Mineralization characteristics of coal fly ash in the transition from non-supercritical CO2 to supercritical CO2

•Supercritical CO2 increases the mineralization efficiency of CaO powder by 2.09 times.•The diffusion depth in block CaO is 1.22 times higher at supercritical pressure.•H2O can activate the adjacent O atoms in CaO and increase adsorption of CO2 by CaO. The accelerated mineralization of fly ash is a potential way to achieve CO2 emissions reduction. However, the slow reaction at the diffusion control stage is the bottleneck. Due to the strong diffusion of supercritical CO2, it has been to strengthen mineralization, but the degree of improvement in mineralization efficiency and the changes that take place in the transition from non-supercritical to supercritical CO2 are not clear. In the mineralization process, CaO in the ash reacts with CO2 to form CaCO3. In order to exclude the influence of other alkaline oxides, powdered and block CaO were used to research the mechanism, and experiments were then carried out on samples of fly ash with different CaO content. Firstly, powdered CaO was used to conduct dry and wet mineralization experiments. In dry mineralization, the mineralization efficiency of the process changes in two stages as the pressure increases, first undergoing a gentle-increase and then a rapid one. Diffusion depth experiments on block CaO show that the supercritical diffusion depth is higher than the non-supercritical diffusion depth by a factor of 1.22, indicating that supercritical CO2 can improve the degree of mineralization in diffusion stage. The presence of water could promote mineralization. In wet mineralization, the efficiency of the process changes in three stages as the pressure increases, with first a gentle increase then a more rapid one and finally an attenuated rate of increase. The supercritical mineralization efficiency at 8 MPa was 55.27%, a factor of 2.09 larger than for non-supercritical mineralization at 3 MPa (26.39%). This is because that supercritical CO2 has increased the diffusion and greatly improved solubility in water. The results of DFT calculations show that H2O can promote the adsorption of CO2 by CaO, which is one of the reasons why water promotes mineralization. Finally, wet mineralization experiments on fly ash show that at 8 MPa, the mineralization efficiencies of HB, SD, SX and YN fly ashes are 1.94, 1.30, 2.03 and 1.62 times their values at 3 MPa. Although the difference in efficiency is affected by the CaO content, supercritical CO2 can effectively improve the mineralization efficiency.