WO2007137068A1 - Fuel cell system and operting method thereof - Google Patents

Fuel cell system and operting method thereof Download PDF

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Publication number
WO2007137068A1
WO2007137068A1 PCT/US2007/069036 US2007069036W WO2007137068A1 WO 2007137068 A1 WO2007137068 A1 WO 2007137068A1 US 2007069036 W US2007069036 W US 2007069036W WO 2007137068 A1 WO2007137068 A1 WO 2007137068A1
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Prior art keywords
fuel cell
partial oxidation
steam
reformed gas
main body
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PCT/US2007/069036
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French (fr)
Inventor
Rhys Foster
Douglas S. Schmidt
Norman F. Bessette
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Acumentrics Corporation
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Publication of WO2007137068A1 publication Critical patent/WO2007137068A1/en

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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0606—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants
    • H01M8/0612—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants from carbon-containing material
    • H01M8/0618—Reforming processes, e.g. autothermal, partial oxidation or steam reforming
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    • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/32—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air
    • C01B3/34—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents
    • C01B3/38—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts
    • C01B3/382—Multi-step processes
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    • C01B3/02—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/32—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air
    • C01B3/34—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents
    • C01B3/38—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts
    • C01B3/384—Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts the catalyst being continuously externally heated
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    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
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    • H01M8/04097—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with recycling of the reactants
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    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H01M8/04268—Heating of fuel cells during the start-up of the fuel cells
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    • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
    • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
    • H01M8/04746—Pressure; Flow
    • H01M8/04753—Pressure; Flow of fuel cell reactants
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    • C01B2203/025—Processes for making hydrogen or synthesis gas containing a partial oxidation step
    • C01B2203/0261—Processes for making hydrogen or synthesis gas containing a partial oxidation step containing a catalytic partial oxidation step [CPO]
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    • C01B2203/08—Methods of heating or cooling
    • C01B2203/0805—Methods of heating the process for making hydrogen or synthesis gas
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    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
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    • C01B2203/0822—Methods of heating the process for making hydrogen or synthesis gas by combustion of fuel the fuel containing hydrogen
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    • C01B2203/0827—Methods of heating the process for making hydrogen or synthesis gas by combustion of fuel at least part of the fuel being a recycle stream
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    • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/12—Feeding the process for making hydrogen or synthesis gas
    • C01B2203/1205—Composition of the feed
    • C01B2203/1211—Organic compounds or organic mixtures used in the process for making hydrogen or synthesis gas
    • C01B2203/1235—Hydrocarbons
    • C01B2203/1241—Natural gas or methane
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    • C01B2203/142—At least two reforming, decomposition or partial oxidation steps in series
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    • C01B2203/142—At least two reforming, decomposition or partial oxidation steps in series
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
    • H01M2008/1293—Fuel cells with solid oxide electrolytes
    • H—ELECTRICITY
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    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0662—Treatment of gaseous reactants or gaseous residues, e.g. cleaning
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
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    • Y02P20/141—Feedstock

Definitions

  • the present invention relates to a fuel cell system that generates electricity using hydrocarbon fuel and an operating method thereof.
  • fuel cell systems that generate electricity by using hydrocarbon fuel (city gas, for example)
  • electricity is generated from hydrogen and carbon monoxide by producing hydrogen and carbon monoxide through steam reforming operations.
  • a fuel cell system is a solid oxide fuel cell system, which uses solid oxides (ceramics) with oxygen ion conductivity as electrolytes.
  • This solid oxide fuel cell (SOFC) has the characteristic that its operating temperature (approximately 700 to 1000 0 C) is much higher than those of other fuel cells.
  • fuel cell systems e.g., SOFC systems
  • SOFC systems that perform steam reforming operations by circulating fuel exhaust containing steam resulting from the generation of electricity and mixing it with hydrocarbon fuel
  • an inert gas such as nitrogen gas or a reducing gas such as hydrogen gas is generally circulated to the fuel electrode of the fuel cell in order to protect the fuel electrode from oxidation.
  • conventional fuel cell systems typically include a gas supply system (e.g., gas cylinder, etc.) for providing gases such as inert gas and a steam generator for supplying steam during activation and shut down.
  • a gas supply system e.g., gas cylinder, etc.
  • gases such as inert gas
  • a steam generator for supplying steam during activation and shut down.
  • Japanese Unexamined Patent Application Publication 2005-293951 which is hereby incorporated herein by reference in its entirety, describes technology for preventing anode oxidation using partial oxidation-reformed gas, but uses only steam- reformed gas as the fuel gas for generating electricity and performs steam reforming operations using steam fed from the outside.
  • the present inventors conducted dedicated research and discovered that in a fuel cell system that performs steam reforming operations by recycling fuel that contains steam following electricity generation and mixing it with hydrocarbon fuel, fuel cell stacks can be heated to a temperature at which electricity can be generated while protecting the fuel electrode from oxidation without installing a supply device for gases such as inert gas, and electricity generation can be started without installing a steam generator as a result of installing a partial oxidation reformer for partial oxidation-reforming hydrocarbon fuel and generating electricity using a partial oxidation-reformed gas produced by the partial oxidation reformer as a reducing gas for preventing the oxidation of the fuel electrode when heating fuel cell stacks and using a partial oxidation-reformed gas when starting to generate electricity, and the inventors completed the present invention based on this finding.
  • embodiments of the present invention generate electricity using hydrocarbon fuel by heating stacks to a temperature at which electricity can be generated while protecting the fuel electrode of the fuel cell from oxidation and starting the generation of electricity, without the use of a gas supply system for supplying gases such as inert gas or a steam generator.
  • a fuel cell system comprising a steam reformer that produces steam-reformed gas containing carbon monoxide and hydrogen by steam-reforming hydrocarbon fuel; a partial oxidation reformer that produces partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming said hydrocarbon fuel; a fuel cell main body that generates electricity using said steam-reformed gas and/or said partial oxidation-reformed gas; and a circulating means that returns fuel exhaust containing steam discharged from said fuel cell main body to said steam reformer.
  • the fuel cell system may be further equipped with a control means that controls the amount of said partial oxidation-reformed gas supplied to said fuel cell main body from said partial oxidation reformer and the amount of said steam-reformed gas supplied to said fuel cell main body from said steam reformer, wherein said control means is characterized in that it controls in such a way that when said fuel cell main body is activated, the control means produces said partial oxidation-reformed gas by partial oxidation-reforming said hydrocarbon fuel with said partial oxidation reformer, and heats said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation-reformed gas to a fuel electrode of said fuel cell main body; when said fuel cell main body starts generating electricity, the control means generates electricity using said partial oxidation-reformed gas while supplying said partial oxidation- reformed gas from said partial oxidation reformer to said fuel cell main body; and when said fuel cell main body is steadily generating electricity, the control means feeds said fuel exhaust and said hydro
  • control means may be characterized in that it controls in such a way that when the fuel cell system is in said state in which it is steadily generating electricity, at the same time as or after it starts generating electricity with said partial oxidation-reformed gas by supplying said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer, the control means stops the supply of said steam-reformed gas from said steam reformer to said fuel cell main body, stops generating electricity with said partial oxidation-reformed gas after it stops the supply of said steam-reformed gas, supplies said partial oxidation-reformed gas from said partial oxidation reformer to said fuel electrode until the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize, and stops the supply of said partial oxidation-reformed gas from said partial oxidation reformer to said fuel cell main body after the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize.
  • an operating method for a fuel cell system characterized in that it produces partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming hydrocarbon fuel with a partial oxidation reformer; starts generating electricity with said fuel cell main body using said partial oxidation- reformed gas after heating said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation-reformed gas to a fuel electrode of the fuel cell main body; feeds fuel exhaust containing steam formed by this generation of electricity and said hydrocarbon fuel to a steam reformer, and produces steam-reformed gas containing carbon monoxide and hydrogen by steam- reforming said hydrocarbon fuel with said steam reformer, and steadily generates electricity by supplying said steam-reformed gas to said fuel cell main body from said steam reformer while reducing the amount of said partial oxidation-reformed gas supplied from said partial oxidation reformer to said fuel cell main body.
  • the operating method may be characterized in that when in the state in which it generates electricity by supplying said steam- reformed gas from said steam reformer to said fuel cell main body, at the same time as or after it starts generating electricity with said partial oxidation-reformed gas by supplying said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer, it stops the supply of said steam-reformed gas to said fuel cell main body from said steam reformer, stops generating electricity with said partial oxidation-reformed gas after it stops the supply of said steam-reformed gas, supplies said partial oxidation-reformed gas from said partial oxidation reformer to said fuel electrode until the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize, and stops the supply of said partial oxidation-reformed gas from said partial oxidation reformer to said fuel cell main body after the temperature of the fuel cell main body drops below a temperature low enough such that said fuel electrode will not oxidize.
  • an exemplary fuel cell system for generating electricity using hydrocarbon fuel and an operating method thereof it is possible to heat stacks to a temperature at which electricity can be generated while protecting the fuel electrode of a fuel cell from oxidation without supplying an inert gas and to start generating electricity without supplying steam from the outside when electricity generation is begun by using a partial oxidation-reformed gas produced by partial oxidation- reforming hydrocarbon fuel to protect the fuel electrode and as the fuel gas when starting to generate electricity with the fuel cell.
  • the fuel cell system can be simpler and less expensive than convention fuel cell systems.
  • the fuel cell main body may include one or more tubular anode-supported solid oxide fuel cells having an inner anode through which through which the steam-reformed gas and partial oxidation-reformed gas flow and from which the fuel exhaust is collected for recirculation to a steam reformer.
  • Fig. 1 is a schematic block diagram showing the constitution of the fuel cell system of an embodiment of the present invention.
  • Fig. 2A is a schematic block diagram showing the activation method of the fuel cell system of this embodiment, and is a diagram that shows the operation when the fuel cell main body is heated.
  • Fig. 2B is a schematic block diagram showing the activation method of the fuel cell system of this embodiment, and is a diagram that shows the operation of the fuel cell main body when it starts to generate electricity.
  • Fig. 2C is a schematic block diagram showing the activation method of the fuel cell system of this embodiment, and is a diagram that shows the operation of the fuel cell main body when it is steadily generating electricity.
  • Fig. 3 A is a schematic block diagram showing the shutdown method of the fuel cell system of this embodiment, and is a diagram that shows the operation at the time of the activation of the partial oxidation reformer.
  • Fig. 3B is a schematic block diagram showing the shutdown method of the fuel cell system of this embodiment, and is a diagram that shows the operation when electricity generation is stopped in the fuel cell main body.
  • Fig. 3C is a schematic block diagram showing the shutdown method of the fuel cell system of this embodiment, and is a diagram that shows the operation when the fuel cell main body is cooled.
  • Fig. 4 is a schematic block diagram showing the constitution of a conventional fuel cell system.
  • Fig. 5A is a schematic block diagram showing the activation method of a conventional fuel cell system, and is a diagram that shows the operation when the fuel cell main body is heated.
  • Fig. 5B is a schematic block diagram showing the activation method of a conventional fuel cell system, and is a diagram that shows the operation of the fuel cell main body when it starts to generate electricity.
  • Fig. 5C is a schematic block diagram showing the activation method of a conventional fuel cell system, and is a diagram that shows the operation of the fuel cell main body when it is steadily generating electricity.
  • Fig. 4 is a schematic block diagram schematically showing the constitution of conventional fuel cell system 100.
  • fuel cell system 100 is primarily equipped with fuel supply means 112, inert gas supply means 114, steam supply means 116, steam reformer 120, fuel cell main body 130, high-temperature blower 140, cathode air supply means 150, and fuel cell main body heating means 160.
  • Fuel supply means 112 is a means for adjusting the flow rate of fuel circulated to steam reformer 120, and it is connected to steam reformer 120. Specifically, this may be a combination of a flow meter and a motor-operated valve (neither shown in the figure) or a mass flow controller (not shown in the figure), for example. Hydrocarbon fuel (gases containing lower alkanes such as methane, ethane, propane or butane) such as city gas, natural gas or coal gas can be used as the fuel supplied by fuel supply means 112.
  • Inert gas supply means 114 supplies an inert gas such as N2 to anode 134 in order to prevent the oxidation of anode 134, which is a fuel electrode, when heating fuel cell main body 130 to a temperature at which electricity can be generated (approximately 700 0 C or higher) or when cooling from the state in which it steadily generates electricity at the time of fuel cell activation and shutdown.
  • anode 134 which is a fuel electrode
  • Steam supply means 116 supplies steam for steam-reforming the hydrocarbon fuel described above to the hydrocarbon fuel when electricity starts to be generated in fuel cell main body 130, after it is heated to a temperature at which electricity can be generated.
  • Steam reformer 120 is equipped with, for example, a steam reforming catalyst (not shown in the figure) contained in the main body of a cylindrical reformer (not shown in the figure), and a means for heating the steam reforming catalyst (not shown in the figure).
  • a steam reforming catalyst not shown in the figure
  • the catalyst heating means when using an SOFC stack as fuel cell main body 130, for example, a method in which the catalyst heating means is installed in the vicinity of the SOFC main body operating at a high temperature (approximately 700 to 1000 0 C) and heat is received from the SOFC main body, or a method in which the catalyst is heated from the outside of steam reformer 120 could be used. It is also sometimes possible to steam-reform fuel with the anode material of the SOFC main body, and therefore it is not absolutely necessary to install steam reformer 120 in certain embodiments.
  • This steam reformer 120 reacts the hydrocarbon fuel supplied from fuel supply means 112 with the fuel exhaust following the generation of electricity containing steam supplied from steam supply means 116 or steam supplied through high- temperature blower 140 on the steam reforming catalyst, and it thus produces hydrogen-rich steam-reformed gas, as shown in reaction formula 1 below.
  • Steam- reformed gas is a hydrogen-rich gas that has carbon monoxide and hydrogen as its principle components, and is used as the fuel gas of anode 134 when generating electricity in fuel cell main body 130.
  • steam reformer 120 may comprise a preliminary reformer (not shown in the figure) that performs steam reforming reactions primarily with high- order hydrocarbons such as ethane, propane or butane, and a steam reformer (not shown in the figure) that performs steam reforming reactions primarily with methane.
  • the temperature at which steam reforming reactions with high-order hydrocarbons are performed (approximately 550 0 C) is lower than the temperature at which steam reforming reactions with methane are performed (approximately 700 0 C), and this may lead to a situation in which carbon is generated when steam reforming reactions with high-order hydrocarbons are performed at the temperature used to perform steam reforming reactions with methane. Therefore, the generation of carbon may be prevented by performing steam reforming reactions with methane using a steam reformer after performing steam reforming reactions with high-order hydrocarbons such as ethane, propane or butane using a preliminary reformer.
  • Fuel cell main body 130 comprises, for example, a stack made of multiple cells. Each cell comprises, for example, electrolyte 132, anode 134, which is a fuel electrode formed on one of the sides of electrolyte 132, and cathode 134, which is an air electrode formed on the other side of electrolyte 132.
  • a solid oxide fuel cell (SOFC) for example, can be used as fuel cell main body 130.
  • YSZ yttria stabilized zirconia
  • electrolyte 132 a cermet of nickel and YSZ is often used as anode 134 (fuel electrode), and a lanthanum manganite oxide is widely used as cathode 136 (air electrode).
  • High-temperature blower 140 is an example of a circulating means for circulating fuel exhaust following electricity generation containing steam produced on the anode 134 side by the generation of electricity in fuel cell main body 130 such that it mixes with the hydrocarbon fuel, in order to supply, to the hydrocarbon fuel, steam for the steam reformation that is performed in steam reformer 120.
  • a high-temperature blower capable of blowing high-temperature exhaust an example of this circulating means is an ejector driven by pressure when supplying fuel.
  • the fuel exhaust following electricity generation has as its principle components steam and carbon dioxide, which are produced by the reactions shown in reaction formulas 2 and 3 below at anode 134, and it also contains components such as un-reacted hydrogen and carbon monoxide.
  • Cathode air supply means 150 is a means for supplying and adjusting the flow rate of air circulated to cathode 136 of fuel cell main body 130. Specifically, this may be a combination of a blower and a flow meter, or a combination of a compressor and a mass flow controller, for example. Air supplied from cathode air supply means 150 is heated by heat exchanger 164 described below and is fed to cathode 136.
  • Fuel cell main body heating means 160 is a means for heating fuel cell main body 130 to a temperature at which electricity can be generated (approximately 700 to 1000 0 C in the case of an SOFC) when fuel cell system 100 is activated, and for heating fuel cell main body 130 during electricity generation.
  • a method for combusting air supplied from cathode air supply means 150 with a device such as a burner and circulating the combustion gas to the cathode 136 side of fuel cell main body 130, or a method for heating the system from the outside with a device such as an electric heater, for example, can be used.
  • fuel cell main body heating means 160 combusts air supplied from cathode air supply means 150 with activation burner 166, while supplying fuel from the outside, and circulates the combusted gas to the cathode 136 side of fuel cell main body 130.
  • fuel cell main body heating means 160 heats air supplied from cathode air supply means 150 with heat exchanger 164 using the combustion heat produced when the fuel exhaust following electricity generation in fuel cell main body 130 was combusted with combustion part 162, as well as the sensible heat of the anode exhaust and cathode exhaust, and then circulates the heated air to the cathode 136 side of fuel cell main body 130.
  • the system is designed such that, when steadily generating electricity, the temperature of fuel cell main body 130 can be maintained with the heat from fuel cell main body 130 produced when electricity is generated, so the combustion part 162 and the heat exchanger 164 may be omitted from certain embodiments.
  • FIG. 5A through 5C are block diagrams showing the activation method of conventional fuel cell system 100;
  • Fig. 5 A schematically shows the operation when fuel cell main body 130 is being heated
  • Fig. 5B schematically shows the operation of fuel cell main body 130 when it starts to generate electricity
  • Fig. 5C schematically shows the operation of fuel cell main body 130 when it is steadily generating electricity.
  • the flow of operations from the activation of fuel cell system 100 until the time of steady electricity generation will be described below.
  • the system heats the air supplied from cathode air supply means 150 (hereinafter sometimes referred to as "cathode air") while supplying an inert gas such as nitrogen gas (N2) from inert gas supply means 114 to anode 134 in order to prevent the oxidation of anode 134 when a temperature conducive to the oxidation of anode 134 is reached (approximately 200 0 C or higher), and it then heats fuel cell main body 130 to a temperature at which electricity can be generated (approximately 700 0 C) by circulating the heated cathode air to the cathode 136 side.
  • cathode air an inert gas such as nitrogen gas (N2)
  • Activation burner 166 is installed on the cathode air line, and the cathode air is heated by directly combusting the cathode air with activation burner 166 while supplying fuel from the outside.
  • an inert gas such as nitrogen gas was supplied from a separately installed inert gas supply means 114 when heating fuel cell main body 130.
  • the produced oxide ions are supplied to anode 134 through electrolyte 132.
  • the steam-reformed gas containing hydrogen and carbon monoxide produced by steam reformer 120 is supplied to anode 134, and electricity generating reactions (an oxidation reaction of hydrogen and carbon monoxide at anode 134 and a reduction reaction of oxygen at cathode 136) take place in fuel cell main body 130 as a result of the supplied steam-reformed gas reacting with the oxide ions supplied from cathode 136 through electrolyte 132.
  • electricity generating reactions an oxidation reaction of hydrogen and carbon monoxide at anode 134 and a reduction reaction of oxygen at cathode 136) take place in fuel cell main body 130 as a result of the supplied steam-reformed gas reacting with the oxide ions supplied from cathode 136 through electrolyte 132.
  • steam and carbon dioxide are generated from anode 134.
  • steam was supplied from a separately installed steam supply means 116 when starting to generate electricity.
  • the system returns fuel exhaust (a gas that has steam and carbon dioxide as its principle components) discharged from anode 134 to steam reformer 120 and circulates the fuel exhaust while controlling high-temperature blower 140 by fixing the current level once it has generated electricity up to a current level between a fraction of the rated current and the rated current using steam-reformed gas. It then mixes the fuel exhaust supplied from high- temperature blower 140 with the hydrocarbon fuel supplied from fuel supply means 112 and produces steam-reformed gas by performing a steam reforming reaction in steam reformer 120.
  • fuel exhaust a gas that has steam and carbon dioxide as its principle components
  • the conventional fuel cell system 100 described above included inert gas supply means 114 and steam supply means 116 in order to activate and shut down the system, and the installation of these components made the overall fuel cell system 100 complex and caused increases in cost.
  • the fuel cell system of the present invention includes a partial oxidation reformer that partial oxidation-reforms hydrocarbon fuel; generates electricity using a partial oxidation-reformed gas produced by the partial oxidation reformer as a reducing gas to prevent the oxidation of the fuel electrode when heating fuel cell stacks; and uses a partial oxidation-reformed gas when starting to generate electricity.
  • Fuel cell system 1 of an embodiment of the present invention will be described in detail hereinafter.
  • Fig 1 (a) is a schematic block diagram showing the constitution of fuel cell system 1 of an embodiment of the present invention
  • Fig. 1 (b) is a schematic block diagram showing the constitution of steam reformer 20 of this embodiment.
  • fuel cell system 1 is equipped with partial oxidation reformer 10, fuel supply means 12, partial oxidation air supply means 14, steam reformer 20, fuel cell main body 30, high-temperature blower 40, cathode air supply means 50, and fuel cell main body heating means 60.
  • Partial oxidation reformer 10 is equipped with a partial oxidation reforming catalyst (not shown in the figure) contained in the main body of a cylindrical reformer (not shown in the figure), for example, and a catalyst heating means (not shown in the figure) for heating the partial oxidation reforming catalyst in order to start partial oxidation reactions.
  • the partial oxidation reformer 10 and/or the partial oxidation reforming catalyst may be heated with combusted gas by combusting hydrocarbon fuel supplied from fuel supply means 12 described below and air supplied from partial oxidation air supply means 14 with an igniter (such as a burner).
  • the partial oxidation reformer 10 and/or the partial oxidation reforming catalyst may be heated from the outside with a device such as an electric heater.
  • a device such as an electric heater.
  • This partial oxidation reformer 10 reacts the hydrocarbon fuel supplied from fuel cell supply means 12 with the air supplied from partial oxidation air supply means 14 on the partial oxidation reforming catalyst, and it thus produces hydrogen- rich partial oxidation-reformed gas, as shown in reaction formula 4 below.
  • Partial oxidation-reformed gas is hydrogen-rich gas that has carbon monoxide and hydrogen as its principle components, and in fuel cell system 1 of this embodiment, it is used as the reducing gas for preventing the oxidation of the fuel electrode when fuel cell main body 30 is heated to a temperature at which electricity can be generated, and as the fuel gas of anode 34 when starting to generate electricity in fuel cell main body 30.
  • Fuel supply means 12 typically includes a means for adjusting the flow rate of fuel circulated to partial oxidation reformer 10 and fuel circulated to steam reformer 20. Specifically, this may be, for example, a combination of a flow meter and a motor-operated valve (neither shown in the figure), or a mass flow controller (not shown in the figure). Hydrocarbon fuel (gases containing lower alkanes such as methane, ethane, propane or butane) such as city gas, natural gas or coal gas can be used as the fuel supplied by fuel supply means 12.
  • Partial oxidation air supply means 14 typically includes a means for supplying and adjusting the flow rate of air circulated to partial oxidation reformer 10. Specifically, this may be, for example, a combination of a blower and a flow meter (neither shown in the figure) or a combination of a compressor and a mass flow controller (neither shown in the figure).
  • Steam reformer 20 is equipped with, for example, a steam reforming catalyst (not shown in the figure) contained in the main body of a cylindrical reformer (not shown in the figure), and a means for heating the steam reforming catalyst (not shown in the figure).
  • the steam reformer 20 and/or the catalyst heating means may be positioned in proximity to the fuel cell main body 30 so that heat from fuel cell main body 30, which operates at a high temperature (approximately 700 to 1000 0 C), can be used to heat the steam reforming catalyst.
  • the steam reforming catalyst may be heated in other ways, such as, for example, by an external outside of steam reformer 20.
  • This steam reformer 20 reacts the hydrocarbon fuel supplied from fuel supply means 12 with the fuel exhaust following the generation of electricity containing steam supplied through high-temperature blower 40 described below on the steam reforming catalyst, and it thus produces hydrogen-rich steam-reformed gas, as shown in reaction formula 1 below.
  • Steam-reformed gas is a hydrogen-rich gas that has carbon monoxide and hydrogen as its principle components, and is used as the fuel gas of anode 34 when generating electricity in fuel cell main body 30 (primarily in the steady electricity generating state).
  • steam reformer 20 of this embodiment performs steam reforming operations using fuel exhaust containing steam supplied through a fuel exhaust circulating means (high-temperature blower 40 in this embodiment) as a steam source rather than steam fed from an external steam generator.
  • steam reformer 20 may comprise a preliminary reformer 22 that performs steam reforming reactions primarily with high-order hydrocarbons such as ethane, propane or butane, and a steam reformer 24 that performs steam reforming reactions primarily with methane, as shown in Figure 1 (b).
  • the temperature at which steam reforming reactions with high-order hydrocarbons are performed (approximately 550 0 C) generally is lower than the temperature at which steam reforming reactions with methane are performed (approximately 700 0 C), and this can result in a situation in which carbon is generated when steam reforming reactions with high-order hydrocarbons are performed at the temperature used to perform steam reforming reactions with methane. Therefore, the generation of carbon may be prevented by performing steam-reforming reactions with methane using steam reformer 24 after performing steam reforming reactions with high-order hydrocarbons such as ethane, propane or butane using preliminary reformer 22.
  • the temperature at which steam reformer 24 performs steam-reforming reactions is approximately 700 0 C, and this is roughly the same as the temperature at which electricity-generating reactions are performed in anode 34 (for example, 700 to 1000 0 C) when an SOFC is used as fuel cell main body 30.
  • Nickel is frequently used as a steam reforming catalyst, and a metal with nickel is often used as the principle component for anode 34, so it is sometimes possible to steam-reform hydrocarbon fuel directly on anode 34, in which case it is not absolutely necessary to install steam reformer 24 in certain embodiments.
  • the temperature at which preliminary reformer 22 performs steam reforming operations is approximately 550 0 C, which is lower than the temperature at which electricity generating reactions are performed in anode 24, so the steam reforming operations performed on high-order hydrocarbons with preliminary reformer 22 generally cannot be performed directly on anode 34.
  • Fuel cell main body 30 comprises, for example, a stack made of multiple cells.
  • each cell comprises electrolyte 32, anode 34, which is a fuel electrode on one of the sides of electrolyte 32, and cathode 36, which is an air electrode on the other side of electrolyte 32.
  • the fuel cells may be solid oxide fuel cells (SOFCs).
  • YSZ yttria stabilized zirconia
  • electrolyte 32 a cermet of nickel and YSZ is often used as anode 34 (fuel electrode), and a lanthanum manganite oxide is widely used as cathode 36 (air electrode).
  • a circulating means is included for circulating fuel exhaust following electricity generation containing steam produced on the anode 34 side as a result of the generation of electricity in fuel cell main body 30 such that it mixes with the hydrocarbon fuel, in order to supply, to the hydrocarbon fuel, steam for the steam reformation that is performed in steam reformer 20 (i.e., in lieu of steam fed from an external steam generator).
  • the circulating means includes high-temperature blower 40, although other types of circulating means may be used such as, for example, an ejector driven by pressure when supplying fuel.
  • the fuel exhaust following electricity generation has as its principle components steam and carbon dioxide produced by the reactions shown in reaction formulas 2 and 3 below, at anode 34, and it also contains components such as un- reacted hydrogen and carbon monoxide.
  • Cathode air supply means 50 is a means for supplying and adjusting the flow rate of air circulated to cathode 36 of fuel cell main body 30.
  • cathode air supply means 50 may include, for example, a combination of a blower and a flow meter, or a combination of a compressor and a mass flow controller. Air supplied from cathode air supply means 50 is heated by heat exchanger 64 described below and is fed to cathode 36.
  • Fuel cell main body heating means 60 is a means for heating fuel cell main body 30 to a temperature at which electricity can be generated (approximately 700 to 1000 0 C in the case of an SOFC) when fuel cell system 1 is activated, and for heating fuel cell main body 30 during electricity generation.
  • a means for combusting air supplied from cathode air supply means 50 with a device such as a burner and circulating said combustion gas to the cathode 36 side of fuel cell main body 30, or a means for heating the system from the outside with a device such as an electric heater, for example, can be used.
  • fuel cell main body heating means 60 combusts air supplied from cathode air supply means 50 with activation burner 66, while supplying fuel from the outside, and circulates the combusted gas to the cathode 36 side of fuel cell main body 30.
  • fuel cell main body heating means 60 heats air supplied from cathode air supply means 50 with heat exchanger 64 using the combustion heat produced when the fuel exhaust following electricity generation in fuel cell main body 30 was combusted with combustion part 62, as well as the sensible heat of the anode exhaust and cathode exhaust, and then circulates the heated air to the cathode 36 side of fuel cell main body 30.
  • the system is designed such that, when steadily generating electricity, the temperature of fuel cell main body 30 can be maintained with the heat from fuel cell main body 30 produced when electricity is generated, so the combustion part 62 and the heat exchanger 64 may be omitted from certain embodiments.
  • Fig. 2A through 2C are schematic block diagrams showing the activation method of fuel cell system 1 of an embodiment of the present invention; Fig. 2A schematically shows the operation when fuel cell main body 30 is being heated, Fig. 2B schematically shows the operation of fuel cell main body 30 when it starts to generate electricity, and Fig. 2C schematically shows the operation of fuel cell main body 30 when it is steadily generating electricity.
  • Fig. 2A schematically shows the operation when fuel cell main body 30 is being heated
  • Fig. 2B schematically shows the operation of fuel cell main body 30 when it starts to generate electricity
  • Fig. 2C schematically shows the operation of fuel cell main body 30 when it is steadily generating electricity.
  • the flow of operations of fuel cell system 1 from activation until steady electricity generation will be described below.
  • hydrocarbon fuel for example, city gas with methane as the principle component
  • fuel supply means 12 and air predominantly N2 and O2
  • partial oxidation reformer 10 has a burner function (for example, an igniter) enabling the combustion of gas passing through this reformer 10 to heat the catalyst.
  • the gas circulated to partial oxidation reformer 10 has the theoretical combustion ratio of the air and hydrocarbon fuel in order to reduce the activation time of partial oxidation reformer 10.
  • reaction formula 5 when, for example, methane is used as the hydrocarbon fuel, the flow rates of the hydrocarbon fuel and air are adjusted such that the ratio of methane to oxygen is 1 :2, which is the theoretical ratio at which methane should be completely combusted.
  • the hydrocarbon fuel and air supplied to partial oxidation reformer 10 are combusted using an igniter, and this combusted gas directly heats the partial oxidation reforming catalyst.
  • the ratio of hydrocarbon fuel and air is adjusted such that it becomes the ratio of a partial oxidation reforming reaction, as shown in reaction formula 4 below (this being an example of a case in which the fuel is methane).
  • partial oxidation reactions occur on the partial oxidation reforming catalyst.
  • the catalyst temperature further increases and thus allows partial oxidation reactions to continue, so, at some point, combustion of the hydrocarbon fuel and air with an igniter may be terminated.
  • the amounts of hydrocarbon fuel and air used are determined based on the structure of fuel cell main body 30 and the capacity of the flow path of anode 34 such that a flow rate that does not allow anode 34 to be contaminated with oxygen from the outside can be ensured.
  • partial oxidation-reformed gas containing carbon monoxide and hydrogen produced by partial oxidation reformer 10 is supplied to anode 34 so that the oxidation of anode 34 can be prevented when a temperature conducive to the oxidation of anode 34 is reached (approximately 200 0 C or higher).
  • the system heats the air supplied from cathode air supply means 50 (hereinafter sometimes referred to as "cathode air") while preventing the oxidation of anode 34 by supplying partial oxidation-reformed gas to anode 34 as described above, and it heats fuel cell main body 30 to a temperature at which electricity can be generated (for example, approximately 700 0 C) by circulating the heated cathode air to the cathode 36 side.
  • Activation burner 66 is installed on the cathode air line, and the cathode air is heated by directly combusting the cathode air with activation burner 66 while supplying fuel from the outside.
  • fuel cell system 1 of this embodiment it is possible to heat fuel cell main body to a temperature at which electricity can be generated while preventing the oxidation of anode 34, even without separately installing a supply device for inert gas or reducing gas, such as the inert gas supply means 114 installed in the conventional fuel cell system 100 described above, by installing partial oxidation reformer 10 and using partial oxidation-reformed gas produced by partial oxidation-reforming hydrocarbon fuel with partial oxidation reformer 10 as a reducing gas for preventing the oxidation of anode 34 when heating fuel cell main body 30.
  • a supply device for inert gas or reducing gas such as the inert gas supply means 114 installed in the conventional fuel cell system 100 described above
  • the system supplies partial oxidation-reformed gas produced by partial oxidation reformer 10 to anode 34 of fuel cell main body 30 and starts to generate electricity with this partial oxidation-reformed gas, as shown in Fig. 2B.
  • the flow rates of the hydrocarbon fuel and the air supplied to partial oxidation reformer are adjusted to levels sufficient for electricity generation in fuel cell main body 30.
  • the supply of partial oxidation-reformed gas can be performed with a method in which the temperature of fuel cell main body 30 is visually confirmed with a device such as a thermometer installed inside fuel cell main body 30, for example, and the partial oxidation-reformed gas is supplied when a predetermined temperature at which electricity can be generated (for example, 700 0 C) is reached.
  • a predetermined temperature at which electricity can be generated for example, 700 0 C
  • an SOFC capable of generating electricity at approximately 700 to 1000 0 C is used as an example of fuel cell main body 30, but other types of fuel cells that operate at lower or higher temperatures may alternatively be used (e.g., an SOFC using different electrolyte, anode, or cathode materials may operate at approximately 600 0 C, for example).
  • a temperature lower than 700-1000 0 C could also be satisfactorily used in certain embodiments, as in fuel cell main body 30 of this embodiment.
  • partial oxidation-reformed gas containing hydrogen and carbon monoxide produced by partial oxidation reformer 10 is supplied to anode 34, and electricity generating reactions (an oxidation reaction of hydrogen and carbon monoxide at anode 34 and a reduction reaction of oxygen at cathode 36) take place in fuel cell main body 30 as a result of the supplied partial oxidation-reformed gas reacting with the oxide ions supplied from cathode 36 through electrolyte 32. As a result of these electricity-generating reactions, hydrogen and carbon dioxide are generated from anode 34.
  • the system returns fuel exhaust (gas with steam and carbon dioxide as its principle components) discharged from anode 34 to steam reformer 20 and circulates the fuel exhaust while controlling high-temperature blower 40 by fixing the current level once it has generated electricity up to a current level between a fraction of the rated current and the rated current using partial oxidation-reformed gas, and it then mixes the fuel exhaust supplied from high-temperature blower 40 with the hydrocarbon fuel supplied from fuel supply means 12, as shown in Fig. 2B. From the perspective of preventing carbon precipitation, it is preferable for the amount of fuel exhaust that is circulated to be at least 3 when expressed as the ratio (steam)/(carbon).
  • CO2 in the exhaust from anode 34 actually contributes to the steam reforming reactions performed by steam reformer 20 in this embodiment as an oxidant in hydrocarbon fuel reforming reactions, as shown in reaction formula 6.
  • the circulation of fuel exhaust may be suitably executed from the stage at which partial oxidation reformer 10 is operated (see (A) above).
  • steam-reformed gas is produced by performing steam- reforming reactions with steam reformer 20.
  • the steam-reformed gas that is produced is supplied to anode 34, and electricity generating reaction (steady electricity generation) is performed at anode 34.
  • electricity generating reaction steady electricity generation
  • the amount of hydrocarbon fuel supplied to steam reformer 20 is gradually increased to a sufficient level as the current is fixed to a level approximately 1/3 of the rated current, and at the same time, the amounts of hydrocarbon fuel and air supplied to partial oxidation reformer 10 are gradually reduced to zero.
  • an SOFC can be used as fuel cell main body 30, and in this case, steam reformer 20 is preferably installed in the vicinity of fuel cell main body 30, so a temperature and an amount of heat enabling steam reforming operations are supplied as a result of the rise in temperature and electricity generation of fuel cell main body 30.
  • the temperature of steam reformer 20 is preferably 700 0 C or higher when reforming methane and approximately 550 0 C when reforming high-order hydrocarbons of ethane or greater.
  • the system is controlled to simultaneously circulate fuel exhaust while switching from partial oxidation-reformed gas to steam- reformed gas. (F) Steady electricity generation by steam-reformed gas
  • fuel exhaust containing steam is produced by the generation of electricity in fuel cell main body 30 while in the steady electricity generation mode, so electricity can be continuously generated in fuel cell main body 30 by steam-reforming the hydrocarbon fuel using this fuel exhaust.
  • Fig. 3A through 3C are schematic block diagrams showing the shutdown method of fuel cell system 1 of this embodiment; Fig. 3 A schematically shows the operation at the time of the activation of partial oxidation reformer 10, Fig. 3B schematically shows the operation when electricity generation is stopped in fuel cell main body 30, and Fig. 3C schematically shows the operation when fuel cell main body 30 is cooled. The flow of operations from the state in which fuel cell system 1 steadily generates electricity until the time of system shutdown will be described below.
  • Fig. 3A in the state in which the system steadily generates electricity with steam-reformed gas by supplying steam-reformed gas from steam reformer 20 to fuel cell main body 30, it activates partial oxidation reformer 10 by circulating hydrocarbon fuel from fuel supply means 12 and air from partial oxidation air supply means 14 to partial oxidation reformer 10.
  • the system adjusts the flow rate of partial oxidation-reformed gas by gradually increasing the hydrocarbon fuel and air supplied to partial oxidation reformer 10 to amounts sufficient for electricity generation while reducing and fixing the current to a level approximately 1/3 of the rated current.
  • the system switches from electricity generation using steam- reformed gas to electricity generation using partial oxidation-reformed gas by gradually reducing the amount of hydrocarbon fuel supplied to steam reformer 20 until it reaches zero.
  • the supply of hydrocarbon fuel to steam reformer 20 for steady electricity generation may be stopped at the same time as or after the adjustment of the flow rate of partial oxidation-reformed gas.
  • electricity is generated using a combination of steam-reformed gas and partial oxidation-reformed gas.
  • an SOFC can be used as fuel cell main body 30, and the partial oxidation reformer 10 is preferably installed in the vicinity of fuel cell main body 30 in a position at which a partial oxidation reforming catalyst temperature of 300 0 C can be maintained during the steady state, and therefore partial oxidation reforming reactions begin immediately on the partial oxidation catalyst when the hydrocarbon fuel and air are circulated to partial oxidation reformer 10 at the flow rates sufficient for partial oxidation reforming operations.
  • the circulation of fuel exhaust by high-temperature blower 40 may be terminated after the system switches from electricity generation using steam-reformed gas to electricity generation using partial oxidation-reformed gas.
  • the system stops generating electricity in the state in which it is generating electricity using partial oxidation-reformed gas from partial oxidation reformer 10 after stopping the supply of steam-reformed gas to fuel cell main body 30 from steam reformer 20.
  • it stops generating electricity by gradually reducing the current until it reaches zero, while circulating an amount of partial oxidation-reformed gas sufficient for electricity generation to anode 34.
  • the amounts of hydrocarbon fuel and air supplied to partial oxidation reformer 10 are determined from the structure of fuel cell main body 30 and the capacity of the flow path of anode 34 such that a flow rate that does not allow anode 34 to be contaminated with oxygen from the outside can be ensured.
  • the system cools fuel cell main body by circulating cathode air to fuel cell main body 30 (here, because the system is cooling fuel cell main body 30, the cathode air is not heated by heat exchanger 64 or activation burner 66).
  • the system continues to supply (circulate) partial oxidation-reformed gas from partial oxidation reformer 10 to anode 34 until the temperature of fuel cell main body 30 drops to a temperature low enough that anode 34 (fuel electrode) will not oxidize (approximately 200 0 C).
  • fuel cell system 1 is shut down by stopping the supply of cathode air from cathode air supply means 50 and the supply of partial oxidation-reformed gas from partial oxidation reformer 10 to fuel cell main body 30 (in other words, the circulation of hydrocarbon fuel and air to partial oxidation reformer 10 is terminated).
  • methane is steam-reformed by steam reformer 20
  • the present invention is not limited to this case, and methane may alternatively be steam-reformed by direct cell-reforming methane at anode 34.
  • high-temperature blower 40 was used as a circulating means
  • the present invention is not limited to this case, and a device such as an ejector, for example, may be used as the circulating means of the present invention.
  • heat exchanger 64 was installed as a means for heating the cathode air, it is not absolutely necessary to install heat exchanger 64 in certain embodiments that are capable of maintaining the temperature of fuel cell main body 30 during electricity generation.
  • partial oxidation-reformed gas supplied from partial oxidation reformer 10 is supplied to anode 34 through steam reformer 20
  • the system may alternatively be configured such that the partial oxidation-reformed gas supplied from partial oxidation reformer 10 is directly supplied to anode 34 without passing through steam reformer 20.
  • a single fuel supply means 12 supplies hydrocarbon fuel to both partial oxidation reformer 10 and steam reformer 20
  • the system may alternatively be configured such that separate fuel supply means supply hydrocarbon fuel to partial oxidation reformer 10 and steam reformer 20, respectively.
  • a single fuel supply means supplies hydrocarbon fuel to both the partial oxidation reformer and the steam reformer, as in the embodiment described above.
  • embodiments of the present invention are particularly well-suited for use with tubular anode-supported solid oxide fuel cells of the types described in Applicant's United States Published Patent Application No. 2002/0028367 entitled Electrode- Supported Solid State Electrochemical Cell, which was published on March 7, 2002 and is hereby incorporated herein by reference in its entirety.
  • Such tubular anode-supported solid oxide fuel cells may be configured such that the anode is the innermost layer of the tubular fuel cell, which facilitates distribution of fuel through the fuel cell, facilitates collection and recirculation of exhaust gases from the fuel cell, and facilitates isolation of the anode from external sources of oxygen (e.g., air) so as to reduce the likelihood of oxygen coming into contact with the anode (e.g., in a fuel cell having an outer anode, fuel is typically circulated around the anode within an enclosure, so a breach of the enclosure could expose the anode to surrounding air, which could result in oxidation of the anode).
  • oxygen e.g., air

Abstract

A fuel cell system and an operating method for a fuel cell system produce partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming hydrocarbon fuel with a partial oxidation reformer, start generating electricity with a fuel cell main body using said partial oxidation-reformed gas after heating said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation-reformed gas to a fuel electrode of said fuel cell main body, feed fuel exhaust containing steam formed by this generation of electricity and said hydrocarbon fuel to a steam reformer, produce steam-reformed gas containing carbon monoxide and hydrogen by steam-reforming said hydrocarbon fuel with said steam reformer, and steadily generate electricity by supplying said steam-reformed gas to said fuel cell main body from said steam reformer while reducing the amount of said partial oxidation-reformed gas supplied to said fuel cell main body from said partial oxidation reformer.

Description

FUEL CELL SYSTEM AND OPERATING METHOD THEREOF
PRIORITY
This PCT patent application claims priority from Japanese Patent Application No. 2006-136748 filed on May 16, 2006, which is hereby incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates to a fuel cell system that generates electricity using hydrocarbon fuel and an operating method thereof.
BACKGROUND OF THE INVENTION
In fuel cell systems that generate electricity by using hydrocarbon fuel (city gas, for example), electricity is generated from hydrogen and carbon monoxide by producing hydrogen and carbon monoxide through steam reforming operations. One example of such a fuel cell system is a solid oxide fuel cell system, which uses solid oxides (ceramics) with oxygen ion conductivity as electrolytes. This solid oxide fuel cell (SOFC) has the characteristic that its operating temperature (approximately 700 to 10000C) is much higher than those of other fuel cells.
In recent years, fuel cell systems (e.g., SOFC systems) that perform steam reforming operations by circulating fuel exhaust containing steam resulting from the generation of electricity and mixing it with hydrocarbon fuel have been developed (for example, see S. Veyo, author, "Westinghouse 100 kW SOFC Demonstration Status," which is hereby incorporated herein by reference in its entirety). In such fuel cell systems, at the time of fuel cell activation, when heating stacks to a temperature at which electricity can be generated (approximately 7000C), an inert gas such as nitrogen gas or a reducing gas such as hydrogen gas is generally circulated to the fuel electrode of the fuel cell in order to protect the fuel electrode from oxidation. In addition, when starting to generate electricity, steam is generally supplied from an outside steam source for steam-reforming the hydrocarbon fuel used to generate electricity. Similar functions are generally also performed when shutting down the fuel cell. Therefore, conventional fuel cell systems typically include a gas supply system (e.g., gas cylinder, etc.) for providing gases such as inert gas and a steam generator for supplying steam during activation and shut down.
Japanese Unexamined Patent Application Publication 2005-293951, which is hereby incorporated herein by reference in its entirety, describes technology for preventing anode oxidation using partial oxidation-reformed gas, but uses only steam- reformed gas as the fuel gas for generating electricity and performs steam reforming operations using steam fed from the outside.
SUMMARY OF THE INVENTION
Looking to reduce cost and complexity of the fuel cell system, the present inventors conducted dedicated research and discovered that in a fuel cell system that performs steam reforming operations by recycling fuel that contains steam following electricity generation and mixing it with hydrocarbon fuel, fuel cell stacks can be heated to a temperature at which electricity can be generated while protecting the fuel electrode from oxidation without installing a supply device for gases such as inert gas, and electricity generation can be started without installing a steam generator as a result of installing a partial oxidation reformer for partial oxidation-reforming hydrocarbon fuel and generating electricity using a partial oxidation-reformed gas produced by the partial oxidation reformer as a reducing gas for preventing the oxidation of the fuel electrode when heating fuel cell stacks and using a partial oxidation-reformed gas when starting to generate electricity, and the inventors completed the present invention based on this finding. Thus, embodiments of the present invention generate electricity using hydrocarbon fuel by heating stacks to a temperature at which electricity can be generated while protecting the fuel electrode of the fuel cell from oxidation and starting the generation of electricity, without the use of a gas supply system for supplying gases such as inert gas or a steam generator.
In accordance with one aspect of the invention there is provided a fuel cell system comprising a steam reformer that produces steam-reformed gas containing carbon monoxide and hydrogen by steam-reforming hydrocarbon fuel; a partial oxidation reformer that produces partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming said hydrocarbon fuel; a fuel cell main body that generates electricity using said steam-reformed gas and/or said partial oxidation-reformed gas; and a circulating means that returns fuel exhaust containing steam discharged from said fuel cell main body to said steam reformer.
In an alternative embodiment, the fuel cell system may be further equipped with a control means that controls the amount of said partial oxidation-reformed gas supplied to said fuel cell main body from said partial oxidation reformer and the amount of said steam-reformed gas supplied to said fuel cell main body from said steam reformer, wherein said control means is characterized in that it controls in such a way that when said fuel cell main body is activated, the control means produces said partial oxidation-reformed gas by partial oxidation-reforming said hydrocarbon fuel with said partial oxidation reformer, and heats said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation-reformed gas to a fuel electrode of said fuel cell main body; when said fuel cell main body starts generating electricity, the control means generates electricity using said partial oxidation-reformed gas while supplying said partial oxidation- reformed gas from said partial oxidation reformer to said fuel cell main body; and when said fuel cell main body is steadily generating electricity, the control means feeds said fuel exhaust and said hydrocarbon fuel to said steam reformer, produces said steam-reformed gas by steam-reforming said hydrocarbon fuel with said steam reformer, and generates electricity using said steam-reformed gas while supplying said steam-reformed gas from said steam reformer to said fuel cell main body, while reducing the amount of said partial oxidation-reformed gas supplied to said fuel cell main body from said partial oxidation reformer.
In still another embodiment, the control means may be characterized in that it controls in such a way that when the fuel cell system is in said state in which it is steadily generating electricity, at the same time as or after it starts generating electricity with said partial oxidation-reformed gas by supplying said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer, the control means stops the supply of said steam-reformed gas from said steam reformer to said fuel cell main body, stops generating electricity with said partial oxidation-reformed gas after it stops the supply of said steam-reformed gas, supplies said partial oxidation-reformed gas from said partial oxidation reformer to said fuel electrode until the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize, and stops the supply of said partial oxidation-reformed gas from said partial oxidation reformer to said fuel cell main body after the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize.
In accordance with another aspect of the invention there is provided an operating method for a fuel cell system characterized in that it produces partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming hydrocarbon fuel with a partial oxidation reformer; starts generating electricity with said fuel cell main body using said partial oxidation- reformed gas after heating said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation-reformed gas to a fuel electrode of the fuel cell main body; feeds fuel exhaust containing steam formed by this generation of electricity and said hydrocarbon fuel to a steam reformer, and produces steam-reformed gas containing carbon monoxide and hydrogen by steam- reforming said hydrocarbon fuel with said steam reformer, and steadily generates electricity by supplying said steam-reformed gas to said fuel cell main body from said steam reformer while reducing the amount of said partial oxidation-reformed gas supplied from said partial oxidation reformer to said fuel cell main body.
In an alternative embodiment, the operating method may be characterized in that when in the state in which it generates electricity by supplying said steam- reformed gas from said steam reformer to said fuel cell main body, at the same time as or after it starts generating electricity with said partial oxidation-reformed gas by supplying said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer, it stops the supply of said steam-reformed gas to said fuel cell main body from said steam reformer, stops generating electricity with said partial oxidation-reformed gas after it stops the supply of said steam-reformed gas, supplies said partial oxidation-reformed gas from said partial oxidation reformer to said fuel electrode until the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize, and stops the supply of said partial oxidation-reformed gas from said partial oxidation reformer to said fuel cell main body after the temperature of the fuel cell main body drops below a temperature low enough such that said fuel electrode will not oxidize.
Thus, in an exemplary fuel cell system for generating electricity using hydrocarbon fuel and an operating method thereof, it is possible to heat stacks to a temperature at which electricity can be generated while protecting the fuel electrode of a fuel cell from oxidation without supplying an inert gas and to start generating electricity without supplying steam from the outside when electricity generation is begun by using a partial oxidation-reformed gas produced by partial oxidation- reforming hydrocarbon fuel to protect the fuel electrode and as the fuel gas when starting to generate electricity with the fuel cell. Because such embodiments to not include a gas supply system for supplying gases such as inert gas or a steam generator, the fuel cell system can be simpler and less expensive than convention fuel cell systems.
In any of the above embodiments, the fuel cell main body may include one or more tubular anode-supported solid oxide fuel cells having an inner anode through which through which the steam-reformed gas and partial oxidation-reformed gas flow and from which the fuel exhaust is collected for recirculation to a steam reformer.
BRIEF DESCRIPTION OF THE DRAWINGS
Suitable modes for carrying out the present invention will be described in detail hereafter with reference to the attached drawings. In this specification and in the drawings, redundant explanations are omitted by using the same symbols for components with essentially the same configurations and structures. The foregoing and advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein:
Fig. 1 is a schematic block diagram showing the constitution of the fuel cell system of an embodiment of the present invention.
Fig. 2A is a schematic block diagram showing the activation method of the fuel cell system of this embodiment, and is a diagram that shows the operation when the fuel cell main body is heated.
Fig. 2B is a schematic block diagram showing the activation method of the fuel cell system of this embodiment, and is a diagram that shows the operation of the fuel cell main body when it starts to generate electricity.
Fig. 2C is a schematic block diagram showing the activation method of the fuel cell system of this embodiment, and is a diagram that shows the operation of the fuel cell main body when it is steadily generating electricity.
Fig. 3 A is a schematic block diagram showing the shutdown method of the fuel cell system of this embodiment, and is a diagram that shows the operation at the time of the activation of the partial oxidation reformer. Fig. 3B is a schematic block diagram showing the shutdown method of the fuel cell system of this embodiment, and is a diagram that shows the operation when electricity generation is stopped in the fuel cell main body.
Fig. 3C is a schematic block diagram showing the shutdown method of the fuel cell system of this embodiment, and is a diagram that shows the operation when the fuel cell main body is cooled.
Fig. 4 is a schematic block diagram showing the constitution of a conventional fuel cell system.
Fig. 5A is a schematic block diagram showing the activation method of a conventional fuel cell system, and is a diagram that shows the operation when the fuel cell main body is heated.
Fig. 5B is a schematic block diagram showing the activation method of a conventional fuel cell system, and is a diagram that shows the operation of the fuel cell main body when it starts to generate electricity.
Fig. 5C is a schematic block diagram showing the activation method of a conventional fuel cell system, and is a diagram that shows the operation of the fuel cell main body when it is steadily generating electricity.
DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
(Constitution of a conventional fuel cell system)
First, the constitution of a conventional fuel cell system 100 will be described based on Fig. 4. Fig. 4 is a schematic block diagram schematically showing the constitution of conventional fuel cell system 100.
As shown in Fig. 4, fuel cell system 100 is primarily equipped with fuel supply means 112, inert gas supply means 114, steam supply means 116, steam reformer 120, fuel cell main body 130, high-temperature blower 140, cathode air supply means 150, and fuel cell main body heating means 160.
Fuel supply means 112 is a means for adjusting the flow rate of fuel circulated to steam reformer 120, and it is connected to steam reformer 120. Specifically, this may be a combination of a flow meter and a motor-operated valve (neither shown in the figure) or a mass flow controller (not shown in the figure), for example. Hydrocarbon fuel (gases containing lower alkanes such as methane, ethane, propane or butane) such as city gas, natural gas or coal gas can be used as the fuel supplied by fuel supply means 112.
Inert gas supply means 114 supplies an inert gas such as N2 to anode 134 in order to prevent the oxidation of anode 134, which is a fuel electrode, when heating fuel cell main body 130 to a temperature at which electricity can be generated (approximately 7000C or higher) or when cooling from the state in which it steadily generates electricity at the time of fuel cell activation and shutdown.
Steam supply means 116 supplies steam for steam-reforming the hydrocarbon fuel described above to the hydrocarbon fuel when electricity starts to be generated in fuel cell main body 130, after it is heated to a temperature at which electricity can be generated.
Steam reformer 120 is equipped with, for example, a steam reforming catalyst (not shown in the figure) contained in the main body of a cylindrical reformer (not shown in the figure), and a means for heating the steam reforming catalyst (not shown in the figure). As said catalyst heating means, when using an SOFC stack as fuel cell main body 130, for example, a method in which the catalyst heating means is installed in the vicinity of the SOFC main body operating at a high temperature (approximately 700 to 10000C) and heat is received from the SOFC main body, or a method in which the catalyst is heated from the outside of steam reformer 120 could be used. It is also sometimes possible to steam-reform fuel with the anode material of the SOFC main body, and therefore it is not absolutely necessary to install steam reformer 120 in certain embodiments.
This steam reformer 120 reacts the hydrocarbon fuel supplied from fuel supply means 112 with the fuel exhaust following the generation of electricity containing steam supplied from steam supply means 116 or steam supplied through high- temperature blower 140 on the steam reforming catalyst, and it thus produces hydrogen-rich steam-reformed gas, as shown in reaction formula 1 below. Steam- reformed gas is a hydrogen-rich gas that has carbon monoxide and hydrogen as its principle components, and is used as the fuel gas of anode 134 when generating electricity in fuel cell main body 130.
CH4 + H2O → CO + 3H2 ... (reaction formula 1) In addition, steam reformer 120 may comprise a preliminary reformer (not shown in the figure) that performs steam reforming reactions primarily with high- order hydrocarbons such as ethane, propane or butane, and a steam reformer (not shown in the figure) that performs steam reforming reactions primarily with methane. The temperature at which steam reforming reactions with high-order hydrocarbons are performed (approximately 5500C) is lower than the temperature at which steam reforming reactions with methane are performed (approximately 7000C), and this may lead to a situation in which carbon is generated when steam reforming reactions with high-order hydrocarbons are performed at the temperature used to perform steam reforming reactions with methane. Therefore, the generation of carbon may be prevented by performing steam reforming reactions with methane using a steam reformer after performing steam reforming reactions with high-order hydrocarbons such as ethane, propane or butane using a preliminary reformer.
Fuel cell main body 130 comprises, for example, a stack made of multiple cells. Each cell comprises, for example, electrolyte 132, anode 134, which is a fuel electrode formed on one of the sides of electrolyte 132, and cathode 134, which is an air electrode formed on the other side of electrolyte 132. A solid oxide fuel cell (SOFC), for example, can be used as fuel cell main body 130. In this SOFC, in the state in which fuel cell main body 130 is at a temperature of 700 to 10000C, when steam-reformed gas containing hydrogen and carbon monoxide produced by steam reformer 120 is supplied to the anode 134 (fuel electrode) side and air is supplied from cathode air supply means 150 described below to the cathode 136 (air electrode) side, electricity flows to an external circuit (not shown in the figure) that electrically connects anode 134 and cathode 136. YSZ (yttria stabilized zirconia) is frequently used as electrolyte 132, a cermet of nickel and YSZ is often used as anode 134 (fuel electrode), and a lanthanum manganite oxide is widely used as cathode 136 (air electrode).
High-temperature blower 140 is an example of a circulating means for circulating fuel exhaust following electricity generation containing steam produced on the anode 134 side by the generation of electricity in fuel cell main body 130 such that it mixes with the hydrocarbon fuel, in order to supply, to the hydrocarbon fuel, steam for the steam reformation that is performed in steam reformer 120. In addition to a high-temperature blower capable of blowing high-temperature exhaust, an example of this circulating means is an ejector driven by pressure when supplying fuel. The fuel exhaust following electricity generation has as its principle components steam and carbon dioxide, which are produced by the reactions shown in reaction formulas 2 and 3 below at anode 134, and it also contains components such as un-reacted hydrogen and carbon monoxide.
CO + O2" → CO2 + 2e" ... (reaction formula 2)
H2 + O2" → H2O + 2e" ... (reaction formula 3)
Cathode air supply means 150 is a means for supplying and adjusting the flow rate of air circulated to cathode 136 of fuel cell main body 130. Specifically, this may be a combination of a blower and a flow meter, or a combination of a compressor and a mass flow controller, for example. Air supplied from cathode air supply means 150 is heated by heat exchanger 164 described below and is fed to cathode 136.
Fuel cell main body heating means 160 is a means for heating fuel cell main body 130 to a temperature at which electricity can be generated (approximately 700 to 10000C in the case of an SOFC) when fuel cell system 100 is activated, and for heating fuel cell main body 130 during electricity generation. Specifically, a method for combusting air supplied from cathode air supply means 150 with a device such as a burner and circulating the combustion gas to the cathode 136 side of fuel cell main body 130, or a method for heating the system from the outside with a device such as an electric heater, for example, can be used.
At the time of activation, fuel cell main body heating means 160 combusts air supplied from cathode air supply means 150 with activation burner 166, while supplying fuel from the outside, and circulates the combusted gas to the cathode 136 side of fuel cell main body 130. At the time of electricity generation, fuel cell main body heating means 160 heats air supplied from cathode air supply means 150 with heat exchanger 164 using the combustion heat produced when the fuel exhaust following electricity generation in fuel cell main body 130 was combusted with combustion part 162, as well as the sensible heat of the anode exhaust and cathode exhaust, and then circulates the heated air to the cathode 136 side of fuel cell main body 130. Ordinarily, the system is designed such that, when steadily generating electricity, the temperature of fuel cell main body 130 can be maintained with the heat from fuel cell main body 130 produced when electricity is generated, so the combustion part 162 and the heat exchanger 164 may be omitted from certain embodiments.
(Operating method of a conventional fuel cell system)
Next, the operating method for conventional fuel cell system 100 will be described based on Fig. 5A through 5C. Fig. 5A through 5C are block diagrams showing the activation method of conventional fuel cell system 100; Fig. 5 A schematically shows the operation when fuel cell main body 130 is being heated, Fig. 5B schematically shows the operation of fuel cell main body 130 when it starts to generate electricity, and Fig. 5C schematically shows the operation of fuel cell main body 130 when it is steadily generating electricity. The flow of operations from the activation of fuel cell system 100 until the time of steady electricity generation will be described below.
First, when heating fuel cell main body 130 to a temperature at which electricity can be generated, as shown in Fig. 5A, the system heats the air supplied from cathode air supply means 150 (hereinafter sometimes referred to as "cathode air") while supplying an inert gas such as nitrogen gas (N2) from inert gas supply means 114 to anode 134 in order to prevent the oxidation of anode 134 when a temperature conducive to the oxidation of anode 134 is reached (approximately 2000C or higher), and it then heats fuel cell main body 130 to a temperature at which electricity can be generated (approximately 7000C) by circulating the heated cathode air to the cathode 136 side. Activation burner 166 is installed on the cathode air line, and the cathode air is heated by directly combusting the cathode air with activation burner 166 while supplying fuel from the outside. As described above, with conventional fuel cell system 100, an inert gas such as nitrogen gas was supplied from a separately installed inert gas supply means 114 when heating fuel cell main body 130.
Next, when starting to generate electricity with fuel cell main body 130 after heating it, as shown in Fig. 5B, electricity generation is begun with steam-reformed gas after fuel cell main body 130 reaches a temperature at which electricity can be generated. At this time, the flow rates of the hydrocarbon fuel inputted into steam reformer 120 from fuel supply means 112 and the steam inputted into steam reformer 120 from steam supply means 116 are adjusted such that stable steam reforming operations are possible, and the reformed gas that is produced is an amount required for electricity generation. Air heated by heat exchanger 164 is supplied to cathode 136 from cathode air supply means 150, and oxide ions (O^) are produced as a result of a reduction reaction of the oxygen contained in the air supplied to cathode 136. The produced oxide ions are supplied to anode 134 through electrolyte 132. Meanwhile, the steam-reformed gas containing hydrogen and carbon monoxide produced by steam reformer 120 is supplied to anode 134, and electricity generating reactions (an oxidation reaction of hydrogen and carbon monoxide at anode 134 and a reduction reaction of oxygen at cathode 136) take place in fuel cell main body 130 as a result of the supplied steam-reformed gas reacting with the oxide ions supplied from cathode 136 through electrolyte 132. As a result of these electricity-generating reactions, steam and carbon dioxide are generated from anode 134. As described above, with conventional fuel cell system 100, steam was supplied from a separately installed steam supply means 116 when starting to generate electricity.
Next, when steadily generating electricity after starting to generate electricity with fuel cell main body 130, as shown in Fig. 5C, the system returns fuel exhaust (a gas that has steam and carbon dioxide as its principle components) discharged from anode 134 to steam reformer 120 and circulates the fuel exhaust while controlling high-temperature blower 140 by fixing the current level once it has generated electricity up to a current level between a fraction of the rated current and the rated current using steam-reformed gas. It then mixes the fuel exhaust supplied from high- temperature blower 140 with the hydrocarbon fuel supplied from fuel supply means 112 and produces steam-reformed gas by performing a steam reforming reaction in steam reformer 120. It is generally unnecessary to supply steam from steam supply means 116 while fuel exhaust containing steam is supplied from high-temperature blower 140, so the supply of steam from steam supply means 116 may be stopped. The steam-reformed gas that is produced is once again supplied to anode 134, and an electricity generating reaction (steady electricity generation) is performed at anode 134. In this way, by circulating fuel exhaust with high-temperature blower 140, electricity can be continuously generated in fuel cell main body 130 while in the steady electricity generation mode, without supplying steam from steam supply means 116. When shutting down fuel cell system 100, a shutdown operation is performed with a method that essentially is the reverse of operations for the activation of fuel cell system 100 described above. Therefore, as with the activation of fuel cell system 100, the supply of an inert gas from a separately installed inert gas supply means 114 and the supply of steam from steam supply means 116 was also used when shutting down fuel cell system 100.
Thus, the conventional fuel cell system 100 described above included inert gas supply means 114 and steam supply means 116 in order to activate and shut down the system, and the installation of these components made the overall fuel cell system 100 complex and caused increases in cost.
Therefore, in order to heat the fuel cell main body and start generating electricity in the fuel cell main body without installing inert gas supply means 114 or steam supply means 116, the fuel cell system of the present invention includes a partial oxidation reformer that partial oxidation-reforms hydrocarbon fuel; generates electricity using a partial oxidation-reformed gas produced by the partial oxidation reformer as a reducing gas to prevent the oxidation of the fuel electrode when heating fuel cell stacks; and uses a partial oxidation-reformed gas when starting to generate electricity. Fuel cell system 1 of an embodiment of the present invention will be described in detail hereinafter.
(Constitution of a fuel cell system of an embodiment of the present invention)
First, the constitution of fuel cell system 1 of an embodiment of the present invention will be described based on Fig. 1. Fig 1 (a) is a schematic block diagram showing the constitution of fuel cell system 1 of an embodiment of the present invention, and Fig. 1 (b) is a schematic block diagram showing the constitution of steam reformer 20 of this embodiment.
As shown in Fig. 1 (a), fuel cell system 1 is equipped with partial oxidation reformer 10, fuel supply means 12, partial oxidation air supply means 14, steam reformer 20, fuel cell main body 30, high-temperature blower 40, cathode air supply means 50, and fuel cell main body heating means 60.
Partial oxidation reformer 10 is equipped with a partial oxidation reforming catalyst (not shown in the figure) contained in the main body of a cylindrical reformer (not shown in the figure), for example, and a catalyst heating means (not shown in the figure) for heating the partial oxidation reforming catalyst in order to start partial oxidation reactions. In one exemplary embodiment, the partial oxidation reformer 10 and/or the partial oxidation reforming catalyst may be heated with combusted gas by combusting hydrocarbon fuel supplied from fuel supply means 12 described below and air supplied from partial oxidation air supply means 14 with an igniter (such as a burner). In another exemplary embodiment, the partial oxidation reformer 10 and/or the partial oxidation reforming catalyst may be heated from the outside with a device such as an electric heater. In addition, it generally is preferable for partial oxidation reformer 10 to be installed in a position at which the temperature of the partial oxidation reforming catalyst can be maintained at approximately 3000C with the heat of fuel cell main body 30 when in the steady operation mode so that partial oxidation reforming reactions can occur by simply circulating the partial oxidation air and the partial oxidation fuel to partial oxidation reformer 10 in amounts necessary for partial oxidation reforming operations.
This partial oxidation reformer 10 reacts the hydrocarbon fuel supplied from fuel cell supply means 12 with the air supplied from partial oxidation air supply means 14 on the partial oxidation reforming catalyst, and it thus produces hydrogen- rich partial oxidation-reformed gas, as shown in reaction formula 4 below. Partial oxidation-reformed gas is hydrogen-rich gas that has carbon monoxide and hydrogen as its principle components, and in fuel cell system 1 of this embodiment, it is used as the reducing gas for preventing the oxidation of the fuel electrode when fuel cell main body 30 is heated to a temperature at which electricity can be generated, and as the fuel gas of anode 34 when starting to generate electricity in fuel cell main body 30.
CH4 + 1/202 → CO + 2H2 ... (reaction formula 4)
Fuel supply means 12 typically includes a means for adjusting the flow rate of fuel circulated to partial oxidation reformer 10 and fuel circulated to steam reformer 20. Specifically, this may be, for example, a combination of a flow meter and a motor-operated valve (neither shown in the figure), or a mass flow controller (not shown in the figure). Hydrocarbon fuel (gases containing lower alkanes such as methane, ethane, propane or butane) such as city gas, natural gas or coal gas can be used as the fuel supplied by fuel supply means 12. Partial oxidation air supply means 14 typically includes a means for supplying and adjusting the flow rate of air circulated to partial oxidation reformer 10. Specifically, this may be, for example, a combination of a blower and a flow meter (neither shown in the figure) or a combination of a compressor and a mass flow controller (neither shown in the figure).
Steam reformer 20 is equipped with, for example, a steam reforming catalyst (not shown in the figure) contained in the main body of a cylindrical reformer (not shown in the figure), and a means for heating the steam reforming catalyst (not shown in the figure). The steam reformer 20 and/or the catalyst heating means may be positioned in proximity to the fuel cell main body 30 so that heat from fuel cell main body 30, which operates at a high temperature (approximately 700 to 10000C), can be used to heat the steam reforming catalyst. Additionally or alternatively, the steam reforming catalyst may be heated in other ways, such as, for example, by an external outside of steam reformer 20.
This steam reformer 20 reacts the hydrocarbon fuel supplied from fuel supply means 12 with the fuel exhaust following the generation of electricity containing steam supplied through high-temperature blower 40 described below on the steam reforming catalyst, and it thus produces hydrogen-rich steam-reformed gas, as shown in reaction formula 1 below. Steam-reformed gas is a hydrogen-rich gas that has carbon monoxide and hydrogen as its principle components, and is used as the fuel gas of anode 34 when generating electricity in fuel cell main body 30 (primarily in the steady electricity generating state). As described above, steam reformer 20 of this embodiment performs steam reforming operations using fuel exhaust containing steam supplied through a fuel exhaust circulating means (high-temperature blower 40 in this embodiment) as a steam source rather than steam fed from an external steam generator.
CH4 + H2O → CO + 3H2 ... (reaction formula 1)
In addition, steam reformer 20 may comprise a preliminary reformer 22 that performs steam reforming reactions primarily with high-order hydrocarbons such as ethane, propane or butane, and a steam reformer 24 that performs steam reforming reactions primarily with methane, as shown in Figure 1 (b). The temperature at which steam reforming reactions with high-order hydrocarbons are performed (approximately 5500C) generally is lower than the temperature at which steam reforming reactions with methane are performed (approximately 7000C), and this can result in a situation in which carbon is generated when steam reforming reactions with high-order hydrocarbons are performed at the temperature used to perform steam reforming reactions with methane. Therefore, the generation of carbon may be prevented by performing steam-reforming reactions with methane using steam reformer 24 after performing steam reforming reactions with high-order hydrocarbons such as ethane, propane or butane using preliminary reformer 22.
Here, the temperature at which steam reformer 24 performs steam-reforming reactions is approximately 7000C, and this is roughly the same as the temperature at which electricity-generating reactions are performed in anode 34 (for example, 700 to 10000C) when an SOFC is used as fuel cell main body 30. Nickel is frequently used as a steam reforming catalyst, and a metal with nickel is often used as the principle component for anode 34, so it is sometimes possible to steam-reform hydrocarbon fuel directly on anode 34, in which case it is not absolutely necessary to install steam reformer 24 in certain embodiments. It should be noted, however, that the temperature at which preliminary reformer 22 performs steam reforming operations is approximately 5500C, which is lower than the temperature at which electricity generating reactions are performed in anode 24, so the steam reforming operations performed on high-order hydrocarbons with preliminary reformer 22 generally cannot be performed directly on anode 34.
Fuel cell main body 30 comprises, for example, a stack made of multiple cells. In this embodiment, each cell comprises electrolyte 32, anode 34, which is a fuel electrode on one of the sides of electrolyte 32, and cathode 36, which is an air electrode on the other side of electrolyte 32. The fuel cells may be solid oxide fuel cells (SOFCs). In this SOFC, in the state in which fuel cell main body 30 is at a temperature of, for example, 700 to 10000C, when partial oxidation-reformed gas containing hydrogen and carbon monoxide produced by partial oxidation reformer 10 and/or steam-reformed gas containing hydrogen and carbon monoxide produced by steam reformer 20 is supplied to the anode 34 (fuel electrode) side, and air is supplied from cathode air supply means 50 described below to the cathode 36 (air electrode) side, electricity flows to an external circuit (not shown in the figure) that electrically connects anode 34 and cathode 36. YSZ (yttria stabilized zirconia) is frequently used as electrolyte 32, a cermet of nickel and YSZ is often used as anode 34 (fuel electrode), and a lanthanum manganite oxide is widely used as cathode 36 (air electrode).
As described above, a circulating means is included for circulating fuel exhaust following electricity generation containing steam produced on the anode 34 side as a result of the generation of electricity in fuel cell main body 30 such that it mixes with the hydrocarbon fuel, in order to supply, to the hydrocarbon fuel, steam for the steam reformation that is performed in steam reformer 20 (i.e., in lieu of steam fed from an external steam generator). In this exemplary embodiment, the circulating means includes high-temperature blower 40, although other types of circulating means may be used such as, for example, an ejector driven by pressure when supplying fuel. The fuel exhaust following electricity generation has as its principle components steam and carbon dioxide produced by the reactions shown in reaction formulas 2 and 3 below, at anode 34, and it also contains components such as un- reacted hydrogen and carbon monoxide.
CO + O2" → CO2 + 2e" ... (reaction formula 2)
H2 + O2" → H2O + 2e" ... (reaction formula 3)
Cathode air supply means 50 is a means for supplying and adjusting the flow rate of air circulated to cathode 36 of fuel cell main body 30. Specifically, cathode air supply means 50 may include, for example, a combination of a blower and a flow meter, or a combination of a compressor and a mass flow controller. Air supplied from cathode air supply means 50 is heated by heat exchanger 64 described below and is fed to cathode 36.
Fuel cell main body heating means 60 is a means for heating fuel cell main body 30 to a temperature at which electricity can be generated (approximately 700 to 10000C in the case of an SOFC) when fuel cell system 1 is activated, and for heating fuel cell main body 30 during electricity generation. Specifically, a means for combusting air supplied from cathode air supply means 50 with a device such as a burner and circulating said combustion gas to the cathode 36 side of fuel cell main body 30, or a means for heating the system from the outside with a device such as an electric heater, for example, can be used. At the time of activation, fuel cell main body heating means 60 combusts air supplied from cathode air supply means 50 with activation burner 66, while supplying fuel from the outside, and circulates the combusted gas to the cathode 36 side of fuel cell main body 30. At the time of electricity generation, fuel cell main body heating means 60 heats air supplied from cathode air supply means 50 with heat exchanger 64 using the combustion heat produced when the fuel exhaust following electricity generation in fuel cell main body 30 was combusted with combustion part 62, as well as the sensible heat of the anode exhaust and cathode exhaust, and then circulates the heated air to the cathode 36 side of fuel cell main body 30. Ordinarily, the system is designed such that, when steadily generating electricity, the temperature of fuel cell main body 30 can be maintained with the heat from fuel cell main body 30 produced when electricity is generated, so the combustion part 62 and the heat exchanger 64 may be omitted from certain embodiments.
(Activation method of the fuel cell system of an embodiment of the present invention)
The constitution of fuel cell system 1 of an embodiment of the present invention has been described above based on Fig. 1. Next, the activation method of fuel cell system 1 of an embodiment of the present invention will be described based on Fig. 2A through 2C. Fig. 2A through 2C are schematic block diagrams showing the activation method of fuel cell system 1 of an embodiment of the present invention; Fig. 2A schematically shows the operation when fuel cell main body 30 is being heated, Fig. 2B schematically shows the operation of fuel cell main body 30 when it starts to generate electricity, and Fig. 2C schematically shows the operation of fuel cell main body 30 when it is steadily generating electricity. The flow of operations of fuel cell system 1 from activation until steady electricity generation will be described below.
(A) Operation of the partial oxidation reformer
First, as shown in Fig. 2A, hydrocarbon fuel (for example, city gas with methane as the principle component) from fuel supply means 12 and air (predominantly N2 and O2) from partial oxidation air supply means 14 are supplied to partial oxidation reformer 10, and partial oxidation reforming reactions are performed in partial oxidation reformer 10. In this exemplary embodiment, partial oxidation reformer 10 has a burner function (for example, an igniter) enabling the combustion of gas passing through this reformer 10 to heat the catalyst. When partial oxidation reforming reactions are started, the gas circulated to partial oxidation reformer 10 has the theoretical combustion ratio of the air and hydrocarbon fuel in order to reduce the activation time of partial oxidation reformer 10. In other words, as shown in reaction formula 5 below, when, for example, methane is used as the hydrocarbon fuel, the flow rates of the hydrocarbon fuel and air are adjusted such that the ratio of methane to oxygen is 1 :2, which is the theoretical ratio at which methane should be completely combusted. The hydrocarbon fuel and air supplied to partial oxidation reformer 10 are combusted using an igniter, and this combusted gas directly heats the partial oxidation reforming catalyst. After the partial oxidation reforming catalyst is heated to around 3000C, the ratio of hydrocarbon fuel and air is adjusted such that it becomes the ratio of a partial oxidation reforming reaction, as shown in reaction formula 4 below (this being an example of a case in which the fuel is methane). As a result, partial oxidation reactions occur on the partial oxidation reforming catalyst. Here, because the partial oxidation reforming reactions are exothermal reactions, the catalyst temperature further increases and thus allows partial oxidation reactions to continue, so, at some point, combustion of the hydrocarbon fuel and air with an igniter may be terminated. The amounts of hydrocarbon fuel and air used are determined based on the structure of fuel cell main body 30 and the capacity of the flow path of anode 34 such that a flow rate that does not allow anode 34 to be contaminated with oxygen from the outside can be ensured.
(Theoretical combustion) CH4 + 2C>2 → CO2 + 2H2O ... (reaction formula 5)
(Partial oxidation reforming) CH4 + I/2O2 → CO + 2H2 ... (reaction formula 4)
(B) Heating the fuel cell main body 30
Next, as shown in Fig. 2A, partial oxidation-reformed gas containing carbon monoxide and hydrogen produced by partial oxidation reformer 10 is supplied to anode 34 so that the oxidation of anode 34 can be prevented when a temperature conducive to the oxidation of anode 34 is reached (approximately 2000C or higher). The system heats the air supplied from cathode air supply means 50 (hereinafter sometimes referred to as "cathode air") while preventing the oxidation of anode 34 by supplying partial oxidation-reformed gas to anode 34 as described above, and it heats fuel cell main body 30 to a temperature at which electricity can be generated (for example, approximately 7000C) by circulating the heated cathode air to the cathode 36 side. Activation burner 66 is installed on the cathode air line, and the cathode air is heated by directly combusting the cathode air with activation burner 66 while supplying fuel from the outside.
As described above, with fuel cell system 1 of this embodiment, it is possible to heat fuel cell main body to a temperature at which electricity can be generated while preventing the oxidation of anode 34, even without separately installing a supply device for inert gas or reducing gas, such as the inert gas supply means 114 installed in the conventional fuel cell system 100 described above, by installing partial oxidation reformer 10 and using partial oxidation-reformed gas produced by partial oxidation-reforming hydrocarbon fuel with partial oxidation reformer 10 as a reducing gas for preventing the oxidation of anode 34 when heating fuel cell main body 30.
(C) Starting electricity generation with partial oxidation-reformed gas
Next, when fuel cell main body 30 reaches a temperature at which electricity can be generated, the system supplies partial oxidation-reformed gas produced by partial oxidation reformer 10 to anode 34 of fuel cell main body 30 and starts to generate electricity with this partial oxidation-reformed gas, as shown in Fig. 2B. Here, the flow rates of the hydrocarbon fuel and the air supplied to partial oxidation reformer are adjusted to levels sufficient for electricity generation in fuel cell main body 30. In addition, the supply of partial oxidation-reformed gas can be performed with a method in which the temperature of fuel cell main body 30 is visually confirmed with a device such as a thermometer installed inside fuel cell main body 30, for example, and the partial oxidation-reformed gas is supplied when a predetermined temperature at which electricity can be generated (for example, 7000C) is reached. In this embodiment, an SOFC capable of generating electricity at approximately 700 to 10000C is used as an example of fuel cell main body 30, but other types of fuel cells that operate at lower or higher temperatures may alternatively be used (e.g., an SOFC using different electrolyte, anode, or cathode materials may operate at approximately 6000C, for example). There is no particular restriction regarding the temperature at which electricity can be generated in this invention, and a temperature lower than 700-10000C could also be satisfactorily used in certain embodiments, as in fuel cell main body 30 of this embodiment.
At this time, inside fuel cell main body 30, air heated by heat exchanger 64 is supplied from cathode air supply means 50 to cathode 36, and oxide ions (O^-) are produced as a result of a reduction reaction of the oxygen contained in the air supplied to cathode 36. The produced oxide ions are supplied to anode 34 through electrolyte 32. Meanwhile, partial oxidation-reformed gas containing hydrogen and carbon monoxide produced by partial oxidation reformer 10 is supplied to anode 34, and electricity generating reactions (an oxidation reaction of hydrogen and carbon monoxide at anode 34 and a reduction reaction of oxygen at cathode 36) take place in fuel cell main body 30 as a result of the supplied partial oxidation-reformed gas reacting with the oxide ions supplied from cathode 36 through electrolyte 32. As a result of these electricity-generating reactions, hydrogen and carbon dioxide are generated from anode 34.
As described above, with fuel cell system 1 of this embodiment, it is possible to start generating electricity without feeding steam from the outside with a separately installed steam generator, such as the steam supply means 116 installed in the conventional fuel cell system 100 described above, by installing partial oxidation reformer 10 and using partial oxidation-reformed gas produced by partial oxidation- reforming hydrocarbon fuel with partial oxidation reformer 10 as a fuel gas when starting to generate electricity with fuel cell main body 30.
(D) Circulation of fuel exhaust
Next, after starting to generate electricity with fuel cell main body 30, the system returns fuel exhaust (gas with steam and carbon dioxide as its principle components) discharged from anode 34 to steam reformer 20 and circulates the fuel exhaust while controlling high-temperature blower 40 by fixing the current level once it has generated electricity up to a current level between a fraction of the rated current and the rated current using partial oxidation-reformed gas, and it then mixes the fuel exhaust supplied from high-temperature blower 40 with the hydrocarbon fuel supplied from fuel supply means 12, as shown in Fig. 2B. From the perspective of preventing carbon precipitation, it is preferable for the amount of fuel exhaust that is circulated to be at least 3 when expressed as the ratio (steam)/(carbon). Here, CO2 in the exhaust from anode 34 actually contributes to the steam reforming reactions performed by steam reformer 20 in this embodiment as an oxidant in hydrocarbon fuel reforming reactions, as shown in reaction formula 6. In addition, from the perspective of preventing fuel backflow and facilitating control, the circulation of fuel exhaust may be suitably executed from the stage at which partial oxidation reformer 10 is operated (see (A) above).
CH4 + CO2 → 2CO + 2H2 ... (reaction formula 6)
(E) Switching from partial oxidation-reformed gas to steam-reformed gas
As described above, steam-reformed gas is produced by performing steam- reforming reactions with steam reformer 20. The steam-reformed gas that is produced is supplied to anode 34, and electricity generating reaction (steady electricity generation) is performed at anode 34. Specifically, the amount of hydrocarbon fuel supplied to steam reformer 20 is gradually increased to a sufficient level as the current is fixed to a level approximately 1/3 of the rated current, and at the same time, the amounts of hydrocarbon fuel and air supplied to partial oxidation reformer 10 are gradually reduced to zero.
In this embodiment, an SOFC can be used as fuel cell main body 30, and in this case, steam reformer 20 is preferably installed in the vicinity of fuel cell main body 30, so a temperature and an amount of heat enabling steam reforming operations are supplied as a result of the rise in temperature and electricity generation of fuel cell main body 30. The temperature of steam reformer 20 is preferably 7000C or higher when reforming methane and approximately 5500C when reforming high-order hydrocarbons of ethane or greater. The system is controlled to simultaneously circulate fuel exhaust while switching from partial oxidation-reformed gas to steam- reformed gas. (F) Steady electricity generation by steam-reformed gas
With a method similar to the one described in (E) above, once the system switches from electricity generation using partial oxidation-reformed gas to electricity generation using steam-reformed gas, it generates electricity (steady electricity generation) while adjusting the flow rate of the hydrocarbon fuel supplied from fuel supply means 12 and the amount of circulated fuel exhaust supplied through high- temperature blower 40 until the rated current is reached, as shown in Fig. 2C.
In this way, due to the circulation of fuel exhaust with high-temperature blower 40 (or other circulation means), fuel exhaust containing steam is produced by the generation of electricity in fuel cell main body 30 while in the steady electricity generation mode, so electricity can be continuously generated in fuel cell main body 30 by steam-reforming the hydrocarbon fuel using this fuel exhaust.
(Shutdown method for the fuel cell system of an embodiment of the present invention)
The activation method of fuel cell system 1 of this embodiment was described above based on Fig. 2A through 2C. Next, the shutdown method of this fuel cell system 1 embodiment will be described next based on Fig. 3A through 3C. Fig. 3A through 3C are schematic block diagrams showing the shutdown method of fuel cell system 1 of this embodiment; Fig. 3 A schematically shows the operation at the time of the activation of partial oxidation reformer 10, Fig. 3B schematically shows the operation when electricity generation is stopped in fuel cell main body 30, and Fig. 3C schematically shows the operation when fuel cell main body 30 is cooled. The flow of operations from the state in which fuel cell system 1 steadily generates electricity until the time of system shutdown will be described below.
(G) Switching from steam-reformed gas to partial oxidation-reformed gas
First, as shown in Fig. 3A, in the state in which the system steadily generates electricity with steam-reformed gas by supplying steam-reformed gas from steam reformer 20 to fuel cell main body 30, it activates partial oxidation reformer 10 by circulating hydrocarbon fuel from fuel supply means 12 and air from partial oxidation air supply means 14 to partial oxidation reformer 10. Next, the system adjusts the flow rate of partial oxidation-reformed gas by gradually increasing the hydrocarbon fuel and air supplied to partial oxidation reformer 10 to amounts sufficient for electricity generation while reducing and fixing the current to a level approximately 1/3 of the rated current. The system switches from electricity generation using steam- reformed gas to electricity generation using partial oxidation-reformed gas by gradually reducing the amount of hydrocarbon fuel supplied to steam reformer 20 until it reaches zero. Here, the supply of hydrocarbon fuel to steam reformer 20 for steady electricity generation may be stopped at the same time as or after the adjustment of the flow rate of partial oxidation-reformed gas. At the point at which electricity generation using partial oxidation-reformed gas is begun, electricity is generated using a combination of steam-reformed gas and partial oxidation-reformed gas.
As discussed above, an SOFC can be used as fuel cell main body 30, and the partial oxidation reformer 10 is preferably installed in the vicinity of fuel cell main body 30 in a position at which a partial oxidation reforming catalyst temperature of 3000C can be maintained during the steady state, and therefore partial oxidation reforming reactions begin immediately on the partial oxidation catalyst when the hydrocarbon fuel and air are circulated to partial oxidation reformer 10 at the flow rates sufficient for partial oxidation reforming operations. The circulation of fuel exhaust by high-temperature blower 40 (or other circulation means) may be terminated after the system switches from electricity generation using steam-reformed gas to electricity generation using partial oxidation-reformed gas.
(H) Termination of electricity generation
As shown in Fig. 3B, the system stops generating electricity in the state in which it is generating electricity using partial oxidation-reformed gas from partial oxidation reformer 10 after stopping the supply of steam-reformed gas to fuel cell main body 30 from steam reformer 20. In other words, it stops generating electricity by gradually reducing the current until it reaches zero, while circulating an amount of partial oxidation-reformed gas sufficient for electricity generation to anode 34. After the current reaches zero, the amounts of hydrocarbon fuel and air supplied to partial oxidation reformer 10 are determined from the structure of fuel cell main body 30 and the capacity of the flow path of anode 34 such that a flow rate that does not allow anode 34 to be contaminated with oxygen from the outside can be ensured.
(I) Cooling of fuel cell main body 30
Next, as shown in Fig. 3C, the system cools fuel cell main body by circulating cathode air to fuel cell main body 30 (here, because the system is cooling fuel cell main body 30, the cathode air is not heated by heat exchanger 64 or activation burner 66). The system continues to supply (circulate) partial oxidation-reformed gas from partial oxidation reformer 10 to anode 34 until the temperature of fuel cell main body 30 drops to a temperature low enough that anode 34 (fuel electrode) will not oxidize (approximately 2000C).
(J) Shutdown of fuel cell system 1
After the temperature of fuel cell main body 30 is cooled to a temperature low enough that anode 34 will not oxidize, fuel cell system 1 is shut down by stopping the supply of cathode air from cathode air supply means 50 and the supply of partial oxidation-reformed gas from partial oxidation reformer 10 to fuel cell main body 30 (in other words, the circulation of hydrocarbon fuel and air to partial oxidation reformer 10 is terminated).
As described above, with fuel cell system 1 of this embodiment, it is no longer necessary to install a supply device for gases such as inert gas or a steam generator for activation and shutdown, such as the inert gas supply means 114 or steam supply means 116 installed in conventional fuel cell system 100, so the overall fuel cell system 1 can be simplified and its cost can be reduced.
A suitable embodiment of the present invention was described above with reference to the attached drawings, but needless to say, the present invention is not limited to this example. Based on the above teachings, a person with ordinary skill in the art could conceive of various variations or modifications, and it is understood that these obviously fall under the technical scope of the present invention.
For example, while it was described in the above embodiment that methane is steam-reformed by steam reformer 20, the present invention is not limited to this case, and methane may alternatively be steam-reformed by direct cell-reforming methane at anode 34.
As another example, while it was described in the above embodiment that high-temperature blower 40 was used as a circulating means, the present invention is not limited to this case, and a device such as an ejector, for example, may be used as the circulating means of the present invention.
As an additional example, while it was described in the above embodiment that heat exchanger 64 was installed as a means for heating the cathode air, it is not absolutely necessary to install heat exchanger 64 in certain embodiments that are capable of maintaining the temperature of fuel cell main body 30 during electricity generation.
As another example, while it was described in the above embodiment that partial oxidation-reformed gas supplied from partial oxidation reformer 10 is supplied to anode 34 through steam reformer 20, the system may alternatively be configured such that the partial oxidation-reformed gas supplied from partial oxidation reformer 10 is directly supplied to anode 34 without passing through steam reformer 20.
As another example, while it was described in the above embodiment that a single fuel supply means 12 supplies hydrocarbon fuel to both partial oxidation reformer 10 and steam reformer 20, the system may alternatively be configured such that separate fuel supply means supply hydrocarbon fuel to partial oxidation reformer 10 and steam reformer 20, respectively. However, from the perspective of simplifying the overall fuel cell system and reducing costs, it is preferable to configure the system such that a single fuel supply means supplies hydrocarbon fuel to both the partial oxidation reformer and the steam reformer, as in the embodiment described above.
It should be noted that embodiments of the present invention are particularly well-suited for use with tubular anode-supported solid oxide fuel cells of the types described in Applicant's United States Published Patent Application No. 2002/0028367 entitled Electrode- Supported Solid State Electrochemical Cell, which was published on March 7, 2002 and is hereby incorporated herein by reference in its entirety. Such tubular anode-supported solid oxide fuel cells may be configured such that the anode is the innermost layer of the tubular fuel cell, which facilitates distribution of fuel through the fuel cell, facilitates collection and recirculation of exhaust gases from the fuel cell, and facilitates isolation of the anode from external sources of oxygen (e.g., air) so as to reduce the likelihood of oxygen coming into contact with the anode (e.g., in a fuel cell having an outer anode, fuel is typically circulated around the anode within an enclosure, so a breach of the enclosure could expose the anode to surrounding air, which could result in oxidation of the anode).
The present invention may be embodied in other specific forms without departing from the true scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive.

Claims

We claim:
1. A fuel cell system comprising: a steam reformer that produces steam-reformed gas containing carbon monoxide and hydrogen by steam-reforming hydrocarbon fuel; a partial oxidation reformer that produces partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming said hydrocarbon fuel; a fuel cell main body that generates electricity using said steam-reformed gas and/or said partial oxidation-reformed gas; and a circulating means that returns fuel exhaust containing steam discharged from said fuel cell main body to said steam reformer.
2. A fuel cell system according to Claim 1 further equipped with a control means that controls the amount of said partial oxidation-reformed gas supplied to said fuel cell main body from said partial oxidation reformer and the amount of said steam- reformed gas supplied to said fuel cell main body from said steam reformer, wherein: said control means is characterized in that it controls in such a way that when said fuel cell main body is activated, said control means produces said partial oxidation-reformed gas by partial oxidation-reforming said hydrocarbon fuel with said partial oxidation reformer and heats said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation- reformed gas to a fuel electrode of said fuel cell main body; when said fuel cell main body starts generating electricity, said control means generates electricity using said partial oxidation-reformed gas while supplying said partial oxidation-reformed gas from said partial oxidation reformer; and when said fuel cell main body is steadily generating electricity, said control means feeds said fuel exhaust and said hydrocarbon fuel to said steam reformer, produces said steam-reformed gas by steam- reforming said hydrocarbon fuel with said steam reformer, and generates electricity using said steam-reformed gas while supplying said steam-reformed gas to said fuel cell main body from said steam reformer, while reducing the amount of said partial oxidation-reformed gas supplied from said partial oxidation reformer to said fuel cell main body.
3. A fuel cell system according to Claim 2, wherein said control means is characterized in that it controls in such a way that when the fuel cell system is in said state in which it is steadily generating electricity, at the same time as or after it starts generating electricity with said partial oxidation-reformed gas by supplying said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer, the control means stops the supply of said steam-reformed gas from said steam reformer to said fuel cell main body, stops generating electricity with said partial oxidation-reformed gas after it stops the supply of said steam-reformed gas, supplies said partial oxidation-reformed gas from said partial oxidation reformer to said fuel electrode until the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize, and stops the supply of said partial oxidation-reformed gas from said partial oxidation reformer to said fuel cell main body after the temperature of said fuel cell main body drops to a temperature low enough such that said fuel electrode will not oxidize.
4. A fuel cell according to any of claims 1-3, wherein the fuel cell main body includes at least one tubular anode-supported solid oxide fuel cell having an inner anode layer through which the steam-reformed gas and partial oxidation-reformed gas flow and from which the fuel exhaust is collected.
5. An operating method for a fuel cell system characterized in that it produces partial oxidation-reformed gas containing carbon monoxide and hydrogen by partial oxidation-reforming hydrocarbon fuel with a partial oxidation reformer, starts generating electricity with a fuel cell main body using said partial oxidation-reformed gas after it heats said fuel cell main body to a temperature at which it can generate electricity while supplying said produced partial oxidation-reformed gas to a fuel electrode of said fuel cell main body, feeds fuel exhaust containing steam formed by this generation of electricity and said hydrocarbon fuel to a steam reformer, produces steam-reformed gas containing carbon monoxide and hydrogen by steam-reforming said hydrocarbon fuel with said steam reformer, and steadily generates electricity by supplying said steam-reformed gas to said fuel cell main body from said steam reformer while reducing the amount of said partial oxidation-reformed gas supplied to said fuel cell main body from said partial oxidation reformer.
6. An operating method for a fuel cell system according to Claim 5 characterized in that when in the state in which it generates electricity by supplying said steam- reformed gas to said fuel cell main body from said steam reformer, at the same time as or after it starts generating electricity with said partial oxidation-reformed gas by supplying said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer, stops the supply of said steam-reformed gas to said fuel cell main body from said steam reformer, stops generating electricity with said partial oxidation-reformed gas after it stops the supply of said steam-reformed gas, supplies said partial oxidation-reformed gas to said fuel electrode from said partial oxidation reformer until the temperature of said fuel cell main body drops to a temperature at which said fuel electrode would oxidize, and stops the supply of said partial oxidation-reformed gas to said fuel cell main body from said partial oxidation reformer after the temperature of said fuel cell main body drops below a temperature at which said fuel electrode would oxidize.
7. A fuel cell according to any of claims 5-6, wherein the fuel cell main body includes at least one tubular anode-supported solid oxide fuel cell having an inner anode layer through which the steam-reformed gas and partial oxidation-reformed gas flow and from which the fuel exhaust is collected.
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