--- title: "Combined cycle advantages and three designs" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-advantages-designs" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-advantages-designs site: BCT Engineering --- # Combined cycle advantages and three designs Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2074 Chaitra, 10 marks - Video: [Combined Steam And Gas Turbine Power Plant](https://www.youtube.com/watch?v=uD0kH-0HME8) — BEST MECHANICAL ENGINEERING - 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-advantages-designs - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question What are the advantages of gas and steam combined cycle? Briefly explain the three popular designs of the combination cycles. ## Short answer Combined cycle gives 50-60% efficiency, lower fuel cost, capital cost, construction time, cooling water and emissions, plus fast start and flexibility. Three popular designs: unfired or supplementary-fired HRSG; exhaust used as combustion air in a fired boiler; exhaust used to heat feed water. ## Answer A gas and steam combined cycle joins a gas turbine (topping) cycle with a steam (bottoming) cycle. The GT exhaust heat produces steam, so the same fuel produces more power. #### Advantages - **High overall efficiency** (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so **lower fuel cost per kWh**. - **Lower capital cost per kW** than coal or nuclear plants; the gas turbine part is compact and factory built. - **Short construction time** (2-3 years); can be built in phases (gas turbine runs first in open cycle). - **Quick start-up and good load-following**, so useful for intermediate and peak loads as well as base load. - **Less cooling water** needed, since only the steam part (about one-third of output) has a condenser. - **Lower emissions** of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling. - **Less space** and smaller staff; high reliability and availability. - **Flexible:** supplementary firing, cogeneration (process steam), repowering of old steam plants. #### Three popular designs #### Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG) ```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 |<---+ +-----------+ ``` The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%). #### Design 2: Exhaust used as combustion air in a fired boiler ```text air fuel |1 | v v [C]--2-->[CC]--3-->[GT]==> G1 | 4 hot exhaust (~15% O2) v used as combustion air fuel --> +-----------------+ a +----+ | Fired steam |---->| ST |==> G2 stack <----| boiler | +----+ +-----------------+ | b ^ d v | +------+ c +-----------+ +--| Pump |<----| Condenser | +------+ +-----------+ ``` The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output. #### Design 3: Exhaust heats the feed water ```text air fuel | 1 | v v [C]--2-->[ CC ]--3-->[GT]==> G1 || | 4 exhaust gas ++=====shaft=========+ v +--------------------------+ stack <-5| Feed water heater (gas | | to water heat exchanger) | +--------------------------+ ^ d | e hot feed water | v +------+ +----------+ a +----+ | Pump | | Steam |---->| ST |==> G2 +------+ | boiler | +----+ ^ c +----------+ | b | ^ fuel v | +-----------------+ +-----------| Condenser | +-----------------+ ``` The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat. #### Comparison | Design | Fuel in boiler | Main feature | |---|---|---| | Unfired / supp. fired HRSG | None / small | Highest efficiency (55-60%), simple | | Exhaust-fired boiler | Full firing | Large steam output, GT exhaust replaces air preheater | | Feed water heating | Full firing | Small gain, easy repowering of existing plant | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a combined cycle allow shorter construction time and phased building? A. The steam turbine is built first and run in open cycle B. The gas turbine can be installed and run first in open cycle C. No boiler is ever required D. Both turbines share one shaft Answer: B. The compact factory-built gas turbine runs first in open cycle while the steam part is completed (2-3 years total). (Key point: Construction in phases) 2. In Design 1 (HRSG), where is the steam raised? A. In a fully fired conventional boiler B. In the combustion chamber C. In a heat recovery boiler using GT exhaust, optionally with duct burner D. In the condenser Answer: C. The GT exhaust raises all the steam in the HRSG; supplementary fuel via a duct burner is optional. (Key point: Unfired/supplementary-fired HRSG design) 3. Why are emissions per kWh lower in a combined cycle? A. Higher efficiency and usually natural gas fuel, with no ash B. It burns more coal C. It rejects all exhaust to the condenser D. Its stack is taller Answer: A. Less fuel per kWh and clean gas fuel give lower CO2, SO2 and particulates, with no ash handling. (Key point: Emissions advantage) 4. Which design gives the highest efficiency (55-60%) and is simplest? A. Feed water heating design B. Exhaust-fired boiler design C. All three are equal D. Unfired / supplementary-fired HRSG design Answer: D. The HRSG design uses no (or little) boiler fuel and recovers most exhaust heat, giving the highest efficiency. (Key point: Comparison of designs) ## Close variants set in other exams - [Combined cycle merits, demerits and designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-merits-designs) — Power Plant Equipment, chapter 8, asked 2× ## Related in Power Plant Equipment - [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 - [Combined plant introduction and popular designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-intro-designs) — Power Plant Equipment, chapter 8 - [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 - [Combined cycle components and waste heat use](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-waste-heat-utilization) — Power Plant Equipment, chapter 8 ## Related in other subjects - [Gas vs vapor cycles; Otto cycle efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Agas-vs-vapor-cycles-otto-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6 - [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 - [Three-process gas cycle: net work, heat, type](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Athree-process-cycle-pv-sketch) — Fundamental of Thermodynamics & Heat Transfer, chapter 4, asked 2× - [Three-process gas cycle: net work and heat](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Athree-process-cycle-pv-const-800kpa) — Fundamental of Thermodynamics & Heat Transfer, chapter 4