Molten Salt-Promoted MgO-Based Sorbents for Intermediate-Temperature CO2 Capture: Performance Assessment for Post-Combustion Applications and Mechanistic Role of LiNO3

Climate change mitigation requires effective CO2 capture technologies, with MgO-based sorbents emerging as promising candidates for intermediate-temperature (200-400 °C) applications due to their high theoretical capacity (24.8 mmol/g), thermal stability, and cost-effectiveness. While practical performance is severely limited by rapid formation of dense MgCO3 surface layer that blocks further CO2 diffusion, molten nitrate can prevent its formation and significantly enhance MgO’s carbonation. This thesis systematically investigates molten salt-promoted MgO-based sorbents for CO2 capture using both fixed-bed reactor and thermogravimetric analysis, addressing critical performance gaps between laboratory and practical conditions. The fixed-bed experiments demonstrate the thermodynamic constraints of MgO/MgCO3 equilibrium at reduced CO2 partial pressures. Among the tested alkali metal salts, Na-salts exhibited the highest initial uptake, but poor stability compared to K-salts, while LiNO3 leads to significant performance fluctuations during cycling. Herein, this study further investigates the individual effects of lithium nitrate (LiNO3) and its transformation product, lithium carbonate (Li2CO3), on MgO carbonation from both kinetic and thermodynamic aspects. Results show that Li+ (from LiNO3) inserts into MgO/MgCO3 lattices, slowing late-stage diffusion and elevating the MgO/MgCO3 equilibrium CO2 pressure. In contrast, Li2CO3 acts as a nucleation seed and stabilizer, outperforming LiNO3 by up to 159% at 0.2 atm CO2 and sustaining 33% MgO conversion after 20 cycles, 23% higher than Na2CO3-promoted analogues. Furthermore, mass-spectrometric tracking of NOx evolution demonstrates that MgCO3 drastically lowers LiNO3 thermostability, triggering denitration well below typical regeneration temperature (~400 °C), leading to our conclusion that LiNO3-rich sorbents cannot avoid gradual transformation to Li2CO3 during operation. Overall, the work reconciles conflicting literature by showing that Li2CO3 rather than LiNO3 drives performance gains. Therefore, we recommend substituting Li2CO3 for LiNO3 in molten salt modified MgO to achieve durable, high-capacity, and environmentally friendly sorbents.