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NEET 2023
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What is biomass? Explain the principle and working of a biogas plant using a labelled schematic diagram.

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Biomass is biological material obtained from living or recently living organisms that can be processed into electricity, fuel and heat. Biomass has been used for thousands of years and there are many forms available which are used in a number of ways. For example, in a wood fire the wood that is burning is a biomass fuel. Biomass is also used in building materials, composting and as in Drax’s case in the electricity industry. The materials that Drax use include sustainable wood-based products, forestry residues and residual agricultural products, such as sunflower seed husks and peanut husks.


Biogas is produced as a result of anaerobic fermentation of biomass in the presence of water.

Construction Back to Top

The biogas plant is a brick and cement structure having the following five sections:

  • Mixing tank present above the ground level.
  • Inlet chamber: The mixing tank opens underground into a sloping inlet chamber.
  • Digester: The inlet chamber opens from below into the digester which is a huge tank with a dome like ceiling. The ceiling of the digester has an outlet with a valve for the supply of biogas.
  • Outlet chamber: The digester opens from below into an outlet chamber.
  • Overflow tank: The outlet chamber opens from the top into a small over flow tank.


Fixed Dome type Biogas Plant

Working Back to Top

  • The various forms of biomass are mixed with an equal quantity of water in the mixing tank. This forms the slurry.
  • The slurry is fed into the digester through the inlet chamber.
  • When the digester is partially filled with the slurry, the introduction of slurry is stopped and the plant is left unused for about two months.
  • During these two months, anaerobic bacteria present in the slurry decomposes or ferments the biomass in the presence of water.
  • As a result of anaerobic fermentation, biogas is formed, which starts collecting in the dome of the digester.
  • As more and more biogas starts collecting, the pressure exerted by the biogas forces the spent slurry into the outlet chamber.
  • From the outlet chamber, the spent slurry overflows into the overflow tank.
  • The spent slurry is manually removed from the overflow tank and used as manure for plants.
  • The gas valve connected to a system of pipelines is opened when a supply of biogas is required.
  • To obtain a continuous supply of biogas, a functioning plant can be fed continuously with the prepared slurry.
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Biomass refers to those living and non-living organic materials that can be used as sources of energy in the form of fuel. Some examples of biomass fuels are biogas, wood, crops, and organic garbage. The chemical energy in biomass is released as heat on burning.

Bio-mass refers to the organic fuel obtained from plants and animal wastes like wood, cow-dung, residue after harvesting the crop, vegetable waste and sewage, etc. It has been used as a fuel for a long time. These fuels, however, do not produce much heat on burning and a lot of smoke is given out when they are burnt.


When substance likes cow-dung, vegetable waste etc undergoes anaerobic (in absence of air) decomposition then these complex substances are broken down into simpler substances and gases like methane, Carbon dioxide etc are produced.


The plant has a dome-like structure built with bricks. A slurry of cow-dung and water is made in the mixing tank from where it is fed into the digester. The digester is a sealed chamber in which there is no oxygen. Anaerobic micro-organisms that do not require oxygen decompose or break down complex compounds of the cow-dung slurry. It takes a few days for the decomposition process to be complete and generate gases like methane, carbon dioxide, hydrogen and hydrogen sulphide. The bio-gas is stored in the gas tank above the digester from which they are drawn through pipes for use.

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