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Friday, January 22, 2010

Hydrogen Safety Part II

Hydrogen is odorless, colorless and tasteless, so most human senses won't help to detect a leak. For these and other reasons, industry often uses hydrogen sensors to help detect hydrogen leaks and has maintained a high safety record using them for decades. By comparison, natural gas is also odorless, colorless and tasteless, but industry adds a sulfur-containing odorant, called mercaptan, to make it detectable by people.

Currently, all known odorants contaminate fuel cells (a popular application for hydrogen) and create complications for food applications, like hydrogenating oils. However, given hydrogen's tendency to rise quickly, a leak would most likely rise above where any human nose might smell it, collecting briefly on the ceiling and then moving towards the corners. Today, researchers are investigating other methods that might be used for hydrogen detection like tracers and advanced sensors.

Hydrogen flames have low radiant heat.

Hydrogen combustion primarily produces heat and water. Due to the absence of carbon and the presence of heat absorbing water vapor created when hydrogen burns, a hydrogen fire has significantly less radiant heat compared to a hydrocarbon fire. Since the flame emits low levels of heat near the flame (the flame itself is just as hot), the risk of secondary fires is lower. This fact has a significant impact for the public and rescue workers.


Like any flammable fuel, hydrogen can combust. But hydrogen's buoyancy, diffusivity and small molecular size make it difficult to containand create a combustible situation. In order for a hydrogen fire to occur, an adequate concentration of hydrogen, the presence of an ignition source and the right amount of oxidizer (like oxygen) must be present at the same time. 

Hydrogen has a wide flammability range (4- 74% in air) and the energy required to ignite hydrogen (0.02mJ) can be very low. However, at low concentrations (below 10%) the energy required to ignite hydrogen is higher-- similar to the energy required to ignite natural gas and gasoline in their respective flammability ranges--making hydrogen realistically more difficult to ignite near the lower flammability limit. On the other hand, if conditions exist where the hydrogen concentration increases toward the stoichiometric(most easily ignited) mixture of 29% hydrogen (in air), the ignition energy drops to about one fifteenth of that required to ignite natural gas (or one tenth for gasoline).

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