Studies on sulfur releasing and migration during thermal pyrolysis of anaerobic digestate

Municipal food waste undergoes anaerobic fermentation for decomposing into usable biogas, and then partly into solid waste, anaerobic digestate. The thermal pyrolysis applications of digestate could further utilize its organic chemical energy. In this article, the pyrolysis processes of digestate and straws were respectively investigated in a drop-tube furnace at different temperatures. And sulfur migration during pyrolysis was analyzed by using all-sulfur test and X-ray photoelectron spectroscopy (XPS). Furthermore, thermo-gravimetric mass spectrometry (TG-MS) was used for tracking the gas-phase releasing of organic and inorganic sulfur. The main results include: 1) Higher sulfur content of 0.56% in the digestate waste due to anaerobic digestion process, as different from sulfur About 0.2% exists in the straw. 2) The sulfur in the digestate is released by about 60% and 85% at 400°C and 600°C, respectively. However, most of the sulfur in the biomass straw remains in the char. 3) During pyrolysis, the sulfur migration possess three processes: ① Sulphate generation within digestate occurs mainly at 200-400°C, while biomass straw mainly at 400-600°C; ② Sulphate undergoes thermal decomposition under 600-1000°C; and ③ The macromolecular organic sulfur (incl. sulphone, sulphoxide, thiophene) is dominant at 1000°C. It provides a certain mechanistic understanding of the sulfur releasing and migration process of anaerobic digestate.