Subcritical and supercritical water gasification of humic acid as a model compound of humic substances in sewage sludge

Abstract

Humic acid is a model compound of sewage sludge that occurs as a result of decomposing organic matter in wastewater. In this study, humic acid gasification was performed at variable subcritical and supercritical water temperatures (325–600 °C), feed concentration (10–25 wt%) and reaction time (30–90 min). High H2 yield of 0.79 mol/kg was obtained at 600 °C, 15 wt% humic acid and 75 min without any catalyst. Catalysts such as K2CO3, Ni/Al2O3–SiO2, FeCl3 and ZnCl2 were examined to enhance H2 production and humic acid degradation. While H2 yield increased exponentially with rising FeCl3 loading, Ni/Al2O3–SiO2 and K2CO3 reduced H2 yield due to bimolecular condensation and catalyst deactivation. Maximum yield of H2 (4.09 mol/kg) and total gases (6.20 mol/kg) were obtained with 15 wt% humic acid and 15 wt% FeCl3 at 600 °C and 75 min. Elevated temperatures and high FeCl3loading promoted the degradation of humic acid with higher gas yields and fragmented surface morphology in char residues.