Combined-Cycle Generation. Combined-cycle generation is a configuration using both gas turbines and

steam generators. In a combined-cycle gas turbine (CCGT), the hot exhaust gases of a gas turbine are used to provide all, or a portion of, the heat source for the boiler, which produces steam for the steam generator turbine. This combination increases the thermal efficiency over a coal- or oil- fueled steam

generator. The system has an efficiency of about 54 percent, and the fuel consumption is approximately 25 percent lower. Combined-cycle systems may have multiple gas turbines driving one steam turbine.

• Gas Turbine Plus Unfired Steam Generator: A steam generator is installed at the discharge of a gas turbine to recover the heat in the gas turbine exhaust so as to create steam in the steam generator. The fuel is fired only in the gas turbine.

• Gas Turbine Plus Supplementary-Fired Steam Generator: A portion of the oxygen in the gas turbine exhaust is used to support further combustion in a supplementary firing system in the connecting duct between the gas turbine and the steam generator.

• Gas Turbine Plus Furnace-Fired Steam Generator: This generator is the same as the gas turbine plus supplementary-fired steam generator, except that essentially all of the oxygen from the gas turbine exhaust is used to support further combustion.

• Supercharged Furnace-Fired Steam Generator Plus Gas Turbine: A steam generator is placed between the air compressor and the gas turbine. The air compressor is used to pressurize the boiler where the fuel is fired. The products of combustion that have been cooled within the boiler are then discharged through a gas turbine.

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