Glossary
Cement
Sulphur, chloride and alkali cycles
Also known as sulphur cycle, chloride cycle, alkali cycle, volatile cycles.
The sulphur, chloride and alkali cycles describe how volatile species evaporate from the rotary-kiln burning zone, rise with the gas flow, condense in the cooler preheater above, return to the kiln in the descending raw meal, and recirculate. Each cycle has its own behaviour and operational consequences.
The three cycles
| Cycle | Volatile species | Condensation window | Operational consequence |
|---|---|---|---|
| Sulphur cycle | SO₂, SO₃, alkali sulphates | 800–1,000 °C | Sticky alkali-sulphate coatings in preheater stages 4–5 |
| Chloride cycle | KCl, NaCl | 700–900 °C | Aggressive sticky coatings; primary driver of kiln-inlet snowmen |
| Alkali cycle | K₂O, Na₂O | wide | Sets cement chemistry; affects strength development |
Why the cycles matter operationally
All three cycles concentrate volatiles in the gas-phase recirculation loop unless something extracts them. Conventional cement raw materials and fossil fuels carry modest loadings; alternative fuels — especially RDF, SRF and TDF — add substantially more chlorine, sulphur and sometimes alkali.
When a cycle saturates:
- Chloride cycle — heavy kiln-inlet build-up; kiln stop unavoidable
- Sulphur cycle — preheater coatings; cyclone pluggage
- Alkali cycle — clinker quality issues; cement performance drift
Cycle management
- Chloride bypass — extracts a slipstream of gas from the kiln inlet to remove chlorine
- Raw-material substitution — selecting lower-Cl/-S/-alkali raw materials
- Fuel blending — controlling AFR chlorine and sulphur content
- Sonic horns and air cannons on the preheater and kiln inlet to keep accumulating coatings under control
Related terms
Related terms
- Preheater towerA preheater tower is a vertical stack of cyclone separators that pre-heats raw meal with kiln exhaust gas before it enters the rotary kiln. The most fouling-prone section of any cement plant.
- Kiln-inlet ring and snowmanA snowman is a massive accretion at the cement kiln inlet that can completely block the gas path. Driven by sulphur and chloride cycles, intensified by alternative fuels.
- Alternative fuelAlternative fuels (AFR) replace fossil fuel in cement kilns. They cut CO2 emissions and waste-disposal cost but increase chlorine, sulphur and alkali loading in the kiln gas.
- Chloride bypassA chloride bypass extracts a slipstream of kiln gas before the preheater to remove chlorine from the recirculating Cl cycle. Essential at high TSR; the bypass duct itself fouls heavily.