Degassing below the surface of the lake
being 25 times less soluble in water than carbon dioxide, its ex-solution should
occur before that of CO2. In fact, if the two gases behave independently of each
other, the first bubbles of methane should appear at a depth of about 120m whereas
those of carbon dioxide should form only at about a depth of 15 metres.
by positioning the separator at this depth, one should obtain pure methane, the
carbon dioxide staying dissolved in the water. This theoretical shortcut is obviously
wrong, since it does not take into account the establishment of a balance between
chemical types : when a bubble forms, all of the dissolved gas spreads in the
bubble until a balance of partial pressures with the water is obtained. The moment
a bubble of methane appears, the instability thus created transforms the other
dissolved elements into gas until the balance of partial pressures is achieved
both inside the bubble and in the liquid.
This important observation is
nevertheless difficult to quantify precisely : the calculation of the behaviour
of diphasic fluid moving in a degassing column is in fact difficult to make, given
the great number of parameters which come into play. It is, however, recognised
that in proceeding with the separation of gas and liquid under pressure, that
is to say submerging the separator at a certain depth, the richness of the methane
in the mixture will be clearly greater than that obtained with a separator on
the surface. The conclusions of the majority of projects during the past thirty
years point in that direction.
On the other hand, from the fact that a large
percentage of the carbon dioxide stays in solution when you operate at depth,
it follows that the volume of gas needing treatment is smaller. Successive washing
operations can thus be recalculated in order to obtain a gas even richer in methane.
Unfortunately there are two negative points to consider.
as the volume of gas resulting from the ex-solution is less, the driving force
of the auto-pumping is equally diminished.
- Secondly, the separation of
liquid and gas at depth increases the quantity of methane remaining in solution
in the water, giving rise to a loss in the total quantity of potentially recuperable
gas flow and criteria for chosing energy
The diminution of gas flow by
volume according to the increase in the depth of the separator is of major importance
in the overall concept of the system. In fact the efficacity of the washing and
its cost in energy depends directly on the volume of gas to be treated. By decreasing
this quantity of gas, the quantity of washing water needed is decreased similarly,
this leading to a gain in the dimensions of the washing column.
the quantities of gas to be washed it seems that a minimum depth of 10 m is necessary.
But the deeper the separation of gas and liquid takes place, the more methane,
which remains in solution, is lost. The depth will thus largely be limited by
this phenomenon of the non-ex-solution of methane. For the loss to be minimised,
it seems that a maximum of 30 m would represent the limit of exploitation.
is interesting to evaluate the variations in energy production of the system according
to the depth of installation of the separator. This criteria clearly shows the
need not to try to work too deeply. In fact a calculation based on gas and liquid
flow allows an estimation of 13% potential energy loss for an increase in depth
of 5 m.
If the extraction and washing of the gas under the lake surface
seems the best solution, it would nevertheless be wise not to exagerate the benefits
of such a method. Losses in extractible methane are such that energy production
is seriously penalised.