• • At 700 °C, fly ash achieved a peak dechlorination efficiency of 93.33%, outperforming red mud (88.61%), but both declined with temperature, dropping to 65.6% and 58.27% at 900 °C, respectively, indicating a temperature-dependent operational window.
• • Alkali (NaOH) modification of fly ash significantly enhanced high-temperature performance: at 800 °C, efficiency rose from 71.9% (unmodified) to 94.98%, a 23.08% absolute improvement, enabling effective HCl capture at higher temperatures.
• • The modification mechanism involved increased surface roughness and porosity, as confirmed by SEM, and disruption of Si-O-Si and Si-O-Al networks, which exposed active sites and increased contact area for HCl adsorption.
• • Both fly ash and red mud demonstrated stable dechlorination across 600–900 °C, validating their feasibility as low-cost, abundant alternatives to conventional calcium-based agents, which suffer from severe efficiency losses above 700 °C.