Re: Can helium be mixed with hydrogen to form a non-explosive mix?

When fuel and air are mixed together and exposed to a source of ignition, 
combustion takes place.  High school chemistry students are exposed to the 
concept of a stoichiometric mixture, the combination of fuel and air that 
produces water vapor and carbon dioxide as byproducts.

Since your intention is to produce essentially a non-flammable mixture of 
helium and hydrogen, the number you are looking for is the "Lower 
Flammability Limit" or "Lower Explosive Limit."  This value can be 
determined theoretically, since there is just sufficient energy produced 
by the combustion of hydrogen to heat up the mixture of oxygen, nitrogen, 
and in your case helium, to the combustion temperature of hydrogen.  The 
value is often determined experimentally, simply by igniting a mixture of 
hydrogen and helium in air, and then reducing the amount of hydrogen until 
the flame goes out.

Using these approaches, the lower flammability limit of hydrogen in air is 
4%.  There is also an upper flammability limit of hydrogen of 75%.  The US 
Department of Energy is doing quite a bit of work currently to promote 
hydrogen fuel cells in vehicles, so safe storage of hydrogen is an 
important issue.  Their web site, 
http://www.eere.energy.gov/hydrogenandfuelcells/codes/
provides an 
interesting comparison of the lower and upper flammability limits for 
several fuels.  Because of the upper flammability limit, a tank containing 
greater than 74% hydrogen with the balance air will not support 
combustion.  However, between 4 and 74%, the same tank becomes a potential 
bomb.

When helium is added to the hydrogen, it adsorbs some of the heat of 
combustion so that more hydrogen is needed to support combustion.  In 
looking for the lower flammability limit for helium-hydrogen mixtures, I 
came across this very interesting material safety data sheet: 
http://www.mwsc.com/MSDS/56.PDF.
 Under Section 14, Transportation, the 
sheet states mixtures of 8.7% or less hydrogen in helium may be shipped as 
non-flammable gas.  Another MSDS sheet,
http://www.vngas.com/pdf/g116.pdf
 
provides confirmation that mixtures of 8.5% hydrogen in helium are being 
shipped as non-flammable gas.  If theoretical calculations and 
experimental tests show that you can go to a higher hydrogen concentration 
before you have a flammable mixture, you still would have to convince the 
government (Department of Transportation, Federal Aviation Administration) 
that it was a non-flammable mixture.

So, 8.5% hydrogen in helium appears to be non-flammable, whereas anything 
above 8.7% is flammable.  Eight percent hydrogen really doesn't provide 
much in the way of cost savings.  You probably ought to consider another 
question concerning your airship business.  Are the cost savings by using 
a 8.5% mixture of hydrogen worth all the trouble and effort it will take 
to convince your customers that the mixture is safe, as opposed to 
avoiding the safety questions altogether by using 100% helium?  As my wife 
tells me; you need to pick your battles.

Thanks for your question, and I hope your plans take flight.