Abstract:
A fuel processor system capable of circulating fuel processor system gases, such as reformate, anode exhaust, and/or combustor exhaust, through the fuel processor to provide a number of distinct advantages. The fuel processor system having a plurality of fuel cells discharging an H 2 -containing anode effluent and an O 2 -containing cathode effluent. A fuel processor is also provided for converting a hydrogen-containing fuel to H 2 -containing reformate for fueling the plurality of fuel cells. A catalytic combustor is positioned in series downstream from the plurality of fuel cells and a vaporizer reactor is coupled to the catalytic combustor. A bypass passage is finally provided that interconnects an outlet of at least one of the group consisting of the fuel processor, the plurality of fuel cells, the catalytic combustor, and the vaporizer reactor to the inlet of the fuel processor. The bypass passage is operable to circulate a fuel processor system gas to the inlet of the fuel processor.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention generally relates to fuel processors and, more particularly, relates to a fuel processor system having gas recirculation for improved startup, shut down, turn down, and transient operation.  
         BACKGROUND OF THE INVENTION  
         [0002]    As is well known to those skilled in the art, in order to heat rapidly the mass of a fuel processor to its proper operating temperature during a startup cycle, it is preferable to provide the largest possible heating gas flow therethrough. However, using fuel rich-combustion gas flow may exceed the temperature limits in the earlier stages of the fuel processor, thereby requiring additional stages to fully heat the remaining stages of the fuel processor.  
           [0003]    During a shut down cycle, it is desirable to remove water from the fuel processor so that the water does not condense onto the catalysts when the fuel processor completely cools, which may damage the catalysts. Furthermore, it is also desirable to stop the fuel processor in a pressurized state so that when the fuel processor cools and the gases contract, the pressure the fuel processor remains above atmospheric pressure so that air is not drawn into the fuel processor. Conventional shut down methods cannot continue operating without water injection, as the ATR catalyst would get too hot.  
           [0004]    During a turn down cycle, it is preferable to circulate a larger flow so that the residence times within the reactors are more constant. However, in conventional fuel processors, as the power level is turned down the flow is thus reduced and the residence times in each reactor increases. This increase in residence times may lead to auto-ignition in the inlet, reverse water gas shift in the PrOx, cell reversal in fuel cell stack due to non-uniform flow distribution of hydrogen containing reformate, and water collection in fuel cell stack.  
           [0005]    During a transient cycle, it is preferable to have a constant flow through the reactors such that the pressure in the reactors remains generally constant, thereby minimizing the lag in transient response associated with filling or venting volumes of the fuel processor.  
           [0006]    Accordingly, there exists a need in the relevant art to provide a fuel processor that is capable of rapid thermal start without the complexity of multiple stages or risk of oxygen exposure. Furthermore, there exists a need in the relevant art to provide a fuel processor that, during shut down, is capable of minimizing water in the reformate and be shut down at an elevated pressure to minimize condensation on the catalyst and air ingestion upon cooling. Still further, there exists a need in the relevant art to provide a fuel processor that, during turn down, is capable of minimizing auto-ignition in the inlet, reverse water gas shift in the PrOx, cell reversal in fuel cell stack due to non-uniform flow distribution of hydrogen containing reformate, and water collection in fuel cell stack. Yet further, there exists a need in the relevant art to provide a fuel processor that, during transient operation, is capable of maintaining a generally constant flow rate through to the fuel processor to minimize the lag time associated with filling or venting volumes of the fuel processor. Still further, there exists a need in the relevant art to provide a fuel processor that is capable of operating without water injection.  
         SUMMARY OF THE INVENTION  
         [0007]    A fuel processor system capable of recirculating fuel processor system gases, such as reformate, anode exhaust, and/or combustor exhaust, through the fuel processor to provide a number of distinct advantages is provided. A fuel processor is also provided for converting a hydrogen-containing fuel to H 2 -containing reformate. The fuel processor system may also include a plurality of fuel cells discharging an H 2 -containing anode effluent and an O 2 -containing cathode effluent. A catalytic combustor is positioned in series downstream from the plurality of fuel cells and a vaporizer reactor is coupled to the catalytic combustor. A bypass passage interconnects an outlet of at least one of the group consisting of the fuel processor, the fuel cell, the catalytic combustor, and the vaporizer reactor to the inlet of the fuel processor. The bypass passage is operable to recirculate a fuel processor system gas to the inlet of the fuel processor.  
           [0008]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a schematic view illustrating a fuel processor system according to a first embodiment of the present invention;  
         [0011]    [0011]FIG. 2 is a schematic view illustrating a fuel processor system according to a second embodiment of the present invention;  
         [0012]    [0012]FIG. 3 is a schematic view illustrating a fuel processor system according to a third embodiment of the present invention; and  
         [0013]    [0013]FIG. 4 is a schematic view illustrating a fuel processor system according to a fourth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For example, the present invention is hereafter described in the context of a fuel cell fueled by reformed gasoline. However, it is to be understood that the principles embodied herein are equally applicable to fuel cells fueled by other reformable fuels. Furthermore, the present invention hereafter described in the context of a self contained fuel cell system having a reforming system and a fuel cell system. However, it is to be understood that the principles embodied herein are equally applicable to a reforming system only.  
         [0015]    Referring to FIG. 1, a fuel processor system, generally indicated as  10 , according to a first embodiment of the present invention is illustrated, which provides rapid startup capabilities. Fuel processor system  10  generally includes a fuel processor  12 , a fuel cell stack  14 , a catalytic combustor reactor  16 , and a vaporizer reactor  18 . Fuel processor  12  would typically include a primary reactor  12 . 2  such as a steam reformer or an autothermal reformer, a water gas shift (WGS) reactor  12 . 4  and a preferential oxidation (PrOx) reactor  12 . 6 .  
         [0016]    Fuel processor system  10  is arranged such that a first fuel inlet stream  20  and a first water inlet stream  22  are introduced into fuel processor  12  to produce a reformate stream  24  according to conventional principles. During a startup cycle, an anode bypass valve  26  directs reformate stream  24  to an anode bypass passage  28 . It is necessary to initially bypass fuel cell stack  14  until “stack grade” (having CO content less than about 100 ppm) reformate is produced. In order to produce such stack grade reformate, it is necessary to heat the various components of fuel processor system  10  to their respective operating temperatures. Recirculated reformate in passage  30  from anode bypass passage  28  is drawn into a recirculation compressor  32  together with a first inlet air stream  34 .  
         [0017]    First fuel inlet stream  20  is then introduced into fuel processor  12 . Reactions may be initiated in fuel processor  12  via a spark lit burner or by an electrically heated catalyst section (not shown). Heat produced by the reaction of first fuel inlet stream  20  and first inlet air stream  34  warms fuel processor  12 . First fuel inlet stream  20  and first inlet air stream  34  are introduced in proportions slightly rich of stoichiometric. This ensures that there is no excess oxygen, which could damage the catalysts within fuel processor  12 . Ordinarily, reactions near stoichiometric conditions produce damagingly high temperatures; however, with a large excess of recirculated reformate  30  acting as a diluent, the gas temperature within fuel processor  12  is maintained at an appropriate level.  
         [0018]    A portion, generally indicated at  36 , of the flow through anode bypass passage  28  is exhausted to catalytic combustor reactor  16 . Under steady flow, this exhausted reformate  36  is equal to the total mass flow of first fuel inlet stream  20 , first inlet air stream  34 , first water inlet stream  22  and vaporizer steam  38  that passes through fuel processor  12 . This exhausted reformate  36  is reacted with a second inlet air stream  40  in catalytic combustor reactor  16 . Second inlet air stream  40  is directed to catalytic combustor reactor  16  via a stack air compressor  42 , a cathode bypass valve  44 , a cathode bypass passage  46 , and an exhaust passage  48 . Second inlet air stream  40  is bypassed around fuel cell stack  14  during startup to prevent drying of the membranes within fuel cell stack  14 . Heat from the reaction in catalytic combustor reactor  16  is integrated back into fuel processor  12  by vaporizing second water inlet stream  50  in vaporizer reactor  18  to produce vaporizer steam  38 , which typically is delivered to the PrOx-vaporizer or steam lines within fuel processor  12 . Exhaust gases from combustor  16  exits vaporizer reactor  18  through exhaust outlet  66 .  
         [0019]    During the startup cycle, the fuel and air are completely consumed (stoichiometric conditions) for maximum heat release within fuel processor system  12  for rapid heating without excessively high temperatures. However, it is important to note that the temperature within the PrOx  12 . 6  may initially be relatively high at about 357° C. However, once the PrOx is heated, normal operation is such that cooling of the PrOx according to conventional methods can be used.  
         [0020]    Referring again to FIG. 1, once the various reactors within fuel processor  12  are warmed to their operating temperature, anode bypass valve  26  routes reformate stream  24  to fuel cell stack  14  via passage  52 . Second inlet air stream  40  is then directed by cathode bypass valve  44  to the cathode side of fuel cell stack  14  via passage  54 . The hydrogen from reformate stream  24  reacts with the oxygen from second air inlet stream  40  across a membrane electrode assembly within fuel cell stack  14  to produce electricity. Anode exhaust or stack effluent  56  from the anode side of fuel cell stack  14  includes a portion of hydrogen that is directed back to catalytic combustor reactor  16  to provide heat recovered in vaporizer  18 . Cathode exhaust  58  from the cathode side of fuel cell stack  14  includes oxygen also for use in catalytic combustor reactor  16 . Anode exhaust  56  and cathode exhaust  58  are combined in exhaust passage  48  and react in catalytic combustor reactor  16 . Vaporizer reactor  18  continues to provide vaporizer steam  38  to fuel processor  12 . Note that the PrOx air, within fuel processor  12 , is drawn from recirculation compressor  32  which contains only first inlet air stream  34  when anode bypass valve  26  directs reformate stream  24  to fuel cell stack  14 . Preferably, a reformate check valve  60  is disposed in exhausted reformate passage  36  to ensure that anode exhaust  56  and cathode exhaust  58  in exhaust passage  48  are not drawn into fuel processor  12  by recirculation compressor  32 .  
         [0021]    As is well known in the art, catalysts, such as that which is often used in water gas shift reactors (i.e. CuZn), are often sensitive to oxygen and condensed water. Therefore, this is particularly important after shut down when the fuel processor cools and any water vapor condenses. That is, the reformate gases within fuel processors often have a very high water (steam) content (typically 30%), which condense when the fuel processor cools after shut down. Additionally, as the fuel processor cools the condensation of water and the cooling of gases within the fuel processor may cause a reduction in gas pressure sufficient to pull a vacuum even if valves at the inlet and exit seal a fuel processor. At this point, any leaks present in the various valves, fittings, or flanges may allow air into the fuel processor and potentially damage the water gas shift catalyst. Therefore, additional features are illustrated in FIG. 2 to address these shut down issues.  
         [0022]    The fuel processor system  10 ′, shown in FIG. 2, is the same as that described in reference to FIG. 1, where like reference numerals are used to indicate like components. Referring to FIG. 2, a recirculation valve  102  is positioned in recirculated reformate passage  30  and an exhaust valve  104  is positioned in exhaust reformate passage  36 . Recirculation valve  102  and exhaust valve  104  are used in conjunction to control the recirculation ratio (i.e., the ratio of recirculated reformate stream to the total reformate stream). That is, by opening recirculation valve  102  the flow of recirculated reformate  30  is increased, while opening exhaust valve  104  the flow of recirculated reformate  30  is decreased. Furthermore, opening both valves  102 ,  104  decreases the pressure within fuel processor  12 . Recirculation valve  102  and/or exhaust valve  104  may be closed to prevent anode exhaust  56  and cathode exhaust  58  from being drawn into fuel processor  12  by recirculation compressor  32 .  
         [0023]    The transition to normal operation for fuel processor system  10 ′, shown in FIG. 2, is the same as described in reference to FIG. 1.  
         [0024]    Fuel processor system  10 ′, shown in FIG. 2, further provide a means to shut down fuel processor  12  without water condensation or air ingestion. For shut down, reformate stream  24  is circulated to anode bypass passage  28  via anode bypass valve  26 . Exhaust valve  104  remains closed to cause higher pressures within fuel processor  12 . Recirculation valve  102  is then slightly opened to maximize pressure within the capacity of recirculation compressor  32 . During shut down, water is condensed and separated from reformate stream  24  in a condenser  106 , which is connected to the system coolant loop (not shown). In normal operation, condenser  106  is used as an anode pre-cooler before fuel cell stack  14 .  
         [0025]    To further increase the pressure within fuel processor  12  during shut down, recirculation compressor  32  draws in first inlet air stream  34 . Preferably, the inlet to recirculation compressor  32  and the downstream side of circulation valve  102  are small in volume such that after recirculation compressor  32  is stopped, the pressure will remain high. Subsequently, the oxygen within first inlet air stream  34  will react with the hydrogen in recirculated reformate  30  within fuel processor  12  to produce additional heat, thereby increasing the pressure within fuel processor  12 . However, if necessary, additional fuel from first fuel inlet stream  20  may be added during shut down to consume the oxygen in first inlet air stream  34  in order to provide sufficient reactants (H 2  and CO) within fuel processor  12 . An oxygen sensor  108  is used in the fuel processor  12  as feedback to ensure that excess oxygen is not present. If the pressure within fuel processor  12  is higher than a predetermined level, exhaust valve  104  may be opened to reduce such pressure.  
         [0026]    Once the water has been condensed from reformate stream  24  and a high pressure condition has been achieved within fuel processor  12 , fuel processor air mass flow controller  62  is closed to seal the inlet, anode bypass valve  26  remains in the bypass position, and exhaust valve  104  remains closed to seal the exit. Recirculation compressor  32  is then stopped. The resident gases within fuel processor  12  are dry and at an elevated pressure, which is desired for shut down condition, particularly with a CuZn water gas shift catalyst.  
         [0027]    During the shut down cycle, the fuel and air are completely consumed (stoichiometric conditions) without water injection and without excessively high reactor temperatures to allow the gases to be dried by condenser  106 .  
         [0028]    As is well known in the art, conventional fuel processors suffer from various disadvantages when operating at reduce power and reduced flow, such as auto-ignition in the inlet, reverse water gas shift in the PrOx, cell reversal in the fuel cell stack, and water collection in the fuel cell stack. Furthermore, the transition between power levels are often slow to react due to the time necessary to pressurize or vent reactor volumes so as to achieve steady flow conditions at the new power level.  
         [0029]    Within the primary reactor temperatures in the inlet region increase such that there is a limited amount of time before undesirable auto-ignition of the fuel will occur. As the flow through the fuel processor is reduced at low power, the residence time within the inlet is increased. Thus, the rate of reduction in flow and power is limited by the auto-ignition condition in the inlet.  
         [0030]    Within the PrOx reactor, after the oxygen is consumed, reformate that is exposed to catalyst will undergo reverse water gas shift reactions, thereby consuming desirable H 2  and creating undesirable CO. At reduced flow, the oxygen is consumed earlier in the PrOx reactor, thereby leaving a larger section of catalyst and a longer residence time for reverse water gas shift reactions to occur.  
         [0031]    Within the fuel cell stack, the current flow through each fuel cell is limited by the fuel cell provided the lowest quantity of H 2 . That is, the fuel cell with the lowest H 2  flow limits the current through all of the remaining fuel cells. Therefore, a portion of the available quantity of H 2  (typically 10 to 20%) leaves the fuel cell stack unused. At reduced flows, the portion of H 2  leaving the fuel cell stack needs to be higher for stable operation, which is likely the result of less uniform flow distribution at reduced flows. Also contributing to the minimum flow for stable fuel cell stack operation is the need to clear condensed water to prevent it from collecting in and blocking passages within the gas distribution plates.  
         [0032]    In conventional systems, the flow rate through the fuel processor system varies with power level, thus the associated pressure drop necessitates a change in reactor pressure between power levels. However, a change in reactor pressure requires time for flow to fill or vent to the downstream reactors in order to achieve the steady pressure at the new power level. The numerous aforementioned disadvantages are overcome in the present invention by maintaining a higher flow rate, even during low power operation, by recirculating gases through the fuel processor and stack.  
         [0033]    Fuel processor system  10 ″, shown in FIG. 3, illustrates a system having reformate circulation through the fuel processor for startup, means for water condensation and pressurization for shut down, and circulation through the fuel processor and anode for turn down and transients. The fuel processor system  10 ″, shown in FIG. 3, is the same as that described in reference to FIGS. 1 and 2, where like reference numerals are used to indicate like components.  
         [0034]    More particularly, for startup, anode bypass valve  26  directs reformate stream  24  to anode bypass passage  28 . First fuel inlet stream  20  is introduced into fuel processor  12 . First inlet air stream  34  is delivered to fuel processor  12  by a fuel processor air compressor  202 . FIG. 3 shows first inlet air stream  34  being delivered to three locations in fuel processor  12  in the form of POx air stream  204 , start air stream  206  and PrOx air stream  208 . POx and PrOx air streams  204 ,  208  would normally be part of fuel processor  12 . Heat produced by the reactions of fuel inlet stream  20  and inlet air stream  34  warms fuel processor  12 . By staging the inlet air to provide multiple heating locations, the startup time is reduced by improving heat distribution within fuel processor  12 .  
         [0035]    To initiate reactions in each of these locations, a spark lit burner or an electrically heated catalyst section (not shown) is used. The overall oxygen to carbon (o/c) ratio (i.e. ratio of first inlet air stream  34  to first fuel inlet stream  20 ) is introduced in proportions slightly rich of stoichiometric to ensure that no excess oxygen is present, which could damage the catalyst within fuel processor  12 . The recirculated reformate  30  acts as a diluent so that all the available first inlet air stream  34  is reacted without excessively high temperatures within fuel processor  12 .  
         [0036]    Exhaust reformate passage  36  is employed to exhaust excess reformate to catalytic combustor reactor  16 . Under steady flow, this exhausted reformate in passage  36  is equal to the total mass flow of first fuel inlet stream  20 , first inlet air stream  34 , first water inlet stream  22  and vaporizer steam  38  that passes through fuel processor  12 . This exhausted reformate in passage  36  is reacted with second inlet air stream  40  in catalytic combustor reactor  16 . Second inlet air stream  40  is directed to catalytic combustor reactor  16  via stack air compressor  42 , cathode bypass valve  44 , cathode bypass passage  46 , and exhaust passage  48 . Second inlet air stream  40  is bypassed around fuel cell stack  14  during startup to prevent drying of the membranes within fuel cell stack  14 . Heat from the reaction in catalytic combustor reactor  16  is integrated back into fuel processor  12  by vaporizing second water inlet stream  50  in vaporizer reactor  18  to produce vaporizer steam  38 , which typically is delivered to the PrOx-vaporizer or steam lines within fuel processor  12 . An anode check valve  210  and a cathode check valve  212  are shown to prevent back flow of reformate exhaust  48  into fuel cell stack  14 . Preferably, a reformate check valve  60  is also disposed in exhausted reformate passage  36  to ensure that anode exhaust  56  and cathode exhaust  58  in exhaust passage  48  are not drawn into fuel processor  12  by recirculation compressor  32 .  
         [0037]    Once the various reactors within fuel processor  12  are warmed to their operating temperature, anode bypass valve  26  routes reformate stream  24  to fuel cell stack  14  via anode inlet passage  52 . Second inlet air stream  40  is then directed by cathode bypass valve  44  to the cathode side of fuel cell stack  14  via cathode inlet passage  54 . The hydrogen from reformate stream  24  reacts with the oxygen from second air inlet stream  40  across a membrane electrode assembly within fuel cell stack  14  to produce electricity. Anode exhaust or stack effluent  56  from the anode side of fuel cell stack  14  includes a portion of hydrogen that is directed back to catalytic combustor reactor  16  where it is oxidized to provide heat. Cathode exhaust  58  from the cathode side of fuel cell stack  14  includes oxygen which may also be used in catalytic combustor reactor  16 . Anode exhaust  56  and cathode exhaust  58  are combined in exhaust passage  48  and react in catalytic combustor reactor  16 . Vaporizer reactor  18  continues to provide vaporizer steam  38  to fuel processor  12 .  
         [0038]    A back pressure regulator  214  is used to set the pressure within fuel processor system  10 ″, while recirculation compressor  32  determines the amount of reformate recirculated. As additional flow from first fuel inlet stream  20 , first inlet air stream  34 , first water inlet stream  22 , and vaporizer steam  38  is added to fuel processor  12 , additional reformate flow will split to exhausted reformate passage  36  to maintain the system pressure. Therefore, at high power, the system  10 ″ operates at a low recirculation ratio, whereby a larger portion of reformate stream  24  is “fresh” having a relatively high H 2  content. At low power, the system  10 ″ operates at a high recirculation ratio, whereby a larger portion of reformate stream  24  is re-circulated and having a relatively low H 2  content. It is important to note that recirculation compressor  32  according to the present embodiment need only overcome the pressure drop through fuel processor  12  and fuel cell stack  14  during normal operation, unlike the system shown in FIG. 2 where the pressure would drop to atmospheric pressure downstream of recirculation valve  102  to allow first inlet air stream  34  to be drawn in. To this end, fuel processor system  10 ″ illustrated in FIG. 3 requires an additional fuel processor air compressor  202 . Alternatively, stack air compressor  42  can be used to deliver air to fuel processor  12 .  
         [0039]    As best seen in FIG. 3, fuel processor system  10 ″ maintains a flow rate that is approximately equal to a fuel processor system operating at an optimum power level. This higher flow rate helps overcome many of the disadvantages described above.  
         [0040]    During the shut down cycle of fuel processor system  10 ″, anode bypass valve  26  routes reformate stream  24  to anode bypass passage  28 . Second inlet air stream  40  is then directed by cathode bypass valve  44  through cathode bypass passage  46  to catalytic combustor reactor  16 . This will provide air to catalytic combustor reactor  16  to react with any exhausted reformate in passage  36  from the recirculation loop.  
         [0041]    Backpressure regulator  214  is adjusted to indirectly produce the highest possible pressure within the capacity of recirculation compressor  32 . As reformate stream  24  recirculates through fuel processor  12 , water is condensed and separated in condenser  106 .  
         [0042]    To further increase the pressure within fuel processor  12  prior to shut down, fuel processor air compressor  202  draws in first inlet air stream  34 . Subsequently, the oxygen within first inlet air stream  34  will react with the hydrogen in circulated reformate  30  within fuel processor  12  to produce additional heat, thereby increasing the pressure within fuel processor  12 . However, if necessary, additional fuel from first fuel inlet stream  20  may be added during shut down to consume the oxygen in first inlet air stream  34  in order to provide sufficient reactants (H 2  and CO) within fuel processor  12 . An O 2  sensor  108  is used in fuel processor  12  as feedback to ensure that excess oxygen is not present.  
         [0043]    Once the water has been condensed from reformate stream  24  and a high pressure condition has been achieved within fuel processor  12 , fuel processor air mass flow controllers  216 ,  218 ,  220  and stack air mass flow controller  64  are closed to seal the inlets, anode bypass valve  26  and cathode bypass valve  44  remain in the bypass position, and back pressure regulator  214  remains closed to seal the exit. Recirculation compressor  32 , fuel processor air compressor  202 , and stack air compressor  42  are stopped. The resident gases within fuel processor  12  are dry and at an elevated pressure, which is desired for shut down condition, particularly with a CuZn water gas shift catalyst.  
         [0044]    Yet another alternative system is illustrated in FIG. 4 wherein a compressor may be eliminated from the fuel processor system, generally indicated at  10 ″′. Fuel processor system  10 ″′ is operated at sub-atmospheric pressures such that potential for air ingestion exists. Otherwise, the startup, shut down, turn down and transient operation are similar to fuel processor system  10 ″ illustrated in FIG. 3. An additional benefit of fuel processor system  10 ″′ is that a recirculated exhaust  302  can be made inert by providing just enough cathode exhaust  58  to catalytic combustor reactor  16  using a combustor air mass flow controller  304  for stoichiometric operation in catalytic combustor reactor  16 .  
         [0045]    A cathode back pressure regulator  306  is needed to match the pressure set by a back pressure regulator  308  downstream of catalytic combustor reactor  16  to ensure cathode exhaust  58  can be directed to catalytic combustor reactor  16 . An O 2  sensor  310  may be used in exhaust  312  to ensure stoichiometric operation.  
         [0046]    A unique capability of the aforementioned systems is the potential to operate without water addition. This is an advantage for a system that is to be started in ambient temperatures below 0° C., where water is not available. Because the system  10 ″′ operates at a high recirculation, this mode of operation is relatively inefficient at about 62%, however it may be used for short duration.  
         [0047]    It should be understood that features of the fuel processor systems illustrated in FIGS.  1 - 4  can be combined as needed for system requirements. For example, PrOx air  208  may preferably be delivered from stack air compressor  42 . That is, various combinations of the various systems described herein might be made depending upon the specific application.  
         [0048]    As should be appreciated from the foregoing discussion, the fuel processor systems of the present invention all include recirculation of fuel processor gases, such as reformate, anode exhaust, or combustor exhaust. This feature provides numerous advantages that are not present in conventional fuel processor systems. For example, the fuel processor systems of the present invention are capable of providing a large mass flow rate through the fuel processor to aid in heating the fuel processor components to the proper operating temperatures during startup. Moreover, during shut down, the fuel processor systems of the present invention enable the fuel processor to run dry and condense water from the reformate to avoid condensation on the catalysts and subsequently be shut down at an elevated pressure to prevent air ingestion upon cooling of the fuel processor. Still further, during turn down, the fuel processor systems of the present invention enable higher flow rates through the fuel processor and fuel cell stack to avoid auto-ignition in the inlet, reverse water gas shift in the PrOx, cell reversal in the fuel cell stack, and water collection in the fuel cell stack, all of which occur at reduced flow rates. During transient response, the fuel processor systems of the present invention, by circulating gases, enables the flow rate and pressure in the fuel processor to remain nearly constant, thereby minimizing the lag in transient response associated with filling or venting volumes in the fuel processor system. The ability to use recirculated gases, which contain water vapor as a product of reaction, enables the fuel processor to run without water injection. The fuel processor systems of the present invention enable rapid thermal start of the fuel processor without the complexity of multiple stages or risk of oxygen exposure.  
         [0049]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.