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 :
- (a)1 and 3
- (b)2 and 3
- (c)3 and 4
- (d)1 and 2
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.
- (b)2 and 3 — 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)3 and 4 — 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)1 and 2 — 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.
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.
The question is really testing whether you can keep the standard fuel gases apart, and the whole item turns on one distractor: butane. Fix the set once — biogas is methane plus carbon dioxide; LPG is butane plus propane; CNG is essentially methane; water gas is carbon monoxide plus hydrogen; coal gas is hydrogen, methane and carbon monoxide. Note also that the question says 'major constituent', so the small amounts of hydrogen sulphide and water vapour in biogas are irrelevant to the answer.
- Biogas is roughly 50-75% methane by volume, with most of the remainder carbon dioxide and only traces of hydrogen sulphide, water vapour, hydrogen and nitrogen
- LPG is mainly butane and propane; CNG is mainly methane; water gas is carbon monoxide and hydrogen; coal gas is hydrogen, methane and carbon monoxide
- Because carbon dioxide does not burn, raw biogas has a lower calorific value than natural gas; scrubbing out the carbon dioxide and hydrogen sulphide upgrades it to Compressed Bio-Gas (CBG), which is close to CNG in composition
- CBG is promoted through the SATAT initiative (Sustainable Alternative Towards Affordable Transportation, 2018) of the Ministry of Petroleum and Natural Gas, and household/community biogas through the GOBARdhan initiative
- Methane is itself a potent greenhouse gas — far stronger than carbon dioxide over a hundred-year horizon — which is why capturing it from dung and waste, instead of letting it escape, counts as climate mitigation
Learn the block, not the single line — UPPSC and UPSC both build questions by swapping one row's constituents into another row.
- Choosing butane because LPG is the fuel gas one uses at home — butane belongs to LPG, not to biogas
- Assuming carbon monoxide must be present because dung 'burns incompletely' — CO belongs to water gas, producer gas and coal gas, not to the product of anaerobic digestion
- Treating biogas as pure methane and forgetting that carbon dioxide is the second major component, which is exactly why raw biogas burns with a lower calorific value
UPPSC asks it flat — 'the main constituent of X gas is' or 'which two are the major constituents' — while UPSC has historically wrapped the same content in a match list of fuel gases against constituents, or approached methane from the greenhouse-gas and source side.
Match List I (Fuel Gases) with List II (Major Constituents) and select the correct answer using the codes given below: List I (A) CNG (B) Coal gas (C) LPG (D) Water gas List II 1. Carbon monoxide, Hydrogen 2. Butane, Propane 3. Methane, Ethane 4. Hydrogen, Methane, Carbon monoxide Codes:
- (a) A 2 B 1 C 3 D 4
- (b) A 3 B 4 C 2 D 1
- (c) A 2 B 4 C 3 D 1
- (d) A 3 B 1 C 2 D 4
Answer(b) A 3 B 4 C 2 D 1
Exactly the same concept, in match-list form: fuel gases set against their major constituents. It fixes the whole comparison set — CNG as methane and ethane, coal gas as hydrogen/methane/carbon monoxide, LPG as butane and propane, water gas as carbon monoxide and hydrogen — which is the block of knowledge this UPPSC question samples from.
Assertion (A): The main constituent of the Liquefied Petroleum Gas is methane. Reason (R): Methane can be used directly for burning in homes and factories where it can be supplied through pipelines.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is not the correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer(d) A is false but R is true
The same butane-versus-methane confusion tested from the other side: the assertion that methane is the main constituent of LPG is false, because LPG is propane and butane. A candidate who has this stored cannot fall for options (b) or (d) here.
Which one of the following is the main constituent of LPG?
- (a) Hexane
- (b) Pentane
- (c) Butane
- (d) Methane
Answer(c) Butane
Same concept, same exam — the constituent of a common fuel gas, and specifically the butane/LPG pairing that is the trap in this 2025 question.
The gas, which is emitted in the paddy fields and increases the earth's temperature is
- (a) Nitrogen
- (b) Carbon dioxide
- (c) Carbon monoxide
- (d) Methane
Answer(d) Methane
The production side of the same chemistry: methane is generated in waterlogged paddy fields by the same anaerobic decomposition of organic matter that a biogas digester is built to harness.
- practice — not a real PYQ
The gas produced in a biogas plant is chiefly the result of the action of:
- (a)aerobic bacteria in the presence of oxygen
- (b)methanogenic micro-organisms in the absence of oxygen
- (c)yeast fermenting sugars
- (d)photosynthetic algae in sunlight
Answer(b) methanogenic micro-organisms in the absence of oxygen — biogas generation is anaerobic digestion, which is why the digester is kept sealed.
- practice — not a real PYQ
Which one of the following is the main constituent of Compressed Natural Gas (CNG)?
- (a)Butane
- (b)Propane
- (c)Methane
- (d)Carbon monoxide
Answer(c) Methane — CNG is compressed natural gas, which is predominantly methane; butane and propane are the LPG constituents.