A laboratory-scale study of the aqueous mineral carbonation of coal fly ash for CO2 sequestration

Abstract

Mineral sequestration of waste materials provides a promising method for CO2 sequestration, due to its potential as a finishing step in industries which produce CO2 and alkaline solid by-products. However, a number of challenges in mineral carbonation that remain to be resolved, including overcoming the slow kinetics of mineral–fluid reactions, dealing with the large volume of source material required, and reducing the energy needed to hasten the carbonation process. In order to overcome the slow reaction kinetics, experiments on accelerated carbonation are being conducted worldwide. As a result, studies of the operational parameters of the carbonation reaction are progressing. The present study examined the effect of two operational parameters on the mineralization of Australian coal fly ashes for CO2 sequestration at laboratory scale. In this study, carbonation tests were carried out for three Australian coal fly ash samples (S1, S2, S3) inside a continuously stirred reaction chamber. Different water-to-solid ratios (from 0.1 to 1) and reaction temperatures (20–80 °C) were tested under a moderate initial CO2 gas pressure of 3 MPa, and the pressure drop due to carbonation with time was recorded until a constant pressure was achieved at the end of each test. The quantity of CO2 stored in each test was estimated by applying ideal gas law to the test conditions. The formation of carbonates during testing was confirmed by performing micro-structural analysis using scanning electron microscopy. According to the results, a 0.2–0.3 water-to-solid mix ratio recorded the highest sequestration potential for all three fly ashes, and was identified as the optimum for mineralization. The increase of reaction temperature resulted in a faster rate of initial CO2 transfer into the fly ash material but did not have a significant impact on the overall sequestration. Of the three tested ashes, S3 ash sample showed the highest sequestration potential of 27.05 kg of CO2 per ton of fly ash under test conditions. The results confirm the possibility of manipulating the water-to-solid mix ratio and the reaction temperature to enhance the carbonation reaction for mineral CO2 sequestration.