Hydrogen Safety
Hydrogen is a fuel. And, as is the nature of all other fuels, has some degree of danger associated with it. Hydrogen is no more or less hazardous than the more conventional fuels such as gasoline (petrol), natural gas, etc. As a matter of fact, some of the peculiar properties of hydrogen actually provide safety benefits compared to petrol and other fuels [1]. Hydrogen is flammable, like other fuels, and must therefore be handled responsibly; as it can behave dangerously under certain conditions. In order to handle hydrogen safely, it is necessary for the user to have some understanding of its behaviour and to observe the relevant guidelines.
Odour, Colour and Taste
Hydrogen is an odourless, colourless and tasteless gas. This makes it difficult to rely on human senses for its detection in the event of a leak. The practice, therefore, is to use hydrogen sensors to detect leaks. Natural gas, by comparison, is also odourless, colourless and tasteless; therefore use is made of odorants called mercaptans (sulphur-containing compounds) to make it detectable by smell. Presently, the use of odorants in hydrogen buffering systems is not common as all known odorants have the ability to contaminate fuel cells. However, hydrogen being the lightest gas has the tendency to rise very quickly above levels where they can be detected by smell. Currently, research is focused on the use of tracers and advanced sensors for the detection of hydrogen leaks.
Diffusivity
Hydrogen is the lightest gas in the universe and diffuses rapidly (about 3.8 times faster than natural gas). Because of this, any leakages of hydrogen tends to be quickly diluted to non-flammable concentrations. Density difference between air and hydrogen (buoyancy) causes hydrogen to rise upward at a speed of 20ms-1 (45mph); i.e. 2 times faster than helium and 6 times faster than natural gas [1]. This means that hydrogen hardly lingers near a leak, unless it is confined by a roof or a poorly ventilated space. This behaviour of hydrogen is often taken into account in the design of hydrogen systems, so that designs are such as to encourage hydrogen to escape upward and away from the user in the event of an unexpected release.
Radiant Heat
Hydrogen burns with a nearly colourless flame, emitting low levels of radiation and producing water and heat as products. Because of the absence of carbon and the presence heat-absorbing water vapour, a hydrogen flame has a much lower radiant heat compared to a hydrocarbon flame. This means that a hydrogen fire is much less likely to start a secondary fire, a fact which has significant implications for the public and rescue workers.
Ignition
Hydrogen will burn in the presence of an oxidizer such as oxygen; but its buoyancy, diffusivity and small molecular size make containment and combustion difficult. Combustion cannot occur unless there is an adequate concentration of hydrogen, the presence of an ignition source and the right amount of oxygen. As shown in Table 1, hydrogen has a wide flammability range in air and a relatively small amount of energy is required for its ignition.
The energy required to ignite hydrogen at low concentrations (below 10%) is, however, higher and is similar to the energy required to ignite gasoline (petrol) and natural gas in their respective flammability ranges. This makes it more difficult to ignite hydrogen near its lower flammability limit. On the other hand, at concentrations close to the stoichiometric mixture of 29% hydrogen in air, the ignition energy reduces sharply to about one fifteenth of that required to ignite natural gas (or one tenth of that required to ignite gasoline).
Explosion
Pure hydrogen cannot explode. For explosion to take place, oxygen must be present in a concentration of at least 10% (or air in a concentration of at least 41%). However, hydrogen has been known to form explosive mixtures at concentrations of 18.3 – 59%. Gasoline, by comparison, has a much greater potential for explosion since explosive mixtures are formed at much lower concentrations (1.1 – 3.3%). Furthermore, hydrogen is much less likely to explode in open air because of its high buoyancy and diffusivity. This contrasts sharply with much heavier gases such as natural gas and gasoline vapour which carry a greater danger of explosion because they hover close to the ground and do not disperse quickly enough.
Asphyxiation
All gases, with the exception of oxygen, carry the danger of asphyxiation. With hydrogen, however, this danger is much less because of its high buoyancy and diffusivity and is much less likely to be confined close to the ground where asphyxiation might occur.
Toxicity
Hydrogen is neither toxic nor poisonous and does not contaminate groundwater. Hydrogen is not an atmospheric pollutant; therefore accidental releases do not pose a danger to the environment.
Hydrogen Codes and Standards
Codes and standards have been developed for the building, installation and handling of hydrogen systems. These codes and standards, if carefully observed, have the advantage of making the handling of hydrogen systems much safer. The following websites provide detailed information on hydrogen safety, codes and standards: www.HydrogenSafety.info, www.fuelcellstandards.com and www.eere.energy.gov/hydrogenandfuelcells/codes.
Reference:
[1] http://www.hydrogenassociation.org/general/factSheet_safety.pdf
Bibliography:
[1] www.HydrogenSafety.info
[2] www.fuelcellstandards.com
[3] www.eere.energy.gov/hydrogenandfuelcells/codes
[4] http://www.hydrogensociety.net/hydrogen_safety_issues.htm
