Which one of the following gases is released from the bio-gas plant is used as a fuel gas?

The correct answer is C) Methane. Biogas is produced when methanogenic bacteria break down cattle dung, kitchen waste and crop residue in an oxygen-free digester. Methane makes up roughly 50 to 75 per cent of biogas, with carbon dioxide as the main balance, and it is the component that burns with a clean blue flame and gives the fuel its value.

• Butane, propane and ethane are wrong because they are not products of anaerobic digestion. Butane and propane come mainly from petroleum refining and form LPG. Ethane is a constituent of natural gas and refinery streams. Aspirants often link biogas with LPG or natural gas because all three are sold as cooking fuels, but their sources are different.

The core concept is anaerobic digestion and the composition of biogas. It connects to the energy and environment sections of the syllabus, and in Mains it links to decentralised energy, waste-to-energy and the Gobar-Dhan scheme for converting cattle waste into biogas and bio-CNG.

Trap: choosing a hydrocarbon that you associate with household cooking gas, instead of the gas that the digester actually produces.