A vertical kiln (also known as a shaft kiln) is a type of industrial furnace used for producing lime (calcium oxide, CaO) by calcining limestone (calcium carbonate, CaCO₃). It is widely used in small to medium-scale lime production due to its compact design, fuel efficiency, and relatively low investment cost compared to rotary kilns.
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Types of Vertical Lime Kilns:
1. Mixed-Feed Shaft Kiln
– Fuel (coal, gas, or biomass) is mixed with limestone before feeding.
– Simple operation but lower energy efficiency.
2. Parallel-Flow Regenerative (PFR) Kiln
– Uses two shafts that alternate between heating and calcining.
– Highly energy-efficient with better lime quality.
3. Annular Shaft Kiln
– Features an annular combustion chamber around the limestone bed.
– Good heat transfer and uniform burning.
4. Single Shaft Kiln
– Traditional design with direct or indirect heating.
– Suitable for small-scale production.
Working Principle:
1. Feeding: Crushed limestone (20–60 mm size) is fed from the top.
2. Preheating Zone: Limestone is heated by rising hot gases (~900°C).
3. Calcination Zone: At 900–1200°C, CaCO₃ decomposes into CaO + CO₂.
4. Cooling Zone: Burnt lime cools before being discharged at the bottom.
5. Exhaust Gases: Flue gases exit through the top for heat recovery or scrubbing.
Advantages of Vertical Lime Kilns:
✔ Compact design, suitable for small/mid-scale plants (~50–400 tons/day).
✔ Lower capital cost than rotary kilns.
✔ Better heat recovery in regenerative designs (PFR kilns).
✔ Can use solid fuels like coal or biomass.
Disadvantages:
✖ Limited production capacity compared to rotary kilns.
✖ Requires high-quality limestone with uniform size.
✖ Higher maintenance due to mechanical wear in moving parts.
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Applications of Lime Produced:
– Steelmaking (fluxing agent)
– Construction (mortar, plaster)
– Water treatment (pH adjustment)
– Chemical industry (precipitated calcium carbonate, PCC)