Lessons Learned from a Hydrogen Explosion
Comparison with Other Flammable Materials
Hydrogen is lighter than air and diffuses rapidly — 3.8 times faster than natural gas — which means that when released, it dilutes quickly into a nonflammable concentration.
Hydrogen rises two times faster than helium and six times faster than natural gas at a speed of almost 45 mph (65.6 feet/second). Therefore, unless a roof, a poorly ventilated room, or some other structure contains the rising gas, the laws of physics prevent hydrogen from lingering near a leak (or near people using hydrogen-filled equipment). Simply stated, to become a fire hazard, hydrogen must first be confined; however, because hydrogen is the lightest element in the universe, it is very difficult to confine. Industry takes these properties into account when designing structures in which hydrogen will be used. The designs help hydrogen escape up and away from the user in case of an unexpected release.
Hydrogen is odorless, colorless, and tasteless, so human senses won’t detect a leak. However, given hydrogen’s tendency to rise quickly, a hydrogen leak indoors would briefly collect on the ceiling and eventually move toward the corners. For that and other reasons, industry often uses hydrogen sensors to help detect hydrogen leaks and has maintained a high safety record using them for decades.
Combustion
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 with a hydrocarbon fire. Because a hydrogen fire emits low levels of heat near the flame (the flame itself is just as hot), the risk of secondary fires is lower.
Like any flammable substance, hydrogen can combust. But hydrogen’s buoyancy, diffusivity, and small molecular size make it difficult to contain and 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% to 74% in air), and the energy required to ignite hydrogen (0.02 mJ) can be very low. However, at low concentrations (below 10%) the energy required to ignite hydrogen is high — similar to the energy required to ignite natural gas and gasoline in their respective flammability ranges — making hydrogen realistically more difficult to ignite near its lower flammability limit.
Explosion
An explosion cannot occur in a tank or any contained location that contains only hydrogen. An oxidizer such as oxygen must be present in a concentration of at least 10% pure oxygen or 41% air. Hydrogen can be explosive at concentrations of 18.3% to 59%. Although this range is wide, it is important to remember that gasoline can present a greater danger than hydrogen because the potential for explosion occurs with gasoline at much lower concentrations: 1.1% to 3.3%. Furthermore, there is very little likelihood that hydrogen will explode in open air due to its tendency to rise quickly. This is the opposite of what we find for heavier gases such as propane or gasoline fumes, which hover near the ground, creating a greater danger for explosion.
Asphyxiation
With the exception of oxygen, any gas can cause asphyxiation. In most scenarios, hydrogen’s buoyancy and diffusivity make hydrogen unlikely to be confined where asphyxiation might occur.
Toxicity/Poison
Hydrogen is nontoxic and nonpoisonous. It will not contaminate groundwater (it’s a gas under normal atmospheric conditions), nor will a release of hydrogen contribute to atmospheric pollution. Hydrogen does not create “fumes.”
More safety information about hydrogen can be found at the following websites: www.hydrogensafety.info and www.hydrogenassociation.org.