NOVELTY - The fuel cell system comprises a valve assembly that is formed to individually control flow rate of water supplied through multiple valves, a fuel reformer connected with first and second heat exchangers through a water pump, a low temperature natural gas field ring reactor located in the first heat exchanger, where the ring reactor reduces carbon monoxide generated in an auto thermal reforming reaction so as to generate reformate hydrogen and an operating temperature of the ring reactor is lower. The system heat-exchanges the hydrogen generated in the fuel reformer and a fuel cell stack.    USE - The fuel cell system is useful for producing natural gas, methane, gasoline and diesel.    ADVANTAGE - The fuel cell system can enhance selective catalytic reduction and operational cost reduction of the fuel reformer with reduced blower and pump malfunction during summer and winter seasons, and effectively controls a load of the fuel reformer.    DETAILED DESCRIPTION - The fuel cell system comprises a valve assembly that is formed to individually control flow rate of water supplied through multiple valves, a fuel reformer connected with first and second heat exchangers through a water pump, a low temperature natural gas field ring reactor located in the first heat exchanger, where the ring reactor reduces carbon monoxide generated in an auto thermal reforming reaction so as to generate reformate hydrogen and an operating temperature of the ring reactor is lower. The system heat-exchanges the hydrogen generated in the fuel reformer and a fuel cell stack in order to generate electricity using anode tail-gas oxidizer so that the air is supplied to a blower through a valve assembly. A third heat exchanger rises temperature of a reactant that is supplied to the fuel reformer. A control unit controls opening and closing of the valves. A temperature sensor is installed at an entrance of the ring reactor. A fourth heat exchanger controls flow rate of the supplied water using the valve assembly. A de-vulcanizer is formed to remove sulfur in auto thermal reforming reaction. A fifth heat exchanger controls the flow rate of the supplied water with the valve assembly. A selective oxidation part is arranged between the fifth heat exchanger and fuel cell stack. A mixer is formed in order to mix the reactant.