TGA, fixed bed and fluidised bed reactor for MgO-based CO2 capture

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

“Another important aspect of current research activities on MgO-based sorbents is the reliance on thermogravimetric analyzers (TGA) to assess the performance of newly developed sorbents. The use of TGAs has also been criticized in the context of CaO-based CO₂ sorbents, largely because the sorbents are not exposed to mechanical stress during the assessment of their cyclic stability, which is, however, relevant should they be used industrially in (circulating) fluidized bed reactors [3,64]. TGAs are important tools for the early performance assessment of new sorbent developments and kinetic investigations. Only a fraction of the CO₂ entering the apparatus reacts with the sorbent, which is sufficient to induce a rapid weight increase as the sorbent carbonates but does not provide any information on how efficiently CO₂ is removed from the gas stream. Recent work by Chen et al. found that the CO₂ capture efficiency of an AMS-promoted MgO sorbent is <10% in a packed bed reactor operated at 325 °C, 80 vol.% CO₂ at 1 atm and a rather long gas–solid contact time [65]. Thus, the observed CO₂ capture efficiency was much lower than the theoretical CO₂ capture efficiency of 89% under these conditions (Figure 1b). Yet, the sorbent had a CO₂ uptake of more than 0.5 g CO₂ per g sorbent (measured both in the TGA and the packed bed reactor), which is among the highest reported [31,66, 67, 68]. For a similar material composition, Hu et al. observed in their packed bed reactor for sorption enhanced WGS that a pressure of at least 8 atm is required to remove >95% of the CO₂ from the gas stream through the MgO-based sorbent [60]. Both studies show that an excellent performance in the TGA (i.e. the fast transitioning between the oxide and carbonate states) does not necessarily translate into a good performance for CO₂ removal from a gas stream – the main objective of any CO₂ capture technology. The reason may, again, be that practically a much higher driving force (p_CO2 – p_CO2,eq) for CO₂ sorption is required for these materials than anticipated from thermodynamic considerations and that much greater gas–solid contact times are needed than anticipated from experience with other noncatalytic gas–solid reactions [69]. Analysis of breakthrough curves, as commonly used in the assessment of adsorbents at low temperatures, would often be more helpful in understanding whether new sorbent formulations are indeed as promising as often claimed [70, 71, 72, 73]. An exception may be the application of thermochemical energy storage [74, 75, 76, 77], for which the transition between the oxide and carbonate states determines the reaction enthalpy, and the CO₂ capture efficiency is of secondary importance.”