---
title: "Methods to utilize waste heat in combined plants"
subject: "Power Plant Equipment"
chapter: 8
key: "semester_7/power_plant_equipment:combined-waste-heat-methods"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/combined-waste-heat-methods
site: BCT Engineering
---

# Methods to utilize waste heat in combined plants

Power Plant Equipment · Chapter 8: Combined Power Plant

- Asked in: 2071 Shrawan, 4 marks; 2070 Asar, 4 marks (asked 2 times)
- 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-methods
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-8

## Question

List the common methods employed to utilize waste heat in combined power plants.

## Short answer

Common methods: unfired or supplementary fired HRSG; exhaust-fired (fully fired) boiler; feed water heating; pressurised (supercharged) boiler; cogeneration; and a regenerator (recuperator) to preheat compressor air.

## Answer

In a combined plant the **waste heat** of the topping cycle (mainly gas turbine exhaust at 450-600 C, still containing about 15% oxygen) is recovered to produce more power or useful heat.

Common methods of utilising the gas turbine exhaust (waste) heat:

1. **Heat recovery steam generator (HRSG), unfired:** exhaust passes through an economiser, evaporator and superheater and raises steam for a steam turbine. Simplest and most common; steam turbine gives about one-third of the plant output.
2. **Supplementary fired HRSG:** extra fuel is burnt in a duct burner in the exhaust (it still contains about 15-16% O2) to raise gas temperature and steam output/temperature; increases output and flexibility.
3. **Exhaust-fired (fully fired) boiler:** GT exhaust is used as hot **combustion air** for a conventional fired steam boiler, replacing the air preheater.
4. **Feed water heating:** exhaust heats the boiler feed water in a gas-to-water heat exchanger (economiser), replacing some or all bled-steam feed heaters of a steam plant.
5. **Pressurised (supercharged) boiler:** combustion takes place under pressure in the boiler furnace; after raising steam, the gases expand in a gas turbine that drives the compressor.
6. **Cogeneration / process heat:** exhaust or exhaust-raised steam is used for industrial process heating, district heating or absorption cooling.
7. **Regenerator / recuperator:** exhaust preheats the compressed air of the gas turbine itself (regenerative gas turbine).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why can a duct burner burn extra fuel in the gas turbine exhaust of a supplementary fired HRSG?
   A. The exhaust still contains about 15-16 % oxygen
   B. The exhaust is pure fuel vapour
   C. The exhaust is at condenser pressure
   D. The exhaust is cold and needs heating
   Answer: A. Gas turbines run with excess air, so the exhaust still has enough oxygen to support extra combustion. (Key point: Supplementary firing)

2. In an exhaust-fired (fully fired) boiler scheme, how is the gas turbine exhaust used?
   A. As hot combustion air, replacing the air preheater
   B. To drive the compressor directly
   C. To cool the condenser
   D. To dry the fuel only
   Answer: A. The oxygen-rich hot exhaust serves as combustion air for a conventional boiler. (Key point: Exhaust-fired boiler)

3. What distinguishes a pressurised (supercharged) boiler method?
   A. Combustion occurs under pressure and the gases then expand in a gas turbine driving the compressor
   B. Steam is expanded in the compressor
   C. Exhaust is vented without use
   D. Water is heated without any fuel
   Answer: A. Gases leave the pressurised furnace after raising steam and expand in a gas turbine that drives the compressor. (Key point: Pressurised boiler)

4. About how much of plant output does the steam turbine give in an unfired HRSG combined plant?
   A. About one-third
   B. About 90 %
   C. About 5 %
   D. About two-thirds
   Answer: A. The unfired HRSG scheme is simplest and the steam turbine gives roughly one-third of total output. (Key point: Unfired HRSG)

## Related in Power Plant Equipment

- [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×
- [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
- [Advantages of combined power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages) — Power Plant Equipment, chapter 8
- [Combined plant heating feed water with exhaust](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-layout) — Power Plant Equipment, chapter 8, asked 2×

## Related in other subjects

- [Cogeneration in biomass-based power plants](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Acogeneration-biomass-plant) — Power Plant Design, chapter 1