Na2CO3-modified CaO-based CO2 sorbents: the effects of structure and morphology on CO2 uptake

Abstract

Calcium looping (CaL) is a CO₂ capture technique based on the reversible carbonation/calcination of CaO that is considered promising to reduce anthropogenic CO₂ emissions. However, the rapid decay of the CO₂ uptake of CaO over repeated cycles of carbonation and calcination due to sintering limits its implementation at the industrial scale. Thus, the development of material design strategies to stabilize the CO₂ uptake capacity of CaO is paramount. The addition of alkali metal salts to CaO has been proposed as a strategy to mitigate the rapid loss of its cyclic CO₂ uptake capacity. However, there are conflicting results concerning the effect of the addition of alkali metal carbonates on the structure and CO₂ capacity of CaO. In this work, we aim at understanding the effect of the addition of Na₂CO₃ to CaO on the sorbent's structure and its CO₂ uptake capacity. We demonstrate that under industrially-relevant conditions the addition of as little as 1 wt% of Na₂CO₃ reduces severely the CO₂ uptake of CaO. Combining TGA, XAS and FIB-SEM analysis allowed us to attribute the performance degradation to the formation of the double salt Na₂Ca(CO₃)₂ that induces strong sintering leading to a significant loss in the sorbent's pore volume. In addition, during the carbonation step the formation of a dense layer of Na₂Ca(CO₃)₂ that covers unreacted CaO prevents its full carbonation to CaCO₃.