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So, my planet is a Halogen world (like Stephen Gillet's Clorox), it has an atmospheric pressure of 7.54 atm, with it being composed by: 93.09% N2 6.569% O2 0.201% Cl2 0.101% Ar 0.039% others (CO2, Ne etc.)

While i know in Earth's atmosphere the amounts of chlorine would weaken most fires im still unsure if the bigger partial pressure of O2 (which is roughly 0.49 atm in my planet) would change anything (for reference, Earth's partial pressure of O2 is roughly 0.21 atm)

Basically, how would fires turn out in my planet's atmosphere?

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    $\begingroup$ Why are the numbers so specific? $\endgroup$ Commented May 10 at 1:35
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    $\begingroup$ Well, its just me being weird about it (i like broken numbers since it feels somewhat more real i guess) $\endgroup$
    – UltraLore
    Commented May 10 at 1:48

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This is a weirdly difficult question to answer.

The value you're probably interested in is called limiting oxygen concentration. Wikipedia, quoting from "An Introduction to Fire Dynamics" says

The effect of increasing the concentration of inert gas can be understood by viewing the inert as thermal ballast that quenches the flame temperature to a level below which the flame cannot exist

The fact that your atmosphere has a higher oxygen partial pressure, and therefore can supply a lot more oxygen to a reaction is offset by the increased amount of inert gas and its ability to pull heat out of a reaction, and a reaction that cools too much simply smolders and stops.

In Hyperbaric and hypobaric chamber fires: a 73-year analysis (which makes for slightly gristly reading) states that fire risk increases with pressure even if oxygen concentration remains unchanged (this should be unsurprising... it is why bellows work, for example) but then goes on to reference "Region of non-combustion in nitrogen-oxygen and helium-oxygen diving atmospheres" from Industrial & Engineering Chemistry Process Design and Development volume 7, 1968, which does not appear to have an online copy. That work tested combustion of stuff like paper at increased air pressures and different oxygen percentages, suggesting that at concentrations of 6% and under combustion simply didn't procede. The implication is that this is true at increased pressures, but it doesn't actually outright state that this is so.

In Inerting biomass dust explosions under hyperbaric working conditions (I can only read the abstract, and haven't looked at the whole paper)

Contrary to the trend indicated by other research (usually for coal), it has been found for biomass that an increase of the initial pressure yields an increase in the limiting oxygen concentration.

The limiting oxygen concentration varies depending on temperature and pressure and the fuels involved, but an increase in pressure seems likely to decrease the LOC for a bunch of combustibles. This means that the 6.5% oxygen with your world's 7.5x pressure will do a better job of sustaining fires than the equivalent concentration at sea level on Earth would, but in general I think you'll find that fires are harder to light and flames are smaller and are extinguished more quickly than on Earth. How much less flamey they'll be would be straightfoward to establish with an experiment, but unless you have friends in a University chemistry lab that might be awkward to do.

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Your planet has 4x less oxygen than Earth' Example case, Fire under various oxygen percentages enter image description here

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    $\begingroup$ Does that still apply when pressure is 7.5 x Earth (sea level) normal, though? The percentage of oxygen in the atmosphere is less, but the raw amount of oxygen in any given volume is actually higher - that's the point of the OP's question. $\endgroup$ Commented May 10 at 4:00
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    $\begingroup$ It's all about partial pressure of O2, not percentage. @LazyReader $\endgroup$ Commented May 10 at 5:07
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    $\begingroup$ Pure oxygen at high pressures can create combustion in the presence of oils and grease. Other materials may catch fire spontaneously with pure oxygen at high pressures. (Source) The reason @Escapeddentalpatient. tells us partial pressure is everything is because despite the lower percentage of the whole, the amount of oxygen available for combustion in a cubic meter has doubled. $\endgroup$
    – JBH
    Commented May 10 at 8:04

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