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 900 °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. Co-pyrolysis of DSS-CR blends showed synergistic effects, particularly in 50–90 % CR blends at 900 °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. • DSS, CR and their blends heated to 500 - 900 °C for 10 minutes under N 2 to yield pyrolytic-products. • DSS produced tars at 500 °C containing higher oxygenated-compounds contents. • CR-derived tars exhibited a higher aliphatic and aromatic hydrocarbons contents. • Temperature elevation enhances deoxygenation, aromatization, and hydrocarbon formation. • Feedstock blending enables compositional tailoring and selectivity of pyrolysis products. • The integration of DSS and CR in pyrolysis processes offers a route for a circular economy.