2.3.4 Consequences of a fire / explosion
2.3.4.1 Invisible flame
Hydrogen burns with an invisible flame making it difficult to detect a hydrogen fire. This apparent low emissivity of hydrogen flames (total heat flux radiated) may reduce the heat transfer by radiation to objects near the flame, thus reducing the risks of secondary ignition and burns. However, such effects have not been fully quantified and further work is needed in this area.2.3.4.2 Rapid burning rate
The maximum burning velocity of a hydrogen-air mixture is about eight times greater than those for natural gas and propane air mixtures. The high burning velocity of hydrogen makes it difficult to confine or arrest hydrogen flames and explosions, particularly in closed environments. In its favour, however, this rapid rate of deflagration means that hydrogen fires transfer less heat to the surroundings than other gaseous fuel fires, thereby reducing the risk of creating secondary fires in neighbouring materials. Another downside of a higher burning velocity of hydrogenis that for a given scenario hydrogen would result in higher explosion pressures and rates of pressure rise than other fuels.
2.3.4.3 Possibility of detonation
Hydrogen/air mixtures have a greater propensity to detonate than mixtures of air with other more common flammable fuels. Detonations cause much more damage and are far more dangerous than ordinary explosions (deflagrations). However, due to the rapid dispersal characteristics of hydrogen, this is only likely to occur in a confined or congested space.
Table 1 Characteristics of hydrogen, dry natural gas and gaseous propane
| Property | Dry natural gas (methane) | LPG (propane) | Hydrogen |
| Density (Kg/m3) * | 0.65 | 1.88 | 0.090 |
| Diffusion coefficient in air (cm2/s) * | 0.16 | 0.12 | 0.61 |
| Viscosity (g/cm-s x 10-5) * | 0.651 | 0.819 | 0.083 |
| Ignition energy in air (mJ) | 0.29 | 0.26 | 0.02 |
| Ignition limits in air (vol %) | 5.3 – 15.0 | 2.1 – 9.5 | 4.0 – 75.0 |
| Auto ignition temperature (C) | 540 | 487 | 585 |
| Specific heat at constant pressure (J/gK) | 2.22 | 1.56 | 14.89 |
| Flame temperature in air (C) | 1875 | 1925 | 2045 |
| Quenching gap (mm) * | 2 | 2 | 0.6 |
| Thermal energy radiated from flame to surroundings (%) | 10-33 | 10 - 50 | 5-10 |
| Detonability limits (vol % in air) | 6.3-13.5 | 3.1 – 7.0 | 13-65 |
| Maximum burning velocity (m/s) | 0.43 | 0.47 | 2.6 |