Report - Agence canadienne d'évaluation environnementale

Assessing the risks related to the project
(400 ºC). Correspondingly, it is more difficult for LNG to auto-ignite, compared to
flammable petroleum products, as well as other hydrocarbons (butane: 430 ºC,
propane: 468 ºC).
Natural gas can only explode when pressurized inside a confined space, with the
presence of oxygen and a heat source. For natural gas to be flammable, it must be
concentrated in the ambient air within a range between 5 to 15%, on a volume basis,
with the presence of oxygen required for ignition. When the concentration is greater
than 15%, i.e. the upper flammability limit, then oxygen concentration is insufficient for
ignition. Concentration levels below 5% are insufficient to cause flammability, i.e. this
is the lower flammability limit. In the same context, the flammability range for propane
is between 2.2 and 9.5% and between 1.3 and 7.1 for gasoline.
Moreover, in confined spaces and with the absence of a thermal source that could
cause an explosion, natural gas will help decrease the oxygen concentration, resulting
in an asphyxiant environment.
♦ Finding – The Panel notes that the physicochemical properties of liquefied natural gas
make it relatively more difficult to ignite than other much more commonly used and
better-known hydrocarbons.
The thermal properties of liquefied natural gas
Leaks or spills from an LNG tanker tank, land pipeline or storage facilities in an LNG
terminal could result in certain physical phenomena, which are examined in the
following sections.
Spills over water and the forming of a liquid pool
According to the impact study, a spill could occur after an accident with an LNG
tanker. Thus, a collision or grounding may result in a spill with a large surface pool of
LNG. In the event of large spills, the thermal contribution from the air may be
insufficient to ensure LNG evaporation, which would lead to a pool forming on the
water (PR3.3.2, F-2, p. 97 and 98).
When in contact with water, the LNG pool would evaporate quickly, thereby creating a
natural gas cloud that would be dispersed at a rate depending on the speed and
direction of the wind. As the wind would disperse the gas cloud, the cloud would
extend and mix with the air depending on atmospheric conditions. Through thermal
exchanges, the cloud would finally reach a density comparable to air, depending on
the ambient temperature, and could then be dispersed according to prevailing air
116 Rabaska Project – Implementation of an LNG Terminal and Related Infrastructure