Lime kiln
A lime kiln is a kiln used to produce quicklime by the calcination of limestone (calcium carbonate). The chemical equation for this reaction is:
This reaction takes place at 900°C (at which temperature the partial pressure of CO2 is 1 atmosphere), but a temperature around 1000°C (at which temperature the partial pressure of CO2 is 3.8 atmospheres[1]) is usually used to make the reaction proceed quickly.[2] Excessive temperature is avoided because it produces unreactive, "dead-burned" lime.
Early lime use
Because it is so readily made by heating limestone, lime must have been known from the earliest times, and all the early civilizations used it in building mortars and as a stabilizer in mud renders and floors.[3] Knowledge of its value in agriculture is also ancient, but agricultural use only became widely possible when the use of coal made it cheap[4] in the coalfields in the late 13th century, and an account of agricultural use was given in 1523.[5] The earliest descriptions of limekilns differ little from those used for small-scale manufacture a century ago[citation needed]. Because land transportation of minerals like limestone and coal was difficult in the pre-industrial era, they were distributed by sea, and lime was most often manufactured at small coastal ports[citation needed]. Many preserved kilns are still to be seen on quaysides around the coasts of Britain.
Early kilns
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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 under-burned (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, near 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.[6]
Sets of seven kilns were common. A loading gang and an unloading gang would work the kilns in rotation through the week.
The large kiln at Crindledykes near Haydon Bridge, Northumbria, was one of more than 300 in the county. It was unique to the area in having four draw arches to a single pot. As production was cut back, the two side arches were blocked up, but were restored in 1989 by English Heritage.
The development of the national rail network increasingly made the local small-scale kilns unprofitable, and they gradually died out through the 19th century, replaced by larger industrial plants. At the same time, new uses for lime in the chemical, steel and sugar industries led to large-scale plants. These also saw the development of more efficient kilns.
A 'lazy kiln' was the name given to a lime kiln which only saw intermittent use.[7]
Modern kilns
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Shaft kilns
The theoretical heat (the standard enthalpy) of reaction required to make high-calcium lime is around 3.15 MJ per kg of lime, so the batch kilns were only around 20% efficient. The key to development in efficiency was the invention of continuous kilns, avoiding the wasteful heat-up and cool-down cycles of the batch kilns. The first were simple shaft kilns, similar in construction to blast furnaces. These are counter-current shaft kilns. Modern variants include regenerative and annular kilns. Output is usually in the range 100-500 tonnes per day.
Counter-current shaft kilns
The fuel is injected part-way up the shaft, producing maximum temperature at this point. The fresh feed fed in at the top is first dried then heated to 800°C, where de-carbonation begins, and proceeds progressively faster as the temperature rises. Below the burner, the hot lime transfers heat to, and is cooled by, the combustion air. A mechanical grate withdraws the lime at the bottom. A fan draws the gases through the kiln, and the level in the kiln is kept constant by adding feed through an airlock. As with batch kilns, only large, graded stone can be used, in order to ensure uniform gas-flows through the charge. The degree of burning can be adjusted by changing the rate of withdrawal of lime. Heat consumption as low as 4 MJ/kg is possible.
Regenerative kilns
These typically consist of a pair of shafts, operated alternately. In shaft A, combustion air and fuel are added near the top and pass downward, cross to shaft B and pass upward to exhaust. The direction of flow is reversed periodically (typically 5-10 times per hour). The cycling produces a long zone of constant, relatively low temperature (around 950°C) that is ideal for lime quality.
Annular kilns
These contain a concentric internal cylinder. This gathers pre-heated air from the cooling zone, which is then used to pressurize the middle annular zone of the kiln. Air spreading outward from the pressurized zone causes counter-current flow upwards, and co-current flow downwards. This again produces a long, relatively cool calcining zone.
Rotary kilns
Rotary kilns started to be used for lime manufacture at the start of the 20th century and now account for a large proportion of new installations. The early use of simple rotary kilns had the advantages that a much wider range of limestone size could be used, from fines upwards, and undesirable elements such as sulfur can be removed. On the other hand, fuel consumption was relatively high because of poor heat exchange compared with shaft kilns, leading to excessive heat loss in exhaust gases. Modern installations partially overcome this disadvantage by adding a preheater, which has the same good solids/gas contact as a shaft kiln, but fuel consumption is still somewhat higher. In the design shown, a circle of shafts (typically 8-15) is arranged around the kiln riser duct. Hot limestone is discharged from the shafts in sequence, by the action of a hydraulic "pusher plate". Kilns of 1000 tonnes per day output are typical.
Exhibits
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A lime kiln erected at Dudley, West Midlands (formerly Worcestershire, England, in 1842 survives to this day as part of the Black Country Living Museum which opened in 1976, although the kilns were last used during the 1920s. It is now among the last still existing in a region which was dominated by coalmining and limestone mining for generations until the 1960s.[1]
Gas cleaning
All the above kiln designs produce exhaust gas that carries an appreciable amount of dust. Lime dust is particularly corrosive. Equipment is installed to trap this dust, typically in the form of electrostatic precipitators or bag filters. The dust usually contains a high concentration of elements such as alkali metals, halogens and sulfur.
Carbon dioxide emissions
The lime industry is a significant carbon dioxide emitter. The manufacture of one tonne of calcium oxide involves decomposing calcium carbonate, with the formation of 785 kg of CO2 in some applications, such as when used as mortar; this CO2 is later re-adsorbed as the mortar goes off. Additionally, if the heat supplied to form the lime (3.75 MJ/kg in an efficient kiln) is obtained by burning fossil fuel it will release CO2: in the case of coal fuel 295 kg/t; in the case of natural gas fuel 206 kg/t. The electric power consumption of an efficient plant is around 20 kWh per tonne of lime. This additional input is the equivalent of around 20 kg CO2 per tonne if the electricity is coal-generated. Thus total emission may be around 1 tonne of CO2 for every tonne of lime even in efficient industrial plants, but is typically 1.3 t/t [2]. However, if the source of heat energy used in its manufacture is a fully renewable source, such as solar, wind, hydro or even nuclear, and depending on the ultimate use of the lime there may be no net emission of CO2.[citation needed] Less energy is required in production per weight than portland cement, primarily because a lower temperature is required.
See also
References
- ^ CRC Handbook of Chemistry and Physics, 54th Ed, p F-76
- ^ Parkes, G.D. and Mellor, J.W. (1939). Mellor's Modern Inorganic Chemistry London: Longmans, Green and Co.
- ^ P. C. Hewlett (Ed), (1998). Lea's Chemistry of Cement and Concrete: 4th Ed, Arnold, ISBN 0-340-56589-6, Chapter 1
- ^ Colin Platt, (1978). Medieval England, BCA, ISBN 0-7100-8815-9, pp 116-7
- ^ Sir Anthony Fitzherbert, Boke of Husbandrye, 1523
- ^ Griffith, R. S. Ll. (1971). Annery Kiln, Weare Gifford. Grenville College project. Supervisor Mr. B. D. Hughes.
- ^ Hood, James (1928). Kilmarnock Water and Craufurdland Kilm Glenf Ramb Soc. Annals. 1919 - 1930. P. 126.
- ^ Stoddart, John (1800), Remarks on Local Scenery and Manners in Scotland. Pub. Wiliam Miller, London. Facing P. 212.