Combined power generation system and driving method thereof

A method of driving a combined power generation system comprising a gas turbine, a steam turbine, a heat recovery steam generator (HRSG), and an ammonia decomposer configured to decompose ammonia and supply ammonia-decomposed gases to a combustor of a gas turbine, the method comprising:

• an exhaust gas supply step of supplying exhaust gases generated in the gas turbine to the ammonia decomposer and the HRSG;
• an ammonia decomposition step of decomposing ammonia using heat of the exhaust gases supplied from the gas turbine to generate the ammonia-decomposed gases;
• a decomposed gas supply step of supplying the decomposed gases generated in the ammonia decomposition step to the combustor;
• and
• a combustion and power generation step of combusting the decomposed gases in the combustor to generate exhaust gases to allow a generator to rotate,
• wherein:
• the low-pressure section comprises a condensate preheater configured to heat condensate, and a low-pressure evaporator configured to heat feedwater and convert the feedwater into steam,
• in the exhaust gas supply step, after heat exchange between the exhaust gases discharged from the gas turbine and the exhaust gases discharged from the ammonia decomposer in a heat exchanger, the exhaust gases discharged from the gas turbine are transferred to the ammonia decomposer, and the exhaust gases discharged from the ammonia decomposer are transferred to the HRSG;
• and
• in the ammonia decomposition step, the exhaust gases are transferred upstream of the low-pressure evaporator with respect to a flow direction of the exhaust gases and the exhaust gases having been used in the ammonia decomposition are transferred to a low pressure section of the HRSG.