Evaluation of the performance of two rural biogas units of Indian and Chinese design
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In the context of endeavours to develop biogas technologies appropriate to Egyptian rural areas, Indian and Chinese designs appear to be the foremost candidates. The aim of the present work is to assess the performance of two family-sized prototype units, one Indian and one Chinese version, with particular reference to their potential suitability to prevailing village conditions. The Chinese unit used is 5 m3 in size and is almost an exact duplicate of the original Chinese shallow and wide cylindrical model. The second prototype, on the other hand, is a modified version of the horizontal vertical Indian model. It holds about 7 m3 of slurry. Both units were operated over sufficiently long periods to allow an evaluation under reasonably comparable conditions which fairly closely represented those anticipated in the actual village circumstances. Water-buffalo dung at a solids concentration of 9-11% was used as feedstock. The experimental program encompassed the study of seasonal variations of digester temperature, as well as the effect of the inherent design features of the digesters, operating temperature, retention time and internal flow patterns on the gas production rate. The slurry temperatures obtained in the digesters during one year of operation reflect the effect of continuous seasonal variation of ambient temperature. The temperature curve exhibits a minimum value during February and a maximum during August. The slurry temperature changed over the range from 17 to 29C. The gas production rate increased exponentially with increase in temperature in the range studied. The gas production rates from both the Chinese and Indian-type digesters are almost the same under the same operating conditions. Internal mixing and flow patterns in the digesters were evaluated by a novel tracer technique. A large amount of dead space was noticed in both types. The mean actual retention times obtained using the pulse tracer technique were less than half the theoretical retention time. Two new digester designs are proposed to overcome most of the difficulties and limitations associated with the Chinese and Indian technologies in the rural Egyptian setting. 1981.
