A fuel cell system includes a reformer for generating hydrogen gas from fuel containing hydrogen using a chemical catalytic reaction and thermal energy. At least one electricity generator generates electrical energy by an electrochemical reaction of the hydrogen gas and oxygen. A fuel supply assembly supplies fuel to the reformer, and an oxygen supply assembly supplies oxygen to the at least one electricity generator. A heat exchanger is connected to the reformer and to the at least one electricity generator. The heat exchanger supplies thermal energy of the reformer, during initial operation of the system, to the at least one electricity generator so as to pre-heat the at least one electricity generator.