An analysis of bio-digester substrate heating methods: A review

This review paper studied substrate heating methods currently used in Bio-digesters with a view to assess their effectiveness in establishing and maintaining a stable temperature in a bio-digester. Stable temperature is known to enhance biogas productivity and process stability. Four heating approaches, on-vessel; floor heating; in-vessel and ex-vessel, were critically analyzed. The analysis focused on their potential to distribute heat and temperature uniformly in the whole volume of digester slurry, the ultimate effect on the methane content and quantity of the biogas produced and applicability to small scale household digesters. The study revealed that the In-vessel heating approach has the best heat and temperature distribution and that since the microbial activities responsible for methane production are sensitive to temperature fluctuations, has the highest potential to produce high quantity and quality biogas. The heating approach can also be easily applied to household biodigesters. It was also revealed that both insulation and substrate heating are necessary for digester temperature stability. The limiting factor in substrate heating in biodigesters is the heating cost. This can be minimized by using heat recovered from CHP plants and solar radiation, in case of non-electricity generating plants. Water, because of its high specific heat capacity, was recommended as a better medium of heat transfer than air in case of indirect heating. To enhance efficiency in biogas production, even household digesters should be designed with some insulation and heating facility regardless of the intended use of the produced gas.