(433c) Development of CO2 Capture and Utilization Technologies for Alkaline Wastes for Building Waste-to-Resource Supply Chain: Theoretical Consideration

In this study, accelerated carbonation of alkaline solid wastes using a rotating packed bed (RPB) for “CO₂ capture” and their utilization as “green construction materials” have been developed for economic feasibility and environmental sustainability.  The CO₂ capture capacity and overall energy consumption of the developed carbonation processes were critically assessed from the engineering and environmental aspects.  The challenges of CO₂ capture by carbonation of alkaline waste were to accelerate the reaction and exploit the heat of reaction to minimize energy and material losses.  Therefore, the objectives of this research work were to evaluate the performance of RPB process by introducing life cycle assessment (LCA) and response surface methodology (RSM) for minimizing the environmental impacts and maximizing the overall CO₂ capture capacity.  

In addition, a prediction model for carbonation within an RPB was developed by combining the process chemistry, reaction kinetics, and mass transfer.  The enhancement factors for mass transfer coefficient in case of carbonation of different types of wastes within an RPB were determined accordingly.  The important factors include particle size of solid waste, reaction temperature, rotation speed, and liquid-to-solid (L/S) ratio.  According to the observations of TGA, XRD and SEM-EDX, the alkaline wastes were found to be successfully carbonated with CO₂ in an RPB, where calcite (CaCO₃) was identified as the main product.  

The LCA was utilized to determine the net performance of carbon offsetting by considering (1) the energy consumption and carbon emission associated with RPB operation; (2) benefits of offering alternative materials for cement or concrete; (3) the CO₂ caused by raw material pre-processing & conditioning of carbonate product for final products.  The functional unit is set to be capture of 1 kg CO₂ by carbonation processes within the system boundary while carrying out the LCA.  Energy consumption of the overall processes including grinding, sieving, pressuring, heating, pumping, and rotating processes was measured.  A balance for material and energy flows is drawn up for calculating and analyzing the investigated system, which consists of all input (e.g., land, energy, raw materials, auxiliary agents, etc.) and output (e.g., products, emissions, waste, etc.)  

Furthermore, the developed RPB process were designed and analyzed by RSM using Design Expert statistical software to visualize the effects of operating parameters on the carbonation conversion of solid wastes for overall maximization.  Two types of response parameters including the carbonation conversion of alkaline waste and environmental impact were systematically evaluated in this study.  The factors affecting the above two types of responses were coded with low and high levels using central composite design (CCD) method.  The carbonation conversion and environmental impact could be maximized and minimized, respectively, by observation of the location of the saddle point on the 3D-surface imaging from different sets of response surfaces.  With the results of the prediction model, LCA and RSM, the best achievable technology (BAT) for carbonation using an RPB was proposed for system optimization.  It was thus concluded that accelerated carbonation of alkaline wastes using an RPB is an effective and efficient method for CO₂ capture and utilization due to its higher mass transfer rate and carbonation conversion.