A precalciner is a key unit in a modern dry process cement plant, working alongside the suspension preheaters. It completes the decomposition of carbonate in raw material before it enters the rotary kiln. Its main function is to decompose 85% - 95% of CaCO₃ inside the calciner. This is achieved through high-temperature gas flow, fuel combustion, and strong gas-solid mixing.
In older kiln systems without a pre calciner, most decomposition occurred inside the rotary kiln. This resulted in large kilns, high energy use, and limited production. However, the precalciner in cement plant moves the decomposition process outside the kiln. The rotary kiln then focuses mainly on the sintering stage, which greatly improves the overall system performance.
The pre calciner works by creating a gas-solid suspension. This allows fast mixing, heat transfer, and decomposition between raw meal and high temperature gas. Its key mechanisms include:
Special flow designs, such as swirling or spouted flow, disperse raw meal inside the calciner. This increases gas-solid contact, enabling fast and even heat transfer.
Fuel is injected through burners and mixes with hot tertiary air from the grate cooler. The suspended raw meal quickly absorbs the released heat. This maintains the precalciner temperature required for decomposition.
The main carbonate component decomposes at about 850-900°C:
CaCO₃ → CaO + CO₂
This endothermic reaction requires high heat input. The precalciner supplies the required decomposition energy through fuel combustion and high temperature tertiary air.
Through these combined processes, most carbonate decomposition finishes before the material enters the kiln. This supports efficient and stable kiln operation.
Raw meal enters the calciner through the inlet duct, usually connected to the discharge pipe of the upstream preheater. The design aims for uniform and stable feeding to avoid blockage or surging. This ensures sufficient mixing with the gas stream to prepare the material for subsequent decomposition reactions.
Fuel lances or nozzles atomize and inject pulverized coal or alternative fuels into the combustion zone. Their design affects atomization quality, combustion speed, and flame shape. Stable temperature distribution and high combustion efficiency depend on proper lance performance.
This duct connects the clinker cooler to the calciner and delivers tertiary air. It provides oxygen for fuel combustion and carries significant sensible heat. This heat is essential to maintain the calciner thermal balance and stable high temperature environment.
The vessel serves as the main chamber for decomposition reactions. Its geometry and dimensions determine the internal gas flow patterns. These patterns govern gas-solid mixing efficiency, material residence time, and overall decomposition performance.
Ducts at the upper section of the calciner connect to the final preheater stage or kiln inlet chamber. They guide partially decomposed raw meal and hot gases to the next step. The design keeps flow resistance low and prevents material buildup. It also spreads raw meal evenly to improve heat transfer and avoid local overheating.
The inner walls of the calciner use refractory and wear-resistant linings. These linings resist high temperatures, chemical corrosion and particle erosion. Material type and thickness match the thermal and mechanical conditions in each zone. Proper selection supports reliable and stable long term operation of the calciner.
The precalciner is a key unit in the cement kiln system. Different fuels, process requirements, and emission standards determine the design of the precalciner. Common classifications include:
| Type | Structural Features | Operating Characteristics |
| Cyclone calciner （swirl-flow calciner） | Cylindrical vessel with gas entering tangentially or from the top, which creates a swirling gas flow inside the furnace. | Strong swirling flow disperses raw meal. Mixing is intense and uniform. The structure is relatively simple. |
| Spouted calciner | Lower section shaped with a constricted bottom and nozzles. High-velocity tertiary air enters from below. | Air jets lift and suspend the raw meal, producing strong turbulence. Kiln exhaust provides preheating. Fuel burns in the tertiary air stream. Temperature remains stable and suitable for pulverized coal. |
| Fluidized Bed Calciner | Base equipped with air caps or a gas distribution plate delivering secondary air (200-250°C). | Injected air forms a fluidized bed. Fuel and raw meal react within this layer. Heat transfer is efficient, temperature uniform, and operation stable. Well suited to low-reactivity, low-calorific fuels. |
| Calciner with preheating chamber | Structure includes a small preheating chamber and a main calcining chamber. | Fuel ignites and stabilizes in the preheating chamber. Hot gases then enter the calcining chamber to mix with raw meal. Staged combustion improves performance with low-calorific fuels and achieves a high decomposition rate. |
Suspension efficient heat exchange and strong mixing improve heat use and reduce unnecessary heat loss inside the kiln.
Since most decomposition is completed outside, the kiln focuses on sintering. A modern precalciner kiln can increase output by 30%-60% compared with older dry process kilns.
The kiln temperature field becomes more stable. This supports kiln coating formation and extends refractory service life.
Stable decomposition reduces under-burning and over-burning. The sintering process becomes more consistent.
Staged combustion inside the cement kiln precalciner creates a reducing zone that converts NOx into nitrogen. This supports in-furnace NOx reduction.
The calciner has an independent combustion system. Operators can precisely adjust combustion parameters online, which simplifies control of the overall clinkering system.
The precalciner cement kiln is a core component of modern dry-process clinker production. It improves thermal efficiency, increases kiln capacity, and enhances environmental performance. As fuel diversification and environmental requirements continue to rise, precalciner combustion control and system optimization will keep advancing. This will support more efficient, stable, and low-carbon cement production.
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