Abstract:
A fuel cell system includes a fuel cell stack that generates electrical energy during operation by reacting two streams of reactant gases. The fuel cell stack also produces a fuel cell exhaust stream. An oxidizer unit is positioned to receive the fuel cell exhaust stream. The oxidizer unit oxidizes at least a part of the fuel cell exhaust stream in an oxidizing gas stream during operation. A temperature sensor is positioned to sense a temperature of the oxidizer unit and an input system provides the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream during operation. The input system controls the amount of the oxidizing gas stream in excess of the stoichiometric amount provided to the oxidizer unit in response to the temperature of the oxidizer unit.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to tail gas oxidizer units in a fuel cell system.  
         BACKGROUND  
         [0002]    Fuel cells generate electrical power by reacting two fuel gas streams with each other. One of the gas streams is referred to as an anode gas while the other is referred as a cathode gas. Certain fuel cells use a stream of gas that is rich in hydrogen as the anode gas and an air stream as the cathode gas. When the fuel cell is in use, the hydrogen in the anode gas reacts with oxygen in the cathode gas to generate electrical power. Exhaust gases exiting the fuel cell may include un-reacted fuel gases, impurities contained within the fuel gas streams, and chemical products of the reactions in the fuel cell.  
           [0003]    Multiple fuel cells are typically arranged in a stack. Fuel cell stacks are normally part of a system, known as a fuel cell system, that includes a fuel processor or reformer for generating one of the fuel gas streams. For example, the fuel cell system that includes the fuel cell of the example above may also include a reformer that reacts a hydrocarbon, such as methane, with water to produce the hydrogen rich stream. Certain fuel cell systems also include an anode tail gas oxidizer unit (ATO) where the exhaust gases from the fuel cell are, for example, reacted with oxygen to eliminate environmentally unfriendly chemicals from the exhaust.  
         SUMMARY  
         [0004]    In general one aspect of the invention relates to a fuel cell system that includes a fuel cell stack, which generates electrical energy during operation by reacting two reactant gases. The fuel cell stack also produces a fuel cell exhaust stream. An oxidizer unit is positioned to receive the fuel cell exhaust stream. The oxidizer unit oxidizes at least a part of the fuel cell exhaust stream in an oxidizing gas stream, such as air, during operation. A temperature sensor is positioned to sense a temperature of the oxidizer unit and an input system provides the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream during operation. The input system controls the amount of the oxidizing gas stream in excess of the stoichiometric amount provided to the oxidizer unit in response to the temperature of the oxidizer unit.  
           [0005]    Embodiments of the invention may include one or more of the following features. The input system controls the amount of the oxidizing gas stream in response to the temperature of the oxidizer unit to maintain the temperature of the oxidizer unit at a target temperature. The input system includes a source, such as a blower, to provide the oxidizing gas stream, and a controller to control the amount of the oxidizing stream provided by the source in response to the temperature of the oxidizer unit. The temperature sensor generates a temperature signal corresponding to the temperature of the oxidizer unit and the controller includes a processor programmed to generate a control signal based on the temperature signal. The source provides the oxidizing gas stream in response to the control signal.  
           [0006]    The controller stores a stoichiometric table for determining a stoichiometric amount of the oxidizing gas stream and uses the stoichiometric table when generating the control signal to direct the source to provide the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream. A meter measures an amount of electrical power generated by the system and generates a corresponding load signal. The stoichiometric table relates the load signal to a blower control signal that causes the blower to provide the oxidizer unit with the stoichiometric amount of the oxidizing gas stream. The controller uses the load signal and the stoichiometric table when generating the control signal to direct the source to provide the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream.  
           [0007]    In general, another general aspect of the invention relates to a method that includes generating electrical energy in a fuel cell stack by reacting two reactant gas streams to produce a fuel cell exhaust stream, oxidizing at least a part of the fuel cell exhaust stream using an oxidizing gas stream in an oxidizer unit, sensing a temperature of the oxidizer unit, providing the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream, and controlling the amount of the oxidizing gas stream in excess of the stoichiometric amount provided to the oxidizer unit in response to the temperature of the oxidizer unit.  
           [0008]    Embodiments of the aspect of the invention may include one or more of the following features. The amount of the oxidizing gas stream is controlled in response to the temperature of the oxidizer unit to maintain the temperature of the oxidizer unit at a target temperature. A temperature signal corresponding to the temperature of the oxidizer unit is generated and a control signal is generated based on the temperature signal. The oxidizing stream is provided in response to the control signal.  
           [0009]    A stoichiometric table for determining a stoichiometric amount of the oxidizing gas stream is stored and used when generating the control signal to provide the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream. An amount of electrical power generated by the system is measured and a load signal corresponding to the amount of electrical power is generated. The stoichiometric table relates the load signal to the stoichiometric amount of the oxidizing gas stream and the control signal is generated based on the load signal and stoichiometric table to provide the oxidizer unit with at least the stoichiometric amount of the oxidizing gas stream.  
           [0010]    Among other advantages, controlling the temperature of the anode tail gas oxidizer unit by, for example, maintaining the temperature at a target operating temperature controls the amounts of environmentally unfriendly chemicals in the oxidizer unit exhaust. Thus, the invention can be used to keep the amounts of environmentally unfriendly chemicals in the oxidizer exhaustbelow a threshold value.  
           [0011]    The details of one or more embodiments of the invention are set forth in the accompanying drawing and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawing, and from the claims. 
       
    
    
     DESCRIPTION OF THE DRAWING  
       [0012]    [0012]FIG. 1 is a block diagram of a fuel cell system. 
     
    
     DETAILED DESCRIPTION  
       [0013]    As shown in FIG. 1, a fuel cell system  10  for supplying electrical energy to a load  12  has a negative electrical terminal  16  and a positive electrical terminal  17  for connecting to corresponding terminals of the load. Load  12  typically includes a power conditioning system and a circuit to which electrical appliances and/or equipment are connected. When fuel cell system  10  is in use, it delivers electrical energy to load  12  by generating a potential difference between terminals  16  and  17 . A power meter  13  measures the rate at which electrical energy is delivered to the load  12  (“electrical power”) and generates a load signal  15  that corresponds to the measured electrical power.  
         [0014]    Fuel cell system  10  includes a fuel cell stack  22  that generates the electrical energy using a hydrogen-rich gas stream  34  produced by a reformer  20 . Fuel cell stack  22  also produces anode exhaust  14 , which contains residual amounts hydrogen gas from the hydrogen rich stream  34  and hydrocarbons from reformer  20 . An anode tail gas oxidizer unit  24  promotes an oxidation reaction between components of the anode exhaust  14  and air  26  to produce oxidizer exhaust  30 . Air  26  may be referred to as the ATO oxidizing stream. An air blower  31  provides the air  26  to the anode tail gas oxidizer unit  24 . At least a portion of the air  26  may also come from the fuel cell cathode exhaust from air stream  44 . Other ATO oxidant sources are possible. A temperature sensor  32 , such as a thermocouple, senses the temperature within the anode tail gas oxidizer unit  24  and generates a temperature signal  34 . A controller  36 , which generates a control signal  38 , controls how much air  26  the blower  31  provides to anode tail gas oxidizer unit  24 . Controller  36  controls blower  31  to provide air  26  in excess of the amount needed to oxidize the hydrocarbons and hydrogen in anode exhaust  14 . Controller  36  controls the temperature of anode exhaust  14  by controlling the amount of air  26  that is flowed into the anode tail gas oxidizer unit  24 . Increasing the flow of air  26  into the anode tail gas oxidizer unit  24  beyond the stoichiometric amount tends to lower the temperature of the unit  24  by carrying heat away from the oxidizer unit  24 .  
         [0015]    A reformer  20  reacts a hydrocarbon  40 , such as methane, with steam  42  and oxygen  43  to generate a hydrogen-rich stream of gas  34 , which, for example, contains about thirty percent hydrogen gas. Since the reformer  20  is not completely efficient at converting the hydrocarbon into hydrogen, the hydrogen rich stream  34  also contains residual amounts of the hydrocarbon  40 . Typically, more than two percent of the hydrogen rich stream  34  is composed of residual hydrocarbons. The hydrogen rich stream  34  may also contain residual amounts of carbon monoxide, 30 ppm for example.  
         [0016]    Fuel cell stack  22  is, for example, a stack of proton exchange membrane fuel cells, each of which reacts some of hydrogen-rich stream  32  (anode gas) with a stream of air  44  (cathode gas) to generate the electrical energy. The reaction in the fuel cell stack  22  also produces anode exhaust  14 . Anode exhaust  40  contains residual amounts of un-reacted hydrogen from the hydrogen rich stream  34  in addition to the previously described residual amounts of the hydrocarbons  36 . For example, anode exhaust  40  may contain ten percent or more un-reacted hydrogen gas.  
         [0017]    Anode tail gas oxidizer unit  24  exposes anode exhaust  14  from fuel cell stack  22  to air  26  in the presence of a catalyst  46 , such as a platinum or a palladium matrix, which promotes oxidation and produces oxidizer exhaust  30 . If the oxidation process occurs at temperatures that are too high (e.g., over 800° C.), it produces undesirable products that are harmful if released to the environment. Oxidation at even greater temperatures may result in damage to the ATO catalyst (by sintering, for example). On the other hand, if the oxidation process occurs at lower temperatures, more benign oxidation products are produced. The catalyst allows the exhaust  14  to be oxidized at lower temperatures than would be possible in alternate oxidation apparatus, such as a flame combustion system. However, if temperature of the catalyst is too low (e.g., below 500° C.), the catalyst may not effectively oxidize the hydrocarbons and the hydrogen in anode exhaust  14 . Anode tail gas oxidizer unit  24  typically operates at a temperature between 500° C. and 800° C. The oxidation process generates heat, thereby raising the temperature of oxidizer exhaust  30 . The oxidizer exhaust  30  may be directed to a heat extraction device  48  that extracts heat from the exhaust  30 , for example, for use in a component of the fuel cell system  10 , such as reformer  20 .  
         [0018]    The amount of heat produced in the anode tail gas oxidizer unit  24  depends on the amount of air  26  provided by the blower  31 . For example, if the blower  31  does not provide enough air to oxidize all of the un-oxidized hydrocarbons and hydrogen in the anode exhaust  14 , only part of the exhaust  14  is oxidized yielding only part of the heating value of the exhaust. As the blower  31  provides more air  26 , more of the anode exhaust  14  is oxidized yielding more of the heating value of the exhaust  14  and resulting in a higher temperature in the anode tail gas oxidizer unit  24 . There is a certain amount of air  26 , known as the stoichiometric amount that is theoretically just enough to oxidize all of the oxidizable components of anode exhaust  14 . When the blower  31  provides the stoichiometric amount of air  26 , the anode tail gas oxidizer unit  24  yields a maximum amount of energy, resulting in a maximum temperature within the oxidizer unit  24 .  
         [0019]    As the blower  31  provides air  26  in excess of the stoichiometric amount, the heat produced by the anode tail gas oxidizer unit  24  remains constant because the additional air does not oxidize any further components of the anode exhaust  14 . However, since the excess air is cooler than the temperature of the oxidizer unit  24 , some of the heat produced in the anode tail gas oxidizer unit  24  heats the excess air and is carried away, thereby lowering the temperature of anode tail gas oxidizer unit  24  and catalyst  44 . Thus, providing excess amounts of air  26  to the oxidizer unit  24  lowers the temperature in the oxidizer unit  24 .  
         [0020]    Controller  36  stores a program  52 , a target temperature of the catalyst  44 , and a stoichiometric table  54  relating the load signal  15  to a control signal  38  required to cause the blower  31  to provide the stoichiometric amount of air  26  to anode tail gas oxidizer unit  24 . Controller  36  includes a processor  50 , which executes program  52  to generate the control signal  38  that controls how much air blower  31  provides to the anode tail gas oxidizer unit  24 . Controller  36  uses stoichiometric table  54 , load signal  15  and temperature signal  34  from sensor  32  positioned within the anode tail gas oxidizer unit to determine the amount of air  26  in excess of the stoichiometric amount that blower  31  should provide to anode tail gas oxidizer unit  24  to achieve the target temperature.  
         [0021]    The data in stoichiometric table  54  are obtained during calibration tests that are performed before the fuel cell is deployed. During the calibration tests, the fuel cell stack  22  is operated at different power loads and samples of anode exhaust  14  are collected for each value of the load signal  15  measured by power meter  13 . Each of the samples is analyzed in a gas chromatograph to determine the amounts of hydrocarbons and hydrogen in the sample. Based on the amount of hydrocarbons and hydrogen in the samples, the stoichiometric amount of air is determined. Knowing the stoichiometric amount of air that must be introduced into the oxidizer unit  24 , the speed of the blower  31  and the control signal  38  required to establish that speed are determined. Corresponding values of the load signal  15  and the control signal  38  are tabulated in stoichiometric table  54 .  
         [0022]    During operation, the processor  50  uses the load signal  15  and stoichiometric table  54  to look-up the stoichiometric control signal  38 , which drives the blower to provide the stoichiometric amount of air. Processor  50  also monitors the temperature signal  34 , which indicates the temperature within the anode tail gas oxidizer unit  24 , and compares that temperature with the target temperature  56  of the catalyst  46 . Typically, the temperature within anode tail gas oxidizer unit  24  will be higher than the target temperature  56  when blower  51  provides the stoichiometric amount of air. To lower the temperature within the oxidizer unit  24  to the target temperature  56 , processor  50  drives the blower  31  to provide air  26  in excess of the stoichiometric amount. The excess air carries heat out of the anode tail gas oxidizer unit  24 , thereby lowering the temperature within the oxidizer unit.  
         [0023]    Processor  50  controls the amount of excess air provided by blower  31  based on the difference between the target temperature  56  and the temperature within the anode tail gas oxidizer unit  24 . In this way, processor  50  controls blower  31  to maintain the temperature within anode tail gas oxidizer unit  24  at the target temperature. The invention may also be implemented in other embodiments having other control arrangements and hardware and software configurations. For example, in a simplified form, the invention may be implemented as a simple feedback loop between the ATO temperature and the oxidizing gas supply.  
         [0024]    It will be understood that various modifications may be made to the embodiment described above without departing from the spirit and scope of the invention. For example, though we have described a hydrogen/oxygen fuel cell, the ideas presented here have applicability to other fuel cell systems.  
         [0025]    Accordingly, other embodiments are within the scope of the following claims.