Assessment of the Effect of Process Conditions and Material Characteristics of Alkali Metal Salt Promoted MgO-Based Sorbents on Their CO2 Capture Performance

CO2 capture using alkali metal salt (AMS)-promoted MgO-based sorbents at intermediate temperatures (300-500 degrees C) has gained increased interest recently. The prospects of such materials for CO2 capture were assessed in this work. We investigated the most reactive MgO-based sorbents that have been reported in the literature (i.e., MgO promoted with a combination of various AMS (including NaNO3, LiNO3, K2CO3, and Na2CO3)), and examined how particle size (from powder to pelletized 500 mu m particles) and reaction conditions (calcination/carbonation temperature and partial pressure of CO2) affect the cyclic CO2 uptake using a thermogravimetric analyzer (TGA) at ambient pressure. The TGA results showed that the CO2 uptake performance of the sorbents decreased significantly after pelletization, losing 74% of its initial capacity. However, the CO2 uptake capacity of the pelletized sorbents continued to increase over 100 cycles and reached a value (similar to 0.46 g(CO2)/g(sorbent)) close to that of the powdery sample (similar to 0.53 g(CO2)/g(sorbent)). Analysis via X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscope (SEM), N2 physisorption, and in situ XRD suggests that the increase in CO2 uptake was related to a change of the nature of the alkali species within the molten phase that is reflected by their recrystallization behavior when cooling them down to room temperature, and appeared to be affected by the CO2 partial pressure present during carbonation. Finally, the CO2 capture performance of the best-performing sorbents was evaluated in a packed bed reactor, in order to assess whether the most reactive sorbents are capable of removing a significant amount of CO2 from a gas stream at ambient pressure. The CO2 uptake of the sorbents in the packed bed experiments was very close to that in the TGA experiments; however, the CO2 capture efficiency was less than 10%, which currently appears too low for an industrial postcombustion CO2 capture process to be viable. New material developments should not only focus on improving the rate of formation of MgCO3 from MgO but also assess whether CO2 removal with such sorbents is actually feasible.