CO₂ Uptake and Cyclic Stability of MgO-Based CO₂ Sorbents Promoted with Alkali Metal Nitrates and Their Eutectic Mixtures

Abstract

CO2 capture and storage (CCS) is a technological solution to stabilize or even reduce the atmospheric concentration of the greenhouse gas CO2, to mitigate climate change. In this context, MgO is a promising solid CO2 sorbent, as the energy penalty sorbent regeneration is comparatively small, but it requires the addition of promoters, typically alkali metal nitrates, to yield acceptable kinetics. Under operating conditions, the promoters are in a molten state. The main objectives of this work are (i) to assess experimentally the validity of different reaction mechanisms for the CO2 uptake of promoted MgO that are currently debated in literature and (ii) to elucidate the processes that lead to sorbent deactivation. Our experimental results support the mechanism in which the dissolution of MgO in the molten nitrate promoter is the rate-limiting step for carbonation. We were able to establish a direct correlation between the solubility of MgO in the promoter and the initial rate of carbonation. In addition, a systematic study of a large number of promoter compositions (mixtures of LiNO3, NaNO3, KNO3) indicate that promoters with a lower melting point exhibit higher CO2 uptakes, presumably due to their lower viscosity and, thus, higher ion mobility at a given temperature. Concerning the cyclic stability of promoted MgO, a decay of its CO2 uptake with number of carbonation/calcination cycles is ascribed only partially to sintering. Instead, the surface migration of the promoter was identified as an at least equally relevant deactivation mechanism. Importantly, it was also found that the CO2 uptake of the deactivated sorbent can be restored to a large extent with a simple hydration step.

NaNO3‐Promoted Mesoporous MgO for High‐Capacity CO2 Capture from Simulated Flue Gas with Isothermal Regeneration

“…At 280 °C, the sorption demonstrated a prolonged induction period throughout the entire 12 h of CO 2 exposure, showing only 0.18 mmol g −1 uptake, and never quite showing a significant increase in uptake rate that was observed at the other temperatures tested. These results differed from previous studies that were performed at high concentrations of CO 2 (above molar concentration of 85 % CO 2 ) in that similar materials studied by others typically showed the highest CO 2 capacity in the temperature range between 300 °C and 350 °C …”

“…The different, advantageous behavior observed from the partially desorbed sample also led us to re‐examine the hypotheses about the nature of the rate limiting step in the CO 2 sorption using molten alkali‐metal‐promoted MgO sorbents. Until now, the two most supported suggestions regarding the rate limiting step in CO 2 sorption have been: 1) dissolution of MgO in the NaNO 3 medium to form [Mg 2+ ⋅⋅⋅O 2− ] ionic pairs or 2) formation of carbonate ions in the molten medium owing to the high solubility of O 2− ions in NaNO 3 medium . As previously observed in Figure S5, a significantly shortened induction period was observed for CO 2 sorption under exposure to pure CO 2 than under 10 % CO 2 /He.…”

“…The bands at 865 and 1296 cm −1 were immediately observed, likely representing surface unidentate carbonates formed upon CO 2 exposure . The peaks at 1088, 884 and 746 cm −1 were assigned to ν 1 , ν 2 , ν 4 modes of bulk carbonate ions . These three bands were not clearly observed until approximately 50 min after CO 2 exposure, indicating that some time was needed until a bulk carbonate was formed.…”

“…Although the reason for the induction period at the initial stage of CO 2 sorption remained unclear, they hypothesized that the CO 2 initially dissolved in the molten NaNO 3 medium, and then interacted with oxide ions (O 2− ) within the nitrate medium to form carbonate ions (CO 3 2− ). The formed carbonate ions have improved contact with MgO than when no nitrate medium was present (e.g., in Na‐free materials) and readily react with MgO and initiates nucleation of MgCO 3 …”

“…Its potential as a promising candidate for CO 2 sorption grew even further following a report that porous MgO promoted by molten alkali metals enabled higher CO 2 sorption capacity and faster uptake kinetics in an intermediate temperature range (250–400 °C) than a bare MgO sorbent . As a result, there has been a growing interest in using such molten‐alkali‐metal‐promoted MgO for CO 2 capture in an intermediate temperature range . Zhang et al.…”