Systematic Approach to Development and Optimization of High-Gravity Carbonation (HiGCarb) Process for Carbon Capture and Utilization

Accelerated carbonation of alkaline solid wastes is an attractive and promising method for CO2 capture and resource utilization. In this study, high-gravity carbonation (HiGCarb) process using alkaline wastes, i.e., basic oxygen furnace slag (BOFS) and cold-rolling mill wastewater (CRW), for CO2 mineralization and utilization was evaluated using field operation data from the steelmaking industry. In this study, the objectives are to (1) set-up a quality assurance and quality control (QA/QC) program for carbonation conversion of alkaline solid wastes; (2) evaluate the effect of key operating factors on the carbonation conversion of BOFS by the HiGCarb process for CO2 fixation; (3) develop integrated prediction models for the HiGCarb process by combining process chemistry, reaction kinetics and mass transfer; (4) quantify the environmental benefits and impacts of the HiGCarb process via a life cycle assessment (LCA); and (5) establish a 3E (Engineering, Environmental and Economic) triangle model for system optimization. 1. Establishment of QA/QC Program for Carbonation Conversion of Alkaline Solid Wastes The evaluation criteria of CaCO3 content in alkaline solid wastes and the way to interpret thermal analysis profiles were found to be quite different among the literature. In this research, an integrated thermal analyses for determining carbonation parameters such as carbonation degree and CaCO3 content in BOFS were proposed based on thermogravimetric (TG), derivative thermogravimetric (DTG), and differential scanning calorimetry (DSC) analyses. Different quantities of reference CaCO3 standards, carbonated BOFS samples and synthetic CaCO3/BOFS mixtures were prepared for evaluating the data quality of the proposed method using TG/DTG analysis. The results indicate that the CaCO3 contents in BOFS determined by the modified method using TG/DTG can be consistent with those obtained by DSC analysis. Moreover, the evolved gas analysis was performed by mass spectrometer (MS) and Fourier transform infrared spectroscopy (FTIR) for detection of the gaseous compounds released during heating, in order to further confirm the TG/DTG/DSC results. Lastly, the decomposition kinetics (i.e., apparent activation energy, kinetic exponent and pre-exponential factor) and thermodynamics (i.e., changes of entropy, enthalpy, and Gibbs free energy) of CaCO3 in BOFS was evaluated using Arrhenius equation and Kissinger equation, and compared to those reported in the literature. 2. Performance Evaluation of HiGCarb Process for Carbon Capture and Utilization The effect of key operating factors including rotation speed, liquid-to-solid ratio, gas flow rate, and slurry flow rate on CO2 removal efficiency was studied. The results indicated that maximal CO2 removal of 97.3% was achieved using BOFS at a gas-to-liquid (G/L) ratio of 40, with a capture capacity of 165 kg CO2 per day. In addition, the BOFS product with different carbonation degrees was used as supplementary cementitious materials in cement mortar at various substitution ratios (i.e., 0, 10 and 20%). The performance of the BOFS/cement mortar, including physico-chemical properties, morphology, mineralogy, compressive strength and autoclave soundness, was evaluated. The results indicated that the BOFS mortar with a higher carbonation degree (i.e., 48%) exhibited a higher mechanical strength in the early stage, compared to pure Portland cement mortar, and possessed superior soundness to fresh BOFS mortar, suggesting its suitability for use as high-early strength cement. 3. Development of Integrated Prediction Models for HiGCarb Process To establish the kinetic and mass transfer models for HiGCarb process, the process chemistry of accelerated carbonation for BOFS with CRW was evaluated using quantitative X-ray diffraction (QXRD) via Rietveld refinement. In addition, the leaching behavior of various metal ions from BOFS matrix into different types of liquid agents (reaction kinetics) was studied. Moreover, the reaction kinetics of accelerated carbonation for BOFS/CRW in the HiGCarb process was determined by introducing the surface coverage model. The reaction kinetics for carbonation with different alkaline wastes in various types of reactors (e.g., HiGCarb, slurry reactor, and autoclave reactor) was compared accordingly. Furthermore, the mass transfer characteristics such as the overall gas-phase mass transfer coefficient (KGa) and height of a transfer unit (HTU) were illustrated based on theoretical theory. The results indicated that the HTU value of HiGCarb was 7.8–28.0 cm with L/S ratios between 13.3 and 20.0, which was superior to that of conventional reactors. It suggests that the reaction mechanisms, kinetics, and mass transfer of accelerated carbonation of BOFS in the HiGCarb process should be well interpreted and expressed by the developed models. 4. Quantification of Environmental Benefits and Impacts via Life-cycle Assessment (LCA) To critically evaluate the benefits of integrating the HiGCarb process in the steelmaking industry, the performance before (i.e., business as usual, BAU) and after integration of HiGCarb process was evaluated. Significant environmental benefits can be realized by establishing the waste-to-resource supply chain between the steelmaking and cement industries, i.e., from waste treatment to cement production. The power consumption of the main unit operation (and/or equipment) for the HiGCarb process such as BOFS grinding, stirring, blowers, air compressors, pumps, and RPB reactor were evaluated. According to the results of the life-cycle assessment, the net CO2 capture amount by the HiGCarb process was 282 kg-CO2/t-BOFS, accompanied by a CO2 avoidance of 997 kg-CO2/t-BOFS due to the product utilization. 5. System Optimization by 3E (Engineering, Environmental and Economic) Triangle Model The HiGCarb process was comprehensively evaluated according to engineering, environmental, and economic (3E) criteria using a cradle-to-gate life-cycle approach. The CO2 source for HiGCarb can be introduced directly from the industrial stacks, eliminating the need for additional CO2 capture, concentration, and transportation prior to the HiGCarb process. In addition, the reacted product is suited as cement substitution material, avoiding environmental burden from the cement industry, also an intensive CO2 emission source. In this study, nine scenarios were selected based on the overall CO2 capture performance in flue gas via the HiGCarb process. A total of 16 key performance indicators were selected by Delphi method for evaluating the HiGCarb process via the 3E triangle model. According to the results of 3E triangle model, an increase in CO2 capture performance should simultaneously reduce the potential costs and environmental impacts, which make integration of the HiGCarb process into the steelmaking industry more economically viable and environmentally friendly.