Char and tar formation during hydrothermal treatment of sewage sludge in subcritical and supercritical water: Effect of organic matter composition and experiments with model compounds

Abstract

Abstract Hydrothermal treatment of sewage sludge is considered to be a promising environmentally friendly technology which can transform the organic matter in sludge into clean energy. However, the char and tar produced by side reactions will inhibit reaction. Five model compounds (humic acid, glutamic acid, glycerol, guaiacol, glucose) representing the main organic components (humus, proteins, lipids, lignin, and carbohydrates) in sewage sludge were treated in an autoclave at 300–400 °C for 30 min, the influence of the organic component on char/tar formation was studied. The char yield of humic acid was highest, up to 69.8(±3.1)%, followed by glucose. The tar yield of glycerol and guaiacol was highest in subcritical water and supercritical water, respectively. To further evaluate the utilization value of char/tar or to give a reasonable strategy for char/tar inhibition, the char/tar was characterized by elemental analysis, fourier transform infrared spectroscopy, thermosgravimetric analyzer, scanning electron microscope, and gas chromatography-mass spectrometer. The char/tar arising from the different model compounds exhibit differences in properties such as calorific value, thermal stability, and surface structure, which were due to the different pathways involved in their formation. The results provide the underlying insights needed to guide follow-up treatments or utilization of the char and tar produced during hydrothermal treatment of sewage sludge.