Accelerated Carbonation of Steel Slags Using CO2 Diluted Sources: CO2 Uptakes and Energy Requirements

Accelerated carbonation has been proposed as an effective method for ex situ carbon dioxide sequestration. This process has been typically applied to alkaline earth minerals, such as serpentine, olivine and wollastonite; despite these materials may exhibit a high carbon sequestration potential, energy intensive operating conditions are typically required to accelerate reaction kinetics in order to make carbonation feasible for an industrial application. A promising alternative feedstock could be represented by alkaline industrial residues characterized by high calcium or magnesium (hydr)oxide or silicate contents. These materials in fact prove to be more reactive than minerals, especially at mild operating conditions. Furthermore, carbonation has shown to affect some of the properties of the tested materials (i.e.: main mineralogy, porosity and mobility of specific elements) improving their long-term technical performance and/or environmental behavior. Thus, the application of this treatment to alkaline industrial residues may be indeed considered as a CO₂ storage and utilization option, since accelerated carbonation may be also employed as a valorization technique for widening the reuse options of these materials and also to achieve other environmental benefits. 

Steel manufacturing plants are typical examples of industrial sites in which relevant flows of both CO₂ and alkaline industrial residues are generated and therefore represent one of the potentially most interesting contexts for the application of accelerated carbonation. Several studies have recently shown that a number of different types of steel slag present a significant reactivity with CO₂, allowing to achieve, for specific process routes and operating conditions, relevant CO₂ uptakes. Some of the types of residues generated in steel manufacturing plants such as BOF, EAF and argon oxygen decarburization (AOD) slag, are typically not valorized and generally landfilled, or employed only for low-end applications, owing for the significant content of free calcium and magnesium (hydr)oxides that may result in poor volumetric stability and hence in a low technical performance in construction applications.

Recently different efforts were carried out to scale-up accelerated carbonation of industrial residues from the laboratory scale to the demonstration or industrial one in order to assess the viability of applying this process as an effective technique for reducing anthropogenic CO₂ emissions. However, the uncertainty on the feasibility of accelerated carbonation, especially as far as material and energy requirements are concerned, has hindered so far its application at larger scale. Specifically, since in order to achieve significant CO₂ uptakes in reasonable timeframes, milling of the residues and operation of the carbonation reactor at enhanced temperature and CO₂ pressure are necessary, the process presents an associated energy penalty which should be assessed and accounted for. Furthermore, a way for reducing the overall energy requirements of CCS is to directly perform the carbonation step using diluted CO₂ sources, thus lumping capture and storage in a single step. In this way the capture step, that as an example for a 20 MW power plant yields an energy penalty of around 4 MW, could be avoided.

With this purpose we have performed several carbonation tests on different types of steel manufacturing slags employing CO₂ diluted sources, and based on the CO₂ uptakes achieved in these experiments we have estimated the associated energy requirements. Carbonation experiments were performed on different types of BOF slag applying either the slurry phase (L/S=5 l/kg) or the wet route (L/S = 0.3-0.4 l/kg) using gas mixtures containing 40% CO₂ and 60% N₂ vol. to simulate a syngas resulting from gasification processes, and 10% CO₂ + 90% N₂ vol. to simulate the gaseous effluent from combustion processes. For the slurry-phase route, the experimental results yielded the following CO₂ uptakes: at  10% CO₂ concentration, from a minimum of 6.7% (attained at 20 °C and 1 bar total pressure) to a maximum of 40% (100 °C, 10 bar); at 40% CO₂ concentration, from a minimum of 10.7% (20 °C, 1 bar) to a maximum of 52.6% (100 °C, 5 bar). As for the wet route, experiments were performed on different types of BOF slags using a gas mixtures containing 40% of carbon dioxide, leading to CO₂ uptakes in the range of 6 -20 % at 50 °C and 7 bar total pressure.

Based on the results of these tests, the energy requirements associated to the carbonation process were estimated considering the CO₂ emissions of a medium size power plant (20 MW). To this aim, two carbonation process layouts, corresponding to the slurry and wet routes, were considered and the mass and energy balance for each unit operations of the process was performed. Specifically, CO₂ compression, milling of the residues, mixing of residues and water, the pumping of liquids or slurries and the heating to the reaction temperature, together with the duties related to mixing in the carbonation reactor were considered for the slurry phase route. Furthermore, in the proceedings paper the influence of the characteristics of the residues (i.e. main mineralogy and particles size) and of the applied operating conditions (i.e. temperature, CO₂ pressure and liquid to solid ratio), on the feasibility of the carbonation process in terms of energy requirements will be discussed.