Utilization of low-calcium fly ash via direct aqueous carbonation with a low-energy input: Determination of carbonation reaction and evaluation of the potential for CO sequestration and utilization.

Environmental impacts from coal-fired power generation that produces large amounts of CO and fly ash are of great interest. To reduce negative environmental impacts, fly ash utilization was investigated via a direct aqueous carbonation with a low-energy input in which the alkali calcium content in the fly ash reacted with CO to form carbonate. Raw fly ash was characterized to understand the potential for direct aqueous carbonation of fly ash. The performance of the fly ash as a calcium source for direct aqueous carbonation at atmospheric pressure was investigated for different solid-liquid ratios and introduced CO concentrations. Variations in fly ash elemental composition, reaction solution pH, CO concentration in the reactor outlet, CO uptake efficiency, CaCO content and degree of carbonation were used to illustrate this process reaction. The maximum CO uptake efficiency was ~0.016 g-CO/g-fly ash. This value was compared with previous studies, and the CO uptake efficiency was comparable despite the use of a low-energy input method, i.e., direct aqueous carbonation with atmospheric pressure and unconcentrated CO. The calculated maximum degree of carbonation was 31.0%, which corresponds to 0.0063 g-CO/g-fly ash. Carbonated product characterization confirmed the carbonation reaction mechanism and safety for further utilization. A comparison of CO uptake efficiency in this work with previous work, and considering the energy input and reactive species content, is provided. An assessment of the CO reduction potential is provided.