In a combined cycle plant, which statement correctly describes how the gas turbine exhaust contributes to the steam cycle?

• A. Gas turbine exhaust is vented directly to atmosphere.
• B. The steam turbine cycle provides exhaust heat to the gas turbine.
• C. The gas turbine exhaust feeds a heat recovery steam generator (HRSG) to produce steam that drives the steam turbine.
• D. The exhaust heat is used to power the electrical generator directly.

Answer: (C)

The essential idea being tested is how waste heat from the gas turbine is used in a combined cycle. In such a plant, the hot exhaust from the gas turbine is not released to the atmosphere; it flows through a heat recovery steam generator (HRSG). The HRSG captures that heat to turn water into steam. This steam then drives a steam turbine, which is connected to a generator, producing additional electricity. The combination of the gas turbine cycle and the steam turbine cycle is what makes the plant more efficient by using the same fuel to generate power in two stages.

Venting the exhaust directly to the atmosphere wastes energy, so that’s not how a combined cycle operates. The steam cycle provides heat to the gas turbine is reversed; the gas turbine exhaust feeds the HRSG, not the other way around. Using exhaust heat to power the electrical generator directly isn’t how the system works either; electricity is produced by turning turbines, not by using exhaust heat to drive the generator without a turbine.