Heterogeneous kinetics of coal char gasification and combustion

Lead removal from flue gas continues to be an important environmental issue. Adding sorbents in the furnace is a reliable method to control lead emission. Modified kaolin sorbent developed by a combined method of thermal pre-activation and phosphate impregnation was first applied to capture lead from flue gas. Modified kaolin exhibited higher PbCl2 removal performance than raw kaolin at 800–1100 ℃. The best adsorption performance of raw and modified kaolin was obtained at 1000 ℃ with adsorption efficiencies of 76.72 % and 85.87 %, respectively. The adsorption capacity of modified kaolin was about 283.51 mg/g at 1000 ℃. Lead aluminosilicate and lead phosphate were produced in the adsorption reaction. O2 was conducive to the conversion of PbCl2 to PbO, and directly participated in the formation of lead aluminosilicate· H2O was in favor of the adsorption reaction, and reduced the energy barrier for dechlorination of PbCl2, and promoted the eutectic melting process. SO2 and NO inhibited PbCl2 removal due to the competitive adsorption between SO2/NO and PbCl2 at the active sites. High concentration of HCl can obviously suppress the reaction of PbCl2 and modified kaolin. Quantum chemistry calculations were conducted to identify the active sites, and to uncover the molecular-level interaction of PbCl2 and modified kaolin. Theoretical results manifested that PbCl2 adsorption was chemisorbed on kaolin (0 0 1) surface. The adsorption energies of PbCl2 on raw kaolin and modified kaolin were − 124.60 and − 205.90 kJ/mol, respectively. Al, O and P atoms of modified kaolin were identified as the active sites for PbCl2 adsorption.