Inside the PFR lime kiln
A twin-shaft regenerative kiln uses alternating firing and heat recovery to convert limestone into quicklime. Change the view to compare conventional operation with the EcoKiln principle.
Two shafts, alternating firing
Overview
The two shafts work together: one fires lime while hot gas preheats the limestone bed in the other. Their roles alternate about every 10–15 minutes.
Illustrative flow principle, not a plant design. EcoKiln flows depend on configuration.
Conventional firing
Fuel and combustion air enter the firing shaft. Hot combustion gas passes through the connecting channel to the other shaft, preheating its limestone bed. The shafts alternate roles. CaCO₃ releases CO₂ as it becomes CaO; cooling air moves upward through the hot lime and contributes to the gas system, diluting the CO₂-rich process gas.
A CO₂-richer off-gas
In the EcoKiln concept, cooling air is extracted before it mixes with the combustion gas. Flue gas with added oxygen replaces combustion air, reducing nitrogen dilution. The separated cooling-air heat can be recovered. Maerz reports over 95% CO₂ in the off-gas for the full concept; this is not a guaranteed value for every kiln or mode.
Flue gas for lance cooling
As a separate, site-specific option, recirculated flue gas can be routed to lance cooling instead of fresh air. The lance circuit then brings less fresh nitrogen into the kiln gas; cooling duty, gas temperature, lance protection and pressure drop must be checked for the actual installation. This is distinct from the burner oxidant loop.
Oxygen + recirculated flue gas
In oxyfuel operation, a portion of flue gas is recycled and supplemented with oxygen for combustion. The recycled CO₂-rich gas dilutes the oxygen and helps control flame and bed temperatures; merely adding oxygen to air is a different mode. Flue-gas drying may remove water before further processing.