According to the embodiment of the present invention, there are provided a first stage auxiliary burner configured to heat up the exhaust gas in the upstream side of the superheater,

a second stage auxiliary burner configured to heat up the exhaust gas in the upstream side of the evaporator, a fuel supply system configured to distribute fuel so as to be supplied to the first stage auxiliary burner and the second stage auxiliary burner. Distribution of fuel charged to each of the first stage auxiliary burner and the second stage auxiliary burner is controlled in accordance with a predetermined distribution ratio of each charging quantity to whole charging quantity in all the range thereof.

To provide a combined cycle plant, a control device thereof, and a steam turbine startup method that are aimed at improving the operability of a combined cycle plant by allowing a quick change of the output. A combined cycle plant is provided with: a gas turbine having a compressor, a combustor, and a turbine; a supplementary firing burner that raises the temperature of exhaust gas of the gas turbine; a heat recovery steam generator that generates steam using exhaust heat of the exhaust gas; a steam turbine that is driven by steam generated by the heat recovery steam generator; and a control device that changes both an output of the combustor and an output of the supplementary firing burner when an output of the combined cycle plant is to be changed.

A combined-cycle power plant comprises a gas turbine engine for generating exhaust gas, an electric generator driven by the gas turbine engine, a steam generator receiving the exhaust gas to heat water and generate steam, and a duct burner system configured to heat exhaust gas in the steam generator before generating the steam and that comprises a source of hydrogen fuel, a fuel distribution manifold to distribute the hydrogen fuel in a duct of the steam generator, and an igniter to initiate combustion of the hydrogen fuel in the exhaust gas. A method for heating exhaust gas in a steam generator for a combined-cycle power plant comprises directing combustion gas of a gas turbine engine into a duct, introducing hydrogen fuel into the duct, combusting the hydrogen fuel and the combustion gas to generate heated gas, and heating water in the duct with the heated gas to generate steam.