The Calcium Carbonate Looping (CCL) process addressed in the SCARLET project is a promising post combustion CO2 capture technology using limestone as solid sorbent. The CO2 capture is based on the following reversible, exothermic reaction that takes place in the carbonator:
The figure shows the principle of the carbonate looping process. The CO2 is absorbed and captured by lime (CaO) from the fluegas entering the carbonator. CO2 depleted flue gas leaves the reactor. The converted limestone (CaCO3) is transferred to a second reactor. In this reactor called calciner CaCO3 is calcined and CO2 is released. Because of the fact that the CO2 release in the calciner is an endothermic reaction, it is necessary to provide heat by combusting fuel with oxygen. The temperatures in the reactors are 600-750 °C in the carbonator and around 900 °C in the calciner, the reactions take place at atmospheric pressure. The solids are recycled by establishing a closed loop between both reactors. The CO2 leaves the process as a gas flow that is further processed in downstream unit operations (purification, compression) before being sent to storage or for utilization. The circulation of solid material between both reactors is associated with losses and deactivation of the solid sorbent. Due to this fact, a make-up flow of fresh limestone (CaCO3) is fed to the process.
The low price and high availability of limestone in various locations on earth is an advantage of the carbonate looping process in comparison with other technological solutions. Furthermore the process is associated with rather low efficiency penalties of approximately 5-7% points. These features lead to relatively low CO2 avoidance costs below 20€/tCO2. Hence, the carbonate looping process provides an efficient and economic alternative to other CO2 capture technologies, in particular as a retrofit of existing power plants.
A large scale carbonate looping unit will most probably be realized as system of two coupled circulating fluidised bed reactors. The advantage of a circulating fluidised bed reactor is a highly efficient contact of small solid particles and large gas volumes in the process.