Waste-heat uses in steam generators: In a thermal power plant, which equipment recover sensible heat from outgoing flue gases under normal operation?

Introduction / Context:
Thermal power plants aim to extract as much useful energy as possible from fuel. Downstream of the furnace, heat-transfer surfaces recover waste heat from flue gases to raise efficiency. This question probes recognition of the multiple surfaces that benefit from the same flue-gas energy stream.

Given Data / Assumptions:

• A boiler/steam generator with furnace, convection pass, and gas exit.
• Standard heat surfaces: economiser, superheater, and air preheater.
• Normal, steady firing without bypasses.

Concept / Approach:
The flue-gas sensible heat is used serially: the superheater elevates steam temperature above saturation; the economiser preheats feedwater; the air preheater warms combustion air. All three are positioned to tap the gas stream, subject to metallurgy and fouling constraints, thus all qualify as waste-heat recovery components.

Step-by-Step Solution:
Identify each exchanger’s duty that relies on flue gas: superheater (steam), economiser (feedwater), air preheater (combustion air).Confirm all take heat from the same gas path.Hence, select “All (a), (b) and (c)”.

Verification / Alternative check:
Typical boiler heat-balance diagrams show a cascade of heat transfer from hottest gases to superheater, then economiser, and finally air preheater before stack release.

Why Other Options Are Wrong:
Any subset omits a standard flue-gas-heated surface present in most utility boilers.Limiting to two devices ignores the superheater’s clear use of flue-gas heat.

Common Pitfalls:
Assuming “waste heat” only means low-grade heat. In boilers, even high-temperature superheaters are part of the same waste-heat recovery chain.

Final Answer:
All (a), (b) and (c)