Thermodynamics of MgO based CO2 capture

https://doi.org/10.1016/j.cogsc.2022.100645

“Here, we point out some aspects of recent research activities on MgO-based sorbents, which, in our opinion, are misleading regarding the practical use of these materials. There is experimental evidence that the promotion with AMS does not only improve the carbonation kinetics of MgO but also affects the thermodynamic equilibrium between MgO and MgCO₃, in particular when Mg-double carbonates are involved in the carbonation reaction [45, 46, 47]. The thermodynamic equilibrium defines the principal application range of MgO sorbents [48], and Figure 1a plots the equilibrium partial pressure of CO₂ (p_CO2,eq) as a function of temperature for different carbonation reactions with MgO. As observed previously [35,49], the thermodynamic data is inconsistent for the reaction MgO + CO₂ →MgCO₃, with the p_CO2,eq differing by an order of magnitude when comparing data using HSC Chemistry (V6.1) and FactSage (V6) databases. The database from HSC Chemistry reported by Duan et al. is the same as that provided by McBride et al. [50], and overestimates the equilibrium partial pressure, as is evident from numerous experimental studies that have shown that MgCO₃ does form from MgO in pure CO₂ at atmospheric pressure at > 300 °C [51∗∗, 52, 53, 54] (although this should have been ruled out when relying on that thermodynamic data). Equilibrium partial pressures reported by Knight et al. [17] or Zevenhoven et al. [55,56] are in line with those from the FactSage database, and they appear to represent better the thermodynamic equilibrium of MgO and MgCO₃ [57]. Duan et al. have used ab initio calculations to assess the thermodynamic equilibrium also for Mg-double carbonates, for example, using Na₂CO₃ as the second carbonate (MgO + Na₂CO₃+CO₂→Na₂Mg(CO₃)₂) [49,58]. They found that the p_CO2,eq at a given temperature is lowered significantly compared to the reaction MgO + CO₂→MgCO₃, which offers interesting possibilities to tune the operating window of MgO-based sorbents such that it fits practical needs (note that in such sorbent materials the theoretical CO₂ sorption capacity is reduced roughly by the weight fraction of the second carbonate). Their reference calculation for MgO + CO₂→MgCO₃ was in agreement with that using the HSC Chemistry database, implying the results should be interpreted with care. Indeed, the same authors observed that the ab initio calculations underestimated the experimentally determined p_CO2,eq by almost an order of magnitude [59], which emphasizes the need for more experimental studies on the thermodynamic properties of Mg-double carbonates to define their theoretical operating window. Hu et al. made similar experimental observations when promoting MgO with a mixture of alkali metal salts (LiNO₃, KNO₃, Na₂CO₃ and K₂CO₃, ca. 27 wt.% in total) and CaCO₃ (ca. 5 wt.%) [60]. They found that the AMS-promoted sorbent had a lower p_CO2,eq than MgO for temperatures >350 °C, thereby increasing the driving force (p_CO2 – p_CO2,eq) for CO₂ sorption for a given combination of T and p_CO2.”

“Figure 1. (a) Equilibrium partial pressure of CO₂, p_CO2,eq, as a function of temperature T for different carbonation reactions with MgO. (b) Maximal CO₂ capture efficiency, (p_CO2 – p_CO2,eq)/p_CO2, as a function of the partial pressure of CO₂ in the gas, p_CO2, and temperature, T. The solid line indicates p_CO2,eq(T) for the reaction MgO + CO₂→MgCO₃ using thermodynamic data reported by Zevenhoven et al. [56].”

“The implication of the thermodynamic equilibrium on the maximal CO₂ capture efficiency (with (p_CO2 – p_CO2,eq)/p_CO2 being a measure of how much CO₂ can theoretically be removed from a gas stream) is illustrated in Figure 1b for the reaction MgO + CO₂→MgCO₃. Any CO₂ capture technology (apart from those that can potentially run from abundant renewable energy such as direct air capture) requires costs and energy, and so the reaction conditions should be chosen such that the theoretical CO₂ capture efficiency is maximized. Reasonably high CO₂ capture efficiencies (>75%) require temperatures as low as 300 °C if CO₂ removal from a gas stream containing 15 vol.% CO₂ is targeted (as in postcombustion CO₂ capture scenarios [3]); CO₂ capture efficiencies >95% would require temperatures <265 °C. Thus far, there are hardly any studies that have experimentally demonstrated the feasibility of rapid CO₂ sorption for such low p_CO2 [61,62], suggesting that the practical driving force for CO₂ sorption is much higher, requiring either a greater p_CO2 or a lower temperature (which is kinetically even less favorable). The same applies to AMS-promoted sorbents, which have not been shown to affect the p_CO2,eq significantly for temperatures <350 °C unless Mg-double carbonates are the main product. Thus, MgO-based sorbents, whether promoted or not, cannot be used as high-temperature sorbents for the efficient CO₂ removal from gas streams with low p_CO2, and they should not be considered as prospective sorbents in postcombustion schemes in the context of carbon capture and storage (CCS). For such applications, good sorbents would need to function as concentrators, that is, they would absorb most of the CO₂ from a gas stream diluted in CO₂, and then release a stream of almost pure CO₂, ideally suitable for sequestration upon regeneration at a higher temperature. This is not possible with MgO-based sorbents operating under conditions (T ≥ 300 °C), facilitating the formation of stable carbonates [13,63].”