Gasification of uric acid in supercritical water: Reaction characteristics, nitrogen transformation mechanisms, and kinetics analysis

Supercritical water gasification (SCWG) of biomass waste as a new clean waste conversion technology is increasingly valued worldwide attributable to the unique properties of supercritical water (water: T > 374 °C, P > 22.1 MPa). However, the fate of conventionally contaminated elements such as nitrogen in this process still warrants further research to control potential contamination. In this work, uric acid was used as the model compound of nitrogenous components in chicken manure to investigate the transformation mechanism of nitrogen during the SCWG process. The effects of different temperatures (550–650 °C) and residence times (2–30 min) on the gasification characteristic of uric acid and the behaviour of nitrogen were experimentally explored. The results indicated that the higher temperatures and longer residence time significantly facilitated the uric acid gasification and nitrogen transformation. The maximum nitrogen conversion efficiency was 84.83%, and the maximum yield of NH₃, H₂, and CO₂ reached 20.20 mol/kg, 23.78 mol/kg, and 20.10 mol/kg at 650 °C and 30 min, respectively. Subsequently, the reaction pathways of nitrogen in uric acid were proposed in detail according to the product analysis. Finally, a kinetic model was developed to describe the SCWG process, especially the reaction kinetic of nitrogenous products. The results revealed that the hydrolysis reactions of uric acid led to a rapid increase in NH₃ yield in the initial stage of the SCWG process. As the process continued, the increase in NH₃ was mainly due to the deamidation reaction of small-molecule nitrogenous intermediates. Notably, NOx, NO₂ ⁻, and NO₃ ⁻ were not detected in products, validating the inhibitory effects of the reducing environment of supercritical water on the generation of nitrogen oxides.