Effects of fly ash properties on carbonation efficiency in CO2 mineralisation

Abstract

CO2 mineralisation by industrial wastes is a promising option for mitigating carbon emissions safely and permanently with low material cost. But there is still absence of a detailed understanding on how fly ash properties affect the carbonation reactions. To fill this knowledge gap, five coal combustion fly ashes, Beijing (BJ), Wuhai (WH), Hazelwood (HW), Yallourn (YA) and Loy Yang (LY) ashes, from China and Australia were selected for carbonation studies. Experiments were performed in a batch reactor at 40 and 140 °C with 20 bar initial CO2 pressure, 200 g/L solid to liquid ratio, 450 rpm stirring rate to compare the carbonation performance of the five fly ashes and the effect of fly ash properties on carbonation reactions. Then BJ, YA and HW ashes were then selected for further study in a wide temperature range (40–220 °C) because of their higher CO2 sequestration capacity than the other two ashes. Quantitative X-ray diffractometry (XRD) with Rietveld refinement was used to characterize the crystalline and amorphous phases in fresh and carbonated fly ashes. Scanning electron microscopy (SEM) with energy dispersive spectrometry (EDS) were used to characterize morphological and elemental properties of fresh and carbonated fly ash samples. Compared to LY and WH ashes, BJ, YA and HW ashes displayed much higher CO2 sequestration capacity due to the higher fraction of reactive Ca/Mg-bearing crystalline phases, including lime (CaO) and portlandite (Ca(OH)2) in BJ ash, periclase (MgO) and srebrodolskite (Ca2Fe2O5) in YA ash, and periclase and brucite (Mg(OH)2) in HW ash. Compared to YA and WH ashes, BJ ash displayed faster kinetics of carbonation reactions because the reactant phases of BJ ash were mainly Ca-bearing phases which have higher reactivity with CO2 than Mg-bearing phases. Also, particle size and morphological analyses indicated that the reacted particles displayed a lower porosity and pore area than the fresh sample due to the new precipitates not only depositing on the active surface, but also filling the pores of the fly ash particles, which was responsible for the reduced kinetics with time.