Abstract:
The control system ( 10 ) utilizes an oxygen sensor ( 78 ) to sense an oxygen concentration within a burner exhaust ( 66 ) of a fuel processing system ( 40 ), wherein the burner device ( 44 ) utilizes an anode exhaust stream from a fuel cell ( 12 ) to supply heat to a reformer ( 48 ). If the anode utilization by the fuel cell ( 12 ) anode ( 14 ) exceeds an acceptable range, less hydrogen is available for the burner device ( 44 ) and more oxygen will therefore be sensed by the oxygen sensor. An oxygen sensor controller ( 80 ), in response to the increase in sensed oxygen, increases flow of a fuel feedstock ( 42 ) into the reformer ( 48 ) to provide more hydrogen fuel to the anode ( 14 ) to thereby return anode utilization to an acceptable anode utilization range. An opposite control sequence occurs if anode utilization falls below the acceptable range.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the disclosure especially relates to a system and method for controlling use of a hydrogen-rich fuel at an anode of a fuel cell power plant that utilizes fuel produced by a reformer within a fuel processing system of the plant. 
       BACKGROUND ART 
       [0002]    Fuel cells are well known and are commonly used to produce electrical current from a hydrogen-rich fuel stream and an oxygen-containing oxidant stream to power electrical apparatus. Fuel cells are typically arranged in a cell stack assembly having a plurality of fuel cells arranged with common manifolds and other components such as a fuel processing system, controllers and valves, etc. to form a fuel cell power plant. In such a fuel cell power plant of the prior art, it is well known that fuel is produced by the fuel processing system reformer and the resulting hydrogen-rich fuel stream flows from the reformer through a fuel inlet line into anode flow fields of the fuel cells. An oxygen stream simultaneously flows through cathode flow fields of the fuel cells to produce electricity. 
         [0003]    Fuel cell power plants are known to provide electricity for differing types of apparatus. For example, many efforts are being undertaken to produce a fuel cell power plant utilizing “proton exchange membrane” (PEM) electrolyte fuel cells to power transportation vehicles. Fuel cells utilizing phosphoric acid electrolytes are also known to power stationary electricity generating plants. In such fuel cell power plants, it is known to use a fuel processing system having a reformer that undergoes an endothermic reaction therefore requiring addition of heat, such as a catalytic steam reformer. 
         [0004]    One source of heat for such reformers is provided by excess hydrogen leaving fuel cells of the plant within an anode exhaust stream. The anode exhaust stream is directed to a burner device and ignited. In a phosphoric acid electrolyte fuel cell (“PAFC”) power plant, heat from the ignited exhaust is transferred by way of a convection and conduction to the reformer catalyst to supply energy to the endothermic reacting reformer. In a proton exchange membrane electrolyte (“PEM”) power plant, heat from the ignited exhaust may be used to heat a water supply into steam which is then directed into a reformer to transform a hydrocarbon fuel feedstock into hydrogen gas and carbon by-products. The hydrogen gas is then directed through a fuel inlet line into anode flow fields of the fuel cells. As described in U.S. Pat. No. 6,818,336 that issued on Nov. 16, 2004 to Isom et al., which patent is owned by the owner of all rights in the present invention, it is known to control flow of the fuel feedstock into a reformer as a function of properties of steam within the reformer and the power demand on the fuel plant. 
         [0005]    For fuel cell power plants that are configured to operate as long-term stationary power plants, efficient control of a rate of flow of the fuel feedstock into the fuel processing system and the resultant flow of hydrogen into the fuel cells of the plant requires precise management as a result of unpredictable disturbances that affect such power plants. A fundamental disturbance affecting load-following fuel cell power plants is a change in power demand. A change in power demand produces a change in the current drawn from the fuel cells of the plant, thus changing the optimal flow of hydrogen. 
         [0006]    To react to changes in power demand, it is known that a fuel flow set point can be varied based on the current drawn from the fuel cells. This basic control mechanism is necessary, but not sufficient, to adequately control the flow of hydrogen-rich reformate fuel to fuel cell anodes. The basic control is not adequate because there are other disturbances that affect the power plant, even when power demand is constant. 
         [0007]    One such disturbance is the fluctuating fuel heating value of a fuel such as natural gas. For example, if a fuel cell power plant has an expected operational duration of ten years, it is known that the heating value of natural gas supplied to the plant will vary significantly over such a ten year span. 
         [0008]    A second common disturbance includes changes in fuel processing system hydrogen conversion efficiency. As an example, in a catalytic steam reformer, it is known that the effectiveness of the catalysts deteriorates over any given reformer life span. A third disturbance giving rise to current transients is changes in a steam-carbon ratio within the reformer. 
         [0009]    Such changes may arise from variations in performance of a steam ejector resulting from mechanical degradation of the ejector or other performance variations of the ejector and related apparatus. 
         [0010]    As a fuel cell power plant operates, for purposes of power plant efficiency, transient performance, and reformer durability, it is important to maintain anode utilization within a certain optimal performance range. For an exemplary fuel cell power plant, that optimal anode utilization range is between about 78 percent (%) and about 84%. (For purposes herein, by the phrase “anode utilization” it is meant that hydrogen fuel at the anode catalyst is dissociated into hydrogen ions and electrons. For example, if the anode utilization is 82%, that means 82% of the supply of hydrogen fuel is transformed into water at the cathode, while the remaining 18% of hydrogen fuel passes out of the anode flow field within the anode exhaust stream.) For most fuel cells it is known that having the anode utilization exceed the optimal range gives rise to damage to the anode catalyst and/or support materials for the catalyst. In contrast, operating the fuel cell at an anode utilization below the optimal range causes loss of valuable hydrogen fuel. 
         [0011]    An effort to maintain anode utilization within the optimal range while the fuel cell power plant experiences one or more of the disturbances described above has produced acceptable results. In a PAFC power plant, this effort includes careful monitoring of a temperature within a reformer that receives heat from the ignited fuel cell anode exhaust. A feedback system for a PEM power plant is described in the aforesaid U.S. Pat. No. 6,818,336. If the anode utilization of the fuel cells exceeds the optimal range, more hydrogen will be used at the anode and therefore less hydrogen will be in the anode exhaust stream. Consequently, the amount of energy in the burner device from the exhaust stream will be less and temperature sensors within the reformer will sense a decrease in the temperature within the reformer. The decrease in the sensed temperature within the reformer is then communicated to a controller which increases a rate of flow of the fuel feedstock into the reformer. This in turn provides a greater amount of hydrogen fuel to the anode flow field to bring the anode utilization back down within the optimal range. 
         [0012]    Similarly, if the anode utilization of the fuel cells decreases below the optimal range, less hydrogen fuel is used at the anode and therefore more hydrogen is within the anode exhaust stream that is fed to the burner device. Consequently, because the burner device has more fuel a temperature within the reformer will increase. The reformer temperature sensors will then communicate the increase in temperature to the controller, which in turn decreases the rate of flow of the fuel feedstock into the reformer. This provides less hydrogen fuel to the anode flow field to bring the anode utilization back up to within the optimal range. 
         [0013]    In manufacture of a fuel cell power plant including such a reformer temperature control system for anode utilization, the power plant is typically tuned during factory testing by establishing a reformer temperature set point as a function of fuel cell current using sensitive (e.g., gas chromatography) measurements to establish reformer temperatures that correspond to anode utilization within an optimal range. 
         [0014]    While the reformer temperature control system provides acceptable operation of the fuel cell power plant, the system involves great costs and requires enormous care. For example, if the fuel cell power plant is designed and manufactured to have a ten-year life span, the temperature sensors within the reformer must generate precise readings within exceedingly harsh conditions for the entire ten years. It is known that steam temperatures passing through stainless steel tubes in a catalytic bed within a catalytic steam reformer often exceed 650 degrees Celsius (C.°), while reformer temperatures may be below freezing when the plant is not operating and exposed to ambient conditions. Additionally, acceptable temperature sensors are typically threaded into such tube assemblies within sealed reformer containers with wire communication links passing through the containers. If such a temperature sensor malfunctions, it is extremely costly and disruptive to operation of the power plant to remove and replace the broken sensor within the complex reformer of the fuel processing system. Moreover, such sensors are necessarily very expensive. 
         [0015]    Consequently, there is a need for a fuel cell power plant that includes an efficient and inexpensive system for control of anode utilization during steady-state operation of the plant as well as during electrical current transients resulting from various types of disturbances. 
       SUMMARY 
       [0016]    The disclosure is directed to an anode utilization control system for a fuel cell power plant for generating electrical current from an oxidant stream and a hydrogen-rich fuel stream. The system includes at least one fuel cell including an anode catalyst and a cathode catalyst secured to opposed sides of an electrolyte, an anode flow field defined in fluid communication with the anode catalyst and with a fuel inlet line for directing flow of the hydrogen-rich fuel stream from the fuel inlet line adjacent the anode catalyst and out of the anode flow field through an anode exhaust. The fuel cell also includes a cathode flow field defined in fluid communication with the cathode catalyst and with a source of the oxidant for directing flow of the oxidant stream from an oxidant inlet line adjacent the cathode catalyst and out of the cathode flow field through a cathode exhaust. 
         [0017]    The power plant also includes a fuel processing system for generating the hydrogen-rich fuel stream from a fuel feedstock. The fuel processing system has a burner device configured to transmit heat to an endothermic reacting reformer by either transmitting heat directly into the reformer by conduction and convection through a heat transfer line from fuel cell anode exhaust ignited within the burner device, or by igniting fuel cell anode exhaust within the burner device to generate steam within a boiler of the burner device and directing the steam from the boiler through a steam transfer line into the reformer that is secured in fluid communication with the steam transfer line. A fuel feedstock inlet directs the fuel feedstock into the reformer to be reformed into the hydrogen-rich fuel stream. The reformer is also secured in fluid communication with the fuel inlet line for directing the reformed hydrogen-rich fuel stream through the fuel inlet line into the fuel cell. A burner feed line is secured in fluid communication between the burner device and the anode exhaust for directing an anode exhaust stream into the burner device to be burned and out of the burner device through the reformer, and out of the reformer through a burner exhaust. Heat is transferred by way of conduction from the burner device to either the endothermic reacting reformer directly, or to generate steam within a steam generator component of the burner device, which steam is then directed to the endothermic reacting reformer. 
         [0018]    A key component of the system is an oxygen sensor that is secured in fluid communication with the burner exhaust for sensing a concentration of oxygen within the burned anode exhaust stream passing out of the burner device and reformer through the burner exhaust. An oxygen sensor controller is also secured in communication between the oxygen sensor and a fuel flow control valve that is secured to the fuel feedstock inlet line. The oxygen sensor controller is configured to selectively control flow of the fuel feedstock into the reformer in response to sensed oxygen concentrations within the burned anode exhaust stream. 
         [0019]    The anode utilization control system may be used to maintain anode utilization by the fuel cell within an optimal anode utilization range. As described above, “anode utilization” means that hydrogen fuel at the anode catalyst is dissociated into hydrogen ions and electrons. (For purposes of efficiency herein, the phrase “anode catalyst” is used interchangeably with the word “anode” to mean the same fuel cell component.) If the anode utilization of the fuel cell exceeds the optimal range, more hydrogen will be used at the anode and therefore less hydrogen will be at the anode exhaust stream. Therefore, the amount of energy directed to the reformer burner device from the anode exhaust stream will decrease so that less oxygen is consumed in burning the hydrogen fuel within the burner device. Consequently, an amount of oxygen within the burner exhaust will increase. The oxygen sensor will sense this increase and then communicate to the oxygen sensor controller to increase a rate of flow of the fuel feedstock into the reformer. In contrast, if the anode utilization of the fuel cell decrease below the optimal range less hydrogen fuel is used at the anode and therefore more hydrogen is within the anode exhaust stream that is fed into the burner device. Consequently because more hydrogen is available within the burner device, more oxygen will be consumed and the amount of oxygen within the burner exhaust will decrease. The oxygen sensor will sense this decrease and communicate to the oxygen sensor controller to decrease the rate of flow of the fuel feedstock into the reformer. An exemplary oxygen sensor may be a wide-range air fuel sensor that utilizes a Nernst cell to generate a voltage responsive to an oxygen concentration within the burner exhaust. 
         [0020]    Use of the present oxygen sensor and oxygen sensor controller to control anode utilization provides many benefits. First, the oxygen sensor may be placed at a location adjacent the burner exhaust that is much easier to access for purposes of efficiency of installation, maintenance and replacement of the sensor without significant disruption to the operation of the power plant. Second, the oxygen sensor provides rapid indications of changes in anode utilization without a need to await for subsequent changes in temperatures within the reformer. Additionally, use of the present oxygen sensor and oxygen sensor controller to control anode utilization reduces or eliminates any need for time-consuming and expensive factory-tuning of a schedule for reformer temperature set point as function of fuel cell current. 
         [0021]    Perhaps most importantly, experimentation with the present anode utilization control system demonstrates that adjusting flow of fuel feedstock into the reformer to maintain a constant burner exhaust oxygen concentration while holding electrical current produced by the power plant fixed and while holding burner air fixed, effectively maintains anode utilization within a optimal utilization band, and thereby eliminates any transient impact resulting from disturbances in fuel heating value of the fuel feedstock, hydrogen production efficiency the fuel processing system, and/or a changes in a steam carbon ratio within the fuel processing system. 
         [0022]    Accordingly, it is a general purpose of the present disclosure to provide an anode utilization control system for a fuel cell power plant utilizing a reformer produced fuel that overcomes deficiencies of the prior art. 
         [0023]    It is a more specific purpose to provide an anode utilization control system for a fuel cell power plant that increases operating efficiencies of the power plant and decrease manufacture and maintenance costs of the plant. 
         [0024]    These and other purposes and advantages of the present anode utilization control system for a fuel cell power plant will become more readily apparent when the following description is read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWING 
         [0025]      FIG. 1  is a simplified schematic representation of an anode utilization control system for a fuel cell power plant of the present disclosure. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    Referring to the drawings in detail, an anode utilization system for a fuel cell power plant is shown in  FIG. 1  and is generally designated by the reference numeral  10 . The system or power plant  10  includes at least one fuel cell  12  including an anode catalyst  14  and a cathode catalyst  16  secured to opposed sides of an electrolyte  18 , an anode flow field  20  defined in fluid communication with the anode catalyst  14  and with a fuel inlet line  22  for directing flow of a hydrogen-rich fuel stream from the fuel inlet line  22  and through the anode flow field  20 , adjacent the anode catalyst  14  and out of the anode flow field  20  through an anode exhaust  24  and anode exhaust valve  25 . The fuel inlet line  22  includes a fuel inlet valve  23  for selectively controlling flow of the fuel into the anode flow field  20 . 
         [0027]    The fuel cell  12  also includes a cathode flow field  26  defined in fluid communication with the cathode catalyst  16  and with an oxidant source  28  for directing flow of an oxidant stream from an oxidant inlet line  30  through the cathode flow field  26  adjacent the cathode catalyst  16  and out of the cathode flow field  26  through a cathode exhaust  34  and cathode exhaust valve  36 . An oxidant inlet valve  32  is secured to the oxidant inlet line  30  for selectively controlling flow of the oxidant stream from the oxidant source  28  into the cathode flow field  26 . 
         [0028]    The power plant  10  also includes a fuel processing system  40  for generating the hydrogen-rich fuel stream from a fuel feedstock  42  stored within a fuel feedstock source  43 . The fuel processing system  40  has a burner device  44  configured to transmit heat to an endothermic reacting reformer  48  by either transmitting heat directly into the reformer  48  by conduction and convection through a heat transfer line  46  from fuel cell anode exhaust ignited within the burner device  44 , or by igniting the fuel cell anode exhaust within the burner device  44  to generate steam within a boiler  45  of the burner device  44  and directing the steam from the boiler through a steam transfer line  49  into the reformer  48  that is secured in fluid communication with the steam transfer line  49 . For example, if the fuel processing system  40  is for a PAFC system, the ignited fuel cell anode exhaust stream would transfer heat directly through the heat transfer line  46 . If the fuel processing system was configured for a PEM system, the ignited fuel cell anode exhaust stream would generate steam within the boiler  45  that would be transferred to the reformer  48  through the steam transfer line  49 . The reformer  48  may be any heated reformer means  48  for generating a hydrogen-rich fuel stream from a fuel feedstock  42 , wherein the reformer  48  requires heat, such as a catalytic steam reformer. The reformer  48  includes a heat-exchange component  50  secured in heat exchange relationship with a fuel passage component  52  of the reformer  48 . The heat-exchange component  50  may be configured to consist of a plurality of tubes (not shown) through which steam passes to a reformer steam exhaust  47 , and wherein the tubes are surrounded by a catalyst bed (not shown). 
         [0029]    A fuel feedstock inlet line  54  directs the fuel feedstock  42  through a fuel pump  55  configured to operate alone or in conjunction with a feedstock fuel flow control valve  56  into the reformer  48  to be reformed into the hydrogen-rich fuel stream. The fuel flow control valve  56  and fuel pump  55  may be any flow control device means  56 ,  55  capable of performing the described function of pumping the fuel feedstock  42  into the reformer  48  at variable rates, such as a standard impeller pump, centrifugal pump, gravity head and/or a pressurized container  43  and control valve  56 , etc. The steam inlet line  49  from the boiler  45  is also secured in fluid communication with the fuel feedstock inlet line  54  down stream from the flow control device  55 ,  56  to supply steam to the reformer  48 . Additionally, for some embodiments, such as a PAFC system, a steam or water source  68  may be secured in fluid communication through a second steam inlet line  80  with the fuel feedstock inlet line  54  down stream from the flow control device  55 ,  56  to supply steam to the reformer  48 . In such high temperature PAFC systems, the steam source  68  may be system coolers (not shown) so that no boiler  45  is required. The reformer  48  is also secured in fluid communication with the fuel inlet line  22  for directing the reformed hydrogen-rich fuel stream through the fuel inlet line  22  into the fuel cell  12 . The fuel processing system  40  may also include further components secured in fluid communication with the fuel inlet line  22 , such as a shift converter  58  and an a selective oxidizer  60  to further condition the fuel stream and process by-products thereof. 
         [0030]    A burner feed line  62  is secured in fluid communication between the burner device  44  and the anode exhaust  24  for selectively directing flow of an anode exhaust stream from the anode exhaust  24  into the burner device  44  to be burned. The burned anode exhaust stream is then directed to flow out of the burner device  44  through a heat transfer line  46  into and through the heat exchange component  50  of the reformer  52 . A burner exhaust  66  directs flow of the burned anode exhaust stream out of the reformer  48  and/or out of the plant  10 . Additionally, a burner air supply  72  may provide air at pre-determined fixed or variable rates through a burner air supply control valve  74  secured on a burner air feed line  76  that is secured in fluid communication between the burner air supply  72  and the burner device  44 . 
         [0031]    The anode utilization control system  10  also includes an oxygen sensor  78  that is secured in fluid communication with the burner exhaust  66  for sensing a concentration of oxygen within the burned anode exhaust stream passing out of the burner exhaust  66 . An oxygen sensor controller  80  is also secured in communication, such as through communication lines  82 ,  84  between the oxygen sensor  78  and the fuel flow control valve  56  that is secured in fluid communication with the fuel feedstock inlet line  54 . The oxygen sensor controller  80  is configured to selectively control flow of the fuel feedstock  42  out of the fuel feedstock source  43  and into the reformer  48  in response to oxygen concentrations sensed by the oxygen sensor  78  within the burned anode exhaust stream passing through the burner exhaust  66 . The oxygen sensor controller may achieve such control of the rate of flow of the fuel feedstock into the reformer  48  by executing control over the fuel flow control valve  56  and/or the fuel pump  55 . 
         [0032]    It is noted that the communication lines  82 ,  84  between the oxygen sensor control  80  and the fuel flow control valve  56  and/or fuel pump  55  may be traditional electric transmission lines, or in contrast may be any technology capable of signaling sensed information, such as wireless transmission, mechanical signals and mechanical or manual actuators, electro-mechanical apparatus, etc. The oxygen sensor controller  80  may be any oxygen sensor controller  80  capable of performing the functions described herein. For example, the controller  80  may be a computer, a micro-computer, electro-mechanical switches, manually operated actuators activated to response to visual indicators, such as gauges, lights, etc. The oxygen sensor  78  may be any oxygen sensor capable of measuring a concentration of oxygen within a burned anode exhaust stream within a tolerance of plus or minus 5.0%. An exemplary oxygen sensor  78  may be a wide-range air fuel sensor that utilizes a Nernst cell to generate a voltage responsive to changes in oxygen concentration within the burner exhaust  66 . A example of an acceptable oxygen sensor is an oxygen sensor sold by the model name “Lambda Sensor” in the “LRU” product line, manufactured by the Robert Bosch LLC company of 2800 S. 25th Ave. Broadview, Ill. 60155. 
         [0033]    Exemplary tests have been performed using the anode utilization control system  10  of the present invention to establish the value of adjusting flow of the fuel feedstock  42  into the reformer  48  to maintain a constant burner device  44  exit oxygen concentration while holding fuel cell electrical current output constant and while providing a constant supply of air to the burner device  44 . These tests are documented in Table 1, and they provide evidence that maintaining a constant burner exhaust oxygen concentration with electrical current output constant and while providing a constant supply of air to the burner device  44  effectively eliminates any electrical current transients resulting from; a. disturbances in fuel heating value; b. disturbances in fuel processing system  40  production efficiency; and, c. disturbances in a steam-carbon ratio. The fuel cell subject to analysis in Table 1 has an optimal anode utilization range of between about 78% and 84%. (For purposes herein, the word “about” is to mean plus or minus 15%). In all scenarios, the air supplied to the burner device is maintained at a constant flow rate. In an embodiment of the present system  10 , air would be supplied to the burner device at a constant rate as a function of current. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Example of how fuel flow feedback using burner oxygen concentration 
               
               
                 measurements effectively rejects disturbances 
               
               
                 to maintain correct anode utilization 
               
             
          
           
               
                   
                 Parameters 
               
             
          
           
               
                   
                   
                 Fuel 
                 Hydrogen 
                 Steam 
                   
                 Oxygen 
                   
               
               
                   
                   
                 LHV, 
                 production 
                 carbon 
                 Anode 
                 Concentration 
                 Fuel flow, 
               
               
                 # 
                 Scenario 
                 kJ/gmol 
                 efficiency 
                 ratio 
                 utilization 
                 leaving burner 
                 gmol/s 
               
               
                   
               
             
          
           
               
                 1 
                 Base scenario 
                 802.10 
                 113% 
                 3.25 
                 80.0% 
                 3.0% 
                 1.451 
               
               
                 2a 
                 Disturbance in fuel 
                 866.27 
                 113% 
                 3.25 
                 74.1% 
                 0.2% 
                 1.451 
               
               
                   
                 LHV, uncorrected 
               
               
                 2b 
                 Disturbance in fuel 
                 866.27 
                 113% 
                 3.25 
                 79.8% 
                 3.0% 
                 1.348 
               
               
                   
                 LHV, corrected by 
               
               
                   
                 control scheme 
               
               
                 3a 
                 Disturbance in 
                 802.10 
                 102% 
                 3.25 
                 89.0% 
                 6.8% 
                 1.450 
               
               
                   
                 hydrogen production 
               
               
                   
                 efficiency, 
               
               
                   
                 uncorrected 
               
               
                 3b 
                 Disturbance in 
                 802.10 
                 102% 
                 3.25 
                 80.3% 
                 3.0% 
                 1.606 
               
               
                   
                 hydrogen production 
               
               
                   
                 efficiency, corrected 
               
               
                   
                 by control scheme 
               
               
                 4a 
                 Disturbance in 
                 802.10 
                 113% 
                 4 
                 80.0% 
                 2.6% 
                 1.451 
               
               
                   
                 steam-carbon, 
               
               
                   
                 uncorrected 
               
               
                 4b 
                 Disturbance in steam- 
                 802.10 
                 113% 
                 4 
                 80.8% 
                 3.0% 
                 1.436 
               
               
                   
                 carbon, corrected by 
               
               
                   
                 control scheme 
               
               
                   
               
             
          
         
       
     
         [0034]    Reviewing Table 1, we see a “Base scenario” identified by reference numeral  1  in the column on the left side of the Table. This Base scenario identifies a fuel heating value (“Fuel LHV”) of 802.10 kJ/gmol; a “Hydrogen production efficiency” of the fuel processing system of 113%; a “Steam carbon ratio” of 3.25; an “Anode utilization” of 80.0%; an “Oxygen concentration leaving burner exhaust” of 3.0%; and, a feedstock “fuel flow” of 1.451 gmol/s. In the Scenarios that follow in Table 1 the three above described disturbances are evaluated. 
         [0035]    In Scenario  2   a  we see an uncorrected disturbance in the fuel heating value from 802.10 kJ/gmol to 866.27 kJ/gmol which results in an anode utilization of 74.1%, well outside the optimal anode utilization range. In scenario  2   b  we see that the fuel flow has been changed from 1.451 gmol/s to 1.348 gmol/s to produce a base scenario oxygen concentration of 3.0%. This results in producing an anode utilization of 79.8%, which is back within the optimal range. 
         [0036]    In scenario  3   a , we see a disturbance in the hydrogen production efficiency of the fuel processing system declining from the base scenario of 113% to 102%. This results in an anode utilization of 89.0%, well above the optimal range. Scenario  3   b , shows an increase in the fuel flow from the  3   a  scenario of 1.451 gmol/s to 1.606 gmol/s, which again produces a base scenario oxygen concentration of 3.0%. This also results in an anode utilization of 80.3%, which is back within the optimal range. 
         [0037]    In scenario  4   a , Table 1 shows a disturbance in the steam carbon ratio from the base scenario of 3.25 to a ratio of 4, which, while keeping the anode utilization within the optimal range, nonetheless results in a decrease in oxygen concentration leaving the burner device from the base scenario of 3.0% to 2.6%. In scenario  4   b , we see that by the automated expedient of decreasing the fuel flow from the  4   a  scenario of 1.451 gmol/s to 1.436 gmol/s in order to maintain the oxygen concentration leaving the burner device at 3.0%, the present invention only changes the anode utilization from 80.0% to only 80.8%, which is also well within the optimal range of anode utilization. 
         [0038]    Scenarios of  4   a  and  4   b  show that while a significant change in steam carbon ratio may not directly force the anode utilization outside an optimal range, nonetheless, the control scheme of this system  10  of adjusting fuel flow to maintain a constant oxygen concentration leaving the burner exhaust  66  results in keeping the anode utilization within the optimal range, while simultaneously decreasing fuel flow to the reformer to thereby more efficiently use the fuel. Therefore, the exemplary data presented in Table 1 clearly establish that by sensing oxygen concentration within the burner exhaust  66  and by adjusting a rate of flow of fuel feedstock into the reformer to constantly maintain an oxygen concentration of about 3.0% in the burner exhaust, the present system  10  effectively rejects any negative impact on anode utilization resulting from the three described, common disturbances. 
         [0039]    In use of the present anode utilization control system for a fuel cell power plant  10 , prior to initiating ordinary operation and during factory testing, the power plant  10  would be tuned to establish an optimal oxygen concentration set point within the burner exhaust  66  (e.g., such as 3.0% in Table 1) that will maintain the anode utilization within a predetermined optimal anode utilization range for the power plant  10  while the plant  10  experiences disturbances in fuel heating value, fuel processing system hydrogen production efficiency and/or steam to carbon ratios. (For purposes herein the phrase “optimal anode utilization range” is to mean a range of hydrogen use at an anode catalyst that causes no damage to the anode catalyst and related support materials and that also efficiently utilizes the hydrogen fuel.) 
         [0040]    The oxygen sensor controller  80  would also be calibrated or otherwise controlled to adjust the flow rate of fuel feedstock  42  into the reformer  48  in response to the sensed oxygen concentrations by the oxygen sensor  78  in order to maintain the oxygen concentration within the burner exhaust  66  at about the predetermined oxygen concentration set point. This results in the anode utilization remaining within the predetermined optimal anode utilization range during ordinary operation of the fuel cell power plant  10 . 
         [0041]    The present disclosure also includes a method of controlling anode utilization in the fuel cell power plant  10 . The method includes directing flow of a hydrogen-rich fuel stream adjacent an anode catalyst  14  of a fuel cell and out of the fuel cell  12  as an anode exhaust stream while directing flow of an oxidant stream adjacent a cathode catalyst  16  of the fuel cell and out of the fuel cell  12 ; directing flow of some or all of the anode exhaust stream into a burner device  44  of a fuel processing system  40  and burning the anode exhaust stream within the burner device  44  to transmit heat to an endothermic reacting reformer  48  by either transmitting heat directly into the reformer  48  through conduction and convection, or by generating steam within a boiler  45  adjacent the burner device  44  and directing the heated steam into the reformer  48 ; directing a fuel feed stock  42  into a reformer  48  to reform the fuel feedstock  42  into the hydrogen-rich fuel stream; sensing an oxygen concentration within the burned anode exhaust stream passing out of the burner device  44 ; and, adjusting a rate of flow of the fuel feedstock  42  into the reformer  48  in response to the sensed oxygen concentration within the burned anode exhaust stream. Additionally, the method may also include first establishing an optimal oxygen concentration set point for the fuel cell power plant  10  to maintain anode utilization within a predetermined optimal anode utilization range for the power plant  10  while the plant  10  experiences disturbances in fuel heating value, fuel processing system hydrogen production efficiency and/or steam to carbon ratios. Then, the rate of flow of the fuel feedstock  42  into the reformer  48  may be adjusted in response to the sensed oxygen concentrations in the burned anode exhaust stream to maintain the oxygen concentration at about the optimal oxygen concentration set point. 
         [0042]    Use of the oxygen sensor  78  to monitor oxygen concentrations within the burner exhaust  66  may also be utilized for other valuable aspects of increasing efficient operation of the fuel cell power plant  10 . For example, the oxygen concentration may be utilized along with other power plant operation parameters to tune or determine a set point for fuel flow rates. Additionally, a fuel flow rate set point may based upon measured current from the fuel cell  12 , and then that set point can be modified based upon feedback from the oxygen sensor  78 . Also, the fuel flow set point established by the actual fuel cell  12  current modified by the oxygen sensor  78  feedback may also be further modified by establishing a multiplication factor which is a function of how far the actual oxygen concentration measurement is from an oxygen measurement set point, and multiplying the set point based on actual current by the multiplication factor. The multiplication factor may also be restricted to be within a specific optimal range. 
         [0043]    While the above disclosure has been presented with respect to the described and illustrated embodiments of the anode utilization control system for a fuel cell power plant  10 , it is to be understood that the disclosure is not to be limited to those alternatives and described embodiments. For example, the system  10  may be utilized with fuel cells employing phosphoric acid electrolytes, proton exchange membrane (“PEM”) electrolytes, or other known electrolytes. Additionally, the system  10  may include other features for system protection, such as temperature sensors (not shown) monitoring temperatures of the reformer  48  and linked to alarms in the event the temperature of the reformer  48  exceeds a predetermined upper safety limit. Accordingly, reference should be made primarily to the following claims rather than the forgoing description to determine the scope of the disclosure.