Combustion in Reducing Atmosphere?

[stevebowers]

On reading a ternary diagram, Grant has this to say;

Drawing a line from the 100% oxygen corner to the passive agent axis gives you the gas mixes possible around a pure oxygen jet in the atmospheric mix of your choice. If the atmosphere is less than 7% methane (more than 93% inert), the line never intersects the combustion zone, so that's the cut-off you were looking for in the OP. Slightly more than 7% methane will allow combustion, but it might be so marginal it was difficult to sustain as a flame.
An atmosphere of 11% methane (89% inert) crosses the stoichiometric point for air/methane mixtures, so a pure oxygen jet in that atmosphere should produce combustion similar to burning a pure methane jet in air.
And more than 11% methane in your atmosphere and you're going to get good sustained combustion friom your scenario, unless you have very low pressures in your mix.

note that Titan only has a 1.4% methane content in its atmosphere, according to Wiki, so no flames there.