Fire & Explosion Hazards
Hydrogen used in fuel cells can cause fires and explosions if the hydrogen is not handled properly. Hydrogen is a colorless, odorless, and tasteless gas. While this is also true of natural gas and propane, a sulfur-containing odorant is added to the latter gases so that a leak can be detected. However, this is not the case with hydrogen. Hydrogen is a very light gas, which means that there are no known odorants that can be added to it that are light enough to diffuse at the same rate as hydrogen. In other words, even if an odorant could be added to hydrogen, by the time a worker smelled the odorant, the hydrogen concentration might have already exceeded the lower flammability limit of hydrogen. Therefore, at the present time, it is very hard to tell if a hydrogen leak has occurred.
Hydrogen Fires
- Hydrogen is flammable at concentrations between 4% and 75% in air, which is a very wide range compared to other common fuels.
- The hydrogen concentration could easily reach the lower flammability limit (4%) if there were a leak in a confined space with no ventilation. An outdoor leak would simply rise quickly and diffuse.
- Hydrogen burns with a pale blue flame that is almost invisible during daylight hours, so fires are almost impossible to see with the naked eye.
- Hydrogen fires have low radiant heat, so you cannot sense the presence of a flame until you are very close to it (or even in it).
- Combustion cannot occur in a tank that contains only hydrogen. Oxygen (or air) and an ignition source are required for combustion to occur.
In addition, hydrogen flames radiate little infrared heat but substantial ultraviolet (UV) radiation. This means that when someone is very close to a hydrogen flame, there is little sensation of heat, making inadvertent contact with the flame a significant concern. UV overexposure is also a concern, since it can result in sunburn-like effects.
If a large cloud of hydrogen gas comes into contact with an ignition source, ignition will result with the flame flashing back to the source of the hydrogen. In open spaces with no confinement, flames will propagate through a flammable hydrogen-air cloud at several meters per second, and even more rapidly if the cloud is above ambient temperature. The result is a rapid release of heat, and the combustion product is steam. Hydrogen combustion is more rapid than combustion of other fuels. A hydrogen cloud will burn within seconds, and all of the energy of the cloud will be released.
If hydrogen gas mixtures enter confined regions, ignition can result in flame acceleration and generation of high pressures that are capable of exploding buildings and throwing shrapnel. Flammable mixtures of hydrogen in confinements such as pipes or ducts, if ignited, will readily result in accelerated flames.
A leak in a pressurized (≥200 psia) hydrogen storage system will result in a jet that may extend for some meters. If ignited, the jet flame can cause serious damage to anything it encounters.
To learn more about Hydrogen Fuel Cell Safety visit our Hydrogen Fuel Cell Safety Online Training web page.
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