Methods to utilize waste heat in combined plants

In a combined plant the waste heat of the topping cycle (mainly gas turbine exhaust at 450-600 C, still containing about 15% oxygen) is recovered to produce more power or useful heat.

Common methods of utilising the gas turbine exhaust (waste) heat:

1. Heat recovery steam generator (HRSG), unfired: exhaust passes through an economiser, evaporator and superheater and raises steam for a steam turbine. Simplest and most common; steam turbine gives about one-third of the plant output.
2. Supplementary fired HRSG: extra fuel is burnt in a duct burner in the exhaust (it still contains about 15-16% O2) to raise gas temperature and steam output/temperature; increases output and flexibility.
3. Exhaust-fired (fully fired) boiler: GT exhaust is used as hot combustion air for a conventional fired steam boiler, replacing the air preheater.
4. Feed water heating: exhaust heats the boiler feed water in a gas-to-water heat exchanger (economiser), replacing some or all bled-steam feed heaters of a steam plant.
5. Pressurised (supercharged) boiler: combustion takes place under pressure in the boiler furnace; after raising steam, the gases expand in a gas turbine that drives the compressor.
6. Cogeneration / process heat: exhaust or exhaust-raised steam is used for industrial process heating, district heating or absorption cooling.
7. Regenerator / recuperator: exhaust preheats the compressed air of the gas turbine itself (regenerative gas turbine).