Thermal efficiency of blast-furnace stoves (hot-blast regenerators) used to preheat air for ironmaking is typically closest to which value?

Introduction / Context:
Blast-furnace stoves recover heat from coke-oven/cleaned BF gas to deliver hot blast air, improving furnace productivity and fuel rate. Their thermal efficiency indicates how effectively exhaust heat is stored and reused.

Given Data / Assumptions:

• Modern checker-brick regenerators with cyclic firing/air modes.
• Well-maintained stoves and correct cycling.

Concept / Approach:
Thermal efficiency reflects the fraction of fuel energy recovered into the hot blast. With optimized cycles, good checker condition, and proper firing control, efficiencies near 80% are achievable.

Step-by-Step Solution:
Recognize typical practice: hot blasts often 900–1250°C using regenerators.Waste-heat storage and retrieval via checker bricks approach high regeneration effectiveness.Industry benchmarks place overall thermal efficiency commonly around 80% under good operation.

Verification / Alternative check:
Energy balances on stove cycles (firing vs. blasting) show high sensible heat recovery when flue losses are minimized and checkers are clean.

Why Other Options Are Wrong:
20% or 40% imply poor recovery and fouled checkers.
60% is low for well-run stoves; modern practice is higher.

Common Pitfalls:
Confusing instantaneous effectiveness with long-term average efficiency; poor cycling or checker fouling reduces efficiency below benchmarks.

Final Answer:
80%