Thermodynamic Properties of CO_2 Capture Reaction by Solid Sorbents: Theoretical Predictions and Experimental Validations

摘要

It is generally accepted that current technologies for capturing CO_2are still too energyintensive. Hence, there is a critical need for development of new materials that can captureCO_2 reversibly with acceptable energy costs. Accordingly, solid sorbents have been proposedto be used for CO_2 capture applications through a reversible chemical transformation. Bycombining thermodynamic database mining with first principles density functional theory andphonon lattice dynamics calculations, a theoretical screening methodology to identify the mostpromising CO_2 sorbent candidates from the vast array of possible solid materials has beenproposed and validated. The calculated thermodynamic properties of different classes of solidmaterials versus temperature and pressure changes were further used to evaluate theequilibrium properties for the CO_2 adsorption/desorption cycles. According to the requirementsimposed by the pre- and post- combustion technologies and based on our calculatedthermodynamic properties for the CO_2 capture reactions by the solids of interest, we were ableto screen only those solid materials for which lower capture energy costs are expected at thedesired pressure and temperature conditions. These CO_2 sorbent candidates were furtherconsidered for experimental validations. In this presentation, we first introduce our screeningmethodology with validating by solid dataset of alkali and alkaline metal oxides, hydroxidesand bicarbonates which thermodynamic properties are available. Then, by studying a series oflithium silicates, we found that by increasing the Li_2O/SiO_2 ratio in the lithium silicates theircorresponding turnover temperatures for CO_2 capture reactions can be increased. Compared toanhydrous K_2CO_3, the dehydrated K_2CO_3?1.5H_2O can only be applied for post-combustionCO_2 capture technology at temperatures lower than its phase transition (to anhydrous phase)temperature, which depends on the CO_2 pressure and the steam pressure with the best rangebeing P_(H_2O)≤1.0 bar. Above the phase-transition temperature, the sorbent will be regeneratedinto anhydrous K_2CO_3. Our theoretical investigations on Na-promoted MgO sorbents revealedthat the sorption process takes place through formation of the Na_2Mg(CO_3)_2 double carbonatewith better reaction kinetics over porous MgO, that of pure MgO sorbent. The experimentalsorption tests also indicated that the Na-promoted MgO sorbent has high reactivity andcapacity towards CO_2 sorption and can be easily regenerated either through pressure ortemperature swing processes.