Maximum soot is released from

Concept — Soot is the solid, carbon-rich aerosol left behind when a carbon-bearing fuel burns incompletely. It is a heterogeneous material rather than a single substance: its refractory, strongly light-absorbing fraction is what emission inventories isolate and report as black carbon, which is why black-carbon figures are the standard quantitative proxy for how sooty a combustion source is. Soot is born in the oxygen-starved pockets of a flame, where heavy hydrocarbon molecules pyrolyse into solid carbon faster than the oxygen present can burn that carbon away. Two properties of a combustion system therefore govern how much of it is released: how completely the fuel and the air are mixed before ignition begins, and how carbon-rich the fuel molecule is (long-chain or aromatic molecules with a low hydrogen-to-carbon ratio yield the most).

Application — A diesel engine is a compression-ignition engine. Liquid diesel is injected as a fine spray straight into air that compression has already heated past the fuel's auto-ignition temperature, so the fuel begins to burn while it is still mixing with that air. Combustion therefore starts around fuel-rich droplet cores that oxygen has not yet reached, which is exactly the oxygen-starved condition the concept above identifies as soot-forming. Diesel is also a heavy distillate, roughly C10 to C20 That combination is why a poorly maintained diesel truck or bus trails visible black smoke, and why diesel exhaust carries the heaviest soot — black-carbon — load of the four sources offered here, both per unit of fuel energy burnt and in national emission totals. The comparison basis matters, and here it is soot specifically: a large coal boiler can emit more TOTAL particulate mass, but most of that mass is mineral fly ash rather than carbon soot.

Contrast — why the other three sources listed release less soot:

• Petrol vehicles: a spark-ignition engine vaporises petrol and premixes it with air before the spark fires, so the charge is close to homogeneous when combustion begins, and petrol is a lighter distillate of roughly C4 to C12. Its characteristic pollutants are carbon monoxide and unburnt hydrocarbons rather than carbon particulate.

• CNG vehicles: compressed natural gas is over 90 per cent methane, CH4 — a single-carbon molecule with no carbon-to-carbon bonds and the highest hydrogen-to-carbon ratio of any hydrocarbon fuel, so the chemistry that builds soot precursors is essentially absent. It also enters the cylinder as a gas and mixes fully with air before ignition.

• Thermal Power Plants: pulverised-coal boilers are deliberately run with substantial excess air and are fitted with electrostatic precipitators. Their dominant particulate output is mineral fly ash formed from the non-combustible ash content of coal, alongside sulphur dioxide and oxides of nitrogen, rather than unburnt carbon soot.

Cross-check — Policy history points the same way. Delhi's court-mandated 2001-02 conversion of its buses, autorickshaws and taxis away from diesel and on to CNG was undertaken specifically to cut suspended particulate matter, and ambient particulate levels in the city were measured to fall over that period. The black-carbon benefit of that switch is an estimated, engine-level result rather than a before-and-after ambient measurement, so it should not be read as a direct citywide soot measurement — but the direction is unambiguous: a programme designed to cut particulate and soot emissions moved vehicles off diesel, which identifies diesel as the soot-heavy fuel in that comparison.

Note on the key — some previous-year solution compilations for this 2009 paper circulate Thermal Power Plants instead, and no official UGC answer key exists for that session. The emissions evidence is what settles it: a coal boiler runs with excess air and its dominant particulate output is mineral fly ash, whereas soot proper is unburnt carbon, and Indian black-carbon inventories place the transport sector roughly ten times above the power sector.

Hence, among the sources offered, the maximum soot is released from diesel vehicles.