Boiler performance: The maximum practical thermal efficiency of a modern boiler system may be approximately what percentage under well-optimised conditions?

Introduction / Context:
Thermal efficiency for boilers relates useful heat transferred to the working fluid versus the chemical energy in the fuel. Modern well-designed boilers with effective heat recovery and good combustion control can reach high efficiency values, important for fuel economy and emissions reduction.

Given Data / Assumptions:

• “Maximum” refers to practical, field-achievable efficiencies (not ideal Carnot or laboratory-only figures).
• Proper excess air control, clean heat-transfer surfaces, economisers/air-preheaters in service.
• Measured on a higher heating value basis or comparable convention.

Concept / Approach:
Typical packaged fire-tube or water-tube boilers with economisers can exceed 85% and, with condensing or advanced recovery, approach about 90% (or slightly above, depending on basis). Options far below this represent poorly tuned or obsolete systems; far above suggests misinterpretation of basis or heat pumps/combined cycles rather than simple boilers.

Step-by-Step Solution:
Compare options with industry benchmarks: 10% and 25% are unrealistic for any functioning boiler.65% represents older or badly operated units without recovery.90% aligns with modern, well-optimized installations.

Verification / Alternative check:
Plant audits and energy balance calculations with economiser outlet stack temperatures often demonstrate efficiencies in the high 80s to about 90%, validating the selection.

Why Other Options Are Wrong:
10%/25%: Far below practical operation.65%: Too low for modern optimized systems.80%: Achievable but not “maximum practical”; 90% is a better top-end estimate for conventional boilers.

Common Pitfalls:
Confusing boiler efficiency with overall plant heat-rate or with combined heat and power system efficiencies, which can be higher due to useful heat recovery beyond the boiler boundary.

Final Answer:
90%