Design of Scalable Biogas Digester for the Developing World

By: Tiffany Cheng, Thomas DavisDawn Schmidt, Kyle Schroeder, Andrew Wu

BME 2723/16/10

Advisors:Dr. Dave Owens – Owen Graduate School of Management Dr. Paul King – Vanderbilt University School of Engineering

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

What is a BioGas Digester?

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

Overall Design Specifications• Retail price $89 or less (present value)

- Financed through Grameen Bank- 8% housing loans leading to $96 paid in equal

installments over a 12 month period ($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

Biogas Digesters Worldwide

http://www.snisd.org.cn/images/05126b.JPGhttp://www.water-technology.net/projects/reading_sewage/images/Island-Road-2.jpg

Floating Drum Egg Shaped

http://www.saintsfarm.org/photos/biogas_digester_2_large.jpg http://www.inforse.dk/asia/images/M_III_biogas3.jpg

Plastic Bag Fixed Dome

Brainstorming

Portable Small Digester

Mixing in Digester

Heating by Compost

Materials

Improve C:N ratio

Specific Design Criteria

• Cost• Material availability• Efficiency of gas production • Longevity• Ingenuity• Maintenance• Ease of Use• Modular• Production

Pairwise Comparison Matrix

  Cost Material

Efficiency of

gas Longevity Ingenuity MaintenanceEase of

use Modular Production Total

Cost - 1 1 1 1 1 1 1 1 8

Material Availability 0 - 1 1 1 1 0 0 1 5

Efficiency of gas product 0 0 - 1 1 0 0 1 1 4

Longevity 0 0 0 - 1 0 0 1 1 3

Ingenuity 0 0 0 0 - 0 0 0 0 0

Maintenance 0 0 1 1 1 - 0 1 1 5

Ease of Use 0 1 1 1 1 1 - 1 1 7

Modular 0 1 0 0 1 0 0 - 1 3

Production 0 0 0 0 1 0 0 0 - 1

3 Proposed Designs

1.39

m 2.4m

2.4m

0.15

m

Brick and Mortar

PVC Hooks

Brick Plastic

Hybrid

Full-Scale Brick Design• Masonry and cement• Readily available materials• Requires sealant

http://www.journeytoforever.org/biofuel_library/methane_nepal.html

Full-Scale Plastic Design• Plastic• Easier to install• Mass producible

Full-Scale Hybrid DesignTop View Side View

Plastic Cover

Brick Digester Tank

Decision for Choosing Design Criteria

Criteria Brick Plastic Hybrid

Quality Weight Value Total Value Total Value Total

Cost 8 3 24 4 32 3 24

Ease of Use 7 3 21 3 21 3 21

Material Availability 5 5 25 3 15 4 20

Maintenance 5 2 10 4 20 5 25

Efficiency of Gas Product 4 3 12 3 12 3 12

Modular 3 1 3 5 15 3 9

Longevity 3 5 15 4 12 2 6

Production 1 4 4 2 2 3 3

Ingenuity 0 1 0 3 0 3 0

Total 114 129 120

Cost Analysis of 3 Proposed Designs

  Brick Plastic Hybrid

Materials $40.44 $3.95 $81.66

Manufacturing $0 TBD $0

Transportation ~$0 variable ~$0

Installation $6.72 $0.96 $3.84

Sealant variable $0 $0

Total $47.16 + sealant

$4.91 + manufacturing and transportation cost $85.50

Properties of Various Plastics

 High Density Polyethylene

Polyvinyl Chloride

Polypropylene UV Stabilized

Polyethylene Terephthalate

  HDPE PVC PP-UV PET

Durability        

Fresh water Very Good Very Good Very Good Very Good

Weak alkalis Very Good Very Good Very Good Good

UV radiation Good Very Good Good Good

Thermal properties        

Max service temperature (°C) 110 50 56.2 55

Mechanical Properties        

Yield Strength (MPa) 19.3 41.4 23.6 50

Relative Permeability to PVDC (Saran)        

CO2 121 34.5 317 5.28

CH4 [Estimated] 148.23 42.26 388.33 6.47

Price (USD/kg) 2.16 1.52 2.14 1.61

Polyethylene Terephthalate (PET) • Lowest permeability• Highest yield strength

– Tests showed that a 2L PET bottle can withstand at least 6 atm pressure

• Inexpensive material

Next Step: Prototype Testing• Test biogas production in large (5 gallon) PET

bottles• Analyze biogas

– Composition• Dr. Debelak’s gas chromatography

– Quantity of production• Dr. Speece’s Wet Tip Gas Meter

Further Designs and Experiments

• PET 2L Bottles Testing– Determine maximum pressure– Determine fatigue limit– Quantify gas flow generated at given pressures

• Accessory Design and Testing– Sealants– Water traps– PVC pipes

• Estimation of per unit cost of manufacturing at various production levels