Hydrogen flammability with elevation?

1. Dec 10, 2011

zmorris

Hi All, I've googled everywhere trying to find out if the combustion/flammability of hydrogen is reduced at all for low pressure at high altitudes. There is data for hydrogen at high pressures, but I've been unable to find any data for low pressures.

For example, if a hydrogen balloon stays above 100,000 feet (10 mbar), is there a risk of explosion?

I have a hunch that it's still flammable, because the ratio of hydrogen to oxygen is going to be fairly constant.

If you know of data for other gasses like methane, hydrogen would probably be similar.

I'm just asking because helium is becoming rare/expensive as a lifting gas. I realize that leakage is also a problem.

Zack

2. Dec 12, 2011

bigfooted

Hi,

From measurements we know that when the ambient pressure and/or temperature goes up(down), the laminar flame speed of the mixture goes up(down) and the flammability range increases(decreases).

When pressure and temperature go down, there is a point where the flame speed is zero for any mixture ratio, so it will not burn.
For hydrogen-air, this happens at approximately 1.3 mbar (at room temperature) (Thomson and Enloe,Combustion and Flame, 1966). For most hydrocarbon mixtures, this value is higher (Arnaldos et al. Chem. Engin. Sci. 56, 2001 ). So hydrogen is among the most dangerous mixtures in terms of flammability limits.

When the temperature goes down as well, the flammability limit range decreases as well, from 300 K to 250K the lower flammability limit changes from 4.13 volume% to 4.88 volume% (Karim et al. Cryogenics 24, 1984).

So I think that yes, at 100,000 ft you can still have a flammable mixture, but the range of mixture ratios for which hydrogen-air will burn will be quite narrow.

3. Apr 30, 2012

peterdaol

That was my question as well, I hope somebody knowledgeable will answer it. My guess is there is no risk.
At 100000 feet, ambient pressure is 1/100th atmosphere. The air is so thin there is so little oxygen to support any kind of combustion. Hydrogen is flammable between 4% to 75% concentration. Note the rather low upper limit of 75%. Suppose your pure 100% hydrogen tank is leaking, there is just too little oxygen (1/100) in the vicinity to dilute the concentration down to 75%. The leaked hydrogen will only raise and dissipate upward where there is even less oxygen.

4. May 1, 2012

Staff: Mentor

This is most likely for 1 atm, generalization for other pressures is unfounded.

Besides, the conclusion that at high elevations there is not enough oxygen is rather off. Yes, air has much lower pressure, but so does the hydrogen, so during a leak the volume in which the ratio is between the upper and lower explosive limits stays the same. There is much less energy stored in that volume, but that's another thing.