Why is "residence time" important for fuel gases in combustion?

The concept of "residence time" in the context of combustion is crucial as it refers to the amount of time that fuel gases spend in the combustion chamber before exiting. Sufficient residence time is necessary to ensure that the fuel has the opportunity to mix thoroughly with the oxidizer (usually air) and undergo complete combustion.

When fuel gases are provided adequate residence time, it enhances the likelihood of all the hydrocarbons breaking down and interacting with oxygen to produce primarily carbon dioxide and water, rather than intermediate products such as carbon monoxide or unburned hydrocarbons. This thorough interaction reduces pollution and increases the efficiency of the combustion process.

In contrast, if the residence time is too short, there may not be enough time for all fuel particles to combust completely, leading to incomplete combustion, which can result in increased emissions and wasted fuel energy. Thus, ensuring a sufficient residence time is fundamental for achieving complete combustion, making it the correct answer for the importance of residence time in fuel gases during combustion.