NOVELTY - A fuel cell system comprises a fuel cell unit (1) and a gas-generating system (2) comprising a first reforming reactor (4) for producing a first reformate R1 with an outlet temperature T1,out, a second reforming reactor (7) for producing a second reformate R2 with a second outlet temperature T2,out that is below T1,out, and a mixer (9) that supplies fuel(s) to the first reformate (R1) between the first reactor outlet (5) and the second reactor inlet (6).    USE - For use in mobile systems.    ADVANTAGE - The fuel cell system works largely without heat exchange during reforming and the subsequent gas purification. The reforming reactors can be constructed simply and compactly. The manufacturing costs of the reactors can be reduced. The system has fewer components. The pressure loss in the gas-generating system is reduced.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for the operation of a fuel cell system comprising supplying fuel(s) with a first inlet temperature T1,in to a first reforming reactor; carrying out an auto-thermal reforming reaction in the first reforming reactor, in which the reaction proceeds in thermodynamic equilibrium to form R1 with T1,out higher than T1,in; mixing R1 with a fuel having a lower temperature than T1.out to form a mixture having a second lower inlet temperature T2,in; and supplying the mixture to a second reforming reactor in which, under adiabatic conditions, steam reforming and a hydrogen shift reaction take place to produce R2 with T2,out that is below T1,out of R1.    DESCRIPTION OF DRAWING(S) - The figure shows a section from a fuel cell system.    Fuel cell unit (1)    Gas-generating system (2)    First reactor inlet (3)    First reforming reactor (4)    First reactor outlet (5)    Second reactor inlet (6)    Second reforming reactor (7)    Second reactor outlet (8)    Mixer (9)    Gas purification system (10)    Heat exchanger (11)    First step of two-step gas purification system (12)    Afterburner (15)    Expander (16)    Reformate cooling unit (19)    Air supply (20)