Double sodium salt-promoted mesoporous MgO sorbent with high CO₂ sorption capacity at intermediate temperatures under dry and wet conditions

Mesoporous MgO·Na₂CO₃·NaNO₃ composites were prepared using supercritical drying of methanol for CO₂ capture in power plants at an intermediate temperature range between 250 and 450 °C. The effects of the molar ratio of salt, temperature, and gas composition on the CO₂ sorption were investigated under dry and wet conditions in order to clarify the sorption mechanisms and roles of NaNO₃ and Na₂CO₃. The composites exhibited excellent sorption capacities of 56.0 wt.% at 325 °C in pure CO₂ and 50.8 wt.% at 275 °C in a wet gas mixtures (10% CO₂, 2.5% H₂O, and balanced N₂). The CO₂ sorption mechanism was dominated through the formation of MgCO₃ and Na₂Mg(CO₃)₂ with Na₂CO₃ working as a CO₂ carrier, while NaNO₃ functioned as a reaction promoter. Under wet conditions, the formation of Mg(OH)₂ resulted in fast sorption rates and high capacities even at low CO₂ concentrations in the gas feedstock. One phase (liquid) condition of NaNO₃ and water vapor during a sorption and regeneration cycle resulted in a high stability of the sorbent. The working capacity in a 14 cycle test under N₂ regeneration (10 min) at 450 °C was 31.8 wt.% at 325 °C in pure dry CO₂ and 29.4 wt.% at 275 °C with a wet CO₂ mixture.