Waste-heat recovery limits: Metallic recuperators are generally avoided when hot flue-gas temperatures exceed approximately what value (°C) because of severe oxidation/corrosion at higher temperatures?

Introduction / Context:
Recuperators transfer sensible heat from hot flue gases to incoming combustion air. Material selection limits the maximum gas temperature that metallic heat exchangers can reliably withstand without unacceptable corrosion, oxidation, or creep. Above certain temperatures, ceramic recuperators are preferred.

Given Data / Assumptions:

• Common metallic recuperator alloys suffer accelerated oxidation/sulfidation at very high furnace exhaust temperatures.
• Industry practice uses ceramics when temperatures move substantially beyond the safe range for metals.

Concept / Approach:
While exact limits depend on alloy, fuel sulfur, and gas chemistry, a widely cited practical threshold is about 750 °C. Above this, corrosion and high-temperature strength issues make metallic recuperators uneconomical or unreliable; ceramic units are specified instead.

Step-by-Step Solution:
List candidate thresholds: 350, 750, 1050, 1250, 900 °C.Map to practice: 350 °C is too low; many metallic units routinely handle higher. 1050–1250 °C are typically ceramic territory.Select the commonly accepted limit for metals: ~750 °C.

Verification / Alternative check:
Manufacturer datasheets and furnace design handbooks show metallic recuperators rated below about 750–800 °C gas inlets, with ceramics used above that range, corroborating the selection.

Why Other Options Are Wrong:
350 °C: far too conservative.900/1050/1250 °C: too high for typical metallic recuperators in corrosive furnace atmospheres.

Common Pitfalls:
Ignoring fuel-bound sulfur and ash, which accelerate attack; even within “metallic” temperature limits, gas chemistry can force more conservative designs.

Final Answer:
750 °C