In-depth study of the sulfur migration and transformation during hydrothermal carbonization of sewage sludge

This study investigated the effect of temperature on sulfur transformation behavior during the hydrothermal carbonization of raw sludge, water washed sludge and the corresponding filtrate solution at 110–235 °C. The different sulfur species in the various products were determined, primarily focusing on the sulfur species present in the aqueous product. At 110 °C, 31.9% of sulfur migrated to the aqueous product, primarily in the form of sulfone-S and sulfoxide-S. As the temperature increased to 160 °C, the hydrolysis of aliphatic-S and aromatic-S was enhanced, and the former species further transformed to other organic-S or SO42-S in the aqueous product. High temperature is beneficial to the formation of sulfate-S and thiophene-S/aromatic-S in the aqueous and hydrochar products, along with the release of H2S and CH3SH. This caused a retention of only 29.2% sulfur (i.e., sulfate-S) in the hydrochar at 235 °C, with a large amount of sulfur migrating to the aqueous product (51.1%), followed by the gas product (9.1%). Among all the sulfur species in raw sludge, aliphatic-S was the most thermally unstable. SO42-S and sulfur-containing gas were primarily formed from the decomposition of water-insoluble aliphatic-S, while part of thiophene-S/aromatic-S in aqueous products was generated from water-soluble sulfone-S and sulfoxide-S at high temperatures of ≥160 °C. The results indicate that 210 °C is the optimal temperature for preparing of sludge-based fuel during hydrothermal carbonization, and water washing pre-treatment is a good strategy for enhancing the removal of sulfur in hydrochar and reducing the release of sulfur-containing odorous gas.