How does coal form and what are coal ranks?

Coal is the compressed remains of Carboniferous swamp forests that grew about 300 million years ago, at a time when fungi and bacteria had not yet evolved the enzymes needed to break down lignin. Fallen trees sank into stagnant acidic swamp water without rotting, accumulated as thick peat, were buried under hundreds of metres of sand and mud, and were slowly transformed by pressure and heat. Coal rank grades the result by carbon purity: lignite, sub-bituminous, bituminous and anthracite.

Coal's story begins in the Carboniferous, roughly 300 million years ago, in a greenhouse world unlike any other era: vast swamp forests, an atmosphere at up to 35% oxygen that let dragonflies grow to the size of hawks, and wildfires that swept the swamps often enough that their charcoal layers are still found inside coal seams today. The dominant plants were Lepidodendron trees reaching 60 feet and giant ferns. To a chemist the end product is one of Earth's most complex substances: a disordered carbon network that is physically heterogeneous, varying sharply within a single seam, and containing trace amounts of nearly the entire periodic table — mercury, sulphur, arsenic, even radioactive isotopes such as uranium and thorium.

What made coal possible was not only the abundance of giant trees but a biological gap: the planet's fungi and bacteria had not yet evolved the specific enzymes needed to digest lignin, the structural glue of wood. When the trees fell they sank into stagnant, acidic swamp water and did not decay, piling up layer on layer into thick carbon-rich peat, a wet precursor to coal. Over millions of years, as plates moved and seas advanced and retreated, those peat layers were buried under hundreds of metres of sand and mud. The resulting pressure and the Earth's internal heat began coalification: moisture, oxygen and hydrogen were squeezed out, and carbon atoms bonded into ever denser, more complex networks.

The compression ratio of coal formation is roughly 20:1 — every metre of coal seam required about 20 metres of dense prehistoric vegetation. Picture a forest six storeys high pressed into three feet of rock. That compression concentrated the energy of millions of square miles of ancient sunlight into a form that can be transported and burned easily, at an energy density wood or draft animals could never reach, which is why coal became the miracle fuel of the industrial age. Global proved reserves are estimated at about 1.07 trillion tonnes, roughly 139 years at current consumption rates.

Rank determines more than plant efficiency; it determines the chemistry of the air everyone breathes. Bituminous coal is the workhorse industrial rank, and when heated in the absence of air it becomes coke, a pure porous carbon that is indispensable for chemically reducing iron ore in a blast furnace — without it the skyscraper cities of the twentieth century could not have been built. On climate, coal is the most carbon-intensive of the major stationary fuels: about 26.8 tonnes of carbon released per terajoule of energy produced, against 20.0 for oil and 15.3 for natural gas (IEA, World Energy Outlook 2023).

Three hundred million years of sunlight and organic matter, compressed into a stone you can hold in your hand — a physical fact that ends in a few seconds of combustion inside a furnace.

This question is covered in depth in The Full Spectrum Every Energy Source Explained — A Singapore Perspective，第三章 3.1