Combined Cycle
A combined-cycle power plant cascades a Brayton-cycle gas turbine with a Rankine-cycle steam plant. Air is compressed and fuel is burned to produce hot gas for the gas turbine. The gas turbine exhaust still contains usable thermal energy, so it passes through a heat-recovery steam generator (HRSG) that produces steam for a steam turbine. The steam is then condensed and returned to the HRSG. Fuel is supplied to the main combustor, while recovered exhaust heat drives the bottoming cycle without a separate steam boiler. Lower stack temperature increases heat recovery, but the gas temperature must stay above the minimum pinch-point difference and the acid-dew-point limit. Overall performance depends on gas-turbine exhaust flow and temperature, HRSG pinch, steam pressure and temperature, condenser pressure, and component pressure losses.
Knowledge Check
1.What is the normal energy-flow sequence in an unfired combined-cycle plant?
2.Why can a combined-cycle plant have a higher efficiency than a gas turbine alone?
3.A plant produces 100 MW from its gas turbine and 50 MW from its steam turbine. Pumps and other auxiliaries consume 2 MW. What is the net plant output?
4.If the combined-cycle plant delivers 148 MW of net power while the fuel energy input is 400 MW, what is its overall efficiency?
5.Gas enters an HRSG at 600°C and leaves at 120°C. For ṁ_g = 100 kg/s and an average c_p,g = 1.1 kJ/(kg·K), approximately how much heat is recovered?
6.Using the idealized cascade relation η_cc = η_GT + η_bottom(1 − η_GT), what is the combined efficiency when η_GT = 0.35 and the bottoming stage converts 30% of the rejected energy into work?
7.What does the HRSG pinch point represent?
8.What is the primary thermodynamic benefit of reducing the HRSG stack temperature?
9.Why does lowering steam-turbine condenser pressure generally increase combined-cycle output?
10.What is the effect of supplementary firing in the HRSG?