Abstract:
An improved fuel processor thermal management system for use with a fuel cell is disclosed. The process includes supplying an air stream and a fuel stream into a auto thermal reactor (ATR) and forming reformate gas therein. Then, preferentially oxidizing the reformate gas and the air stream in the preferential oxidizer reactor (PrOx). The temperature of the preferential oxidizer reaction is controlled with a water stream by vaporizing the water stream to form a first portion of vaporized water. Then, reacting the air stream with the reformate gas exiting the PrOx is reached in a fuel cell to form an anode exhaust stream which is subsequently combined with the air stream to heat the water stream to form a second portion of vaporized water. The first portion of vaporized water and the second portion of vaporized water form a steam fluid. The steam fluid heats the auto thermal reactor and the air stream prior to entering the ATR and the reformate gas prior to entering the water shift gas reactor (WGS) to control the temperature of the reformate gas.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a thermal management system for the processing of fuel for fuel cells.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fuel cells are a leading alternate fuel powerplant candidates for both portable and stationary electrical power generation. A fuel cell is an electrochemical energy converter consisting of two electrodes which sandwich an electrolyte. In one form being developed for both portable and stationary applications, an ion-conducting polymer electrolyte membrane (PEM) is disposed between two electrode layers to form a membrane electrode assembly (MEA). The MEA is typically porous and electrically conductive to promote the desired electrochemical reaction from two reactants. One reactant, oxygen or air, passes over one electrode and hydrogen, the other reactant, passes over the other electrode to produce electricity, water and heat. Typical PEM fuel cells with membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994 and assigned to the General Motors Corporation.  
           [0003]    For vehicular applications, it is desirable to use a liquid fuel such as a low molecular weight alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the fuel for the vehicle because of the ease of onboard storage of liquid fuels and the existence of a nationwide infrastructure for supplying liquid fuels. However, liquid fuels must be dissociated to release their hydrogen content from the liquid fuel prior to use in a fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, also known as a reformer, that in conjunction with thermal energy and a suitable catalyst, yields a reformate gas including N 2 , H 2 O, CO 2 , H 2  and CO.  
           [0004]    The heat required to produce sufficient hydrogen varies with the energy demand required by the fuel cell system at any given moment in time. Accordingly, the heating system for the reformer must be capable of operating over a wide range of energy output. Heating a reformer with heat generated externally is generally known in the prior art. One such reformer is described in U.S. Pat. No. 4,650,727 to Vanderborgh. The reformate exiting the reformer, however, may contain undesirably high concentrations of carbon monoxide (CO) most of which must be removed (i.e., to a concentration of less than about 50 ppm) to prevent poisoning of the fuel cell&#39;s anode.  
           [0005]    It is known that the CO level of the reformate/effluent exiting a reformer can be reduced by utilizing a well-known “water gas shift” (WGS) reaction where water (i.e., in the form of steam) is added to the reformate/effluent exiting the reformer in the presence of a suitable catalyst. This lowers the carbon monoxide content of the reformate/effluent gas.  
           [0006]    However, some CO (i.e., about 0.5 mole % or more) still survives the shift reaction. Hence, shift reactor effluent gases include hydrogen, carbon dioxide, water and carbon monoxide. If the shift reaction is not sufficient to reduce the CO content of the reformate to a satisfactory level (i.e., to below about 50 ppm), it may be necessary to further remove carbon monoxide from the hydrogen-rich reformate stream exiting the shift reactor prior to supplying the effluent gas to the fuel cell. It is known to further reduce the CO content of H 2 -rich reformate gas exiting the shift reactor by a preferential oxidation or PrOx reaction effected in a suitable reactor operated at temperatures which promote the preferential oxidation of the CO by air in the presence of the H 2 , but without consuming/oxidizing substantial quantities of the H 2  or triggering the so-called “reverse water gas shift” (RWGS) reaction.  
           [0007]    The preferential oxidation process is described in a paper entitled “Methanol Fuel Processing for Low Temperature Fuel Cells” published in the Program and Abstracts of the 1988 Fuel Cell Seminar, Oct. 23-26, 1988, Long Beach, Calif., and in U.S. Pat. No. 5,271,916, issued to Vanderborgh et. al. Preferential oxidation reactors may be either adiabatic (i.e. where the temperature of the reactor is allowed to rise during oxidation of the CO) or isothermal (i.e. where the temperature the reactor is maintained substantially constant during oxidation of the CO). The adiabatic preferential oxidation process is sometimes effected by means of a number of sequential stages, which progressively reduce the CO content in stages, and requires careful temperature control, to prevent the reverse water gas shift reaction which counterproductively consumes H 2  and produces more CO. The isothermal process can effect the same CO reduction as the adiabatic process, but in fewer stages and without concern for the reverse water gas shift reaction if the reactor temperature can be kept low enough, and O 2  depletion near the exit of the reactor unit can be avoided.  
           [0008]    One known isothermal reactor is essentially a catalyzed heat exchanger having a thermally conductive barrier or wall that separates the heat exchanger into a first channel through which the H 2 -rich gas to be decontaminated (i.e. CO removed) passes, and a second channel through which a coolant flows to maintain the temperature of the reactor substantially constant within a defined working range. The barrier wall has a catalyzed first surface confronting the first channel for promoting the CO+O 2  reaction, and an uncatalyzed second surface confronting the second channel for contacting the coolant therein to extract heat from the catalyzed first surface through the barrier. Therefore, it has been found that the proper control of the fuel processor for fuel cells requires the thermal management of the water gas shift and the preferential oxidation reactors such that the reactors (primarily WGS and PrOx) are operated within their preferred temperature ranges. This means removing heat from the reformate stream entering the water gas shift and preferential oxidation reactors and in some cases removing the heat of reaction within the reactors (by means of a catalyzed heat exchanger).  
           [0009]    Conventional fuel processor systems have little or no thermal management. One system uses high temperature oil to remove the heat rejected by the preferential oxidation reactor and uses an air-to-oil heat exchanger to reject this heat to the ambient environment. Another system utilizes the heat from the reactors and heat exchangers with high temperature oil. Such systems require additional hardware, add an additional large thermal mass, are complex and add volume to the fuel processor, as well as additional control and maintenance issues.  
           [0010]    Therefore, there is a need for a fuel processor thermal management system that does not add additional mass, complexity and volume to the fuel cell thermal system and utilizes one of the process fluid streams as a heat transfer medium to control the fuel processor.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention seeks to improve the thermal management of a fuel processor by utilizing ATR process water for the thermal media. There are several advantages including a minimal parasitic pumping power requirement for the media since water can be pumped to a high pressure in liquid form, prior to its vaporization. Additionally, significant heat absorption is available with a relatively low mass flow rate by using the high latent heat energy of water. Water also has a higher sensible heat capacity and thermal conductivity compared to other known process fluids used in fuel cell systems.  
           [0012]    The present invention is directed to a thermal management process that is adapted for use with a fuel processor for a fuel cell. The fuel processor system having an auto thermal reformer, a water gas shift reactor, a preferential oxidation reactor, a first air (ATR) stream, a fuel stream and a first (ATR) vaporized water stream. The process includes supplying the air, vaporized water and fuel streams into the auto thermal reformer (ATR). The ATR effluent is fed into the water gas shift (WGS) reactor with a second (WGS) vaporized water stream. The WGS effluent is fed into the preferential oxidation reactor (PrOx) with a second (PrOx) air stream. Control of the temperature of the PrOx is performed through vaporization of the water streams to form a first portion of vaporized water. The PrOx effluent and a third (stack) air stream are fed to the fuel cell stack. The anode exhaust stream is combined with a fourth (combustor) air stream which is fed to the combustor. The combustor exhaust heats a third vaporized water stream to form a second portion of vaporized water. The first portion of vaporized water and the second portion of vaporized water forming a steam fluid. The ATR effluent (i.e. the reformate gas exiting the ATR) gives up heat to the steam and air streams prior to entering the WGS. In this way, the temperature of the ATR effluent is conditioned for further reformation in the system, and the steam and air streams being sent to the ATR inlet are preheated to maximize reformer efficiency.  
           [0013]    The present invention provides independent temperature control of each chemical reactor resulting in minimum reactor size and maximum performance throughout turndown and transients, with maximum utilization of waste heat for vaporization and preheating of the auto thermal reformer air, water, and fuel to minimize auto thermal reformer air requirements (o/c ratio) and thereby maximize fuel processor efficiency. In addition, the present invention accomplishes fuel processor thermal management with increased flexibility, lower mass and volume and potentially lower maintenance than a fuel processor thermal management system that uses a separate heat carrier loop (such as oil).  
           [0014]    For a more complete understanding of the invention, its objects and advantages, reference should be made to the following specification and to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The drawings which form an integral part of the specification, are to be read in conjunction therewith.  
         [0016]    [0016]FIG. 1 is a schematic diagram of the preferred embodiment of the process according to the present invention; and  
         [0017]    [0017]FIG. 2 is a schematic diagram of an alternate embodiment of the process according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The present invention is directed to a fuel cell system having a fuel processor as shown in FIG. 1 and generally designated by the reference numeral  8 . The system  8  includes a primary reactor  10 , a water gas shift (WGS) reactor  12 , a preferential oxidation (PrOx) reactor  14 , a fuel cell  16  and a catalytic combustor  18 . The system  8  may best be understood with a description of the formation and flow of the reformate gas stream there through.  
         [0019]    Initially, ambient air is pumped by compressor  20  through line  22  into heat exchanger  24  where the air is heated by steam flowing through line  26  into bundle  28 . The heated air flows from heat exchanger  24  through line  30  into mixing volume  32 . Steam, flowing through line  34 , mixes with the heated air in mixing volume  32  to form a heated air/steam mixture. The heated air/steam mixture flows through line  36  into the inlet  38  of primary reactor  10 . Fuel, typically in the form of gasoline, flows through line  40  and is mixed with the heated air/steam mixture in the inlet  38  to form a fuel/air/steam mixture.  
         [0020]    The fuel/air/steam mixture enters into an auto thermal reformer or ATR  42  through line  44  where the mixture is catalytically reacted to form a hydrogen rich gas which is discharged through line  46 . The reformate gas comprises primarily hydrogen (H 2 ) and carbon dioxide (CO 2 ) but also includes nitrogen (N 2 ), carbon monoxide (CO), water (H 2 O), and methane (CH 4 ). To minimize the amount of methane formed in ATR  42 , the temperature of the reformate gas is generally kept to a range between 700° C. and 750° C. as the gas stream exits ATR  42 . In this regard, the temperature of the reformate gas flowing out of ATR  42  is a function of the amount of air used in ATR  42 , that is, the oxygen to carbon (O/C) ratio and the temperature of the air, fuel and water that is introduced into ATR  42 . Because a higher O/C ratio generally results in a lower efficiency of ATR  42 , it is preferable to preheat the air, steam and fuel before entering ATR  42 . In order to control the temperature of reformate gas exhausted from ATR  42 , a heat exchanger  48  is used. Heat exchanger  48  has a bundle  50  which is in close contact with the reformate gas. Steam in bundle  50  absorbs heat from the reformate gas in line  46  and exists heat exchanger  48  into passage  34  which is connected via mixing volume  32  as described earlier. The reformate gas exits heat exchanger  48  through line  52 . Optionally, another heat exchanger  54  may be utilized downstream of heat exchanger  48  and performs a similar function. With the addition of heat exchanger  54 , a 2-stage heat control system of the reformate gas flowing through the primary reactor  10  is provided. Heat exchanger  54  has a bundle  56  with steam which is in close contact with the reformate gas to control the temperature thereof.  
         [0021]    Reformate gas exits heat exchanger  54  through line  58  and is connected to a mixing volume  60 . Ambient water is pumped in through line  62 , mixed with the reformate gas and discharged through line  64  into the WGS reactor  12 . Preferably, water is sprayed into mixing volume  60  by means of conventional water injection nozzles. The water mixes with and controls the temperature of the reformate gas entering WGS reactor  12 . The use of water at this point also aids in controlling the temperature of the reformate gas under transient conditions or unexpected temperature excursions of ATR  42  that could potentially damage the WGS reactor  12  or could result in an increased formation of CO therein. The addition of ambient water into the reformate gas has a further benefit of increasing the steam to carbon (S/C) ratio in the WGS reactor  12 , thereby having the desirable effect of converting more CO and water to CO 2  and H 2 .  
         [0022]    The WGS reactor  12  includes a medium temperature shift (MTS) reactor  66 , a heat exchanger  68  and a low temperature shift (LTS) reactor  70 . Alternately, a high temperature shift (HTS) reactor could be used in place of the MTS reactor  66 . As used herein, an HTS reactor operates in about the range of  400  to 550° C., MTS reactor operates in the range of about 300 to 400° C. and an LTS reactor operates in the range of about 200 to 300° C. The reformate gas stream passes through MTS reactor  66  to reduce the CO level of the gas and is discharged through line  72 . Adjacent to MTS reactor  66  is a heat exchanger  68  to control the temperature of the reformate gas within the water gas shift reactor  12 . Heat exchanger  68  transfers heat from the reformate gas to steam flowing through bundle  74 . The cooled reformate gas is discharged through line  76  into mixing volume  78  where it is mixed with ambient water injected into the reformate gas through line  80  to further cool the reformate gas. The reformate gas is discharged from mixing volume  78  through line  82  into the LTS reactor  70 . If the secondary cooling is not required, mixing volume  78  can be eliminated. Reformate gas passes through the LTS reactor  70  to further reduce the carbon monoxide level therein and is discharged through line  84 . After passing through the WGS reactor  12 , the reformate gas flows into the preferential oxidation (PrOx) reactor  14  which includes a unit reactor  86  to further reduce the carbon monoxide in the reformate gas to an acceptable level (i.e., below 50 ppm). In order to optimize the performance of unit reactor  86 , a heat exchanger  88  is installed between LTS reactor  70  and unit reactor  86 . Heat exchanger  88  is used to control the temperature of the reformate gas. Specifically, the temperature of the reformate gas exiting the WGS reactor  12  is generally in the range of about 250 to 400° C. depending on the type of WGS reactor used. However, the desired temperature of reformate gas prior to entering unit reactor  86  is in the range of about 150 to 200° C. Heat exchanger  88  cools the reformate gas by inputting a mixture of liquid water and water vapor having a high vapor quality (i.e., in the range of 0.7 to 1.0) through line  90  into a bundle  92 . The vaporized water is heated to achieve a slightly super-heated, high quality water vapor (i.e. about 150° C. and 0.7 to 1.0 vapor quality) and discharged through line  94 . As used herein, vapor quality refers to the mass fraction that is a vapor (i.e., steam). High vapor quality refers to the condition where the liquid water has been almost completely vaporized to its gaseous state.  
         [0023]    The reformate gas flows from heat exchanger  88  through line  96  to the PrOx reactor  86  and is discharged through line  98  where it enters heat exchanger  100 . Water is provided at line  102  and flows through bundle  104  of heat exchanger  100 . This heat transfer step provides preheating of the water in line  148  utilized by heat exchange element  150  of unit reactor  86  to reduce the possibility that the PrOx reaction will be quenched or stopped by over cooling the catalyst within PrOx reactor  86 . The water flowing through bundle  104  further cools the reformate gas to a temperature of approximately 90° C. before entering the fuel cell stack  16 . Heat exchanger  100  is also utilized to reduce the heat rejection load of the fuel cell stack coolant, and thereby reduce the size and fan requirements of the fuel cell cooling system.  
         [0024]    The reformate gas enters into the anode side of fuel cell  16  through line  106 . Air enters the cathode side of fuel cell  16  through line  108 . The reformate gas and air react in fuel cell  16  to produce electricity and water vapor in a conventional manner. Any unused reformate gas exits fuel cell  16  through the anode exhaust line  110 . Unused air and water vapor exits the fuel cell  16  through the cathode exhaust line  112 . The anode exhaust in line  110  flows into a mixing volume  114  where it is mixed with compressed air provided through line  116  to form an anode exhaust/air mixture which is discharged through line  118 . In this regard, the cathode exhaust line  112  may be coupled to mixing volume  114  to provide the required air.  
         [0025]    The cathode exhaust/air mixture enters into a catalytic element  120  of combustor  18  where the mixture is catalyzed to form hot gases. A vaporizer  122  in the form of a heat exchanger is fluidly coupled to the catalytic element  120  and extracts heat from the hot gases generated thereby. Specifically, water passes through line  124  into bundle  126  where the heat is transferred from the hot exhaust gases to the water which is discharged in the form of steam through line  128 . The exhaust from combustor  120  flows through vaporizer  122  which is preferably restricted by a flow restrictor such as a valve (not shown) to maintain the pressure of the reformate gas at an absolute pressure of between approximately 1 to 7 atmospheres and preferably at about  3  atmospheres.  
         [0026]    The steam in line  94  from heat exchanger  88  and in line  128  from heat exchanger  122  are combined in mixing volume  130 . These combined steam flows are discharged through line  132  which is fluidly coupled with bundle  74  of heat exchanger  68  in the WGS reactor  12 . There heat from the water gas shift reaction is transferred to the steam and discharged in line  134 . Pressure regulator  136  coupled to line  134  operates to maintain the steam pressure in the PrOx reactor  14  and combustor  18  at a substantially constant pressure level. Steam exiting pressure regulator  136  through  138  enters mixing volume  140  where it is combined with water provided through line  142 . The outlet of mixing volume  140  is coupled to bundle  56  of heat exchanger  54  through line  144 .  
         [0027]    In order to better understand the thermal process management of the present invention, an exemplary thermal balance at full power condition of fuel cell  16  will now be described in relation first to the thermal condition of the reformate gas, and then of the water/steam loop. In this regard, the approximate reformate temperature drops and corresponding heat removal rates for WGS reactor  12  and PrOx reactor  14  are set forth in the table below.  
                                                                 WGS   PrOx                                    Reformate temperature drop   750 to 330° C.   330 to 170° C.       Heat removal from reformate   0.194 kW/kWH 2     0.080 kW/kWH 2         Heat of reaction (to be removed)   0.025 kW/kWH 2     0.143 kW/kWH 2         Total heat removed   0.219 kW/kWH 2     0.223 kW/kWH 2                    
 
         [0028]    In addition to the heat being removed from the reformate stream, excess hydrogen from fuel cell  16  is typically converted to thermal energy by a catalytic combustor  18 . Depending on the anode stoicheometry of the fuel cell stack  16 , the additional heat from the excess hydrogen can be approximately 0.08 to 0.18 kW/kWH 2 . Thus, for every 2 kW of H 2  chemical energy produced, more than 1 kW of thermal energy is produced. This represents a significant limitation to the overall fuel processor efficiency if this thermal energy is under utilized.  
         [0029]    The optimization of thermal management has other benefits in the fuel processor system. The temperature of the reformate gas exiting ATR  42  is generally kept near 750° C. to minimize the amount of methane formed in ATR  42 . As previously mentioned, this temperature, in turn, is a function of the air consumed in ATR  42  or O/C ratio. Because a higher O/C ratio generally leads to a lower ATR efficiency, the preferred approach is to preheat the air, steam and fuel entering the primary reactor  10 . For steam reforming fuel processing systems, a ratio of the steam to carbon (S/C) ratio is used as a control parameter. Because a higher O/C ratio or S/C ratio generally relates to lower reformer efficiencies, the preferred approach is to preheat the air, steam and fuel prior to primary reformation. For example, with an S/C ratio of 2.8, about 0.23 kW/kWH 2  is required to vaporize the water and about 0.21 kW/kWH 2  are required to heat the fuel, steam and air mixture to a temperature of about 500° C. Hence, of the approximately 0.54 kW/kWH 2  of heat available, approximately 0.44 kW/kWH 2  or 81% can be utilized by the system to increase the fuel processor efficiency.  
         [0030]    With this understanding of the importance of thermal balancing, the operation of fuel cell reforming system  8  will now be described. Air enters into heat exchanger  24  where the air is heated by steam flowing through bundle  28  to form heated air at a temperature of approximately 450° C. The heated air exits heat exchanger  24  through line  30  into mixing volume  32  where it mixes with steam from line  34  to form a heated air/steam mixture of approximately 625° C. The heated air/steam mixture flows out of mixing volume  32  through line  36 .  
         [0031]    The mixture in line  36  flows into inlet plenum  38 . In plenum  38 , fuel enters through line  40  where it is mixed with the heated air/steam mixture to form a heated air/steam/fuel mixture at approximately 500° C. The air/steam/fuel mixture flows through line  44  into ATR  42  and is discharged as reformate gas through line  46  at a temperature of approximately 760° C. The reformate gas then flows into heat exchanger  48  where it is cooled to a temperature of approximately 550° C. and discharged through line  52 .  
         [0032]    The reformate gas next flows into a second heat exchanger  54  and is further cooled to approximately 400° C. The reformate gas is discharged through line  58  into a mixing volume  60  where ambient water from line  62  is sprayed into mixing volume  60  and cools the reformate gas stream to a temperature of approximately 330° C. The reformate gas/water stream enters into HTS reactor  66  where the carbon monoxide level is reduced causing the temperature of the reformate gas to increase slightly to a temperature of approximately 380° C. After exiting the HTS reactor  66 , the reformate gas flows through line  72  into heat exchanger  68  where the reformate gas is again cooled to a temperature of approximately 300° C. Next, the reformate gas is discharged through line  76  to flow into mixing volume  78  where it is mixed with water from line  80  in mixing volume  78  and cooled to a temperature of approximately 280° C. The reformate gas is discharged from mixing volume  78  through line  82  into LTS reactor  70  where the level of CO is reduced and the reformate gas is discharged through line  84 .  
         [0033]    Next, the reformate gas enters heat exchanger  88  where it is cooled to a temperature in the range of approximately 150 to 200° C. prior to entering PrOx reactor  86  through line  96 . Air is provided to PrOx reactor  86  through line  146  where it is combined with the reformate gas to further reduce the carbon monoxide to an acceptable level. From PrOx reactor  86 , the reformate gas flows through line  98  into heat exchanger  100  where it is cooled to a temperature of approximately 90° C.  
         [0034]    The reformate gas is discharged from heat exchanger  100  through line  106  to the anode side of the fuel cell  16 . Air is supplied to the cathode side of fuel cell  16  through line  108 . The reformate gas and air are reacted in fuel cell  16  by the MEA to generate electrical power. The unused reformate gas exits through the anode exhaust line  110  to mixing volume  114 . Likewise, the excess air or cathode exhaust exits through line  112 . At mixing volume  114 , air supplied through line  116  is combined with the anode exhaust stream and discharged through line  118  into catalytic combustor  120  where the mixture is burned to form hot gases. The heat generated in combustor  120  are recovered in vaporizer  122  before being exhausted to the atmosphere.  
         [0035]    The operation of the heat transfer water/steam loop will now be described in further detail working backwards through fuel processing system  8 . As previously described, vaporizer  122  is a heat exchanger with bundle  126  which extracts heat from the hot gases flowing out of combustor  120 . Water, introduced through inlet line  124  passes through bundles  126  and is discharged out line  128 . The water is pressurized by a pump (not shown) to a pressure preferably between 1 to 7 atmospheres and most preferably at about 3 atmospheres. The water passing through bundle  126  is fully vaporized and super heated at a temperature of approximately 150° C. at about 3 atmospheres. The steam from bundle  126  flows through line  128  to mixing volume  130 .  
         [0036]    A second source of steam is provided to mixing volume  130 . Ambient water enters bundle  104  of heat exchanger  100  through line  102  at a pressure of between 1 to 7 atmospheres and preferably at a pressure of approximately 3 atmospheres. The water absorbs heat from the reformate gas as it passes through heat exchanger  100 . The heated water exits through line  148  and enters bundle  150  of PrOx reactor  86 . In PrOx reactor  86 , the heated water absorbs additional heat from the reformate gas and exits out of PrOx reactor  86  through line  90  having a high vapor quality on the order of 0.7 to 1.0 and preferably about 0.85. Line  90  which is coupled with the bundle  92  associated with heat exchanger  88 . Steam at about 150° C. exits bundle  92 . The steam in line  94  and the steam in line  128  are mixed together in mixing volume  130  and flow therefrom by way of line  132 . At this point, the steam in line  132  may be slightly super heated at a temperature of approximately 150° C. at about 3 atmospheres.  
         [0037]    Steam flowing through line  132  is directed to bundle  74  of WGS heat exchanger  68 . Steam passes through bundle  74  and absorbs additional heat from the reformate gas in WGS reactor  12  and exits via line  134  at a temperature of approximately 350° C. Line  134  is connected to pressure regulator  136  to maintain the pressure of the steam at an elevated pressure, preferably at about 3 atmospheres and is discharged at a pressure slightly greater than 2 atmospheres through line  138  to mixing volume  140 . Dynamic temperature control is provided by pumping and mixing water from line  142  with steam from line  138  in mixing volume  140 . Steam, having an approximate temperature of 350° C., is discharged through line  144  and enters the bundle  56  of heat exchanger  54  to extract heat from the reformate gas in primary reactor  10 . The steam exits heat exchanger  54  through line  26  at a temperature of about 500° C. Line  26  is connected to bundle  28  of heat exchanger  24  where the steam transfers heat to the inlet air as discussed earlier cooling the steam to a temperature of approximately 330° C. whereupon it is discharged from exchanger  24  through line  152 . Line  152  is connected to bundle  50  in heat exchanger  48  where the steam extracts heat from the reformate gas and exits heat exchanger  48  through line  34  at a temperature of approximately 650° C. Line  34  is connected to mixing volume  32  to mix with heated air introduced via line  30  to form the heated mixture of air/steam in line  36  as described earlier.  
         [0038]    The thermal management process of the present invention controls the amount of processed water in the primary reactor and specifically in ATR  42  through two separate steam generation circuits. The first steam generation circuit is defined by vaporizer  122  of combustor  18 . The second steam generation circuit is defined by the PrOx reactor  14  including the unit reactor  86  and heat exchangers  88  and  100 . By utilizing two separate steam generation circuits, the present invention is able to carefully control the operational temperature of PrOx reactor  14  through the second generation circuit, while utilizing the first generation circuit including combustion vaporizer  122  with no control limits to highly vaporize the remaining amount of water. Under typical operational conditions, the second steam generation circuit is able to provide up to 50 percent of the total steam requirements with the balance being provided by the first steam generation circuit. Furthermore, the temperature of PrOx reactor  14  can be controlled preferably between a range of about 100 to 150° C. by use of steam pressure regulator  136  which is coupled to the steam generation circuit downstream of PrOx reactor  14  and combustion vaporizer  122 . Regulator  136  maintains the steam pressure in PrOx reactor  14  at a constant pressure level typically between 1 to 7 atmospheres. Preferably, regulator  136  controls the circuit pressure at about 3 atmospheres.  
         [0039]    Depending on the temperature limits of regulator  136  and other application constraints, regulator  136  can be optionally placed in a variety of places along the steam flow path such as in line  34  between heat exchanger  48  and mixing volume  32  or in line  152  between heat exchanger  24  and heat exchanger  48  or in line  26  between heat exchanger  54  and heat exchanger  24  or in line  132  between mixing volume  130  and heat exchanger  68 .  
         [0040]    Depending on operating conditions, including the level of carbon monoxide in the reformate gas exiting water gas shift reactor  12 , about 20 to 35 percent of the total heat needed by the primary reactor  10  and specifically ATR  42 , is supplied by PrOx reactor  14 . After discharge from WGS reactor  12 , the reformate passes through heat exchanger  88  prior to the PrOx unit reactor  86 )in order to reduce the temperature of the reformate gas from a temperature in the range of 250 to 400° C. to the desired temperature range of 150 to 200° C. After discharge from PrOx unit reactor  86 , the reformate passes through heat exchanger  100  in order to reduce the temperature of the reformate gas from a temperature in the range of 160 to 240° C. to the desired temperature range of 90 to 150° C.  
         [0041]    The heat recovered in heat exchangers  88 ,  100  are used to generate a source of vaporized water or steam. Specifically, heat energy is added to ambient water entering via line  102  within the heat exchanger  100 . Line  148  transfers the heated liquid water through the bundle  150  in PrOx  86  where the heat generated by the chemical reaction therein is transferred to the water to form a high quality water vapor (i.e. a vapor quality between 0.7 and 1.0). The partially vaporized water exits the PrOx via line  90  and enters heat exchanger  88 . The partially vaporized water is heated in bundle  92  and transformed to fully vaporize water or steam exiting heat exchanger  88  via line  94 . The steam in line  94  combines with the steam in line  128  from combustion vaporizer  122  at mixing volume  130  and is discharged through line  132 . The steam in line  132  represents the total process steam for the primary reactor  10  (i.e., the total amount of steam needed relative to the flow of reformate) and is further utilized upstream to cool the reformate gas, thereby adding heat to the process steam passing through heat exchangers  68 ,  54 ,  24  and  48 , respectively.  
         [0042]    During a large up-transient event, the increased reformate gas flow demand causes an initial drop in power. Since the various heat exchangers or vaporizers typically have the slowest response time, it is preferred to increase the PrOx air and water flows before increasing the ATR fuel flow, thereby generating the necessary steam flow to maintain a desired s/c ratio while achieving the increased flow demand. Specifically, increases in the air flow in line  146  and the water flow in line  102  lead the increase in the fuel flow in line  40 . The combustor vaporizer  122  may be operated in a similar fashion to provide increased steam flow though line  128  for accommodating up-transients. Specifically, by increasing the anode stochiometry, additional H 2  in the anode exhaust  110  will be provided to the combustor  120  to provide additional thermal input to be used for vaporization of water stream  124 . The air flow  116  to the combustor  120  would also be increased to maintain the combustor operating temperature. The combustor  120  may also be operated below maximum vaporization capacity so that sufficient thermal mass is available to provide additional vaporization capacity. The increase steam generated by the PrOx heat exchangers  88 ,  100 ,  150  and by the combustor heat exchanger  122  are used to avoid an initial drop in power and to increase efficiency. If the fuel processor is operated with an excess of steam, the increase reformate flow demand may also be accommodated by temporarily increasing the flow rate of the fuel supply stream at a rate greater than that required to maintain the desired S/C ratio. In this mode of operation, the air stream to the PrOx is also increased to so that the PrOx reactor can accommodate the increased CO levels in the reformate stream resulting from the decrease in the efficiency of the WGS reactor due to the lower S/C ratio.  
         [0043]    In addition to combustion vaporizer  122  and PrOx heat exchanger  100 , the thermal management process of the present invention also utilizes pressure regulator  136  to insure that the desired S/C ratio is maintained even under transient conditions. Specifically, pressure regulator  136  insures that the water vaporization temperature does not change by controlling the pressure of the steam at a near constant level even as the pressure within primary reactor  10  changes. Any excess heat is eventually carried out through the exhaust of combustor  18 . When additional water vaporization is required, the needed thermal energy is absorbed from the hot gases in the combustor exhaust gas stream by combustor vaporizer  122 .  
         [0044]    Without regulator  136 , the pressure in the portion of the steam loop that includes combustor vaporizer bundle  122  and PrOx cooling bundle  150  would fluctuate depending upon the power generation of fuel cell  16 . For example, when fuel cell  16  is operating at reduce power, the vapor pressure of the steam drops causing a surge of steam until the vaporized water is cooled to a new vaporization temperature. Alternatively, when fuel cell  16  is operating at maximum power, the vapor pressure rises causing the steam output to be suppressed until the vaporized water is heated to a new vaporization temperature. Further, the steam loop including pressure regulator  136  permits utilization of the steam as an atomizing agent to assist in the distribution of liquid fuel in inlet  38  of primary reactor  10 . This steam atomizer has the advantage of being able to atomize fuel at high inlet temperatures without cooling the air/steam mixture through coupled metering cooling utilized with conventional liquid fuel injector nozzles.  
         [0045]    With reference now to FIG. 2, a second preferred embodiment of the present invention is illustrated and designated by the reference numeral  208 . Where the elements of the second preferred embodiment are the same as those in the first preferred embodiment identical reference number designations will be used. Where the elements of the second preferred embodiment are similar to those in the first preferred embodiment reference numeral designating incremented by  200  will be used. In the second preferred embodiment of the present invention, two heat exchangers have been eliminated in primary reactor  210 . The fuel processor system  208  includes an inlet  238 , ATR  242 , WGS reactor  212 , PrOx reactor  214 , fuel cell  216  and catalytic combustor  218 . Specifically, WGS reactor  212  is a medium temperature shift (MTS) reactor to  66 . In the second preferred embodiment, intake air is pumped from compressor  220  through line  222  into mixing volume  232 . Air combines with steam flowing through line  238  at mixing volume  232  to form an air/steam mixture which flows into line  244 . The air/steam mixture then flows through bundle  256  of heat exchanger  254  where heat is transferred from the reformate gas to the air/steam mixture. The heated air/steam mixture flows through line  226  into primary reactor inlet  238 . Fuel is injected from line  240  into inlet  238  where a fuel/air/steam mixture is formed and input to auto thermal reactor  242  via line  244 .  
         [0046]    The HTS/LTS reactor configuration of the first preferred embodiment have been combined into a medium temperature shift (MTS) reactor  266  for carbon monoxide reduction. In all other aspects, the second preferred embodiment of the present invention operates in a manner similar to that previously described with respect to the first preferred embodiment.  
         [0047]    The present invention has been described in terms of a fuel reforming system in combination with a fuel cell system as preferred embodiments. As such, the preferred embodiments are described as self-contained fuel cell systems particularly suitable for vehicular applications. However, a skilled practitioner will readily recognized that the principles of the present invention are equally applicable to a fuel reforming systems only. As such the present invention is not is not intended to be limited to the preferred embodiments, and is subject to various changes, adaptations and modifications encompassed within the scope of the present invention as set forth hereinafter in the claims.