Energy evaluation for MgO production and CO₂ mineralization processes of carbon recycling technology using waste brine

Abstract

Waste brine produced from the existing fleet of desalination facilities could remove and stabilize ~231 Mt-CO₂/y as Mg-carbonate. The source carbon dioxide can range in concentration from the ambient atmosphere to 100% CO₂, allowing for a range of applications, including as an alternative to underground storage for direct air capture and point source CO₂ capture systems. To achieve rapid CO₂ removal via mineralization into Mg-carbonate, it is necessary to establish a process for thermally decomposing hydrated Mg-chloride extracted from waste brine. When heated above 400–450 °C, hydrated Mg-chloride thermally decomposes into MgO. While this route is operated industrially using fossil fuel burners at >800 °C, we produced MgO from waste brine-derived Mg-chloride dihydrate in an electric, rotary kiln at temperatures <600 °C. In our process, thermal decomposition to MgO reached 96.7% when the kiln was operated at 600 °C. Subsequent gas-solid reaction of MgO with CO₂ in a 30 °C incubator found that MgO produced in the range of 534-571 °C yielded the fastest CO₂ mineralization. To further accelerate the CO₂ mineralization rate, the energy consumption of grinding the MgO to smaller particles sizes and increasing the concentration of CO₂ were evaluated; grinding was found to be substantially less energy intensive than increasing the CO₂ concentration. Optimizing CO₂ mineralization using desalination brine can be achieved by a combination of low-temperature thermal decomposition of hydrated Mg-chloride and fine grinding of the generated MgO; deliberate concentration of CO₂ is not recommended unless case-specific factors such as site footprint require additional mineralization acceleration.