A control unit estimates a discharged fuel gas amount, i.e., an amount of fuel gas discharged from the outlet of a cathode flow field, of a fuel exhaust gas introduced from a communication flow path to the inlet of the cathode flow field and then flowing through a cathode. The control unit calculates an oxygen-containing gas amount necessary for dilution at the time of discharge into the atmosphere, from the estimated discharged fuel gas amount, and sets a discharge amount of the air pump, based on the calculated oxygen-containing gas amount.

A fuel cell system (200) for providing electrical energy. The system (200) comprises a fuel cell stack (201), an anode subsystem (203) with a proportional valve (205) for dosing a volume of gas to be fed to the fuel cell stack (201), a purge valve (207) for discharging gas from the anode subsystem (203) into an exhaust-gas path (209) of the fuel cell system (200), and a control unit (211) for controlling the proportional valve (205) and the purge valve (207). The control unit (211) is configured to use an electrical control current that is fed to the proportional valve (205) to readjust for a purging operation to draw conclusions regarding a hydrogen concentration in a gas that is fed to the purge valve (207), wherein the control unit (211) is furthermore configured to adjust the fuel cell system (200) in a manner dependent on the determined hydrogen concentration.

A fuel cell stack includes a stack body including a plurality of power generation cells stacked together, a stack case storing the stack body, and an auxiliary device case storing a fuel cell auxiliary device. Two exhaust gas openings are provided in an upper part of the stack case. One exhaust gas opening is provided in an upper part of the auxiliary device case. An exhaust duct is connected to the only three exhaust gas openings in total.

A fuel cell system includes a fuel cell stack having a plurality of fuel cells that each contain a plurality of fuel electrodes and air electrodes. The system includes a fuel receiving unit connected to the fuel cell stack, which receives a hydrocarbon fuel from a fuel supply. The system includes a fuel exhaust processing unit fluidly coupled to the fuel cell stack by a slip stream, where the fuel exhaust processing unit processes fuel exhaust from the fuel cell stack, and the slip stream is fluidly connected to an exhaust stream flowing from the fuel cell stack. The fuel processing unit removes a first portion of carbon dioxide (CO.sub.2) from fuel exhaust within the slip stream, outputs the first portion of CO.sub.2 in a first stream, and outputs a second portion of CO.sub.2 remaining from the fuel exhaust in the slip stream into a second stream, which includes hydrogen.

The present disclosure relates to an apparatus and a method for controlling a concentration of exhaust hydrogen in a fuel cell system. The apparatus may include an air exhaust valve for discharging hydrogen from a cathode in a fuel cell stack to an outside environment through an air exhaust line, an air compressor for supplying ambient air to the air exhaust line, an air cut-off valve for blocking air supplied to the cathode, and a controller that opens the air exhaust valve and drives the air compressor when starting to supply hydrogen to the fuel cell stack, and opens the air cut-off valve such that a concentration of the hydrogen discharged from the cathode is reduced by air in the air exhaust line when the hydrogen supply is completed.

Fuel cell systems configured for efficient byproduct recovery and reuse are disclosed herein. In one embodiment, a fuel cell system includes a reformer configured to reform a fuel containing methane (CH.sub.4) with steam to produce a reformed fuel having methane (CH.sub.4), carbon monoxide (CO), and hydrogen (H.sub.2). The fuel cell system also includes a fuel cell configured to perform an electrochemical reaction between a first portion of the reformed fuel and oxygen (O.sub.2) to produce electricity and an exhaust having carbon dioxide (CO.sub.2), water (H.sub.2O), and a second portion of the reformed fuel. The fuel cell system further includes an oxygen enricher configured to generate an oxygen enriched gas and a combustion chamber configured to combust the second portion of the reformed fuel with the oxygen enriched gas.

A fuel cell system includes a fuel cell in which cells are stacked, a voltage sensor that detects a voltage in unit of one or more of the cells, a control unit that determines an operating point of the fuel cell and causes the fuel cell to operate. The control unit causes the fuel cell to operate at a low efficiency operating point having a lower efficiency than an efficiency of a reference operating point in a warm-up operation. In the warm-up operation, the control unit calculates a total number of the cells in which the voltage detected by the voltage sensor is equal to or less than a predetermined first reference voltage and calculates an exhaust hydrogen concentration based on the total number or the cells.

An apparatus and method for reducing an exhaust hydrogen concentration in a fuel cell system includes a first air cut-off valve (ACV) blocking ambient air supplied to a cathode, a second ACV blocking exhaust hydrogen discharged from the cathode, and an air suction valve (ASV) operating in a first mode connecting the cathode and an intake port of an air compressor and in a second mode blocking connection between the cathode and the intake port of the air compressor. The apparatus also includes a controller for operating the ASV in the first mode to store air of the cathode while the first ACV is opened and the second ACV is closed when hydrogen is supplied to an anode, and for operating the ASV in the second mode to discharge ambient air through an exhaust line while the first ACV is opened and the second ACV is opened when ambient air is supplied to the cathode.

Systems and methods are provided for using fuel cell staging to reduce or minimize variations in current density when operating molten carbonate fuel cells with elevated CO.sub.2 utilization. The fuel cell staging can mitigate the amount of alternative ion transport that occurs when operating molten carbonate fuel cells under conditions for elevated CO.sub.2 utilization.

An apparatus and method for reducing an exhaust hydrogen concentration in a fuel cell system includes a first air cut-off valve (ACV) blocking ambient air supplied to a cathode, a second ACV blocking exhaust hydrogen discharged from the cathode, and an air suction valve (ASV) operating in a first mode connecting the cathode and an intake port of an air compressor and in a second mode blocking connection between the cathode and the intake port of the air compressor. The apparatus also includes a controller for operating the ASV in the first mode to store air of the cathode while the first ACV is opened and the second ACV is closed when hydrogen is supplied to an anode, and for operating the ASV in the second mode to discharge ambient air through an exhaust line while the first ACV is opened and the second ACV is opened when ambient air is supplied to the cathode.