Which term refers to the gas expelled through the flue during combustion?

The term that accurately describes the gas expelled through the flue during combustion is "flue gas." Flue gas is the mixture of gases produced when fuel is burned, and it typically contains several components, including carbon dioxide, water vapor, and various other combustion byproducts depending on the type of fuel used. This gas is a critical aspect of the combustion process and is essential for understanding boiler operations, emissions, and efficiency.

Vapor gas is not a standard term used in the industry to describe combustion products. Similarly, while combustion gas might seem like a logical term, it is not as specific as flue gas, which clearly denotes the exhaust produced by the combustion process. Lastly, steam gas is misleading, as it implies that the gas is solely steam, without acknowledging the other components of the flue gas mixture. Understanding these distinctions is crucial for maintaining compliance with environmental regulations and ensuring safe boiler operation.