Analysis of the structural dynamics in a NaNO3-promoted MgO-based CO2-sorbent during cyclic operation: an in situ time-resolved X-ray powder diffraction study

ISSN:0021-8898ISSN:1662-534XISSN:1662-535

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

ISSN:2574-096

CO2 Uptake Potential of Ca-Based Air Pollution Control Residues over Repeated Carbonation–Calcination Cycles

ISSN:0887-0624ISSN:1520-502

Deciphering the structural dynamics in molten salt–promoted MgO-based CO2 sorbents and their role in the CO2 uptake

The development of effective CO2 sorbents is vital to achieving net-zero CO2 emission targets. MgO promoted with molten salts is an emerging class of CO2 sorbents. However, the structural features that govern their performance remain elusive. Using in situ time-resolved powder x-ray diffraction, we follow the structural dynamics of a model NaNO3-promoted, MgO-based CO2 sorbent. During the first few cycles of CO2 capture and release, the sorbent deactivates owing to an increase in the sizes of the MgO crystallites, reducing in turn the abundance of available nucleation points, i.e., MgO surface defects, for MgCO3 growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na2Mg(CO3)2 crystallites that act effectively as seeds for MgCO3 nucleation and growth. Na2Mg(CO3)2 forms due to the partial decomposition of NaNO3 during regeneration at T ≥ 450°C followed by carbonation in CO2.ISSN:2375-254

Peering into Buried Interfaces with X-Rays and Electrons to Unveil MgCO3 Formation During CO2 Capture in Molten Salt Promoted MgO

The addition of molten alkali metal salts drastically accelerates the kinetics of CO2 capture by MgO through the formation of MgCO3. However, the growth mechanism, the nature of MgCO3 formation and the exact role of the molten alkali metal salts on the CO2 capture process remains elusive, holding back the development of more effective MgO-based CO2 sorbents. Here, we unveil the growth mechanism of MgCO3 under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO3. The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO2, a non-crystalline surface carbonate layer of ca. 7 Å thickness forms. In contrast, when MgO(100) is coated with NaNO3 MgCO3 crystals nucleate and growth. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO3. MgCO3 grows epitaxially with respect to MgO(100) and the lattice mismatch between MgCO3 and MgO is relaxed through lattice misfit dislocations. Pyramid shaped pits on the surface of MgO, in the proximity and below the MgCO3 crystals, point to the etching of surface MgO, providing dissolved [Mg2+…O2–] ionic pairs for MgCO3 growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions.</p

CO₂ Uptake Potential of Ca-Based Air Pollution Control Residues over Repeated Carbonation–Calcination Cycles

The operation of dry processes for acid gas removal from flue gas in waste-to-energy plants based on the use of calcium hydroxide as a solid sorbent generates a solid waste stream containing fly ash, unreacted calcium hydroxide, and the products of its reaction with acid pollutants in the flue gas (HCl and SO₂). To date, the fate of the solid waste stream is to be put into a landfill in the absence of commercially viable recycling approaches. The present study investigates the potential of these residues as CO₂ sorbents in the calcium looping process. Samples collected in different waste-to-energy plants were tested over multiple carbonation–calcination cycles, comparing their performance to that of limestone. Although inferior, the CO₂ sorption capacity of the residues resulted in values comparable to that of limestone and that steadily increased for a significant number of cycles. This peculiar behavior was attributed to the presence of a chlorinated phase, which enhances the CO₂ uptake in the diffusion-controlled stage of carbonation by reducing the product layer resistance to CO₂ diffusion. No significant release of acid gases was observed at the characteristic temperatures of calcium looping carbonation