Mechanistic Insights into Temperature-Dependent Ester Formation in Sewage Sludge Pyrolysis Oil

Abstract

Although converting carbonaceous components in sewage sludge into value-added esters via pyrolysis presents significant resource recovery potential, the complex temperature-dependent interconversion and the limited molecular resolution of conventional analytical techniques have impeded mechanistic elucidation and selective regulation. Here, an integrated approach combining Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), paired mass distance (PMD) network analysis, interpretable machine learning, and density functional theory (DFT) calculations was employed to elucidate the transformation network of organooxygen species (OOSs) during sludge pyrolysis and enabled the proposal of a cascade pathway governing ester enrichment. Esters, unsaturated acids, saturated acids, and peptides/amides accounted for over 95.7% of total OOSs. Ester content decreased at 300–400 °C and regenerated at 400–500 °C, consistent with hydrolysis-esterification interconversion, defining a highly reactive temperature window. Within this interval, dominant interconversion pathways of peptides/amides ⇌ acids ⇌ esters were proposed, suggesting a cascade transformation pathway of peptides/amides → unsaturated acids → saturated acids → esters. The process yielded an additional net benefit of 164.6–474.6 CNY/t over conventional sludge disposal, with a carbon mitigation potential of 776.7 kg CO₂e/t. These findings provide a molecular-scale basis for engineering-directed process optimization of pyrolysis systems targeting selective ester production from organic solid wastes.