What does "flue gas" refer to?

Flue gas refers specifically to the mixture of gases that results from the combustion of fuel in a boiler or furnace. When fuel is burned, it reacts with oxygen, leading to the production of various gases such as carbon dioxide, water vapor, nitrogen oxides, and other combustion products. This process is a fundamental aspect of how boilers operate since the heat generated from burning fuel is used to produce steam or heat water.

The other options do not accurately define flue gas. Water vapor produced by condensation pertains to the physical transformation of steam back into water, which is not the primary focus of flue gas. Unburned fuel exiting the boiler describes a scenario of incomplete combustion, which is not typical of the standard definition of flue gas. Lastly, residual heat after energy conversion is related to the energy lost in the combustion process but does not capture the composition of the gases produced during combustion itself. Thus, the correct answer emphasizes the connection between combustion and the resultant gases, making it an important concept in understanding boiler operations and safety.