The selective removal of HCN by Fe2O3 in reforming of syngas from wet sludge pyrolysis

Pyrolysis has emerged as a promising method for treating sewage sludge (SS) while producing hydrogen-rich syngas. However, the process generates hydrogen cyanide (HCN), a highly toxic gas. This study investigates the selective removal of HCN using Fe2O3, a widely used oxygen carrier, through reforming experiments, thermodynamic analysis, and first-principles calculations. The Gibbs free energy minimization approach indicates that HCN oxidation is more likely to form N2 rather than NOx, even with an excess of Fe2O3. Experiments conducted in the fixed-bed reactor revealed that increasing the Fe2O3 to SS mass ratio from 0 to 1:3 completely suppressed HCN release and reduced NH3 yield by 21.0 %, further corroborating the selective oxidation of HCN by Fe2O3. Density functional theory calculations showed that the chemisorbed HCN undergoes a dissociative oxidation process: HCN∗ → CN∗ → NCO∗ → N∗ → N2, with the third step being rate-determining. The presence of H2O in the atmosphere facilitated the oxidation of CN∗ by providing OH∗ instead of surface oxygen. DFT calculations suggest that any potentially formed NO react with NCO∗ produced during HCN dissociation, resulting in N2 as the final nitrogen product with an energy barrier of 61.33 kJ/mol. This study provides fundamental insights into inhibiting nitrogen-containing pollutants during sludge reforming.