      FIG. 3  is a diagram of a combustor, according to an embodiment. 
           [0008]      FIG. 4  is a diagram of a combustor, according to another embodiment. 
           [0009]      FIG. 5  is a flow diagram of a process for operating a combustor, according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    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. 
         [0011]      FIG. 1  is a sectional diagram of a combustor wall  100 , according to an embodiment. The combustor wall  100  includes a conductive wall  102  defining an exterior surface  104  configured to lie adjacent to an outer volume  106  and configured to provide tensile strength to the combustor wall  100 . A thermal insulator  108  defining an interior surface  110  is configured to lie adjacent to a combustion volume  112 . The combustion volume  112  is configured to be heated to an elevated temperature and to carry charged particles  114 . An electrical insulator  116  is disposed between the conductive wall  102  and the thermal insulator  108 . The thermal insulator  108  is configured to thermally insulate the electrical insulator  116  from the combustion volume  112 . The electrical insulator  116  is configured to electrically insulate the conductive wall  102  from the thermal insulator  108  and the combustion volume  112 . 
         [0012]    The outer volume  106  adjacent to the exterior surface  104  of the conductive wall  102  can be atmospheric and/or a water jacket, for example. The conductive wall  102  can be steel or iron and can be electrically grounded. The electrical insulator  116  is contemplated to include several alternative materials. For example, in one embodiment, the electrical insulator  116  was steatite, also referred to as soapstone. Steatite has a relatively low electrical conductivity that is persistent to relatively high temperatures. Low electrical conductivity at high temperatures can be leveraged to reduce the thickness of the thermal insulating layer  108 . Thermal insulating properties of the electrical insulator  116  can similarly be leveraged to reduce the thickness of the thermal insulator  108 . In other embodiments, other electrical insulator materials and structures may be used. For example, some electrically insulating materials may be selected for a relatively high dielectric constant (at least at a modulation frequency of the charged particles  114 ), a melting point or glass transition temperature high enough to avoid degradation, and/or a coefficient of thermal expansion that is relatively well-matched to that of the material in the wall  102  and/or the thermal insulator  108 . For example, the electrical insulator  116  may include one or more of polyether-ether-ketone, polyimide, silicon dioxide, silica glass, alumina, silicon, titanium dioxide, strontium titanate, barium strontium titanate, or barium titanate. More electrically conductive (poorer electrically insulating) material options (such as polyimide, polyether-ether-ketone, silicon dioxide, silica glass, or silicon) may be most appropriate for the electrical insulator  116  for embodiments using lower voltages, greater electrical insulator  116  thicknesses, and/or greater thermal insulator  108  thicknesses. 
         [0013]    The thermal insulator  108  can be a ceramic fiber, a refractory fiber, and/or a refractory ceramic fiber. For example, the thermal insulator  108  can be a vitreous aluminosilicate fiber. For thermal insulator  108  materials that include binder materials that are relatively lower melt point or higher thermal conductivity, the thermal insulator  108  can be heat treated to remove (“burn off”) the binders. The thermal insulator  108  can additionally or alternatively include cordierite (magnesium iron aluminum cyclosilicate), Mullite (a silicate mineral including Al 2 O 3  and SiO 2 , as 2Al 2 O 3 SiO 2  or 3Al 2 O 3 2SiO 2 .), alumina, and/or an aerogel. The thermal insulator  108  can be formed as a honeycomb material or having another structure including air gap thermally insulating features. 
         [0014]      FIG. 2  is a sectional diagram of a combustor wall  200 , according to another embodiment. The thermal insulator  108  can include quiescent air channels  202 . The thermal insulator  108  can be selected to be electrically conductive at elevated temperatures. At least a portion of the thermal insulator  108  can be configured to act as an electrode at elevated temperatures. 
         [0015]      FIG. 3  is a diagram of a combustor  300 , according to an embodiment. The combustor  300  includes a furnace wall  302  defining the combustion volume  112  configured to enclose a combustion reaction  304 . A power supply  306  is configured to output a high voltage. A charger  308  is operatively coupled to the power supply  306  and to the combustion volume  112  and configured to receive the high voltage from the power supply  306  and to cause the combustion reaction  304  to carry a majority charge. The furnace wall  302  includes the conductive wall  102  adjacent to an outside volume  106 , the thermal insulator  108  adjacent to the combustion volume  112 , and the electrical insulator  116  disposed between the thermal insulator  108  and the conductive wall  102 . The conductive wall  102  can define a water jacket. The conductive wall  102  can include steel and/or iron. 
         [0016]    As described above, the electrical insulator  116  can include steatite or another material having suitable properties. The electrical insulator  116  can be configured as a plurality of continuous planes respectively held by gravity adjacent to the conductive wall  102 . Additionally or alternatively, the electrical insulator  116  can be configured as a plurality of tiles. Air gaps between adjacent tiles may provide electrical insulation and reduce the need for close fitting of the tiles. For example, the tiles may be separated from one another by up to 0.25 inch in some installations. In other installations, the tiles are installed within 0.125 inch of one another. In some installations, the tiles are installed within 0.0625 of one another. 
         [0017]    In some embodiments, the electrical insulator  116  can include two or more layers of insulating tiles (e.g., soapstone tiles). Tiles in respective layers can be offset from one another to minimize or eliminate any single gap penetrating the entire thickness of the electrical insulator  116  (e.g., a two-layer field of electrically insulating tiles can include tiles centered on every three- or four-corner abutting location on an underlying layer of electrically insulating tiles. 
         [0018]    The electrical insulator  116  can be adhered to the conductive wall  102  and/or to adjoining electrical insulator layers by adhesive. In an embodiment, the electrical insulator  116  can be affixed to the conductive wall  102 , other layers of the electrical insulator  116 , and/or to the thermal insulator  108  by a cementitious material that acts an adhesive. In another embodiment, the electrical insulator  116  can be affixed to the conductive wall  102 , other layers of the electrical insulator  116 , and/or to the thermal insulator  108  by an adhesive material. In another embodiment, the electrical insulator  116  can be affixed to the conductive wall  102 , other layers of the electrical insulator  116 , and/or to the thermal insulator  108  by nonconductive hardware. For example, alumina screws or posts (including tensile reinforced alumina screws or posts) can mechanically adjoin the electrical insulator  116  to the conductive wall  102 , other layers of the electrical insulator  116 , and/or to the thermal insulator  108 . 
         [0019]    The thermal insulator  108  can include a ceramic fiber, a refractory fiber, and/or a refractory ceramic fiber, according to embodiments. The thermal insulator  108  can be held adjacent to the electrical insulator  116  by gravity. Additionally or alternatively, the thermal insulator  108  can be adhered to the electrical insulator  116  by an adhesive and/or by substantially non-conducting fasteners. In an embodiment, the thermal insulator  108  can include a vitreous aluminosilicate fiber. The thermal insulator  108  can be heat treated to remove binders. The thermal insulator  108  can additionally or alternatively include cordierite, Mullite, alumina, and/or an aerogel. The thermal insulator  108  can be formed as a honeycomb or porous material. 
         [0020]    The thermal insulator  108  can be configured, under steady-state conditions, to thermally insulate the electrical insulator  116  sufficiently to maintain at least a 700° F. difference between the combustion volume  112  and the electrical insulator  116 . In some embodiments, the thermal insulator  108  may be configured to maintain a 1700° F. difference (steady-state) between the combustion volume  112  and the electrical insulator  116 . 
         [0021]    In one embodiment, the electrical insulator is configured to inhibit leakage current between the charger  308  and outer wall  102  at elevated temperatures. For example, the electrical insulator is configured to allow a maximum voltage drop across the electrical insulator  116  corresponding to 5% of the voltage between the outer wall  102  and the charger  308 . Thus, if 40 kV are applied between the charger  308  and the outer wall  102 , then a voltage drop of 2 kV is permitted across the electrical insulator  116 . 
         [0022]    In an embodiment, the electrical insulator  116  maintains at least 10 megaohms resistance between the combustion volume  112  and the furnace wall  302 . In another embodiment, the electrical insulator  116  can maintain at least 100 megaohms of resistance to a grounded furnace wall  102 . The conductive wall  102  can be held at an electrical ground such as earth ground. 
         [0023]    The power supply  306  can be configured to output a high voltage greater than 1000V magnitude. In another embodiment, the power supply  306  can be configured to output a high voltage equal to or greater than 15 kV in magnitude. The power supply  306  can be configured to output a DC voltage and/or output an AC voltage. 
         [0024]    The combustor  300  can be a solid fuel  310  burner. The charger  308  can be configured to output an AC high voltage to the combustion reaction  304 . The combustor  300  can include a conductive grate  312  configured to act as a counter electrode to the charger  308 . The conductive grate  312  can be galvanically isolated. 
         [0025]      FIG. 4  is a diagram of a combustor  400 , according to another embodiment. The combustor  300  can be a gas burner  400 . The combustor  400  can include an electrically grounded fuel nozzle  402 . The combustor  400  can include a second power supply voltage lead  404  configured to carry a voltage. A region  406  of the thermal insulator  108  can be operatively coupled to the second power supply voltage lead  404 . The thermal insulator  108  can become electrically conductive at elevated temperatures responsive to heating by the combustion reaction  304 . At least the region  406  of the thermal insulator  108  can be configured to operate as an electrode upon being heated by the combustion reaction  304 . In the combustor  400 , the second power supply voltage lead  404  can pass through an aperture  408  defined by the conductive wall  102 . 
         [0026]      FIG. 5  is a flow diagram of a process  500  for operating a combustor, according to one embodiment. At  502  a combustion reaction is initiated in a combustion chamber of a combustor. A wall of the combustion chamber includes an outer conductive layer, an inner thermal insulation layer, and an intermediate electrical insulation layer positioned between the outer conductive layer and the thermal insulation layer. The conductive outer layer is coupled to a power supply. A charger electrode may be positioned within the combustion chamber and is coupled to the power supply. Alternatively, a charged particle emission electrode may be positioned within the combustion chamber or may be positioned within a gas stream that enters the combustion chamber, such as a combustion air stream, a flue gas recirculation stream, or a gaseous fuel stream. Additionally or alternatively, the combustion chamber may be at least partly tribo-electrically charged, such as by contact-generated charges carried into the combustion chamber by fuel particles. 
         [0027]    As the combustion reaction proceeds, the temperature of the gas within the combustion chamber rises until it reaches a steady-state temperature. The thermal insulation layer thermally insulates the electrical insulation layer and the conductive outer wall such that the temperature of the electrical insulation layer is significantly lower than the temperature within the combustion chamber. 
         [0028]    At  504  a ground voltage is applied to the conductive outer layer of the wall of the combustion chamber. Alternatively, a voltage other than ground may be applied to the conductive outer layer of the wall of the combustion chamber. Typically, the conductive outer layer may be held at ground by virtue of its continuity with an external environment and/or via a grounded conductor. 
         [0029]    At  506  a high voltage is applied to the charger within the combustion chamber. When the high voltage is applied to the charger in the combustion chamber, gases undergoing the combustion reaction are ionized such that a flame within the combustion chamber carries a majority charge. In this way, the characteristics of the combustion reaction within the combustion chamber can be controlled to have selected characteristics. In one embodiment, the high-voltage applied to the charger is between 1000V and 15,000V. Alternatively, the high-voltage can be higher than 15,000V. The voltage applied to the charger can be an AC voltage, a DC voltage, or any suitable waveform to obtain selected characteristics of the combustion reaction within the combustion chamber. 
         [0030]    According to one embodiment, at high temperatures the thermal insulation layer becomes electrically conductive. A portion of the thermal insulation layer can be used as an electrode for further controlling characteristics of the combustion reaction within the combustion chamber. Thus, in one embodiment the process  500  comprises applying ground voltage or a third voltage to the portion of the thermal insulation layer used as an electrode. The voltage can be applied to thermal insulation layer by passing a conductor through an aperture in the conductive outer layer and the electrical insulation layer to the thermal insulation layer. 
         [0031]    In one embodiment, the process  500  includes cooling the outer conductive layer of the wall of the combustion chamber by passing water along the outside of the conductive layer of the combustion chamber wall. In this case, a water jacket configuration contains the water as it is passed along the outer conductive layer of the wall of the combustion chamber. The water jacket is thermally coupled to the outer conductive layer of the combustion chamber wall such that the water cools the outer conductive layer of the combustion chamber wall. Similarly, the water jacket may act as at least a portion of a thermal load that is heated by the combustion reaction. 
         [0032]    In one embodiment, a conductor is positioned adjacent a bottom portion of the combustion chamber. Therefore, in one embodiment the process  500  includes applying a voltage to the conductor near the bottom of the combustion chamber. In this case, the conductor near the bottom of the combustion chamber acts as a counter electrode to the charger. The conductor at the bottom of the combustion chamber can be connected to ground or can carry any other suitable voltage to influence the combustion reaction. Alternatively, the conductor at the bottom of the combustion chamber can be connected to a voltage through an electrically resistive material. 
         [0033]    In one embodiment the combustor burns liquid or gaseous fuel. In this case the conductor near the bottom of the electrode can be a conductive fuel nozzle from which fuel is output into the combustion chamber. Alternatively, if the combustor burns a solid fuel, the conductor near the bottom of the combustion chamber can be a conductive grid or mesh on which the solid fuel is disposed during combustion and/or during preheating awaiting combustion. 
         [0034]    While the steps of the process  500  have been described as occurring in a particular order, the steps of the process  500  can be performed in different orders then shown in  FIG. 5  and described in the foregoing description. For example, the voltages can be applied to the conductive outer layer and the charger prior to initiation of the combustion reaction, or simultaneous with the initiation of the combustion reaction. Those skilled in the art will understand, in light of the present disclosure, that many other process steps and orders of performing the process steps are possible in accordance with principles of the present disclosure. All such other orders and process steps fall within the scope of the present disclosure. 
         [0035]    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.