Assessment of the Effect of Process Conditions and Material Characteristics of Alkali Metal Salt-Promoted MgO-Based Sorbents on Their CO2 Capture Performance
CO<sub>2</sub> capture using alkali metal salt (AMS)-promoted MgO-based sorbents at intermediate temperatures (300 – 500 °C) has gained increased interest recently. The prospects of such materials for CO<sub>2</sub> 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 (incl. NaNO<sub>3</sub>, LiNO<sub>3</sub>, K<sub>2</sub>CO<sub>3</sub> and Na<sub>2</sub>CO<sub>3</sub>), and examined how particle size (from powder to pelletized 500 μm particles) and reaction conditions (calcination/carbonation temperature, and partial pressure of CO<sub>2</sub>) affect the cyclic CO<sub>2</sub> uptake using a thermogravimetric analyzer (TGA) at ambient pressure. The TGA results showed that the CO<sub>2</sub> uptake of the sorbents decreased significantly after pelletization, losing 74 % of its initial capacity. However, the CO<sub>2</sub> uptake capacity of the pelletized sorbents continued to increase over 100 cycles and reached a value (~ 0.46 g<sub>CO2</sub>/g<sub>sorbent</sub>) close to that of the powdery sample (~ 0.53 g<sub>CO2</sub>/g<sub>sorbent</sub>). Analysis via X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscope (SEM) and N<sub>2</sub> physisorption suggests that the increase in CO<sub>2</sub> uptake was related to a change of the nature of the alkali species within the molten phase that is reflected by their re-crystallization behavior when cooling them down to room temperature, and appeared to be affected by the CO<sub>2</sub> partial pressure present during carbonation. Finally, the CO<sub>2</sub> 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 CO<sub>2</sub> from a gas stream at ambient pressure. The CO<sub>2</sub> uptake of the sorbents in the packed bed experiments was very close to that in the TGA experiments; however, the CO<sub>2</sub> capture efficiency was less than 10 %, which currently appears too low for an industrial post-combustion CO<sub>2</sub> capture process to be viable. New material developments should not only focus on improving the rate of formation of MgCO<sub>3</sub> from MgO, but also assess whether CO<sub>2</sub> removal with such sorbents is actually feasible.
