Gas generating system for a fuel cell system and method for providing hydrogen fuel

The system has one or more selective oxidation stages (3) for reducing the CO component in the gas flow. These stages are thermally coupled to a reformer (1) for hydrocarbon reforming or to a heat exchanger. One or more additional, thermally decoupled selective oxidation stages (2) are arranged before and/or after and/or between the oxidation stages for selective oxidation of carbon monoxide when starting the gas generation system. Independent claims are also included for the following: a method of providing a gas flow containing hydrogen for a fuel cell system.