What are flue gases?

Flue gases refer to the products of combustion that are discharged into the atmosphere from a furnace, boiler, or chimney. When a fuel is burned, it undergoes a chemical reaction with oxygen, producing various byproducts depending on the type of fuel and the combustion conditions. These byproducts typically include carbon dioxide, water vapor, carbon monoxide, nitrogen oxides, sulfur dioxide, and particulate matter, all of which are released as gases.

In this context, the correct understanding of flue gases is that they encompass the entirety of these combustion products, particularly when mixed with the surrounding air. This mixture can contribute to environmental concerns and is subject to regulation to ensure clean air.

The other options do not capture the full scope of what flue gases are. Excess air in combustion chambers primarily refers to the air supply exceeding what is required for complete combustion, which does not encompass the products produced. Water vapor is indeed a component of flue gases but is not the only product formed, thus limiting the definition. Non-toxic gases may be a fraction of the total byproducts, but flue gases can contain harmful constituents and therefore can't be deemed non-toxic overall.