https://www.elgas.com.au/blog/2208-complete-incomplete-combustion-of-lpg-propane-equations
It's not just having propane present, but you also need the right proportion of oxygen.
Too much gas, or too much air, and no boom. So the window for ignition is very small.
.
Propane – LPG – burns within its limits of flammability.
The lower and upper limits of flammability are the percentages of LPG that must be present in an LPG/air mixture.
This means that between 2.15% and 9.6% of the total LPG/air mixture must be LPG in order for it to be combustible.
However, the optimal mixture is 4% LPG/air.
So, 4 parts LPG (propane) to 96 parts air.
With complete combustion of propane, the burner produces a blue flame yielding about 25 MJ/litre or 49 MJ/kg of heat.
So, richer mixtures, those closer to 9.6%, are likely to suffer from incomplete combustion.
It's not just having propane present, but you also need the right proportion of oxygen.
Too much gas, or too much air, and no boom. So the window for ignition is very small.
.