EQP-08 — Preheater tower, calciner, rotary kiln, and clinker cooler
Raw meal from the silo enters the top of the five-stage cyclone preheater and cascades downward through Stages 1 through 5, exchanging heat with hot gases rising from the kiln and calciner. Each stage consists of a gas duct where meal is injected into the hot gas stream, a cyclone where gas-solid separation occurs via centrifugal force, and a discharge pipe directing meal to the next stage below. The counter-current flow configuration is the most thermally efficient arrangement, achieving gas exit temperatures as low as 280-350°C while heating the meal to 800-850°C before it enters the calciner.
The heat transfer is extremely efficient because raw meal particles (mean diameter 40-60 micrometers, specific surface area 5,000-8,000 m²/kg) reach thermal equilibrium with the surrounding gas in 0.1-0.3 seconds. This rapid heat exchange is what makes the cyclone preheater system thermally efficient, with exit gas temperatures as low as 280-350°C at the top stage.
The calciner fires 55-65% of the total kiln fuel, raising the temperature to 900-950°C (with local peaks up to 1,150°C) and achieving approximately 95% calcination of the raw meal. The calcination reaction (CaCO₃ + heat → CaO + CO₂) is strongly endothermic, requiring approximately 425 kcal/kg of clinker — roughly 60% of the total thermal energy consumed in the clinker production process. By performing the majority of calcination in the calciner rather than in the rotary kiln, the kiln length can be significantly reduced, and the kiln's primary function shifts to final clinkering (C₃S formation at 1,450°C). Hot gases from the calciner rise through the preheater stages to heat incoming raw meal. The calciner temperature is controlled by the fuel firing rate and the tertiary air flow from the clinker cooler.
Material enters the rotary kiln at approximately 900°C and 95% calcined. The kiln is a steel cylinder lined with refractory brick, inclined at 3-4% slope, rotating at 2.5-4.0 RPM. Material travels downhill through five distinct thermal zones over the 30-45 minute residence time:
The main burner at the kiln discharge end fires pulverized coal or pet coke with primary air (5-10% of total combustion air) and secondary air preheated to 800-1,100°C from the clinker cooler. The flame core reaches 1,800-2,000°C with a flame length of 12-20 m (adjustable via burner positioning and air momentum). The burning zone temperature of 1,450°C is measured by a radiation pyrometer aimed at the clinker bed. The refractory lining in the burning zone consists of magnesia-spinel bricks, which withstand the high temperature and chemical attack from the liquid clinker phase. The C₃S (alite) formation reaction (C₂S + CaO → C₃S) proceeds only in the presence of the liquid phase (C₃A + C₄AF + impurities), which provides a transport medium for CaO ions to reach C₂S crystals. The final clinker composition is typically 50-70% C₃S, 15-30% C₂S, 5-10% C₃A, and 5-15% C₄AF.
Clinker exits the kiln at 1,200-1,400°C and enters the grate cooler where it is rapidly quenched to 100-150°C. Rapid cooling is essential for two reasons: (1) it prevents the conversion of desirable β-C₂S (which has hydraulic properties) to undesirable γ-C₂S (which has no hydraulic properties), and (2) it fixes the liquid phase as a glass, which is more reactive than crystalline phases. The cooler recuperates heat from the hot clinker into the secondary air (for kiln combustion) and tertiary air (for calciner combustion), achieving thermal efficiencies of 65-75%. A 1% improvement in cooler efficiency reduces the kiln heat consumption by approximately 5-6 kcal/kg clinker. The cooled clinker is conveyed to the clinker storage silo for cement grinding.