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Biogas in Kenya

 Future projects for Biogas

 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 CO2 gas will come out of the water, silently without bubbling. Methane of course does not dissolve in 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.

 
 It may be that the best design of gasholder would be wider than suggested here, to provide a larger surface area to allow the water to absorb CO2 from the biogas.
 How would the the water containing dissolved CO2 be circulated in the growing tunnel? One method might be to use plastic guttering raised above the plants (to prevent rubbish getting into the water). The gas would diffuse through the tunnel. The water could descend by gravity back to the upper level of the gasholder. How fast should the water circulate? That is to be decided by experiment.

Safety
Is there any danger to people working in the tunnel? Probably not but Health and Safety should be consulted.

 Research Projects
Some of this work may have been done already but these are things I would like to know.

1. The quantities of carbon dioxide extractable from a supply of biogas by diffusion.

2. The amount of animal manure needed to produce biogas from a mainly vegetation based digester. This would be of use for the Water Hyacinth Project.)

3. The effect of raised levels of CO2 on crops in a growing tunnel. (There are research results available on this topic, but are they applicable in the various climate types found in Kenya? Perhaps they need to be tested at every contour.)

4. The adjustments necessary for burners and engines to suit various proportions of methane and CO2.

All these might be suitable projects for postgraduate students. However, it is important that the knowledge be applied to the design and manufacture of practical biogas installations. Too often postgraduate research is not applied in the commercial world.

More Information

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Biogas theory

 Home Power magazine

 A British supplier of solar and wind power devices

Refocus: Renewable energy journal

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 Hyacinth Project, Lake Victoria

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