Synthesis of carbon dioxide adsorbents by zeolitization of fly ash

Coal-combusting power plants are simultaneously among the main emitters of carbon dioxide and the main generator of solid by-products, such as fly ash (FA). An attempt to find a common solution for these environmental problems is reported in the present research work. This study describes a synthesis procedure for zeolite X by interaction of FA, originating from Bulgarian lignite coals with sodium hydroxide (NaOH), followed by hydrothermal treatment. The sorption capacity of the so-obtained zeolite with respect to carbon dioxide is estimated by using the thermogravimetric method. Ash residues, containing 52.66 mass% of SiO2 and 23.37 mass% of Al2O3, were subjected to a dual-stage fusion-hydrothermal conversion. Coal ash and NaOH mixture at a ratio of 1:2 was sintered at 550 °C, and after dilution with water under continuous stirring, the reaction mixture was charged into an autoclave for hydrothermal reaction occurring for 2 h at 90 °C. The obtained zeolite X was characterized by scanning electron microscopy and X-ray diffraction, and it was subjected to thermogravimetric analysis before and after exposure to carbon dioxide flow. Determination of its adsorption capacity toward CO2 was carried out at 22 °C, it was measured to be 60 mg g−1 at this temperature, and the adsorption–desorption equilibrium was reached after 1 h under flow conditions (30 mL min−1). The sorption capacity of fly ash-based zeolite was compared to that of a reference zeolite X, synthesized from pure starting materials.