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