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close this bookGATE - 1985/4 - Renewable Energy - Biogas (GTZ GATE, 1985, 56 p.)
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Catchword

Biogas

The fact that usable gases are produced during anaerobic fermentation of organic matter is not a recent discovery.

Biogas was being produced as early as the 1920s in a number of communal sewage farms in central Europe; but the primary consideration was not so much how to obtain additional energy, but rather the problem of rational and hygienic waste disposal.

The first experiments specifically associated with the production and use of biogas were performed at the beginning of the 1950s. Since fossil forms of energy such as natural gas, mineral oil and coal were cheap at that time and the energy supplies of the industrialized nations were centralized, production of biogas was obviously uneconomic. Moreover, the climatic conditions for gas production in the countries in the temperate zone are rather unfavourable, as the necessary bacterial activity only begins at about 15°C.

For about 30 years now, simple biogas plants have also been built in Third-World countries (in particular India and China).

Not until the middle of the 1970s was biogas technology given a new lease of life, due to the sharp rise in energy costs. Since then, a large number of widely differing types of plants have been developed and tested. ranging from electronically controlled and heated large-scale plants to simple small-scale plants for subsistence farmers.

Biogas is a product of the decomposition of organic material by putrefactive bacteria with exclusion of air (anaerobic putrefaction process). The resulting gas is composed of 60-65% methane (CH4) and 30-35% carbon dioxide (CO2). The remainder (less than 2%) is made up of nitrogen, hydrogen, and hydrogen sulphide. The water content in the digester must be at least 50%; the reaction is best at pH values of between 6.8 and 7.6.

The methane content, and thus the calorific value, increase with the duration of the putrefaction process. The longer the fermentable material stays in the digester, the more thoroughly it will be putrefied and the more liquid the sludge will be at the end of production.

Basically, any organic material can be decomposed; inorganic solids are unused ballast materials which are not changed by the process of putrefaction. Pig, cattle and poultry excrement (manure), human faeces, agricultural and abattoir waste are suitable. Purely vegetable matter, such as straw etc., first has to be mechanically chopped.

There are various advantages which favour the introduction and refinement of biogas technology in Third-World countries in particular:

· Biogas is an important renewable source of energy; it can be used decentrally everywhere where organic waste is produced in concentrated form and in appropriate quantities;

· in certain regions, the use of biogas technology can make an important contribution to the protection of natural resources and the environment (e.g., by replacing firewood);

· sludge is a high-grade fertilizer which can to some extent replace expensive mineral fertilizer (in particular nitrogen).

It can now be said of biogas technology that it is "suitable for dissemination", i.e., in future, in the context of Technical Cooperation, more attention will be paid to questions of motivation, training of artisans, and especially the setting-up and promotion of efficient counterpart institutions.

For many people biogas technology still has a negative image ("poor people's technology"); therefore, simple biogas plants must become a symbol of social advancement for "poor people" too. As far as the potential user is concerned, the biogas plant as an investment has to compete with other desirable products and projects. Hence the first question that arises for him is the question of the economic benefit of the plant. So technical adaptation and increasing perfection of biogas plants by the designer are not enough: convincing potential users at the location in question has meanwhile become just as important a task.