TECHNICAL EVALUATION OF DECENTRALIZED HOUSEHOLD BIOGAS DIGESTERS

摘要

In developing countries, many people are forced to invest daily part of their lifetime by collecting firewood to satisfy their cooking, lighting, and heating needs. This is considered as a health and security problem as well as an important determinant of the development of these countries. In highly industrialized countries, new concepts for electric power generation are required, including decentralized approaches, to address the foreseen worldwide increase of electrical power demand. The use of biomass in small local facilities with low power range (decentralized approach) offers an efficient, CO_2-neutral, and environmental friendly production of electricity and heat. The coupling of biogas reactors with fuel flexible micro gas turbines promises a highly efficient use of biomass. In the present work, the suitability and the potential of existing small household digesters for providing sufficient energy a family may need for their daily life (developing countries) or people can make use of in rural areas, e.g. allotment gardens (Germany) was investigated. Global good prospects of this technology were found. An overview of existing small household digesters located mostly in Asia, Africa, and South America was given, referring to parameters identified as necessary for the evaluation of the performance of small fermenters - loading rate, biogas yield, heating value (percentage of methane), and biogas production rate. Two types of fermenters (ACME and Little Green Monster) were identified to be suitable without major modifications, in principle, to ensure a sufficient biogas production even at low temperatures. The potential of transferring the technology of small anaerobic digesters to central European countries was also studied. Considering the (theoretical) biogas yields resulting from the digestion of grass as the major substrate gathered from German allotments, this strategy seems possible. Two types of small scale digesters, flooded and heap digester, were identified as the most suitable ones. The usage of biogas was also addressed; a conversion into electricity was considered as not favorable (Germany) due to the small biogas amounts produced. In addition, biogas mixtures produced were predicted to be burned safe and reliably, in a burner or in a micro gas turbine, according to flame speed calculations applying a validated and reliable chemical-kinetic reaction model. Toxic components present in the biogas are of no influence on the combustion of biogas mixtures, as shown for ammonia contents up to 1000 ppm.