(27b) Theoretical Screening of Mixed Solid Sorbents for CO2 Capture Technology

Carbon dioxide is one of the major combustion products which once released into the air can contribute to global climate change.^1-4 It is generally accepted that current technologies for capturing CO₂including solvent-based (amines) and CaO-based materials are still too energy intensive. One approach to solve such environmental problems is to capture and sequester the CO₂. Hence, there is critical need for development of new materials that can capture and release CO₂ reversibly with acceptable energy costs. In particular, solid oxide sorbent materials have been proposed for capturing CO₂ through a reversible chemical transformation leading primarily to formation of carbonate products. Solid sorbents containing alkali and alkaline earth metals have been reported in several previous studies to be promising candidates for CO₂ sorbent applications due to their high CO₂ absorption capacity at moderate working temperatures.^5-8

During past few years we developed a theoretical methodology to identify promising solid sorbent candidates for CO₂ capture by combining thermodynamic database searching with ab initio thermodynamics obtained based on first-principles density functional theory (DFT) and lattice phonon dynamics.^8-19 The primary outcome of our screening scheme is a list of promising CO₂ sorbents with optimal energy usage. The calculated thermodynamic properties of different classes of solid materials versus temperature and pressure changes were further used to evaluate the equilibrium properties for the CO₂ adsorption/desorption cycles. According to the requirements imposed by the pre- and post- combustion technologies and based on our calculated thermodynamic properties for the CO₂ capture reactions by the solids of interest, we were able to identify only those solid materials for which lower capture energy costs are expected at the desired pressure and temperature conditions. These CO₂ sorbent candidates were further considered for experimental validations.

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, in this study, we demonstrate that by mixing different types of solids it’s possible to shift T_t to the a range of practical operating temperature conditions. 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). Now, 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. An example of this case is represented by Li₂O as A component. 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). In this way, the turnover T and the CO₂ capture capacity of mixtures decrease with decreasing the ratio of Li₂O/SiO₂ or Li₂O/ZrO₂.^8-19

(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. 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.^20-22 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.^20-22

(3)  T_A and T_B are close to each other. In this case, both A and B components are active to capture CO₂, and the CO₂ capacity of the mixture is the summation of those of A and B. As we know another potential advantage of mixing solids is to increase the surface area of the solids in order to have faster reaction rates. Such a mixing scenario doesn’t show too much advantage in shifting the capture temperature, but may enhance the kinetics of the capture process and eventually make the mixtures more efficient. Although there is no such report in literature, we think such an attempt is worthwhile and are working on several doped systems.

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, D. Luebke, J. K. Johnson, H. Pennline, B. Li, D. King and X. F. Wang for their help and colaborations.

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