Design and simulation of a new carbonation device with both steel slag modification and carbon sequestration functions

The steel industry faces the challenges of high energy consumption, high carbon emissions and high solid waste production. At present, the recycling and treatment of steel slag solid waste is the most serious problem, with the utilization rate of steel slag being only 30 %. Steel slag solid waste contains a high content of f-CaO (free calcium oxide), which reacts with water causing expansion and deformation. It is necessary to pre-digest the f-CaO in steel slag to improve the quality of steel slag. Using CO₂ to react with steel slag can not only effectively strengthen the digestion reaction of f-CaO, but also realize the in-situ sequestration of CO₂ emitted from steel mills and reduce carbon emissions from the iron and steel industry. However, there is currently limited development of equipment for the carbonation process of steel slag, and most existing equipment is either fluidized beds or moving beds designed for continuous operation, which have the high particle size requirements for steel slag raw materials and high operational energy consumption. Therefore, in this study, based on the dry carbonation process route of steel slag, the traditional hot smothering process was modified for carbonation. A new type of fixed-bed pressurized reactor for steel slag carbonation with both steel slag modification and in situ carbon sequestration was developed. The physical structure of the reactor was designed in detail, and a numerical model of gas–solid two-phase reaction flow based on the carbonation reaction of steel slag was established. The intermittent dry carbonation process of steel slag in the modified device was simulated, and the effects of slag temperature, gas amount, particle size and reaction time on the carbonation effect of steel slag were studied. The calculation results showed that the increase of initial slag temperature, flue gas inflow and reaction time as well as the decrease of particle size can promote the carbonation reaction of steel slag in different degrees. For the steel slag system with an initial f-CaO content of 4.48 wt%, when the initial temperature of the slag was 873.15 K, and the total amount of flue gas was equivalent to digesting the f-CaO to 1.00 wt%, the f-CaO content of steel slag of all particle sizes was less than 4 wt% after a reaction time of 2.5 h, meeting the requirement for the f-CaO content of steel slag in the Chinese national standard.