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

The multiple influence mechanisms of Fe 2 O 3 on nitrogen evolution during sewage sludge pyrolysis were systematically investigated by correlating temperatures. The results showed that Fe 2 O 3 promoted the pyrolysis of the sludge. The activation energy of the pyrolysis was reduced by 12–110 kJ/mol. Fe 2 O 3 acted oxidatively on sludge nitrogen in the following order Fe 2 O 3 →Fe 3 O 4 ( ca. 528 K)→Fe x N y ( ca. 763 K)→Fe x C ( ca. 1023 K). The oxidation target of Fe 2 O 3 on nitrogen containing compounds varied as a function of temperature. At temperatures of 528 K and 623 K, Fe 2 O 3 hindered the deamination of proteins in char to NH 3 , facilitated the condensation of pyridines to nitrile, and impeded the generation of HCN. At temperatures of 763 K and 1023 K, Fe 2 O 3 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 NH 3 and HCN at the front. Overall, Fe 2 O 3 inhibited the release of NOx precursors through inhibition of deamination, fixation of nitrogen in char and consumption of NH 3 and HCN. This study will establish the effect mechanism of action of Fe 2 O 3 on the nitrogen conversion of sewage sludge pyrolysis at relevant temperatures. It will provide a theoretical foundation for controlling NOx and the targeted regulation NH 3 (carbon-free hydrogen energy carriers) in the high-value conversion of nitrogen-rich biomass.