Environ. Sci. Technol. 2009, 43, 6314–6319
Reaction Mechanisms for Enhancing Mineral Sequestration of CO2 K A R A L E E J A R V I S , † R . W . C A R P E N T E R , * ,†,‡ TODD WINDMAN,§ YOUNGCHUL KIM,‡ RYAN NUNEZ,‡ AND FIRAS ALAWNEH‡ School of Materials, LeRoy Eyring Center for Solid State Science, and Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287
Received November 25, 2008. Revised manuscript received May 4, 2009. Accepted June 12, 2009.
Storage of CO2 through mineral sequestration using olivine has been shown to produce environmentally benign carbonates. However, due to the formation of a rate-limiting reaction product layer, the rate of reaction is insufficient for largescale applications. We report the results of altering the reactant solution composition and the resultant reaction mechanism to enhance the reaction rate. The products were analyzed for total carbon content with thermal decomposition analysis, product phase compositions with Debye-Scherrer X-ray powder diffraction (XRD), surface morphology with scanning electron microscopy (SEM), and composition with energy dispersive X-ray spectroscopy (EDXS). Carbon analysis showed that an increase in bicarbonate ion activity increased the olivine to carbonate conversion rate. The fastest conversion rate, 63% conversion in one hour, occurred in a solution of 5.5 M KHCO3. Additionally, SEM confirmed that when the bicarbonate ion activity was increased, magnesium carbonate product particles significantly increased in both number density and size and the rate passivating-reaction layer exfoliation was augmented.
Introduction Global warming, a current threat to modern society, has been shown to be directly related to human activity (1). The greatest human contributor to global warming, CO2 emissions, comes largely from the combustion of fossil fuels for electrical power (2, 3). Nevertheless, due to the rapidly increasing demand for energy, fossil fuels remain the major source of many countries’ energy (2, 4). Though alternative energy forms are being investigated, none have been developed to meet global energy demands for the near future (3, 4). As a result, options for large-scale CO2 storage through sequestration (ocean, underground, and mineral) have been studied extensively. Mineral sequestration produces stable and environmentally benign products (2, 4-8). However, current processes are too expensive for large-scale implementation (2, 4-8). To make mineral sequestration an economically viable solution for CO2 storage, emphasis has been placed on enhancing reaction rates without requiring additional energy inputs (2). In the United States, the CO2 Mineral Sequestration Working Group, managed by DOE (Fossil Energy) and composed of members from the Albany Research Center (ARC), Argonne * Corresponding author phone: (480) 965- 4549; e-mail: carpenter@ asu.edu. † Science and Engineering of Materials, School of Materials. ‡ LeRoy Eyring Center for Solid State Science. § Department of Chemistry and Biochemistry. 6314
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 16, 2009
National Laboratory, Arizona State University, Los Alamos National Laboratory, the National Energy Technology Laboratory, and Science Application International Corporation has performed a significant amount of research in an attempt to understand the factors that affect mineral sequestration reaction rates. The leading materials for mineral sequestration are various forms of magnesium silicates (4, 8, 9). One such material is olivine, (Mg,Fe)2SiO4. Forsterite, the pure magnesium silicate form of olivine, has an exothermic reaction with CO2 and produces stable carbonates and silica for the sequestration reaction products. Mg2SiO4 + 2CO2 f 2MgCO3 + SiO2 Past work showed that the optimum reaction rate occurred in a solution of 0.64 M NaHCO3 and 1.0 M NaCl reacted with olivine particles less than 37 µm in diameter at 185 °C and 13.5 MPa of CO2 (7). The reaction rate was found to be controlled by a passivating SiO2 reaction layer formed as the olivine dissolved (4). Therefore, emphasis has been placed on destabilizing the reaction layer through either chemical dissolution or mechanical abrasion. In this paper, we show that the rate of reaction can be increased by adjusting the reactant solution composition. Additionally, the reaction surface morphology has been studied to determine the effect of reactant solution chemistry on the formation and destabilization of the passivating reaction product layer and the formation of magnesium carbonate product. It should be noted that the reactor used in this work differs from those used by several other members of the CO2 Mineral Sequestration Working Group. Our extents of reaction were measured in closed systems and will vary compared with data from loop reactors or fluidized bed reactors. Our intent was not to optimize the reactor design but rather to find the optimum solution composition and provide a better understanding of solution chemistry-induced reaction mechanisms for any reactor design.
Experimental Section The olivine used in these studies was collected from the San Carlos Apache Indian Reservation in East Central Arizona. It contained approximately 8% Fe (substituted on the Mg2+ sites in Mg2SiO4) and
