The Heat Recovery Steam Generator (HRSG) is a critically important subsystem of a Combined Cycle or Cogeneration Power Plant. In most of these plants the HRSG uses the exhaust gas from the gas turbine as the energy source for the production of high pressure and temperature steam. The main difference in these plants is that in a combined cycle power plant the steam generated in the HRSG is used solely in the production of power while in a cogeneration plant the steam can be used for process as well as power production. The Combined Cycle Power Plant uses the steam in a large condensing steam turbine that produces about 40% of the power generated at design conditions, while in a cogeneration application it is not uncommon to bleed the steam from an extraction steam turbine for process purposes. In the cogeneration mode if a steam turbine is used these extraction type steam turbines are usually smaller and may be of a backpressure type.
The Combined Cycle Power Plant, in most cases, consists of the combination of the Brayton, and Rankine Cycles is one of the most efficient cycles in operation for practical power generation systems. The Brayton Cycle is the Gas Turbine Cycle and the Rankine Cycle is the Steam Turbine Cycle. In most combined cycle applications the Gas Turbine is the topping cycle and the Steam Turbine is the bottoming cycle. Thermal efficiencies of the combined cycles can reach as high as 60 percent. In the typical combination the gas turbine produces about 60% of the power and the steam turbine about 40%. Individual unit thermal efficiencies of the gas turbine and the steam turbine are between 30%–40%. The steam turbine utilizes the energy in the exhaust gas of the gas turbine as its input energy. The energy transferred to the Heat Recovery Steam Generator (HRSG) by the gas turbine is usually equivalent to about the rated output of the Gas Turbine at design conditions. At off design conditions the Inlet Guide Vanes (IGV) are used to regulate the air so as to maintain a high temperature to the HRSG.
