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

Abstract

CO 2 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 2 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 NaNO 3 , LiNO 3 , K 2 CO 3 , and Na 2 CO 3 )), and examined how particle size (from powder to pelletized 500 μm particles) and reaction conditions (calcination/carbonation temperature and partial pressure of CO 2 ) affect the cyclic CO 2 uptake using a thermogravimetric analyzer (TGA) at ambient pressure. The TGA results showed that the CO 2 uptake performance of the sorbents decreased significantly after pelletization, losing 74% of its initial capacity. However, the CO 2 uptake capacity of the pelletized sorbents continued to increase over 100 cycles and reached a value (∼0.46 g CO 2 /g sorbent ) close to that of the powdery sample (∼0.53 g CO 2 /g sorbent ). Analysis via X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscope (SEM), N 2 physisorption, and in situ XRD suggests that the increase in CO 2 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 CO 2 partial pressure present during carbonation. Finally, the CO 2 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 2 from a gas stream at ambient pressure. The CO 2 uptake of the sorbents in the packed bed experiments was very close to that in the TGA experiments; however, the CO 2 capture efficiency was less than 10%, which currently appears too low for an industrial postcombustion CO 2 capture process to be viable. New material developments should not only focus on improving the rate of formation of MgCO 3 from MgO but also assess whether CO 2 removal with such sorbents is actually feasible. KEYWORDS: CO 2 capture, MgO-based sorbents, Alkali metal salt, CO 2 partial pressure, CO 2 capture efficiency

Unravelling the Mechanism of Intermediate‐Temperature CO2 Interaction with Molten‐NaNO3‐Salt‐Promoted MgO

“…The view of dissolved molecular CO 2 and NO 2 + ion sharing a synergetic promotion effect on the MgO loaded with both alkali metal nitrates and carbonates is consistent with recent experimental studies. [ 8,39–42 ]…”

“…Different from what proposed earlier, [6,7,35,38] that CO 3 2-ions are formed by the reaction of dissolved CO 2 with the O 2ions in MgO, the former result from the conversion of NO + ion sharing a synergetic promotion effect on the MgO loaded with both alkali metal nitrates and carbonates is consistent with recent experimental studies. [8,[39][40][41][42] Overall, the introduction of molten salt changes the original reaction path of MgO with CO 2 by grabbing surface lattice oxygen from MgO, which promotes the rapid generation of O 2− and greatly reduces the relative dissociation energy barrier, from ΔE = 7.15 to 6.20 eV. This in turn greatly improves the efficiency of MgO toward CO 2 adsorption as described in detail in Table S2, Supporting Information, and Figure 7.…”

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

“…Given the lower sorption temperatures of MgO compared to CaO, a popular approach to enhance the kinetics of CO 2 uptake of MgO is its promotion with alkali metal nitrates such as LiNO 3 , NaNO 3 , and KNO 3 . ,− Webley and co-workers , studied adsorbents based on double salts of MgCO 3 with either K 2 CO 3 or Na 2 CO 3 prepared by wet mixing; others investigated also Cs 2 CO 3 or Rb 2 CO 3 . , K–Mg double salts reached a CO 2 uptake of 8 wt % at 375 °C in pure CO 2 , while Na–Mg double salts showed higher CO 2 uptakes reaching between 15 wt % and ∼20.7 wt % . Zhang et al .…”

“…It is interesting to note that the molten alkali salt has been observed to decompose and transform partially over repeated CO 2 capture and regeneration cycles at high temperatures of ca. 450 °C (e.g., NaNO 3 → NaNO 2 + 0.5 O 2 ) , forming double carbonates (e.g., Na 2 Mg­(CO 3 ) 2 ), which in some cases have shown to improve the CO 2 uptake over repeated cycling. , An alternate approach to stabilize the cyclic uptake of AMS-promoted MgO has been shown through the addition of hydrotalcites or TiO 2 . Here, the composite structure (e.g., MgO–TiO 2 ) enhanced the growth of more but small crystallites of MgCO 3 , thereby reducing sintering.…”

Porous CaO–MgO Nanostructures for CO2 Capture

“…Carbon dioxide capture, − utilizing solid sorbents, − has emerged as a sustainable strategy to handle anthropogenic CO 2 emissions. − Characteristics such as tunable wide-range sorption conditions (temperature/CO 2 concentration/chemical compositions), easy regeneration, and cycling stability resulting in high sorption efficiency and durability make solid sorbents a viable option. − Sorbents capture CO 2 molecules via either weak physical (adsorption) or strong chemical (acid–base) interactions. − The low-temperature (<200 °C) amine scrubbing technique (0.5 mol of CO 2 per mole of amine) has been widely adopted in the industry. , While the use of a mixture of pristine amines is an expensive process and involves hazardous byproducts, , the high capture capacity , of amine-impregnated/supported solid sorbents is limited due to the low accessibility of active sites by CO 2 and the leaching of amine over multiple regeneration cycles. ,, Carbon ,, /silica , /zeolite , /graphene/MOF ,,, /COF , /polymer ,, /clay-based materials also sorb CO 2 at low temperatures, and their capacity is largely controlled by the surface area available for weak adsorption . Carbonic anhydrase enzyme-based membranes have garnered interest for reversible CO 2 capture at ambient temperatures. , Layered double hydroxide-derived mixed oxides ,, and MgO − represent intermediate-temperature (200–400 °C) sorbents and mineralize CO 2 through adsorption within the morphological pores of metal oxides. High-temperature (>400 °C) sorbents largely comprise basic CaO-based materials. , CaO, with attributes such as ra...…”