Abstract:
An emergency shutdown apparatus for a solid-oxide fuel cell system, including a fuel cell stack, comprises a reservoir containing a reducing fluid, a valve enabling or preventing flow of the reducing fluid from the reservoir to the fuel cell stack, a timing circuit operating and controlling the valve, and a battery powering the timing circuit. The apparatus for an emergency system shutdown is able to operate independently of the main power plant and does not require any active control from the solid-oxide fuel cell system. The disclosed apparatus is entirely a stand-alone component that may be added to any conventional solid-oxide fuel cell system. The apparatus in accordance with the invention can be recharged, allowing the same hardware to be used over and over, however, a disposable unit could be used if found to be desirable.

Description:
RELATIONSHIP TO GOVERNMENT CONTRACTS 
       [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 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 oxygen and reformed fuel are supplied to the stack; and most particularly, to an apparatus for shutdown of a solid-oxide fuel cell system and a method for anode oxidation prevention within the stack during an emergency system shutdown. 
       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 through 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 during normal system shutdowns. The normal 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, purging the entire SOFC stack with cathode air for cooling prior to extended periods of nonuse results in a degrading and fatiguing oxidation of the anodes since oxygen is allowed in the cavities adjacent to 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 and which over time 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 accepted method for preventing the oxidation of the anodes in a laboratory environment is a process in which the cavities in the anode side of the SOFC stack are continuously purged with a fluid containing no free oxygen during a normal cooling process. 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. where harmful oxidation of the anodes ceases. Though a purge is required, such continuous purging process requires more pressurized reducing gas than can be stored on an onboard system, such as what might be needed in a non-stationary motor vehicle. 
         [0009]    A typical SOFC system requires usually between four to eight hours to cool from its operating temperature to a temperature that will not harm the anode side of the stack. During this time, a continuous purging process will exhaust more reducing gas than a volume of storage bottles, equal to the size of the entire system, can hold. While purging the anode side of the stack during a normal cooling process with a reducing fluid produced by the SOFC system itself, such as utilization of exhaust gas where oxygen has been reduced below ignition concentration, has been proposed, this is not a solution for emergency system shutdowns when an unexpected event requires an immediate shutdown of the entire SOFC system including the use of external power. In such emergency situation, the SOFC system does not have time to execute its normal shutdown process and the anode components of the stack cannot be protected from detrimental oxidation. No protecting mechanisms for the anode side of the stack are currently known for such emergency situation. 
         [0010]    What is needed in the art is a cooling apparatus and strategy that prevents detrimental oxidation of the anode side of the stack from occurring in the event that a sudden and complete emergency shutdown of the SOFC system is necessary. 
         [0011]    It is a principal object of the present invention to provide an apparatus that enables a SOFC system to safely cool down during an emergency system shutdown, while the anodes of the stack are protected from oxidation by a reducing environment. 
         [0012]    It is a further object of the invention to provide an apparatus that requires only a fraction of space required by prior art gas bottles to provide the desired oxygen-free environment. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly described, an apparatus and method for an emergency shutdown of a SOFC system enables prevention of the degrading and fatiguing oxidation of the anodes of the stack in the event that a sudden and complete system shutdown is necessary. 
         [0014]    The unique and novel apparatus utilizes a battery, a timing circuit, a temperature sensor, a reservoir containing a reducing fluid, and a valve. In order to control the flow of reducing fluid to the stack of the SOFC system, the valve is controlled by the timing circuit. Upon a system emergency shutdown, the timing circuit will be enabled and will pulse the valve, allowing an intermittent flow of the reducing fluid. For a prescribed period of time the valve is opened allowing the flow of reducing fluid, and then the valve is closed, blocking the flow of reducing fluid for a prescribed period of time. This flow pulsation may continue until the temperature sensor, located within the system, reaches a prescribed set point that preferably lies below the critical temperature for anode oxidation, or until the reducing fluid reservoir is empty. It is not necessary for the reducing fluid flow to be cut off once the desired temperature of the system is reached. However, a reservoir of reducing fluid may be used multiple times on a single filling if it can be shut off once a safe temperature is achieved. By intermittently providing a reducing fluid to the anode side of the fuel cell stack, the volume of the reducing fluid required per shutdown can be reduced compared to prior art continuously flowing systems, while an oxygen free environment surrounding the anodes of the stack can be maintained. 
         [0015]    Another advantage of the apparatus for an emergency system shutdown in accordance with the invention is the ability to operate independently of the main power plant. The apparatus does not require any active control from the SOFC system, and therefore can be used in an immediate emergency shutdown situation. The disclosed apparatus is entirely a stand-alone component that may be added to any conventional SOFC system. The apparatus in accordance with the invention can be recharged, allowing the same hardware to be used over and over, however, a disposable unit could be used if found to be desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a schematic mechanization diagram of an SOFC power plant in accordance with the invention; and 
           [0018]      FIG. 2  is a schematic chart of an example pulsing strategy in accordance with the invention. 
       
    
    
       [0019]    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 
       [0020]    Referring to  FIG. 1 , a schematic mechanization diagram of a Solid-Oxide Fuel Cell (SOFC) power plant  100  in accordance with the invention shows a SOFC system  110  and an emergency shutdown apparatus  120 . 
         [0021]    The SOFC system  110  includes at least one SOFC stack  112  as well as auxiliary equipment and controls. SOFC stack  112  includes a plurality of solid-oxide fuel cells stacked together in electrical series. Each of the fuel cells includes a cathode  114  and an anode  116 , the plurality of cathodes  114  forming the cathode side of stack  112  and the plurality of anodes  116  forming the anode side of stack  112 . 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  112  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  112  to the anode side of stack  112  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. 
         [0022]    Emergency shutdown apparatus  120  includes a battery  122 , a timing circuit  124 , a valve  126 , and a reservoir  128  holding a reducing fluid  130 . Battery  122  may be any battery suitable as an emergency-stop circuit battery and is used only to power timing circuit  124 . Battery  122  may be a rechargeable battery. Timing circuit  124  operates and controls valve  126 . Valve  126  may be a solenoid actuated valve or any other valve operable by a timing device, such as timing circuit  124 . Valve  126  is connected with reservoir  128  and enables or prevents the flow of reducing fluid  130  from reservoir  128  to SOFC stack  112 . 
         [0023]    Reducing fluid  130  may be a reducing gas, for example a gas mixture containing 95% nitrogen and 5% hydrogen, or a reducing liquid. In any case, reducing fluid  130  may not contain free oxygen. Reducing fluid  130  may be pressurized within reservoir  128 , thereby eliminating a need for a pump to flow the reducing fluid toward stack  112 . The size of reservoir  128  is adapted to hold enough reducing fluid  130  to provide an oxygen-free environment around the anodes  116  of SOFC stack  112  during at least one emergency shutdown of fuel cell system  110 . Reservoir  128  may be refillable after an emergency use and may also be replaceable as a reservoir cartridge. It may further be possible to use reformate as reducing fluid  130  and to fill and refill reservoir  128  with reformate produced during normal operation of SOFC system  110 . In this case a pump or compressor that pumps reformate produced by system  110  into reservoir  128  during normal system operation could be integrated between fuel cell system  110  and emergency shutdown apparatus  120 . 
         [0024]    Emergency shutdown apparatus  120  further includes a relay  132  positioned between battery  122  and timing circuit  124 . Relay  132  is used to activate timing circuit  124  and, therefore, to enable operation of emergency shutdown apparatus  120 . Relay  132  may be a relay that is normally closed but is opened by a signal from a controller  118  of SOFC system  110  when system  110  is in a normal operating state at elevated temperatures. In case of an emergency shutdown of system  110 , this signal is lost and relay  132  closes allowing battery  122  to power timing circuit  124 . 
         [0025]    Emergency shutdown apparatus  120  may further include a temperature sensor  134  that may be used to disable operation of timing circuit  124  and, therefore, of emergency shutdown apparatus  120 . Temperature sensor  134  may be positioned downstream of or within SOFC stack  112 . When temperature sensor  134  reaches a prescribed set point or calibrated threshold, such as a temperature below the oxidation temperature of the anodes  116 , which is typically around about 400° C., relay  132  or a second relay or other type of temperature switch (not shown) may be activated to disable operation of timing circuit  124  and, therefore, to stop flow of reducing fluid  130  to stack  112 . By integrating temperature sensor  134  and by enabling shut off of the flow of reducing fluid  130 , reservoir  128  may be used multiple times on a single filling. Still, it is not necessary or may not be desirable to include temperature sensor  134  to cut off the flow of reducing fluid  130  once a safe temperature of stack  112  is reached, since it is also possible to flow reducing fluid  130  to stack  112  until reservoir  128  is empty. In this case, reservoir  128  will simply empty and battery  122  will drain, neither occurrence being problematic, since the fluid reservoir and battery charge can be replenished after the emergency shutdown procedure is completed. It may further be possible to use a signal from temperature sensor  134  instead of the above described signal from controller  118  to activate operation of apparatus  120 . 
         [0026]    When SOFC system  110  is operated in a mode at elevated temperatures, such as during warm-up, in a power producing mode, or at a hot idle mode, an emergency shutdown of system  110  may be initiated at any time by either controller  118  of system  110  (automatically) or by an operator (manually). Once the emergency shutdown of SOFC system  110  has been initiated, emergency shutdown apparatus  120  is activated. Relay  132  closes thereby enabling battery  122  to power timing circuit  124 . Timing circuit  124  pulses valve  126 , thereby opening valve  126  for a first prescribed period of time and closing valve  126  for a second prescribed period of time in an alternating fashion. As a result, flow of reducing fluid  130  to the anodes  116  of stack  112  is allowed for the first prescribed period of time and flow of reducing fluid  130  to the anodes  116  of stack  112  is blocked for the second prescribed period of time generating a flow pulsation. This flow pulsation of reducing fluid  130  continues until temperature sensor  134  reaches a prescribed set point and disables operation of timing circuit  124  or until reservoir  128  is empty. While it is possible to provide a continuous flow of reducing fluid  130  to stack  112 , a pulsed, intermittent flow is preferred in order to keep the size of reservoir  128  as small as possible. A continuous flow of reducing fluid  130  to stack  112  is not needed to achieve an oxygen-free environment around the anodes  116  of stack  112 . Upon opening of valve  126 , reducing fluid  130  may be provided directly to stack  112  or may be provided to existing conduits of SOFC system  110  upstream of stack  112 . 
         [0027]    Referring to  FIG. 2 , an example of a pulsing strategy  200  is illustrated. As can be seen, valve  126  is operated at a fixed rate. Upon activation of timing circuit  124  at elapsed time  212  of zero minutes, timing circuit  124  provides a voltage  214  of a certain preset value V open  to open valve  124  for a first period of time  220 . Then, voltage  214  drops down to zero volts to close valve  124  for a second period of time  230  creating a flow pulsation. Voltage  214  is shown to alternate between V open  and zero volts at a constant rate until a temperature below the oxidation risk of anodes  116  is reached at an elapsed time t cool . At t cool , temperature sensor  134  sends a signal to deactivate timing circuit  124 . When temperature sensor  134  is not used to deactivate timing circuit  124 , the flow pulsation may continue beyond t cool  until reservoir  128  is empty. 
         [0028]    The first period of time  220  and the second period of time  230  are infinitely variable and may be calibrated as desired for a specific application. The first period of time  220  may be shortened or prolonged to keep valve  124  open for a shorter or longer time, respectively. The second period of time  230  may be shortened or prolonged to keep valve  124  closed for a shorter or longer time, respectively. 
         [0029]    While first period of time  220  and second period of time  230  are shown in  FIG. 2  to be constant over the entire elapsed time  212 , they may be varied over elapsed time  212 . For example, the occurrences of valve  126  openings may be reduced by extending the second periods of time  230  with increasing elapsed time  212 . It may further be possible, to sense the state of reservoir  128  and to adjust the timing of valve  126  accordingly. 
         [0030]    By purging the anode side of stack  112  intermittently with reducing fluid  130  after an emergency shutdown of SOFC system  110 , air and therefore oxygen is prevented from entering the anode side of stack  112  and stack  112  can safely cool down to a temperature where detrimental oxidation of the anodes  116  will not occur anymore, typically below 400° C. By pulsing the flow of reducing fluid  130 , the size of reservoir  128  may be minimized. 
         [0031]    By providing emergency shutdown apparatus  120  as a stand-alone unit that operates independently from the SOFC system  110  and that does not require any active control from SOFC system  110 , an oxygen-free environment needed to prevent damage to stack  112  at oxidation enabling temperatures can be provided in the event that a sudden and complete shutdown of the SOFC system  110  is necessary. 
         [0032]    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.