Supercritical water partial oxidation of indole: Reaction variables, migration pathways, and nitrogen distribution

Supercritical water partial oxidation (SCWPO) is a clean and efficient organic waste resource utilisation method. Indole is an essential intermediate in the thermochemical conversion process of nitrogenous organic waste, and its conversion is often regarded as the rate-limiting step in these processes. However, the detailed reaction mechanism of the SCWPO process of indole is still unclear. This paper deeply analysed the gasification characteristics and the distribution of nitrogen in products during this process by an autoclave reactor. The results revealed that lengthening the reaction time, increasing the temperature, and lowering the feed concentration all promoted the formation of gaseous products. The indole was almost completely converted when the temperature, the reaction time, the feedstock concentrate, and the oxidant equivalent ratio were 750 °C, 30 min, 4 wt%, and 0.5, respectively. The excessive increase in the oxidant equivalent ratio could improve the carbon gasification efficiency and convert more organic nitrogen to N₂ while consuming a significant amount of H₂. Notably, no NOx was detected in the gaseous product, and the thermodynamic equilibrium data supported this conclusion. Finally, an SCWPO mechanism of indole was proposed based on the detailed analysis of experimental liquid intermediate products.