Conversion and migration of iron and nitrogen under co-hydrothermal carbonization of pomelo peel and dyeing sludge

Dyeing sludge contained a high content of iron, which of conversion and migration during co-hydrothermal carbonization (co-HTC) with biomass inevitably influenced the behaviors of other components and vice versa. In this study, the migration and conversion mechanism of Fe and N in the co-HTC process of Pomelo peel (PP) and dyeing sludge (DS) under conditions of various PP/DS mass ratios at 240 °C was analyzed by a series of characterizations. The nitrogen removal efficiency (NRE) of hydrochar was higher than the calculated value, the N content decreased, and some N migrated from the solid phase to liquid phase, showing a positive synergistic effect. The amino-N in the solid was converted into stable N compounds, pyridine-N (N-6), pyrrole-N (N-5), graphitic-N (N-Q) respectively. Cellulose and hemicellulose in PP were completely hydrolyzed to generate hydroxyl (-OH) and aldehyde (-CHO) groups by co-HTC, which promoted the reduction of Fe³⁺ due to the synergistic effect with protein, with the most obvious effect in DP2-8. At the same time, whether the Fe³⁺ migrated to the liquid phase was re-enriched on the solid surface depended on the decomposition and migration rates of secondary hydrochar, the PP ratio in feedstock affected Fe²⁺/Fe³⁺ ratio and Fe³⁺ migration in liquid. The co-HTC treatment of feedstocks enhanced the aromatization degree of hydrochars, resulting in improved combustion characteristics suitable for fuel applications.