Which of the following two are the major constituent of Bio-gas ?
1. Methane
2. Butane
3. Carbon dioxide
4. Carbon monoxide
Select the correct answer from the code given below :

Question

UnlockIAS · Accepted Answer

Correct — A, 1 and 3, i.e. methane and carbon dioxide. Biogas is what comes off organic matter — cattle dung, sewage, crop residue, food waste — when it is digested by methanogenic micro-organisms in the absence of oxygen (anaerobic digestion). The gas that collects is dominated by methane, typically about 50 to 75 per cent by volume, and almost all of the balance is carbon dioxide, roughly a quarter to a half. Everything else — hydrogen sulphide, water vapour, traces of hydrogen and nitrogen — is present only in small amounts. So the two major constituents are methane and carbon dioxide, and option (a) is the only pair that names both.

Why the other options are wrong — (b) Carbon dioxide is right but butane is not. Butane (C4H10) is a petroleum-refinery product and, along with propane, is the main constituent of LPG — the cylinder gas — not of biogas. Anaerobic digestion of dung does not produce butane in any appreciable quantity.  (c) Carbon monoxide is essentially absent from biogas. CO is a product of incomplete combustion and a major component of the manufactured fuel gases — water gas (CO + H2), producer gas and coal gas — but it is not a product of anaerobic digestion. This option also drops methane, the single largest component of biogas.  (d) Methane is correctly included, but butane is the LPG constituent, not a biogas constituent. This is the most tempting wrong option because butane is the fuel gas most students handle every day, so it feels like it must belong on any list of fuel gases.

Concept: Biogas is produced by anaerobic digestion: in a sealed, oxygen-free digester, bacteria first hydrolyse and acidify organic matter and methanogenic archaea then convert the products into methane and carbon dioxide. The Indian household version is the 'gobar gas' plant — either the KVIC floating-drum design or the fixed-dome design — fed on cattle dung, and the spent slurry that comes out is a nitrogen-rich manure, so the plant yields fuel and fertiliser together.

State PCS 2025 Q7 — Science & Technology PYQ with Answer

State PCS 2025 Q7 — Science & Technology PYQ with Answer