Stage-resolved fuel-N conversion and char-tar evolution during microwave and conventional pyrolysis of N-rich sewage sludge

Stage-matched framework reveals fuel-N pathway shifts. • Microwave shifts NH 3 and HCN release to lower temperature regions. • Microwave enriches amines and nitrile precursors in tar at lower temperatures. • Microwave promotes early char deoxygenation and condensation. • Polar O/N activation couples char-N reconstruction with tar-N evolution. Microwave-assisted pyrolysis shows promise for the clean conversion of N-rich biomass, yet its mechanism for regulating nitrogen migration remains unclear. Deashed sewage sludge and protein-derived amino acid models were employed to contrast microwave and conventional pyrolysis. This comparison utilized a reaction-stage-referenced framework based on the characteristic release regions of N-containing gases. Microwave heating did not change the basic nitrogen release pattern, with NH 3 as the dominant product and HCN as the secondary product, but substantially altered the temperature-dependent progression of the fuel-N conversion pathway. Under the present microwave experimental configuration, substantial fuel-N conversion was observed within the measured 190–300 °C range, occurring at lower measured temperatures than under conventional pyrolysis and accompanied by marked changes in O/N-containing structures. Amine intermediates were enriched in tar, and the tar-N fraction increased from about 44% to 87%, driving NH 3 release to lower temperatures. At 300–500 °C, these intermediates further underwent dehydrogenation, cyclization, and nitrilation under microwave effects, forming nitriles and low-carbon mono‑nitrogen compounds and enhancing HCN release around 300 °C. As a result, the characteristic NH 3 and HCN release regions were recorded at lower measured temperatures in the microwave system than in the conventional system. Meanwhile, microwave heating promoted early char deoxygenation and condensation, as reflected by protein-N conversion to pyrrolic-N, pyridinic-N, and nitrile-N, removal of C O and O-C=O groups, reduced interlayer spacing, and enhanced microcrystalline stacking. Tar evolved from heterocyclic and long-chain N-containing compounds toward light aromatics, oxygenated species, and low-carbon nitriles. A three-phase network is proposed wherein tar acts as an intermediate pool connecting char-N reconstruction with NH 3 /HCN release. This study provides mechanistic insights into fuel-N migration during microwave and conventional pyrolysis of N-rich sewage sludge. Download: Download high-res image (180KB) Download: Download full-size image