NH 3 Oxidation Pathways in the Presence of Sewage Sludge Ash: Focusing on the Mechanistic Roles of CaO and Fe 2 O 3

In recent years, the cocombustion of zero-carbon fuel NH3 with biomass-derived sewage sludge (SS) represents a promising pathway to reduce CO2 emissions. Ash produced during sewage sludge combustion has been identified as a major factor influencing the NH3 oxidation pathway. As pure reagents fail to reproduce the realistic effects of complex mineral interfaces in sewage sludge ash, a fixed-bed reactor system coupled to an Fourier-transform infrared (FTIR) gas analyzer was therefore established in this study. In combination with NH3-TPD, H2-TPR, electron paramagnetic resonance spectroscopy (EPR), X-ray photoelectron spectroscopy (XPS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterizations, the oxidation behavior of NH3 in the presence of authentic sewage sludge ash under different conditions was systematically investigated, with particular emphasis on elucidating the decisive roles of CaO and Fe2O3. The presence of SSA increased the NH3 conversion and shifted the oxidation pathways. CaO provided abundant Lewis acid sites that strongly adsorbed and activated NH3 to form NHX species, which preferentially reacted with O2, thereby governing the NO pathway under most conditions. At higher NH3 concentrations, Fe2O3 sustained Fe3+/Fe2+ redox and lattice-oxygen migration, promoting coupling between NHX and NO to N2. These results establish a foundation for the large-scale, efficient, and clean deployment of NH3 as a zero-carbon fuel.