Process Simulations Reveal the Carbon Dioxide Removal Potential of a Process That Mineralizes Industrial Waste Streams via an Ion Exchange-Based Regenerable pH Swing

The sequestration of CO2 within stable mineral carbonates (e.g., CaCO3) represents an attractive emissions-reduction strategy because it offers an energy efficient, environmentally benign, and leakage-free alternative to geological storage. However, the pH levels of aqueous streams equilibrated with CO2-containing gas streams (pH ∼ 4) are lower than the pH required for carbonate precipitation (pH > 8). Thus, the use of regenerable ion exchange materials is proposed to induce alkalinity in CO2-containing aqueous streams to achieve the pH required for mineralization without the addition of expensive stoichiometric reagents such as caustic soda (e.g., NaOH). Herein, geochemical and process-modeling software was used to identify the optimum thermodynamic conditions and to quantify the energy intensity and CO2 reduction potential of a process that sequesters CO2 (dissolved in wastewater) as solid calcium carbonate (CaCO3). CaCO3 yields were maximized when the initial calcium to CO2 ratio in the aqueous phase was 1:1. The energy intensity of the process (0.22–2.10 MW·h/t of CO2 removed) was dependent on the concentration of CO2 in the gas phase (i.e., 5–50 vol %) and the produced water composition, with the nanofiltration and reverse osmosis steps used to recover magnesium and sodium ions requiring the most energy (0.07–0.80 MW·h/t of CO2 removed). Energy consumption was minimized under conditions where CaCO3 yields were maximized for all produced water compositions and CO2 concentrations. The ratio of net CO2 to gross CO2 removal for the process ranged from 0.05 to 0.90, indicating a net CO2 reduction across all conditions studied. The results from these studies indicate that ion exchange processes can be used as alternatives to the addition of stoichiometric bases to provide alkalinity for the precipitation of CaCO3 at the CO2 concentrations studied, thereby opening a pathway toward sustainable and economic mineralization processes.