Synthesis of SiO2-Modified CaO from the Aluminum Leaching Residue from Coal Ash for Its High CO2 Absorption Capacity

Carbon capture and storage (CCS) is a promising technology for mitigating CO₂ emissions, and Ca-based adsorbents are regarded as potential candidates for CO₂ capture. This study focuses on the synthesis of Ca-based adsorbents using an aluminum (Al) leaching residue from coal ash. Through hydrothermal treatment of the Al leaching residue, tobermorite with a high Ca ratio was synthesized. By employing CO₂-pressurized leaching followed by calcination at 900 °C, the fibrous SiO₂-modified CaO structure was synthesized. The synthesis of SiO₂-modified CaO and its CO₂ absorption property were investigated by TG (Thermogravimetry), SEM (Scanning Electron Microscopy), ICP (Inductively Coupled Plasma), and TEM (Transmission Electron Microscopy). The conversion of SiO₂-modified CaO was 93.8% during the first CO₂ adsorption, and the CaO conversion rate can still be maintained at 70.3% after 10 cycles. In comparison, pure CaO exhibited only a 42.9% conversion rate after 10 cycles. Due to the inhibiting effect of SiO₂ on sintering, the synthesized SiO₂-modified CaO showed a higher CO₂ absorption amount after 10 cycles. After 15 cycles, the absorption amounts of pure CaO and Ca-based adsorbents were 30.7 and 32.5 g of CO₂/(100 g of absorbent), respectively. After cycling, SiO₂-modified CaO still had a fibrous structure, and it had a smaller diameter and higher specific surface area than pure CaO. The special structure of tobermorite can inhibit the sintering of SiO₂-modified CaO and improve its cyclic stability in the process of CO₂ absorption at high temperature. The synthesis of SiO₂-modified CaO by using the Al leaching residue of coal ash offers novel approaches for treating coal-based wastes containing significant amounts of Ca and Si.