Melanoidins-derived active-N species alleviate endogenous minerals-initiated inhibitory effect during pyrolysis of waste activated sludge: Promoting methane and short-chain hydrocarbons recovery

Melanoidins (MLD) by-products and Fe₂O₃-endogenous minerals represent an irreversible inhibitory effect on waste activated sludge (WAS) resource regeneration during pyrolysis. However, MLD-Fe₂O₃ interactions for positive regulation of pyrolysis reactivity of organic components are overlooked. Here, MLD by-products are recycled to promote high-value gas recovery from WAS pyrolysis exposed to environmental level Fe₂O₃. 2.5–5 mg/g DS MLD weakens the inhibition of Fe₂O₃ at 500–700 °C, and alters Fe₂O₃-mediated methane catalytic cracking and methane dry reforming pathways, thereby enhancing CH₄ and C2–C3 hydrocarbon yields by 40.84% and 29.66–73.34%, respectively. The superior activity of MLD was confirmed by the formation of anionic, unsaturated, heterocyclic, and nitrogen-containing polymeric structures. MLD accelerated the thermal degradation kinetics by destabilizing the binding force of Fe₂O₃, which degraded 45.41% of WAS-endogenous organic carbon and inorganic components. Critically, active pyrrolic-N released from MLD improved the morphology and catalytic activity of biochar, further promoting the functional bond cleavage of volatiles and the cracking of N-containing components (labile protein-N and stable pyridinic-N) in the solid-phase, finally facilitating the degradability of biochar-derived soluble organics in the aqueous-phase. The above findings are attributed to the catalytic activity of biochar-endogenous Fe-containing species elicited by MLD. In addition, MLD exhibited a positive effect on the initial and secondary cracking of heavy components into light components in condensable phase, promoting the fracture of long-chain N-/O-/S-containing components and the formation of unsaturated aromatic hydrocarbons. This work provides a new avenue for the recycle of sludge-derived by-products and the upcycling of waste resources.