Risks of LNG
by Cliff Goudey, Bailey Island

 

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.