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 Scalable – reduce the cost per unit by mass producting

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 For our project, we will be focusing on Bangladesh. The standard of living in Banglaldesh is one of the bottom 25% of the world. The Rezas are a typically family from Bangulash. The family has 6 family members, two parents, 4 children and one cow. The parents make about 45 dollars per month in US dollar, and spend 10 dollars on Petroleum fuel. And they would also spend 2 weeks per year collecting straw and burn them as fuel source. Few months ago, the Rezas noticed that since their neighboring family had installed a biogas digester with the help of NGO. From then on, the Rezas observed that the neighboring family had additional money to purchase new clothing. And they are really interested in having a biogas digester as well to improve their standard of living. So The Rezas talked to the neighbors to learn more about biogas. “With the help of Non-Governmental Organizations, the neighboring family installed a biogas digester. The Rezas noticed that a few months later the neighboring family had additional money to purchase new clothing. The Rezas talked to the neighbors to learn more about biogas.” http://www.1dollaraday.net/Images/Photos/Profiles/ClientProfiles_clip_image002.jpg The family is looking for new opportunities to improve their standard of living. 2

What is a BioGas Digester? The neighboring family explained that the biogas digester converted cow manure into biogas which is used as cooking fuel. Water is mixed with manure to create a slurry, which is then fed into a large sealed tank where bacteria anaerobically digest the organic matter to produce biogas. Biogas is composed of about 65% methane, 25% carbon dioxide and 5% other gasses. With the help of Non-Governmental Organizations, the neighboring family installed a biogas digester. The Rezas noticed that a few months later the neighboring family had additional money to purchase new clothing. The Rezas talked to the neighbors to learn more about biogas. The neighboring family explained that the biogas digester converted cow manure into biogas which is used as cooking fuel. Water is mixed with manure to create a slurry, which is then fed into a large sealed tank where bacteria anaerobically digest the organic matter to produce biogas. Biogas is composed of about 65% methane, 25% carbon dioxide and 5% other gasses. Usually 1kg of manure produces 360 liters of biogas which is almost enough to cook for one person for a day.(1) However the Biogas digester right now are too expensive To do: Cite source Put in picture of digester for steps (water +manure  slurry) Tiffany 1)Unicef Article Saubolle, B., & Bachmann, A. (1983). Fuel Gas from Cowdung. Kathmandu: Sahayogi Press. Previous slide: Converts cow manure into biogas 65% Methane 25% Carbon dioxide 10% Other gases Methane can be used for cooking 1kg manure  360L of biogas almost enough to cook for one person for one day (Saubolle & Bachmann, 1983) 3

Biological Processes

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.

Neighbor’s Digester Cost $200 Expensive materials Hard to install Requires specialist Pros: Requires little maintenance. During the hot months, the neighbor’s two cows produce enough manure to cook for 14 people. Cons: NGO helped pay for a $200 digester It uses expensive materials (concrete) Takes about a week to install Requires specialist for installation.

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 Goal: To Design an Affordable BioGas Digester for the Developing World To do: Cite Unicef 7

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 The first step is to develop an understanding of current approaches Floating Drum More expensive High level of maintenance Short expected lifetime http://www.snisd.org.cn/images/05126b.JPG Fixed Dome Challenging construction Frequent gas leaks Fluctuating gas pressure Gas production not immediately visible  http://www.inforse.dk/asia/images/M_III_biogas3.jpg Egg shaped (industrial) Difficult construction http://www.water-technology.net/projects/reading_sewage/images/Island-Road-2.jpg Plastic Bag Short lifespan http://www.saintsfarm.org/photos/biogas_digester_2_large.jpg Plastic Cylinder To Do: Import pictures and add some cons on why we don’t like some of them. http://www.saintsfarm.org/photos/biogas_digester_2_large.jpg http://www.inforse.dk/asia/images/M_III_biogas3.jpg

Portable Small Digester Brainstorming Improve C:N ratio Mixing in Digester Heating by Compost Portable Small Digester Improve C:N ratio  Cow manure has good enough C:N ratio already. If we were to add more additive, it will effect the pH.(optimum pH=6.8-7.8 )The anaerobic bacteria is really sensitive to the pH. Thus it will require a lot of technical knowledge to maintain the digester in good working condition Optimum 30:1 Mixing Since we are building a small digester, mixing is not as important. However, inserting a stick through the exit or inlet from time to time would be good to break up the slum. Heating by compost  Increase heating wont effect much ? (Graph) Temp= around 35 Portable small digester  prone to error, Transporting. Impossible to mass produce for everyone. Greatest opportunity to improve the cost efficieny… is material choice. Materials

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

Pairwise Comparison Matrix   Cost Material Efficiency of gas Longevity Ingenuity Maintenance Ease of use Assembly Production Total - 1 8 Material Availability 5 Efficiency of gas product 4 3 Ease of Use 7 Pairwise Comparison Matrix 1) determine qualitatively which criteria are more important - i.e. establish a ranking of the criteria, and 2) assign each criterion a quantitative weight so that the qualitative ranking is satisfied.

Our Full-Scale Designs Brick Design Masonry and cement Readily available materials Requires sealant Plastic Design Polyethylene Easier to install Mass producible Mention for the brick design, we would have compost around the structure to warm it. Also put dome above ground so compost can be next to the digesting portion of the digester (maybe redraw) Say: Both Cylindrical, underground, with compost around outside. Mention that we are waiting on feedback from contacts in Bangladesh on pricing for Brick vs Cement. We are also looking into the cost to develop a mold for the plastic model To do: Make drawing(s) http://www.journeytoforever.org/biofuel_library/methane_nepal.html

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

Decision for Choosing Design Criteria Brick Plastic Hybrid Quality Weight Value total Cost 8 3 24 4 32 Ease of Use 7 21 28 Material Availability 5 25 15 20 Maintenance 2 10 Efficiency of Gas Product 12 Assembly 1 9 Longevity 6 Production Ingenuity Total 114 121 135

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

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

PV = nRT (Assume T is a constant) 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

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.

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

Future Work Building Testing 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 http://www.wettipgasmeter.com/images/meter_photo_500.jpg