Power generation system and operation method thereof

A power generation system comprising a fuel cell system including a fuel cell for generating electric power using a fuel gas and an oxidizing gas and a casing for accommodating the fuel cell, a gas supply device, and a controller;

• the power generation system further comprising:
• a combustion device;
• an exhaust passage placed to provide communication between the casing and an exhaust port of the combustion device and configured to exhaust an exhaust gas exhausted from the fuel cell system and an exhaust gas exhausted from the combustion device, to atmosphere, through an opening leading to the atmosphere;
• a gas passage which is provided with the gas supply device and used to supply a gas supplied from the gas supply device to the exhaust passage;
• and
• a back-flow preventing device placed in the gas passage or the exhaust passage and including a valve seat and a valve element;
• wherein the back-flow preventing device is configured such that the valve element moves away from the valve seat to permit the gas to flow in a direction from the casing to the opening of the exhaust passage, and the valve element contacts the valve seat to inhibit the gas from flowing in a direction from the opening of the exhaust passage to the casing and as a result to prevent the exhaust gas exhausted from the combustion device from flowing into the casing;
• wherein the exhaust passage has a shape in which its upstream end branches into at least two parts to be connected to the exhaust port of the combustion device and to the casing;
• wherein the back-flow preventing device is placed in a location closer to the casing than a point where the upstream end of the exhaust passage branches;
• and
• wherein during a shut-down state or at start-up of the fuel cell system, the controller executes an operation for relieving the state in which the valve element remains incapable of moving away from the valve seat in such a manner that the gas supply device is operated so that a differential pressure between an upstream side and a downstream side of the back-flow preventing device reaches a value which is equal to or greater than a predetermined value which can relieve the state in which the valve element remains incapable of moving away from the valve seat in the back-flow preventing device.