Early Kilns
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 to 2.5 inch) lumps - fine stone was rejected. Successive dome-shaped layers of coal and limestone 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 coal 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).
The lime production was sometimes at an industrial scale. One example at Annery in North Devon, England, near Great Torrington, was made up of three kilns grouped together in an 'L' shape and was situated beside the Torrington canal and the River Torridge to bring in the limestone and coal, and to transport away the calcined lime in the days before properly metalled roads existed.
Sets of seven kilns were common. A loading gang and an unloading gang would work the kilns in rotation through the week.
A 'lazy kiln' was how kilns were called if they were only used rarely.
Read more about this topic: Lime Kiln
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