NOVELTY - The method involves moisturizing process gas before supplying to fuel cells by a liquid ring compressor (2), and intaking process flue gas by the compressor. The flue gas, before supplied to the process gas, is directed through a liquid separator (3) for separation of product water from the flue gas.    USE - Used for operating a fuel cell system e.g. polymer electrolyte membrane (PEM) fuel cell, that is utilized for generating electric current in a vehicle.    ADVANTAGE - The process gas is moisturized by the liquid ring compressor, which intakes the process flue gas, so the dual function of the compressor simplifies the design of the fuel cell system and also controls and regulates the process gas supply to the fuel cell in a simple manner. The method increases the speed of the compressor to increase the flue gas stream within a short time, thus increasing the supply of the flue gas to the process gas and/or increasing the pressure level in the fuel cell, and hence enabling high dynamic of the fuel cell system during the power output. The intake speed of the compressor enables the intake of the process flue gas supply to the different size and/or number of the fuel cells in a simple manner, thus standardizing the fuel cell system extensively, and adapting to different number and/or size of the fuel cells in a simple and flexible manner. The process flue gas, before supplied to the process gas, is directed through the liquid separator for separation of the product water from the process flue gas, thus increasing efficiency of the fuel cell system. The method enables the operation of the system with good utilization of the process gas.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a fuel cell system comprising a set of fuel cells. The flue gas is supplied to the process gas before the moisturization by the compressor, and a portion of the flue gas is discharged from a fuel cell system depending on a ratio of inert gas in the process gas.    DESCRIPTION OF DRAWING(S) - The drawing shows a fuel cell system.    Fuel cell system (1)    Liquid ring compressor (2, 2`)    Liquid separator (3, 3`)    Control valve (8)    Fuel cell block (10)