Rotary kiln for limestone calcination — typical thermal efficiency The overall thermal efficiency of a rotary kiln used for limestone (CaCO3) calcination is typically closest to:

Introduction / Context:
Limestone calcination (CaCO3 → CaO + CO2) in rotary kilns is an energy-intensive operation. Efficiency reflects how much of the fuel energy ends up as useful process heat for calcination and sensible heating of solids, rather than being lost in exhaust, radiation, and shell losses.

Given Data / Assumptions:

• Conventional long/medium rotary kilns without state-of-the-art waste-heat recovery or precalciner sections.
• Typical industrial operating practice and insulation quality.

Concept / Approach:
Rotary kilns have substantial gas exit losses and shell radiation due to large hot surfaces and counter-current flow. Without extensive recovery, practical thermal efficiencies are modest. Values around 35–50% are common, making 40% a representative figure for many installations.

Step-by-Step Solution:
Account for energy sinks: endothermic calcination duty, sensible heating of solids, gas exit losses, shell radiation, dust carryover.Benchmark industry ranges: conventional kilns often near ~40% overall efficiency.Select the closest option reflecting typical practice.

Verification / Alternative check:
Process audits commonly report large exhaust losses unless recuperators or preheaters are added; upgrades can lift efficiency but baseline figures center near 40%.

Why Other Options Are Wrong:

• 20%: too low for standard operation.
• 65% or 80%: more representative of highly optimized systems (preheaters, tight heat recovery), not typical baseline kilns.

Common Pitfalls:

• Confusing instantaneous combustion efficiency with overall kiln thermal efficiency.

Final Answer:
40%