Analysis of the synchronous enhancement in CO2 sequestration by steel slag using steam in flue gas: performance and mechanism

The utilization of a large amount of discarded steel slag to capture and sequester CO₂ in industrial flue gases, with the process being enhanced by the accompanying steam in the emissions, is very promising. To provide fundamental data for the formation of more practical industrial guidelines in the future, this paper systematically investigates the effects of steam on the CO₂ sequestration performance of steel slag and its underlying mechanisms through a combination of experimental analysis and theoretical calculations. The experimental results demonstrate that the CO₂ sequestration capacities initially rise and then decrease as the steam content increases, reaching an optimal CO₂ uptake of 26 g/kg and a carbonation conversion of 7.03 % with 28.6 vol% of steam at 700 °C with 1 h duration time. This is an encouraging increase from the 17.72 g/kg CO₂ uptake and 4.79 % carbonation conversion seen without steam. The macroscopic mechanism of steam-enhanced CO₂ sequestration, as revealed by the characterization of carbonated slag, suggests that the high energy barriers for the migration of calcium and oxygen ions in the gas-solid reaction make the nucleation of CaCO₃ quite difficult. However, ion migration becomes easier in the presence of steam, thus promoting the nucleation and growth of CaCO₃. Theoretical calculations have analyzed the microscopic mechanism, suggesting that water molecules decompose and adsorb on the surface of the slag particles at suitable temperatures. The resulting proton defects use calcium vacancy as channels to achieve high mobility within the solid, thus enhancing the internal electronic activity of the solid.