Abstract:
One embodiment of the present invention is a unique reducing gas generator. Another embodiment is a unique method for generating a reducing gas. Other embodiments include apparatuses, systems, devices, hardware, methods, and combinations for generating reducing gas. Further embodiments, forms, features, aspects, benefits, and advantages of the present application will become apparent from the description and figures provided herewith.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is continuation-in-part of U.S. patent application Ser. No. 12/554,460, entitled Apparatus For Generating A Gas Which May Be Used For Startup And Shutdown Of A Fuel Cell, filed on Sep. 4, 2009 now U.S. Pat. No. 8,668,752 and U.S. patent application Ser. No. 12/554,039, entitled Method For Generating A Gas Which May Be Used For Startup And Shutdown Of A Fuel Cell, filed on Sep. 4, 2009, now U.S. Pat. No. 8,597,841 each of which is incorporated herein by reference. 
    
    
     GOVERNMENT RIGHTS IN PATENT 
     This invention was made with Government support under DE-FC26-08NT01911 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to reducing gas, and more particularly, to systems and methods for generating a reducing gas. 
     BACKGROUND 
     Systems and methods for generating a reducing gas remain an area of interest. Some existing systems have various shortcomings, drawbacks, and disadvantages relative to certain applications. Accordingly, there remains a need for further contributions in this area of technology. 
     SUMMARY 
     One embodiment of the present invention is a unique reducing gas generator. Another embodiment is a unique method for generating a reducing gas. Other embodiments include apparatuses, systems, devices, hardware, methods, and combinations for generating reducing gas. Further embodiments, forms, features, aspects, benefits, and advantages of the present application will become apparent from the description and figures provided herewith. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description herein makes reference to the accompanying drawings, wherein like reference numerals refer to like parts throughout the several views, and wherein: 
         FIG. 1  schematically depicts a fuel cell system in accordance with an embodiment of the present invention. 
         FIG. 2  schematically depicts the fuel cell system of  FIG. 1  in greater detail, including a reducing gas generator in accordance with an embodiment of the present invention. 
         FIGS. 3A-3D  are a flowchart depicting a method for startup and shutdown of a fuel cell using a reducing gas generator in accordance with an embodiment of the present invention. 
         FIG. 4  is a plot depicting catalytic conversion parameters in a catalytic reactor of a reducing gas generator in accordance with an embodiment of the present invention. 
         FIGS. 5A and 5B  schematically illustrates some aspects of non-limiting examples of an oxidant system in accordance with embodiments of the present invention. 
         FIG. 6  illustrates the flammables content in a reformed gas plotted against oxygen percentage at constant methane conversion. 
     
    
    
     DETAILED DESCRIPTION 
     For purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nonetheless be understood that no limitation of the scope of the invention is intended by the illustration and description of certain embodiments of the invention. In addition, any alterations and/or modifications of the illustrated and/or described embodiment(s) are contemplated as being within the scope of the present invention. Further, any other applications of the principles of the invention, as illustrated and/or described herein, as would normally occur to one skilled in the art to which the invention pertains, are contemplated as being within the scope of the present invention. 
     Referring now to the figures, and in particular,  FIG. 1 , a schematic of a fuel cell system  10  in accordance with an embodiment of the present invention is depicted. Fuel cell system  10  includes one or more of a fuel cell  12 , and includes a reducing gas generator  14 . Fuel cell system  10  is configured to provide power to an electrical load  16 , e.g., via electrical power lines  18 . In the present embodiment, fuel cell  12  is a solid oxide fuel cell (SOFC), although it will be understood that the present invention is equally applicable to other types of fuel cells, such as alkali fuel cells, molten-carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), and proton exchange membrane (PEM) fuel cells. In the present embodiment, fuel cell system  10  is suitable, but not limited to, use in a fuel cell turbine hybrid system where high-pressure feed streams are employed. 
     Reducing gas generator  14  of the present embodiment is configured to generate a reducing gas having a combustibles content (which is primarily hydrogen—H 2  and carbon monoxide—CO) that may be varied within a compositional range of approximately 3% combustibles content to approximately 45% combustibles content. In other embodiments, different compositional ranges may be employed, for example, a range of approximately 2% combustibles content to approximately 50% combustibles content in some embodiments, and approximately 1% combustibles content to approximately 60% combustibles content in other embodiments. As set forth below, reducing gas generator  14  of the present embodiment is tailored to yield a start gas in the form of a reducing gas having a primary function of protecting the anode of fuel cell  12  from oxidation during startup of fuel cell  12 , e.g., during system heat-up prior to power generation. As power generation is started, the reducing gas is transitioned off. 
     In the embodiment of  FIG. 1 , various features, components and interrelationships therebetween of aspects of an embodiment of the present invention are depicted. However, the present invention is not limited to the particular embodiment of  FIG. 1  and the components, features and interrelationships therebetween as are illustrated in  FIG. 1  and described herein. For example, other embodiments encompassed by the present invention, the present invention being manifested by the principles explicitly and implicitly described herein via the present Figures and Detailed Description and set forth in the Claims, may include a greater or lesser number of components, features and/or interrelationships therebetween, and/or may employ different components and/or features having the same and/or different nature and/or interrelationships therebetween, which may be employed for performing similar and/or different functions relative to those illustrated in  FIG. 1  and described herein. 
     Referring now  FIG. 2 , fuel cell  12  and reducing gas generator  14  are described in greater detail. Fuel cell  12  includes at least one each of an anode  20 , an electrolyte  22 , a cathode  24 , and a reformer  26 . Anode  20 , electrolyte  22  and cathode  24  are considered part of fuel cell  12 . Reformer  26  is an internal steam reformer that receives steam as a constituent of a recycled fuel cell product gas stream, and heat for operation from fuel cell  12  electro chemical reactions. Reducing gas generator  14  is not a part of fuel cell  12 , but rather, is configured for generating gases for use in starting up and shutting down fuel cell  12 . 
     Anode  20  is electrically coupled to electrical load  16  via electrical power line  18 , and cathode  24  is also electrically coupled to electrical load  16  via the other electrical power line  18 . Electrolyte  22  is disposed between anode  20  and cathode  24 . Anode  20  and cathode  24  are electrically conductive, and are permeable to oxygen, e.g., oxygen ions. Electrolyte  22  is configured to pass oxygen ions, and has little or no electrical conductivity, e.g., so as to prevent the passage of free electrons from cathode  24  to anode  20 . 
     Reformer  26  is coupled to anode  20 , and is configured to receive a fuel and an oxidant and to reform the fuel/oxidant mixture into a synthesis gas (syngas) consisting primarily of hydrogen (H 2 ), carbon monoxide (CO), as well as other reformer by-products, such as water vapor in the form of steam, and other gases, e.g., nitrogen and carbon-dioxide (CO 2 ), methane slip (CH 4 ), as well as trace amounts of hydrocarbon slip. In the present embodiment, the oxidant employed by fuel cell  12  during normal operations, i.e., in power production mode to supply electrical power to electrical load  16 , is air, and the fuel is natural gas, although it will be understood that other oxidants and/or fuels may be employed without departing from the scope of the present invention. 
     The synthesis gas is oxidized in an electro-chemical reaction in anode  20  with oxygen ions received from cathode  24  via migration through electrolyte  22 . The electro-chemical reaction creates water vapor and electricity in a form of free electrons on the anode that are used to power electrical load  16 . The oxygen ions are created via a reduction of the cathode oxidant using the electrons returning from electrical load  16  into cathode  24 . 
     Once fuel cell  12  is started, internal processes maintain the required temperature for normal power generating operations. However, in order to start the fuel cell, the primary fuel cell system components must be heated, including anode  20 , electrolyte  22 , cathode  24  and reformer  26 . 
     In addition, some fuel cell  12  components may be protected from damage during the start-up, e.g., due to oxidation. For example, anode  20  may be subjected to oxidative damage in the presence of oxygen at temperatures above ambient but below the normal operating temperature of fuel cell  12  in the absence of the synthesis gas. Also, reformer  26  may need a specific chemistry, e.g. H 2 O in the form of steam in addition to the heat provided during start-up of fuel cell  12 , in order to start the catalytic reactions that generate the synthesis gas. Further, it is desirable that fuel cell  12  be started in a safe manner, e.g., so as to prevent a combustible mixture from forming during the starting process. Thus, it may be desirable to purge anode  20  with a nonflammable reducing gas during the initial startup as the temperature of anode  20  increased. In one aspect, a characteristic of reducing gas generator  14  is that the reducing gas may be made sufficiently dilute in combustibles to prevent the potential formation of a flammable (i.e., potentially explosive) mixture upon mixing with air. This may be desirable during the low temperature portion of heat-up of fuel cell  12  where any combustibles mixing with air are below auto-ignition temperature, and therefore, can potentially build up to form dangerous quantities of potentially pressurized flammable gases within the vessel that contains fuel cell  12 . 
     The reducing gas strength for protecting anode  20  of fuel cell  12  from oxygen migration can be quite high, e.g., up to 45% combustibles content in the present embodiment, up to 50% in other embodiments, and up to 60% combustibles content in still other embodiments. Mechanisms that cause the migration of oxygen through electrolyte  22  to the anode  20  side of the fuel cell  12  are often temperature dependent and include oxygen permeation through electrolyte  22  or oxygen transfer induced by short circuit currents. Also, physical leakage mechanisms may become worse with temperature as materials differentially expand. Thus, the ability of reducing gas generator  14  to increase combustibles content at high fuel cell  12  temperatures during startup may be particularly useful in protecting anode  20  from oxidation damage. 
     From a safety perspective, it may be possible to step to a greater reducing strength at higher temperatures during fuel cell  12  startup, since the possibility of mixing the reducing gas with a pressurized volume of air to form an combustible mixture in or near fuel cell  12  is reduced if the reducing gas is above auto-ignition temperature, because the reducing gas would tend to immediately burn upon mixing with air. In addition, this may prevent build-up of a flammable mixture that can potentially deflagrate if the mixture were to suddenly come in contact with an ignition source, since any such mixture would tend to burn immediately when above the auto-ignition temperature, rather than build up a large quantity of the mixture. 
     Thus, in some embodiments, it may be desirable to operate reducing gas generator  14  in a manner by which the reducing gas is initially weakly reducing and well below the flammability limit, e.g., 3% combustibles content in the present embodiment, although other values may be employed, for example, 2% combustibles content in some embodiments and 1% combustibles content or less in other embodiments. In still other embodiments, the combustibles content may be greater than 3%. The combustibles content may subsequently be changed to a strongly reducing (i.e., higher combustibles) condition (higher reducing strength) when temperature conditions in fuel cell  12 , e.g., anode  20 , are high enough to ensure that the reducing gas is far above its lower flammability limit. For example, the strongly reducing condition may be up to 45% combustibles content in the present embodiment, up to 50% combustibles content in other embodiments, and up to 60% combustibles content or greater in yet other embodiments, depending upon the conditions in fuel cell  12 . The increased energy input to the system with a stronger reducing gas may be offset by decreasing fuel flow to the fuel cell power plant&#39;s Off-Gas Burner for such plants so equipped. 
     Accordingly, embodiments of the present invention may employ reducing gas generator  14  to generate a purging gas to purge fuel cell  12  of oxidants, in particular, cathode  24 , as well as to generate a safe gas, i.e., a weak reducing gas having a relatively low level of combustibles. 
     In addition, embodiments of the present invention may also employ reducing gas generator  14  to produce a variable-reducing-strength reducing gas. The reducing gas composition provided by reducing gas generator  14  may also be configured to contain adequate steam to initiate the operation of the internal reformer  26  as the normal fuel cell  12  fuel stream flow, e.g., natural gas, is started. Accordingly, the reducing gas supplied to fuel cell  12  from reducing gas generator  14  may be considered a transition gas as power production by fuel cell  12  is ramped up. Additionally, reducing gas generator  14  of the present embodiment may be capable of rapid start-up, e.g., for protecting anode  20  in the event of emergency fuel cell  12  shutdown events, for example, by maintaining certain elements of reducing gas generator  14  at elevated temperatures in order to speed up initiation of the catalytic reactions that yield the reducing gas. 
     In the present embodiment, as illustrated in  FIG. 2 , reducing gas generator  14  includes a fuel system  28 , an oxidant system  30 , a merging chamber  32 , and a catalytic reactor  34  having a catalyst  36 . In the present embodiment, the outputs of fuel system  28  and oxidant system  30  are combined in merging chamber  32  and directed to fuel cell  12  via catalytic reactor  34  to selectively provide purging gas, safe gas, and variable strength reducing gas to anode  20  and reformer  26 . 
     In the embodiment depicted in  FIG. 2 , various features, components and interrelationships therebetween of aspects of an embodiment of the present invention are depicted. However, the present invention is not limited to the particular embodiment of  FIG. 2  and the components, features and interrelationships therebetween as are illustrated in  FIG. 2  and described herein. For example, other embodiments encompassed by the present invention, the present invention being manifested by the principles explicitly and implicitly described herein via the present Figures and Detailed Description and set forth in the Claims, may include a greater or lesser number of components, features and/or interrelationships therebetween, and/or may employ different components and/or features having the same and/or different nature and/or interrelationships therebetween, which may be employed for performing similar and/or different functions relative to those illustrated in  FIG. 2  and described herein. 
     In any event, in the embodiment of  FIG. 2 , fuel system  28  includes a fuel input  38 , a pressure regulator  40 , a sulfur capture sorbent  42 , a fuel flow controller  44 , and a variable position/output fuel control valve  46 . Fuel input  38  is configured to receive a hydrocarbon fuel, e.g., natural gas, and serves as a source of the hydrocarbon fuel used by reducing gas generator  14 . Pressure regulator  40  is fluidly coupled to fuel inlet  38 , and regulates the pressure of the hydrocarbon fuel. Sulfur capture sorbent  42  is fluidly coupled to pressure regulator  40 , and is configured to capture sulfur from the fuel stream received from pressure regulator  40 . Fuel flow controller  44  and fuel control valve  46  are coupled to the output of sulfur capture sorbent  42 , and are configured to control the amount of fuel delivered to merging chamber  32 . 
     Oxidant system  30  functions as an oxidant source for reducing gas generator  14 , and includes an air intake  48 , an air compressor  50  as a pressurized air source, a pressure regulator  52 , a nitrogen generator  54  having a nitrogen separation membrane  56 , a variable position/output air control valve  58 , an air flow controller  60 , a variable position/output oxidant control valve  62 , an oxidant flow controller  64  and an oxygen sensor  66 . 
     Air intake  48  may be any structure or opening capable of providing air, and is fluidly coupled to air compressor  50 , which compresses ambient air received from the atmosphere. Pressure regulator  52  is fluidly coupled to air compressor  50 , and regulates the air pressure delivered to reducing gas generator  14 . Air control valve  58  is part of an air charging system structured to variably add air to the nitrogen-rich gas received from nitrogen generator  54  to yield an oxidant having a variable O 2  content. 
     The O 2  content may be sensed by oxygen sensor  66 , which may be used by the control system of reducing gas generator  14  to vary the O 2  content of the oxidant supplied to merging chamber  32 . For example, under normal operating conditions, the O 2  content is controlled based on a control temperature, e.g., the temperature of catalyst  36  in the present embodiment, although other temperatures may be used in other embodiments, e.g., the temperature of the reducing gas output by reducing gas generator  14 . However, during startup of reducing gas generator  14 , oxygen sensor  66  may be used to provide feedback until the temperature is available as a feedback. The amount or flow of the oxidant having the variable O 2  content is controlled by oxidant control valve  62  and oxidant flow controller  64 . 
     Nitrogen generator  54  is configured to generate a nitrogen-rich stream, which may be used as a purging gas, and which may also be combined with air to form a low oxygen (O 2 ) content oxidant stream, e.g., a nitrogen-diluted air stream, used by reducing gas generator  14  to form a reducing gas. The purity of the nitrogen-rich stream may vary with the needs of the particular application, for example, and may consist essentially of nitrogen. Alternatively, it is considered that in other embodiments, other gases may be employed in place of or in addition to nitrogen, such as argon or helium, for use as a purging gas and/or as a constituent of a low O 2  content oxidant stream, e.g., as a dilutant (diluent) of air. As used herein, “low O 2  content oxidant” means that the oxygen content of the oxidant stream is less than that of atmospheric air under the same pressure and temperature conditions. 
     Nitrogen generator  54  and air control valve  58  are fluidly coupled in parallel to pressure regulator  52 , and receive pressurized air from air compressor  50  for use in reducing gas generator  14  operations. Nitrogen generator  54  has an output  54 A, e.g., an opening or passage structured to discharge the products of nitrogen generator  54 . Nitrogen generator  54  is structured to receive air from air intake  48 , extract oxygen (O 2 ) from the air, and to discharge the balance in the form of a nitrogen-rich gas from the outlet. The extracted O 2  is discharged from nitrogen generator  54  to the atmosphere in the present embodiment, although it will be understood that in other embodiments, the extracted O 2  may be employed for other purposes related to fuel cell  12  and/or reducing gas generator  14 , e.g., as part of an oxidant stream. 
     Nitrogen separation membrane  56  of nitrogen generator  54  is configured to separate oxygen out of the air received from air intake  48 , and provides the nitrogen-rich stream, which is then combined with the air supplied by air control valve  58  to yield the low O 2  content oxidant, which is delivered to oxidant control valve  62 . Oxidant control valve  62  is fluidly coupled to the outputs of both nitrogen generator  54  and air control valve  58 . Oxygen sensor  66 , which may be in the form of an O 2  analyzer, is fluidly coupled downstream to oxidant control valve  62 , and provides a control signal via control line  68  that communicatively couples oxygen sensor  66  with air flow controller  60 . Air flow controller  60  provides control signals to air control valve  58  to control the amount of air added to the nitrogen-rich stream based on the control input from oxygen sensor  66 . 
     Merging chamber  32  is in fluid communication with the output of nitrogen generator  54  and fuel input  38 , and is structured to receive and combine the hydrocarbon fuel and nitrogen-rich gas and discharge a feed mixture containing both the fuel and the oxidant including the nitrogen-rich gas to catalytic reactor  34 . Catalytic reactor  34  is structured to receive the feed mixture and to catalytically convert the feed mixture into a reducing gas. The form of merging chamber  32  is a simple plumbing connection joining the oxidant stream with the fuel stream in the present embodiment, although any arrangement that is structured to combine an oxidant stream with a fuel stream may be employed without departing from the scope of the present invention. For example, a dedicated mixing chamber having swirler vanes to mix the streams may be employed. 
     Reducing gas generator  14  includes a start control valve  69  having a valve element  70  and a valve element  72 ; and a feed mixture heater  74 , which may be used to start the process of generating reducing gas. In one form, valve elements  70  and  72  are part of a combined valving element. The inlets of valve elements  70  and  72  are fluidly coupled to merging chamber  32  downstream thereof. The outlet of valve element  70  is fluidly coupled to catalytic reactor  34  for providing the feed mixture to catalyst  36  of catalytic reactor  34 . The outlet of valve element  72  is fluidly coupled to the inlet of feed mixture heater  74 . In one form, start control valve  69  is a three-way valve that operates valve elements  70  and  72  to direct flow entering valve  69  into catalytic reactor  34  directly or via feed mixture heater  74 . It is alternatively considered that other valve arrangements may be employed, such as a pair of individual start control valves in place of start control valve  69  with valve elements  70  and  72 . 
     Feed mixture heater  74  includes a heating body  76  and a flow coil  78  disposed adjacent to heating body  76 . The outlet of feed mixture heater  74  is fluidly coupled to catalytic reactor  34  for providing heated feed mixture to catalyst  36  of catalytic reactor  34 . In the normal operating mode, valve elements  70  and  72  direct all of the feed mixture directly to the catalytic reactor  34 . In the startup mode, the feed mixture is directed through feed mixture heater  74 . In one form, all of the feed mixture is directed through feed mixture heater  74 , although in other embodiments, lesser amounts may be heated. 
     Feed mixture heater  74  is configured to “light” the catalyst  36  of catalytic reactor  34  (initiate the catalytic reaction of fuel and oxidant) by heating the feed mixture, which is then supplied to catalytic reactor  34 . In one form, the feed mixture is heated by feed mixture heater  74  to a preheat temperature above the catalyst light-off temperature of the feed mixture (the catalyst light-off temperature is the temperature at which reactions are initiated over the catalyst, e.g., catalyst  36 ). Once catalyst  36  is lit, the exothermic reactions taking place at catalyst  36  maintain the temperature of catalytic reactor  34  at a controlled level, as set forth below. Also, once catalyst  36  is lit it may no longer be necessary to heat the feed mixture, in which case valve elements  70  and  72  are positioned to direct all of the feed mixture directly to the catalytic reactor  34 , bypassing feed mixture heater  74 . 
     In order to provide for a quick supply of reducing gas in the event of a sudden shutdown of fuel cell  12 , heating body  76  is configured to continuously maintain a temperature sufficient to light catalyst  36  during normal power production operations of fuel cell  12 . That is, while fuel cell  12  is operating in power production mode to supply power to electrical load  16 , which is the normal operating mode for fuel cell  12 , heating body  76  maintains a preheat temperature sufficient to heat the feed mixture in order to be able to rapidly light the catalyst for startup of reducing gas generator  14  so that reducing gas may be supplied to fuel cell  12  during shutdown. 
     In addition, one or more catalyst heaters  80  are disposed adjacent to catalytic reactor  34 , and are configured to heat catalyst  36  and maintain catalyst  36  at a preheat temperature that is at or above the catalyst light-off temperature for the feed mixture supplied to catalytic reactor  34 . This preheat temperature is maintained during normal operations of fuel cell  12  in power production mode in the event of a sudden need for reducing gas, e.g., in the event of the need for a shutdown of fuel cell  12 . 
     In other embodiments, it is alternatively considered that another heater  82  may be used in place of or in addition to heaters  74  and  80 , e.g., a heater  82  positioned adjacent to catalytic reactor  34  on the upstream side. Such an arrangement may be employed to supply heat more directly to catalyst  36  in order to initiate catalytic reaction of the feed mixture in an upstream portion of catalytic reactor  34 . 
     In the present embodiment, heaters  74 ,  80  and  82  are electrical heaters, although it is alternatively considered that in other embodiments, indirect combustion heaters may be employed in addition to or in place of electrical heaters. Also, although the present embodiment employs both feed mixture heater  74  and heaters  80  to rapidly light the feed mixture on the catalyst, it is alternatively considered that in other embodiments, only one such heater may be employed, or a greater number of heaters may be employed, without departing from the scope of the present invention. 
     A control temperature sensor  84  is positioned adjacent catalyst  36  of catalytic reactor  34 , and is structured to measure the temperature of catalyst  36 . In one form, control temperature sensor  84  is structured to provide a signal indicating the temperature of a portion of catalyst  36  via a sense line  92  that communicatively couples air flow controller  60  with control temperature sensor  84 . The control temperature is a temperature employed by control system  96  in regulating the output of reducing gas generator  14 . Air flow controller  60  is configured to direct the operations of air control valve  58  based on the signal received from control temperature sensor  84  in conjunction with the signal received from oxygen sensor  66 . In another form, other temperatures may be sensed for purposes of controlling reducing gas generator  14 . For example, in one such embodiment, the temperature of the reducing gas produced by reducing gas generator  14 , e.g., as output by catalytic reactor  34 , may be measured and used as a control temperature feedback to direct the operations of air control valve  58 . 
     A reducing gas combustibles detection sensor  86 , which in the present embodiment is in the form of a hydrogen (H 2 ) sensor or H 2  analyzer, is configured to determine the quantity of one or more combustibles, e.g., percent mole, present in the reducing gas output by catalytic reactor  34 . In other embodiments, reducing gas combustibles detection sensor  86  may be in the form of a carbon monoxide (CO) sensor or analyzer in addition to or in place of the H 2  sensor/analyzer. In any case, a control line  94  communicatively couples fuel flow controller  44  and reducing gas combustibles detection sensor  86 . Reducing gas combustibles detection sensor  86  is configured to supply a signal reflecting the combustibles content of the reducing gas to fuel flow controller  44 . Fuel flow controller  44  is configured to control the amount of fuel delivered to merging chamber  32 . 
     The reducing gas output by catalytic reactor  34  is cooled using a heat exchanger  88 . In one form, heat exchanger  88  is an indirect heat exchanger. In other embodiments, other types of heat exchangers may be employed. In one form, reducing gas combustibles detection sensor  86  is positioned downstream of heat exchanger  88 . In other forms, reducing gas combustibles detection sensor  86  may positioned in other locations, for example, upstream of heat exchanger  88  or inside of or mounted on heat exchanger  88 . 
     The pressure output of catalytic reactor  34  is maintained by a backpressure regulator  90  downstream of heat exchanger  88 . Heat exchanger  88  maintains the temperature of the reducing gas downstream of catalytic reactor  34  at a suitable level to prevent damage to backpressure regulator  90 . In one form, the reducing gas is cooled to between 100° C. and 150° C. using cooling air. In other embodiments, other suitable fluids may be used as the heat sink, and other temperatures may be used. In one form, a control loop (not shown) may be used to control the temperature of the reducing gas exiting heat exchanger  88  by varying the flow of cooling air or other cooling fluid. 
     The output of reducing gas generator  14  is fluidly coupled to catalytic reactor  34 , and is in fluid communication with anode  20 , e.g., either directly or via reformer  26 . The output of backpressure regulator  90  serves as a reducing gas output in the present embodiment, and is operative to direct the reducing gas to anode  20  and reformer  26 . The “reducing gas output” is the output of reducing gas generator  14  that discharges the product of reducing gas generator  14  into fuel cell  12 , and may be one or more of any opening or passage structured to discharge the products of reducing gas generator  14 . 
     Fuel flow controller  44 , air flow controller  60  and oxidant flow controller  64  form a control system  96  that is structured to control the temperature and chemical makeup of the product mixture supplied from catalytic reactor  34  based on the signals output by oxygen sensor  66  (during startup in the present embodiment), control temperature sensor  84  and reducing gas combustibles detection sensor  86 . In particular, air control valve  58  is controlled by air flow controller  60  to regulate the O 2  content of the oxidant stream supplied to merging chamber  32 , e.g., the amount of O 2  expressed as a mole percentage of the O 2  in the oxidant stream. Oxidant control valve  62  is controlled by oxidant flow controller  64  to regulate flow of the oxidant stream formed of nitrogen-rich gas and air supplied to merging chamber  32 . Fuel control valve  46  is controlled by fuel flow controller  44  to regulate the amount of hydrocarbon fuel supplied to merging chamber  32 . 
     Thus, in the present embodiment, control system  96  is configured to control the oxygen (O 2 ) content of the oxidant stream, and to also control the oxidant/fuel ratio of the feed mixture, which is defined by a ratio of the amount of the oxidant in the feed mixture to the amount of hydrocarbon fuel in the feed mixture, e.g., the mass flow rate of the oxidant stream relative to the mass flow rate of the hydrocarbon fuel stream. In particular, the O 2  content of the oxidant stream supplied to merging chamber  32  is controlled by air control valve  58  via the output of air flow controller  60  based on the signal received from oxygen sensor  66 . In addition, the oxidant/fuel ratio of the feed mixture supplied to catalytic reactor  34  is controlled by fuel control valve  46  and oxidant control valve  62  under the direction of fuel flow controller  44  and oxidant flow controller  64 , respectively. In one form, the flow of reducing gas output by reducing gas generator  14  is controlled by oxidant control valve  62 , e.g., including an offset or other compensation to account for the amount of fuel in the feed mixture, whereas the oxidant/fuel ratio is then controlled using fuel control valve  46 . In other embodiments, other control schemes may be employed. 
     In the present embodiment, each of fuel flow controller  44 , air flow controller  60  and oxidant flow controller  64  are microprocessor-based, and execute program instructions in the form of software in order to perform the acts described herein. However, it is alternatively contemplated that each such controller and the corresponding program instructions may be in the form of any combination of software, firmware and hardware, and may reflect the output of discreet devices and/or integrated circuits, which may be co-located at a particular location or distributed across more than one location, including any digital and/or analog devices configured to achieve the same or similar results as a processor-based controller executing software or firmware based instructions, without departing from the scope of the present invention. Further, it will be understood that each of fuel flow controller  44 , air flow controller  60  and oxidant flow controller  64  may be part of a single integrated control system, e.g., a microcomputer, without departing from the scope of the present invention. 
     In any event, control system  96  is configured to execute program instructions to both vary the O 2  content of the oxidant stream and vary the oxidant/fuel ratio of the feed mixture while maintaining a selected temperature of the reducing gas in order to achieve a selected combustibles content at desired flow rate. The flow rate may be varied, e.g., depending upon the particular application or operational phase. Control system  96  varies the O 2  content of the oxidant stream and the oxidant/fuel ratio of the feed mixture based on the output of control temperature sensor  84 , oxygen sensor  66  and reducing gas combustibles detection sensor  86 . 
     Reducing gas generator  14  may be employed during startup and shutdown of fuel cell  12 , e.g., to provide reducing gas of various reducing strengths, including reducing gas in the form of a safe (non-flammable) gas, and in some embodiments, to provide a purging gas with no combustibles. 
     The reducing gas is generated by combining the nitrogen-rich stream with air supplied via air control valve  58  to form the oxidant stream, which is regulated by oxidant control valve  62  and combined with the hydrocarbon fuel supplied via fuel control valve  46  to form the feed mixture that is catalytically converted in catalytic reactor  34  into the reducing gas. As set forth herein, the O 2  content of the oxidant stream and the oxidant fuel ratio of the feed mixture are varied by control system  96  in order to both regulate the control temperature, e.g., at catalytic reactor  34 , while also controlling the reducing strength of the reducing gas to achieve the selected combustibles content at the desired flow rate. 
     The combustibles content may be selected in order to provide the appropriate reducing gas chemical configuration during various phases in the fuel cell  12  startup and shut down processes. In the present embodiment, control system  96  is structured to maintain the control temperature, e.g., the catalyst  36  temperature, while varying the combustibles content. For example, the reducing strength may be varied from weakly reducing, i.e., a low reducing strength, for purposes of forming a safe gas, to a high reducing strength having greater combustibles content. The combustibles content is primarily in the form of hydrogen (H 2 ) and carbon monoxide (CO). 
     The safe gas may be supplied to fuel cell  12  during ramp up to fuel cell  12  operating temperature. In one form, the reducing gas may be supplied to fuel cell  12  in the form of a safe gas to transition reformer  26  into service. In another form, as the operating temperature of fuel cell  12  increases, e.g., the temperature of anode  20  and reformer  26 , the strength of the reducing gas may be increased by increasing the combustibles content of the reducing gas, which may thus protect anode  20  at the higher temperatures at which a significant amount of oxidation damage may otherwise occur, e.g., due to oxygen migration through electrolyte  22  or other leakages. In addition, as anode  20  (and/or reformer  26 , in some embodiments) approaches normal operating temperatures, the combustibles content of the reducing gas may be further increased to achieve combustibles content levels similar to that of the synthesis gas that is produced by reformer  26  during normal power generation operations of fuel cell  12 , which may help initiate the normal electrical power-producing reactions of anode  20 . In embodiments where supplied to reformer  26 , this may help initiate the normal operating catalytic reactions of reformer  26 . 
     Regarding the purging gas, in some embodiments, a noncombustible purging gas may be generated by nitrogen generator  54  in the form of a nitrogen-rich stream, e.g., consisting primarily of nitrogen, which may supplied to fuel cell  12  via back pressure regulator  90 , although other plumbing schemes to direct the output of nitrogen generator  54  to fuel cell  12  may alternatively be employed. In one form, the purging gas may be supplied to fuel cell  12 , e.g., to purge one or more of cathode  24  and/or other fuel cell  12  components, e.g., when a cold start of fuel cell  12  is desired. In another form, the purging gas may be supplied to fuel cell  12  to purge fuel cell  12  before maintenance. In yet another form, nitrogen generator  54  and/or a second nitrogen generator may be employed to create a purge gas. For example, in the event of a loss of the power plant&#39;s main air supply during an emergency shut-down, a nitrogen rich cathode purge may be supplied to cathode  24  with, e.g., using nitrogen generator  54  and/or a second nitrogen generator, while nitrogen generator  54  is used to generate the reducing gas supplied to the anode  20  loop. Such embodiments may be used to ensure that “safe” non-flammable mixtures reside in the fuel cell  12  vessel. 
     Having thus described exemplary means for varying the combustibles content of the reducing gas output by catalytic reactor  34  while maintaining a constant reducing gas output temperature from catalytic reactor  34 , including means for varying the O 2  content in oxidant supplied to merging chamber  32  and means for varying the oxidant/fuel ratio of feed mixture exiting merging chamber  32 , an exemplary embodiment of a method for generating a purging gas and a reducing gas for startup and shutdown of a fuel cell is described as follows. The exemplary embodiment is described with respect to  FIGS. 3A-3D , which form a flowchart having control blocks B 100 -B 146  depicting a method for starting up and shutting down a fuel cell. Although a particular sequence of events is illustrated and described herein, it will be understood that the present invention is not so limited, and that other sequences having the same or different acts in lesser or greater numbers and in the same or different order may be employed without departing from the scope of the present invention. 
     Referring now to  FIG. 3A , at block B 100 , a command to start fuel cell  12  is received by control system  96 , e.g., via an operator of fuel cell  12 . 
     At block B 102 , a bypass system  98  is engaged. Bypass system  98  opens a vent line to vent the output of reducing gas generator  14 , and closes the flowpath to fuel cell  12 . The output of reducing gas generator is vented until the control loop, e.g., control system  96 , holds process parameters within their prescribed bounds, at which point bypass system  98  closes the vent line and opens the flowpath to fuel cell  12 . 
     At block B 104 , air is supplied to reducing gas generator  14 , e.g., via air intake  48 , by initiating operation of air compressor  50 . 
     At block B 106 , air compressor  50  compresses the air received from air intake  48 . In one form, the air is compressed to a pressure in a range from 5 bar absolute to  14  bar absolute. In other embodiments, the pressure of the compressed air may fall within a different range, for example, in a range from 2 bar absolute to 25 bar absolute in some embodiments, and in other embodiments, 1 bar absolute to 30 bar absolute. The pressure supplied by air compressor  50  may vary, for example, depending upon the characteristics of nitrogen separation membrane  56  and nitrogen generator  54 . 
     At block B 108 , the nitrogen-rich gas stream is generated in nitrogen generator  54  of reducing gas generator  14  by supplying the compressed air to nitrogen separation membrane  56 . The O 2  removed from the air by nitrogen separation membrane  56  as a byproduct of the nitrogen generation process is directed offboard, e.g., for use elsewhere, or simply vented, whereas the resulting nitrogen-rich stream is directed toward oxidant control valve  62 . In the present embodiment, the nitrogen-rich stream contains oxygen, albeit at levels lower than that of ambient air. In other embodiments, the nitrogen stream may consist essentially of nitrogen (e.g., &lt;1% O 2 ). 
     At block B 110 , compressed air is added to the nitrogen-rich stream in a controlled manner by air control valve  58  under the direction of air flow controller  60  to form a low oxygen (O 2 ) content oxidant stream, i.e., an oxidant stream having less O 2  than ambient atmospheric air. 
     At block B 112 , a flow of hydrocarbon fuel to reducing gas generator  14  is initiated by fuel control valve  46  under the direction of fuel flow controller  44 . Fuel flow may be initially set to a default value anticipated to achieve the desired combustibles content of the reducing gas and the control temperature, and may be subsequently adjusted. 
     At block B 114 , the oxidant stream is combined with the hydrocarbon fuel stream in merging chamber  32  to form the feed mixture having an oxidant/fuel ratio, e.g., defined by a ratio of the mass flow rate of the oxidant stream in the feed mixture to the mass flow rate of the hydrocarbon fuel stream in the feed mixture. 
     Referring now to  FIG. 3B , at block B 116 , heating devices are operated at a temperature at or above the catalyst light-off temperature of the feed mixture, and the heat output by the heating devices is supplied to the feed mixture. In one form, the heating devices are turned on immediately after receiving the command to start the fuel cell  12 , e.g., immediately after block B 100 . In other embodiments, the heating devices may be turned on at other times suitable to the application, e.g., depending upon how much time it takes the heaters to reach the desired temperature. In the present embodiment, the heating devices are feed mixture heater  74  and heater  80 , although in other embodiments, only one heater may be employed or a plurality of heaters may be employed in place of or in addition to one or both of feed mixture heater  74  and heater  80 . The types or forms of heaters used in other embodiments may vary with the needs of the application. 
     Heating body  76  and flow coil  78  are maintained at or above the catalyst light-off temperature of the feed mixture. The heat from heating body  76  and flow coil  78  is supplied to the feed mixture by diverting feed mixture through feed mixture heater  74 , in particular, flow coil  78 . In one form, all of the feed mixture is diverted through feed mixture heater  74 . In another form, a portion of the feed mixture is diverted through feed mixture heater  74 . The feed mixture is diverted to flow coil  78  by controlling the output of start control valve  69  to operate valve elements  70  and  72 . The resulting heated feed mixture is directed to catalyst  36  of catalytic reactor  34  to help initiate the catalytic reactions that yield reducing gas. Once the catalytic reactions in catalytic reactor  34  have been started, three-way start control valve  69  is re-oriented to direct all of the feed mixture directly to catalytic reactor  34 , bypassing feed mixture heater  74 . While the present application is described using a feed mixture heater  74  with heating body  76  and flow coil  78 , it will be understood that other types of heaters may be employed in embodiments that utilize a flow mixture heater. 
     Heater  80  of the present embodiment is in the form an electric band heater, and maintains catalyst  36  at or above the catalyst light-off temperature of the feed mixture, thereby promoting rapid lighting (hence, re-lighting) of catalyst  36 . It will be understood that other types of heaters may be employed without departing from the scope of the present invention. 
     In other embodiments, heater  82  may be employed to heat catalyst  36  at or near the location where the feed mixture is supplied to catalyst  36  in order to initiate the catalytic reactions. In various other embodiments, one or more heaters  82  may be used in place of or in addition to heaters  74  and  80 . 
     At block B 118 , the heated feed mixture is directed to catalyst  36 , where catalytic reactions are initiated. In one form, the catalytic reactions are initiated based on the heat received from feed mixture heater  74 . In various other forms, the reactions may be initiated based on heat received from feed mixture heater  74  and/or heater  80  and/or heater  82 ). 
     At block B 120 , the feed mixture is catalytically converted to reducing gas in catalytic reactor  34  of reducing gas generator  14 . 
     At block B 122 , the O 2  content of the oxidant stream and the oxidant/fuel ratio of the feed mixture are each controlled by control system  96  to maintain the selected control temperature of the reducing gas and to yield the reducing gas in the form of a safe gas. In one form, the O 2  content of the oxidant stream is controlled by air flow controller  60  directing the operations of air control valve  58 , although in other embodiments, the O 2  content of the oxidant stream may be controlled differently. In one form, the oxidant/fuel ratio is controlled by fuel flow controller  44  directing the operations of respective fuel control valve  46 , although in other embodiments, the oxidant/fuel ratio may be controlled differently. Prior to reaching the control temperature, control of the O 2  content may be based on the output of oxygen sensor  66 . Once a temperature indicating catalytic combustion is achieved, the control algorithm switches to feedback based on control temperature sensor  84 . The control temperature in some embodiments may be, for example, a function of reducing gas flow rate (catalyst load), time at service, or some other operating parameter. In other embodiments, the output of either or both of oxygen sensor  66  and control temperature sensor  84  may be employed during system startup and/or normal operation. 
     The flow rate of the feed mixture is controlled primarily by oxidant flow controller  64  directing the operations of oxidant control valve  62 . In the form of a safe gas, i.e., a weakly reducing gas mixture, the reducing gas may have a combustibles content (e.g., predominantly CO+H 2 ) of approximately 4.5%. Other reducing gases having greater or lesser percentages of combustibles content may be employed without departing from the scope of the present invention. 
     Because the mass flow of the feed mixture is based predominantly on the flow rate of the oxidant flow stream, the total flow of the feed mixture, and hence the reducing gas output by reducing gas generator  14 , is based primarily on the flow rate of the oxidant control flow stream as governed by oxidant flow controller  64 . The selected control temperature in the present embodiment is 800° C., which is measured at one of the hottest points in catalyst  36 , and which in the present embodiment yields a bulk average temperature of 770° C. The selected temperature in the present embodiment is a predetermined temperature value selected based on life considerations for components of reducing gas generator  14  and fuel cell  12 , as well as catalytic conversion efficiency. Other temperature values and measurement locations may be employed in other embodiments. 
     At block B 124 , bypass system  98  is disengaged from the bypass mode, and the reducing gas in the form of a safe gas is thus directed from reducing gas generator  14  to anode  20  of fuel cell  12 . In other embodiments, the safe gas may be directed to reformer  26 . 
     Referring now to  FIG. 3C , a block B 126  is illustrated. In one form, block B 126  is bypassed, and process flow proceeds directly to block B 128 . In another form, at block B 126  the O 2  content of the oxidant stream and the oxidant/fuel ratio of the feed mixture are controlled to selectively vary the reducing strength of the reducing gas by selectively varying the combustibles content of the reducing gas while maintaining the selected temperature of the reducing gas of block B 122 . As set forth above with respect to block B 122 , in one form, the O 2  content of the oxidant stream is controlled by air flow controller  60  directing the operations of air control valve  58 . In other forms, the O 2  content of the oxidant stream may be controlled differently. In one form, the oxidant/fuel ratio is primarily controlled by fuel flow controller  44 , and the reducing gas flow is primarily controlled by oxidant flow controller  64  directing the operations of oxidant control valve  62 . In other forms, the oxidant/fuel ratio and reducing gas flow rate may be controlled differently. 
     Control of the O 2  content of the oxidant stream and of the oxidant/fuel ratio of the feed mixture to selectively vary the reducing strength of the reducing gas while maintaining the selected temperature and flow rate of the reducing gas output by catalytic reactor  34  in the present embodiment is now described. 
     Reducing gas generator  14  catalytically converts the low O 2  content oxidant and hydrocarbon fuel to form the reducing gas with sufficient combustibles content to protect fuel cell anode  20  of fuel cell  12  during start-up and shutdown of the fuel cell system  10  power plant. By adjusting the O 2  content of the oxidant gas in combination with changing the oxidant/fuel ratio, the reducing gas strength may be changed while the catalyst operating temperature is held constant, e.g., at an ideal conversion temperature. This temperature is sensed by control temperature sensor  84  and used as input to control system  96  for use in maintaining the output temperature of catalytic reactor  34  at the selected temperature. 
     Referring now to  FIG. 4 , an example of catalytic reactor  34  parameters is depicted. The illustrated parameters include oxidant stream mass flow rate  100 ; hydrocarbon fuel stream mass flow rate  102 ; percent (%) stoichiometric air  104 , which represents the percentage amount of air in the oxidant stream relative to the amount of air required for complete combustion of the hydrocarbon fuel stream; and the oxygen/carbon ratio (O 2 /C)  106 . In the plot of  FIG. 4 , the abscissa is H 2  content of the reducing gas, the left-hand ordinate is in units of percent and also grams per second (g/s), against which % stoichiometric air  104  and oxidant stream mass flow rate  100  are plotted. The right-hand ordinate is in units of both molar fraction and g/s, against which O 2 /C ratio  106  and hydrocarbon fuel stream mass flow rate  102  are plotted. 
       FIG. 4  illustrates catalytic reactor  34  operating parameters over a reducing gas compositional range of 2% to 20% H 2  and 1% to 10% CO (3% to 30% CO+H2). To produce higher combustibles content (CO+H 2 ), the O 2  content in the oxidant is raised. At a constant oxidant/fuel ratio of the feed mixture, e.g., air to fuel ratio, raising the O 2  content in the oxidant stream reduces combustibles and raises operating temperature. However, in the present embodiment, as the O 2  content in the oxidant stream is increased, the oxidant/fuel ratio of the feed mixture is simultaneously decreased, i.e., made more fuel rich, in order to achieve higher combustibles content at the same operating temperature. 
     By varying both the O 2  content in the oxidant stream and the oxidant/fuel ratio of the feed mixture, a broad range of reducing gas strengths may be achieved at a selected catalyst operating temperature, e.g., 770° C. in the present embodiment. For example, in one form, the range may extend from a reducing gas strength that represents normal operating conditions for reformer  26  (˜45% CO+H 2 ) to weakly reducing conditions (˜3% CO+H 2 ). In other forms, different ranges may be employed, e.g., as set forth herein. 
     As 20% H 2  content in the reducing gas is approached, conditions in catalytic reactor  34  may approach that normally occurring in reformer  26  in power production mode as the oxidant approaches air with respect to % O 2  content and the O 2  to C molar ratio reaches 0.65. As the reducing gas becomes richer in combustibles, the fuel flow may increase by a factor of about 4 at 20% H 2  relative to weakly reducing conditions. The percentage of the fuel burned may decrease significantly as conditions approach those in the reformer  26 . The temperature may be sustained because the lower percentage of combustion oxygen is offset by the combination of the elevated fuel flow rate and the decreased heat dissipation through less N 2  dilution in the oxidant. Thus, even though the O 2  concentration in the oxidant increases for increased reducing strength, as a percentage of oxygen required to completely consume the fuel, the oxygen level decreases. In the present embodiment, percent CO content is about ½ of the percent of H 2  content at the desired operating temperature, and hence the combustibles content of the reducing gas is approximately 1.5 times the percent of H 2  content in the reducing gas. While described in the present application with respect to a fuel cell system, it will be understood that reducing gas generator  14  is equally applicable to other systems, such as systems for generating reducing gas for other purposes. 
     Referring again to  FIG. 3C , at block B 128 , the reducing gas is supplied to reformer  26 , and to anode  20 , e.g., via reformer  26 . 
     At block B 130 , a transition of fuel cell  12  into power production mode is initiated, which includes supplying to fuel cell  12  flows of the primary fuel and the primary oxidant that are normally provided to fuel cell  12  for operation in power production mode, in contrast to the oxidant and hydrocarbon fuel provided to reducing gas generator  14  to generate reducing gas for use during startup or shutdown of fuel cell  12 . The transition into power production mode also includes heating portions of fuel cell  12 , including anode  20  and reformer  26 , to normal operating temperature in a controlled fashion so as to reduce mechanical stresses that might result from thermal gradients within and between such components. The heating of fuel cell  12  may be performed prior to, during and after the provision of reducing gas to fuel cell  12 , and may be performed until satisfactory operating temperatures in such portions, e.g., anode  20  and reformer  26 , are achieved. During the transition into power production mode, bypass system  98  may be transitioned into bypass mode. 
     At block B 132 , fuel cell  12  is operated in power production mode, i.e., normal operating mode, to supply power to electrical load  16 . 
     At block B 134 , the airflow and fuel flow supplied to reducing gas generator  14  are terminated, ending the production of reducing gas by reducing gas generator  14 . 
     Referring now to  FIG. 3D , at block B 136 , the temperature of the heating device is maintained at or above the temperature required to initiate catalytic reaction of the feed mixture at catalyst  36 . This temperature is maintained during operation of the fuel cell in the power production mode, e.g., in order to provide for rapid restart of reducing gas generator  14 , including rapid restart of catalyst  36 , in the event of a need to shut down fuel cell  12 . 
     At block B 138 , a command to shut down fuel cell  12  from the power production mode is received by control system  96 , e.g., via a human input or an automated process. It will be noted that in some embodiments, block B 136  may be performed subsequent to receiving the command to shut down fuel cell  12 . For example, in some embodiments, the heating device may be not be heated to a temperature at or above the catalytic light-off temperature until the command to shutdown fuel cell  12  is received. 
     At block B 140 , reducing gas generator  14  generates reducing gas in response to the command, e.g., by performing some or all of the actions indicated above with respect to blocks B 102  to B 128 , including controlling the O 2  content of the oxidant stream and the oxidant/fuel ratio of the feed mixture to selectively vary the reducing strength of the reducing gas by selectively varying the combustibles content of the reducing gas to a desired level while maintaining a selected temperature, e.g., the selected temperature of block B 122 , above. 
     At block B 142 , the reducing gas generated by reducing gas generator  14  is supplied to anode  20  of fuel cell  12  by disengaging bypass system  98  from the bypass mode. This may help to prevent oxidation damage to anode  20  during shutdown of fuel cell  12 . Initially, the reducing gas may have a high reducing strength, which may be decreased as the temperature of fuel cell  12  decreases. 
     At block B 144 , a transition of fuel cell  12  out of the power production mode is initiated, including gradually reducing the flow to anode  20  of the primary fuel that is normally provided during operation in power production mode. 
     At block B 146 , the airflow and fuel flow supplied to reducing gas generator  14  are terminated, ending the production of reducing gas by reducing gas generator  14 . Block B 146  may be executed after anode  20  is sufficiently cooled to a temperature at which oxidative damage is not a concern, which may vary with the materials used to manufacture anode  20 . 
     A reducing gas generator in accordance with some embodiments of the present application may include a compressed air supply that feeds a polymer nitrogen-separation membrane, which uses the high pressure to segregate oxygen from nitrogen across a polymer fiber. Such embodiments may preclude the need for bottled nitrogen. In other embodiments, other nitrogen sources may be employed. The product gas is a nitrogen-rich stream that is depleted in oxygen. A variable-position bypass valve may divert a relatively small stream of the feed air around the nitrogen generator for blending with the nitrogen-rich stream. In some embodiments, the bypass airflow is directly proportional to the final oxygen content of the blended streams. The blended stream of nitrogen-rich product gas and bypass air may be referred to as an oxidant stream, which passes through a flow control device that sets the flow of oxidant to the process. The bypass valve controls the proportions of bypass air and nitrogen-rich gas to achieve the desired oxygen content of the oxidant stream. 
     A relatively small quantity of hydrocarbon fuel may be metered into the oxidant stream through a flow control device. In a steady state flow mode, the premixed oxidant and fuel blend is fed directly into a catalytic reactor that converts the feed mixture into the reducing gas. Compared with ordinary combustion in air, the reduced oxygen content oxidant stream may translate to less fuel per unit combustibles yield in the reducing gas. Thus, the required chemical energy input (i.e., the thermal load due to the input of fuel) per unit production of combustibles (e.g., H 2  and CO) may also be decreased, and therefore, less heat may need to be extracted from the process gas to cool the product stream to a required temperature. The nitrogen dilution of the oxidant stream may also decrease the reaction temperature into the range that may be preferable for the catalyst, and may not exceed the material limits in the downstream heat exchanger. In contrast to embodiments of the present invention, a reactor designed for combustion with normal air (in contrast to the nitrogen-rich oxidant employed in embodiments of the present invention) at the required scale might be complex, and might require cooling jackets that would likely require a liquid coolant, or otherwise a very high volumetric flow of coolant gas, and therefore, would have a relatively large heat duty in order to protect reactor materials from excessive temperature. In contrast, the catalytic reactor of some embodiments of the present invention may be designed to operate at a lower temperature without the need for external cooling. 
     Fuel oxidation with an oxygen-depleted oxidant may yield a given range of combustibles concentration (or molar flow) over a much wider range of air to fuel ratio relative to ordinary combustion with air, which makes control of the combustibles content easier to achieve. 
     Thermocouple(s) may monitor the exit temperature at the catalyst exit. The thermocouple may act as the control input for the air bypass valve. If the exit temperature were to fall too far below the set point, a control signal would open the bypass by some amount since an oxidant stream having a higher proportion of O 2  elevates the exit temperature (by oxidizing more fuel) and vice versa. The set point temperature is set high enough to achieve complete conversion of the flammable feed mixture to the equilibrated gas composition, but not too high as to approach the operational material limit temperatures for either the catalyst or the downstream heat exchanger. 
     An oxygen sensor  66  may measure the oxygen content on a volume basis of the oxidant stream downstream of the mix point for the bypass air and the nitrogen-rich stream exiting the nitrogen generator. An alternative embodiment may employ the measured oxygen concentration rather than the exit temperature to position air bypass control valve so that the exit temperature is maintained to a set point value. This may be preferable at start-up before a representative steady state reactor exit temperature is available to set the bypass valve position. 
     The oxygen sensor may be a small zirconia sensor maintained at a high temperature, e.g., around 600° C. for some embodiments, which develops a Nernst potential when exposed to oxygen, which is related to the oxygen content of the gas. The sensor can be located in-situ. However, the sensor may alternatively be submerged in a controlled small slip stream that is blown down off the main process line through a critical flow orifice. The control software may dictate the relationship between the deviation of the measured oxygen content from the targeted value, and the incremental amount the bypass valve is opened as a result. The sensor may have a rapid response to changes in the oxygen content of the process gas, and therefore, the optimized tuning parameters on the air bypass valve control loop may provide more reliable control over a broader range of conditions. 
     The downstream heat exchanger cools the reducing gas to a temperature that is required for introduction of the reducing gas into the downstream process. A temperature control loop may vary a flow of cooling air or other cooling medium to the heat exchanger based on the deviation of the catalyst exit temperature from the temperature set point of the outlet gas. The heat exchanger may be a compact alloy steel or ceramic design to withstand the temperature of the gas exiting the catalyst. 
     A hydrogen or combustibles sensor may extract a slipstream of the process gas downstream of the heat exchanger to measure the percent by volume hydrogen or combustibles as a constituent of the reducing gas. The control software may compare the measured % H 2  to a set point value, and based on the difference sends a control signal to fuel control valve. If the measured % H 2  deviates too far below the set point, the fuel feed would be increased, and vice versa. The control software may dictate the relationship between the deviation of the measured % H 2  with the targeted % H 2 , and the incremental amount the fuel valve is opened or closed. 
     One approach for continuously measuring hydrogen uses a thermal conductivity hydrogen sensor calibrated over the permissible range of hydrogen content for the reducing gas. Similar to the oxygen sensor, a critical flow orifice may be used as a relatively inexpensive and simple way to meter a very small slipstream of the reducing gas at the correct sample gas flow to the sensor. 
     A method for rapid restart of the catalyst from a standby condition to bring the reducing gas generator back on-line as quickly as possible for unforeseen events within the fuel cell system that will require an immediate supply of safe reducing gas may also be provided by embodiments of the present invention. A rapid restart capability may avoid the need for a bottled storage of reducing-gas necessary to bridge the gap between the time that the gas is demanded and the time required to bring the reducing gas generator on-line. A rapid restart method may employ a heater with a high thermal mass located just upstream of the catalyst reactor and, e.g., a pair of valves or a three-way valve for diverting feed mixture flow through the heater. During normal operation the valve directs the mixture directly into the catalytic reactor, bypassing the heater. At start-up, flow may be diverted through the heater. In the absence of flow, e.g., under idle conditions of the reducing gas generator, the heater is continuously supplied sufficient power to sustain the metal at the desired preheat temperature while balancing a relatively small heat loss, and thus, this power demand may be small. Within the heater, a flow coil may be engulfed with a metallic body. The heater may contain sufficient thermal mass so that when flow is initiated upon a re-start attempt, the process stream immediately acquires the targeted ignition temperature. 
     Such a design may be relatively safe because it may achieve good electrical isolation between the flammable mixture and the power supply that acts on the metallic body. Prior to a re-start sequence, the heater regulates power to the internal metal to the required temperature prior to the introduction of flow, and must only maintain power to offset heat loss through the surrounding insulation at this condition. 
     On a start-up attempt, power may be immediately ramped up to sustain or elevate the set-point preheat temperature until reaction of the catalyst feed mixture is achieved. Once this is achieved, e.g., as indicated by a sufficient rise in temperature at the catalyst exit, the flow may be diverted around the ignition heater directly into the catalyst (normal operating flow mode) to prevent overheating of the catalyst. 
     To further promote rapid re-start, band heaters may provide an additional heat source. The band heaters may surround the catalyst reactor to hold the catalyst at or above the catalyst light-off temperature before flow is initiated at start-up. Prior to start-up, the band heaters would preferably provide the energy to offset heat loss through the insulation surrounding the band heaters. Once the catalyst is lit, the band heaters may turn off as the skin temperature rises above the set point temperature of the heaters. Power to the heater may be either turned off or turned down to sustain the heater&#39;s thermal mass at the temperature set point for the next restart. 
     Other alternative embodiments would simplify the heat-up scheme by employing a closely coupled heater at the catalyst inlet. This approach may use a low thermal mass heater that would locally initiate reaction near the front side of the catalyst by close thermal coupling, which in such embodiments may potentially reduce the reducing gas generator&#39;s part count and cost. 
     In an additional embodiment, the reducing gas generator may replace the internal reformer for the fuel cell system for those embodiments where the reducing gas generator is structured to produce a reducing gas that is suitable for power production in the fuel cell system. In some such embodiments, the reduced gas generator may be used for producing a reducing gas of one composition for startup and shutdown of the fuel cell system, and for producing a reducing gas of an alternate composition for the normal operation of the fuel cell system. 
     Referring to  FIGS. 5A and 5B , some aspects of non-limiting examples of a reducing gas generator  214  in accordance with embodiments of the present invention are schematically depicted. In the embodiments depicted in  FIGS. 5A and 5B , various features, components and interrelationships therebetween of aspects of embodiments of the present invention are depicted. However, the present invention is not limited to the particular embodiments of  FIGS. 5A and 5B  and the components, features and interrelationships therebetween as are illustrated in  FIGS. 5A and 5B  and described herein. For example, other embodiments encompassed by the present invention, the present invention being manifested by the principles explicitly and implicitly described herein via the present Figures and Detailed Description and set forth in the Claims, may include a greater or lesser number of components, features and/or interrelationships therebetween, and/or may employ different components and/or features having the same and/or different nature and/or interrelationships therebetween, which may be employed for performing similar and/or different functions relative to those illustrated in  FIGS. 5A and 5B  and described herein. 
     In some reducing gas generator embodiments, it is desirable to increase the flammables content (concentration) of the reducing gas, which may also be referred to as a reformed fuel, than that afforded by some previously described embodiments. The flammables (also referred to as combustibles) content in the reformed gas varies with the oxygen (O 2 ) content (concentration) present in the oxidant supplied with the hydrocarbon fuel to the reformer. For example, some previously described embodiments employed air control valve  58  to variably add air to the nitrogen-rich gas received from nitrogen generator  54  to yield an oxidant having a variable oxygen content ranging from, for example and without limitation, 5% to approximately 21% by volume. In such embodiments, the flammables content of the reformed gas discharged by catalytic reactor  34 , which is a reducing gas, varies with the amount of oxygen provided in the oxidant. The inventor has determined that an oxygen-enriched oxidant having a greater oxygen content than air may be employed to yield a higher flammability content in the reformed gas exiting catalytic reactor  34  than that achieved by using air or nitrogen-enriched air having a lower oxygen content than air as the oxidant. 
     Accordingly, in some embodiments,  214  reducing gas generator includes an oxidant system  230  configured to provide an oxidant with an oxygen content greater than that of ambient atmospheric air. In one form, oxidant system is configured to provide the oxidant without the use of stored oxygen, e.g., bottled oxygen or other forms of compressed or liquefied oxygen. Reducing gas generator  214  is configured to provide or discharge a reducing gas  215  having an expanded range of flammables content relative to the reducing gas provided by reducing gas generator  14 , based on using the oxidant discharged by oxidant system  230 . Reducing gas  215  may be supplied, in various embodiments, to other systems, such as piston engines, gas turbine engines, fuel cell systems and/or other systems that employ reducing gas. In some embodiments, oxidant system  230  is configured to provide an oxidant with the oxygen content at a selected value in a range having a maximum value that exceeds the oxygen content of air, e.g., in the range of approximately 21% to 40% oxygen by volume in some embodiments, and approximately 21% to 50% oxygen by volume or greater in other embodiments. In some embodiments, oxidant system  230  is configured to provide a variable oxygen content in the oxidant in a range having a maximum value that exceeds the oxygen content of air, e.g., in the range of approximately 21% to 40% oxygen by volume in some embodiments, and approximately 21% to 50% oxygen by volume or greater in other embodiments. In some embodiments, oxidant system  230  is configured to vary the oxygen content in a range extending from below the oxygen content of ambient atmospheric air to an oxygen content above that of ambient atmospheric air e.g., in the range of approximately 5% to 40% oxygen by volume in some embodiments, and approximately 5% to 50% oxygen by volume or greater in other embodiments or lesser in still other embodiments. In some embodiments, oxidant system  230  is used in place of oxidant system  30  in reducing gas generator  14  to yield a reducing gas generator  214  configured to discharge a reducing gas having a higher flammables content than reducing gas generator  14 . Oxidant system  230  has many of the same components described above with respect to oxidant system  30 , which perform the same or similar functions as those described above with respect to oxidant system  30  and reducing gas generator  14 . 
     In one form, reducing gas generator  214  employs the same components to perform the same or a similar function as that described above with respect to reducing gas generator  14 , most of which are not illustrated in  FIG. 5  for purposes of clarity, except that oxidant system  30  is replaced with an oxidant system  230 . In other embodiments, reducing gas generator  214  may include only one or more of the components described above with respect to reducing gas generator  14  and/or may include components not described above with respect to reducing gas generator  14 . In some embodiments, any of the same components as described above with respect to gas generator  14  may provide the same and/or a different function in reducing gas generator  214 . 
     Although the component identified with element number  34  has been referred to as a “catalytic reactor,” it will be understood by those having ordinary skill in the art that catalytic reactor  34  is one form of a reformer. Hence, catalytic reactor  34  may also be referred to as “reformer  34 .” It will also be understood by those having ordinary skill in the art that one or more other reformer types may be employed in addition to or in place of a catalytic reactor in some embodiments of the present invention. 
     In one form, oxidant system  230  includes an air intake  48  (which in various may or may not be pressurized, e.g., may or may not be provided with pressurized air); a compressor  50 ; a valve  52 , e.g., a pressure regulator; a nitrogen generator or separator  54  having a nitrogen separation membrane  56 , a valve  58 , for example and without limitation, a gas flow control valve; a merge chamber  232 ; a controller  60 , for example and without limitation, a gas flow controller; a valve  62 , for example and without limitation, an oxidant flow control valve; a controller  64 , for example and without limitation, an oxidant flow controller; and an oxygen sensor  66 . The output of oxidant system  230  is discharged to merge chamber  32 . In one form, each of merge chamber  32 , air intake  48 , compressor  50 , valve  52 , nitrogen generator or separator  54  with nitrogen separation membrane  56 , controller  60 , valve  62 , controller  64  and oxygen sensor  66  are each same or similar and configured to perform the same or similar function as set forth above with respect to oxidant system  30  and reducing gas generator  14 , and hence are described using the same reference characters (element numbers). In other embodiments, oxidant system  230  may include only one or more of the components described above with respect to oxidant system  30  and/or one or more of such components may perform a different function; and/or oxidant system  230  may include components not described above with respect to oxidant system  30 . For example, in some embodiments, valves  52  and  62 , and controller  64  may be replaced by a flow sensor that controls the speed of compressor  50 . It will be understood that in some embodiments, other types of nitrogen extraction systems may be employed in addition to or in place of nitrogen separation membrane  56 . Oxidant system  230  also includes a valve  234 , for example and without limitation, a back-pressure regulating valve, although other valve types may be employed in other embodiments of the present invention. 
     Compressor  50  is in fluid communication with air intake  48 . Valve  52  is in fluid communication with compressor  50  and nitrogen separator  54  on the high pressure side  236  of nitrogen separation membrane  56  (as in reducing gas generator  14 ), and is configured to control the air flow delivered to nitrogen separator  54 . Nitrogen separation membrane  56  configured to extract nitrogen from the air supplied thereto, and to discharge the balance of the air supplied as an oxygen-rich gas having a greater oxygen content than ambient atmospheric air, wherein the oxygen-rich gas forms at least a part of the oxidant discharged by oxidant system  230 . Hence, nitrogen generator  54  is also configured extract oxygen from air in the form of an oxygen-rich gas, and to discharge an oxygen-rich gas with the extracted oxygen to form at least a part of the oxidant. Nitrogen generator  54  is also configured to discharge a nitrogen-rich gas, the nitrogen-rich gas having a nitrogen content greater than that of ambient atmospheric air, e.g., in terms of percentage by volume. 
     Valve  58  is coupled to a merge chamber  232 , which has structural attributes similar to those described above with respect to merge chamber  32 . Merging chamber  232  is also in fluid communication with nitrogen separator  54  on the low pressure side  238  of nitrogen separation membrane  56 , which provides an oxygen-rich gas, e.g., oxygen-enriched air. 
     Merging chamber  32  is configured to receive the hydrocarbon fuel and the oxidant discharged from oxidant system  230 , and to discharge a feed stream containing both the hydrocarbon fuel and the oxidant. Controller  60  is operably coupled to valve  58  and configured to operate valve  58 . Valve  62  is in fluid communication with merge chamber  32  and configured to discharge an oxidant (stream) to merge chamber  32 . Controller  64  is operably coupled to valve  62  and configured to operate valve  62 . Oxygen sensor  66  is configured to sense the oxygen content of the oxidant discharged from valve  62 . 
     Valve  234  is in fluid communication with nitrogen separator  54  on the high pressure side  236 , and with valve  58 . Excess nitrogen-rich gas is vented, e.g., to atmosphere or a component or system requiring nitrogen rich gas. Valve  234  is determines much excess nitrogen-rich gas is vented from oxidant system  230 . In one form, valve  234  regulates back pressure against the high pressure side  236  of nitrogen separator  54 , and against valve  58 . In one form, the amount of excess nitrogen-rich gas that is vented increases with increasing oxygen content in the oxidant discharged by oxidant system  230 . The back-pressure maintained by valve  234  determines, at least in part, how much oxygen-rich gas is discharged by low pressure side  238  of nitrogen separator  54 . 
     Valve  58  is configured to control the amount of nitrogen-rich gas from nitrogen separator  54  that is supplied to merge chamber  232 . In one form, the output of low pressure side  236  of nitrogen separator  54  is supplied directly to merging chamber  232  for combining the oxygen-rich gas from low pressure side  236  of nitrogen separator  54  with the nitrogen-rich gas supplied by high pressure side  236  of nitrogen separator  54  to yield an oxidant (stream). Valve  62  and controller  64  are configured to control how much oxidant is supplied to merge chamber  32  for combining with a gaseous hydrocarbon fuel, such as natural gas or compressed natural gas (CNG), for use in reformer  34 . Reformer  34  is in fluid communication with merging chamber  32 , and is configured to receive the feed stream from merging chamber  32 , to reform the feed mixture into a reducing gas, and to discharge the reducing gas. 
     Low pressure side  238  of nitrogen separator  54  is configured to discharge the oxygen-rich gas with an oxygen content greater than ambient atmospheric, for example and without limitation, up to 40% oxygen content by volume in some embodiments, and up to 50% or more oxygen content by volume in other embodiments. By mixing the oxygen-rich gas with nitrogen rich gas, the resultant oxygen content of the oxidant discharged by oxidant system  230  may be reduced, e.g., from a maximum value. Hence, the oxidant discharged by oxidant system  230  of oxidant system may have a maximum value for oxygen content greater than that of air, up to 40% oxygen content by volume in some embodiments, and up to 50% or more oxygen content by volume in other embodiments. 
     In some embodiments, a lower oxygen content may also be obtained, e.g., down to 5% or less oxygen by volume. Referring to  FIG. 5B , in some embodiments, as set forth above, oxidant system  230  may be configured to provide an oxidant having an oxygen content less than that of ambient atmospheric air, e.g., to 5% or less, for example, by including some additional aspects of oxidant system  30 . For example, in some embodiments, oxidant system  230  may also include a second instance of valve  58  and controller  60 , referred to herein as valve  258  and controller  260 , in fluid communication between the discharge of valve  52  and merging chamber  232 . Controller  260  is coupled to oxygen sensor  66 , and is configured to operate valve  260  to control a flow of pressurized air from compressor  50  and valve  52  to merging chamber  232 . In addition, such embodiments of oxidant system  230  may include a valve  201 , for example and without limitation, a shutoff valve; a valve  203 , for example and without limitation, a bypass valve; and a valve  205 , for example and without limitation, a three-way valve. In order to output an oxidant having an oxygen content approximately 21% or less by volume, valve  201  is closed to prevent the venting of nitrogen-rich gas from high pressure side  236  of nitrogen separator  54 . In addition, valve  203  is opened, and valve  58  is closed, thereby shunting the output of high pressure side  236  of nitrogen separator  54  (nitrogen-rich gas) directly to merging chamber  232 . Also, valve  205  is switched vent the output of low pressure side  238  of nitrogen separator  54 , e.g., to atmosphere or an application that employs an oxygen-rich gas. In order to output an oxidant having an oxygen content approximately 21% or greater by volume, valve  201  is opened to allow the venting of nitrogen-rich gas from high pressure side  236  of nitrogen separator  54  via a valve  234 . In addition, valve  203  is closed, and valve  58  is opened, thereby directing the output of high pressure side  236  of nitrogen separator  54  (other than that which is vented) through valve  58  to merging chamber  232 . Also, valve  205  is switched supply the output of low pressure side  238  of nitrogen separator  54  to merging chamber  232 . 
     In some embodiments, one or more of compressor  50 , and valves  52 ,  234 ,  58  and  62  may be adjusted or controlled, manually or automatically, to provide an oxidant having an oxygen content selectable from, for example and without limitation, the range of approximately 21% to 40% oxygen by volume in some embodiments, and approximately 21% to 50% oxygen by volume or greater in other embodiments. In some embodiments, one or more of compressor  50 , and valves  52 ,  234 ,  58  and  62 , as well as valves,  201 ,  203 ,  205 ,  258  and  260  may be adjusted or controlled, manually or automatically, to provide an oxidant having an oxygen content selectable from the range of, for example and without limitation, the range of approximately 5% to 40% oxygen by volume in some embodiments, and approximately 5% to 50% oxygen by volume or greater in other embodiments. In other embodiments, one or more of compressor  50 , and valves  52 ,  234 ,  58  and  62 , and in some embodiments, one or more of valves,  201 ,  203 ,  205 ,  258  and  260  as well, may be adjusted or controlled, manually or automatically to provide a variable oxygen content in the oxidant supplied by oxidant system  230 , i.e., that varies within a range, “on the fly,” e.g., to meet some demand, such as a desired flammables content of the reducing gas discharged by reducing gas generator  214 . In various embodiments, the range may be, for example and without limitation, approximately 21% to 40% oxygen by volume in some embodiments, and approximately 21% to 50% oxygen by volume or greater in other embodiments, or may be from approximately 5% to 40% oxygen by volume in some embodiments, and approximately 5% to 50% oxygen by volume or greater in other embodiments. In other embodiments, other suitable ranges may be selected. 
     The reducing gas exiting reformer  34  includes flammables, including primarily hydrogen (H 2 ) and carbon monoxide (CO), and some methane slip, e.g., on the order of approximately 1%, and trace amounts of higher hydrocarbon slip, such as ethane. The reducing gas also includes also contains other gases, e.g., including nitrogen, carbon dioxide (CO 2 ) and water vapor (steam). 
     Referring to  FIG. 6 , a non-limiting example of a plot  106  of percent flammables output by a reformer, such as reformer  34 , vs. percent oxygen in the oxidant supplied to the reformer, at constant methane conversion, i.e., at a constant percentage of methane in the reducing gas discharged by reformer  34 , is depicted. The plot of  FIG. 6  is based on thermodynamic equilibrium process simulation calculations. From the plot of  FIG. 6 , it is seen that the flammables content (percent flammables) of the reducing gas increases with increasing oxygen in the oxidant supplied to as part of the feed stream provided to reformer  34 . The oxygen/carbon ratio in the plot of  FIG. 6  is varies between approximately 0.6 (e.g., at 50% oxygen by volume) to 0.7 (e.g., at 21% oxygen by volume). The flammables content of  FIG. 6  varies from approximately 45% by volume at approximately 21% oxygen content by volume in the oxidant to approximately 80% by volume at 50% oxygen content by volume in the oxidant. 
     By providing an oxidant having a greater oxygen content than that of ambient atmospheric air, the amount of flammables in the reducing gas discharged by reformer  34  may be greater than that capable of being generated using an oxygen content equivalent to that of air. In addition, by varying the oxygen content, e.g., in one or more of the ranges set forth above, the flammables content of the reducing gas  215  discharged by reducing gas generator may be varied over a substantial range. For example and without limitation, in some embodiments, approximately 45% to 70% flammables content by volume, in other embodiments, approximately 45% to 80% flammables content by volume; in yet other embodiments, approximately near 0% to 70% flammables content by volume; and in still other embodiments, in yet other embodiments, approximately near 0% to 80% flammables content by volume. 
     In some embodiments, the reducing gas is generated by generating an oxidant with oxidant system  230  having an oxygen content greater than that of ambient atmospheric air, forming a feed stream with the oxidant and a hydrocarbon fuel; and reforming the feed stream, e.g., in reformer  34 , e.g., by directing the feed stream to catalyst  36 ; and catalytically converting the feed stream into a reducing gas. In some embodiments, the oxygen content of the oxidant may be varied or selected within a range, e.g., as set forth above. In one form, the generating of the oxidant includes supplying pressurized air to nitrogen separation membrane  56 ; extracting an oxygen-rich gas using nitrogen separation membrane  56 ; and forming the oxidant at least in part using the oxygen-rich gas. In some embodiments, the oxidant may be provided having a selectable oxygen content in the range of approximately 21% to 40% 21% to 40% oxygen by volume, and approximately 21% to 50% oxygen by volume or greater in other embodiments. In some embodiments, the oxidant may be provided having a selectable oxygen content in the range of approximately 5% to 40% oxygen by volume in some embodiments, and approximately 5% to 50% oxygen by volume or greater in other embodiments. 
     In some embodiments, the reducing gas may be generated by using oxidant system  230  to generate an oxidant having a selectable oxygen content, wherein a maximum oxygen content of the oxidant exceeds that of ambient atmospheric air; using reformer  34  to reform a hydrocarbon fuel with the oxidant to produce reducing gas  215 ; and discharging reducing gas  215  from reformer  34 . In some embodiments, the oxidant may also be generated to have an oxygen content less than that of ambient atmospheric air. 
     Embodiments of the present invention include a reducing gas generator, comprising: an oxidant system configured to generate from air an oxidant having a variable oxygen content, and configured to provide an oxygen content of the oxidant at a selected value in a range from the oxygen content of ambient atmospheric air to greater than that of ambient atmospheric air; a merging chamber in fluid communication with the oxidant system and a source of a hydrocarbon fuel, wherein the merging chamber is configured to receive the hydrocarbon fuel and the oxidant and to discharge a feed stream containing both the hydrocarbon fuel and the oxidant; and a reformer in fluid communication with the merging chamber, wherein the reformer is configured to receive the feed stream from the merging chamber, to reform the feed stream into a reducing gas, and to discharge the reducing gas. 
     In a refinement, the oxidant system includes a nitrogen separator having a nitrogen separation membrane configured to extract nitrogen from air supplied thereto, and to discharge the balance of the air supplied as an oxygen-rich gas, wherein the oxygen-rich gas forms at least a part of the oxidant. 
     In another refinement, the oxygen-rich gas has a higher oxygen content than ambient atmospheric air. 
     In yet another refinement, the oxygen-rich gas has an oxygen content in the range of approximately 21% to 50% by volume. 
     In still another refinement, the nitrogen separator is also configured to discharge a nitrogen-rich gas, the nitrogen-rich gas having a nitrogen content greater than that of ambient atmospheric air. 
     In yet still another refinement, the reducing gas generator further comprises at least one valve configured to combine the nitrogen-rich gas with the oxygen-rich gas to form the oxidant. 
     In a further refinement, the reducing gas generator is configured to generate a reducing gas having a flammables content in the range of approximately 0% to 80% by volume. 
     In a yet further refinement, the reformer is a catalytic reactor. 
     In a still further refinement, the reducing gas generator is configured to generate a reducing gas having a flammables content in the range of approximately 0% to 80% by volume. 
     Embodiments of the present invention include a reducing gas generator, comprising: an oxidant system configured to provide an oxidant, and configured to provide an oxygen content of the oxidant having a value that exceeds the oxygen content of ambient atmospheric air, wherein the oxidant system is configured to provide the oxidant without the use of stored oxygen; and a reformer configured to receive the oxidant from the oxidant source, to receive a hydrocarbon fuel, to reform the oxidant and fuel into a reducing gas, and to discharge the reducing gas. 
     In a refinement, the oxidant system is configured generate the oxidant from ambient atmospheric air. 
     In another refinement, the oxidant system is configured to provide a variable oxygen content in the oxidant in a range having a maximum value that exceeds the oxygen content of air. 
     In yet another refinement, the oxidant system is configured to provide a selectable oxygen content of the oxidant in a range of approximately 21% to 50% by volume. 
     In still another refinement, the oxidant system is configured to provide a selectable oxygen content in the oxidant in a range of approximately 5% to 50% by volume. 
     In yet still another refinement, the oxidant system includes a nitrogen generator having a nitrogen separation membrane operable to extract nitrogen from air, and wherein the nitrogen generator is configured to discharge the balance of the air supplied thereto as an oxygen-rich gas, wherein the oxygen-rich gas forms at least a part of the oxidant. 
     In a further refinement, the nitrogen generator is also configured to discharge a nitrogen-rich gas, the nitrogen-rich gas having a nitrogen content greater than that of ambient atmospheric air. 
     In a yet further refinement, the reducing gas generator further comprises at least one valve configured to mix the nitrogen-rich gas with the oxygen-rich gas to form the oxidant. 
     Embodiments of the present invention include a method of generating a reducing gas, comprising: generating an oxidant having an oxygen content greater than that of ambient atmospheric air without the use of stored oxygen; forming a feed stream with the oxidant and a hydrocarbon fuel; and reforming the feed stream. 
     In a refinement, the method further comprises varying the oxygen content of the oxidant. 
     In another refinement, the reforming of the feed stream includes directing the feed stream to a catalyst; and catalytically converting the feed stream into a reducing gas. 
     In yet another refinement, the generating of the oxidant includes supplying pressurized air to a nitrogen separation membrane; extracting an oxygen-rich gas using the nitrogen separation membrane; and forming the oxidant at least in part of the oxygen-rich gas. 
     In still another refinement, the generating of the oxidant includes providing a selectable oxygen content of the oxidant in a range of approximately 21% to 50% oxygen by volume. 
     In yet still another refinement, the generating of the oxidant includes generating the oxidant with the oxygen content of the oxidant being in a range of approximately 5% to 50% oxygen by volume. 
     Embodiments of the present invention include a method of generating a reducing gas, comprising: generating an oxidant having a selectable oxygen content, wherein a maximum oxygen content of the oxidant exceeds that of ambient atmospheric air, wherein the generating is performed without the use of stored oxygen; reforming a hydrocarbon fuel with the oxidant to produce a reducing gas; and discharging the reducing gas from a reformer. 
     In a refinement, the generating of the oxidant includes generating the oxidant with an oxygen content being less than that of ambient atmospheric air. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment(s), but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as permitted under the law. Furthermore it should be understood that while the use of the word preferable, preferably, or preferred in the description above indicates that feature so described may be more desirable, it nonetheless may not be necessary and any embodiment lacking the same may be contemplated as within the scope of the invention, that scope being defined by the claims that follow. In reading the claims it is intended that when words such as “a,” “an,” “at least one” and “at least a portion” are used, there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. Further, when the language “at least a portion” and/or “a portion” is used the item may include a portion and/or the entire item unless specifically stated to the contrary.