TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Device: The filter can be changed. It can be regenerated and the circulation line comprises means for regenerating the filter. The means for regenerating the filter comprise means for performing the regeneration in an underpressure. The means for regenerating the filter comprise means for heating the filter and means for removing impurities detaching during the heating and/or inert gases from the circulation line. The circulation line comprises at least one valve, preferably a three-way valve (17), for removing impurities detaching during the regeneration and/or inert gases from the circulation line. It is fitted in parallel with the anode or cathode of the fuel cell, relative to the direction of flow of the fuel or oxidizer, in such a way that part of the fuel or oxidizer can be circulated along the circulation line past the fuel cell's anode or cathode. The circulation line is fitted to the fuel or oxidizer return line as a closed circulation line. Preferably, 0.1-30%, more preferably 0.5-5% of the fuel or oxidizer is arranged to flow through the filter along the circulation line. The fuel cell is a closed-end hydrogen fuel cell. The filter is arranged to collect at least one of the following: dihydrogen sulfide (H2S), sulfur dioxide (SO2), nitrogen oxides (NOx), and organic hydrocarbons (VOC). At least the anode side is equipped with a circulation line. Both the anode and the cathode sides are equipped with a circulation line. The system further comprises common means for regenerating the filters of the anode and cathode sides simultaneously and preferably at a common under- or overpressure. Preferred Method: The method further comprises using for filtering the impurities a regenerable filter, which at desired intervals is regenerated, preferably at a raised temperature and/or reduced pressure and the impurities detaching during regeneration are removed from the circulation. It further comprises removing the impurities arising during regeneration through a valve arranged in the circulation line. The circulation line is arranged in such a way that the part of the fuel is circulated past the anode or cathode along a bypass feed line parallel to the anode or cathode, or along a closed loop in flow connection with the anode or cathode.