--- title: "Combined cycle components and waste heat use" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-waste-heat-utilization" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-waste-heat-utilization site: BCT Engineering --- # Combined cycle components and waste heat use Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2071 Chaitra, 6 marks - Video: [Gas Turbines and Combined Cycle Power Plants Explained (Combustion Turbines)](https://www.youtube.com/watch?v=N-V_uGeCFNg) — saVRee - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-waste-heat-utilization - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question Draw the basic component of a gas and steam turbine combined cycle. Explain how the waste heat is utilized in this cycle. ## Short answer Basic components: compressor, combustion chamber, gas turbine and generator; HRSG with economiser, evaporator, superheater; steam turbine, generator, condenser, feed pump. Waste heat: 500-600 C exhaust with 15-16% oxygen passes through HRSG, superheating steam, evaporating water, preheating feed water; stack gas leaves at 100-150 C. Steam produces about one-third of output without extra fuel; overall efficiency 50-60%. ## Answer #### Basic components of a gas and steam turbine combined cycle ```text air 1 fuel | | v v +------+ 2 +-----------+ 3 +------+ | Comp |---------->| Combustion|------>| GT |===> G1 +------+ | chamber | +------+ || +-----------+ || | ++=============== shaft =============++ | 4 hot exhaust v (~500-600 C) stack <-- 5 --+-------------------------------+ | HRSG (economiser, | | evaporator, superheater) | +-------------------------------+ ^ d | a steam | v +------+ +------+ | Pump | | ST |===> G2 +------+ +------+ ^ c | b | +-----------+ | +---| Condenser |<---+ +-----------+ ``` - **Gas turbine plant:** compressor (Comp), combustion chamber, gas turbine (GT), generator G1. - **Heat recovery steam generator (HRSG):** economiser, evaporator and superheater in the exhaust duct; optional duct burner. - **Steam plant:** steam turbine (ST), generator G2, condenser, feed pump. #### How the waste heat is utilised 1. The gas turbine exhaust leaves at about 500-600 C and still contains 15-16% oxygen; in a simple gas turbine plant this heat (about 60-65% of fuel energy) would go to the atmosphere. 2. In the HRSG the exhaust gives up this heat in stages: first superheating the steam, then evaporating water, and finally preheating feed water in the economiser. Gas leaves the stack at about 100-150 C. 3. The steam produced drives the steam turbine and generates about one-third of the total plant output without any extra fuel. 4. If more steam is needed, extra fuel is burnt in the exhaust (supplementary firing), using its remaining oxygen. Thus the heat rejected by the gas turbine cycle becomes the heat input of the steam cycle, and the overall efficiency rises to about 50-60%: $$ \eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST} $$ ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In which order does the exhaust gas give up heat in the HRSG? A. Economiser, evaporator, superheater B. Evaporator, superheater, economiser C. Superheater, evaporator, economiser D. Superheater, economiser, evaporator Answer: C. Hottest gas first superheats steam, then evaporates water, and finally preheats feed water in the economiser. (Key point: Order of heat extraction in HRSG) 2. Why can extra fuel be burnt in the exhaust duct (supplementary firing)? A. Exhaust still contains 15-16% oxygen B. Exhaust is pure fuel gas C. Exhaust contains no oxygen but is very hot D. Steam supplies the oxygen Answer: A. Gas turbine exhaust is lean and retains much oxygen, so a duct burner can burn extra fuel to raise more steam. (Key point: Supplementary firing) 3. Roughly what share of total plant output does the steam turbine produce without any extra fuel? A. About 90% B. About one-third C. None D. About 5% Answer: B. Steam from waste heat generates about one-third of the combined plant's output. (Key point: Share of output from waste heat) ## Related in Power Plant Equipment - [Methods to utilize waste heat in combined plants](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-methods) — Power Plant Equipment, chapter 8, asked 2× - [Working of combined gas-steam plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-gas-steam-working) — Power Plant Equipment, chapter 8 - [Waste heat methods and feed water heating layout](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-feedwater) — Power Plant Equipment, chapter 8, asked 2× - [Open cycle gas turbine with intercooling](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-intercooling-sketch) — Power Plant Equipment, chapter 6 - [Open cycle gas turbine on heavy oil](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-heavy-oil-capacity) — Power Plant Equipment, chapter 6 - [Three popular combined cycle designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-three-designs) — Power Plant Equipment, chapter 8 ## Related in other subjects - [Combined cycle gas turbine versus hydro](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-vs-hydro) — Power Plant Design, chapter 1 - [Closed gas turbine cycle components; thermal efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aclosed-gas-turbine-cycle-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [Modifications of the basic gas turbine cycle](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Avariations-basic-gas-turbine-cycle) — Turbo Machines, chapter 6 - [Actual gas turbine cycle with component efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aactual-brayton-cycle-numerical) — Turbo Machines, chapter 3