Enhancing the intermediate-temperature CO2 capture efficiency of mineral MgO via molten alkali nitrates and CaCO3: Characterization and sorption mechanism

Abstract

Carbonate looping using MgO-based sorbents has recently aroused the scientific interest as an auspicious technology for intermediate temperature (200−400 °C) CO2 capture, with the main hindrance being the slow sorption kinetics of MgO. This study investigated the preparation of MgO-based sorbents derived from mineral magnesite and promoted with a mixture of Li, Na and K nitrates, which by shifting into a molten state can reinforce the CO2 capture kinetics. Mineral limestone was also tested as promoter, which enabled the formation of CaMg(CO3)2 as another carbonate product except MgCO3. In-situ X-ray diffraction was employed to interpret the sorption mechanism of materials with different promoter contents and under various operating conditions. Even though CaMg(CO3)2 required a minimum temperature of 300 °C for formation in contrast to MgCO3, it generally displayed a faster nucleation rate, which was profitable mainly when applying a low CaCO3 to MgO molar ratio. However, the CaMg(CO3)2 decomposition demanded higher temperatures than MgCO3, which were associated with more pronounced sintering of the sorbent and elevated energy requirements of the process. The increase of the alkali nitrate content led to a deterioration of the pore network, but also enabled a faster generation and growth of carbonate products. Lowering the CO2 concentration of gas feedstock for more realistic carbonate looping applications required a slightly milder operating temperature for efficient CO2 sorption to enhance both CO2 solubility and carbonation kinetic driving force. The results proved the feasibility of mineral ores as sustainable low-cost precursors for intermediate-temperature CO2 capture.