Effect of modified Ca-Al adsorbents on heavy metal fixation during co-combustion of coal and coconut shells: Experiments and simulations

As an alternative fuel, bio-waste coconut shells have shown promise in reducing pollution during the co-combustion process. In this study, a novel metal-mineral adsorbent, Ca-Al (Ca₅Al₆O₁₄), was prepared and physically (ultrasonic) and chemically (ethylene glycol) modified to enhance its adsorption properties. Thermal adsorption experiments investigated the effect of the adsorbents on the fixation of six heavy metals (HMs): Cr, Cu, Mn, Ni, Pb, and Zn. XRD, SEM, BET, and XPS were used to analyze the adsorbents and the adsorption mechanism was investigated by combining lattice oxygen concentration and Factsage simulation calculations. The ecological risk assessment of heavy metals was used to comprehensively evaluate the fixation effects of the three Ca-Al adsorbents at different additive levels. The results showed that the modification significantly changed the morphological characteristics and oxygen activity of the Ca-Al adsorbents, increased the lattice oxygen and chemisorbed oxygen concentration, and laid the foundation for promoting the chemisorption process in the fixation of heavy metals. In combination with the E_r (environmental risk factors), adding all three adsorbents reduced R_i (Risk index). Among them, Ca-Al (EG) at 3% had the best effect on R_i. 3% Ca-Al (EG) reduced the E_r value of Ni by 49.02%. 5% Ca-Al (EG) reduced the E_r value of Cr by 86.01%. 5% Ca-Al (UM) had the best effect on Mn, with a reduction of 46.13%. The addition of 10% Ca-Al (UM) reduced E_r of Ni by 50.43%. Considering the practical application in coal-fired power plants, Ca-Al(EG), which exhibits a higher fixation rate at small additions, is more suitable.