Recycling melanoidin by-products to promote green hydrogen and short-chain gaseous hydrocarbons recovery from thermal-hydrolyzed sludge pyrolysis: Unveiling the key role of active N species

With limited organic components conversion and deteriorated product quality, melanoidins (MLD) by-products represent an immense obstacle for the recycling of waste activated sludge resources during thermal hydrolysis and pyrolysis. This study taps MLD with underappreciated catalytic activity, eliciting the pyrolysis reactivity of low temperature-thermal hydrolyzed sludge. Notably, the optimum catalytic activity of 2.5 mg/g dry sludge (DS) MLD at 500 °C was confirmed, while 5–10 mg/g DS MLD exhibited activity-limited levels. 2.5 mg/g DS MLD mediated the methane catalytic cracking and methane dry reforming pathways, promoting hydrogen yield increased by 108.47 %, C₂–C₃ hydrocarbon (C₂H₄, C₂H₆, and C₃H₈) yields augmented by 15.87 %–325.77 %. The positive effect of MLD on gas production is weakened at a high temperature of 700 °C. The catalytic activity of MLD was demonstrated by the formation of aliphatic alkane and nitrogen-containing structures, in which active pyridinic-N specifically induced the transformation of protein-N and pyrrolic-N in biochar into unsaturated aromatic N in bio-oil, thereby evolving into small-molecule reducing gas. Critically, MLD triggered the synergistic activity of Si-, Al-, and Fe-containing minerals by modulating biochar's tighter aromatic porous structure and electron-accepting capacity, which accelerates the degradation of macromolecular proteins and polysaccharides by cleaving hydroxyl, carbonyl, ether bonds, and methylene, further improving the degradability of derived soluble organics. Finally, MLD facilitated the transformation of heavy components into light components, evolved into organic (aliphatic hydrocarbon, pyridine-N/pyrrole-N/nitrile-N) and aqueous compounds (aldehydes, acids, alcohols, and phenols), generating hydrogen cracked from protein and carbohydrate, and lipid and lignin contributed C₂–C₃ hydrocarbons via fragmentation and cyclization. This study opens a new avenue for finding the catalytic mechanism of MLD by-products and promoting the recovery of high-value energy during pyrolysis.