Pilot Study on Mechanical Force-Enhanced High-Pressure Carbonation of Fly Ash in a 300 MW Power Plant

Laboratory-scale experiments have shown that supercritical CO 2 combined with mechanical force can simultaneously achieve fly ash carbonation and heavy metal stabilization. However, the reaction characteristics and influencing factors at a larger scale remain unclear. In this study, a pilot-scale experimental platform with an ash treatment capacity of 1 t/d was established in a 300 MW coal-fired power unit. The results show that the maximum carbonation efficiency reached 58.34%, and the trend of carbonation efficiency with varying operating parameters differed from that observed in laboratory experiments. After a 1000-fold scale-up, three main reasons account for the discrepancies. First, for mechanical modification, ball milling was used in laboratory experiments, whereas a grinding disc was employed in pilot tests. Characterization experiments indicate that ball milling is more effective. Second, in the pilot-scale reactor, a dead zone appeared at the bottom, which hindered gas–solid mixing. Third, the increase in scale led to a larger absolute amount of water and a lower surface-area-to-volume ratio. The increased water volume enhances CO 2 dissolution and Ca 2+ leaching but, under poor mixing conditions, may also cause local acidification and thicker water films that hinder gas diffusion, thereby reducing carbonation efficiency. These conditions favor the dissolution of a large amount of CO 2 into the water phase in pilot experiments, as described by Henry’s law. However, when the flow field deteriorates, local acidification is more likely to occur in large-scale reactors, leading to the redissolution of previously formed carbonates into soluble HCO 3 – . Therefore, the net carbonation efficiency under high-pressure conditions decreases. Separately, despite the lower efficiency, enhanced carbonation under pilot-scale conditions still improved the stability of heavy metals, similar to the laboratory-scale observations.