UPPSC 2022 Prelims GS Paper-I · Question 104 of 150

Correct — C, Methane. Compressed natural gas is nothing more than natural gas squeezed into a cylinder, typically at 200 to 250 bar, so its composition is the composition of natural gas — and natural gas is overwhelmingly methane, CH4, usually quoted at around 95 per cent, with small amounts of ethane, propane and butane and traces of carbon dioxide, nitrogen, hydrogen sulphide and helium. Compression changes the pressure and the volume, not the chemistry. Methane is the first and simplest alkane, one carbon and four hydrogens, and because it is lighter than air a CNG leak rises and disperses instead of pooling at floor level — the standard safety argument for CNG over LPG in enclosed spaces, and the reason Delhi's public transport fleet was converted to CNG under Supreme Court orders from 2001-02.

• (a)Propane — Propane, C3H8, is one of the two gases that make up LPG, the cylinder gas used for cooking. It occurs in natural gas only in small quantities and is separated out at the processing plant; it is not what a CNG cylinder is filled with.
• (b)Butane — The classic confusion, and the option that catches most candidates. Butane, C4H10, is the main constituent of LPG — UPPSC keyed exactly that in 2021. LPG comes from refining petroleum and is liquefied under modest pressure; CNG comes from natural gas and stays a gas under high pressure. Different source, different molecule.
• (d)Ethane — The most defensible wrong answer. Ethane, C2H6, genuinely is present in natural gas and is the second alkane in the series — but only as a minor component. The question asks what CNG is mainly composed of, and that is methane by a wide margin.

The gaseous fuels an exam asks about separate cleanly by source and by molecule. Natural gas is extracted from underground reservoirs and is mainly methane; compressed to high pressure it becomes CNG, and cooled to about minus 162 degrees Celsius it becomes LNG. LPG is a refinery product, a mixture of propane and butane, liquefied under relatively low pressure because those larger molecules condense easily. Biogas is produced by anaerobic digestion of organic waste and is a mixture of methane and carbon dioxide. Coal gas and water gas are older manufactured gases containing hydrogen and carbon monoxide. Fix the molecule to the source and the whole family becomes a single fact rather than four.

Every wrong option here is a real hydrocarbon, so the question is testing one distinction: natural gas versus petroleum gas. If the fuel comes out of a gas field, think methane; if it comes out of a refinery and arrives in a cylinder for the kitchen, think propane and butane. A second cue is physical: methane is lighter than air, so CNG leaks upward, while LPG is heavier than air and settles — which is why LPG leaks are the more dangerous indoors and why CNG was the fuel chosen for buses in a crowded city. Candidates who recall only 'hydrocarbon gas' will hover between ethane and methane; the tie-breaker is that ethane is a minor component of natural gas, never the main one.

• CNG is natural gas compressed to roughly 200-250 bar; its main constituent is methane (CH4), commonly quoted at about 95 per cent of natural gas
• Natural gas also contains small amounts of ethane, propane and butane, plus carbon dioxide, nitrogen, hydrogen sulphide and helium
• LPG, by contrast, is mainly propane and butane — a petroleum-refinery product, liquefied under modest pressure
• Methane is lighter than air, so CNG leaks disperse upward; LPG is heavier than air and pools at floor level
• Delhi's public transport fleet was converted to CNG under Supreme Court orders from 2001-02; city gas distribution and CNG retailing are regulated by the Petroleum and Natural Gas Regulatory Board

• Answering 'butane' because the household cylinder is the more familiar fuel — that cylinder is LPG, not CNG
• Assuming compression changes composition; CNG and piped natural gas are the same gas at different pressures
• Confusing CNG with LNG — LNG is the same gas liquefied by deep cooling, not by compression

UPPSC asks this family as one-line recall — the main constituent of CNG, of LPG, of biogas — and has asked all three in different years, so learn them as a set. UPSC prefers the match-the-list form, pairing four fuel gases with their constituents, or a statement set on CNG policy, hydrogen-enriched CNG and the regulator.

• practice — not a real PYQ

  Compared with LPG, a leak of CNG is generally considered less hazardous indoors because

  • (a)CNG is heavier than air and settles harmlessly
  • (b)CNG is lighter than air and disperses upward
  • (c)CNG does not burn at atmospheric pressure
  • (d)CNG is stored at a lower pressure than LPG

  Answer(b) CNG is lighter than air and disperses upward — methane's low density means a leak rises and dilutes, whereas LPG's propane and butane are heavier than air and pool at floor level.

• practice — not a real PYQ

  City gas distribution, including the retailing of CNG, is regulated in India by

  • (a)Central Pollution Control Board
  • (b)Bureau of Energy Efficiency
  • (c)Petroleum and Natural Gas Regulatory Board
  • (d)Oil and Natural Gas Corporation

  Answer(c) Petroleum and Natural Gas Regulatory Board — the PNGRB regulates refining, transportation, distribution and marketing of petroleum products and natural gas, including city gas distribution networks and CNG retailing.