|MadSci Network: Physics|
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.
Try the links in the MadSci Library for more information on Physics.