Combustion Efficiency and Waste Heat

A modern gas burner converts 98% of its fuel's chemical energy into heat. Yet the power plant built around that burner turns only 40-60% of that heat into electricity. Two efficiencies are at work, and confusing them leads to bad policy conclusions.

Combustion efficiency measures how completely fuel burns: what fraction of its chemical energy actually becomes heat rather than escaping as unburned hydrocarbons or carbon monoxide. Modern power plants achieve 97-99%. Thermal efficiency measures something harder: how much of that heat becomes useful work (electricity). This second number is capped by the Carnot limit, which says no heat engine can exceed 1 - T_cold/T_hot, where temperatures are in Kelvin. The gap between combustion efficiency and thermal efficiency is waste heat, and the First Law guarantees every joule is accounted for.

Trace the energy balance. A coal plant burns 100 MW of fuel. Combustion efficiency is 98%, delivering 98 MW of heat. Thermal efficiency is 36%, producing 35 MW of electricity. The remaining 63 MW exits as waste heat through the cooling tower and exhaust stack.

Apply the Carnot ceiling. Steam enters the turbine at 540 degrees C (813 K), exhaust cooling at 30 degrees C (303 K). Carnot limit: 1 - 303/813 = 63%.

Why does the actual 36% fall so far below the 63% Carnot limit?

Account for real-world losses. Friction, turbulence, heat leaks through insulation, and auxiliary power for pumps and fans generate entropy beyond the theoretical minimum. Real plants achieve 50-80% of their Carnot ceiling.

This energy balance explains why every thermal plant needs cooling infrastructure, and why raising steam temperatures is the primary engineering lever for reducing fuel consumption per kilowatt-hour.

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