High-temperature reaction chemistry of chromium and chlorine over Fe2O3 within sludge-incinerated fly ash

Abstract

Fe2O3 is one of the main compositions of sludge-incinerated fly ash and shows reactivity for chromium removal from flue gas in the incinerator. Experiments and density functional theory (DFT) calculations were performed to investigate the high-temperature reaction chemistry of chromium and chlorine over Fe2O3 within fly ash. Fe2O3 exhibits outstanding chromium removal efficiency of 86.29 % in N2 atmosphere. The presence of HCl obviously inhibits chromium removal, the inhibition effect increases with increasing HCl concentration. Fe-Cr composite oxides are the main products of chromium-Fe2O3 high-temperature reaction. Chromium within the reaction products in N2 or HCl-containing atmosphere mainly exists in the form of Cr3+. O2 addition leads to the formation of a small amount of Cr6+. Fe2O3 exhibits excellent inhibition performance of chromium oxidation. DFT calculations suggest that CrCl3 and CrO2Cl2 are chemically adsorbed on Fe2O3 surface. Both O and Fe atoms of Fe2O3 surface serve as active sites for chromium adsorption. HCl is dissociatively adsorbed on the Fe2O3 surface, and reacted with chromium to form CrCl3. CrCl3 formation on the Fe2O3 surface is dominated by a four-step reaction pathway: Cr(ads) → CrCl(ads) → CrCl2(ads) → CrCl3(ads) → CrCl3. CrCl formation is the rate-determining step with the energy barrier of 39.51 kJ/mol.