Strategic valorization of sewage sludge via modified HZSM-5 catalyzed hydrothermal carbonization: Optimized pyrolysis of derived hydrochar

The global waste management crisis requires innovative valorization techniques to handle various complex waste streams. This study advances an innovative catalyst-mediated hydrothermal carbonization (HTC) process of sewage sludge (SS), followed by strategic pyrolysis (Py) of derived hydrochar (HC) into valuable energy carriers. At concentrations ranging from 1–7 wt.% for the modified HZSM-5 catalysts Pt/H₂SO₄/HZSM-5 (X) and H₂SO₄/HZSM-5 (Y), a comprehensive analysis of derived catalyzed HC was performed. XPS analysis indicated that the HC-X7 (7% loading of X) attained superior carbon (C) functionality, exhibiting 64.37% C–C bonds and an increased C content of 51.75%, thereby demonstrating improved aromatization efficiency compared to Y-based HC. Thermogravimetric profiles revealed distinct decomposition stages (56°C–800°C), with X-based HC demonstrating regulated degradation pathways characterized by well-defined DTG peaks. Temperature-dependent pyrolysis (500°C–900°C) of catalyzed HC yielded syngas rich in H₂ with a H₂/CO ratio of 0.57 at 900°C, while 700°C resulted in the maximum production of aromatic compounds with minimal oxygenated species. At 500°C, HC-X7 achieved exceptional H₂ yields (63.85 vol%), and at 900°C, the highest CO production (57.79 vol%), respectively, enabling precise control of the H₂/CO ratio. The X catalyst promoted selective hydrogenation pathways to inhibit the formation of polyaromatic hydrocarbons successfully. This integrated HTC-pyrolysis pathway transforms problematic waste into clean energy carriers, balancing an eco-friendly environment.