Optimizing Ex-Situ Carbon Sequestration by Modeling Rock-CO2(G) Reaction with Basalts of Varying Composition from Nevada

Sturmer, Daniel M.¹; Price, Jonathan G.²; Tempel, Regina N.^1
1 Geological Sciences and Engineering, University of Nevada, Reno, Reno, NV.
² Nevada Bureau of Mines and Geology, University of Nevada, Reno, Reno, NV.

Mineral carbonation is a process whereby carbon dioxide reacts with minerals or rocks to store the carbon permanently in synthetic minerals. The amount of carbon sequestered generally increases with Mg and Fe content in a rock, thereby focusing most mineral carbonation studies on ultramafic rocks. Although Nevada has minimal concentrations of ultramafic rocks, it has large volumes of mafic volcanic and plutonic rocks. Thus, the purpose of this study is to model the rock-CO₂(g) reaction for basalts from Nevada at various temperatures using the EQ3/6 reaction path code. Preliminary work has tested carbonation reaction sensitivity to changes in temperature and basalt composition.

This study modeled experimental conditions for ex-situ wet mineral carbonation of forsterite (O’Connor et al., 2002). The experimental procedure from that study used a solution of 1.0 molar NaCl and 0.64 molar NaHCO₃, and reacted it with forsterite (crushed to 37 μm) at 185°C in the presence of 15 MPa CO₂(g). In the modeled system, a similar solution containing sodium, chloride, and bicarbonate ions was reacted with a basalt with CO₂ fugacity fixed at 150 bars. CIPW norms calculated for several Nevada basalts served as preliminary input basalt mineralogies. Models were run from 0 to 200°C at 25°C intervals for each basalt and assuming arbitrary kinetics.

Mafic rock-CO₂ reactions maximized carbon sequestration at 25-50°C, but were fairly insensitive to temperature below 100°C. However, low-temperature mafic rock-CO₂ reactions resulted in 3-5 times as much product volume as high-temperature reactions. In the models, carbon was sequestered in four phases: siderite, magnesite, dolomite, and dawsonite (stable below 150°C). Dawsonite has a much larger molar volume than other product minerals, resulting in the increased product volume with lower reaction temperature.

The models will be improved by incorporating true kinetics and observed basalt mineralogies. True kinetics models track the reaction path in time, an important consideration for optimizing reaction conditions. Thin section point counts from basalts collected in Nevada will be used for input mineralogies. Basalt glass presents a unique opportunity for carbonation as it is metastable at surface conditions and should react more quickly with CO₂(g) than minerals. Ultimately, these models will be used to assist experimental design for small-scale experimental carbonation of several Nevada basalts.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009