Investigations of the Mechanisms that Govern Carbon Dioxide Sequestration via Aqueous Olivine Mineral Carbonation

摘要

Coal, in particular, and fossil fuels, in general, are well positioned to supply the world's energy needs for centuries to come if the environmental challenges associated with anthropogenic carbon dioxide emissions can be overcome. Carbon dioxide sequestration is being actively pursued as an option to reduce CO_2 emissions, while still enjoying the advantages of low-cost fossil fuel energy. Mineral carbonation is an intriguing CO_2 sequestration candidate technology, which provides environmentally benign and geologically stable CO_2 disposal in the form of mineral carbonates. Importantly, such disposal bypasses many long-term storage problems by (ⅰ) providing permanent containment, (ⅱ) avoiding adverse environmental consequences, and (ⅲ) essentially eliminating the need for continuous site monitoring. The primary challenge for viable sequestration process development is reducing process cost. Enhancing carbonation rates is crucial to reducing cost. This is the primary focus of the CO_2 Mineral Sequestration Working Group managed by Fossil Energy at DOE. Carbonation of the widely occurring mineral olivine (e.g., forsterite, Mg_2SiO_4) is a leading process candidate, which converts CO_2 into the environmentally benign mineral magnesite (MgCO_3). As olivine carbonation is exothermic, it offers intriguing low-cost potential. Recent studies at the Albany Research Center have found aqueous-solution carbonation is particularly promising. Cost-effectively enhancing carbonation reactivity is central to reducing process cost. Many of the mechanisms that impact reactivity occur at the solid/solution interface. Understanding these mechanisms is central to the engineering of processes to enhance carbonation reactivity and lower cost. Herein, we describe our investigations of mineral carbonation reaction mechanisms for a model phase-pure olivine. Aqueous-solution olivine carbonation was discovered to be a complex process associated with passivating silica layer formation and cracking, silica surface migration, olivine etch pit formation, transfer of the Mg and Fe in the olivine into the product carbonate, and the nucleation and growth of magnesite crystals on/in the silica/olivine reaction matrix. These phenomena occur in concert with the large solid volume changes that accompany the carbonation process, which can substantially impact carbonation reactivity.