Which condition can lead to the formation of yellow-tipped flames in a combustion process?

The formation of yellow-tipped flames during combustion is primarily indicative of insufficient primary air. In a combustion system, primary air is necessary for the initial mixing and burning of fuel. When there is not enough primary air, the combustion process becomes incomplete, leading to a greater proportion of unburned fuel particles. This incomplete combustion can cause the flame to burn cooler and produce luminous soot particles, which glow yellow when heated, resulting in yellow-tipped flames.

Adequate primary air is crucial for achieving the correct stoichiometric balance for complete combustion, where all the fuel is consumed, resulting in a clean blue flame rather than a yellow one. Insufficient primary air allows for the buildup of partially oxidized compounds, contributing to both inefficiency and the characteristic yellow flame coloration.

In contrast, excessive primary air can create a cooler flame that lacks sufficient fuel and may also lead to incomplete combustion, but it typically does not result in yellow-tipped flames like those produced by insufficient primary air. Insufficient secondary air primarily affects the latter stages of combustion, impacting the complete oxidation of products rather than the initial flame color. Low gas flow rates can also affect flame characteristics, but they are less directly correlated with the yellow flame color compared to the primary air supply.