In-Situ Catalytic Pyrolysis of Polystyrene with Sewage Sludge-Derived Catalysts: Sewage Sludge Ash, Sewage Sludge Char, and Fe-Impregnated Acid-Leached Sewage Sludge Char

Sewage sludge (SS)-derived materials were investigated as low-cost heterogeneous catalytic materials for polystyrene (PS) pyrolysis, focusing on the effects of thermal treatment, HCl leaching, and Fe impregnation. Sewage sludge ash (SSA), sewage sludge char (SSC), and HCl-leached Fe-impregnated sewage sludge char (Fe-LSSC) were evaluated as in-situ catalytic materials. Catalyst-to-PS ratios of 1:1–1:10 were investigated in a horizontal semi-batch tubular reactor operated in an in-situ catalyst–polymer contact mode at 800 °C. Thermal pyrolysis produced 63.9 wt.% PyOil and 12.6 wt.% PyGas. SSA increased the PyOil yield to 78.0 wt.% at 1:10 and promoted lighter aromatics; at 1:1, light aromatics increased to 66.8 wt.%, BTEX from 5.8 to 24.58 wt.%, and PAHs decreased to 11.8 wt.%. SSC showed a weaker, non-monotonic effect, with PyOil decreasing to 54.4 wt.% and PyChar increasing to 4.7 wt.% at 1:2. Fe-LSSC produced the strongest changes, increasing PyGas to 28.0 wt.% at 1:1 and H2 to 75.3 vol.% at 1:2. The gasoline-range fraction reached 82.1 wt.% at 1:2, while PAHs decreased to 8.79 wt.% at 1:1. Catalyst reuse altered gas composition, whereas thermal regeneration partially restored catalytic behavior. Overall, SS-derived materials exhibited distinct, loading-dependent catalytic functions, while HCl leaching and Fe impregnation enhanced the conversion of PS pyrolysis vapors toward lighter aromatic and gaseous products.