Development of Molten Salt Promoted Metal Oxide based Absorbents for CO2 Separation

CO2 capture and storage from both power generation and industrial activity is a central strategy for stabilization of atmospheric greenhouse gas concentrations to avoid drastic climate change. The deployment of fully integrated commercial CO2 capture and storage schemes is hindered by the considerable cost of current CO2 capture technologies. In pre-combustion (gasification or natural gas reforming) systems, capture of CO2 at warm temperatures (250-400 °C) with solid absorbents can provide a lower energy penalty than the use of low-temperature liquid absorbers, by avoiding the need to cool and reheat the gas stream. To date, efficient regenerable CO2 solid absorbents applicable at warm temperatures are still greatly desired. In this thesis, a molten salt promoting effect was discovered that can significantly facilitate the CO2 reaction with bulk metal oxides. This leads to the invention of a series of molten salt promoted metal oxide or metal oxide contained double salt absorbents with superior performance, applicable to different warm temperature windows. A facile preparation procedure utilizing ball milling was developed to prepare these absorbent materials. The roles of each individual component in the absorbent mixture were discussed in the exemplary system of NaNO3 promoted MgO-Na2CO3. For this same system, the chemistry was tuned for optimal performance, which was demonstrated in a fixed bed reactor. NaNO3 promoted MgO was chosen as the basic material system to study the origin of the significant promotion effects of molten salts on metal oxides. Comprehensive experimental and computational calculation results reveal that this facilitation originates from the capability of molten nitrate to dissolve bulk MgO. Dynamic MgO dissolution/precipitation equilibrium in molten nitrate provides activated MgO species accessible to CO2 at gas-solid-liquid triple phase boundaries. This proposed reaction mechanism is also applicable to other systems composed of different molten salts with other basic metal oxides or double salts, inspiring the design of absorbents that require activation of the bulk material. It is also proposed here that molten NaNO3 acts as a phase transfer catalyst in the gas-solid reaction between CO2 and MgO, by converting the solid reaction environment into liquid and providing an alternate reaction pathway to traditional gas-solid reactions