How a kiln works

The basics of lime burning:

The lime burning process requires enough heat to be transferred to the limestone in order to decompose the calcium and magnesium carbonates. Heat transfer for lime burning can be divided into three main stages;

1. Preheating zone - limestone is heated to approximately 800°C by direct contact with gases leaving the calcining zone (i.e. the products of combustion, excess and CO₂ given off from the calcination).
2. Calcining zone - fuel is burnt in preheated air from the cooling zone and (depending on kiln type) in additional ‘combustion’ air added with fuel. This produces heat at above 900°C and turns limestone into quicklime and CO₂.
3. Cooling zone - quicklime leaving the calcining zone at 900°C is cooled by direct contact with ‘cooling’ air.

The zones are illustrated in Fig 1 for a vertical shaft kiln:

Fig 1. Cross-section of a vertical shaft kiln: (a) preheating zone; (b) calcining zone; (c) cooling zone.

There are a large variety of kiln designs available all over the world. The choice of lime kiln is extremely important for lime producers. The selected kiln must be suitable for burning the selected feedstone and for producing the required quality of quicklime. Generally speaking, it must have sufficiently low capital and operating costs to produce quicklime at a competitive price. Its capacity must also be appropriate for the market requirements.

There are three main types of lime kilns being used in the UK today – Shaft kilns, Rotary kilns and Maerz (parallel flow regenerative) kilns.

Shaft Kilns:

(See above for basic description of how ‘traditional’ shaft kilns work).

(See Fig 2).

Fig 2. Example of a vertical shaft kiln in use

Modern shaft kilns – acceptable sizes for the feedstone range from a minimum of 20 mm to a top size of up to 175 mm and even up to 350 mm for design B. Reported long term net heat usages at design output for these types of kiln generally lie in the range 1,000 to 1,200 kcal/kg. Some kilns are suitable for operation on gaseous (see Fig 3), liquid and solid fuels, while the options for others are more restricted.

Fig 3. Cross sections of an HPK gas-fired shaft kiln, incorporating internal arches: (a) limestone; (b) quicklime; (c) natural gas; (d) primary air; (e) recycled kiln gas; (f) cooling air; (g) exhaust gases.

Rotary Kilns:

The traditional/long rotary kiln (see Fig 4 and 5) consists of a rotating cylinder (110 to 140 m long) inclined at an angle of 3 to 4 to the horizontal. Limestone is fed into the upper ‘back end’ and fuel plus combustion air is fired into the lower ‘front end’. Quicklime is then discharged from the kiln into a lime cooler, where it is used to pre-heat the combustion air.

Fig 4. Cross section of a long rotary kiln: (a) limestone; (b) exhaust gases; (c) refractory trefoils; (d) kiln shell; (e) fuel plus secondary air; (f) lime cooler; (g) cooling air; (h) quicklime.

Maerz parallel-flow regenerative kilns:

The parallel-flow regenerative kiln (see Fig 6 and Fig 7) consists of two interconnected vertical cylindrical shafts. Batches of limestone are charged alternately to each shaft, the burden is drawn downwards through a preheating/regenerative heat exchange zone, past the fuel lances and into the calcining zone. From there the quicklime passes into the cooling zone.

The operation of the kiln consists of two equal stages, of 8 to 15 min duration at full output. In the first stage fuel is injected through the lances in shaft 1 and burns in the combustion air blown down that shaft. The heat released is partly absorbed the calcination of limestone in shaft 1. Air is then blown into the base of each shaft to cool the lime. The cooling air in shaft 1, together with the combustion gases and CO₂ from calcination, pass through the interconnecting cross-duct into shaft 2 at about 1050°C. In shaft 2, the gases from shaft 1 mix with the cooling air blown into the base of shaft 2 and pass upwards. This then heats the stone in the preheating zone of that shaft.

After 8 to 15 minutes, the second stage commences. The fuel and air flows to shaft 1 are stopped, and ‘reversal’ occurs. After charging limestone to shaft 1, fuel and air are injected to shaft 2 and the exhaust gases are vented from the top of shaft 1.

Fig 6. Cross section of a parallel-flow regenerative kiln: (a) fuel; (b) combustion air; (c) cooling air; (d) kiln gases; (e) cross-duct; (f) shaft 1; (g) shaft 2.

Fig 7. A pair of Maerz parallel-flow regenerative kilns.