Phosphorus Transformation in Hydrothermal Pretreatment and Steam Gasification of Sewage Sludge

Sewage sludge is an important pool of phosphorus (P), while thermal disposal results in the enrichment of P into a final solid product. Hydrothermal treatment combined with steam gasification is an effective method to produce high-quality syngas from sewage sludge. This study discussed the transformation of P during this integrated thermochemical process based on thermodynamic calculation and the Standards, Measurements and Testing Programme analysis of solid products. The results showed that P was enriched in hydrochar with the concentration above 14 mg/g. By the hydrothermal pretreatment, organic phosphorus (OP) was significantly removed from the solid as a result of destruction, while inorganic phosphorus (IP) was mostly recovered. Part of OP was converted to IP in disposal from 220 to 240 °C. At 260 °C, destructed OP was emitted to the gaseous phase in the form of phosphorus oxide and, thus, not captured by minerals. In addition, inorganic phosphorus associated with oxides and hydroxides of Al, Fe, an...