CO₂ absorption and desorption characteristics of MgO-based absorbent promoted by triple eutectic alkali carbonate

Carbon capture and sequestration is emerging as a promising technology to mitigate the greenhouse effect by reducing CO₂ emissions. Of a number of metal oxides applied as CO₂ absorbents, MgO is a potential material that can operate in a relatively low elevated temperature range (200-500 °C), namely, intermediate-temperatures. In the present research, we investigated the characteristics of CO₂ absorption and desorption on MgO-based absorbents promoted by molten alkali metal carbonate that has a melting point of 397 °C (eutectic molar ratio of Li₂CO₃ : Na₂CO₃ : K₂CO₃ = 0.435 : 0.315 : 0.250). These absorbents absorb CO₂ in two steps with the first step being very fast and large in capacity in comparison with other related absorbents in the literature and the second step being much slower than the first one. The overall capacity can be as large as 77% MgO conversion, much larger than other carbonate-promoted MgO absorbents in the literature. The fast first step of CO₂ absorption is associated with the reaction of highly basic sites on the MgO surface formed through the interaction between the carbonates and MgO upon pre-treatment. Detailed analyses via in situ XRD revealed that MgCO₃ and a new phase, probably a double carbonate between Mg and alkali ions, are formed as the carbonation products. A detailed mechanism is proposed based on the experimental data, which highlights the unique properties of the molten alkali carbonate as a dissolution medium for CO₂ and MgO and even the product MgCO₃.