Catalyzed hydrothermal carbonization of sewage sludge: structural modification of hydrochar and its derived selective pyrolytic product distribution

Abstract

The pressing demand for sustainable alternatives to fossil fuels coupled with environmental risks associated with inappropriate sewage sludge (SS) disposal calls for innovative valorization strategies that transform waste into value-added products. This study introduces a novel approach by directly incorporating zeolite catalysts (HZSM-5 and USY) into the hydrothermal carbonization (HTC) of SS, followed by pyrolysis (Py) of the derived hydrochar (HC). Insights into derived HC are explored through comprehensive characterization, such as morphology, crystallinity, functionality, and thermal analysis. HZSM-5 significantly reduced the activation energy of HC from 27 to 5.5 kJ mol⁻¹, while increasing the structural disorder (ID/IG0.73). The selective production of CO and H₂ was achieved through temperature-dependent pyrolysis between 500 and 900 °C. HZSM-5 facilitated an increase in CO production to 54.18%, whereas USY boosted CO yield up to 35.6%. The optimal product distribution was achieved by strategically incorporating zeolite catalysts, allowing for precise control of N and O functionality and promoting selective syngas and chemical precursors yield. This innovative catalyst-mediated HTC-Py cascade offers unique control over pyrolytic products by introducing an efficient pathway for transforming problematic SS into green energy carriers, thus bridging the gap between environmental sustainability and feasible industrial utilization.