The present invention relates to a fuel cell system using an anode off gas (AOG) excellent in burner combustion efficiency of a reformer. The fuel cell system according to the present invention includes a stream reformer, a stack, and a phase separator. The phase separator is connected to an outlet of an anode to receive AOG, and includes a gas outlet and a moisture outlet. The gas outlet can be connected to a burner unit of the steam reformer to supply moisture-free AOG to the burner unit of the reformer. In addition, the fuel cell system according to the present invention can further include a heat exchanger in which air for heat-exchange with AOG is transferred to the phase separator, so that the air can be preheated and supplied to the burner unit of the reformer or a cathode of the stack.
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