Integrating hydrothermal carbonization and chemical leaching to recover biogenic carbon from sewage sludge

With more than 10 million dry tons annually produced in the EU and a carbon content of approximately 30% db, sewage sludge (SS) can be a strategic source of biogenic carbon. However, the high moisture content and large amount of ash are strong barriers to sustainable valorisation. This study aims to assess the potential of hydrothermal carbonisation (HTC) as a sustainable alternative to sludge drying. Furthermore, it aims to test the integration of HTC and chemical leaching to convert sewage sludge into a leached hydrochar (LHC) useable as an alternative to fossil coal in steelmaking, and to extract inorganic compounds. Seven HTC tests were conducted at temperatures ranging from 180 to 270 °C and a residence time of 20 and 120 min. Hydrochar was chemically leached with HNO₃ at 70 °C for 1 h. On average, a solid mass yield of 62% was obtained by the HTC tests; the residence time didn't significantly affect the process performance and the hydrochar composition. In all tests, a significant portion of sludge carbon (between 25% and 38.5%) was found as TOC in the aqueous phase. After leaching, the ash content in the LHC was reduced by half. Extraction efficiencies of >95% for P and Ca and >80% for Fe were achieved. The LHC was more stable than raw hydrochar when subjected to gasification with CO₂ above 800 °C. Moreover, LHC showed potential as reducing agent in blast furnaces, with an average replacement ratio of 40%.