A method of operating a fuel cell system including a fuel cell stack having an anode-side and a cathode-side, said anode side having an inlet for receiving fuel and an exit for discharging exhaust fuel gas, and said cathode-side having an inlet for receiving oxidant gas and an exit for discharging exhaust oxidant gas, a burner, and a mixer and eductor, the method comprising the steps of: supplying oxidant gas from an oxidant supply to the mixer and eductor with a controllable flow rate, the mixer and eductor being adapted for operating on and mixing the exhaust fuel gas from said exit of said anode-side and the oxidant gas supplied in the supplying step in the mixer and eductor and delivering the resultant gas mixture to the burner for burning oxidant and exhaust fuel gas and for delivering the resultant gas to the inlet of said cathode-side; characterized by the mixer and eductor being adapted for operating on and mixing the exhaust fuel gas and oxidant supply such that the difference between the pressure of said exhaust fuel gas at the exit of the anode-side and the pressure of the oxidant gas at the inlet of the cathode-side is reduced; the mixer and eductor being adapted for operating on and mixing the exhaust fuel gas and oxidant supply gas by: increasing the velocity and decreasing the pressure of the incoming oxidant gas using a pinch area of the mixer and eductor, thereby creating a high pressure oxidant stream which produces a vacuum for the exhaust fuel gas; mixing the high pressure oxidant stream and the exhaust fuel gas in a mixing and recovery area of the mixer and eductor following the pinch area and increasing the pressure of the resultant mixture and controlling the pinch area and the flow rate of the oxidant supply so as to provide said reduction in pressure difference, so that the gas pressure at the anode-side exit is made substantially equal to the gas pressure at the cathode-side inlet. supplying oxidant gas from an oxidant supply to the mixer and eductor with a controllable flow rate, the mixer and eductor being adapted for operating on and mixing the exhaust fuel gas from said exit of said anode-side and the oxidant gas supplied in the supplying step in the mixer and eductor and delivering the resultant gas mixture to the burner for burning oxidant and exhaust fuel gas and for delivering the resultant gas to the inlet of said cathode-side; characterized by the mixer and eductor being adapted for operating on and mixing the exhaust fuel gas and oxidant supply such that the difference between the pressure of said exhaust fuel gas at the exit of the anode-side and the pressure of the oxidant gas at the inlet of the cathode-side is reduced; the mixer and eductor being adapted for operating on and mixing the exhaust fuel gas and oxidant supply gas by: increasing the velocity and decreasing the pressure of the incoming oxidant gas using a pinch area of the mixer and eductor, thereby creating a high pressure oxidant stream which produces a vacuum for the exhaust fuel gas; mixing the high pressure oxidant stream and the exhaust fuel gas in a mixing and recovery area of the mixer and eductor following the pinch area and increasing the pressure of the resultant mixture and controlling the pinch area and the flow rate of the oxidant supply so as to provide said reduction in pressure difference, so that the gas pressure at the anode-side exit is made substantially equal to the gas pressure at the cathode-side inlet. increasing the velocity and decreasing the pressure of the incoming oxidant gas using a pinch area of the mixer and eductor, thereby creating a high pressure oxidant stream which produces a vacuum for the exhaust fuel gas; mixing the high pressure oxidant stream and the exhaust fuel gas in a mixing and recovery area of the mixer and eductor following the pinch area and increasing the pressure of the resultant mixture and controlling the pinch area and the flow rate of the oxidant supply so as to provide said reduction in pressure difference, so that the gas pressure at the anode-side exit is made substantially equal to the gas pressure at the cathode-side inlet.