Coupling of hydrothermal pretreatment and supercritical water gasification of sewage sludge for hydrogen production

In this study, hydrothermal pretreatment and supercritical water gasification were coupled to form a combined process for the treatment of dewatered sludge for hydrogen production. First, the effects of varying hydrothermal pretreatment conditions on the transformation of organic matter in sludge were studied. Results showed that about 31% of the carbon in sludge was transferred into liquid products at 250 °C for 60 min, which were considered to be the optimal pretreatment conditions considering both the hydrothermal pretreatment effects and the energy consumption requirements. The organic matter components were determined, showing that 87% of the carbohydrate components in sludge were transformed during the process of hydrothermal pretreatment, with 49% of crude proteins and 62% of humus remaining in the solid phase products. During the subsequent process of supercritical water gasification, AlCl₃, KOH, K₂CO₃ and CaO were selected as catalysts. Compared with directly catalyzed supercritical water gasification of sludge, the integrated process was found to improve H₂ selectivity, H₂ yield and energy recovery. Moreover, the use of AlCl₃ as a catalyst showed the highest H₂ yield and energy recovery. The H₂ yield and the energy recovery increased by 45.1% and 13.2%, respectively.