Patent Publication Number: US-2023132692-A1

Title: Catalytic heating systems comprising dual-mode liquid fuel vaporizers and methods of operating thereof

Description:
BACKGROUND 
     Many types of vehicles, especially medium-duty and heavy-duty vehicles, are shifting towards electrification including batteries and fule cells. In particular, fuel cells offer high electrochemical conversion efficiency and zero emissions. Several types of fuel cells operate at high temperatures, such as molten carbonate fule cells and solid oxide fuel cells (SOFCs). For example, the SOFC operating range can be from 600° C. to over 900° C. Heating SOFCs to their operating temperatures is typically performed with electric heaters or combustors. However, using electric heaters is expensive and inefficient. On other hand, combustors cause emissions. Catalytic heaters, described herein, provide a low emissions solution (e.g., a zero-emission solution) to heat up fuel cells up to their operating temperature. Specifically, catalytic heaters provide more control over fuel oxidation conditions. However, various operating controls in these catalytic heaters are needed to ensure efficient operations with minimal emissions. 
     SUMMARY 
     Described herein are catalytic heating systems, comprising catalytic reactors and dual-mode fuel evaporators, and methods of operating such systems. A dual-mode fuel evaporator is thermally coupled to a catalytic reactor and comprises an electric heater used for preheating the evaporator to at least a fuel-flow threshold temperature. Upon reaching this threshold, the liquid fuel, such as ethanol or methanol, is flown into the evaporator and evaporates therein, forming vaporized fuel. The vaporized fuel is mixed with oxidant, and the mixture is flown into the catalytic reactor where the vaporized fuel undergoes catalytic exothermic oxidation. At least some heat, generated in the catalytic reactor, is transferred to the evaporator and used for the evaporation of additional fuel. When the evaporator reaches or exceeds its operating threshold, the electric heater can be turned off and all heat is supplied to the evaporator from the catalytic reactor. 
     In some examples, a method of operating a catalytic heating system comprising a catalytic reactor and a dual-mode fuel evaporator, thermally coupled to the catalytic reactor, is provided. The method comprises heating the dual-mode fuel evaporator using an electrical heater of the dual-mode fuel evaporator until the temperature of the dual-mode fuel evaporator reaches or exceeds a fuel-flow threshold. When the temperature of the dual-mode fuel evaporator reaches or exceeds the fuel-flow threshold, the method proceeds with flowing a liquid fuel into the dual-mode fuel evaporator thereby causing the liquid fuel to evaporate and generate vaporized fuel. The method comprises mixing the vaporized fuel with oxidant thereby forming a vaporized fuel-oxidant mixture flowing the vaporized fuel-oxidant mixture into the catalytic reactor thereby causing catalytic exothermic oxidation of the vaporized fuel in the catalytic reactor and producing heat, at least partially transferred from the catalytic reactor to the dual-mode fuel evaporator. 
     In some examples, the method further comprises monitoring the temperature of the dual-mode fuel evaporator while the heat, produced in the catalytic reactor, at least partially transferred to the dual-mode fuel evaporator from the catalytic reactor. The method also comprises turning off the electrical heater when the temperature of the dual-mode fuel evaporator reaches or exceeds an evaporator operating threshold, higher than the fuel-flow threshold. 
     In some examples, turning off the electrical heater comprises gradually reducing the power output of the electrical heater as the temperature of the dual-mode fuel evaporator approaches the evaporator operating threshold. 
     In some examples, heating the dual-mode fuel evaporator using the electrical heater further comprises heating the catalytic reactor by heat transfer from the dual-mode fuel evaporator to the catalytic reactor. The liquid fuel is flown into the dual-mode fuel evaporator when a temperature of the catalytic reactor reaches or exceeds a fuel-receiving threshold. 
     In some examples, the method further comprises monitoring the temperature of the catalytic reactor while the heat, at least partially transferred to the dual-mode fuel evaporator from the catalytic reactor. The method also comprises turning off the electrical heater when the temperature of the catalytic reactor reaches or exceeds a reactor operating threshold. 
     In some examples, the dual-mode fuel evaporator further comprises a heater coupler, attached to and in direct contact with the catalytic reactor and comprising an evaporation surface. The dual-mode fuel evaporator comprises an evaporator chamber, extending away from the evaporation surface. The dual-mode fuel evaporator also comprises an evaporator inlet, receiving and directing the liquid fuel to the evaporation surface. In more specific examples, the electrical heater extends through the heater coupler. In the same or other examples, the electrical heater comprises multiple heating elements, evenly distributed throughout the heater coupler. The evaporator inlet can comprise a fogging nozzle. 
     In some examples, the catalytic reactor comprises an enclosure and a catalyst, positioned within the enclosure. The heater coupler at least partially surrounds the enclosure, positioned between the catalyst and the heater coupler. In more specific examples, the heater coupler fully surrounds the enclosure of the catalyst reactor. 
     In some examples, the fuel-flow threshold is at least 30° C. higher than the boiling temperature of the liquid fuel. In the same or other examples, the liquid fuel is selected from the group consisting of ethanol, methanol, and biodiesel. 
     In some examples, the method further comprises heating the oxidant, before mixing the vaporized fuel with the oxidant. 
     Also provided is a catalytic heating system comprising a catalytic reactor and a dual-mode fuel evaporator, thermally and fluidically coupled to the catalytic reactor and comprising an electric heater and a thermocouple. The catalytic heating system further comprises a liquid fuel supply, fluidically coupled to the dual-mode fuel evaporator. The catalytic heating system also comprises a system controller, communicatively coupled to the dual-mode fuel evaporator and liquid fuel supply. The system controller is configured to control the dual-mode fuel evaporator to turn on the electrical heater until the temperature of the dual-mode fuel evaporator, provided from the thermocouple, reaches or exceeds a fuel-flow threshold. The system controller is also configured to control the liquid fuel supply to flow a liquid fuel into the dual-mode fuel evaporator when the temperature of the dual-mode fuel evaporator, provided from the thermocouple, reaches or exceeds the fuel-flow threshold. 
     In some examples, the catalytic heating system further comprises an additional catalytic reactor, fluidically coupled to the catalytic reactor and positioned downstream relative to the catalytic reactor such that the catalytic heating system is a dual-stage catalytic heating system. 
     In some examples, the dual-mode fuel evaporator further comprises a heater coupler, attached to and in direct contact with the catalytic reactor and comprising an evaporation surface, wherein the electrical heater extends through the heater coupler. The dual-mode fuel evaporator also comprises an evaporator chamber, extending away from the evaporation surface, and an evaporator inlet, receiving and directing the liquid fuel to the evaporation surface. In more specific examples, the catalyst reactor comprises an enclosure and a catalyst, positioned within the enclosure. The heater coupler at least partially surrounds the enclosure, positioned between the catalyst and the heater coupler. For example, the heater coupler fully surrounds the enclosure of the catalyst reactor. In some examples, the electrical heater comprises multiple heating elements, evenly distributed throughout the heater coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a graph illustrating a fuel-vapor concentration profile and an emissions profile for a liquid fuel model. 
         FIG.  2 A  is a cross-sectional view of a catalytic heating system comprising a catalytic reactor and a dual-mode fuel evaporator, in accordance with some examples. 
         FIG.  2 B  is a block diagram of the catalytic heating system (as in  FIG.  2 A ) showing additional components, in accordance with some examples. 
         FIG.  2 C  is a perspective view of a dual-mode fuel evaporator, attached and thermally coupled to a catalytic reactor, in accordance with some examples. 
         FIG.  2 D  is a cross-sectional view of the dual-mode fuel evaporator and the catalytic reactor in  FIG.  2 C , in accordance with some examples. 
         FIG.  2 E  is a perspective view of a dual-mode fuel evaporator as a standalone component, in accordance with some examples. 
         FIG.  2 F  is a cross-sectional view of a dual-mode fuel evaporator fully enclosing a catalytic reactor, in accordance with some examples. 
         FIG.  2 G  is a block diagram illustrating a system controller interacting with various other components of the catalytic heating system, in accordance with some examples. 
         FIG.  3    is a block diagram of a vehicle comprising a catalytic heating system, in accordance with some examples. 
         FIG.  4    is a process flowchart corresponding to a method of operating a catalytic heating system comprising a catalytic reactor and a dual-mode fuel evaporator, in accordance with some examples. 
         FIG.  5 A  is a temperature profile of the dual-mode fuel evaporator during the operation of the catalytic heating system, in accordance with some examples. 
         FIG.  5 B  is a cross-sectional view of the dual-mode fuel evaporator showing the heat supplied solely by the evaporator&#39;s electrical heater during the initial preheating stage, in accordance with some examples. 
         FIG.  5 C  is a cross-sectional view of the dual-mode fuel evaporator showing the heat supplied by the evaporator&#39;s electrical heater and by the catalytic reactor, in accordance with some examples. 
         FIG.  5 D  is a cross-sectional view of the dual-mode fuel evaporator showing the heat supplied solely by the catalytic reactor during the post-preheating operation, in accordance with some examples. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are outlined to provide a thorough understanding of the presented concepts. The presented concepts may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail to not unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific embodiments, it will be understood that these embodiments are not intended to be limiting. 
     INTRODUCTION 
     As noted above, vehicles use various types of heaters for interior heating, window defrosting, and even powertrain preheating (e.g., warming up battery packs). Conventional resistive heaters and fuel-fired heaters present various issues (e.g., range reduction, pollution), which makes them unsuitable for some types of vehicles (e.g., electrical vehicles and, in particular, medium and heavy-duty electrical trucks). Unlike fuel-fired heaters, catalytic heaters provide more control of the fuel oxidation conditions. For example, catalytic heaters can operate at reduced combustion temperatures, which generally correspond to less emission. However, direct fuel oxidation, especially, direct oxidation of renewable fuels (e.g., ethanol, methanol) can be challenging. 
     When a mixture of fuel and oxidant reaches a catalytic reactor (in a catalytic heating system), the fuel undergoes catalytic exothermic oxidation producing heat. The uniformity of the fuel-oxidant mixture entering the catalyst reactor and while contacting the catalyst in this reactor is critical for reducing emissions. It should be noted that the uniformity of this fuel-oxidant mixture is critical throughout the entire catalytic reactor. For example, a catalytic reactor can have multiple individual channels, e.g., forming a honeycomb-like structure and coated with the catalyst. The same fuel-oxidant mixture needs to enter each channel to maximize the utilization of the catalyst surface. Besides the uniformity, the fuel-oxidant mixture also needs to have the same space velocity. 
     Liquid fuels, such as ethanol and methanol, are inexpensive, easy to handle and store. Furthermore, liquid fuels have a high energy density. For example, the energy density of ethanol is 27 MJ/kg. All these characteristics make liquid fuels particularly attractive for catalytic heater applications. However, liquid fuels tend to generate some undesirable emissions, such as NOx, CO, PM, and HC. Atomizing liquid fuels helps to reduce these emissions, but further improvements are needed for many green-tech applications. 
     Specifically, atomizing the liquid fuel typically involves passing the fuel through a fogging nozzle or a fuel injector thereby reducing the droplet size. However, each small droplet needs to vaporize to react with the oxidant.  FIG.  1    is a schematic illustration of a liquid fuel droplet model and corresponding vapor fuel and emission concentration profiles. The model illustrates that the liquid fuel drop forms a boundary vapor layer with varying fuel concentration, i.e., the concentration is smaller further away from the droplet due to the spatial distribution of the vapor fuel as well as fuel consumption. At the same time, ful oxidation can occur across this entire vapor layer. At some concentration ranges, oxidation can cause the undesirable formation of NOx and other emissions. 
     It has been found that vaporizing liquid fuels before forming fuel-oxidant mixtures and contacting the catalyst surface with these mixtures can help to avoid undesirable fuel concentrations that cause emissions. For example, switching from atomized liquid ethanol to vaporized ethanol in a catalytic heater helped to reduce carbon monoxide (CO) emission in a 2-stage catalytic heater from more than 6 g/mi to 0.05 g/mi. As a reference, the California Air Resource Board (CARB) carbon monoxide emission limit for ultra-low emission vehicles (ULEV) is 1.7 g/mi. In other words, vaporized fuels allow achieving very uniform fuel distribution in fuel-oxidant mixtures, down to molecular level. This mixture uniformity promotes the desired catalytic oxidation of the fuel and minimizes emissions. 
     However, the vaporization of liquid fuels requires heat. While the heat, produced by a catalytic heating system, can be used for fuel vaporization, this heat is not immediately available (e.g., when the catalytic heating system is turned and still in the cold state). Due to the system&#39;s thermal mass, it takes some time (e.g., a few minutes) for the system to warm up and reach the level at which the heat can be supplied for fuel vaporization. At the same time, generating emissions, while the system is warming up and while the fuel is not vaporized, is not desirable. 
     Catalytic heating systems, described herein, comprise catalytic reactors and dual-mode fuel evaporators, thermally coupled to the catalytic reactors. A dual-mode fuel evaporator comprises an electric heater used for preheating the evaporator before the liquid fuel is supplied therein. For example, the overall system operation may start with turning on the electric heater of the dual-mode fuel evaporator before supplying any fuel. The evaporator is allowed to heat until the evaporator&#39;s temperature is at or above a fuel-flow threshold. For example, the fuel-flow threshold is higher than the fuel boiling point, e.g., at least 20° C. higher, at least 40° C. higher, at least 60° C. higher, or even at least 100° C. higher. This temperature margin (above the fuel boiling point) ensures that the vaporized fuel does not condense back to its liquid form once the fuel comes in contact with colder components of the catalytic heating system (e.g., while the system is still warming up) and also when the vaporized fuel is mixed with the oxidant, which can be supplied at a lower temperature. In some examples, the oxidant is also preheated to prevent fuel condensation. For example, another electrical heater can be positioned in an oxygen supply line. It should be noted that the fuel-flow threshold is below the coking temperature to prevent fuel decomposition. 
     Once the temperature of the dual-mode fuel evaporator is at or above a fuel-flow threshold, the liquid fuel is flown into the evaporator. The liquid fuel evaporates and forms vaporized fuel, which is mixed with the oxidant. The mixture is flown into the catalytic reactor where the vaporized fuel undergoes catalytic exothermic oxidation, generating heat. At least some of this generated heat is eventually transferred to the evaporator and used for the evaporation of additional fuel. It should be noted that once the heat is generated in the catalytic reactor, it may take some time for this heat transfer from the catalytic reactor to the evaporator to occur. The heat generated in the catalytic reactor initially heats various internal components of the catalytic reactor, which can have significant thermal mass. Furthermore, some heat will be carried out by exhaust (e.g., into a heat-recovery device). During this transition period, the electric heater can be used to maintain the temperature of the dual-mode fuel evaporator at or above a fuel-flow threshold. After some initial period, the catalytic reactor becomes sufficiently hot and is capable of transferring heat to the evaporator. The electric heater is turned off and further fuel vaporization utilizes the heat transferred from the catalytic reactor. 
     Overall, the dual-mode fuel evaporator not only vaporizes the fuel for more efficient mixing with the oxidant and catalytic reactor operation (e.g., catalyst conversion of the fuel) but also provides the seamless transition from one heating mode (using the electric heater of the evaporator) to another heating mode (using heat transferred from the catalytic reactor). In other words, the dual-mode fuel evaporator can utilize, as needed, two different heat sources: (1) its own electric heater and (2) the catalytic reactor. The first heat source (the electric heater) is independently controlled. The second heat source depends on vaporized fuel supplied to the catalytic reactor. 
     Examples of Catalytic Heating Systems 
       FIG.  2 A  is a cross-sectional schematic view of catalytic heating system  100 , in accordance with some examples.  FIG.  2 B  is a block diagram of catalytic heating system  100  in  FIG.  2 A , illustrating additional components. Specifically,  FIGS.  2 A and  2 B  illustrate catalytic heating system  100  comprising dual-mode fuel evaporator  170  and two catalytic reactors, i.e., catalytic reactor  110  and additional catalytic reactor  120 . This example of catalytic heating system  100  may be referred to as a two-stage catalytic heating system. Furthermore, in this example, dual-mode fuel evaporator  170  is thermally coupled to catalytic reactor  110 . However, in other examples, dual-mode fuel evaporator  170  can be thermally coupled to additional catalytic reactor  120 . In further examples, dual-mode fuel evaporator  170  can be coupled to both catalytic reactors. In either case, dual-mode fuel evaporator  170  is fluidically coupled catalytic reactor  110  to supply vaporized fuel  191  into catalytic reactor  110 . Finally, catalytic heating system  100  with only one catalytic reactor is also within the scope. 
     Referring to  FIGS.  2 A and  2 B , dual-mode fuel evaporator  170  is thermally and fluidically coupled to catalytic reactor  110 . Dual-mode fuel evaporator  170  can also be fluidically coupled to liquid fuel supply  130 , which stores and delivers liquid fuel  190  into dual-mode fuel evaporator  170 . For example, liquid fuel supply  130  comprises fuel storage (e.g., tank), fuel filter, and/or fuel delivery device (e.g., a fuel pump, a fuel compressor). Depending on the integration of catalytic heating system  100 , one or more of these components can be also parts of another system, e.g., a vehicle. For example, the fuel storage can be a vehicle fuel tank, the fuel filter can be a vehicle fuel filter, and the fuel delivery device can be a vehicle fuel pump. Some aspects of this integration are described below with reference to  FIG.  3   . Some examples of liquid fuel  190  include but are not limited methanol, ethanol, isopropanol, and renewable equivalents thereof (e.g., bio-methanol, bio-ethanol). In some examples, liquid fuel supply  130  comprises a replaceable cartridge. Unlike conventional fuel tanks, replaceable cartridges do not require any specific emission controls. A replaceable cartridge comprises a connecting port for connecting to the fuel line of catalytic heating system  100 . For example, a replaceable cartridge can be plugged into a fuel canister shell of liquid fuel supply  130 . The shell can be mounted on a vehicle and would protect the replaceable cartridge from road hazards. 
     Once liquid fuel  190  reaches dual-mode fuel evaporator  170 , liquid fuel  190  evaporates thereby forming vaporized fuel  191 . Vaporized fuel  191  is then mixed with oxidant  192 , flown from oxidant supply  140 . For example, catalytic reactor  110  further comprises fuel inlet  111  and oxidant inlet  112 . Fuel inlet  111  is fluidically coupled to dual-mode fuel evaporator  170 , while oxidant inlet  112  is fluidically coupled to oxidant supply  140 . Furthermore, catalytic reactor  110  comprises fuel-oxidant mixer  113  to which fuel inlet  111  and oxidant inlet  112  are connected. Some examples of fuel-oxidant mixer  113  are injectors, jets, showerheads, nozzles (e.g., swirl nozzle), Venturi devices, and the like. 
     Oxidant supply  140  can be an air intake, compressor, and the like. In some examples, oxidant  192  is oxygen in the air, which is obtained from the environment. In some examples, oxidant storage is used when the ambient air is not available, e.g., mining applications, underwater applications, and the like. When catalytic heating system  100  is a two-stage system, oxidant supply  140  is configured to supply oxidant  192  into catalytic reactor  110  (to be mixed with vaporized fuel  191  and form vaporized fuel-oxidant mixture  193 ) and also to supply additional oxidant  194  to additional catalytic reactor  120  (to be mixed with syngas  195  produced in catalytic reactor  110  and to form syngas-oxidant mixture  197 ). In other words, oxidant supply  140  is a dual supply of oxidant  192  and additional oxidant  194 . The flow rates of oxidant  192  and additional oxidant  194  can be independently controlled. Furthermore, in some examples, oxidant supply  140  comprises an oxidant-supply heater for heating oxidant  192 , e.g., to prevent recondensation of the fuel when vaporized fuel-oxidant mixture  193  is formed. For example, oxidant  192  can be heated to a temperature above the fuel boiling temperature. 
     Vaporized fuel-oxidant mixture  193  is directed to catalytic reactor  110 , which comprises catalyst  114 . Vaporized fuel  191  (in the mixture) catalytically oxides upon contacting catalyst  114  and forms exhaust  196  (in a single-stage catalytic system) or syngas  195  (in a two-stage catalytic system). This catalytic oxidation is an exothermic reaction, producing heat that heats catalytic reactor  110  and is partially removed by exhaust  196  or syngas  195 . 
     In a two-stage catalytic system, catalytic reactor  110  can be operated at a lower temperature than additional catalytic reactor  120 . For example, catalytic reactor  110  can be maintained from 500° C. to 700° C., while additional catalytic reactor  120  can be maintained from 600° C. and 800° C. One having ordinary skill in the art would understand that these operating target temperatures depend on the fuel type, oxidant type, fuel-oxidant ratios, catalysts, and other like conditions. The temperature is controlled by controlling the fuel flow rate and the oxidant flow rates. It should be noted that these flow rates also control the fuel-rich and fuel-lean conditions in catalytic reactor  110  and additional catalytic reactor  120 , respectively. 
     Catalyst  114  is specifically selected to achieve catalytic exothermic oxidation of vaporized fuel  191 . Some examples of materials suitable for catalyst  114  include, but are not limited to rhodium, ceria, platinum, and palladium. For example, when catalytic heating system  100  comprises two catalytic reactors, catalyst  114  of catalytic reactor  110  can comprise rhodium and ceria, while catalyst of additional catalytic reactor  120  can comprise platinum and palladium. 
     In some examples, catalyst  114  is arranged as a layer (e.g., formed in part by catalyst particles) on a support structure (e.g., such as metallic support). Lower operating temperatures of these reactors allow using metallic support, rather than ceramic support that is common in a conventional catalytic converter. In comparison to ceramic supports, metallic supports are more robust to vibration and temperature fluctuations. Furthermore, metallic supports have better thermal conductivity, which is important for catalyst preheating and maintaining uniform temperature throughout the entire catalyst. The metallic supports may be specifically configured to balance the flow rate through the reactor and the operating surface area. 
     Before vaporized fuel-oxidant mixture  193  is introduced into catalytic reactor  110 , the temperature of catalyst  114  needs to be brought to an operating range. For example, catalytic reactor  110  can be equipped with reactor preheater  119 . In some examples, dual-mode fuel evaporator  170  can be used for preheating catalyst  114 , in addition to or instead of reactor 
     When catalytic reactor  110  produces syngas  195 , syngas  195  is then combined with additional oxidant  194  and directed to additional catalytic reactor  120 , which forms exhaust  196 . Similar to catalytic reactor  110 , additional catalytic reactor  120  comprises additional fuel inlet  121  (which may be also referred to as a syngas inlet) and additional oxidant inlet  122 . Additional fuel inlet  121  is fluidically coupled to an outlet of catalytic reactor  110 , while additional oxidant inlet  122  is fluidically coupled to oxidant supply  140 . Furthermore, additional catalytic reactor  120  comprises additional fuel-oxidant mixer  123  to which additional fuel inlet  121  and additional oxidant inlet  122  are connected. 
     Referring to  FIGS.  2 A- 2 F , dual-mode fuel evaporator  170  comprises electric heater  174  such as an automotive glow plug, a hot plate, a heat tape, or a heater cartridge. Electric heater  174  can receive electric power  182  from battery  180  or some other source (e.g., an electric generator). The power supply to electric heater  174  is controlled by system controller  150 , various aspects of which are described below with reference to  FIG.  2 G . In some examples, electric heater  174  comprises multiple elements spread apart within dual-mode fuel evaporator  170  to provide uniform heating. For example, an electric glow plug having a power of 50 W-200 W can be used as electric heater  174 . Multiple electric glow plugs and/or a higher power heaters can be used to expedite the initial heating process. 
     Referring to  FIG.  2 D , in some examples, dual-mode fuel evaporator  170  comprises thermocouple  177  for measuring the temperature of dual-mode fuel evaporator  170 . The temperature input is provided to system controller  150 . This evaporator temperature is used to determine when liquid fuel  190  can be flown into dual-mode fuel evaporator  170 , e.g., when the temperature of dual-mode fuel evaporator  170  reaches or exceeds a fuel-flow threshold. 
     Furthermore, this temperature is used to determine when electric heater  174  of dual-mode fuel evaporator  170  can be turned off, e.g., when the temperature of dual-mode fuel evaporator  170  reaches or exceeds an evaporator operating thresholds. Various examples of these thresholds are described below with reference to  FIGS.  4  and  5 A . 
     Referring to  FIGS.  2 C- 2 E , in some examples, dual-mode fuel evaporator  170  further comprises heater coupler  172 . Heater coupler  172  can be shaped like a saddle and used for positioning and thermally coupling to catalytic reactor  110 . For example,  FIG.  2 C  illustrates catalytic reactor  110  comprising cylindrical enclosure  115  with catalyst  114 , positioned within enclosure  115 . Heater coupler  172  conforms to at least a portion of this enclosure  115  thereby ensuring the thermal coupling. Specifically, heater coupler  172  at least partially surrounds enclosure  115 , positioned between catalyst  114  and heater coupler  172 . In some examples, heater coupler  172  fully surrounds enclosure  115  of catalyst heater  110  as, e.g., is shown in  FIG.  2 F . 
     Heater coupler  172  comprises evaporation surface  173 . During the operation of dual-mode fuel evaporator  170 , liquid fuel  190  is directed at evaporation surface  173  where liquid fuel  190  is vaporized. In some examples, thermocouple  177  is configured to measure the temperature of evaporation surface  173 , e.g., to ensure that this temperature reaches or exceeds the fuel-flow threshold. 
     Dual-mode fuel evaporator  170  further comprises evaporator chamber  171 , extending away from evaporation surface  173 . Evaporator chamber  171  captures any vaporized fuel  191  and directs vaporized fuel  191  to evaporator outlet  179 , which is fluidically coupled with catalytic reactor  110  or, more specifically, to fuel inlet  111  of catalytic reactor  110 . In some examples, evaporator chamber  171  and heater coupler  172  are formed from stainless steel. In some examples, evaporator chamber  171  is insulated to prevent heat losses to the environment and reduce the overall heating requirements. 
     Dual-mode fuel evaporator  170  also comprises evaporator inlet  175 , receiving and directing liquid fuel  190  to evaporation surface  173 . For example, evaporator inlet  175  can comprise fogging nozzle  176 . Other types of inlets are also within the scope. 
     Catalytic heating system  100  generates heat due to the exothermic oxidation of the fuel and provides for one or more ways to recover this heat (e.g., for various needs in a vehicle). For example, catalytic heating system  100  comprises a heat exchanger for receiving exhaust  196  and recovering heat from exhaust  196  (e.g., to transfer the heat to thermal fluid supplied to other systems). 
     System Controller Examples 
     In some examples, catalytic heating system  100  comprises system controller  150 , communicatively coupled to at least dual-mode fuel evaporator  170  and liquid fuel supply  130  as, e.g., is schematically shown in  FIG.  2 G . In more specific examples, system controller  150  is communicatively coupled to catalytic reactor  110 , additional catalytic reactor  120 , oxidant supply  140 , and/or external systems  200  (e.g., vehicle&#39;s electronic control unit (ECU) and the like). 
     System controller  150  is configured to control various operations of dual-mode fuel evaporator  170 . Specifically, system controller  150  is configured to turn on electrical heater  174  until the temperature of dual-mode fuel evaporator  170  reaches or exceeds the fuel-flow threshold. The fuel-flow threshold is determined by the type of liquid fuel  190  used for powering catalytic heating system  100 . Furthermore, system controller  150  is configured to turn off electrical heater  174 , e.g., when the temperature of dual-mode fuel evaporator  170  reaches or exceeds the evaporator operating threshold. This turning off function may be binary or gradual. For example, system controller  150  can gradually reduce the power supplied electrical heater  174  as the temperature of dual-mode fuel evaporator  170  approaches the evaporator operating threshold. 
     System controller  150  is also configured to control various operations of liquid fuel supply  130 . Specifically, system controller  150  is configured to flow liquid fuel  190  into dual-mode fuel evaporator  170  when the temperature of dual-mode fuel evaporator  170  reaches or exceeds the fuel-flow threshold. 
     System controller  150  comprises processor  152  and memory  154 , storing various operating parameters of catalytic heating system  100 . Some examples of these parameters are the fuel-flow temperature threshold for dual-mode fuel evaporator  170 , the operating temperature threshold for dual-mode fuel evaporator  170 , and the like. Various other controlled operating parameters are also within the scope, such as flow rates of various components at various stages, temperatures of various components at various stages, and the like. Processor  152  uses various inputs to system controller  150  as well as the operating parameters in memory  154  to determine operating instructions for various components of catalytic heating system  100 , such as power settings for catalyst heaters and delivery devices and/or valve positions. 
     System Integration Examples 
       FIG.  3    is a block diagram illustrating various examples of integrating catalytic heating system  100  into vehicle  300 . While this integration is shown in the context of vehicle, one having ordinary skill in the art would understand that integrating catalytic heating system  100  can be similarly integrated into other systems, requiring heat, such as power plants (e.g., utilizing fuel cells), residential and commercial heating-ventilation-air conditioning (HVAC) systems) and the like. Unlike conventional heaters, catalytic heating system  100  produces heat in a cost-efficient, energy-efficient, and low-emission manner. 
     For example, catalytic heating system  100  can be used to supply heat to interior  310  of vehicle  300  (e.g., to maintain the interior temperature at the desired level, to defrost windows, and the like). Specifically, the thermal fluid is pumped between catalytic heating system  100  (wherein the thermal fluid is heated) and a radiator (wherein the thermal fluid is cooled to heat air). The radiator may be coupled to an air blower, which directs the heated air from the radiator into the vehicle cabin. In the same or other examples, catalytic heating system  100  can be used to supply heat to powertrain system  320  or, more specifically, to battery pack  330 . One having ordinary skill in the art would appreciate that the operating temperature of battery pack  330  can deviate from the environmental temperature (e.g., vehicle  300  is parked outside during a winter night). To achieve the efficient operation of battery pack  330 , thermal fluid can be pumped between catalytic heating system  100  and battery pack  330  to bring battery pack  330  to the operating temperature. 
     In some examples, catalytic heating system  100  is used for preheating fuel cell  335 , which can be a part vehicle&#39;s drivetrain system  320 , integrated into other systems or applications, or be a standalone component. As noted above, several types of fuel cells operate at high temperatures, such as molten carbonate fuel cells and solid oxide fuel cells (SOFCs), e.g., from 600° C. to over 900° C. for SOFCs. Catalytic heating system  100  can be used for preheating fuel cell  335  to its operating temperature. 
     In some examples, vehicle  300  is equipped with fuel tank  340 , e.g., an internal combustion vehicle, a plug-in hybrid vehicle. Fuel tank  340  can be used to supply the fuel to catalytic heating system  100 , effectively eliminating the need for a separate fuel supply at the heating system level. A similar integration can be used on the oxidant supply side. 
     Examples of Operating Catalytic Heating Systems 
       FIG.  4    is a process flowchart corresponding to method  400  of operating catalytic heating system  100 , in accordance with some examples. Various aspects of catalytic heating system  100  are described above with reference to  FIGS.  2 A- 2 G . Specifically, catalytic heating system  100  comprises dual-mode fuel evaporator  170  equipped with electrical heater  174 . 
     Method  400  comprises (block  410 ) heating dual-mode fuel evaporator  170  using electrical heater  174  of dual-mode fuel evaporator  170 . This heating operation continues until the temperature of dual-mode fuel evaporator  170  reaches or exceeds the fuel-flow threshold as shown by decision block  430 . In some examples, electrical heater  174  is kelp on until the temperature of dual-mode fuel evaporator  170  reaches or exceeds the evaporator operating threshold as shown by decision block  475 . 
     The temperature profile of dual-mode fuel evaporator  170  is shown in  FIG.  5 A . The heating of dual-mode fuel evaporator  170  is initiated at t 1 . At this point, the starting temperature of dual-mode fuel evaporator  170  can be at an ambient level. At this point, the heating is performed using only electrical heater  174 . This heating continues until t 2 , at which point the temperature of dual-mode fuel evaporator  170  reaches the fuel-flow threshold.  FIG.  56    is a schematic cross-sectional illustration of dual-mode fuel evaporator  170  showing heat generated by two elements of electrical heater  174  and flowing to evaporation surface  173 . It should be noted that some heat, produced by electrical heater  174 , also flows to catalytic reactor  110  or, more specifically, to enclosure  115  of catalytic reactor  110 . 
     Returning to  FIG.  4   , in some examples, (block  410 ) heating dual-mode fuel evaporator  170  using electrical heater  174  further comprises (block  415 ) heating catalytic reactor  110 . As noted above with reference to  FIG.  5 B , at least some heat generated by electrical heater  174  of dual-mode fuel evaporator  170  can be transferred to catalytic reactor  110 , which is thermally coupled to dual-mode fuel evaporator  170 . This heat transfer (from dual-mode fuel evaporator  170  to catalytic reactor  110 ) should be distinguished from the heat transfer to dual-mode fuel evaporator  170  (from catalytic reactor  110 ) during the stable-state operation of catalytic heating system  100 . It should be noted that liquid fuel  190  is flown into dual-mode fuel evaporator  170  when the temperature of catalytic reactor  110  reaches or exceeds the fuel-receiving threshold. The heating of catalytic reactor  110  can be performed solely by electrical heater  174  of dual-mode fuel evaporator  170 . Alternatively, the heating of catalytic reactor  110  can be performed by electrical heater  174  of dual-mode fuel evaporator  170  as well as a dedicated heater of catalytic reactor  110 . In some examples, heating of catalytic reactor  110  using electrical heater  174  of dual-mode fuel evaporator  170  is negligible or not performed and most or all heating is performed by the dedicated heater of catalytic reactor  110 . In further examples, dual-mode fuel evaporator  170  can be at least partially heated using the dedicated heater of catalytic reactor  110 . 
     When the temperature of dual-mode fuel evaporator  170  reaches or exceeds the fuel-flow threshold, method  400  proceeds with (block  440 ) flowing liquid fuel  190  into dual-mode fuel evaporator  170  thereby causing liquid fuel  190  to evaporate and generate vaporized fuel  191 . It should be noted that the fuel-flow threshold is above the evaporation temperature of liquid fuel  190 . The fuel-flow threshold depends on the type of liquid fuel  190  as well as other factors, e.g., the thermal mass of other components of catalytic heating system  100 , the current temperature of the other components, various characteristics of oxidant  192  (e.g., temperature, flow rate, specific heat). The fuel-flow threshold is selected to ensure that vaporized fuel  191  does not recondense after vaporized fuel  191  leaves dual-mode fuel evaporator  170 . In some examples, the fuel-flow threshold is dynamically controlled based on various conditions and parameters listed above. In some examples, the fuel-flow threshold is higher than the fuel boiling point, e.g., at least 20° C. higher, at least 40° C. higher, at least 60° C. higher, or even at least 100° C. higher. In more specific examples, methanol&#39;s boiling point is about 65° C. The fuel-flow threshold can be at least 100° C., at least 125° C., or at least 150° C. when methanol is used as fuel. Ethanol&#39;s boiling point is about 78° C. n this example, the fuel-flow threshold can be at least 110° C., at least 135° C., or at least 150° C. 
     Method  400  proceeds with (block  450 ) mixing vaporized fuel  191  with oxidant  192  thereby forming vaporized fuel mixture  193 . For example, vaporized fuel  191  and oxidant  192  can be supplied to fuel-oxidant mixer  113  of catalytic reactor  110  as described above. 
     Method  400  proceeds with (block  460 ) flowing vaporized fuel-oxidant mixture  193  into catalytic reactor  110  thereby causing catalytic exothermic oxidation of vaporized fuel  191  and producing heat. This heat is at least partially transferred to dual-mode fuel evaporator  170 . It should be noted that when vaporized fuel-oxidant mixture  193  is initially flown into catalytic reactor  110 , electrical heater  174  of dual-mode fuel evaporator  170  can continue to operate. In other words, dual-mode fuel evaporator  170  can be heated from two sources during this period: (1) electrical heater  174  and (2) catalytic reactor  110 . 
     Referring to  FIG.  5 A , vaporized fuel-oxidant mixture  193  is flown into catalytic reactor  110  at t 2 . The temperature of dual-mode fuel evaporator  170  at this point is at the fuel-flow threshold and is not sufficient for electrical heater  174  to be turned off. Due to the two heat sources working simultaneously, the heating rate of dual-mode fuel evaporator  170  between t 2  and t 3  is faster than between t 1  and t 2 , at least in this example. This combination heating continues until t 3 , at which point dual-mode fuel evaporator  170  reaches the evaporator operating threshold, and electrical heater  174  can be turned off.  FIG.  5 C  is a schematic cross-sectional view of dual-mode fuel evaporator  170  with electrical heater  174  still no and while the heat is also being transferred from catalytic reactor  110 . 
     Returning to  FIG.  4   , in some examples, method  400  comprises (block  470 ) monitoring the temperature of dual-mode fuel evaporator  170  while the heat, produced by catalytic reactor  110 , at least partially transferred to dual-mode fuel evaporator  170  from catalytic reactor  110 . This temperature monitoring operation is similar to the one described above with reference to block  420 . In fact, the evaporator temperature monitoring can be one continuous operation. The evaporator temperature is supplied to system controller  150 , which determines at which point the fule can be flown into dual-mode fuel evaporator  170  and, at which point, electrical heater  174  can be turned off. 
     Method  400  also comprises (block  490 ) turning off electrical heater  174  when (decision block  475 ) the temperature of dual-mode fuel evaporator  170  reaches or exceeds an evaporator operating threshold. The evaporator operating threshold is higher than the fuel-flow threshold, e.g., 30° C. higher, 50° C. higher, or even 100° C. higher. The evaporator operating threshold can be also referred to as an evaporator self-heating threshold. This threshold is sufficiently high that the heat from electrical heater  174  is no longer needed. In some examples, the power output of electrical heater  174  can be gradually reduced as the temperature of dual-mode fuel evaporator  170  approaches the evaporator operating threshold. 
     Referring to  FIG.  5 A , electrical heater  174  is turned off at t 3 . Further heating of dual-mode fuel evaporator  170  is performed by catalytic reactor  110 . At some point, schematically shown as t 4 , the temperature of dual-mode fuel evaporator  170  can reach a steady state, e.g., all heat received from catalytic reactor  110  is either consumed for fuel vaporization or lost to the environment.  FIG.  5 D  is a schematic cross-sectional view of dual-mode fuel evaporator  170  with electrical heater  174  turned off and heat is being transferred from catalytic reactor  110 . 
     Returning to  FIG.  4   , in some examples, the temperature of catalytic reactor  110  can be used as feedback for controlling the operation of dual-mode fuel evaporator  170 . In these examples, method  400  may comprise (block  480 ) monitoring the temperature of catalytic reactor  110  while the heat, generated by catalytic reactor  110 , at least partially transferred to dual-mode fuel evaporator  170  from catalytic reactor  110 . Method  400  then proceeds with (block  490 ) turning off electrical heater  174  (decision block  485 ) when the temperature of catalytic reactor  110  reaches or exceeds a reactor operating threshold. 
     CONCLUSION 
     Although the foregoing concepts have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatuses. Accordingly, the present embodiments are to be considered illustrative and not restrictive.