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DEVELOPMENT OF AN AFFORDABLE FAMILY-SCALE BIOGAS GENERATOR
Impact/Purpose:
We will design a simple scalable biodigester system that will: a) generate enough biogas to cook a meal per day for a family of 6, b) be constructed of local or scrap materials, c) be flexible in the type of wastes it uses and d) be inexpensive enough to be affordable by the least affluent members of society with no technical training. The digester will be disseminated as a kit containing components not readily available and understandable assembly instructions with information regarding the best waste sources and mixes thereof. The kit will be constructed by local artisans as a cottage industry. Benefits of replacing wood and dung with biogas for cooking include: 1) reduced pressure on local forests; 2) reduced disease caused by pathogens in unused dung and indoor air pollutants from combustion of wood and dung for cooking and 3) increased revenues through cottage industry and increased ecotourism. Local agriculture will benefit from use of the spent slurry as a nutrient-rich soil amendment.
Description:
From laboratory experiments we calculated that our system would have to deliver 262 liters/hr of biogas to cook a meal. Biogas produced by slurries of various wastes was measured with a two liter bench-top digester system designed by the team. Gas volume was measured by displacement of water from a second flask attached to the generator. The results of duplicate batches of the various single wastes and mixtures run for 28 days revealed that cow manure produced the most biogas (7.34 liters), followed by goat manure (6.78 liters), green waste (3.25 liters), food waste (2.98 liters), and paper (2.98 liters). Mixtures of goat and cow manure produced more biogas (11.2 liters) than either waste alone as did mixtures of food and manure (10.3 liters). Interestingly, though paper was least effective at producing biogas alone, mixtures of small amounts of paper with manures produced more biogas than the same amount of manure alone. Spent cow manure slurry was analyzed for nutrients with the following results: B = 2 mg/kg; Fe = 272 mg/kg; P = 270 mg/kg; K = 170 mg/kg Zn = 7.1 mg/kg; pH = 7.1; Total N = 0.04; C:N = 22.8. This suggests that growth tests of the spent slurry are warranted in Phase II.
A prototype biogas burner was designed that will serve as a model for construction of a burner that can be made of scrap or local materials during Phase II. Burner parameters were determined by calculations using Bernoulli's equations augmented by laboratory experiments. It was determined that at our expected delivery pressure of 0.04 psi, the following values were found most effective: Orifice Area = 3.67 mm2; Air Inlet Area = 19.2 mm2; Tubular Mixing Chamber: Length = 10 cm, Diameter = 1.5 cm. Because we plan to examine the use of plastic garbage bags as bladders for digestion and gas storage, we tested five commercially available bags for strain in response to increasing stress and ultimate rupture strength. The results for maximum strain and failure strength are as follows: Ralph's Value Kitchen bags (0.4 mil): 327% elongation, 0.7 kg failure; Hefty white bags (0.9 mil): 570% elongation, 1.2 kg failure; Ralph's Value Black (0.5 mil): 468% elongation, 0.7 kg failure, Rite Aid Waste Basket (0.6 mil): 400% elongation, 0.8 kg failure; and Pro Value Clear (0.55 mil): 464% elongation, 1.55 kg failure. All were able to contain the maximum pressure (1.8 psi) generated by our bench-top generators.