Which of the following options does not involve use of carbon in any form for energy production ?
A. Wood
B. Biogas
C. Geothermal source
D. Coal

Question

UnlockIAS · Accepted Answer

Correct — option (3), Only C. Of the four sources listed, three release energy by oxidising carbon-containing fuel, and only one — geothermal energy — does not. Statement A, wood, is a biomass fuel: it is mostly cellulose and lignin, both carbon-based organic compounds, and burning it releases energy by oxidising that carbon to carbon dioxide, exactly like any other combustion fuel. Statement B, biogas, is produced by anaerobic digestion of organic (carbon-containing) waste and is chiefly a mixture of methane (CH4) and carbon dioxide; burning biogas for energy is, again, the combustion of a carbon compound, methane. Statement D, coal, is the most carbon-dense of all — it is essentially fossilised, carbon-rich plant matter, and coal-fired power generation is textbook carbon combustion. Statement C, geothermal energy, works on an entirely different physical principle: it taps heat that already exists inside the Earth — from the residual heat of planetary formation and from the radioactive decay of elements in the Earth's core and mantle — and brings it to the surface as steam or hot water to drive turbines. No fuel is burned and no carbon compound is oxidised to release this energy; the energy is thermal and geological in origin, not chemical. Because A, B and D all involve carbon combustion while C alone does not, the correct set is 'Only C'.

Why the other options are wrong — (a) This selects wood and biogas as the two sources that do not involve carbon, when in fact both are carbon-based combustion fuels — wood is burned cellulose and lignin, and biogas is burned methane, a single-carbon compound. This option also omits the one source that genuinely does not involve carbon, geothermal energy (C), so it gets both the composition of the included sources and the identity of the correct answer wrong.  (b) This correctly includes C (geothermal, which indeed does not involve carbon) but incorrectly pairs it with B, biogas — which is chiefly methane, a carbon compound, and is burned to release energy exactly like any other combustible gas. Because biogas does involve carbon, pairing it with geothermal produces a set that is only half right.  (d) This names coal alone, but coal is precisely the source that most obviously involves carbon — it is a carbon-rich fossil fuel formed from ancient plant matter, and its combustion is the classic example of carbon-based energy production. Selecting only D also misses that A (wood) and B (biogas) are equally carbon-based, and it misses the actual non-carbon source, C (geothermal), entirely.

Concept: Energy sources can be grouped by whether they release energy through the chemical combustion of carbon-containing fuel or through some other physical process. Wood, biogas and coal all belong to the first group: each is, chemically, a store of carbon compounds (cellulose/lignin in wood, methane in biogas, largely elemental and hydrocarbon-bound carbon in coal), and each yields usable energy by being oxidised, releasing carbon dioxide as a combustion product. Geothermal energy belongs to a different category altogether — it is a form of thermal energy harvested directly from the Earth's internal heat, generated by the planet's residual formation heat and by ongoing radioactive decay deep in the crust and mantle. Geothermal plants use naturally occurring or engineered steam and hot water to spin turbines, with no fuel combustion and no carbon compound consumed in the energy-release step itself.

State PCS 2025 Q65 — Science & Technology PYQ with Answer

State PCS 2025 Q65 — Science & Technology PYQ with Answer