Patent Publication Number: US-2022235931-A1

Title: Fuel-Flexible Combustor

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 63/139,587, filed 20 Jan. 2021, the entire content of which is incorporated herein by reference. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with government support under contract number W911QX-17-P-0157 awarded by the United States Army. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     The ability to efficiently generate thermal energy via combustion of hydrocarbon fuels and to transfer that thermal energy in a controlled manner across specific interfaces facilitates the development of advanced power generation and heating systems. Power-generation technologies, such as thermoelectrics, thermophotovoltaics, thermionics, and fuel cells, all benefit from precise delivery of thermal and radiative energy to defined surface geometries at specific surface temperatures and heat fluxes. 
     For instance, thermoelectric modules, which rely upon the conduction of thermal energy from the hot side of the module to the cold side of the module to generate electricity via the Seebeck effect, typically have an optimum hot-side operating temperature, below which the power output and efficiency of the thermoelectric module are reduced and above which the module is damaged. Combustion devices employed to provide thermal energy for thermoelectric-module operation deliver the thermal energy to the module at the optimum temperature, with as uniform a surface temperature as possible, and through a surface that matches the geometry of the hot side of the thermoelectric module to ensure good thermal contact between the hot combustor surface and the hot side of the thermoelectric module. 
     Similarly, thermophotovoltaic systems, which rely upon the irradiation of semiconductor cells with infrared energy to generate electricity via electron/hole pair separation, typically have an optimum irradiation flux, below which the power output and efficiency of the semiconductor cells are reduced and above which the cells are damaged. Combustion devices employed to provide the radiative energy for thermophotovoltaic cell operation deliver the radiative energy to the cells at the optimum flux, with as uniform a surface flux as possible, and in a geometry that maximizes the fraction of radiation emitted from the combustor surface that is intercepted by the thermophotovoltaic cells. 
     Existing systems and apparatuses for converting hydrocarbon fuels to thermal energy suitable for use in power generation systems are typically limited to narrow operating ranges with regards to parameters, such as equivalence ratio, fuel throughput, temperature, pressure, type of fuel, etc. Efficient combustion of liquid fuels (in particular, high-flash-point hydrocarbon fuels) is particularly challenging, as vaporization and thorough mixing of the fuel vapor with oxidant is generally required to ensure complete fuel combustion. Consequently, there is a need for compact and efficient combustors that convert liquid hydrocarbon fuels into thermal energy over a wide range of operating conditions that can be delivered to thermal-to-electric converters in a highly controlled manner. 
     SUMMARY 
     The following description relates to a method and apparatus for liquid-hydrocarbon combustion, and more particularly, to an integration of several components for converting liquid-hydrocarbon fuels into thermal energy that can be efficiently directed to thermal-to-electric conversion devices. 
     The liquid-hydrocarbon fuel is vaporized into air by passing the fuel and the air through a vaporizer. After vaporization, the mixed hydrocarbon-fuel vapor and air passes into a catalytic combustor that converts the hydrocarbon-fuel vapor and air into a carbon-dioxide-and-water-vapor-containing exhaust while simultaneously generating thermal energy and transferring that energy through the walls of the combustor. The hot exhaust passes into a recuperator where thermal energy from the exhaust is transferred to an air stream to produce a heated air stream that can be fed to the vaporizer. 
     Vaporization of the liquid-hydrocarbon fuel is conducted within a vaporizer that contains a porous structure to aid in vaporization. The porous structure comprises a random or ordered three-dimensional array of interconnected pores distributed within an array of interconnected solids, such as a reticulated foam, packed fibers, and the like. When the liquid hydrocarbon is brought into contact with the porous structure, the liquid hydrocarbon is wicked into the porous structure via capillary action, which is dependent upon the surface tension of the porous structure, the surface tension of the hydrocarbon, the pore dimensions of the porous structure, and the overall porosity of the porous structure. The liquid hydrocarbon within the porous structure evaporates into air that simultaneously flows through the porous structure, with the porous structure providing a large specific surface area from which mass transfer of the hydrocarbon from the liquid phase to the vapor phase takes place. Following evaporation, the hydrocarbon vapor and air are progressively mixed as the stream passes through the tortuous features of the porous structure. 
     Combustion of the hydrocarbon vapor and air mixture is initiated via contact of the mixture with a catalyst located within a combustor enclosure. The catalyst promotes heterogeneous oxidation of the hydrocarbon, resulting in the consumption of hydrocarbon and oxygen and the production of thermal energy along with primarily carbon dioxide and water vapor, which are exhausted from the combustor. The catalyst can be coated on selected surface areas of the combustor enclosure but not on others in order to selectively generate thermal energy at those surface areas and to promote the transfer of that thermal energy out of the combustor enclosure via those surface areas. 
     A recuperator is used to transfer a portion of the thermal energy retained within the hot combustor exhaust back into the air stream that is fed to the vaporizer. Efficient transfer of thermal energy within the recuperator is accomplished by passing the hot exhaust stream across heat-exchange surfaces, the opposite sides of which are exposed to the flowing air stream. Routing of exhaust-stream and air-stream flows within the recuperator may be accomplished via a variety of heat-exchanger geometries, including shell and tube, parallel plate, and plate fin, among others. 
     Thermal-energy recuperation and hydrocarbon vaporization may also be conducted simultaneously by locating the vaporizer within the recuperator to form a single vaporizer/recuperator device in which thermal energy is transferred from the hot exhaust to the cooler hydrocarbon and air streams as the liquid hydrocarbon is evaporated and mixed into the air. In this instance, in addition to providing evaporation and mixing functionality, the porous structure within the vaporizer/recuperator also enhances the transfer of thermal energy into the hydrocarbon-air mixture by increasing the thermal conductivity on the vaporization side of the vaporizer/recuperator and by promoting hydrocarbon-air stream turbulence as the stream flows through the tortuous pathways within the porous structure. 
     The apparatus and methods described herein offer many advantages over existing approaches. The fuel-flexible combustor apparatus and methods can operate on a variety of liquid hydrocarbon fuels, such as heating oil, diesel, jet fuel, gasoline, kerosene, naphtha, alcohols, and the like, over a wide range of fuel throughputs. The vaporizer readily evaporates the liquid-hydrocarbon fuel into the air stream at low temperatures, minimizing the risk of coke and residue formation, and simultaneously mixes the evaporated-hydrocarbon vapor into the air stream, reducing the likelihood of coke-forming, fuel-rich pockets within the combustor. The use of a heterogeneous or homogeneous/heterogeneous combustion process enables operation over a much wider range of equivalence ratios than a homogeneous combustion process. The use of a homogeneous/heterogeneous combustion process provides dual heat-generation mechanisms that can be used to control where thermal energy is generated within the combustor and where it is transferred out of the combustor. Configuring the vaporizer within the recuperator enables effective recovery of waste heat from the combustor exhaust while simultaneously preparing the hydrocarbon-fuel-air mixture for efficient combustion within the combustor. The combustor apparatus can also be light-weight. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, described below, like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular principles of the methods and apparatus characterized in the Detailed Description. 
         FIG. 1  is a conceptual drawing showing the relationship between selected elements of an embodiment of the apparatus 
         FIG. 2  is another conceptual drawing showing the relationship between selected elements of an embodiment of the apparatus. 
         FIG. 3  is a cross-sectional side view of an embodiment of the apparatus. 
         FIG. 4  is a cross-sectional side view of another cylindrical embodiment of the apparatus. 
         FIG. 5  is a cross-sectional side view of another planar embodiment of the apparatus. 
         FIG. 6  is a magnified photographic image of an exemplification of the porous structure. 
     
    
    
     DETAILED DESCRIPTION 
     The foregoing and other features and advantages of various aspects of the invention(s) will be apparent from the following, more-particular description of various concepts and specific embodiments within the broader bounds of the invention(s). Various aspects of the subject matter introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the subject matter is not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     Unless otherwise herein defined, used or characterized, terms that are used herein (including technical and scientific terms) are to be interpreted as having a meaning that is consistent with their accepted meaning in the context of the relevant art and are not to be interpreted in an idealized or overly formal sense unless expressly so defined herein. For example, if a particular composition is referenced, the composition may be substantially (though not perfectly) pure, as practical and imperfect realities may apply; e.g., the potential presence of at least trace impurities (e.g., at less than 1 or 2%) can be understood as being within the scope of the description. Likewise, if a particular shape is referenced, the shape is intended to include imperfect variations from ideal shapes, e.g., due to manufacturing tolerances. Percentages or concentrations expressed herein can be in terms of weight or volume. Processes, procedures and phenomena described below can occur at ambient pressure (e.g., about 50-120 kPa—for example, about 90-110 kPa) and temperature (e.g., −20 to 50° C.—for example, about 10-35° C.) unless otherwise specified. 
     Although the terms, first, second, third, etc., may be used herein to describe various elements, these elements are not to be limited by these terms. These terms are simply used to distinguish one element from another. Thus, a first element, discussed below, could be termed a second element without departing from the teachings of the exemplary embodiments. 
     Spatially relative terms, such as “above,” “below,” “left,” “right,” “in front,” “behind,” and the like, may be used herein for ease of description to describe the relationship of one element to another element, as illustrated in the figures. It will be understood that the spatially relative terms, as well as the illustrated configurations, are intended to encompass different orientations of the apparatus in use or operation in addition to the orientations described herein and depicted in the figures. For example, if the apparatus in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term, “above,” may encompass both an orientation of above and below. The apparatus may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     Further still, in this disclosure, when an element is referred to as being “on,” “connected to,” “coupled to,” “in contact with,” etc., another element, it may be directly on, connected to, coupled to, or in contact with the other element or intervening elements may be present unless otherwise specified. 
     The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of exemplary embodiments. As used herein, singular forms, such as “a” and “an,” are intended to include the plural forms as well, unless the context indicates otherwise. Additionally, the terms, “includes,” “including,” “comprises” and “comprising,” specify the presence of the stated elements or steps but do not preclude the presence or addition of one or more other elements or steps. 
     “Air” typically refers to a gas comprising approximately 78% nitrogen, approximately 21% oxygen, and approximately 1% argon, and other components in lesser amounts. As used herein, “air” may more generally refer to any gas in which nitrogen and oxygen are the two most prevalent components. 
     Now, referring to  FIGS. 1-6 , features and details of the fuel-flexible combustor apparatus and method are described. The same numeral present in different figures represents the same item. Particular embodiments are detailed, below, for the purpose of illustration and not as limitations of the invention. 
     Selected components of the apparatus are illustrated and labeled in  FIGS. 1 and 2 . These FIGURES offer a simplified view of the more-detailed illustration of the cylindrical exemplification presented in  FIG. 3  and are provided for ease of illustrating broader aspects of the apparatus and method. The apparatus in  FIG. 1  includes the principal components through which the liquid-hydrocarbon fuel  12  (and, later, its oxidation products) is passed. Those components, through which the liquid-hydrocarbon fuel  12  (and, later, its oxidation products) passes in sequence, are the vaporizer  100 , combustor  102  and recuperator  101 . The vaporizer  100  and recuperator  101  are combined in  FIG. 2  because the structure of the recuperator  101  is intertwined with the vaporizer  100  in various embodiments of the apparatus, as shown in the other FIGURES. In the various embodiments, the recuperator  101  includes the structures that transfer heat from the combustor  102  to the vaporizer  100 . 
     In the illustrated embodiment shown in  FIG. 3 , the hydrocarbon fuel  12  enters the vaporizer  100  through the fuel inlet tube  104  and wicks into the porous structure  106 . Air  14  enters the vaporizer  100  through the air inlet tube  108  and flows through the porous structure  106 . The recuperator  101  includes the concentric annular channel  122  formed by the cylindrical shell  123  encircling the inner channel that contains the porous structure  106  and through which the fuel  12  and air  14  flow. Exhaust  16  from the combustor  102  flows through the annular channel  122 , allowing heat from the exhaust  16  to flow through the wall separating the annular channel from the inner channel, into the air  14  and fuel  12  mixture, and into the porous structure  106  in the vaporizer  100 . 
     The hydrocarbon fuel  12  and air  14  flowing through the porous structure  106  form a well-mixed hydrocarbon-vapor/air stream that flows to the combustor  102  via the hydrocarbon/air transfer line  110 . The hydrocarbon-vapor/air stream flows around the heat shield  112  and into the annular combustion zone  114  defined by the walls of the inner liner  116  and the combustor  102 . The walls of the inner liner  116  and combustor  102  typically comprise a high-temperature metallic alloy, such as stainless steel, an austenitic nickel-chromium-based alloy (available as INCONEL alloy from Special Metals Corporation), a nickel-molybdenum alloy (available as HASTELLOY alloy from Haynes International), an iron-chromium alloy (available as FECRALLOY alloy from Goodfellow), etc. A combustion catalyst  118  is present on the interior surface of the combustor  102  within the annular combustion zone  114 . Exemplary combustion catalysts  118  that may be used include noble-metal (such as Pt, Pd, Rh, etc.) or transition-metal-oxide (such as Mn 2 O 3 , Fe 2 O 3 , NiO, Co 3 O 4 , etc.) species supported on high-surface-area inorganic supports (such as Al 2 O 3 , SiO 2 , etc.). 
     The combustion catalyst  118  can be coated on selected surface areas of the combustor enclosure  103  but not on others in order to selectively generate thermal energy at those surface areas and to promote the transfer of that thermal energy out of the combustor enclosure  103  via those surface areas. Because the thermal energy is simultaneously generated and transferred out of the combustor at these surface areas, the surface temperature is much lower than the adiabatic flame temperature of the hydrocarbon-air mixture. The surface temperature can be readily controlled by manipulating the catalytic oxidation rate (for instance, by manipulating the hydrocarbon-air equivalence ratio, the combustor pressure, the rate of hydrocarbon and air mass transfer to the combustor-enclosure surface, the catalyst activity, the catalyst loading on the combustor enclosure surface, etc.) and manipulating the surface heat-transfer rate (for instance, by manipulating the composition of the combustor-enclosure wall, the thickness of the combustor-enclosure wall, the emissivity of the combustor-enclosure surface, the properties of the fluid external to the combustor-enclosure wall, etc.). A high catalytic-oxidation rate and a low heat-transfer rate will produce a higher surface temperature, while a low catalytic-oxidation rate and a high heat-transfer rate will produce a lower surface temperature. 
     Beyond the selective application of the catalyst  118  on selected surface areas of the combustor enclosure  103 , as described above, varying the degree of catalyst loading (i.e., the mass of catalyst per unit area of surface) across the surface areas is a particularly useful tool for more-precisely controlling the magnitude and spatial distribution of the surface temperature, as the loading of the catalyst  118  can be increased (i.e., from left-to-right in the configuration and orientation of  FIG. 3 ) as the hydrocarbon is depleted from the hydrocarbon-air mixture in order to maintain a constant specific hydrocarbon oxidation rate (i.e., moles of hydrocarbon oxidized per unit time per unit area of surface) along the catalyst-coated surface. 
     In addition to catalytic hydrocarbon oxidation at the catalyst-coated combustor surface areas, at higher temperatures, homogeneous (non-catalytic, flame) hydrocarbon combustion may also occur within the internal volume of the combustor  102 . Thus, at these higher temperatures, the generation of thermal energy proceeds via mixed homogeneous-heterogeneous oxidation processes. In this case, the location of thermal-energy generation within the combustor is governed by both the location of the catalytic coating on the combustor surfaces and the location at which the homogeneous combustion flame is stabilized within the combustor volume. The location at which the homogeneous combustion flame is stabilized within the combustor enclosure  103  may be controlled by manipulating the geometry of the combustor  102 , the location of flame stabilizing elements within the combustor  102 , and the location of the catalyst  118 , among other features. 
     The combustion catalyst  118  can also be coated on selected surface areas of the inner liner  116  but not on others in order to generate additional thermal energy and thereby increase the combustor temperature. 
     In the annular combustion zone  114 , the hydrocarbon vapor is oxidized to generate thermal energy that is transferred out of the combustor along with a hot combustion gas that exits the annular combustion zone  114  through an outlet  120 . Exemplary reactions that may occur within the annular combustion zone  114  include C n H m +(m/4+n) O 2 →n CO 2 +(m/2) H 2 O, C n H m +(m/4+n/2) O 2 →n CO+(m/2) H 2 O, and CO+(½) O 2 →CO 2 . 
     The hot combustion gas enters the annular channel  122  that forms the recuperator  101  where the hot combustion gas provides thermal energy to the hydrocarbon vapor-air stream within the porous structure  106 . The combustion gas exhaust  16  exits through the exhaust tube  124 . Thermal insulation  126  is positioned to minimize thermal-energy transfer to the environment from surfaces other than those surfaces adjacent to the annular combustion zone  114 , while the heat shield  112  is positioned to minimize thermal energy transfer from the annular combustion zone  114  to other upstream components within the combustor  102  and vaporizer  100 . 
     A heater  128  is positioned within the volume confined by the inner liner  116  to provide thermal energy during combustor startup. The heater  128  is typically an electrically-resistive heating element. Thermal energy is transferred from the heater  128  to the walls of the inner liner  116  and to the combustion catalyst  118  to heat the combustion catalyst  118  to a temperature at which heterogeneous oxidation of the hydrocarbon vapor may proceed. Simultaneously, thermal energy is transferred from the heater  128  to the porous structure  106  via air flowing from the outlet  120  to the exhaust tube  124  to assist with hydrocarbon fuel vaporization. 
     Another cylindrical embodiment of the apparatus is shown in  FIG. 4 . In this embodiment, the vaporizer  100  is located adjacent to the combustor  102  in order to increase thermal integration and reduce the size and mass of the cylindrical fuel-flexible combustor. In this exemplification, the annular channel  122  serves as the recuperator  101 , facilitating the transfer of heat from the exhaust  16  to the fuel/air mixture  12 / 14  before the fuel/air mixture  12 / 14  flows into the combustor  102 . Heat shield  112  is positioned to minimize thermal-energy transfer from the annular combustion zone  114  to the hydrocarbon/air transfer line  110  and modulate thermal-energy transfer to the vaporizer  100 . 
     A planar embodiment of the apparatus is shown in  FIG. 5 . In this embodiment, the recuperator  101  is located adjacent to the combustor  102  in order to provide a planar fuel-flexible combustor in which the divider  132  separates the recuperator  101  from the combustor  102 . The recuperator  101  includes, in particular, the exhaust chamber  134 , the lower wall/boundary of which is adjacent the porous structure  106 , facilitating the transfer of heat from exhaust  16  flowing through the exhaust chamber  134  (before its discharge) to the porous structure  106  and to the mixture of fuel  12  and air  14  flowing therethrough. In the illustrated embodiment, the hydrocarbon fuel  12  enters the vaporizer  100  through the fuel inlet tube  104  and wicks into the porous structure  106 . Air  14  enters the vaporizer  100  through the air inlet tube  108  and flows through the porous structure  106 . The hydrocarbon fuel  12  and air  14  flow through the porous structure  106  to form a well-mixed hydrocarbon-vapor/air stream that flows to the combustor  102  via the hydrocarbon/air transfer line  110 . The hydrocarbon-vapor/air stream flows into the planar combustion zone  130  defined by the walls of the divider  132  and the combustor  102 . The walls of the divider  132  and combustor  102  are typically composed of a high-temperature metallic alloy, such as stainless steel, an austenitic nickel-chromium-based alloy (available as INCONEL alloy from Special Metals Corporation), a nickel-molybdenum alloy (available as HASTELLOY alloy from Haynes International), an iron-chromium alloy (available as FECRALLOY alloy from Goodfellow), etc. A combustion catalyst  118  is present on the interior surface of the combustor  102  within the planar combustion zone  130 . Exemplary combustion catalysts  118  that may be used include noble-metal or transition-metal-oxide species supported on high-surface-area inorganic supports. The combustion catalyst  118  can also be coated on selected surface areas of the divider  132  but not on others in order to generate additional thermal energy within the planar combustion zone and thereby increase the combustor temperature. 
     In the planar combustion zone  130 , the hydrocarbon vapor is oxidized to generate thermal energy that is transferred out of the combustor  102  along with a hot combustion gas that exits the planar combustion zone  130  through an outlet  120 . Exemplary reactions that may occur within the planar combustion zone  130  include C n H m +(m/4+n) O 2 →n CO 2 +(m/2) H 2 O, C n H m +(m/4+n/2 O 2 )→n CO+(m/2) H 2 O, and CO+(½) O 2 →CO 2 . 
     The hot combustion gas enters the planar exhaust chamber  134  of the recuperator  101  where it transfers thermal energy to the hydrocarbon vapor-air stream within the porous structure  106  of the vaporizer  100 . The combustion gas exhaust  16  exits through the exhaust tube  124 . Thermal insulation  126  is positioned to minimize thermal-energy transfer to the environment from surfaces other than those surfaces adjacent to the planar combustion zone  130 . 
     A heater  128  is positioned adjacent to the divider  132  to provide thermal energy during combustor startup. Thermal energy is transferred from the heater  128  to the divider  132  and to the combustion catalyst  118  to heat the combustion catalyst  118  to a temperature at which heterogeneous oxidation of the hydrocarbon vapor may proceed. Simultaneously, thermal energy is transferred from the heater  128  to the porous structure  106  via air flowing from the outlet  120  to the exhaust tube  124  to assist with hydrocarbon-fuel vaporization. 
     An exemplary method for operating the fuel-flexible combustor includes feeding a liquid hydrocarbon fuel  12  into the fuel inlet tube  104  and air  14  into the air inlet tube  108 . The liquid hydrocarbon fuel  12  and air  14  are fed to the fuel-flexible combustor at an equivalence ratio between about 0.3 and 1.0. The liquid hydrocarbon fuel in this and other exemplifications can comprise any of the following: heating oil, diesel, jet fuel, gasoline, kerosene, naphtha, alcohols, and the like. 
     The porous structure  106  distributes the liquid hydrocarbon fuel and assists with evaporation of the liquid hydrocarbon and mixing of the hydrocarbon vapor with the air. The porous structure  106  comprises a metal (stainless steel, aluminum, copper, etc.) or ceramic (alumina, zirconium oxide, silicon carbide, etc.) reticulated foam or fibrous network that operates at a pressure between about 0 kPa and 200 kPa and a temperature between about 0° C. and 600° C. The reticulated foam or fibrous network comprises two scales of porosity, a finer scale, comprising pores with average dimensions less than about 0.3 mm—e.g., less than about 0.1 mm, and, in more-particular embodiments, less than about 0.05 mm that retain liquid hydrocarbons under the influence of capillary pressure and that promote the spread of the liquid hydrocarbons out across the surface of the porous structure  106 , and a coarser scale, comprising interconnected pores with average dimensions greater than about 0.5 mm—e.g., greater than about 1 mm, and in more-particular embodiments, greater than about 2 mm that provide a pathway for the flow of air and hydrocarbon vapor through the porous structure  106 . A magnified image of a porous structure is presented in  FIG. 6 , showing both finer-scale pores  202  and coarser-scale pores  204 . 
     A reticulated foam or fibrous network comprising of two scales of porosity may be formed by a number of methods. For example, a reticulated foam may be formed by coating a reticulated polyurethane foam with a ceramic or metal composition, followed by burning out the polyurethane substrate. The resulting ceramic or metal foam possesses coarser pores derived from the pores of the original reticulated foam and finer pores derived from voids formed upon polyurethane burnout. A fibrous network may be formed by using a knitted or woven fabric. The coarser pores arise from the spaces between the knitted or woven yarn, while the finer pores arise from voids located between the interlocked fibers of the yarn. Numerous other methods may also be used to form a reticulated foam or fibrous network comprising two scales of porosity. 
     The well-mixed hydrocarbon vapor-air stream is fed to the combustor  102  via the hydrocarbon-air transfer line  110  that operates at a pressure between about 0 kPa and 200 kPa and a temperature between about 0° C. and 600° C.—e.g., about 100 to 400° C., and, in more-particular embodiments, about 200 to 300° C. Within the combustor  102 , the well-mixed hydrocarbon-vapor/air stream contacts the combustion catalyst  118  at a pressure between about 0 kPa and 200 kPa and at a temperature above about 200° C., which is sufficient to initiate heterogeneous oxidation of the hydrocarbon vapor with oxygen contained in the air at the surface of the combustion catalyst  118  and generate hot combustion gas. Homogeneous oxidation of the hydrocarbon vapor with oxygen contained in the air may also be initiated within the combustor along surfaces with temperatures greater than about 600° C. The surfaces defining the annular combustion zone  114  or the planar combustion zone  130  operate at a temperature between about 200° C. and 1500° C. and are typically composed of a high-temperature metallic alloy, as discussed above. 
     The distance between the surfaces defining the annular combustion zone  114  or the planar combustion zone  130  is typically about 0.5 to 5 mm—e.g., about 1 to 4 mm, and, in more-particular embodiments, about 2 to 3 mm. Smaller distances will tend to promote heterogeneous oxidation processes over homogeneous oxidation processes, as the surface-area-to-volume ratio of the combustion zone  130  increases as the distance decreases. 
     The distance between the surface of the heat shield  112  and the wall of the combustor  102  in the exemplification of  FIG. 3  is typically smaller than the distance between the surfaces defining the annular combustion zone  114  in order to prevent propagation of the homogeneous oxidation process into the hydrocarbon/air transfer line  110 . The distance between the surface of the heat shield  112  and the wall of the combustor  102  is typically about 0.2 to 4 mm—e.g., about 0.5 to 3 mm, and, in more-particular embodiments, about 1 to 2 mm. 
     The hot combustion gas is fed to the recuperator/vaporizer  100  at a pressure between about 0 kPa and 200 kPa and at a temperature between about 200° C. and 1500° C. Within the recuperator/vaporizer  100 , thermal energy is transferred from the hot combustion gas to the hydrocarbon-vapor/air stream, and the combustion gas exhaust  16  exits through the exhaust tube  124  at a pressure between about 0 kPa and 200 kPa and at a temperature between about 100° C. and 800° C. 
     In describing embodiments of the invention, specific terminology is used for the sake of clarity. For purposes of description, each specific term is intended to at least include all technical and functional equivalents that operate in a similar manner to accomplish a similar purpose. 
     Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Further, where parameters for various properties are specified herein for embodiments of the invention, those parameters can be adjusted up or down by 1/100 th , 1/50 th , 1/20 th , 1/10 th , ⅕ th , ⅓ rd , ½, ⅔ rd , ¾ th , ⅘ th , 9/10 th , 19/20 th , 49/50 th , 99/100 th , etc. (or up by a factor of 1, 2, 3, 4, 5, 6, 8, 10, 20, 50, 100, etc.), or by rounded-off approximations thereof, unless otherwise specified. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various substitutions and alterations in form and details may be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention; and all embodiments of the invention need not necessarily achieve all of the advantages or possess all of the characteristics described above. Additionally, steps, elements and features discussed herein in connection with one embodiment can likewise be used in conjunction with other embodiments. The contents of all references, including reference texts, journal articles, patents, patent applications, etc., cited throughout this application are hereby incorporated by reference in their entirety. All appropriate combinations of embodiments, features, characterizations, components and methods of those references and the present disclosure may be selected for inclusion in embodiments of the invention. Still further, the components and methods identified in the Background section are integral to this disclosure and can be used in conjunction with or substituted for components and methods described elsewhere in the disclosure within the scope of the invention.