Cement production occurs by clinkering a raw material typically including limestone CaCO3 (as a source of calcium oxide CaO), and clay, marl or shale (as a source of SiO2, Al2O3, Fe2O3) and typically also other materials such as sand.
A coupled carbon capture and cement production occurs over the following steps:                Sorbent preparation; material milling and heat-treatment, such as the calcination of limestone to produce CaO (allowing capture of the chemically associated CO2 during calcination in the calciner reactor);        Exposure of the sorbent powder to flue gas (possibly generated for the above mentioned heat treatment but also possibly from an external process) in order to convert CaO to CaCO3 by lowering the concentration of CO2 in the combustion flue gas through gas solid contact in the carbonator reactor;        Regeneration of the sorbent materials, in particular the restoration of the CO2 capture capacity (quantity CO2/quantity sorbent);        A sorbent purge to close the material balance of the cycle and remove deactivated sorbent from the system which is utilized for cement production.        
Considering the above steps, FIG. 1 shows a simplified block flow diagram representing a state of the art process scheme for cement production coupled with a carbon dioxide capture process. It includes a calciner 1, a carbonator 2 and a kiln 3.
Raw material containing limestone CaCO3 is supplied from a raw material supply 4 into the calciner 1 where the make-up sorbent (mainly CaCO3) is calcined (decomposed by heat) along with CaCO3 from the carbonator according to the reaction:CaCO3→CaO+CO2 
generating carbon dioxide CO2 and carbon dioxide lean sorbent CaO. The lean sorbent CaO is supplied to the carbonator 2 where carbon dioxide CO2 is captured from flue gas 5 according to the carbonation reactionCaO+CO2→CaCO3.
Gas 6 deprived from carbon dioxide CO2 is discharged from the carbonator 2. The calcium carbonate CaCO3 generated at the carbonator 2 is supplied back into the calciner 1 allowing the release of captured Carbon dioxide and sorbent regeneration.
The carbon containing flue gas 5 may be attributed to a variety of thermally driven processes, such as power generation but a quantity of the flue gas will always be attributed to the cement process. At the kiln 3 flue gas is generated by combustion of a fuel with air; in addition carbon dioxide (CO2) is released during the clinkering reactions due to the residual carbon content of the feed material leaving the carbonator 2 on route to the kiln 3.
FIG. 6 shows that after some adsorption/desorption cycles, the calcium oxide CaO loses its capacity to adsorb carbon dioxide CO2; for this reason it must be purged.
According to the scheme of FIG. 1, purging is made by discharging calcined raw material containing calcium carbonate CaCO3 from the carbonator 2 and supplying it to the kiln 3 for calcination and thus clinkering.
As mentioned this scheme has the drawback that calcium carbonate CaCO3 formed in the carbonator is calcined at the kiln 3 which requires additional fuel combustion and produces additional CO2 associated with the chemically bound CO2 captured from the flue gas at the carbonator 2. This causes an unnecessary carbon dioxide circulation between the kiln and the carbonator and consequently increased energy consumption and equipment dimensions and costs.
FIG. 2 shows a second simplified block flow diagram representing a state of the art process scheme for cement production. It is similar to the scheme of FIG. 1 and same numbers indicates same or similar components. The scheme of FIG. 2 differs from the scheme of FIG. 1 in that purging is made by discharging raw material containing calcium oxide CaO from the calciner 1 and supplying it to the kiln 3.
This second scheme reduces the amount of circulating carbon dioxide, because a reduced amount of carbon dioxide is released at the kiln (because additional calcination of CaCO3 associated with carbon dioxide captured from the flue gas is avoided); nevertheless fresh and highly active calcium oxide CaO (sorbent) that can be most efficiently used for carbon capture is purged unselectively together with deactivated or spent sorbent (i.e. sorbent that has a reduced capacity to adsorb carbon dioxide).