Fuel supply method, fuel supply system, fuel combustion system provided with fuel supply system, and gas turbine plant

A fuel supply system comprising:

• a main ammonia line connected to an ammonia tank configured to store liquid ammonia;
• a main ammonia pump provided on the main ammonia line, the main ammonia pump configured to pressurize the liquid ammonia from the ammonia tank;
• a vaporizer connected to an end of the main ammonia line, the vaporizer configured to heat and vaporize the liquid ammonia via heat exchange between a heating medium and the liquid ammonia pressurized by the main ammonia pump;
• a gaseous ammonia line connected to the vaporizer, the gaseous ammonia line configured to guide gaseous ammonia, which is the ammonia vaporized by the vaporizer, as fuel to a combustor of a gas turbine;
• a liquid ammonia line configured to guide liquid ammonia, which is the liquid ammonia pressurized by the main ammonia pump and that has not undergone heat exchange with the heating medium at the vaporizer, as fuel to the combustor;
• and
• a switching device configured to switch an ammonia supply state between a plurality of states including a first state in which the gaseous ammonia is guided from the gaseous ammonia line to the combustor and a second state in which the liquid ammonia is guided from the liquid ammonia line to the combustor.