Design and Fabrication of a Modified Portable Biogas Digester for Renewable Cooking-Gas Production

This research paper focuses on re-engineering design and fabrication of a modified potable biogas digester for the production of biogas as a renewable energy source for domestic use. Digesters used around the world are commonly big in size. There is need to modify and re-engineer an existing biogas digester to fit in to modern day design for better efficiency, portability and safety. Floating drum and Flexible balloon are digesters to be modified as a thin walled pressure vessel with radius-thickness ratio greater than 10. ASME codes and standards were used to carry out the sizing calculations, thickness and pressure calculations for the cylindrical pressure vessel shells and also calculations for the storage tank hemisphere shell. AISI 304 (Chromium-Nickel steel) is used for the vessel shell and the maximum allowable stress is 137 MPa. Weld efficiency (85%), corrosion allowance (0.02mm), of 24 (for digester), of 18 (for collector) and of 20 (storage tank). 17 kg of cow dung and 34 kg of water with temperature of 32oC were the raw materials used for this research to produce a cumulative gas volume of 0.1243 m3 for 30 days. Furthermore, a bike pump is modified and used to increase methane gas pressure from 4.903 kPa to 345 kPa to suit the modern day gas cookers design and storage tank. Unlike other biogas plant, an electronic smoke alarm detector (model: Ei100) is placed 300 mm below the digester top for effective fire protection. Series of tests were performed to ensure that the constructed prototype met the specifications/standards. Such test include, smoke detector test, pressure testing, gas leak test, and a unit test run also confirmed that the aim of research was achieved.