2.2.2 Fire and explosion hazards

The estimation of hazards and hazard levels is essential to the consideration of accidental consequences, e.g. overpressures, thermal radiation, the throw of debris or missiles, and the damage level or the vulnerability of the receiving objects. In chemical fires/explosions that are usually exothermal oxidation reactions, a great proportion of the combustion energy is carried by the developing blast wave uniformly distributed in all directions [7].

Many flammable gases are widely in use today, such as methane, propane etc. Without appropriate measures being taken, a gas release and subsequent fire and explosion can occur. Hydrogen has some significantly different properties from these more commonly used gases which need to be fully appreciated to achieve comparable levels of safety.

Hydrogen for use in fuel cells may be stored in a number of ways:

• As a compressed gas - normally in conventional gas cylinders at a pressure of 200bar, but this pressure may be increased in specialist applications to increase energy storage density.
• As a cryogenic liquid - hydrogen is stored as a liquid below -250 oC therefore, consideration should be given to cold burns, condensation of oxygen-enriched atmospheres, and the way in which a liquid spill may develop into a flammable cloud. It should be appreciated that the vapour produced by a liquid spill will not initially be buoyant due to its low temperature.
• Complex hydrides are also used as a hydrogen storage medium, generally based on sodium aluminium hydrides or similar materials. These materials are flammable solids and can react violently with water to produce hydrogen and a corrosive aqueous solution. Hydride storage systems can be suitably designed to avoid these hazards.