Under normal pressure, the decomposition temperature of limestone is 898°C, and it decomposes rapidly when the temperature is higher than 925°C. When the calcinations temperature is 900°C, the limestone decomposition rate is 3.3mm/h; 1000°C is 6.6mm/h; 1100°C is 14mm/h. It can be seen that increasing the calcinations temperature can accelerate the decomposition of limestone. However, when the calcinations temperature is greater than 1100°C, overburning is likely to occur, the lime crystal size increases rapidly, the lime activity becomes worse, the digestion time increases, and the product quality decreases. Therefore, the calcination temperature should be controlled at about 1050°C in actual production.
The calcination rate of limestone depends on the particle size of limestone, the larger the particle size, the slower the calcination rate. Spherical or cubic limestone has the shortest calcination time. The decomposition of calcium carbonate in limestone is carried out layer by layer from the surface to the inside. The thermal conductivity of quicklime is smaller than that of limestone. The thicker the lime layer, the worse the thermal conductivity and the longer the heat transfer time; and the more difficult it is to decompose CO₂ as it goes inside. Escape, resulting in the formation of lime that has been in a high temperature state for a long time, which will gradually increase the CaO crystals and decrease the decomposition rate. It can be seen that the decomposition time of large particle size limestone is longer than that of small particle size, and calcination is also more difficult.
In the process of limestone combustion, the proportion of fuel is the key to the decomposition of limestone. The proportion should be adjusted timely and reasonably according to the limestone particle size, fuel particle size, water content, kiln shutdown time, lime quality and output changes. Generally, the proportion of anthracite is 2% higher than that of coke.
The decomposition rate of calcium carbonate=decomposed calcium carbonate/input calcium carbonate*100%.
The higher the CO₂ content in the kiln gas, the shorter the carbonization time required for the lime milk and the greater the capacity of the carbonization tower. Conditions for maintaining high CO₂ concentration in kiln gas:
The main components of kiln gas are: CO₂, CO and O₂, which should be controlled to ϕ(CO₂)=40～42%, ϕ (CO)<0.4, and ϕ(O₂)≤1.0 respectively.
When the CO₂ content is lower than 35%, the amount of raw burn in the produced lime increases; when the CO content is high, it indicates that the fuel is not completely burned, and it is also a feature of nodulation in the kiln; when the O₂ concentration is too high, it indicates If the air supply is too large, it will increase the lime over-burning rate and heat loss, which is not conducive to calcinations.