The CaL process (as shown in the figure below) consists of two fluidized bed reactors connected by solid transport pipes and makes use of the reversible carbonation reaction of CaO and the subsequent calcination of the CaCO3 formed. A CO2-lean gas exits the carbonator and is released to the atmosphere. The produced CaCO3 is transported to the regenerator where the calcination reaction takes place in order to regenerate the CaO and produce a pure CO2 stream. The CaO produced is transported back to the carbonator to further capture flue gas CO2, while the CO2 released from the regenerator can be directed to purification, compression and storage.
The Calcium Looping process schematic
(SG: Steam Generator, ASU: Air Separation Unit) (Hawthorne; 2009)
A unique advantage of the CaL process is that extra power is generated from the CaL system. The power increase of a CaL DFB system applied to a state of the art coal power plant can reach up to 50% Moreover, due to the efficient utilization of heat in the steam cycle the efficiency penalty on the overall system (initial power plant and CaL DFB system) has been calculated to be lower than 10% by numerous studies , while it can be as low as 4-6 % under optimum steam cycle configurations. In addition, the calculated CO2 avoidance costs for the CaL process is less than 20 €/t. The cost of electricity for the CaL process is shown to be approx. 40 €/MWh. (Romeo;2008, Poboss;2008, Hawthorne;2009).
The above exceptional values of the CaL process lie on a number of technical aspects related mainly to the sorbent and reactor type used. Firstly, limestone is well geographically distributed and low cost. Moreover, the CaO purge removed for the DFB system can be utilized for cement production. In addition, the CaO purge can be utilized in FB combustors due to improved SOX capture properties compared to “fresh” limestone.
As noted, the CaL process benefits largely on the fact that it is based on the established Fluidized Bed Combustion (FBC) technology. Similarities exist between the Circulating Fluidized bed Combustor (CFBC) and the CFB carbonator which aid reactor design. Moreover, the regenerator is in essence an oxy-fuel CFBC.