If the phenomenon is not controlled, a veritable explosion
of a mixture of gas and water takes place at the surface. This is exactly what
happens when you uncork a bottle of champagne or lemonade : by causing pressure
to fall inside the bottle, bubbles of dissolved gas form suddenly and rise to
the surface, drawing liquid up with them.
It is also, on a vast scale, what
caused the Lake Nyos (Cameroon) catastrophe on the night of 21 August 1986. The
saturation limit was exceeded ; the ex-solution process was triggered off and
spread to the whole lake through a chain reaction. A gas explosion threw a column
of water more than 80 m high into the air. The enormous quantity of carbon dioxide
thus liberated, being denser than air, then 'flowed' into the surrounding valleys
and asphyxiated all forms of life as far as 30 km from the lake. 1800 people thus
died in this catastrophe.
Some years after this disaster, our company proposed
a system designed to rid Lake Nyos of its deadly gas. This was to reproduce, in
a controlled fashion, the phenomenon of ex-solution and in so doing release into
the atmosphere harmless quantities of carbon dioxide gas.
In order to control
the ex-solution we proposed setting up in the lake a piece of equipment which
we called an 'auto-siphon'. This technique consists of placing a polyethylene
tube vertically in the lake. A mechanical pump draws the water up to the top of
the column. The liquid taken from the deep water in the lake (rich in dissolved
gas) rises in the column. Pressure drops and the water reaches the saturation
limit. When this is reached, bubbles begin to form and rise naturally in the column.
New bubbles form, drawing up the liquid. Once the process has been started, the
work of the pump becomes superfluous and it can be stopped. A jet of water spurts
from the top of the column and dissipates inoffensive quantities of carbon dioxide
in the atmosphere. Lakes Nyos and Monoun in Cameroon are thus equipped with degassing
columns. The water jet at Lake Nyos rises 50 m from the lake surface ; that at
Monoun about 10 m.
This degassing programme for Lake Nyos and Lake Monoun
(MNDP : Nyos and Monoun Degassing Program) was entrusted to our company by the
government and continues successfully to this day. Lake Monoun should be gas-free
in 3 years. At Lake Nyos a single column is currently working. The Cameroon government
hasplaced a request with the European Community for five more columns. If this
comes to pass the lake should be completely safe within about 5 years.
degassing programme, which has been spread out over 10 years, has permitted us
to develop a technology and computerised calculation methods in a previously unexplored
domain. It is this scientific knowledge, the computer skills and the technical
tools which we wish to bring to the project of extracting methane from Lake Kivu.
The ~ 8 m-high steady jet flushing out the 65 m long pipe moored vertically in
the middle of lake Monoun, April 1992. (photo: J. Grangeon).
B. The 21 m-high
soda fountain at Lake Nyos, March 1995: an experiment very close to the operational
scale (photo: B. Canet).
C. The February 2001 full strength 50 m-high jet
of Lake Nyos, (Photo: M. Halbwachs). Due to its ca. 0.1 density, the discharged
fluid is harmless and silent, despite the high exit velocity (> 100 km/h).
The 8 m-high jet at Lake Monoun, February 2003 (photo: S. Chikhi).