What does a yellow flame indicate in a digester gas burner?

A yellow flame in a digester gas burner is indicative of poor gas quality. This color often signifies incomplete combustion, which can occur when there is an insufficient supply of oxygen or a high concentration of impurities in the gas. When gas quality is low, it typically contains a higher level of contaminants or is not suitable for efficient burning, leading to a yellow or orange flame rather than the desired blue flame that indicates complete and efficient combustion.

In contrast, a blue flame usually signifies normal combustion and good gas quality, where the gas is burning efficiently with proper air mixing. Low gas production may not necessarily reveal itself through flame color but rather through the volume of gas being produced. High gas quality, on the other hand, would generally result in blue flames, reflecting a clean burn.