Combustion in a Methane Atmosphere?

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Esquire
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Combustion in a Methane Atmosphere?

Post by Esquire »

Does anybody happen to know what, if anything, burns in a methane-based atmosphere? Alternatively, if combustion doesn't work outside an Earthlike environment, what analogous chemical processes might exist? This is half idle curiosity and half story research; I don't know enough chemistry to ask Google intelligent questions.
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Re: Combustion in a Methane Atmosphere?

Post by SpottedKitty »

Don't forget, burning needs oxygen to work. If you put something burning into a pure methane atmosphere, it goes out. If you try to start something burning in a pure methane atmosphere, it won't.

Note that "something burning" can be confusing if you try it with something like strike-anywhere safety matches. These get round the restriction because they include their own oxygen in the chemicals they're made out of.
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Re: Combustion in a Methane Atmosphere?

Post by Sea Skimmer »

Sounds like you really need to just review the fundamentals of combustion on wikipedia and it'd make sense to you. Fire is fuel, oxygen and an ignition source. It will work anywhere given a viable fuel and oxygen mixture. Inside gunpowder cartridges, in a cloud orbiting mars ect. Methane is a fuel. The earth's atmosphere does not contain any element that is a fuel in large quantities, most of it is just nitrogen. It does contain about 21% oxygen.

If you have an atmosphere based on methane combustion would depend on supplying enough oxygen to create a localized ignitable mixture. Too much or too little methane will not support combustion, the fuel absorbs too much heat to sustain the chain reaction or not enough fuel is present to make enough heat in the first place.

If the atmosphere also had enough oxygen to support say, human life, you might end up at risk of setting the entire atmosphere on fire. However that would never be possible to maintain for long because static electric would ignite it one way or another in short order. Then you'd have a new atmosphere with much more free carbon in it, and far less hydrogen.
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Re: Combustion in a Methane Atmosphere?

Post by Napoleon the Clown »

Fluorine makes most things quite happy to catch on fire, even in absence of oxygen. Fire is, at its most basic, rapid oxidation. Which is what fluorine does to things. Some fluorine compounds will make asbestos burn quite cheerfully. So yeah, a methane atmosphere wouldn't be an issue for fluorine to make fires.

Of course, fluorine is even more insanely reactive than oxygen. So, um, good having any floating around that hasn't already done hilarious things.
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Re: Combustion in a Methane Atmosphere?

Post by Sea Skimmer »

Yes oxidizer would be better a term then oxygen for what I said. But you already hit the problem of why I didn't really care. Such compounds are generally either solid, liquid or so reactive that they have serious trouble existing in a pure free floating form. Fluorine for example will react with water and steal its hydrogen, release oxygen and become hydrogen fluoride. IIRC more water then makes it into hydrofluoric acid. So it couldn't be a significant part of an earth like or at all wet world's atmosphere out of hand. It'd be all reacted into that stuff and require electrolysis to break it back down. If were aiming for remotely close to earth like its going to mean water and oxygen is the only thing I can think of that's going to work at that point for supporting fire in the atmosphere. But perhaps I am wrong on that, chem is not my think.

It'd be kind awesome though if we had oceans of liquid oxidizer and a flammable atmosphere churning up a constant firestorm that only exists at sea level, but kinda not so plausible this would actually occur to be able to last long. And it'd certainly be getting far away from the original point.
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Re: Combustion in a Methane Atmosphere?

Post by Coop D'etat »

A methane atmosphere is a reducing one so any free oxidizer will be consumed. Atmosphericly that means the free O2 is quickly converted to C02 or H20 although Oxygen could also be found in rocks depending on conditions. So to get fire you have to invert your Earth-based, oxidizing enviroment thinking. On earth to get a fire you add fuel (typically a reduced substance which is not energetically stable under earth like conditions) and then a spark to get the reaction going and you are cooking. In a methane atmosphere, to get a fire what you do is introduce free oxygen, which has essentially becomes your "fuel for the fire," because the oxidizer is the limiting reagent, not the reduced compound.
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Re: Combustion in a Methane Atmosphere?

Post by LaCroix »

Also, methane stochimetry says that to achieve combustion, you'd need a localized mixture of ~6 % methane to "air" or, 2 O2 per methane molecule. And that's a pretty narrow window, you need to really hit it within 3-5% to achieve combustion.

Methane is lighter than air or O2, so the O2 (if venting from the ground) would pool pretty fast, concentrating as it goes. This would lead to a layer of O2 underneath the methane layer if there is no weind to stirr. There could be 'freak events' in the turbulent outer layer of that bubble if the timing is right, but other than this, it would be pretty hard to cause such a combustion. IF there is wind, it would cause a very quick dissolution of the bubble, which again, renders the mixture innert.
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Re: Combustion in a Methane Atmosphere?

Post by Sky Captain »

Would it be possible to run combustion engines in methane atmosphere? For example you want to send aircraft to explore planet with methane atmosphere. Fit it with liquid oxygen tanks and jet engines. Would it work?
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Re: Combustion in a Methane Atmosphere?

Post by Borgholio »

Sky Captain wrote:Would it be possible to run combustion engines in methane atmosphere? For example you want to send aircraft to explore planet with methane atmosphere. Fit it with liquid oxygen tanks and jet engines. Would it work?
In principle, if you could keep the ratio correct, it would burn for sure. The real question is, can you run a high performance aircraft engine on methane fuel? I don't know if methane provides enough energy for that.
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Re: Combustion in a Methane Atmosphere?

Post by Sea Skimmer »

Oh yes you can, in fact turbine engines would rather run on a methane gas fuel then a liquid oil based hydrocarbon in some respects, and Soviets did an enormous amount of work on natural gas and hydrogen powered airliners, look up Tu-155.

The problem in real life is storing the methane on the aircraft, because you need large high pressure cryogenic cooled tanks which cost you weight and volume (adding drag) compared to jet fuel tanks. The Soviets were interested mainly because they wanted to export more oil make hard currency. This project got as far as the Tu-155 actually being flown for extensive tests, but it had the fuel tanks displacing the passenger space. New designs with giant bubble roofs (rather like an A380 but single passenger deck) would have been needed for service operations. That was money the Soviets rather didn't have handy before the cold war imploded, and independent Russia was left with a large surplus of oil!

LOX has a lot of the same storage problems liquified natural gas does (pure gas storage would suck) but you would need far less LOX then you would need methane so it would be much easier in that respect to find volume to put it in.

As far as energy density goes as far as I can tell any flammable liquid or gas will work fine in a subsonic engine, as will a fair number of solid fuels provided they are either turned into dust or suspended in a liquid. See the infamous boron based cancer zip fuel for where this avenue leads....

Supersonic engines can or can not involve higher temperatures which begin to create problems. SR-71 for example could not run on natural jet fuel, it had to use fuel blended from purified feed stocks because in normal jet fuel certain impurities would turn into coke on the turbine blades and wreck the engine. Around mach 7-8 life gets so hot that no hydrocarbon liquid will work because they will actually coke up in the fuel tanks and lines before it even reaches the engine, and you need pure hydrogen fuel (not sure where methane ranks on this), which will work up to around mach 12-14 before it just doesn't have the ommmph to go faster in a scramjet. Notionally pulse detonation engines could go twice as fast, but this assumes crazy high pressures and a lot of other guesswork.
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