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Design of Scalable Biogas Digester for the Developing World

Design of Scalable Biogas Digester for the Developing World. By: Tiffany Cheng, Thomas Davis Dawn Schmidt, Kyle Schroeder, Andrew Wu BME 272 1/21/10 Advisors: Dr. Dave Owens – Owen Graduate School of Management Dr. Paul King – Vanderbilt University School of Engineering. Meet the Rezas.

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Design of Scalable Biogas Digester for the Developing World

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  1. Design of Scalable Biogas Digester for the Developing World By: Tiffany Cheng, Thomas Davis Dawn Schmidt, Kyle Schroeder, Andrew Wu BME 272 1/21/10 Advisors: Dr. Dave Owens – Owen Graduate School of Management Dr. Paul King – Vanderbilt University School of Engineering

  2. Meet the Rezas • 6 members • Two parents, four children • 1 cow (60-65% of families own at least 1 cow in rural Bangladesh) • Make $45 per month • Spend $10 on petroleum fuel per month • Spend 2 weeks per year collecting additional fuel • Interested in neighbor’s biogas digester • To improve their standard of living

  3. What is a BioGas Digester?

  4. Biological Processes

  5. Biogas Production Technology: An Indian Perspective (Nagamani, B. and K. Ramasamy, 1999) On average, a cow in India produces 3.6m3 or 3600 L of biogas per day. This yields approximately 76,280 BTU/animal/day.

  6. Neighbor’s Digester • Cost $200 • Expensive materials • Hard to install • Requires specialist

  7. Overall Design Specifications • Costs $89 or less (present value) -Calculated using Grameen Bank’s simple interest 8% housing loans with $8 monthly payments • Produces 2800 liters of biogas/day • 15,000 Kcal (59,500 BTU) • Cooks for 6 people • Lasts at least 5 years • Easy to install

  8. Biogas Digesters Worldwide • Floating Drum • Egg Shaped • http://www.water-technology.net/projects/reading_sewage/images/Island-Road-2.jpg • http://www.snisd.org.cn/images/05126b.JPG • Fixed Dome • Plastic Bag http://www.saintsfarm.org/photos/biogas_digester_2_large.jpg http://www.inforse.dk/asia/images/M_III_biogas3.jpg

  9. Brainstorming Improve C:N ratio Mixing in Digester Heating by Compost Portable Small Digester Materials

  10. Specific Design Criteria • Cost • Material Availability • Efficiency of gas product • Longevity • Ingenuity • Maintenance • Ease of Use • Assembly • Production

  11. Pairwise Comparison Matrix

  12. Our Full-Scale Designs Brick Design • Masonry and cement • Readily available materials • Requires sealant Plastic Design • Polyethylene • Easier to install • Mass producible http://www.journeytoforever.org/biofuel_library/methane_nepal.html

  13. Hybrid Digester Design Top View Side View Plastic Cover Brick Digester Tank

  14. Decision for Choosing Design Criteria

  15. Hybrid Digester Design: Bottom PVC Hooks 0.15m 2.4m 2.4m 1.39m Brick and Mortar

  16. Black Plastic Cover PVC Elbow PVC Pipe 2.3m Glue Plastic Cover 2.3m

  17. Slurry Exit PV = nRT(Assume T is a constant) Plastic Cover Gas Produced Digester • Some biogas is produced • Plastic Cover is not yet taught • Volume Increased • Pressure remain about constant. • No biogas produce • No pressure difference

  18. Calculation for Required Pressure to Supply Biogas Stove Plastic Cover Gas Produced Hagen-Poiseuille Equation • Qv =  ΔP/R = πD4ΔP/(128μL) • u = 1.71X10-5 Pa s at 30 °C • L ~ 20m • D = 1/2" PVC pipe for flow 200 - 450 L/hr • Qv = 200 - 450 L/hr • ΔP = 29.76 Pa – 66.95 Pa • The above pressure range is needed to achieve flow rates of 200 L/hr and 450 L/hr, respectively. ρgh Digester • Plastic Cover is taught • Volume Increased • Pressure Increased • Height Difference in Slurry Levels.

  19. Next steps for Plastic Covering • Identify ideal plastic covering material • Determine how to deliver relatively constant pressure • Protect digester from rain • Use this added layer to provide solar heating • Prevent damage at digester-plastic interface to improve lifespan of cover

  20. Future Work • Building • Scale down the hybrid design • Order parts for the digester • Testing • Measure amount of gas produced • Dr. Speece’s Wet Tip Gas Meter • Measure composition of produced gas • Dr. Debelak’s Gas Chromatography http://www.wettipgasmeter.com/images/meter_photo_500.jpg

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