Coal gas

a 19th century technology

Coal gas was used for lighting, heating and cooking. In 1802 William Murdock installed gas burners

at James Watts'(steam engine) factory, near Manchester. In 1812, gas lighting was used in the streets of London. In England objectors to gaslight argued that it was destroying the whaling industry, because demand for whale oil for lighting was declining.

How was coal gas produced?

The coal, inside airtight retorts, was heated by a coke furnace. Air was forced through the coke to produce carbon monoxide which burned around the retorts. The temperature was high enough to drive off all volatiles over a period of hours.

In air cooled condensers, tar and water (with dissolved ammonia) formed. These liquids were pumped to the liquid separator. The gaseous impurities were HCN: 0.1%, H2S: 1.3%, CS2: 0.04%The gas purification section of the plant produced gas suitable for heating and lighting. The gas was scrubbed to remove the last traces of ammonia.

Carbon disulfide was removed by reaction with hydrogen using a nickel catalyst.

CS2 + 2H2 2H2S + C

CS2 + 4H2 2H2S + CH4

Next the gas was passed through iron(III) oxide removing hydrogen sulfide,

Fe2O3 + 3H2S Fe2S3 + 3H2O

and removing hydrogen cyanide (as Prussian blue).

Fe2O3 + 6HCN 2FeC3N3 + 3H2O

Prussian blue, a valuable pigment, was washed out with water.

The remaining material was sold to sulfuric acid plants.

2Fe2S3 + 9O2 2Fe2O3 + 6SO2

The liquid separator produced ammonia solution and coal tar. Nearly all the ammonia was converted into fertilizer (sulfate of ammonia).

2NH3(aq) + H2SO4(aq) (NH4)2SO4(aq)

Benzene, naphthalene and other aromatic organics were distilled from the tar.
The remaining viscous liquid was used for road making.
In the gas holders, the gas was stored till required.
This gas consisted of hydrogen 48%, methane 32%, carbon monoxide 8%, ethylene (ethene) 2%.
Sisley's "The Gasometers* at Clichy" , in the National Gallery of Victoria, Melbourne, gives us an idea of the importance of coal gas 100 years ago.

*gasometer: popular name for gas holder.