Solid oxides, hydroxides and carbonates

The use of solid inorganic bases, rather than their solutions, for ultradilute CO2

removal provides a conceptually straightforward path to CO2 capture from air and can offer effective sorbents for power plants flue gasses treatment; the reactions for different metals cycles are the same involved in water solution processes but, in these cases, take place on the solid surface, commonly in presence of moisture.

Direct carbonation of quicklime (CaO) could be achieved at temperatures between 300 and 450 °C while higher temperatures, often near to 900 °C, are involved in desorption, as thermodynamically favour the formed limestone (CaCO3) decomposition. Despite being not necessary, the presence of water vapor, increases the extent and the rate of carbonation, due to the formation of a film on the solid surface and the adsorption of CO2 by OH⁻ groups. The use of Ca(OH)2, as expected, offers a faster carbonation reaction with a higher degree of conversion, even without the presence of moisture; for this reason, the carbonation step can be carried out at slightly lower temperatures, ranging from 200 to 425 °C, but the sorbent regeneration still relies on a calcination step (97).

The main drawbacks of these solutions are that the CO2 solid bulk diffusion from the surface result to be strongly limiting the practical achievement of maximum uptake capacity, which also suffers from a marked reduction by subsequent cycles, and its, and gas/solid contact enhancement would emphasise material attrition issues. Moreover, high temperatures are involved both in the desorption, to reverse the strong binding of CO2 commonly achieved through calcination, as in the adsorption step, to assure effective reaction rates. Anyway, the low cost of the limestone sorbent, its reactivity toward both CO2 and SO2 and its resistance to high temperatures, indeed required for the process, make it particularly suitable for post-combustion capture, as neither cooling nor desulfurization pre-treatment would be needed.

Despite Na-based thermochemical cycles are favourable processes in solution and can offer energy benefits also in solid systems, the significantly slow reaction rates during carbonation and subsequent large mass flow rates render

55 the associated processes inefficient respect to those applying Ca-based cycles.

Indeed, during carbonation at 25 °C, NaOH was found to reach only 9%

conversion after 4 h, while Na2CO3 in water-saturated air achieved only 3.5%

conversion after 2 h. The closing of the cycles could be carried out easier, starting from NaHCO3 as well as Na2CO3, that reached completion after heating at 90−200 °C for 3 min, and at 1000−1400 °C for 15 min, respectively (98).

Similar thermochemical cycles based on lithium can be also exploited, using its zirconates (Li2ZrO3) and silicates (Li4SiO4), that absorb CO2 through the reactions (18), (19), involving the formation of a lithium carbonate solid phase.

(18) 𝐿𝑖₂𝑍𝑟𝑂₃+ 𝐶𝑂₂ ⇄ 𝐿𝑖₂𝐶𝑂₃+ 𝑍𝑟𝑂₂ (19) 𝐿𝑖₄𝑆𝑖𝑂₄+ 𝐶𝑂₂ ⇄ 𝐿𝑖₂𝑆𝑖𝑂₃+ 𝐿𝑖₂𝐶𝑂₃

Lithium zirconate offers reaction reversibility within the temperature range of 450 to 590 °C, allowing improved thermal management by swinging the temperature in a narrow window for adsorption and regeneration cycles;

moreover, the addition of other alkali metal oxides or carbonates, such as K2CO3, forms binary or ternary eutectic compositions, which improve CO2 adsorption by a molten carbonate film on the surface of the zirconate particles (99).

Lithium silicate exhibits higher CO2 capacity than its zirconate, up to 10 times with respect to other oxides values, and could offer great advantages in post-combustion capture, as an alternative carbonation process to Ca cycles, like shown in Figure 9. Indeed, it offers rapid adsorption kinetics, stability and operation at elevated temperatures below 720 °C; in addition, reaction reversibility at temperatures slightly above, allows to achieve great thermal efficiency. However, the overall rate of CO2 uptake, which exhibits first order kinetics with respect to CO2 partial pressure, seems to be controlled by a highly activated and sluggish surface reaction, so the introduction of lattice defects doping the adsorbents with many different hetero metal atoms, including Al, Ti, V, Fe, Ge, Mg and Nd, is a promising solution for promoting the adsorption capacity of Li4SiO4 (100).

56 Figure 9:Comparison of Ca-based and Li-based cycles for CO2 capture from

flue gases. (100)

Chemical adsorption of CO2 with a dry regenerable alkali-metal (Li, Na, K) carbonate-based solid sorbent exploits reaction (16), listed above for sodium, to absorb CO2, with moisture, through bicarbonate formation at 60 to 110 °C and extract it heating the bicarbonate at 100 to 200 °C. Among different alkali metal, K2CO3 performs the best out of the three carbonates, showing a wide carbonation temperature range where the sorbent efficiency is 100%, while NaCO3 could be even used as it offers better economics. Due to solid mass transfer limitations these sorbents should be often dispersed on different, active or less, supports;

despite CO2 uptake capacity of the sorbent is higher as the higher is the carbonate loading, above an optimum value, excess carbonate would block the support micropores, restricting CO2 supply at the active reaction sites, with a reduction of adsorption kinetics (101).

Support materials could be both hydrophilic, like silica gel, alumina and vermiculite, and hydrophobic, such as different kinds of ACs; as discussed above, the porous support matrix is often a sorbent by itself, and can show different synergic, as well as undesired, effects when combined with an active phase. Porous alumina support, impregnated with the active phase, offers the highest dynamic capacity, which however decreases heavily after the first cycle, while AC-impregnated sorbent systems allow a completely reversible adsorption and regeneration. The main cause of CO2 uptake capacity loss in carbonates supported on alumina or other metal oxides, such as MgO, is attributed to the

57 formation of KAl(CO3)2(OH)2, K2Mg(CO3)2, and K2Mg(CO3)2 3 4(H2O) phases during carbonation, which are not converted to the original K2CO3 phase after regeneration (102).

In summary, the high CO2 capture capacity and the favourable process temperatures could make supported carbonates a better solution than metal oxides and hydroxides, also cheaper than many other sorbents, for post-combustion CO2 capture as for DAC. Anyway, to be commercially viable, the long-term stability and persistence performance of these sorbents under real operating conditions have yet to be established.