A two-step hydrothermal synthesis of fly-ash-based ZSM-5 zeolite and its catalytic properties in NO removal

Abstract

The pursuit of carbon peaking and carbon neutrality goals necessitates flexible operation and deep peaking of coal-fired units, thus placing heightened demands on nitrogen oxide (NO_X) emission control. Fly ash, a solid waste rich in silicon (Si) and aluminum (Al), should be fully leveraged, and the high-value synthesis of zeolite represents a viable utilization method. In this context, fly ash-based zeolite was prepared as a catalyst to facilitate the generation of active oxides from H₂O₂, combined with alkali solution absorption, to achieve efficient denitrification. This approach effectively transforms ‘waste to pollution’ mitigation. The optimal synthesis conditions for FA-ZSM-5 zeolite were established using the alkali fusion hydrothermal synthesis method, with fly ash as the raw material. The specified conditions were NaOH/FA = 1.2, SiO₂/Al₂O₃ = 40, H₂O/Al₂O₃ = 1325, TPAOH/Al₂O₃ = 10, ZSM-5-seed addition of 5 %, and a hydrothermal temperature and time of 140 °C for 24 h. Experimental parameters were meticulously investigated to ascertain their impact on nitric oxide (NO) removal efficiency. The highest removal efficiency was achieved under specific conditions: a reaction temperature of 140 °C, an H₂O₂ concentration of 6 %, a catalyst dose of 0.3 g, an NO concentration of 600 mg/m³, and an SO₂ concentration of 1000 mg/m³. Finally, analysis of the catalytic oxidation mechanism revealed that H₂O₂, when catalyzed by FA-ZSM-5 zeolite, generates ·OH radicals with strong oxidizing properties, thereby facilitating the oxidative removal of NO.