(48a) Computational Modeling of Mixture Solids for CO2 Capture Sorbents

Nowadays, the burning of fossil fuels is still the main energy source for the world’s economy. One consequence of the use of these fuels is the emission of huge quantities of CO₂ into the atmosphere creating environmental problems such as global climate warming.^1-3 In order to solve such problems, there is a need to reduce CO₂ emissions into atmosphere by capturing and sequestrating CO₂.^4-6

For a given CO₂ capture process, its optimal working conditions (CO₂ pressures of pre- and after-capture, absorption/desorption temperature range (ΔT_o), etc.) were fixed. However, at a given CO₂ pressure, the turnover temperature (T_t) of an individual solid capture CO₂ reaction is fixed. Such T_t may be outside the operating temperature range ΔT_o for a particularly capture technology. In order to adjust T_t to fit the practical working through reversible chemical transformations ΔT_o, we have demonstrated that by mixing different types of solids it’s possible to shift T_t to the practical operating ΔT_o range.^7-9 Generally, when we mix two solids A and B to form a new sorbent C, the turnover temperature of the newly resulted system (T_C) is located between those of A and B (T_A, T_B). Here it was assumed that A is a strong CO₂ sorbent while B is a weak CO₂ sorbent and T_A>T_B. Also, we assumed that the desired operating temperature T_O is between T_A and T_B (T_A>T_O>T_B). Depending on the properties of A and B, we have typically three scenarios to synthesize the mixing sorbent C:

(1) T_A>>T_B and the A component is the key part to capture CO₂. In this case since T_A is higher than T_O, by mixing B into A will decrease the turnover T_C of the C solid to values closer to T_o. For example, Li₂O is a very strong CO₂ sorbent which forms Li₂CO₃. However, its regeneration from Li₂CO₃ only can occur at very high temperature (T_A). In order to move its T_A to lower temperatures, one can mix some weak CO₂ sorbents (such as SiO₂, ZrO₂). Our results showed that in this way, the turnover T and the CO₂ capture capacity of mixtures decrease with decreasing the ratio of Li₂O/SiO₂ or Li₂O/ZrO₂.^7-13

(2) T_A>>T_B and B component is the key part to capture CO₂. In this case, since T_B is lower than T_O, by mixing A into B will increase the turnover temperature T_C of the C solid to values closer to T_o. For example, pure MgO (as B component) has a very high theoretical CO₂ capture capacity.¹⁴ However, its turnover temperature (250 °C) is lower than the required temperature range of 300-470 °C used in warm gas clean up technology and its practical CO₂ capacity is very low, and therefore, it cannot be used directly as a CO₂ sorbent in this technology. Our experimental and theoretical results showed that by mixing alkali metal oxides M₂O (M=Na, K, Cs, Ca) or carbonates (M₂CO₃) into MgO, the calculated results showed that the corresponding newly formed mixing systems have higher turnover temperatures making them useful as CO₂ sorbents through the reaction MgO + CO₂ + M₂CO₃ = M₂Mg(CO₃)₂.^15-17

(3) Both A and B components are active to capture CO₂. In this case, the CO₂ capacity of the mixture is the summation of those of A and B. Li₂MSiO₄ (M=Mg, Ca, etc.) and M_2-aN_aZrO₃ (M, N=Li, Na, K) belong to this category. Obviously, those doped systems can be treated as mixing of three solids (Li₂O:MO:SiO₂, M₂O:N₂O:ZrO₂). In this study, we focus on this type of mixing sorbents: Na_2-¨»M_¨»ZrO₃ (M=Li, Na, K, ¨»=0, 0.5, 1.0, 1.5, 2.0).¹⁸

Our obtained results showed that by changing the mixing ratio of solid A and solid B to form mixed solid C it’s possible to shift the turnover T_t of the newly formed solid C to fit the practical CO₂ capture technologies. When mixing SiO₂ or ZrO₂ into the strong Li₂O sorbent, one can obtain a series of lithium silicates (or zirconates) with T_t lower than that of pure Li₂O. By mixing oxides (Na₂O, K₂O, CaO) or their corresponding carbonates into MgO, the obtained mixtures exhibit different thermodynamic behaviors and their T_t are higher than that of pure MgO. Such results can be used to provide insights for designing new CO₂ sorbents. Therefore, although one single material taken in isolation might not be an optimal CO₂ sorbent to fit the particular needs to operate at specific temperature and pressure conditions, by mixing or doping two or more materials to form a new material, our results showed that it is possible to synthesize new CO₂ sorbent formulations which can fit the industrial needs. Our results also show that computational modeling can play a decisive role for identifying materials with optimal performance.

The author thanks Drs. D. C. Sorescu, B. Zhang, K. Zhang, S. X. Li, J. K. Johnson, H. Pennline, B. Li, D. King and X. F. Wang for their help and collaborations.

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