Use of Reactive Species in Water for CO 2 Mineralization Juan Ma and Roe-Hoan Yoon* Center for Advanced Separation Technologies, Virginia Polytechnic Institute and State University, 146 Holden Hall, Blacksburg, Virginia 24061, United States ABSTRACT: Various carbon capture and sequestration (CCS) technologies have been developed to address the issues concerning climate changes associated with anthropogenic CO 2 emissions. In the present work, possibilities of mineralizing CO 2 with the reactive species, such as Mg 2+ ions, present in nature, such as seawater and produced water, have been explored. Laboratory tests conducted with solutions containing 1400 ppm of Mg 2+ ions showed that nesquehonite [Mg(OH)- (HCO 3 )·2H 2 O] is formed upon CO 2 injection to the solution at an atmospheric pressure. The results showed that, for mineralization to occur, the pH should be raised above 6.8, as predicted from thermodynamics. Kinetic studies conducted at different temperatures showed that the nesquehonite formation involves an activation energy of 66.7 kJ/mol, which can be overcome by increasing the mass- and heat-transfer efficiency as well as the operative temperature. On the basis of the kinetics data obtained at a low agitation speed, the number and volume of the mineralization reactors required to capture CO 2 emitted from a 600 MW coal power plant have been determined. In addition, the amount of alkalis needed to raise the pH for precipitation and, subsequently, to obtain the natural pH of seawater has been estimated. 1. INTRODUCTION According to the Fourth Intergovernmental Panel on Climate Change (IPCC) Assessment Report (AR4), 1 global warming is unequivocal, as evidenced by increasing air and ocean temperatures, melting of artic snow and ice, rising sea levels, and frequent extreme weather events. The major culprit of these problems is considered to be the CO 2 emitted from burning fossil fuels. The report suggests four abatement options, which include fuel switch, energy efficiency, renewable and nuclear energies, and CO 2 capture and sequestration (CCS). The last is essential for continued use of fossil energy for electricity and heat generation. At present, fossil energy accounts for 67% of electricity generated globally and 90% of the 33.9 gigatonnes of CO 2 emitted in 2011. 2 In the CCS option, CO 2 is separated from flue gas and compressed to a liquid state (capture), transported via a pipeline (transport), and pumped underground for storage (storage). Engineering studies suggest that the capture portion of the process will account for approximately 90% of the additional costs associated with CCS. 3 The storage sites include oil and gas wells, deep saline aquifers, and unminable coal seams. Dependent upon the nature of the storage site, CO 2 is stored as compressed gas, liquid, or supercritical fluid. Supercritical CO 2 is immiscible with water, easy to pump because of low viscosity, and has a high affinity for hydrophobic substances, such as coal and oil, a property that is important for the enhanced oil recovery (EOR) and methane (CH 4 ) recovery from unminable coal seams. The four different geological formations noted above have an estimated capacity of ∼2000 gigatonnes of CO 2 globally. In addition, CO 2 can be stored in deep oceans, where it becomes heavier than water and stays at the bottom. Estimated costs for the capture, transport, and storage are $20-95, $1-10, and $0.5-10/tonne of CO 2 without including long-term monitor- ing costs, respectively. 4,5 The additional costs associated with CCS cause the electricity costs to increase by ∼76%, which may decrease in time to ∼35% via “learning-by-doing”. Many investigators explored the possibility of reacting CO 2 with basic minerals and storing the resulting carbonates (or minerals). Because carbonates are stable on a geological time scale, they can be stored permanently under ambient conditions with minimal precautions. Ultramafic rocks, such as basalt and peridotite, may be used for the mineralization. The major components of these rocks are olivine (Mg 2 SiO 4 ), pyroxene (MgSi 2 O 6 ), and calcium feldspar, of which magnesium (Mg), iron (Fe), and calcium (Ca) are the most important reactive species that can form carbonates. Basalt is the common extrusive igneous rock formed on the crustal surface; however, its reaction rate is slow because of its fine-grained texture. It has been shown that the reaction rate of coarse-grained peridotite is fast enough for CO 2 storage by in situ mineralization. In this approach, supercritical CO 2 is injected through bore holes drilled into the intrusive igneous rocks. 6 The reaction rate can be accelerated once the carbonization, which is exothermic, is initiated and the temperature reaches 150-200 °C. Lackner et al. 7,8 developed an ex situ process of mineralizing CO 2 , in which magnesium hydroxide [Mg(OH) 2 ] is extracted from olivine by heat and acid (HCl) treatments and the hydroxide is contacted with CO 2 gas at a high temperature (140-300 °C) and pressure to form magnesium carbonates. However, the process is energy-intensive, generating 4 times as much CO 2 as that to be captured. 9,10 O’Connor et al. 11-13 developed a less costly process, in which ultramafic rocks are “activated” such that reactive species, such as Mg 2+ , Ca 2+ , and Fe 2+ species, are more readily released into aqueous media. Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: February 1, 2013 Revised: April 15, 2013 Published: April 23, 2013 Article pubs.acs.org/EF © 2013 American Chemical Society 4190 dx.doi.org/10.1021/ef400201a | Energy Fuels 2013, 27, 4190-4198