A fuel cell system, comprising: a fuel cell; a liquid fuel supply for providing liquid fuel; an evaporator configured for evaporating the liquid fuel to fuel vapor and an upstream liquid-conduit to the liquid fuel supply for receiving the liquid fuel from the liquid fuel supply by the evaporator; a reformer for catalytic conversion of the fuel vapor to syngas; a vapor-conduit for providing the fuel vapor from the evaporator to the reformer; a syngas-conduit from the reformer to the fuel cell for providing syngas from the reformer to the fuel cell; a burner configured for catalytic burning of fuel vapor or rest gas from the fuel cell and for providing heated exhaust gas from the burning; the burner having a gas exhaust for exhaust of burned gas, wherein the gas exhaust is in flow-communication with the reformer through an exhaust gas flow path for heating the reformer by the exhaust gas and where the burner comprise a catalytic monolith which is arranged in extension and down-stream of a mixing chamber in which air is mixed with evaporated fuel or rest gas prior to entering the monolith; wherein the mixing chamber is surrounded by a sleeve, which comprises a plurality of openings (29A, 29B) around the mixing chamber for supply of fuel vapor through the openings (29A, 29B) in the startup phase and for supply of rest gas through the openings (29A, 29B) during normal operation, wherein the openings (29A, 29B) are arranged as a first and a second set of openings (29A, 29B), each set (29A, 29B) being distributed in a plane perpendicular to a longitudinal axis of the sleeve, where the plane of the first set of openings has a distance to the plane of the second set of openings, wherein the system is configured for transport of fuel vapor into the mixing chamber only through the first set of openings and transport of rest gas into the mixing chamber only through the second set of openings. a fuel cell; a liquid fuel supply for providing liquid fuel; an evaporator configured for evaporating the liquid fuel to fuel vapor and an upstream liquid-conduit to the liquid fuel supply for receiving the liquid fuel from the liquid fuel supply by the evaporator; a reformer for catalytic conversion of the fuel vapor to syngas; a vapor-conduit for providing the fuel vapor from the evaporator to the reformer; a syngas-conduit from the reformer to the fuel cell for providing syngas from the reformer to the fuel cell; a burner configured for catalytic burning of fuel vapor or rest gas from the fuel cell and for providing heated exhaust gas from the burning; the burner having a gas exhaust for exhaust of burned gas, wherein the gas exhaust is in flow-communication with the reformer through an exhaust gas flow path for heating the reformer by the exhaust gas and where the burner comprise a catalytic monolith which is arranged in extension and down-stream of a mixing chamber in which air is mixed with evaporated fuel or rest gas prior to entering the monolith; wherein the mixing chamber is surrounded by a sleeve, which comprises a plurality of openings (29A, 29B) around the mixing chamber for supply of fuel vapor through the openings (29A, 29B) in the startup phase and for supply of rest gas through the openings (29A, 29B) during normal operation,