Coal
Compressed plant matter that became rock — the fuel the industrial revolution ran on, and still the reducing agent that most of the world's steel is made with.
Coal is plant matter that accumulated faster than it could rot, was buried, and was cooked by pressure and heat until most of what was not carbon had been driven out of it. The ranks — peat, lignite, bituminous, anthracite — are stages of that process, and each has more carbon and less water and volatile matter than the last.
Its industrial significance is not only as a fuel. Coke, made by heating coal without air, is what reduces iron oxide to iron in a blast furnace, and there is no established substitute at scale. A world that stops burning coal for electricity still has to decide what to do about steel.
How it forms
Nearly all the world's coal formed in two intervals, and the larger of them is named after it: the Carboniferous, roughly 360 to 300 million years ago, when equatorial swamp forests buried organic matter on a scale never repeated.
Why it was never repeated is genuinely argued about. One long-standing explanation is that lignin-degrading fungi had not yet evolved, so dead wood simply accumulated; another is that the tectonic and climatic conditions — vast subsiding tropical basins — happened to coincide. The evidence has moved back and forth.
Either way the consequence is the same as for banded iron formation: a resource produced by a process that ran once and stopped, and whose geography is therefore fixed by events hundreds of millions of years ago.
Economic significance
Coal made the industrial revolution possible in a specific sense: it removed the ceiling that charcoal imposed. An iron industry running on charcoal is limited by how fast forests grow, and Britain had largely run out of trees before it worked out how to smelt with coke.
It remains around a third of world electricity generation and the largest single source of carbon dioxide emissions from any human activity. The metallurgical use is the harder problem: hydrogen direct reduction can in principle replace coke, and doing so would mean rebuilding the world's steel industry rather than adjusting it.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Carbon — element · the defining constituent, rising from about sixty per cent in lignite to over ninety in anthracite as the other elements are driven off
- Hydrogen — element · in the volatile matter, and what distinguishes a coal that makes gas from one that makes coke
- Sulfur — element · as pyrite and organic sulfur, and the reason coal burning acidified rainfall across industrial Europe and North America
is an input to
- Smelting — process · as coke: heating coal without air drives off the volatiles and leaves the carbon that reduces iron oxide in a blast furnace, which is what replaced charcoal and lifted the ceiling on iron production
- Pyrolysis — process · to make coke, and the coal gas that came off it lit European cities for a century as the by-product
is commonly confused with
- Jet — material · and it is coal — the gem-quality end of lignite. The difference between jet and the lignite in a power station is the burial history of the particular wood, not the chemistry
succeeded
- Charcoal — material · as coke, at Coalbrookdale from 1709. English ironmaking was constrained by the growth rate of woodland rather than by ore or demand, and coke is what uncoupled the two
is an alternative to
- Charcoal — material · as a smelting fuel, and still chosen over coke where the impurities matter — silicon metal, some ferroalloys, and charcoal-fired steelmaking in Brazil
- Peat — material · as a domestic and station fuel where there was no coal — Ireland, the Hebrides, Finland and the Baltic — and it is worse than coal per unit of energy once the emissions from draining the bog are counted
is sourced from
- Peat — material · every coal seam in the world was a peat bog. Bury it, compress it, heat it, and it passes through lignite and bituminous coal to anthracite, losing water and volatiles at each step
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing