Editing Lime kiln (section)

==Modern kilns==
{{Unreferenced section|date=September 2007}}
[[File:LDLimeShaftKilnBasic.jpg|thumb|170px|left|Cross section of simple shaft kiln]]

===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 furnace]]s. 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&nbsp;°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 (mechanical)|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, but 4.5 to 5 MJ/kg is more typical. Due to temperature peak at the burners up to 1200&nbsp;°C in a shaft kiln conditions are ideal to produce medium and hard burned lime.

[[File:LDLimeShaftKilnRegen.jpg|thumb|200px|right|Gas flows in two cycles of operation of regenerative shaft kilns]]

====Regenerative kilns====
These typically consist of a pair of shafts, operated alternately. First, when shaft A is the "primary" and B the "secondary" shaft, the combustion air is added from the top of shaft A, while fuel somewhat below via burner lances. The flame is top-bottom. The hot gases pass downward, cross to shaft B via the so-called "channel" and pass upward to exhaust of shaft B. At same time in both shafts cooling air is added from the bottom to cool the lime and to make exhaust of gases via the bottom of the kiln impossible via maintaining a positive pressure. The combustion air and cooling air leave the kiln jointly via exhaust on top of shaft B, preheating the stone. The direction of flow is reversed periodically (typically 5–10 times per hour) shaft A and B changing the role of "primary" and "secondary" shaft. The kiln has three zones: preheating zone on the top, burning zone in the middle, and cooling zone close to the bottom. The cycling produces a long burning zone of constant, relatively low temperature (around 950&nbsp;°C) that is ideal for the production of high quality soft burned reactive lime. With exhaust gas temperatures as low as 120&nbsp;°C and lime temperature at kiln outlet in 80&nbsp;°C range the heat loss of the regenerative kiln is minimal, fuel consumption is as low as 3.6 MJ/kg. Due to these features the regenerative kilns are today mainstream technology under conditions of substantial fuel costs. Regenerative kilns are built with 150 to 800 t/day output, 300 to 450 being typical.

[[File:LDLimeShaftKilnAnnul.jpg|thumb|170px|left|Gas flows in an annular shaft kiln]]

====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. Fuel consumption is in 4 to 4.5 MJ/kg range and the lime is typically medium burned.

[[File:LDLimeRotaryPH.jpg|thumb|200px|right|Rotary kiln with preheater: hot gas flows]]

===Rotary kilns===
{{Main|Rotary kiln}}
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 if energy costs are less important. 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. Old fashioned "long" rotary kilns operate at 7 to 10 MJ/kg. 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, typically in range of 4.5 to 6 MJ/kg. 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. The rotary kiln is the most flexible of any lime kilns able to produce soft, medium, or hard burned as well as dead-burned lime or dolime.

===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 precipitator]]s or bag filters. The dust usually contains a high concentration of elements such as [[alkali metal]]s, [[halogen]]s and sulfur.