Abstract: The invention concerns a method of operating a fuel cell system (1) in fuel cell mode, the fuel cell system (1) comprising at least one fuel cell (2) comprising an anode (2a), a cathode (2b) and an electrolyte (2c) provided between the anode (2a) and the cathode (2b), the fuel cell (2) being arranged for an internal reformation of hydrocarbon compounds inside the anode (2a), an anode exhaust conduit (4) connecting the anode outlet (2d) and a fuel conduit (6) and comprising a methanation unit (3a), a water-gas shift reactor (3b), and a water vapor condenser or water removal unit (12), the anode exhaust conduit (4) or the fuel conduit (6) comprising a carbon dioxide separation unit (11), and the fuel conduit (6) connecting a fuel cell system inlet (7) and an anode inlet (2e), the method comprising the steps of a) feeding feed gas into the fuel conduit (6) via the fuel cell system inlet (7), the feed gas comprising hydrocarbon compounds, b) controlling the methanation unit (3a) to produce methane and controlling

Abstract: A fuel cell system arranged for the conversion of pure hydrogen comprising a) at least one fuel cell comprising an anode, a cathode and an electrolyte, and arranged for an internal reformation of methane, b) a fuel conduit connecting a fuel conduit inlet with an anode inlet, c) an anode exhaust conduit connecting an anode outlet and a methanation unit capable of producing methane from anode exhaust, and d) a methanation unit exhaust conduit connecting a methanation unit exit and the fuel conduit, and e) a water removal and/or water condenser unit coupled to the methanation unit exhaust conduit, wherein the fuel introduced into an inlet of the fuel conduit is pure hydrogen, and the amount of methane produced in the methanation unit is equal to the amount of methane reformed inside of the fuel cell so that the content of methane cycling through the fuel cell system is constant.