The development of biogas technology in India is of great importance considering the growing shortage of traditional sources of fuel like wood, oil, coal etc. Biogas is a renewable source of fuel energy, and uses organic wastes for generation, and thus helps preservation of forest wealth and reduction in indiscriminate felling of trees. Biogas is a non-polluting and smokeless source for cooking in villages.

It is estimated that about 980 million tonnes of cattle dung is produced in the country annually. This is sufficient to produce over 41,000 million cubic metres of biogas per annum. This is equivalent to 25,848 million litres of kerosene, 25,536 million litres of diesel, 29,500 million litres of petrol and 196 MW of electric power. There are about 150 million hens and 1,700 million broilers that generate 3.6 million tonnes per annum of waste in India. This plus the agricultural wastes of rice, maize, sugarcane and potatoes can produce more biogas. Thus, there is huge potential to utilise this biomass to produce cooking gas and also generate electricity.

The National Project on Biogas Development (NPBD) installed 24,306 family size biogas plants between 1981 and 1994. The slow progress of installation was mainly due to shortage of trained technical staff, masons and lack of awareness among the people. The Regional Biogas Development & Training Centre (RBDTC) in Ludhiana started functioning in 1994-95 and around 130,000 family size biogas plants have been installed up to March 2012 which covered about 31.50 per cent of total estimated potential.

Biogas is obtained by anaerobic digestion/fermentation of cellulose containing organic materials like cattle dung, poultry droppings, pig excreta, human excreta and crop residues, perishable food products and kitchen wastes etc. The decomposition results in the production of gases like methane (50 to 65%), carbon dioxide (30 to 40%), hydrogen (1 to 5%), nitrogen (1%) and hydrogen sulphide (0.1%) and water vapours (0.1%) at normal temperature and pressure. Of these, carbon dioxide and nitrogen are neither combustible nor support combustion. However, methane is highly combustible and is responsible for burning of biogas into a non-luminous flame. One cubic metre of biogas can help us save 3.50 kg of wood, 0.62 litre of kerosene oil, 0.43 kg of LPG.

Many farmers have adopted dairy farming as an alternative to exclusive crop farming. However, most of the dairy farms fail to keep their surroundings free from obnoxious odour generated by semisolid and liquid organic waste. This attracts flies and rodents and causes health problems for humans and animals. Biogas plant is the best remedy to make good use of the waste coming out of the dairy sheds while also generating fuel and energy to add into profitability.

Design and Construction of Biogas Plant

The Punjab Agricultural University (PAU) has designed a large biogas plant, having gas holder that is fixed and is hemispherical in shape. A hemispherical dome is structurally safe and crack-free because whole of the dome is under compressive force and there is no tensile force in any part. The construction of dome is also easy. A sketch of this biogas plant (PAU Janata Model) is shown in Figure 1. However, the actual dimensions will vary depending upon the capacity of biogas plant in cubic metres as given in Table 1. Accordingly the materials required for construction and the cost of construction will group as indicated in Table 2.

Figure 1. Dimensions of new modified PAU Janata Model biogas plant.

Table 1. Dimensions of new modified PAU Janata Model biogas plants.

Dimensions (feet) Symbol Capacity of biogas plant (m3)
25 50 75 100 125
Diameter of digester D 13.5 15.5 19 20.5 23.5
Inner radius of digester R 6.75 7.75 9.5 10.25 11.75
Depth of digester H 11.5 14.5 14.5 14.5 14
Depth of digester up to smaller portion of outlet chamber H1 5.75 7.25 7.25 7.75 7
Height of smaller portion of outlet chamber H2 9.5 12 13.75 15 15.75
Length of bigger portion of outlet chamber M 14 18 22 25 31
Width of bigger portion of outlet chamber N 10 13 17 19 20
Diameter of mixing tank R 5 8 8 8 8
Height of mixing tank P 2 2 3 3 3

 

Table 2. Materials and cost for construction of different capacities of Janata Model biogas plants.

Material Units Capacity of biogas plant (m3)
25 50 75 100 125
Bricks Number 10,000 13,500 21,000 27,000 32,000
Cement Bags 110 150 190 220 275
Brick ballast m3 2.5 3.25 4.5 5.25 6.5
Stone ballast m3 8 9.5 11.5 13 16
Sand m3 11.5 16 27 32 40
Inlet pipe (PVC) having 300 mm diameter M 6 6 6 6.25 6.5
Steel required for slabs to cover the outlet chamber Kg 200 300 450 550 700
Mixing arrangement for feeding material (lump sum) Rs. 8,000 10,000 20,000 27,000 30,000
Gas pipe line system (lump sum) Rs. 5,000 10,000 18,000 22,000 25,000
Cost of digging and filling the pit (lump sum) Rs. 35,000 35,000 40,000 54,000 70,000
Cost of shuttering required for construction work (lump sum) Rs. 20,000 30,000 40,000 50,000 65,000
Cost of labour to construct the biogas plant (lump sum) Rs. 40,000 45,000 55,000 63,000 75,000

The total area required for a biogas plant of 25m3 will be 1,300 square feet and for a capacity of 125m3 the requirement will be 3,145 square feet (14.50 marla).

 

ECONOMICS OF BIOGAS PLANTS

This has been worked out on the basis of daily production of 280 cubic metres of gas which is possible only on a large size institutional biogas plant where adequate quantity of dung is made available daily and throughout the year.

To work out the economics of biogas plants, it is necessary to consider two aspects—one is the production of cooking gas and the other is generation of electricity.

A. Biogas plant for cooking gas

Capital cost: This can be established with capital cost of Rs. 28 lakhs as shown below:

                                                                                 Amount (Rs.)

  • Cost of civil construction                                                                              12,75,000
  • Cost of tubewell and slurry disposal system                                              6,00,000
  • Cost of gas pipeline & other gas delivery arrangements                          2,25,000
  • Cost of tractor and trolley                                                                              7,00,000

Total cost                                                        28,00,000

The operating cost of biogas plant for cooking gas is given the Table 3.

Table 3. Operating cost of biogas plant for cooking gas.

Annual working costs Use of dung as
Manure (Rs.) Fuel (Rs.)

Civil construction work (25 year life)

Depreciation @ 4%

 51,000 51,000
Interest on capital @ 10% 5,100 5,100
Tubewell and slurry disposal system (10 year life) Depreciation @ 10% 60,000 60,000
Interest on capital @ 10% 6,000 6,000

Gas pipeline and appliances (20 year life)

Depreciation @ 5%

 11,250 11,250
Interest on capital @ 10% 1,125 1,125

Tractor and trolley (20 year life)

Depreciation @ 5%

 35,000 35,000
Interest on capital @ 10% 3,500 3,500
Maintenance (1% of total cost) 28,000 28,000
Cost of 3 labourers/annum @ Rs. 10,000 per person per month 3,60,000 3,60,000
P.O.L. expenses/repair of tractor/trolley 1,00,000 1,00,000
Cost of dung as manure (766T) @ Rs. 125/T 3,19,375 –
Cost of dung (dung cakes) as fuel in terms of LPG equivalent – 8,10,000
Total cost 10,07,900 14,98,525

Assuming that the biogas plant will produce 280 cubic metres of gas per day the annual income has been worked out as under:

Annual income

Income from biogas in terms of LPG

1 m3 of biogas = 0.43 kg of LPG

Total amount of biogas produced during one year = 280 x 365 = 1,02,200 m3

About 20% of biogas deducted from total production (due to operational faults, etc)

Net amount of biogas produced = (1,02,200 x 80)/100 = 81,760 m3

Amount of biogas in terms of LPG = 81,760 x 0.43 = 35,156.8 kg

Cost of 1 kg of LPG = Rs. 30.28

So cost of biogas in terms of LPG = 35,156 kg.8 x Rs. 30.28 = Rs. 10,64,548

Income from manure = Rs. 7,66,000

Total income = Rs. 18,30,548

On the basis of this annual income, the profitability and per cent return on investment will be as under:

Profitability and per cent return on investment

Use of dung as

Income

(Rs. )

Cost

(Rs.)

Annual profit

(Rs)

 % return on investment*
Farm manure 18,30,548 10,07,900 8,22,648 29.38
Fuel 18,30,548 14,98,525 3,32,023 11.85

* Rs. 8,22,648/28,00,000 x 100 = 29.38

Rs. 3,32,023/28,00,000 x 100 = 11.85

Average annual profit = (Rs. 8,22,648 + Rs. 3,22,023)/2 = Rs. 5,77,335

       Payback period of the plant = 28,00,000/5,77,335.5 = 4.84 years = 5 years (say)

B. Biogas plant for electricity generation

                                                                     Amount (Rs)

  • Cost of civil construction 12,75,000
  • Cost of gas holder, tubewell and slurry disposal system                                   6,00,000
  • Cost of gas pipeline & other gas delivery arrangements                                    2,25,000
  • Cost of tractor and trolly                                                                                           7,00,000
  • T&P construction cost and cost of room generator                                             7,00,000
  • Cost of 100% biogas based engine genset & accessories                                    2,00,000

Total cost                                                          37,00,000

The operating cost of biogas plant for generation of electricity is given in Table 4.

Table 4. Operating cost of biogas plant for electricity generation.

Annual working costs

Use of dung as
Manure (Rs. )

Fuel (Rs.)

Civil construction work (25 year life)

Depreciation @ 4%

51,000

51,000
Interest on capital @ 10%

5,100

5,100
Tubewell and slurry disposal system (10 year life)

60,000

60,000
Depreciation @ 10%

6,000

6,000
Gas pipe line and appliances (20 year life) Depreciation @ 5%

11,250

11,250
Interest on capital @ 10%

1,125

1,125

Tractor trolley (20 year life)

Depreciation @ 5%

35,000

35,000
Interest on capital @10%

3,500

3,500
Maintenance (1% of total cost)

28,000

28,000
T&P construction and generator room (25 year life) Depreciation @ 4%

4,000

4,000
Interest on capital @ 10%

400

400
Cost of 100% biogas based engine genset and accessories (10 year life)

70,000

70,000
Depreciation @ 10%

7,000

7,000
Cost of 3 labourers/annum @ Rs. 10,000 per person per month

3,60,000

3,60,000
P.O.L. expenses/repair of tractor/trolley

1,00,000

1,00,000
Cost of dung as manure (766 tonnes) @ Rs. 125/tonne

3,19,375

–
Cost of dung (dung cakes) as fuel in terms of LPG equivalent

–

8,10,900

Tractor trolley (20 year life)

Depreciation @ 5%

35,000

35,000

Total cost

 10,89,300

15,79,925

Assuming that the biogas plant will produce 280 cubic metres of gas per day, the following factors are considered:

Electricity generation from biogas plant

  • Total gas available = 280 m3/day = 280,000 l/day
  • Taking gas consumption rate = 373 lit/hp/hr
  • Engine is assumed to run for 24 hrs
  • Consumption of biogas in engine

I HP = 746W (output)

Efficiency of engine = 40%

For 1 HP = 746/0.40 = 1865 W = 1.865 kW

1 HP/hr = 1.865 kW hr

1 kW/hr = 3.6 MJ

So, 1 HP/hr = 3.6 MJ x 1.865

1 m3 of biogas = 18 MJ

Therefore for 1.865 x 3.6 MJ of energy biogas consumption

= (1.865 x 3.6)/18 = 0.373 m3 = 373 litres

  • Required HP of engine = 2,80,000/(24 x 373) = 31.27 HP

Take 35 HP engine alternator giving 26 kVA (under full load) capacity of generator

kW = kVA x power factor = 26 x 0.85 = 22.1

Power factor is taken to be 85% because load of generator is not described

  • Yearly generated units = 24 x 365 x 22.1 = 1,93,596 units
  • Utilisation factor = 95% (5% losses are considered)
  • Total generated units = 1,93,596 x 0.95 = 1,83,916 units

The annual income on the basis of above generation has been worked out as under:

Annual income

Income from biogas in terms of electricity generation

Cost of electricity/unit = Rs. 6.00 per unit

Units of electricity generated = 1,83,916 units

Therefore cost of biogas as electricity = 1,83,916 x 6 = Rs. 11,03,496

Income from manure @ Rs. 1,000 per annum = Rs. 7,66,000

Total annual income = Cost of dung as manure + Cost of biogas in terms of electricity = Rs. (11,03,496 + 7,66,000) = Rs. 18,69,496

On the basis of this annual income the profitability and per cent return on investment will be as under:

Profitability and percentage return on investment

Use of dung as

Income

(Rs.)

Cost

(Rs.)

Annual profit

(Rs.)

% return on investment*

Farm year manure

 18,69,496 10,89,300 7,80,196 21.08

Fuel

 18,69,496 15,79,925 2,89,571

7.82

* Rs. 7,80,196/37,00,000 x 100 = 21.08

Rs. 2,89,571/37,00,000 x 100 = 7.82

Average annual profit = Rs. 7,80,196 + 2,89,571)/2 = Rs. 5,34,883

Payback period of the plant = 37,00,000/5,34,883 = 6.917 years = 7 years (say)

Thus it can be concluded that the large capacity institutional/community biogas plants are economically feasible not only for production of cooking gas but also for electricity generation. Further, in the global context their contribution in producing clean energy from non-conventional source is beyond any doubt having impact on clean environment. Therefore, while establishing large size commercial dairy farms, a positive approach to establish large capacity biogas plant may also be adopted. This also applies to those agencies that have huge quantities of biomass available and want to have outlet for its economic disposal. However, this would require location-specific study covering several factors to determine the quantity and quality of dung/biomass that can produce the required amount of biogas on year round basis, to decide the plant size accordingly.