Unveiling the complex effect of Fe2O3 on NOx precursors evolution mechanism during sludge protein pyrolysis based on product characteristics

Abstract

The multiple influence mechanisms of Fe2O3 on nitrogen evolution during sewage sludge pyrolysis were systematically investigated by correlating temperatures. The results showed that Fe2O3 promoted the pyrolysis of the sludge. The activation energy of the pyrolysis was reduced by 12–110 kJ/mol. Fe2O3 acted oxidatively on sludge nitrogen in the following order Fe2O3→Fe3O4 (ca. 528 K)→FexNy (ca. 763 K)→FexC (ca. 1023 K). The oxidation target of Fe2O3 on nitrogen containing compounds varied as a function of temperature. At temperatures of 528 K and 623 K, Fe2O3 hindered the deamination of proteins in char to NH3, facilitated the condensation of pyridines to nitrile, and impeded the generation of HCN. At temperatures of 763 K and 1023 K, Fe2O3 facilitated the cleavage of amines, heterocycles, and nitrile in char. Additionally, it promoted the breaking of chains and cyclization of long chain amines in tar. This process theoretically leads to the production of a great amount of NH3 and HCN at the front. Overall, Fe2O3 inhibited the release of NOx precursors through inhibition of deamination, fixation of nitrogen in char and consumption of NH3 and HCN. This study will establish the effect mechanism of action of Fe2O3 on the nitrogen conversion of sewage sludge pyrolysis at relevant temperatures. It will provide a theoretical foundation for controlling NOx and the targeted regulation NH3 (carbon-free hydrogen energy carriers) in the high-value conversion of nitrogen-rich biomass.