CO2 Capture by Energy-Efficient Carbon Mineralization

CO2 is typically captured via amine absorption; however, the regeneration of amine through thermal stripping is energy-intensive. We assess the performance of amine regeneration by coal fly ash and basalt as they mineralize CO2 into carbonates. Fly ash is rich in calcium and magnesium (25% and 5%, respectively), whereas basalt contains moderate amounts (7% and 3%), but is abundant. N-methyldiethanolamine (MDEA, a tertiary amine) and piperazine (PZ, a secondary diamine) are used as amine solvents. First, a 15 wt% solution of MDEA or PZ in water was reacted with a simulated flue gas (10% CO2, 90% N2) to absorb the CO2. The solution was then reacted with either fly ash or basalt with a solid-to-liquid ratio of 350 g/L, under 1 atm and 40°C. CO2-loading and pH were monitored over time, and the reaction was stopped once CO2-loading stabilized or reached zero. The regenerated solvent was analyzed using 13C NMR, while the precipitates were characterized using thermogravimetric analysis (TGA) to quantify carbonate formation. Scanning electron microscopy (SEM) was also used to show morphological changes. Reacting CO2-saturated amines with fly ash or basalt resulted in solvent regeneration and carbonate formation. With fly ash, MDEA was fully regenerated within about 19.5 hours, while PZ stabilized at 60% regeneration after 123 hours. For basalt, MDEA and PZ stabilized at 43% and 20% regeneration after 120 and 24 hours, respectively. 13C NMR showed that tertiary amines capture CO2 as bicarbonate, whereas secondary amines form both carbamates and bicarbonate. Since only bicarbonate can react with divalent cations to form carbonates, carbamate stability likely accounts for the lower regeneration observed with PZ compared to MDEA. SEM imaging revealed carbonate deposits on the original fly ash and basalt grains. TGA suggests CO2 is primarily stored as simple carbonates in fly ash, while basalt forms products with greater thermal stability. This study demonstrates that coal fly ash can be used for the chemical regeneration of CO2-saturated tertiary amines.