What should the carbon dioxide content in the flue gases of an efficiently fired boiler measure approximately?

• A. 3%
• B. 6%
• C. 9%
• D. 12%

Answer: (D)

An efficiently fired boiler typically has a carbon dioxide content in the flue gases that indicates proper combustion and efficiency levels. For a well-tuned and fully operational boiler, the expected carbon dioxide content is generally around 9% to 12% by volume. This range is indicative of complete combustion of fuel, meaning that most of the fuel has been converted into carbon dioxide, with minimal unburned hydrocarbons or carbon monoxide present.

When combustion is efficient, carbon dioxide levels increase, demonstrating that more fuel is being converted into energy rather than being lost as unburned fuel or emissions. If the carbon dioxide content were significantly lower, it would imply incomplete combustion, leading to inefficiencies and possibly dangerous emissions like carbon monoxide.

In this context, the measurement of approximately 12% represents a boiler operating at or near its optimal efficiency, making it the correct answer for indicating a well-functioning system.