An understanding of the properties of hydrogen is critical for the proper design of a facility or workspace. A workspace can be configured to mitigate hazards by understanding and taking advantage of some of the characteristics of hydrogen.
Designers and operators of hydrogen storage facilities must be aware that hydrogen's flammability range is very wide compared to other fuels. Additionally, under optimal combustion conditions (at a 29% hydrogen-to-air volume ratio), the energy required to initiate hydrogen combustion is much lower than that required for other common fuels (e.g., a small spark).
| Property | Hydrogen H2 |
Methane CH4 |
Gasoline |
|---|---|---|---|
| Normal boiling point1 (NBP) [°C] | -253 | -162 | 37 - 205 |
| Physical state at 25°C, 1 atm | Gas | Gas | Liquid |
| Heating Values2 LHV (kJ/g) HHV (kJ/g) |
120 142 |
50 55.5 |
44.5 48 |
| Flammability limits [vol% in air] | 4.0-75 | 5.3-15 | 1.0-7.6 |
| Molecular weight | 2.02 | 16.0 | ~107 |
| Flame temperature in air3 [°C] | 2045 | 1875 | 2200 |
| Minimum ignition energy4 [mJ] | 0.02 | 0.29 | 0.24 |
| Quenching distance [mm] | 0.64 | 2.0 | 2.0 |
| Density at NBP (g/L) | 70.8 | 423 | ~700 |
| Vapor specific gravity at 25°C, 1atm (air=1) | 0.070 | 0.54 | 3.7 |
1The boiling point at 1atm pressure
2Heating values are the energy, per gram of fuel, generated by a combustion reaction. The higher heating value (HHV) is obtained when all of the water formed by combustion is liquid. The lower heating value (LHV) is obtained when all of the water formed by combustion is vapor.
3Experimentally determined flame temperatures are shown in the table. These values do not differ significantly from theoretical adiabatic flame temperatures. See Ref. [3] for discussion.
4In air at 1 atm pressure
For any incident involving hydrogen, keep in mind the properties of hydrogen and watch for potential ignition sources that can ignite a hydrogen leak:
- electrical (e.g., static electricity, electric charge from operating equipment)
- mechanical (e.g., impact, friction, metal fracture)
- thermal (e.g., open flame, high-velocity jet heating, hot surfaces, vehicle exhaust)
There should be no grass or shrubs planted near areas where hydrogen potentially may be released to prevent the need for using powered garden tools in the area. According to NFPA 55, both compressed gaseous hydrogen storage vessels and liquid hydrogen storage vessels must be located at least 50 feet from combustible materials.
Mixtures near optimal combustion conditions should be considered prone to spontaneous ignition.
CGA G-5, Hydrogen
CGA H-4 Terminology Associated with Hydrogen Fuel Technologies
B. Lewis and G. von Elbe, Combustion, Flames and Explosions of Gases, 3rd ed., Academic Press, Orlando, 1987, pg. 717.
Babrauskas, Vytenis. "Ignition Handbook" Fire Science Publishers, Issaquah, WA.
J. Hord, Is Hydrogen Safe? National Bureau of Standards (NBS) Technical Note 690, October 1976.
F.J. Edeskuty and W.F. Stewart, Safety in the Handling of Cryogenic Fluids, Plenum Press, New York, 1996, pg. 102.