Abstract:
A fuel cell system includes a plurality of fuel cells arranged into a stack. A combustor receives a cathode exhaust flow from the fuel cell cathodes and a flow of fuel. The fuel is oxidized in the combustor by the cathode exhaust to produce a mixed exhaust flow. A cathode air flow path extends between a fresh air source and the fuel cell cathodes. An air preheater is arranged along the cathode air flow path, cathode air passing through the air preheater being heated therein by a flow of uncombusted anode exhaust from the fuel cell anodes. A cathode recuperator is arranged along the cathode air flow path, cathode air passing through the cathode recuperator being heated therein by the mixed exhaust flow. Water passing through a vaporizer is converted to steam by heat from the mixed exhaust flow and directed from the vaporizer to the fuel cell anodes.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of prior-filed co-pending U.S. patent application Ser. No. 11/124,810 filed on May 9, 2005, the entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention is generally directed to fuel cells and more specifically to high temperature fuel cell systems and their operation. 
         [0003]    Fuel cells are electrochemical devices which can convert energy stored in fuels to electrical energy with high efficiencies. High temperature fuel cells include solid oxide and molten carbonate fuel cells. These fuel cells may operate using hydrogen and/or hydrocarbon fuels. There are classes of fuel cells, such as the solid oxide regenerative fuel cells, that also allow reversed operation, such that oxidized fuel can be reduced back to unoxidized fuel using electrical energy as an input. 
         [0004]    In a high temperature fuel cell system such as a solid oxide fuel cell (SOFC) system, an oxidizing flow is passed through the cathode side of the fuel cell while a fuel flow is passed through the anode side of the fuel cell. The oxidizing flow is typically air, while the fuel flow is typically a hydrogen-rich gas created by reforming a hydrocarbon fuel source. The fuel cell, operating at a typical temperature between 750° C. and 950° C., enables the transport of negatively charged oxygen ions from the cathode flow stream to the anode flow stream, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor and/or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical side of the fuel cell through an electrical circuit completed between anode and cathode, resulting in an electrical current flow through the circuit. {Text} 
       SUMMARY 
       [0005]    According to one aspect of the invention, a fuel cell system is provided and includes a fuel cell stack, a cathode recuperator heat exchanger adapted to heat an air inlet stream using heat from a fuel cell stack cathode exhaust stream, and an air preheater heat exchanger which is adapted to heat the air inlet stream using heat from a fuel cell stack anode exhaust stream. 
         [0006]    In one aspect, the air preheater heat exchanger is located upstream of the cathode recuperator heat exchanger, such that the air inlet stream is first heated by the anode exhaust stream followed by being heated by the cathode exhaust stream prior to being provided into the fuel cell stack. 
         [0007]    According to one aspect, the cathode recuperator heat exchanger is adapted to provide the cathode exhaust stream exiting the cathode recuperator heat exchanger at a temperature of at least 200° C., the fuel cell stack and the cathode recuperator heat exchanger are located inside a hot box, and the air preheater heat exchanger and an air blower which provides the air inlet stream are located outside the hot box. 
         [0008]    In accordance with one aspect of the invention, a fuel cell system includes a fuel cell stack, a first means for heating an air inlet stream using heat from a fuel cell stack cathode exhaust stream, and a second means for heating the air inlet stream using heat from a fuel cell stack anode exhaust stream. 
         [0009]    According to one aspect, the second means is located upstream of the first means, such that the air inlet stream is first heat by the anode exhaust stream followed by being heated by the cathode exhaust stream prior to being provided into the fuel cell stack. 
         [0010]    In accordance with one aspect of the invention, a method is provided for operating a fuel cell system. The method includes the steps of heating an air inlet stream being directed to a fuel cell stack using heat from a fuel cell stack anode exhaust stream, and heating the air inlet stream using heat from a fuel cell stack cathode exhaust stream. 
         [0011]    In one aspect, the air inlet stream is first heated by the anode exhaust stream followed by being heated by the cathode exhaust stream prior to being provided into the fuel cell stack. 
         [0012]    In accordance with one aspect of the invention, a fuel cell system includes a fuel cell stack, and a first means for heating an air inlet stream using heat from a fuel cell stack cathode exhaust stream, wherein the cathode exhaust stream has a temperature of at least 200° C. after exiting the first means. 
         [0013]    In a further aspect, the fuel cell system includes a second means for heating the air inlet stream using heat from a fuel cell stack anode exhaust stream. 
         [0014]    In yet a further aspect, the cathode exhaust stream has a temperature of about 200° C. to about 230° C. after exiting the first means, and the second means is located upstream of the first means, such that the air inlet stream is first heated by the anode exhaust stream followed by being heated by the cathode exhaust stream prior to being provided into the fuel cell stack. 
         [0015]    In accordance with one aspect of the invention, a method is provided for operating a fuel cell system. The method includes the steps of providing an air inlet stream into a first heat exchanger, providing a fuel cell stack cathode exhaust stream into the first heat exchanger to heat the air inlet stream, wherein the cathode exhaust stream has a temperature of at least 200° C. after exiting the first heat exchanger, and providing the air inlet stream from the first heat exchanger into a fuel cell stack. 
         [0016]    In a further aspect, the method further heating the air inlet stream using heat from a fuel cell stack anode exhaust stream. 
         [0017]    Other objects, features, and advantages of the invention will become apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a plot of temperature versus heat for fluid flow in a system of a comparative example. 
           [0019]      FIGS. 2 and 3  are schematics of fuel cell systems according to the first preferred embodiment of the present invention.  FIG. 2  is a system components and flow diagram and  FIG. 3  shows the schematic of the heat exchanger network for the fuel cell system. 
           [0020]      FIGS. 4 ,  5 ,  6  and  8  are plots of temperature versus heat for various fluid flows in systems of the preferred embodiments of the present invention. 
           [0021]      FIG. 7  shows the schematic of the heat exchanger network for the fuel cell system of the third preferred embodiment of the present invention. 
           [0022]      FIG. 9  shows a somewhat diagrammatic representation of an integrated fuel humidifier assembly of the invention. 
           [0023]      FIG. 10  is a somewhat diagrammatic representation illustrating the flow paths of the assembly of  FIG. 9 . 
           [0024]      FIG. 11  is a partially exploded perspective view of one embodiment of the assembly of  FIG. 9 . 
           [0025]      FIG. 12  is a plan view of a heat exchanger plate of the assembly of  FIG. 11 . 
           [0026]      FIG. 13  is a partial, exploded perspective view of a heat exchanger plate pair for use in one embodiment of the assembly of  FIG. 9 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    In order to maintain the SOFC at its elevated operating temperature, the anode and cathode flow streams exiting the fuel cell typically transfer heat to the incoming flows through a series of recuperative heat exchangers. In a comparative example, this can include the process of transferring heat to a liquid water source in order to generate steam for steam reforming of a hydrocarbon fuel in order to generate the hydrogen-rich reformate flow. 
         [0028]    For example, the cathode heat may be recuperatively transferred from the cathode exhaust flow stream to the incoming cathode air, while the anode heat is partially recuperatively transferred from the anode exhaust to the incoming humidified fuel, such as natural gas, which feeds the steam reformer, and partially transferred to the water to generate the water vapor being provided into the fuel to humidify the fuel. In addition, the water vapor within the anode exhaust may be recaptured to serve either wholly or in part as the water source for the steam reformer. 
         [0029]    The inventors discovered that a thermodynamic analysis of the system in which the anode (i.e., fuel side) exhaust stream is used to heat the humidified fuel and to evaporate the water reveals that there will be more energy available in the anode exhaust exiting the fuel cell than is required to be transferred to the incoming humidified fuel (i.e., water and fuel). However, a sizable portion of both the heat available in the anode exhaust and the heat required for the feed is in the form of latent heat. The result is that, while there is sufficient energy available in the anode exhaust, attempts to transfer the heat from the anode exhaust to the water and natural gas via a heat exchanger, in which the heat is transferred by convection from the anode exhaust stream to a thermally conductive surface separating the exhaust stream and one or more of the incoming fluids, and from said surface to the one or more of the incoming fluids, may not be commercially practical. 
         [0030]    The above described problem is illustrated in  FIG. 1 , which shows the plot of temperature versus heat transferred for the anode exhaust and the water. The conditions in  FIG. 1  assume a 400° C. anode exhaust temperature entering an evaporator (i.e., vaporizer) from a water-gas shift reactor, and a hypothetical counter flow evaporator capable of achieving full vaporization of the water, with minimal superheat. 
         [0031]    As can be seen in  FIG. 1 , the condensing of water vapor from the fully saturated anode exhaust and the isothermal vaporization of the water causes the temperature of the heat rejecting anode exhaust to drop below the temperature of the heat receiving water for a substantial portion of the heat duty (i.e., the water curve is located above the anode exhaust curve for Q values of about 1,100 to about 1750 W). As a result, achieving the required heat transfer between the fluids solely by use of typical heat exchangers may not be feasible for the conditions assumed in  FIG. 1 , since the transfer of heat in a typical heat exchanger requires the temperature of the thermally conductive separating material to be less than the local bulk fluid temperature of the heat rejecting fluid, and higher than the local bulk fluid temperature of the heat receiving fluid. 
         [0032]    Therefore, an additional heating source may be needed to evaporate sufficient water to satisfy the amount of steam required for methane reformation, which can be as high as 1.5 kW in a system with 6.5 kW electrical output. This additional heating source reduces system efficiency. 
         [0033]    The inventors discovered that the cathode (i.e., air side) exhaust may be used to evaporate water being provided into the fuel and/or to heat the fuel being provided into the system. By using this alternative approach to the recapture of heat energy in the SOFC fuel cell system, the entire thermodynamic potential of the exhaust gases can be recaptured for preheating of the fuel cell feeds without mass transfer devices such as an enthalpy wheel, or additional heat sources. However, in some systems utilizing this alternative approach, it still may be desirable to utilize mass transfer devices such as an enthalpy wheel, or additional heat sources. The system where the cathode exhaust is used to vaporize water for humidifying the fuel and/or used to heat incoming fuel is also be capable of being passively controlled. However, in some systems where the cathode exhaust is used to vaporize water for humidifying the fuel and/or used to heat incoming fuel, it may be desirable to utilize active control. 
         [0034]      FIGS. 2 and 3  illustrate a fuel cell system  1  according to a first preferred embodiment of the invention. Preferably, the system  1  is a high temperature fuel cell stack system, such as a solid oxide fuel cell (SOFC) system or a molten carbonate fuel cell system. The system  1  may be a regenerative system, such as a solid oxide regenerative fuel cell (SORFC) system which operates in both fuel cell (i.e., discharge) and electrolysis (i.e., charge) modes or it may be a non-regenerative system which only operates in the fuel cell mode. 
         [0035]    The system  1  contains one or more high temperature fuel cell stacks  3 . The stack  3  may contain a plurality of SOFCs, SORFCs or molten carbonate fuel cells. Each fuel cell contains an electrolyte, an anode electrode on one side of the electrolyte in an anode chamber, a cathode electrode on the other side of the electrolyte in a cathode chamber, as well as other components, such as separator plates/electrical contacts, fuel cell housing and insulation. In a SOFC operating in the fuel cell mode, the oxidizer, such as air or oxygen gas, enters the cathode chamber, while the fuel, such as hydrogen or hydrocarbon fuel, enters the anode chamber. Any suitable fuel cell designs and component materials may be used. 
         [0036]    The system  1  also contains a heat transfer device  5  labeled as a fuel humidifier in  FIG. 2 . The device  5  is adapted to transfer heat from a cathode exhaust of the fuel cell stack  3  to evaporate water to be provided to the fuel inlet stream and to also mix the fuel inlet stream with steam (i.e., the evaporated water). Preferably, the heat transfer device  5  contains a water evaporator (i.e., vaporizer)  6  which is adapted to evaporate water using the heat from the cathode exhaust stream. The evaporator  6  contains a first input  7  operatively connected to a cathode exhaust outlet  9  of the fuel cell stack  3 , a second input  11  operatively connected to a water source  13 , and a first output  15  operatively connected to a fuel inlet  17  of the stack  3 . The heat transfer device  5  also contains a fuel—steam mixer  8  which mixes the steam or water vapor, provided into the mixer  8  from the first output  15  of the evaporator  6  through conduit  10 , and the input fuel, such as methane or natural gas, provided from a fuel inlet  19 , as shown in  FIG. 3 . 
         [0037]    The term “operatively connected” means that components which are operatively connected may be directly or indirectly connected to each other. For example, two components may be directly connected to each other by a fluid (i.e., gas and/or liquid) conduit. Alternatively, two components may be indirectly connected to each other such that a fluid stream passes between the first component to the second component through one or more additional components of the system. 
         [0038]    The system  1  also preferably contains a reformer  21  and a combustor  23 . The reformer  21  is adapted to reform a hydrocarbon fuel to a hydrogen containing reaction product and to provide the reaction product to the fuel cell stack  3 . The combustor  23  is preferably thermally integrated with the reformer  21  to provide heat to the reformer  21 . The fuel cell stack  3  cathode exhaust outlet  9  is preferably operatively connected to an inlet  25  of the combustor  23 . Furthermore, a hydrocarbon fuel source  27  is also operatively connected to the combustor  23  inlet  25 . 
         [0039]    The hydrocarbon fuel reformer  21  may be any suitable device which is capable of partially or wholly reforming a hydrocarbon fuel to form a carbon containing and free hydrogen containing fuel. For example, the fuel reformer  21  may be any suitable device which can reform a hydrocarbon gas into a gas mixture of free hydrogen and a carbon containing gas. For example, the fuel reformer  21  may reform a humidified biogas, such as natural gas, to form free hydrogen, carbon monoxide, carbon dioxide, water vapor and optionally a residual amount of unreformed biogas by a steam methane reformation (SMR) reaction. The free hydrogen and carbon monoxide are then provided into the fuel inlet  17  of the fuel cell stack  3 . Preferably, the fuel reformer  21  is thermally integrated with the fuel cell stack  3  to support the endothermic reaction in the reformer  21  and to cool the stack  3 . The term “thermally integrated” in this context means that the heat from the reaction in the fuel cell stack  3  drives the net endothermic fuel reformation in the fuel reformer  21 . The fuel reformer  21  may be thermally integrated with the fuel cell stack  3  by placing the reformer and stack in the same hot box  37  and/or in thermal contact with each other, or by providing a thermal conduit or thermally conductive material which connects the stack to the reformer. 
         [0040]    The combustor  23  provides a supplemental heat to the reformer  21  to carry out the SMR reaction during steady state operation. The combustor  23  may be any suitable burner which is thermally integrated with the reformer  21 . The combustor  23  receives the hydrocarbon fuel, such as natural gas, and an oxidizer (i.e., air or other oxygen containing gas), such as the stack  3  cathode exhaust stream, through inlet  25 . However, other sources of oxidizer besides the cathode exhaust stream may be provided into the combustor. The fuel and the cathode exhaust stream (i.e., hot air) are combusted in the combustor to generate heat for heating the reformer  21 . The combustor outlet  26  is operatively connected to the inlet  7  of the heat transfer device  5  to provide the cathode exhaust mixed with the combusted fuel components from the combustor to the heat transfer device  5 . While the illustrated system  1  utilizes a cathode exhaust flow in the heat transfer device  5  that has passed through a combustor, it may be desirable in some systems to utilize a cathode exhaust flow in the heat transfer device  5  that has not been passed through a combustor. 
         [0041]    Preferably, the supplemental heat to the reformer  21  is provided from both the combustor  23  which is operating during steady state operation of the reformer (and not just during start-up) and from the cathode (i.e., air) exhaust stream of the stack  3 . Most preferably, the combustor  23  is in direct contact with the reformer  21 , and the stack  3  cathode exhaust is configured such that the cathode exhaust stream contacts the reformer  21  and/or wraps around the reformer  21  to facilitate additional heat transfer. This lowers the combustion heat requirement for SMR. 
         [0042]    Preferably, the reformer  21  is sandwiched between the combustor  23  and one or more stacks  3  to assist heat transfer. When no heat is required by the reformer, the combustor unit acts as a heat exchanger. Thus, the same combustor  23  may be used in both start-up and steady-state operation of the system  1 . 
         [0043]    The system  1  also includes a fuel preheater heat exchanger (i.e., anode recuperator)  29  which is adapted to heat the fuel inlet stream using heat from the fuel cell stack  3  anode exhaust stream exiting from the stack  3  anode exhaust outlet  31 . The system  1  further includes a cathode recuperator heat exchanger  33  which is adapted to heat an air inlet stream from an air blower  35  using heat from the cathode exhaust stream exiting the stack  3  cathode exhaust outlet  9 . Preferably, the cathode exhaust stream mixed with the combusted fuel components from combustor  23  outlet  26  are provided into the cathode recuperator  33  to heat the air inlet stream. The cathode exhaust stream mixed with the combusted fuel components are then provided to the evaporator  6  of the heat transfer device  5  to evaporate the water to steam, which will then be provided into the fuel inlet stream heading into the reformer  21 . 
         [0044]    Preferably, the fuel cell stack  3 , the reformer  21 , the combustor  23 , the fuel preheater heat exchanger  29  and the cathode recuperator heat exchanger  33  are located in a hot box  37 . Preferably, the cathode recuperator heat exchanger  33  is intentionally undersized to ensure that the temperature of the cathode exhaust stream exiting the heat exchanger  33  is sufficiently high to allow the heat transfer device  5  to evaporate the water to steam via transfer of heat from the cathode exhaust stream. For example, in one highly preferred embodiment, the cathode recuperator heat exchanger preferably has a size below a predetermined size, such that the cathode exhaust stream exits the cathode recuperator heat exchanger at a temperature of at least 200° C., such as 200° C. to 230° C., for example about 210° C. In this highly preferred embodiment, the cathode exhaust stream may enter the cathode recuperator heat exchanger  33  at a temperature of at least 800° C., such as about 800° C. to about 850° C., for example about 820° C. The cathode recuperator heat exchanger  33  is intentionally undersized to have an exchange rate of about 10 to 12 kW, such as about 11 kW for this highly preferred embodiment. In contrast, a full sized heat exchanger for the highly preferred embodiment may have an exchange rate of about 16 kW. While specific temperatures and heat exchange rates have been described for one highly preferred embodiment, it should be understood that the exit and entrance temperatures and heat exchange rates will be highly dependent upon the particular parameters of each specific application, and accordingly, it should be understood that no limitations to specific exit and entrance temperatures or heat exchange rates are intended unless specifically recited in the claims. 
         [0045]    The system  1  also preferably contains an air preheater heat exchanger  39  which is adapted to preheat the air inlet stream from the air blower  35  using heat from an anode exhaust stream exiting from the stack anode outlet  31 . Preferably, the air blower provides an air inlet stream into the system  1  which comprises at least 2.5 times, such as 2.5 to 6.5 times, preferably 3 to 4.5 times as much air as required for the fuel cell stack  3  to generate electricity. For example, the blower  35  may preheat the air inlet stream to about 50° C. The slightly preheated inlet air stream is then provided from the blower into the air preheater heat exchanger  39  where it is preheated to about 100° C. to about 150° C., such as about 140° C., for example. This preheated air inlet stream then enters the cathode recuperator heat exchanger  33  at about 100° C. to about 150° C. and exits the heat exchanger  33  at about 700° C. to about 750° C., such as about 720° C. Since the preheated air inlet stream enters the cathode recuperator heat exchanger  33  at a temperature above room temperature, the cathode exhaust stream can exit the heat exchanger  33  at a temperature above 200° C. Thus, the air preheater heat exchanger  39  sufficiently preheats the air inlet stream to allow the use of an undersized cathode recuperator heat exchanger  33 , which reduces the overall system manufacturing cost. 
         [0046]    Preferably, the air preheater  39  is located outside the hot box  37  and upstream of the cathode recuperator  33 , such that the air inlet stream is first heated by the anode exhaust stream in the air preheater  39 , followed by being heated by the cathode exhaust stream in the cathode recuperator  33 . Thus, the air inlet stream provided into the cathode inlet  41  of the stack  3  is heated by both the anode and cathode exhaust streams from the stack  3 . 
         [0047]    The system  1  optionally contains a water gas shift reactor  43  which is adapted to convert at least a portion of water vapor in the fuel cell stack anode exhaust stream into free hydrogen. Thus, the inlet  45  of the reactor  43  is operatively connected to the stack anode outlet  31 , and the outlet  47  of the reactor  43  is operatively connected to an inlet  49  of the air preheater  39 . The water-gas shift reactor  43  may be any suitable device which converts at least a portion of the water exiting the fuel cell stack  3  fuel exhaust outlet  31  into free hydrogen. For example, the reactor  43  may comprise a tube or conduit containing a catalyst which converts some or all of the carbon monoxide and water vapor in the anode exhaust stream into carbon dioxide and hydrogen. The catalyst may be any suitable catalyst, such as an iron oxide or a chromium promoted iron oxide catalyst. 
         [0048]    The system  1  also optionally contains a condenser  51  adapted to condense water vapor in the anode exhaust stream into liquid water, preferably using an ambient air flow as a heatsink. The system  1  also optionally contains a hydrogen recovery system  53  adapted to recover hydrogen from the anode exhaust stream after the anode exhaust stream passes through the condenser  51 . The hydrogen recovery system may be a pressure swing adsorption system or another suitable gas separation system, for example. Preferably, the air preheater  39  partially condenses the water vapor in the anode exhaust stream prior to the anode exhaust stream entering the condenser  51  to reduce the load on the condenser  51 . Thus, the outlet  55  of the air preheater  39  is operatively connected to the inlet  57  of the condenser  51 . A first outlet  59  of the condenser  51  provides hydrogen and other gases separated from the water to the hydrogen recovery system  53 . A second outlet  61  of the condenser  51  provides water to an optional water purification system  63 . The water from the purification system  63  is provided to the evaporator  6  which comprises a portion of the heat transfer device  5 , through inlet  11 . 
         [0049]    The system  1  also optionally contains a desulfurizer  65  located in the path of the fuel inlet stream from the fuel source  27 . The desulfurizer  65  removes some or all of the sulfur from the fuel inlet stream. The desulfurizer  65  preferably comprises the catalyst, such as Co—Mo or other suitable catalysts, which produces CH.sub.4 and H.sub.2S gases from hydrogenated, sulfur containing natural gas fuel, and a sorbent bed, such as ZnO or other suitable materials, for removing the H.sub.2S gas from the fuel inlet stream. Thus, a sulfur free or reduced sulfur hydrocarbon fuel, such as methane or natural gas, leaves the desulfurizer  65 . 
         [0050]    A method of operating the system  1  according to a first preferred embodiment of the present invention is described with reference to  FIGS. 2 and 3 . 
         [0051]    The air inlet stream is provided from the air blower  35  into the air preheater  39  through conduit  101 . The air inlet stream is preheated in the air preheater  39  by exchanging heat with the anode exhaust stream coming from the water-gas shift reactor  43 . The preheated air inlet stream is then provided into the cathode recuperator  33  through conduit  103 , where the air inlet stream is heated to a higher temperature by exchanging heat with the cathode exhaust stream. The air inlet stream is then provided into the cathode inlet  41  of the stack  3  through conduit  105 . 
         [0052]    The air then exits the stack  3  cathode outlet  9  as the cathode exhaust stream. The cathode exhaust stream wraps around the reformer  21  and enters the combustion zone of the combustor  23  through conduit  107  and inlet  25 . Desulfurized natural gas or another hydrocarbon fuel is also supplied from the fuel inlet  27  through conduit  109  into the combustor  23  inlet  25  for additional heating. The exhaust stream from the combustor  23  (i.e., cathode exhaust stream) then enters the cathode recuperator through conduit  111  where it exchanges heat with the incoming air. 
         [0053]    The cathode exhaust stream is then provided into the evaporator  6  of the heat transfer device  5  through conduit  113 . The rest of the heat left in the cathode exhaust stream is then extracted in the evaporator  6  for evaporating water for steam methane reformation before venting out through exhaust conduit  115 . 
         [0054]    On the fuel side, the hydrocarbon fuel inlet stream enters the desulfurizer  65  from the fuel source  27 , such as a gas tank or a valved natural gas pipe. The desulfurized fuel inlet stream (i.e., desulfurized natural gas) then enters the fuel mixer  8  of the heat transfer device  5  through conduit  117 . In the mixer  8 , the fuel is mixed with purified steam from the evaporator  6 . 
         [0055]    The steam/fuel mix is then provided into the fuel preheater  29  through conduit  119 . The steam/fuel mix is then heated by exchanging heat with the anode exhaust stream in the fuel preheater  29  before entering the reformer through conduit  121 . The reformate then enters the stack  3  anode inlet  17  from the reformer  21  through conduit  123 . 
         [0056]    The stack anode exhaust stream exists the anode outlet  31  and is provided into the fuel preheater  29  through conduit  125 , where it heats the incoming fuel/steam mix. The anode exhaust stream from the hot box  37  then enters the water gas shift reactor  43  through conduit  127 . The anode exhaust stream from reactor  43  is then provided into the air preheater  39  through conduit  129 , where it exchanges heat with the air inlet stream. The anode exhaust stream is then provided into the condenser  51  through conduit  131 , where water is removed from the anode exhaust stream and recycled or discharged. For example, the water may be provided into the water purifier  63  through conduit  133 , from where it is provided into the evaporator through conduit  135 . Alternatively, water may be provided into the purifier  63  through a water inlet  137 , such as a water pipe. The hydrogen rich anode exhaust is then provided from the condenser  51  through conduit  139  into the hydrogen purification system  53 , where hydrogen is separated from the other gases in the stream. The other gases are purged through purge conduit  141  while hydrogen is provided for other uses or storage through conduit  143 . 
         [0057]    Thus, as described above, the fluid streams in the system  1  exchange heat in several different locations. The cathode exhaust stream is wrapped around the steam methane reformer  21  to supply the endothermic heat required for reformation. Then, natural gas or other hydrocarbon fuel is added directly to the cathode exhaust stream passing through the combustor  23  as needed to satisfy the overall heat requirement for reformation. Heat from the high-temperature exhaust exiting the combustor  23  (containing the cathode exhaust stream and the combusted fuel components, referred to as “cathode exhaust stream”) is recuperated to the incoming cathode air (i.e., air inlet stream) in the cathode recuperator  33 . The heat from the anode exhaust stream exiting the anode side of the fuel cell stack  3  is first recuperated to the incoming anode feed (i.e., the fuel inlet stream) in the fuel preheater  29  and then recuperated to the incoming cathode feed (i.e., the air inlet stream) in the air preheater  39 . 
         [0058]    Preferably, the air supplied to the fuel cell stack  3  from air blower  35  is provided in excess of the stoichiometric amount required for fuel cell reactions, in order to cool the stack and take away the heat produced by the stack. The typical ratio of air flow to stoichiometric amount is in excess of 4, such as 4.5 to 6, preferably about 5. This leads to substantially higher mass flow of cathode air than anode gas (i.e., fuel). Consequently, if the cathode exhaust stream only heats the air inlet stream, then the amount of heat which is transferred between the cathode exhaust and air inlet streams is significantly higher than that which is transferred between the anode exhaust and fuel inlet streams, typically by a factor of approximately 3. 
         [0059]    The inventors discovered that rather than transferring all of the heat which is recaptured from the cathode exhaust stream directly to the incoming air, the system  1  transfers only a portion of the cathode exhaust stream heat to the incoming air inlet stream and uses the remainder of the available cathode exhaust stream heat for complete vaporization of the water in the evaporator  6 . 
         [0060]    Thus, before the air inlet stream is heated to the appropriate fuel cell temperature, it is preheated by the anode exhaust stream in the air preheater  39 . This preheating ensures that the air inlet stream has a sufficiently high temperature when entering the cathode recuperator  33  to ensure that the recuperator  33  can raise the temperature of the air inlet stream to the appropriate fuel cell temperature 
         [0061]      FIGS. 4 and 5  show graphs of the fluid temperature vs. the heat transferred for the evaporator  6  (i.e., the water vaporizer), and the air preheater  39 , respectively, for one analyzed embodiment. As can be seen from the graphs in  FIGS. 4 and 5 , the thermodynamic cross-over shown in  FIG. 1  is eliminated. This removes the need for either a humidity exchanger or a supplemental heater which consumes additional fuel. 
         [0062]    In a heat exchanger, the “temperature approach” is defined as the smallest temperature difference between the two fluid streams at any location in the heat exchanger. As can be seen in  FIGS. 4 and 5 , both of the heat exchangers (i.e., the evaporator  6  and the air preheater  39 ) have a very small temperature approach, located away from either end of the heat exchanger at the point where the two-phase region begins. It is advantageous to maximize the temperature approach in each heat exchanger, since the rate of heat transfer between the fluids will decrease as the local temperature difference between the streams decreases, leading to a need for a larger heat exchanger to transfer the required heat. 
         [0063]    If the portion of total cathode air preheat which occurs in the cathode recuperator  33  is decreased, the temperature approach will increase in the evaporator  6 . However, the temperature approach will decrease in the air preheater  39 . Conversely, if the portion of total cathode air preheat which occurs in the cathode recuperator  33  is increased, the temperature approach will increase in the air preheater  39 . However, the temperature approach will decrease in the evaporator  6 . Of the total cathode heat duty, there will then be some optimum percentage which should be transferred within the cathode recuperator  33  in order to maximize the temperature approach in both the evaporator  6  and the air preheater  39 . 
         [0064]    The inventors also discovered that by using the cathode exhaust stream for vaporizing the water, the amount of superheat in the steam exiting the evaporator  6  is very sensitive to the temperature and mass flow rate of the cathode exhaust stream entering the evaporator. This can be seen in  FIG. 6 , which shows the impact of a 4.5% increase in cathode exhaust stream mass flow (with the cathode exhaust stream temperature into the evaporator remaining unchanged) on the resulting humidified natural gas temperature. 
         [0065]    The temperature of the humidified natural gas entering the fuel preheater  29  can be seen to increase by 28° C. due to this slight increase in cathode exhaust stream flow rate. This increase in temperature will result in a higher anode exhaust stream temperature exiting the fuel preheater, and subsequently a higher temperature exiting the water gas shift reactor  43  and entering the air preheater  39 . This in turn leads to an increase in the cathode air preheat, which will tend to increase the temperature of the cathode exhaust stream entering the evaporator  6 , thereby exacerbating the problem. The humidified natural gas temperature will continue to ratchet up, resulting in system stability problems, unless the inlet air flow rate is controlled. Thus, the cathode air (i.e., inlet air) flow rate needs to be controlled because it is one of the prime means of controlling the system  1 . 
         [0066]    In a second preferred embodiment, the previously mentioned potential stability problems may be reduced or eliminated by having an adjustable cathode exhaust bypass around the evaporator  6 , through which a small portion of the cathode exhaust stream could be diverted in order to control the cathode exhaust flow rate through the evaporator  6 . This solution uses active control of the fluid flow rate. 
         [0067]    In a third preferred embodiment, a passive approach is used to reduce or eliminate the previously mentioned potential stability problems without the need for additional monitoring and control. The inventors have discovered that a temperature of the humidified natural gas entering the fuel preheater  29  can be made to be relatively insensitive to changes in the cathode exhaust stream flow rate and/or temperature by limiting the potential for increased superheat in the evaporator through a temperature pinch. 
         [0068]      FIG. 7  illustrates the heat exchanger portion of the system of the third preferred embodiment. The other parts of the system of the third preferred embodiment are the same as those of the first preferred embodiment shown in  FIGS. 2 and 3 . 
         [0069]    As shown in  FIG. 7 , the direction of the water flow through the evaporator  6  is concurrent, rather than counter-current, with the flow of the cathode exhaust stream through the evaporator  6 . Rather than having the temperature approach in the evaporator  6  located at the onset of the two-phase flow region, it is shifted to the end of the heat transfer region of the evaporator  6 , where the temperature approach will “pinch” to a value of zero or closely approaching zero. No heat transfer between the streams will occur after this point, and the two fluids will exit at or near a common temperature. The cathode exhaust stream flow rate may need to be increased slightly in order to ensure that the heat capacity in the cathode exhaust stream is sufficient to achieve full vapor quality in the water. The water (i.e., steam) will then exit the evaporator  6  with some amount of superheat. The cathode exhaust stream exiting the evaporator  6  can then be used to preheat the fuel, such as natural gas in a second fuel preheater  67 . Since the fuel inlet stream has a very small flow rate compared to the cathode exhaust stream, it is quite easy to achieve 100% effective heat transfer and preheat the fuel inlet stream to the same temperature as the water vapor and cathode exhaust stream exiting the evaporator. 
         [0070]    Thus, as shown in  FIG. 7 , the system of the third preferred embodiment also contains the second fuel preheater  67 . The fuel preheater  67  includes a first input  69  operatively connected to a cathode exhaust outlet  9  of the fuel cell stack  3 , a second input  71  operatively connected to the fuel source  27 , and a first output  73  operatively connected to the fuel inlet conduit  17 . The second fuel preheater  67  is adapted to transfer heat from the cathode exhaust stream of the fuel cell stack to the fuel inlet stream being provided to the fuel cell stack  3 . The evaporator  6  in the third preferred embodiment comprises a concurrent flow or “co-flow” evaporator in which the cathode exhaust stream and the water are adapted to flow in a same direction, and an output of the evaporator is operatively connected to an inlet of the fuel preheater  67  such that the cathode exhaust stream flows from the evaporator  6  into the second fuel preheater  67 . 
         [0071]    Thus, the water and the cathode exhaust stream are preferably provided into the same side of the evaporator and flow concurrent to each other. The water is converted to steam in the evaporator  6  and is provided into the steam/fuel mixer  8 . The cathode exhaust stream is provided from the evaporator into the second fuel preheater heat exchanger  67  where it heats the inlet fuel flow which is then provided through the mixer  8  and the first fuel preheater heat exchanger (anode recuperator  29 ) into the stack  3 . 
         [0072]    The system of the third preferred embodiment is substantially insensitive to variations in cathode exhaust stream temperature and mass flow.  FIG. 8  shows that, for one analyzed embodiment, the humidified natural gas temperature entering the anode recuperator (i.e., first fuel preheater)  29  will increase by less than 7° C. due to a 6.8% increase in cathode exhaust stream mass flow in the system of the third preferred embodiment. Such a small temperature rise should not cause the temperature ratcheting described above, and therefore will result in system stability without the need for active control of the inlet air and/or cathode exhaust stream flow. 
         [0073]    Thus, in the preferred embodiments of the present invention, water is evaporated using the heat from cathode exhaust stream. The air heat exchanger (i.e., cathode recuperator) is undersized so that the hot stream exits it at a high temperature of at least 200° C., such as 200° C. to 230° C. Air is fed into the system at a stoic of 2.5 and above to have enough exhaust heat for evaporating water needed for steam methane reformation. Preferably, between 2.5 and 6.5 times, more preferably between 3 and 4.5 times as much air is provided into the fuel cell stack as required for the fuel cell stack to generate electricity. The inlet air entering the cathode recuperator is preheated in the air preheater using the anode exhaust stream to reduce the load on the cathode recuperator. Water from the anode exhaust stream is partially condensed in the air pre-heater to reduce load in the anode condenser. 
         [0074]    With reference to  FIG. 9 , the fuel humidifier  5  is preferably provided in the form of an integrated assembly  200  that includes, as a single integrated unit, the water evaporator  6 , a fuel heater or preheater, such as the fuel preheater  67 , and the fuel/steam mixer  8  connected to both the water evaporator  6  to receive steam therefrom and the fuel heater  67  to receive heated fuel therefrom. The water evaporator  6  preferably includes a water flow path  202  in heat transfer relation with a heat carrying fluid flow path  204 , which in the illustrated system is a cathode exhaust gas flow path, while the fuel heater includes a fuel flow path  206  also in heat transfer relation with the heat carrying fluid flow path  204 , which again is the cathode exhaust gas flow path  204  for the illustrated system. The fuel/steam mixer  8  is connected to both the water flow path  202  to receive steam therefrom and to the fuel flow path  206  to receive heated fuel therefrom. As seen in  FIG. 9 , the fuel preheater  67  is preferably located downstream from the water evaporator  6  with respect to the heat carrying fluid flow path  204 . However, in some applications, it may desirable for the fuel preheater  67  to be located upstream from the water evaporator  6  with respect to the heat carrying fluid flow path  204 . 
         [0075]    With reference to  FIG. 10 , in one preferred embodiment, the water flow path  202  preferably includes a plurality of parallel water flow passages  210 , the fuel flow path  206  includes a plurality of parallel fuel flow passages  212  and the heat carrying fluid flow path  204  includes a plurality of parallel heat carrying fluid flow passages  214  interleaved with the water flow passages  210  in the water evaporator  6  and interleaved with the fuel flow passages  212  in the fuel heater  67 . In further reference to  FIG. 10 , the fuel/steam mixture  8  preferably is in the form of a manifold or plenum  216  that is connected to all of the water and fuel flow passages  210  and  212 . 
         [0076]    It is preferred that each of the water flow passages  210  include a liquid pressure drop inlet region  220  that provides a greater pressure drop than the remainder  222  of the water flow passage  210  to help ensure proper distribution of the water flow to all of the water flow passages  210 . However, while the regions  220  are preferred, in some applications it may be desirable for the water flow passages  210  to be free of any such regions  220 . 
         [0077]    It is also preferred that each of the regions  220  be thermally isolated from the heat carrying fluid flow path  206  by a thermal break, shown schematically at  224 . The thermal break  224  acts to reduce conduction of heat to the pressure drop inlet regions  220  and preferably prevents or limits any vaporization of the water flow in the regions  220 . 
         [0078]    As seen in both  FIGS. 9 and 10 , the water flow and the heat carrying fluid flows have a concurrent flow relationship through the integrated assembly  200 , the advantages of which were previously discussed herein and which include providing stability for the associated system because of the temperature pinch and making the system less sensitive to changes in the flow rate of the heat carrying fluid, as well as temperature changes in the heat carrying fluid. While the concurrent flow arrangement is preferred, in some applications it may be desirable for the flow to be arranged so as to provide a counter-current relationship, which can possibly allow for a lower flow rate and/or inlet temperature for the heat carrying fluid flow in comparison to the concurrent flow relationship, or a higher humidified fuel outlet temperature. 
         [0079]      FIG. 11  shows one preferred embodiment of the integrated fuel humidifier assembly  200 . This embodiment utilizes a so-called stacked plate construction and includes a plurality of water/fuel plates or sheets  228  interleaved with a plurality of heat carrying fluid plates or frames  230 , with each of the water/fuel plates defining one of the water flow passages  210  and one of the fuel flow passages  212 , and each of the heat carrying fluid plates  230  defining one of the heat carrying fluid passages  214 . 
         [0080]    Each of the water/fuel plates  228  further includes a water/fuel mixing chamber  232  that is open to both of the passages  210  and  212  to receive steam and heated fuel, respectively, therefrom. Each of the heat carrying fluid plates  230  also includes a water/fuel mixing chamber  234  that is closed from the heat carrying fluid flow passage  214 . The chambers  232  and  234  are aligned to form the water/fuel mixing plenum  216  that extends through all of the plates  228  and  230 . 
         [0081]    Each of the water/fuel plates  228  further includes a pair of heat carrying fluid bypass openings  238  and  240  that are closed to the passages  210  and  212  in the water/fuel plate  228 . The openings  238  and  240  in each of the plates  228  are aligned with the opposite ends, respectively, of the heat carrying fluid flow passages  214  in the heat carrying fluid plates  230  to form a heat carrying fluid inlet manifold  242  and a heat carrying fluid exit manifold  244 , respectively, that extend through all of the plates  228  and  230  to direct the heat carrying fluid into and out of, respectively, the passages  214 . 
         [0082]    Each of the water/fuel plates  228  also includes a water inlet opening  246 , with the openings  246  being aligned with each other and a water bypass opening  250  in each of the heat carrying fluid plates  230  to form a water inlet manifold  252  that extends through all of the plates  228  and  230 . 
         [0083]    Each of the heat carrying fluid plates includes a fuel bypass opening  254 , with the openings  254  aligned with an end of the fuel flow passage  212  in each of the water/fuel plates  228  opposite from the chamber  232  to form a fuel inlet plenum or manifold  256  that extends through all of the plates  228  and  230  to supply fuel to each of the passages  212 . 
         [0084]    The assembly  200  also includes separator sheets  260  that are interleaved between each of the plates  228  and  230  in order to seal their respective flow passages from each other, as is known in stacked plate heat exchanger constructions. Each of the separator sheets  260  has openings  262 ,  264 ,  268 ,  270  and  272  that are aligned with and correspond to the chambers  232  and  234 , the bypass openings  238 , the bypass openings  240 , the water inlet openings  246  and bypass openings  250 , and the fuel bypass openings  254 , respectively. 
         [0085]    The assembly  200  also includes a pair of end plates  280  and  282  that sandwich the plates  228  and  230  and sheets  260  to seal the assembly  200  in a fluid tight manner. The end plate  280  includes a heat carrying fluid inlet connection or port  284  that is aligned with the heat carrying fluid inlet manifold  242  to direct heat carrying fluid thereto, and a humidified fuel outlet connection or port  286  that is aligned with the water/fuel mixing plenum  236  at an end of the plenum  236  opposite from the openings to the passages  210  and  212  to direct humidified fuel from aligned with the fuel manifold  256  to supply the fuel flow thereto, and a heat carrying fluid outlet connection or port  292  that is aligned with the outlet manifold  244  to direct heat carrying fluid therefrom. 
         [0086]    As best seen in  FIG. 12 , the passage  210  is defined by a continuous slot that extends from the water inlet opening  246  to the water/fuel mixing chamber  232 , with the slot being open to both faces of the plate  228 . Similarly, the fuel passages  212  is defined by a continuous slot that extends from the fuel inlet manifold  256  to the water/fuel mixing chamber  232 , again with the slot being open to the opposing faces of the water/fuel plate  228 . With reference to both  FIGS. 11 and 12 , the pressure reduction region  220  of the passage  210  is defined by a portion of the slot that is formed in a tight serpentine pattern with a relatively narrow slot width, which together provide a tortuous flow path. The water passage  210  then continues to a more open region of the slot where vaporization of the water occurs. In this regard, the initial length of the slot adjacent the pressure reduction region  220  has a reduced width in order to avoid separation of the water flow as it moves from the pressure reduction region  220  to the remainder  222  of the flow passage  210 , with the passage  210  widening further as it extends to the chamber  232 . 
         [0087]    As best seen in  FIG. 12 , each of the water/fuel plates  228  also includes the thermal break  224  in the form of a slit or slot  300  that extends for the length of the pressure drop inlet region  220  between the pressure reduction region  220  and the remainder  222  of the water flow passage  210 . As seen in  FIG. 11 , each of the heat carrying fluid plates  230  includes a corresponding slit or slot  302 , each of the separator sheets  260  includes a corresponding slit or slot  304 , and each of the end plates  282  includes a corresponding slit or slot  306 , with all of the slits  300 , 302 , 304 , 306  being aligned throughout the stack to form a plenum  308  that extends through the stack and is open to atmosphere. As previously discussed, the thermal break  224  acts to minimize conduction of heat to the pressure drop inlet region  220  and preferably prevents or limits any vaporization of the water flow in the pressure reduction region  220  to ensure that the water flow remains in the liquid phase in the pressure reduction region  220 . This is desirable because if the water is allowed to evaporate, a high pressure drop could be produced in the narrow passages of the pressure drop inlet region  220  and that pressure drop could dominate. While the thermal break  224  is preferred, in some applications it may be desirable not to have the thermal break  224  in the assembly  200 . 
         [0088]    As seen in both  FIGS. 11 and 12 , the flow passage  210  directs the water flow in a globally concurrent flow relationship with the heat carrying fluid flow in the passage  214 , but is formed with a serpentine configuration so as to provide localized cross flow with respect to the heat carrying fluid flow in the passage  214 , thereby improving the transfer of heat to the water while still providing the desired concurrent flow relationship. 
         [0089]    Preferably, each of the flow passages  214  includes extended surfaces, which in the illustrated embodiment are shown in the form of a fin or turbulator insert  310 , many suitable types of which are known. Extended surfaces may also be provided in the flow passages  210  and  212 , but are not shown in the illustrated embodiment. 
         [0090]    With reference to  FIG. 13 , a water/fuel plate pair  312  is shown to illustrate one alternate embodiment for forming the water flow passage  210 . Each plate  314 ,  316  of the plate pair  312  includes a plurality of discrete slots  318  that are arranged so as to overlie portions of corresponding discrete slots  318  in the opposite plate to form the water flow passage  210 , with the water flowing from one of the slots  318  in one of the plates  314 ,  316  to a corresponding slot  318  in the opposite plate  314 ,  316  and then from that corresponding slot  318  back to a second corresponding slot  318  in the first plate  314 ,  316  and so forth until the water flows into the water/fuel mixer  8 . The pressure reduction region  220  in this embodiment is defined by multiple ones of the slots  318 , each of a relatively narrow width and short length, thereby requiring multiple changes in flow direction and providing the tortuous flow path. For the particular arrangement of slots in  FIG. 13 , the water flow passage  210  is divided into three parallel legs  320 , but it should be understood that such a configuration is optional and will be highly dependent upon the requirements of each application. It should also be appreciated that a plurality of the plate pairs  312  of appropriate shape and size could be substituted for the water/fuel plates  228  in the embodiment shown in  FIGS. 11 and 12 . 
         [0091]    While a couple of preferred embodiments for the assembly  200  have been shown and described in connection with  FIGS. 11-13 , it should be understood that any suitable heat exchanger construction can be utilized to form the assembly  200 , including, for example, plate and bar type constructions, drawn cup constructions, nested plate constructions, and constructions that incorporate discrete heat transfer tubes. It should also be appreciated that the particular type of heat exchanger construction employed will be highly dependent upon the particular requirements of the system in which the integrated humidifier assembly  200  is employed. In this regard, it should be understood that while the integrated fuel humidifier assembly  200  has been described herein in connection with the fuel cell system  1 , the integrated fuel humidifier assembly may find use in many other types of systems, and that no limitation to a fuel cell system is intended unless expressly recited in the claims. 
         [0092]    While the integrated assembly  200  may be made utilizing any suitable material for the particular application, when employed in the fuel cell system  1  it is preferred that the sheets  260  and plates  228 ,  230 ,  280 , and  282  be formed from stainless steel or another suitable corrosion-resistant alloy and be nickel-brazed or brazed using another suitable corrosion-resistant brazing alloy. 
         [0093]    The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The description was chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.