Ammonia (NH3) generated during sewage sludge gasification adversely affects syngas quality and causes catalyst poisoning in downstream processes. Previous studies indicate that sewage sludge-derived char holds potential for in situ NH3 removal. However, the underlying mechanism, particularly the synergistic effect between surface oxygen-containing functional groups and the main ash phases, remains unclear. In this study, fixed-bed experiments, multicharacterization analyses, and theoretical calculations were employed to elucidate the catalytic decomposition mechanism of NH3 over char at 200–900 °C. At low temperatures (200–500 °C), NH3 conversion is limited to 16.4% due to insufficient activation of the active sites. In contrast, at 900 °C, active sites are adequately activated, achieving a peak conversion of 99.2%. It was revealed that the formation of double hydrogen bonds between NH3 and the oxygen-containing functional groups inhibits the loss of these oxygen-containing functional groups, thereby promoting NH3 adsorption and subsequent decomposition. This effect is further strengthened by Fe2P/Fe, with Fe2P primarily facilitating adsorption and Fe contributing to catalytic decomposition. During the decomposition step mediated by Fe2P/Fe, electrons from N–H cleavage transfer to *H via the functional groups and carbon substrate, thereby promoting H2 formation. These findings provide theoretical support for optimizing the NH3 removal process in gasification.
Li et al. (Thu,) studied this question.