Here’s a question that has come up on the OA worldbuilding group, the answer to which I cannot guess.
If you have a moon or planet like Titan with an atmosphere of nitrogen/carbon dioxide then it might support combustion if there is a substantial amount of methane in the atmosphere. On such a world you could set fire to an oxygen jet (for instance) like a Bunsen burner.
But what is the critical level of methane that would allow a flame? 2%, 5%, 10%, more? Is this affected by the total pressure of the atmosphere and/or the partial pressure of methane? How about the ambient temperature?
Perhaps you could get a flame by playing an oxygen jet onto the ethane seas, if they are not too impure.
On a planet that’s all fuel, the “flammable” substance is the oxidizer. Amusingly, this makes me wonder what an “oxygen flame” looks like in a purely combustible environment.
At first principles, just invert the equations. You know fuel and a 21% oxygen environment supports fire. So 100% oxygen + 20% fuel should also work just fine. The CO2 shouldn’t matter at all, the equilibria is so in favor of combustion that the reverse reaction will be rare.
02 has a heavier molecular weight than methane, so the flame is going to be closer to what it looks like when you burn a heavier substance than methane on earth. Probably more like a propane fire.
Partial pressures are the only pressures that matter, of course. I meant at 1 atmosphere, but in actuality, it matters much more what the partial pressure of the fuel is on Titan.