Energy and Environment I: The Sun and its Spectrum, the Fossil Fuels — Coal, Petroleum and Natural Gas — with Shale Oil, Coal Bed Methane and Gas Hydrates, Gross and Net Calorific Value, and Nuclear Fission, Fusion, Fuels and Reactors

Coal forms from plant matter buried in swamps and transformed by heat and pressure over millions of years, a process called coalification. Its rank rises from peat through lignite (brown coal) and sub-bituminous and bituminous coal to anthracite: with rising rank, carbon content increases from about 60% to over 90%, moisture and volatile matter decrease, and calorific value increases. Coal is characterised by proximate analysis (moisture, volatile matter, ash and fixed carbon, determined by heating) and ultimate analysis (the elemental composition, carbon, hydrogen, nitrogen, sulphur and oxygen). Most Indian coal is bituminous to sub-bituminous coal of Gondwana age, from the Damodar, Mahanadi, Son and Godavari valleys, with high ash, commonly 30 to 45%, and low sulphur, usually below 1%; lignite is mined at Neyveli in Tamil Nadu and in Gujarat and Rajasthan. High ash means large volumes of fly ash and a need for particulate control; low sulphur means less sulphur dioxide per tonne than many imported coals.

Petroleum forms from marine plankton buried in fine sediments, where heat in the "oil window" (roughly 60 to 150 °C) converts the organic matter first to kerogen and then to oil and, at higher temperature, to gas; it migrates upward until trapped beneath an impermeable cap rock. Crude oil is refined by fractional distillation into liquefied petroleum gas, petrol, naphtha, kerosene and aviation fuel, diesel, fuel oil, lubricants and bitumen, in order of rising boiling point. Crude with little sulphur is "sweet" and with much sulphur "sour". Natural gas, largely methane, is the cleanest-burning fossil fuel: it emits almost no sulphur dioxide or particulate matter and, per unit of energy, a little over half the carbon dioxide of coal, because much of its energy comes from oxidising hydrogen to water rather than carbon to CO₂. Methane itself is a potent greenhouse gas, so leaks from wells and pipelines erode that advantage.

The calorific value of a fuel is the heat released by burning a unit mass (or volume) completely. When a fuel containing hydrogen burns, water is formed, and any moisture in the fuel is evaporated. The gross calorific value (GCV), or higher heating value, assumes that this water is condensed and its latent heat recovered; it is what a bomb calorimeter measures. The net calorific value (NCV), or lower heating value, assumes that the water leaves as vapour, as it does from most boilers and engines, and so is smaller: NCV = GCV − (mass of water per kg of fuel × latent heat of vaporisation, about 2.44 MJ/kg at 25 °C), where the water comes from both the hydrogen (9 kg of water for each kg of hydrogen, since 2 g of hydrogen gives 18 g of water) and the fuel's own moisture.

A worked case: a fuel with a GCV of 25 MJ/kg contains 6% hydrogen and 10% moisture. Water formed per kg = 9 × 0.06 + 0.10 = 0.64 kg, so NCV = 25 − 0.64 × 2.44 = 25 − 1.56 ≈ 23.4 MJ/kg. The gap is largest for wet, hydrogen-rich fuels such as fresh biomass and municipal waste, and for natural gas, whose NCV is about 10% below its GCV, and smallest for dry anthracite. The GCV of a fuel can be estimated from its ultimate analysis by Dulong's formula, GCV ≈ 33.8C + 144(H − O/8) + 9.4S MJ/kg, with C, H, O and S as mass fractions; the term (H − O/8) counts only the hydrogen not already combined with the fuel's own oxygen.

In fission, a heavy nucleus such as uranium-235 absorbs a slow (thermal) neutron and splits into two lighter fragments, releasing about 200 MeV and two or three new neutrons; if on average at least one of these causes another fission, a self-sustaining chain reaction results. Fissile nuclides, uranium-235, plutonium-239 and uranium-233, fission with slow neutrons; fertile nuclides, uranium-238 and thorium-232, do not, but absorb neutrons and are converted into fissile plutonium-239 and uranium-233 respectively. Natural uranium is only about 0.7% uranium-235, the rest uranium-238; light-water reactors need it enriched to about 3 to 5%, while heavy-water reactors can use natural uranium. In fusion, light nuclei combine: deuterium and tritium fuse to helium-4 and a neutron, releasing 17.6 MeV. Fusion needs temperatures of the order of 100 million K to overcome electrostatic repulsion, and the international ITER tokamak under construction in France aims to demonstrate net fusion energy; fusion produces no long-lived fission products, but it is not yet a source of power.

The main reactor types differ in moderator and coolant. The pressurised water reactor (PWR), the commonest in the world, uses ordinary water under pressure as both moderator and coolant, with a separate steam generator; the Russian-designed VVER units at Kudankulam are of this kind. The boiling water reactor (BWR) lets the water boil in the core; India's first nuclear power station, Tarapur (1969), began with two BWRs. The pressurised heavy water reactor (PHWR), of the CANDU family, uses heavy water and natural uranium and is the mainstay of India's fleet. Fast breeder reactors have no moderator, are cooled by liquid sodium and produce more fissile material than they consume. The graphite-moderated, water-cooled RBMK was the Chernobyl design. India's three-stage programme, conceived by Homi Bhabha, matches this to its resources, modest uranium but large thorium reserves in the monazite sands of Kerala and Odisha: PHWRs burn natural uranium and produce plutonium; fast breeders use that plutonium and breed more, converting thorium to uranium-233; and a third stage is to run on thorium and uranium-233. Radioactive waste, especially high-level waste from spent fuel, remains the unresolved environmental problem, to be isolated for thousands of years in deep geological repositories.