1

IntroductionIn the late seventies RCE Industrieofenbau

Engineering GmbH, Radenthein, Austria, devel-oped a high temperature shaft kiln for the pro-duction of top quality basic and non-basicrefractory products. Since then numerous kilnshave been supplied and built world-wide makingRCE the market leader for this type of shaft kiln.

Based on this design and in response to mar-ket requirements RCE subsequently designed andpatented a high efficiency single shaft kiln forthe calcination of limestone, dolomite andmagnesite (see Fig. 1). The main features of thewidely recognised “RCE High Temperature ShaftKiln” and the experience gained in many years ofoperating practice were incorporated.

Main features of the RCE Lime Kiln• A wide range of product reactivity (compared

to other shaft kiln types) due to easy adjustability of operating temperatures

• Large stone size range• High flexibility in adjusting the burning

temperatures• Low impurity level in the product, e.g. sulphur

or fluorine• Suitable for gaseous, liquid and solid fuels• Low investment costs per ton of product due

to high specific output per m2 of shaft cross section.

Process descriptionThe raw material to be calcined is charged at

the top of the kiln. It is then pre-heated by thecombustion gases in counter-current beforereaching the burning zone. In the burning zone the material is cal-cined by means of fuel introduced together with primary combus-tion air through radially arranged burner lances. Cooling air isintroduced at the discharge table of the kiln to cool the product ina counter-current manner. The heated cooling air travels upwardsto the burning zone and is used there as secondary combustion air.From the top of the kiln the exhaust gases are directed to a dustcleaning system.

Kiln plant descriptionThe screened raw material is

transported to the kiln top by abucket elevator and subsequentlycharged into the kiln feed hopperby means of a conveyor belt. Thefeed hopper has to be kept full atall times so it is equipped with alevel sensor. From this hopper theraw material is distributed uni-formly over the cross section of thekiln by a static distributor.

The kiln can optionally beequipped with an air sealed charg-ing system if a high CO2-content inthe exhaust gases is required.

The shaft kiln basically consistsof a cylindrical steel shell made ofmild steel which is lined with highquality basic and non-basic re-fractories.

The bottom section of the kiln isequipped with the dischargemechanism. In order to operate thekiln with a constant production ratethe discharge mechanism comprisesa speed controlled rotary dischargetable which can vary the amount ofproduct discharged from the kiln.The discharge table is connected toa valve system with an intermediateweighing hopper. This hopper con-trols the preset production rate bycontrolling the speed of the rotarydischarge table.

The calcined product is finally discharged from the weighinghopper by means of a vibrating feeder. The subsequent transportof the product to the storage bin depends on the local require-ments of the customer.

Cooling air is introduced to the kiln in the area immediatelybelow the discharge table. The main volume is fed in at the periph-ery of the shaft whereas the remaining volume is introduced at thecentre. This enables an optimum cooling air distribution through-out the cross section of the kiln. The cooling air is supplied by ablower driven by a variable speed motor, and adjustment of the airquantity is achieved by changing the blower speed.

The required thermal energy is introduced through radiallyarranged burner lances (see Fig. 2) situated at one or more burner levels. This ensures sufficient flexibility in adjusting thetemperature profile to the requirements of the specific feed ma-terial and the required product quality. Fuel and combustion air isevenly distributed throughout the kiln by means of a definednumber of burners arranged at the burner level. Two air blowers

RCE Lime Kiln

Fig. 2: Patented Burner System

Fig. 1: RCE Lime Kiln

2

are used to supply primary combustion air, and adjustment of theair quantity is achieved by varying the blower speed.

As shown in Fig. 3, the burner lances protrude into the hot bedof material at different depths to enable an optimum fuel distributionover the whole cross section of the kiln. The distance of the burnertips from the centre line of the kiln can easily be adjusted with a spe-cially designed quick-action mounting and fastening device.

In case of gas firing, the mixing of gas and combustion air isaccomplished within the burner lances by means of gas nozzles ateach burner. Combustion air and gas is fed in by means ofvertically mounted header pipes to the burner level ring lines whichin turn distribute the air and the gas to the single burners. Acentral gas control and safety train assures constant and safesupply of the gas.

In case of liquid fuel operation the constant supply of a definedquantity of fuel to each burner is guaranteed by allocating onedosing device to several burners. Combustion air for the burners isfed through vertically mounted header pipes to the ring lines at the

burner level which then distribute the combustion air to the singleburners.

For solid fuel firing a static fuel distribution system is used. Thissystem can be applied in all cases where solid fuels can bepulverised and readily fluidised. The fuel, e.g. coal, lignite or pet-coke, is stored in a bin to allow batch discharge into the weighhopper located underneath. The outlet cone of the storage bin isequipped with a fluidising device operated with compressed air. Forsafety reasons an inerting gas system, using nitrogen or carbondioxide, is provided which will be activated automatically wheneverrequired. The flow rate of the fuel is controlled by measuring theweight difference of the fuel content in the weighing hopper inregular time intervals. The fuel is fed in by means of compressedair through metering valves to the static distributor from where itis conveyed to the individual burner lances.

The nature of protruding burner operation requires the use of athermal oil cooling circuit. A heat exchanger is installed to main-tain the thermal oil temperature constant. The system consists ofa loading/unloading tank, a loading/unloading pump station,circulation pumps, an expansion vessel and the heat exchanger.

The exhaust gases leave the kiln at the top and are cleaned ina bag filter before being exhausted to the atmosphere by aninduced draught fan.

Principal layout and performance dataThe present range of RCE Lime Kilns, an example is shown in

Fig. 4, comprises the following standard types:

Kiln type: H1 to H4Daily output (tons): 25 to 200Outside diameter (m): 2.3 to 3.2Kiln shell height (m): approx. 10Total kiln height (m): approx. 20Stone grain size (mm): min. 10 to max. 80Fuels: gaseous, liquid and

pulverised solid fuelsSpecific heat consumption (depending on type of fuel and required reactivity of product)- burned lime (kJ/kg of product): 3800 to 4800

(kcal/kg of product): 900 to 1150Residual CO2- medium burned lime/dolime (%): < 2- hard burned lime/dolime (%): < 1Reactivity t60 according to DIN/EN 459-2 (min.): 2 to 20

Highlights and referencesSeveral RCE Lime Kilns characterised by their compact design

and high specific production capacity have been built in recentyears. Uniform product quality, with low residual CO2, and re-activity adjustable to all market requirements make this kiln type avaluable alternative to the rotary kilns which have been commonlyused, up to now, for this application.

Fig. 4: RCE Lime Kiln Type H2

Fig. 3: Burner Arrangement