Report - Agence canadienne d'évaluation environnementale

Assessing the risks related to the project
turbulence. Generally speaking, an LNG cloud floats, i.e. it is lighter than air.
However, the very cold temperature of the cloud, and the presence of heavier
hydrocarbons, could reduce its initial buoyancy. The cloud could then become
sufficiently diluted close to its source before becoming lighter than air, and thereby not
present any further risks of ignition (ibid., p. 100 and 101).
Flash fires
A cloud of dispersing methane gas becomes flammable if its concentration falls within
the lower and upper flammability limits. Most ignited clouds become ignited at their
perimeter when they meet an ignition source. If a cloud ignites, it undergoes a “back
propagation” phenomenon of flames towards the source, and a flash fire occurs
throughout the entire flammable part of the cloud. The cloud then burns at the upper
flammability limit until the hydrocarbon source is depleted. The flashback will occur
almost always up to the source, and the pool will ignite. However, the cold methane,
heavy with condensed humidity from the water vapour in the atmosphere, is not very
flammable, and experience has shown that several cloud fires go out on their own
(ibid., p. 101 and 102).
Most of LNG fires propagate at a relatively slow speed, of 10 to 20 m/s up to the
source, and burn the flammable substance at a relatively low rate, while emitting a
limited thermal radiation. The greatest portion of thermal energy emitted is absorbed
by the flue gases that would probably be emitted over the cloud. Several flash fires
have broken out after industrial accidents in refineries or chemical product plants.
Their consequences have been properly modeled with respect to the scope of the
lower flammability limit area during the cloud’s spread. In these cases, the fires are an
alarming situation for anyone caught inside the flammable cloud. According to the
proponent, significant impacts outside of the flammability area have rarely been
recorded, an opinion that was shared by the Environment Canada representative
(ibid., p. 101 and 102; DA86.1, p. 52):
A flashback could result in serious consequences for whoever would be in its
path, within the flames, but it would pose few problems through heat flux for
whoever would be in the vicinity.
(Mr. Robert Reiss, DQ75.1)
♦ Finding – The Panel notes that flash fires which can occur in natural gas clouds
present a low risk for public exposure outside of the cloud’s flammability area.
Pool fires on the water
In the wake of a spill, an LNG pool can ignite after a flash fire, burning until it is
depleted. According to the information provided by the proponent, a pool that has
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