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There are some developments that would be useful if
more biogas is to be used profitably.
1. Carbon dioxide
Biogas contains methane and carbon dioxide. This carbon dioxide
does not add to the proportion in the world's atmosphere - it
does not make the climate problem worse. This is because the
vegetable matter that is being digested in the tanks comes from
plants that made use of existing carbon dioxide in the atmosphere
to build sugars and other carbohydrates in their tissues. Thus
this is a local cycle.
However, it would be useful to separate carbon dioxide from
the methane and use the methane alone. It would then have a similar
quality to natural gas.
Carbon dioxide itself can have a commercial value. However,
probably the apparatus to extract all the carbon dioxide to sell
is not worth installing in small plants, but some of the CO2
can be separated with very simple procedures.
One of these would be to circulate the water from the gasholder
through channels in a greenhouse tunnel. This gas dissolves in
water especially when under pressure. The operating pressure
for biogas machinery of the type I have described here is quite
low. Nevertheless I have shown in Kenya that there is some uptake
of the gas into water at that pressure. If there is an outside
surface of the water exposed to normal atmospheric pressure,
some of the gas will come out of the water.
Some types of plants growing in a greenhouse tunnel benefit
from an increased proportion of carbon dioxide.
It would be easiest to do this if the gasholder is sunk in
the ground with the top of that tank at normal ground level.
A simple pump could extract water from the bottom of the gasholder
tank, send it down channels in the greenhouse tunnel and return
it to the top of the gasholder tank.
This would be worth doing as an experimental procedure to
discover how much of the gas actually comes off, and how much
can be retained in the greenhouse. Another set of experiments
can investigate the effect of the gas on plants. Much of this
research has of course already been done.
The gas remaining in the gasholder should have a higher proportion
of methane in it than comes from the digester. This will have
implications for the design of burners and carburettors for engines.
The less CO2 there is, the faster the flame will propagate.
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