CHEMICAL DATA NOTEBOOK SERIES #63: METHANE/NATURAL GAS
HAZARDOUS MATERIALS
Methane is a flammable, nontoxic, colorless, odorless, tasteless gas that can exist as a gas under low or high pressure or as a cryogenic liquid. It is the major ingredient of natural gas, which is a mixture of several gases— the exact composition depends on where in the earth the natural gas is found. Natural gas is the end product of anaerobic (in the absence of oxygen) decomposition of organic materrial and is found in concentrations or deposits by itself, in conjunction with petroleum, and/or in deposits near coal; petroleum and coal also are remains of decomposed organic matter.
Since natural gas ranges in concentration from 70 to 95 percent methane (depending on the deposit), the two materials are very similar in those properties important to emergency responders. Consequently, this articletreats methane and natural gas as essentially the same substance. Although there are some differences, some of which are technically important, they behave so similarly that they will be treated as one substance, even though reference is made to both materials.
Natural gas is used chiefly as a fuel and is the most commonly compressed gas used in the world today. Hie most common container in which natural gas is found is a pipeline, a method of transporting the gas that many people, including firefighters, seem to forget is a container and should be treated as such. It may be under relatively high pressure in the pipeline or at pressures as low as four ounces psi. It also may be liquefied at extremely low temperatures and shipped or stored in giant cryogenic containers called Dewar flasks. Natural gas is a mixture of gases and is not a chemical compound.
Methane, on the other hand, is a pure substance, a chemical compound. Its major use is to manufacture specific chemicals such as acetylene, ammonia, carbon tetrachloride, chloroform, ethanol (ethyl alcohol), methanol (methyl alcohol), methyl chloride, and methylene chloride. Methane and natural gas each are used to produce carbon black—by burning them under specific conditions. Different grades of this pure form of carbon are produced by altering the conditions under which the gas is burned, usually with the minimum amount of oxygen required for combustion.
Both natural gas and methane burn relatively cleanly, since short-chain hydrocarbons are involved. Methane, a pure compound, is the simplest of all the hydrocarbons and burns the cleanest of all the hydrocarbons—that is, at the proper mixture with air, it evolves the least carbon and carbon monoxide of all the common fuels.
PROPERTIES
Methane is a flammable gas with an ignition temperature of 1,000°F and a flammable range of from 5 to 15 percent in air. Since it is a gas, it technically has no flash point, since gases are already in the proper physical state to burn. However, since methane may be liquefied, some references list – 306°F as its flash point, since at this temperature the liquefied gas produces vapors sufficient to form an ignitablc mixture with the air. This means that for any situation and any temperature at which human life exists, methane will be ready to burn. Methane’s vapor density is 0.55, and its molecular weight is 16. Its boiling point is — 259°F, its freezing point is — 296.5°F, and it is slightly soluble in water. Its molecular formula is CH4.
Natural gas is a flammable gas with an ignition temperature of between 900°F and 1,100°F, and its flammable range is from 3.8 to 17.0 percent in air. Liquefied natural gas (LNG) has a specific gravity of 0.45, and its vapor density (at about -263°F) is 0.834. Its boiling point is — 263°F (some references have reported values as low as -289°F), and it is slightly soluble in water. Since natural gas is a mixture, it has no molecular or structural formula.
Methane has few synonyms, among which are fire damp, marsh gas, and methyl hydride. Natural gas also is known as coal damp, marsh gas, and swamp gas. Liquefied natural gas has no synonyms.
HAZARDS
The major hazard of both methane and natural gas is flammability. As far as fire and explosion are concerned, the properties of both gases can be considered the same. Strictly and scientifically speaking, methane and natural gas are two separate and distinct substances, and although their chemical and physical properties are similar, there are differences. In emergency situations, however, they can be treated as the same substance, using the most conservative values for their properties (i.e., a flammable range of from 3-8 to 17.0 percent).
The unfortunate factor about flammable gases is that emergency responders, as well as ordinary citizens, believe that the major hazard of a flammable gas is that it will burn. The truth is that in any release of a flammable gas, fire is the secondary hazard. The real primary hazard is the tremendous explosion that occurs when an accumulation of flammable gas within its flammable range is ignited. Most loss of life and property occurs with this explosion, with more injury, death, and damage done by the ensuing fire. Emergency responders must be aware of the danger of a killer explosion when dealing with a released flammable gas.
Methane’s extremely low vapor density of 0.55 means that it is considerably lighter than air and disperses very quickly. Any indoor release of methane or natural gas results in fairly quick concentration of the gas near the ceiling—the first area to be in the flammable range. This lightness actually is not a hazard but makes the gas a little safer than denser gases, especially those gases that have vapor densities of 1.5 or higher (propane’s vapor density is 1.5, while the vapor density of butane is 2.0). These “dense” gases, once released, tend to “hang together” and flow along low spots in the terrain.
Any gas escaping from a container under pressure will be very cold (Charles’ gas law says that as the pressure of a gas drops, its absolute temperature drops), and its vapor density at lower temperatures is higher than a gas at higher temperatures (cold gases are more dense than warm gases). Methane or natural gas always is lighter than air, but the difference lessens as the gas gets colder. If the escaping gas originates from liquid methane or liquefied natural gas, the gas will be near the boiling point and will not disperse as fast as it would if it were warmer.
Liquefied natural gas and liquefied methane have a few more hazards in addition to their flammability. The first hazard is the extremely low temperature of the liquefied gas. At temperatures near — 259°F (and lower), the liquid poses an extreme hazard to any living thing it contacts. Contact of the cryogenic liquid with human skin causes an immediate probability of severe frostbite and possible irreversible damage to the tissue.
A second hazard is the tremendous liquid-to-gas ratio of the cryogenic liquid. Gases are liquefied for economic reasons, since much more of the gas can be stored in the same volume as a liquid than as a gas. In the case of methane or natural gas, one cubic foot of the liquid produces 635 cubic feet of gas at normal atmospheric pressure. Therefore, even the smallest release of the liquefied gas produces large quantities of flammable gas. The probability of a violent explosion is very high whenever a leak of liquefied flammable gas occurs.
Natural gas is nontoxic. The movie and television portrayals of depressed people killing themselves by turning on the gas in an unlit oven and then being overcome by the gas are pure fiction. It is possible, of course, to become unconscious and die from lack of oxygen if the natural gas displaces enough oxygen in the air to a level below that necessary to sustain life, but it would take a considerable amount of gas to do that, especially since it rises rapidly to the ceiling and becomes concentrated from the ceiling down. Exposures of humans to concentrations as high as 9 percent without injury have been reported. Higher concentrations may cause headaches, but the oxygen must be reduced significantly before unconsciousness and death occur.
Unfortunately, those attempting suicide by this method usually feel no ill effects after a few minutes, often get nervous about their decision, and light a cigarette to calm their nerves.
The resulting explosion usually does the job. Unfortunately, other people also may be killed, especially in an apartment building. If the person doesn’t smoke or even before he/she lias time to light up, an electrical appliance that switches on automatically (like a refrigerator) or some other ignition source may provide the energy required to ignite the gas. The nontoxicity of methane and natural gas does not make them any less dangerous once they reach the flammable range, contact an ignition source, and explode.
Another hazard of methane and natural gas is that they are odorless. If natural gas were delivered to homes and/or factories or other occupancies in its odorless state, many more explosions would occur, as the gas would build up to its lower flammable limit undetected. Therefore, a warning odor must be built into the gas before the gas reaches that lower limit. A chemical compound, belonging to a family of chemicals called mercaptans, is added by the utility company or seller of the gas so that the detection of natural gas can occur at 1 percent concentration in air, well below the lower flammable limit. This chemical is so powerful that only a very small amount of it is needed to produce the common odor of natural gas. This odorant is related to the chemical present in the working end of a skunk, which should be an indication of its power.
Unfortunately, vandals and terrorists have learned of the mercaptan chemical used and have taken great delight in releasing it in small amounts. This, of course, duplicates the odor of a large amount of released natural gas and can cause panic in public places. Emergency responders must react to these situations as though there really is a leak, since they cannot be sure the odor is not caused by a gas leak.
Methane and natural gas are not known as reactive gases. Indeed, these gases are quite stable. However, violent reactions are possible if these gases come in contact with certain chemicals. Methane and natural gas react with all oxidizers, but the reaction is most violent with powerful oxidizers such as bromine pentafluoride, chlorine, chlorine dioxide, chlorine trifluoride, fluorine, iodine heptafluoride, liquid oxygen, and oxygen difluoride. Methane and natural gas never must be stored in the same area where oxidizing agents may be present.
In some incidents, methane may not be leaking from a container but may be leaking from an old landfill where garbage, grass clippings, yard waste, animal bodies, and other organic materials are decomposing. Dangerous buildups of gas may occur undetected, since no powerful odorant will have been added to this gas. Ignition of collections of this gas can cause powerful explosions. Modern sanitary landfills containing the same type of refuse usually do not generate methane, since nothing really decomposes in them.
NONFIRE RELEASE
Accidental releases of methane and natural gas may occur in several ways, depending on the physical state of the released material and the type of container from which it is released. In any event, any release of these gases can produce death and major destruction, and therefore such releases must trigger the local emergency response activities as mandated and coordinated by the local Emergency Response Committee (ERC) in the jurisdiction in which the accidental release occurs.
Natural gas is the most common compressed gas in the world simply because of its use as a fuel. Most students of hazardous materials fail to consider natural gas in its most common container—-the pipeline. Transmission pipelines carrying the gas from its production sites to points of distribution crisscross the United States from the Southwest (the production and collection points) to the populous Midwest and East. Some natural gas is used in the Northeast and Southeast, but fuel and oil and/or LPG (propane and butane, the common liquid petroleum gases) are used as fuels in large quantities in these parts of the country. The point is that large containers of natural gas are buried in and pass through many areas of die country and many fire departments and other emergency responders may not be aware of their presence and/or exact locations. All fire departments must know the exact locations of and types of gas transmission lines in their jurisdictions, and each apparatus must carry special wrenches so that valves can be turned to stop the flow of gas where a leak might occur or to turn off the gas service to a burning building.
In any release of methane or natural gas, approach from upwind, consider wind shifts, and eliminate all possible ignition sources near the leak. Consider evacuation in an area anywhere from one-half mile to two miles around the release, depending on the nature and size of the release. The danger is not toxicity but the possibility of a destructive explosion on ignition of the gas.
Many releases of natural gas occur when construction equipment ruptures a natural gas line. If a hole is being dug and a line is broken and if the escaping gas has not been ignited by the sparks created by the equipment hitting against the pipe, chances are the gas will escape without igniting. However, there is always the possibility of ignition by static discharge—the static electricity created by the rush of gas. Also, if gas is forced through the soil at high pressure, a static discharge may be created and ignition can occur.
The highest gas pressures may be found in transmission lines or in distribution “main” lines. This pressure is stepped down at local distribution points and reduced again before it enters an occupancy where it will be used as a fuel. In the average home utilizing natural gas, the pressure at the appliance may be only one to four ounces psi. If the pressure-reduction device outside the occupancy fails, 3/4to 1-inch-high pilot flames (where they still exist) may become twoto four-foot torches.
In the event of a leak of natural gas in a building, eliminate all ignition sources. This includes not only all open flames but also all electrical appliances that produce a spark on starting or stopping, including telephones and doorbells. If the main electrical source can be interrupted without causing an electrical discharge at any point in the house, this should be done. Ventilation fans used to remove gas from a building must be explosionproof, as must radios, flashlights, and other battery-powered items.
A leak of liquefied natural gas evolves tremendous amounts of natural gas vapors. The instant the liquid contacts the air, gas begins evolving. The speed with which the gas evolves depends on the manner in which the liquid is escaping and what it contacts. Liquids need energy to evaporate, and the liquefied natural gas is so cold that it contains very little energy. Therefore, the liquid must absorb energy from its surroundings to provide enough energy to evaporate, which is why cryogenic gases arccalled “self-insulating”—they don’t possess enough energy to cause much evaporation. If the liquid contacts a metal (which is a good conductor of heat energy), the liquid warms up relatively quickly and boils (a liquid evaporates fastest at its boiling point). If the liquid contacts the ground (a relatively poor conductor of heat energy), the liquid evaporates slowly. If liquid natural gas contacts water, the water freezes almost instantly. In any event, gas generation is not as rapid as with liquefied propane and/or butane, since these gases have considerably higher boiling points (see “Liquefied Petroleum Gas,” Fire Engineering, January 1989). However, one cubicfoot of liquid natural gas produces more than twice as many cubic feet of gas than does a cubic foot of liquefied propane.
Contain liquid natural gas releases by constructing dikes around the spill, creating a containment pond, or digging a containment pit to capture the spilled liquid. Any equipment used in this activity must be sparkproof and explosionproof.
If liquid natural gas reaches a pond, lake, or moving waterway of any kind, the first water touched by the liquefied gas will freeze quickly. Large amounts of liquefied gas will create large chunks of ice. However, water has a great capacity for holding energy, and it will begin to warm up the liquefied gas rather quickly, causing rapid boiling and large amounts of evolved gas. The liquid evaporates much faster in water (if there is enough of it or the water is moving rapidly) than if spilled on land. Notify all downstream users of the water immediately.
If liquefied natural gas enters a sewer, a very dangerous citywide condition can develop. Any water in the sewer will freeze on contact with the liquefied gas, and the liquefied gas will begin to evaporate slowly. Even though the gas is lighter than air, its generation in large quantities will force it through the sewer system, causing a potential for an explosion that could destroy a very large area. Notify all sewage treatment facilities immediately, and eliminate all ignition sources from areas where the gas might leave the sewer.
IDENTIFICATION NUMBERS AND RATINGS
CAS
(Chemical Abstract Services)
Methane-74-82-8
STCC
(Standard Transportation Commodity Code)
Methane—4905755
UN/NA
(United Nations/North America)
Methane—1971, pressurized gas; 1972, liquefied gas Natural gas—1971, pressurized gas; 1972, liquefied gas
CHRIS
(Chemical Hazard Response Information System)
Methane—MTH
DOT
(U.S. Department of Transportation)
Methane—Flammable gas
NFPA 704 Rating Methane—1-4-0
Natural gas—1-4-0, gas; 3-4-1, liquefied gas
IMO
(International Maritime Organization)
Methane—Flammable gas, 2.0
FIRE SCENARIO
If a natural gas or methane container of any type is leaking and the gas is burning, do not extinguish the flame unless the flow of gas can be stopped immediately. The proper way to fight any gas fire is to stop the flow of gas. If the leaking container is isolated from people and property, no system is threatened, and the environment is not being harmed, the best tactic may be to allow the fire to continue to burn until the product is used up or the flow can be stopped safely. The extinguishment of the flame of burning natural gas as it escapes from a broken pipeline can cause the buildup of flammable gas and a possible devastating explosion.
If the container is releasing liquefied natural gas and it has ignited, the same rule applies. The flow of fuel must be stopped before the flame is extinguished, and the remaining released gas should be allowed to burn off if it poses no life danger and does not threaten to cause the leaking container to BLEVE. Since all cryogenic containers are giant insulated bottles, a flame contacting the outer skin may not affect the gas inside over a short period of time. Likewise, water applied to a cryogenic container will not keep the contents cool because of the insulation effect and because the contained liquid is already more than 400°F colder than the water. However, water applied near the leak will freeze almost instantly, and the leak may be plugged. This may or may not be desirable, depending on the amount of pressure rise of the gas within the container. Water applied to any pool of liquefied natural gas will warm up the water rapidly (as the water freezes), causing very rapid evolution of explosive gas.
PROTECTIVE CLOTHING AND EQUIPMENT
No special clothing is required for contact with methane and/or natural gas, but if an emergency responder enters an atmosphere containing either of these gases within its flammable range, protective clothing that will protect him/her from burning gas should be worn. There is no protective clothing to prevent harm from an explosion of either of these gases. Even though both gases are nontoxic, positive-pressure, self-contained breathing apparatus should be worn to provide emergency responders with the proper oxygen level. Where the liquefied gas might be encountered, protective clothing, gloves, shoes, and eye and face protection must be worn to prevent contact with tlte skin. Any article of clothing that comes in contact with the liquefied gas will freeze, stiffen, and shatter at the slightest pressure.
FIRST AID
For inhalation, move the victim to fresh air and keep him/her quiet and warm. Administer artificial respiration if breathing becomes difficult or has stopped. Seek medical attention immediately.
For skin contact with the liquefied gas, remove all contaminated clothing carefully to avoid damage to the skin and wash affected areas gently with large amounts of cool to lukewarm water. Never use hot water where frostbite may have occurred. Immediate medical attention is necessary.
For eye contact, flush the eyes immediately with cool to lukewarm water for at least 15 minutes, occasionally lifting the eyelids. Again, seek medical attention immediately.
