|GATE - 1/82 - Appropriate Technology - by whom? for whom? and how? (GTZ GATE, 1982, 36 p.)|
|GATE seconding expert team on biogas support for interested partner countries in the Third World|
Biogas is a mixture of gases: methane (CH4, c. 60-65%), carbondioxide (CO2, c. 30-35%) and small amounts (less than 2%) of nitrogen (N2), hydrogen (H2) and hydrogen sulphide (H2S). The higher the methane content of the gas, the higher its calorific value.
Biogas is formed by the anaerobic decomposition of organic materials (proteins, carbohydrates, fats, etc.). It is a bacterial process of putrefaction which takes place sealed from the oxygen of the air. Decomposition must take place in a gaslight, airtight container; the rotting material must have a high water content (over 50%) and a pH value of between 6.8 and 7.6. Maximum output of gas is achieved if a temperature of about 30° to 37° is maintained.
In principle, any organic matter can be used. Suitable materials occurring in agriculture are: Pig manure (semiliquid), poultry manure, straw, agro-industrial residues such as coffee pulp, sisal residues, etc. Residues from slaughter-houses and sludge from sewerage works may also be used.
The use of biogas plants has four advantages of roughly equal weight:
1. The production of biogas as a source of energy (with a calorific value of c. 5100 kcal/Nm³);
2. the production of valuable natural fertiliser (compared with stable manure compost, the nitrogen content may be as much as five times higher); A biogas plant containing a large fermentation room and gas container in Latin America.
3. the treatment of manure in a biogas plant helps to improve sanitary conditions (killing pathogenic germs, reduction of pests);
4. from the environmental point of view, a biogas plant offers substantial advantages, for example, in eliminating undesirable smells. Provided decomposition was complete, a practically odourless sludge can be raked out.
The technique of producing
biogas has been known for decades. It was used principally in industrial countries for the purpose of sewage disposal in large communal plants (energy production normally played only a secondary role).
At the beginning of the fifties, a series of experiments was
on the use of biogas plants in other fields, including agriculture. However, at this stage, they were found to be not economical given the extremely low cost of oil. Consequently, with very few exceptions, operations were discontinued. Interestingly enough, about the same time, the first attempt to introduce biotechnology in China on a large scale failed.
Since the mid-seventies, with energy prices rising, biogas technology has again become the object of attention. In the meantime, many different types of plant have been developed which are currently being put through rigorous testing. A wide spectrum of technologies is involved, from electronically operated plants to the very simplest kind. The size of the plants varies from a few cubic metres to several thousand cubic metres of decomposition capacity. The price of a plant varies from about 1000 DM for the smallest plant, to several million DM for the large plants.