Abstract:
An apparatus and method for a normal system shutdown of a SOFC system implements a control strategy that utilizes existing system hardware and operating processes already used during normal operation of the SOFC system. The control strategy enables the SOFC system to generate the fluid needed for prevention of oxidation during the cooling process of the anode side of the SOFC stack by converting the conventional system fuel supply for delivery of a reducing fluid to the anode side of the SOFC stack during normal system shutdown thereby preventing subjecting the hardware to cyclic stress that typically occurs during oxidation. The control strategy further enables the SOFC system to control the temperature gradient that exists across the system hardware thereby eliminating induction of thermal stress on the hardware, hence prolonging the life of the system hardware.

Description:
GOVERNMENT INTEREST 
       [0001]    The present invention was supported in part by a U.S. Government Contract, No. DE-FC2602NT41246. The United States Government may have rights in the present invention. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to hydrogen/oxygen fuel cells having a solid-oxide electrolyte layer separating an anode layer from a cathode layer; more particularly, to fuel cell assemblies and systems including a plurality of individual fuel cells in a stack wherein air and reformed fuel are supplied to the stack; and most particularly, to an apparatus and method for anode oxidation prevention within the stack during normal system shutdown and a stack cooling strategy. 
       BACKGROUND OF THE INVENTION 
       [0003]    Fuel cells, which generate electric current by the electrochemical combination of hydrogen and oxygen, are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a Solid-Oxide Fuel Cell (SOFC). SOFC systems derive electrical power though a high-efficiency conversion process from a variety of fuels including natural gas, liquefied petroleum gas, ethanol, and other hydrocarbon and non-hydrocarbon fuels. Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. 
         [0004]    Each O 2  molecule is split and reduced to two O −2  anions catalytically by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. 
         [0005]    When hydrogen as a feed stock for the fuel cell is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the reformate gas includes CO which is converted to CO 2  at the anode via an oxidation process similar to that performed on the hydrogen. A single fuel cell is capable of generating a relatively small amount of voltage and wattage and, therefore, in practice it is known to stack a plurality of fuel cells together in electrical series. 
         [0006]    Reformed gasoline is a commonly used feed stock in automotive fuel cell applications. Reformate gas is typically the effluent from a catalytic liquid or gaseous hydrogen oxidizing reformer and is often referred to as “fuel gas” or “reformate”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of about 700° C. to about 900° C. 
         [0007]    Since optimum fuel consumption and electrical generation, and therefore optimum efficiency of a SOFC stack, are reached at relatively high operating temperatures, a cooling process needs to be performed prior to normal system shutdowns. The system shutdown is a period that occurs, for example, prior to an extended duration of nonuse. Typically, the SOFC stack is cooled with air utilizing the cathode airflow. Due to the relative high operating temperature of the SOFC stack, typically about 750° C. and higher, and the chemical composition of the anodes, which are the system&#39;s functional and most vulnerable components, purging the entire SOFC stack with cathode air for cooling results in a degrading and fatiguing oxidation of the anodes. The anode side of the fuel cell is, in part, nickel. At temperatures above about 400° C., and in the presence of free oxygen, nickel oxide is formed, which may lead to deterioration of the SOFC stack over time, which may cause failure of the SOFC stack. Therefore, it is harmful to the SOFC stack when oxygen is allowed in the cavities adjacent to the plurality of anodes. 
         [0008]    The currently used method for preventing the oxidation of the anodes is a process in which the cavities in the anode side of the SOFC stack are purged with a fluid containing no free oxygen during system cool down. For example, a blend of bottled reducing gas may be flowed through the anode side of the SOFC stack while the system cools from its operating temperature to a temperature below about 400° C. when harmful oxidation of the anodes ceases. This process requires a reservoir to store, means to pressurize, and hardware to meter the reducing fluid, in addition to hardware and reservoirs required for normal system operation. As a secondary issue, the fluid necessary to perform this purging process is currently not commercially available. Furthermore, a purging process with such reducing fluid may subject the system hardware to an uncontrolled thermal gradient, and therefore may induce unnecessary stress on the anode side of the stack. 
         [0009]    What is needed in the art is a cooling strategy that eliminates the need for supplementary system hardware and mitigates the risk of subjecting the SOFC system to degradation mechanisms during normal system shutdowns. 
         [0010]    It is a principal object of the present invention to provide a SOFC stack cooling strategy for normal system shutdown that utilizes existing system hardware and conventional fuel supply to provide the oxygen free environment required to cool down the system to a point that the functional components of the SOFC system are no longer at risk of degradation. 
       SUMMARY OF THE INVENTION 
       [0011]    Briefly described, an apparatus and method for a normal system shutdown of a SOFC system implements a control strategy that utilizes component hardware already available for normal operation of the SOFC system. This unique and novel control strategy enables the SOFC system to generate the fluid needed for prevention of oxidation during the cooling process of the anode side of the SOFC stack by converting the conventional system fuel supply for the delivery of a reducing fluid to the anode side of the SOFC stack during normal system shutdown. Purging the anode side of the SOFC stack is accomplished with a reducing fluid that is generated by using existing system hardware and the conventional fuel supply. As a result, the anode side of the stack is protected from oxidation and the cyclic stress that such oxidation would subject the hardware to is prevented, thereby prolonging the life of the SOFC system. 
         [0012]    An additional benefit of the invention lies in the system&#39;s ability to control the temperature gradient that exists across the system hardware. The undesirable thermal stress that is currently induced on the hardware during a normal system shutdown when a prior art additional reducing fluid from an additional reservoir is used may therefore be eliminated. Accordingly, the apparatus and method in accordance with the invention not only prevents a potentially detrimental oxidation to susceptible system components from occurring, such as the anodes of the SOFC system, but it also eliminates the need for a currently used second reducing fluid stored in a second reservoir and a currently used secondary purging process of the anode side of the SOFC stack. 
         [0013]    The control strategy in accordance with the invention allows the cooling rate of the SOFC stack to be controlled during a normal system shutdown by an existing control system, as well as provides the oxygen free environment needed to prevent damage from oxidation to the stacks in the SOFC system. Accordingly, cooling the SOFC stack with a controlled temperature gradient to a temperature below the critical temperature for detrimental oxidation is enabled while a reducing environment on the anode side of the stack is maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is a schematic mechanization diagram of an SOFC system in accordance with the invention; and 
           [0016]      FIG. 2  is a schematic flow chart of a cooling strategy for the SOFC system in accordance with the invention. 
       
    
    
       [0017]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring to  FIG. 1 , a schematic mechanization diagram of an SOFC system  100  in accordance with the invention is illustrated. The SOFC system  100  includes at least one SOFC stack  110  as well as auxiliary equipment and controls. SOFC stack  110  includes a plurality of solid-oxide fuel cells  112  stacked together in electrical series. Each of the fuel cells  112  includes a cathode  114  and an anode  116 , the plurality of cathodes  114  forming the cathode side of stack  110  and the plurality of anodes  116  forming the anode side of stack  110 . Because each anode  116  and cathode  114  must have a free space for fluid passage over its surface, the cathode side and the anode side of stack  110  are typically separated by perimeter spacers which are selectively vented to permit fluid flow to the anodes  116  and cathodes  114  as desired but which also form seals on the axial surfaces to prevent fluid leakage from the cathode side of stack  110  to the anode side of stack  110  and vise versa. Thus, all of the cathodes  114  are in parallel pneumatic flow and all of the anodes  116  are in parallel pneumatic flow. SOFC stack  110  is electrically connected to a DC/AC inverter  118  to convert a voltage generated by fuel cells  112  to application power  108  usable by an external load. 
         [0019]    Filtered air  120  entering SOFC system  100  at or near ambient temperature may be preheated to accommodate and regulate the temperature of SOFC stack  110  and is, therefore, controllably passed through a cathode air heat exchanger  122  ahead of stack  110  using hot exhaust stream  128  as a heat source. Filtered air  120  may also be used to cool electronics of an electronic control system  140 , which may include, for example, an internal bus power unit  142 , a controller  144 , and a plurality of sensors and actuators  146 . Air  120  is further passed through a cathode/reformate equalizer heat exchanger  124  before entering SOFC stack  110 . Within stack  110 , air  120  is provided to the surfaces of the cathodes  114 . The total of incoming air  120  is divided among the plurality of cathodes  114  such that each increment of air passes over only a single cathode  114  and then is collected in an air exhaust manifold. The relatively hot spent air  121  coming from cathode  114  may be first utilized by a main system burner  126 . The heat of the hot exhaust stream  128  coming from main burner  126  may be utilized by a main fuel reformer  134  as well as the cathode air heat exchanger  122  before exiting system  100 . 
         [0020]    Fuel  130 , for example gasoline, natural gas, liquefied petroleum gas, ethanol, and other hydrocarbon and non-hydrocarbon fuels, is controllably provided to system  100  by a fuel feed pump  131  that draws fuel  130  from a storage tank. Fuel  130  is combined with a portion of filtered air  120  and in some occasions with anode tail gas  138  in an air/fuel/recycle preparation unit  132  before it is vaporized and fed to the main fuel reformer  134 . Main fuel reformer  134  may derive the heat needed for the reforming processes from the hot exhaust stream  128  coming from main system burner  126 . Main fuel reformer  134  reforms fuel  130  to, principally, hydrogen and carbon monoxide. The effluent exiting main fuel reformer  134 , reformate  135 , is passed through a hydrocarbon cracker  136  where it may be further processed before being fed to the anodes  116  in SOFC stack  110 . Reformate  135  is passed through cathode/reformate equalizer heat exchanger  124  before entering hydrocarbon cracker  136 . Cathode/reformate equalizer heat exchanger  124  is utilized to bring the temperature of the reformate  135  coming from main fuel reformer  134  and the temperature of incoming air  120  to be fed to the cathodes  114  (cathode air) as close together as possible. 
         [0021]    Main fuel reformer  134  and hydrocarbon cracker  136  are used in varying capacity based on the operating point of system  100 . During low power operation of system  100 , air  120  and fuel  130  are processed by main fuel reformer  134  and the effluent (reformate  135 ) passes through hydrocarbon cracker  136  with little or no further processing. Little or no chemical reaction takes place within hydrocarbon cracker  136  in this case. During medium power operation of system  100 , some filtered air  120 , fuel  130 , and anode tail gas  138  (recycle) is processed by the main fuel reformer  134 , however with the addition of recycled anode tail gas  138 , a higher level of H 2 O and CO 2  is contained in the reformate  135 . When this reformate  135  is blended with unprocessed fuel  130  before entering hydrocarbon cracker  136 , the H 2 O, CO 2 , and unprocessed fuel  130  react as they pass through hydrocarbon cracker  136 . The chemical reactions that take place in hydrocarbon cracker  136  are more efficient than those that take place in main fuel reformer  134 , thus boosting the overall efficiency of system  100 . During high power operation of system  100 , all of the fuel  130  entering system  100  may be processed by hydrocarbon cracker  136  and only the anode tail gas  138  may pass through main fuel reformer  134 , using main fuel reformer  134  only as a conduit for the tail gas. During normal system shutdown, no chemical reaction takes place in hydrocarbon cracker  136  and hydrocarbon cracker  136  is used only as a conduit for feeding the reformate  135  formed in main fuel reformer  134  to the anodes  116  of stack  110 . 
         [0022]    The total reformate  135  entering the stack  110  assembly is divided among the plurality of anodes  116  such that each increment of reformate  135  passes over only a single anode  116  and is then collected in the reformate exhaust manifold. Unconsumed fuel  130  from the anodes  116  is fed to main system burner  126  where the fuel is combined with air  120  coming from the cathodes  114  and is burned. The hot burner gases, hot exhaust stream  128 , may be passed through a cleanup catalyst in main fuel reformer  134  and may then be passed through the hot side of cathode heat exchanger  122  to heat the incoming air  120  before being exhausted from system  100 . Unconsumed fuel  130  from the anodes  116  in the form of anode tail gas  138  may be cooled and fed via anode tail gas pump  148  to air/fuel/recycle preparation unit  132  for recycling. 
         [0023]    The electronic control system  140  is utilized to control the flow of air  120  and fuel  130 , as well as an anode tail gas pump  148  that provides cooled anode tail gas  138  (recycle) to air/fuel/recycle preparation unit  132 . Individual flow controllers that are controlled by controller  144  may be included in the air circuit and in the fuel circuit. A flow controller  152  as shown in  FIG. 1  is integrated in the air circuit and controls the flow of filtered air  120  to cathode heat exchanger  122  and air/fuel/recycle preparation unit  132 . A flow controller  154  is shown integrated in a primary fuel circuit and controls the flow of fuel  130  to air/fuel/recycle preparation unit  132  and to main fuel reformer  134 . A flow controller  156  is shown integrated in a secondary fuel circuit and controls a flow of unprocessed fuel  130  to be blended with reformate  135  immediately upstream of hydrocarbon cracker  136 . 
         [0024]    Referring to  FIG. 2 , a cooling strategy  200  for normal system shutdown of the SOFC system  100  shown in  FIG. 1  in accordance with the invention is illustrated. Cooling strategy  200  may be applied when system  100  is in a hot idle or hot operating state  210 . In the hot idle state system  100  is not producing power but has been driven up to a relatively hot operating temperature; and in the hot operating state system  100  is producing power at the relatively high operating temperature. When a user or an onboard diagnostic system, which may be part of the electronic control system  140 , requests a shutdown of system  100  in a step  220 , the following steps  230  and  240  occur in a coordinated fashion. In a step  230  the rate at which fuel  130  is provided to system  100  is reduced. As a result, the amount of reformate  135  produced by main fuel reformer  134  is also reduced. To use as little fuel  130  as possible, the fuel rate of the reformer  134  may be reduced to its minimum-operating limit even though this is not required. At the same time, an external load using application power  108  is removed from system  100  and parasitic loads are placed on external power support, in a step  240 . The external power support may be provided, for example, by an existing external power supply that is used during start up of system  100 . 
         [0025]    In a following step  250 , the electronic control system  140  inverts a desired temperature control strategy in order to start the cooling process of SOFC stack  110 . This control strategy may include the request of a new target temperature for SOFC stack  110 . Such target temperature is preferably a temperature below the oxidation temperature of the anodes  116 . Additional software for calibration of system  100  during cool down of stack  110  may be installed in the already existing system controller  144  in a step  260 . In a following step  270 , a control algorithm holds an inlet temperature of air  120  provided to the cathodes  114  below an outlet temperature of the anodes  116 . The software implemented in controller  144  in step  250  adjusts the temperature of the air  120  provided to the cathodes  114  and the temperature of the reformate  135  fed to the anodes  116  of stack  110  in order to cool stack  110  and system  100  until an oxygen-safe temperature for anodes  116  is reached in a step  280 . Until the oxygen-safe temperature is reached in step  280 , reformate  135  is fed to the anodes  116  to avoid formation of free oxygen around anodes  116 . 
         [0026]    By purging the anode side of stack  110  with reformate  135  during the cool down of stack  110 , air  120  used for cooling the cathode side of stack  110  is prevented from entering the anode side of stack  110  and the need for a currently used supplementary reducing fluid can be eliminated. When stack  110  and, therefore system  100 , reaches a temperature below the oxidation risk of anodes  116 , the supply of fuel  130  to system  100  is stopped, and accordingly the production of reformate  135  in fuel reformer  134  and feeding of reformate  135  to the anodes  116  of stack  110  is stopped. System  100  may be cooled down to a standby state temperature by supplying air  120  alone to stack  110  in a last step  290 . As can be seen, only a primary fuel circuit including flow controller  154 , main fuel reformer  134 , cathode/reformate equalizer heat exchanger  124 , SOFC stack  110  and an air circuit including flow control  152  and cathode air heat exchanger  122  are used by cooling strategy  200  for normal system shutdown of the SOFC system  100 . 
         [0027]    As illustrated in  FIG. 2 , the ability of SOFC system  100  to control the cathode air temperature allows a controlled cool down of SOFC stack  110  upon request, which may be manual or automatic. Accordingly system  100  is able to control the temperature gradient that exists across stack  110  eliminating potential induction of thermal stress within stack  110  thereby prolonging the life of stack  110 . The cooling strategy  200  for normal system shutdown of the SOFC system  100  enables system  100  to generate the fluid used to prevent the oxidation of the anodes  116  during the cool down of stack  110  by converting the conventional system fuel  130  to a reducing fluid. This protects the anode side of stack  110  from oxidation and from the cyclic stress that the oxidation subjects the anodes  116  to, hence prolonging the operational life of stack  110  and system  100 . Accordingly, cooling strategy  200  allows for the cooling rate of stack  110  to be controlled by the conventional control system  140  of system  100 , and also provides the oxygen free environment needed to prevent damage to stack  110  at oxidation enabling temperatures. 
         [0028]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.