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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) is usually used to make the reaction proceed quickly. Excessive temperature is avoided because it produces unreactive, "dead-burned" lime.
Additional recommended knowledge
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. Knowledge of its value in agriculture is also ancient, but agricultural use only became widely possible when the use of coal made it cheap. in the coalfields in the late 13th century, and an account of agricultural use was given in 1523. The earliest descriptions of limekilns differ little from those used for small-scale manufacture a century ago. 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. Many preserved kilns are still to be seen on quaysides around the coasts of Britain.
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 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 in North Devon, near Torrington, was made up of four kilns grouped together in a square and it was situated beside the Torrington canal to bring in the limestone and transport away the 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. The development of the rail network 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.
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.
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.
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 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.
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 addition, the heat supplied to form the lime (3.75 MJ/kg in an efficient kiln) is obtained by burning fuel which generates further 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 is equivalent to around 20 kg CO2 per tonne if the electricity is coal-generated. Thus total emission is around 1 tonne of CO2 for every tonne of lime in the most efficient plant, but is typically 1.3 t/t. In some applications, however, lifetime CO2 emissions are reduced because slaked lime reacts with atmospheric CO2, removing it from the atmosphere.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Lime_kiln". A list of authors is available in Wikipedia.|