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Hydrogen safety

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Hydrogen safety covers the safe production, handling and use of hydrogen. Hydrogen poses unique challenges due to its ease of leaking, low-energy ignition, wide range of combustible fuel-air mixtures, buoyancy, and its ability to embrittle metals that must be accounted for to ensure safe operation. Liquid hydrogen poses additional challenges due to its increased density and the extremely low temperatures needed to keep it in liquid form.

Hydrogen is used as a feedstock in industrial processes including production of ammonia and methanol, and cooling electric generators in power stations.

Summary

Although hydrogen has many useful properties, some have serious safety implications:

  • Colorless, tasteless and odorless
  • Extremely reactive with oxygen and other oxidizers
  • Low ignition energy (an order of magnitude lower than most other gases)
  • High flame temperature (of the common gases, only Acetylene and Propane burn hotter)
  • Invisible flame in daylight conditions (due to the flame's low emissivity)
  • Negative Joule-Thomson coefficient; leaking gas warms itself and may spontaneously ignite
  • Small molecular size promotes leaks and diffusion
  • Very wide range between lower and upper flammability limits in air mixtures
  • Very high flame speed
  • Can diffuse into or react with certain metals, embrittling them
  • The cryogenic liquid at 20K is even colder than frozen nitrogen, oxygen or argon
  • Does not support life (can asphyxiate)

On the other hand, in the context of safety, hydrogen's considerable buoyancy and lack of toxicity other than as an asphyxiant work in its favor.

Hydrogen codes and standards

Hydrogen codes and standards are codes and standards (RCS) for hydrogen fuel cell vehicles, stationary fuel cell applications and portable fuel cell applications.

Additional to the codes and standards for hydrogen technology products, there are codes and standards for hydrogen safety, for the safe handling of hydrogen[1] and the storage of hydrogen.

Guidelines

The current ANSI/AIAA standard for hydrogen safety guidelines is AIAA G-095-2004, Guide to Safety of Hydrogen and Hydrogen Systems.[2] As NASA has been one of the world's largest users of hydrogen, this evolved from NASA's earlier guidelines, NSS 1740.16 (8719.16).[3] These documents cover both the risks posed by hydrogen in its different forms and how to ameliorate them.

Ignition

Main article: Minimum Ignition Energy
  • "Hydrogen-air mixtures can ignite with very low energy input, 1/10 that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."
  • "Although the autoignition temperature of hydrogen is higher than those for most hydrocarbons, hydrogen's lower ignition energy makes the ignition of hydrogen–air mixtures more likely. The minimum energy for spark ignition at atmospheric pressure is about 0.02 millijoules."

Mixtures

  • "The flammability limits based on the volume percent of hydrogen in air at 14.7 psia (1 atm, 101 kPa) are 4.0 and 75.0. The flammability limits based on the volume percent of hydrogen in oxygen at 14.7 psia (1 atm, 101 kPa) are 4.0 and 94.0."
  • "The limits of detonability of hydrogen in air are 18.3 to 59 percent by volume"[4]
  • "Flames in and around a collection of pipes or structures can create turbulence that causes a deflagration to evolve into a detonation, even in the absence of gross confinement."

(For comparison: Deflagration limit of gasoline in air: 1.4–7.6%; of acetylene in air,[5] 2.5% to 82%)

Leaks

  • Leakage, diffusion, and buoyancy: These hazards result from the difficulty in containing hydrogen. Hydrogen diffuses extensively, and when a liquid spill or large gas release occurs, a combustible mixture can form over a considerable distance from the spill location.
  • Hydrogen, in both the liquid and gaseous states, is particularly subject to leakage because of its low viscosity and low molecular weight (leakage is inversely proportional to viscosity). Because of its low viscosity alone, the leakage rate of liquid hydrogen is roughly 100 times that of JP-4 fuel, 50 times that of water, and 10 times that of liquid nitrogen.
  • Hydrogen leaks can support combustion at very low flow rates, as low as 4 micrograms/s.[6]

Liquid hydrogen

  • "Condensed and solidified atmospheric air, or trace air accumulated in manufacturing, contaminates liquid hydrogen, thereby forming an unstable mixture. This mixture may detonate with effects similar to those produced by trinitrotoluene (TNT) and other highly explosive materials"

Liquid Hydrogen requires complex storage technology such as the special thermally insulated containers and requires special handling common to all cryogenic substances. This is similar to, but more severe than liquid oxygen. Even with thermally insulated containers it is difficult to keep such a low temperature, and the hydrogen will gradually leak away. (Typically it will evaporate at a rate of 1% per day.[7])

Prevention

Hydrogen collects under roofs and overhangs, where it forms an explosion hazard; any building that contains a potential source of hydrogen should have good ventilation, strong ignition suppression systems for all electric devices, and preferably be designed to have a roof that can be safely blown away from the rest of the structure in an explosion. It also enters pipes and can follow them to their destinations. Hydrogen pipes should be located above other pipes to prevent this occurrence. Hydrogen sensors allow for rapid detection of hydrogen leaks to ensure that the hydrogen can be vented and the source of the leak tracked down. As in natural gas, an odorant can be added to hydrogen sources to enable leaks to be detected by smell. While hydrogen flames can be hard to see with the naked eye, they show up readily on UV/IR flame detectors.

Incidents

Hydrogen has been portrayed in the popular press as a relatively more dangerous fuel, and hydrogen in fact has the widest explosive/ignition mix range with air of all gases except acetylene. However this is mitigated by the fact that hydrogen rapidly rises and disperses before ignition, and unless the escape is in an enclosed, unventilated area, it is unlikely to be serious.

Demonstrations have shown that a fuel fire in a hydrogen-powered vehicle can burn out completely with little damage to the vehicle, in stark contrast to the expected result in a gasoline-fueled vehicle.[8]

In a more recent event, an explosion of compressed hydrogen during delivery at the Muskingum River Coal Plant (owned and operated by AEP) caused significant damage and killed one person.[9][10] For more information on incidents involving hydrogen, visit the US DOE's Hydrogen Incident Reporting and Lessons Learned page.[11]

During the 2011 Fukushima nuclear emergency, four reactor buildings were damaged by hydrogen explosions. Exposed Zircaloy cladded fuel rods became very hot and reacted with steam, releasing hydrogen.[12][13] Safety devices that normally burn the generated hydrogen failed due to loss of electric power. To prevent further explosions, vent holes were opened in the top of the remaining reactor buildings.

See also

References

  1. ^ HySafe Initial Guidance for Using Hydrogen in Confined Spaces. (PDF) . Retrieved on 2012-07-13.
  2. ^ "AIAA G-095-2004, Guide to Safety of Hydrogen and Hydrogen Systems" (PDF). AIAA. Retrieved 2008-07-28.
  3. ^ Gregory, Frederick D. (February 12, 1997). "Safety Standard for Hydrogen and Hydrogen Systems" (PDF). NASA. Retrieved 2008-05-09.
  4. ^ Lewis, Bernard; Guenther, von Elbe (1961). Combustion, Flames and Explosions of Gases (2nd ed.). New York: Academic Press, Inc. p. 535. ISBN 978-0124467507.
  5. ^ MSHA – Safety Hazard Information – Special Hazards of Acetylene. Msha.gov. Retrieved on 2012-07-13.
  6. ^ M.S. Butler, C.W. Moran, Peter B. Sunderland, R.L. Axelbaum, Limits for Hydrogen Leaks that Can Support Stable Flames, International Journal of Hydrogen Energy 34 (2009) 5174–5182.
  7. ^ Peter Kushnir. Hydrogen As an Alternative Fuel . PB 700-00-3. Vol. 32, Issue 3, May–June 2000. almc.army.mil.
  8. ^ "Hydrogen Car Fire Surprise". January 18, 2003. Retrieved 2008-05-09.
  9. ^ Williams, Mark (January 8, 2007). "Ohio Power Plant Blast Kills 1, Hurts 9". Associated Press. Retrieved 2008-05-09.
  10. ^ "Muskingum River Plant Hydrogen Explosion January 8, 2007" (PDF). American Electric Power. November 11, 2006. Archived from the original (PDF) on 2008-04-09. Retrieved 2008-05-09.
  11. ^ "Hydrogen Incident Reporting and Lessons Learned". h2incidents.org.
  12. ^ Nuclear Fuel Behaviour in Loss-of-coolant Accident (LOCA) Conditions (PDF). Nuclear Energy Agency, OECD. 2009. p. 140. ISBN 978-92-64-99091-3.
  13. ^ Hydrogen explosions Fukushima nuclear plant: what happened?. Hyer.eu. Retrieved on 2012-07-13.

External links

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