Systematic understanding of char-volatile evolution and interaction mechanism during sewage sludge pyrolysis through in-situ tracking solid-state reaction and products fate

The complexity of sludge components and the heterogeneity of pyrolysis products make it challenging to trace char-volatile evolutions and interaction mechanisms during pyrolysis. Herein, we systematically dissected the solid-state reactions and volatile dynamic variations via in-situ infrared/mass spectral probes coupled signal amplification techniques. The identification of hidden reactions was further enhanced by comparing the discrepancies in the pyrolysis of three systems: raw sludge, sludge-extracted organics, and pseudo-components of organics. A three-stage sludge pyrolysis of bond cleavage (α = 0.2–0.5), intermediates diffusion (α = 0.5–0.7), and interface interaction (α = 0.7–0.8) was proposed through solid-state reaction tracing, and the pyrolysis reaction was found to be dominated by the first two stages. The generation of reactive intermediates accelerated the collision frequency between reactants, which increased the order of solid-state reactions and raised the energy barrier from 148 to 180–261–297 kJ/mol. The temperature-response sequence of the major pyrolysis volatiles was H2O/CO2/furans/alcohols (<250 °C), amine-N/acids/ketones/esters (250–350 °C), heterocyclic-N/phenols/C2–3 (300–400 °C), CH4/aromatics/nitrile-N (350–450 °C), and CO/HCN (>450 °C). The temperature-dependent evolution of these volatiles was consistent with the variations of chars in terms of pyrolysis behaviors, reaction models, and surface characteristics. The comprehensive understanding of the staged pyrolysis pathways and the char-volatile interaction mechanisms may provide critical information for pyrolysis procedure design and product targeted regulation.