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 This decomposition process releases gases like methane, CO2, hydrogen sulfide

and hydrogen.

 These gases are drawn via pipes which are transmitted to a turbine for the
production of electricity.

Composition of Biogas:

Biogas is mainly composed methane (up to 75%), CO 2 (25%) and traces of other
gases such as nitrogen and hydrogen. Whereas methane is a high value calorific

fuel, carbon dioxide is an inert gas. Biogas is prepared in biogas plants which are of

two types: (i) Fixed Dome Type (ii) Floating Gas Holder Type.

Advantages of Biogas: -

i) A biogas plant, being quite simple, can easily be built in rural areas. A small plant

using dung from 3 to 4 heads of cattle is capable of supplying biogas for 6 hours

daily for cooking purposes.

ii) Biogas is a clean fuel that burns without smoke and leaves no ash.

iii) The main constituent of biogas, i.e., ethane has a higher calorific value (55kj/g)

that of petrol (50kj/g).

iv) The spent slurry, being rich in nitrogen and phosphorus, is good manure.

v) By using biogas, firewood is saved, and deforestation is reduced.

Geothermal energy

 There is a huge amount of heat trapped inside the earth. Molten rocks from

Earth’s core sometimes come up due to geological changes and get trapped in

hotspots. Harnessing this heat energy is called geothermal energy.
 Any underground water present gets heated due to the hotspots and gets

converted to steam which escapes from the surface of the earth as hot springs.