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DESIGN OF BIOGAS PLANT
Biogas system design or Bio Gas Plant for cooking for a family of six members is considered here. The system design includes the estimation of total gas required, amount of feedstock (or dung), required and the number of animals required to have feedstock of a given amount.
Following assumptions are made for the design:
About 350-450 liters of biogas required per day per person for cooking (in engine about 450 liters/hp/hour of biogas is required)
Average production of dung per animal per day:
- Cow– 10 kg/day
- Bullock – 14 kg/day
- Buffalo – 15 kg/day
Other biomass night soil, sugar cane bagasse, maize straw, etc. can also be used)
- 1000 liters of gas is equivalent to 1 m3 of gas.
- Average gas production from dung is about 40 liters/kg of fresh dung.
- Retention period of dung slurry in the digester is 50 days.
AMOUNT OF GAS REQUIRED PER DAY
I. A number of family members =6 (adult). Considering 400 litre/day/person for cooking, total gas required = 6 × 400 = 2400 litres/day or about 2.4 m3 gas/day.
II. A number of animals required to fulfill daily gas requirement = Amount of gas produced from a kg of fresh dung = 40 liters/kg.
III. The total amount of dung required = Total gas required/Gasper kg of dung
= 2400/40 = 60 kg.
IV. Thus, in order to have 60 kg of dung no. of cows required = in order to have 60 kg of dung no. of cows required = 60/10 = 6 cows.
DESIGN OF DIGESTER AND GAS HOLDER
- In order to make a slurry, water should be added to an equal amount of dung.
- Total mass of slurry – Dung+ Water = 60+60 = 120 kg.
- The volume of slurry per day (specific gravity of slurry is about 1090 kg/m3)= Total mass of slurry/Specific gravity= 120/1090 = 0.11 m3
- Retention period of slurry = 50 days. Total volume of the digester = per day volume of slurry × retention period = 0.11 × 50 = 5.5 m3 ~ 6 m3.
- Dimensions of the digester: Depth to diameter ratio should be between 1 and 1.3
- Gasholder tank dimension: The volume of the gas holder tank should be about 60% of the per day gas volume.
- Various possible costs incurred in the manufacturing of a biogas plant is given below: