Direct Air Capture: Effects of Low-Purity CO2 and Geological Depths on Post-Capture Compression Energy and Storage

Abstract

Phreeqc software has been successfully employed to study the behaviour of the gypsum rock-water system under different conditions and various levels of impurities present in the injected CO2 stream. The dissolution of gypsum leads to the increase in water quantity because of release of hydration water molecules. Under the conditions at 1km geological depth, an anhydrite (CaSO4) produced remained dissolved in the solution while the pH slightly drops owing to sulphate ions generation. As the pressure and temperature increase at 1.5km depth, more gypsum dissolves until at 2.5km depth where excess rise in temperature inhibits gypsum dissolution. The injection of pure CO2 leads to the production of calcite in the solution, which continues to grow as more CO2 is injected. But, the presence of impurity such as N2 in the CO2 stream inhibits formation of calcite, owing to continuous dissolution/ionization of calcium. Other carbonate crystals were similarly affected while the pH continues to drop drastically. It is inferred that the presence of impurities such as N2 in the CO2 stream complicates carbonation mechanisms and inhibits formations of the carbonate crystals.