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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. This
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. |
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