Combined gas/steam cycle power generation from a user’s viewpoint

Gas turbines have been used since the 1940's to provide electric power at times of peak demand, but this is due to low initial cost and short starting times. Historically low efficiency and a higher quality fuel requirement has kept the gas turbine on the high end of the load curve. The combined cycle uses a gas turbine as the prime engine, then uses an exhaust heat recovery boiler to raise steam for a steam turbine cycle. High efficiencies are achievable and the gas turbines can now b u m fuels from natural gas through to crude oil and coal via gasification. High efficiency, short construction time,, competitive installation costs and smaller plant power ratings, has led to increasing and extensive use of the combined cycle. During the next two decades large numbers of gas turbines will be purchased for all load curve requirements with combined cycles in the mid to base load range. Gas turbines and combined cycles are studied in a holistic way, from performance theory to issues of procurement and maintenance. Gas turbines are discussed more fully than steam turbines as they are a less mature technology and dictate the operation of the combined cycle. A mathematical, model of a hypothetical engine is used to find performance trends. This model includes the gas turbine, single pressure boiler and steam turbine. Off design of the gas turbine is studied using the Cranfield program Turbomatch. A mathematical model of the off design of the boiler is developed to give off design trends for the combined cycle. Other issues discussed are applications, fuels, rating, operation, maintenance and various configurations. Past development, current state-of-the-art and future prospects are also discussed.