Evaluating pyrolytic synergy in co-processing South African sewage sludge and fine coal rejects

The disposal of large quantities of South African sewage sludge (DSS) and fine-coal rejects (CR) presents significant environmental and health challenges. This study investigated the co-pyrolysis of DSS and CR to explore their potential for waste valorization and clean energy production. The results demonstrated that pyrolysis temperature and feedstock composition significantly influenced the nature and yield of pyrolysis products. DSS produced oxygenated compounds—mainly fatty acids and phenols—at 500 °C due to its high lipid and protein content. These compounds were reduced at 700 °C through decarboxylation and aromatization reactions, aided by mineral matter. CR-derived tars exhibited higher aliphatic and aromatic hydrocarbon contents, originating from its aromatic carbon structure and secondary cracking. Temperature elevation from 500 °C to 700 °C led to increased yields of non-condensable gases, particularly H₂ and C₂–C₆ hydrocarbons. DSS-derived gases were rich in CO₂ at both temperatures due to its high oxygen content, while CR favoured the formation of H₂, CH₄, and light hydrocarbons under high-temperature conditions, reflecting intensified gas-phase reforming and cracking reactions. Co-pyrolysis of DSS-CR blends showed synergistic effects, particularly in 50–90% CR blends at 700 °C, which yielded light hydrocarbons and hydrogen-rich gas suitable for energy applications. This approach offers a sustainable waste management strategy supporting a circular economy model.