**6.2 Combined cycle power plant**

The combined cycle for power generation offers efficiencies of about 58–60% (electricity only) in theory. It combines the gas-turbine (GT) based Brayton cycle (which has simple cycle efficiency of 18–27%) as the front end followed by a Rankine cycle at the back end. The GT fuel has to necessarily be in the vapor

phase—natural gas, synthesis gas from coal, vaporized diesel, etc. The hot GT exhaust containing about 15% O2 issued as combustion air for a downstream heat recovery steam generator (HRSG) which uses the HP steam to make additional power by the Rankine cycle.
