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.

A plant includes a fuel supply line for supplying high-pressure fuel gas; and at least one expander disposed in the fuel supply line and configured to extract power from the high-pressure fuel gas by expanding the high-pressure fuel gas.

A fuel cell system and a control method thereof are provided. In the system, an oxygen sensor is mounted on the anode inlet side and the anode outlet side of a fuel cell stack to measure an oxygen concentration. Based on the measured oxygen concentration, a control operation is performed on the fuel cell system to reduce the oxygen concentration on the anode side. Accordingly, the irreversible deterioration of the fuel cell occurring due to the reverse voltage of the cell during driving of the fuel cell vehicle and the cathode carbon corrosion occurring due to the inflow of air during parking are effectively reduced, thereby increasing the durability of the fuel cell and the fuel cell vehicle.

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 method for estimating a hydrogen concentration of a fuel cell includes estimating an initial amount of gas included at an anode side of a fuel cell, calculating a crossed over amount of gas between the anode side and a cathode side of the fuel cell and an amount of gas purged from the anode side to the outside from time when an initial amount of gas is predicted to a present time, and estimating a current hydrogen concentration at the anode side based on the predicted initial amount of gas, the calculated amount of crossed over gas, and the amount of purged gas.

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 device is improved for operating conditions during a partial load operation. The fuel cell device comprises a cell stack formed by electrically connecting fuel cells for generating power by fuel gas and oxygen-containing gas; a fuel gas supply unit for supplying the fuel gas to the fuel cells; and a power adjustment unit for adjusting the amount of current that is supplied to an external load and a controller for controlling the fuel gas supply unit and the power adjustment unit. The controller adjusts, during the partial load operation of the fuel cell device and when the fuel gas supplied to the cell stack is at a low flow rate. The relationship between a fuel utilization rate of the cell stack and the amount of power generated by the cell stack can be nonlinear.

In various aspects, systems and methods are provided for operating a molten carbonate fuel cell, such as a fuel cell assembly, with increased production of syngas while also reducing or minimizing the amount of CO.sub.2 exiting the fuel cell in the cathode exhaust stream. This can allow for improved efficiency of syngas production while also generating electrical power.

Systems and methods are provided for integrating a chemical looping combustion system with molten carbonate fuel cells to provide improved operation of the molten carbonate fuel cells when using the exhaust from a gas turbine or other electrical power generation device as the CO.sub.2 source for the MCFC cathodes. This integration can be accomplished by using metal oxide in the chemical looping combustion system to oxidize the anode output flow from the MCFCs. This can reduce or minimize the number of separations that need to be performed in order to process the concentrated CO.sub.2 present within the anode exhaust. By reducing, minimizing, or eliminating the CO and H.sub.2 in the anode exhaust, the need to perform more costly separations (such as cryogenic separation or amine washing) to obtain a high purity CO.sub.2 product stream can be reduced or minimized. Optionally, the cathode exhaust from the molten carbonate fuel cells can be used as an oxygen-containing stream for regeneration of the metal oxide.

Systems and methods are provided for operating molten carbonate fuel cells to allow for periodic regeneration of the fuel cells while performing elevated CO.sub.2 capture. In some aspects, periodic regeneration can be achieved by shifting the location within the fuel cells where the highest density of alternative ion transport is occurring. Such a shift can result in a new location having a highest density of alternative ion transport, while the previous location can primarily transport carbonate ions. Additionally or alternately, periodic regeneration can be performed by modifying the input flows to the fuel cell and/or relaxing the operating conditions of the fuel cell to reduce or minimize the amount of alternative ion transport.