--- title: "Combined cycle merits, demerits and designs" subject: "Power Plant Equipment" chapter: 8 key: "semester_7/power_plant_equipment:combined-merits-designs" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-merits-designs site: BCT Engineering --- # Combined cycle merits, demerits and designs Power Plant Equipment · Chapter 8: Combined Power Plant - Asked in: 2075 Chaitra, 10 marks - Video: [Advantages Of Combined Cycle Power Plant](https://www.youtube.com/watch?v=TIc08Hz1geQ) — 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-merits-designs - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8 ## Question What are the advantages and disadvantages of gas and steam combined cycle? Briefly discuss the popular designs of the combination cycles. ## Short answer Advantages: 50-60% efficiency, lower fuel cost, low capital cost, short construction, quick start, less cooling water, lower emissions, less space. Disadvantages: needs clean costly fuel, output falls at high ambient temperature, complex controls, frequent gas turbine hot-part inspection, limited unit size. Popular designs: HRSG, exhaust as combustion air in fired boiler, exhaust feed water heating. ## Answer A gas and steam combined cycle uses the exhaust heat of a gas turbine to generate steam for a steam turbine, so overall efficiency becomes 50-60%. #### 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. #### Disadvantages - Needs **clean, costly fuel** (natural gas or distillate oil); not suited to coal unless gasified. - **Output and efficiency fall with high ambient temperature and altitude** (gas turbine takes less mass of air). - **More complex** plant and controls; two kinds of machines to maintain. - Gas turbine hot parts need frequent inspection; high-temperature materials are costly. - Unit size limited by available gas turbine sizes. - Part-load efficiency falls when the gas turbines run at reduced load. #### Popular designs of combination cycles #### 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. | Design | Fuel in boiler | Main feature | |---|---|---| | Unfired / supp. fired HRSG | None / small | Highest efficiency, simple | | Exhaust-fired boiler | Full firing | Large steam output, exhaust as air | | Feed water heating | Full firing | Repowering of old steam plants | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does a combined cycle have a lower fuel cost per kWh than a gas or steam plant alone? A. It uses cheaper coal as fuel B. Its overall efficiency of 50-60% is higher than 35-40% (steam) or 30-38% (gas) C. It has no condenser D. It runs only at peak load Answer: B. Recovering GT exhaust heat for steam raises overall efficiency, so less fuel per unit of energy. (Key point: Efficiency advantage) 2. Why does a combined cycle need less cooling water than a steam plant of equal output? A. Only the steam part (about one-third of output) has a condenser B. It uses air cooling for the gas turbine and no condenser at all C. The gas turbine produces no waste heat D. Steam is not condensed Answer: A. Only about one-third of the output comes from the steam turbine, so only that portion rejects heat in a condenser. (Key point: Cooling water requirement) 3. Why do output and efficiency of the plant fall at high ambient temperature and altitude? A. The steam turbine cannot condense steam B. Boiler tubes corrode faster C. The gas turbine takes in a smaller mass of air D. The HRSG stops working Answer: C. Less dense air means less mass flow through the gas turbine, reducing its output. (Key point: Disadvantage with ambient conditions) 4. In the design where GT exhaust is used as combustion air in a fired boiler, what does the exhaust replace? A. The steam turbine B. The condenser C. The feed pump D. The air preheater and forced draft air supply Answer: D. The hot, oxygen-rich exhaust (about 15% O2) serves as combustion air, replacing the preheater and forced draft air. (Key point: Design 2 exhaust-fired boiler) 5. In the feed-water heating design, the GT exhaust mainly replaces A. The boiler's furnace B. Bled-steam feed water heaters C. The steam turbine D. The gas compressor Answer: B. The exhaust heats the condensate/feed water, so extracted bled steam is no longer needed; the boiler is still fired. (Key point: Feed water heating design) ## Close variants set in other exams - [Combined cycle advantages and three designs](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages-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 - [Combined cycle combinations and advantages](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-combinations-advantages) — Power Plant Equipment, chapter 8 - [Reactors in stations: merits and demerits](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactors-merits-demerits) — Power Plant Equipment, chapter 4 ## 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 - [Operational characteristics of combined cycle plant](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Accgt-operational-characteristics) — Power Plant Design, chapter 1 - [Cofferdam types, uses, merits and demerits](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Acofferdam-types-merits) — Foundation Engineering, chapter 5, asked 7×