Performance assessment of producing Mg(OH)₂ for CO₂ mineral sequestration

This study presents the energy and environmental performance assessment of producing magnesium hydroxide (Mg(OH)(2)) from Mg-silicates for CO2 mineral sequestration applied to a natural gas combined cycle (NGCC) power plant. Mg(OH)(2) produced via a closed loop reaction of serpentinite and ammonium sulfate (AS), precipitation of Mg(OH)(2) and AS looping/recovery binds CO2 into a thermodynamically stable, environmentally benign and leak-free magnesium carbonate (MgCO3). We used results from laboratory, modeling and life cycle assessment (LCA) studies to determine the extent to which magnesium (Mg) from serpentinite rock can be converted to Mg(OH)(2), the effects of reaction parameters, scalability and the associated life cycle greenhouse gas emissions (GHGs). We found that reaction temperature positively affects Mg extraction from serpentinite, reaching a maximum yield at different temperatures depending on the reaction time. Also, the reactor properties affect the extraction results as the optimal extraction yield and conditions reported for different reactors differ. While the process of producing Mg(OH)(2) is promising, it also possesses a level of energy and environmental burden that cannot be ignored when considering large scale implementation. At 100% conversion and recovery of reagent, the CO2 mineralization process has a life cycle global warming potential (GWP) of 433 kg CO2 equivalents per ton CO2 (CO(2)e/t-CO2). This value increases by 82, 7 and 0.4 kg CO(2)e/t-CO2 for every %-point efficiency loss of AS recovery, Mg(OH)(2) production and Mg(OH)(2) carbonation respectively. Mineral sequestration applied to the 555 MW NGCC plant reduces its net plant efficiency from 50.2% to 38.6%-points (an energy penalty of 30%) but avoids 51% of the GHG emissions to the atmosphere. The results from this study are timely, and could have significant implications on mineral sequestration methods that consider the exothermic nature of the overall mineral carbonation chemistry beneficial.