Transportation Safety

In addition to the dangers posed by the potential rupture of hydrogen fuel tanks or pipelines, there are other dangers of using hydrogen that may not be present with other fuels. For example, the fact that hydrogen gas is light will cause it to rise to the ceiling of a room or tunnel in which there is a hydrogen leak. The stratification of the gas creates the conditions necessary for a combustion wave that can propagate a flame under ignition at several times the normal rate of combustion. This poses a threat to ventilation systems in tunnels and building that are not design to clear out a light gas such as hydrogen that would collect along tops of rooms.

In addition, a phenomenon known as auto-ignition can create additional risk of combustion and explosion. Auto-ignition occurs first due to hydrogen’s low molecular mass; it will quickly mix with oxygen (and nitrogen) in the air to form a combustible mixture once a leak has occurred in a storage chamber. Second, because hydrogen must be stored at tremendous pressures to overcome its low energy density, a shock wave may form when a leak occurs because of the supersonic speed at which the gas exits through the leak. The pressure of the shock wave combined with the mixing effects means that hydrogen can rapidly ignite around a leak with no outside spark.

Source: Frederick Dryer, Department of Mechanical and Aerospace Engineering (personal conversation).