Potential utilization of fly ash for CO2 sequestration and acid mine drainage (AMD) wastewater treatment

Abstract

The continued use of fossil fuels to meet the world’s energy demands contributes largely to the emissions of greenhouse gases. In South Africa, a considerable amount of carbon dioxide (CO2) is emitted annually from burning coal to produce electricity. To mitigate the adverse environmental effects arising from the continual increase in CO2 concentration (MtCO2) in the atmosphere, which in turn gives rise to global warming, this work investigated carbon capture and storage (CCS) using accelerated mineral carbonation (MC). Accelerated MC is emerging as a promising technology for the permanent storage of CO2 based on its high storage potential (> 10 000 Gt of CO2) because of the abundance of natural silicates worldwide. The accelerated MC process involves reacting captured CO2 from a CO2 emission source with an alkaline-rich feedstock to produce a mineral carbonate, thereby storing the CO2 permanently. Despite the high storage potential displayed by MC, the relatively high energy consumption (MW) and costs ($/tonne CO2) associated with the process continue to hinder its widespread implementation. Hence, the focus for accelerated MC in this work was the optimization of several process parameters and the use of coal fly ash (CFA) to improve the carbonation performance thereby reducing the energy consumption (MW) of the process. A preliminary study was first conducted to determine the amount of calcium (Ca2+) leachable from the fly ash (FA). The parameters investigated for the calcium (Ca2+) extraction study were the temperature (0C), time (min), and particle size (𝜇𝑚). Conditions resulting in the maximum concentration of calcium (Ca2+) extracted were considered the optimum conditions from the leaching study and these process conditions were subsequently used for the carbonation process. The optimum temperature was determined to be 70 0C and the optimum reaction times were 30 min, 90 min, and 120 min due to the lower calcium (Ca2+) concentration obtained after 60 min, which from the Dixon’s Q-Test proved to be an outlier potentially caused by errors while conducting the experiment. The particle size (𝜇𝑚) was not considered for carbonation experiments based on the trade-off between the maximum calcium (Ca2+) leached and the potential additional energy requirement (MW) due to sieving. Following the carbonation experiments, the carbonation performance was measured through the percentage of CaCO3 formed, the carbonation efficiency CE (%), and the maximum CO2 storage capacity (kg/kg fly ash). It was found that the maximum % CaCO3 formed after a reaction time of 120 min at 4 Mpa from direct carbonation with AMD wastewater was higher compared to the maximum % CaCO3 formed of 2.43 % from direct aqueous carbonation under the same conditions of time (min) and CO2 pressure (Mpa). This was attributed to the lower S/L ratio of 0.2 g/mL used for experiments involving AMD wastewater, which improved the calcium (Ca2+) extraction, the additional calcium (Ca2+) concentration of 362.5 ppm from the AMD wastewater, and the increase in the stirring speed from 100 rpm up to 400 rpm upon process optimization.

The highest CE (%) was 63 % and was achieved from direct carbonation with AMD wastewater after 120 min of direct carbonation at 4 Mpa, followed by 53.9 % achieved from indirect carbonation with AMD under the same process conditions. The higher CE (%) through the direct carbonation route was possibly due to the continued extraction of calcium (Ca2+) from the fly ash during the direct carbonation reaction. A lower CE (%) was achieved when pure water was used as the reaction solvent compared to when AMD wastewater was utilized, again attributed to the lower S/L ratio, the additional calcium (Ca2+) concentration provided by the AMD wastewater, as well as the higher stirring speed (rpm) used for the AMD carbonation study. The CE (%) achieved from direct aqueous carbonation was 29.4 % while the CE (%) achieved from the indirect aqueous route was found to be 35.2 % after 120 min of carbonation at 4 Mpa. The higher CE (%) achieved from indirect aqueous carbonation compared to direct aqueous carbonation was due to the higher stirring rate (rpm) used for indirect aqueous carbonation, which improved calcium (Ca2+) extraction. The maximum CO2 storage capacity which gave an indication of the maximum CO2 storage potential of the fly ash per 1 kg of fly ash used, was also measured, and it was found to be 0.026 kg/kg fly ash which was expected for fly ash material with a lower calcium oxide (CaO wt. %) content of 4.06 wt. % such as the one used in this study. The study demonstrated a relatively effective storage of CO2 considering the lower CaO (wt. %) in the fly ash used. From material balance considerations, there was a higher concentration of undissolved CO2 (i.e., Σ𝐶𝑂!(#$%) 22.89 g) in the liquid phase from the total CO2 introduced into the reaction system (i.e., Σ𝐶𝑂!('() 36.97 g). The mass of dissolved CO2 was 14.08 g, and 2.08 g was stored as CaCO3. 0.068 MW was consumed from the process, and a high amount of the energy from the total energy output was due to the power for heating, which suggested that lower temperatures (0C) could be applied for the carbonation process. The energy consumption of 0.068 MW was relatively low due to the non-pre-treatment (i.e., crushing, grinding, sieving, etc.) of the FA. Effective neutralization of the AMD was achieved after 120 min of carbonation at a CO2 pressure of 4 Mpa for both direct and indirect carbonation with AMD. A pH of 7.1 was achieved under these conditions, which was close to a neutral pH of 7. The percentage (%) removal of most toxic elements was close to 100 % in all cases investigated, which suggests that most of the concentrations after treatment met the target water quality range (TWQR).