Synthesis of zeolites from circulated fluidized bed coal fly ash

Abstract

Circulating fluidized bed combustion (CFBC) is known for its ability to significantly reduce the emission of NO_x and SO₂ from coal combustion. However, the fly ash produced from CFBC (CFBFA) is impractical for use as a construction material due to its low pozzolanic activity. The present work evaluated two methods for zeolite synthesis from CFBFA, namely a low-temperature hydrothermal method and an alkali fusion followed by hydrothermal method, in order to provide another method to recycle CFBFA. Various synthesis conditions were systematically investigated to optimize the process. The low-temperature hydrothermal method led to the formation of a mixture of zeolites A and X, and the proportion of zeolite X increased with an increase in the NaOH/CFBFA ratio. However, with the assistance of ageing and seeding techniques, zeolitization of CFBFA was enhanced and zeolite A with a little zeolite X as an impurity was obtained. The alkali fusion followed by hydrothermal method, on the other hand, enabled the formation of high-quality and pure phase zeolite A (497 m² g⁻¹ BET surface area) when the NaOH/CFBFA ratio was 0.9 and fusion at 300 °C for 1 h was applied.