The calcination of limestone is a simple chemical process:
CaCO3 + heat = CaO + CO2
The calcination temperature depends upon several factors. At the atmospheric pressure and with a CO2 concentration of 25%, the dissociation starts at about 810°C. Some components in fact (e.g. NaHCO3 or KHCO3) dissociate at lower temperatures (200-300°C) already in the preheating zone; some other like NaCO3 or K2CO3 dissociate at 800-900°C in the calcining zone, this forming Na2O and K2O which sublime to Na and K vapours at approx. 1200°C. However experience and a well proven design are the secrets for a successful lime burning project as several factors influence the choice of a lime kiln technology: the available raw material quality, the fuel and the required characteristics of the finished product.
The new TWIN-C Lime Kiln is Cimprogetti’s answer to the industry’s demand for a modern and flexible kiln that can be easily integrated into complex limestone and/or lime processing systems. The proven twin-cylinder arrangement and the application of state of the art design technologies make it possible to supply the perfect kiln for lime processing plants wherealternately soft burned or medium burned lime and/or a continuous flow of exhaust gases are required. The TWIN-C Kiln is a valuable choice in all cases where a superior lime quality is required along with a continuous flow of exhaust gases with high CO2-content.
Daily output (tons)
100 to 600
Stone grain size (mm)
30 to 120 (ratio 1:2 to 1:3)
Fuels
gaseous, liquid and pulverised solid fuels
Heat consumption (kJ/kg of product) (kcal/kg of product)
approx. 3520 approx. 840
Reactivity of lime
very high to medium
Operating principles of an annular kiln
The pre-weighed limestone is charged into the kiln through a rotary hopper.
The charging system is air tight to avoid bleeding air to dilute the exhaust gases, thus decreasing their relative CO2-content. A high CO2-content in the exhaust gases is important for several chemical applications, e.g. PCC or soda ash production.
The charged limestone moves downwards and is pre-heated in counter-current by the hot gases coming from the upper burning zone. The flow of exhaust gases through the central top bell and through the outer annular channel is carefully controlled in order to preheat the limestone uniformly.
After preheating, the limestone is calcined in two distinct zones. The upper counter-current zone is situated between the top of the inner cylinder and the lower level of the combustion chambers. The lower one is a co-current zone located between the lower combustion chambers and the suction ports of the inner cylinder. The exhaust gases moving upwards through the preheating zone are sucked off at the top of the kiln by an ID-fan. At the lower end of the countercurrent burning zone the limestone is only partially calcined and the material can withstand the exposure to the high temperature in the lower combustion chambers without being over-burned. The material is eventually softly calcined to its final quality by the hot gases flowing in co-current through the lower burning zone.After final calcination, the lime enters the cooling zone located below the suction ports of the inner cylinder where it is cooled down in counter-current by the air sucked in at the kiln bottom. Finally, hydraulically driven drawers discharge the quick lime into the bottom hopper from where it is fed to the finished product transport line. The kiln works under negative pressure. The main induced draft which sucks in the lime cooling air at the kiln bottom and removes the exhaust gases from the top of the kiln is produced by an ID-fan. This induced draft creates the counter-current flow in the cooling zone, in the counter-current section of the calcining zone and in the preheating zone. The external subsonic Venturi ejectors equipped with HPC (High Pressure Combustors-Ejectors) act as a “gas jet pump” sucking the gas stream of the co-current burning zone and the lime cooling air stream through the suction ports located near the bottom of the inner cylinder. The two streams are mixed while entering the inner cylinder from the bottom, leave the inner cylinder at the top and are re-circulated into the lower combustion chambers. Twin-C kilns can be fired with natural gas, heavy or light fuel oil as well as with a broad range of lean gases. Mixed feed firing with coke or anthracite in lumps for a part of the total heat input required is also possible.
Limestone calcination
