Insights into Carbonation Kinetics of Fly Ash from Victorian Lignite for CO2 Sequestration

Mineral carbonation of fly ash can both capture and store CO2 permanently in a single process without long-term monitoring. Previous studies indicate that fly ash with high calcium and magnesium contents exhibit promising CO2 fixation capability. However, the reaction mechanisms and kinetics involved in the carbonation reaction of fly ash is still not fully understood. In this study, a typical Victorian brown coal fly ash from Hazelwood power plant was selected to sequestrate CO2 in a direct aqueous carbonation process. Experiments were conducted in a vessel reactor at various temperatures (40, 50, 60, and 70 °C), stirring rates (900, 1050, 1200, and 1350 rpm), and CO2 pressures (3, 4, 5, 6, and 7 bar) to investigate the reaction kinetics and identify the rate-limiting steps of carbonation. The results show that both the carbonation rate and the maximum carbonation efficiency could be improved by optimizing parameters and by the introduction of NaHCO3. Also, the complex effects of the operating parameters on the carbonation rate and the maximum carbonation efficiency were investigated. The kinetic data can be well fitted by the surface coverage model with the R2 ≥ 0.98, indicating that the carbonation of fly ash can be physically expressed by this model. The maximum carbonation efficiency of fly ash could also be well-predicted by the model. In addition, the assumed mechanisms of the carbonation reaction were validated by particle size, surface area, and porosity changes of the fly-ash particles after carbonation reactions. The observation of scanning electron microscope equipped with energy-dispersive X-ray spectroscopy before and after carbonation also confirmed that the newly formed precipitates were not only deposited on the active surface but also filled the pores of the fly-ash particles.