Exceeding 100% CO2 capture efficiencies in a calcium looping system using Ca(OH)2 as a polishing sorbent: Experimental results from a 1.7 MWth pilot plant

Calcium Looping using circulating fluidized bed technology (CFB-CaL) is increasingly being developed to address the decarbonization challenges of hard-to-abate industrial sectors. This study experimentally demonstrates the potential to reach extremely high CO₂ capture efficiencies when using Ca(OH)₂ as an additional sorbent. Recent experimental campaigns conducted at the 1.7 MW_th La Pereda pilot plant tested a novel strategy that thermally decouples the carbonator into two distinct regions. In the lower solid-dense zone, CaO coming from the calciner captures 85–95% of the CO₂ in the flue gas. In the upper solids-lean section, heat exchangers reduce the temperature below 550 °C, overcoming equilibrium limitations while powdered Ca(OH)₂ is introduced as a polishing sorbent to further enhance CO₂ removal. Experiments have been carried out by feeding Ca(OH)₂ into the CFB carbonator of La Pereda pilot plant during short periods of time to facilitate the data interpretation. Despite the limitation of the facility to achieve sufficient Ca(OH)₂ residence time, the experimental results confirm that CO₂ capture efficiencies above 99%, including values >100% under some conditions, can be achieved. A basic plug flow model that incorporates known kinetic limitations in the carbonation of Ca(OH)₂ is consistent with the observations. In addition, the hydration of the CaO purge to produce Ca(OH)₂, together with complementary strategies such as recycling carbonated fly ash, emerges as a promising route to expand the operating window and reduce sorbent demand in future implementations. Results presented in this work can facilitate future scale up of this approach to attain the “zero emission” concept in industries implementing CO₂ capture by Calcium Looping.