Transcript Bag Biodigester

Bag Biodigester:
Conceptual Design
Engineers Without Borders
University of Minnesota
Leo A. Kucek
13 Nov 2008
Bag Biodigester:
Background
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Developed in 1960s, Taiwan
Long PVC cylinder
Tested in Nepal by GGC at Butwal, 1986
Reqirements:
– PVC bag is easily available
– Pressure inside the digester is increased and
– Welding facilities are easily available
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Background
• Polyethylene (PE) tubular biodigester
technology:
– Cheap and simple
– Small-scale farmers
– Low cost of the installation
– Robust: rural or urban areas, both in low and hilly
lands
http://www.fao.org/WAICENT/FAOINFO/AGRICULT/AGA/AGAP/FRG/Recycle/biodig/manual.htm#Introduction
Bag Biodigester:
Alternatives
• Plug flow digester:
– trench lined with concrete or an impermeable membrane
– Reactor covered with flexible cover gas holder anchored to the ground
– South Africa, 1957.
• Anaerobic filter:
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1950's
dilute and soluble waste water, low solids
Simple, reduces the reactor volume
packing medium: stones, plastic, coral, mussel shells, reeds, etc.
Methane producing bacteria form film on the large surface of the packing
medium
– "fixed film“
• Upflow anaerobic sludge blanket:
– 1980, the Netherlands
– Methane producing bacteria in granules of sludge blanket (floor)
– Feed enters from bottom; biogas is produced during liquid upflow
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Reaction
• Stage 1: Hydrolysis
– Waste: carbohydrates, lipids, proteins and inorganic materials
– Solubilize large molecules into simpler ones (via extracellular bacterial
enzyme)
– Example: cellulose (polymer of glucose)  glucose (via cellulolytic bacteria)
• Stage 2: Acidification
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Monomer (e.g. glucose) from Stage 1 fermented under anaerobic condition
Produce various acids (via acid-forming bacterial enzymes)
C6 broken down  more reduced state (acids)
Products: acetic acid, propionic acid, butyric acid and ethanol
• Stage 3: Methanization:
– Acids produced in Stage 2 converted to methane by methanogenic bacteria
– Several reactions, several by-products
– Desired product: Methane (CH4)
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Biogas
• Biogas Composition:
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– 50 -70 % CH4
– 30 to 40 percent CO2
– Other gases: H2 (5-10), N2 (1-2), H2O (0.3), H2S (Trace)
Ignition temperature: 650 - 750 °C
Odourless, colourless gas
Clear blue flame (similar to that of LPG gas)
20 MJ/m3
60 percent efficiency in a conventional biogas
stove.
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Sizing
• 0.025 m3 biogas/ kg Human Waste
• Assuption: C/N ~ 20 (Sawdust?)
• Design Specification: Heat 5 L of water from 25°C
to 100°C per home (30 homes)
• 5 L / home x 30 homes = 150 L
• H100°C – H25°C = 419.06-104.86 kJ/kg = 314.2 kJ/kg
• Necessary heat = 48,000 kJ / 0.6 = 80 MJ
• Average density: 0.98 kg/L
http://www.thermexcel.com/english/tables/eau_atm.htm
Bag Biodigester:
Sizing
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Necessary heat: 80 MJ
Biogas heat content: 20 MJ/ m3
Biogas produced: 4 m3
Biogas to human dung ratio: 0.025 m3/ kg
Amount of human dung necessary: 160 kg
Bag Biodigester:
Sizing
• Case Study #1:
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10 fattening pigs
Digester volume required: 4 m3 (VLiq)
PE Bags: 80, 125, 200 cm (D)
D = 0.80 m
Cross-Sectional Area = π(D/2)2 = 0.5 m2
On average, total volume is 80% liquid
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VTotal= VLiq + VGas
VLiq = 0.80 Vtotal
VGas = (1-0.80)/0.80(VLiq) = 1 m3
Vtotal = π(D/2)2L = 5m3  L = 10 m
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D
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Sizing
• Case Study #1, (cont.):
– Ditch for biodigester:
W1
– W1 = 90 cm
– W2 = 70 cm
– H = 90 cm
– LD = 10 m
H
LD
W2
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Materials
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Case Study #1 (cont.):
– Bag
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PE: 80 cm ID, 100 m roll, 50 kg / 100 m roll
200-250 μm thickness (calibre) (UV)
– Ceramic tubes (2)
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Ltubes = 75 to 100 cm, Dtubes = 15 cm (ID)
– Plastic (PVC) Hosepipe
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DHosepipe = 12.5mm (ID) (Length is site-specific (to cooking site))
– Others:
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2 PVC adapters (male and female) of 12.5mm internal diameter.
2 rubber washers (from car inner tube) of 7cm diameter and 1mm thickness with a 12.5 mm diameter
central hole.
2 rigid plastic (perspex) washers of 10 cm diameter and a central hole of 12.5mm.
2 m of PVC pipe of 12.5mm internal diameter.
4 used inner tubes (from bicycle, motor cycle or motor car) cut into bands 5 cm wide.
1 transparent plastic bottle.
1 PVC elbow of 12.5mm internal diameter.
3 PVC "T" pieces of 12.5mm internal diameter.
1 tube of PVC cement.
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
Materials
• Case Study #2
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5.5 m of a strong, flexible, plastic sheet (> 2.8 mm width)
4 m x 3" PVC tubing (feed and exit tubes)
2 m3 sand, 1 m3 rock (mix w/ cement for walls)
9 50-kg sacks of cement (walls & floor)
60 cement blocks (12 cm X 20 cm X 40 cm): three rows for pins and
hangers
– 1 x 1/2" PVC tubing (rectangular frame, circumference of 16.6 m)
– PVC tubing to cooking site
– Others (minor):
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Rebar
Tubes, fittings
Rope
Gallon jugs
Hardware
http://www.ruralcostarica.com/biodigester.html
Bag Biodigester
Materials
• Other costs to consider:
– Fencing
– Housing (roof)
– Additional PVC piping
– Maintenance: Joe the Plumber!
Bag Biodigester:
Final Remarks
• Dry human waste: expect 1 month
fermentation until any biogas produced
• Concerns about pressure?
• Case Study #1: Payback in approximately 1
year (price of liquid fuel, no odors from pigs)
• Case Study #2: ~$300 Materials Cost
http://www.fao.org/sd/EGdirect/EGre0022.htm
Bag Biodigester:
The Next Step
• Economic data:
– Materials & labor
– Financial sustainability ($/fuel)
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Effects of changing reactor size
Feed & product composition
Separations?
Safety?