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Much has been said by LNG proponents regarding the
exceptional safety of their product. They have explained its exceptional
safety based, in part, on its limited flammability ratio (5 to 15 % air
mixture) and that is it lighter than air. They have suggested that
because of these properties, a leak of LNG would most likely diffuse
harmlessly up into the atmosphere. Is there a scientific basis for this
claim? Let’s look.
Natural gas (mostly methane) is a flammable gas and
behaves in ways that are dictated by its chemical composition. Compared
to other flammable gases with which we are more familiar, its most unique
property is its low boiling point (minus 259 degrees F.).
Flammability Limits
Below is a table of flammability limits of various
gases. The values are shown as a percentage volume in air. Below the
range indicated the mixture is too lean to ignite. Above that range the
mixture is too rich to ignite.
|
Gas |
Flammability Limits |
|
Hydrogen |
4.0 - 75.0 |
|
Acetone |
2.6 - 13 |
|
Methane |
5.0 - 15.0 |
|
Ethane |
3.0 - 12.5 |
|
Propane |
2.1 - 9.5 |
|
Butane |
1.9 - 8.5 |
|
Jet fuel
(JP-4) |
1.3 - 8.0 |
|
Gasoline |
1.4 - 7.6 |
These values
can be verified at several web sites including:
http://www.jlab.org/,
http://www.methanol.org/,
and
http://www.delphian.com.
Though the
other gases can ignite at lower concentrations, methane has a broader
range of flammability than competing fuels such as propane or gasoline.
From this standpoint, it is far more likely to occur at a flammable ratio
than propane, gasoline, or jet fuel.
Density
Proponents of
LNG have suggested that a spill of liquid methane would be a harmless
event because it is lighter than air and would it would quickly rise into
the atmosphere. Let’s look at the facts.
The density of
a gas depends on its
chemical
composition and its temperature. If this were not so, hot air balloons
would need wheels. Here is a table of densities for common gases.
|
Gas |
Density
(kg/cu m) |
Specific
Gravity |
|
Air |
129 |
1.00 |
|
Hydrogen |
0.09 |
0.07 |
|
Helium |
0.18 |
0.14 |
|
Methane |
0.72 |
0.56 |
|
Ethane |
1.35 |
1.05 |
|
Propane |
2.01 |
1.56 |
|
Butane |
2.69 |
2.09 |
|
LPG
(average) |
2.35 |
1.82 |
|
Acetylene |
1.17 |
0.91 |
This table
makes methane look pretty good – a little more than half as heavy as air.
You can also see that methane is eight times heavier than hydrogen and
four times heavier than helium.
However, these
values apply only when the gas is at normal temperatures. At lower
temperatures methane gas contracts in volume and becomes more dense. For
example:
|
Temp (Deg.
F.) |
Density |
Specific
Gravity |
|
59 |
0.72 |
0.56 |
|
-260 |
1.926 |
1.50 |
Why does this
matter? Because when methane boils off from LNG, the gas is still at
roughly the same temperature as the liquid – just like when water boils.
This is why the gas coming off a spill of LNG stays close to the ground or
close to the water – it can be 50% heavier than the surrounding air.
The resulting
vapor cloud will travel with the wind until it finds an ignition source.
This is especially important during a large release of LNG, as would occur
with a catastrophic containment failure.
The cold,
growing vapor cloud remains denser that the surrounding air. At its
center, the methane concentration is well above the flammability limit so
ignition can only occur at its perimeter. Within the inner portion of
such a vapor cloud a person would be asphyxiated. If the cloud passes
without ignition, then resuscitation might be possible. If the perimeter
ignites while someone is in the center, then they would probably die.
Eventually the
methane cloud will heat up – absorbing heat from the air as it mixes and
picking up heat from the land or water as it is driven along by the wind.
The distance such a vapor cloud travels and remains flammable depends on
many factors including the size of the spill, the boil off rate, ambient
temperature, wind and wind turbulence, topography, and objects in its
path.
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