DEVELOPMENT OF EFFICIENT HEATING SYSTEM FOR BIO -GAS DIGESTERS

Abstract

Ever since the energy crisis of seventies, there has been a growing interest in developing alternative non-conventional sources of energy around the world. Bio-gas is one such source. The fact that the bio-gas can be obtained by the digestion of bio-degradable materials under anaerobic conditions is known for a long time. Anaerobic digestion of organic matter not only produces a clean source of energy, but also yields a soil_ conditioner and fertilizer, and has a potential of minimizing the transmission of some diseases. • In a bio-gas system, the gas production rate is the most important design parameter. The factors affecting the gas production rate include temperature, retention time, design of digester and feed materials characteristics; the digester contents temperature stabilization being dominating one. Generally during cold nights the ambient temperature falls to 5°C to 10°C. The average subsoil temperature around the bio-gas plant reaches, 18 to 20°C while slurry temperature may attain 15°C to 17°C. The technology for heating the slurry and reducing the heat losses from a plant suited to the rural areas has been termed high-rate digester technology. Thus the optimization of digester contents temperature level is essential in order to increase gas production rate and improve the system's economics. Considering all these factors-an efficient bio-gas digester has been designed and fabricated of digester volume 0.214 m3 in which a heating coil of 0.314 m2 heat transfer area has been provided in order to maintain both mesophilic (optimum at 35°C) and thermophilic (optimum at 55°C) temperature range. The overall heat transfer coefficient, the heat transfer rate and the time required to heat up the slurry (water) in the digester from a given initial temperature (varying from 25 °C to 34 °C) upto 35 °C are determined experimentally. The effect of the digester temperature, flow rate of the heating medium (hot water) over the overall heat transfer coefficient and the heating time are also investigated experimentally. The results show that the overall heat transfer coefficient increases as the flow rate increases. The heating time is also predicted through the expressions derived and with the help of a software developed for the purpose. It is compared with the experimentally observed value. It is found that there is good matching with the experimental value. (