HAZARDOUS MATERIALS
CHEMICAL DATA NOTEBOOK SEMES #90: HYDROGEN
Hydrogen is a highly flammable, nontoxic, colorless, odorless, tasteless gas. It is used as a fuel and as an intermediate in the production of ammonia, aniline, catalysts, dyes, flavors and fragrances, inorganic acids, methanol (methyl alcohol), pesticides, and other important chemicals. It also is used to hydrogenate animal and vegetable oils to cause them to become solids. It is used in welding, in petroleum refining, and to prevent metallic oxidation. Hydrogen is available as a compressed gas and as a liquefied, cryogenic material. The gas is shipped in cylinders and tube trailers, and the liquid is shipped in insulated tanks. The properties and hazards given below are for the compressed (gaseous) form. The section on liquefied hydrogen describes different properties and hazards.
PROPERTIES
Hydrogen is an extremely flammable gas. Its flammable range is from four to 75 percent and its ignition temperature is 1,06()°F. As a gas, it is in the proper state to burn and therefore has no flash point. It has a molecular weight of two and a vapor density of 0.07. Its freezing point is — 434.6°F, its boiling point is — 423°F, and it is slightly soluble in water. Its molecular formula is H2.
HAZARDS
Hydrogen’s major hazard is its extreme flammability. It has the second widest flammable range of any common flammable gas (the widest flammable range belongs to acetylene at 2.5 to 82 percent). This wide flammable range means that in any confined space, especially near the ceiling, it would be very difficult to find hydrogen outside its flammable range once the lower flammable limit had been reached. Its ignition temperature of 1,060°F may seem high, but this is well within the range of any normal ignition source.
Another hazard of hydrogen is the very’ high heat content of its flame. When hydrogen burns, its flame temperature is 3,700°F. This is the second highest flame temperature of the common flammable gases (the highest flame temperature, again, belongs to acetylene, at 4,217°F). The very high flame temperature of acetylene is veryeasy to explain. Acetylene, chemical formula C2H2, contains a triple covalent bond into which is packed a tremendous amount of energy (see Fire Engineering, October 1990). When acetylene burns, this triple bond breaks and releases all the energy stored within it. Hydrogen has just a single bond connecting the two atoms of hydrogen that make up its very simple molecular structure. From where, then, does the searing heat come when hydrogen burns?
The answer is in the way hydrogen burns. Most materials, when they burn, liberate heat and light in quantities that generate a considerable amount of heat, which, when coupled with the liberation of enough light energy, produces a bright, visible flame. Hydrogen, in contrast, burns with an almost invisible flame, converting almost all the energy released in combustion (oxidation) as heat energy. This almost invisible flame is the third hazard of hydrogen.
When emergency responders answer an alarm involving leaking hydrogen, the odds are that the hydrogen is burning, but no one can see the flame. (The assumption is that the leak is in a well-lit area inside or outside on a sunny day. At night, and in poorly lit conditions, the flame may be visible.) The factors that increase the chance that hydrogen is burning as it leaks are that the hydrogen molecule is the smallest of all molecules (hydrogen atoms are the smallest of all the elements) and that the heat of friction ignites the tiny molecules as they escape from the container. The small size of the molecule is an additional hazard of hydrogen; special valves and gaskets are required to form a tight seal so the hydrogen molecules cannot pass through.
Hydrogen is generated by the action of several ac ids on certain metals. In any situation where released inorganic acids contact metals and severely corrode them, hydrogen is released from the reaction.
Hydrogen also is generated by acid storage batteries. Any time the battery is charged, hydrogen is generated. No one should ever smoke when working under the hood of a car or when the hood is up and the battery is charging.
Hydrogen also may be released from water under certain conditions. Anytime hydrolysis occurs, hydrogen and oxygen are released from the water. If this occurs under conditions of extreme heat, as would be the case when water is being applied to extinguish a fire in a coal pile, steam explosions will occur. They will be followed by explosions as the fire’s heat rips apart the water molecule, igniting the hydrogen and violently reuniting it with the oxygen that was simultaneously released.
Hydrogen was the fuel used in the German dirigible Hindenburg and many other dirigibles and blimps in the pre-World War II era. American lighter-than-air ships used helium, which does not burn (and which, in fact, is totally inert). The Americans would not share their small supply of helium, so the infinitely more dangerous hydrogen w’as used by other countries. The story is an old one. As the Hindenburg was docking in New Jersey after a transatlantic run, it somehow caught on fire. The hydrogen burned fiercely, and many of the passengers and crew members were killed or seriously burned. There were some survivors. After that tragic accident, which was totally predictable because of the flammability hazards of hydrogen, the future of dirigibles as large passenger-carrying vehicles was doomed. The U.S. Navy’ continued to use blimps and dirigibles, but with helium for lifting power.
If the vapor density of a gas is less than 1 (that of air), the gas will rise; if its vapor density is greater than 1, it will sink in air. If you know the molecular weight of a gas or a liquid that generates vapor, you can calculate the vapor density simply by’ dividing the material’s molecular weight by 29, which is the “average molecular weight” of air. Even though air is a mixture and there are no molecules of air, the weighted average weight of air can be calculated by adding the product of the molecular weight of each gas multiplied by its percentage in air. When hydrogen’s molecular weight (exactly 2.016) is divided by 29, the vapor density of hydrogen works out to be approximately 0.07. Helium’s vapor density is approximately 0.14, or twice the density of hydrogen. Even at twice the vapor density (or one-half the lifting power) of hydrogen, helium is very efficient for lifting air bags, balloons, blimps, and dirigibles; its safety as far as combustion is concerned makes it ideal for these purposes.
Liquid hydrogen is the fuel used in the U.S. space shuttle series, and the oxidizer is liquid oxygen. The explosion of the space shuttle Challenger was caused by flame burning through a gasket from the solid fuel engines and through the liquid fuel/oxidizer compartments. The violent explosion was caused by the ignition of very flammable hydrogen gas in the presence of pure oxygen.
This is a lesson many responders must learn. When flammable gases are ignited, an explosion is the very first event to occur. If the sound of the ignition of natural gas in the burner of a gas kitchen range could be amplified, the sound would be that of a miniature explosion. The larger the volume of gas, the more violent the explosion. Most firefighters expect a fire when a flammable gas is ignited; but when a large volume of gas ignites and explodes, there is a fire only if all the fuel has not been blown away.
Hydrogen is a stable material, but it will react violently in the presence of or when in contact with certain other materials, including the halogens (fluorine, chlorine, bromine, and iodine), lithium, finely powdered metals, and strong oxidizing agents, particularly nitrogen trifluoride and oxygen difluoride.
When hydrogen burns, water is the only combustion product. However, hydrogen burns so hot that if a continuous supply of hydrogen is burning, the resulting liberation of energy would be sufficient to cause the nitrogen in the atmosphere to oxidize, forming the family of toxic gases known as the nitrogen oxides (NOx).
Liquid hydrogen presents at least tw o other hazards in addition to flammability. At its boiling point (to remain a liquid, the gas must remain below its boiling point) of — 423°F, liquid hydrogen is cold enough to cause permanent damage to any tissue contacting the liquid. The — 423°F boiling point is only 36.6°F from absolute zero! If any liquid is released, it will begin to boil instantly, since anything it touches certainly will be warmer than it is. Large quantities of hydrogen gas will be generated, at least for as long as w hatever the liquid hydrogen has contacted gives up its heat. Subsequent generation of hydrogen gas from the liquid will be somewhat slower after that but still will produce dangerous quantities of the gas.
This represents another hazard of the liquid: its tremendous vapor to liquid ratio of 850 to one. This means that one cubic foot of liquid hydrogen will produce 850 cubic feet of gas as it evaporates. Therefore, even small containers of liquid hydrogen will generate large amounts of explosive, flammable gas. The failure of any container holding liquid hydrogen could be disastrous.
NONFIRE RELEASE
Any release of hydrogen is dangerous, but as is true for any hazardousmaterials incident, circumstances determine the severity of the hazard. The form of the materials when released (gas or liquid), the surroundings (inside or outside), the environment (populated or nonpopulated area), the amount of material released, the terrain, the weather, the wind direction, and the temperature—among other factors —all affect the hazards involved and dictate the type of response.
Obviously, if the quantity of hydrogen released is large, or populated areas are threatened w ith damage, the community’s emergency response plan would have to be activated. In this case, it probably would not require the presence of environmental experts, since hydrogen is nonpolluting and mitigation efforts probably would not harm the environment. Of course, if liquid hydrogen is released and the release causes a secondary problem that releases toxic or polluting materials, the expertise of the environmental authorities would be required.
Any container of hydrogen threatened by heat or flame must be kept as cool as possible by the application of water by unmanned appliances placed as far away as possible. The container holding liquid hydrogen (called a Dewar flask) is highly insulated, and water will not cool the interior. Any water that gets near the pressure-relief valve will freeze instantly, and this ice formation then may cause the pressure-relief valve to malfunction. There may be no effective way to protect a Dewar flask from fire, except for preventing the fire from approaching the container. If nothing can be done to stop the fire from approaching, withdrawal is the proper strategy. Eventual failure of the container will be spectacular and highly dangerous.
Firefighters are taught to plug leaks in tanks by driving wooden plugs or some other sparkproof materials into the leaking tank. This procedure might be too dangerous for a compressed hydrogen tank, since the friction of the escaping molecules in even the slightest leak could cause the hydrogen to ignite. No protective suit could protect firefighters caught in the intense heat of hydrogen flames.
The very tiny size of hydrogen molecules, plus their extremely low vapor density, means that any escaping hydrogen will rise and disperse rapidly. Even though the gas will be very cold as it escapes (the gas from boiling hydrogen starts out as low as — 423°F) and its vapor density will be momentarily higher than 0.07 (cold gases are more dense than warm gases), it still will rise in air. In most cases, where life is not immediately threatened, it may be wise to allow the hydrogen gas to escape, dispersing and rising harmlessly away from the release.
If efforts are made to stop the leak and salvage the cargo, experts should always perform such actions. This means that employees of the company that sold the hydrogen would perform the salvage operations; they will be properly educated, trained, and equipped. Firefighters never should be involved in salvage or cleanup operations. Instead, they should secure the area, withdraw, and prevent unauthorized personnel from entering the area.
FIRE SCENARIO
Any hydrogen leaking as a gas may be burning even though the flame may not be visible. In the case of containers of hydrogen in well-lit areas or outside in the sunlight, approach the leak in an extremely cautious manner if fire is suspected and no flame is visible. The approaching responder should carry in front a straw broom or rolled-up newspaper and move it from side to side while watching to see if it breaks into flame. Or, the responder could toss small amounts of sawdust, soil, or other organic material in front while approaching. Any invisible flame will become immediately visible as the new materials contact it.
Carbon dioxide, dry chemicals, foam, water fog, and water spray may be used to fight small hydrogen fires. The extinguishing agent and method of application chosen depend on the ty pe of the release, the size of the release, its proximity’ to populated areas, the weather, the amount of extinguishing agent available, wind conditions, the terrain, and other factors. Do not extinguish flames from leaking gases unless the flow of gas can he stopped immediately after the flame has been extinguished.
In many cases, the proper action may be to withdraw and allow the fire to burn out. If the flame is impinging on the leaking container or on another one containing hydrogen (or many other materials), a BLEVE (boilingliquid, expanding-vapor explosion) might occur. This can happen even in liquid hydrogen containers because the impinging flame will be hot enough to burn through the outer metal skin of the insulating layer.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
1333-74-0
STCC
(Standard Transportation Commodity Code)
4905746
RTECS
(Registry of Toxic Effects of Chemical Substances) MW8900000
UN/NA
(United Nations/North America)
1049, compressed gas 1966, liquid
CHRIS
(Chemical Hazard Response Information System) HXX
DOT
(U.S. Department of Transportation)
2.1, flammable gas
NEPA 704 Rating
0-4-0
IMO
(International Maritime Organization)
2.1, flammable gas
Hydrogen is an extremely dangerous material and should be handled as such. The procedures for fighting fires in leaking hydrogen containers may be different than those used for other gases because of hydrogen’s special properties.
PROTECTIVE CLOTHING AND EQUIPMENT
No special protective clothing is specified for hydrogen, since no toxicity is associated with the gas. However, under special conditions, hydrogen can asphyxiate, simply by displacing oxygen from the air, if respiratory protection is not used. Protecting against the liquefied gas is almost impossible, since any contact with liquid hydrogen will embrittle any substance to the point where it will break and fall apart almost immediately. Ordinary turnout gear, because of its thickness, offers more protection than any encapsulating suit. But once wet with liquid hydrogen, the clothing will disintegrate at the slightest movement. It also will be covered with a rapidly boiling liquid that will be producing great quantities of highly flammable and explosive gas.
SYNONYMS
Hydrogen has no real synonyms but may be listed on shipping or storage documents as the following:
- compressed hydrogen
- hydrogen, compressed
- hydrogen, liquefied
- liquefied hydrogen
- molecular hydrogen
Normal self-contained breathing apparatus offers protection against asphyxia, but all tubes and hoses will become brittle and disintegrate if contacted by liquid hydrogen.
Chemical goggles and face pieces will provide short-term eye protection against splashes of liquid hydrogen; but once contacted by the liquid, the goggles and face piece will become opaque and fall apart.
FIRST AID
No ill effects can be expected from eye or skin contact with hydrogen gas at ambient temperatures. However, gas leaking from a liquid hydrogen container will be extremely cold, and direct contact with this gas can cause frostbite to the skin and possible eye damage. In case of contact with the liquid or extremely cold gas, flush the affected area with tepid water until the skin temperature returns to normal.
Should inhalation interrupt breathing or make it difficult, trained personnel must begin artificial respiration immediately. Keep the victim quiet and warm and summon immediate medical attention.
Ingestion of liquid hydrogen is highly unlikely.
