TECHNOLOGY FOCUS - COMPUTING AND CONTROL - Preferred Components: The computer program adapted to be downloaded to an arrangement or a fuel cell system or one of its components when run on a computer which is connected to the internet.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The initial step precedes the first stage, where hydrocarbon fuel and air is fed into the fuel reformer as a lean mixture, and the fuel reformer is catalytically started and heated by an exothermic reaction. In the fourth stage, the hydrogen which is generated by the ATR process is purified before supplying it to the fuel cell, by especially reducing the amount of carbon monoxide. The transition phase precedes the steady state operation, in which a portion of the hydrogen which is generated by the ATR process is supplied to the fuel cell and another portion is converted into water steam, which is fed back into the fuel reformer. The catalytic combustion reactor is used for conducting the second stage, where in the steady state operation, excess hydrogen which is generated by the fuel cell is supplied to the combustion reactor for catalytically converting the same into water steam, which is fed as a heat source to a heat exchanger in which water which is generated by the fuel cell is evaporated to steam which is fed into the fuel reformer. In the first stage, oxygen is fed into the fuel reformer in the form of ambient air, where the lambda value of the air to fuel mixture is 0.1-0.7. In the initial step, oxygen is fed into the fuel reformer in the form of ambient air, where the lambda value of the air to fuel mixture is 4-8. The arrangement comprises: a gas clean-up reactor between an outlet of the fuel reformer and an inlet of the combustion reactor for reducing the amount of carbon monoxide from the product of the fuel reformer; a control unit (1d) for controlling the ratio of air to fuel supplied into the fuel reformer; a control unit for controlling inlet valves of the fuel reformer so that the first stage and/or the initial step is realized; and a fuel reformer which comprises a catalyst with a multipurpose catalyst material or at least two one-purpose catalyst materials, for conducting endothermic steam reforming and exothermic oxidation processes. In the fuel cell system: hydrogen generated by the fuel cell is supplied via a separator to the combustion reactor for generating heat for evaporating water of the fuel cell; and water generated by the fuel cell is supplied via a separator into a vessel for generating steam for feeding the fuel reformer.