Magnesium Hydroxide Sorbents for Combined Carbon Dioxide Capture and Storage in Energy Conversion Systems

Ever increasing anthropogenic emissions of greenhouse gas carbon dioxide (CO₂) are considered as the main driving force behind climate change on Earth. Despite the incentives surrounding carbon-free energy, it will take decades until significant market penetration is achieved. In the meantime, while carboneous fossilized energy sources continue to dominate the energy blend and worldwide energy demand is continuously increasing, carbon capture and storage (CCS) can offer an immediate solution to fight climate change. Several high temperature CO₂ capture technologies are under development (e.g. chemical looping of calcium sorbents is predicted to provide zero emission energy from coal). Calcium sorbents must be recycled given its natural state in the Earth's crust as calcium carbonate. Looping raises concerns about the energy intensive sorbent regeneration and ultimate fate of the separated CO₂ as well as the degradation of the sorbent material. Unlike their Ca-based counterparts, Mg-bearing sorbents, derived from silicate minerals and industrial wastes, can act as combined carbon capture and storage media in various energy conversion systems. The magnesium carbonate formed during the carbon mineralization process is recognized as the most safe and permanent method to store anthropogenic CO₂. Despite the benefits of Mg carbon mineralization, the reactions experience limitations in terms of kinetics and overall conversion, depending on the reaction system, and the mechanisms are not well understood, especially at high temperature and pressure conditions. Mg(OH)₂ carbonation in the slurry phase is known to occur spontaneously and recent results show improved gas-solid carbonation with comparable materials in the presence of H₂O vapor. The pathways of H₂O enhanced gas-solid Mg(OH)₂ carbonation were investigated at elevated temperatures and CO₂ pressures (up to 400 °C and 15 atm). Attributed to the fast formation of hydrated carbonate intermediates, carbonation conversion showed dramatic increase with increasing H₂O loading. Still, carbonation of Mg(OH)₂ in a gas-solid system has largely demonstrated limited reaction kinetics and overall conversion. The gas-solid limitations and the enhanced effect of steam motivated an in-depth study of slurry phase Mg(OH)₂ carbonation. The literature lacked an investigation of Mg(OH)₂ slurry carbonation at elevated temperature, thus this study examined carbonation at moderate temperature and CO₂ pressure (up to 200 ºC and 15 atm). The reaction conditions responsible for hydrated and anhydrous carbonate product phases were evaluated and carbonate formation kinetics were investigated. Reaction temperature was found to be the dominant parameter driving the formation of specific carbonate phases. Anhydrous carbonate is most desirable from a carbon storage perspective, due to its magnesium efficiency, omission of additional crystal waters and thermodynamic stability; therefore solution additives were investigated for their role in bypassing formation of metastable intermediates. The use of MgCO₃ seed particles gave the best result, producing 100% anhydrous carbonate at 150 ºC where it was not observed previously. After the detailed study of Mg(OH)₂ carbonation in both gas-solid and slurry arrangements, its integration with an energy conversion process, the water gas shift reaction (WGSR), was explored to increase the sustainability of carboneous energy sources. The removal of CO₂ by the carbonation reaction enhanced hydrogen yield of the WGSR as the equilibrium of the gas phase reaction was shifted towards products. Unexpectedly, a side reaction was exposed, which converted CO to aqueous formate ion and limited the overall production of hydrogen. Overall, this study explored the fundamental chemistry relating to carbonate phase formation mechanisms and kinetics during the carbonation reaction of Mg(OH)₂ in various reactor systems.