HOW TO DESIGN A BIO GAS PLANT--- SOLVED EXAMPLE

 FLOW CHART

ALL ABOUT DESIGN OF BIO GAS PLANT

The two commonly used types of bio-gas plants are:

a) Floating drum type, and

b) Fixed dome type.

The commonly used model of bio-gas plants are: 

a) Floating drum design:

i) KVIC model, 

ii) Pre-fabricated ferro-cement digester model, and 

iii) Pragati model. 

b) Fixed dome type: 

i) Janta model, and 

ii) Deenbandhu model.

Source: https://archive.org/stream/gov.in.is.9478.1989/is.9478.1989_djvu.txt

Design Parameters taken for  Bio Methanation

·         Feed Substrate Total Solid Concentration(TSC):  8-9 % (For Cow dung)

·         Ratio of Dung to Water: 1:1

·         Bio Gas produced : 0.06 cu mtr / kg dung (Summer 47 degree)

·         0.03 cu mtr / kg dung (winter 8 degree)

·         Temperature : 35 degree centigrade

·         PH – 7-8

·         Retention Time : 30 days (For temp 25-35 Degree Cent)

·         Depth of the plant is between 4 to 6 m according to the size

·         Depth to diameter ratio between 1.0 to 1.3

·         When the digester diameter exceeds 1.6 m, a partition wall is provided in the digester

·         Average gas production from dung may be taken as 40 lit/kg. of fresh dung

·         One Cu. m gas is equivalent to 1000 litres

DESIGN EXAMPLE OF BIO GAS PLANT
http://archive.unu.edu/unupress/unupbooks/80362e/80362E0j.htm
FIG. 3. Chinese Biogas Plant Design
The digester is of standard KIVC design, consisting of a cylindrical underground chamber using 23-cm (9 in.) brick walls and a concrete floor. It has two standard 10-cm (4 in.) cement household pipes for the inlet and outlet. A feed trough, slurry pit, and soaking pit for the digested slurry are provided. Figure 1 shows the details. The only departure from the standard design is provision of a water trough to hold the gas holder (as explained below).
The gas holder consists of a geodesic dome made of wood, to which a vinyl balloon is secured. The balloon is made of heat-sealed vinyl fabric available on the market. The whole assembly sits inside a water trough that serves two purposes: it prevents gas leakage through the water seal if filled with 20 to 30 cm of water, and it helps to anchor the balloon. Hooks around the gas dome also help to secure the structure so that it does not blow off under pressure. The dome struts and hubs were made as shown in figure 2A and B.

Design of Biogas Plant
Number of cows	4
Assuming 1 cow produces	10 kg of dung/day
Amount of dung produced by 4 cows	40 kg
Amount of gas produced by 1 kg of dung	0.05 m�
Amount of gas produced by 40 kg of dung	2 m�
Daily requirement of gas for cooking and lighting
for 1 person	0.5 to 0.6 m�
2 m� of gas per day will provide cooking and lighting for	2/ 0.6 to 2/0.5= 3 or 4 persons

The volume of the fermentation well should be at least 30 times as large as the daily input. Since manure is usually retained in the fermentation well for about six weeks, it is desirable for the well to be about 45 times the volume of the daily input.

Using a 1:1 ratio of cow dung and water:
Daily input of cow dung	40 kg
Daily input of water	40 kg
Total input	80 kg
Volume of the well required
(45 times the daily input)	80 x 45 = 3,600 kg
100 kg of dung and water occupy	1 m�
3,600 kg of dung and water occupy	3.6 m�
Digester tank capacity required	3.6 m�

The gas holder volume should be enough for 60 to 70 per cent of one day's production.

70 % of 2 m� gas	[70 x 2] /100 = 1.4 m�
Digester tank capacity	3.6 m�
Gas holder capacity required	1.4 m�

Size of the Digestion Tank
Assume 1.75 m as the internal diameter of the digestion tank.

The depth required will be	1.5 m
Using a 20 cm thick wall, the external diameter will be	1.75+0.2+0.2m = 2.15 m

Size of the Gas Holder
A hemispherical PVC balloon is used as the gas collecter.

Assuming diameter of the dome to be	1.9 m
Volume of the dome (half sphere)	1.795 m�

Design of Dome to Support the Gas Holder
Type	2 frequency dome,Class I,
	Method I
Diameter of dome	1.95 m
Radius of dome	0.975 m = 38.38 in.
Length of struts (including hubs)
Long struts	radius of dome x 0.618= 23.75 in.
Short struts	radius of dome x 0.5465= 21 in.
Distance from centre of hub to centre of hole at end of strut	2.75 in.
Length from centres of holes at each end of strut to ends of strut	1.5 in.
Actual length of long struts	23.75 in. - (2 x 2.75 in.)
	+ (2 x 1.5 in.) = 21.25 in.
Hole-to-hole distance	18.25 in.
Actual length of short struts	21 in. - (2 x 2.75 in.)+ (2 x 1.5 in.) = 18.5 in.
Hole-to-hole distance	15.5 in.
Number of long struts required	35
Number of short struts required	30
Number of five-element hubs required	6
Number of six-element hubs required	20

How much Biogas can I produce?

The following is a calculator for estimating the amount of biogas your operation can produce. The calculator is a guideline only and should not be used for design purposes.

Choose the biogas production number that applies to your operation...
Example: 600 sow farrow to finish operation, choose Farrow to Finish 

Hogs

Cubic metres biogas per hog per year

Farrow to Finish

720

Farrow to Wean

222

Farrowing

174

Weaner

24

Feeder

78

Dairy

Cubic metres biogas per cow per year

Freestall

860

Multiply the number of animals by biogas production number...
Example: 600 hogs x 720 m³ biogas / hog / yr = 432000 m³ biogas / year

Multiply the result by the numbers below for cogeneration of electricity and heat...

____________ x 1.7 kWh/ m³ biogas = _________ kWh of electricity per year
____________ x 7.7 MJ/ m³ biogas = _________ MJ of heat per year

Multiply the result by the numbers below for heat production using boiler....
____________ x 15 MJ/ m³ biogas = _________ MJ of heat per year