Semi-Continuous Ex Situ Carbon Dioxide Mineralization in Produced Water for Calcite Production

Flue gas, a high concentration point CO₂ waste stream created across many industries, offers an opportunity for capturing CO₂ at rates much greater than direct air capture. In this work, flue gas is contacted with produced water, a brine waste produced from hydraulic fracturing (fracking) that contains high concentrations of Ca²⁺ ions, to mineralize CO₂, forming calcium carbonate (CaCO₃) [1]. This mineralization route simultaneously sequesters CO₂ into a valuable commodity product, mitigates two common waste streams, and can utilize “off-the-shelf” technologies including common crystallization reactors for mineralization and a tandem chlor-alkali cell to provide hydroxide. Herein the design and operational conditions of the crystallizer are studied, and the chlor-alkali cell is simulated with a concentrated NaOH stream.

Crystallizer parameters including CO₂ concentration in flue gas, temperature, and reaction pH were studied in a two-level design of experiments. The CaCO₃ precipitation rate and the material properties of the collected product were characterized. Rates as high as 16.59 ± 0.37 µmol.L^-1.s^-1 were sustained in a bench scale (1 L) crystallizer allowing full recovery (~100% yield) of Ca²⁺ as CaCO₃. The reactor design also influences the crystal structure of the collected precipitate. Examples of designs that will produce a majority aragonite (orthorhombic) structure as opposed to the more stable and valuable calcite (trigonal) structure will be shown. Effects of reactor parameters on particle size were also studied. Overall, this study of CO₂ mineralization utilizing flue gas and produced water is aimed at providing the necessary information to scale this process to a pilot scale (200 L crystallizer) capable of mineralizing >10 kg CO₂/day as well as informing techno-economic analyses of this promising process.

[1] Zhu, B.; Wilson, S.; Zeyen, N.; Raudsepp, M. J.; Zolfaghari, A.; Wang, B.; Rostron, B. J.; Snihur, K. N.; von Gunten, K.; Harrison, A. L.; Alessi, D. S. Unlocking the Potential of Hydraulic Fracturing Flowback and Produced Water for CO2 Removal via Mineral Carbonation. Applied Geochemistry 2022, 142, 105345. https://doi.org/10.1016/j.apgeochem.2022.105345.