NRG COSIA Carbon XPRIZE: carbon-dioxide mineralization in recycled concrete wash water

Wash water is generated as a waste stream from ready-mixed-concrete production. Reuse of the water as mixture water is limited, in practice, by the negative material performance impacts associated with the water chemistry and properties; the effects are intensified with increasing content of suspended solids and age. However, this waste material can be used as a beneficial additive to concrete by profiting from the cementitious properties of the suspended solids, if variability can be reduced. A method of stabilizing this material is through CO2 treatment. The added CO2 is mineralized through a reaction with the calcium from the cement particles. This provides a calcium-carbonate coating that prevents further cement hydration, making the material predictable. This has been shown to alleviate concerns with set acceleration and inconsistencies in compressive strength. A method of CO2 treatment was tested as part of the NRG COSIA Carbon XPRIZE at a site in Calgary, Alberta. The slurry for the treatment was provided by a local concrete plant and had a specific gravity of 1.15. The simulated wash water was treated in 1000-L quantities with each treatment mineralizing an average of 40 kg of CO2. The system ran for 1600 hours of operation over 127 treatment cycles and converted 14.5 tonnes of CO2 at an average mineralization efficiency of 80%. The treated slurry was used as an additive in >300 batches of concrete where the concrete met the necessary requirements for fresh properties and setting time, while achieving a strength benefit. Replacement of 5% and 10% of batch water with treated slurry (9.4 and 18.8 kg slurry/m3 concrete) showed a strength benefit of 3% and 6% compared to a reference. The technology was selected as the winner of the NRG COSIA Carbon XPRIZE (Track B: Natural Gas) in April 2021.

Implementation of Ion Exchange Processes for Carbon Dioxide Mineralization Using Industrial Waste Streams

Sequestration of CO2 within stable mineral carbonates (e.g., CaCO3) represents an attractive emission reduction strategy because it offers a leakage-free alternative to geological storage of CO2 in an environmentally benign form. However, the pH of aqueous streams equilibrated with gaseous streams containing CO2 (pH < 4) are typically lower than that which is required for carbonate precipitation (pH > 8). Traditionally, alkalinity is provided by a stoichiometric reagent (e.g., NaOH) which renders these processes environmentally hazardous and economically unfeasible. This work investigates the use of regenerable ion-exchange materials to induce alkalinity in CO2-saturated aqueous solutions such that the pH shift required for mineralization occurs without the need for stoichiometric reagents. Na+-H+ exchange isotherms (at [H+] = 10−8–10−1 M) and rates were measured for 13X and 4A zeolites and TP-207 and TP-260 organic exchange resins in batch equilibrium and fixed-bed exchange experiments, respectively. At solutions equilibrated with CO2 at 1.0 atm (pH = 3.9), H+ exchange capacities for the materials were similar (1.7–2.4 mmol H+/g material) and resulted in pH increases from 3.9 to greater than 8.0. Multi-component mixtures using Ca2+ and Mg2+ cations (at 10−3–10−1 M) in CO2-saturated water were used to probe competitive ion exchange. The presence of divalent cations in solution inhibited H+ exchange, reducing capacities to as low as 0.2 mmol H+/g for both resins and zeolites. Dynamic H+ exchange capacities in fixed-bed ion exchange columns were similar to equilibrium values for resins (∼1.5 mmol/g) and zeolites (∼0.8 mmol/g) using inlet solutions that were equilibrated with gaseous streams of CO2 at 1.0 atm. However, exchange kinetics were limited by intraparticle diffusion as indicated by the increased rate parameters with increasing inlet flow rates (20–160 cm3 min−1). Experimental calcite precipitation from mixing the alkaline CO32−-rich water solution obtained from the ion-exchange column with a simulated liquid waste stream solution achieved thermodynamic maximum yields. The results from these studies indicate that ion exchange processes can be used as an alternative to the addition of stoichiometric bases to induce alkalinity for the precipitation of CaCO3, thereby opening a pathway toward sustainable and economic mineralization processes.

Microwave roasting of blast furnace slag for carbon dioxide mineralization and energy analysis

This paper highlights the potential of microwave roasting in solid-waste treatment and carbon dioxide storage.