Calciner

A calciner is the combustion chamber in a modern cement preheater tower where raw meal is pre-calcined - the endothermic CaCO3 to CaO + CO2 reaction is driven to ~90% completion - before the meal enters the rotary kiln. Calciners can be inline (placed in the kiln-riser gas path) or separate (a dedicated combustion chamber receiving tertiary air through a dedicated tertiary air duct).

AFR firing

Calciners are the dominant firing location for alternative fuels (RDF, SRF, TDF, sewage sludge). They tolerate variable-quality waste fuels better than the main kiln burner because residence time is longer and temperatures are lower. Cement plants targeting high thermal substitution rates (TSR) concentrate their AFR firing in the calciner.

Fouling

AFR firing in the calciner raises the chlorine and sulphur loading of the gas reaching the preheater cyclones above. This intensifies the build-up problem in the lower preheater stages and the kiln riser, driving the need for chloride bypass and active sonic-horn cleaning.

Cleaning

Sonic horns are mounted on calciner walls and on the calciner outlet to the preheater stage 5 cyclone, keeping the gas path free of the alkali coatings that accumulate at high AFR rates.

Process role

The calciner shifts most of the limestone decomposition duty out of the rotary kiln. Raw meal enters from the lower cyclone stages and meets hot kiln exhaust, tertiary air and fuel. The main reaction is calcium carbonate to calcium oxide plus carbon dioxide. Because this reaction absorbs heat, the calciner is deliberately designed as a high-residence-time combustion and solids suspension zone rather than a simple duct.

Modern preheater kilns commonly target a high degree of calcination before the meal reaches the kiln inlet. This reduces thermal load in the kiln burning zone, improves kiln stability and increases clinker production for a given shell size. The trade-off is that a larger fraction of total fuel is burned in a dusty, chemically recirculating gas path where build-up is harder to inspect.

Operating mechanisms and failure modes

Good calciner operation depends on fuel dispersion, oxygen availability, meal distribution and gas residence time. Poor mixing leaves unburned fuel, carbon monoxide, local reducing zones or hot streaks. These conditions can disturb the sulphur, chloride and alkali cycles and make coatings more tenacious in the kiln riser and lower cyclones.

High alternative-fuel firing adds ash, chlorine, moisture and variable particle size. Coarse or wet fuel can fall out before complete burnout, while fine chlorine-bearing material volatilises and condenses higher in the tower. Operators watch oxygen, carbon monoxide, calciner outlet temperature, cyclone pressure drop and kiln inlet build-up as a combined picture rather than as independent signals.

Design and acoustic-cleaning context

Inline calciners sit directly in the kiln gas stream and are compact, while separate-line calciners use a dedicated tertiary-air route and can offer more control of combustion staging. In both cases, access for lances, inspection doors, refractory repair and cleaning equipment matters because build-up can grow rapidly once a rough deposit has formed.

Sonic horns are normally treated as a build-up prevention tool, not a substitute for combustion control. They are placed where low-frequency energy can keep sticky alkali salts from consolidating on walls, expansion joints, meal shelves and outlet transitions. The best results come when horn sequencing is coordinated with stable fuel feed, adequate chloride bypass operation and disciplined refractory inspection.

Operating checks

Calciner performance is watched through outlet temperature, oxygen, carbon monoxide, cyclone differential pressure, feed rate, fuel split and kiln inlet conditions. A stable calciner keeps raw meal suspended long enough for heat transfer while avoiding local reducing zones that raise build-up risk. Operators also watch the tertiary air damper position, alternative-fuel feed consistency and meal distribution, because a small change in any of these can move the hot zone and create coating on the calciner roof, walls or outlet throat.

Maintenance inspections focus on refractory condition, burner tips, meal splash, poke holes, tertiary air duct build-up and false-air leaks. Build-up that narrows the outlet raises gas velocity into the lower cyclone and can trigger a sequence of cyclone pluggage, kiln back-end instability and emergency cleaning. Sonic horns are useful where the deposits are still friable and cyclic loading can stop them maturing into hard rings. They are not a substitute for correcting excess chloride, poor alternative-fuel preparation, inadequate oxygen or an overloaded bypass system.