Making Coal Gas for Lighting — The Gasworks That Lit the Nineteenth Century

Before electricity, the brightest, most modern light a city could have came piped from a gasworks. Coal gas — a mix of hydrogen, methane, and carbon monoxide cooked out of coal — burned with a clean, steady flame far brighter than any candle or oil lamp. From the 1810s, gas mains snaked under the streets of London and then every industrial city, lighting factories, shops, theatres, and homes.

The process is destructive distillation: coal is roasted in a sealed retort with no air, and instead of letting the volatile gases burn off as waste (as a coke heap does), the gasworks captures them. The raw gas is cooled to drop out the coal tar, washed to remove ammonia, and purified over lime to take out the foul-smelling sulphur. The clean gas is stored in a great floating gasholder and piped out to burners.

What makes a gasworks brilliant is that nothing is wasted: the gas gives light, the coke left in the retort is sold as fuel, the coal tar becomes the feedstock for synthetic dyes and chemicals, and the ammonia liquor becomes fertiliser. One load of coal yields four products — the same destructive distillation that makes coke, run the other way round to harvest the gas.