Cement Kilns

Precalciner kilns

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A twin-string four-stage air-separate precalciner kiln layout

This is one of an infinite variety of arrangements possible in a precalciner system, the common feature being the burning of part - and sometimes the large majority - of the kiln's fuel in the preheater in a vessel designed for that purpose.

Today, nearly all new kilns installed have precalciners. They represent the final improvement in efficiency, and hold out the prospect for nearly total elimination of heat wastage from cement kilns.

The precalciner system is a suspension preheater in which, in addition to the kiln flame, extra fuel is burned in the base of the preheater. The first kilns of this kind operated in Japan in the late 1960s. These systems allow more thermal processing to be accomplished efficiently in the preheater.

The idea of the precalciner emerged naturally during investigation of the ways in which suspension preheater systems can be made more thermally efficient. The main usage of heat in a dry process kiln is in the decomposition of calcium carbonate in the temperature range 650-1050°C. It was found that well-operated suspension preheaters could raise the feed temperature to around 750°C, at which the carbonates might be 10% decomposed. This led to attempts to increase the amount of decarbonation in the preheater by injecting extra fuel at its base. The provision of an aerodynamically designed vessel in which this could be done constituted the first precalciner.

There are two types: so-called “air-through” and “air-separate”. In the former, all the combustion air passes through the kiln, and because there is a limit to the amount of excess air that can be pulled through the burning zone, only around 25% of the fuel can be burned in the preheater. The air-separate system is the most sophisticated: extra combustion air ("tertiary air") for the calciner is extracted from the cooler, and this allows up to 70% or more of the fuel to be burned in the calciner. This allows a large output to be produced by a relatively small kiln. The feed in this case is completely calcined before it enters the kiln, which solely performs the sintering process, for which the rolling action of the rotary kiln is indispensible.

Not all precalciner kilns are strictly speaking "dry process", since modifications have been devised allowing filter cake (Semi-Wet Process) or even slurry (Wet Process - e.g. Rugby and Tunstead) to be fed to the system. Naturally, the more water is added to the system, the higher is the energy consumption, but the flexibility of the precalciner arrangement at least allows this energy to be transferred with a minimum of collateral waste.

yearkiln process stages
1977Platin A2AT4
1983Limerick A6AT4
1983Ribblesdale A7AS4
1985Cauldon B1AS4
1985Ketton A8AS4
1986Dunbar B1AS4
1989Derrylin A1AT4
1998Platin A2 (converted)AS4
2000Ballyconnell A1AS5
2002Rugby A7AS3
2002Kinnegad A1AS4
2004Tunstead A2AS4
2005Padeswood A4AS6
2009Platin A3AS5

As with conventional suspension preheater kilns, the gases leaving the top of the preheater are used for drying raw materials. Some heat is still wasted, because rawmills tend to run intermittently, since they are always designed with "overtaking capacity" to avoid restricting the output of the kiln. While the rawmill is stopped, the hot gas has to be run to waste, usually with injection of water in a conditioning tower. With this limitation, the number of cyclone stages used depends on the amount of drying capacity needed: if the raw material is very wet, a smaller number of stages is employed so that ample waste heat will be available. If the raw material is dry (and reliably so, year-round) then a larger number of stages can be used, minimising the amount of heat leaving the kiln system. Here, again, the number of stages that can be employed is limited by the escalating fan-power requirement offset against the diminishing returns from adding an extra stage, despite continual improvement in "low-pressure drop" cyclone design. In Britain, the maximum number of stages employed is six, at Padeswood.

It is not intended to treat these modern systems in any detail because

  • it is the policy of this website not to describe currently operating systems in detail in the public version of the site
  • the various systems are too idiosyncratic to describe on a common basis
  • much of the current technology is proprietary
  • excellent general-interest accounts can be found in equipment suppliers' websites.

© Dylan Moore 2011: last edit 26/08/2014.