Contribution of iron to the energetics of CO2 sequestration in Mg–silicates-based rock

The main purpose of this paper is to investigate the contribution of iron to the energy requirements of a process for producing magnesium hydroxide {Mg(OH)2} from alkaline-earth Mg–silicate rock that contains iron, such as serpentinite. Once produced Mg(OH)2 could be used to sequester carbon either by direct mineralization at a power plant or from the air, or as a means to deliver alkalinity to the ocean thus tending to restore oceanic pH and sequester atmospheric carbon. Fe-containing by-products obtained from producing Mg(OH)2 are considered to be beneficial as secondary raw materials for iron-and steel-making industries. It has been proposed that this could further reduce CO2 emissions as well as raw material costs. However, this study hypothesized that the extent of this benefit, if any, would depend on energy intensity of reactions involving iron compounds. Using Aspen Plus® software, the contribution of iron to the energy input requirement of CO2 sequestration was modeled. Results obtained showed that the extraction of iron from Mg–silicate minerals could present a significant energy penalty to the mineralization process. Exergy analysis shows that at the experimental optimal temperature of 400 °C, the energy penalties of having iron oxide (FeO), hematite (Fe2O3) and magnetite (Fe3O4) as dominant iron compounds results are (for 10 wt.% Fe in the rock) an increase of 0.3 GJ/t CO2 (7%), 0.7 GJ/t CO2 (20%) and 2.2 GJ/t CO2 (60%) respectively when compared to an iron-free base case. Recovery of input raw material, ammonium sulfate (AS) by evaporative crystallization is a major energy intensive step in this process. However, our model applied mechanical vapor recompression (MVR), which resulted in a significant reduction in energy demand. It can be concluded that the benefit of producing useful Fe by-products comes with an energy penalty, the extent of which varies with the form of Fe compound in the mineral. The findings in this paper are useful in determining which Mg–silicate-based rocks would be energy efficient for use.