Lime Kilns

by Gethin Thomas September. 02, 2020 390 views

[28-365] 2nd September 2020- Lime Kilns were once an important part of the everyday Devon scene, they were scattered around the coast and although most of them could be found near river mouths and estuaries, some were set up inland, near farms. They were used to produce quick-lime through the calcination of limestone (calcium carbonate). From the earliest period onwards, supplies of limestone were carried by sea from Torquay or Plymouth along the south coast and up the estuaries on which numerous lime kilns were built.

Mentions of lime appear in the Devonshire medieval records and Risdon Tristram in his “The Choreographical Description or Survey of the County of Devon” written before 1630 but not published until 1811 says, “Of late a new invention has sprung up and been practised by burning lime and incorporating it for a season with earth and then spread upon arable land hath produced a plentiful increase of all sorts of grain among us where formerly such never grew in all mean’s memory”.

The common feature of early kilns was an egg-cup shaped burning chamber, with an air inlet at the base (the "eye"), constructed of brick. Limestone was crushed (often by hand) to fairly uniform 20–60 mm (1–2 1⁄2 in) lumps – fine stone was rejected. Successive dome-shaped layers of limestone and wood or coal were built up in the kiln on grate bars across the eye. When loading was complete, the kiln was kindled at the bottom, and the fire gradually spread upwards through the charge. When burnt through, the lime was cooled and raked out through the base. Fine ash dropped out and was rejected with the "riddlings".

Only lump stone could be used, because the charge needed to "breathe" during firing. This also limited the size of kilns and explains why kilns were all much the same size. Above a certain diameter, the half-burned charge would be likely to collapse under its own weight, extinguishing the fire. So kilns always made 25–30 tonnes of lime in a batch. Typically the kiln took a day to load, three days to fire, two days to cool and a day to unload, so a one-week turnaround was normal. The degree of burning was controlled by trial and error from batch to batch by varying the amount of fuel used. Because there were large temperature differences between the center of the charge and the material close to the wall, a mixture of underburned (i.e. high loss on ignition), well-burned and dead-burned lime was normally produced. Typical fuel efficiency was low, with 0.5 tonnes or more of coal being used per tonne of finished lime (15 MJ/kg).

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