Green Synthesis of Nanosilica from Coal Fly Ash and Its Stabilizing Effect on CaO Sorbents for CO2 Capture

High-temperature sorption of CO2 via calcium looping has wide applications in postcombustion carbon capture, sorption-enhanced hydrogen production, and inherent energy storage. However, fast deactivations of CaO sorbents and low CO2 uptake in the fast carbonation stage are major drawbacks of this technology. For the first time, we developed a green approach through the reuse of nanosilica derived from coal fly ash (CFA) to enhance both the cyclic CO2 uptakes and the sorption kinetics of CaO sorbents. The as-synthesized nanosilica-supported CaO sorbent showed superior cyclic stability even under realistic carbonation/calcination conditions, and maintained a final CO2 uptake of 0.20 g(CO2) g(sorbent)-1 within short carbonation time, markedly increased by 155% over conventional CaO sorbent. Significantly, it also exhibited very fast sorption rate and could achieve almost 90% of the total CO2 uptake within ∼20 s after the second cycle, which is critical for practical applications. These positive effects were attributed to the formation of larnite (Ca2SiO4) and the physical nanostructure of silica, which could yield and keep abundant reactive small pores directly exposed to CO2 throughout multiple cycles. The proposed strategy, integrating the on-site recycling of CFA, appears to be promising for CO2 abatement from coal-fired power plants.