Hydrothermal carbonization of model compounds of municipal sludge: Effects of reaction conditions and aqueous phase recirculation

In this study, lauric acid (fatty acid), glutamic acid (amino acid), and cellulose (carbohydrate) were employed as model compounds of municipal sludge (MSM) to investigate the effects of hydrothermal carbonization (HTC) conditions (temperature and time) and aqueous phase recirculation (APR) on the properties of hydrochar and process liquid. Increasing temperature and prolonging reaction time promoted the formation and transformation of organic acids and nitrogenous compounds, leading to a decrease in hydrochar yield. At 270 °C for 60 min, the hydrochar exhibited a higher N/C ratio (0.019), indicating enhanced nitrogen retention. This condition was selected for four cycles of APR. With increasing recirculation cycles, the hydrochar N/C ratio (0.019–0.050) further decreased, while the yield (24.3 %–22.5 %) slightly declined. APR enhanced reactions such as aromatization and Maillard reaction, increasing the content of O-containing functional groups in the hydrochar and accumulating more N-derived compounds, thereby accelerating the HTC process. Under APR, the N-doped hydrochar demonstrates improved potential as a functional carbon-based material, and the HTC liquid rich in organic matter shows broad prospects for use as aqueous carbon source.